Regulation and Maintenance of Cardiac Muscle Sarcomere Integrity

描述(申请人提供)：心肌肌节是一个复杂和高度有序的收缩和调节蛋白质的集合，旨在产生力量。为了维持肌节的功能，需要蛋白质的精确周转，以平衡新的蛋白质合成和并入肌节，以及磨损或受损蛋白质的去除和降解。鉴于蛋白质周转率的异质性，单个肌节成分的独立周转同时保持肌节结构的功能完整性的机制尚不清楚。一些研究表明，包括肌动蛋白和肌球蛋白在内的许多肌丝蛋白与细胞质前体池处于动态平衡状态，这表明磨损的肌丝蛋白作为其前体不断地被替换到一个原本完整的肌节中。然而，随着蛋白质大小的增加，出现了几个问题，使得简单的肌节蛋白交换变得不太可能。首先，维持高分子量蛋白质前体的成本增加了心肌细胞的能量成本。另一个问题是，与在现有肌节中不断循环的较小肌丝蛋白不同，肌原纤维大分子复合体的周转可能需要部分或完全分解肌节。这些考虑将我们的注意力集中在调节巨型肌丝蛋白titin周转的分子事件上。肌动蛋白是肌节复合体不可或缺的一部分，起着(1)分子模板的作用，肌球蛋白和其他结构和信号蛋白围绕着它进行组装，(2)给心脏带来肌原纤维硬度的分子弹簧。因此，我们假设肌动蛋白的降解将引发肌节的局部解体。根据初步数据，我们认为肌动蛋白的氧化损伤通过钙蛋白酶和泛素-蛋白酶体系统触发了顺序的降解。我们认为肌节机械传感器在肌动蛋白降解过程中被激活，并移位到细胞核以激活肌动蛋白基因的转录。我们认为，Titin mRNA是以肌节为靶点的，并且肌节的局部合成伴随着肌节的重组。我们最后提出，心肌肥厚可以调节肌动蛋白的转录和翻译途径，导致肌节的净增加。提出的实验目标将使我们能够评估和完善这一模型，并促进我们对肌丝肌节翻转复杂的生理和时间方面的理解。

DESCRIPTION (provided by applicant): 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.