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

Severe muscle injury is hard to heal and always results in a poor prognosis. Recent studies found that extracellular vesicle-based therapy has promising prospects for regeneration medicine, however, whether extracellular vesicles have therapeutic effects on severe muscle injury is still unknown. Herein, we extracted apoptotic extracellular vesicles derived from mesenchymal stem cells (MSCs-ApoEVs) to treat cardiotoxin induced tibialis anterior (TA) injury and found that MSCs-ApoEVs promoted muscles regeneration and increased the proportion of multinucleated cells. Besides that, we also found that apoptosis was synchronized during myoblasts fusion and MSCs-ApoEVs promoted the apoptosis ratio as well as the fusion index of myoblasts. Furthermore, we revealed that MSCs-ApoEVs increased the relative level of creatine during myoblasts fusion, which was released via activated Pannexin 1 channel. Moreover, we also found that activated Pannexin 1 channel was highly expressed on the membrane of myoblasts-derived ApoEVs (Myo-ApoEVs) instead of apoptotic myoblasts, and creatine was the pivotal metabolite involved in myoblasts fusion. Collectively, our findings firstly revealed that MSCs-ApoEVs can promote muscle regeneration and elucidated that the new function of ApoEVs as passing inter-cell messages through releasing metabolites from activated Pannexin 1 channel, which will provide new evidence for extracellular vesicles-based therapy as well as improving the understanding of new functions of extracellular vesicles.
Creatine/metabolism* ; Extracellular Vesicles ; Muscle, Skeletal/metabolism* ; Myoblasts/metabolism* ; Regeneration ; Connexins/metabolism*

OBJECTIVES: To study the relationship between skeletal muscle mass index (SMI) and metabolic phenotypes of obesity in adolescents, and to provide a basis for the prevention and control of adolescent obesity and related metabolic diseases. METHODS: A total of 1 352 adolescents aged 12 to 18 years were randomly selected by stratified cluster sampling in Yinchuan City from October 2017 to September 2020, and they were surveyed using questionnaires, physical measurements, body composition measurements, and laboratory tests. According to the diagnostic criteria for metabolic abnormalities and the definition of obesity based on the body mass index, the subjects were divided into four metabolic phenotypes: metabolically healthy normal weight, metabolically healthy obesity, metabolically unhealthy normal weight, and metabolically unhealthy obesity. The association between SMI and the metabolic phenotypes was analyzed using multivariate logistic regression. RESULTS: The SMI level in the metabolically unhealthy normal weight, metabolically healthy obesity, and metabolically unhealthy obesity groups was lower than that in the metabolically healthy normal weight group (P<0.001). Multivariate logistic regression analysis showed that after adjusting for gender and age, a higher SMI level was a protective factors for adolescents to develop metabolic unhealthy normal weight, metabolically healthy obesity, and metabolically unhealthy obesity phenotypes (OR=0.74, 0.60, and 0.54, respectively; P<0.001). CONCLUSIONS Increasing SMI can reduce the risk of the development of metabolic unhealthy/obesity.
Adolescent ; Humans ; Body Mass Index ; Metabolic Syndrome/metabolism* ; Muscle, Skeletal/metabolism* ; Obesity, Metabolically Benign/diagnosis* ; Pediatric Obesity ; Phenotype ; Risk Factors ; Child

Age-dependent loss of skeletal muscle mass and function is a feature of sarcopenia, and increases the risk of many aging-related metabolic diseases. Here, we report phenotypic and single-nucleus transcriptomic analyses of non-human primate skeletal muscle aging. A higher transcriptional fluctuation was observed in myonuclei relative to other interstitial cell types, indicating a higher susceptibility of skeletal muscle fiber to aging. We found a downregulation of FOXO3 in aged primate skeletal muscle, and identified FOXO3 as a hub transcription factor maintaining skeletal muscle homeostasis. Through the establishment of a complementary experimental pipeline based on a human pluripotent stem cell-derived myotube model, we revealed that silence of FOXO3 accelerates human myotube senescence, whereas genetic activation of endogenous FOXO3 alleviates human myotube aging. Altogether, based on a combination of monkey skeletal muscle and human myotube aging research models, we unraveled the pivotal role of the FOXO3 in safeguarding primate skeletal muscle from aging, providing a comprehensive resource for the development of clinical diagnosis and targeted therapeutic interventions against human skeletal muscle aging and the onset of sarcopenia along with aging-related disorders.
Animals ; Humans ; Sarcopenia/metabolism* ; Forkhead Box Protein O3/metabolism* ; Muscle, Skeletal/metabolism* ; Aging/metabolism* ; Primates/metabolism*

Skeletal muscle is one of the most important organs in animal, and the regulatory mechanism of skeletal muscle development is of great importance for the diagnosis of muscle-related diseases and the improvement of meat quality of livestock. The regulation of skeletal muscle development is a complex process, which is regulated by a large number of muscle secretory factors and signaling pathways. In addition, in order to maintain steady-state and maximum use of energy metabolism in the body, the body coordinates multiple tissues and organs to form the complex and sophisticated metabolic regulation network, which plays an important role for the regulation of skeletal muscle development. With the development of omics technologies, the underlying mechanism of tissue and organ communication has been deeply studied. This paper reviews the effects of crosstalk among adipose tissue, nerve tissue and intestinal tissue on skeletal muscle development, with the aim to provide a theoretical basis for targeted regulation of skeletal muscle development.
Animals ; Muscle, Skeletal/metabolism* ; Adipose Tissue/metabolism* ; Signal Transduction

The growth and development of skeletal muscle is an important factor affecting pork production and quality, which is elaborately regulated by many genetic and nutritional factors. MicroRNA (miRNA) is a non-coding RNA with a length of about 22 nt, which binds to the 3'UTR sequence of the mRNA of the target genes, and consequently regulates its post-transcriptional expression level. In recent years, a large number of studies have shown that miRNAs are involved in various life processes such as growth and development, reproduction, and diseases. The role of miRNAs in the regulation of porcine skeletal muscle development was reviewed, with the hope to provide a reference for the genetic improvement of pigs.
Swine ; Animals ; MicroRNAs/metabolism* ; Muscle, Skeletal/metabolism* ; Muscle Development/genetics*

Titin, the largest known protein in the body expressed in three isoforms (N2A, N2BA and N2B), is essential for muscle structure, force generation, conduction and regulation. Since the 1950s, muscle contraction mechanisms have been explained by the sliding filament theory involving thin and thick muscle filaments, while the contribution of cytoskeleton in force generation and conduction was ignored. With the discovery of insoluble protein residues and large molecular weight proteins in muscle fibers, the third myofilament, titin, has been identified and attracted a lot of interests. The development of single molecule mechanics and gene sequencing technology further contributed to the extensive studies on the arrangement, structure, elastic properties and components of titin in sarcomere. Therefore, this paper reviews the structure, isforms classification, elastic function and regulatory factors of titin, to provide better understanding of titin.
Connectin/genetics* ; Muscle Proteins/metabolism* ; Protein Isoforms/genetics* ; Sarcomeres/metabolism* ; Muscle Fibers, Skeletal/metabolism*

Objective: To study the effects of vibration on the expression of mitochondrial fusion and fission genes and ultrastructure of skeletal muscle in rabbits. Methods: Thirty-two 3.5-month-old New Zealand rabbits were randomly divided into low-intensity group, medium-intensity group, high-intensity group and control group, with 8 rabbits in each group. The rabbits in the experimental group were subjected to hind limb vibration load test for 45 days. The vibration intensity of the high intensity group was 12.26 m/s(2), the medium intensity group was 6.13 m/s(2), and the low intensity group was 3.02 m/s(2) according to the effective value of weighted acceleration[a(hw (4))] for 4 hours of equal energy frequency. The control group was exposed to noise only in the same experimental environment as the medium-intensity group. The noise levels of each group were measured during the vibration load experiment. After the test, the mRNA expression of mitochondrial fusion gene (Mfn1/Mfn2) and fission gene (Fis1, Drp1) by RT-PCR in the skeletal muscles were measured and the ultrastructure of the skeletal muscles were observed in high intensity group. Results: The mRNA expression of mitochondrial in the skeletal muscle tissues of control group, low intensity group, medium intensity group and high intensity group were Mfn1: 3.25±1.36, 3.85±1.90, 4.53±2.31 and 11.63±7.68; Mfn2: 0.68±0.25, 1.02±0.40, 0.94±0.33 and 1.40±0.45; Fis1: 1.05±0.62, 1.15±0.59, 1.53±1.06 and 2.46±1.51 and Drp1: 3.72±1.76, 2.91±1.63, 3.27±2.01 and 4.21±2.46, respectively. Compared with the control group, the expressions of Mfn1 mRNA, Mfn2 mRNA and Fis1 mRNA in the high-intensity group increased significantly (P<0.05) , and the expressions of Mfn2 mRNA in the medium-intensity group and the low-intensity group increased significantly (P<0.05) . Compared with the control group, the ultrastructure of skeletal muscle of high intensity group showed mitochondrial focal accumulation, cristae membrane damage, vacuole-like changes; Z-line irregularity of muscle fibers, and deficiency of sarcomere. Conclusion: Vibration must be lead to the abnormal mitochondrial morphology and structure and the disorder of energy metabolism due to the expression imbalance of mitochondrial fusion and fission genes in skeletal muscles of rabbits, which may be an important target of vibration-induced skeletal muscle injury.
Animals ; Hindlimb/metabolism* ; Mitochondria/metabolism* ; Mitochondrial Dynamics ; Mitochondrial Proteins/pharmacology* ; Muscle, Skeletal ; Rabbits ; Vibration/adverse effects*

OBJECTIVE: To observe the effect of mibefradil on skeletal muscle mass, function and structure in obese mice. METHODS: Fifteen 6-week-old C57BL/6 mice were randomized equally into normal diet group (control group), high-fat diet (HFD) group and high-fat diet +mibefradil intervention group (HFD +Mibe group). The grip strength of the mice was measured using an electronic grip strength meter, and the muscle content of the hindlimb was analyzed by X-ray absorptiometry (DXA). Triglyceride (TG) and total cholesterol (TC) levels of the mice were measured with GPO-PAP method. The cross-sectional area of the muscle fibers was observed with HE staining. The changes in the level of autophagy in the muscles were detected by Western blotting and immunofluorescence assay, and the activation of the Akt/mTOR signaling pathway was detected with Western blotting. RESULTS: Compared with those in the control group, the mice in HFD group had a significantly greater body weight, lower relative grip strength, smaller average cross sectional area of the muscle fibers, and a lower hindlimb muscle ratio (P < 0.05). Immunofluorescence assay revealed a homogenous distribution of LC3 emitting light red fluorescence in the cytoplasm in the muscle cells in HFD group and HFD+Mibe group, while bright spots of red fluorescence were detected in HFD group. In HFD group, the muscular tissues of the mice showed an increased expression level of LC3 II protein with lowered expressions of p62 protein and phosphorylated AKT and mTOR (P < 0.05). Mibefradil treatment significantly reduced body weight of the mice, lowered the expression level of p62 protein, and increased forelimb grip strength, hindlimb muscle ratio, cross-sectional area of the muscle fibers, and the expression levels of LC3 II protein and phosphorylated AKT and mTOR (P < 0.05). CONCLUSION Mibefradil treatment can moderate high-fat diet-induced weight gain and improve muscle mass and function in obese mice possibly by activating AKT/mTOR signal pathway to improve lipid metabolism and inhibit obesityinduced autophagy.
Animals ; Body Weight ; Diet, High-Fat ; Mibefradil/metabolism* ; Mice ; Mice, Inbred C57BL ; Mice, Obese ; Muscle, Skeletal/metabolism* ; Proto-Oncogene Proteins c-akt/metabolism* ; TOR Serine-Threonine Kinases/metabolism*

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