Protein Quality Control Systems of the Thick Filament Protein Myosin in Zebrafish Skeletal Muscle Assembly and Maintenance

Skeletal and cardiac muscle function is dependent on the proper assembly and maintenance of the smallest contractile unit of striated muscle, the sarcomere. The sarcomere is composed of hundreds of proteins and associated factors, each of which must be properly folded, inserted into the sarcomere, and monitored for damage. Systems that perform each of these roles are critical for muscle homeostasis are called protein quality control systems. Although protein quality control systems have been widely studied in neurogenerative diseases, they are understudied in muscle. Here I hypothesize that different protein quality control systems respond to the thick filament protein, myosin, when it is damaged before the sarcomere has assembled, or after sarcomere assembly is complete. Using zebrafish as a model system, I show that the mutant steif fails to assemble myosin thick filaments, while the mutant herzschlag assembles thick filaments that degenerate due to contraction induced damage. Using immunofluorescence, hematoxylin and eosin staining, and transmission electron microscopy, I show that herzschlag mutants have a specific loss of slow muscle tissue. By pharmacologically inhibiting the proteasome, I show that this loss is dependant on the ubiquitin proteasome system. Similar analysis in steif mutants reveals that these fish differ in their ability to tolerate proteasome inhibition, and qPCR of protein quality control factors shows different expression profiles in these fish. Together, this work suggests that different protein quality control mechanisms respond to myosin damage depending on the developmental stage in which it occurs. This work has implications in the treatment of striated muscle myopathies, specifically in the identification of druggable therapeutic targets.