SMART approaches for genome-wide analyses of skeletal muscle stem and niche cells.

Muscle stem cells (MuSCs) also called satellite cells are the building blocks of skeletal muscle, the largest tissue in the human body which is formed primarily of myofibers. While MuSCs are the principal cells that directly contribute to the formation of the muscle fibers, their ability to do so depends on critical interactions with a vast array of nonmyogenic cells within their niche environment. Therefore, understanding the nature of communication between MuSCs and their niche is of key importance to understand how the skeletal muscle is maintained and regenerated after injury. MuSCs are rare and therefore difficult to study in vivo within the context of their niche environment. The advent of single-cell technologies, such as switching mechanism at 5' end of the RNA template (SMART) and tagmentation based technologies using hyperactive transposase, afford the unprecedented opportunity to perform whole transcriptome and epigenome studies on rare cells within their niche environment. In this review, we will delve into how single-cell technologies can be applied to the study of MuSCs and muscle-resident niche cells and the impact this can have on our understanding of MuSC biology and skeletal muscle regeneration.

Recapitulating human myogenesis ex vivo using human pluripotent stem cells.

Human pluripotent stem cells (hPSCs) provide a human model for developmental myogenesis, disease modeling and development of therapeutics. Differentiation of hPSCs into muscle stem cells has the potential to provide a cell-based therapy for many skeletal muscle wasting diseases. This review describes the current state of hPSCs towards recapitulating human myogenesis ex vivo, considerations of stem cell and progenitor cell state as well as function for future use of hPSC-derived muscle cells in regenerative medicine.

High mobility group box 2 regulates skeletal muscle development through ribosomal protein S6 kinase 1.

HMGB2, a DNA-binding protein, highly expresses during embryogenesis and plays an important role in development of some organs and tissues. However, it remains to be further investigated weather HMGB2 influences muscle development. In this work, we identified HMGB2 as an essential factor in myogenesis. Compared to wild type (WT) mice, body weights of systemic hmgb2 homozygous knockout (hmgb2-/- ) mice especially males were reduced. Diameter and cross-section area of tibialis anterior (TA) muscle fibers as well as expression of Myogenin and MyHC were all decreased in hmgb2-/- mice. CTX injury model revealed that HMGB2 was required for satellite cell proliferation and muscle regeneration. Moreover, HMGB2 interacted with S6K1 and regulated the kinase activity of S6K1 during cell proliferation. Knockdown and inactivation of S6K1 in C2C12 cells both resulted in impaired proliferation and differentiation. Furthermore, expression of cyclin D1 and Myf5 were both decreased when HMGB2 or S6K1 were knocked down and kinase activity of S6K1 was inhibited. These results indicate that HMGB2 is required for skeletal muscle development and regeneration, and HMGB2 maintains proliferation of myoblasts through regulating kinase activity of S6K1.

miR-206 family is important for mitochondrial and muscle function, but not essential for myogenesis in vitro.

miR-206, miR-1a-1, and miR-1a-2 are induced during differentiation of skeletal myoblasts and promote myogenesis in vitro. miR-206 is required for skeletal muscle regeneration in vivo. Although this miRNA family is hypothesized to play an essential role in differentiation, a triple knock-out (tKO) of the three genes has not been done to test this hypothesis. We report that tKO C2C12 myoblasts generated using CRISPR/Cas9 method differentiate despite the expected derepression of the miRNA targets. Surprisingly, their mitochondrial function is diminished. tKO mice demonstrate partial embryonic lethality, most likely due to the role of miR-1a in cardiac muscle differentiation. Two tKO mice survive and grow normally to adulthood with smaller myofiber diameter, diminished physical performance, and an increase in PAX7 positive satellite cells. Thus, unlike other miRNAs important in other differentiation pathways, the miR-206 family is not absolutely essential for myogenesis and is instead a modulator of optimal differentiation of skeletal myoblasts.

The regulatory role of Myomaker and Myomixer-Myomerger-Minion in muscle development and regeneration.

Skeletal muscle plays essential roles in motor function, energy, and glucose metabolism. Skeletal muscle formation occurs through a process called myogenesis, in which a crucial step is the fusion of mononucleated myoblasts to form multinucleated myofibers. The myoblast/myocyte fusion is triggered and coordinated in a muscle-specific way that is essential for muscle development and post-natal muscle regeneration. Many molecules and proteins have been found and demonstrated to have the capacity to regulate the fusion of myoblast/myocytes. Interestingly, two newly discovered muscle-specific membrane proteins, Myomaker and Myomixer (also called Myomerger and Minion), have been identified as fusogenic regulators in vertebrates. Both Myomaker and Myomixer-Myomerger-Minion have the capacity to directly control the myogenic fusion process. Here, we review and discuss the latest studies related to these two proteins, including the discovery, structure, expression pattern, functions, and regulation of Myomaker and Myomixer-Myomerger-Minion. We also emphasize and discuss the interaction between Myomaker and Myomixer-Myomerger-Minion, as well as their cooperative regulatory roles in cell-cell fusion. Moreover, we highlight the areas for exploration of Myomaker and Myomixer-Myomerger-Minion in future studies and consider their potential application to control cell fusion for cell-therapy purposes.

Making muscle: skeletal myogenesis in vivo and in vitro.

Skeletal muscle is the largest tissue in the body and loss of its function or its regenerative properties results in debilitating musculoskeletal disorders. Understanding the mechanisms that drive skeletal muscle formation will not only help to unravel the molecular basis of skeletal muscle diseases, but also provide a roadmap for recapitulating skeletal myogenesis in vitro from pluripotent stem cells (PSCs). PSCs have become an important tool for probing developmental questions, while differentiated cell types allow the development of novel therapeutic strategies. In this Review, we provide a comprehensive overview of skeletal myogenesis from the earliest premyogenic progenitor stage to terminally differentiated myofibers, and discuss how this knowledge has been applied to differentiate PSCs into muscle fibers and their progenitors in vitro.

Nonthermal atmospheric plasma enhances myoblast differentiation by eliciting STAT3 phosphorylation.

The use of nonthermal atmospheric plasma (NTP) in the biomedical field has recently expanded into cell death induction in cancer, infection prevention, inflammation treatment, and wound-healing enhancement. NTP has been demonstrated to enhance skin and muscle regeneration, but its effects on tissue regeneration, following deep tissue or muscle damage, remains underinvestigated. In this study, we determined the effects of NTP on muscle differentiation and the mechanisms of NTP's contribution to differentiation and regeneration. NTP treatment enhanced cell differentiation in primary normal human skeletal muscle myoblast cells and increased the relative expression of mRNA levels of MyoD which is one of the earliest markers of myogenic commitment, and myogenin, which are important transcription factors required for myogenic differentiation. Furthermore, NTP treatment induced increases in the levels of myosin heavy chain, a differentiated muscle-specific protein, and in myotube formation of myoblasts. We observed that signal transducer and activator of transcription 3 (STAT3) activation induced by NTP treatment affects the myogenic differentiation. In addition, STAT3 phosphorylation was also enhanced by NTP treatment in injured animal muscle. These findings indicate that NTP could enhance musculoskeletal differentiation by acting as an external stimulus for myoblast differentiation, suggesting its treatment potential in promoting regeneration of damaged muscle.-Park, J. K., Kim, Y. S., Kang, S. U., Lee, Y. S., Won, H.-R., Kim, C.-H. Nonthermal atmospheric plasma enhances myoblast differentiation by eliciting STAT3 phosphorylation.

Rainbow trout slow myoblast cell culture as a model to study slow skeletal muscle, and the characterization of mir-133 and mir-499 families as a case study.

Muscle fibres are classified as fast, intermediate and slow. In vitro myoblast cell culture model from fast muscle is a very useful tool to study muscle growth and development; however, similar models for slow muscle do not exist. Owing to the compartmentalization of fish muscle fibres, we have developed a slow myoblast cell culture for rainbow trout (Oncorhynchus mykiss). Slow and fast muscle-derived myoblasts have similar morphology, but with differential expression of slow muscle markers such as slow myhc, sox6 and pgc-1α We also characterized the mir-133 and mir-499 microRNA families in trout slow and fast myoblasts as a case study during myogenesis and in response to electrostimulation. Three mir-133 (a-1a, a-1b and a-2) and four mir-499 (aa, ab, ba and bb) paralogues were identified for rainbow trout and named base on their phylogenetic relationship to zebrafish and Atlantic salmon orthologues. Omy-mir-499ab and omy-mir-499bb had 0.6 and 0.5-fold higher expression in slow myoblasts compared with fast myoblasts, whereas mir-133 duplicates had similar levels in both phenotypes and little variation during development. Slow myoblasts also showed increased expression for omy-mir-499b paralogues in response to chronic electrostimulation (7-fold increase for omy-mir-499ba and 2.5-fold increase for omy-mir-499bb). The higher expression of mir-499 paralogues in slow myoblasts suggests a role in phenotype determination, while the lack of significant differences of mir-133 copies during culture development might indicate a different role in fish compared with mammals. We have also found signs of sub-functionalization of mir-499 paralogues after electrostimulation, with omy-mir-499b copies more responsive to electrical signals.

A "noisy" electrical stimulation protocol favors muscle regeneration in vitro through release of endogenous ATP.

An in vitro system of electrical stimulation was used to explore whether an innovative "noisy" stimulation protocol derived from human electromyographic recordings (EMGstim) could promote muscle regeneration. EMGstim was delivered to cultured mouse myofibers isolated from Flexor Digitorum Brevis, preserving their satellite cells. In response to EMGstim, immunostaining for the myogenic regulatory factor myogenin, revealed an increased percentage of elongated myogenin-positive cells surrounding the myofibers. Conditioned medium collected from EMGstim-treated cell cultures, promoted satellite cells differentiation in unstimulated myofiber cell cultures, suggesting that extracellular soluble factors could mediate the process. Interestingly, the myogenic effect of EMGstim was mimicked by exogenously applied ATP (0.1 µM), reduced by the ATP diphosphohydrolase apyrase and prevented by blocking endogenous ATP release with carbenoxolone. In conclusion, our results show that "noisy" electrical stimulations favor muscle progenitor cell differentiation most likely via the release of endogenous ATP from contracting myofibres. Our data also suggest that "noisy" stimulation protocols could be potentially more efficient than regular stimulations to promote in vivo muscle regeneration after traumatic injury or in neuropathological diseases.

Snail regulates MyoD binding-site occupancy to direct enhancer switching and differentiation-specific transcription in myogenesis.

In skeletal myogenesis, the transcription factor MyoD activates distinct transcriptional programs in progenitors compared to terminally differentiated cells. Using ChIP-Seq and gene expression analyses, we show that in primary myoblasts, Snail-HDAC1/2 repressive complex binds and excludes MyoD from its targets. Notably, Snail binds E box motifs that are G/C rich in their central dinucleotides, and such sites are almost exclusively associated with genes expressed during differentiation. By contrast, Snail does not bind the A/T-rich E boxes associated with MyoD targets in myoblasts. Thus, Snai1-HDAC1/2 prevent MyoD occupancy on differentiation-specific regulatory elements, and the change from Snail to MyoD binding often results in enhancer switching during differentiation. Furthermore, we show that a regulatory network involving myogenic regulatory factors (MRFs), Snai1/2, miR-30a, and miR-206 acts as a molecular switch that controls entry into myogenic differentiation. Together, these results reveal a regulatory paradigm that directs distinct gene expression programs in progenitors versus terminally differentiated cells.

First Identification of RNA-Binding Proteins That Regulate Alternative Exons in the Dystrophin Gene.

The Duchenne muscular dystrophy (DMD) gene has a complex expression pattern regulated by multiple tissue-specific promoters and by alternative splicing (AS) of the resulting transcripts. Here, we used an RNAi-based approach coupled with DMD-targeted RNA-seq to identify RNA-binding proteins (RBPs) that regulate splicing of its skeletal muscle isoform (Dp427m) in a human muscular cell line. A total of 16 RBPs comprising the major regulators of muscle-specific splicing events were tested. We show that distinct combinations of RBPs maintain the correct inclusion in the Dp427m of exons that undergo spatio-temporal AS in other dystrophin isoforms. In particular, our findings revealed the complex networks of RBPs contributing to the splicing of the two short DMD exons 71 and 78, the inclusion of exon 78 in the adult Dp427m isoform being crucial for muscle function. Among the RBPs tested, QKI and DDX5/DDX17 proteins are important determinants of DMD exon inclusion. This is the first large-scale study to determine which RBP proteins act on the physiological splicing of the DMD gene. Our data shed light on molecular mechanisms contributing to the expression of the different dystrophin isoforms, which could be influenced by a change in the function or expression level of the identified RBPs.

Regulation and phylogeny of skeletal muscle regeneration.

One of the most fascinating questions in regenerative biology is why some animals can regenerate injured structures while others cannot. Skeletal muscle has a remarkable capacity to regenerate even after repeated traumas, yet limited information is available on muscle repair mechanisms and how they have evolved. For decades, the main focus in the study of muscle regeneration was on muscle stem cells, however, their interaction with their progeny and stromal cells is only starting to emerge, and this is crucial for successful repair and re-establishment of homeostasis after injury. In addition, numerous murine injury models are used to investigate the regeneration process, and some can lead to discrepancies in observed phenotypes. This review addresses these issues and provides an overview of some of the main regulatory cellular and molecular players involved in skeletal muscle repair.

Anti-diabetic effect and mechanism of Kursi Wufarikun Ziyabit in L6 rat skeletal muscle cells.

Kursi Wufarikun Ziyabit (KWZ) is a traditional prescription that used in folk tea drinking for its health care effect in treatment of type 2 diabetes mellitus (T2DM) in central Asia. However, the underlying mechanism of KWZ in T2DM has not been investigated extensively. This study designed to observe the effect of KWZ on glucose consumption and assess the molecular mechanism on associated proteins in insulin signaling and ER stress pathway in L6 rat skeletal muscle cells. The results showed that, KWZ exhibited proteins of PTP-1B and α-glycosidase inhibitory activity in vitro. No cytotoxicity of KWZ was found on L6 cell line. The best effect of glucose consumption of cells was shown at 6.25 µg/mL after KWZ treatment for 12 h. Expression of PTP-1B protein was inhibited by KWZ in L6 moytubes. PI3K-dependent Akt phosphorylation was found to be activated by KWZ. Moreover, the insulin-mediated induction of IRS-1 and GSK-3 were also activated by KWZ. Western blot results indicated that KWZ significantly improved the levels of ER stress proteins, which reduced the expression of GRP78, enhanced the expression of the PERK, eIF2α and XBP1s. The activation of PERK/eIF2α was likely consequence of GRP78 inhibition, and this might be beneficial for improving the stability of ER and alleviating insulin resistance. These results suggest that KWZ might be serving as the potential drug for the prevention and treatment of T2DM.

Activation of autophagy by globular adiponectin is required for muscle differentiation.

Regulated autophagy is a critical component for a healthy skeletal muscle mass, such that dysregulation of the autophagic processes correlates with severe myopathies. Thus, defining the biological molecules involved in the autophagic processes within skeletal muscle is of great importance. Here we demonstrate that globular adiponectin (gAd) activates autophagy in skeletal muscle myoblasts via an AMPK-dependent mechanism. Activation of autophagy through gAd promotes myoblast survival and apoptosis inhibition during serum starvation and the gAd-activated autophagy orchestrates the myogenic properties of the hormone. Consistent with this conclusion, inhibition of gAd-activated autophagy by both a pharmacological (chloroquine) or siRNA approach greatly inhibited muscle differentiation, as demonstrated by reductions in myosin heavy chain expression and myotube formation. Further support for the role of adiponectin in autophagy comes from the skeletal muscles of adiponectin KO mice which display decreased LC3 II expression and a myopathic phenotype (heterogeneous fiber sizes, numerous central nuclei). Overall, these findings demonstrate that gAd activates autophagy in myoblasts and that gAd-activated autophagy drives the myogenic properties of this hormone.

Biocompatible Elastic Conductive Films Significantly Enhanced Myogenic Differentiation of Myoblast for Skeletal Muscle Regeneration.

The key factor in skeletal muscle tissue engineering is regeneration of the functional skeletal muscles. Materials that could promote the myoblast proliferation and myogenic differentiation are promising candidates in skeletal muscle tissue engineering. Herein, we developed an elastic conductive poly(ethylene glycol)-co-poly(glycerol sebacate) (PEGS) grafted aniline pentamer (AP) copolymer that could promote the formation of myotubes by differentiating the C2C12 myoblast cells. The results of hydration behavior and water contact angle suggested that by adjusting the poly(ethylene glycol) (PEG) and AP content, this film showed a proper surface hydrophilicity for cell attachment. Additionally, these films showed tunable conductivity and mechanical properties that can be altered by changing the AP content. The maximum conductivity of the films was 1.84 × 10-4 S/cm and the Young's modulus of these films ranged from 14.58 ± 1.35 MPa to 24.62 ± 0.61 MPa. Our findings indicate that the PEGS-AP films promote the proliferation and myogenic differentiation of C2C12 cells, suggesting that they are promising biomaterials for skeletal muscle tissue engineering.

Plasticity of the Muscle Stem Cell Microenvironment.

Satellite cells (SCs) are adult muscle stem cells capable of repairing damaged and creating new muscle tissue throughout life. Their functionality is tightly controlled by a microenvironment composed of a wide variety of factors, such as numerous secreted molecules and different cell types, including blood vessels, oxygen, hormones, motor neurons, immune cells, cytokines, fibroblasts, growth factors, myofibers, myofiber metabolism, the extracellular matrix and tissue stiffness. This complex niche controls SC biology-quiescence, activation, proliferation, differentiation or renewal and return to quiescence. In this review, we attempt to give a brief overview of the most important players in the niche and their mutual interaction with SCs. We address the importance of the niche to SC behavior under physiological and pathological conditions, and finally survey the significance of an artificial niche both for basic and translational research purposes.

Tumor necrosis factor-alpha inhibits differentiation of myogenic cells in human urethral rhabdosphincter.

OBJECTIVES: To examine the inhibitory effects of tumor necrosis factor-α on myogenic differentiation of human urethral rhabdosphincter cells. METHODS: A rhabdosphincter sample was obtained from a patient who underwent total cystectomy. To expand the lifespan of the primary cultured cells, rhabdosphincter myogenic cells were immortalized with mutated cyclin-dependent kinase 4, cyclin D1 and telomerase. The differential potential of the cells was investigated. The transfected human rhabdosphincter cells were induced for myogenic differentiation with recombinant human tumor necrosis factor-α and/or the tumor necrosis factor-α antagonist etanercept at different concentrations, and activation of signaling pathways was monitored. RESULTS: Human rhabdosphincter cells were selectively cultured for at least 40 passages. Molecular analysis confirmed the expression of myosin heavy chain, which is a specific marker of differentiated muscle cells, significantly increased after differentiation induction. Although tumor necrosis factor-α treatment reduced the myosin heavy chain expression in a concentration-dependent manner, etanercept inhibited this suppression. Tumor necrosis factor-α suppressed phosphorylation of protein kinase B and p38, whereas etanercept pretreatment promoted phosphorylation and myosin heavy chain expression in a concentration-dependent manner. CONCLUSIONS: Tumor necrosis factor-α inhibits differentiation of urethral rhabdosphincter cells in part through the p38 mitogen-activated protein kinase and phosphoinositide 3-kinase pathways. Inhibition of tumor necrosis factor-α might be a useful strategy to treat stress urinary incontinence.

Skeletal Muscle Microvasculature: A Highly Dynamic Lifeline.

Skeletal muscle is highly irrigated by blood vessels. Beyond oxygen and nutrient supply, new vessel functions have been identified. This review presents vessel microanatomy and functions at tissue, cellular, and molecular levels. Mechanisms of vessel plasticity are described during skeletal muscle development and acute regeneration, and in physiological and pathological contexts.

Rapid depletion of muscle progenitor cells in dystrophic mdx/utrophin-/- mice.

Duchenne muscular dystrophy (DMD) patients lack dystrophin from birth; however, muscle weakness becomes apparent only at 3-5 years of age, which happens to coincide with the depletion of the muscle progenitor cell (MPC) pools. Indeed, MPCs isolated from older DMD patients demonstrate impairments in myogenic potential. To determine whether the progression of muscular dystrophy is a consequence of the decline in functional MPCs, we investigated two animal models of DMD: (i) dystrophin-deficient mdx mice, the most commonly utilized model of DMD, which has a relatively mild dystrophic phenotype and (ii) dystrophin/utrophin double knock-out (dKO) mice, which display a similar histopathologic phenotype to DMD patients. In contrast to age-matched mdx mice, we observed that both the number and regeneration potential of dKO MPCs rapidly declines during disease progression. This occurred in MPCs at both early and late stages of myogenic commitment. In fact, early MPCs isolated from 6-week-old dKO mice have reductions in proliferation, resistance to oxidative stress and multilineage differentiation capacities compared with age-matched mdx MPCs. This effect may potentially be mediated by fibroblast growth factor overexpression and/or a reduction in telomerase activity. Our results demonstrate that the rapid disease progression in the dKO model is associated, at least in part, with MPC depletion. Therefore, alleviating MPC depletion could represent an approach to delay the onset of the histopathologies associated with DMD patients.

Construction and myogenic differentiation of 3D myoblast tissues fabricated by fibronectin-gelatin nanofilm coating.

In this study, we used a recently developed approach of coating the cells with fibronectin-gelatin nanofilms to build 3D skeletal muscle tissue models. We constructed the microtissues from C2C12 myoblasts and subsequently differentiated them to form muscle-like tissue. The thickness of the constructs could be successfully controlled by altering the number of seeded cells. We were able to build up to ∼76 µm thick 3D constructs that formed multinucleated myotubes. We also found that Rho-kinase inhibitor Y27632 improved myotube formation in thick constructs. Our approach makes it possible to rapidly form 3D muscle tissues and is promising for the in vitro construction of physiologically relevant human skeletal muscle tissue models.