The cardiac sarcomere is a complex and highly ordered ensemble of contractile and regulatory proteins designed to generate force. To maintain functional sarcomeres, precise turnover of proteins is required that balances new protein synthesis and incorporation into the sarcomere with removal and degradation of worn out or damaged proteins. Given the heterogeneity in protein turnover rates, the mechanisms by which independent turnover of the individual sarcomere components occurs while maintaining the functional integrity of the sarcomere structure, is not well understood. Several studies have shown that a number of myofilament proteins, including actin and myosin, are in kinetic equilibrium with a cytoplasmic precursor pool, suggesting continual replacement of worn out myofilament proteins for their precursors into an otherwise intact sarcomere. As the size of the protein increases, however, several problems arise that make simple sarcomere protein exchange improbable. For one, the maintenance of a precursor pool of high molecular weight proteins comes at an increasing energetic cost to the myocyte. Another problem is that unlike smaller myofilament proteins that are continually recycled in the existing sarcomere, turnover of myofibrillar macromolecular complexes likely requires either partial or complete disassembly of the sarcomere. These considerations have focused our attention on the molecular events regulating the turnover of the giant myofilament protein titin. Titin is an integral part of the sarcomere complex, serving as (1) a molecular template around which the myosins and other structural and signaling proteins assemble, and (2) a molecular spring to impart myofibrillar stiffness to the heart. We therefore postulate that the degradation of titin will trigger local disassembly of the sarcomere. Based on preliminary data, we propose that oxidative damage to titin triggers sequential degradation by the calpains and ubiquitin-proteasome system. We propose that sarcomere mechanosensors are activated during the process of titin degradation, and translocate to the nucleus to activate titin gene transcription. We propose that titin mRNA is targeted to the sarcomere and that localized titin synthesis occurs with concurrent reassembly of the sarcomere. We finally propose that cardiac hypertrophy modulates titin transcription and translation pathways leading to net sarcomere addition. The proposed experimental aims will allow us to evaluate and refine this model, and advance our understanding of the complex physiological and temporal aspects of myofilament sarcomere turnover.
The cardiac sarcomere is the basic contractile unit of the heart and is composed of an ensemble of myofilament and regulatory proteins designed to generate force. It is generally assumed that maintenance of functioning sarcomeres requires precise control of synthesis, assembly, and degradation of myofilament proteins, however, the mechanisms that mediate turnover of large integrated myofilament proteins, such as titin, have not been looked at. Understanding the molecular mechanisms of titin protein turnover will provide insight into the regulatory processes of sarcomere maintenance, and may ultimately lead to novel therapeutic approaches for heart failure cardiomyopathies.
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