The Role of Irisin in Initiating Resorption During the Skeletal Response to Exercise

鸢尾素是一种新的信号肽，由细胞膜结合蛋白Fndc5（纤连蛋白III型）水解而成 含结构域的蛋白质5），首先由我们的合作者描述，在运动过程中从肌肉释放， 在米色脂肪组织中诱导产热程序。越来越多的工作凸显了另一个关键 Irisin在调节运动对骨的影响中的作用。早期的工作描述了一种合成代谢作用， 间歇性低剂量的鸢尾素刺激皮质骨形成并防止卸载诱导的骨丢失 在体内，和血清衍生的鸢尾素从运动小鼠增强成骨细胞在体外。我们集团 采用遗传学方法进一步阐明了鸢尾素在骨骼重塑中的作用，证明 肌肉中Fndc5的强制表达显著减少骨形成和成骨细胞数量， 增加硬化蛋白（SOST）表达和破骨细胞数量。Fndc5基因缺失 完全防止卵巢切除术引起的骨丢失，以维持骨细胞陷窝为标志， 阻断骨吸收，不增加核因子κ-β受体激活剂配体（RANKL） 表达，尽管长期雌激素剥夺。类似地，在野生型小鼠中短期输注鸢尾素 导致硬化素和RANKL的血清水平更高。这些结果突出了鸢尾素信号传导的广度 对骨的影响，以及进一步了解其对再吸收和重塑的总体影响的必要性。我们现在 有额外的证据表明鸢尾素通过整合素α V β 5直接向破骨细胞发出信号，刺激分化 和再吸收。目前的工作致力于描绘破骨细胞的完整信号通路， 采用新的体内模型来测试在骨骼对以下物质的反应的背景下对再吸收的这种调节： 锻炼的我们已经开发了一种新的条件性小鼠Fndc5等位基因，并产生了骨骼肌- 特异性靶向无效，我们将使用它来测试肌肉来源的鸢尾素对骨骼对运动的反应的影响。 与此同时，我们将利用体外技术来确认鸢尾素的假定受体，并表征 破骨细胞的细胞内信号转导。全面评估鸢尾素对骨骼的影响 吸收，我们还将研究鸢尾素对破骨细胞的间接影响，通过调节， 骨细胞最后，由于骨细胞对机械刺激的反应是骨细胞在骨形成过程中功能的一个关键方面， 运动，我们将研究鸢尾素信号和机械敏感性之间的潜在相互作用。我们预测 鸢尾素的多效性作用-它可能是一种偶联因子，通过直接刺激成骨细胞和 破骨细胞，也作为一个独特的作用，作为一个反调节激素，以维持钙稳态， 增加再吸收。通过聚焦破骨细胞，这项工作试图阐明鸢尾素在启动骨形成中的作用。 吸收，更好地了解其对整体重塑的影响。这些见解都提供了一个更大的 了解骨骼对运动的反应，但为解决骨退行性疾病提供信息， 利用天然信号通路。

Irisin is a novel signaling peptide proteolyzed from the cell membrane-bound protein Fndc5 (fibronectin type III domain-containing protein 5), first described by our collaborators to release from muscle during exercise and induce a thermogenic program in beige adipose tissue. A growing body of work has highlighted another key role for irisin in mediating the effect of exercise on bone. Early work described an anabolic action, with intermittent low doses of irisin stimulating cortical bone formation and preventing unloading-induced bone loss in vivo, and serum-derived irisin from exercised mice enhancing osteoblastogenesis in vitro. Our group has employed a genetic approach to further elucidate the role of irisin in skeletal remodeling, demonstrating that forced expression of Fndc5 in muscle markedly reduced bone formation and osteoblast numbers while increasing sclerostin (SOST) expression and osteoclast numbers. Global genetic deletion of Fndc5 conferred complete protection against ovariectomy-induced bone loss, marked by maintenance of osteocyte lacunae and blocked bone resorption with no increases in Receptor Activator of Nuclear Factor Kappa-β Ligand (RANKL) expression despite prolonged estrogen deprivation. Similarly, short term irisin infusions in wild type mice resulted in higher serum levels of sclerostin and RANKL. These results highlight the breadth of irisin’s signaling effects in bone, and the need to further understand its overall impact on resorption and remodelling. We now have additional evidence that irisin signals directly to the osteoclast via integrin αVβ5, stimulating differentiation and resorption. The present work endeavors to delineate the full signaling pathway in the osteoclast, while employing novel in vivo models to test this regulation of resorption in the context of skeletal response to exercise. We have developed a novel conditional mouse allele of Fndc5 and generated a skeletal muscle- specific targeted null, which we will use to test the effect of muscle-derived irisin on bone response to exercise. In parallel, we will utilize in vitro techniques to confirm the putative receptors for irisin and characterize intracellular signal transduction in the osteoclast. To comprehensively assess irisin’s impact on bone resorption, we will also investigate indirect effects of irisin on the osteoclast, through regulation by the osteocyte. Finally, as osteocyte response to mechanical cues is a key aspect of this cell’s function during exercise, we will investigate the potential interplay between irisin signaling and mechanosensitivity. We predict pleiotropic effects of irisin – it may be a coupling factor by directly stimulating both the osteoblast and osteoclast, and also serve a unique role as a counter regulatory hormone to maintain calcium homeostasis by increasing resorption. By focusing on the osteoclast, this work seeks to elucidate irisin’s role in initiating bone resorption and better understand its influence on remodeling as a whole. Such insights both provide a greater understanding of skeletal response to exercise but inform efforts to address bone degenerative diseases by taking advantage of native signaling pathways.