Molecular clock and skeletal muscle weakness

描述(由申请人提供)：骨骼肌无力长期以来一直被认为是导致衰老和其他疾病的发病率和死亡率的原因。来自人类和啮齿动物不同虚弱模型的单纤维研究表明，肌丝蛋白表达、翻译后修饰和肌节组织的变化与作用力减少有关。我们最近在新出现的昼夜节律和骨骼肌分子时钟领域的工作，有可能为我们提供对虚弱机制的洞察。最近，我们建立了一个诱导系小鼠，在该系小鼠中，BMal1仅在他莫昔芬治疗后的成年骨骼肌中缺失(iMSBMal1-/-)。我们观察到笼子活动和主动轮子活动的进行性下降，我们确定肌肉较弱，最大力量和被动张力显着下降。为了确定这个模型的缺陷的分子机制(S)，我们已经确定了肌肉重要的转录因子(SRF，Sox6和Tead1)的昼夜表达，我们发现这些基因在iMSBMal1-/-小鼠的肌肉中的表达受到干扰。这些观察结果支持我们的假设，即骨骼肌中的分子时钟直接调节一系列重要转录因子(肌肉时钟控制的基因)的表达，当分子时钟被破坏时，这将导致对肌节基因表达、肌节结构和肌肉机械功能的下游影响。除了对分子钟的研究外，我的实验室和其他人已经证明，有计划的体力活动可以作为骨骼肌分子钟的环境非光时间提示。这一发现突显了一种新的身体活动机制，并为我们的第二个假设提供了基础：锻炼时间将作为一种治疗干预，减缓肌肉衰老过程中肌肉无力的进展。这两个假设将在以下三个具体目标中得到检验。具体目的1：确定肌源性转录因子SRF、Sox6、Tead1是否是骨骼肌中直接的分子时钟控制基因。具体目的2：确定iSMBMal1-/-小鼠骨骼肌纤维主动张力和被动张力降低的肌小节变化。具体目标3：确定运动时间是否改变了肌肉无力随年龄增长的进展速度。这些研究的结果将为研究昼夜节律紊乱和肌肉无力之间的联系提供新的见解。由于衰老和许多慢性疾病已知会扰乱分子时钟功能，这些发现也可能为治疗策略提供新的靶点。

DESCRIPTION (provided by applicant): Skeletal muscle weakness has long been known to contribute to morbidity and mortality in aging and other diseases. Single fiber studies from different models of weakness in humans and rodents have demonstrated reduced force implicating alterations in myofilament protein expression, post-translational modifications and sarcomere organization. Our recent work in the emerging area of circadian rhythms and the molecular clock in skeletal muscle holds potential to provide insight into mechanisms of weakness. Most recently we generated an inducible line of mice in which Bmal1 is deleted only in adult skeletal muscle following tamoxifen treatment (iMSBmal1-/-). We observed progressive declines in cage activity and voluntary wheel activity and we determined that the muscles were weak with significant declines in maximum force and passive tension. To begin to identify the molecular mechanism(s) underlying weakness in this model, we have identified circadian expression of transcription factors important for muscle (Srf, Sox6 and Tead1) and we found that expression of these genes was disrupted in the muscle of iMSBmal1-/- mice. These observations support our hypothesis that the molecular clock in skeletal muscle directly regulates expression of a network of important transcription factors (muscle clock controlled genes) and when the molecular clock is disrupted, this leads to downstream effects on sarcomere gene expression, sarcomere structure and muscle mechanical function. In addition to work on the molecular clock, my lab and others have demonstrated that scheduled physical activity can function as an environmental non-photic time cue for the skeletal muscle molecular clock. This discovery highlights a new mechanism for physical activity and provides the basis for our second hypothesis: Time of exercise will act as a therapeutic intervention to slow the progression of muscle weakness in aging. These two hypotheses will be tested in the following three specific aims. Specific Aim 1: To determine whether the myogenic transcription factors, Srf, Sox6, Tead1 are direct molecular clock controlled genes in skeletal muscle. Specific Aim 2: To determine the sarcomeric changes through which active and passive tension is reduced in skeletal muscle fibers of iSMBmal1-/- mice. Specific Aim 3: To determine whether time of exercise modifies the rate of progression of muscle weakness with aging. The results of these studies will provide new insight into mechanisms that link disrupted circadian rhythms and muscle weakness. Since aging and many chronic diseases are known to disrupt molecular clock function these findings may also provide novel new targets for therapeutic strategies.