Cellular and molecular mechanisms of skeletal muscle homeostasis during hibernation

骨骼肌是人体最大的器官，约占体重的50%。骨骼肌是促进健康生活的几个方面所必需的，包括运动，产热和葡萄糖稳态。骨骼肌质量的损失可能是多种情况的结果，包括衰老，营养剥夺和长期不活动。这反过来又增加了病理性骨折、功能恶化、住院和死亡率的发生率。冬眠是一些哺乳动物在长时间低温和食物供应不足的情况下生存的重要适应策略。值得注意的是，虽然冬眠的动物可以几个月不活动，在这段时间里不吃东西，但骨骼肌萎缩很小。这与非冬眠的哺乳动物形成鲜明对比，比如老鼠，在仅仅12天的肢体固定后，它们的肢体肌肉量就会减少近一半。当前建议的主要目的是采用一种全新的和创新的方法来阐明涉及保存关键骨骼肌质量和功能的细胞和分子机制。我们将全面表征这些机制在13线地松鼠，专性冬眠进入冬眠，在冬季静止状态。作为冬眠研究的长期模型，13纹地鼠提供了一个强大的比较模型，可能对肌肉稳态的基本过程产生有价值的见解。为了解决我们研究计划的主要目标，我们将我们的研究分为四个不同但互补的项目。我们的第一个项目将旨在确定两个明确定义的运动诱导信号通路在防止萎缩过程中的关键组成部分。与不冬眠的哺乳动物相比，冬眠的地松鼠肌肉中的两条通路同时活跃。在我们的第二个项目中，我们将研究冬眠期间肌肉保护对NAD（烟酰胺腺嘌呤二核苷酸）的依赖，NAD是整个细胞中几种酶促反应所必需的重要分子。由于肌肉萎缩与线粒体功能障碍有关，而NAD对线粒体正常功能和能量产生至关重要，因此我们将验证在冬眠期间保持线粒体内足够的NAD水平对保持肌肉质量至关重要的假设。我们的第三个项目将旨在描述驻留骨骼肌干细胞（称为卫星细胞）在损伤后休眠肌肉启动的修复过程中的作用。这些研究将侧重于再生能力和纤维化，因为我们之前观察到，与活动松鼠相比，受伤的冬眠松鼠肌肉再生明显较慢且缺乏纤维化。最后，我们将通过比较活动地松鼠和冬眠地松鼠的血浆代谢物谱来验证循环因子（骨骼肌外部的）有助于冬眠期间肌肉质量维持的假设。总之，这些项目将为冬眠地松鼠维持骨骼肌质量的机制提供新的见解和相关特征。此外，它们将有可能提高我们对非冬眠哺乳动物骨骼肌保存和再生的生物学机制的理解。

Skeletal muscle is the largest organ in the human body, comprising ~50% of body mass. Skeletal muscle is necessary to promote several facets of a healthy life, including locomotion, heat production and glucose homeostasis. Loss of skeletal muscle mass can be the result of a variety of conditions including aging, nutrient deprivation and prolonged immobility. This, in turn, increases the incidence of pathologic fractures, functional deterioration, institutionalization and mortality. Hibernation is an important adaptation strategy among some mammals that allow for survival during prolonged cold temperatures and scarce food supply. Remarkably, although the hibernating animal can be inactive for several months and does not consume food during this time, there is minimal skeletal muscle atrophy. This is in stark contrast to non-hibernating mammals such as mice that can lose almost half of their limb muscle mass after just 12 days of limb immobilization.The primary objective of the current proposal is to employ an entirely novel and innovative approach to elucidate the cellular and molecular mechanisms involved in preservation of critical skeletal muscle mass and function. We will comprehensively characterize these mechanisms in the 13-lined ground squirrel, an obligate hibernator that enters into a torpid, immobilized state during the winter. As a long-standing model for studies of hibernation, the 13-lined ground squirrel presents a powerful comparative model that may yield valuable insights into fundamental processes of muscle homeostasis.In order to address the primary objective of our research program, we will organize our studies into four distinct but complimentary projects. Our first project will aim to identify the critical components of two well-defined exercise-induced signaling pathways during protection against atrophy. It has been demonstrated that both pathways are active in the muscle of hibernating ground squirrels simultaneously, in contrast to non-hibernating mammals. In our second project, we will investigate the dependence of muscle protection during hibernation on NAD (nicotinamide adenine dinucleotide), a vital molecule that is required in several enzymatic reactions throughout the cell. Since muscle atrophy has been associated with mitochondrial dysfunction, and NAD is critical to proper mitochondrial function and energy production, we will test the hypothesis that maintaining adequate NAD levels within the mitochondria during hibernation is critical to preserving muscle mass. Our third project will aim to delineate the role of resident skeletal muscle stem cells, known as satellite cells, in the repair process that is initiated by hibernating muscle following injury. These studies will focus on regeneration capacity and fibrosis, since we have previously observed that injured hibernating squirrel muscle regenerates notably slower and lacks fibrosis when compared with active squirrels. Finally, we will test the hypothesis that circulating factors, extrinsic to skeletal muscle, contribute to the maintenance of muscle mass during hibernation by comparing plasma metabolite profiles between active and hibernating ground squirrels. Together, these projects will provide novel insights and relevant characterization of the mechanisms utilized by hibernating ground squirrels to maintain skeletal muscle mass. They will moreover carry the potential to improve our understanding of the biological mechanisms involved in skeletal muscle preservation and regeneration in non-hibernating mammals.