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

Irisin 是一种新型信号肽，由细胞膜结合蛋白 Fndc5（纤连蛋白 III 型）水解而成。 含有结构域的蛋白质 5)，首先由我们的合作者描述为在运动过程中从肌肉中释放， 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 鸢尾素在调节运动对骨骼的影响中的作用。早期的工作描述了合成代谢作用， 间歇性低剂量鸢尾素刺激皮质骨形成并防止卸载引起的骨质流失 体内的鸢尾素和来自运动小鼠的血清来源的鸢尾素在体外增强成骨细胞生成。我们组有 采用遗传学方法进一步阐明鸢尾素在骨骼重塑中的作用，证明 Fndc5在肌肉中的强制表达显着减少了骨形成和成骨细胞数量，而 增加硬化蛋白（SOST）表达和破骨细胞数量。 Fndc5 的全局基因缺失 完全防止卵巢切除术引起的骨质流失，其特点是维持骨细胞腔隙和 阻止骨吸收，但核因子 Kappa-β 配体受体激活剂 (RANKL) 不增加 尽管长期缺乏雌激素，但仍表达。同样，对野生型小鼠进行短期鸢尾素输注 导致血清硬化素和 RANKL 水平升高。这些结果凸显了鸢尾素信号传导的广度 对骨骼的影响，以及需要进一步了解其对吸收和重塑的总体影响。我们现在 还有其他证据表明鸢尾素通过整合素 αVβ5 直接向破骨细胞发出信号，刺激分化 和吸收。目前的工作致力于描绘破骨细胞中的完整信号通路，同时 采用新颖的体内模型来测试骨骼反应背景下的吸收调节 锻炼。我们开发了一种新的 Fndc5 条件小鼠等位基因，并产生了骨骼肌- 特定目标无效，我们将用它来测试肌肉来源的鸢尾素对骨骼运动反应的影响。 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 破骨细胞中的细胞内信号转导。全面评估鸢尾素对骨骼的影响 吸收，我们还将研究鸢尾素通过调节破骨细胞的间接影响 骨细胞。最后，由于骨细胞对机械信号的反应是该细胞功能的一个关键方面 练习中，我们将研究鸢尾素信号传导和机械敏感性之间潜在的相互作用。我们预测 鸢尾素的多效性作用——它可能是一个耦合因子，直接刺激成骨细胞和 破骨细胞，还作为反调节激素发挥独特的作用，通过以下方式维持钙稳态 增加吸收。通过关注破骨细胞，这项工作旨在阐明鸢尾素在启动骨形成中的作用 吸收并更好地了解其对整个重塑的影响。这些见解都提供了更大的 了解骨骼对运动的反应，并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.