Age-Dependent Regulation of Muscle Stem Cell Homeostasis

描述（由申请人提供）：由于骨骼肌功能丧失而导致的活动能力丧失是衰老的不可避免的结果，导致生活质量下降和需要住院或家庭护理的发病率增加，从而显着增加医疗保健费用。随着我们人口的老龄化，肌肉功能的严重丧失（称为肌肉减少症）估计会造成 xxx 的损失。肌肉减少症与肌肉萎缩、肌球蛋白同种型的变化、收缩力的降低和收缩速度的降低有关。这些复杂的生理变化已有详细记录，但造成这些变化的机制尚不清楚。人们普遍认为再生能力的丧失伴随着骨骼肌萎缩。通常认为负责肌肉修复的细胞是卫星细胞，因其解剖位置位于骨骼肌肌纤维质膜和基底膜之间而得名。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。尽管从老年和年轻肌肉中分离出的卫星细胞似乎在行为上有所不同，但尚不清楚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否随着骨骼肌老化而减少或保持不变的问题，存在相互矛盾的报道。卫星细胞数量如何维持尚不清楚。包括我们自己在内的几个研究小组已经确定了卫星细胞的子集，这些卫星细胞的行为类似于干细胞，能够更新卫星细胞库并致力于肌生成。卫星细胞是否是由分层干细胞产生的，经历不对称分裂产生定型成肌细胞并通过对称分裂进行自我更新和扩展，还是由等能干细胞的共同池随机产生，目前尚不清楚。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在久坐、自愿运动和受伤肌肉的年轻和老年小鼠中比较卫星细胞的周转和扩张。为了实现这些目标，我们建议：（1）建立骨骼肌中临时灵活的谱系追踪系统，（2）比较年轻和老年久坐小鼠、运动动物和受伤肌肉的卫星细胞更新和克隆扩增，以及（3）比较移植干细胞的胫骨前肌的卫星干细胞更新和克隆扩增。这些方法将使我们能够使用组合方法通过实验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。

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.