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潜在机制骨骼肌的萎缩、虚弱和损伤被广泛认为是年龄相关性身体虚弱的主要原因，但对肌肉和肌腱细胞外基质（ECM）随年龄变化的重要性了解甚少。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，肌腱肌肉端的刚度显著增加，但骨端的变化很小。TA肌腱的力学性能随着老化而发生广泛的变化，而总胶原含量没有重大变化。肌腱功能特性的区域性变化加上潜在结构的区域性变化，可能导致迄今为止生物化学和生物力学特性之间缺乏明确的关系。因此，我们的目标是澄清潜在的机制（i）沿着肌腱的机械性能的区域差异和（ii）随着年龄的增长，机械性能的变化，并确定肌腱的变化对肌肉功能和损伤的敏感性的影响。工作假设是：（i）肌腱机械性能的区域差异是由于胶原含量和蛋白聚糖表达的区域变化导致ECM结构的差异，以及（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，导致组织整体上更硬和更少的可伸展性，并增加了收缩引起的肌肉纤维损伤的可能性。将对成年（8个月）、中年（24个月）和老年（33个月）Fisher X Brown Norway大鼠的胫骨前肌（TA）肌腱单位（MTU）和透化单肌纤维进行实验。具体目标是：1。确定老化对MTU被动力学性能的影响以及收缩期间肌腱刚度和肌纤维长度之间的相互作用，2.阐明了沿着肌腱力学性能的区域差异以及随着老化发生的肌腱力学性能变化的潜在机制，以及3.评估先前主动缩短对由来自成年和老年大鼠肌肉的透化单纤维的延长收缩（LC）引起的力缺陷的影响。为了实现这些目标，我们的方法将是（i）在无激活的拉伸期间以及在有和无拉伸的肌肉激活期间沿着MTU进行应力-应变关系的高分辨率光学测量，（ii）对肌腱的蛋白质和基因表达、胶原含量和交联以及ECM结构进行全面的区域分析，以建立有意义的结构-功能关系，以及（iii）应用，从大鼠的TA肌肉最大程度地活化透化的单纤维，预拉伸缩短运动，其在幅度和速度上跨越在目的1中建立的肌腱的伸展性质，以确定对肌肉功能的影响和对由肌腱性质的变化可避免地产生的纤维的机械环境的改变的损伤敏感性。公共卫生相关性。 与公共卫生的相关性是基于对肌肉和肌腱损伤的易感性增加的影响，肌肉和肌腱损伤是随着身体虚弱的衰老而发展的贡献者。虚弱导致不动和福尔斯，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肌腱和肌纤维的机械性能随年龄的变化对肌肉骨骼损伤易感性增加的贡献。阐明肌肉-肌腱单位的结构和功能随年龄增长而发生变化的潜在机制将提高延缓虚弱发作和进展的可能性。

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.