Phosphoinositide 3-kinase induces the transcriptional activity of MEF2 proteins during muscle differentiation

Tanshinone IIA promotes the proliferation and differentiation ability of primary muscle stem cells via MAPK and Akt signaling

Salvia miltiorrhiza Bunge is a widely-used traditional Chinese medicine to treat a variety of diseases including muscle disorders. The underlying pharmacological mechanisms of which active component and how it functions are still unknown. Tanshinone IIA (Tan IIA) is the main active lipophilic compound in Salvia miltiorrhiza Bunge. Muscle stem cells (MuSCs) play a crucial role in maintaining healthy physiological function of skeletal muscle. For the purpose of this study, we investigated the effects of Tan IIA on primary MuSCs as well as mechanism. The EdU staining, cell counts assay and RT-qPCR results of proliferative genes revealed increased proliferation ability of MuSCs after Tan IIA treatment. Immunofluorescent staining of MyHC and RT-qPCR results of myogenic genes found Tan IIA contributed to promoting differentiation of MuSCs. In addition, enrichment analysis of RNA-seq data and Western blot assay results demonstrated activated MAPK and Akt signaling after treatment of Tan IIA during proliferation and differentiation. The above proliferative and differentiative phonotypes could be suppressed by the combination of MAPK inhibitor U0126 and Akt inhibitor Akti 1/2, respectively. Furthermore, HE staining found significantly improved myofiber regeneration of injured muscle after Tan IIA treatment, which also contributed to muscle force and running performance recovery. Thus, Tan IIA could promote proliferation and differentiation ability of MuSCs through activating MAPK and Akt signaling, respectively. These beneficial effects also significantly contributed to muscle regeneration and muscle function recovery after muscle injury.

Signaling pathways of adipose stem cell-derived exosomes promoting muscle regeneration

ABSTRACT

Severe muscle injury is still a challenging clinical problem. Exosomes derived from adipose stem cells (ASC-exos) may be a potential therapeutic tool, but their mechanism is not completely clear. This review aims to elaborate the possible mechanism of ASC-exos in muscle regeneration from the perspective of signal pathways and provide guidance for further study. Literature cited in this review was acquired through PubMed using keywords or medical subject headings, including adipose stem cells, exosomes, muscle regeneration, myogenic differentiation, myogenesis, wingless/integrated (Wnt), mitogen-activated protein kinases, phosphatidylinositol-4,5-bisphosphate 3-kinase/protein kinase B (PI3K/Akt), Janus kinase/signal transducers and activators of transcription, and their combinations. We obtained the related signal pathways from proteomics analysis of ASC-exos in the literature, and identified that ASC-exos make different contributions to multiple stages of skeletal muscle regeneration by those signal pathways.

Betalain exerts cardioprotective and anti-inflammatory effects against the experimental model of heart failure

Betalain is a natural plant pigment known to elicit various biological activities. However, studies on the protective effect of betalain against heart failure have not reported yet. The experimental model of heart failure was created in Wistar rats using isoproterenol (ISO). The animals were randomly assigned into four groups such as sham-control, ISO-induced heart failure, betalain pretreated before ISO induction (50 mg/kg/day), and betalain drug control group were maintained for 6 weeks. At the end of the experimental period, anti-oxidant enzymes, inflammatory markers, matrix proteins, cardiac-specific markers, and micro RNAs were elucidated using RT-PCR, and ELISA analysis. The results demonstrated that the rats induced with ISO displayed an abnormality in cardiac functions, increased oxidative stress markers (p < 0.01), inflammatory cytokines (p < 0.01) while abrogated the expression of miR-18a, and increased miR-199a. While betalain pre-treated rats prevented the cardiac failure significantly (p < 0.01) with improved anti-oxidant enzymes, abrogated the inflammatory signals with restored matrix proteins, cardiac biomarker genes, and attenuated miR-423 and miR-27 compared to heart failure rats. The results of the study suggest that the betalain treatment protected the hearts from failing via microRNA mediated activation the anti-inflammatory signaling and restoring the matrix protein modulation.

N6-methyladenine demethylase ALKBH1 inhibits the differentiation of skeletal muscle

DNA N⁶-methyladenine (N6-mA) was recently recognized as a new epigenetic modification in mammalian genome, and ALKBH1 was discovered as its demethylase. Knock-out mice studies revealed that ALKBH1 was indispensable for normal embryonic development. However, the function of ALKBH1 in myogenesis is largely unknown. In this study, we found that N6-mA showed a steady increase, going along with a strong decrease of ALKBH1 during skeletal muscle development. Our results also showed that ALKBH1 enhanced proliferation and inhibited differentiation of C2C12 cells. Genome-wide transcriptome analysis and reporter assays further revealed that ALKBH1 accomplished the differentiation inhibiting function by regulating a core set of genes and multiple signaling pathways, including increasing chemokine (C-X-C motif) ligand 14 (CXCL14) and activating ERK signaling. Taken together, our results demonstrated that ALKBH1 is critical for the myogenic differentiation of C2C12 cells, and suggested that N6-mA might be a new epigenetic mechanism for the regulation of myogenesis.

Akirin1 promotes myoblast differentiation by modulating multiple myoblast differentiation factors

Akirin1 is found to be involved in myoblast differentiation. However, the mechanism by which the Akirin1 gene regulates myoblast differentiation still remains unclear. In the present study, we found that ectopic expression of Akirin1 promoted myoblast differentiation by increasing the expression of myogenic regulatory factor (MRF) 4 (MRF4) and myocyte enhancer factor 2B (MEF2B) mRNA. Additionally, we showed that ectopic Akirin1 induced cell cycle arrest by up-regulating p21 mRNA. To further uncover the mechanism by which Akirin1 promotes myoblast differentiation, we showed that the enhanced Akirin1 increased the mRNA expression of P38α. Importantly, the enhanced MRF4 expression by Akirin1 can be abrogated by treatment of SB203580, a p38 inhibitor. Similarly, we found that enhanced MEF2B expression by Akirin1 can be abrogated by treatment with LY294002, a PI3K inhibitor. Together, our results indicate that Akirin1 promotes myoblast differentiation by acting on the p38 and PI3K pathways and subsequently inducing the expression of myoblast differentiation factors.

p110α of PI3K is necessary and sufficient for quiescence exit in adult muscle satellite cells

Adult mouse muscle satellite cells (MuSCs) are quiescent in uninjured muscles. Upon injury, MuSCs exit quiescence in vivo to become activated, re-enter the cell cycle to proliferate, and differentiate to repair the damaged muscles. It remains unclear which extrinsic cues and intrinsic signaling pathways regulate quiescence exit during MuSC activation. Here, we demonstrated that inducible MuSC-specific deletion of p110α, a catalytic subunit of phosphatidylinositol 3-kinase (PI3K), rendered MuSCs unable to exit quiescence, resulting in severely impaired MuSC proliferation and muscle regeneration. Genetic reactivation of mTORC1, or knockdown of FoxOs, in p110α-null MuSCs partially rescued the above defects, making them key effectors downstream of PI3K in regulating quiescence exit. c-Jun was found to be a key transcriptional target of the PI3K/mTORC1 signaling axis essential for MuSC quiescence exit. Moreover, induction of a constitutively active PI3K in quiescent MuSCs resulted in spontaneous MuSC activation in uninjured muscles and subsequent depletion of the MuSC pool. Thus, PI3K-p110α is both necessary and sufficient for MuSCs to exit quiescence in response to activating signals.

MEF2 signaling and human diseases

The members of myocyte Enhancer Factor 2 (MEF2) protein family was previously believed to function in the development of heart and muscle. Recent reports indicate that they are also closely associated with development and progression of many human diseases. Although their role in cancer biology is well established, the molecular mechanisms underlying their action is yet largely unknown. MEF2 family is closely associated with various signaling pathways, including Ca²⁺ signaling, MAP kinase signaling, Wnt signaling, PI3K/Akt signaling, etc. microRNAs also contribute to regulate the activities of MEF2. In this review, we summarize the known molecular mechanism by which MEF2 family contribute to human diseases.

AKT2 deficiency induces retardation of myocyte development through EndoG-MEF2A signaling in mouse heart

Protein kinase B2 (AKT2) is implicated in diverse process of cardiomyocyte signaling including survival and metabolism. However, the role of AKT2 in myocardium development and the signaling pathway is rarely understood. Therefore, we sought to determine the effect of AKT2 deletion on heart development and its downstream targets. By using experimental animal models and neonatal rat cardiomyocytes (NRCMs), we observed that AKT2 deficiency induces retardation of heart development and increased systemic blood pressure (BP) without affecting cardiac function. Further investigation suggested that deficiency of AKT2 in myocardium results in diminished MEF2A abundance, which induced decreased size of cardiomyocytes. We additionally confirmed that EndoG, which is also regulated by AKT2, is a suppressor of MEF2A in myocardium. Finally, our results proved that AKT2 deficiency impairs the response to β-adrenergic stimuli that normally causes hypertrophy in cardiomyocytes by downregulating MEF2A expression. Our data are the first to show the important role of AKT2 in determining the size of myocardium, its deficiency causes retardation of cardiomyocyte development. We also proved a novel pathway of heart development involving EndoG and MEF2A regulated by AKT2.

Activated Integrin-Linked Kinase Negatively Regulates Muscle Cell Enhancement Factor 2C in C2C12 Cells

Our previous study reported that muscle cell enhancement factor 2C (MEF2C) was fully activated after inhibition of the phosphorylation activity of integrin-linked kinase (ILK) in the skeletal muscle cells of goats. It enhanced the binding of promoter or enhancer of transcription factor related to proliferation of muscle cells and then regulated the expression of these genes. In the present investigation, we explored whether ILK activation depended on PI3K to regulate the phosphorylation and transcriptional activity of MEF2C during C2C12 cell proliferation. We inhibited PI3K activity in C2C12 with LY294002 and then found that ILK phosphorylation levels and MEF2C phosphorylation were decreased and that MCK mRNA expression was suppressed significantly. After inhibiting ILK phosphorylation activity with Cpd22 and ILK-shRNA, we found MEF2C phosphorylation activity and MCK mRNA expression were increased extremely significantly. In the presence of Cpd22, PI3K activity inhibition increased MEF2C phosphorylation and MCK mRNA expression indistinctively. We conclude that ILK negatively and independently of PI3K regulated MEF2C phosphorylation activity and MCK mRNA expression in C2C12 cells. The results provide new ideas for the study of classical signaling pathway of PI3K-ILK-related proteins and transcription factors.

Akt-mediated phosphorylation controls the activity of the Y-box protein MSY3 in skeletal muscle

BACKGROUND

The Y-box protein MSY3/Csda represses myogenin transcription in skeletal muscle by binding a highly conserved cis-acting DNA element located just upstream of the myogenin minimal promoter (myogHCE). It is not known how this MSY3 activity is controlled in skeletal muscle. In this study, we provide multiple lines of evidence showing that the post-translational phosphorylation of MSY3 by Akt kinase modulates the MSY3 repression of myogenin.

METHODS

Skeletal muscle and myogenic C2C12 cells were used to study the effects of MSY3 phosphorylation in vivo and in vitro on its sub-cellular localization and activity, by blocking the IGF1/PI3K/Akt pathway, by Akt depletion and over-expression, and by mutating potential MSY3 phosphorylation sites.

RESULTS

We observed that, as skeletal muscle progressed from perinatal to postnatal and adult developmental stages, MSY3 protein became gradually dephosphorylated and accumulated in the nucleus. This correlated well with the reduction of phosphorylated active Akt. In C2C12 myogenic cells, blocking the IGF1/PI3K/Akt pathway using LY294002 inhibitor reduced MSY3 phosphorylation levels resulting in its accumulation in the nuclei. Knocking down Akt expression increased the amount of dephosphorylated MSY3 and reduced myogenin expression and muscle differentiation. MSY3 phosphorylation by Akt in vitro impaired its binding at the MyogHCE element, while blocking Akt increased MSY3 binding activity. While Akt over-expression rescued myogenin expression in MSY3 overexpressing myogenic cells, ablation of the Akt substrate, (Ser126 located in the MSY3 cold shock domain) promoted MSY3 accumulation in the nucleus and abolished this rescue. Furthermore, forced expression of Akt in adult skeletal muscle induced MSY3 phosphorylation and myogenin derepression.

CONCLUSIONS

These results support the hypothesis that MSY3 phosphorylation by Akt interferes with MSY3 repression of myogenin circuit activity during muscle development. This study highlights a previously undescribed Akt-mediated signaling pathway involved in the repression of myogenin expression in myogenic cells and in mature muscle. Given the significance of myogenin regulation in adult muscle, the Akt/MSY3/myogenin regulatory circuit is a potential therapeutic target to counteract muscle degenerative disease.

Role of phosphoinositide 3-OH kinase p110β in skeletal myogenesis

Phosphoinositide 3-OH kinase (PI3K) regulates a number of developmental and physiologic processes in skeletal muscle; however, the contributions of individual PI3K p110 catalytic subunits to these processes are not well-defined. To address this question, we investigated the role of the 110-kDa PI3K catalytic subunit β (p110β) in myogenesis and metabolism. In C2C12 cells, pharmacological inhibition of p110β delayed differentiation. We next generated mice with conditional deletion of p110β in skeletal muscle (p110β muscle knockout [p110β-mKO] mice). While young p110β-mKO mice possessed a lower quadriceps mass and exhibited less strength than control littermates, no differences in muscle mass or strength were observed between genotypes in old mice. However, old p110β-mKO mice were less glucose tolerant than old control mice. Overexpression of p110β accelerated differentiation in C2C12 cells and primary human myoblasts through an Akt-dependent mechanism, while expression of kinase-inactive p110β had the opposite effect. p110β overexpression was unable to promote myoblast differentiation under conditions of p110α inhibition, but expression of p110α was able to promote differentiation under conditions of p110β inhibition. These findings reveal a role for p110β during myogenesis and demonstrate that long-term reduction of skeletal muscle p110β impairs whole-body glucose tolerance without affecting skeletal muscle size or strength in old mice.

Inositol 1,4,5 trisphosphate receptor 1 is a key player of human myoblast differentiation

Cytosolic Ca(2+) signals are fundamental for the early and late steps of myoblast differentiation and are, as in many cells, generated by Ca(2+) release from internal stores as well as by plasma membrane Ca(2+) entry. Our recent studies identified the store-operated Ca(2+) channels, Orai1 and TRPC1&C4, as crucial for the early steps of human myogenesis and for the late fusion events. In the present work, we assessed the role of the inositol-1,4,5 tris-phosphate receptor (IP3R) type 1 during human myoblast differentiation. We demonstrated, using siRNA strategy that IP3R1 is required for the expression of muscle-specific transcription factors such as myogenin and MEF2 (myocyte enhancer factor 2), and for the formation of myotubes. The knockdown of IP3R1 strongly reduced endogenous spontaneous Ca(2+) transients, and attenuated store-operated Ca(2+) entry. As well, two Ca(2+)-dependent key enzymes of muscle differentiation, NFAT and CamKII are down-regulated upon siIP3R1 treatment. On the contrary, the overexpression of IP3R1 accelerated myoblasts differentiation. These findings identify Ca(2+) release mediated by IP3R1 as an essential mechanism during the early steps of myoblast differentiation.

JAK-STAT pathway and myogenic differentiation

Myogenic differentiation plays an important role in muscle regeneration and is regulated by two transcription factor families, MRFs and MEF2, which induce differentiation of myoblasts through expression of the muscle-specific gene, myogenin. In addition, many intracellular signaling pathways are also involved in myogenic differentiation, including p38 MAPK, ERK/MAPK and PI3K/AKT. The JAK-STAT pathway is activated by various cytokines and positively or negatively regulates the differentiation of myoblasts. JAK1 plays a notable role in proliferation; whereas, JAK2 and JAK3 function mainly in differentiation. The STATs, molecules downstream of JAK, regulate myogenesis. With JAK1, STAT1 promotes proliferation, while STAT3 has a dual effect on proliferation and differentiation. The JAK-STAT negative regulator, SOCS, is also associated with myogenesis; although, its role is controversial. In this review, we will discuss the role of the JAK-STAT pathway on myogenic differentiation.

Methylcobalamin promotes proliferation and migration and inhibits apoptosis of C2C12 cells via the Erk1/2 signaling pathway

Methylcobalamin (MeCbl) is a vitamin B12 analog that has some positive effects on peripheral nervous disorders. Although some previous studies revealed the effects of MeCbl on neurons, its effect on the muscle, which is the final target of motoneuron axons, remains to be elucidated. This study aimed to determine the effect of MeCbl on the muscle. We found that MeCbl promoted the proliferation and migration of C2C12 myoblasts in vitro and that these effects are mediated by the Erk1/2 signaling pathway without affecting the activity of the Akt signaling pathway. We also demonstrated that MeCbl inhibits C2C12 cell apoptosis during differentiation. Our results suggest that MeCbl has beneficial effects on the muscle in vitro. MeCbl administration may provide a novel therapeutic approach for muscle injury or degenerating muscle after denervation.

Transcription factors in heart: promising therapeutic targets in cardiac hypertrophy

Regulation of gene expression is central to cell growth, differentiation and diseases. Context specific and signal dependent regulation of gene expression is achieved to a large part by transcription factors. Cardiac transcription factors regulate heart development and are also involved in stress regulation of the adult heart, which may lead to cardiac hypertrophy. Hypertrophy of cardiac myocytes is an outcome of the imbalance between prohypertrophic factors and anti-hypertrophic factors. This is initially a compensatory mechanism but sustained hypertrophy may lead to heart failure. The growing knowledge of transcriptional control mechanisms is helpful in the development of novel therapies. This review summarizes the role of cardiac transcription factors in cardiac hypertrophy, emphasizing their potential as attractive therapeutic targets to prevent the onset of heart failure and sudden death as they can be converging targets for current therapy.

TRAF6 promotes myogenic differentiation via the TAK1/p38 mitogen-activated protein kinase and Akt pathways

p38 mitogen-activated protein kinase (MAPK) is an essential kinase involved in myogenic differentiation. Although many substrates of p38 MAPK have been identified, little is known about its upstream activators during myogenic differentiation. TRAF6 is known to function in cytokine signaling during inflammatory responses. However, not much is known about its role in myogenic differentiation and muscle regeneration. We showed here that TRAF6 and its intrinsic ubiquitin E3 ligase activity are required for myogenic differentiation. In mouse myoblasts, knockdown of TRAF6 compromised the p38 MAPK and Akt pathways, while deliberate activation of either pathway rescued the differentiation defect caused by TRAF6 knockdown. TAK1 acted as a key signal transducer downstream of TRAF6 in myogenic differentiation. In vivo, knockdown of TRAF6 in mouse muscles compromised the injury-induced muscle regeneration without impairing macrophage infiltration and myoblast proliferation. Collectively, we demonstrated that TRAF6 promotes myogenic differentiation and muscle regeneration via the TAK1/p38 MAPK and Akt pathways.

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.

Muscle-specific expression of insulin-like growth factor 1 improves outcome in Lama2Dy-w mice, a model for congenital muscular dystrophy type 1A

MDC1A, the second most prevalent form of congenital muscular dystrophy, results from laminin-α2 chain deficiency. This disease is characterized by extensive muscle wasting that results in extremely weak skeletal muscles. A large percentage of children with MDC1A are faced with respiratory as well as ambulatory difficulties. We investigated the effects of overexpressing insulin-like growth factor-1 (IGF-1) as a potential therapeutic target for the disease in the Lama2(Dy-w) mouse, a model that closely resembles human MDC1A. IGF-1 transgenic Lama2(Dy-w) mice showed increased survivability, body weight and muscle weight. In addition, these mice showed better ability to stand up on their hind limbs: a typical exploratory behavior seen in healthy mice. Histology and immunohistochemistry analyses revealed increased regenerative capacity and proliferation in IGF-1 transgenic Lama2(Dy-w) muscles. Western blot analysis showed increased phosphorylation of Akt and ERK1/2, both known to enhance myogenesis. Additionally, we saw increases in the expression of the regeneration markers MyoD, myogenin and embryonic myosin (myosin heavy chain 3, MYH3). We conclude that overexpression of IGF-1 in Lama2(Dy-w) mice increases lifespan and improves their overall wellbeing mainly through the restoration of impaired muscle regeneration, as fibrosis or inflammation was not impacted by IGF-1 in this disease model. Our results demonstrate that IGF-1 has a promising therapeutic potential in the treatment of MDC1A.

Arsenic inhibits myogenic differentiation and muscle regeneration

BACKGROUND

The incidence of low birth weights is increased in offspring of women who are exposed to high concentrations of arsenic in drinking water compared with other women. We hypothesized that effects of arsenic on birth weight may be related to effects on myogenic differentiation.

OBJECTIVE

We investigated the effects of arsenic trioxide (As2O3) on the myogenic differentiation of myoblasts in vitro and muscle regeneration in vivo.

METHODS

C2C12 myoblasts and primary mouse and human myoblasts were cultured in differentiation media with or without As2O3 (0.1-0.5 microM) for 4 days. Myogenic differentiation was assessed by myogenin and myosin heavy chain expression and multinucleated myotube formation in vitro; skeletal muscle regeneration was tested using an in vivo mouse model with experimental glycerol myopathy.

RESULTS

A submicromolar concentration of As2O3 dose-dependently inhibited myogenic differentiation without apparent effects on cell viability. As2O3 significantly and dose-dependently decreased phosphorylation of Akt and p70s6k proteins during myogenic differentiation. As2O3-induced inhibition in myotube formation and muscle-specific protein expression was reversed by transfection with the constitutively active form of Akt. Sections of soleus muscles stained with hematoxylin and eosin showed typical changes of injury and regeneration after local glycerol injection in mice. Regeneration of glycerol-injured soleus muscles, myogenin expression, and Akt phosphorylation were suppressed in muscles isolated from As2O3-treated mice compared with untreated mice.

CONCLUSION

Our results suggest that As2O3 inhibits myogenic differentiation by inhibiting Akt-regulated signaling.

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.

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.

Engineered early embryonic cardiac tissue increases cardiomyocyte proliferation by cyclic mechanical stretch via p38-MAP kinase phosphorylation

Cardiomyocyte (CM) transplantation is one therapeutic option for cardiac repair. Studies suggest that fetal CMs display the best cell type for cardiac repair, which can finitely proliferate, integrate with injured host myocardium, and restore cardiac function. We have recently developed an engineered early embryonic cardiac tissue (EEECT) using embryonic cardiac cells and have shown that EEECT contractile properties and cellular proliferative response to cyclic mechanical stretch stimulation mimic developing fetal myocardium. However, it remains unknown whether cyclic mechanical stretch-mediated high cellular proliferation activity within EEECT reflects CM or non-CM population. Studies have shown that p38-mitogen-activated protein kinase (p38MAPK) plays an important role in both cyclic mechanical stretch stimulation and cellular proliferation. Therefore, in the present study, we tested the hypothesis that cyclic mechanical stretch (0.5 Hz, 5% strain for 48 h) specifically increases EEECT CM proliferation mediated by p38MAPK activity. Cyclic mechanical stretch increased CM, but not non-CM, proliferation and increased p38MAPK phosphorylation. Treatment of EEECT with the p38MAPK inhibitor, SB202190, reduced CM proliferation. The negative CM proliferation effects of SB202190 were not reversed by concurrent stretch stimulation. Results suggest that immature CM proliferation within EEECT can be positively regulated by mechanical stretch and negatively regulated by p38MAPK inhibition.

Different roles of H-ras for regulation of myosin heavy chain promoters in satellite cell-derived muscle cell culture during proliferation and differentiation

The effect of constitutively activated proto-oncogene H-ras (H-rasQ61L) on the regulation of myosin heavy chain (MHC) promoter activities was investigated in rabbit satellite cell-derived muscle cell culture during the proliferation stage and early and later stages of differentiation, respectively. During proliferation, overexpression of H-rasQ61L did not affect basal level of activity of the slow MHCI/beta or the fast MHCIId/x promoter luciferase reporter gene construct in transient transfection assays. By contrast, H-rasQ61L affected both MHC promoter activities during differentiation, and this effect changes from inactivation after 2 days to activation after 4 days of differentiation. The activating effect of H-rasQ61L on both MHC promoters after 4 days of differentiation was significantly reduced by LY-294002, a specific inhibitor of the phosphoinositol-3-kinase (PI3K), a downstream target of Ras. Furthermore, the protein kinase Akt (protein kinase B), a downstream target of PI3k, was activated 4 days after initiation of differentiation in myotubes overexpressing H-rasQ61L. By contrast, inhibition of another Ras downstream pathway, mitogen-activated protein kinase kinase 1/2-extracellular signal-regulated protein kinase 1/2 (MKK1/2-ERK1/2-MAPK), increased activities of both MHC promoters, indicating a suppressive role of this pathway. Moreover, the Ras-PI3K-Akt signaling pathway is involved in the activation of MHCI/beta and IId/x promoters in a later stage of differentiation of muscle cells, presumably by a known inhibiting effect of activated Akt on the MKK1/2-ERK1/2-MAPK pathway. The experiments demonstrate that during differentiation of muscle cells activated H-ras is an important regulator of MHC isoform promoter function with opposite effects during early and later stages.

A-type lamins and signaling: the PI 3-kinase/Akt pathway moves forward

Lamin A/C is a nuclear lamina constituent mutated in a number of human inherited disorders collectively referred to as laminopathies. The occurrence and significance of lamin A/C interplay with signaling molecules is an old question, suggested by pioneer studies performed in vitro. However, this relevant question has remained substantially unanswered, until data obtained in cellular and organismal models of laminopathies have indicated two main aspects of lamin A function. The first aspect is that lamins establish functional interactions with different protein platforms, the second aspect is that lamin A/C activity and altered function may elicit different effects in different cells and tissue types and even in different districts of the same tissue. Both these observations strongly suggest that signaling mechanisms targeting lamin A/C or its binding partners may regulate such a plastic behavior. A number of very recent data show involvement of kinases, as Akt and Erk, or phosphatases, as PP1 and PP2, in lamin A-linked cellular mechanisms. Moreover, altered activation of signaling in laminopathies and rescue of the pathological phenotype in animal models by inhibitors of signaling pathways, strongly suggest that signaling effectors related to lamin A/C may be implicated in the pathogenesis of laminopathies and may represent targets of therapeutic intervention. In face of such an open perspective of basic and applied research, we review current evidence of lamin A/C interplay with signaling molecules, with particular emphasis on the lamin A-Akt interaction and on the biological significance of their relationship.

Sod2 overexpression preserves myoblast mitochondrial mass and function, but not muscle mass with aging

Mice lacking superoxide dismutase-2 (SOD2 or MnSOD) die during embryonic or early neonatal development, with diffuse superoxide-induced mitochondrial damage. Although stem and progenitor cells are exquisitely sensitive to oxidant stress, they have not been well studied in MnSOD2-manipulated mouse models. Patterns of proliferation and differentiation of cultured myoblasts (muscle progenitor cells), PI3-Akt signaling during differentiation, and the maintenance of mitochondrial mass with aging using myoblasts from young (3-4 week old) and aged (27-29 months old) MnSOD2-overexpressing (Sod2-Tg) and heterozygote (Sod2(+/-)) mice were characterized by us. Overexpression of MnSOD2 in myoblasts had a protective effect on mitochondrial DNA abundance and some aspects of mitochondrial function with aging, and preservation of differentiation potential. Sod2 deficiency resulted in defective signaling in the PI3-Akt pathway, specifically impaired phosphorylation of Akt at Ser473 and Thr308 in young myoblasts, and decreased differentiation potential. Compared with young myoblasts, aged myoblast Akt was constitutively phosphorylated, unresponsive to mitogen signaling, and indifferent to MnSOD2 levels. These data suggest that specific sites in the PI3K-Akt pathway are more sensitive to increased superoxide levels than to the increased hydrogen peroxide levels generated in Sod2-transgenic myoblasts. In wild-type myoblasts, aging was associated with significant loss of mitochondrial DNA relative to chromosomal DNA, but MnSOD2 overexpression was associated with maintained myoblast mitochondrial DNA with aging.

Identification and characterization of a novel gene, dapr, involved in skeletal muscle differentiation and protein kinase B signaling

The phosphatidylinositol 3-kinase (PI3K) signaling pathway has been associated with a variety of cellular functions ranging from cell cycle regulation to tissue development. Although years of research have extensively characterized this signaling pathway, little is known as to how specific cellular events are coordinated by its activation. Here we demonstrate that Dapr (differentiation-associated protein), a novel protein, appears to focus one aspect of this pathway by acting as a putative scaffold protein during skeletal muscle differentiation. We present for the first time a description of this protein using in silico analysis. dapr was discovered through a previous study employing chromatin immunoprecipitation and CpG microarray analysis experiments as being regulated by myocyte-enhancing factor 2, a key transcription factor involved in the differentiation of skeletal muscle tissue. In this study we show that during the course of differentiation, Dapr binds to the PI3K signaling pathway member protein kinase B (PKB). In C2C12 myoblast cells before differentiation Dapr is localized to the cytosol, migrating with PKB to the membrane after initiation of muscle differentiation. Knockdown of Dapr by RNAi resulted in inhibition of myotube formation. Our findings indicate that Dapr is a key component required by myoblasts for orchestrating their differentiation during myogenesis. Furthermore, it appears that Dapr is involved in the PI3K signaling cascade, potentially acting as a scaffold protein for PKB and coordinating its compartmentalization during differentiation.

JAK2/STAT2/STAT3 are required for myogenic differentiation

Skeletal muscle satellite cell-derived myoblasts are mainly responsible for postnatal muscle growth and injury-induced regeneration. However, the cellular signaling pathways that control proliferation and differentiation of myoblasts remain poorly defined. Recently, we found that JAK1/STAT1/STAT3 not only participate in myoblast proliferation but also actively prevent them from premature differentiation. Unexpectedly, we found that a related pathway consisting of JAK2, STAT2, and STAT3 is required for early myogenic differentiation. Interference of this pathway by either a small molecule inhibitor or small interfering RNA inhibits myogenic differentiation. Consistently, all three molecules are activated upon differentiation. The pro-differentiation effect of JAK2/STAT2/STAT3 is partially mediated by MyoD and MEF2. Interestingly, the expression of the IGF2 gene and the HGF gene is also regulated by JAK2/STAT2/STAT3, suggesting that this pathway could also promote differentiation by regulating signaling molecules known to be involved in myogenic differentiation. In summary, our current study reveals a novel role for the JAK2/STAT2/STAT3 pathway in myogenic differentiation.

Lamin A Ser404 is a nuclear target of Akt phosphorylation in C2C12 cells

Akt/PKB is a central activator of multiple signaling pathways coupled with a large number of stimuli. Although both localization and activity of Akt in the nuclear compartment are well-documented, most Akt substrates identified so far are located in the cytoplasm, while nuclear substrates have remained elusive. A proteomic-based search for nuclear substrates of Akt was undertaken, exploiting 2D-electrophoresis/MS in combination with an anti-Akt phosphosubstrate antibody. This analysis indicated lamin A/C as a putative substrate of Akt in C2C12 cells. In vitro phosphorylation of endogenous lamin A/C by recombinant Akt further validated this result. Moreover, by phosphopeptide analysis and point mutation, we established that lamin A/C is phosphorylated by Akt at Ser404, in an evolutionary conserved Akt motif. To delve deeper into this, we raised an antibody against the lamin A Ser404 phosphopeptide which allowed us to determine that phosphorylation of lamin A Ser404 is triggered by the well-known Akt activator insulin, and is therefore to be regarded as a physiological response. Remarkably, expression of S404A lamin A in primary cells from healthy tissue caused the nuclear abnormalities that are a hallmark of Emery-Dreifuss muscular dystrophy (EDMD) cells. Indeed, it is known that mutations at several sites in lamin A/C cause autosomal dominant EDMD. Very importantly, we show here that Akt failed to phosphorylate lamin A/C in primary cells from an EDMD-2 patient with lamin A/C mutated in the Akt consensus motif. Together, our data demonstrate that lamin A/C is a novel signaling target of Akt, and implicate Akt phosphorylation of lamin A/C in the correct function of the nuclear lamina.

The chemokine SDF1 controls multiple steps of myogenesis through atypical PKCζ

Mice deficient in the SDF1-chemokine-receptor CXCR4, exhibit severe defects of secondary limb myogenesis. To further elucidate the role of SDF1 in muscle development, we have now analyzed putative effects of this chemokine on proliferation, migration and myogenic differentiation of mouse C2C12 myogenic progenitor/myoblast cells. In addition, we have characterized the signaling pathways employed by SDF1-CXCR4 to control myogenesis. We found that SDF1 stimulates proliferation and induces migration of C2C12 cells with a potency similar to that of FGF2 and HGF, which both represent prototypical extracellular regulators of myogenesis. In addition, SDF1 inhibits myogenic differentiation in both C2C12 cells and primary myoblasts, as assessed by MyoD, myosin heavy chain and/or myogenin expression. Regarding signaling pathways, C2C12 cells responded to SDF1 with activation (phosphorylation) of Erk and PKCζ, whereas even after prolonged SDF1 treatment for up to 120 minutes, levels of activated Akt, p38 and PKCα or PKCβ remained unaffected. Preventing activation of the classic MAP kinase cascade with the Erk inhibitor UO126 abolished SDF1-induced proliferation and migration of C2C12 cells but not the inhibitory action of SDF1 on myogenic differentiation. Moreover, the effects of SDF1 on proliferation, migration and differentiation of C2C12 cells were all abrogated in the presence of myristoylated PKCζ peptide pseudosubstrate and/or upon cellular depletion of PKCζ by RNA interference. In conclusion, our findings unravel a previously unknown role of CXCR4-PKCζ signaling in myogenesis. The potent inhibitory effects of SDF1 on myogenic differentiation point to a major function of CXCR4-PKCζ signaling in the control of secondary muscle growth.

JAK1-STAT1-STAT3, a key pathway promoting proliferation and preventing premature differentiation of myoblasts

Skeletal muscle stem cell-derived myoblasts are mainly responsible for postnatal muscle growth and injury-induced muscle regeneration. However, the cellular signaling pathways controlling the proliferation and differentiation of myoblasts are not fully understood. We demonstrate that Janus kinase 1 (JAK1) is required for myoblast proliferation and that it also functions as a checkpoint to prevent myoblasts from premature differentiation. Deliberate knockdown of JAK1 in both primary and immortalized myoblasts induces precocious myogenic differentiation with a concomitant reduction in cell proliferation. This is caused, in part, by an accelerated induction of MyoD, myocyte enhancer-binding factor 2 (MEF2), p21Cip1, and p27Kip1, a faster down-regulation of Id1, and an increase in MEF2-dependent gene transcription. Downstream of JAK1, of all the signal transducer and activator of transcriptions (STATs) present in myoblasts, we find that only STAT1 knockdown promotes myogenic differentiation in both primary and immortalized myoblasts. Leukemia inhibitory factor stimulates myoblast proliferation and represses differentiation via JAK1-STAT1-STAT3. Thus, JAK1-STAT1-STAT3 constitutes a signaling pathway that promotes myoblast proliferation and prevents premature myoblast differentiation.

Sonic hedgehog promotes proliferation and differentiation of adult muscle cells: Involvement of MAPK/ERK and PI3K/Akt pathways

Sonic hedgehog (Shh) has been reported to act as a mitogen and survival factor for muscle satellite cells. However, its role in their differentiation remains ambiguous. Here, we provide evidence that Shh promotes the proliferation and differentiation of primary cultures of chicken adult myoblasts (also termed satellite cells) and mouse myogenic C2 cells. These effects are reversed by cyclopamine, a specific chemical inhibitor of the Shh pathway. In addition, we show that Shh and its downstream molecules are expressed in adult myoblast cultures and localize adjacent to Pax7 in muscle sections. These gene expressions are regulated during postnatal muscle growth in chicks. Most importantly, we report that Shh induces MAPK/ERK and phosphoinositide 3-kinase (PI3K)-dependent Akt phosphorylation and that activation of both signaling pathways is essential for Shh's signaling in muscle cells. However, the effect of Shh on Akt phosphorylation is more robust than that on MAPK/ERK, and data suggest that Shh influences these pathways in a manner similar to IGF-I. By exploiting specific chemical inhibitors of the MAPK/ERK and PI3K/Akt signaling pathways, UO126 and Ly294002, respectively, we demonstrate that Shh-induced Akt phosphorylation, but not that of MAPK/ERK, is required for its promotive effects on muscle cell proliferation and differentiation. Taken together, we suggest that Shh acts in an autocrinic manner in adult myoblasts, and provide first evidence of a role for PI3K/Akt in Shh signaling during myoblast differentiation.

Insulin-like growth factor 1 and muscle growth: implication for satellite cell proliferation

Insulin-like growth factor 1 (IGF-1) has been shown to rescue the aging-related or inactivity-induced loss of muscle mass through the activation of satellite cells. However, the signalling pathways and the mechanism by which IGF-1 affects satellite cells have not been not completely identified. The purpose of the present review is to provide current understanding of the cellular and molecular events underlying IGF-1 induced proliferation of satellite cells.

MyoD synergizes with the E-protein HEB beta to induce myogenic differentiation

The MyoD family of basic helix-loop-helix transcription factors function as heterodimers with members of the E-protein family to induce myogenic gene activation. The E-protein HEB is alternatively spliced to generate alpha and beta isoforms. While the function of these molecules has been studied in other cell types, questions persist regarding the molecular functions of HEB proteins in skeletal muscle. Our data demonstrate that HEB alpha expression remains unchanged in both myoblasts and myotubes, whereas HEB beta is upregulated during the early phases of terminal differentiation. Upon induction of differentiation, a MyoD-HEB beta complex bound the E1 E-box of the myogenin promoter leading to transcriptional activation. Importantly, forced expression of HEB beta with MyoD synergistically lead to precocious myogenin expression in proliferating myoblasts. However, after differentiation, HEB alpha and HEB beta synergized with myogenin, but not MyoD, to activate the myogenin promoter. Specific knockdown of HEB beta by small interfering RNA in myoblasts blocked differentiation and inhibited induction of myogenin transcription. Therefore, HEB alpha and HEB beta play novel and central roles in orchestrating the regulation of myogenic factor activity through myogenic differentiation.

The calcineurin pathway links hyperpolarization (Kir2.1)-induced Ca2+ signals to human myoblast differentiation and fusion

In human myoblasts triggered to differentiate, a hyperpolarization, resulting from K+ channel (Kir2.1) activation, allows the generation of an intracellular Ca2+ signal. This signal induces an increase in expression/activity of two key transcription factors of the differentiation process, myogenin and MEF2. Blocking hyperpolarization inhibits myoblast differentiation. The link between hyperpolarization-induced Ca2+ signals and the four main regulatory pathways involved in myoblast differentiation was the object of this study. Of the calcineurin, p38-MAPK, PI3K and CaMK pathways, only the calcineurin pathway was inhibited when Kir2.1-linked hyperpolarization was blocked. The CaMK pathway, although Ca2+ dependent, is unaffected by changes in membrane potential or block of Kir2.1 channels. Concerning the p38-MAPK and PI3K pathways, their activity is present already in proliferating myoblasts and they are unaffected by hyperpolarization or Kir2.1 channel block. We conclude that the Kir2.1-induced hyperpolarization triggers human myoblast differentiation via the activation of the calcineurin pathway, which, in turn, induces expression/activity of myogenin and MEF2.

The p38 MAPK signaling pathway: a major regulator of skeletal muscle development

Skeletal muscle development is regulated by extracellular growth factors that transmit largely unknown signals into the cell affecting the muscle-transcription program. One intracellular signaling pathway activated during the differentiation of myogenic cell lines is p38 mitogen-activated protein kinase (MAPK). As a result of modifying the activity of p38 in myoblasts, the pathway proved essential for the expression of muscle-specific genes. P38 affects the activities of transcription factors from the MyoD and MEF2 families and participates in the remodeling of chromatin at specific muscle-regulatory regions. P38 cooperates with the myogenic transcription factors in the activation of a subset of late-transcribed genes, hence contributing to the temporal expression of genes during differentiation. Recent developmental studies with mouse and Xenopus embryos, substantiated and further extended the essential role of p38 in myogenesis. Evidence exists supporting the crucial role for p38 signaling in activating MEF2 transcription factors during somite development in mice. In Xenopus, p38 signaling was shown to be needed for the early expression of Myf5 and for the expression of several muscle structural genes. The emerging data indicate that p38 participates in several stages of the myogenic program.

S100B stimulates myoblast proliferation and inhibits myoblast differentiation by independently stimulating ERK1/2 and inhibiting p38 MAPK

The Ca2+-modulated protein of the EF-hand type, S100B, was shown to inhibit rat L6 myoblast differentiation and myotube formation by interacting with a high affinity with an unidentified receptor (Sorci et al., 2003). We show here that S100B independently inhibits the MKK6-p38 MAPK pathway and stimulates the Ras-MEK-ERK1/2 pathway. The inhibitory effect of S100B on p38 MAPK translates into a defective induction of the muscle-specific transcription factor myogenin and the antiproliferative factor p21(WAF1), while S100B's stimulatory effect on ERK1/2 results in stimulation of myoblast proliferation via cyclin D1 induction and Rb phosphorylation and protection against apoptosis via activation of NF-kappaB transcriptional activity. Also, the S100B's effects that are mediated by the Ras-MEK-ERK1/2 pathway that is, stimulation of proliferation and protection against apoptosis, depend on reactive oxygen species production, being inhibited by antioxidants, while the S100B inhibitory effect on the MKK6-p38 MAPK pathway is not. We propose that S100B might participate in the regulation of myoblast differentiation by stimulating myoblast proliferation, protecting myoblasts against apoptosis, and modulating myotube formation.

Tumor Necrosis Factor-like Weak Inducer of Apoptosis Inhibits Skeletal Myogenesis through Sustained Activation of Nuclear Factor-κB and Degradation of MyoD Protein

In this study we have investigated the effect and the mechanisms by which tumor necrosis factor-like weak inducer of apoptosis (TWEAK) modulates myogenic differentiation. Treatment of C2C12 myoblasts with TWEAK inhibited their differentiation evident by a decrease in the expression of creatine kinase, myosin heavy chain-fast twitch, myogenin, and the formation of multinucleated myotubes. TWEAK also inhibited the differentiation of mouse primary myoblasts. Conversely, the proliferation of C2C12 myoblasts and the expression of a cell-cycle regulator cyclin D1 were increased in response to TWEAK treatment. Inhibition of cellular proliferation using hydroxyurea only partially reversed the inhibitory effect of TWEAK on myogenic differentiation. Treatment of C2C12 myoblasts with TWEAK resulted in the activation of nuclear factor-kappaB (NF-kappaB), the (IkappaB) IkappaB kinase (IKK) complex, and the phosphorylation and degradation of IkappaBalpha protein. Inhibition of NF-kappaB activity by overexpression of a dominant negative mutant of IkappaBalpha (IkappaBalphaDeltaN) significantly increased the myogenic differentiation in TWEAK-treated C2C12 cultures. Furthermore, overexpression of a dominant negative mutant of IKKbeta (IKKbetaK44A) but not IKKalpha (IKKalphaK44M) reversed the inhibitory effect of TWEAK on myogenesis. TWEAK inhibited the expression of myogenic regulatory factors MyoD and myogenin and also induced the degradation of MyoD protein. Finally, inhibition of NF-kappaB activation through overexpression of IKKbetaK44A prevented the degradation of MyoD protein. Overall, our data suggest that TWEAK inhibits myogenesis through the activation of NF-kappaB signaling pathway and degradation of MyoD protein.

Signaling to the chromatin during skeletal myogenesis: Novel targets for pharmacological modulation of gene expression

Cellular differentiation entails an extensive reprogramming of the genome toward the expression of discrete subsets of genes, which establish the tissue-specific phenotype. This program is achieved by epigenetic marks of the chromatin at particular loci, and is regulated by environmental cues, such as soluble factors and cell-to-cell interactions. How the intracellular cascades convert the myriad of external stimuli into the nuclear information necessary to reprogram the genome toward specific responses is a question of biological and medical interest. The elucidation of the signaling converting cues from outside the cells into chromatin modifications at individual promoters holds the promise to unveil the targets for selective pharmacological interventions to modulate gene expression for therapeutic purposes. Enhancing muscle regeneration and preventing muscle breakdown are important goals in the therapy of muscular diseases, cancer-associated cachexia and aging-associated sarcopenia. We will summarize the recent progress of our knowledge of the regulation of gene expression by intracellular cascades elicited by external cues during skeletal myogenesis. And will illustrate the potential importance of targeting the chromatin signaling in regenerative medicine--e.g. to boost muscle regeneration.

Suppressor of cytokine signaling 1 suppresses muscle differentiation through modulation of IGF-I receptor signal transduction

Suppressor of cytokine signaling (SOCS) 1 was initially identified as an intracellular negative feedback regulator of the JAK-STAT signal pathway. Recently, it has been suggested that SOCS1 affects signals of growth factors and hormones. One of them, SOCS1, is also known to be involved in auto-regulation of IRS-1-mediated signaling. However, the mechanism(s) of SOCS1 induction by insulin-like growth factor (IGF)-I and a role of SOCS1 on IGF-I receptor-mediated signaling are not clarified. Here, we investigate SOCS1 on muscle differentiation. We found that muscle differentiation was suppressed in SOCS1 stable transformant C2C12 myoblasts, while it was promoted in SOCS1-deficient myoblasts. Additionally, SOCS1 augmented MEK phosphorylation and reduced Akt phosphorylation induced by IGF-I. Then, SOCS1 stable transformant C2C12 myoblasts, infected with adenovirus bearing constitutively active Akt, have the ability to differentiate again. Collectively, these findings suggest that SOCS1 suppresses muscle differentiation through negative feedback regulation of IGF-I receptor-mediated signaling.

Static stretch promotes MEF2A nuclear translocation and expression of neonatal myosin heavy chain in C2C12myocytes in a calcineurin- and p38-dependent manner

Although the effects of mechanical stimuli have been studied extensively in fully differentiated skeletal muscle and have been shown to promote changes in phenotype, including altered myosin heavy chain isoform expression, the effects of a change in mechanical environment have been poorly studied at earlier stages of skeletal muscle differentiation. In particular, the early events elicited by mechanical stimuli upon differentiating myocytes have not been investigated. In the present study, the effect of static stretch on the activation of transcriptional factors MEF2A and NFATc1, which have been shown to be involved in the differentiation and phenotype regulation of skeletal muscle, have been examined. Furthermore, putative second messenger signaling pathways that could be involved in the dephosphorylation and hence activation of these factors were also examined. We have demonstrated that static stretch application produces a robust increase in p38 phosphorylation preceding MEF2A, but not NFATc1, nuclear translocation as well as deactivation of GSK-3β via its phosphorylation. Using SB-203580 and cyclosporine A drugs to inhibit both p38- or/and calcineurin-dependent signals, respectively, we have shown that MEF2A phosphorylation and subsequent nuclear translocation are regulated by p38 and calcineurin in a biphasic, time-dependent manner. Moreover, we also present evidence for another kinase that is involved in the stretch-related signal triggering MEF2A hyperphosphorylation, impairing its nuclear translocation, and that is related to p38. Finally, we have shown that static stretch application overnight promotes neonatal myosin heavy chain expression, which is inhibited by an inactivation of both p38 and calcineurin.

The p38 pathway regulates Akt both at the protein and transcriptional activation levels during myogenesis

The molecular signalling pathways governing skeletal muscle differentiation remain unclear. Recent work has demonstrated that both the phosphatidylinositol 3-kinase (PI3K)/Akt and p38 pathways play important roles in myogenesis. Here, we describe the interactions between these pathways in C2C12 cells. Overall, our results suggest that Akt acts downstream of p38 in myogenic cell differentiation. Activating the p38 pathway results in the concurrent activation of Akt; conversely, activating Akt does not affect p38. We have analysed Akt messenger RNA and protein levels in a C2C12 cell line stably expressing a dominant negative (DN) form of the p38 activator MKK3. Compared to control cells, this cell line exhibits reduced levels of Akt messenger RNA and total protein. In addition, blocking the p38 pathway during differentiation inhibits Akt activation. Our results show for the first time that p38 can directly affect Akt at the transcriptional level as well as at the protein activation level during myogenic differentiation.

A MyoD-generated feed-forward circuit temporally patterns gene expression during skeletal muscle differentiation

The development and differentiation of distinct cell types is achieved through the sequential expression of subsets of genes; yet, the molecular mechanisms that temporally pattern gene expression remain largely unknown. In skeletal myogenesis, gene expression is initiated by MyoD and includes the expression of specific Mef2 isoforms and activation of the p38 mitogen-activated protein kinase (MAPK) pathway. Here, we show that p38 activity facilitates MyoD and Mef2 binding at a subset of late-activated promoters, and the binding of Mef2D recruits Pol II. Most importantly, expression of late-activated genes can be shifted to the early stages of differentiation by precocious activation of p38 and expression of Mef2D, demonstrating that a MyoD-mediated feed-forward circuit temporally patterns gene expression.

FGF and PI3 kinase signaling pathways antagonistically modulate sex muscle differentiation in C. elegans

Myogenesis in vertebrate myocytes is promoted by activation of the phosphatidyl-inositol 3'-kinase (PI3 kinase) pathway and inhibited by fibroblast growth factor (FGF) signaling. We show that hyperactivation of the Caenorhabditis elegans FGF receptor, EGL-15, similarly inhibits the differentiation of the hermaphrodite sex muscles. Activation of the PI3 kinase signaling pathway can partially suppress this differentiation defect, mimicking the antagonistic relationship between these two pathways known to influence vertebrate myogenesis. When ectopically expressed in body wall muscle precursor cells, hyperactivated EGL-15 can also interfere with the proper development of the body wall musculature. Hyperactivation of EGL-15 has also revealed additional effects on a number of fundamental processes within the postembryonic muscle lineage, such as cell division polarity. These studies provide important in vivo insights into the contribution of FGF signaling events to myogenesis.

RhoA controls myoblast survival by inducing the phosphatidylinositol 3-kinase-Akt signaling pathway

The small GTPase RhoA regulates the expression of the myogenic transcription factor, MyoD, and the transcription of muscle-specific genes. We report that RhoA also affects the survival of differentiating myoblasts. Two signaling pathways, extracellular signal-regulated kinase (ERK) and phosphatidylinositol 3-kinase (PI3-K)-Akt, are involved in myoblast survival. Here, we show that inhibition of RhoA prevents the phosphorylation of Akt, but does not affect the phosphorylation of ERK. Constitutive expression of an active form of Akt prevents apoptosis in myoblasts treated with the Rho inhibitor C3-transferase. These results indicate that RhoA functions to prevent myoblast death by inducing the PI3-K-Akt pathway.

Differential regulation of the phosphoinositide 3-kinase and MAP kinase pathways by hepatocyte growth factor vs. insulin-like growth factor-I in myogenic cells

Hepatocyte growth factor (HGF) promotes the proliferation of adult myoblasts and inhibits their differentiation, whereas insulin-like growth factor I (IGF-I) enhances both processes. Recent studies indicate that activation of the phosphoinositide 3'-kinase (PI3K) pathway promotes myoblast differentiation, whereas activation of the mitogen-activated protein kinase/extracellular signal-regulated protein kinase (MAPK/ERK) promotes proliferation and inhibits their differentiation. This simple model is confounded by the fact that both HGF and IGF-I have been shown to activate both pathways. In this study, we have compared the ability of HGF and IGF-I to activate PI3K and MAPK/ERK in i28 myogenic cells. We find that, although the two stimuli result in comparable recruitment of the p85alpha subunit of PI3K into complexes with tyrosine-phosphorylated proteins, the p85beta regulatory subunit and p110alpha catalytic subunit of PI3K are preferentially recruited into these complexes in response to IGF-I. In agreement with this observation, IGF-I is much more potent than HGF in stimulating phosphorylation of Akt/PKB, a protein kinase downstream of PI3K. In contrast, MAPK/ERK phosphorylation was higher in response to HGF and lasted longer, relative to IGF-I. Moreover, the specific PI3K inhibitor, Wortmannin, abolished MAPK/ERK and Elk-1 phosphorylation in HGF-treated cells, suggesting the requirement of PI3K in mediating the HGF-induced MAPK pathway. UO126, a specific MAPK pathway inhibitor, had no effect on PI3K activity or Akt phosphorylation, implying that at least in muscle cells, the MAPK/ERK pathway is not required for HGF-induced PI3K activation. These results provide a biochemical rationale for the previous observations that HGF and IGF-I have opposite effects on myogenic cells, consistent with studies linking PI3K activation to differentiation and MAPK/ERK activation to proliferation in these cells. Moreover, the finding that PI3K activity is required for HGF-induced MAPK activation suggests its additional role in proliferation, rather than exclusively in the differentiation of adult myoblasts.

L6 myoblast differentiation is modulated by Cdk5 via the PI3K–AKT–p70S6K signaling pathway

Cdk5 regulates myogenesis but the signaling cascade through which Cdk5 modulates this process remains to be characterized. Here, we investigated whether PI3K, Akt, p70S6K, p38 MAPK, p44/42 MAPK, and Egr-1 serve as upstream regulators of Cdk5 during L6 myoblast differentiation. Upon serum reduction, we found that besides elevated expression of Cdk5 and its activator, p35, and increased Cdk5/p35 activity, Egr-1, Akt, p70S6K, and p38 MAPK activity were upregulated in differentiating L6 cells. However, p44/42 MAPK was downregulated and SAPK/JNK was unaffected. LY294002, a PI3K inhibitor, blocked the activation of Akt and p70S6K, indicating that Akt and p70S6K activation is linked to PI3K activation. The lack of LY294002 effect on p38 MAPK suggests that p38 MAPK activation is not associated with PI3K activation. Rapamycin, a specific inhibitor of FRAP/mTOR (the upstream kinase of p70S6K), also blocked p70S6K activation, indicating the involvement of FRAP/mTOR activation. LY294002 and rapamycin also blocked the enhancement of Egr-1 level, Cdk5 activity, and myogenin expression, suggesting that upregulation of these factors is coupled to PI3K-p70S6K activation. Overexpression of dominant-negative-Akt also reduced Cdk5/p35 activity and myogenin expression, indicating that the PI3K-p70S6K-Egr-1-Cdk5 signaling cascade is linked to Akt activation. SB2023580, a p38 MAPK inhibitor, had no effect on p70S6K, Egr-1, or Cdk5 activity, suggesting that p38 MAPK activation lies in a pathway distinct from the PI3K-Akt-p70S6K-Egr-1 pathway that we identify as the upstream modulator of Cdk5 activity during L6 myoblast differentiation.