c-Myb inhibits myoblast fusion

Skeletal Muscle Regeneration in Cardiotoxin-Induced Muscle Injury Models

Skeletal muscle injuries occur frequently in daily life and exercise. Understanding the mechanisms of regeneration is critical for accelerating the repair and regeneration of muscle. Therefore, this article reviews knowledge on the mechanisms of skeletal muscle regeneration after cardiotoxin-induced injury. The process of regeneration is similar in different mouse strains and is inhibited by aging, obesity, and diabetes. Exercise, microcurrent electrical neuromuscular stimulation, and mechanical loading improve regeneration. The mechanisms of regeneration are complex and strain-dependent, and changes in functional proteins involved in the processes of necrotic fiber debris clearance, M1 to M2 macrophage conversion, SC activation, myoblast proliferation, differentiation and fusion, and fibrosis and calcification influence the final outcome of the regenerative activity.

Muscle-Specific Cellular and Molecular Adaptations to Late-Life Voluntary Concurrent Exercise

Murine exercise models can provide information on factors that influence muscle adaptability with aging, but few translatable solutions exist. Progressive weighted wheel running (PoWeR) is a simple, voluntary, low-cost, high-volume endurance/resistance exercise approach for training young mice. In the current investigation, aged mice (22-mo-old) underwent a modified version of PoWeR for 8 wk. Muscle functional, cellular, biochemical, transcriptional, and myonuclear DNA methylation analyses provide an encompassing picture of how muscle from aged mice responds to high-volume combined training. Mice run 6-8 km/d, and relative to sedentary mice, PoWeR increases plantarflexor muscle strength. The oxidative soleus of aged mice responds to PoWeR similarly to young mice in every parameter measured in previous work; this includes muscle mass, glycolytic-to-oxidative fiber type transitioning, fiber size, satellite cell frequency, and myonuclear number. The oxidative/glycolytic plantaris adapts according to fiber type, but with modest overall changes in muscle mass. Capillarity increases markedly with PoWeR in both muscles, which may be permissive for adaptability in advanced age. Comparison to published PoWeR RNA-sequencing data in young mice identified conserved regulators of adaptability across age and muscles; this includes Aldh1l1 which associates with muscle vasculature. Agrn and Samd1 gene expression is upregulated after PoWeR simultaneous with a hypomethylated promoter CpG in myonuclear DNA, which could have implications for innervation and capillarization. A promoter CpG in Rbm10 is hypomethylated by late-life exercise in myonuclei, consistent with findings in muscle tissue. PoWeR and the data herein are a resource for uncovering cellular and molecular regulators of muscle adaptation with aging.

SMaRT lncRNA controls translation of a G-quadruplex-containing mRNA antagonizing the DHX36 helicase

Guanine‐quadruplexes (G4) included in RNA molecules exert several functions in controlling gene expression at post‐transcriptional level; however, the molecular mechanisms of G4‐mediated regulation are still poorly understood. Here, we describe a regulatory circuitry operating in the early phases of murine muscle differentiation in which a long non‐coding RNA (SMaRT) base pairs with a G4‐containing mRNA (Mlx‐γ) and represses its translation by counteracting the activity of the DHX36 RNA helicase. The time‐restricted, specific effect of lnc‐SMaRT on the translation of Mlx‐γ isoform modulates the general subcellular localization of total MLX proteins, impacting on their transcriptional output and promoting proper myogenesis and mature myotube formation. Therefore, the circuitry made of lnc‐SMaRT, Mlx‐γ, and DHX36 not only plays an important role in the control of myogenesis but also unravels a molecular mechanism where G4 structures and G4 unwinding activities are regulated in living cells.

c-Myb regulates tumorigenic potential of embryonal rhabdomyosarcoma cells

Rhabdomyosarcomas (RMS) are a heterogeneous group of mesodermal tumors, the most common sub-types are embryonal (eRMS) and alveolar (aRMS) rhabdomyosarcoma. Immunohistochemical analysis revealed c-Myb expression in both eRMS and aRMS. c-Myb has been reported to be often associated with malignant human cancers. We therefore investigated the c-Myb role in RMS using cellular models of RMS. Specific suppression of c-Myb by a lentiviral vector expressing doxycycline (Dox)-inducible c-Myb shRNA inhibited proliferation, colony formation, and migration of the eRMS cell line (RD), but not of the aRMS cell line (RH30). Upon c-Myb knockdown in eRMS cells, cells accumulated in G0/G1 phase, the invasive behaviour of cells was repressed, and elevated levels of myosin heavy chain, marker of muscle differentiation, was detected. Next, we used an RD-based xenograft model to investigate the role of c-Myb in eRMS tumorigenesis in vivo. We found that Dox administration did not result in efficient suppression of c-Myb in growing tumors. However, when c-Myb-deficient RD cells were implanted into SCID mice, we observed inefficient tumor grafting and attenuation of tumor growth during the initial stages of tumor expansion. The presented study suggests that c-Myb could be a therapeutic target in embryonal rhabdomyosarcoma assuming that its expression is ablated.

The Expression of c-Myb Correlates with the Levels of Rhabdomyosarcoma-specific Marker Myogenin

The transcription factor c-Myb is required for modulation of progenitor cells in several tissues, including skeletal muscle and its upregulation is observed in many human malignancies. Rhabdomyosarcomas (RMS) are a heterogeneous group of mesodermal tumors with features of developing skeletal muscle. Several miRNAs are downregulated in RMS, including miR-150, a negative regulator of c-Myb expression. Using the C2C12 myoblast cell line, a cellular model of skeletal muscle differentiation, we showed that miR-150 controls c-Myb expression mainly at the level of translation. We hypothesized that a similar mechanism of c-Myb regulation operates in RMS tumors. We examined expression of c-Myb by immunohistochemistry and revealed c-Myb positivity in alveolar and embryonal tumors, the two most common subgroups of RMS. Furthermore, we showed direct correlation between c-Myb production and myogenin expression. Interestingly, high myogenin levels indicate poor prognosis in RMS patients. c-Myb could, therefore, contribute to the tumor phenotype by executing its inhibitory role in skeletal muscle differentiation. We also showed that c-Myb protein is abundant in migratory C2C12 myoblasts and its ectopic expression potentiates cell motility. In summary, our results implicate that metastatic properties of some RMS subtypes might be linked to c-Myb function.

Dynamic transcriptome profiles of skeletal muscle tissue across 11 developmental stages for both Tongcheng and Yorkshire pigs

Background

The growth and development of skeletal muscle directly impacts the quantity and quality of pork production. Chinese indigenous pig breeds and exotic species vary greatly in terms of muscle production and performance traits. We present transcriptome profiles of 110 skeletal muscle samples from Tongcheng (TC) and Yorkshire (YK) pigs at 11 developmental periods (30, 40, 55, 63, 70, 90, and 105 days of gestation, and 0, 1, 3, and 5 weeks of age) using digital gene expression on Solexa/Illumina’s Genome Analyzer platform to investigate the differences in prenatal and postnatal skeletal muscle between the two breeds.

Results

Muscle morphological changes indicate the importance of primary fiber formation from 30 to 40 dpc (days post coitus), and secondary fiber formation from 55 to 70 dpc. We screened 4,331 differentially expressed genes in TC and 2,259 in YK (log₂ ratio >1 and probability >0.7). Cluster analysis showed different gene expression patterns between TC and YK pigs. The transcripts were annotated in terms of Gene Ontology related to muscle development. We found that the genes CXCL10, EIF2B5, PSMA6, FBXO32, and LOC100622249 played vital roles in the muscle regulatory networks in the TC breed, whereas the genes SGCD, ENG, THBD, AQP4, and BTG2 played dominant roles in the YK breed. These genes showed breed-specific and development-dependent differential expression patterns. Furthermore, 984 genes were identified in myogenesis. A heat map showed that significantly enriched pathways (FDR <0.05) had stage-specific functional regulatory mechanisms. Finally, the differentially expressed genes from our sequencing results were confirmed by real-time quantitative polymerase chain reaction.

Conclusions

This study detected many functional genes and showed differences in the molecular mechanisms of skeletal muscle development between TC and YK pigs. TC pigs showed slower muscle growth and more complicated genetic regulation than YK pigs. Many differentially expressed genes showed breed-specific expression patterns. Our data provide a better understanding of skeletal muscle developmental differences and valuable information for improving pork quality.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1580-7) contains supplementary material, which is available to authorized users.