Controlled differentiation of myoblast cells into fast and slow muscle fibers

R-spondin3 is a myokine that differentiates myoblasts to type I fibres

Muscle fibres are broadly categorised into types I and II; the fibre-type ratio determines the contractile and metabolic properties of skeletal muscle tissue. The maintenance of type I fibres is essential for the prevention of obesity and the treatment of muscle atrophy caused by type 2 diabetes or unloading. Some reports suggest that myokines are related to muscle fibre type determination. We thus explored whether a myokine determines whether satellite cells differentiate to type I fibres. By examining the fibre types separately, we identified R-spondin 3 (Rspo3) as a myokine of interest, a secreted protein known as an activator of Wnt signalling pathways. To examine whether Rspo3 induces type I fibres, primary myoblasts prepared from mouse soleus muscles were exposed to a differentiation medium containing the mouse recombinant Rspo3 protein. Expression of myosin heavy chain (MyHC) I, a marker of type I fibre, significantly increased in the differentiated myotubes compared with a control. The Wnt/β-catenin pathway was shown to be the dominant signalling pathway which induces Rspo3-induced MyHC I expression. These results revealed Rspo3 as a myokine that determines whether satellite cells differentiate to type I fibres.

Tent5a modulates muscle fiber formation in adolescent idiopathic scoliosis via maintenance of myogenin expression

OBJECTIVE

Paravertebral muscle asymmetry may be involved in the pathogenesis of adolescent idiopathic scoliosis (AIS), and the Tent5a protein was recently identified as a novel active noncanonical poly(A) polymerase. We, therefore, explored the function of the AIS susceptibility gene Tent5a in myoblasts.

MATERIALS AND METHODS

RNA-seq of AIS paravertebral muscle was performed, and the molecular differences in paravertebral muscle were investigated. Twenty-four AIS susceptibility genes were screened, and differential expression of Tent5a in paravertebral muscles was confirmed with qPCR and Western blot. After the knockdown of Tent5a, the functional effects of Tent5a on C2C12 cell proliferation, migration, and apoptosis were detected by Cell Counting Kit-8 assay, wound-healing assay, and TUNEL assay, respectively. Myogenic differentiation markers were tested with immunofluorescence and qPCR in vitro, and muscle fiber formation was compared in vivo.

RESULTS

The AIS susceptibility gene Tent5a was differentially expressed in AIS paravertebral muscles. Tent5a knockdown inhibited the proliferation and migration of C2C12 cells and inhibited the maturation of type I muscle fibers in vitro and in vivo. Mechanistically, the expression of myogenin was decreased along with the suppression of Tent5a.

CONCLUSIONS

Tent5a plays an important role in the proliferation and migration of myoblasts, and it regulates muscle fiber maturation by maintaining the stability of myogenin. Tent5a may be involved in the pathogenesis of AIS by regulating the formation of muscle fiber type I.

Nanopattern surface improves cultured human myotube maturation

BACKGROUND

In vitro maturation of human primary myoblasts using 2D culture remains a challenging process and leads to immature fibers with poor internal organization and function. This would however represent a valuable system to study muscle physiology or pathophysiology from patient myoblasts, at a single-cell level.

METHODS

Human primary myoblasts were cultured on 800-nm wide striated surface between two layers of Matrigel, and in a media supplemented with an inhibitor of TGFβ receptor. Gene expression, immunofluorescence, and Ca²⁺ measurements upon electrical stimulations were performed at various time points during maturation to assess the organization and function of the myotubes.

RESULTS

We show that after 10 days in culture, myotubes display numerous functional acetylcholine receptor clusters and express the adult isoforms of myosin heavy chain and dihydropyridine receptor. In addition, the myotubes are internally well organized with striations of α-actinin and STIM1, and occasionally ryanodine receptor 1. We also demonstrate that the myotubes present robust Ca²⁺ responses to repetitive electrical stimulations.

CONCLUSION

The present method describes a fast and efficient system to obtain well matured and functional myotubes in 2D culture allowing thorough analysis of single-cell Ca²⁺ signals.

Leucine regulates porcine muscle fiber type transformation via adiponectin signaling pathway

Leucine can promote slow-twitch muscle fibers formation, and this effect may be mediated by AMPK signaling pathway. In addition, adiponectin (AdipoQ) plays an important role in regulation of muscle fiber type transformation. AdipoQ is located in the upstream of AMPK and its secretion can be regulated by leucine. Therefore, the aim of this study was to explore whether leucine affects muscle fiber type transformation through AdipoQ signaling pathway. Our data showed that 4 mM leucine significantly increased protein expression levels of slow MyHC, Myoglobin, Troponin I-SS, AdipoQ, AdipoR1, phospho-AMPK (p-AMPK) and PGC-1α and mRNA expression levels of AMPKα2, PGC-1α, AdipoQ and AdipoR1, and significantly decreased fast MyHC protein expression. In addition, 4 mM leucine significantly increased the SDH activity while significantly decreased the LDH activity. However, knockdown of AdipoR1 expression by AdipoR1-siRNA abolished leucine-induced upregulation of protein expressions of slow MyHC, AdipoR1, p-AMPK, PGC-1α and NRF1, mRNA expressions of MyHC I, MyHC IIa, AdipoR1, AMPKα2 and PGC-1α, ATP5G, TFAM and NRF1, and mtDNA level, as well as downregulation of protein expression of fast MyHC and mRNA expression of MyHC IIb. Together, our data revealed that leucine promotes muscle fiber type transformation from fast-twitch to slow-twitch through AdipoQ signaling pathway.

Functional myotube formation from adult rat satellite cells in a defined serum-free system

This manuscript describes the development of a culture system whereby mature contracting myotubes were formed from adult rat derived satellite cells. Satellite cells, extracted from the Tibialis Anterior of adult rats, were grown in defined serum-free growth and differentiation media, on a nonbiological substrate, N-1[3-trimethoxysilyl propyl] diethylenetriamine. Myotubes were evaluated morphologically and immunocytochemically, using MyHC specific antibodies, as well as functionally using patch clamp electrophysiology to measure ion channel activity. Results indicated the establishment of the rapid expression of adult myosin isoforms that contrasts to their slow development in embryonic cultures. This culture system has applications in the understanding and treatment of age-related muscle myopathy, muscular dystrophy, and for skeletal muscle engineering by providing a more relevant phenotype for both in vitro and in vivo applications.

Expression profile of IGF-I-calcineurin-NFATc3-dependent pathway genes in skeletal muscle during early development between duck breeds differing in growth rates

The insulin-like growth factor I (IGF-I)-calcineurin (CaN)-NFATc signaling pathways have been implicated in the regulation of myocyte hypertrophy and fiber-type specificity. In the present study, the expression of the CnAα, NFATc3, and IGF-I genes was quantified by RT-PCR for the first time in the breast muscle (BM) and leg muscle (LM) on days 13, 17, 21, 25, and 27 of embryonic development, as well as at 7 days posthatching (PH), in Gaoyou and Jinding ducks, which differ in their muscle growth rates. Consistent expression patterns of CnAα, NFATc3, and IGF-I were found in the same anatomical location at different development stages in both duck breeds, showing significant differences in an age-specific fashion. However, the three genes were differentially expressed in the two different anatomical locations (BM and LM). CnAα, NFATc3, and IGF-I messenger RNA (mRNA) could be detected as early as embryonic day 13 (ED13), and the highest level appeared at this stage in both BM and LM. Significant positive relationships were observed in the expression of the studied genes in the BM and LM of both duck breeds. Also, the expression of these three genes showed a positive relationship with the percentage of type IIb fibers and a negative relationship with the percentage of type I fibers and type IIa fibers. Our data indicate differential expression and coordinated developmental regulation of the selected genes involved in the IGF-I-calcineurin-NFATc3 pathway in duck skeletal muscle during embryonic and early PH growth and development; these data also indicate that this signaling pathway might play a role in the regulation of myofiber type transition.

Thyroid hormone regulates muscle fiber type conversion via miR-133a1

Thyroid hormone promotes slow-to-fast muscle fiber type conversion by inducing miR-133a1 and thereby repressing the expression of the slow muscle determinant TEAD1.

It is known that thyroid hormone (TH) is a major determinant of muscle fiber composition, but the molecular mechanism by which it does so remains unclear. Here, we demonstrated that miR-133a1 is a direct target gene of TH in muscle. Intriguingly, miR-133a, which is enriched in fast-twitch muscle, regulates slow-to-fast muscle fiber type conversion by targeting TEA domain family member 1 (TEAD1), a key regulator of slow muscle gene expression. Inhibition of miR-133a in vivo abrogated TH action on muscle fiber type conversion. Moreover, TEAD1 overexpression antagonized the effect of miR-133a as well as TH on muscle fiber type switch. Additionally, we demonstrate that TH negatively regulates the transcription of myosin heavy chain I indirectly via miR-133a/TEAD1. Collectively, we propose that TH inhibits the slow muscle phenotype through a novel epigenetic mechanism involving repression of TEAD1 expression via targeting by miR-133a1. This identification of a TH-regulated microRNA therefore sheds new light on how TH achieves its diverse biological activities.

In vitro characterization of proliferation and differentiation of pig satellite cells

Skeletal muscle contains various muscle fiber types exhibiting different contractile properties based on the myosin heavy chain (MyHC) isoform profile. Muscle fiber type composition is highly variable and influences growth performance and meat quality, but underlying mechanisms regulating fiber type composition remain poorly understood. The aim of the present work was to develop a model based on muscle satellite cell culture to further investigate the regulation of adult MyHC isoforms expression in pig skeletal muscle. Satellite cells were harvested from the mostly fast-twitch glycolytic longissimus (LM) and predominantly slow-twitch oxidative rhomboideus (RM) muscles of 6-week-old piglets. Satellite cells were allowed to proliferate up to 80% confluence, reached after 7 day of proliferation (D7), and then induced to differentiate. Kinetics of proliferation and differentiation were similar between muscles and more than 95% of the cells were myogenic (desmin positive) at D7 with a fusion index reaching 65 ± 9% after 4 day of differentiation. One-dimensional SDS polyacrylamide gel electrophoresis revealed that satellite cells from both muscles only expressed the embryonic and fetal MyHC isoforms in culture, without any of the adult MyHC isoforms that were expressed in vivo. Interestingly, triiodothyronine (T3) induced de novo expression of adult fast and α-cardiac MyHC in vitro making our culture system a valuable tool to study de novo expression of adult MyHC isoforms and its regulation by intrinsic and/or extrinsic factors.

Inverse regulation of the cytosolic Ca²⁺ buffer parvalbumin and mitochondrial volume in muscle cells via SIRT1/PGC-1α axis

Skeletal muscles show a high plasticity to cope with various physiological demands. Different muscle types can be distinguished by the force, endurance, contraction/relaxation kinetics (fast-twitch vs. slow-twitch muscles), oxidative/glycolytic capacity, and also with respect to Ca²⁺-signaling components. Changes in Ca²⁺ signaling and associated Ca²⁺-dependent processes are thought to underlie the high adaptive capacity of muscle fibers. Here we investigated the consequences and the involved mechanisms caused by the ectopic expression of the Ca²⁺-binding protein parvalbumin (PV) in C2C12 myotubes in vitro, and conversely, the effects caused by its absence in in fast-twitch muscles of parvalbumin null-mutant (PV⁻/⁻) mice in vivo. The absence of PV in fast-twitch muscle tibialis anterior (TA) resulted in an increase in the peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) and of its positive regulator, the deacetylase sirtuin 1 (SIRT1). TA muscles from PV⁻/⁻ mice also have an increased mitochondrial volume. Mild ionophore treatment of control (PV-devoid) C2C12 myotubes causing a moderate elevation in [Ca²⁺](c) resulted in an increase in mitochondrial volume, together with elevated PGC-1α and SIRT1 expression levels, whilst it increased PV expression levels in myotubes stably transfected with PV. In PV-expressing myotubes the mitochondrial volume, PGC-1α and SIRT1 were significantly lower than in control C2C12 myotubes already at basal conditions and application of ionophore had no effect on either one. SIRT1 activation causes a down-regulation of PV in transfected myotubes, whilst SIRT1 inhibition has the opposite effect. We conclude that PV expression and mitochondrial volume in muscle cells are inversely regulated via a SIRT1/PGC-1α signaling axis.

The application of three-dimensional collagen-scaffolds seeded with myoblasts to repair skeletal muscle defects

Three-dimensional (3D) engineered tissue constructs are a novel and promising approach to tissue repair and regeneration. 3D tissue constructs have the ability to restore form and function to damaged soft tissue unlike previous methods, such as plastic surgery, which are able to restore only form, leaving the function of the soft tissue often compromised. In this study, we seeded murine myoblasts (C2C12) into a collagen composite scaffold and cultured the scaffold in a roller bottle cell culture system in order to create a 3D tissue graft in vitro. The 3D graft created in vitro was then utilized to investigate muscle tissue repair in vivo. The 3D muscle grafts were implanted into defect sites created in the skeletal muscles in mice. We detected that the scaffolds degraded slowly over time, and muscle healing was improved which was shown by an increased quantity of innervated and vascularized regenerated muscle fibers. Our results suggest that the collagen composite scaffold seeded with myoblasts can create a 3D muscle graft in vitro that can be employed for defect muscle tissue repair in vivo.

Systems-wide proteomic analysis in mammalian cells reveals conserved, functional protein turnover

The turnover of each protein in the mammalian proteome is a functionally important characteristic. Here, we employed high-resolution mass spectrometry to quantify protein dynamics in nondividing mammalian cells. The ratio of externally supplied versus endogenous amino acids to de novo protein synthesis was about 17:1. Using subsaturating SILAC labeling, we obtained accurate turnover rates of 4106 proteins in HeLa and 3528 proteins in C2C12 cells. Comparison of these human and mouse cell lines revealed a highly significant turnover correlation of protein orthologs and thus high species conservation. Functionally, we observed statistically significant trends for the turnover of phosphoproteins and gene ontology categories that showed extensive covariation between mouse and human. Likewise, the members of some protein complexes, such as the proteasome, have highly similar turnover rates. The high species conservation and the low complex variances thus imply great regulatory fine-tuning of protein turnover.

Characterization of in vitro cultured myoblasts isolated from duck (Anas platyrhynchos) embryo

Myoblasts isolated from duck embryonic muscle were purified and in vitro cultured. External characteristics were observed by using the immunofluorescence technique, and growth curve of duck embryonic myoblasts was established after measuring with the MTT method. Moreover, mRNA expression of three marker genes, the Desmin, the muscle creatine kinase (Mck) and the troponin C (Tnnc), which could reflect the development status of myofibers, were detected each 24 h for cultured cells by using the qPCR technique. Results showed that the in vitro cultured duck myoblasts went through a series of developmental stages, including the proliferation of myoblasts, the differentiation of multi-nuclei myotubes, and the formation of myofiber. The cultured duck embryonic myoblasts entered into a logarithmic stage approximately on the fourth day after seeding. Accompanying with its progressive growth before entering into the logarithmic phase, the myoblasts also showed some differentiation phenomena, reflected by a low expression level of Desmin and high expression level of the Mck and Tnnc genes. During the rapid growth of the logarithmic phase, there was a high expression of the Desmin gene, and a low expression level of the Mck gene and the Tnnc gene in the cultured myoblasts. The expression profiles of the three marker genes for muscle development could be used for distinguishing the different developmental stages of in vitro cultured myoblasts at the molecular level, which would be more accurate and more feasible than observing the external characteristics of the cultured cells.

Electric pulse stimulation of cultured murine muscle cells reproduces gene expression changes of trained mouse muscle

Adequate levels of physical activity are at the center of a healthy lifestyle. However, the molecular mechanisms that mediate the beneficial effects of exercise remain enigmatic. This gap in knowledge is caused by the lack of an amenable experimental model system. Therefore, we optimized electric pulse stimulation of muscle cells to closely recapitulate the plastic changes in gene expression observed in a trained skeletal muscle. The exact experimental conditions were established using the peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha) as a marker for an endurance-trained muscle fiber. We subsequently compared the changes in the relative expression of metabolic and myofibrillar genes in the muscle cell system with those observed in mouse muscle in vivo following either an acute or repeated bouts of treadmill exercise. Importantly, in electrically stimulated C2C12 mouse muscle cells, the qualitative transcriptional adaptations were almost identical to those in trained muscle, but differ from the acute effects of exercise on muscle gene expression. In addition, significant alterations in the expression of myofibrillar proteins indicate that this stimulation could be used to modulate the fiber-type of muscle cells in culture. Our data thus describe an experimental cell culture model for the study of at least some of the transcriptional aspects of skeletal muscle adaptation to physical activity. This system will be useful for the study of the molecular mechanisms that regulate exercise adaptation in muscle.

MyoD expression profile and developmental differences of leg and breast muscle in Peking duck (Anas platyrhynchos Domestica) during embryonic to neonatal stages

In order to investigate the developmental differences between the duck breast muscle and leg muscle tissues during the embryonic stage to neonatal stages, as well as the expression profile of MyoD between the two muscle tissues, the morphologic characteristics in the two muscle tissues during duck embryo stages at E14, E18, E22, E27 and D7 were compared through the muscle paraffin sections. The coding domain sequence of duck MyoD gene was cloned, and then the expression of MyoD in duck leg muscle and breast muscle during embryo stage on E10, E14, E18, E22, E27 and D7 was detected using qRT-PCR method. Results showed that the developmental status of the duck breast muscle in embryonic phrases lag behind that of leg muscle. The CDS of duck MyoD gene consists of 894 nucleotides, and showed relatively high similarity with the gene of other species. The MyoD mRNA expressed in both kinds of muscle tissues and the expression profile had a similar trend, although the expression level of MyoD in the breast muscle was significantly higher than that in the leg muscle at each developmental stages (p<0.05). Results suggested that MyoD might have potential functions in controlling muscle fiber phenotype during the secondary myogenesis of muscle development. These fundamental works may provide some valuable clues for knowing the roles of MyoD in the myogenesis and the muscle fiber type differentiation in birds.