Structural Dynamics of Stretch Activation in Muscle

描述（由申请人提供）：本研究的长期目标是了解拉伸肌肉使其拉得更用力的分子机制。所有横纹肌都通过肌球蛋白马达蛋白周期性地拉动肌动蛋白丝来收缩。在松弛的肌肉中，肌钙蛋白-原肌球蛋白复合体在空间上阻止肌球蛋白与肌动蛋白相互作用.在神经原性收缩中，肌钙蛋白结合释放的钙，从而解除原肌球蛋白的空间阻滞，使肌球蛋白能够结合并拉动肌动蛋白。通过一种未知的机制，拉伸肌肉会使其拉得更用力，如果预先拉伸松弛的肌肉会导致后续收缩中更大的力量，则该过程称为“长度依赖性激活”，或者如果拉伸部分激活的肌肉会导致力量延迟上升，则称为“拉伸激活”。长度依赖性激活是心脏的Frank-Starling定律的基础心肌的基本特征，其允许其通过在增加舒张充盈后增加收缩力来逐搏匹配输入和输出体积。伸展激活是大多数昆虫飞行肌肉的基本特征，这使得它能够在飞行过程中机械地触发收缩。所有的横纹肌，无论是脊椎动物还是昆虫， 长度依赖性激活和牵张激活，但目前尚不知道这两个过程是否反映了相同的潜在现象，也不是已知的分子机制，尽管几十年的研究。目前的数据表明，昆虫飞行肌肉的牵张激活是由原肌球蛋白控制的，类似于神经源性钙激活，但由肌球蛋白-肌钙蛋白连接介导，当肌肉被拉伸时，肌球蛋白-肌钙蛋白连接传递应变。该项目是有史以来第一次系统地比较脊椎动物心脏，慢骨骼，快骨骼和昆虫飞行肌肉的长度依赖性激活和拉伸激活，以确定这两种激活模式是否是所有肌肉类型的单一过程的不同表现。从牛心肌和昆虫飞行肌的初步数据表明，长度依赖性激活和牵张激活是一个单一的过程。所有四种肌肉类型之间的肌钙蛋白交换将决定通过生理反应判断的长度依赖性激活和牵张激活的类型特异性要求，以及通过电子显微镜判断的肌球蛋白-肌钙蛋白连接的类型特异性要求。兔腰大肌牵张激活的实时X射线衍射图像将揭示脊椎动物横纹肌牵张激活和长度依赖性激活的分子机制，揭示分子结构变化的顺序，并与昆虫飞行肌进行比较。了解长度依赖性激活、牵张激活和肌球蛋白-肌钙蛋白桥的作用的分子基础对于详细了解正常肌肉功能是必要的，这反过来又是了解这些机制如何在人类疾病中缺乏的必要先决条件，包括心脏病、肌肉肌病、肌肉损伤和肌肉减少症。 公共卫生相关性：该项目旨在了解拉动肌肉使其更难拉回的分子机制。这种反应在心肌中尤其发达，在那里它一次又一次地调节收缩力。对这一过程的更深入了解可能会导致更好地预防和治疗心脏病。

DESCRIPTION (provided by applicant): The long-term objective of this research is to understand the molecular mechanism by which stretching a muscle makes it pull harder. All striated muscles contract via myosin motor proteins that cyclically pull on actin filaments. In a relaxed muscle, myosin is sterically blocked from interacting with actin by the troponin- tropomyosin complex. In a neurogenic contraction, troponin binds released calcium, which relieves the steric blocking by tropomyosin and enables myosin to bind to and pull on actin. By an unknown mechanism, stretching a muscle makes it pull harder, a process known as "length dependent activation" if pre-stretching a relaxed muscle leads to greater force in subsequent contractions, or as "stretch activation" if stretching a partially activated muscle leads to a delayed rise in force. Length dependent activation is an essential feature of cardiac muscle underlying the Frank-Starling Law of the Heart, which allows it to match the input and output volumes beat-to-beat by increasing systolic force after increased diastolic filling. Stretch activation is an essential feature of most insect flight muscles, which allows it to mechanically trigger contraction during flight. All striated muscles, vertebrate or insect, show varying degrees of length dependent activation and stretch activation, but it is currently not known whether these two processes reflect the same underlying phenomenon, nor is the molecular mechanism for either known, despite decades of research. Current data from insect flight muscle suggest that stretch activation is controlled by tropomyosin, similar to neurogenic calcium-activation but mediated by myosin-troponin connections that transmit strain when the muscle is stretched. This project is the first ever systematic comparison of length dependent activation and stretch activation in vertebrate cardiac, slow skeletal, fast skeletal, and insect flight muscles, to determine whether the two modes of activation are different manifestations of a single process for all muscle types. Preliminary data from bovine cardiac muscle and insect flight muscle suggest that length dependent activation and stretch activation are a single process. Troponin-exchange among all four muscle types will determine the type-specific requirements for length dependent activation and stretch activation as judged by the physiological responses, and for myosin-troponin connections as judged by electron microscopy. Real-time X-ray diffraction movies of stretch activation in rabbit psoas muscle will elucidate the molecular mechanisms of stretch activation and length dependent activation in vertebrate striated muscle by revealing the sequence of molecular structural changes, to be compared side by side with insect flight muscle. Understanding the molecular basis of length dependent activation, stretch activation, and the action of myosin-troponin-bridges is necessary for a detailed mechanistic understanding of normal muscle function, which in turn is an essential prerequisite for understanding how these mechanisms are deficient in human disease, including heart disease, muscle myopathies, muscle injuries, and sarcopenia. PUBLIC HEALTH RELEVANCE: This project seeks to understand the molecular mechanism by which pulling on a muscle makes it pull back harder. This response is especially well developed in heart muscle where it modulates the force of contraction beat by beat. A deeper understanding of this process may lead to better prevention of and treatment for heart disease.