Regulation of myogenic cell proliferation and differentiation during mammalian skeletal myogenesis

Mammalian skeletal myogenesis is a complex process that allows precise control of myogenic cells' proliferation, differentiation, and fusion to form multinucleated, contractile, and functional muscle fibers. Typically, myogenic progenitors continue growth and division until acquiring a differentiated state, which then permanently leaves the cell cycle and enters terminal differentiation. These processes have been intensively studied using the skeletal muscle developing models in vitro and in vivo, uncovering a complex cellular intrinsic network during mammalian skeletal myogenesis containing transcription factors, translation factors, extracellular matrix, metabolites, and mechano-sensors. Examining the events and how they are knitted together will better understand skeletal myogenesis's molecular basis. This review describes various regulatory mechanisms and recent advances in myogenic cell proliferation and differentiation during mammalian skeletal myogenesis. We focus on significant cell cycle regulators, myogenic factors, and chromatin regulators impacting the coordination of the cell proliferation versus differentiation decision, which will better clarify the complex signaling underlying skeletal myogenesis.

Requirement for PINCH in skeletal myoblast differentiation

PINCH, an adaptor of focal adhesion complex, plays essential roles in multiple cellular processes and organogenesis. Here, we ablated PINCH1 or both of PINCH1 and PINCH2 in skeletal muscle progenitors using MyoD-Cre. Double ablation of PINCH1 and PINCH2 resulted in early postnatal lethality with reduced size of skeletal muscles and detachment of diaphragm muscles from the body wall. PINCH mutant myofibers failed to undergo multinucleation and exhibited disrupted sarcomere structures. The mutant myoblasts in culture were able to adhere to newly formed myotubes but impeded in cell fusion and subsequent sarcomere genesis and cytoskeleton organization. Consistent with this, expression of integrin β1 and some cytoskeleton proteins and phosphorylation of ERK and AKT were significantly reduced in PINCH mutants. However, N-cadherin was correctly expressed at cell adhesion sites in PINCH mutant cells, suggesting that PINCH may play a direct role in myoblast fusion. Expression of MRF4, the most highly expressed myogenic factor at late stages of myogenesis, was abolished in PINCH mutants that could contribute to observed phenotypes. In addition, mice with PINCH1 being ablated in myogenic progenitors exhibited only mild centronuclear myopathic changes, suggesting a compensatory role of PINCH2 in myogenic differentiation. Our results revealed a critical role of PINCH proteins in myogenic differentiation.

Long non-coding RNA Gm10561 promotes myogenesis by sponging miR-432

Skeletal myogenesis is a highly ordered process finely regulated by various factors. Long non-coding RNAs play an important regulatory role in myogenesis via multiple mechanisms. In this study, we identified the lncRNA Gm10561, which was upregulated during myogenic differentiation and is highly expressed in skeletal muscle. Knockdown of Gm10561 inhibited the proliferation and differentiation of C2C12 myoblasts in vitro and muscle growth in vivo. Overexpression of Gm10561 promoted the proliferation and differentiation of both C2C12 myoblasts and porcine muscle satellite cells. Notably, lncRNA Gm10561 is localized in the cytoplasm and competitively bound to miR-432, which directly targets MEF2C and E2F3. It was confirmed that lncRNA Gm10561 regulates the proliferation and differentiation of myoblasts by acting as a sponge of miR-432 to modulate MEF2C and E2F3 expression. Thus, the lncRNA-Gm10561-miR-432-MEF2C/E2F3 axis plays an important role in myogenesis.

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.

Histone variants in skeletal myogenesis

Histone variants regulate chromatin accessibility and gene transcription. Given their distinct properties and functions, histone varint substitutions allow for profound alteration of nucleosomal architecture and local chromatin landscape. Skeletal myogenesis driven by the key transcription factor MyoD is characterized by precise temporal regulation of myogenic genes. Timed substitution of variants within the nucleosomes provides a powerful means to ensure sequential expression of myogenic genes. Indeed, growing evidence has shown H3.3, H2A.Z, macroH2A, and H1b to be critical for skeletal myogenesis. However, the relative importance of various histone variants and their associated chaperones in myogenesis is not fully appreciated. In this review, we summarize the role that histone variants play in altering chromatin landscape to ensure proper muscle differentiation. The temporal regulation and cross talk between histones variants and their chaperones in conjunction with other forms of epigenetic regulation could be critical to understanding myogenesis and their involvement in myopathies.

Disabled-2: a positive regulator of the early differentiation of myoblasts

Dab2 is an adaptor protein and a tumor suppressor. Our previous study has found that Dab2 was expressed in early differentiating skeletal muscles in mouse embryos. In this study, we determined the role of Dab2 in the skeletal muscle differentiation using C2C12 myoblasts in vitro and Xenopus laevis embryos in vivo. The expression of Dab2 was increased in C2C12 myoblasts during the formation of myotubes in vitro. Knockdown of Dab2 expression in C2C12 myoblasts resulted in a reduction of myotube formation, whereas the myotube formation was enhanced upon overexpression of Dab2. Re-expression of Dab2 in C2C12 myoblasts with downregulated expression of Dab2 restored their capacity to form myotubes. Microarray profiling and subsequent network analyses on the 155 differentially expressed genes after Dab2 knockdown showed that Mef2c was an important myogenic transcription factor regulated by Dab2 through the p38 MAPK pathway. It was also involved in other pathways that are associated with muscular development and functions. In Xenopus embryos developed in vivo, XDab2 was expressed in the myotome of somites where various myogenic markers were also expressed. Knockdown of XDab2 expression with antisense morpholinos downregulated the expression of myogenic markers in somites. In conclusion, this study is the first to provide solid evidence to show that Dab2 is a positive regulator of the early myoblast differentiation.

Pluripotent stem cell-derived skeletal muscle fibers preferentially express myosin heavy-chain isoforms associated with slow and oxidative muscles

BACKGROUND

Skeletal muscle function is essential for health, and it depends on the proper activity of myofibers and their innervating motor neurons. Each adult muscle is composed of different types of myofibers with distinct contractile and metabolic characteristics. The proper balance of myofiber types is disrupted in most muscle degenerative disorders, representing another factor compromising muscle function. One promising therapeutic approach for the treatment of these diseases is cell replacement based on the targeted differentiation of pluripotent stem cells (PSCs) towards the myogenic lineage. We have previously shown that transient induction of Pax3 or Pax7 in PSCs allows for the generation of skeletal myogenic progenitors endowed with myogenic regenerative potential, but whether they contribute to different fiber types remains unknown.

RESULTS

Here, we investigate the fiber type composition of mouse PSC-derived myofibers upon their transplantation into dystrophic and non-dystrophic mice. Our data reveal that PSC-derived myofibers express slow and oxidative myosin heavy-chain isoforms, along with developmental myosins, regardless of the recipient background. Furthermore, transplantation of the mononuclear cell fraction re-isolated from primary grafts into secondary recipients results in myofibers that maintain preferential expression of slow and oxidative myosin heavy-chain isoforms but no longer express developmental myosins, thus indicating postnatal composition.

CONCLUSIONS

Considering oxidative fibers are commonly spared in the context of dystrophic pathogenesis, this feature of PSC-derived myofibers could be advantageous for therapeutic applications.

MiR-17 and miR-19 cooperatively promote skeletal muscle cell differentiation

Skeletal myogenesis is a highly coordinated process that involves cell proliferation, differentiation and fusion controlled by a complex gene regulatory network. The microRNA gene cluster miR-17–92 has been shown to be related to this process; however, the exact role of each cluster member remains unclear. Here, we show that miR-17 and miR-20a could effectively promote the differentiation of both C2C12 myoblasts and primary bovine satellite cells. In contrast, miR-18a might play a negative role in C2C12 cell differentiation, while miR-19 and miR-92a had little influence. Transcriptome and target analyses revealed that miR-17 could act on Ccnd2, Jak1 and Rhoc genes that are critical for cell proliferation and/or fusion. Notably, the addition of miR-19 could reverse the lethal effect of miR-17 and could thus facilitate the maturation of myotubes. Furthermore, by co-injecting the lentiviral shRNAs of miR-17 and miR-19 into mouse tibialis anterior muscles, we demonstrated the wound healing abilities of the two miRNAs. Our findings indicate that in combination with miR-19, miR-17 is a potent inducer of skeletal muscle differentiation.

Lentivirus-Mediated RNAi in Skeletal Myogenesis

RNA interference (RNAi) has greatly facilitated investigation of gene functions in vitro as well as in vivo. Recombinant lentivirus is widely used to deliver small hairpin RNA (shRNA) because of its high transduction capacity into diverse cell types and tissues. Here, we describe methods of lentivirus-mediated delivery of shRNA for the study of skeletal muscle cell differentiation in vitro and injury-induced muscle regeneration in mice.

Recapitulating muscle disease phenotypes with myotonic dystrophy 1 induced pluripotent stem cells: a tool for disease modeling and drug discovery

Myotonic dystrophy 1 (DM1) is a multisystem disorder primarily affecting the central nervous system, heart and skeletal muscle. It is caused by an expansion of the CTG trinucleotide repeats in the 3' untranslated region of the DMPK gene. Although patient myoblasts have been used for studying the disease in vitro, the invasiveness as well as the low accessibility to muscle biopsies motivate the development of alternative reliable myogenic models. Here, we established two DM1 induced pluripotent stem (iPS) cell lines from patient-derived fibroblasts and, using the PAX7 conditional expression system, differentiated these into myogenic progenitors and, subsequently, terminally differentiated myotubes. Both DM1 myogenic progenitors and myotubes were found to express the intranuclear RNA foci exhibiting sequestration of MBNL1. Moreover, we found the DM1-related mis-splicing, namely BIN1 exon 11 in DM1 myotubes. We used this model to test a specific therapy, antisense oligonucleotide treatment, and found that this efficiently abolished RNA foci and rescued BIN1 mis-splicing in DM1 iPS cell-derived myotubes. Together, our results demonstrate that myotubes derived from DM1 iPS cells recapitulate the critical molecular features of DM1 and are sensitive to antisense oligonucleotide treatment, confirming that these cells can be used for in vitro disease modeling and candidate drug testing or screening.This article has an associated First Person interview with the first author of the paper.

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.

Gene expression profiling discerns molecular pathways elicited by ligand signaling to enhance the specification of embryonic stem cells into skeletal muscle lineage

Regulation of lineage specification and differentiation in embryonic stem (ES) cells can be achieved through the activation of endogenous signaling, an avenue for potential application in regenerative medicine. During vertebrate development, retinoic acid (RA) plays an important role in body axis elongation and mesoderm segmentation in that graded exposure to RA provides cells with positional identity and directs commitment to specific tissue lineages. Nevertheless, bexarotene, a clinically approved rexinoid, enhances the specification and differentiation of ES cells into skeletal myocytes more effectively than RA. Thus profiling the transcriptomes of ES cells differentiated with bexarotene or RA permits the identification of different genetic targets and signaling pathways that may contribute to the difference of bexarotene and RA in efficiency of myogenesis. Interestingly, bexarotene induces the early expression of a myogenic progenitor marker, Meox1, while the expression of many RA targets is also enhanced by bexarotene. Several signaling molecules involved in the progression of myogenic specification and commitment are differentially regulated by bexarotene and RA, suggesting that early targets of rexinoid allow the coordinated regulation of molecular events which leads to efficient myogenic differentiation in ES cells.

Axial and limb muscle development: dialogue with the neighbourhood

Skeletal muscles are part of the musculoskeletal system which also includes nerves, tendons, connective tissue, bones and blood vessels. Here we review the development of axial and limb muscles in amniotes within the context of their surrounding tissues in vivo. We highlight the reciprocal dialogue mediated by signalling factors between cells of these adjacent tissues and developing muscles and also demonstrate its importance from the onset of muscle cell differentiation well into foetal development. Early embryonic tissues secrete factors which are important regulators of myogenesis. However, later muscle development relies on other tissue collaborators, such as developing nerves and connective tissue, which are in turn influenced by the developing muscles themselves. We conclude that skeletal muscle development in vivo is a compelling example of the importance of reciprocal interactions between developing tissues for the complete and coordinated development of a functional system.

Activation of histamine H3 receptor decreased cytoplasmic Ca(2+) imaging during electrical stimulation in the skeletal myotubes

Histamine is a neurotransmitter and chemical mediator in multiple physiological processes. Histamine H3 receptor is expressed in the nervous system, heart, and gastrointestinal tract; however, little is known about H3 receptor in skeletal muscle. The aim of this study was to investigate the role of H3 receptor in skeletal myotubes. The expression of H3 receptor and myosin heavy chain (MHC), a late myogenesis marker, was assessed by real-time PCR and immunostaining in C2C12 skeletal myogenesis and adult mid-urethral skeletal muscle tissues. H3 receptor mRNA showed a significant increase upon differentiation of C2C12 into myotubes: 1-, 26-, 91-, and 182-fold at days 0, 2, 4, and 6, respectively. H3 receptor immunostaining in differentiated C2C12 cells and adult skeletal muscles was positive and correlated with that of MHC. The functional role of H3receptor in differentiated myotubes was assessed using an H3 receptor agonist, (R)-a-methylhistamine ((R)-α-MeHA). Ca(2+) imaging, stimulated by electric pacing, was decreased by 55% after the treatment of mature C2C12 myotubes with 1μM (R)-α-MeHA for 10min and 20min, while treatment with 100nm (R)-α-MeHA for 5min caused 45% inhibition. These results suggested that H3 receptor may participate in the maintenance of the relaxed state and prevention of over-contraction in mature differentiated myotubes. The elucidation of the role of H3R in skeletal myogenesis and adult skeletal muscle may open a new direction in the treatment of skeletal muscle disorders, such as muscle weakness, atrophy, and myotonia in motion systems or peri-urethral skeletal muscle tissues.

Flt3L is a novel regulator of skeletal myogenesis

Various cues initiate multiple signaling pathways to regulate the highly coordinated process of skeletal myogenesis. Myoblast differentiation comprises a series of ordered events starting with cell cycle withdrawal and ending with myocyte fusion, with each step probably controlled by multiple extracellular signals and intracellular signaling pathways. Here we report the identification of Fms-like tyrokine kinase 3 ligand (Flt3L) signaling as a novel regulator of skeletal myogenesis. Flt3L is a multifunctional cytokine in immune cells, but its involvement in skeletal muscle formation has not been reported. We found that Flt3L is expressed in C2C12 myoblasts, with levels increasing throughout differentiation. Knockdown of Flt3L, or its receptor Flt3, suppresses myoblast differentiation, which is rescued by recombinant Flt3L or Flt3, respectively. Differentiation is not rescued, however, by recombinant ligand when the receptor is knocked down, or vice versa, suggesting that Flt3L and Flt3 function together. Flt3L knockdown also inhibits differentiation in mouse primary myoblasts. Both Flt3L and Flt3 are highly expressed in nascent myofibers during muscle regeneration in vivo, and Flt3L siRNA impairs muscle regeneration, validating the physiological significance of Flt3L function in myogenesis. We have identified a cellular mechanism for the myogenic function of Flt3L, as we show that Flt3L promotes cell cycle exit that is necessary for myogenic differentiation. Furthermore, we identify Erk as a relevant target of Flt3L signaling during myogenesis, and demonstrate that Flt3L suppresses Erk signaling through p120RasGAP. In summary, our work reveals an unexpected role for an immunoregulatory cytokine in skeletal myogenesis and a new myogenic pathway.