Deciphering the role of RANK/RANKL/OPG in skeletal muscle

核因子κB受体激活剂RANK、核因子κB配体受体激活剂RANK/RANKL/OPG以及骨保护素途径的发现是骨生物学和骨疾病研究的重要进展之一。RANK/RANKL/OPG是调节破骨细胞分化和功能的关键细胞因子系统。RANKL由成骨细胞表达，其受体RANK表达于破骨前细胞。RANK/RANKL相互作用诱导多核成熟破骨细胞的形成，最终导致骨吸收。然而，第三个主角OPG也是由成骨细胞产生的，对破骨细胞前分化具有抑制作用。OPG通过与RANKL结合，抑制RANKL/RANK相互作用和随后的破骨细胞生成。OPG基因缺失的小鼠患有骨质疏松症的事实证明了OPG的生理重要性。OPG还可以作为肿瘤坏死因子相关的凋亡诱导配体的诱饵受体，通过阻断RANK/RANKL相互作用的促凋亡效应来提高细胞存活率。因此，OPG是一种非常有效的抗吸收和抗细胞凋亡剂。然而，我们最近在Am上发表的文章J.帕索尔。(2015)令人信服地表明，OPG-FC完全恢复了MDX小鼠快抽动的指长伸肌功能，显著改善了慢抽动的比目鱼肌和横隔肌的功能，表明OPG不仅是骨骼的保护者，也是骨骼肌的保护者。 在这个提案中，我们将使用遗传学和药理学的方法来确定RANK/RANKL/OPG三联体是如何调节骨骼肌的表型、肌肉功能、SERCA-2a含量和SERCA活性的。我们还积累了大量证据支持RANKL与肌肉等级结合的观察结果，但RANKL/OPG也有自己的肌肉受体直接作用于骨骼肌。因此，我们打算在未来5年实现三个具体目标。 (1)了解培养的肌细胞中OPG、RANKL、机械负荷与细胞外钙离子的相互作用。 (2)探讨细胞外钙离子对营养不良肌肉等级缺陷小鼠肌肉功能的影响。 (3)证明RANKL通过RANKL/RANK相互作用非依赖机制影响C2C12肌管中SERCA-2a的含量。 总之，这一基础研究方案强调了骨骼肌和骨骼之间的相互协作作用，并有望为RANK/RANKL/OPG途径开辟新的生理机制。

The discovery of RANK/RANKL/OPG the receptor-activator of nuclear factor κB (RANK), the receptor-activator of nuclear factor κB ligand (RANKL), and the osteoprotegerin (OPG) pathway has been among the most important advances in bone biology and disease. RANK/RANKL/OPG form a key cytokine system for regulating osteoclast differentiation and function. RANKL is expressed by osteoblasts while RANK, its receptor, is expressed on pre-osteoclastic cells. The RANK/RANKL interaction induces the formation of multinucleated mature osteoclasts, ultimately causing bone resorption. However, the third protagonist, OPG, is also produced by osteoblasts and exerts an inhibitory effect on pre-osteoclastic differentiation. OPG, by binding to RANKL, inhibits the RANKL/RANK interaction and the subsequent osteoclastogenesis. The fact that OPG-null mice are osteoporotic is testimony to the physiological importance of OPG. OPG also serves as a decoy receptor for the tumour necrosis factor-related apoptosis-inducing ligand and increases cell survival by blocking the pro-apoptotic effects of the RANK/RANKL interaction. OPG is thus a very efficient anti-resorptive and anti-apoptotic agent. However, our recent publication in Am. J. Pathol. (2015) convincingly showed that OPG-Fc completely restores the function of fast-twitch extensor digitorum longus muscles and significantly improves the function of slow-twitch soleus and diaphragm muscles in mdx mice, indicating that OPG is not only a protector of bone but also skeletal muscles. In this proposal, we will use genetic and pharmacological approaches to determine how RANK/RANKL/OPG triad, known for its role in bone regulation, regulates skeletal muscle phenotype, muscle function and SERCA-2a content and SERCA activity. We have also accumulated substantial evidence to support the observation that RANKL binds to muscle RANK but RANKL/OPG would also have its own muscle receptor acting directly on skeletal muscle. We thus intend to pursue three specific aims over the next 5 years. (1) To understand the interplay between OPG, RANKL, mechanical loading and extracellular Ca2+ in muscle cells in culture. (2) To determine how extracellular Ca2+ influences muscle performance in dystrophic muscle RANK deficient mice. (3) To demonstrate that RANKL influences SERCA-2a content in C2C12 myotubes through a RANKL/RANK interaction-independent mechanism. Together, this basic research proposal highlights the mutual cooperative interactions between skeletal muscle and bone and should open new physiological mechanisms for RANK/RANKL/OPG pathway.