DNA demethylation enhances myoblasts hypertrophy during the late phase of myogenesis activating the IGF-I pathway

Considerations for enhanced mesenchymal stromal/stem cell myogenic commitment in vitro

The generation of tissue from stem cells is an alluring concept as it holds a number of potential applications in clinical therapeutics and regenerative medicine. Mesenchymal stromal/stem cells (MSCs) can be isolated from a number of different somatic sources, and have the capacity to differentiate into adipogenic, osteogenic, chondrogenic, and myogenic lineages. Although the first three have been extensively investigated, there remains a paucity of literature on the latter. This review looks at the various strategies available in vitro to enhance harvested MSC commitment and differentiation into the myogenic pathway. These include chemical inducers, myogenic-enhancing cell culture substrates, and mechanical and dynamic culturing conditions. Drawing on information from embryonic and postnatal myogenesis from somites, satellite, and myogenic progenitor cells, the mechanisms behind the chemical and mechanical induction strategies can be studied, and the sequential gene and signaling cascades can be used to monitor the progression of myogenic differentiation in the laboratory. Increased understanding of the stimuli and signaling mechanisms in the initial stages of MSC myogenic commitment will provide tools with which we can enhance their differentiation efficacy and advance the process to clinical translation.

5-Aza-2'-deoxycytidine-mediated DNA hypomethylation with and without concurrent insulin resistance suppresses myotube mitochondrial capacity

Type 2 diabetes is characterized by elevated blood glucose and reduced insulin sensitivity in target tissues. Moreover, reduced mitochondrial metabolism and expressional profile of genes governing mitochondrial metabolism (such as peroxisome proliferator-activated receptor gamma coactivator 1-alpha [PGC-1α]) are also reduced during insulin resistance. Epigenetic regulation via DNA methylation of genes including PGC-1α may contribute to diminished mitochondrial capacity, while hypomethylation of PGC-1α (such as that invoked by exercise) has been associated with increased PGC-1α expression and favorable metabolic outcomes. The purpose of the present report is to characterize the effects of DNA hypomethylation on myotube metabolism and expression of several related metabolic targets. C2C12 myotubes were treated with 5-Aza-2'-deoxycytidine (5-Aza) for either 24 or 72 h both with and without hyperinsulinemic-induced insulin resistance. Mitochondrial and glycolytic metabolism were measured via oxygen consumption and extracellular acidification rate, respectively. Metabolic gene and protein expression were assessed via quantitative real time polymerase chain reaction and western blot analysis, respectively. Though expression of PGC-1α and other related targets remained unaltered, insulin resistance and 5-Aza treatment significantly reduced mitochondrial metabolism. Similarly, peak glycolytic metabolism was diminished by 5-Aza-treated cells, while basal glycolytic metabolism was unaltered. 5-Aza also reduced the expression of branched-chain amino acid (BCAA) catabolic components, however BCAA utilization was enhanced during insulin resistance with 5-Aza treatment. Together the present work provides proof-of-concept evidence of the potential role of DNA methylation in the regulation of mitochondrial metabolism and the potential interactions with insulin resistance in a model of skeletal muscle.

Myostatin Mutation Enhances Bovine Myogenic Differentiation through PI3K/AKT/mTOR Signalling via Removing DNA Methylation of RACK1

Myostatin (MSTN) is a negative regulator of skeletal muscle development and plays an important role in muscle development. Fluctuations in gene expression influenced by DNA methylation are critical for homeostatic responses in muscle. However, little is known about the mechanisms underlying this fluctuation regulation and myogenic differentiation of skeletal muscle. Here we report a genome-wide analysis of DNA methylation dynamics in bovine skeletal muscle myogenesis after myostatin editing. We show that, after myostatin editing, an increase in TETs (DNA demethylases) and a concomitant increase in the receptor for activated C kinase 1 (RACK1) control the myogenic development of skeletal muscle. Interestingly, enhancement of PI3K/AKT/mTOR signaling by RACK1 appears to be an essential driver of myogenic differentiation, as it was associated with an increase in myogenic differentiation marker factors (MyHC and MyoG) during muscle differentiation. Overall, our results suggest that loss of myostatin promotes the myogenic differentiation response in skeletal muscle by decreasing DNA methylation of RACK1.

MSTN Mutant Promotes Myogenic Differentiation by Increasing Demethylase TET1 Expression via the SMAD2/SMAD3 Pathway

Myostatin (MSTN) is mostly expressed in skeletal muscle and plays crucial roles in the negative regulation of muscle mass development. The methylation and demethylation of myogenesis-specific genes are major regulatory factors in muscle satellite cell differentiation. The present study was designed to investigate the mechanism of myogenic differentiation regulated by MSTN mutation (MT) and the methylation/demethylation state of downstream genes. The results showed that, in the MSTN^-/+ satellite cells, a higher myotube fusion index and a larger myotube length were observed compared to the wild type controls; the genes associated with myogenesis were all up-regulated compared to the WT controls. The methylation of the promoters and gene bodies of PAX3, PAX7, MyoD, and MyoG were all down-regulated, while the expression of the key demethylase TET1 was significantly promoted. ChIP-qPCR was used to demonstrate that the SMAD2/SMAD3 complex combined with the promoter of TET1 to inhibit the activity of TET1 promoter, indicating that MSTN may regulate TET1 via SMAD2/SMAD3. The overexpression of TET1 in wild type cells promoted myogenic differentiation, increased the myotube index, and reduced the methylation of the associated genes. On the contrary, the knockdown of TET1 in the MSTN mutant cells resulted in the opposite phenomena as in the overexpressed cells. In conclusion, the myostatin mutant showed an increased transcriptional activity of TET1, inducing higher levels of demethylation and improving the transcriptional activity levels of myogenic differentiation-associated genes. The binding of SMAD2/SMAD3 directly to the TET1 promoter region indicated that the MSTN mutant demethylated the myogenesis-specific genes by up-regulating TET1, which is directly controlled by SMAD2/SMAD3.

(Epi)genetic Modifications in Myogenic Stem Cells: From Novel Insights to Therapeutic Perspectives

The skeletal muscle is considered to be an ideal target for stem cell therapy as it has an inherent regenerative capacity. Upon injury, the satellite cells, muscle stem cells that reside under the basal lamina of the myofibres, start to differentiate in order to reconstitute the myofibres while maintaining the initial stem cell pool. In recent years, it has become more and more evident that epigenetic mechanisms such as histon modifications, DNA methylations and microRNA modulations play a pivatol role in this differentiation process. By understanding the mechanisms behind myogenesis, researchers are able to use this knowledge to enhance the differentiation and engraftment potential of different muscle stem cells. Besides manipulation on an epigenetic level, recent advances in the field of genome-engineering allow site-specific modifications in the genome of these stem cells. Combining epigenetic control of the stem cell fate with the ability to site-specifically correct mutations or add genes for further cell control, can increase the use of stem cells as treatment of muscular dystrophies drastically. In this review, we will discuss the advances that have been made in genome-engineering and the epigenetic regulation of muscle stem cells and how this knowledge can help to get stem cell therapy to its full potential.

From Nutrient to MicroRNA: a Novel Insight into Cell Signaling Involved in Skeletal Muscle Development and Disease

Skeletal muscle is a remarkably complicated organ comprising many different cell types, and it plays an important role in lifelong metabolic health. Nutrients, as an external regulator, potently regulate skeletal muscle development through various internal regulatory factors, such as mammalian target of rapamycin (mTOR) and microRNAs (miRNAs). As a nutrient sensor, mTOR, integrates nutrient availability to regulate myogenesis and directly or indirectly influences microRNA expression. MiRNAs, a class of small non-coding RNAs mediating gene silencing, are implicated in myogenesis and muscle-related diseases. Meanwhile, growing evidence has emerged supporting the notion that the expression of myogenic miRNAs could be regulated by nutrients in an epigenetic mechanism. Therefore, this review presents a novel insight into the cell signaling network underlying nutrient-mTOR-miRNA pathway regulation of skeletal myogenesis and summarizes the epigenetic modifications in myogenic differentiation, which will provide valuable information for potential therapeutic intervention.

Transcriptional activity of acetylcholinesterase gene is regulated by DNA methylation during C2C12 myogenesis

The expression of acetylcholinesterase (AChE), an enzyme hydrolyzes neurotransmitter acetylcholine at vertebrate neuromuscular junction, is regulated during myogenesis, indicating the significance of muscle intrinsic factors in controlling the enzyme expression. DNA methylation is essential for temporal control of myogenic gene expression during myogenesis; however, its role in AChE regulation is not known. The promoter of vertebrate ACHE gene carries highly conserved CG-rich regions, implying its likeliness to be methylated for epigenetic regulation. A DNA methyltransferase inhibitor, 5-azacytidine (5-Aza), was applied onto C2C12 cells throughout the myotube formation. When DNA methylation was inhibited, the promoter activity, transcript expression and enzymatic activity of AChE were markedly increased after day 3 of differentiation, which indicated the putative role of DNA methylation. By bisulfite pyrosequencing, the overall methylation rate was found to peak at day 3 during C2C12 cell differentiation; a SP1 site located at -1826bp upstream of mouse ACHE gene was revealed to be heavily methylated. The involvement of transcriptional factor SP1 in epigenetic regulation of AChE was illustrated here: (i) the SP1-driven transcriptional activity was increased in 5-Aza-treated C2C12 culture; (ii) the binding of SP1 onto the SP1 site of ACHE gene was fully blocked by the DNA methylation; and (iii) the sequence flanking SP1 sites of ACHE gene was precipitated by chromatin immuno-precipitation assay. The findings suggested the role of DNA methylation on AChE transcriptional regulation and provided insight in elucidating the DNA methylation-mediated regulatory mechanism on AChE expression during muscle differentiation.

Metformin Treatment Prevents Sedentariness Related Damages in Mice

Metformin (METF), historical antihyperglycemic drug, is a likely candidate for lifespan extension, treatment and prevention of sedentariness damages, insulin resistance, and obesity. Skeletal muscle is a highly adaptable tissue, capable of hypertrophy response to resistance training and of regeneration after damage. Aims of this work were to investigate METF ability to prevent sedentariness damage and to enhance skeletal muscle function. Sedentary 12-week-old C57BL/6 mice were treated with METF (250 mg/kg per day, in drinking water) for 60 days. METF role on skeletal muscle differentiation was studied in vitro using murine C2C12 myoblasts. Muscular performance evaluation revealed that METF enhanced mice physical performance (Estimated VO2max). Biochemical analyses of hepatic and muscular tissues indicated that in liver METF increased AMPK and CAMKII signaling. In contrast, METF inactivated ERKs, the principal kinases involved in hepatic stress. In skeletal muscle, METF activated AKT, key kinase in skeletal muscle mass maintenance. In in vitro studies, METF did not modify the C2C12 proliferation capacity, while it positively influenced the differentiation process and myotube maturation. In conclusion, our novel results suggest that METF has a positive action not only on the promotion of healthy aging but also on the prevention of sedentariness damages.

Potential therapeutic role of L-carnitine in skeletal muscle oxidative stress and atrophy conditions

The targeting of nutraceutical treatment to skeletal muscle damage is an emerging area of research, driven by the need for new therapies for a range of muscle-associated diseases. L-Carnitine (CARN) is an essential nutrient and plays a key role in mitochondrial β-oxidation and in the ubiquitin-proteasome system regulation. As a dietary supplement to improve athletic performance, CARN has been studied for its potential to enhance β-oxidation. However, CARN effects on myogenesis, mitochondrial activity, and hypertrophy process are not completely elucidated. This in vitro study aims to investigate CARN role on skeletal muscle remodeling, differentiation process, and myotubes formation. We analyzed muscle differentiation and morphological features in C2C12 myoblasts exposed to 5 mM CARN. Our results showed that CARN was able to accelerate C2C12 myotubes formation and induce morphological changes, characterizing the start of hypertrophy process. In addition, CARN improved AKT activation and downstream cellular signaling pathways involved in skeletal muscle atrophy process prevention. Also, CARN positively regulated the pathways involved in oxidative stress defense. In this work, we provide an interesting novel mechanism of the potential therapeutic use of CARN to treat pathological conditions characterized by skeletal muscle morphological and functional impairment, oxidative stress production, and atrophy process in aging.