Regulation and Maintenance of Cardiac Muscle Sarcomere Integrity

描述（由申请人提供）：心脏肌节是一种复杂且高度有序的收缩和调节蛋白集合体，旨在产生力。为了维持功能性肌节，需要精确的蛋白质周转，以平衡新蛋白质的合成和并入肌节与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，在维持肌节结构功能完整性的同时，单个肌节组分的独立周转发生的机制还不清楚。几项研究表明，许多肌丝蛋白，包括肌动蛋白和肌球蛋白，与细胞质前体池处于动力学平衡，这表明磨损的肌丝蛋白不断取代其前体进入一个完整的肌节。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质交换变得不可能。首先，维持高分子量蛋白质的前体库对肌细胞来说是一个不断增加的能量消耗。另一个问题是，与在现有肌节中不断回收的较小肌丝蛋白不同，肌原纤维大分子复合物的周转可能需要肌节的部分或完全分解。这些考虑使我们的注意力集中在调节巨肌丝蛋白肌联蛋白周转的分子事件上。肌联蛋白是肌节复合物的组成部分，用作（1）肌球蛋白和其他结构和信号蛋白围绕其组装的分子模板，以及（2）赋予心脏肌原纤维硬度的分子弹簧。因此，我们假设肌联蛋白的降解将引发肌节的局部解体。基于初步的数据，我们提出，氧化损伤肌联蛋白触发顺序降解的钙蛋白酶和泛素-蛋白酶体系统。我们认为肌小节机械感受器在肌联蛋白降解过程中被激活，并移位到细胞核中激活肌联蛋白基因的转录。我们建议肌联蛋白mRNA的靶向肌节和局部肌联蛋白的合成发生与同时重组的肌节。我们最后提出，心肌肥大调节肌联蛋白的转录和翻译途径，导致净肌节添加。提出的实验目标将使我们能够评估和完善该模型，并促进我们对肌丝肌节更替的复杂生理和时间方面的理解。 公共卫生相关性：心肌肌节是心脏的基本收缩单位，由肌丝和调节蛋白组成，旨在产生力。一般认为，维持功能性肌节需要精确控制肌丝蛋白的合成、组装和降解，然而，介导大型整合肌丝蛋白（如肌联蛋白）周转的机制尚未研究。了解肌联蛋白蛋白周转的分子机制将提供深入了解肌节维持的调节过程，并可能最终导致心力衰竭心肌病的新的治疗方法。

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. PUBLIC HEALTH RELEVANCE: 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.