Stress-activated protein kinase-2/p38 and a rapamycin-sensitive pathway are required for C2C12 myogenesis

Application of the [γ-32P] ATP kinase assay to study anabolic signaling in human skeletal muscle

AMPK (AMP-dependant protein kinase)-mTORC1 (mechanistic target of rapamycin in complex 1)-p70S6K1 (ribosomal protein S6 kinase 1 of 70 kDa) signaling plays a crucial role in muscle protein synthesis (MPS). Understanding this pathway has been advanced by the application of the Western blot (WB) technique. However, because many components of the mTORC1 pathway undergo numerous, multisite posttranslational modifications, solely studying the phosphorylation changes of mTORC1 and its substrates may not adequately represent the true metabolic signaling processes. The aim of this study was to develop and apply a quantitative in vitro [γ-(32)P] ATP kinase assay (KA) for p70S6K1 to assess kinase activity in human skeletal muscle to resistance exercise (RE) and protein feeding. In an initial series of experiments the assay was validated in tissue culture and in p70S6K1-knockout tissues. Following these experiments, the methodology was applied to assess p70S6K1 signaling responses to a physiologically relevant stimulus. Six men performed unilateral RE followed by the consumption of 20 g of protein. Muscle biopsies were obtained at pre-RE, and 1 and 3 h post-RE. In response to RE and protein consumption, p70S6K1 activity as assessed by the KA was significantly increased from pre-RE at 1 and 3 h post-RE. However, phosphorylated p70S6K1(thr389) was not significantly elevated. AMPK activity was suppressed from pre-RE at 3 h post-RE, whereas phosphorylated ACC(ser79) was unchanged. Total protein kinase B activity also was unchanged after RE from pre-RE levels. Of the other markers we assessed by WB, 4EBP1(thr37/46) phosphorylation was the only significant responder, being elevated at 3 h post-RE from pre-RE. These data highlight the utility of the KA to study skeletal muscle plasticity.

A zebrafish embryo culture system defines factors that promote vertebrate myogenesis across species

Ex vivo expansion of satellite cells and directed differentiation of pluripotent cells to mature skeletal muscle have proved difficult challenges for regenerative biology. Using a zebrafish embryo culture system with reporters of early and late skeletal muscle differentiation, we examined the influence of 2,400 chemicals on myogenesis and identified six that expanded muscle progenitors, including three GSK3β inhibitors, two calpain inhibitors, and one adenylyl cyclase activator, forskolin. Forskolin also enhanced proliferation of mouse satellite cells in culture and maintained their ability to engraft muscle in vivo. A combination of bFGF, forskolin, and the GSK3β inhibitor BIO induced skeletal muscle differentiation in human induced pluripotent stem cells (iPSCs) and produced engraftable myogenic progenitors that contributed to muscle repair in vivo. In summary, these studies reveal functionally conserved pathways regulating myogenesis across species and identify chemical compounds that expand mouse satellite cells and differentiate human iPSCs into engraftable muscle.

Angiotensin II promotes cardiac differentiation of embryonic stem cells via angiotensin type 1 receptor

As embryonic stem cells (ESCs) represent an attractive candidate cell source for obtaining cardiomyocytes to be used in cell replacement therapy, it is thus of considerable importance to understand the mechanism by which cardiac differentiation is regulated. In previous studies, we have shown that angiotensin type 1 receptor (AT1R) expressed in cardiomyocytes derived from mouse embryonic stem cells. However, little is known about the role of AT1R in cardiac differentiation, which plays a key role in cardiac physiology and pharmacology. In the present study, we demonstrated that AT1R agonist significantly enhanced cardiac differentiation as determined by increased percentage of beating embryoid bodies and a higher expression level of cardiac markers. On the contrary, AT1R agonist stimulated differentiation was reversed in the presence of AT1R antagonist. In addition, by administering selective inhibitors we found that the effect of AT1R was driven via extracellular-signal regulated kinase, c-Jun NH2-terminal kinase and p38 mitogen-activated protein kinase pathways. These findings suggest that AT1R signaling plays a key role in cardiac differentiation of ESCs.

A p38MAPK-p53 cascade regulates mesodermal differentiation and neurogenesis of embryonic stem cells

Embryonic stem cells (ESCs) differentiate in vivo and in vitro into all cell lineages, and they have been proposed as cellular therapy for human diseases. However, the molecular mechanisms controlling ESC commitment toward specific lineages need to be specified. We previously found that the p38 mitogen-activated protein kinase (p38MAPK) pathway inhibits neurogenesis and is necessary to mesodermal formation during the critical first 5 days of mouse ESC commitment. This period corresponds to the expression of specific master genes that direct ESC into each of the three embryonic layers. By both chemical and genetic approaches, we found now that, during this phase, the p38MAPK pathway stabilizes the p53 protein level and that interfering directly with p53 mimics the effects of p38MAPK inhibition on ESC differentiation. Anti-p53 siRNA transient transfections stimulate Bcl2 and Pax6 gene expressions, leading to increased ESC neurogenesis compared with control transfections. Conversely, p53 downregulation leads to a strong inhibition of the mesodermal master genes Brachyury and Mesp1 affecting cardiomyogenesis and skeletal myogenesis of ESCs. Similar results were found with p53^−/− ESCs compared with their wild-type counterparts. In addition, knockout p53 ESCs show impaired smooth muscle cell and adipocyte formation. Use of anti-Nanog siRNAs demonstrates that certain of these regulations result partially to p53-dependent repression of Nanog gene expression. In addition to its well-known role in DNA-damage response, apoptosis, cell cycle control and tumor suppression, p53 has also been involved in vivo in embryonic development; our results show now that p53 mediates, at least for a large part, the p38MAPK control of the early commitment of ESCs toward mesodermal and neural lineages.

Mitogen-activated protein kinase-activated protein kinases 2 and 3 regulate SERCA2a expression and fiber type composition to modulate skeletal muscle and cardiomyocyte function

The mitogen-activated protein kinase (MAPK)-activated protein kinases 2 and 3 (MK2/3) represent protein kinases downstream of the p38 MAPK. Using MK2/3 double-knockout (MK2/3(-/-)) mice, we analyzed the role of MK2/3 in cross-striated muscle by transcriptome and proteome analyses and by histology. We demonstrated enhanced expression of the slow oxidative skeletal muscle myofiber gene program, including the peroxisome proliferator-activated receptor gamma (PPARγ) coactivator 1α (PGC-1α). Using reporter gene and electrophoretic gel mobility shift assays, we demonstrated that MK2 catalytic activity directly regulated the promoters of the fast fiber-specific myosin heavy-chain IId/x and the slow fiber-specific sarco/endoplasmic reticulum Ca(2+)-ATPase 2 (SERCA2) gene. Elevated SERCA2a gene expression caused by a decreased ratio of transcription factor Egr-1 to Sp1 was associated with accelerated relaxation and enhanced contractility in MK2/3(-/-) cardiomyocytes, concomitant with improved force parameters in MK2/3(-/-) soleus muscle. These results link MK2/3 to the regulation of calcium dynamics and identify enzymatic activity of MK2/3 as a critical factor for modulating cross-striated muscle function by generating a unique muscle phenotype exhibiting both reduced fatigability and enhanced force in MK2/3(-/-) mice. Hence, the p38-MK2/3 axis may represent a novel target for the design of therapeutic strategies for diseases related to fiber type changes or impaired SERCA2 function.

Improved regenerative myogenesis and muscular dystrophy in mice lacking Mkp5

Duchenne muscular dystrophy (DMD) is a degenerative skeletal muscle disease caused by mutations in dystrophin. The degree of functional deterioration in muscle stem cells determines the severity of DMD. The mitogen-activated protein kinases (MAPKs), which are inactivated by MAPK phosphatases (MKPs), represent a central signaling node in the regulation of muscle stem cell function. Here we show that the dual-specificity protein phosphatase DUSP10/MKP-5 negatively regulates muscle stem cell function in mice. MKP-5 controlled JNK to coordinate muscle stem cell proliferation and p38 MAPK to control differentiation. Genetic loss of Mkp5 in mice improved regenerative myogenesis and dystrophin-deficient mdx mice lacking Mkp5 exhibited an attenuated dystrophic muscle phenotype. Hence, enhanced promyogenic MAPK activity preserved muscle stem cell function even in the absence of dystrophin and ultimately curtailed the pathogenesis associated with DMD. These results identify MKP-5 as an essential negative regulator of the promyogenic actions of the MAPKs and suggest that MKP-5 may serve as a target to promote muscle stem cell function in the treatment of degenerative skeletal muscle diseases.

Identification of Map4k4 as a novel suppressor of skeletal muscle differentiation

Myoblast differentiation into mature myotubes is a critical step in the development and repair of human skeletal muscle. Here we show that small interfering RNA (siRNA)-based silencing of the Ste20-like mitogen-activated protein 4 kinase 4 (Map4k4) in C2C12 myoblasts markedly enhances expression of myogenic differentiation genes, myoblast fusion, and myotube diameter. In contrast, adenovirus-mediated expression of native Map4k4 in C2C12 cells attenuates each of these processes, indicating that Map4k4 is a negative regulator of myogenic differentiation and hypertrophy. Expression of a Map4k4 kinase-inactive mutant enhances myotube formation, suggesting that the kinase activity of Map4k4 is essential for its inhibition of muscle differentiation. Map4k4 regulation of myogenesis is unlikely to be mediated by classic mitogen-activated protein kinase (MAPK) signaling pathways, because no significant difference in phosphorylation of extracellular signal-regulated kinase (ERK), p38, or c-Jun N-terminal kinase (JNK) is observed in Map4k4-silenced cells. Furthermore, silencing of these other MAPKs does not result in a hypertrophic myotube phenotype like that seen with Map4k4 depletion. Uniquely, Map4k4 silencing upregulates the expression of the myogenic regulatory factor Myf5, whose depletion inhibits myogenesis. Furthermore, Myf5 is required for enhancement of myotube formation in Map4k4-silenced cells, while Myf5 overexpression rescues Map4k4-mediated inhibition of myogenic differentiation. These results demonstrate that Map4k4 is a novel suppressor of skeletal muscle differentiation, acting through a Myf5-dependent mechanism.

Mammalian target of rapamycin (mTOR) signaling network in skeletal myogenesis

Mammalian (or mechanistic) target of rapamycin (mTOR) regulates a wide range of cellular and developmental processes by coordinating signaling responses to mitogens, nutrients, and various stresses. Over the last decade, mTOR has emerged as a master regulator of skeletal myogenesis, controlling multiple stages of the myofiber formation process. In this minireview, we present an emerging view of the signaling network underlying mTOR regulation of myogenesis, which contrasts with the well established mechanisms in the regulation of cell and muscle growth. Current questions for future studies are also highlighted.

Role of phosphatidylinositol 3,4,5-trisphosphate (PIP3) 5-phosphatase skeletal muscle- and kidney-enriched inositol polyphosphate phosphatase (SKIP) in myoblast differentiation

Insulin-like growth factors (IGFs) are essential for the development, regeneration, and hypertrophy of skeletal muscles. IGF-II promotes myoblast differentiation through phosphatidylinositol 3-kinase (PI 3-kinase), Akt, and mTOR signaling. Here, we report that skeletal muscle- and kidney-enriched inositol polyphosphate phosphatase (SKIP) negatively regulates myogenesis through inhibition of IGF-II production and attenuation of the IGF-II-Akt-mTOR signaling pathway. We also demonstrate that SKIP expression, which was markedly elevated during differentiation, was controlled by MyoD in C2C12 cells. Expression of SKIP inhibited IGF-II at the transcription level. These results indicate that SKIP regulates MyoD-mediated muscle differentiation. Silencing of SKIP increased IGF-II transcription and myoblast differentiation. Furthermore, knockdown of SKIP resulted in thick myotubes with a larger number of nuclei than that in control C2C12 cells. Taken together, these data indicate that SKIP controls the IGF-II-PI 3-kinase-Akt-mTOR auto-regulation loop during myogenesis. Our findings identify SKIP as a key regulator of muscle cell differentiation.

p38γ Mitogen-activated protein kinase signals through phosphorylating its phosphatase PTPH1 in regulating ras protein oncogenesis and stress response

Phosphatase plays a crucial role in determining cellular fate by inactivating its substrate kinase, but it is not known whether a kinase can vice versa phosphorylate its phosphatase to execute this function. Protein-tyrosine phosphatase H1 (PTPH1) is a specific phosphatase of p38γ mitogen-activated protein kinase (MAPK) through PDZ binding, and here, we show that p38γ is also a PTPH1 kinase through which it executes its oncogenic activity and regulates stress response. PTPH1 was identified as a substrate of p38γ by unbiased proteomic analysis, and its resultant phosphorylation at Ser-459 occurs in vitro and in vivo through their complex formation. Genetic and pharmacological analyses showed further that Ser-459 phosphorylation is directly regulated by Ras signaling and is important for Ras, p38γ, and PTPH1 oncogenic activity. Moreover, experiments with physiological stimuli revealed a novel stress pathway from p38γ to PTPH1/Ser-459 phosphorylation in regulating cell growth and cell death by a mechanism dependent on cellular environments but independent of canonical MAPK activities. These results thus reveal a new mechanism by which a MAPK regulates Ras oncogenesis and stress response through directly phosphorylating its phosphatase.

Targeted functional analysis of p300 coactivator in Wnt/β-catenin signaling pathway using phosphoproteomic and biochemical approaches

Both p300 and β-catenin are transcriptional activators and phosphoproteins, and play a central role in Wnt/β-catenin-dependent transcriptional regulation. The minimum β-catenin binding domain of p300 has been mapped to the N-terminus 1-111 amino acids. Here, we performed phosphoproteomic analysis for the critical binding region using LC-MS/MS approach to investigate potential phosphosites that may affect the binding affinity. By implementing TiO(2)-based phosphopeptide affinity purification followed by LC-MS/MS analysis with both collision-induced dissociation (CID) and electron transfer dissociation (ETD) methods, two unique phosphosites Ser12 and Ser89 were identified, of which, phosphorylation at Ser12 is novel. Functional studies aided by site-directed mutagenesis, co-immunoprecipitation and mammalian two-hybrid assay have concluded that phosphorylation at Ser12 critically mediates the binding ability of p300 with β-catenin. Further studies utilizing specific MAPK inhibitors suggest that the p38 MAPK activation is the upstream signal required for Ser12 phosphorylation. The transcriptional roles of p300/β-catenin complex in myoblast differentiation are discussed.

TGF-β-activated kinase 1 (TAK1) and apoptosis signal-regulating kinase 1 (ASK1) interact with the promyogenic receptor Cdo to promote myogenic differentiation via activation of p38MAPK pathway

p38MAPK plays an essential role in the transition of myoblasts to differentiated myotubes through the activation of MyoD family transcription factors. A promyogenic cell surface molecule, Cdo, promotes myogenic differentiation mainly through activation of the p38MAPK pathway. Two MAP3Ks, TAK1 and ASK1, can activate p38MAPK via MKK6 in various cell systems. Moreover TAK1 has been shown to promote myogenic differentiation via p38MAPK activation. In this study, we hypothesized that TAK1 and ASK1 might function as MAP3Ks in Cdo-mediated p38MAPK activation during myoblast differentiation. Both ASK1 and TAK1 were expressed in myoblasts and interacted with the cytoplasmic tail of Cdo and a scaffold protein, JLP. The depletion of TAK1 or ASK1 in C2C12 cells decreased myoblast differentiation, whereas overexpression of TAK1 or ASK1 in C2C12 cells enhanced myotube formation. In agreement with this, overexpression of ASK1 or TAK1 resulted in enhanced p38MAPK activation, and their knockdown inhibited p38MAPK in C2C12 cells. Overexpression of TAK1 or ASK1 in Cdo(-/-) myoblasts and Cdo-depleted C2C12 cells restored p38MAPK activation as well as myotube formation. Furthermore, ASK1 and TAK1 compensated for each other in p38MAPK activation and myoblast differentiation. Taken together, these findings suggest that ASK1 and TAK1 function as MAP3Ks in Cdo-mediated p38MAPK activation to promote myogenic differentiation.

Urothelial tumor initiation requires deregulation of multiple signaling pathways: implications in target-based therapies

Although formation of urothelial carcinoma of the bladder (UCB) requires multiple steps and proceeds along divergent pathways, the underlying genetic and molecular determinants for each step and pathway remain undefined. By developing transgenic mice expressing single or combinatorial genetic alterations in urothelium, we demonstrated here that overcoming oncogene-induced compensatory tumor barriers was critical for urothelial tumor initiation. Constitutively active Ha-ras (Ras*) elicited urothelial hyperplasia that was persistent and did not progress to tumors over a 10 months period. This resistance to tumorigenesis coincided with increased expression of p53 and all pRb family proteins. Expression of a Simian virus 40 T antigen (SV40T), which disables p53 and pRb family proteins, in urothelial cells expressing Ras* triggered early-onset, rapidly-growing and high-grade papillary UCB that strongly resembled the human counterpart (pTaG3). Urothelial cells expressing both Ras* and SV40T had defective G(1)/S checkpoint, elevated Ras-GTPase and hyperactivated AKT-mTOR signaling. Inhibition of the AKT-mTOR pathway with rapamycin significantly reduced the size of high-grade papillary UCB but hyperactivated mitogen-activated protein kinase (MAPK). Inhibition of AKT-mTOR, MAPK and STAT3 altogether resulted in much greater tumor reduction and longer survival than did inhibition of AKT-mTOR pathway alone. Our studies provide the first experimental evidence delineating the combinatorial genetic events required for initiating high-grade papillary UCB, a poorly defined and highly challenging clinical entity. Furthermore, they suggest that targeted therapy using a single agent such as rapamycin may not be highly effective in controlling high-grade UCB and that combination therapy employing inhibitors against multiple targets are more likely to achieve desirable therapeutic outcomes.

New Insights into the p38γ and p38δ MAPK Pathways

The mammalian p38 mitogen-activated protein kinases (MAPKs) family is composed of four members (p38α, p38β, p38γ, and p38δ), which are very similar in amino acid sequence but differ in their expression patterns. This suggests that they may have specific functions in different organs. In the last years most of the effort has been centred on the study of the function of the p38α isoform, which is widely referred to as p38 in the literature. However, the role that other p38 isoforms play in cellular functions and their implication in some of the pathological conditions have not been precisely defined so far. In this paper we highlight recent advances made in defining the functions of the two less studied alternative p38MAPKs, p38γ and p38δ. We describe that these p38MAPKs show similarities to the classical p38α isoform, although they may play central and distinct role in certain physiological and pathological processes.

Satellite cell count, VO(2max) , and p38 MAPK in inactive to moderately active young men

Satellite cells (SCs) are responsible for muscle repair following strenuous exercise or injury. SC responses to intervention have been studied, but most studies do not discuss or take into account the substantial variability in SC number among young individuals. We hypothesized that an active lifestyle reflected in higher VO(2max) may be associated with greater SC number. As training alters basal p38-mitogen-activated protein kinase (MAPK) activity, which is associated with SC proliferation, SC count may also correlate with this stress signaling kinase. Muscle biopsies from vastus lateralis of eight male participants were analyzed for fiber type, myogenin, and p38/phospho-p38 MAPK using SDS-PAGE and Western blotting. Immunofluorescence was used to detect Pax7(+) SCs. Two weeks following the biopsy, subjects underwent an incremental treadmill test to determine VO(2max) . A strong positive correlation (P = 0.0087) was found between the number of Pax7(+) nuclei and VO(2max) . Pax7(+) cell number correlated negatively with phospho-p38/p38 MAPK (P = 0.0006), but had no correlation with fiber type or myogenin. SC number is proportional to VO(2max) , and hence it can be postulated that higher levels of physical activity activate SC proliferation but not fusion, underlining the relevance of exercise in stimulating SC pool size even without injury.

The myogenic kinome: protein kinases critical to mammalian skeletal myogenesis

Myogenesis is a complex and tightly regulated process, the end result of which is the formation of a multinucleated myofibre with contractile capability. Typically, this process is described as being regulated by a coordinated transcriptional hierarchy. However, like any cellular process, myogenesis is also controlled by members of the protein kinase family, which transmit and execute signals initiated by promyogenic stimuli. In this review, we describe the various kinases involved in mammalian skeletal myogenesis: which step of myogenesis a particular kinase regulates, how it is activated (if known) and what its downstream effects are. We present a scheme of protein kinase activity, similar to that which exists for the myogenic transcription factors, to better clarify the complex signalling that underlies muscle development.

p38γ promotes breast cancer cell motility and metastasis through regulation of RhoC GTPase, cytoskeletal architecture, and a novel leading edge behavior

Understanding the molecular alterations that confer cancer cells with motile, metastatic properties is needed to improve patient survival. Here, we report that p38γ motogen-activated protein kinase regulates breast cancer cell motility and metastasis, in part, by controlling expression of the metastasis-associated small GTPase RhoC. This p38γ-RhoC regulatory connection was mediated by a novel mechanism of modulating RhoC ubiquitination. This relationship persisted across multiple cell lines and in clinical breast cancer specimens. Using a computational mechanical model based on the finite element method, we showed that p38γ-mediated cytoskeletal changes are sufficient to control cell motility. This model predicted novel dynamics of leading edge actin protrusions, which were experimentally verified and established to be closely related to cell shape and cytoskeletal morphology. Clinical relevance was supported by evidence that elevated expression of p38γ is associated with lower overall survival of patients with breast cancer. Taken together, our results offer a detailed characterization of how p38γ contributes to breast cancer progression. Herein we present a new mechanics-based analysis of cell motility, and report on the discovery of a leading edge behavior in motile cells to accommodate modified cytoskeletal architecture. In summary, these findings not only identify a novel mechanism for regulating RhoC expression but also advance p38γ as a candidate therapeutic target.

Raptor and Rheb negatively regulate skeletal myogenesis through suppression of insulin receptor substrate 1 (IRS1)

The mammalian target of rapamycin (mTOR) is essential for skeletal myogenesis through controlling distinct cellular pathways. The importance of the canonical mTOR complex 1 signaling components, including raptor, S6K1, and Rheb, had been suggested in muscle maintenance, growth, and metabolism. However, the role of those components in myogenic differentiation is not entirely clear. In this study we have investigated the functions of raptor, S6K1, and Rheb in the differentiation of C2C12 mouse myoblasts. We find that although mTOR knockdown severely impairs myogenic differentiation as expected, the knockdown of raptor, as well as Rheb, enhances differentiation. Consistent with a negative role for these proteins in myogenesis, overexpression of raptor or Rheb inhibits C2C12 differentiation. On the other hand, neither knockdown nor overexpression of S6K1 has any effect. Moreover, the enhanced differentiation elicited by raptor or Rheb knockdown is accompanied by increased Akt activation, elevated IRS1 protein levels, and decreased Ser-307 (human Ser-312) phosphorylation on IRS1. Finally, IRS1 knockdown eliminated the enhancement in differentiation elicited by raptor or Rheb knockdown, suggesting that IRS1 is a critical mediator of the myogenic functions of raptor and Rheb. In conclusion, the Rheb-mTOR/raptor pathway negatively regulates myogenic differentiation by suppressing IRS1-PI3K-Akt signaling. These findings underscore the versatility of mTOR signaling in biological regulations and implicate the existence of novel mTOR complexes and/or signaling mechanism in skeletal myogenesis.

mTOR and the differentiation of mesenchymal stem cells

The mammalian target of rapamycin (mTOR), an evolutionarily conserved serine-threonine protein kinase, belongs to the phosphoinositide 3-kinase (PI3K)-related kinase family, which contains a lipid kinase-like domain within their C-terminal region. Recent studies have revealed that mTOR as a critical intracellular molecule can sense the extracellular energy status and regulate the cell growth and proliferation in a variety of cells and tissues. This review summarizes our current understanding about the effects of mTOR on cell differentiation and tissue development, with an emphasis on the lineage determination of mesenchymal stem cells. mTOR can promote adipogenesis in white adipocytes, brown adipocytes, and muscle satellite cells, while rapamycin inhibits the adipogenic function of mTOR. mTOR signaling may function to affect osteoblast proliferation and differentiation, however, rapamycin has been reported to either inhibit or promote osteogenesis. Although the precise mechanism remains unclear, mTOR is indispensable for myogenesis. Depending on the cell type, rapamycin has been reported to inhibit, promote, or have no effect on myogenesis.

Low-molecular-weight fucoidan regulates myogenic differentiation through the mitogen-activated protein kinase pathway in C2C12 cells

Low-molecular-weight fucoidan (LMWF) has been broadly studied in recent years due to its numerous biological properties. Nevertheless, there have been no reports about the effects of LMWF on myogenic differentiation (MyoD). The objective of the present study was to demonstrate the impact of LMWF on myogenesis in C2C12 cells. The ultimate aim was to establish whether LMWF regulates myogenesis similar to heparin as a partial regulator of myogenesis. LMWF was prepared at a minimal size by ultra-filtration compared with crude fucoidan. We treated C2C12 cells with LMWF and/or heparin during MyoD. The data from the present study are the first to suggest that LMWF suppresses the expression of the myogenic regulatory factors and the myocyte enhancer factors as well as the morphological changes that occur during differentiation. Additionally, the expression of the mitogen-activated protein kinase (MAPK) family was significantly inhibited by LMWF when compared with controls. The LMWF-treated group showed significantly decreased expression of reactive oxygen species (ROS) enzymes compared with control cells. Heparin was used as a positive control because it has been reported to activate MyoD. Taken together, these results suggest that LMWF might regulate MyoD through the MAPK pathway and by regulating ROS activity in C2C12 cells.

Attenuation of CHOP-mediated myocardial apoptosis in pressure-overloaded dominant negative p38α mitogen-activated protein kinase mice

BACKGROUND/AIMS

Pressure overload stimulation is known to elicit disturbances in the endoplasmic reticulum (ER), which leads to ER stress (ERS). p38 mitogen-activated protein kinase (MAPK) plays an important role in mediating apoptotic processes, however, the roles of this kinase in activating ERS-initiated apoptosis in pressure-overloaded hearts are largely unknown.

METHODS

We clarified the role of p38α MAPK in ERS-associated apoptosis by subjecting transgenic mice displaying cardiac specific dominant negative (DN) mutant p38α MAPK over-expression to seven day pressure overload.

RESULTS

Seven days pressure overload resulted in the same extent of cardiac hypertrophy and ERS in the wild-type (WT) and DN p38α mice compared with the sham mice. It also activated inositol-requiring enzyme (Ire)-1α and its downstream molecule, tumor necrosis factor receptor (TNFR)-associated factor (TRAF)2 in the WT and DN p38α mice compared with the sham mice. Interestingly, increased myocardial apoptosis and the up-regulation of CCAAT/enhancer binding protein homology protein (CHOP) expression compared with those in the sham mice were found in the aortic-banded WT mice, but not in the DN p38α mice.

CONCLUSION

Partial inhibition of p38α protein blocked the activation of CHOP-mediated apoptotic processes during pressure overload by partially inhibiting signaling from the Ire-1α/TRAF2 to its down-stream molecule, CHOP.

Phospholipase D regulates myogenic differentiation through the activation of both mTORC1 and mTORC2 complexes

How phospholipase D (PLD) is involved in myogenesis remains unclear. At the onset of myogenic differentiation of L6 cells induced by the PLD agonist vasopressin in the absence of serum, mTORC1 complex was rapidly activated, as reflected by phosphorylation of S6 kinase1 (S6K1). Both the long (p85) and short (p70) S6K1 isoforms were phosphorylated in a PLD1-dependent way. Short rapamycin treatment specifically inhibiting mTORC1 suppressed p70 but not p85 phosphorylation, suggesting that p85 might be directly activated by phosphatidic acid. Vasopressin stimulation also induced phosphorylation of Akt on Ser-473 through PLD1-dependent activation of mTORC2 complex. In this model of myogenesis, mTORC2 had a positive role mostly unrelated to Akt activation, whereas mTORC1 had a negative role, associated with S6K1-induced Rictor phosphorylation. The PLD requirement for differentiation can thus be attributed to its ability to trigger via mTORC2 activation the phosphorylation of an effector that could be PKCα. Moreover, PLD is involved in a counter-regulation loop expected to limit the response. This study thus brings new insights in the intricate way PLD and mTOR cooperate to control myogenesis.

Apoptosis signal-regulating kinase 1 is an intracellular inducer of p38 MAPK-mediated myogenic signalling in cardiac myoblasts

Myogenic differentiation is an essential process for the myogenesis in response to various extracellular stimuli. p38 MAPK is a core signalling molecule in myogenic differentiation. The activation of p38 MAPK is required for myogenic differentiation; however, the mechanism for this activation remains undefined. ASK1 is a member of the MAP3K family that activates both JNK and p38 MAPK pathways in response to an array of stresses such as oxidative stress, endoplasmic reticulum stress and calcium influx. Here, we reported that TNFα was significantly released from H9c2 cardiac myoblast in differentiation medium. Furthermore, the oxidant H(2)O(2) acted as a messenger in the TNFα signalling pathway to disrupt the complex of ASK1-Trx, which was followed by the activation of ASK1 in cardiac myogenic differentiation. Subsequently, the activated ASK1 stimulated MKK3/6-p38MAPK signalling cascade to induce specific myogenic differentiation. In addition, exogenous TNFα added to the medium at physiological levels enhanced the ASK1-p38 MAPK signalling pathway through the increased generation of H(2)O(2). Interestingly, inhibition of p38 MAPK abrogated the production of H(2)O(2), suggesting that there might be a positive feedback loop in the myogenic-redox signalling pathway. These results indicate that ASK1 is a new intracellular regulator of activation of the p38 MAPK in cardiac myogenic differentiation.

Secretome Analysis of Skeletal Myogenesis Using SILAC and Shotgun Proteomics

Myogenesis, the formation of skeletal muscle, is a multistep event that commences with myoblast proliferation, followed by cell-cycle arrest, and finally the formation of multinucleated myotubes via fusion of mononucleated myoblasts. Each step is orchestrated by well-documented intracellular factors, such as cytoplasmic signalling molecules and nuclear transcription factors. Regardless, the key step in getting a more comprehensive understanding of the regulation of myogenesis is to explore the extracellular factors that are capable of eliciting the downstream intracellular factors. This could further provide valuable insight into the acute cellular response to extrinsic cues in maintaining normal muscle development. In this paper, we survey the intracellular factors that respond to extracellular cues that are responsible for the cascades of events during myogenesis: myoblast proliferation, cell-cycle arrest of myoblasts, and differentiation of myoblasts into myotubes. This focus on extracellular perspective of muscle development illustrates our mass spectrometry-based proteomic approaches to identify differentially expressed secreted factors during skeletal myogenesis.

JAK/STAT signaling and human in vitro myogenesis

BACKGROUND

A population of satellite cells exists in skeletal muscle. These cells are thought to be primarily responsible for postnatal muscle growth and injury-induced muscle regeneration. The Janus kinase/signal transducers and activators of transcription (JAK/STAT) signaling cascade has a crucial role in regulating myogenesis. In rodent skeletal muscle, STAT3 is essential for satellite cell migration and myogenic differentiation, regulating the expression of myogenic factors. The aim of the present study was to investigate and compare the expression profile of JAK/STAT family members, using cultured primary human skeletal muscle cells.

RESULTS

Near confluent proliferating myoblasts were induced to differentiate for 1, 5 or 10 days. During these developmental stages, members of the JAK/STAT family were examined, along with factors known to regulate myogenesis. We demonstrate the phosphorylation of JAK1 and STAT1 only during myoblast proliferation, while JAK2 and STAT3 phosphorylation increases during differentiation. These increases were correlated with the upregulation of genes associated with muscle maturation and hypertrophy.

CONCLUSIONS

Taken together, these results provide insight into JAK/STAT signaling in human skeletal muscle development, and confirm recent observations in rodents.

Early response to ErbB2 over-expression in polarized Caco-2 cells involves partial segregation from ErbB3 by relocalization to the apical surface and initiation of survival signaling

In several human cancers, ErbB2 over-expression facilitates the formation of constitutively active homodimers resistant to internalization which results in progressive signal amplification from the receptor, conducive to cell survival, proliferation, or metastasis. Here we report on studies of the influence of ErbB2 over-expression on localization and signaling in polarized Caco-2 and MDCK cells, two established models to study molecular trafficking. In these cells, ErbB2 is not over-expressed and shares basolateral localization with ErbB3. Over-expression of ErbB2 by transient transfection resulted in partial separation of the receptors by relocalization of ErbB2, but not ErbB3, to the apical surface, as shown by biotinylation of the apical or basolateral surfaces. These results were confirmed by immunofluorescence and confocal microscopy. Polarity controls indicated that the relocalization of ErbB2 is not the result of depolarization of the cells. Biotinylation and confocal microscopy also showed that apical, but not basolateral ErbB2 is activated at tyrosine 1139. This phosphotyrosine binds adaptor protein Grb2, as confirmed by immunoprecipitation. However, we found that it does not initiate the canonical Grb2-Ras-Raf-Erk pathway. Instead, our data supports the activation of a survival pathway via Bcl-2. The effects of ErbB2 over-expression were abrogated by the humanized anti-ErbB2 monoclonal antibody Herceptin added only from the apical side. The ability of apical ErbB2 to initiate an altered downstream cascade suggests that subcellular localization of the receptor plays an important role in regulating ErbB2 signaling in polarized epithelia.

Myogenesis and rhabdomyosarcoma the Jekyll and Hyde of skeletal muscle

Rhabdomyosarcoma, a neoplasm composed of skeletal myoblast-like cells, represents the most common soft tissue sarcoma in children. The application of intensive chemotherapeutics and refined surgical and radiation therapy approaches have improved survival for children with localized disease over the past 3 decades; however, these approaches have not improved the dismal outcome for children with metastatic and recurrent rhabdomyosarcoma. Elegant studies have defined the molecular mechanisms driving skeletal muscle lineage commitment and differentiation, and the machinery that couples differentiation with irreversible cell proliferation arrest. Further, detailed molecular analyses indicate that rhabdomyosarcoma cells have lost the capacity to fully differentiate when challenged to do so in experimental models. We review the intersection of normal skeletal muscle developmental biology and the molecular genetic defects in rhabdomyosarcoma with the underlying premise that understanding how the differentiation process has gone awry will lead to new treatment strategies aimed at promoting myogenic differentiation and concomitant cell cycle arrest.

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.

TIMP3: a physiological regulator of adult myogenesis

Myogenic differentiation in adult muscle is normally suppressed and can be activated by myogenic cues in a subset of activated satellite cells. The switch mechanism that turns myogenesis on and off is not defined. In the present study, we demonstrate that tissue inhibitor of metalloproteinase 3 (TIMP3), the endogenous inhibitor of TNFalpha-converting enzyme (TACE), acts as an on-off switch for myogenic differentiation by regulating autocrine TNFalpha release. We observed that constitutively expressed TIMP3 is transiently downregulated in the satellite cells of regenerating mouse hindlimb muscles and differentiating C2C12 myoblasts. In C2C12 myoblasts, perturbing TIMP3 downregulation by overexpressing TIMP3 blocks TNFalpha release, p38 MAPK activation, myogenic gene expression and myotube formation. TNFalpha supplementation at a physiological concentration rescues myoblast differentiation. Similarly, in the regenerating soleus, overexpression of TIMP3 impairs release of TNFalpha and myogenic gene expression, and delays the formation of new fibers. In addition, downregulation of TIMP3 is mediated by the myogenesis-promoting microRNA miR-206. Thus, TIMP3 is a physiological regulator of myogenic differentiation.

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.

Abl promotes cadherin-dependent adhesion and signaling in myoblasts

Cell-cell contact promotes myogenic differentiation but the mechanisms that regulate this phenomenon are not well understood. Cdo (also known as Cdon), an Ig superfamily member, functions as a component of cell surface complexes to promote myogenic differentiation through activation of p38alpha/beta MAP kinase. We recently showed that N-cadherin ligation activated p38alpha/beta in a Cdo-dependent manner, whereas N-cadherin ligation-dependent activation of ERK MAP kinase was not affected by loss of Cdo. The non-receptor tyrosine kinase Abl associates with Cdo during myoblast differentiation and is necessary for full activition of p38alpha/beta during this process. The Abl SH3 domain binds to a PxxP motif in the Cdo intracellular domain, and both these motifs are required for their promyogenic activity. Here we show that Abl is necessary for p38alpha/beta activation initiated by N-cadherin ligation, but in contrast to Cdo, Abl is also required for N-cadherin-dependent ERK activation. Moreover, Abl is required for efficient cadherin-mediated myoblast aggregation via modulation of RhoA-ROCK signaling. Therefore, Abl regulates N-cadherin-mediated p38alpha/beta activation by multiple mechanisms, more generally through regulation of cell-cell adhesion and specifically as a component of Cdo-containing complexes. The role of Cdo as a multifunctional coreceptor with roles in several pathways is also discussed.

Mammalian target of rapamycin regulates miRNA-1 and follistatin in skeletal myogenesis

Mammalian target of rapamycin (mTOR) has emerged as a key regulator of skeletal muscle development by governing distinct stages of myogenesis, but the molecular pathways downstream of mTOR are not fully understood. In this study, we report that expression of the muscle-specific micro-RNA (miRNA) miR-1 is regulated by mTOR both in differentiating myoblasts and in mouse regenerating skeletal muscle. We have found that mTOR controls MyoD-dependent transcription of miR-1 through its upstream enhancer, most likely by regulating MyoD protein stability. Moreover, a functional pathway downstream of mTOR and miR-1 is delineated, in which miR-1 suppression of histone deacetylase 4 (HDAC4) results in production of follistatin and subsequent myocyte fusion. Collective evidence strongly suggests that follistatin is the long-sought mTOR-regulated fusion factor. In summary, our findings unravel for the first time a link between mTOR and miRNA biogenesis and identify an mTOR-miR-1-HDAC4-follistatin pathway that regulates myocyte fusion during myoblast differentiation in vitro and skeletal muscle regeneration in vivo.

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.

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.

TGF-beta receptors, in a Smad-independent manner, are required for terminal skeletal muscle differentiation

Skeletal muscle differentiation is strongly inhibited by transforming growth factor type beta (TGF-beta), although muscle formation as well as regeneration normally occurs in an environment rich in this growth factor. In this study, we evaluated the role of intracellular regulatory Smads proteins as well as TGF-beta-receptors (TGF-beta-Rs) during skeletal muscle differentiation. We found a decrease of TGF-beta signaling during differentiation. This phenomenon is explained by a decline in the levels of the regulatory proteins Smad-2, -3, and -4, a decrease in the phosphorylation of Smad-2 and lost of nuclear translocation of Smad-3 and -4 in response to TGF-beta. No change in the levels and inhibitory function of Smad-7 was observed. In contrast, we found that TGF-beta-R type I (TGF-beta-RI) and type II (TGF-beta-RII) increased on the cell surface during skeletal muscle differentiation. To analyze the direct role of the serine/threonine kinase activities of TGF-beta-Rs, we used the specific inhibitor SB 431542 and the dominant-negative form of TGF-beta-RII lacking the cytoplasmic domain. The TGF-beta-Rs were important for successful muscle formation, determined by the induction of myogenin, creatine kinase activity, and myosin. Silencing of Smad-2/3 expression by specific siRNA treatments accelerated myogenin, myosin expression, and myotube formation; although when SB 431542 was present inhibition in myosin induction and myotube formation was observed, suggesting that these last steps of skeletal muscle differentiation require active TGF-beta-Rs. These results suggest that both down-regulation of Smad regulatory proteins and cell signaling through the TGF-beta receptors independent of Smad proteins are essential for skeletal muscle differentiation.

MAP kinase phosphatase-1 deficiency impairs skeletal muscle regeneration and exacerbates muscular dystrophy

In skeletal muscle, the mitogen-activated protein kinase (MAPK) phosphatase-1 (MKP-1) is a critical negative regulator of the MAPKs. Since the MAPKs have been reported to be both positive and negative for myogenesis, the physiological role of MKP-1 in skeletal muscle repair and regeneration has remained unclear. Here, we show that MKP-1 plays an essential role in adult regenerative myogenesis. In a cardiotoxin-induced muscle injury model, lack of MKP-1 impaired muscle regeneration. In mdx mice, MKP-1 deficiency reduced body weight, muscle mass, and muscle fiber cross-sectional area. In addition, MKP-1-deficient muscles exhibit exacerbated myopathy accompanied by increased inflammation. Lack of MKP-1 compromised myoblast proliferation and induced precocious differentiation, phenotypes that were rescued by pharmacological inhibition of p38alpha/beta MAPK. MKP-1 coordinates both myoblast proliferation and differentiation. Mechanistically, MyoD bound to the MKP-1 promoter and activated MKP-1 expression in proliferating myoblasts. Later, during myogenesis, MyoD uncoupled from the MKP-1 promoter leading to the down-regulation of MKP-1 and facilitation of promyogenic p38alpha/beta MAPK signaling. Hence, MKP-1 plays a critical role in muscle stem cells and in the immune response to coordinate muscle repair and regeneration.

PTPH1 dephosphorylates and cooperates with p38gamma MAPK to increase ras oncogenesis through PDZ-mediated interaction

Protein phosphatases are believed to coordinate with kinases to execute biological functions, but examples of such integrated activities, however, are still missing. In this report, we have identified protein tyrosine phosphatase H1 (PTPH1) as a specific phosphatase for p38gamma mitogen-activated protein kinase (MAPK) and shown their cooperative oncogenic activity through direct binding. p38gamma, a Ras effector known to act independent of its phosphorylation, was first shown to require its unique PDZ-binding motif to increase Ras transformation. Yeast two-hybrid screening and in vitro and in vivo analyses further identified PTPH1 as a specific p38gamma phosphatase through PDZ-mediated binding. Additional experiments showed that PTPH1 itself plays a role in Ras-dependent malignant growth in vitro and/or in mice by a mechanism depending on its p38gamma-binding activity. Moreover, Ras increases both p38gamma and PTPH1 protein expression and there is a coupling of increased p38gamma and PTPH1 protein expression in primary colon cancer tissues. These results reveal a coordinative oncogenic activity of a MAPK with its specific phosphatase and suggest that PDZ-mediated p38gamma/PTPH1 complex may be a novel target for Ras-dependent malignancies.

Hypoxia converts the myogenic action of insulin-like growth factors into mitogenic action by differentially regulating multiple signaling pathways

Insulin-like growth factors (IGFs) stimulate myoblast proliferation and differentiation. It remains elusive how these mutually exclusive cellular responses are elicited by the same growth factor. Here we report that whereas IGF promotes myoblast differentiation under normoxia, it stimulates proliferation under hypoxia. Hypoxia activates the HIF-1 transcriptional program and knockdown of HIF-1alpha changes the mitogenic action of IGF into myogenic action under hypoxia. Conversely, overexpression of HIF-1alpha abolishes the myogenic effect of IGF under normoxia. Under normoxia, IGF activates the Akt-mTOR, p38, and Erk1/2 MAPK pathways. Hypoxia suppresses basal and IGF-induced Akt-mTOR and p38 activity, whereas it enhances and prolongs IGF-induced Erk1/2 activation in a HIF-1-dependent fashion. Activation of Akt-mTOR and p38 promotes myogenesis, and p38 also inhibits proliferation. Activation of Erk stimulates myoblast proliferation but inhibits differentiation. These results suggest that hypoxia converts the myogenic action of IGFs into mitogenic action by differentially regulating multiple signaling pathways via HIF-1-dependent mechanisms. Our findings provide a mechanistic explanation for the paradoxical actions of IGFs during myogenesis and reveal a novel mechanism by which cells sense and integrate growth factor signals and oxygen availability in their microenvironments.

Yap is a novel regulator of C2C12 myogenesis

The expression, regulation and function of mammalian Hippo pathway members in skeletal muscle is largely unknown. The aim of this study was thus to test the hypothesis that core members of the mammalian Hippo pathway are expressed in skeletal muscle and that the transcriptional co-factor Yap, a core member of the Hippo pathway, regulates C2C12 myogenesis. We found that the major components of the mammalian Hippo pathway including Yap are all expressed in skeletal muscles, C2C12 myoblasts and myotubes. In C2C12 myoblasts, Yap Ser127 phosphorylation is low and Yap localises to nuclei. Upon differentiation, Yap Ser127 phosphorylation increases approximately 20-fold and Yap translocates from the nucleus to the cytosol. To test whether the observed increase of Yap Ser127 phosphorylation is required for differentiation we overexpressed hYAP1 S127A, a mutant that can not be phosphorylated at Ser127, in C2C12 myoblasts. We found that overexpression of hYAP S127A prevented myotube formation, whereas the overexpression of wildtype hYAP1 or empty vector had no effect. In addition, more hYAP1 S127A overexpressing cells progressed through the S phase of the cell cycle and the expression of MRFs (myogenin, Myf5), Mef2c and cell cycle regulators (p21, cyclin D1) was significantly changed when compared to wildtype hYAP1 and empty vector overexpressing cells. This data suggests that the phosphorylation of Yap at Ser127 leads to a changed expression of MRFs and cell cycle regulators and is required for C2C12 myoblasts to differentiate into myotubes.

The role of p38alpha mitogen-activated protein kinase gene in the HELLP syndrome

Mitogen-activated protein kinase (MAPK) p38alpha was shown to be implicated in the organogenesis of the placenta, and such placental alteration is crucial for the development of hemolysis, elevated liver enzymes, and low platelets (HELLP) syndrome. We aimed to analyze for the first time human placental expression of MAPK p38alpha in pregnancies complicated by HELLP. The placental expression of MAPK p38alpha was investigated by semiquantitative polymerase chain reaction using cDNA extracted from placental tissue of 15 pregnancies with HELLP syndrome and 15 gestational age-matched controls. Seven patients with HELLP also had intrauterine fetal growth restriction (IUGR). In placenta from pregnancy complicated by HELLP, the expression of MAPK p38alpha is significantly decreased compared to the group with normal pregnancy (p < 0.001), while no difference was found between the HELLP and HELLP with IUGR subpopulations. Our study shows for the first time that MAPK p38alpha is expressed in the human placenta. Pregnancies with placental dysfunction and hypertensive complications are characterized by a significantly decreased expression of MAPK p38alpha. Our observations suggest that p38 MAPK signaling may be essential in placental angiogenesis and functioning.

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.

p38-{gamma}-dependent gene silencing restricts entry into the myogenic differentiation program

The regenerative capacity of muscle is regulated by p38-γ, which phosphorylates MyoD and leads to formation of a complex that represses myogenin transcription.

The mitogen-activated protein kinase p38-γ is highly expressed in skeletal muscle and is associated with the dystrophin glycoprotein complex; however, its function remains unclear. After induced damage, muscle in mice lacking p38-γ generated significantly fewer myofibers than wild-type muscle. Notably, p38-γ-deficient muscle contained 50% fewer satellite cells that exhibited premature Myogenin expression and markedly reduced proliferation. We determined that p38-γ directly phosphorylated MyoD on Ser199 and Ser200, which results in enhanced occupancy of MyoD on the promoter of myogenin together with markedly decreased transcriptional activity. This repression is associated with extensive methylation of histone H3K9 together with recruitment of the KMT1A methyltransferase to the myogenin promoter. Notably, a MyoD S199A/S200A mutant exhibits markedly reduced binding to KMT1A. Therefore, p38-γ signaling directly induces the assembly of a repressive MyoD transcriptional complex. Together, these results establish a hitherto unappreciated and essential role for p38-γ signaling in positively regulating the expansion of transient amplifying myogenic precursor cells during muscle growth and regeneration.

Expression of caspase family and muscle- and apoptosis-specific genes during skeletal myogenesis in mouse embryo

The caspases (Casps) are a family of cysteine proteases that are known to regulate apoptotic signaling. Apoptosis by activation of Casp is strongly associated with embryonal development and regeneration in many organs, therefore indicating that disorders caused by homozygous mutation in Casp genes can result in embryonic lethality. In the present study, the authors investigated the causative relationship between skeletal myogenesis and the activation of Casps by analyzing their dynamics during mouse embryogenesis. Individual myogenetic tissues were obtained from C57BL/6 mouse embryos aged 12.5-17.5 days post-conception (dpc), and the expression of Casps was analyzed by histochemical and molecular biological methods. Immunoreactions for Casp-3, -9 and -12 were detected first in myoblasts, increasing according to embryonal development, as a result of which myoblasts differentiated into myotube cells. On the other hand, the immunoreaction for ssDNA, which is well-known as an apoptosis marker, was little detected during the skeletal myogenesis. Quantification analysis for Casp mRNA expression by RT-PCR as well as by in situ hybridization showed a peak at 15.5 dpc but a decrease at 17.5 dpc. Similar dynamics were detected for Myod1 mRNA, one of the muscle regulatory factors, but not for Fasl, Bax and Rock1, apoptosis-associated factors during skeletal myogenesis. These results suggest that the activation of Casps in skeletal myogenesis is deeply associated with myoblast differentiation, but not directly related to apoptosis.

Glycogenome expression dynamics during mouse C2C12 myoblast differentiation suggests a sequential reorganization of membrane glycoconjugates

Background

Several global transcriptomic and proteomic approaches have been applied in order to obtain new molecular insights on skeletal myogenesis, but none has generated any specific data on glycogenome expression, and thus on the role of glycan structures in this process, despite the involvement of glycoconjugates in various biological events including differentiation and development. In the present study, a quantitative real-time RT-PCR technology was used to profile the dynamic expression of 375 glycogenes during the differentiation of C2C12 myoblasts into myotubes.

Results

Of the 276 genes expressed, 95 exhibited altered mRNA expression when C2C12 cells differentiated and 37 displayed more than 4-fold up- or down-regulations. Principal Component Analysis and Hierarchical Component Analysis of the expression dynamics identified three groups of coordinately and sequentially regulated genes. The first group included 12 down-regulated genes, the second group four genes with an expression peak at 24 h of differentiation, and the last 21 up-regulated genes. These genes mainly encode cell adhesion molecules and key enzymes involved in the biosynthesis of glycosaminoglycans and glycolipids (neolactoseries, lactoseries and ganglioseries), providing a clearer indication of how the plasma membrane and extracellular matrix may be modified prior to cell fusion. In particular, an increase in the quantity of ganglioside G_M3 at the cell surface of myoblasts is suggestive of its potential role during the initial steps of myogenic differentiation.

Conclusion

For the first time, these results provide a broad description of the expression dynamics of glycogenes during C2C12 differentiation. Among the 37 highly deregulated glycogenes, 29 had never been associated with myogenesis. Their biological functions suggest new roles for glycans in skeletal myogenesis.

mTOR regulates skeletal muscle regeneration in vivo through kinase-dependent and kinase-independent mechanisms

Rapamycin-sensitive signaling is required for skeletal muscle differentiation and remodeling. In cultured myoblasts, the mammalian target of rapamycin (mTOR) has been reported to regulate differentiation at different stages through distinct mechanisms, including one that is independent of mTOR kinase activity. However, the kinase-independent function of mTOR remains controversial, and no in vivo studies have examined those mTOR myogenic mechanisms previously identified in vitro. In this study, we find that rapamycin impairs injury-induced muscle regeneration. To validate the role of mTOR with genetic evidence and to probe the mechanism of mTOR function, we have generated and characterized transgenic mice expressing two mutants of mTOR under the control of human skeletal actin (HSA) promoter: rapamycin-resistant (RR) and RR/kinase-inactive (RR/KI). Our results show that muscle regeneration in rapamycin-administered mice is restored by RR-mTOR expression. In the RR/KI-mTOR mice, nascent myofiber formation during the early phase of regeneration proceeds in the presence of rapamycin, but growth of the regenerating myofibers is blocked by rapamycin. Igf2 mRNA levels increase drastically during early regeneration, which is sensitive to rapamycin in wild-type muscles but partially resistant to rapamycin in both RR- and RR/KI-mTOR muscles, consistent with mTOR regulation of Igf2 expression in a kinase-independent manner. Furthermore, systemic ablation of S6K1, a target of mTOR kinase, results in impaired muscle growth but normal nascent myofiber formation during regeneration. Therefore, mTOR regulates muscle regeneration through kinase-independent and kinase-dependent mechanisms at the stages of nascent myofiber formation and myofiber growth, respectively.