Mechanisms underlying mechanical properties of muscle-tendon units

骨骼肌的萎缩、无力和损伤被广泛认为是与年龄相关的身体虚弱的主要原因，但随着年龄的增长，肌肉和肌腱的细胞外基质（ECM）发生变化的重要性却知之甚少。初步数据显示，随着年龄的增长，肌腱的力学性能发生了巨大的变化，肌腱肌肉末端的刚度明显增加，但骨端变化很小。随着年龄的增长，TA肌腱的力学性能发生了广泛的变化，而总胶原蛋白含量却没有大的变化。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迄今为止缺乏生物化学和生物力学特性之间的明确关系。因此，我们的目标是阐明(i)肌腱力学性能的区域差异和（ii）随年龄增长力学性能变化的机制，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)肌腱力学性能的区域差异是由于胶原含量和蛋白多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，可扩展性更低，增加了收缩性肌肉纤维损伤的可能性。实验将在成年（8个月）、中年（24个月）和老年（33个月）Fisher X Brown Norway大鼠的胫骨前肌（TA）肌腱单元（MTU）和渗透性单肌纤维上进行。具体目标是：1。2.确定老化对mtu被动力学性能的影响以及收缩过程中肌腱刚度与肌纤维长度的相互作用；2 .阐明肌腱力学性能区域差异的机制，以及随年龄增长而发生的肌腱力学性能变化；评估先前主动缩短对成年和老年大鼠肌肉渗透性单纤维延长收缩（LCs）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)在无激活拉伸和有和没有拉伸的肌肉激活期间沿着mtu进行高分辨率的应力-应变关系光学测量，（ii）对肌腱的蛋白质和基因表达，胶原含量和交联以及ECM结构进行彻底的区域分析，以建立有意义的结构-功能关系，以及（iii）应用，最大限度地激活来自大鼠TA肌的渗透性单纤维，预拉伸缩短了在幅度和速度上跨越的运动，以及在Aim 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改变对肌肉功能和损伤易感性的影响。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，作为身体虚弱随着年龄增长的因素。虚弱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，肌腱和肌肉纤维的力学性能变化对肌肉骨骼损伤的易感性增加的贡献。阐明肌肉肌腱结构和功能随年龄变化的潜在机制，将有助于延缓衰弱的发生和发展。

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.