Age-Dependent Regulation of Muscle Stem Cell Homeostasis

描述（由申请人提供）：骨骼肌功能丧失导致的活动能力丧失是衰老的必然结果，导致生活质量降低和需要住院或家庭护理的发病率增加，显著增加了医疗保健费用。肌肉功能的严重丧失，称为肌肉减少症，估计随着我们的一般人群年龄的增长而花费xxx。肌肉减少症与肌肉萎缩、肌球蛋白同种型变化、收缩力降低和收缩速度减慢有关。这些复杂的生理变化是有据可查的，但负责这些变化的机制还不清楚。再生能力的丧失通常被认为伴随着骨骼肌萎缩。通常认为负责肌肉修复的细胞是卫星细胞，因其在骨骼肌肌纤维的质膜和基底膜之间的解剖位置而得名。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。虽然从老年和年轻肌肉中分离的卫星细胞似乎在行为上有所不同，但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。随着骨骼肌年龄的增长，卫星细胞的数量是减少还是保持不变，这一问题引起了争论。卫星小区号码如何维持尚不清楚。包括我们自己在内的几个研究小组已经鉴定出卫星细胞的亚群，它们的行为与干细胞相似，能够更新卫星细胞库并致力于肌生成。卫星细胞是由经历不对称分裂以产生定向成肌细胞并通过对称分裂自我更新和扩增的分级干细胞产生的，还是由共同的等能干细胞库随机产生的尚不清楚。如果没有对卫星细胞自我更新的基本了解，就很难推断出老化的环境并试图解释再生能力的丧失。我们将使用这些方法比较久坐不动、自愿运动和肌肉受伤的年轻和老年小鼠卫星细胞的周转和扩增。为了实现这些目标，我们建议：（1）建立一个系统的时间灵活的谱系追踪骨骼肌，（2）比较卫星细胞周转和克隆扩张的年轻和老年小鼠久坐，运动的动物和受伤的肌肉，（3）比较卫星干细胞周转和克隆扩张的干细胞移植胫骨前肌。这些方法将使我们能够实验性地解决卫星细胞更新的机制，使用组合的方法，允许通过病毒感染和干细胞移植进行时间灵活的谱系追踪。

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.