MiR-320-3p Regulates the Proliferation and Differentiation of Myogenic Progenitor Cells by Modulating Actin Remodeling

Skeletal myogenesis is essential for the maintenance of muscle quality and quantity, and impaired myogenesis is intimately associated with muscle wasting diseases. Although microRNA (miRNA) plays a crucial role in myogenesis and relates to muscle wasting in obesity, the molecular targets and roles of miRNAs modulated by saturated fatty acids (SFA) are largely unknown. In the present study, we investigated the role of miR-320-3p on the differentiation of myogenic progenitor cells. Palmitic acid (PA), the most abundant dietary SFA, suppressed myogenic factors expression and impaired differentiation in C2C12 myoblasts, and these effects were accompanied by CFL2 downregulation and miR-320-3p upregulation. In particular, miR-320-3p appeared to target CFL2 mRNA directly and suppress the expression of CFL2, an essential factor for filamentous actin (F-actin) depolymerization. Transfection of myoblasts with miR-320-3p mimic increased F-actin formation and nuclear translocation of Yes-associated protein 1 (YAP1), a key component of mechanotransduction. Furthermore, miR-320-3p mimic increased myoblast proliferation and markedly impeded the expression of MyoD and MyoG, consequently inhibiting myoblast differentiation. In conclusion, our current study highlights the role of miR-320-3p on CFL2 expression, YAP1 activation, and myoblast differentiation and suggests that PA-inducible miR-320-3p is a significant mediator of muscle wasting in obesity.

Interferon-gamma mediates skeletal muscle lesions through JAK/STAT pathway activation in inclusion body myositis

Dysimmune and Inflammatory Myopathies (DIMs) are acquired idiopathic myopathy associated with immune response dysregulation. Inclusion Body Myositis (IBM), the most common DIMs, is characterized by endomysial infiltrates of cytotoxic T lymphocytes CD8, muscle type II-interferon (IFNγ) signature, and by the lack of response to immunomodulatory therapies. We showed that IBM was pathologically characterized by the presence of chronic degenerative myopathic features including myofiber atrophy, fibrosis, adipose involution, and the altered functions of skeletal muscle stem cells. Here, we demonstrated that protracted systemic exposure to IFNγ delayed muscle regeneration and led to IBM-like muscular degenerative changes in mice. In vitro, IFNγ treatment inhibited the activation, proliferation, migration, differentiation, and fusion of myogenic progenitor cells and promoted their senescence through JAK-STAT-dependent activation. Finally, JAK-STAT inhibitor, ruxolitinib abrogated the deleterious effects of IFNγ on muscle regeneration, suggesting that the JAK-STAT pathway could represent a new therapeutic target for IBM.

Resolvin-D2 targets myogenic cells and improves muscle regeneration in Duchenne muscular dystrophy

Lack of dystrophin causes muscle degeneration, which is exacerbated by chronic inflammation and reduced regenerative capacity of muscle stem cells in Duchenne Muscular Dystrophy (DMD). To date, glucocorticoids remain the gold standard for the treatment of DMD. These drugs are able to slow down the progression of the disease and increase lifespan by dampening the chronic and excessive inflammatory process; however, they also have numerous harmful side effects that hamper their therapeutic potential. Here, we investigated Resolvin-D2 as a new therapeutic alternative having the potential to target multiple key features contributing to the disease progression. Our in vitro findings showed that Resolvin-D2 promotes the switch of macrophages toward their anti-inflammatory phenotype and increases their secretion of pro-myogenic factors. Moreover, Resolvin-D2 directly targets myogenic cells and promotes their differentiation and the expansion of the pool of myogenic progenitor cells leading to increased myogenesis. These effects are ablated when the receptor Gpr18 is knocked-out, knocked-down, or blocked by the pharmacological antagonist O-1918. Using different mouse models of DMD, we showed that Resolvin-D2 targets both inflammation and myogenesis leading to enhanced muscle function compared to glucocorticoids. Overall, this preclinical study has identified a new therapeutic approach that is more potent than the gold-standard treatment for DMD.

Integrative molecular roadmap for direct conversion of fibroblasts into myocytes and myogenic progenitor cells

Transient MyoD overexpression in concert with small molecules treatment reprograms mouse fibroblasts into induced myogenic progenitor cells (iMPCs). However, the molecular landscape and mechanisms orchestrating this cellular conversion remain unknown. Here, we undertook an integrative multi-omics approach to delineate the process of iMPC reprogramming in comparison to myogenic transdifferentiation mediated solely by MyoD. Utilizing transcriptomics, proteomics and genome-wide chromatin accessibility assays, we unravel distinct molecular trajectories which govern the two processes. Notably, iMPC reprogramming is characterized by gradual upregulation of stem and progenitor cell markers, unique signaling pathways, chromatin remodelers and cell cycle regulators which manifest via rewiring of the chromatin in core myogenic promoters. Furthermore, we determine that only iMPC reprogramming is mediated by Notch pathway activation, which is indispensable for iMPC formation and self-renewal. Collectively, this study charts divergent molecular blueprints for myogenic transdifferentiation or reprogramming and underpins the heightened capacity of iMPCs in capturing myogenesis ex vivo.

Dissecting dual roles of MyoD during lineage conversion to mature myocytes and myogenic stem cells

The generation of myotubes from fibroblasts upon forced MyoD expression is a classic example of transcription factor-induced reprogramming. We recently discovered that additional modulation of signaling pathways with small molecules facilitates reprogramming to more primitive induced myogenic progenitor cells (iMPCs). Here, we dissected the transcriptional and epigenetic dynamics of mouse fibroblasts undergoing reprogramming to either myotubes or iMPCs using a MyoD-inducible transgenic model. Induction of MyoD in fibroblasts combined with small molecules generated Pax7+ iMPCs with high similarity to primary muscle stem cells. Analysis of intermediate stages of iMPC induction revealed that extinction of the fibroblast program preceded induction of the stem cell program. Moreover, key stem cell genes gained chromatin accessibility prior to their transcriptional activation, and these regions exhibited a marked loss of DNA methylation dependent on the Tet enzymes. In contrast, myotube generation was associated with few methylation changes, incomplete and unstable reprogramming, and an insensitivity to Tet depletion. Finally, we showed that MyoD's ability to bind to unique bHLH targets was crucial for generating iMPCs but dispensable for generating myotubes. Collectively, our analyses elucidate the role of MyoD in myogenic reprogramming and derive general principles by which transcription factors and signaling pathways cooperate to rewire cell identity.

Hypoxic Signaling in Skeletal Muscle Maintenance and Regeneration: A Systematic Review

In skeletal muscle tissue, oxygen (O2) plays a pivotal role in both metabolism and the regulation of several intercellular pathways, which can modify proliferation, differentiation and survival of cells within the myogenic lineage. The concentration of oxygen in muscle tissue is reduced during embryogenesis and pathological conditions. Myogenic progenitor cells, namely satellite cells, are necessary for muscular regeneration in adults and are localized in a hypoxic microenvironment under the basal lamina, suggesting that the O2 level could affect their function. This review presents the effects of reduced oxygen levels (hypoxia) on satellite cell survival, myoblast regeneration and differentiation in vertebrates. Further investigations and understanding of the pathways involved in adult muscle regeneration during hypoxic conditions are maybe clinically relevant to seek for novel drug treatments for patients with severe muscle damage. We especially outlined the effect of hypoxia-inducible factor 1-alpha (HIF1A), the most studied transcriptional regulator of cellular and developmental response to hypoxia, whose investigation has recently been awarded with the Nobel price.

Regulation of Skeletal Muscle Satellite Cell Differentiation by Omega-3 Polyunsaturated Fatty Acids: A Critical Review

Skeletal muscle is composed of multinuclear cells called myofibres, which are formed by the fusion of myoblasts during development. The size of the muscle fiber and mass of skeletal muscle are altered in response to several pathological and physiological conditions. Skeletal muscle regeneration is primarily mediated by muscle stem cells called satellite cells (SCs). In response to injury, these SCs replenish myogenic progenitor cells to form new myofibers to repair damaged muscle. During myogenesis, activated SCs proliferate and differentiate to myoblast and then fuse with one another to form muscle fibers. A reduced number of SCs and an inability to undergo myogenesis may contribute to skeletal muscle disorders such as atrophy, cachexia, and sarcopenia. Myogenic regulatory factors (MRF) are transcription factors that regulate myogenesis and determines whether SCs will be in the quiescent, activated, committed, or differentiated state. Mitochondria oxidative phosphorylation and oxidative stress play a role in the determination of the fate of SCs. The potential activation and function of SCs are also affected by inflammation during skeletal muscle regeneration. Omega-3 polyunsaturated fatty acids (PUFAs) show promise to reduce inflammation, maintain muscle mass during aging, and increase the functional capacity of the muscle. The aim of this critical review is to highlight the role of omega-3 PUFAs on the myogenic differentiation of SCs and pathways affected during the differentiation process, including mitochondrial function and inflammation from the current body of literature.

Notch Signaling Slows Down the Progression of Embryonic Myogenic Differentiation in Landrace

BackgroundDelving into porcine embryonic myogenesis is the key to elucidating the complex regulation of breed-specific differences in growth performance and meat production. Increasing evidence proved that pigs with less meat production showed more intense embryonic myogenesis, but little is known about the underlying mechanisms. ResultsIn this study, we confirmed that the differentiation process of myogenic progenitors in Lantang pig (LT, fat) was faster than that in Landrace pig (LR, lean), which resulted in more differentiated myoblasts (Pax7-/MyoD+) but less myogenic progenitors (Pax7+/MyoD-) in LT at 35 days post-conception (35dpc). Additionally, in vitro, embryonic myogenic progenitors isolated from LT showed stronger differentiation capacity with earlier expression of MyoD. Furthermore, the expression levels of genes related to Notch signaling in LR progenitor cells were significantly higher than that of LT, while there was no significant difference between the two breeds in gene expression levels of Wnt and Akt/mTOR pathway. Inhibition of Notch signaling or knockdown of Pax7 promotes myogenic differentiation of primary progenitor cells or myoblasts, while activation of Notch signaling or overexpression of Pax7 has the opposite effects.ConclusionsMyogenic differentiation is more rapid in LT than that in LR at 35dpc. Mechanically, Notch signaling facilitates maintenance of myogenic progenitor cells and antagonizes myogenic differentiation by promoting Pax7 expression but preventing MyoD expression in LR.