Inclusion Body Myositis: A Degenerative Muscle Disease Associated with Intra‐Muscle Fiber Multi‐Protein Aggregates, Proteasome Inhibition, Endoplasmic Reticulum Stress and Decreased Lysosomal Degradation

Sporadic inclusion body myositis (s‐IBM), the most common muscle disease of older persons, is of unknown cause, and there is no enduring treatment. Abnormal accumulation of intracellular multi‐protein inclusions is a characteristic feature of the s‐IBM phenotype, and as such s‐IBM can be considered a “conformational disorder,” caused by protein unfolding/misfolding combined with the formation of inclusion bodies. Abnormal intracellular accumulation of unfolded proteins may lead to their aggregation and inclusion body formation. The present article is focusing on the multiple proteins that are accumulated in the form of aggregates within s‐IBM muscle fibers, and it explores the most recent research advances directed toward a better understanding of mechanisms causing their impaired degradation and abnormal aggregation. We illustrate that, among other factors, abnormal misfolding, accumulation and aggregation of proteins are associated with their inadequate disposal—and these factors are combined with, and perhaps provoked by, an aging intracellular milieu. Other concurrent and possibly provocative phenomena known within s‐IBM muscle fibers are: endoplasmic reticulum stress and unfolded protein response, mitochondrial abnormalities, proteasome inhibition, lysosome abnormality and endodissolution. Together, these appear to lead to the s‐IBM‐specific vacuolar degeneration, and muscle fiber atrophy, concluding with muscle fiber death.