Abstract
Denervation induces skeletal muscle atrophy, which primarily impairs oxidative slow twitch fibers. The underlying mechanism of this phenomenon, however, remains to be addressed. We hypothesize that denervation‐induced fiber‐specific atrophy may result from the distinct activities of different signaling pathways that are involved in protein synthesis and degradation in fast‐ and slow‐twitch fibers. In this study, 1‐month‐old male mice were subjected to unilateral sciatic denervation for 4 days. Fast‐twitch muscle extensor digitorum longus (EDL) and slow‐twitch muscle soleus were collected from the denervated side and the control side of hind limbs. Total and phosphorylated protein levels of key factors of major signaling pathways in these tissues were determined using western blot assay. Our data showed that total AKT and FoxO3 protein levels were upregulated in denervated muscles as compared with control sides. Phosphorylation of AKT and FoxO3 were proportionally enhanced in denervated EDL but not soleus, indicating AKT activation drives phosphorylation of FoxO3 in EDL but not in soleus upon denervation. As a result, FoxO3‐targeted atrogenes MurF1 and Atrogin1 protein abundances were reduced in denervated EDL but not altered in soleus. In consistent with this change, polyubiquitination were significantly increased in denervated soleus, but only a slight increase in ubiquitination was found in denervated EDL. Autophagy marker LC3 protein level was significantly increased in both muscle types, but in greater extent in EDL after denervation. IRS1 protein level and active ERK were reduced in both muscles upon denervation, which might contribute to the upregulation of total AKT protein level and FoxO3 abundance in EDL and soleus. Total and phosphorylated AMPK protein levels were increased in denervated soleus but not in EDL. Overall, these data reveal that the key signaling pathways that regulate protein synthesis and degradation are more sensitive in soleus than EDL in response to denervation.
Highlights
Skeletal muscle composes distinct muscle fibers, which are defined by myosin heavy chain (MyHC) isoforms
Physiological Reports published by Wiley Periodicals, Inc. on behalf of The Physiological Society and the American Physiological Society
Total AKT protein abundance was enhanced in denervated muscles, with a sixfold increase in extensor digitorum longus (EDL) but only a threefold increase in soleus in response to denervation (Fig. 1B left panel)
Summary
Skeletal muscle composes distinct muscle fibers, which are defined by myosin heavy chain (MyHC) isoforms. Soleus is the typical muscle with slow-twitch fibers, and extensor digitorum longus (EDL) is predominantly composed of fasttwitch glycolytic fibers (Schiaffino and Reggiani 2011). Skeletal muscle denervation, such as in trauma and aging, causes muscle atrophy, denervation-induced muscle atrophy features greater extent of muscle loss in type I fiber than type II type (Macpherson et al 2011; Wang and Pessin 2013). The underlying mechanism by which type I fiber is more prone to atrophy remains to be fully understood. A thorough understanding of this mechanism is critical for development of a strategy to prevent the detrimental consequences of denervation-induced muscle atrophy
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