Regulation of skeletal muscle gene expression by p38 MAP kinases

p38α kinase governs muscle strength through PGC1α in mice

AIMS

Skeletal muscle, with its remarkable plasticity and dynamic adaptation, serves as a cornerstone of locomotion and metabolic homeostasis in the human body. Muscle tissue, with its extraordinary capacity for force generation and energy expenditure, plays a fundamental role in the movement, metabolism, and overall health. In this context, we sought to determine the role of p38α in mitochondrial metabolism since mitochondrial dynamics play a crucial role in the development of muscle-related diseases that result in muscle weakness.

METHODS

We conducted our study using male mice (MCK-cre, p38αMCK-KO and PGC1α MCK-KO) and mouse primary myoblasts. We analyzed mitochondrial metabolic, physiological parameters as well as proteomics, western blot, RNA-seq analysis from muscle samples.

RESULTS

Our findings highlight the critical involvement of muscle p38α in the regulation of mitochondrial function, a key determinant of muscle strength. The absence of p38α triggers changes in mitochondrial dynamics through the activation of PGC1α, a central regulator of mitochondrial biogenesis. These results have substantial implications for understanding the complex interplay between p38α kinase, PGC1α activation, and mitochondrial content, thereby enhancing our knowledge in the control of muscle biology.

CONCLUSIONS

This knowledge holds relevance for conditions associated with muscle weakness, where disruptions in these molecular pathways are frequently implicated in diminishing physical strength. Our research underscores the potential importance of targeting the p38α and PGC1α pathways within muscle, offering promising avenues for the advancement of innovative treatments. Such interventions hold the potential to improve the quality of life for individuals affected by muscle-related diseases.

Effects of rumen metabolite butyric acid on bovine skeletal muscle satellite cells proliferation, apoptosis and transcriptional states during myogenic differentiation

Butyric acid, a pivotal short-chain fatty acid in rumen digestion, profoundly influences animal digestive and locomotor systems. Extensive research indicates its direct or indirect involvement in the growth and development of muscle and fat cells. However, the impact of butyric acid on the proliferation and differentiation of bovine skeletal muscle satellite cells (SMSCs) remains unclear. This study aimed to elucidate the effects of butyrate on SMSCs proliferation and differentiation. After isolating, SMSCs were subjected to varying concentrations of sodium butyrate (NaB) during the proliferation and differentiation stages. Optimal treatment conditions (1 mM NaB for 2 days) were determined based on proliferative force, cell viability, and mRNA expression of proliferation and differentiation marker genes. Transcriptome sequencing was employed to screen for differential gene expression between 1 mM NaB-treated and untreated groups during SMSCs differentiation. Results indicated that lower NaB concentrations (≤1.0 mM) inhibited proliferation while promoting differentiation and apoptosis after a 2-day treatment. Conversely, higher NaB concentrations (≥2.0 mM) suppressed proliferation and differentiation and induced apoptosis. Transcriptome sequencing revealed differential expression of genes(ND1, ND3, CYTB, COX2, ATP6, MYOZ2, MYOZ3, MYBPC1 and ATP6V0A4,etc.) were associated with SMSCs differentiation and energy metabolism, enriching pathways such as Oxidative phosphorylation, MAPK, and Wnt signaling. These findings offer valuable insights into the molecular mechanisms underlying butyrate regulation of bovine SMSCs proliferation and differentiation, as well as muscle fiber type conversion in the future study.

A shift in chromatin binding of phosphorylated p38 precedes transcriptional changes upon oxidative stress

P38 mitogen-activated protein kinases are key in the regulation of the cellular response to stressors. P38 is known to regulate transcription, mRNA processing, stability, and translation. The transcriptional changes mediated by phosphorylated p38 (P-p38) in response to extracellular stimuli have been thoroughly analyzed in many tissues and organisms. However, the genomic localization of chromatin-associated P-p38 remains poorly understood. Here, we analyze the chromatin binding of activated P-p38 and its role in the response to reactive oxygen species (ROS) in Drosophila S2 cells. We found that P-p38 is already bound to chromatin in basal conditions. After ROS exposure, chromatin-associated P-p38 relocates towards genes involved in the recovery process. Our findings highlight the role of P-p38 dynamic chromatin binding in orchestrating gene expression responses to oxidative stress.

Changes of mRNA, miRNA and lncRNA expression contributing to skeletal muscle differences between fetus and adult Mongolian horses

The growth and development of myofibers, as the fundamental units comprising muscle tissue, and their composition type are indeed among the most crucial factors influencing skeletal muscle types. Muscle fiber adaptation is closely associated with alterations in physiological conditions. Muscle fiber types undergo dynamic changes in fetus and adult horses. Our aim is to investigate the mechanisms influencing the differences in muscle fiber types between fetal and adult stages of Mongolian horses. The study investigated the distribution of muscle fiber types within longissimus dorsi muscle of fetus and adult Mongolian horses. A total of 652 differentially expressed genes (DEGs), 476 Differentially expressed lncRNAs (DELs), and 174 Differentially expressed miRNAs (DEMIRs) were identified using deep RNA-seq analysis. The results of functional analysis reveal the transformations in muscle fiber type from the fetal to adult stage in Mongolian horses. The up-regulated DEGs were implicated in the development and differentiation of muscle fibers, while down-regulated DEGs were associated with muscle fiber contraction, transformation, and metabolism. Additionally, connections between non-coding RNA and mRNA landscapes were identified based on their functional alterations, some non-coding RNA target genes may be associated with immunity. These data have broadened our understanding of the specific roles and interrelationships among regulatory molecules involved in Mongolian horse development, this provides new perspectives for selecting and breeding superior individuals and for disease prevention.

RASopathies - what they reveal about RAS/MAPK signaling in skeletal muscle development

RASopathies are rare developmental genetic syndromes caused by germline pathogenic variants in genes that encode components of the RAS/mitogen-activated protein kinase (MAPK) signal transduction pathway. Although the incidence of each RASopathy syndrome is rare, collectively, they represent one of the largest groups of multiple congenital anomaly syndromes and have severe developmental consequences. Here, we review our understanding of how RAS/MAPK dysregulation in RASopathies impacts skeletal muscle development and the importance of RAS/MAPK pathway regulation for embryonic myogenesis. We also discuss the complex interactions of this pathway with other intracellular signaling pathways in the regulation of skeletal muscle development and growth, and the opportunities that RASopathy animal models provide for exploring the use of pathway inhibitors, typically used for cancer treatment, to correct the unique skeletal myopathy caused by the dysregulation of this pathway.

Therapeutic applications of biological macromolecules and scaffolds for skeletal muscle regeneration: A review

Skeletal muscle (SM) mass and strength maintenance are important requirements for human well-being. SM regeneration to repair minor injuries depends upon the myogenic activities of muscle satellite (stem) cells. However, losses of regenerative properties following volumetric muscle loss or severe trauma or due to congenital muscular abnormalities are not self-restorable, and thus, these conditions have major healthcare implications and pose clinical challenges. In this context, tissue engineering based on different types of biomaterials and scaffolds provides an encouraging means of structural and functional SM reconstruction. In particular, biomimetic (able to transmit biological signals) and several porous scaffolds are rapidly evolving. Several biological macromolecules/biomaterials (collagen, gelatin, alginate, chitosan, and fibrin etc.) are being widely used for SM regeneration. However, available alternatives for SM regeneration must be redesigned to make them more user-friendly and economically feasible with longer shelf lives. This review aimed to explore the biological aspects of SM regeneration and the roles played by several biological macromolecules and scaffolds in SM regeneration in cases of volumetric muscle loss.

Molecular Regulation of Porcine Skeletal Muscle Development: Insights from Research on CDC23 Expression and Function

Cell division cycle 23 (CDC23) is a component of the tetratricopeptide repeat (TPR) subunit in the anaphase-promoting complex or cyclosome (APC/C) complex, which participates in the regulation of mitosis in eukaryotes. However, the regulatory model and mechanism by which the CDC23 gene regulates muscle production in pigs are largely unknown. In this study, we investigated the expression of CDC23 in pigs, and the results indicated that CDC23 is widely expressed in various tissues and organs. In vitro cell experiments have demonstrated that CDC23 promotes the proliferation of myoblasts, as well as significantly positively regulating the differentiation of skeletal muscle satellite cells. In addition, Gene Set Enrichment Analysis (GSEA) revealed a significant downregulation of the cell cycle pathway during the differentiation process of skeletal muscle satellite cells. The protein-protein interaction (PPI) network showed a high degree of interaction between genes related to the cell cycle pathway and CDC23. Subsequently, in differentiated myocytes induced after overexpression of CDC23, the level of CDC23 exhibited a significant negative correlation with the expression of key factors in the cell cycle pathway, suggesting that CDC23 may be involved in the inhibition of the cell cycle signaling pathway in order to promote the differentiation process. In summary, we preliminarily determined the function of CDC23 with the aim of providing new insights into molecular regulation during porcine skeletal muscle development.

Comparison of secreted miRNAs and proteins during proliferation and differentiation of bovine satellite cells in culture implies potential roles in regulating myogenesis

Cultivated meat is an emerging new technology to produce sustainable meat for the future. The common approach for cultivated meat, is the isolation of satellite cells from donor animals, followed by in vitro proliferation and differentiation into primitive muscle fibers. The transformation of satellite cells into myofibers is tightly orchestrated by intra-cellular signaling, while the inter-cellular signaling is less well understood. Thus, the current study was conducted to map the secretion of potential signaling molecules (MicroRNAs and proteins) during proliferation and differentiation. Primary cultures of satellite cells were grown to 50% and 80% confluence, representing the proliferative phase or serum-starved for 1 and 3 days to induce differentiation. Post incubation in FBS-free media, the media were collected and analyzed for miRNA and protein content using gene-arrays and LC-MS/MS, respectively. When comparing the miRNA secretome at 50% and 80% confluence, we observed four differentially expressed miRNA, while only five were differentially expressed when comparing Day 1 to Day 3. A subsequent in silico analysis suggested that pathways of importance for myogenesis, e.g., MAPK and AMPK signaling, could be regulated by the secreted miRNAs. In addition, >300 proteins were secreted, including insulin-like growth factor 1 binding proteins 2, 3, 4, 5 and 6. In conclusion, this study demonstrated differential secretion of several miRNAs and proteins during both proliferation and differentiation of bovine satellite cells in vitro.

Vitamin A injection at birth improves muscle growth in lambs

Vitamin A and its metabolite, retinoic acid (RA) play important roles in regulating skeletal muscle development. This study was conducted to investigate the effects of early intramuscular vitamin A injection on the muscle growth of lambs. A total of 16 newborn lambs were given weekly intramuscular injections of corn oil (control group, n = 8) or 7,500 IU vitamin A palmitate (vitamin A group, n = 8) from birth to 3 wk of age (4 shots in total). At 3 wk of age and weaning, biceps femoris muscle samples were taken to analyze the effects of vitamin A on the myogenic capacity of skeletal muscle cells. All lambs were slaughtered at 8 months of age. The results suggest that vitamin A treatment accelerated the growth rate of lambs and increased the loin eye area (P < 0.05). Consistently, vitamin A increased the diameter of myofibers in longissimus thoracis muscle (P < 0.01) and increased the final body weight of lambs (P < 0.05). Vitamin A injection did not change the protein kinase B/mammalian target of rapamycin and myostatin signaling (P > 0.05). Moreover, vitamin A upregulated the expression of PAX7 (P < 0.05) and the myogenic marker genes including MYOD and MYOG (P < 0.01). The skeletal muscle-derived mononuclear cells from vitamin A-treated lambs showed higher expression of myogenic genes (P < 0.05) and formed more myotubes (P < 0.01) when myogenic differentiation was induced in vitro. In addition, in vitro analysis showed that RA promoted myogenic differentiation of the skeletal muscle-derived mononuclear cells in the first 3 d (P < 0.05) but not at the later stage (P > 0.05) as evidenced by myogenic gene expression and fusion index. Taken together, neonatal intramuscular vitamin A injection promotes lamb muscle growth by promoting the myogenic potential of satellite cells.

DNA repair byproduct 8-oxoguanine base promotes myoblast differentiation

Muscle contraction increases the level of reactive oxygen species (ROS), which has been acknowledged as key signaling entities in muscle remodeling and to underlie the healthy adaptation of skeletal muscle. ROS inevitably endows damage to various cellular molecules including DNA. DNA damage ought to be repaired to ensure genome integrity; yet, how DNA repair byproducts affect muscle adaptation remains elusive. Here, we showed that exercise elicited the generation of 8-oxo-7,8-dihydroguanine (8-oxoG), that was primarily found in mitochondrial genome of myofibers. Upon exercise, TA muscle's 8-oxoG excision capacity markedly enhanced, and in the interstitial fluid of TA muscle from the post-exercise mice, the level of free 8-oxoG base was significantly increased. Addition of 8-oxoG to myoblasts triggered myogenic differentiation via activating Ras-MEK-MyoD signal axis. 8-Oxoguanine DNA glycosylase1 (OGG1) silencing from cells or Ogg1 KO from mice decreased Ras activation, ERK phosphorylation, MyoD transcriptional activation, myogenic regulatory factors gene (MRFs) expression. In reconstruction experiments, exogenously added 8-oxoG base enhanced the expression of MRFs and accelerated the recovery of the injured skeletal muscle. Collectively, these data not only suggest that DNA repair metabolite 8-oxoG function as a signal entity for muscle remodeling and contribute to exercise-induced adaptation of skeletal muscle, but also raised the potential for utilizing 8-oxoG in clinical treatment to skeletal muscle damage-related disorders.

LncRNA XLOC_015548 affects the proliferation and differentiation of myoblasts via the MAPK signaling pathway

In recent years, an increasing number of studies have reported that long non-coding RNAs (lncRNAs) play essential regulatory roles in myogenic differentiation. In this study, a specific LncRNA XLOC_015548 (Lnc000280) was identified. However, little research has explored its mechanism of action by constructing XLOC_015548 gene editing cell models. In this study, relevant sequences were obtained according to the RNA-seq results. Subsequently, XLOC_015548 knockdown and over-expression lentiviral vectors were constructed, and the C2C12 myoblast cell line was transfected to prepare the XLOC_015548 gene-edited myoblast model. The in vitro analysis revealed that over-expression of XLOC_015548 significantly promoted the proliferation and differentiation of myoblasts and the formation of myotubes, whereas the opposite result was obtained in the knockdown group. XLOC_015548 regulated myogenic differentiation and affected the expression of myogenic differentiation regulators such as Myod, myogenin, and MyHC. Regarding the signaling pathway, we found that XLOC_015548 correlated with the phosphorylation level of MAPK/MEK/ERK pathway proteins. And the degree of phosphorylation was positively correlated with the protein expression of myogenic differentiation regulators. In conclusion, a new gene-edited myoblast model was constructed based on the lncRNA regulator XLOC_015548. The in vitro cell experiments verified that XLOC_015548 had regulatory effects on muscle growth and myoblast differentiation. These findings provide a laboratory foundation for the clinical application of lncRNAs as regulatory factors in the treatment of disuse muscle atrophy.

P2Y1R and P2Y2R: potential molecular triggers in muscle regeneration

Muscle regeneration is indispensable for skeletal muscle health and daily life when injury, muscular disease, and aging occur. Among the muscle regeneration, muscle stem cells' (MuSCs) activation, proliferation, and differentiation play a key role in muscle regeneration. Purines bind to its specific receptors during muscle development, which transmit environmental stimuli and play a crucial role of modulator of muscle regeneration. Evidences proved P2R expression during development and regeneration of skeletal muscle, both in human and mouse. In contrast to P2XR, which have been extensively investigated in skeletal muscles, the knowledge of P2YR in this tissue is less comprehensive. This review summarized muscle regeneration via P2Y1R and P2Y2R and speculated that P2Y1R and P2Y2R might be potential molecular triggers for MuSCs' activation and proliferation via the p-ERK1/2 and PLC pathways, explored their cascade effects on skeletal muscle, and proposed P2Y1/2 receptors as potential pharmacological targets in muscle regeneration, to advance the purinergic signaling within muscle and provide promising strategies for alleviating muscular disease.

Lactate Activates AMPK Remodeling of the Cellular Metabolic Profile and Promotes the Proliferation and Differentiation of C2C12 Myoblasts

Lactate is a general compound fuel serving as the fulcrum of metabolism, which is produced from glycolysis and shuttles between different cells, tissues and organs. Lactate is usually accumulated abundantly in muscles during exercise. It remains unclear whether lactate plays an important role in the metabolism of muscle cells. In this research, we assessed the effects of lactate on myoblasts and clarified the underlying metabolic mechanisms through NMR-based metabonomic profiling. Lactate treatment promoted the proliferation and differentiation of myoblasts, as indicated by significantly enhanced expression levels of the proteins related to cellular proliferation and differentiation, including p-AKT, p-ERK, MyoD and myogenin. Moreover, lactate treatment profoundly regulated metabolisms in myoblasts by promoting the intake and intracellular utilization of lactate, activating the TCA cycle, and thereby increasing energy production. For the first time, we found that lactate treatment evidently promotes AMPK signaling as reflected by the elevated expression levels of p-AMPK and p-ACC. Our results showed that lactate as a metabolic regulator activates AMPK, remodeling the cellular metabolic profile, and thereby promoting the proliferation and differentiation of myoblasts. This study elucidates molecular mechanisms underlying the effects of lactate on skeletal muscle in vitro and may be of benefit to the exploration of lactate acting as a metabolic regulator.

Muscle cell-derived cytokines in skeletal muscle regeneration

Regeneration of the mammalian adult skeletal muscle is a well-orchestrated process regulated by multiple proteins and signalling pathways. Cytokines constitute a major class of regulators of skeletal myogenesis. It is well established that infiltrating immune cells at the site of muscle injury secrete cytokines, which play critical roles in the myofibre repair and regeneration process. In the past 10-15 years, skeletal muscle itself has emerged as a prolific producer of cytokines. Much attention in the field has been focused on the endocrine effects of muscle-secreted cytokines (myokines) on metabolic regulation. However, ample evidence suggests that muscle-derived cytokines also regulate myogenic differentiation and muscle regeneration in an autocrine manner. In this review, we survey cytokines that meet two criteria: (a) evidence of expression by muscle cells; (b) evidence demonstrating a myogenic function. Dozens of cytokines representing several major classes make up this group, and together they regulate all steps of the myogenic process. How such a large array of cytokines coordinate their signalling to form a regulatory network is a fascinating, pressing question. Functional studies that can distinguish the source of the cytokines in vivo are also much needed in order to facilitate exploration of their full therapeutic potential.

Anti‑oncogenic and pro‑myogenic action of the MKK6/p38/AKT axis induced by targeting MEK/ERK in embryonal rhabdomyosarcoma

Insights into the molecular and cellular biology of embryonal rhabdomyosarcoma (ERMS), an aggressive paediatric tumour, are required in order to identify new targets for novel treatments that may benefit patients with this disease. The present study examined the functional effects of MKK3 and MKK6, two upstream kinases of p38, and found that the ectopic expression of MKK6 led to rapid p38 activation and the myogenic differentiation of ERMS cells, whereas MKK3 failed to induce differentiation, while maintaining the proliferation state. Myogenin and myosin heavy chain were induced in MKK6‑overexpressing ERMS cells and were inhibited by the p38 inhibitor, SB203580. The expression of Myc and ERK‑PO4 increased under the effect of SB203580, whereas it decreased in MKK6‑overexpressing cells. AKT activation was part of the myogenic program triggered by MKK6 overexpression alone. To the best of our knowledge, the present study demonstrates, for the first time, that the endogenous MKK6 pathway may be recovered by MEK/ERK inhibition (U0126 and trametinib) and that it concomitantly induces the reversal of the oncogenic pattern and the induction of the myogenic differentiation of ERMS cell lines. The effects of MEK/ERK inhibitors markedly increase the potential clinical applications in ERMS, particularly on account of the MEK inhibitor‑induced early MKK6/p38 axis activation and of their anti‑oncogenic effects. The findings presented herein lend further support to the antitumour effects of MKK6; MKK6 may thus represent a novel target for advanced personalised treatments against ERMS.

Regulation of A-to-I RNA editing and stop codon recoding to control selenoprotein expression during skeletal myogenesis

Selenoprotein N (SELENON), a selenocysteine (Sec)-containing protein with high reductive activity, maintains redox homeostasis, thereby contributing to skeletal muscle differentiation and function. Loss-of-function mutations in SELENON cause severe neuromuscular disorders. In the early-to-middle stage of myoblast differentiation, SELENON maintains redox homeostasis and modulates endoplasmic reticulum (ER) Ca2+ concentration, resulting in a gradual reduction from the middle-to-late stages due to unknown mechanisms. The present study describes post-transcriptional mechanisms that regulate SELENON expression during myoblast differentiation. Part of an Alu element in the second intron of SELENON pre-mRNA is frequently exonized during splicing, resulting in an aberrant mRNA that is degraded by nonsense-mediated mRNA decay (NMD). In the middle stage of myoblast differentiation, ADAR1-mediated A-to-I RNA editing occurs in the U1 snRNA binding site at 5' splice site, preventing Alu exonization and producing mature mRNA. In the middle-to-late stage of myoblast differentiation, the level of Sec-charged tRNASec decreases due to downregulation of essential recoding factors for Sec insertion, thereby generating a premature termination codon in SELENON mRNA, which is targeted by NMD.

Transcription Regulation of Tceal7 by the Triple Complex of Mef2c, Creb1 and Myod

Simple Summary

We have previously reported a striated muscle-specific gene during embryogenesis, Tceal7. Our studies have characterized the 0.7 kb promoter of the Tceal7 gene, which harbors important E-box motifs driving the LacZ reporter in the myogenic lineage. However, the underlying mechanism regulating the dynamic expression of Tceal7 during skeletal muscle regeneration is still elusive. In the present work, we have defined a cluster of Mef2#3–CRE#3–E#4 motifs through bioinformatic analysis and transcription assays. Our studies suggested that the triple complex of Mef2c, Creb1 and Myod binds to the Mef2#3–CRE#3–E#4 cluster region, therefore driving the dynamic expression of Tceal7 during skeletal muscle regeneration. The novel mechanism may throw new light on understanding transcription regulation in skeletal muscle myogenesis.

Abstract

Tceal7 has been identified as a direct, downstream target gene of MRF in the skeletal muscle. The overexpression of Tceal7 represses myogenic proliferation and promotes cell differentiation. Previous studies have defined the 0.7 kb upstream fragment of the Tceal7 gene. In the present study, we have further determined two clusters of transcription factor-binding motifs in the 0.7 kb promoter: CRE#2–E#1–CRE#1 in the proximal region and Mef2#3–CRE#3–E#4 in the distal region. Utilizing transcription assays, we have also shown that the reporter containing the Mef2#3–CRE#3–E#4 motifs is synergistically transactivated by Mef2c and Creb1. Further studies have mapped out the protein–protein interaction between Mef2c and Creb1. In summary, our present studies support the notion that the triple complex of Mef2c, Creb1 and Myod interacts with the Mef2#3–CRE#3–E#4 motifs in the distal region of the Tceal7 promoter, thereby driving Tceal7 expression during skeletal muscle development and regeneration.

MEK-inhibitor-mediated rescue of skeletal myopathy caused by activating Hras mutation in a Costello syndrome mouse model

Costello syndrome (CS) is a congenital disorder caused by heterozygous activating germline HRAS mutations in the canonical Ras/mitogen-activated protein kinase (Ras/MAPK) pathway. CS is one of the RASopathies, a large group of syndromes caused by mutations within various components of the Ras/MAPK pathway. An important part of the phenotype that greatly impacts quality of life is hypotonia. To gain a better understanding of the mechanisms underlying hypotonia in CS, a mouse model with an activating Hras^G12V allele was utilized. We identified a skeletal myopathy that was due, in part, to inhibition of embryonic myogenesis and myofiber formation, resulting in a reduction in myofiber size and number that led to reduced muscle mass and strength. In addition to hyperactivation of the Ras/MAPK and PI3K/AKT pathways, there was a significant reduction in p38 signaling, as well as global transcriptional alterations consistent with the myopathic phenotype. Inhibition of Ras/MAPK pathway signaling using a MEK inhibitor rescued the Hras^G12V myopathy phenotype both in vitro and in vivo, demonstrating that increased MAPK signaling is the main cause of the muscle phenotype in CS.

Summary: A Costello syndrome (CS) mouse model carrying a heterozygous Hras p.G12V mutation was utilized to investigate Ras pathway dysregulation, revealing that increased MAPK signaling is the main cause of the muscle phenotype in CS.

Dyrk1b promotes autophagy during skeletal muscle differentiation by upregulating 4e-bp1

Rare gain of function mutations in the gene encoding Dyrk1b, a key regulator of skeletal muscle differentiation, have been associated with sarcopenic obesity (SO) and metabolic syndrome (MetS) in humans. So far, the global gene networks regulated by Dyrk1b during myofiber differentiation have remained elusive. Here, we have performed untargeted proteomics to determine Dyrk1b-dependent gene-network in differentiated C2C12 myofibers. This analysis led to identification of translational inhibitor, 4e-bp1 as a post-transcriptional target of Dyrk1b in C2C12 cells. Accordingly, CRISPR/Cas9 mediated knockout of Dyrk1b in zebrafish identified 4e-bp1 as a downstream target of Dyrk1b in-vivo. The Dyrk1b knockout zebrafish embryos exhibited markedly reduced myosin heavy chain 1 expression in poorly developed myotomes and were embryonic lethal. Using knockdown and overexpression approaches in C2C12 cells, we found that 4e-bp1 enhances autophagy and mediates the effects of Dyrk1b on skeletal muscle differentiation. Dyrk1bR102C, the human sarcopenic obesity-associated mutation impaired muscle differentiation via excessive activation of 4e-bp1/autophagy axis in C2C12 cells. Strikingly, the defective muscle differentiation in Dyrk1bR102C cells was rescued by reduction of autophagic flux. The identification of Dyrk1b-4e-bp1-autophagy axis provides significant insight into pathways that are relevant to human skeletal muscle development and disorders.

FTO-mediated demethylation of GADD45B promotes myogenesis through the activation of p38 MAPK pathway

N6-methyladenosine (m⁶A) modification plays a critical role in mammalian development. However, the role of m⁶A in the skeletal muscle development remains largely unknown. Here, we report a global m⁶A modification pattern of goat skeletal muscle at two key development stages and identified that the m⁶A modification regulated the expression of the growth arrest and DNA damage-inducible 45B (GADD45B) gene, which is involved in myogenic differentiation. We showed that GADD45B expression increased during myoblast differentiation, whereas the downregulation of GADD45B inhibits myogenic differentiation and mitochondrial biogenesis. Moreover, the expression of GADD45B regulates the expression of myogenic regulatory factors and peroxisome proliferator-activated receptor gamma coactivator 1 alpha by activating the p38 mitogen-activated protein kinase (MAPK) pathway. Conversely, the inactivation of p38 MAPK abolished the GADD45B-mediated myogenic differentiation. Furthermore, we found that the knockdown of fat mass and obesity-associated protein (FTO) increases GADD45B m⁶A modification and decreases the stability of GADD45B mRNA, which impairs myogenic differentiation. Our results indicate that the FTO-mediated m⁶A modification in GADD45B mRNA drives skeletal muscle differentiation by activating the p38 MAPK pathway, which provides a molecular mechanism for the regulation of myogenesis via RNA methylation.

ZSWIM8 is a myogenic protein that partly prevents C2C12 differentiation

Cell adhesion molecule-related/downregulated by oncogenes (Cdon) is a cell-surface receptor that mediates cell–cell interactions and positively regulates myogenesis. The cytoplasmic region of Cdon interacts with other proteins to form a Cdon/JLP/Bnip-2/CDC42 complex that activates p38 mitogen-activated protein kinase (MAPK) and induces myogenesis. However, Cdon complex may include other proteins during myogenesis. In this study, we found that Cullin 2-interacting protein zinc finger SWIM type containing 8 (ZSWIM8) ubiquitin ligase is induced during C2C12 differentiation and is included in the Cdon complex. We knocked-down Zswim8 in C2C12 cells to determine the effect of ZSWIM8 on differentiation. However, we detected neither ZSWIM8-dependent ubiquitination nor the degradation of Bnip2, Cdon, or JLP. In contrast, ZSWIM8 knockdown accelerated C2C12 differentiation. These results suggest that ZSWIM8 is a Cdon complex-included myogenic protein that prevents C2C12 differentiation without affecting the stability of Bnip2, Cdon, and JLP.

A non-translational role of threonyl-tRNA synthetase in regulating JNK signaling during myogenic differentiation

Aminoacyl-tRNA synthetases (aaRSs) are house-keeping enzymes that are essential for protein synthesis. However, it has become increasingly evident that some aaRSs also have non-translational functions. Here we report the identification of a non-translational function of threonyl-tRNA synthetase (ThrRS) in myogenic differentiation. We find that ThrRS negatively regulates myoblast differentiation in vitro and injury-induced skeletal muscle regeneration in vivo. This function is independent of amino acid binding or aminoacylation activity of ThrRS, and knockdown of ThrRS leads to enhanced differentiation without affecting the global protein synthesis rate. Furthermore, we show that the non-catalytic new domains (UNE-T and TGS) of ThrRS are both necessary and sufficient for the myogenic function. In searching for a molecular mechanism of this new function, we find the kinase JNK to be a downstream target of ThrRS. Our data further reveal MEKK4 and MKK4 as upstream regulators of JNK in myogenesis and the MEKK4-MKK4-JNK pathway to be a mediator of the myogenic function of ThrRS. Finally, we show that ThrRS physically interacts with Axin1, disrupts Axin1-MEKK4 interaction and consequently inhibits JNK signaling. In conclusion, we uncover a non-translational function for ThrRS in the maintenance of homeostasis of skeletal myogenesis and identify the Axin1-MEKK4-MKK4-JNK signaling axis to be an immediate target of ThrRS action.

BMP2 downregulates urokinase-type plasminogen activator via p38 MAPK: Implications in C2C12 cells myogenic differentiation

Bone morphogenetic protein (BMP)2 strongly affects the differentiation program of myoblast cells by inhibiting myogenesis and inducing osteogenic differentiation. In turn, extracellular matrix (ECM) proteinases, such as urokinase-type plasminogen activator (uPA), can influence the fate of muscle stem cells by participating in ECM reorganization. Although both BMP2 and uPA have antagonistic roles in muscles cells differentiation, no connection between them has been elucidated so far. This study aims to determine whether BMP2 regulates uPA expression in the myogenic C2C12 cell line and its impact on muscle cell fate differentiation. Our results showed that BMP2 did not modify C2C12 cell proliferation in a growth medium or myogenic differentiation medium. Although BMP2 inhibited myogenesis and induced osteogenesis, these effects were achieved with different doses of BMP2. Low concentrations of BMP2 blocked myogenesis, while a higher concentration was needed to induce osteogenesis. Reduced uPA expression was noticed alongside myogenic inhibition at low concentrations of BMP2. BMP2 activated p38 MAPK signaling to inhibit uPA activity. Furthermore, ectopic human uPA expression reduced BMP2's ability to inhibit the myogenic differentiation of C2C12 cells. In conclusion, BMP2 inhibits uPA expression through p38 MAPK and in vitro myogenesis at non-osteogenic concentrations, while uPA ectopic expression prevents BMP2 from inhibiting myogenesis in C2C12 cells.

Genome-Wide Association Study Identifies 12 Loci Associated with Body Weight at Age 8 Weeks in Korean Native Chickens

Meat from Korean native chickens (KNCs) has high consumer demand; however, slow growth performance and high variation in body weight (BW) of KNCs remain an issue. Genome-wide association study (GWAS) is a powerful method to identify quantitative trait-associated genomic loci. A GWAS, based on a large-scale KNC population, is needed to identify underlying genetic mechanisms related to its growth traits. To identify BW-associated genomic regions, we performed a GWAS using the chicken 60K single nucleotide polymorphism (SNP) panel for 1328 KNCs. BW was measured at 8 weeks of age, from 2018 to 2020. Twelve SNPs were associated with BW at the suggestive significance level (p < 2.95 × 10⁻⁵) and located near or within 11 candidate genes, including WDR37, KCNIP4, SLIT2, PPARGC1A, MYOCD and ADGRA3. Gene set enrichment analysis based on the GWAS results at p < 0.05 (1680 SNPs) showed that 32 Gene Ontology terms and two Kyoto Encyclopedia of Genes and Genomes pathways, including regulation of transcription, motor activity, the mitogen-activated protein kinase signaling pathway, and tight junction, were significantly enriched (p < 0.05) for BW-associated genes. These pathways are involved in cell growth and development, related to BW gain. The identified SNPs are potential biomarkers in KNC breeding.

p38 MAPKs - roles in skeletal muscle physiology, disease mechanisms, and as potential therapeutic targets

p38 MAPKs play a central role in orchestrating the cellular response to stress and inflammation and in the regulation of myogenesis. Potent inhibitors of p38 MAPKs have been pursued as potential therapies for several disease indications due to their antiinflammatory properties, although none have been approved to date. Here, we provide a brief overview of p38 MAPKs, including their role in regulating myogenesis and their association with disease progression. Finally, we discuss targeting p38 MAPKs as a therapeutic approach for treating facioscapulohumeral muscular dystrophy and other muscular dystrophies by addressing multiple pathological mechanisms in skeletal muscle.

Vitamin D Supplementation and Impact on Skeletal Muscle Function in Cell and Animal Models and an Aging Population: What Do We Know So Far?

Aging is associated with impairment in skeletal muscle mass and contractile function, predisposing to fat mass gain, insulin resistance and diabetes. The impact of Vitamin D (VitD) supplementation on skeletal muscle mass and function in older adults is still controversial. The aim of this review was to summarize data from randomized clinical trials, animal dietary intervention and cell studies in order to clarify current knowledge on the effects of VitD on skeletal muscle as reported for these three types of experiments. A structured research of the literature in Medline via PubMed was conducted and a total of 43 articles were analysed (cells n = 18, animals n = 13 and humans n = 13). The results as described by these key studies demonstrate, overall, at cell and animal levels, that VitD treatments had positive effects on the development of muscle fibres in cells in culture, skeletal muscle force and hypertrophy. Vitamin D supplementation appears to regulate not only lipid and mitochondrial muscle metabolism but also to have a direct effect on glucose metabolism and insulin driven signalling. However, considering the human perspective, results revealed a predominance of null effects of the vitamin on muscle in the ageing population, but experimental design may have influenced the study outcome in humans. Well-designed long duration double-blinded trials, standardised VitD dosing regimen, larger sample sized studies and standardised measurements may be helpful tools to accurately determine results and compare to those observed in cells and animal dietary intervention models.

Moderators of skeletal muscle maintenance are compromised in sarcopenic obese mice

The purpose of this study was to determine whether sarcopenic obesity accelerates impairments in muscle maintenance through the investigation of cell cycle progression and myogenic, inflammatory, catabolic and protein synthetic signaling in mouse gastrocnemius muscles. At 4 weeks old, 24 male C57BL/6 mice were fed either a high fat diet (HFD, 60 % fat) or normal chow (NC, 17 % fat) for either 8-12 weeks or 21-23 months. At 3-4 months or 22-24 months the gastrocnemius muscles were excised. In addition, plasma was taken for C2C12 differentiation experiments. Mean cross-sectional area (CSA) was reduced by 29 % in aged HFD fed mice compared to the aged NC mice. MyoD was roughly 50 % greater in the aged mice compared to young mice, whereas TNF-α and IGF-1 gene expression in aged HFD fed mice were reduced by 52 % and 65 % in comparison to aged NC fed mice, respectively. Myotubes pretreated with plasma from aged NC fed mice had 14 % smaller myotube diameter than their aged HFD counterparts. Aged obese mice had greater impairments to mediators of muscle maintenance as evident by reductions in muscle mass, CSA, along with alterations in cell cycle regulation and inflammatory and insulin signaling.

N6-methyladenine demethylase ALKBH1 inhibits the differentiation of skeletal muscle

DNA N⁶-methyladenine (N6-mA) was recently recognized as a new epigenetic modification in mammalian genome, and ALKBH1 was discovered as its demethylase. Knock-out mice studies revealed that ALKBH1 was indispensable for normal embryonic development. However, the function of ALKBH1 in myogenesis is largely unknown. In this study, we found that N6-mA showed a steady increase, going along with a strong decrease of ALKBH1 during skeletal muscle development. Our results also showed that ALKBH1 enhanced proliferation and inhibited differentiation of C2C12 cells. Genome-wide transcriptome analysis and reporter assays further revealed that ALKBH1 accomplished the differentiation inhibiting function by regulating a core set of genes and multiple signaling pathways, including increasing chemokine (C-X-C motif) ligand 14 (CXCL14) and activating ERK signaling. Taken together, our results demonstrated that ALKBH1 is critical for the myogenic differentiation of C2C12 cells, and suggested that N6-mA might be a new epigenetic mechanism for the regulation of myogenesis.

The functional analysis of transiently upregulated miR-101 suggests a "braking" regulatory mechanism during myogenesis

Skeletal muscle differentiation is a highly coordinated process that involves many cellular signaling pathways and microRNAs (miRNAs). A group of muscle-specific miRNAs has been reported to promote myogenesis by suppressing key signaling pathways for cell growth. However, the functional role and regulatory mechanism of most non-muscle-specific miRNAs with stage-specific changes during differentiation are largely unclear. Here, we describe the functional characterization of miR-101a/b, a pair of non-muscle-specific miRNAs that show the largest change among a group of transiently upregulated miRNAs during myogenesis in C2C12 cells. The overexpression of miR-101a/b inhibits myoblast differentiation by suppressing the p38/MAPK, Interferon Gamma, and Wnt pathways and enhancing the C/EBP pathway. Mef2a, a key protein in the p38/MAPK pathway, was identified as a direct target of miR-101a/b. Interestingly, we found that the long non-coding RNA (lncRNA) Malat1, which promotes muscle differentiation, interacts with miR-101a/b, and this interaction competes with Mef2a mRNA to relieve the inhibition of the p38/MAPK pathway during myogenesis. These results uncovered a "braking" role in differentiation of transiently upregulated miRNAs and provided new insights into the competing endogenous RNA (ceRNA) regulatory mechanism in myoblast differentiation and myogenesis.

Epigenetic modifications in muscle regeneration and progression of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a multisystemic disorder that affects 1:5000 boys. The severity of the phenotype varies dependent on the mutation site in the DMD gene and the resultant dystrophin expression profile. In skeletal muscle, dystrophin loss is associated with the disintegration of myofibers and their ineffective regeneration due to defective expansion and differentiation of the muscle stem cell pool. Some of these phenotypic alterations stem from the dystrophin absence-mediated serine-threonine protein kinase 2 (MARK2) misplacement/downregulation in activated muscle stem (satellite) cells and neuronal nitric oxide synthase loss in cells committed to myogenesis. Here, we trace changes in DNA methylation, histone modifications, and expression of regulatory noncoding RNAs during muscle regeneration, from the stage of satellite cells to myofibers. Furthermore, we describe the abrogation of these epigenetic regulatory processes due to changes in signal transduction in DMD and point to therapeutic treatments increasing the regenerative potential of diseased muscles based on this acquired knowledge.

Understanding the common mechanisms of heart and skeletal muscle wasting in cancer cachexia

Cachexia is a severe complication of cancer that adversely affects the course of the disease, with currently no effective treatments. It is characterized by a progressive atrophy of skeletal muscle and adipose tissue, resulting in weight loss, a reduced quality of life, and a shortened life expectancy. Although the cachectic condition primarily affects the skeletal muscle, a tissue that accounts for ~40% of total body weight, cachexia is considered a multi-organ disease that involves different tissues and organs, among which the cardiac muscle stands out for its relevance. Patients with cancer often experience severe cardiac abnormalities and manifest symptoms that are indicative of chronic heart failure, including fatigue, shortness of breath, and impaired exercise tolerance. Furthermore, cardiovascular complications are among the major causes of death in cancer patients who experienced cachexia. The lack of effective treatments for cancer cachexia underscores the need to improve our understanding of the underlying mechanisms. Increasing evidence links the wasting of the cardiac and skeletal muscles to metabolic alterations, primarily increased energy expenditure, and to increased proteolysis, ensuing from activation of the major proteolytic machineries of the cell, including ubiquitin-dependent proteolysis and autophagy. This review aims at providing an overview of the key mechanisms of cancer cachexia, with a major focus on those that are shared by the skeletal and cardiac muscles.

A skeleton muscle model using GelMA-based cell-aligned bioink processed with an electric-field assisted 3D/4D bioprinting

The most important requirements of biomedical substitutes used in muscle tissue regeneration are appropriate topographical cues and bioactive components for the induction of myogenic differentiation/maturation. Here, we developed an electric field-assisted 3D cell-printing process to fabricate cell-laden fibers with a cell-alignment cue. Methods: We used gelatin methacryloyl (GelMA) laden with C2C12 cells. The cells in the GelMA fiber were exposed to electrical stimulation, which induced cell alignment. Various cellular activities, such as cell viability, cell guidance, and proliferation/myogenic differentiation of the microfibrous cells in GelMA, were investigated in response to parameters (applied electric fields, viscosity of the bioink, and encapsulated cell density). In addition, a cell-laden fibrous bundle mimicking the structure of the perimysium was designed using gelatin hydrogel in conjunction with a 4D bioprinting technique. Results: Cell-laden microfibers were fabricated using optimized process parameters (electric field intensity = 0.8 kV cm-1, applying time = 12 s, and cell number = 15 × 106 cells mL-1). The cell alignment induced by the electric field promoted significantly greater myotube formation, formation of highly ordered myotubes, and enhanced maturation, compared to the normally printed cell-laden structure. The shape change mechanism that involved the swelling properties and folding abilities of gelatin was successfully evaluated, and we bundled the GelMA microfibers using a 4D-conceptualized gelatin film. Conclusion: The C2C12-laden GelMA structure demonstrated effective myotube formation/maturation in response to stimulation with an electric field. Based on these results, we propose that our cell-laden fibrous bundles can be employed as in vitro drug testing models for obtaining insights into the various myogenic responses.

Comprehensive Analysis of the Proteome and PTMomes of C2C12 Myoblasts Reveals that Sialylation Plays a Role in the Differentiation of Skeletal Muscle Cells

The C2C12 myoblast is a model that has been used extensively to study the process of skeletal muscle differentiation. Proteomics has advanced our understanding of skeletal muscle biology and also the differentiation process of skeletal muscle cells. However, there is still no comprehensive analysis of C2C12 myoblast proteomes, which is important for the understanding of key drivers for the differentiation of skeletal muscle cells. Here, we conducted multidimensional proteome profiling to get a comprehensive analysis of proteomes and PTMomes of C2C12 myoblasts with a TiSH strategy. A total of 8313 protein groups were identified, including 7827 protein groups from nonmodified peptides, 3803 phosphoproteins, and 977 formerly sialylated N-linked glycoproteins. Integrated analysis of proteomic and PTMomic data showed that almost all of the kinases and transcription factors in the muscle cell differentiation pathway were phosphorylated. Further analysis indicated that sialylation might play a role in the differentiation of C2C12 myoblasts. Further functional analysis demonstrated that C2C12 myoblasts showed a decreased level of sialylation during skeletal muscle cell differentiation. Inhibition of sialylation with the sialyltransferase inhibitor 3Fax-Neu5Ac resulted in the lower expression of MHC and suppression of myoblast fusion. In all, these results indicate that sialylation has an effect on the differentiation of skeletal muscle cells.

[Roles of p38 MAPK signaling in the skeletal muscle formation, regeneration, and pathology]

Sarcopenia and frailty in aging, or cancer cachexia shows an abnormal decrease in skeletal muscle mass and muscle strength. However, the underlying mechanisms are not clear, and the promising drug seeds have not been discovered. The formation of skeletal muscle occurs not only during embryonic development but also in adulthood, and the muscle can be regenerated even if it is damaged by exercise overload or physical injury. Although p38MAPK is ubiquitous among tissues and transmits signal of inflammation and environmental stress into the nucleus, it has been revealed that this kinase is deeply involved in maintaining skeletal muscle homeostasis. Knowledge of p38MAPK accumulated so far suggests that it not only functions as an on-off switch for gene expression, but also it balances cell proliferation and differentiation of progenitor cells to properly respond to muscle damage and repair muscle according to its surrounding environmental cues. In addition, its role in cell fusion to induce myotube formation has been recently revealed. On the other hand, it has been pointed out that in aging and chronic inflammation, excessive enhancement of the p38MAPK activity may disrupt skeletal muscle homeostasis and lead to muscle pathology. Interestingly, animal models have shown that pharmacological manipulation of p38MAPK activity can re-activate aged muscle satellite cells, suggesting the possibility of plastically manipulating skeletal muscle aging. Furthermore, it has become possible to track the dynamics of intracellular signaling of skeletal muscle cells or muscle progenitor cells in time and space by using advanced imaging techniques. In this review, we focus on the functional roles and regulatory mechanism of p38MAPK in skeletal muscle and its relation to the pathology in the context of dysregulation of skeletal muscle formation and regeneration.

Trilobatin ameliorates insulin resistance through IRS-AKT-GLUT4 signaling pathway in C2C12 myotubes and ob/ob mice

Background

Trilobatin, a natural compound, has been found to exhibit anti-diabetic properties in high-fat diet (HFD) and streptozotocin (STZ) induced type 2 diabetic mice. But up to now no research has been reported on the effect of trilobatin on insulin resistance in peripheral tissues. Herein, we determined the effects of trilobatin on insulin resistance in palmitate-treated C2C12 myotubes and ob/ob mice.

Methods

Male ob/ob mice (8-10 weeks) and same background C57BL/6 mice were used to evaluate the role of trilobatin on insulin resistance; protein expression and phosphorylation were measured by western blot; glucose uptake was determined a fluorescent test.

Results

Treatment with trilobatin prevented palmitate-induced insulin resistance by enhancing glucose uptake and the phosphorylation of insulin resistance substrate 1 (IRS1) and protein Kinase B, (PKB/AKT), recovered the translocation of GLUT4 from cytoplasm to membrane, but preincubation with LY294002, an inhibitor of PI3K, blocked the effects of trilobatin on glucose uptake and the distribution of GLUT4 in C2C12 myotubes. Furthermore, administration with trilobatin for 4 weeks significantly improved insulin resistance by decreasing fasting blood glucose and insulin in serum, enhancing the phosphorylation of IRS1 and AKT, and recovering the expression and translocation of GLUT4 in ob/ob mice.

Conclusions

IRS-AKT-GLUT4 signaling pathway might be involved in trilobatin ameliorating insulin resistance in skeletal muscle of obese animal models.

miR-194-5p negatively regulates the proliferation and differentiation of rabbit skeletal muscle satellite cells

Skeletal muscle satellite cells (SMSCs), also known as a multipotential stem cell population, play a crucial role during muscle growth and regeneration. In recent years, numerous miRNAs have been associated with the proliferation and differentiation of SMSCs in a number of mammalian species; however, the regulatory mechanisms of miR-194-5p in rabbit SMSCs still remain scarce. In this study, miR-194-5p was first observed to be highly expressed in the rabbit leg muscle. Furthermore, both the mimics and inhibitor of miR-194-5p were used to explore its role in the proliferation and differentiation of rabbit SMSCs cultured in vitro. Results from both EdU and CCK8 assays showed that miR-194-5p inhibited the proliferation of SMSCs. Meanwhile, Mef2c was identified as a target gene of miR-194-5p based on the dual-luciferase reporter assay results. In addition, upregulation of miR-194-5p decreased the expression levels of Mef2c and MyoG during rabbit SMSCs differentiation on Days 3 and 7 of in vitro culture. Taken together, these data demonstrated that miR-194-5p negatively regulates the proliferation and differentiation of rabbit SMSCs by targeting Mef2c.

Regulation of the Mammalian SWI/SNF Family of Chromatin Remodeling Enzymes by Phosphorylation during Myogenesis

Myogenesis is the biological process by which skeletal muscle tissue forms. Regulation of myogenesis involves a variety of conventional, epigenetic, and epigenomic mechanisms that control chromatin remodeling, DNA methylation, histone modification, and activation of transcription factors. Chromatin remodeling enzymes utilize ATP hydrolysis to alter nucleosome structure and/or positioning. The mammalian SWItch/Sucrose Non-Fermentable (mSWI/SNF) family of chromatin remodeling enzymes is essential for myogenesis. Here we review diverse and novel mechanisms of regulation of mSWI/SNF enzymes by kinases and phosphatases. The integration of classic signaling pathways with chromatin remodeling enzyme function impacts myoblast viability and proliferation as well as differentiation. Regulated processes include the assembly of the mSWI/SNF enzyme complex, choice of subunits to be incorporated into the complex, and sub-nuclear localization of enzyme subunits. Together these processes influence the chromatin remodeling and gene expression events that control myoblast function and the induction of tissue-specific genes during differentiation.

P38α-MAPK Signaling Inhibition Attenuates Soleus Atrophy during Early Stages of Muscle Unloading

To test the hypothesis that p38α-MAPK plays a critical role in the regulation of E3 ligase expression and skeletal muscle atrophy during unloading, we used VX-745, a selective p38α inhibitor. Three groups of rats were used: non-treated control (C), 3 days of unloading/hindlimb suspension (HS), and 3 days HS with VX-745 inhibitor (HSVX; 10 mg/kg/day). Total weight of soleus muscle in HS group was reduced compared to C (72.3 ± 2.5 vs 83.0 ± 3 mg, respectively), whereas muscle weight in the HSVX group was maintained (84.2 ± 5 mg). The expression of muscle RING-finger protein-1 (MuRF1) mRNA was significantly increased in the HS group (165%), but not in the HSVX group (127%), when compared with the C group. The expression of muscle-specific E3 ubiquitin ligases muscle atrophy F-box (MAFbx) mRNA was increased in both HS and HSVX groups (294% and 271%, respectively) when compared with C group. The expression of ubiquitin mRNA was significantly higher in the HS (423%) than in the C and HSVX (200%) groups. VX-745 treatment blocked unloading-induced upregulation of calpain-1 mRNA expression (HS: 120%; HSVX: 107%). These results indicate that p38α-MAPK signaling regulates MuRF1 but not MAFbx E3 ligase expression and inhibits skeletal muscle atrophy during early stages of unloading.

The p38 Pathway: From Biology to Cancer Therapy

The p38 MAPK pathway is well known for its role in transducing stress signals from the environment. Many key players and regulatory mechanisms of this signaling cascade have been described to some extent. Nevertheless, p38 participates in a broad range of cellular activities, for many of which detailed molecular pictures are still lacking. Originally described as a tumor-suppressor kinase for its inhibitory role in RAS-dependent transformation, p38 can also function as a tumor promoter, as demonstrated by extensive experimental data. This finding has prompted the development of specific inhibitors that have been used in clinical trials to treat several human malignancies, although without much success to date. However, elucidating critical aspects of p38 biology, such as isoform-specific functions or its apparent dual nature during tumorigenesis, might open up new possibilities for therapy with unexpected potential. In this review, we provide an extensive description of the main biological functions of p38 and focus on recent studies that have addressed its role in cancer. Furthermore, we provide an updated overview of therapeutic strategies targeting p38 in cancer and promising alternatives currently being explored.

Long acting GLP-1 analog liraglutide ameliorates skeletal muscle atrophy in rodents

BACKGROUND

Skeletal muscle atrophy is characterized by muscle wasting with partial or complete functional loss. Skeletal muscle atrophy severely affects the quality of life and currently, there is no available therapy except for spinal muscular atrophy.

OBJECTIVE

Drug repositioning is a promising strategy that reduces cost and time due to prior availability of safety and toxicity details. Here we investigated myogenic and anti-atrophy effects of glucagon-like peptide-1 (GLP-1) analog liraglutide.

METHODS

We used several in vitro atrophy models in C2C12 cells and in vivo models in Sprague Dawley rats to study Liraglutide's efficacy. Western blotting was used to assess cAMP-dependent signaling pathways specifically activated by liraglutide. Therapeutic efficacy of liraglutide was investigated by histological analysis of transverse muscle sections followed by morphometry. Myogenic capacity was investigated by immunoblotting for myogenic factors.

RESULTS

Liraglutide induced myogenesis in C2C12 myoblasts through GLP-1 receptor via a cAMP-dependent complex network of signaling events involving protein kinase A, phosphoinositide 3-kinase/protein kinase B, p38 mitogen-activated protein kinase and extracellular signal-regulated kinase. Liraglutide imparted protection against freeze injury, denervation, and dexamethasone -induced skeletal muscle atrophy and improved muscular function in all these models. In a therapeutic model, liraglutide restored myofibrillar architecture in ovariectomy-induced atrophy. Anti-atrophy actions of liraglutide involved suppression of atrogene expression and enhancement in expression of myogenic factors.

CONCLUSION

Liraglutide imparted protection and restored myofibrillar architecture in diverse models of muscle atrophy. Given its potent anti-atrophy, and recently reported osteoanabolic effects, we propose liraglutide's clinical evaluation in skeletal muscle atrophy and musculoskeletal disorders associated with diverse pathologies.

Exercise as an anti-inflammatory therapy for cancer cachexia: a focus on interleukin-6 regulation

Cancer cachexia is a complicated disorder of extreme, progressive skeletal muscle wasting. It is directed by metabolic alterations and systemic inflammation dysregulation. Numerous studies have demonstrated that increased systemic inflammation promotes this type of cachexia and have suggested that cytokines are implicated in the skeletal muscle loss. Exercise is firmly established as an anti-inflammatory therapy that can attenuate or even reverse the process of muscle wasting in cancer cachexia. The interleukin IL-6 is generally considered to be a key player in the development of the microenvironment of malignancy; it promotes tumor growth and metastasis by acting as a bridge between chronic inflammation and cancerous tissue and it also induces skeletal muscle atrophy and protein breakdown. Paradoxically, a beneficial role for IL-6 has also been identified recently, and that is its status as a “founding member” of the myokine class of proteins. Skeletal muscle is an important source of circulating IL-6 in people who participate in exercise training. IL-6 acts as an anti-inflammatory myokine by inhibiting TNFα and improving glucose uptake through the stimulation of AMPK signaling. This review discusses the action of IL-6 in skeletal muscle tissue dysfunction and the role of IL-6 as an “exercise factor” that modulates the immune system. This review also sheds light on the main considerations related to the treatment of muscle wasting in cancer cachexia.

Skeletal muscle: A review of molecular structure and function, in health and disease

Decades of research in skeletal muscle physiology have provided multiscale insights into the structural and functional complexity of this important anatomical tissue, designed to accomplish the task of generating contraction, force and movement. Skeletal muscle can be viewed as a biomechanical device with various interacting components including the autonomic nerves for impulse transmission, vasculature for efficient oxygenation, and embedded regulatory and metabolic machinery for maintaining cellular homeostasis. The "omics" revolution has propelled a new era in muscle research, allowing us to discern minute details of molecular cross-talk required for effective coordination between the myriad interacting components for efficient muscle function. The objective of this review is to provide a systems-level, comprehensive mapping the molecular mechanisms underlying skeletal muscle structure and function, in health and disease. We begin this review with a focus on molecular mechanisms underlying muscle tissue development (myogenesis), with an emphasis on satellite cells and muscle regeneration. We next review the molecular structure and mechanisms underlying the many structural components of the muscle: neuromuscular junction, sarcomere, cytoskeleton, extracellular matrix, and vasculature surrounding muscle. We highlight aberrant molecular mechanisms and their possible clinical or pathophysiological relevance. We particularly emphasize the impact of environmental stressors (inflammation and oxidative stress) in contributing to muscle pathophysiology including atrophy, hypertrophy, and fibrosis. This article is categorized under: Physiology > Mammalian Physiology in Health and Disease Developmental Biology > Developmental Processes in Health and Disease Models of Systems Properties and Processes > Cellular Models.

Spatial and temporal changes in myogenic protein expression by the microenvironment after freeze injury

Skeletal muscle has the remarkable capability to regenerate itself following injury. Adult myogenic stem cells (MSCs) are responsible for the repair and regeneration, and their activity is controlled by intrinsic and extrinsic factors. The aim of this study was to examine and compare the expression levels of Pax3, Pax7, MRF and p38 proteins during the course of regeneration and in different areas of the focal freeze-lesion damaged adult rat TA muscle. Using the focal freeze injury model, immunohistochemistry, laser-capture micro-dissection and Western blot analysis were performed. The results show that (1) in the severely damaged area, the focal freeze-lesion injury significantly activated Pax7 and myogenin expression within 7 days and down-regulated Pax3, MyoD and Myf-5 within 1 or 3 days, and (2) the level of the p38 protein was strongly and transiently up-regulated in the whole muscle on day 7 following injury, whereas the level of the pp38 protein was down-regulated within 3 days in the severely damaged and non-damaged areas. These findings indicate that the temporal (e.g. the time course of regeneration) and spatial (e.g. three zones created by the focal freeze-lesion) cues in a regenerating muscle have a significant impact on the activity of the adult MSCs.

Hsp70 Interacts with Mitogen-Activated Protein Kinase (MAPK)-Activated Protein Kinase 2 To Regulate p38MAPK Stability and Myoblast Differentiation during Skeletal Muscle Regeneration

The regenerative process of injured muscle is dependent on the fusion and differentiation of myoblasts derived from muscle stem cells. Hsp70 is important for maintaining skeletal muscle homeostasis and regeneration, but the precise cellular mechanism remains elusive. In this study, we found that Hsp70 was upregulated during myoblast differentiation. Depletion or inhibition of Hsp70/Hsc70 impaired myoblast differentiation. Importantly, overexpression of p38 mitogen-activated protein kinase α (p38MAPKα) but not AKT1 rescued the impairment of myogenic differentiation in Hsp70- or Hsc70-depleted myoblasts. Moreover, Hsp70 interacted with MK2, a substrate of p38MAPK, to regulate the stability of p38MAPK. Knockdown of Hsp70 also led to downregulation of both MK2 and p38MAPK in intact muscles and during cardiotoxin-induced muscle regeneration. Hsp70 bound MK2 to regulate MK2-p38MAPK interaction in myoblasts. We subsequently identified the essential regions required for Hsp70-MK2 interaction. Functional analyses showed that MK2 is essential for both myoblast differentiation and skeletal muscle regeneration. Taken together, our findings reveal a novel role of Hsp70 in regulating myoblast differentiation by interacting with MK2 to stabilize p38MAPK.

Lactate Promotes Myoblast Differentiation and Myotube Hypertrophy via a Pathway Involving MyoD In Vitro and Enhances Muscle Regeneration In Vivo

Lactate is a metabolic substrate mainly produced in muscles, especially during exercise. Recently, it was reported that lactate affects myoblast differentiation; however, the obtained results are inconsistent and the in vivo effect of lactate remains unclear. Our study thus aimed to evaluate the effects of lactate on myogenic differentiation and its underlying mechanism. The differentiation of C2C12 murine myogenic cells was accelerated in the presence of lactate and, consequently, myotube hypertrophy was achieved. Gene expression analysis of myogenic regulatory factors showed significantly increased myogenic determination protein (MyoD) gene expression in lactate-treated cells compared with that in untreated ones. Moreover, lactate enhanced gene and protein expression of myosin heavy chain (MHC). In particular, lactate increased gene expression of specific MHC isotypes, MHCIIb and IId/x, in a dose-dependent manner. Using a reporter assay, we showed that lactate increased promoter activity of the MHCIIb gene and that a MyoD binding site in the promoter region was necessary for the lactate-induced increase in activity. Finally, peritoneal injection of lactate in mice resulted in enhanced regeneration and fiber hypertrophy in glycerol-induced regenerating muscles. In conclusion, physiologically high lactate concentrations modulated muscle differentiation by regulating MyoD-associated networks, thereby enhancing MHC expression and myotube hypertrophy in vitro and, potentially, in vivo.

Expression profiling of lncRNAs and mRNAs reveals regulation of muscle growth in the Pacific abalone, Haliotis discus hannai

Long non-coding RNAs (lncRNAs) are known to play a major role in the epigenetic regulation of muscle development. Unfortunately there is little understanding of the mechanisms with which they regulate muscle growth in abalone. Therefore, we used RNA-seq to study the muscle transcriptomes of six Haliotis discus hannai specimens: three large (L_HD group) and three small (S_HD group). We identified 2463 lncRNAs in abalone muscle belonging to two subtypes: 160 anti-sense lncRNAs and 2303 intergenic lncRNAs (lincRNAs). In the L_HD group, we identified 204 significantly differentially expressed lncRNAs (55 upregulated and 149 downregulated), and 2268 significantly differentially expressed mRNAs (994 upregulated and 1274 downregulated), as compared to the S_HD group. The bioinformatics analysis indicated that lncRNAs were relate to cell growth, regulation of growth, MAPK signaling pathway, TGF-β signaling pathway, PI3K-Akt and insulin signaling pathway, which involved in regulating muscle growth. These findings contribute to understanding the possible regulatory mechanisms of muscle growth in Pacific abalone.