Protein folding factors (chaperones) are required for many diverse cellular functions. In striated muscle, chaperones are required for contractile protein function, as well as the larger scale assembly of the basic unit of muscle, the sarcomere. The sarcomere is complex and composed of hundreds of proteins and the number of proteins and processes recognized to be regulated by chaperones has increased dramatically over the past decade. Research in the past ten years has begun to discover and characterize the chaperones involved in the assembly of the sarcomere at a rapid rate. Because of the dynamic nature of muscle, wear and tear damage is inevitable. Several systems, including chaperones and the ubiquitin proteasome system (UPS), have evolved to regulate protein turnover. Much of our knowledge of muscle development focuses on the formation of the sarcomere but recent work has begun to elucidate the requirement and role of chaperones and the UPS in sarcomere maintenance and disease. This review will cover the roles of chaperones in sarcomere assembly, the importance of chaperone homeostasis and the cooperation of chaperones and the UPS in sarcomere integrity and disease.

The structure and function of the sarcomere of striated muscle is well studied but the steps of sarcomere assembly and maintenance remain under-characterized. With the aid of chaperones and factors of the protein quality control system, muscle proteins can be folded and assembled into the contractile apparatus of the sarcomere. When sarcomere assembly is incomplete or the sarcomere becomes damaged, suites of chaperones and maintenance factors respond to repair the sarcomere. Here we show evidence of the importance of the M-line proteins, specifically myomesin, in the monitoring of sarcomere assembly and integrity in previously characterized zebrafish muscle mutants. We show that myomesin is one of the last proteins to be incorporated into the assembling sarcomere, and that in skeletal muscle, its incorporation requires connections with both titin and myosin. In diseased zebrafish sarcomeres, myomesin1a shows an early increase of gene expression, hours before chaperones respond to damaged muscle. We found that myomesin expression is also more specific to sarcomere damage than muscle creatine kinase, and our results and others support the use of myomesin assays as an early, specific, method of detecting muscle damage.

In order to maintain homeostasis, defective proteins must be identified, removed, and replaced. In skeletal muscle, failures in this process can lead to a variety of incurable muscle diseases (myopathies), many of which are fatal. While aspects of protein turnover have been studied widely, the protein turnover process in skeletal muscle is still incompletely understood, especially with respect to the motor protein, myosin. By studying the assembly and maintenance of the basic unit of striated muscle, the sarcomere, we aim to elucidate the components involved and the mechanism controlling muscle myosin quality control in skeletal muscle.Zebrafish make an excellent model system to study sarcomere protein turnover due to their ability to survive with myopathies that are fatal in other organisms. Using two different zebrafish lines, steif, and herzschlag, we and others show that steif sarcomeres fail to assemble myosin thick filaments due to a loss of a critical component (Etard et al., 2007), while herzschlag sarcomeres degrade over time as a consequence of muscle contractions. Transmission electron microscopy coupled with H&E staining revealed a slow muscle atrophy specific to herzschlag embryos. To identify factors involved in response to myosin damage during assembly or maintenance, we have screened through candidate genes, myosin chaperones and muscle specific E3 enzymes, using in situ hybridization and qPCR. Our data indicates a division of function where specific E3 enzymes and myosin chaperones respond to myosin damage exclusively during assembly, while other E3 enzymes respond to myosin damage during sarcomere maintenance. Pharmaceutical inhibition of the proteasome shifts this response in steif embryos, but not herzschlag, suggesting an interconnected network of protein quality control during myosin assembly. Together, this work helps highlight potential therapeutic targets to treat myosin turnover related myopathies.Support or Funding InformationNSERCThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

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