Effects of Osteocyte Factors, WNT3a and PGE2, on Muscle with Aging

摘要 肌肉减少症是随着年龄增长而发展的骨骼肌质量和力量/功率的进行性损失。 积累的数据表明，这主要是软弱，伴随着肌肉减少症，而不是肌肉的损失 尺寸本身，这会导致残疾。我们是第一个提出骨细胞， 骨细胞向肌肉发出信号，并在此过程中发挥重要作用。我们发现， 来自年轻小鼠（5个月）的骨细胞加速C2 C12细胞和原代成肌细胞的肌生成，而 用来自老龄小鼠（22个月）骨细胞的因子没有观察到效果。我们发现了两个已知的骨细胞 因子，PGE 2和Wnt 3a（即，骨因子），作为骨细胞到肌肉信号传导的潜在介质，因为它们 密切模仿年轻骨细胞可溶性因子对肌生成的影响。Wnt 3a和PGE 2 显著上调协调生长、肥大、线粒体生物合成和Ca 2 +- 肌肉中的释放/摄取。Wnt 3a与β-连环蛋白向细胞核的移位有关， Wnt/β-catenin途径的两个下游基因，Fhl 1和Numb，在肌发生中具有直接作用。我们 基于细胞的结果进一步得到我们的离体、体内和临床研究的支持。有条件删除 骨细胞中β-catenin的一个等位基因导致骨骼肌质量和功能的降低 相反，在Lrp 5G 171 V小鼠中，骨骼肌功能增强， 增加骨骼Wnt/β-catenin信号传导。临床上，我们发现来自健康60岁妇女的血清， 具有高浓度的WNT 3a并增加人原代成肌细胞的肌源性分化， 具有健康妇女的1/10的WNT 3a血清水平的阿尔茨海默病妇女的血清降低 肌生成体外，PGE 2能够通过增强力来反映年轻骨细胞骨因子的作用， 在年轻肌肉中产生，但在老年肌肉中不产生，并且效果与EP 4的表达有关。 肌肉中的受体。脂质组学分析显示，随着年龄的增长， 花生四烯酸途径减少，然而运动拯救这些脂质的水平。这些数据表明 随着年龄的增长，骨骼到肌肉的信号传导是有缺陷的，或者肌肉对PGE 2的反应是受损的，这可能是通过 老年骨骼肌中的EP 4-PGE 2信号传导。这些研究支持了我们的假设， 由于Wnt/β-catenin和PGE 2信号转导受损，信号转导随着衰老而改变， 肌肉萎缩症相关的肌肉无力为了验证这一假设，我们将进行以下研究。目标1： 确定Wnt/β-catenin信号在老化/运动过程中骨-肌肉串扰中的作用。目标2：确定 PGE 2信号在老化/运动过程中骨-肌肉串扰中的作用。这些研究将提供新的 通过骨信号骨因子调节肌肉功能的机制，以及提供一个平台， 在老化/运动期间参与骨-肌肉串扰的分泌因子。这些研究将产生重要的 用于预防和/或治疗“骨质疏松症”的信息。

ABSTRACT Sarcopenia is the progressive loss of skeletal muscle mass and force/power that develops with aging. Accumulating data shows that it is principally the weakness that accompanies sarcopenia, not the loss of muscle size per se, that contributes to disability. We were the first group to propose that bone cells, specifically osteocytes, signal to muscle and play an important role in this process. We discovered that soluble factors from osteocytes from young mice (5 mo) accelerate myogenesis of both C2C12 cells and primary myoblasts, whereas no effect was observed with factors from osteocytes of aged mice (22 mo). We identified two known osteocyte factors, PGE2 and Wnt3a (i.e., osteokines), as potential mediators of osteocyte to muscle signaling, as they closely mimicked the effects of young osteocyte soluble factors on myogenesis. Both Wnt3a and PGE2 significantly upregulated genes that orchestrate growth, hypertrophy, mitochondrial biogenesis, and Ca2+- release/uptake in muscle. Wnt3a was associated with the translocation of β-catenin to nuclei and was linked to two downstream genes of the Wnt/β-catenin pathway, Fhl1 and Numb that have direct roles in myogenesis. Our cell based results were further supported by our ex vivo, in vivo and clinical studies. The conditional deletion of one allele of β-catenin in osteocytes led to a decrease in both muscle mass and function of skeletal muscles during aging, while conversely, skeletal muscle function was enhanced in the Lrp5G171V mouse, a model of increased skeletal Wnt/β-catenin signaling. Clinically, we have found that serum from healthy 60 yr old women, has high concentrations of WNT3a and increases myogenic differentiation of human primary myoblasts, while serum from osteoporotic women, which has 1/10 of the WNT3a serum levels of healthy women, decreases myogenesis. Ex vivo, PGE2 is able to mirror the effects of young osteocyte osteokines by enhancing force generation in young muscles, but not in aged muscles and the effects were linked to the expression of the EP4 receptor in muscles. Lipidomics profiling revealed that with age, levels of key lipid-signaling mediators of the arachidonic acid pathway decrease, however exercise rescues the levels of these lipids. These data suggest that with aging, bone to muscle signaling is defective or muscle response to PGE2 is impaired, potentially through EP4-PGE2 signaling in aged skeletal muscles. These studies support our hypothesis that bone to muscle signaling is altered with aging due to compromised Wnt/β-catenin and PGE2 signaling, contributing to sarcopenia-related muscle weakness. To test this hypothesis, we will conduct the following studies. Aim 1: Determine the role of Wnt/β-catenin signaling in bone-muscle crosstalk during aging/exercise. Aim 2: Determine the role of PGE2 signaling in bone-muscle crosstalk during aging/exercise. These studies will provide new mechanisms for regulation of muscle function by bone-signaling osteokines as well as provide a platform for new secreted factors involved in bone-muscle crosstalk during aging/exercise. These studies will generate important information for prevention and/or treatment of `osteosarcopenia'.