Duchenne muscular dystrophy (DMD) is the most common muscular dystrophy seen in children. In addition to skeletal muscle, DMD also has a significant impact on bone. The pathogenesis of bone abnormalities in DMD is still unknown. Recently, we have identified a novel bone-regulating cytokine, fibroblast growth factor-21 (FGF-21), which is dramatically upregulated in skeletal muscles from DMD animal models. We hypothesize that muscle-derived FGF-21 negatively affects bone homeostasis in DMD. Dystrophin/utrophin double-knockout (dKO) mice were used in this study. We found that the levels of circulating FGF-21 were significantly higher in dKO mice than in age-matched WT controls. Further tests on FGF-21 expressing tissues revealed that both FGF-21 mRNA and protein expression were dramatically upregulated in dystrophic skeletal muscles, whereas FGF-21 mRNA expression was downregulated in liver and white adipose tissue (WAT) compared to WT controls. Neutralization of circulating FGF-21 by i.p. injection of anti-FGF-21 antibody significantly alleviated progressive bone loss in weight-bearing (vertebra, femur, and tibia) and non-weight bearing bones (parietal bones) in dKO mice. We also found that FGF-21 directly promoted RANKL-induced osteoclastogenesis from bone marrow macrophages (BMMs), as well as promoted adipogenesis while concomitantly inhibiting osteogenesis of bone marrow mesenchymal stem cells (BMMSCs). Furthermore, fibroblast growth factor receptors (FGFRs) and co-receptor β-klotho (KLB) were expressed in bone cells (BMM-derived osteoclasts and BMMSCs) and bone tissues. KLB knockdown by small interfering RNAs (siRNAs) significantly inhibited the effects of FGF21 on osteoclast formation of BMMs and on adipogenic differentiation of BMMSCs, indicating that FGF-21 may directly affect dystrophic bone via the FGFRs-β-klotho complex. In conclusion, this study shows that dystrophic skeletal muscles express and secrete significant levels of FGF-21, which negatively regulates bone homeostasis and represents an important pathological factor for the development of bone abnormalities in DMD. The current study highlights the importance of muscle/bone cross-talk via muscle-derived factors (myokines) in the pathogenesis of bone abnormalities in DMD. © 2019 American Society for Bone and Mineral Research.

中文翻译：

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FGF-21 表达增加对 Dystrophin/Utrophin 双基因敲除小鼠的骨稳态产生负面影响。

杜氏肌营养不良症 (DMD) 是儿童中最常见的肌营养不良症。除了骨骼肌，DMD 对骨骼也有显着影响。DMD中骨异常的发病机制仍然未知。最近，我们发现了一种新的骨调节细胞因子，成纤维细胞生长因子 21 (FGF-21)，它在 DMD 动物模型的骨骼肌中显着上调。我们假设肌肉来源的 FGF-21 对 DMD 中的骨稳态产生负面影响。本研究中使用了肌营养不良蛋白/utrophin 双敲除 (dKO) 小鼠。我们发现 dKO 小鼠的循环 FGF-21 水平显着高于年龄匹配的 WT 对照。对 FGF-21 表达组织的进一步测试表明，在营养不良的骨骼肌中，FGF-21 mRNA 和蛋白质表达均显着上调，而与WT对照相比，FGF-21 mRNA表达在肝脏和白色脂肪组织（WAT）中下调。通过腹腔注射抗 FGF-21 抗体中和循环 FGF-21 显着减轻了 dKO 小鼠负重（椎骨、股骨和胫骨）和非负重骨（顶骨）的进行性骨丢失。我们还发现 FGF-21 直接促进 RANKL 诱导的骨髓巨噬细胞 (BMM) 的破骨细胞生成，并促进脂肪生成，同时抑制骨髓间充质干细胞 (BMMSCs) 的成骨。此外，成纤维细胞生长因子受体 (FGFRs) 和辅助受体 β-klotho (KLB) 在​​骨细胞（BMM 衍生的破骨细胞和 BMMSCs）和骨组织中表达。通过小干扰 RNA (siRNA) 敲低 KLB 显着抑制了 FGF21 对 BMM 破骨细胞形成和 BMMSCs 成脂分化的影响，表明 FGF-21 可能通过 FGFRs-β-klotho 复合物直接影响营养不良的骨。总之，这项研究表明，营养不良的骨骼肌表达和分泌显着水平的 FGF-21，它负向调节骨稳态，是 DMD 骨异常发展的重要病理因素。目前的研究强调了通过肌肉衍生因子（肌因子）进行的肌肉/骨骼串扰在 DMD 骨骼异常发病机制中的重要性。© 2019 美国骨与矿物研究学会。表明 FGF-21 可能通过 FGFRs-β-klotho 复合物直接影响营养不良的骨。总之，这项研究表明，营养不良的骨骼肌表达和分泌显着水平的 FGF-21，它负向调节骨稳态，是 DMD 骨异常发展的重要病理因素。目前的研究强调了通过肌肉衍生因子（肌因子）进行的肌肉/骨骼串扰在 DMD 骨骼异常发病机制中的重要性。© 2019 美国骨与矿物研究学会。表明 FGF-21 可能通过 FGFRs-β-klotho 复合物直接影响营养不良的骨。总之，这项研究表明，营养不良的骨骼肌表达和分泌显着水平的 FGF-21，它负向调节骨稳态，是 DMD 骨异常发展的重要病理因素。目前的研究强调了通过肌肉衍生因子（肌因子）进行的肌肉/骨骼串扰在 DMD 骨骼异常发病机制中的重要性。© 2019 美国骨与矿物研究学会。目前的研究强调了通过肌肉衍生因子（肌因子）进行的肌肉/骨骼串扰在 DMD 骨骼异常发病机制中的重要性。© 2019 美国骨与矿物研究学会。目前的研究强调了通过肌肉衍生因子（肌因子）进行的肌肉/骨骼串扰在 DMD 骨骼异常发病机制中的重要性。© 2019 美国骨与矿物研究学会。