Function of the myogenic regulatory factors Myf5, MyoD, Myogenin and MRF4 in skeletal muscle, satellite cells and regenerative myogenesis

Exploring the Role of Skeletal Muscle in Insulin Resistance: Lessons from Cultured Cells to Animal Models

Skeletal muscle is essential to maintain vital functions such as movement, breathing, and thermogenesis, and it is now recognized as an endocrine organ. Muscles release factors named myokines, which can regulate several physiological processes. Moreover, skeletal muscle is particularly important in maintaining body homeostasis, since it is responsible for more than 75% of all insulin-mediated glucose disposal. Alterations of skeletal muscle differentiation and function, with subsequent dysfunctional expression and secretion of myokines, play a key role in the pathogenesis of obesity, type 2 diabetes, and other metabolic diseases, finally leading to cardiometabolic complications. Hence, a deeper understanding of the molecular mechanisms regulating skeletal muscle function related to energy metabolism is critical for novel strategies to treat and prevent insulin resistance and its cardiometabolic complications. This review will be focused on both cellular and animal models currently available for exploring skeletal muscle metabolism and endocrine function.

Skeletal muscle regeneration in facioscapulohumeral muscular dystrophy is correlated with pathological severity

Facioscapulohumeral muscular dystrophy (FSHD) is an autosomal-dominant myopathy characterized by slowly progressive skeletal muscle weakness and wasting. While a regenerative response is often provoked in many muscular dystrophies, little is known about whether a regenerative response is regularly elicited in FSHD muscle, prompting this study. For comparison, we also examined the similarly slowly progressing myotonic dystrophy type 2 (DM2). To first investigate regeneration at the transcriptomic level, we used the 200 human gene Hallmark Myogenesis list. This myogenesis biomarker was elevated in FSHD and control healthy myotubes compared to their myoblast counterparts, so is higher in myogenic differentiation. The myogenesis biomarker was also elevated in muscle biopsies from most independent FSHD, DM2 or Duchenne muscular dystrophy (DMD) studies compared to control biopsies, and on meta-analysis for each condition. In addition, the myogenesis biomarker was a robust binary discriminator of FSHD, DM2 and DMD from controls. We also analysed muscle regeneration at the protein level by immunolabelling muscle biopsies for developmental myosin heavy chain. Such immunolabelling revealed one or more regenerating myofibres in 76% of FSHD muscle biopsies from quadriceps and 91% from tibialis anterior. The mean proportion of regenerating myofibres per quadriceps biopsy was 0.48%, significantly less than 1.72% in the tibialis anterior. All DM2 muscle biopsies contained regenerating myofibres, with a mean of 1.24% per biopsy. Muscle regeneration in FSHD was correlated with the pathological hallmarks of fibre size variation, central nucleation, fibrosis and necrosis/regeneration/inflammation. In summary, the regenerative response in FSHD muscle biopsies correlates with the severity of pathology.

Identifying Codium fragile extract components and their effects on muscle weight and exercise endurance

Codium fragile (CF) is a type of green algae consumed as kimchi in Asia. UPLC-QTOF-MS/MS analysis showed that CF contain lysophosphatidyl choline, canthaxanthin, retinoic acid, α-tocopherol, and unsaturated fatty acids, which reportedly improve skeletal muscle health. However, the effect of CF on skeletal muscle mass and function remains to be elucidated. In mice fed with CF extracts, exercise endurance and muscle weight increased. CF extracts enhanced protein synthesis and myogenic differentiation through the mTORC1 pathway. CF extracts also promoted oxidative muscle fiber formation and mitochondrial biogenesis through the PGC-1α-related signaling pathway. Upregulation of PGC-1α by CF extracts was abolished by EX527 SIRT1 inhibitor treatment. Changed signaling molecules in the CF extracts were partially regulated by canthaxanthin, a new compound in CF extracts, suggesting that canthaxanthin contribute synergistically to the effect of CF extracts. Therefore, CF is a potential food source for sport nutrition or prevention of sarcopenia.

Pathomechanisms and biomarkers in facioscapulohumeral muscular dystrophy: roles of DUX4 and PAX7

Facioscapulohumeral muscular dystrophy (FSHD) is characterised by progressive skeletal muscle weakness and wasting. FSHD is linked to epigenetic derepression of the subtelomeric D4Z4 macrosatellite at chromosome 4q35. Epigenetic derepression permits the distal-most D4Z4 unit to transcribe DUX4, with transcripts stabilised by splicing to a poly(A) signal on permissive 4qA haplotypes. The pioneer transcription factor DUX4 activates target genes that are proposed to drive FSHD pathology. While this toxic gain-of-function model is a satisfying "bottom-up" genotype-to-phenotype link, DUX4 is rarely detectable in muscle and DUX4 target gene expression is inconsistent in patients. A reliable biomarker for FSHD is suppression of a target gene score of PAX7, a master regulator of myogenesis. However, it is unclear how this "top-down" finding links to genomic changes that characterise FSHD and to DUX4. Here, we explore the roles and interactions of DUX4 and PAX7 in FSHD pathology and how the relationship between these two transcription factors deepens understanding via the immune system and muscle regeneration. Considering how FSHD pathomechanisms are represented by "DUX4opathy" models has implications for developing therapies and current clinical trials.

Hypoxia preconditioned bone marrow-derived mesenchymal stromal/stem cells enhance myoblast fusion and skeletal muscle regeneration

Background

The skeletal muscle reconstruction occurs thanks to unipotent stem cells, i.e., satellite cells. The satellite cells remain quiescent and localized between myofiber sarcolemma and basal lamina. They are activated in response to muscle injury, proliferate, differentiate into myoblasts, and recreate myofibers. The stem and progenitor cells support skeletal muscle regeneration, which could be disturbed by extensive damage, sarcopenia, cachexia, or genetic diseases like dystrophy. Many lines of evidence showed that the level of oxygen regulates the course of cell proliferation and differentiation.

Methods

In the present study, we analyzed hypoxia impact on human and pig bone marrow-derived mesenchymal stromal cell (MSC) and mouse myoblast proliferation, differentiation, and fusion. Moreover, the influence of the transplantation of human bone marrow-derived MSCs cultured under hypoxic conditions on skeletal muscle regeneration was studied.

Results

We showed that bone marrow-derived MSCs increased VEGF expression and improved myogenesis under hypoxic conditions in vitro. Transplantation of hypoxia preconditioned bone marrow-derived MSCs into injured muscles resulted in the improved cell engraftment and formation of new vessels.

Conclusions

We suggested that SDF-1 and VEGF secreted by hypoxia preconditioned bone marrow-derived MSCs played an essential role in cell engraftment and angiogenesis. Importantly, hypoxia preconditioned bone marrow-derived MSCs more efficiently engrafted injured muscles; however, they did not undergo myogenic differentiation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02530-3.

Bovine Satellite Cells Isolated after 2 and 5 Days of Tissue Storage Maintain the Proliferative and Myogenic Capacity Needed for Cultured Meat Production

Cultured meat is an emerging alternative food technology which aims to deliver a more ethical, sustainable, and healthy muscle-tissue-derived food item compared to conventional meat. As start-up companies are rapidly forming and accelerating this technology, many aspects of this multi-faceted science have still not been investigated in academia. In this study, we investigated if bovine satellite cells with the ability to proliferate and undergo myogenic differentiation could be isolated after extended tissue storage, for the purpose of increasing the practicality for cultured meat production. Proliferation of bovine satellite cells isolated on the day of arrival or after 2 and 5 days of tissue storage were analyzed by metabolic and DNA-based assays, while their myogenic characteristics were investigated using RT-qPCR and immunofluorescence. Extended tissue storage up to 5 days did not negatively affect proliferation nor the ability to undergo fusion and create myosin heavy chain-positive myotubes. The expression patterns of myogenic and muscle-specific genes were also not affected after tissue storage. In fact, the data indicated a positive trend in terms of myogenic potential after tissue storage, although it was non-significant. These results suggest that the timeframe of which viable myogenic satellite cells can be isolated and used for cultured meat production can be greatly extended by proper tissue storage.

Time Course of Skeletal Muscle miRNA Expression after Resistance, High-Intensity Interval, and Concurrent Exercise

INTRODUCTION

Exercise-induced microRNA (miRNA) expression has been implicated in the regulation of skeletal muscle plasticity. However, the specificity and acute time course in miRNA expression after divergent exercise modes are unknown. In a randomized crossover design, we compared the acute expression profile of eight skeletal muscle miRNAs previously reported to be involved in skeletal muscle development, growth, and maintenance after a bout of either resistance exercise (RE), high-intensity interval exercise (HIIE), and concurrent resistance and high-intensity interval exercises (CE).

METHODS

Nine untrained young men (23.9 ± 2.8 yr, 70.1 ± 14.9 kg, 177.2 ± 3.0 cm, 41.4 ± 5.2 mL·kg-1·min-1) underwent a counterbalanced crossover design in which they performed bouts of RE (2 × 10 repetitions maximum 45° leg press and leg extension exercises), HIEE (12 × 1-min sprints at V˙O2peak with 1-min rest intervals between sprints), and CE (RE followed by HIIE), separated by 1 wk. Vastus lateralis biopsies were harvested immediately before (Pre) and immediately (0 h), 4 h, and 8 h after each exercise bout.

RESULTS

There were similar increases (main effect of time; P < 0.05) in miR-1-3p, miR-133a-3p, miR-133b, miR-181a-3p, and miR-486 expression at 8 h from Pre with all exercise modes. Besides a main effect of time, miR-23a-3p and miR-206 presented a main effect of condition with lower expression after HIIE compared with RE and CE.

CONCLUSIONS

Select miRNAs (miR-1-3p, miR-133a-3p, miR-133b, miR-23a-3p, miR-181a-3p, miR-206, miR-486) do not exhibit an expression specificity in the acute recovery period after a single bout of RE, HIIE, or CE in skeletal muscle. Our data also indicate that RE has a higher effect on the expression of miR-23a-3p and miR-206 than HIIE. As upregulation of these miRNAs seems to be confined to the 8-h period after exercise, this may subsequently affect the expression patterns of target mRNAs forming the basis of exercise-induced adaptive responses.

Betaine Delayed Muscle Loss by Attenuating Samtor Complex Inhibition for mTORC1 Signaling Via Increasing SAM Level

SCOPE

The muscle loss during aging results from the blunt of protein synthesis and poses threat to the elderly health. This study aims to investigate whether betaine affects muscle loss by improving protein synthesis.

METHODS AND RESULTS

Male C57BL/6J mice are raised from age 12 or 15 months. Mice are fed with AIN-93M diet without or with 2% w/v betaine in distilled water as control group or betaine intervention group (Bet), respectively. Betaine supplementation to mice demonstrates better body composition, grip strength, and motor function. Muscle morphology upregulates expression of myogenic regulate factors, and elevates myosin heavy chain and also improves in Bet group. Betaine promotes muscle protein synthesis via tethering mammalian target of rapamycin complex1 protein kinase (mTORC1) on the lysosomal membrane thereby activating mTORC1 signaling. All these effects aforementioned are time-dependent (p < 0.05). Ultrahigh-performance liquid chromatography results show that betaine increases S-adenosyl-l-methionine (SAM) via methionine cycle. SAM sensor-Samtor-overexpression in C2C12 cells could displace mTORC1 from lysosome thereby inhibiting the mTORC1 signaling. Addition of betaine attenuates this inhibition by increasing SAM level and then disrupting interaction of Samtor complex.

CONCLUSIONS

These observations indicate that betaine could promisingly promote protein synthesis to delay age-related muscle loss.

The ESCRT-0 subcomplex component Hrs/Hgs is a master regulator of myogenesis via modulation of signaling and degradation pathways

Background

Myogenesis is a highly regulated process ending with the formation of myotubes, the precursors of skeletal muscle fibers. Differentiation of myoblasts into myotubes is controlled by myogenic regulatory factors (MRFs) that act as terminal effectors of signaling cascades involved in the temporal and spatial regulation of muscle development. Such signaling cascades converge and are controlled at the level of intracellular trafficking, but the mechanisms by which myogenesis is regulated by the endosomal machinery and trafficking is largely unexplored. The Endosomal Sorting Complex Required for Transport (ESCRT) machinery composed of four complexes ESCRT-0 to ESCRT-III regulates the biogenesis and trafficking of endosomes as well as the associated signaling and degradation pathways. Here, we investigate its role in regulating myogenesis.

Results

We uncovered a new function of the ESCRT-0 hepatocyte growth factor-regulated tyrosine kinase substrate Hrs/Hgs component in the regulation of myogenesis. Hrs depletion strongly impairs the differentiation of murine and human myoblasts. In the C2C12 murine myogenic cell line, inhibition of differentiation was attributed to impaired MRF in the early steps of differentiation. This alteration is associated with an upregulation of the MEK/ERK signaling pathway and a downregulation of the Akt2 signaling both leading to the inhibition of differentiation. The myogenic repressors FOXO1 as well as GSK3β were also found to be both activated when Hrs was absent. Inhibition of the MEK/ERK pathway or of GSK3β by the U0126 or azakenpaullone compounds respectively significantly restores the impaired differentiation observed in Hrs-depleted cells. In addition, functional autophagy that is required for myogenesis was also found to be strongly inhibited.

Conclusions

We show for the first time that Hrs/Hgs is a master regulator that modulates myogenesis at different levels through the control of trafficking, signaling, and degradation pathways.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01091-4.

Catalytically active phospholipase A2 myotoxin from Crotalus durissus terrificus induces proliferation and differentiation of myoblasts dependent on prostaglandins produced by both COX-1 and COX-2 pathways

Although crotoxin B (CB) is a well-established catalytically active secretory phospholipase A₂ group IIA (sPLA₂-IIA) myotoxin, we investigated its potential stimulatory effect on myogenesis with the involvement of prostaglandins (PGs) produced by cyclooxygenase (COX)-1 and -2 pathways. Myoblast C2C12 were cultured in proliferation or commitment protocols and incubated with CB followed by lumiracoxib (selective COX-2 inhibitor) or valeryl salicylate (selective COX-1 inhibitor) and subjected to analysis of PG release, cell proliferation and activation of myogenic regulatory factors (MRFs). Our data showed that CB in non-cytotoxic concentrations induces an increase of COX-2 protein expression and stimulates the activity of both COX isoforms to produce PGE₂, PGD₂ and 15d-PGJ₂. CB induced an increase in the proliferation of C2C12 myoblast cells dependent on PGs from both COX-1 and COX-2 pathways. In addition, CB stimulated the activity of Pax7, MyoD, Myf5 and myogenin in proliferated cells. Otherwise, CB increased myogenin activity but not MyoD in committed cells. Our findings evidence the role of COX-1- and COX-2-derived PGs in modulating CB-induced activation of MRFs. This study contributes to the knowledge that CB promote early myogenic events via regulatory mechanisms on PG-dependent COX pathways, showing new concepts about the effect of sPLA₂-IIA in skeletal muscle repair.

Induction of Skeletal Muscle Progenitors and Stem Cells from human induced Pluripotent Stem Cells

Induced pluripotent stem cells (iPSCs) have the potential to differentiate into various types of cells and tissues including skeletal muscle. The approach to convert these stem cells into skeletal muscle cells offers hope for patients afflicted with skeletal muscle diseases such as Duchenne muscular dystrophy (DMD). Several methods have been reported to induce myogenic differentiation with iPSCs derived from myogenic patients. An important point for generating skeletal muscle cells from iPSCs is to understand in vivo myogenic induction in development and regeneration. Current protocols of myogenic induction utilize techniques with overexpression of myogenic transcription factors such as Myod1(MyoD), Pax3, Pax7, and others, using recombinant proteins or small molecules to induce mesodermal cells followed by myogenic progenitors, and adult muscle stem cells. This review summarizes the current approaches used for myogenic induction and highlights recent improvements.

Electrical Stimulation of Cultured Myotubes in vitro as a Model of Skeletal Muscle Activity: Current State and Future Prospects

Skeletal muscles comprise more than a third of human body mass and critically contribute to regulation of body metabolism. Chronic inactivity reduces metabolic activity and functional capacity of muscles, leading to metabolic and other disorders, reduced life quality and duration. Cellular models based on progenitor cells isolated from human muscle biopsies and then differentiated into mature fibers in vitro can be used to solve a wide range of experimental tasks. The review discusses the aspects of myogenesis dynamics and regulation, which might be important in the development of an adequate cell model. The main function of skeletal muscle is contraction; therefore, electrical stimulation is important for both successful completion of myogenesis and in vitro modeling of major processes induced in the skeletal muscle by acute or regular physical exercise. The review analyzes the drawbacks of such cellular model and possibilities for its optimization, as well as the prospects for its further application to address fundamental aspects of muscle physiology and biochemistry and explore cellular and molecular mechanisms of metabolic diseases.

Dact1 is expressed during chicken and mouse skeletal myogenesis and modulated in human muscle diseases

Vertebrate skeletal muscle development and repair relies on the precise control of Wnt signaling. Dact1 (Dapper/Frodo) is an important modulator of Wnt signaling, interacting with key components of the various Wnt transduction pathways. Here, we characterized Dact1 mRNA and protein expression in chicken and mouse fetal muscles in vivo and during the differentiation of chick primary and mouse C2C12 myoblasts in vitro. We also performed in silico analysis to investigate Dact1 gene expression in human myopathies, and evaluated the Dact1 protein structure to seek an explanation for the accumulation of Dact1 protein aggregates in the nuclei of myogenic cells. Our results show for the first time that in both chicken and mouse, Dact1 is expressed during myogenesis, with a strong upregulation as cells engage in terminal differentiation, cell cycle withdrawal and cell fusion. In humans, Dact1 expression was found to be altered in specific muscle pathologies, including muscular dystrophies. Our bioinformatic analyses of Dact1 proteins revealed long intrinsically disordered regions, which may underpin the ability of Dact1 to interact with its many partners in the various Wnt pathways. In addition, we found that Dact1 has strong propensity for liquid-liquid phase separation, a feature that explains its ability to form nuclear aggregates and points to a possible role as a molecular 'on'-'off' switch. Taken together, our data suggest Dact1 as a candidate, multi-faceted regulator of amniote myogenesis with a possible pathophysiological role in human muscular diseases.

Suppression of MyoD induces spontaneous adipogenesis in skeletal muscle progenitor cell culture

The degree of intramuscular adipose tissue accumulation is one of the factors affecting meat quality. Accumulation of adipocytes is also observed under the pathological condition of skeletal muscle such as muscular dystrophy and sarcopenia. The origin of adipocytes seen in skeletal muscle is mesenchymal progenitor cells that can give rise to both adipocytes and fibroblasts. In the present study, we demonstrated that siRNA-mediated suppression of MyoD expression in rat skeletal muscle progenitor cell culture, which comprises both myogenic satellite cells and mesenchymal progenitor cells, resulted in diminished myotube formation and an unexpected spontaneous appearance of white adipocytes. Suppressing myomaker expression also resulted in complete absence of myotube formation without reducing MyoD expression, but no adipogenesis was seen in this scenario, indicating that decline in MyoD expression rather than decreased myotube formation is necessary to induce adipogenesis. In addition, spontaneous adipogenesis induced by suppressing MyoD expression in culture was inhibited by the conditioned medium from control culture, indicating that anti-adipogenic factor(s) are secreted from MyoD-positive myogenic cells. These results indicate the presence of regulatory mechanism on adipogenesis by myogenic cells.

Fast photocurable thiol-ene elastomers with tunable biodegradability, mechanical and surface properties enhance myoblast differentiation and contractile function

Biodegradable elastomers are important emerging biomaterials for biomedical applications, particularly in the area of soft-tissue engineering in which scaffolds need to match the physicochemical properties of native tissues. Here, we report novel fast photocurable elastomers with readily tunable mechanical properties, surface wettability, and degradability. These elastomers are prepared by a 5-min UV-irradiation of thiol-ene reaction systems of glycerol tripentenoate (GTP; a triene) or the combination of GTP and 4-pentenyl 4-pentenoate (PP; a diene) with a carefully chosen series of di- or tri-thiols. In the subsequent application study, these elastomers were found to be capable of overcoming delamination of myotubes, a technical bottleneck limiting the in vitro growth of mature functional myofibers. The glycerol-based elastomers supported the proliferation of mouse and human myoblasts, as well as myogenic differentiation into contractile myotubes. More notably, while beating mouse myotubes detached from conventional tissue culture plates, they remain adherent on the elastomer surface. The results suggest that these elastomers as novel biomaterials may provide a promising platform for engineering functional soft tissues with potential applications in regenerative medicine or pharmacological testing.

Stimulation of myogenesis by ascorbic acid and capsaicin

Myogenesis is a complex process regulated by several factors. This study evaluated the functional interaction between vitamin C and a high dose of capsaicin (a potential endoplasmic reticulum (ER) stress inducer) on myogenesis. After the induction of differentiation, treatment with ascorbic acid or ascorbic acid phosphate (AsAp) alone had minimal effects on myogenesis in C2C12 cells. However, treatment with capsaicin (300 μM) in undifferentiated C2C12 cells increased the expression levels of genes related to ER stress as well as oxidative stress. Myogenesis was effectively enhanced in C2C12 cells treated with a combination of capsaicin (300 μM) for one day before differentiation stimulation and AsAp for four days post-differentiation; subsequently, thick and long myotubes formed, and the expression levels of myosin heavy chain (MYH) 1/2 and Myh1, Myh4, and Myh7 increased. Considering that mild ER stress stimulates myogenesis, AsAp may elicit myogenesis through the alleviation of oxidative stress-induced negative effects in capsaicin-pretreated cells. The enhanced expression of Myh1 and Myh4 coincided with the expression of Col1a1, a type I collagen, suggesting that the fine-tuning of the myogenic cell microenvironment is responsible for efficient myogenesis. Our results indicate that vitamin C is a potential stimulator of myogenesis in cells, depending on the cell context.

Glutamine supplementation stimulates cell proliferation in skeletal muscle and cultivated myogenic cells of low birth weight piglets

Muscle growth of low birth weight (LBW) piglets may be improved with adapted nutrition. This study elucidated effects of glutamine (Gln) supplementation on the cellular muscle development of LBW and normal birth weight (NBW) piglets. Male piglets (n = 144) were either supplemented with 1 g Gln/kg body weight or an isonitrogeneous amount of alanine (Ala) between postnatal day 1 and 12 (dpn). Twelve piglets per group were slaughtered at 5, 12 and 26 dpn, one hour after injection with Bromodeoxyuridine (BrdU, 12 mg/kg). Muscle samples were collected and myogenic cells were isolated and cultivated. Expression of muscle growth related genes was quantified with qPCR. Proliferating, BrdU-positive cells in muscle sections were detected with immunohistochemistry indicating different cell types and decreasing proliferation with age. More proliferation was observed in muscle tissue of LBW-GLN than LBW-ALA piglets at 5 dpn, but there was no clear effect of supplementation on related gene expression. Cell culture experiments indicated that Gln could promote cell proliferation in a dose dependent manner, but expression of myogenesis regulatory genes was not altered. Overall, Gln supplementation stimulated cell proliferation in muscle tissue and in vitro in myogenic cell culture, whereas muscle growth regulatory genes were barely altered.

Gene Expression and Carcass Traits Are Different between Different Quality Grade Groups in Red-Faced Hereford Steers

Simple Summary

Producing a consistent and positive experience for beef consumers is challenging. The gene expression in muscle at harvest may provide insight into better prediction of United States Department of Agriculture (USDA) quality grade. In this pilot study muscle samples were collected at harvest on sixteen steers with a similar background and identical management. Muscle transcripts were sequenced to determine gene expression. Transcripts related to the extracellular matrix, stem cell differentiation, and focal cell adhesions were differentially expressed in muscle tissue from carcasses with differing USDA quality grades. This confirmed the application of this technique to provide insight into muscle development and fat deposition necessary for better prediction and selection to improve consistency and consumer experience.

Abstract

Fat deposition is important to carcass value and some palatability characteristics. Carcasses with higher USDA quality grades produce more value for producers and processors in the US system and are more likely to have greater eating satisfaction. Using genomics to identify genes impacting marbling deposition provides insight into muscle biochemistry that may lead to ways to better predict fat deposition, especially marbling and thus quality grade. Hereford steers (16) were managed the same from birth through harvest after 270 days on feed. Samples were obtained for tenderness and transcriptome profiling. As expected, steaks from Choice carcasses had a lower shear force value than steaks from Select carcasses; however, steaks from Standard carcasses were not different from steaks from Choice carcasses. A significant number of differentially expressed (DE) genes was observed in the longissimus lumborum between Choice and Standard carcass RNA pools (1257 genes, p < 0.05), but not many DE genes were observed between Choice and Select RNA pools. Exploratory analysis of global muscle tissue transcriptome from Standard and Choice carcasses provided insight into muscle biochemistry, specifically the upregulation of extracellular matrix development and focal adhesion pathways and the downregulation of RNA processing and metabolism in Choice versus Standard. Additional research is needed to explore the function and timing of gene expression changes.

HMEJ-mediated site-specific integration of a myostatin inhibitor increases skeletal muscle mass in porcine

As a robust antagonist of myostatin (MSTN), follistatin (FST) is an important regulator of skeletal muscle development, and the delivery of FST to muscle tissue represents a potential therapeutic strategy for muscular dystrophies. The N terminus and FSI domain of FST are the functional domains for MSTN binding. Here, we aimed to achieve site-specific integration of FSI-I-I, including the signal peptide, N terminus, and three FSI domains, into the last codon of the porcine MSTN gene using a homology-mediated end joining (HMEJ)-based strategy mediated by CRISPR-Cas9. Based on somatic cell nuclear transfer (SCNT) technology, we successfully obtained FSI-I-I knockin pigs. H&E staining of longissimus dorsi and gastrocnemius cross-sections showed larger myofiber sizes in FSI-I-I knockin pigs than in controls. Moreover, the Smad and Erk pathways were inhibited, whereas the PI3k/Akt pathway was activated in FSI-I-I knockin pigs. In addition, the levels of MyoD, Myf5, and MyoG transcription were upregulated while that of MRF4 was downregulated in FSI-I-I knockin pigs. These results indicate that the FSI-I-I gene mediates skeletal muscle hypertrophy through an MSTN-related signaling pathway and the expression of myogenic regulatory factors. Overall, FSI-I-I knockin pigs with hypertrophic muscle tissue hold great promise as a therapeutic model for human muscular dystrophies.

Morphological changes and functional circRNAs screening of rabbit skeletal muscle development

BACKGROUND

The temporal expression pattern of circular RNAs (circRNAs) across developmental stages is essential for skeletal muscle growth and functional analysis. However, there are few analyses on the potential functions of circRNAs in rabbit skeletal muscle development.

RESULTS

Initially, the paraffin sections showed extremely significant differences in the diameter, number, area and density of skeletal muscle fibers of the fetus, child, adult rabbit hind legs (P < 0.01). Then, RNA-seq libraries of these three stages were constructed. A total of 481 differentially expressed circRNAs (DE-circRNAs) and 5,658 differentially expressed genes (DEGs) were identified. Subsequently, DE-circRNAs, whose host genes were DEGs or non-DEGs, were analyzed by GO respectively. In the fetus vs. child group, up-regulated DE-circRNAs (whose host genes were DEGs) were related to muscle fiber structure, and down-regulated ones were related to mitosis. The up-regulated DE-circRNAs (whose host genes were non-DEGs) were involved in enzyme activity, methylation and glycosylation, and the down-regulated ones were involved in mitosis and catabolism. In the fetus vs. adult group, the up-regulated DE-circRNAs (whose host genes were DEGs) were related to skeletal muscle basic structure, and the down-regulated ones were also associated with cell proliferation. But the up-regulated DE-circRNAs (whose host genes were non-DEGs) were connected with regulation of histone ubiquitination, chromatin and organelles. The down-regulated DE-circRNAs were connected with the catabolism processes. In addition, novel_circ_0022663 and novel_circ_0005489, which might have coding potential, and novel_circ_0004210 and novel_circ_0001669, which might have miRNA sponge capability, were screened out.

CONCLUSIONS

In this study, hind leg muscles of fetus, child and adult rabbits were collected for paraffin section and RNA-seq to observe the structural changes of skeletal muscle and obtain circRNA expression profiles at different stages. These data provided a catalog of circRNAs related to muscle development in New Zealand rabbits, allowing us to better understand the functional transitions in mammalian muscle development.

The LIM domain protein nTRIP6 modulates the dynamics of myogenic differentiation

The process of myogenesis which operates during skeletal muscle regeneration involves the activation of muscle stem cells, the so-called satellite cells. These then give rise to proliferating progenitors, the myoblasts which subsequently exit the cell cycle and differentiate into committed precursors, the myocytes. Ultimately, the fusion of myocytes leads to myofiber formation. Here we reveal a role for the transcriptional co-regulator nTRIP6, the nuclear isoform of the LIM-domain protein TRIP6, in the temporal control of myogenesis. In an in vitro model of myogenesis, the expression of nTRIP6 is transiently up-regulated at the transition between proliferation and differentiation, whereas that of the cytosolic isoform TRIP6 is not altered. Selectively blocking nTRIP6 function results in accelerated early differentiation followed by deregulated late differentiation and fusion. Thus, the transient increase in nTRIP6 expression appears to prevent premature differentiation. Accordingly, knocking out the Trip6 gene in satellite cells leads to deregulated skeletal muscle regeneration dynamics in the mouse. Thus, dynamic changes in nTRIP6 expression contributes to the temporal control of myogenesis.

Characterization of Optimal Strain, Frequency and Duration of Mechanical Loading on Skeletal Myotubes' Biological Responses

BACKGROUND/AIM

Mechanical loading of differentiated myoblasts in vitro may mimic loading patterns of skeletal muscle in vivo. However, it is still uncharacterized the loading conditions that can produce the most effective muscle cells' biological responses, in vitro. This study investigated the effects of different loading protocols on the expression of myogenic regulatory factors, anabolic, atrophy and pro-apoptotic factors in skeletal myotubes.

MATERIALS AND METHODS

C2C12 myoblasts were differentiated and underwent various stretching protocols by altering their elongation, frequency and duration, utilizing an in vitro cell tension system. The loading-induced expression changes of MyoD, Myogenin, MRF4, IGF-1 isoforms, Murf1, Atrogin, Myostatin, Foxo and Fuca were measured by Real Time-PCR.

RESULTS

Stretching by 2% elongation at 0.25 Hz for 12 h was overall the most effective in inducing beneficial responses.

CONCLUSION

A low strain, low frequency intermediate duration stretching can most effectively up-regulate myogenic/anabolic factors and down-regulate pro-apoptotic and atrophy genes in myotubes.

Transcriptome analysis of embryonic muscle development in Chengkou Mountain Chicken

Background

Muscle is the predominant portion of any meat product, and growth performance and product quality are the core of modern breeding. The embryonic period is highly critical for muscle development, the number, shape and structure of muscle fibers are determined at the embryonic stage. Herein, we performed transcriptome analysis to reveal the law of muscle development in the embryonic stage of Chengkou Mountain Chicken at embryonic days (E) 12, 16, 19, 21.

Results

Diameter and area of muscle fibers exhibited significant difference at different embryonic times(P < 0.01). A total of 16,330 mRNAs transcripts were detected, including 109 novel mRNAs transcripts. By comparing different embryonic muscle development time points, 2,262 in E12vsE16, 5,058 in E12vsE19, 6139 in E12vsE21, 1,282 in E16vsE19, 2,920 in E16vsE21, and 646 in E19vsE21differentially expressed mRNAs were identified. It is worth noting that 7,572 mRNAs were differentially expressed. The time-series expression profile of differentially expressed genes (DEGs) showed that the rising and falling expression trends were significantly enriched. The significant enrichment trends included 3,150 DEGs. GO enrichment analysis provided three significantly enriched categories of significantly enriched differential genes, including 65 cellular components, 88 molecular functions, and 453 biological processes. Through KEGG analysis, we explored the biological metabolic pathways involved in differentially expressed genes. A total of 177 KEGG pathways were enriched, including 19 significant pathways, such as extracellular matrix-receptor interactions. Similarly, numerous pathways related to muscle development were found, including the Wnt signaling pathway (P < 0.05), MAPK signalingpathway, TGF-beta signaling pathway, PI3K-Akt signaling pathway and mTOR signaling pathway. Among the differentially expressed genes, we selected those involved in developing 4-time points; notably, up-regulated genes included MYH1F, SLC25A12, and HADHB, whereas the down-regulated genes included STMN1, VASH2, and TUBAL3.

Conclusions

Our study explored the embryonic muscle development of the Chengkou Mountain Chicken. A large number of DEGs related to muscle development have been identified ,and validation of key genes for embryonic development and preliminary explanation of their role in muscle development. Overall, this study broadened our current understanding of the phenotypic mechanism for myofiber formation and provides valuable information for improving chicken quality.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07740-w.

C2C12 myoblasts are more sensitive to the toxic effects of simvastatin than myotubes and show impaired proliferation and myotube formation

Statins reduce cardiovascular complications in patients with high LDL-cholesterol but are associated with myopathy. We compared the toxicity of simvastatin of C2C12 myoblasts and myotubes. Since myoblasts can proliferate and fuse to myotubes, myoblasts can be considered as satellite cells and myotubes as mature muscle fibers. Simvastatin increased plasma membrane permeability and decreased the cellular ATP content in both myoblasts and myotubes, but with a stronger effect on myoblasts. While insulin prevented cytotoxicity up to 8 h after addition of simvastatin to myotubes, prevention in myoblasts required simultaneous addition. Mevalonate and geranylgeraniol prevented simvastatin-associated cytotoxicity in both myoblasts and myotubes. Simvastatin impaired the phosphorylation of the insulin receptor (IR β), Akt ser473 and S6rp, and increased phosphorylation of AMPK thr172 in both myotubes and myoblasts, which was prevented by insulin and mevalonate. Simvastatin impaired oxygen consumption and increased superoxide production by myoblasts and myotubes and induced apoptosis via cytochrome c release. In addition, simvastatin impaired proliferation and fusion of myoblasts to myotubes by inhibiting the expression of the nuclear transcription factor MyoD and of the metalloprotease ADAM-12. Decreased expression of the proliferation factor Ki-67 and of ADAM-12 were also observed in gastrocnemius of mice treated with simvastatin. In conclusion, myoblasts were more susceptible to the toxic effects of simvastatin and simvastatin impaired myoblast proliferation and myotube formation. Impaired muscle regeneration may represent a new mechanism of statin myotoxicity.

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

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

Controlling cellular organization in bioprinting through designed 3D microcompartmentalization

Controlling cellular organization is crucial in the biofabrication of tissue-engineered scaffolds, as it affects cell behavior as well as the functionality of mature tissue. Thus far, incorporation of physiochemical cues with cell-size resolution in three-dimensional (3D) scaffolds has proven to be a challenging strategy to direct the desired cellular organization. In this work, a rapid, simple, and cost-effective approach is developed for continuous printing of multicompartmental hydrogel fibers with intrinsic 3D microfilaments to control cellular orientation. A static mixer integrated into a coaxial microfluidic device is utilized to print alginate/gelatin-methacryloyl (GelMA) hydrogel fibers with patterned internal microtopographies. In the engineered microstructure, GelMA compartments provide a cell-favorable environment, while alginate compartments offer morphological and mechanical cues that direct the cellular orientation. It is demonstrated that the organization of the microtopographies, and consequently the cellular alignment, can be tailored by controlling flow parameters in the printing process. Despite the large diameter of the fibers, the precisely tuned internal microtopographies induce excellent cell spreading and alignment, which facilitate rapid cell proliferation and differentiation toward mature biofabricated constructs. This strategy can advance the engineering of functional tissues.

Lateral rectus muscle differentiation potential in paralytic esotropia patients

Purpose and background

Recently, we found that maximal medial rectus recession and lateral rectus resection in patients with complete lateral rectus paralysis resulted in a partial restoration of abduction. In an attempt to understand some of the mechanisms involved with this effect we examined gene expression profiles of lateral recti from these patients, with our focus being directed to genes related to myogenesis.

Materials and methods

Lateral recti resected from patients with complete lateral rectus paralysis and those from concomitant esotropia (controls) were collected. Differences in gene expression profiles between these two groups were examined using microarray analysis and quantitative Reverse-transcription PCR (qRT-PCR).

Results

A total of 3056 differentially expressed genes (DEGs) were identified between these two groups. Within the paralytic esotropia group, 2081 genes were up-regulated and 975 down-regulated. The results of RT-PCR revealed that PAX7, MYOG, PITX1, SIX1 and SIX4 showed higher levels of expression, while that of MYOD a lower level of expression within the paralytic esotropia group as compared with that in the control group (p < 0.05).

Conclusion

The decreased expression of MYOD in the paralytic esotropia group suggested that extraocular muscle satellite cell (EOMSCs) differentiation processes were inhibited. Whereas the high expression levels of PAX7, SIX1/4 and MYOG, suggested that the EOMSCs were showing an effective potential for differentiation. The stimulation resulting from muscle surgery may induce EOMSCs to differentiate and thus restore abduction function.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12886-021-01994-4.

Taurine Protects C2C12 Myoblasts From Impaired Cell Proliferation and Myotube Differentiation Under Cisplatin-Induced ROS Exposure

In cancer patients, chemotherapeutic medication induces aberrant ROS (reactive oxygen species) accumulation in skeletal muscles, resulting in myofiber degradation, muscle weakness, and even cachexia, which further leads to poor therapeutic outcomes. Acting as an antioxidant, taurine is extensively used to accelerate postexercise muscle recovery in athletes. The antioxidant effects of taurine have been shown in mature myotubes and myofibers but not yet in myoblasts, the myotube precursor. The proliferation and differentiation ability of myoblasts play a very important role in myofiber repair and regeneration, which is usually impaired during chemotherapeutics in cancer patients as well. Here, we explored the effects of taurine supplementation on C2C12 myoblasts exposed to cisplatin-induced ROS. We found that cisplatin treatment led to dramatically decreased cell viability; accumulated ROS level; down-regulated expressions of MyoD1 (myoblast determination protein 1), myogenin, and MHC (myosin heavy chain); and impaired myotube differentiation in myoblasts. Significantly, taurine supplementation protected myoblasts against cisplatin-induced cell viability decrease, promoted cellular ROS clearance, and, most importantly, preserved the expressions of MyoD1, myogenin, and MHC as well as myotube differentiation ability. We further conducted NMR-based metabolomic analysis to clarify the underlying molecular mechanisms. We identified 14 characteristic metabolites primarily responsible for the discrimination of metabolic profiles between cisplatin-treated cells and normal counterparts, including increased levels of BCAAs (branched-chain amino acids: leucine and isoleucine), alanine, glycine, threonine, glucose, ADP (adenosine diphosphate), phenylalanine, and PC (O-phosphocholine), and decreased levels of lysine, β-alanine, choline, GPC (sn-glycero-3-phosphocholine), and myo-inositol. Evidently, taurine supplementation partially reversed the changing trends of several metabolites (isoleucine, threonine, glycine, PC, β-alanine, lysine, and myo-inositol). Furthermore, taurine supplementation promoted the proliferation and myotube differentiation of myoblasts by alleviating cellular catabolism, facilitating GSH (reduced glutathione) biosynthesis, improving glucose utilization and TCA (tricarboxylic acid) cycle anaplerosis, and stabilizing cellular membranes. Our results demonstrated the protective effects of taurine on cisplatin-impaired myoblasts and elucidated the mechanistic rationale for the use of taurine to ameliorate muscle toxicity in clinical chemotherapy cancer patients.

Muscle regeneration controlled by a designated DNA dioxygenase

Tet dioxygenases are responsible for the active DNA demethylation. The functions of Tet proteins in muscle regeneration have not been well characterized. Here we find that Tet2, but not Tet1 and Tet3, is specifically required for muscle regeneration in vivo. Loss of Tet2 leads to severe muscle regeneration defects. Further analysis indicates that Tet2 regulates myoblast differentiation and fusion. Tet2 activates transcription of the key differentiation modulator Myogenin (MyoG) by actively demethylating its enhancer region. Re-expressing of MyoG in Tet2 KO myoblasts rescues the differentiation and fusion defects. Further mechanistic analysis reveals that Tet2 enhances MyoD binding by demethylating the flanking CpG sites of E boxes to facilitate the recruitment of active histone modifications and increase chromatin accessibility and activate its transcription. These findings shed new lights on DNA methylation and pioneer transcription factor activity regulation.

Histological and transcriptomic analysis of adipose and muscle of dairy calves supplemented with 5-hydroxytryptophan

In mammals, peripheral serotonin is involved in regulating energy balance. Herein, we characterized the transcriptomic profile and microstructure of adipose and muscle in pre-weaned calves with increased circulating serotonin. Holstein bull calves (21 ± 2 days old) were fed milk replacer supplemented with saline (CON, 8 mL/day n = 4) or 5-hydroxytryptophan (5-HTP, 90 mg/day, n = 4) for 10 consecutive days. Calves were euthanized on d10 to harvest adipose and muscle for RNA-Sequencing and histological analyses. Twenty-two genes were differentially expressed in adipose, and 33 in muscle. Notably, Interferon gamma inducible protein-47 was highly expressed and upregulated in muscle and adipose (avg. log FC = 6.5). Enriched pathways in adipose tissue revealed serotonin’s participation in lipid metabolism and PPAR signaling. In muscle, enriched pathways were related to histone acetyltransferase binding, Jak-STAT signaling, PI3K-Akt signaling and cell proliferation. Supplementation of 5-HTP increased cell proliferation and total cell number in adipose and muscle. Adipocyte surface area was smaller and muscle fiber area was not different in the 5-HTP group. Manipulating the serotonin pathway, through oral supplementation of 5-HTP, influences signaling pathways and cellular processes in adipose and muscle related to endocrine and metabolic functions which might translate into improvements in calf growth and development.

A human Myogenin promoter modified to be highly active in alveolar rhabdomyosarcoma drives an effective suicide gene therapy

Rhabdomyosarcoma is a rare childhood soft tissue cancer whose cells resemble poorly differentiated skeletal muscle, expressing myogenic proteins including MYOGENIN. Alveolar rhabdomyosarcoma (ARMS) accounts for ~40% of cases and is associated with a poorer prognosis than other rhabdomyosarcoma variants, especially if containing the chromosomal translocation generating the PAX3-FOXO1 hybrid transcription factor. Metastasis is commonly present at diagnosis, with a five-year survival rate of <30%, highlighting the need for novel therapeutic approaches. We designed a suicide gene therapy by generating an ARMS-targeted promoter to drive the herpes simplex virus thymidine kinase (HSV-TK) suicide gene. We modified the minimal human MYOGENIN promoter by deleting both the NF1 and MEF3 transcription factor binding motifs to produce a promoter that is highly active in ARMS cells. Our bespoke ARMS promoter driving HSV-TK efficiently killed ARMS cells in vitro, but not skeletal myoblasts. Using a xenograft mouse model, we also demonstrated that ARMS promoter-HSV-TK causes apoptosis of ARMS cells in vivo. Importantly, combining our suicide gene therapy with standard chemotherapy agents used in the treatment of rhabdomyosarcoma, reduced the effective drug dose, diminishing deleterious side effects/patient burden. This modified, highly ARMS-specific promoter could provide a new therapy option for this difficult-to-treat cancer.

Obesity-induced alterations to the immunoproteasome: a potential link to intramuscular lipotoxicity

Although the mechanisms are unclear, inflammation and/or lipotoxicity likely contribute to obese muscle pathology. The immunoproteasome is known to respond to inflammation and oxidative damage and may aid muscle regeneration. We sought to determine whether diet-induced obesity (DIO) influences the immunoproteasome subunits LMP7 and MECL-1 in mouse muscle with and without exercise-induced muscle damage (EIMD). Muscle mass, regeneration, macrophage content and lipid peroxidation (8-isoprostane) were also assessed. Sixty male, 4-week-old C57BL/6J mice were fed a high-fat (HFD) or low-fat diet for 12 weeks. Mice were then subdivided into EIMD or no muscle damage (NMD) groups. The gastrocnemius muscle was excised 1 or 5 days after EIMD, producing 6 groups (n = 10/group). Body mass was greater; however, relative gastrocnemius mass was lower in HFD-fed mice. Despite no macrophage or MECL-1 alterations, LMP7 and 8-isoprostane were increased in obese mice in the NMD and 1 day post-EIMD groups. However, 8-isoprostane was reduced in obese mice 5 days post-EIMD, and accompanied by increased muscle LMP7, MECL-1 and macrophage content. Consequently, DIO may impair the immunoproteasome's ability to control muscle lipid peroxidation but is reversed with eccentric exercise. Although muscle regeneration was unchanged, immunoproteasome dysregulation occurs in obese muscle and may contribute to muscle pathology. Novelty: DIO may impair the intramuscular immunoproteasome response to lipid peroxidation. Acute eccentric exercise may protect obese individuals from muscle lipotoxicity via immunoproteasome upregulation.

Dietary Selenium Promotes Somatic Growth of Rainbow Trout (Oncorhynchus mykiss) by Accelerating the Hypertrophic Growth of White Muscle

As a nutritionally essential trace element, selenium (Se) is crucial for fish growth. However, the underlying mechanisms remain unclear. Fish somatic growth relies on the white muscle growth. This study aimed to explore the effects and underlying mechanisms of Se on fish white muscle growth using a juvenile rainbow trout (Oncorhynchus mykiss) model. Fish were fed a basal diet unsupplemented or supplemented with selenium yeast at nutritional dietary Se levels (2 and 4 mg/kg Se, respectively) for 30 days. Results showed that dietary Se supplementation significantly enhanced trout somatic growth. Histological and molecular analysis of trout white muscle tissues at the vent level showed that dietary Se supplementation elevated the total cross-sectional area of white muscle, mean diameter of white muscle fibers, protein content, nuclei number, and DNA content of individual muscle fiber, and suppressed the activities of calpain system and ubiquitin-proteasome pathway. Overall, this study demonstrated that dietary Se within the nutritional range inhibits calpain- and ubiquitin-mediated protein degradation and promotes the fusion of myoblasts into the existed muscle fibers to promote the hypertrophic growth of white muscle, thereby accelerating the somatic growth of rainbow trout. Our results provide a mechanistic insight into the regulatory role of Se in fish growth.

Pediatric Benign Tumors With a Skeletal Muscle Component: Myogenin Expression, Diagnostic Pitfalls, and New Molecular Insights

OBJECTIVES

Benign tumors with skeletal muscle differentiation are rare and their characterization in the literature is limited. We present a series of twelve pediatric benign tumors with rhabdomyomatous differentiation including seven rhabdomyomatous mesenchymal hamartomas, four fetal rhabdomyomas, and one benign triton tumor, analyzing myogenic markers as well as clinicopathologic and molecular features. A review of the literature was also performed with an emphasis on myogenic marker expression and correlation with molecular features.

METHODS AND RESULTS

Cases obtained from three tertiary pediatric hospitals were retrospectively reviewed. Eleven of twelve cases expressed myogenin in rare to greater than 15% of cells. Five of nine cases had rare to 70-80% of cells positive for MyoD1. One fetal rhabdomyoma demonstrated homozygous deletions in ZEB2. The benign triton tumor harbored a CTNNB1 mutation. Review of the literature identified 160 pediatric benign tumors with skeletal muscle differentiation of which 9 reported myogenin positivity.

CONCLUSIONS

Myogenin and MyoD1 may be variably expressed in benign lesions with skeletal muscle differentiation. Recognition of key morphologic features remains critical to diagnose these lesions and, in rhabdomyoma, to exclude malignancy. Our series expands the knowledge of the relationship between rhabdomyoma and rhabdomyosarcoma (RMS) by identifying a shared molecular alteration in ZEB2.

Exploring the ability of low-level laser irradiation to reduce myonecrosis and increase Myogenin transcription after Bothrops jararacussu envenomation

Envenoming caused by snakebites is a very important neglected tropical disease worldwide. The myotoxic phospholipases present in the bothropic venom disrupt the sarcolemma and compromise the mechanisms of energy production, leading to myonecrosis. Photobiomodulation therapy (PBMT) has been used as an effective tool to treat diverse cases of injuries, such as snake venom-induced myonecrosis. Based on that, the aim of this study was to analyze the effects of PBMT through low-level laser irradiation (904 nm) on the muscle regeneration after the myonecrosis induced by Bothrops jararacussu snake venom (Bjssu) injection, focusing on myogenic regulatory factors expression, such as Pax7, MyoD, and Myogenin (MyoG). Male Swiss mice (Mus musculus), 6-8-week-old, weighing 22 ± 3 g were used. Single sub-lethal Bjssu dose or saline was injected into the right mice gastrocnemius muscle. At 3, 24, 48, and 72 h after injections, mice were submitted to PBMT treatment. When finished the periods of 48 and 72 h, mice were euthanized and the right gastrocnemius were collected for analyses. We observed extensive inflammatory infiltrate in all the groups submitted to Bjssu injections. PBMT was able to reduce the myonecrotic area at 48 and 72 h after envenomation. There was a significant increase of MyoG mRNA expression at 72 h after venom injection. The data suggest that beyond the protective effect promoted by PBMT against Bjssu-induced myonecrosis, the low-level laser irradiation was able to stimulate the satellite cells, thus enhancing the muscle repair by improving myogenic differentiation.

Dietary Supplementation With Chlorogenic Acid Derived From Lonicera macranthoides Hand-Mazz Improves Meat Quality and Muscle Fiber Characteristics of Finishing Pigs via Enhancement of Antioxidant Capacity

Chlorogenic acid (CGA), one of the most abundant polyphenol compounds in nature, is regarded as a potential feed additive to promote animal health and enhance the meat products’ quality via its various biological properties. The current study aims: (1) to determine whether dietary CGA supplementation improves meat quality and muscle fiber characteristics, and (2) to ascertain whether the corresponding improvement is associated with enhancing the antioxidant capacity of the finishing pigs. Thirty-two (Large × White × Landrace) finishing pigs with an average initial body weight of 71.89 ± 0.92 kg were allotted to 4 groups, and each was fed diets supplemented with 0, 0.02, 0.04, or 0.08% (weight/weight) of CGA. The meat quality traits, muscle fiber characteristics, and the serum and muscle antioxidant capacity were assessed. Results suggested that, compared with the control group, dietary CGA supplementation at a level of 0.04% significantly decreased the b^∗ value and distinctly increased the inosinic acid content of longissimus dorsi (LD) and biceps femoris (BF) muscles (P < 0.01). Moreover, dietary supplementation with 0.04% of CGA markedly improved the amino acid composition of LD and BF muscles, as well as augmented the mRNA abundance of Nrf-2, GPX-1, MyoD, MyoG, and oxidative muscle fiber (I and IIa) in LD muscle (P < 0.05). This result indicates that a diet supplemented with 0.04% of CGA promotes myogenesis and induces a transformation toward more oxidative muscle fibers in LD muscle, subsequently improving meat quality. Besides, dietary supplementation with 0.02% and 0.04% of CGA notably enhanced the serum GSH-PX level (P < 0.01). Considering all these effects are closely related to the alteration of antioxidant activities of the finishing pigs, the underlying metabolism is likely connected to the boosting of their antioxidant capacity induced by dietary CGA.

More Than One-to-Four via 2R: Evidence of an Independent Amphioxus Expansion and Two-Gene Ancestral Vertebrate State for MyoD-Related Myogenic Regulatory Factors (MRFs)

The evolutionary transition from invertebrates to vertebrates involved extensive gene duplication, but understanding precisely how such duplications contributed to this transition requires more detailed knowledge of specific cases of genes and gene families. Myogenic differentiation (MyoD) has long been recognized as a master developmental control gene and member of the MyoD family of bHLH transcription factors (myogenic regulatory factors [MRFs]) that drive myogenesis across the bilaterians. Phylogenetic reconstructions within this gene family are complicated by multiple instances of gene duplication and loss in several lineages. Following two rounds of whole-genome duplication (2R WGD) at the origin of the vertebrates, the ancestral function of MRFs is thought to have become partitioned among the daughter genes, so that MyoD and Myf5 act early in myogenic determination, whereas Myog and Myf6 are expressed later, in differentiating myoblasts. Comparing chordate MRFs, we find an independent expansion of MRFs in the invertebrate chordate amphioxus, with evidence for a parallel instance of subfunctionalization relative to that of vertebrates. Conserved synteny between chordate MRF loci supports the 2R WGD events as a major force in shaping the evolution of vertebrate MRFs. We also resolve vertebrate MRF complements and organization, finding a new type of vertebrate MRF gene in the process, which allowed us to infer an ancestral two-gene state in the vertebrates corresponding to the early- and late-acting types of MRFs. This necessitates a revision of previous conclusions about the simple one-to-four origin of vertebrate MRFs.

Myogenin suppresses apoptosis induced by angiotensin II in human induced pluripotent stem cell-derived cardiomyocytes

BACKGROUND

Angiotensin II (Ang II), an important component of the renin-angiotensin system (RAS), plays a critical role in the pathogenesis of cardiovascular disorders. In addition, human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) have been considered as a promising platform for studying personalized medicine for heart diseases. However, whether Ang II can induce the apoptosis of hiPSC-CMs is not known.

METHODS

In this study, we treated hiPSC-CMs with different concentrations of Ang II [0 nM (vehicle as a control), 1 nM, 10 nM, 100 nM, 1 μM, 10 μM, 100 μM, and 1 mM] for various time periods (24 h, 48 h, 6 days, and 10 days) and analyzed the viability and apoptosis of hiPSC-CMs.

RESULTS

We found that treatment with 1 mM Ang II for 10 days reduced the viability of hiPSC-CMs by 41% (p = 2.073E-08) and increased apoptosis by 2.74-fold, compared to the control group (p = 6.248E-12). MYOG, which encodes the muscle-specific transcription factor myogenin, was also identified as an apoptosis-suppressor gene in Ang II-treated hiPSC-CMs. Ectopic MYOG expression decreased the apoptosis and increased the viability of Ang II-treated hiPSC-CMs. Further analysis of the RNA sequencing (RNA-seq) data illustrated that myogenin ameliorated Ang II-induced apoptosis of hiPSC-CMs by downregulating the expression of proinflammatory genes.

CONCLUSION

Our findings suggest that Ang II induces the apoptosis of hiPSC-CMs and that myogenin attenuates Ang II-induced apoptosis.

Satellite cells deficiency and defective regeneration in dynamin 2-related centronuclear myopathy

Dynamin 2 (DNM2) is a ubiquitously expressed protein involved in many functions related to trafficking and remodeling of membranes and cytoskeleton dynamics. Mutations in the DNM2 gene cause the autosomal dominant centronuclear myopathy (AD-CNM), characterized mainly by muscle weakness and central nuclei. Several defects have been identified in the KI-Dnm2^R465W/+ mouse model of the disease to explain the muscle phenotype, including reduction of the satellite cell pool in muscle, but the functional consequences of this depletion have not been characterized until now. Satellite cells (SC) are the main source for muscle growth and regeneration of mature tissue. Here, we investigated muscle regeneration in the KI-Dnm2^R465W/+ mouse model for AD-CNM. We found a reduced number of Pax7-positive SCs, which were also less activated after induced muscle injury. The muscles of the KI-Dnm2^R465W/+ mouse regenerated more slowly and less efficiently than wild-type ones, formed fewer new myofibers, and did not recover its normal mass 15 days after injury. Altogether, our data provide evidence that the muscle regeneration is impaired in the KI-Dnm2^R465W/+ mouse and contribute with one more layer to the comprehension of the disease, by identifying a new pathomechanism linked to DNM2 mutations which may be involved in the muscle-specific impact occurring in AD-CNM.

Downregulated hypoxia-inducible factor 1α improves myoblast differentiation under hypoxic condition in mouse genioglossus

The treatment of obstructive sleep apnea-hypopnea syndrome targets the narrow anatomic structure of the upper airway (UA) and lacks an effective therapy for UA dilator muscle dysfunction. Long-term hypoxia can cause damage to UA dilator muscles and trigger a vicious cycle. We previously confirmed that hypoxia-inducible factor 1α (HIF-1α) upregulation mediates muscle fatigue in hypoxia condition, but the underlying mechanism remains to be determined. The present study investigated the intrinsic mechanisms and related pathways of HIF-1α that affect myoblast differentiation, with an aim to search for compounds that have protective effects in hypoxic condition. Differentiation of myoblasts was induced under hypoxia, and we found that hypoxia significantly inhibits the differentiation of myoblasts, damages the ultrastructure of mitochondria, and reduces the expression of myogenin, PGC-1β and pAMPKα1. HIF-1α has a negative regulation effect on AMPK. Downregulation of HIF-1α increases the expression of the abovementioned proteins, promotes the differentiation of myoblasts, and protects mitochondrial integrity. In addition, mitochondrial biogenesis occurs during myogenic differentiation. Inhibition of the AMPK pathway inhibits mitochondrial biogenesis, decreases the level of PGC-1β, and increases apoptosis. Resveratrol dimer can reverse the mitochondrial damage induced by AMPK pathway inhibition and decrease myoblast apoptosis. Our results provided a regulatory mechanism for hypoxic injury in genioglossus which may contribute to the pathogenesis and treatment of OSAHS.

Methods to Mimic In Vivo Muscle Cell Biology in Primary Human Myoblasts Using Quiescence as a Guidepost in Regenerative Medicine Research

Regenerative medicine research and testing of new therapeutics for muscle-related human diseases call for a deeper understanding of how human myoblasts gain and maintain quiescence in vitro versus in vivo. The more closely we can experimentally simulate the in vivo environment, the more relevance in vitro research on myoblasts will have. In this context, isolation of satellite cells from muscle tissue causes activation while myoblasts remain activated in culture, thus not simulating quiescence as in their in vivo niche. Cells synchronized for cell cycle present a good starting point for experimental intervention. In the past, myoblast quiescence has been induced using suspension culture (SuCu) and, recently, by knockout serum replacement (KOSR)-supplemented culture media. We assessed the proportion of cells in G0 and molecular regulators after combining the two quiescence-inducing approaches. Quiescence was induced in primary human myoblasts (PHMs) in vitro using KOSR-treatment for 10 days or suspension in viscous media for 2 days (SuCu), or suspension combined with KOSR-treatment for 2 days (blended method, SuCu-KOSR). Quiescence and synchronization were achieved with all three protocols (G0/G1 cell cycle arrest >90% cells). Fold-change of cell cycle controller p21 mRNA for KOSR and SuCu was 3.23 ± 0.30 and 2.86 ± 0.15, respectively. Since this was already a significant change (p < 0.05), no further change was gained with the blended method. But SuCu-KOSR significantly decreased Ki67 (p = 0.0019). Myogenic regulatory factors, Myf5 and MyoD gene expression in PHMs were much more suppressed (p = 0.0004 and p = 0.0034, respectively) in SuCu-KOSR, compared to SuCu alone. In conclusion, a homogenous pool of quiescent primary myoblasts synchronized in the G0 cell cycle phase was achieved with cells from three different donors regardless of the experimental protocol. Myogenic dedifferentiation at the level of Myogenic Regulatory Factors was greater when exposed to the blend of suspension and serum-free culture. We suggest that this blended new protocol can be considered in future biomedical research if differentiation is detected too early during myoblast expansion. This shall also inform new ways to bridge the in vitro and in vivo divides in regenerative medicine research.

Interactions between Growth of Muscle and Stature: Mechanisms Involved and Their Nutritional Sensitivity to Dietary Protein: The Protein-Stat Revisited

Childhood growth and its sensitivity to dietary protein is reviewed within a Protein-Stat model of growth regulation. The coordination of growth of muscle and stature is a combination of genetic programming, and of two-way mechanical interactions involving the mechanotransduction of muscle growth through stretching by bone length growth, the core Protein-Stat feature, and the strengthening of bone through muscle contraction via the mechanostat. Thus, growth in bone length is the initiating event and this is always observed. Endocrine and cellular mechanisms of growth in stature are reviewed in terms of the growth hormone-insulin like growth factor-1 (GH-IGF-1) and thyroid axes and the sex hormones, which together mediate endochondral ossification in the growth plate and bone lengthening. Cellular mechanisms of muscle growth during development are then reviewed identifying (a) the difficulties posed by the need to maintain its ultrastructure during myofibre hypertrophy within the extracellular matrix and the concept of muscle as concentric "bags" allowing growth to be conceived as bag enlargement and filling, (b) the cellular and molecular mechanisms involved in the mechanotransduction of satellite and mesenchymal stromal cells, to enable both connective tissue remodelling and provision of new myonuclei to aid myofibre hypertrophy and (c) the implications of myofibre hypertrophy for protein turnover within the myonuclear domain. Experimental data from rodent and avian animal models illustrate likely changes in DNA domain size and protein turnover during developmental and stretch-induced muscle growth and between different muscle fibre types. Growth of muscle in male rats during adulthood suggests that "bag enlargement" is achieved mainly through the action of mesenchymal stromal cells. Current understanding of the nutritional regulation of protein deposition in muscle, deriving from experimental studies in animals and human adults, is reviewed, identifying regulation by amino acids, insulin and myofibre volume changes acting to increase both ribosomal capacity and efficiency of muscle protein synthesis via the mechanistic target of rapamycin complex 1 (mTORC1) and the phenomenon of a "bag-full" inhibitory signal has been identified in human skeletal muscle. The final section deals with the nutritional sensitivity of growth of muscle and stature to dietary protein in children. Growth in length/height as a function of dietary protein intake is described in the context of the breastfed child as the normative growth model, and the "Early Protein Hypothesis" linking high protein intakes in infancy to later adiposity. The extensive paediatric studies on serum IGF-1 and child growth are reviewed but their clinical relevance is of limited value for understanding growth regulation; a role in energy metabolism and homeostasis, acting with insulin to mediate adiposity, is probably more important. Information on the influence of dietary protein on muscle mass per se as opposed to lean body mass is limited but suggests that increased protein intake in children is unable to promote muscle growth in excess of that linked to genotypic growth in length/height. One possible exception is milk protein intake, which cohort and cross-cultural studies suggest can increase height and associated muscle growth, although such effects have yet to be demonstrated by randomised controlled trials.

Aged Skeletal Muscle Retains the Ability to Remodel Extracellular Matrix for Degradation of Collagen Deposition after Muscle Injury

Aging causes a decline in skeletal muscle function, resulting in a progressive loss of muscle mass, quality, and strength. A weak regenerative capacity is one of the critical causes of dysfunctional skeletal muscle in elderly individuals. The extracellular matrix (ECM) maintains the tissue framework structure in skeletal muscle. As shown by previous reports and our data, the gene expression of ECM components decreases with age, but the accumulation of collagen substantially increases in skeletal muscle. We examined the structural changes in ECM in aged skeletal muscle and found restricted ECM degradation. In aged skeletal muscles, several genes that maintain ECM structure, such as transforming growth factor β (TGF-β), tissue inhibitors of metalloproteinases (TIMPs), matrix metalloproteinases (MMPs), and cathepsins, were downregulated. Muscle injury can induce muscle repair and regeneration in young and adult skeletal muscles. Surprisingly, muscle injury could not only efficiently induce regeneration in aged skeletal muscle, but it could also activate ECM remodeling and the clearance of ECM deposition. These results will help elucidate the mechanisms of muscle fibrosis with age and develop innovative antifibrotic therapies to decrease excessive collagen deposition in aged muscle.

Nanomedicine for Gene Delivery and Drug Repurposing in the Treatment of Muscular Dystrophies

Muscular Dystrophies (MDs) are a group of rare inherited genetic muscular pathologies encompassing a variety of clinical phenotypes, gene mutations and mechanisms of disease. MDs undergo progressive skeletal muscle degeneration causing severe health problems that lead to poor life quality, disability and premature death. There are no available therapies to counteract the causes of these diseases and conventional treatments are administered only to mitigate symptoms. Recent understanding on the pathogenetic mechanisms allowed the development of novel therapeutic strategies based on gene therapy, genome editing CRISPR/Cas9 and drug repurposing approaches. Despite the therapeutic potential of these treatments, once the actives are administered, their instability, susceptibility to degradation and toxicity limit their applications. In this frame, the design of delivery strategies based on nanomedicines holds great promise for MD treatments. This review focuses on nanomedicine approaches able to encapsulate therapeutic agents such as small chemical molecules and oligonucleotides to target the most common MDs such as Duchenne Muscular Dystrophy and the Myotonic Dystrophies. The challenge related to in vitro and in vivo testing of nanosystems in appropriate animal models is also addressed. Finally, the most promising nanomedicine-based strategies are highlighted and a critical view in future developments of nanomedicine for neuromuscular diseases is provided.

Myogenic Differentiation of Stem Cells for Skeletal Muscle Regeneration

Stem cells have become a hot research topic in the field of regenerative medicine due to their self-renewal and differentiation capabilities. Skeletal muscle tissue is one of the most important tissues in the human body, and it is difficult to recover when severely damaged. However, conventional treatment methods can cause great pain to patients. Stem cell-based tissue engineering can repair skeletal muscle to the greatest extent with little damage. Therefore, the application of stem cells to skeletal muscle regeneration is very promising. In this review, we discuss scaffolds and stem cells for skeletal muscle regeneration and put forward our ideas for future development.

Functional Non-coding RNA During Embryonic Myogenesis and Postnatal Muscle Development and Disease

Skeletal muscle is a highly heterogeneous tissue that plays a crucial role in mammalian metabolism and motion maintenance. Myogenesis is a complex biological process that includes embryonic and postnatal development, which is regulated by specific signaling pathways and transcription factors. Various non-coding RNAs (ncRNAs) account for the majority of total RNA in cells and have an important regulatory role in myogenesis. In this review, we introduced the research progress in miRNAs, circRNAs, and lncRNAs related to embryonic and postnatal muscle development. We mainly focused on ncRNAs that regulate myoblast proliferation, differentiation, and postnatal muscle development through multiple mechanisms. Finally, challenges and future perspectives related to the identification and verification of functional ncRNAs are discussed. The identification and elucidation of ncRNAs related to myogenesis will enrich the myogenic regulatory network, and the effective application of ncRNAs will enhance the function of skeletal muscle.

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.

Morphological study of TNPO3 and SRSF1 interaction during myogenesis by combining confocal, structured illumination and electron microscopy analysis

Transportin3 (TNPO3) shuttles the SR proteins from the cytoplasm to the nucleus. The SR family includes essential splicing factors, such as SRSF1, that influence alternative splicing, controlling protein diversity in muscle and satellite cell differentiation. Given the importance of alternative splicing in the myogenic process and in the maintenance of healthy muscle, alterations in the splicing mechanism might contribute to the development of muscle disorders. Combining confocal, structured illumination and electron microscopy, we investigated the expression of TNPO3 and SRSF1 during myogenesis, looking at nuclear and cytoplasmic compartments. We investigated TNPO3 and its interaction with SRSF1 and we observed that SRSF1 remained mainly localized in the nucleus, while TNPO3 decreased in the cytoplasm and was strongly clustered in the nuclei of differentiated myotubes. In conclusion, combining different imaging techniques led us to describe the behavior of TNPO3 and SRSF1 during myogenesis, showing that their dynamics follow the myogenic process and could influence the proteomic network necessary during myogenesis. The combination of different high-, super- and ultra-resolution imaging techniques led us to describe the behavior of TNPO3 and its interaction with SRSF1, looking at nuclear and cytoplasmic compartments. These observations represent a first step in understanding the role of TNPO3 and SRFSF1 in complex mechanisms, such as myogenesis.

Effect of hydrogen peroxide concentration on the maintenance and differentiation of cultured skeletal muscle cells

We have studied the effects of hydrogen peroxide (H₂O₂) on the differentiation and maintenance of C2C12 myoblasts. The effects of H₂O₂ were evaluated by cell viability, total protein concentration, the relative amount of muscle-related proteins, sarcomere structure, and active tension generation. Oxidative stress is one of the major causes of myopathy after exercise and thus establishing the method to evaluate the effects on muscle function is essential. The primary function of striated muscle is to generate force, thus, the measurement of active tension is important in assessing the effect of chemicals on muscle. Among the indices we tested, the sarcomere structure was the most sensitive to the H₂O₂ exposure while the cell viability was less sensitive. The effects of H₂O₂ on active tension correlated with a decrease in the amount of muscle proteins. In this study, our results showed that the effect of chemicals on muscle should be measured in multiple ways, including active tension generation, for a better understanding of its physiological impact.