Distinct signal transductions in fast- and slow- twitch muscles upon denervation

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.

The Role of AMPK in the Regulation of Skeletal Muscle Size, Hypertrophy, and Regeneration

“…Disuse atrophy of rodent skeletal muscle after 1–4 weeks of hindlimb unloading (HU) has been reported to increase [ 94 ] or decrease AMPK phosphorylation [ 95 , 96 , 97 ], while others observed no effect of HU on AMPKα1 and AMPKα2 activity or on acetyl-CoA carboxylase (ACC; AMPK target and marker of AMPK activity) phosphorylation [ 98 ]. Similarly, AMPK activity is reported by some to increase after 4 and 7 days of denervation in mice and/or rats [ 99 , 100 , 101 , 102 ], while spinal cord transection does not alter AMPKα2 activity in muscle [ 103 ]. In the denervation model, these conflicting results may be due to timing since AMPK phosphorylation in denervated soleus muscles is decreased during at least the first 24 h post-denervation, is not different at 3 days, and is elevated by 7 days post-denervation [ 104 ] compared to control muscles.…”

UCHL1 regulates oxidative activity in skeletal muscle

“…As described previously [ 20 ], mice were anesthetized with isoflurane. Hindlimb muscles were carefully exposed by removing skin.…”

“…As described previously [ 20 , 23 ], muscle tissues were homogenized in 1X RIPA buffer containing freshly added 1% protease inhibitor cocktail (Santa Cruz, Dallas, TX) and 1% phosphatase inhibitor (Research Product International, Mount Prospect, IL). Protein concentration of samples was determined by a standard BCA assay and normalized.…”

AMP-Activated Protein Kinase as a Key Trigger for the Disuse-Induced Skeletal Muscle Remodeling

“…Moreover, the literature reveals contradictory results concerning AMPK Thr172 phosphorylation at various time-points in different models of disuse. It has been shown that four- and seven-day denervation resulted in a significant increase in AMPK phosphorylation in rodent skeletal muscles [ 51 , 52 ], while deletion of AMPKα2 significantly attenuated denervation-induced skeletal muscle wasting and protein degradation [ 51 ]. In human skeletal muscle with recent complete cervical spinal cord injury, AMPKα2 protein abundance decreased by 25% during the first year after injury, without significant change in AMPKα1 content.…”