Mechanisms underlying mechanical properties of muscle-tendon units

描述（由申请人提供）：肌肉肌腱单位机械特性的机制骨骼肌萎缩、无力和损伤被广泛认为是导致与年龄相关的身体虚弱的主要原因，但人们对肌肉和肌腱细胞外基质（ECM）随衰老变化的重要性知之甚少。初步数据显示，随着年龄的增长，肌腱的机械性能发生了巨大的变化，肌腱肌肉端的硬度显着增加，但骨端的变化很小。随着年龄的增长，TA 肌腱的机械性能发生了广泛的变化，但总胶原蛋白含量没有发生重大变化。肌腱功能特性的区域差异加上底层结构的区域差异可能导致生化和生物力学特性之间迄今为止缺乏明确的关系。因此，我们的目标是阐明（i）肌腱机械性能的区域差异和（ii）机械性能随老化而变化的潜在机制，并确定肌腱变化对肌肉功能和损伤易感性的影响。工作假设是：(i) 肌腱机械性能的区域差异是由于胶原蛋白含量和蛋白聚糖表达的区域差异导致 ECM 结构的差异；(ii) 随着年龄的增长，整个肌腱的组成变得与最接近骨骼的区域相似，导致组织整体更硬且延展性更差，并增加了收缩引起的肌纤维损伤的可能性。实验将在成年（8 个月）、中年（24 个月）和老年（33 个月）Fisher X Brown 挪威大鼠的胫骨前肌 (TA) 肌肉肌腱单位 (MTU) 和透化单肌纤维上进行。具体目标是：1. 确定衰老对 MTU 被动机械性能的影响以及收缩过程中肌腱刚度和肌纤维长度之间的相互作用；2. 阐明肌腱机械性能区域差异的潜在机制以及随衰老而发生的肌腱机械性能变化；3. 评估先前主动缩短对成年和老年大鼠肌肉透化单纤维延长收缩 (LC) 引起的力不足的影响。为了实现这些目标，我们的方法将是（i）在没有激活的拉伸过程中以及在有和没有拉伸的肌肉激活过程中沿 MTU 进行应力-应变关系的高分辨率光学测量，（ii）对蛋白质和基因表达、胶原蛋白含量和交联以及肌腱的 ECM 结构进行彻底的区域分析，以建立有意义的结构功能关系，以及（iii）对来自大鼠 TA 肌肉的最大激活的透化单纤维应用在两个幅度上跨越的预拉伸缩短运动速度、目标 1 中建立的肌腱的伸展特性，以确定肌腱特性变化不可避免地造成的纤维机械环境变化对肌肉功能和受伤敏感性的影响。公共卫生相关性。 与公共健康的相关性是基于肌肉和肌腱损伤的易感性增加的影响，因为随着年龄的增长，肌肉和肌腱损伤会导致身体虚弱。虚弱会导致行动不便和跌倒，是限制独立生活能力的关键因素。我们的项目将进一步了解随着年龄的增长，肌腱和肌肉纤维的机械性能变化对肌肉骨骼损伤敏感性增加的影响。阐明肌肉肌腱单位结构和功能随衰老变化的机制将提高延缓衰弱发生和进展的可能性。

DESCRIPTION (provided by applicant): Mechanisms underlying mechanical properties of muscle-tendon units Atrophy, weakness, and injury of skeletal muscle are widely recognized as major contributors to age-related physical frailty, but the importance of changes with aging in extracellular matrix (ECM) of muscle and tendon are poorly understood. Preliminary data show dramatic changes in tendon mechanical properties with aging, with a marked increase in stiffness at the muscle end of the tendon but minimal change at the bone end. The extensive changes with aging in mechanical properties of TA tendons occur with no major change in total collagen content. The regional variation in tendon functional properties coupled with regional variation in underlying structure may contribute to the lack thus far of clear relationships between biochemical and biomechanical properties. Therefore, our goals are to clarify the mechanisms underlying (i) regional differences in mechanical properties along tendons and (ii) changes in mechanical properties with aging, and to determine the impact of tendon changes on muscle function and susceptibility to injury. The working hypotheses are that (i) regional differences in tendon mechanical properties are due to regional variation in collagen content and proteoglycan expression resulting in differences in ECM structure and (ii) with aging, the composition of the entire tendon becomes similar to that of the region nearest bone, causing the tissue to be stiffer and less extensible overall and increasing the likelihood of contraction-induced injury to muscle fibers. Experiments will be performed on tibialis anterior (TA) muscle-tendon units (MTU) and permeabilized single muscle fibers of adult (8 months), middle aged (24 months), and old (33 months) Fisher X Brown Norway rats. The Specific Aims are: 1. determine the impact of aging on passive mechanical properties of MTUs and the interaction between tendon stiffness and muscle fiber length during contraction, 2. elucidate mechanisms underlying the regional differences in mechanical properties along tendons and the changes in tendon mechanical properties that occur with aging, and 3. evaluate the effects of prior active shortening on force deficits caused by lengthening contractions (LCs) of permeabilized single fibers from muscles of adult and old rats. To address the Aims, our approach will be to (i) make high resolution optical measurements of stress-strain relationships along MTUs during stretches without activation and during muscle activation with and without stretch, (ii) perform thorough regional analyses of protein and gene expression, collagen content and cross-linking, and ECM structure of tendons to establish meaningful structure-function relationships, and (iii) apply, to maximally activated permeabilized single fibers from TA muscles of rats, pre-stretch shortening movements that span, in both amplitude and speed, the extension properties of tendons established in Aim 1 to determine the impact on muscle function and susceptibility to injury of alterations in the mechanical environment of fibers that are unavoidably created by changes in tendon properties. PUBLIC HEALTH RELEVANCE. The relevance to public health is based on the impact of increasing susceptibility to muscle and tendon injury as contributors to the development with aging of physical frailty. Frailty causes immobility and falls and is a key factor limiting the ability to live independently. Our project will further understanding of the contribution of changes with aging in the mechanical properties of tendons and muscle fibers to the increased susceptibility to musculoskeletal injury. Clarifying mechanisms underlying changes with aging in the structure and function of muscle-tendon units will improve the likelihood of delaying the onset and progression of frailty.