Fgf21 has been identified as playing a regulatory role in muscle growth and function. Although the mechanisms through which endurance training regulates skeletal muscle have been widely studied, the contribution of Fgf21 remains poorly understood. Here, muscle size and function were measured, and markers of fiber type were evaluated using immunohistochemistry, immunoblots, or qPCR in endurance-exercise-trained wild-type and Fgf21 KO mice. We also investigated Fgf21-induced fiber conversion in C2C12 cells, which were incubated with lentivirus and/or pathway inhibitors. We found that endurance exercise training enhanced the Fgf21 levels of liver and GAS muscle and exercise capacity and decreased the distribution of skeletal muscle fiber size, and fast-twitch fibers were observed converting to slow-twitch fibers in the GAS muscle of mice. Fgf21 promoted the markers of fiber-type transition and eMyHC-positive myotubes by inhibiting the TGF-β1 signaling axis and activating the p38 MAPK signaling pathway without apparent crosstalk. Our findings suggest that the transformation and function of skeletal muscle fiber types in response to endurance training could be mediated by Fgf21 and its downstream signaling pathways. Our results illuminate the mechanisms of Fgf21 in endurance-exercise-induced fiber-type conversion and suggest a potential use of Fgf21 in improving muscle health and combating fatigue.

Background: The gradual rise in global temperatures can affect skeletal muscle development and intestinal microorganisms. However, the influence of microbial metabolites on skeletal muscle homeostasis under heat stress (HS) remains unclear. Methods: C57BL/6J mice were exposed to normal temperature or 40 °C conditions for 3 d, 7 d, or 14 d. The HS 7 d mice also were treated with sodium butyrate (NaB, 200 mg/kg, gavage). Results: Strikingly, the body weight, antioxidative ability (MDA, T-SOD, and GSH-Px), and average cross-sectional area decreased, but the blood glucose and core temperature increased under HS. However, the NaB treatment reversed these effects. Meanwhile, HS also increased the levels of TNF-α and CORT. Additionally, HS led to a reduction in the villus height and an increase in the crypt depth of the intestine. Microbial 16S rRNA sequencing analysis revealed that HS caused gut microbiota dysbiosis. NaB increased the expression of HSP70 under HS, to maintain skeletal muscle homeostasis. HS stimulated the expression of Pax7, which indicates that skeletal muscle homeostasis was disrupted. Meanwhile, the expressions of MyoG and MyoD were decreased under HS. The immunofluorescence results also show that HS triggered a shift from slow muscle fibers (MYH7) to fast muscle fibers (MYH1). However, NaB recovered the expressions of these muscle-related factors. HS inhibited autophagy initiation (mTOR, Beclin1, Atg5, Atg7, and Atg12), the formation (LC3 II/LC3 I) of autophagosomes, and the binding (p62 and LAMP1) of lysosomes to autophagosomes, which were activated by NaB. C2C12 cells were treated with H₂O₂ to simulate skeletal muscle oxidative stress, and treated with NaB in advance. Oxidative stress disrupted the homeostasis of the C2C12 cells, characterized by an increase in Pax 7 and decreases in MyoG and MyoD, but these changes were reversed by the NaB treatment. Meanwhile, NaB was unable to maintain the stable expression of Pax7 when autophagy was inhibited. Conclusions: This suggests that NaB can regulate oxidative stress induced by HS through autophagy to maintain skeletal muscle homeostasis.

Porcine circovirus type 3 (PCV3) infection can cause symptoms similar to those of porcine circovirus type 2 (PCV2) infection, and coinfections with both PCV2 and PCV3 are observed in the swine industry. Consequently, developing chimeric vaccines is essential to prevent and control porcine circovirus infections. In this study, we used both E. coli and mammalian expression systems to express PCV3 Cap (Cap3) and a chimeric gene containing the PCV2-neutralizing epitope within the PCV3 Cap (Cap3-Cap2E), which were assembled into virus-like particle (VLP) vaccines. We found that Cap3 lacking nuclear localization signal (NLS) could not form VLPs, while Cap3 with a His-tag successfully assembled into VLPs. Additionally, the chimeric of PCV2-neutralizing epitopes did not interfere with the assembly process of VLPs. Various immunization approaches revealed that pCap3-Cap2E VLP vaccines were capable of activating high PCV3 Cap-specific antibody levels and effectively neutralizing both PCV3 and PCV2. Furthermore, pCap3-Cap2E VLPs demonstrated a potent ability to activate cellular immunity, protecting against PCV3 infection and preventing lung damage in mice. In conclusion, this study successfully developed a PCV3 Cap VLP vaccine incorporating chimeric PCV2-neutralizing epitope genes, providing new perspectives for PCV3 vaccine development.

Prolonged high-dose administration of synthetic glucocorticoids (GCs) leads to limb muscle atrophy and weakness, yet its underlying mechanisms remain incompletely understood. Muscle fibers and muscle satellite cells (MSCs) are essential for skeletal muscle development and associated pathologies. This study demonstrates that dexamethasone (Dex) induced MSC-derived extracellular vesicles (EVs) impair myogenesis in muscle fiber-like cells (MFLCs) via inducible nitric oxide synthase (iNOS) suppression. High-throughput sequencing revealed a marked upregulation of miR-335-5p in MSC-derived EVs following Dex treatment. Mechanistically, EV miR-335-5p targeted MAPK11, leading to iNOS downregulation and subsequent UPS activation in MFLCs, which directly promoted muscle protein degradation. Collectively, our findings identify the EV miR-335-5p/MAPK11/iNOS axis as a critical mediator of GC-induced muscle atrophy, offering novel insights into therapeutic strategies targeting EV-mediated signaling in muscle wasting disorders.

Skeletal muscle development and physiological homeostasis are profoundly influenced by environmental cues. Among these factors, ambient temperature represents a critical determinant of growth performance and metabolic adaptation in mammals. However, the effects of different ambient temperature ranges on skeletal muscle characteristics and on responses across multiple visceral tissues remain poorly understood. In this study, five ambient temperature conditions (16 °C, 20 °C, 24 °C, 28 °C, and 32 °C) were established to investigate their physiological impacts in a mouse model. Our results demonstrate that ambient temperature markedly influences growth performance and skeletal muscle phenotype. Notably, mice housed at 20 °C showed relatively preserved grip strength and a shift in myofiber cross-sectional area distribution, although these findings did not consistently indicate superior skeletal muscle development across all indices. Further analysis revealed that ambient temperature significantly modulated the expression profiles of myosin heavy chain (MyHC) isoforms in skeletal muscle. Specifically, cold exposure was associated with an upregulation of the slow-twitch-related MyHC I, whereas heat stress correlated with an elevation of the fast-twitch-related MyHC IIb. Functional assessments indicated that exposure to colder or hotter conditions was associated with impaired muscle performance, as reflected by reduced grip strength at 16 °C, 28 °C, and 32 °C, and decreased endurance capacity at 28 °C and 32 °C. Histological analyses of major visceral organs revealed no obvious structural alterations in the heart, liver, spleen, lung, or kidney across temperature conditions. However, exposure to thermal extremes (16 °C and 32 °C) significantly reduced intestinal villus height, suggesting compromised intestinal integrity under temperature stress. Collectively, these findings indicate that ambient temperature is associated with multi-tissue changes in skeletal muscle characteristics, functional performance, and intestinal morphology. This study provides new insights into how environmental temperature modulates tissue adaptation and physiological homeostasis in mammals.