Dystrophin Involved in the Susceptibility of Slow Muscles to Hindlimb Unloading via Concomitant Activation of TGF-β1/Smad3 Signaling and Ubiquitin–Proteasome Degradation in Mice

• Published in: Cell biochemistry and biophysics 2014-11, Vol.70 (2), p.1057-1067
• Main Authors: Zhang, Peng, Li, Wenjiong, Liu, Hongju, Li, Jinglong, Wang, Jing, Li, Yanan, Chen, Xiaoping, Yang, Zhong, Fan, Ming
• Format: Article
• Language: English
• Subjects: Animals; Biochemistry; Biological and Medical Physics; Biomedical and Life Sciences; Biophysics; Biotechnology; Cell Biology; Down-Regulation; Dystrophin - metabolism; Hindlimb Suspension; Life Sciences; Male; Mice; Muscle Fibers, Fast-Twitch - metabolism; Muscle Fibers, Fast-Twitch - pathology; Muscle Fibers, Slow-Twitch - metabolism; Muscle Fibers, Slow-Twitch - pathology; Muscle Proteins - genetics; Muscular Atrophy - etiology; Muscular Atrophy - metabolism; Muscular Atrophy - pathology; Myosin Heavy Chains - genetics; Original Paper; Pharmacology/Toxicology; Proteasome Endopeptidase Complex - metabolism; Proteolysis; Signal Transduction; SKP Cullin F-Box Protein Ligases - genetics; Smad3 Protein - metabolism; Transforming Growth Factor beta1 - metabolism; Tripartite Motif Proteins; Troponin I - genetics; Ubiquitin - metabolism; Ubiquitin-Protein Ligases - genetics
• ISSN: 1085-9195; 1559-0283
• DOI: 10.1007/s12013-014-0023-4

While it is well known that the slow-twitch muscles are vulnerable to microgravity conditions, the molecular and cellular mechanisms underlying this phenomenon remain unknown. Dystrophin, which constitutes an important link between the cytoskeleton and the extracellular matrix, is hypothesized to be involved in force generation and mechanical stabilization of the skeletal muscle. Here we have shown that after a 14-day hindlimb unloading (HU) of the C57BL/10 mice, the expression of dystrophin was significantly down-regulated in the fast-twitch myofibers, while in the slow-twitch myofibers, it was up-regulated. In order to investigate the role of dystrophin in HU-induced susceptibility to muscle atrophy, we compared the degradation signaling mechanisms of slow-twitch soleus muscle in dystrophin-deficient (mdx) and the wild-type (WT) mice. We found that mdx mice manifest less reduction of muscle mass and myofiber cross-sectional area than the control animals. Also, the expression of two ubiquitin ligases (MuRF1, Atrogin-1), which plays a crucial role in the ubiquitin–proteasome-mediated muscular degradation, was significantly down-regulated in soleus muscle of the hindlimb-unloaded mdx mice. In comparison, in the soleus muscle of unloaded WT mice, these ligases were significantly up-regulated. Whereas the hindlimb unloading reduced the expression of transforming growth factor β (TGF-β1)/Smad3 in mdx mice, in WT mice, the expression of this growth factor was augmented in response to unloading. Correspondingly, as a result of HU of the mdx mice, the expression of four subtypes of the myosin heavy chain and troponin I was reduced or it exhibited a delayed slow-to-fast transition. In summary, our results suggest that dystrophin exerts an intermediary and positive role in the disuse atrophy of the slow-twitch muscles. This effect is mediated through the activation of TGF-β1/Smad3 signaling and downstream ubiquitin–proteasome pathway.