Modulating stretch activation to restore muscle and heart function

描述(由申请人提供)：拉伸激活(SA)是一种内在的肌节特性，可以增加肌肉力量、力量和经济性。SA在有节奏地延长和缩短的肌肉类型中最为突出，例如心脏和昆虫飞行肌肉。快速增加具有显著SA的肌肉的长度会导致力在钙激活力之上的延迟跳跃。这种力量增加背后的机制尚不清楚，因此限制了我们对基本肌肉属性的理解。我们的长期目标是应用从学习SA机制如何调节力、功率和肌肉效率中获得的见解来帮助设计恢复受损肌肉功能的方法。这一应用的直接目标是阐明SA背后的肌瘤机制。我们的中心假设是，SA可能是由任何肌节机制引起的，这些机制增加了拉伸后强结合交叉桥的总数。我们认为，至少有两种机制可以实现这一点。在中度SA肌肉类型中，这种增加是基于肌球蛋白的机制，而在高度SA肌肉类型中，这种增加是需要细丝机制的。我们的假设是基于我们的新的初步数据，即肌球蛋白亚型交换增加了最小SA肌肉类型的SA力量的产生，相当于中等SA肌肉类型，以及在高SA昆虫飞行肌肉中，特定的肌钙蛋白C亚型TnC4是SA所必需的。这些发现是由于我们开发了一种新的果蝇肌肉纤维制剂--跳跃肌肉，它允许我们寻找SA功能的获得，而不仅仅是IFM中SA功能的丧失。具体目的1是确定某些肌球蛋白亚型增强SA力产生的动力学和结构机制。我们的工作假设是，对于SA肌球蛋白异构体，肌肉拉伸增加了它们在低力状态下暂时重新连接其他交叉桥的可能性，从而增加了随后可用于转换到高力、肌动蛋白结合状态的交叉桥的数量。我们将检验我们的假设，即PI亲和力和不同版本的肌球蛋白接力螺旋对于改变肌球蛋白对拉伸的敏感性至关重要。目的2是确定细丝蛋白对SA的作用机制。我们将验证我们的工作假设，与SA最小的肌肉中的TNC亚型相比，SA肌肉类型中的TNC亚型在钙结合时不能完全激活细丝。这允许通过拉伸进一步激活细丝。我们将验证这一假设，即肌钙蛋白和原肌球蛋白通过横跨粗细细丝的肌钙蛋白桥直接物理运动而发生进一步的激活。这项拟议的研究具有重要意义，因为它将提供对肌球蛋白、TNC和其他肌肉蛋白亚型启用SA的动力学和结构机制的详细了解。我们将对力量、力量和能量在不同肌肉类型中调节的基本机制有新的见解。阐明SA的机制可能导致恢复或改善肌肉功能的方法。

DESCRIPTION (provided by applicant): Stretch activation (SA) is an intrinsic sarcomeric property that increases muscle force, power and economy. SA is most prominent in muscle types that rhythmically lengthen and shorten such as cardiac and insect flight muscle. Rapidly increasing the length of a muscle with significant SA causes a delayed jump in force above calcium activated force. The mechanisms behind this force increase are not known, thus limiting our understanding of a fundamental muscle property. Our long-term goal is to apply insights gained from learning how SA mechanisms modulate force, power and muscle efficiency to help devise ways to restore impaired muscle function. The immediate objective of this application is to elucidate the sarcomeric mechanisms behind SA. Our central hypothesis is that SA can be caused by any sarcomeric mechanism that increases the total number of strongly bound cross-bridges following stretch. We propose that there are at least two mechanisms by which this occurs. In moderately SA muscle types the increase occurs by a myosin based mechanism, while a thin filament mechanism is required in highly SA muscle types. Our hypotheses are based on our novel preliminary data that a myosin isoform exchange increases SA force generation in a minimally SA muscle type to be equivalent to a moderately SA muscle type, and that a specific troponin C isoform, TnC4, is necessary for SA in highly SA insect flight muscle. These findings were made possible by our development of a new Drosophila muscle fiber preparation, the jump muscle, which allows us to look for gain of SA function and not just loss of SA function in the IFM. Specific aim 1 is to determine the kinetic and structural mechanisms by which some myosin isoforms enhance SA force production. Our working hypothesis is that for SA myosin isoforms, muscle stretch increases their probability of temporarily rejoining other cross-bridges in a low force state, thus increasing the number of cross-bridges available to subsequently transition into a high force, actin bound state. We will test our hypotheses that Pi affinity and different versions of the myosin relay helix are critical for changing the sensitivityof myosin to stretch. Aim 2 is to determine mechanisms by which thin filament proteins contribute to SA. We will test our working hypothesis that TnC isoforms in SA muscle types do not fully activate the thin filament upon calcium binding compared to TnC isoforms from muscles with minimal SA. This allows for further activation of the thin filament by stretch. We will test the hypothesis that further activation occurs by direct physical movement of troponin and tropomyosin by troponin bridges which span the thick and thin filaments. The proposed research is significant because it will provide a detailed understanding of the kinetic and structural mechanisms by which myosin, TnC, and other muscle protein isoforms enable SA. We will gain new insights into fundamental mechanisms by which force, power, and energetics are modulated in different muscle types. Elucidating SA mechanisms may lead to ways of restoring or improving muscle function.