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

RANK/RANKL/OPG-核因子B受体激活剂（RANK）、核因子B配体受体激活剂（RANKL）和骨保护素（OPG）通路的发现是骨生物学和疾病中最重要的进展之一。RANK/RANKL/OPG形成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，而其受体RANK在前成骨细胞上表达。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也由成骨细胞产生，并对前骨细胞分化产生抑制作用。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基因敲除的小鼠是类人猿，这一事实证明了OPG的生理重要性。OPG也可作为肿瘤坏死因子相关凋亡诱导配体的诱饵受体，通过阻断RANK/RANKL相互作用的促凋亡作用增加细胞存活。因此，OPG是一种非常有效的抗再吸收和抗凋亡剂。然而，我们最近的出版物在上午。J. Pathol.（2015）令人信服地表明，OPG-Fc完全恢复了mdx小鼠快缩趾长伸肌的功能，并显著改善了慢缩比目鱼肌和膈肌的功能，表明OPG不仅是骨的保护剂，也是骨骼肌的保护剂。 在本提案中，我们将使用遗传学和药理学方法来确定RANK/RANKL/OPG三联体（以其在骨调节中的作用而闻名）如何调节骨骼肌表型、肌肉功能以及SERCA-2a含量和SERCA活性。我们还积累了大量证据支持RANKL与肌肉RANK结合的观察结果，但RANKL/OPG也有其自身的肌肉受体直接作用于骨骼肌。因此，我们打算在今后五年内实现三个具体目标。 (1)了解OPG、RANKL、机械负荷和培养肌细胞中细胞外Ca 2+之间的相互作用。 (2)确定细胞外Ca 2+如何影响营养不良肌肉RANK缺陷小鼠的肌肉性能。 (3)证明RANKL通过RANKL/RANK相互作用非依赖性机制影响C2 C12肌管中的SERCA-2a含量。 总之，这一基础研究提案突出了骨骼肌和骨之间的相互合作相互作用，并为RANK/RANKL/OPG通路开辟了新的生理机制。

The discovery of RANK/RANKL/OPG the receptor-activator of nuclear factor B (RANK), the receptor-activator of nuclear factor B 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.