Key Laboratory for Space Bioscience and Biotechnology, School of Life Sciences, Northwestern Polytechnical University, Xi'an 710072, Shaanxi, China.
Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, TX 78229, USA.
Cells. 2021 Jan 26;10(2):237. doi: 10.3390/cells10020237.
Bone-muscle crosstalk plays an important role in skeletal biomechanical function, the progression of numerous pathological conditions, and the modulation of local and distant cellular environments. Previous work has revealed that the deletion of connexin (Cx) 43 in osteoblasts, and consequently, osteocytes, indirectly compromises skeletal muscle formation and function. However, the respective roles of Cx43-formed gap junction channels (GJs) and hemichannels (HCs) in the bone-muscle crosstalk are poorly understood. To this end, we used two Cx43 osteocyte-specific transgenic mouse models expressing dominant negative mutants, Δ130-136 (GJs and HCs functions are inhibited), and R76W (only GJs function is blocked), to determine the effect of these two types of Cx43 channels on neighboring skeletal muscle. Blockage of osteocyte Cx43 GJs and HCs in Δ130-136 mice decreased fast-twitch muscle mass with reduced muscle protein synthesis and increased muscle protein degradation. Both R76W and Δ130-136 mice exhibited decreased muscle contractile force accompanied by a fast-to-slow fiber transition in typically fast-twitch muscles. In vitro results further showed that myotube formation of C2C12 myoblasts was inhibited after treatment with the primary osteocyte conditioned media (PO CM) from R76W and Δ130-136 mice. Additionally, prostaglandin E2 (PGE2) level was significantly reduced in both the circulation and PO CM of the transgenic mice. Interestingly, the injection of PGE2 to the transgenic mice rescued fast-twitch muscle mass and function; however, this had little effect on protein synthesis and degradation. These findings indicate a channel-specific response: inhibition of osteocytic Cx43 HCs decreases fast-twitch skeletal muscle mass alongside reduced protein synthesis and increased protein degradation. In contrast, blockage of Cx43 GJs results in decreased fast-twitch skeletal muscle contractile force and myogenesis, with PGE2 partially accounting for the measured differences.
骨-肌串扰在骨骼生物力学功能、许多病理状况的进展以及局部和远处细胞环境的调节中起着重要作用。以前的工作表明，成骨细胞中连接蛋白 (Cx) 43 的缺失，进而导致成骨细胞中的骨细胞，间接损害了骨骼肌的形成和功能。然而，Cx43 形成的缝隙连接通道 (GJ) 和半通道 (HC) 在骨-肌串扰中的各自作用还知之甚少。为此，我们使用两种表达显性负突变体的 Cx43 骨细胞特异性转基因小鼠模型，Δ130-136（GJ 和 HC 功能均受抑制）和 R76W（仅 GJ 功能受阻），以确定这两种类型的 Cx43 通道对相邻骨骼肌的影响。Δ130-136 小鼠中骨细胞 Cx43 GJ 和 HC 的阻断减少了快肌质量，同时降低了肌肉蛋白合成并增加了肌肉蛋白降解。R76W 和 Δ130-136 小鼠均表现出肌肉收缩力下降，同时典型的快肌发生快肌向慢肌的转变。体外结果进一步表明，用 R76W 和 Δ130-136 小鼠的原代骨细胞条件培养基 (PO CM) 处理后，C2C12 成肌细胞的肌管形成受到抑制。此外，转基因小鼠的循环和 PO CM 中的前列腺素 E2 (PGE2) 水平显著降低。有趣的是，向转基因小鼠注射 PGE2 可挽救快肌质量和功能；然而，这对蛋白质合成和降解几乎没有影响。这些发现表明存在一种通道特异性反应：抑制成骨细胞 Cx43 HC 会减少快肌骨骼肌质量，同时降低蛋白质合成并增加蛋白质降解。相比之下，Cx43 GJ 的阻断会导致快肌骨骼肌收缩力和肌生成减少，而 PGE2 部分解释了所测量的差异。
