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

Abstract

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.

Beneficial cilostazol therapeutic effects in mdx dystrophic skeletal muscle

“…32 No differences were found in MAFbx levels in the dystrophic diaphragm muscles of the three experimental groups. Others have also reported unchanged MAFbx levels in mdx mice compared with C57BL/10 mice 33 and in a dog model of Duchenne MD. 34 The mechanism behind the protective effect of cilostazol observed in the dystrophic muscle could be due to its anti-inflammatory and antioxidative activity.…”

“…No differences were found in MAFbx levels in the dystrophic diaphragm muscles of the three experimental groups. Others have also reported unchanged MAFbx levels in mdx mice compared with C57BL/10 mice and in a dog model of Duchenne MD…”

Endotoxin-induced skeletal muscle wasting is prevented by angiotensin-(1–7) through a p38 MAPK-dependent mechanism

“…LPS has been reported to induce the expression of TGF-β1 (transforming growth factor β1) in endothelial cells [74][75][76]. Furthermore, TGF-β1 is capable of inducing skeletal muscle atrophy [77] through UPP activation and the up-regulation of atrogin-1 and MuRF-1 [78]. Ang-(1-7), through the Mas receptor, decreases the expression and activity of TGF-β1 through the inhibition of ROS production [44].…”

Apocynin inhibits the upregulation of TGF-β1 expression and ROS production induced by TGF-β in skeletal muscle cells

“…Thus, our data are in agreement with these reports since they indirectly suggest that the fibrotic effect mediated by TGF-β could require ROS in skeletal muscle. Interestingly, TGF-β also produces skeletal muscle atrophy, as has been recently reported (Mendias et al, 2012;Narola et al, 2013;Ohsawa et al, 2012;Zhang et al, 2014). However, the molecular mechanism and signalling pathways that are involved have not been elucidated yet.…”