Cellular and molecular mechanisms of skeletal muscle homeostasis during hibernation

骨骼肌是人体最大的器官，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、营养缺乏和长期不动。这反过来又增加了病理性骨折、功能恶化、机构化和死亡率的发生率。冬眠是一些哺乳动物的一种重要适应策略，可以在长时间寒冷的温度和稀缺的食物供应中生存。值得注意的是，尽管冬眠动物可以在几个月内不活动，在这段时间内不进食，但骨骼肌萎缩的程度很小。这与非冬眠的哺乳动物形成鲜明对比，例如小鼠，在肢体固定12天后，可以失去几乎一半的肢体肌肉质量。目前建议的主要目标是采用一种全新的创新方法来阐明关键骨骼肌质量和功能保存所涉及的细胞和分子机制。我们将全面描述这些机制在13线地松鼠，专性冬眠，进入一个迟钝，固定状态在冬季。作为一个长期的冬眠研究模型，13线地松鼠提出了一个强大的比较模型，可能会产生有价值的见解肌肉稳态的基本过程。为了解决我们的研究计划的主要目标，我们将组织我们的研究分为四个不同的，但互补的项目。我们的第一个项目将旨在确定两个明确的运动诱导的信号通路的关键组成部分，在防止萎缩。已经证明，这两种途径在冬眠的地松鼠的肌肉中同时活跃，与非冬眠的哺乳动物相反。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）的依赖性，NAD是一种重要的分子，在整个细胞的几种酶促反应中都是必需的。由于肌肉萎缩与线粒体功能障碍有关，而NAD对适当的线粒体功能和能量产生至关重要，因此我们将测试以下假设：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。我们的第三个项目将致力于描述驻留骨骼肌干细胞（称为卫星细胞）在损伤后冬眠肌肉启动的修复过程中的作用。这些研究将集中在再生能力和纤维化，因为我们以前观察到，与活跃的松鼠相比，受伤的冬眠松鼠肌肉再生明显较慢，缺乏纤维化。最后，我们将测试的假设，即循环因素，外在的骨骼肌，有助于维持肌肉质量在冬眠期间通过比较活跃和冬眠的地松鼠之间的血浆代谢产物。总之，这些项目将提供新的见解和冬眠地松鼠利用的机制，以保持骨骼肌质量的相关表征。此外，它们还具有提高我们对非冬眠哺乳动物骨骼肌保存和再生生物学机制的理解的潜力。

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.