Abstract
Desmin is a 53 kDa muscle-specific intermediate filament protein, which forms extrasarcomeric cytoskeleton that interconnects myofibrils and links the contractile apparatus to mitochondria, nuclei and intercellular junctions. Mutations in desmin cause desmin-related myopathy (DRM), a rare and devastating genetic disorder affecting both skeletal and cardiac muscles. DRM is characterized by intrasarcolemmal aggregates of granulofilamentous material containing predominantly desmin. Since that initial discovery, mutations in the small heat shock protein B-crystallin was found to phenocopy DRM caused by desmin mutation, providing the first genetic evidence in support of a functional interaction of B-crystallin and desmin. In this study, we used a variety of biochemical assay to show that B-crystallin can interact with desmin. In addition, we generated polyclonal antibodies specific to the N-terminal fragment of desmin (N-desmin) generated by caspase 6 cleavage. Using this antibody, N-desmin was detected in cells undergoing apoptosis. In vitro assembly studies had demonstrated that the N-desmin was unable to form 10-nm intermediate filaments, but instead form electron-dense aggregates. Coassembly studies showed that N-desmin interfered with the normal assembly of intact desmin in a way that promoted filament aggregation. Taken together, these data suggest caspase-mediated cleavage of desmin produces N-terminal proteolytic fragment that is prone to aggregation. Furthermore, N-desmin and other disease-causing mutations interfere intermediate filaments networks and mitochondrial phenotypes in cells. This study provides a firm and testable foundation to the hypothesis that the N-terminal desmin is pro-apoptotic, affecting mitochondria function that compromises cell viability as key part of the pathophysiology of DRM.