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

摘要 骨质疏松症是骨骼肌质量和力量/力量随着年龄的增长而逐渐减少。 越来越多的数据表明，主要是伴随着骨质疏松症的虚弱，而不是肌肉的丧失 体型本身就是导致残疾的原因之一。我们是第一个提出骨骼细胞的小组，特别是 骨细胞向肌肉发出信号，并在这一过程中发挥重要作用。我们发现，可溶性因子来自于 幼年小鼠的骨细胞(5mo)促进C2C12细胞和原代成肌细胞的肌肉生成，而 老龄小鼠的骨细胞因子(22mo)对其无明显影响。我们鉴定了两个已知的骨细胞 因子，PGE2和WNT3a(即，骨因子)，作为骨细胞到肌肉信号的潜在中介，因为它们 与年轻的骨细胞可溶性因子在肌肉生成中的作用非常相似。Wnt3a和PGE2 显著上调了协调生长、肥大、线粒体生物发生和钙离子的基因- 在肌肉中释放/摄取。WNT3a与β-连环蛋白向细胞核的移位有关，并与 Wnt/β-catenin途径的两个下游基因Fhl1和Numb在肌肉发生中有直接作用。我们的 我们的体外、体内和临床研究进一步支持了基于细胞的结果。有条件地删除 骨细胞中β-连环蛋白的一个等位基因导致肌肉质量和骨骼肌功能的下降 在衰老过程中，相反，Lrp5G171V小鼠的骨骼肌功能得到增强，这是一种 骨骼Wnt/β-连环蛋白信号增强。临床上，我们发现60岁健康女性的血清， 具有高浓度的WNT3a，并促进人原代成肌细胞的肌源性分化，而 骨质疏松女性的血清WNT3a水平是健康女性的十分之一， 肌肉发生。在体外，PGE2能够通过增强力量来反映年轻骨细胞成骨因子的作用 在年轻肌肉中生成，但在老年肌肉中不生成，其影响与EP4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，关键的脂质信号介质水平 花生四烯酸途径减少，但运动挽救了这些脂质的水平。这些数据表明 随着年龄的增长，骨骼到肌肉的信号转导有缺陷，或者肌肉对PGE2的反应受损，可能是通过 衰老骨骼肌中的EP4-PGE2信号转导。这些研究支持了我们的假设，即骨骼到肌肉 由于Wnt/β-连环蛋白和前列腺素E_2信号通路受损，信号会随着年龄的增长而改变，从而导致 骨质疏松症相关的肌肉无力。为了验证这一假设，我们将进行以下研究。目标1： 确定Wnt/β-连环蛋白信号在衰老/运动过程中骨骼-肌肉串扰中的作用。目标2：确定 PGE2信号在衰老/运动过程中骨骼-肌肉串扰中的作用。这些研究将提供新的 骨信号转导的骨因子调节肌肉功能的机制以及为新的 在衰老/运动过程中与骨骼-肌肉串扰有关的分泌因素。这些研究将产生重要的 关于预防和/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'.