Denervation induces skeletal muscle atrophy, accompanied by complex biochemical and physiological changes, with potentially devastating outcomes even an increased mortality. Currently, however, there remains a paucity of effective strategies to treat skeletal muscle atrophy. Therefore, it is required to understand the molecular mechanisms of skeletal muscle atrophy and formulate new treatment strategies. In this study, we investigated the transcriptional profile of denervated skeletal muscle after peripheral nerve injury in rats. The cDNA microarray analysis showed that a huge number of genes in tibialis anterior (TA) muscles were differentially expressed at different times after sciatic nerve transection. Notably, the 24 h of denervation might be a critical time point for triggering TA muscle atrophy. According to the data from self-organizing map (SOM), Pearson correlation heatmap, principal component analysis (PCA), and hierarchical clustering analysis, three nodal transitions in gene expression profile of the denervated TA muscle might partition the period of 0.25 h–28 days post nerve injury into four distinct transcriptional phases. Moreover, the four transcriptional phases were designated as “oxidative stress stage”, “inflammation stage”, “atrophy stage” and “atrophic fibrosis stage”, respectively, which was concluded from Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene ontology (GO) analyses at each transcriptional phase. Importantly, the differentially expressed genes at 24 h post sciatic nerve transection seemed to be mainly involved in inflammation, which might be a critical process in denervation-induced muscle atrophy. Overall, our study would contribute to the understanding of molecular aspects of denervation-induced muscle atrophy, and may also provide a new insight into the time window for targeted therapy.

Skeletal muscle atrophy occurs under various conditions, such as disuse, denervation, fasting, aging, and various diseases. Although the underlying molecular mechanisms are still not fully understood, skeletal muscle atrophy is closely associated with reactive oxygen species (ROS) overproduction. In this study, we aimed to investigate the involvement of ROS in skeletal muscle atrophy from the perspective of gene regulation, and further examine therapeutic effects of antioxidants on skeletal muscle atrophy. Microarray data showed that the gene expression of many positive regulators for ROS production were up-regulated and the gene expression of many negative regulators for ROS production were down-regulated in mouse soleus muscle atrophied by denervation (sciatic nerve injury). The ROS level was significantly increased in denervated mouse soleus muscle or fasted C2C12 myotubes that had suffered from fasting (nutrient deprivation). These two muscle samples were then treated with N-acetyl-L-cysteine (NAC, a clinically used antioxidant) or pyrroloquinoline quinone (PQQ, a naturally occurring antioxidant), respectively. As compared to non-treatment, both NAC and PQQ treatment (1) reversed the increase in the ROS level in two muscle samples; (2) attenuated the reduction in the cross-sectional area (CSA) of denervated mouse muscle or in the diameter of fasted C2C12 myotube; (3) increased the myosin heavy chain (MHC) level and decreased the muscle atrophy F-box (MAFbx) and muscle-specific RING finger-1 (MuRF-1) levels in two muscle samples. Collectively, these results suggested that an increased ROS level was, at least partly, responsible for denervation- or fasting-induced skeletal muscle atrophy, and antioxidants might resist the atrophic effect via ROS-related mechanisms.

Tumor necrosis factor receptor-associated factor 6 (TRAF6), a unique E3 ubiquitin ligase and adaptor protein, is involved in activation of various signaling cascades. Recent studies identify TRAF6 as one of the novel regulators of skeletal muscle atrophy. The role of TRAF6 in glucocorticoid-induced muscle atrophy, however, remains to be elucidated. In this study, we show that TRAF6 and its downstream signaling molecules, muscle atrophy F-box (MAFBx) and muscle ring finger 1 (MuRF1), were all upregulated in dexamethasone-induced atrophy of mouse C2C12 myotubes or mouse tibialis anterior (TA) muscle. To further investigate the role of TRAF6 in dexamethasone-induced muscle atrophy, TRAF6-siRNA was used to transfect cultured C2C12 myotubes or was injected into the TA muscle of mice respectively, and we note that TRAF6 knockdown attenuated dexamethasone-induced muscle atrophy in vitro and in vivo, and concomitantly decreased the expression of MuRF1 and MAFBx. Our findings suggest that a decreased expression of TRAF6 could rescue dexamethasone-induced skeletal muscle atrophy through, at least in part, regulation of the expression of MAFBx and MuRF1.

To have a better insight into the molecular events involved in denervation-induced atrophy and reinnervation-induced regeneration of skeletal muscles, it is important to investigate the changes in expression levels of a great multitude of muscle proteins during the process of denervation-reinnervation. In this study, we employed an experimental model of rat sciatic nerve crush to examine the differentially expressed proteins in the rat gastrocnemius muscle at different time points (0, 1, 2, 3, 4 weeks) after sciatic nerve crush by using two-dimensional gel electrophoresis (2-DE) followed by matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-TOF-MS), collectively referred to as the modern proteomic analysis. The results showed that 16 proteins in the rat gastrocnemius muscle exhibited two distinct types of change pattern in their relative abundance: (1) The relative expression levels of 11 proteins (including alpha actin, myosin heavy chain, etc.) were decreased either within 1 or 2 weeks post-sciatic nerve injury, followed by restoration during the ensuing days until 4 weeks. (2) The other 5 proteins (including alpha enolase, beta enolase, signal peptide peptidase-like 3, etc.) displayed an up-regulation in their relative expression levels within 1 week following sciatic nerve injury, and a subsequent gradual decrease in their relative expression levels until 4 weeks. Moreover, the significance of the changes in expression levels of the 16 proteins during denervation-reinnervation has been selectively discussed.

Skeletal muscle atrophy is associated with pro-inflammatory cytokines. Salidroside is a biologically active ingredient of Rhodiola rosea, which exhibits anti-inflammatory property. However, there is little known about the effect of salidroside on denervation-induced muscle atrophy. Therefore, the present study aimed to determine whether salidroside could protect against denervation-induced muscle atrophy and to clarify potential molecular mechanisms. Denervation caused progressive accumulation of inflammatory factors in skeletal muscle, especially interleukin 6 (IL6) and its receptor, and recombinant murine IL6 (rmIL6) local infusion could induce target muscle atrophy, suggesting that denervation induced inflammation in target muscles and the inflammation may trigger muscle wasting. Salidroside alleviated denervation-induced muscle atrophy and inhibited the production of IL6. Furthermore, the inhibition of phosphorylation of signal transducer and activator of transcription 3 (STAT3), and the decreased levels of suppressor of cytokine signaling (SOCS3), muscle RING finger protein-1 (MuRF1), atrophy F-box (atrogin-1), microtubule-associated protein light chain 3 beta (LC3B) and PTEN-induced putative kinase (PINK1) were observed in denervated muscles that were treated with salidroside. Finally, all of these responses to salidroside were replicated in neutralizing antibody against IL6. Taken together, these results suggest that salidroside alleviates denervation-induced inflammation response, thereby inhibits muscle proteolysis and muscle atrophy. Therefore, it was assumed that salidroside might be a potential therapeutic candidate to prevent muscle wasting.

Skeletal muscle atrophy is a common and debilitating condition that lacks an effective therapy. Oxidative stress and inflammation are two main molecular mechanisms involved in muscle atrophy. In the current study, we want to explore whether and how salidroside, with antioxidant and anti-inflammatory properties, protects against skeletal muscle atrophy induced by denervation. First, oxidative stress and inflammatory response were examined during myotube atrophy induced by nutrition deprivation. The results demonstrated that oxidative stress and inflammatory response were induced in cultured myotubes suffered from nutrition deprivation, and salidroside not only inhibited oxidative stress and inflammatory response but also attenuated nutrition deprivation-induced myotube atrophy, as evidenced by an increased myotube diameter. The antioxidant, anti-inflammatory, and antiatrophic properties of salidroside in cultured myotubes were confirmed in denervated mouse models. The mice treated with salidroside showed less oxidative stress and less inflammatory cytokines, as well as higher skeletal muscle wet weight ratio and larger average cross sectional areas of myofibers compared with those treated with saline only during denervation-induced skeletal muscle atrophy. Moreover, salidroside treatment of denervated mice resulted in an inhibition of the activation of mitophagy in skeletal muscle. Furthermore, salidroside reduced the expression of atrophic genes, including MuRF1 and MAFbx, autophagy genes, including PINK1, BNIP3, LC3B, ATG7, and Beclin1, and transcription factor forkhead box O3 A (Foxo3A), and improved the expression of myosin heavy chain and transcriptional factor phosphorylated Foxo3A. Taken together, these results suggested that salidroside alleviated denervation-induced muscle atrophy by suppressing oxidative stress and inflammation.

Background: The molecular mechanisms underlying denervated skeletal muscle atrophy with concomitant muscle mass loss have not been fully elucidated. Therefore, this study aimed to attain a deeper understanding of the molecular mechanisms underlying denervated skeletal muscle atrophy as a critical step to developing targeted therapy and retarding the concomitant loss of skeletal muscle mass. Methods:We employed microarray analysis to reveal the potential molecular mechanisms underlying denervated skeletal muscle atrophy. We used in vitro and in vivo atrophy models to explore the roles of the interleukin 6 (IL-6), Janus kinase (JAK), and signal transducers and activators of transcription 3 (STAT3) in muscle atrophy.Results: In this study, microarray analysis of the differentially expressed genes demonstrated that inflammation-related cytokines were markedly triggered and IL-6/JAK/STAT3 signaling pathway was strongly activated during denervated skeletal muscle atrophy. The high level of IL-6 enhanced C2C12 myotube atrophy through the activation of JAK/STAT3, while inhibiting JAK/STAT3 pathway by ruxolitinib (a JAK1/2 inhibitor) or C188-9 (a STAT3 inhibitor) significantly attenuated C2C12 myotube atrophy induced by IL-6. Pharmacological blocking of IL-6 by tocilizumab (antibody against IL-6 receptor) and pharmacological/genetic inhibition of JAK/STAT3 pathway by ruxolitinib/C188-9 (JAK/STAT3 inhibitor) and STAT3 short hairpin RNA (shRNA) lentivirus in tibialis anterior muscles could suppress muscle atrophy and inhibit mitophagy, and was accompanied by the decreased expression of atrophic genes (MuRF1 and MAFbx) and autophagy-related genes (PINK1, BNIP3, Beclin 1, ATG7, and LC3B).Conclusions: Taken together, the results suggest that IL-6/JAK/STAT3 pathway may be a principal mediator in denervated skeletal muscle atrophy, meaning targeted therapy against IL-6/JAK/STAT3 pathway might have potential as a therapeutic strategy for prevention of skeletal muscle atrophy.

Various diseases can cause skeletal muscle atrophy, usually accompanied by inflammation, mitochondrial dysfunction, apoptosis, decreased protein synthesis, and enhanced proteolysis. The underlying mechanism of inflammation in skeletal muscle atrophy is extremely complex and has not been fully elucidated, thus hindering the development of effective therapeutic drugs and preventive measures for skeletal muscle atrophy. In this review, we elaborate on protein degradation pathways, including the ubiquitin-proteasome system (UPS), the autophagy-lysosome pathway (ALP), the calpain and caspase pathways, the insulin growth factor 1/Akt protein synthesis pathway, myostatin, and muscle satellite cells, in the process of muscle atrophy. Under an inflammatory environment, various pro-inflammatory cytokines directly act on nuclear factor-κB, p38MAPK, and JAK/STAT pathways through the corresponding receptors, and then are involved in muscle atrophy. Inflammation can also indirectly trigger skeletal muscle atrophy by changing the metabolic state of other tissues or cells. This paper explores the changes in the hypothalamic-pituitary-adrenal axis and fat metabolism under inflammatory conditions as well as their effects on skeletal muscle. Moreover, this paper also reviews various signaling pathways related to muscle atrophy under inflammatory conditions, such as cachexia, sepsis, type 2 diabetes mellitus, obesity, chronic obstructive pulmonary disease, chronic kidney disease, and nerve injury. Finally, this paper summarizes anti-amyotrophic drugs and their therapeutic targets for inflammation in recent years. Overall, inflammation is a key factor causing skeletal muscle atrophy, and anti-inflammation might be an effective strategy for the treatment of skeletal muscle atrophy. Various inflammatory factors and their downstream pathways are considered promising targets for the treatment and prevention of skeletal muscle atrophy.