Structural Dynamics of Stretch Activation in Muscle

描述（由申请人提供）：这项研究的长期目标是了解拉伸肌肉使其更用力的分子机制。所有横纹肌都是通过肌凝蛋白运动蛋白来收缩的，肌凝蛋白运动蛋白循环地拉动肌动蛋白丝。在松弛的肌肉中，肌凝蛋白被肌钙蛋白-原肌凝蛋白复合物立体阻断与肌动蛋白的相互作用。在神经源性收缩中，肌钙蛋白结合释放出钙，从而缓解原肌凝蛋白的空间阻滞，使肌凝蛋白结合并拉动肌动蛋白。通过一种未知的机制，拉伸肌肉会使它更用力，如果预先拉伸放松的肌肉会在随后的收缩中产生更大的力量，那么这个过程被称为“长度依赖激活”，或者如果拉伸部分激活的肌肉会导致力量的延迟上升，那么这个过程被称为“拉伸激活”。长度依赖性激活是心肌的一个基本特征，是Frank-Starling心脏定律的基础，它允许心肌在舒张充盈增加后通过增加收缩力来匹配心跳的输入和输出容量。拉伸激活是大多数昆虫飞行肌肉的基本特征，它允许它在飞行过程中机械地触发收缩。所有脊椎动物或昆虫的横纹肌都表现出不同程度的横纹肌

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.