The shivering and nonshivering thermogenesis in skeletal muscles is important for maintaining body temperature in a cold environment. In addition to nervous-humoral regulation, adipose tissue was demonstrated to directly respond to cold in a cell-autonomous manner to produce heat. However, whether skeletal muscle can directly respond to low temperature in an autoregulatory manner is unknown. Transient receptor potential (TRP) channels TRPM8 and TRPA1 are two important cold sensors. In the current study, we found TRPM8 was expressed in mouse skeletal muscle tissue and C2C12 myotubes by RT-PCR. After exposure to 33 °C for 6 h, the gene expression pattern of C2C12 myotubes was significantly changed which was evidenced by RNA-sequencing. KEGG-Pathway enrichment analysis of these differentially expressed genes showed that low temperature changed several important signaling pathways, such as IL-17, TNFα, MAPK, FoxO, Hedgehog, Hippo, Toll-like receptor, Notch, and Wnt signaling pathways. Protein-protein interaction network analysis revealed that IL-6 gene was a key gene which was directly affected by low temperature in skeletal muscle cells. In addition, both mRNA and protein levels of IL-6 were increased by 33 °C exposure in C2C12 myotubes. In conclusion, our findings demonstrated that skeletal muscle cells could directly respond to low temperature, characterized by upregulated expression of IL-6 in skeletal muscle cells.
Animals ; Cold Temperature ; Interleukin-6/metabolism* ; Mice ; Muscle Fibers, Skeletal/metabolism* ; Muscle, Skeletal/physiology* ; Temperature

Skeletal muscle plays a paramount role in physical activity, metabolism, and energy balance, while its homeostasis is being challenged by multiple unfavorable factors such as injury, aging, or obesity. Exosomes, a subset of extracellular vesicles, are now recognized as essential mediators of intercellular communication, holding great clinical potential in the treatment of skeletal muscle diseases. Herein, we outline the recent research progress in exosomal isolation, characterization, and mechanism of action, and emphatically discuss current advances in exosomes derived from multiple organs and tissues, and engineered exosomes regarding the regulation of physiological and pathological development of skeletal muscle. These remarkable advances expand our understanding of myogenesis and muscle diseases. Meanwhile, the engineered exosome, as an endogenous nanocarrier combined with advanced design methodologies of biomolecules, will help to open up innovative therapeutic perspectives for the treatment of muscle diseases.
Exosomes/physiology* ; Muscle, Skeletal/metabolism* ; Cell Communication ; Homeostasis

Skeletal muscle is the largest organ of human body, which completes 80%-90% of glucose intake stimulated by insulin, and is closely related to the occurrence and development of insulin resistance (IR). Skeletal muscle is one of the main places of lipid metabolism, and lipid metabolites participate in skeletal muscle metabolism as signal molecules. Fatty acids regulate skeletal muscle insulin sensitivity through insulin signaling pathway, inflammatory response and mitochondrial function. Saturated fatty acids (SFAs) induce insulin resistance by impairing insulin signal transduction, inducing mitochondrial dysfunction and inflammatory response, while unsaturated fatty acids reverse the adverse effects of SFAs and ameliorate IR by enhancing insulin signal transduction and anti-inflammatory effect. In addition, disorders of lipid metabolism in skeletal muscle cause accumulation of harmful metabolic intermediates, such as diacylglycerol, ceramide and long-chain acyl-coenzyme A, and induce IR by directly or indirectly damaging insulin signaling pathway. This article reviews the research progress of lipid metabolic intermediates regulating insulin sensitivity in skeletal muscle, which will help to better understand the pathogenesis of diabetes.
Humans ; Insulin Resistance/physiology* ; Muscle, Skeletal/metabolism* ; Insulin/metabolism* ; Lipid Metabolism ; Fatty Acids/metabolism*

To investigate the difference between the functions of oxygen uptake in skeletal muscle and living habits of plateau zokor (Myospalax rufescens baileyi) and plateau pika (Ochotona curzoniac), the microvessel densities (MVD) of skeletal muscle of plateau zokor, plateau pika and Sprague-Dawley (SD) rat were measured by immunohistochemical staining; the numerical density on area (N(A)) of mitochondria, and surface density (S(V), external surface area density of mitochondria per unit volume of skeletal muscle fiber) were obtained by stereo microscope technique; mRNA levels of myoglobin (Mb) in skeletal muscle were determined by real-time PCR, and the contents of Mb protein in skeletal muscle were determined by spectro-photometer. The results showed that MVD, N(A) and S(V) of mitochondria in skeletal muscle of plateau pika were significantly lower than those of plateau zokor and SD rat (P<0.05). The mRNA levels of Mb gene in skeletal muscle of plateau zokor and plateau pika were notably higher than that of SD rat (P<0.05). There were significant differences in the contents of Mb among these three species, and plateau zokor and SD rat presented the highest and the lowest value, respectively (P<0.05). The results suggest that even though plateau zokor inhabits in the hypoxia environment, most of its skeletal muscle fiber are red muscle fiber. While most of skeletal muscle fibers of plateau pika are white muscle fibers. This kind of white muscle has low MVD, N(A) and S(V) of mitochondria and less content of Mb compared with the red one, suggesting it obtains most energy from aerobic oxidation. The above-mentioned differences in skeletal muscles may be related to not only the different species, but also the different living habits of these two high altitude species.
Animals ; Hypoxia ; Lagomorpha ; physiology ; Microvessels ; physiology ; Mitochondria, Muscle ; physiology ; Muscle Fibers, Skeletal ; physiology ; Muscle, Skeletal ; blood supply ; physiology ; Myoglobin ; metabolism ; Oxygen ; metabolism ; RNA, Messenger ; Rats ; Rats, Sprague-Dawley ; Rodentia ; physiology

OBJECTIVETo explore the adaptive mechanism to hypoxia in skeletal muscle of tibetan antelope.

METHODSTibetan sheep which living at the same altitude (4 300 m) with tibetan antelope and low altitude (1 800 m) sheep as control, the content of myoglobin (Mb) and lactic acid (LA), the activity of lactate dehydrogenase (LDH) and malate dehydrogenase (MDH) in skeletal muscles among three animals were analyzed by spectrophotometer.

RESULTSThe content of myoglobin in skeletal muscle of tibetan antelope significantly higher than that of tibetan sheep and low altitude sheep (P < 0.05). And the content of LA in skeletal muscle of tibetan antelope significantly lower than that of tibetan sheep and low altitude sheep (P < 0.05), activity of LDH and MDH in skeletal muscle was significantly lower and higher respectively than that of tibetan sheep and low altitude sheep (P < 0.05). There was no significant difference between tibetan sheep and low altitude sheep.

CONCLUSIONTibetan antelope may improve their ability to get oxygen under hypoxia by increasing the content of myoglobin in skeletal muscle, and the proportion of aerobic metabolism is high in skeletal muscle, it may be relate that with high myoglobin content in skeletal muscle, we suppose that high myoglobin content in skeletal muscle of tibetan antelope might be one of the molecular basis to adapt hypoxia.

Altitude ; Animals ; Antelopes ; metabolism ; physiology ; Hypoxia ; metabolism ; L-Lactate Dehydrogenase ; metabolism ; Malate Dehydrogenase ; metabolism ; Muscle, Skeletal ; metabolism ; Myoglobin ; metabolism

Voltage gated calcium channels (VGCCs) are multi-subunit membrane proteins present in a variety of tissues and control many essential physiological processes. Due to their vital importance, VGCCs are regulated by a myriad of proteins and signaling pathways. Here we review the literature on the regulation of VGCCs by proteolysis of the pore-forming α1 subunit, Ca(v)α(1). This form of regulation modulates channel function and degradation and affects cellular gene expression and excitability. L-type Ca(2+) channels are proteolyzed in two ways, depending on tissue localization. In the heart and skeletal muscle, the distal C-terminus of Ca(v)α(1) is cleaved and acts as an autoinhibitor when it reassociates with the proximal C-terminus. Relief of this autoinhibition underlies the β-adrenergic stimulation-induced enhancement of cardiac and skeletal muscle calcium currents, part of the "fight or flight" response. Proteolysis of the distal C-terminus of L-type channels also occurs in the brain and is probably catalyzed by a calpain-like protease. In some brain regions, the entire C-terminus of L-type Ca(2+) channels can be cleaved by an unknown protease and translocates to the nucleus acting as a transcription factor. The distal C-terminus of P/Q-channel Ca(v)α(1) is also proteolyzed and translocates to the nucleus. Truncated forms of the PQ-channel Ca(v)α(1) are produced by many disease-causing mutations and interfere with the function of full-length channels. Truncated forms of N-type channel Ca(v)α(1), generated by mutagenesis, affect the expression of full-length channels. New forms of proteolysis of VGCC subunits remain to be discovered and may represent a fruitful area of VGCC research.
Animals ; Calcium Channels, L-Type ; metabolism ; Calcium Signaling ; Humans ; Muscle, Skeletal ; physiology ; Proteolysis

The elevated plasma level of thyroxin and/or triiodothyronine in hyperthyroidism not only induces a transition from the innervated slow-twitch muscle fibers to fast-twitch fibers, but also changes the contractile function in transition muscle fibers. So the muscle weakness of thyrotoxic myopathy would relate to alteration in fatigability of tetanic contraction in muscles, especially in slow-twitch fibers. The aim of the present study was to observe the extent of fatigue of soleus in 4-week hyperthyroid rats and elucidate its underlying mechanism. The isolated soleus muscle strips were perfused in Krebs-Henseleit solution with or without an inhibitor of sarcoplasmic reticulum Ca(2+)-ATPase (SERCA), cyclopiazonic acid (CPA). The contractile function of soleus was observed in twitch and intermittent tetanic contraction. The body weight in 4-week hyperthyroid rats decreased as compared with that in the control group [(292±13) g vs (354±10) g], but there was no difference between hyperthyroid and control groups in the wet weight of soleus [(107.3±8.6) mg vs (115.1±6.9) mg]. The time to peak tension (TPT) and time from peak tension to 75% relaxation (TR(75)) in twitch contraction were shortened in the soleus of hyperthyroid rats, and the TR(75) of tetanic contraction was also shortened as compared with that in the control group [(102.8±4.1) ms vs (178.8±15.8) ms]. The optimal stimulation frequency at which a maximal tension of tetanic contraction happened was shifted from 100 Hz in the control group to 140 Hz in hyperthyroid group. The soleus of hyperthyroid rat was easier to fatigue than that of the control rat during intermittent tetanic contraction. The SERCA activity also increased in soleus of hyperthyroid rat. The TR(75) in tetanic contraction was prolonged and showed an increased fatigue resistance in the soleus of control and hyperthyroid groups treated with 1.0 μmol/L CPA. The fatigue resistance of tetanic contraction in the soleus of hyperthyroid rat increased further with 5.0 μmol/L CPA treatment, but the resting tension kept rising. The 10 μmol/L CPA reduced the fatigue resistance of tetanic contraction in the soleus of hyperthyroid rat. The above results demonstrate that the SERCA activity in soleus can also influence the relaxation duration of twitch contraction like that in the myocardium. The SERCA activity in slow-twitch fibers is possibly involved in the regulation of fatigue resistance of intermittent tetanic contraction.
Animals ; Fatigue ; Glucose ; Hyperthyroidism ; enzymology ; In Vitro Techniques ; Muscle Contraction ; Muscle Fibers, Slow-Twitch ; enzymology ; physiology ; Muscle, Skeletal ; enzymology ; physiology ; Rats ; Sarcoplasmic Reticulum Calcium-Transporting ATPases ; metabolism ; Tromethamine

Animals ; Bone and Bones ; metabolism ; Free Radicals ; metabolism ; Male ; Muscle, Skeletal ; metabolism ; Physical Conditioning, Animal ; physiology ; Rats ; Rats, Wistar

OBJECTIVETo study the effects of glucocorticoid on skeletal muscle protein metabolism in burn sepsis and its possible mechanism.

METHODSThe rats were randomly divided into four groups with 15 rats in each group. Group B, 30% TBSA full-thickness burn was produced on the back and endotoxin (6 mg/kg bw) was given intraperitoneally after the injury to simulate burn sepsis. Groups C and D, glucocorticoid receptor antagonist RU38486 (10 mg/kg bw) was given by gavage 2 hours before or 2 hours after burn with endotoxin, respectively. Group A, the rats received only normal saline in same volume as endotoxin. Plasma levels of cortisol were determined with standard procedure. Extensor digitorium longus muscles (EDL) were procured from both legs 12 hours after the injury. After weighing, the proteolytic rate was determined in vitro in an incubation system with oxygen rich environment by high performance liquid chromatography. The gene expressions of ubiquitin, E(2)-14kDa and C2 in the muscles were determined by Northern blot analysis.

RESULTSThe weight of EDL was significantly lower in group B than in group A (t = 9.03, P < 0.01). Although the weight of EDL muscles was also lower in groups C and D than in group A, it was significantly higher than in group B (t = 2.26, 6.42, P < 0.05 or P < 0.01). The concentrations of plasma cortisol were markedly higher in groups B, C and D than in group A (t = 9.03 - 22.94, P < 0.01). A 58.8% (210/357) of the total and 335.5% (4.16/1.24) of myofibrillar proteolytic rate in group B was higher than in group A (t = 36.99 and t = 46.19, P < 0.01), respectively. The total and myofibrillar proteolytic rate in group D was 28.3% (161/567) and 49.6% (2.68/5.40) and in group C 18.9% (108/567) and 23.2% (1.25/5.40), which were lower than those in group B (t = 5.34 approximately 34.68, P < 0.01), respectively. Although the expressions of ubiquitin mRNA (2.4 kb), E(2)-14 kDa mRNA (1.2 kb) and C2 mRNA in groups C and D were significantly higher than in group A, all the values were lower than those in group B (t = 3.22, 11.32, P < 0.01), especially in group C.

CONCLUSIONSThe proteolytic rate of skeletal muscle, especially the myofibrillar proteolytic rate, was enhanced during burn with sepsis. Hypersecretion of glucocorticoid could upgrade the gene expression of ubiquitin system, resulting in hyperdegradation of skeletal muscle protein during burn with sepsis. Glucocorticoid receptor antagonist RU38486 could decrease the hyperdegradation of skeletal muscle during burn with sepsis.

Animals ; Burns ; metabolism ; Gene Expression Regulation ; Glucocorticoids ; physiology ; Hydrocortisone ; blood ; Male ; Mifepristone ; pharmacology ; Muscle Proteins ; metabolism ; Muscle, Skeletal ; metabolism ; Rats ; Rats, Wistar ; Sepsis ; metabolism ; Ubiquitin ; metabolism

The aim of the present study was to investigate the effects of exercises with different durations and intensities on mitochondrial autophagy and FUNDC1 in rat skeletal muscles. Sixty male Sprague-Dawley rats were randomly divided into 2- and 4-week control groups (Con), moderate-intensity exercise groups (M-ex groups, treadmill exercise, 16 m/min, 1 h/d, 6 d/week), and high-intensity exercise groups (Hi-ex groups, treadmill exercise, 35 m/min, 20 min/d, 6 d/week). The bilateral soleus muscles were separated after the intervention, and paraffin sections were prepared for transmission electron microscopy. ELISA method was used to detect the content of citrate synthase (CS). The co-localizations of microtubule-associated protein 1 light chain 3 (LC3)/cytochrome c oxidase IV (COX-IV), FUNDC1/COX-IV and LC3/FUNDC1 were observed by immunofluorescent staining in frozen sections. The skeletal muscle mitochondria were extracted, and the expression of autophagy-related proteins, including AMPKα, p-AMPKα, Unc-51 like kinase 1 (ULK1), FUNDC1, LC3 and p62, were detected by Western blot. The results showed that exercise increased mitochondrial function, i.e. peroxisome proliferator-activated receptor γ co-activator-1α (PGC-1α), COX-I protein expression levels and CS content. There was no difference of mitochondrial function parameters between 2-week M-ex and 2-week Hi-ex groups, while mitochondrial function of 4-weeks Hi-ex group was significantly lower than that of 4-week M-ex group. Under the same exercise intensity, mitochondrial autophagy activation in skeletal muscle of 4-week exercise was higher than that in 2-week exercise group; Under the same duration of exercise, mitochondrial autophagy activation of Hi-ex group was higher than that in M-ex group. Both 2- and 4-week exercise intervention increased LC3/COX-IV, COX-IV/FUNDC1, and FUNDC1/LC3 co-localizations. Exercise increased LC3-II/LC3-I ratio, down-regulated p62 protein expression level, up-regulated FUNDC1, ULK1 protein expression levels and AMPKα phosphorylation, and the changes of these proteins in 4-week Hi-ex group were significantly greater than those in 4-week M-ex group. These results suggest exercise induces mitochondrial autophagy in skeletal muscles, and the activity of autophagy is related to the duration and intensity of exercise. The induction mechanism of exercise may involve the mediation of FUNDC1 expression through AMPK-ULK1 pathway.
Animals ; Autophagy ; Exercise Therapy ; Humans ; Male ; Membrane Proteins/physiology* ; Mitochondria ; Mitochondrial Proteins/physiology* ; Muscle, Skeletal/metabolism* ; Rats ; Rats, Sprague-Dawley