Age-Dependent Regulation of Muscle Stem Cell Homeostasis

描述(由申请人提供)：骨骼肌功能丧失引起的活动能力丧失是衰老的必然结果，导致生活质量下降，需要住院或家庭护理的发病率增加，显著增加了医疗保健成本。严重的肌肉功能丧失，被称为骨质疏松症，随着我们普通人口的老龄化，估计会造成xxx的损失。骨质疏松症与肌肉萎缩、肌球蛋白同工类型改变、收缩力量减弱和收缩速度减慢有关。这些复杂的生理变化被很好地记录下来，但导致这些变化的机制尚不清楚。再生能力的丧失通常被认为伴随着骨骼肌萎缩。通常公认的负责肌肉修复的细胞是卫星细胞，因为它们在解剖上位于骨骼肌肌纤维的质膜和基底膜之间，所以被称为卫星细胞。这些细胞主要处于有丝分裂静止状态，可以被激活、增殖和分化，以维持或修复骨骼肌。尽管从老年和年轻肌肉分离的卫星细胞似乎在行为上有所不同，但目前尚不清楚所观察到的差异是内在的，还是仅仅是对细胞培养压力的不同反应。关于卫星细胞数量是随着骨骼肌年龄的增长而减少还是保持不变，相互矛盾的报告争论不休。卫星细胞数量是如何维持的还不得而知。包括我们自己在内的几个小组已经确定了卫星细胞的亚群，这些细胞具有干细胞的作用，能够更新卫星细胞库并致力于肌肉生成。目前尚不清楚卫星细胞是从经历不对称分裂的分级干细胞产生的，到产生承诺的成肌细胞并通过对称分裂自我更新和扩张，还是由相同能力的共同干细胞池随机产生的。如果没有对卫星细胞自我更新的基本了解，就很难推断老化的环境，也很难解释再生能力的丧失。我们将使用这些方法比较久坐不动、自愿锻炼的年轻和老年小鼠以及受伤肌肉中卫星细胞的周转和扩张。为了实现这些目标，我们建议：(1)在骨骼肌中建立一个暂时灵活的谱系追踪系统，(2)比较年轻和老年小鼠在久坐、运动和受伤肌肉中卫星细胞的周转和克隆扩张，以及(3)比较植入干细胞的胫前肌中卫星干细胞的周转和克隆扩张。这些方法将使我们能够利用一种组合方法在实验上解决卫星细胞更新所涉及的机制，从而允许通过病毒感染和干细胞移植进行临时灵活的谱系追踪。 公共卫生相关性：骨骼肌对呼吸、运动和排泄废物是必不可少的。骨骼肌在衰老过程中的严重丧失是灾难性的，极大地降低了整体生活质量，并招致了巨大的医疗费用。在正常衰老过程中发生的严重肌肉功能丧失称为骨质疏松症。骨质疏松症可能是由于成年肌肉干细胞失去了再生能力和自我更新能力。我们建议进行研究，以更好地了解成年肌肉干细胞是如何受到调控的，方法是使用我们实验室开发的新程序进行谱系追踪分析，以更好地了解肌肉干细胞如何在衰老过程中修复衰退。

DESCRIPTION (provided by applicant): Loss of mobility arising from loss of skeletal muscle function is an inevitable consequence of aging, resulting in a reduction of quality of life and increased morbidity requiring hospitalization or home care significantly raising health care costs. Severe loss of muscle function, termed sarcopenia is estimated to cost xxx as our general population ages. Sarcopenia is associated with muscle atrophy, changes in myosin isotypes, a reduction in contractile force and a diminishment in the speed of contraction. These complex physiological changes are well documented but the mechanisms responsible for these changes are not understood. A loss of regenerative capacity is generally acknowledged to accompany skeletal muscle atrophy. The cells generally acknowledged to be responsible for muscle repair are satellite cells, so named for their anatomical location between the plasma membrane of the skeletal muscle myofiber and the basement membrane. Predominately mitotically quiescent, these cells can be activated, will proliferate and differentiate to either maintain or repair skeletal muscle. Although satellite cells isolated from aged and young muscles appear to differ in their behavior, it is not clear whether the observed differences are intrinsic or simply a different response to the stresses of cell culture. Conflicting reports debate on whether satellite cell numbers decrease or remain unchanged as skeletal muscle ages. How satellite cell numbers are maintained is not known. Several groups including our own have identified subsets of satellite cells that behave as stem cells, capable of renewing the satellite cell pool and commitment to myogenesis. Whether satellite cells are generated from a hierarchical stem cell that under- goes asymmetric division to generated committed myoblasts and self-renew and expands by symmetric division or are generated stochastically by a common pool of equipotent stem cells is not known. Without a basic understanding of satellite cell self-renewal it is difficult to extrapolate to an aged environment and attempt to interpret the loss of regenerative capacity. We will compare the turnover and expansion of satellite cells using these methods in young and aged mice that are sedentary, undergoing voluntary exercise and in injured muscles. To accomplish these goals we propose: (1) to establish a system for temporally flexible lineage-tracing in skeletal muscle, (2) to compare satellite cell turnover and clonal expansion in young and aged mice for sedentary, and exercised animals and in injured muscle, and (3) to compare satellite stem cell turnover and clonal expansion in stem cell-engrafted tibialis anterior muscles. These approaches will allow us to experimentally address the mechanisms involved in satellite cell renewal using a combinatorial approach permitting temporally flexible lineage tracing by viral infection and by stem cell transplantation. PUBLIC HEALTH RELEVANCE: Skeletal muscle is essential for respiration, locomotion, and elimination of waste. Severe loss of skeletal muscle is during aging is catastrophic, dramatically reducing overall quality of life and incurring significant health care costs. Severe loss of muscle function that occurs during normal aging is termed sarcopenia. Sarcopenia is likely due to loss of regenerative capacity and the ability of adult muscle stem cells to renew them- selves. We propose research to better understand how adult muscle stem cells are regulated by performing lineage tracing analyses using novel procedures developed in our laboratory to better understand how muscle stem cells repair declines during aging.