Regulation of skeletal muscle gene expression by p38 MAP kinases

S100B in myoblasts regulates the transition from activation to quiescence and from quiescence to activation and reduces apoptosis

S100B protein activates IKKβ/NF-κB within myoblasts, thereby inhibiting the expression of MyoD and the MyoD-downstream effectors, myogenin and p21(WAF1), and myoblast differentiation. Herein we show that myoblasts downregulate S100B expression once transferred from proliferation medium to differentiation medium via a p38 MAPK-driven transcriptional mechanism as well as a post-translational, proteasome-dependent mechanism, and that myoblasts that have not been committed to differentiation resume expressing S100B once transferred back to proliferation medium. Likewise, myoblasts downregulate S100B expression once transferred to quiescence medium, and interference with S100B downregulation as obtained by stable overexpression of the protein results in reduced acquisition of quiescence and a faster proliferation upon transfer of the cells from quiescence medium to proliferation medium, compared to controls. These latter effects are dependent on S100B-induced activation of JNK. Moreover, S100B reduces myoblast apoptosis in an MEK-ERK1/2, Akt, JNK, and NF-κB-dependent manner. However, myogenin(+) myoblasts (i.e., myocytes) and myotubes abundantly express S100B likely induced by myogenin. Our results suggest that (1) a timely repression of S100B expression is required for efficient myogenic differentiation; (2) S100B plays an important role in the expansion of the activated (i.e., proliferating) myoblast population; (3) under conditions associated with enhanced expression of S100B, the transition from proliferation to quiescence and from quiescence to proliferation might be altered; and (4) S100B exerts different regulatory effects in myoblasts and myocytes/myotubes/myofibers. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Interleukin-10 reduces the pathology of mdx muscular dystrophy by deactivating M1 macrophages and modulating macrophage phenotype

M1 macrophages play a major role in worsening muscle injury in the mdx mouse model of Duchenne muscular dystrophy. However, mdx muscle also contains M2c macrophages that can promote tissue repair, indicating that factors regulating the balance between M1 and M2c phenotypes could influence the severity of the disease. Because interleukin-10 (IL-10) modulates macrophage activation in vitro and its expression is elevated in mdx muscles, we tested whether IL-10 influenced the macrophage phenotype in mdx muscle and whether changes in IL-10 expression affected the pathology of muscular dystrophy. Ablation of IL-10 expression in mdx mice increased muscle damage in vivo and reduced mouse strength. Treating mdx muscle macrophages with IL-10 reduced activation of the M1 phenotype, assessed by iNOS expression, and macrophages from IL-10 null mutant mice were more cytolytic than macrophages isolated from wild-type mice. Our data also showed that muscle cells in mdx muscle expressed the IL-10 receptor, suggesting that IL-10 could have direct effects on muscle cells. We assayed whether ablation of IL-10 in mdx mice affected satellite cell numbers, using Pax7 expression as an index, but found no effect. However, IL-10 mutation significantly increased myogenin expression in vivo during the acute and the regenerative phase of mdx pathology. Together, the results show that IL-10 plays a significant regulatory role in muscular dystrophy that may be caused by reducing M1 macrophage activation and cytotoxicity, increasing M2c macrophage activation and modulating muscle differentiation.

p38 mitogen activated protein kinase controls two successive-steps during the early mesodermal commitment of embryonic stem cells

Embryonic stem (ES) cells differentiate in vitro into all cell lineages. We previously found that the p38 mitogen activated kinase (p38MAPK) pathway controls the commitment of ES cells toward either cardiomyogenesis (p38 on) or neurogenesis (p38 off ). In this study, we show that p38α knock-out ES cells do not differentiate into cardiac, endothelial, smooth muscle, and skeletal muscle lineages. Reexpression of p38MAPK in these cells partially rescues their mesodermal differentiation defects and corrects the high level of spontaneous neurogenesis of knock-out cells. Wild-type ES cells were treated with a p38MAPK-specific inhibitor during the differentiation process. These experiments allowed us to identify 2 early independent successive p38MAPK functions in the formation of mesodermal lineages. Further, the first one correlates with the regulation of the expression of Brachyury, an essential mesodermal-specific transcription factor, by p38MAPK. In conclusion, by genetic and biochemical approaches, we demonstrate that p38MAPK activity is essential for the commitment of ES cell into cardiac, endothelial, smooth muscle, and skeletal muscle mesodermal lineages.

Changes in growth-related kinases in head, neck and limb muscles with age

Sarcopenia coincides with declines in several systemic processes that signal through the MAP kinase and Akt-mTOR-p70S6k cascades typically associated with muscle growth. Effects of aging on these pathways have primarily been examined in limb muscles, which experience substantial activity and neural changes in addition to systemic hormonal and metabolic changes. Head and neck muscles are reported to undergo reduced sarcopenia and disuse with age relative to limb muscles, suggesting muscle activity may contribute to maintaining mass with age. However many head and neck muscles derive from embryonic branchial arches, rather than the somites from which limb muscles originate, suggesting that developmental origin may be important. This study compares the expression and phosphorylation of MAP kinase and mTOR networks in head, neck, tongue, and limb muscles from 8- and 26-month old F344 rats to test the hypothesis that physical activity and developmental origin contribute to preservation of muscle mass with age. Phosphorylation of p38 was exaggerated in aged branchial arch muscles. Phosphorylation of ERK and p70S6k T421/S424 declined with age only in the biceps brachii. Expression of p70S6k declined in all head and neck, tongue and limb muscles although no change in phosphorylation of p70S6k on T389 could be resolved. A systemic change that results in a loss of p70S6k protein expression may reduce the capacity to respond to acute hypertrophic stimuli, while the exaggerated p38 signaling in branchial arch muscles may reflect more active muscle remodeling.

Effects of combined hormone replacement therapy or its effective agents on the IGF-1 pathway in skeletal muscle

OBJECTIVES

To investigate the effects of combined hormone replacement therapy (HRT) and its effective agents on the IGF-1 signaling pathway.

DESIGN AND METHODS

To examine the effects of HRT on skeletal muscle in vivo, we utilized pre- and post-intervention samples from a randomized double blinded trial with 50-57-year-old women. The intervention included the year-long use of either HRT preparation (2 mg 17β-estradiol, E₂; 1mg norethisterone acetate, NETA, n=10) or placebo (CO, n=9). Microarray technology and quantitative PCR (qPCR) were used to study the expression of insulin-like growth factor I (IGF-1) and its splice variants as well as IGF-1 receptor, Akt1, mTOR, FOXO1, FOXO3, atrogin, estrogen receptors and androgen receptor in muscle samples. Serum concentrations of IGF-1, E(2) and testosterone were measured. C2C12 myotubes were fed with E₂ or NETA followed by analyzing the expression of essentially the same gene transcripts as in human samples by qPCR and phosphorylation of Akt and mTOR by Western blotting.

RESULTS

The gene expression of IGF-1 and its splice variant, IGF-1Ec (also known as the mechano growth factor or MGF), mTOR, FOXO3, and AR was up-regulated among the HRT users compared to the CO (P<0.05), while Akt1 was down-regulated (P<0.05). The change in the level of IGF-1Ec transcript correlated positively with muscle size at post-intervention (r=0.5, P<0.05). In C2C12 myotubes, no statistically significant effects of either E₂ or NETA at the level of gene transcripts studied were identified. The amount of phosphorylated Akt appeared to respond to NETA, albeit the response was not statistically significant. Phosphorylation of mTOR did not respond to either of the treatments.

CONCLUSION

Year-long postmenopausal HRT was found to affect the expression of the genes along the IGF-1 signaling cascade reflecting the higher muscle mass compared to the CO women. By using cell culture model we were, however, unable to confirm the possible differential role of E₂ and NETA. It appears that the synchronous presence of both effective agents of the HRT or the presence of yet unidentified microenvironmental factors providing proper paracrine signals naturally existing in the intact muscle tissue is critical for appropriate signaling via sex steroid-IGF-1 axis to occur.

Mechanism underlying long-term regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase during L6 myoblast differentiation

3-Hydroxy 3-methylglutaryl Coenzyme A reductase (HMG-CoAR) and its end-products are crucial for insulin-induced differentiation of fetal rat myoblasts (L6) both at early and terminal stages of development. Inhibition of HMG-CoAR activity and reduction of the enzyme levels impair the expression of L6 differentiation markers and prevent myoblast fusion into multinucleated syncytia. The mechanism underlying the modulation of this crucial enzyme so that muscular differentiation can occur is poorly understood. Thus, the aim of this work was to explore the long-term regulation of HMG-CoAR in an attempt to provide a new molecular basis for the control of muscle development. All experiments were performed in L6 rat myoblasts induced to differentiate utilizing insulin. The results indicate the following: (i) at early stages of L6 differentiation, the increase in HMG-CoAR protein levels is probably due to transcriptional induction and a decrease in the enzyme degradation rate; (ii) the subsequent reduction of HMG-CoAR protein levels is related both to an increased degradation rate and reduced gene transcription, as indicated by the rise of Insig-1 levels and the subsequent decrease in the amount of n-SREBP-1; (iii) in the terminal stages of myogenesis, reduced protein levels of HMG-CoAR could be ascribed to the decrease in gene transcription while its degradation rate is not affected. By highlighting the mechanisms involved in HMG-CoAR long-term regulation during myogenesis, this work provides useful information for searching for tools to improve the regenerative ability of muscle tissue and for the development of new pharmacological treatments of myopathies.

Proline isomerase Pin1 represses terminal differentiation and myocyte enhancer factor 2C function in skeletal muscle cells

Reversible proline-directed phosphorylation at Ser/Thr-Pro motifs has an essential role in myogenesis, a multistep process strictly regulated by several signaling pathways that impinge on two families of myogenic effectors, the basic helix-loop-helix myogenic transcription factors and the MEF2 (myocyte enhancer factor 2) proteins. The question of how these signals are deciphered by the myogenic effectors remains largely unaddressed. In this study, we show that the peptidyl-prolyl isomerase Pin1, which catalyzes the isomerization of phosphorylated Ser/Thr-Pro peptide bonds to induce conformational changes of its target proteins, acts as an inhibitor of muscle differentiation because its knockdown in myoblasts promotes myotube formation. With the aim of clarifying the mechanism of Pin1 function in skeletal myogenesis, we investigated whether MEF2C, a critical regulator of the myogenic program that is the end point of several signaling pathways, might serve as a/the target for the inhibitory effects of Pin1 on muscle differentiation. We show that Pin1 interacts selectively with phosphorylated MEF2C in skeletal muscle cells, both in vitro and in vivo. The interaction with Pin1 requires two novel critical phospho-Ser/Thr-Pro motifs in MEF2C, Ser⁹⁸ and Ser¹¹⁰, which are phosphorylated in vivo. Overexpression of Pin1 decreases MEF2C stability and activity and its ability to cooperate with MyoD to activate myogenic conversion. Collectively, these findings reveal a novel role for Pin1 as a regulator of muscle terminal differentiation and suggest that Pin1-mediated repression of MEF2C function could contribute to this function.

Altered signal transduction pathways and induction of autophagy in human myotonic dystrophy type 1 myoblasts

Congenital myotonic dystrophy type 1 (CDM1) affects patients from birth and is associated with mental retardation and impaired muscle development. CDM1 patients carry 1000-3000 CTG repeats in the DMPK gene and display defective skeletal muscles differentiation, resulting in reduced size of myotubes and decreased number of satellite cells. In this study, human myoblasts in culture deriving from control and DM1 embryos (3200 CTG repeats) were analyzed using both a biochemical and electron microscopic approach, in order to provide new insights into the molecular mechanisms underlying such alteration. Interestingly, electron microscopy analysis showed not only ultrastructural features of abnormal differentiation but also revealed the presence of autophagic vacuoles in DM1 myoblasts not undergoing differentiation. In accordance with the electron microscopic findings, the autophagic markers LC3 and ATG5, but not apoptotic markers, were significantly up regulated in DM1 myoblasts after differentiating medium addition. The induction of autophagic processes in DM1 myoblasts was concomitant to p53 over-expression and inhibition of the mTOR-S6K1 pathway, causatively involved in autophagy. Moreover biochemical alterations of the two main signal transduction pathways involved in differentiation were observed in DM1 myoblasts, in particular decreased activation of p38MAPK and persistent activation of the MEK-ERK pathway. This work, while demonstrating that major signaling pathways regulating myoblasts differentiation are profoundly deranged in DM1 myoblasts, for the first time provides evidence of autophagy induction, possibly mediated by p53 activation in response to metabolic stress which might contribute to the dystrophic alterations observed in the muscles of congenital DM1 patients.

Genetic ablation of TWEAK augments regeneration and post-injury growth of skeletal muscle in mice

Impairment in the regeneration process is a critical determinant for skeletal muscle wasting in chronic diseases and degenerative muscle disorders. Inflammatory cytokines are known to cause significant muscle wasting, however, their role in myofiber regeneration is less clear. In this study we have investigated the role of tumor necrosis factor-like weak inducer of apoptosis (TWEAK) in skeletal muscle regeneration in vivo. Our results show that expression levels of TWEAK and its receptor Fn14 are significantly increased in skeletal muscles of mice after injury. Genetic deletion of TWEAK increased the fiber cross-sectional area and levels of embryonic isoform of myosin heavy chain in regenerating tibial anterior muscle. Conversely, muscle-specific transgenic overexpression of TWEAK reduced the fiber cross-sectional area and levels of the embryonic myosin heavy chain in regenerating muscle. TWEAK induced the expression of several inflammatory molecules and increased interstitial fibrosis in regenerating muscle. Genetic ablation of TWEAK suppressed, whereas overexpression of TWEAK increased, the activation of nuclear factor-kappa B without affecting the activation of Akt or p38 kinase in regenerating myofibers. Primary myoblasts from TWEAK-null mice showed enhanced differentiation in vitro, whereas myoblasts from TWEAK-Tg mice showed reduced differentiation compared with wild-type mice. Collectively, our study suggests that TWEAK negatively regulates muscle regeneration and that TWEAK is a potential therapeutic target to enhance skeletal muscle regeneration in vivo.

Mechanisms and functions of p38 MAPK signalling

The p38 MAPK (mitogen-activated protein kinase) signalling pathway allows cells to interpret a wide range of external signals and respond appropriately by generating a plethora of different biological effects. The diversity and specificity in cellular outcomes is achieved with an apparently simple linear architecture of the pathway, consisting of a core of three protein kinases acting sequentially. In the present review, we dissect the molecular mechanisms underlying p38 MAPK functions, with special emphasis on the activation and regulation of the core kinases, the interplay with other signalling pathways and the nature of p38 MAPK substrates as a source of functional diversity. Finally, we discuss how genetic mouse models are facilitating the identification of physiological functions for p38 MAPKs, which may impinge on their eventual use as therapeutic targets.

Prolonged activation of S6K1 does not suppress IRS or PI-3 kinase signaling during muscle cell differentiation

BACKGROUND

Myogenesis in C2C12 cells requires the activation of the PI3K/mTOR signaling pathways. Since mTOR signaling can feedback through S6K1 to inhibit the activation of PI3K, the aim of this work was to assess whether feedback from S6K1 played a role in myogenesis and determine whether siRNA mediated knockdown of S6K1 would lead to an increased rate of myotube formation.

RESULTS

S6K1 activity increased in a linear fashion following plating and was more than 3-fold higher after Day 3 of differentiation (subconfluent = 11.09 +/- 3.05, Day 3 = 29.34 +/- 3.58). IRS-1 levels tended to increase upon serum withdrawal but decreased approximately 2-fold (subconfluent = 0.88 +/- 0.10, Day 3 = 0.42 +/- 0.06) 3 days following differentiation whereas IRS-2 protein remained stable. IRS-1 associated p85 was significantly reduced upon serum withdrawal (subconfluent = 0.86 +/- 0.07, Day 0 = 0.31 +/- 0.05), remaining low through day 1. IRS-2 associated p85 decreased following serum withdrawal (subconfluent = 0.96 +/- 0.05, Day 1 = 0.56 +/- 0.08) and remained suppressed up to Day 3 following differentiation (0.56 +/- 0.05). Phospho-tyrosine associated p85 increased significantly from subconfluent to Day 0 and remained elevated throughout differentiation. siRNA directed against S6K1 and S6K2 did not result in changes in IRS-1 levels after either 48 or 96 hrs. Furthermore, neither 48 nor 96 hrs of S6K1 knockdown caused a change in myotube formation.

CONCLUSIONS

Even though S6K1 activity increases throughout muscle cell differentiation and IRS-1 levels decrease over this period, siRNA suggests that S6K1 is not mediating the decrease in IRS-1. The decrease in IRS-1/2 associated p85 together with the increase in phospho-tyrosine associated p85 suggests that PI3K associates primarily with scaffolds other than IRS-1/2 during muscle cell differentiation.

SWI/SNF complexes, chromatin remodeling and skeletal myogenesis: it's time to exchange!

Skeletal muscle differentiation relies on the coordinated activation and repression of specific subsets of genes. This reflects extensive changes in chromatin architecture, composition of chromatin-associated complexes and histone modifications at the promoter/enhancer elements of skeletal muscle genes. An early, key event in the activation of muscle-specific gene transcription is the disruption of the repressive conformation imposed by nucleosomes, which impede the access of pioneer transcription factors, such as the muscle-specific basic helix-loop-helix (bHLH) factors MyoD and Myf5, to their DNA-binding sites. This review focuses on our current understanding of the role of the SWI/SNF ATP-dependent chromatin-remodeling complex in the activation of the myogenic program, by inducing conformational changes permissive for muscle-gene expression. Recent findings suggest that specific combinations of individual SWI/SNF components can generate sub-complexes with specialized functions that are engaged at sequential stages of muscle-gene activation--e.g., initial displacement of the nucleosome followed by the loading of the complete myogenic transcriptosome that promotes gene transcription. SWI/SNF composition and function is regulated by the exchange of specific variants of structural sub-units. In turn, an exchange of histone variants and related epigenetic modifications might reflect the impact of distinct SWI/SNF complexes on the architecture and activity of target promoter/enhancer elements. Thus, the SWI/SNF complexes should be regarded not just as simple executors of the program imposed by transcription factors, but as multifaceted "readers" and "shapers" of the chromatin/DNA landscape within target muscle genes along the transition from myoblasts to myotubes.

Cdo interacts with APPL1 and activates Akt in myoblast differentiation

Cdo activates Akt via indirect interaction with APPL1 during myoblast differentiation, and this complex likely mediates some of the promyogenic effect of cell–cell interaction. The promyogenic function of Cdo involves a coordinated activation of p38MAPK and Akt via interaction with scaffold proteins, JLP and Bnip-2 for p38MAPK and APPL1 for Akt.

Cell–cell interactions between muscle precursors are required for myogenic differentiation; however, underlying mechanisms are largely unknown. Promyogenic cell surface protein Cdo functions as a component of multiprotein complexes containing other cell adhesion molecules, Boc, Neogenin and N-cadherin, and mediates some of signals triggered by cell–cell interactions between muscle precursors. Cdo activates p38MAPK via interaction with two scaffold proteins JLP and Bnip-2 to promote myogenesis. p38MAPK and Akt signaling are required for myogenic differentiation and activation of both signaling pathways is crucial for efficient myogenic differentiation. We report here that APPL1, an interacting partner of Akt, forms complexes with Cdo and Boc in differentiating myoblasts. Both Cdo and APPL1 are required for efficient Akt activation during myoblast differentiation. The defective differentiation of Cdo-depleted cells is fully rescued by overexpression of a constitutively active form of Akt, whereas overexpression of APPL1 fails to do so. Taken together, Cdo activates Akt through association with APPL1 during myoblast differentiation, and this complex likely mediates some of the promyogenic effect of cell–cell interaction. The promyogenic function of Cdo involves a coordinated activation of p38MAPK and Akt via association with scaffold proteins, JLP and Bnip-2 for p38MAPK and APPL1 for Akt.

Essential role of p18Hamlet/SRCAP-mediated histone H2A.Z chromatin incorporation in muscle differentiation

The chromatin-remodelling complex SNF2-related CBP activator protein (SRCAP) regulates chromatin structure in yeast by modulating the exchange of histone H2A for the H2A.Z variant. Here, we have investigated the contribution of H2A.Z-mediated chromatin remodelling to mammalian cell differentiation reprogramming. We show that the SRCAP subunit named ZNHIT1 or p18(Hamlet), which is a substrate of p38 MAPK, is recruited to the myogenin promoter at the onset of muscle differentiation, in a p38 MAPK-dependent manner. We also show that p18(Hamlet) is required for H2A.Z accumulation into this genomic region and for subsequent muscle gene transcriptional activation. Accordingly, downregulation of several subunits or the SRCAP complex impairs muscle gene expression. These results identify SRCAP/H2A.Z-mediated chromatin remodelling as a key early event in muscle differentiation-specific gene expression. We also propose a mechanism by which p38 MAPK-mediated signals are converted into chromatin structural changes, thereby facilitating transcriptional activation during mammalian cell differentiation.

NF-kappaB signaling: a tale of two pathways in skeletal myogenesis

NF-kappaB is a ubiquitiously expressed transcription factor that plays vital roles in innate immunity and other processes involving cellular survival, proliferation, and differentiation. Activation of NF-kappaB is controlled by an IkappaB kinase (IKK) complex that can direct either canonical (classical) NF-kappaB signaling by degrading the IkappaB inhibitor and releasing p65/p50 dimers to the nucleus, or causes p100 processing and nuclear translocation of RelB/p52 via a noncanonical (alternative) pathway. Under physiological conditions, NF-kappaB activity is transiently regulated, whereas constitutive activation of this transcription factor typically in the classical pathway is associated with a multitude of disease conditions, including those related to skeletal muscle. How NF-kappaB functions in muscle diseases is currently under intense investigation. Insight into this role of NF-kappaB may be gained by understanding at a more basic level how this transcription factor contributes to skeletal muscle cell differentiation. Recent data from knockout mice support that the classical NF-kappaB pathway functions as an inhibitor of skeletal myogenesis and muscle regeneration acting through multiple mechanisms. In contrast, alternative NF-kappaB signaling does not appear to be required for myofiber conversion, but instead functions in myotube homeostasis by regulating mitochondrial biogenesis. Additional knowledge of these signaling pathways in skeletal myogenesis should aid in the development of specific inhibitors that may be useful in treatments of muscle disorders.

Regulatory interactions between muscle and the immune system during muscle regeneration

Recent discoveries reveal complex interactions between skeletal muscle and the immune system that regulate muscle regeneration. In this review, we evaluate evidence that indicates that the response of myeloid cells to muscle injury promotes muscle regeneration and growth. Acute perturbations of muscle activate a sequence of interactions between muscle and inflammatory cells. The initial inflammatory response is a characteristic Th1 inflammatory response, first dominated by neutrophils and subsequently by CD68(+) M1 macrophages. M1 macrophages can propagate the Th1 response by releasing proinflammatory cytokines and cause further tissue damage through the release of nitric oxide. Myeloid cells in the early Th1 response stimulate the proliferative phase of myogenesis through mechanisms mediated by TNF-alpha and IL-6; experimental prolongation of their presence is associated with delayed transition to the early differentiation stage of myogenesis. Subsequent invasion by CD163(+)/CD206(+) M2 macrophages attenuates M1 populations through the release of anti-inflammatory cytokines, including IL-10. M2 macrophages play a major role in promoting growth and regeneration; their absence greatly slows muscle growth following injury or modified use and inhibits muscle differentiation and regeneration. Chronic muscle injury leads to profiles of macrophage invasion and function that differ from acute injuries. For example, mdx muscular dystrophy yields invasion of muscle by M1 macrophages, but their early invasion is accompanied by a subpopulation of M2a macrophages. M2a macrophages are IL-4 receptor(+)/CD206(+) cells that reduce cytotoxicity of M1 macrophages. Subsequent invasion of dystrophic muscle by M2c macrophages is associated with progression of the regenerative phase in pathophysiology. Together, these findings show that transitions in macrophage phenotype are an essential component of muscle regeneration in vivo following acute or chronic muscle damage.

Cell-fusion-mediated somatic-cell reprogramming: a mechanism for tissue regeneration

Spontaneous cell fusion between two cells of different lineages will originate new hybrid cells that have different features from the original parent cells. It has been shown that injury to a tissue can enhance spontaneous cell-cell fusion events. If one of the parent cells of a cell-cell fusion is highly plastic, such as a stem cell, and the other is a somatic cell, their fusion can be followed by reprogramming events that can generate new hybrid pluripotent cells. These, in turn, have the potential to differentiate and regenerate the damaged tissue. However, if this process is deregulated, this would provide a mechanism for cancer development.

S100B protein in myoblasts modulates myogenic differentiation via NF-kappaB-dependent inhibition of MyoD expression

S100B, a Ca(2+)-binding protein of the EF-hand type, is expressed in myoblasts, the precursors of skeletal myofibers, and muscle satellite cells (this work). S100B has been shown to participate in the regulation of several intracellular processes including cell cycle progression and differentiation. We investigated regulatory activities of S100B within myoblasts by stable overexpression of S100B and by inhibition of S100B expression. Overexpression of S100B in myoblast cell lines and primary myoblasts resulted in inhibition of myogenic differentiation, evidenced by lack of expression of myogenin and myosin heavy chain (MyHC) and absence of myotube formation. S100B-overexpressing myoblasts showed reduced MyoD expression levels and unchanged Myf5 expression levels, compared with control myoblasts, and transient transfection of S100B-overexpressing myoblasts with MyoD, but not Myf5, restored differentiation and fusion in part. The transcriptional activity of NF-kappaB, a negative regulator of MyoD expression, was enhanced in S100B-overexpressing myoblasts, and blocking NF-kappaB activity resulted in reversal of S100B's inhibitory effects. Yin Yang1, a transcriptional repressor that is induced by NF-kappaB (p65) and mediates NF-kappaB inhibitory effects on several myofibrillary genes, also was upregulated in S100B-overexpressing myoblasts. Conversely, silencing S100B expression in myoblast cell lines by RNA interference resulted in reduced NF-kappaB activity and enhanced MyoD, myogenin and MyHC expression and myotube formation. Thus, intracellular S100B might modulate myoblast differentiation by interfering with MyoD expression in an NF-kappaB-dependent manner.

N-cadherin ligation, but not Sonic hedgehog binding, initiates Cdo-dependent p38alpha/beta MAPK signaling in skeletal myoblasts

The p38alpha/beta mitogen-activated protein kinase (MAPK) pathway promotes muscle-specific gene expression and myoblast differentiation but how pathway activity is initiated during these processes is poorly understood. During myoblast differentiation, the intracellular region of the promyogenic cell surface protein Cdo (also known as Cdon) binds to Bnip-2 and JLP, scaffold proteins for Cdc42 and p38alpha/beta MAPK, respectively. The Bnip-2/Cdc42 and JLP/p38alpha/beta complexes associate in a Cdo-dependent manner, resulting in Bnip-2/Cdc42-dependent p38alpha/beta activation and stimulation of cell differentiation. Although the Cdo ectodomain binds to several different proteins, it is unclear how Cdo-dependent p38alpha/beta activation is initiated. In myoblasts, Cdo interacts with the cell-cell adhesion molecule N-cadherin. Cdo also binds directly to the secreted morphogen Sonic hedgehog (Shh) to promote Shh pathway signaling. We report here that N-cadherin ligation activates p38alpha/beta in myoblasts in a Cdo-, Bnip-2-, and JLP-dependent manner. Furthermore, these proteins and activated Cdc42 cluster at sites of N-cadherin ligation. In contrast, neither JLP nor Bnip-2 is associated with Cdo bound to Shh, and Shh does not activate p38alpha/beta in myoblasts. Taken together, these results link cadherin-based cell-cell adhesion to a defined signaling pathway (Cdo --> p38alpha/beta) that directly regulates a cell-type-specific differentiation program. Furthermore, they are consistent with a model whereby Cdo serves as a multifunctional coreceptor with mechanistically distinct roles in multiple signaling pathways.

Distinctive patterns of microRNA expression in extraocular muscles

The extraocular muscles (EOMs) are a unique group of muscles that are anatomically and physiologically distinct from other muscles. We and others have shown that EOMs have a unique transcriptome and proteome. Here we investigated the expression pattern of microRNAs (miRNAs), as they may play a role in generating the unique EOM allotype. We isolated RNA and screened LC Sciences miRNA microarrays covering the sequences of miRBase 10.0 to define the microRNAome of normal mouse EOM and tibialis anterior (TA) limb muscle. Seventy-four miRNAs were found to be differentially regulated (P value <0.05) of which 31 (14 upregulated, 17 downregulated) were differentially regulated at signal strength >500. Muscle-specific miRNAs miR-206 and miR-499 were upregulated and miR-1, miR-133a, and miR-133b were downregulated in EOM. Quantitative PCR (qPCR) analysis was used to validate the differential expression. Bioinformatic tools were used to identify potential miRNA-mRNA-protein interactions and integrate data with previous transcriptome and proteomic profiling data. Luciferase assays using cotransfection of precursor miRNAs with reporter constructs containing the 3'-untranslated region of predicted target genes were used to validate targeting by identified miRNAs. The definition of the EOM microRNAome complements existing transcriptome and proteome data about the molecular makeup of EOM and provides further insight into regulation of muscle genes. These data will also help to further explain the unique EOM muscle allotype and its differential sensitivity to diseases such as Duchenne muscular dystrophy and may assist in development of therapeutic strategies.

Glucocorticoid-induced leucine zipper (GILZ) and long GILZ inhibit myogenic differentiation and mediate anti-myogenic effects of glucocorticoids

Myogenesis is a process whereby myoblasts differentiate and fuse into multinucleated myotubes, the precursors of myofibers. Various signals and factors modulate this process, and glucocorticoids (GCs) are important regulators of skeletal muscle metabolism. We show that glucocorticoid-induced leucine zipper (GILZ), a GC-induced gene, and the newly identified isoform long GILZ (L-GILZ) are expressed in skeletal muscle tissue and in C2C12 myoblasts where GILZ/L-GILZ maximum expression occurs during the first few days in differentiation medium. Moreover, we observed that GC treatment of myoblasts, which increased GILZ/L-GILZ expression, resulted in reduced myotube formation, whereas GILZ and L-GILZ silencing dampened GC effects. Inhibition of differentiation caused by GILZ/L-GILZ overexpression correlated with inhibition of MyoD function and reduced expression of myogenin. Notably, results indicate that GILZ and L-GILZ bind and regulate MyoD/HDAC1 transcriptional activity, thus mediating the anti-myogenic effect of GCs.

Regulation of the intracellular localization of Foxo3a by stress-activated protein kinase signaling pathways in skeletal muscle cells

Muscle atrophy is a debilitating process associated with many chronic wasting diseases, like cancer, diabetes, sepsis, and renal failure. Rapid loss of muscle mass occurs mainly through the activation of protein breakdown by the ubiquitin proteasome pathway. Foxo3a transcription factor is critical for muscle atrophy, since it activates the expression of ubiquitin ligase Atrogin-1. In several models of atrophy, inhibition of the phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway induces nuclear import of Foxo3a through an Akt-dependent process. This study aimed to identify signaling pathways involved in the control of Foxo3a nuclear translocation in muscle cells. We observed that after nuclear import of Foxo3a by PI3K/Akt pathway inhibition, activation of stress-activated protein kinase (SAPK) pathways induced nuclear export of Foxo3a through CRM1. This mechanism involved the c-Jun NH(2)-terminal kinase (JNK) signaling pathway and was independent of Akt. Likewise, we showed that inhibition of p38 induced a massive nuclear relocalization of Foxo3a. Our results thus suggest that SAPKs are involved in the control of Foxo3a nucleocytoplasmic translocation in C2C12 cells. Moreover, activation of SAPKs decreases the expression of Atrogin-1, and stable C2C12 myotubes, in which the p38 pathway is constitutively activated, present partial protection against atrophy.

Transforming growth factor-beta-activated kinase 1 is an essential regulator of myogenic differentiation

Satellite cells/myoblasts account for the majority of muscle regenerative potential in response to injury and muscular adaptation to exercise. Although the ability to influence this process would provide valuable benefits for treating a variety of patients suffering from muscle loss, the regulatory mechanisms of myogenesis are not completely understood. We have tested the hypothesis that transforming growth factor-beta-activated kinase 1 (TAK1) is an important regulator of skeletal muscle formation. TAK1 is expressed in proliferating C2C12 myoblasts, and its levels are reduced upon differentiation of myoblasts into myotubes. In vivo, TAK1 is predominantly expressed in developing skeletal muscle of young mice. However, the expression of TAK1 was significantly up-regulated in regenerating skeletal muscle of adult mice. Overexpression of a dominant negative mutant of TAK1 or knockdown of TAK1 inhibited the proliferation and differentiation of C2C12 myoblasts. TAK1 was required for the expression of myogenic regulatory factors in differentiating myoblasts. Genetic ablation of TAK1 also inhibited the MyoD-driven transformation of mouse embryonic fibroblasts into myotubes. Inhibition of TAK1 suppressed the differentiation-associated activation of p38 mitogen-activated protein kinase (MAPK) and Akt kinase. Overexpression of a constitutively active mutant of MAPK kinase 6 (MKK6, an upstream activator of p38 MAPK) but not constitutive active Akt restored the myogenic differentiation in TAK1-deficient mouse embryonic fibroblasts. Insulin growth factor 1-induced myogenic differentiation was also found to involve TAK1. Collectively, our results suggest that TAK1 is an important upstream regulator of skeletal muscle cell differentiation.

Glucose represses PPARα gene expression via AMP-activated protein kinase but not via p38 mitogen-activated protein kinase in the pancreatic β-cell

BACKGROUND

Peroxisome proliferator-activated receptor α (PPARα) regulates the expression of fatty acid metabolism genes and is thought to play a role in the regulation of insulin secretion and lipid detoxification. We have examined the mechanism whereby glucose decreases PPARα gene expression in the pancreatic β-cell.

METHODS

INS832/13 β-cell and isolated rat islets were incubated at 3 and 20 mM glucose for 18 h in the absence or presence of adenosine monophosphate (AMP)-activated protein kinase (AMPK) activators and inhibitors, as well as p38 mitogen-activated protein kinase (p38 MAPK) inhibitors. In another set of experiments, INS832/13 were infected with an adenovirus expressing a dominant-negative form of AMPK. PPARα expression levels were measured by reverse transcription polymerase chain reaction and Western blot.

RESULTS

Elevated glucose reduced the abundance of the PPARα transcript and protein, and its target genes acyl-coenzyme A (CoA) oxidase (ACO) and uncoupling protein 2 (UCP-2) in INS832/13 β-cell and isolated rat islets. Glucose reduced AMPK activity, while the AMPK activators 5-amino-4-imidazolecarboxamide riboside and metformin increased PPARα expression and suppressed the action of glucose. By contrast, the AMPK inhibitor compound C mimicked the glucose effect. A dominant negative form of AMPKα reduced the PPARα, ACO and UCP-2 transcripts to the same extent as elevated glucose. Pharmacological evidence indicated that glucose-regulated PPARα expression does not involve p38 MAPK, a target of AMPK in several cell types.

CONCLUSIONS

The results indicate that glucose represses PPARα gene expression via AMPK, but not via p38 MAPK in the β-cell.

17β-Estradiol regulates the first steps of skeletal muscle cell differentiation via ER-α-mediated signals

17β-Estradiol (E2) mediates a wide variety of complex biological processes determining the growth and development of reproductive tract as well as nonreproductive tissues of male and female individuals. While E2 effects on the reproductive system, bone, and cardiovascular system are quite well established, less is known about how it affects the physiology of other tissues. Skeletal muscle is a tissue that is expected to be E2 responsive since both isoforms of estrogen receptor (ER-α and ER-β) are expressed. Significant sex-related differences have been described in skeletal muscle, although the role played by E2 and the mechanisms underlying it remain to be determined. Here, we demonstrate that E2 increases the glucose transporter type 4 translocation at membranes as well as the expression of well-known differentiation markers of myogenesis (i.e., myogenin and myosin heavy chain) in rat myoblast cells (L6). These E2-induced effects require rapid extranuclear signals and the presence of ER-α, whereas no contribution of IGF-I receptor has been observed. In particular, ER-α-dependent Akt activation participates in regulating the first step of myogenic differentiation. Moreover, both receptors mediate the E2-induced activation of p38, which, in turn, affects the expression of myogenin and myosin heavy chain. All together, these data indicate that E2 should be included in the list of skeletal muscle trophic factors.

Microrna-221 and microrna-222 modulate differentiation and maturation of skeletal muscle cells

Background

MicroRNAs (miRNAs) are a class of small non-coding RNAs that have recently emerged as important regulators of gene expression. They negatively regulate gene expression post-transcriptionally by translational repression and target mRNA degradation. miRNAs have been shown to play crucial roles in muscle development and in regulation of muscle cell proliferation and differentiation.

Methodology/Principal Findings

By comparing miRNA expression profiling of proliferating myoblasts versus differentiated myotubes, a number of modulated miRNAs, not previously implicated in regulation of myogenic differentiation, were identified. Among these, miR-221 and miR-222 were strongly down-regulated upon differentiation of both primary and established myogenic cells. Conversely, miR-221 and miR-222 expression was restored in post-mitotic, terminally differentiated myotubes subjected to Src tyrosine kinase activation. By the use of specific inhibitors we provide evidence that expression of miR-221 and miR-222 is under the control of the Ras-MAPK pathway. Both in myoblasts and in myotubes, levels of the cell cycle inhibitor p27 inversely correlated with miR-221 and miR-222 expression, and indeed we show that p27 mRNA is a direct target of these miRNAs in myogenic cells. Ectopic expression of miR-221 and miR-222 in myoblasts undergoing differentiation induced a delay in withdrawal from the cell cycle and in myogenin expression, followed by inhibition of sarcomeric protein accumulation. When miR-221 and miR-222 were expressed in myotubes undergoing maturation, a profound alteration of myofibrillar organization was observed.

Conclusions/Significance

miR-221 and miR-222 have been found to be modulated during myogenesis and to play a role both in the progression from myoblasts to myocytes and in the achievement of the fully differentiated phenotype. Identification of miRNAs modulating muscle gene expression is crucial for the understanding of the circuits controlling skeletal muscle differentiation and maintenance.

RNA-binding motif protein 4 translocates to cytoplasmic granules and suppresses translation via argonaute2 during muscle cell differentiation

The RNA-binding motif protein 4 (RBM4) plays multiple roles in mRNA metabolism, including translation control. RBM4 suppresses cap-dependent translation but activates internal ribosome entry site-mediated translation. Because of its high expression level in muscle and heart, we investigated the function of RBM4 in myoblast cells. Here, we demonstrate that RBM4 is phosphorylated and translocates to the cytoplasm in mouse C2C12 cells upon cell differentiation. Notably, RBM4 is transiently deposited into cytoplasmic granules containing microtubule assembly factors as well as poly(A)(+) RNAs. Moreover, RBM4 colocalizes with the components of micro-ribonucleoproteins, including the Argonaute2 (Ago2) protein, during muscle cell differentiation. RBM4 interacts directly with Ago2 and may recruit Ago2 to suppress translation of target mRNAs. Furthermore, RBM4 selectively associates with muscle cell-specific microRNAs and potentiates their translation repression activity by promoting micro-ribonucleoprotein association with target mRNAs. Altogether, our results suggest that RBM4 translocates to the cytoplasm and participates in translation suppression during muscle cell differentiation.

NET37, a nuclear envelope transmembrane protein with glycosidase homology, is involved in myoblast differentiation

The nuclear lamina and its associated proteins are important for nuclear structure and chromatin organization and also have been implicated in the regulation of cell signaling and gene expression. In this study we demonstrate that the lamina-associated nuclear envelope transmembrane protein NET37 is required for myogenic differentiation of C2C12 cells. NET37, a member of glycosidase family 31, is highly expressed in mouse skeletal muscle and is strongly up-regulated during C2C12 differentiation. By protease mapping we show that its glycosidase homology domain is located in the lumen of the nuclear envelope/endoplasmic reticulum. When NET37 is depleted from proliferating myoblasts by RNAi, myogenic differentiation is significantly impaired, and there is a concomitant delay in up-regulation of the late myogenic transcription factor myogenin. We expressed silencing-resistant NET37 mutated at a conserved residue in the glycosidase domain and found that this predicted catalytically inactive protein is unable to support myogenesis in cells depleted of wild type NET37. Therefore, the enzymatic function of NET37 appears to be important for myogenic differentiation. C2C12 cells depleted of NET37 have reduced activation of Akt after shifting to differentiation medium and are defective in insulin like growth factor-II (IGF-II) secretion, an autocrine/paracrine factor involved in Akt activation. We also observed that pro-IGF-II co-immunoprecipitates with NET37. Based on our results, we propose that NET37 has a role in IGF-II maturation in the secretory pathway during myoblast differentiation. The localization of NET37 at the nuclear envelope raises the possibility that it may coordinate myogenic events between the nuclear interior and the endoplasmic reticulum lumen via transmembrane communication.

Signal integration by JNK and p38 MAPK pathways in cancer development

Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK) family members function in a cell context-specific and cell type-specific manner to integrate signals that affect proliferation, differentiation, survival and migration. Consistent with the importance of these events in tumorigenesis, JNK and p38 MAPK signalling is associated with cancers in humans and mice. Studies in mouse models have been essential to better understand how these MAPKs control cancer development, and these models are expected to provide new strategies for the design of improved therapeutic approaches. In this Review we highlight the recent progress made in defining the functions of the JNK and p38 MAPK pathways in different cancers.

Signal integration by JNK and p38 MAPK pathways in cancer development

Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK) family members function in a cell context-specific and cell type-specific manner to integrate signals that affect proliferation, differentiation, survival and migration. Consistent with the importance of these events in tumorigenesis, JNK and p38 MAPK signalling is associated with cancers in humans and mice. Studies in mouse models have been essential to better understand how these MAPKs control cancer development, and these models are expected to provide new strategies for the design of improved therapeutic approaches. In this Review we highlight the recent progress made in defining the functions of the JNK and p38 MAPK pathways in different cancers.

SOCS1, SOCS3, and PIAS1 promote myogenic differentiation by inhibiting the leukemia inhibitory factor-induced JAK1/STAT1/STAT3 pathway

We recently showed that a leukemia inhibitory factor (LIF)-engaged signaling pathway consisting of JAK1, STAT1, and STAT3 plays dual roles in myogenic differentiation: while it participates in myoblast proliferation, it also actively represses differentiation. Downregulation of this pathway is required at the onset of differentiation. However, it remained unclear how this is achieved mechanistically. We now show that SOCS1, SOCS3, and PIAS1 promote myogenic differentiation by specifically inhibiting the LIF-induced JAK1/STAT1/STAT3 pathway via distinct targets; whereas SOCS1 and SOCS3 selectively bind and inhibit JAK1 and gp130, respectively, PIAS1 targets mainly the activated STAT1 and prevents its binding to DNA. We further demonstrated that the SUMO E3-ligase activity of PIAS1 is dispensable for its role in myogenic differentiation. Collectively, our current study revealed a molecular mechanism that explains how the LIF-induced JAK1/STAT1/STAT3 pathway is downregulated upon myogenic differentiation.

The muscle-specific microRNA miR-206 blocks human rhabdomyosarcoma growth in xenotransplanted mice by promoting myogenic differentiation

Many microRNAs (miRNAs), posttranscriptional regulators of numerous cellular processes and developmental events, are downregulated in tumors. However, their role in tumorigenesis remains largely unknown. In this work, we examined the role of the muscle-specific miRNAs miR-1 and miR-206 in human rhabdomyosarcoma (RMS), a soft tissue sarcoma thought to arise from skeletal muscle progenitors. We have shown that miR-1 was barely detectable in primary RMS of both the embryonal and alveolar subtypes and that both miR-1 and miR-206 failed to be induced in RMS cell lines upon serum deprivation. Moreover, reexpression of miR-206 in RMS cells promoted myogenic differentiation and blocked tumor growth in xenografted mice by switching the global mRNA expression profile to one that resembled mature muscle. Finally, we showed that the product of the MET proto-oncogene, the Met tyrosine-kinase receptor, which is overexpressed in RMS and has been implicated in RMS pathogenesis, was downregulated in murine satellite cells by miR-206 at the onset of normal myogenesis. Thus, failure of posttranscriptional modulation may underlie Met overexpression in RMS and other types of cancer. We propose that tissue-specific miRNAs such as miR-1 and miR-206, given their ability to modulate hundreds of transcripts and to act as nontoxic differentiating agents, may override the genomic heterogeneity of solid tumors and ultimately hold greater therapeutic potential than single gene-directed drugs.

Preliminary quantitative profile of differential protein expression between rat L6 myoblasts and myotubes by stable isotope labeling with amino acids in cell culture

Defining the mechanisms governing myogenesis has advanced in recent years. Skeletal-muscle differentiation is a multi-step process controlled spatially and temporally by various factors at the transcription level. To explore those factors involved in myogenesis, stable isotope labeling with amino acids in cell culture (SILAC), coupled with high-accuracy mass spectrometry (LTQ-Orbitrap), was applied successfully. Rat L6 cell line is an excellent model system for studying muscle myogenesis in vitro. When mononucleate L6 myoblast cells reach confluence in culture plate, they could transform into multinucleate myotubes by serum starvation. By comparing protein expression of L6 myoblasts and terminally differentiated multinucleated myotubes, 1170 proteins were quantified and 379 proteins changed significantly in fully differentiated myotubes in contrast to myoblasts. These differentially expressed proteins are mainly involved in inter-or intracellular signaling, protein synthesis and degradation, protein folding, cell adhesion and extracellular matrix, cell structure and motility, metabolism, substance transportation, etc. These findings were supported by many previous studies on myogenic differentiation, of which many up-regulated proteins were found to be involved in promoting skeletal muscle differentiation for the first time in our study. In summary, our results provide new clues for understanding the mechanism of myogenesis.

Cdo binds Abl to promote p38alpha/beta mitogen-activated protein kinase activity and myogenic differentiation

The p38 mitogen-activated protein kinase (MAPK) pathway is required for differentiation of skeletal myoblasts, but how the pathway is activated during this process is not well understood. One mechanism involves the cell surface receptor Cdo (also known as Cdon), which binds to Bnip-2 and JLP, scaffold proteins for Cdc42 and p38, respectively; formation of these complexes results in Bnip-2/Cdc42-dependent activation of p38. It has been reported that the tyrosine kinase Abl promotes myogenic differentiation in a manner dependent on its cytoplasmic localization, but the cytoplasmic signaling proteins with which it interacts to achieve this effect are unidentified. We report that Abl associates with both Cdo and JLP during myoblast differentiation. Abl binds a proline-rich motif in Cdo via its SH3 domain, and these regions of Abl and Cdo are required for their promyogenic effects. Cdo is important for full Abl kinase activity, and Abl is necessary for full activation of p38 MAPK, during myogenic differentiation. As seen with myoblasts depleted of Cdo, the diminished differentiation displayed by Abl-depleted cells is rescued by the expression of an activated form of the immediate upstream p38-activating kinase MAPK kinase 6. Abl's promyogenic effect is therefore linked to a multiprotein cell surface complex that regulates differentiation-dependent p38 activation.

Cardiotrophin-1 maintains the undifferentiated state in skeletal myoblasts

Skeletal myogenesis is potently regulated by the extracellular milieu of growth factors and cytokines. We observed that cardiotrophin-1 (CT-1), a member of the interleukin-6 (IL-6) family of cytokines, is a potent regulator of skeletal muscle differentiation. The normal up-regulation of myogenic marker genes, myosin heavy chain (MyHC), myogenic regulatory factors (MRFs), and myocyte enhancer factor 2s (MEF2s) were inhibited by CT-1 treatment. CT-1 also represses myogenin (MyoG) promoter activation. CT-1 activated two signaling pathways: signal transducer and activator of transcription 3 (STAT3), and mitogen-activated protein kinase kinase (MEK), a component of the extracellular signal-regulated MAPK (ERK) pathway. In view of the known connection between CT-1 and STAT3 activation, we surprisingly found that pharmacological blockade of STAT3 activity had no effect on the inhibition of myogenesis by CT-1 suggesting that STAT3 signaling is dispensable for myogenic repression. Conversely, MEK inhibition potently reversed the inhibition of myotube formation and attenuated the repression of MRF transcriptional activity mediated by CT-1. Taken together, these data indicate that CT-1 represses skeletal myogenesis through interference with MRF activity by activation of MEK/ERK signaling. In agreement with these in vitro observations, exogenous systemic expression of CT-1 mediated by adenoviral vector delivery increased the number of myonuclei in normal post-natal mouse skeletal muscle and also delayed skeletal muscle regeneration induced by cardiotoxin injection. The expression pattern of CT-1 in embryonic and post-natal skeletal muscle and in vivo effects of CT-1 on myogenesis implicate CT-1 in the maintenance of the undifferentiated state in muscle progenitor cells.

Chromatin: the interface between extrinsic cues and the epigenetic regulation of muscle regeneration

Muscle regeneration provides a paradigm by which to study how extrinsic signals coordinate gene expression in somatic stem cells (satellite cells) by directing the genome distribution of chromatin-modifying complexes. Understanding the signal-dependent control of the epigenetic events underlying the transition of muscle stem cells through sequential regeneration stages holds the promise to reveal new targets for selective interventions toward repairing diseased muscles. This review describes the latest findings on how regeneration cues are integrated at the chromatin level to build the transcription network that regulates progression of endogenous muscle progenitors throughout the myogenic program. In particular, we describe how specific epigenetic signatures can confer responsiveness to extrinsic cues on discrete regions of the muscle stem cell genome.

Mouse model of testosterone-induced muscle fiber hypertrophy: involvement of p38 mitogen-activated protein kinase-mediated Notch signaling

As a prerequisite for studies using mutant mice, we established a mouse model for investigating the molecular mechanisms by which testosterone (T) promotes muscle growth. Groups of six adult male mice (C57BL/6) received one of the following treatments: 1) vehicle (sterile distilled water; normal control) and 2) GnRH antagonist with empty (sham control) or 2 cm T- filled implant. Mice were killed 2, 6, and 8 weeks after treatment. T treatment for 8 weeks resulted in a significant (P<0.001) increase in fiber area of gastrocnemius muscles. T-induced fiber-hypertrophy was accompanied by up-regulation of the Notch ligand Delta 1 and activation of Notch signaling, as evidenced by increase in activated forms of Notch 1 and Notch 2. Consistent with this, we also observed an increase in the number of proliferating cell nuclear antigen (PCNA)-positive nuclei in muscles of T-treated mice, indicating that activation of Notch signaling enhanced cell proliferation. T supplementation not only triggered p38 mitogen-activated protein kinase (MAPK) activation but also concurrently inhibited c-Jun NH(2)-terminal kinase (JNK) activation within 2 weeks of treatment. Concomitant administration of SB203580, a p38 MAPK inhibitor, effectively blocked T-induced activation of Notch signaling and significantly (P<0.001) suppressed PCNA levels. Together, our results indicate that T induces muscle fiber hypertrophy through activation of Notch signaling and the inactivation of JNK together with the activation of p38 MAPK may be critical for T-induced activation of Notch signaling and, as a consequence, muscle fiber hypertrophy.

Cdo promotes neuronal differentiation via activation of the p38 mitogen-activated protein kinase pathway

Neural basic helix-loop-helix transcription factors (bHLHs) control many aspects of neurogenesis, such as proliferation, fate determination, and differentiation. We have previously shown that the promyogenic cell surface receptor Cdo modulates the Cdc42 and p38 mitogen-activated protein kinase (MAPK) pathways via a direct association with two scaffold-type proteins, JLP and Bnip-2, to regulate activities of myogenic bHLH factors and myogenic differentiation. We report here that Cdo uses similar regulatory mechanisms to promote neuronal differentiation. Expression of JLP, a scaffold protein for p38MAPK, and Bnip-2, a regulator of Cdc42, is increased during differentiation of C17.2 neural precursor cells and P19 embryonal carcinoma cells. These molecules regulate Cdc42 and p38MAPK activities, which increase in a Cdo-dependent manner during neuronal differentiation of C17.2 cells and retinoic acid-treated P19 cells. Furthermore, enhancement or reduction of Cdc42 and p38MAPK activities enhances or reduces, respectively, neuronal differentiation of these cell lines. Cdc42 and p38MAPK activities also promote heterodimerization of neurogenin1 and E47, suggesting that one way they promote neurogenesis is via regulation of neural bHLH factor activities. These results imply that a conserved intracellular signaling mechanism initiated by Cdo regulates the activities of tissue-specific bHLH factors and therefore functions as a key regulator of differentiation of several different cell lineages.

Stem cells and plasticity of skeletal muscle cell differentiation: potential application to cell therapy for degenerative muscular diseases

Degenerative muscular diseases, such as muscular dystrophies, have been the representative targets of regenerative cell therapy. Although satellite cells play central roles in skeletal muscle regeneration that intrinsically occurs after muscle injury, their application to cell therapy is confronted by difficulties. Other stem cells expected to be applicable to cell therapy include muscle-resident stem cells and nonmuscle-resident stem cells. Moreover, dedifferentiated cells of skeletal muscle might provide unique system for cell therapy. Terminally differentiated myotubes have plasticity of differentiation and dedifferentiate under certain experimental conditions, including the expression of SV40 large T antigen or the homeobox gene Msx1. The dedifferentiated cells exhibit multipotency to transdifferentiate into multiple mesenchymal origin cells. In addition, fibroblasts or undifferentiated myoblasts treated with a drug acquire multipotency. These cells may open new doors in cell therapy.

A non-apoptotic role for caspase-9 in muscle differentiation

Caspases, a family of cysteine proteases most often investigated for their roles in apoptosis, have also been demonstrated to have functions that are vital for the efficient execution of cell differentiation. One such role that has been described is the requirement of caspase-3 for the differentiation of skeletal myoblasts into myotubes but, as yet, the mechanism leading to caspase-3 activation in this case remains elusive. Here, we demonstrate that caspase-9, an initiator caspase in the mitochondrial death pathway, is responsible for the activation of caspase-3 in differentiating C2C12 cells. Reduction of caspase-9 levels, using an shRNA construct, prevented caspase-3 activation and inhibited myoblast fusion. Myosin-heavy-chain expression, which accompanies myoblastic differentiation, was not caspase-dependent. Overexpression of Bcl-xL, a protein that inhibits caspase-9 activation, had the same effect on muscle differentiation as knockdown of caspase-9. These data suggest that the mitochondrial pathway is required for differentiation; however, the release of cytochrome c or Smac (Diablo) could not be detected, raising the possibility of a novel mechanism of caspase-9 activation during muscle differentiation.

Regenerative pharmacology in the treatment of genetic diseases: the paradigm of muscular dystrophy

Current evidence supports the therapeutic potential of pharmacological interventions that counter the progression of genetic disorders by promoting regeneration of the affected organs or tissues. The rationale behind this concept lies on the evidence that targeting key events downstream of the genetic defect can compensate, at least partially, the pathological consequence of the related disease. In this regard, the beneficial effect exerted on animal models of muscular dystrophy by pharmacological strategies that enhance muscle regeneration provides an interesting paradigm. In this review, we describe and discuss the potential targets of pharmacological strategies that promote regeneration of dystrophic muscles and alleviate the consequence of the primary genetic defect. Regenerative pharmacology provides an immediate and suitable therapeutic opportunity to slow down the decline of muscles in the present generation of dystrophic patients, with the perspective to hold them in conditions such that they could benefit of future, more definitive, therapies.

A Cdo-Bnip-2-Cdc42 signaling pathway regulates p38alpha/beta MAPK activity and myogenic differentiation

The p38α/β mitogen-activated protein kinase (MAPK) pathway promotes skeletal myogenesis, but the mechanisms by which it is activated during this process are unclear. During myoblast differentiation, the promyogenic cell surface receptor Cdo binds to the p38α/β pathway scaffold protein JLP and, via JLP, p38α/β itself. We report that Cdo also interacts with Bnip-2, a protein that binds the small guanosine triphosphatase (GTPase) Cdc42 and a negative regulator of Cdc42, Cdc42 GTPase-activating protein (GAP). Moreover, Bnip-2 and JLP are brought together through mutual interaction with Cdo. Gain- and loss-of-function experiments with myoblasts indicate that the Cdo–Bnip-2 interaction stimulates Cdc42 activity, which in turn promotes p38α/β activity and cell differentiation. These results reveal a previously unknown linkage between a cell surface receptor and downstream modulation of Cdc42 activity. Furthermore, interaction with multiple scaffold-type proteins is a distinctive mode of cell surface receptor signaling and provides one mechanism for specificity of p38α/β activation during cell differentiation.

MURC, a muscle-restricted coiled-coil protein, is involved in the regulation of skeletal myogenesis

Skeletal myogenesis is a multistep process by which multinucleated mature muscle fibers are formed from undifferentiated, mononucleated myoblasts. However, the molecular mechanisms of skeletal myogenesis have not been fully elucidated. Here, we identified muscle-restricted coiled-coil (MURC) protein as a positive regulator of myogenesis. In skeletal muscle, MURC was localized to the cytoplasm with accumulation in the Z-disc of the sarcomere. In C2C12 myoblasts, MURC expression occurred coincidentally with myogenin expression and preceded sarcomeric myosin expression during differentiation into myotubes. RNA interference (RNAi)-mediated knockdown of MURC impaired differentiation in C2C12 myoblasts, which was accompanied by impaired myogenin expression and ERK activation. Overexpression of MURC in C2C12 myoblasts resulted in the promotion of differentiation with enhanced myogenin expression and ERK activation during differentiation. During injury-induced muscle regeneration, MURC expression increased, and a higher abundance of MURC was observed in immature myofibers compared with mature myofibers. In addition, ERK was activated in regenerating tissue, and ERK activation was detected in MURC-expressing immature myofibers. These findings suggest that MURC is involved in the skeletal myogenesis that results from modulation of myogenin expression and ERK activation. MURC may play pivotal roles in the molecular mechanisms of skeletal myogenic differentiation.

Specific requirements of MRFs for the expression of muscle specific microRNAs, miR-1, miR-206 and miR-133

The expression of three microRNAs, miR-1, miR-206 and miR-133 is restricted to skeletal myoblasts and cardiac tissue during embryo development and muscle cell differentiation, which suggests a regulation by muscle regulatory factors (MRFs). Here we show that inhibition of C2C12 muscle cell differentiation by FGFs, which interferes with the activity of MRFs, suppressed the expression of miR-1, miR-206 and miR-133. To further investigate the role of myogenic regulators (MRFs), Myf5, MyoD, Myogenin and MRF4 in the regulation of muscle specific microRNAs we performed gain and loss-of-function experiments in vivo, in chicken and mouse embryos. We found that directed expression of MRFs in the neural tube of chicken embryos induced ectopic expression of miR-1 and miR-206. Conversely, the lack of Myf5 but not of MyoD resulted in a loss of miR-1 and miR-206 expression. Taken together our results demonstrate differential requirements of distinct MRFs for the induction of microRNA gene expression during skeletal myogenesis.

Muscling in on microarrays

Adaptations that are the result of exercise require a multitude of changes at the level of gene expression. The mechanisms involved in regulating these changes are many, and can occur at various points in the pathways that affect gene expression. The completion of the human genome sequence, along with the genomes of related species, has provided an enormous amount of information to help dissect and understand these pathways. High-throughput methods, such as DNA microarrays, were the first on the scene to take advantage of this wealth of information. A new generation of microarrays has now taken the next step in revealing the mechanisms controlling gene expression. Analysis of the regulation of gene expression can now be profiled in a high-throughput fashion. However, the application of this technology has yet to be fully realized in the exercise physiology community. This review will highlight some of the latest advances in microarrays and briefly discuss some potential applications to the field of exercise physiology.

Myocyte enhancer factor 2A is transcriptionally autoregulated

MEF2 (myocyte enhancer factor 2) proteins are a small family of transcription factors that play pivotal roles in striated muscle differentiation, development, and metabolism, in neuron survival and synaptic formation, and in lymphocyte selection and activation. Products of the four mammalian MEF2 genes, MEF2A, MEF2B, MEF2C, and MEF2D, are expressed with overlapping but distinct temporospatial patterns. Toward analysis of MEF2A functions and the determinants of its regulated expression, we have mapped and begun studies of the transcriptional control regions of this gene. Heterogeneous 5'-untranslated regions of MEF2A mRNAs result from use of alternative promoters and splicing patterns. The two closely approximated TATA-less promoters are approximately 65 kb upstream of the exon containing the sole initiation codon. Ribonuclease protection and primer extension assays show that each promoter is active in various adult tissues. A canonical MEF2 site overlies the major promoter 1 transcription start site. This element specifically binds MEF2 factors, including endogenous nuclear MEF2A according to chromatin immunoprecipitation studies, and is critical to MEF2A transcription in myocytes. The site exerts reciprocal control of the alternative promoters, silencing promoter 1 and activating promoter 2 under some conditions. Erk5 and p38 MAPK signaling stimulate MEF2A expression by activating both promoters from the MEF2 element. MEF2A transcription is therefore subject to positive or negative regulation by its protein products, depending on signaling activities that influence MEF2 factor trans-activity. The sole MEF2 gene of the cephalochordate amphioxus has a similar regulatory region structure, suggesting that this mode of autoregulatory control is conserved among higher metazoan MEF2 genes.

The proinflammatory cytokine tumor necrosis factor-alpha increases the amount of glucose transporter-4 at the surface of muscle cells independently of changes in interleukin-6

TNFalpha is a proinflammatory cytokine secreted by macrophages in response to bacterial infection. Recently new evidence has emerged suggesting that stressed or injured myocytes produce TNFalpha that then acts as an autocrine and/or paracrine mediator. TNFalpha receptors types 1 and 2 are present in skeletal muscle cells, and muscle cells can secrete, in addition to TNFalpha, other cytokines such as IL-1beta or IL-6. Furthermore, the plasma concentration of TNFalpha is elevated in insulin-resistant states associated with obesity and type 2 diabetes. Here we show that TNFalpha increased the amount of glucose transporter (GLUT)-4 at the plasma membrane and also glucose uptake in the L6 muscle cell line stably expressing GLUT4 tagged with the c-myc epitope. Regardless of the state of differentiation of the L6 cells, TNFalpha did not affect the rate of proliferation or of apoptosis. The stimulatory effects of TNFalpha on cell surface GLUT4 and glucose uptake were blocked by nuclear factor-kappaB and p38MAPK pathway specific inhibitors (Bay 11-7082 and SB220025), and these two pathways were stimulated by TNFalpha. Furthermore, although TNFalpha increased IL-6 mRNA and protein expression, IL-6 did not mediate the effects of TNFalpha on cell surface GLUT4 levels, which also did not require de novo protein synthesis. The results indicate that TNFalpha can stimulate glucose uptake in L6 muscle cells by inducing GLUT4 translocation to the plasma membrane, possibly through activation of the nuclear factor-kappaB and p38MAPK signaling pathways and independently of the production of IL-6.

NF-kappaB signaling in skeletal muscle: prospects for intervention in muscle diseases

Muscle remodeling is an important physiological process that promotes adaptive changes in cytoarchitecture and protein composition after exercise, aging, or disease conditions. Numerous transcription factors have been reported to regulate skeletal muscle homeostasis. NF-κB is a major pleiotropic transcription factor modulating immune, inflammatory, cell survival, and proliferating responses; however, its role in muscle development, physiology, and disease has just started to be elucidated. The current review article aims to summarize the literature on the role of NF-κB signaling in skeletal muscle pathophysiology, investigated over the last years using in vitro and more recently in vivo systems. Understanding the exact role of NF-κB in muscle cells will allow better therapeutic manipulations in the setting of human muscle diseases.

Functional interdependence at the chromatin level between the MKK6/p38 and IGF1/PI3K/AKT pathways during muscle differentiation

During muscle regeneration, the mechanism integrating environmental cues at the chromatin of muscle progenitors is unknown. We show that inflammation-activated MKK6-p38 and insulin growth factor 1 (IGF1)-induced PI3K/AKT pathways converge on the chromatin of muscle genes to target distinct components of the muscle transcriptosome. p38 alpha/beta kinases recruit the SWI/SNF chromatin-remodeling complex; AKT1 and 2 promote the association of MyoD with p300 and PCAF acetyltransferases, via direct phosphorylation of p300. Pharmacological or genetic interference with either pathway led to partial assembly of discrete chromatin-bound complexes, which reflected two reversible and distinct cellular phenotypes. Remarkably, PI3K/AKT blockade was permissive for chromatin recruitment of MEF2-SWI/SNF complex, whose remodeling activity was compromised in the absence of MyoD and acetyltransferases. The functional interdependence between p38 and IGF1/PI3K/AKT pathways was further established by the evidence that blockade of AKT chromatin targets was sufficient to prevent the activation of the myogenic program triggered by deliberate activation of p38 signaling.