Cdo interacts with APPL1 and activates Akt in myoblast differentiation

Alteration of N-glycosylation of CDON promotes H2O2-induced DNA damage in H9c2 cardiomyocytes

Protein glycosylation is involved in DNA damage. Recently, DNA damage has been connected with the pathogenesis of heart failure. Cell adhesion associated, oncogene regulated (CDON), considered as an N-linked glycoprotein, is a transmembrane receptor for modulating cardiac function. But the role of CDON and its glycosylation in DNA damage remains unknown. In this study, we found that the knockdown of CDON caused DNA double-strand breaks as indicated by an increase in phosphorylated histone H2AX (γH2AX) protein level, immunofluorescent intensity of γH2AX and tail DNA moment in H9c2 cardiomyocytes. Conversely, overexpression of CDON led to decreasing DNA damage induced by hydrogen peroxide (H2O2) and upregulating the expression of genes related to DNA repair pathways-homologous recombination (HR) and non-homologous end joining (NHEJ). Moreover, we expressed nine predicted N-glycosylation site mutants in H9c2 cells prior to treatment with H2O2. The results showed that mutation of N-glycosylation sites (N99Q, N179Q, and N870Q) increased the accumulation of DNA damage and downregulated the expression of HR-related genes, demonstrating that CDON N-glycosylation on DNA damage is site-specific and these specific N-glycan sites may regulate HR repair-related transcript abundance of genes. Our data highlight that N-glycosylation of CDON is critical to cardiomyocyte DNA lesion. It may uncover the potential strategies targeting DNA damage pathway in heart disease.

Saururus chinensis (Lour.) Baill. extract promotes skeletal muscle cell differentiation by positively regulating mitochondrial biogenesis and AKT/mTOR signaling in vitro

Promotion of myoblast differentiation by activating mitochondrial biogenesis and protein synthesis signaling pathways provides a potential alternative strategy to balance energy and overcome muscle loss and muscle disorders. Saururus chinensis (Lour.) Baill. extract (SCE) has been used extensively as a traditional herbal medicine and has several physiological activities, including anti‑asthmatic, anti‑oxidant, anti‑inflammatory, anti‑atopic, anticancer and hepatoprotective properties. However, the effects and mechanisms of action of SCE on muscle differentiation have not yet been clarified. In the present study, it was investigated whether SCE affects skeletal muscle cell differentiation through the regulation of mitochondrial biogenesis and protein synthesis in murine C2C12 myoblasts. The XTT colorimetric assay was used to determine cell viability, and myosin heavy chain (MyHC) levels were determined using immunocytochemistry. SCE was applied to C2C12 myotube at different concentrations (1, 5, or 10 ng/ml) and times (1,3, or 5 days). Reverse transcription‑quantitative PCR and western blotting were used to analyze the mRNA and protein expression change of factors related to differentiation, mitochondrial biogenesis and protein synthesis. Treatment of C2C12 cells with SCE at 1,5, and 10 ng/ml did not affect cell viability. SCE promoted C2C12 myotube formation and significantly increased MyHC expression in a concentration‑ and time‑dependent manner. SCE significantly increased the mRNA and protein expression of muscle differentiation‑specific markers, such as MyHC, myogenic differentiation 1, myogenin, Myogenic Factor 5, and β‑catenin, mitochondrial biosynthesis‑related factors, such as peroxisome proliferator‑activated receptor‑gamma coactivator‑1α, nuclear respirator factor‑1, AMP‑activated protein kinase phosphorylation, and histone deacetylase 5 and AKT/mTOR signaling factors related to protein synthesis. SCE may prevent skeletal muscle dysfunction by enhancing myoblast differentiation through the promotion of mitochondrial biogenesis and protein synthesis.

Macamide, a component of maca (Lepidium meyenii Walp) lipophilic extract, enhances myogenic differentiation via AKT/p38 signaling and attenuates dexamethasone-induced muscle atrophy

Maca (Lepidium meyenii) is a plant that grows in the central Andes region of Peru, and it has been reported to have various bioactive functions, such as improving or preventing osteoporosis, sexual dysfunction, and memory impairment. In this study, maca roots of various colors (yellow, red, or black) were extracted using different polar solvents (PE, HEX, or BuOH) to compare their effects on muscle differentiation. Among them, the red maca lipophilic extract, which showed the most effectiveness, was chosen for further investigation. Our results show that RMLE enhances muscle differentiation by inducing MyoD-E2A heterodimerization through the activation of the AKT/p38 pathway. Additionally, RMLE attenuated dexamethasone-induced muscle atrophy by inhibiting nuclear translocation of FoxO3a and expression of E3-ligase (MAFbx and MURF1) in vitro and in vivo. Therefore, based on these results suggest that lipophilic extract of maca, which can abundantly contain nonpolar compounds, macamides, can enhance the functional properties of maca in alleviating muscle homeostasis.

Inonotus obliquus upregulates muscle regeneration and augments function through muscle oxidative metabolism

Skeletal muscle wasting related to aging or pathological conditions is critically associated with the increased incidence and prevalence of secondary diseases including cardiovascular diseases, metabolic syndromes, and chronic inflammations. Much effort is made to develop agents to enhance muscle metabolism and function. Inonotus obliquus (I. obliquus; IO) is a mushroom popularly called chaga and has been widely employed as a folk medicine for inflammation, cardiovascular diseases, diabetes, and cancer in Eastern Europe and Asia. However, its effect on muscle health has not been explored. Here, we aimed to investigate the beneficial effect of IO extract in muscle regeneration and metabolism. The treatment of IO in C2C12 myoblasts led to increased myogenic differentiation and alleviation of dexamethasone-induced myotube atrophy. Network pharmacological analysis using the identified specific chemical constituents of IO extracts predicted protein kinase B (AKT)-dependent mechanisms to promote myogenesis and muscle regeneration. Consistently, IO treatment resulted in the activation of AKT, which suppressed muscle-specific ubiquitin E3 ligases induced by dexamethasone. IO treatment in mice improved the regeneration of cardiotoxin-injured muscles accompanied by elevated proliferation and differentiation of muscle stem cells. Furthermore, it elevated the mitochondrial content and muscle oxidative metabolism accompanied by the induction of peroxisome proliferator-activated receptor γ coactivator α (PGC-1α). Our current data suggest that IO is a promising natural agent in enhancing muscle regenerative capacity and oxidative metabolism thereby preventing muscle wasting.

Regulation of Phosphoinositide Signaling by Scaffolds at Cytoplasmic Membranes

Cytoplasmic phosphoinositides (PI) are critical regulators of the membrane-cytosol interface that control a myriad of cellular functions despite their low abundance among phospholipids. The metabolic cycle that generates different PI species is crucial to their regulatory role, controlling membrane dynamics, vesicular trafficking, signal transduction, and other key cellular events. The synthesis of phosphatidylinositol (3,4,5)-triphosphate (PI3,4,5P3) in the cytoplamic PI3K/Akt pathway is central to the life and death of a cell. This review will focus on the emerging evidence that scaffold proteins regulate the PI3K/Akt pathway in distinct membrane structures in response to diverse stimuli, challenging the belief that the plasma membrane is the predominant site for PI3k/Akt signaling. In addition, we will discuss how PIs regulate the recruitment of specific scaffolding complexes to membrane structures to coordinate vesicle formation, fusion, and reformation during autophagy as well as a novel lysosome repair pathway.

Saponin extract from Achyranthes bidentata Blume alleviates disuse-induced muscle atrophy through PI3K/Akt signaling pathway

ETHNOPHARMACOLOGICAL RELEVANCE

The roots of Achyranthes bidentata Blume are one of the regularly used herbal drugs in Chinese medicine, and has been applied for strengthening the muscle and bone for a long time. However, its effect on muscle remains unclear.

AIM OF THE STUDY

This paper aims to explore the anti-muscle atrophy effect of A. bidentata, and to clarify the possible signaling pathways involved.

MATERIALS AND METHODS

The saponin extract of the roots of A. bidentata (ABSE) was prepared and analyzed, and its activity on myoblast differentiation was assayed with C2C12 cell culture. ABSE was then orally administered at dosage of 35, 70 and 140 mg/kg/day to disuse-induced muscle atrophy mice. The studies on mice body weight and muscle quality were conducted, and Western blot was used for exploring the possible signaling pathways involved in the muscle protective action aided with transcriptome analysis.

RESULTS

The total saponin content of ABSE was 59.1%. ABSE promoted the C2C12 cells differentiation to myotube in C2C12 differentiation assay. Further study with disuse-induced muscle atrophy mice model demonstrated that ABSE significantly increased muscle fiber diameter as well as the proportion of slow muscle fibers. Possible mechanism study aided with transcriptome analysis revealed that ABSE alleviated muscle atrophy at least through activation of PI3K/Akt pathway in vivo & vitro.

CONCLUSIONS

The saponin extract of the root of A. bidentata (ABSE) has a protective effect on muscle atrophy, and showed a considerable potential in prevention and treatment of muscle atrophy.

MicroRNA-668-3p inhibits myoblast proliferation and differentiation by targeting Appl1

BACKGROUND

Skeletal muscle is the largest tissue in the body, and it affects motion, metabolism and homeostasis. Skeletal muscle development comprises myoblast proliferation, fusion and differentiation to form myotubes, which subsequently form mature muscle fibres. This process is strictly regulated by a series of molecular networks. Increasing evidence has shown that noncoding RNAs, especially microRNAs (miRNAs), play vital roles in regulating skeletal muscle growth. Here, we showed that miR-668-3p is highly expressed in skeletal muscle.

METHODS

Proliferating and differentiated C2C12 cells were transfected with miR-668-3p mimics and/or inhibitor, and the mRNA and protein levels of its target gene were evaluated by RT‒qPCR and Western blotting analysis. The targeting of Appl1 by miR-668-3p was confirmed by dual luciferase assay. The interdependence of miR-668-3p and Appl1 was verified by cotransfection of C2C12 cells.

RESULTS

Our data reveal that miR-668-3p can inhibit myoblast proliferation and myogenic differentiation. Phosphotyrosine interacting with PH domain and leucine zipper 1 (Appl1) is a target gene of miR-668-3p, and it can promote myoblast proliferation and differentiation by activating the p38 MAPK pathway. Furthermore, the inhibitory effect of miR-668-3p on myoblast cell proliferation and myogenic differentiation could be rescued by Appl1.

CONCLUSION

Our results indicate a new mechanism by which the miR-668-3p/Appl1/p38 MAPK pathway regulates skeletal muscle development.

Ginsenoside compound K ameliorates palmitate-induced atrophy in C2C12 myotubes via promyogenic effects and AMPK/autophagy-mediated suppression of endoplasmic reticulum stress

Background

Compound K (CK) is among the protopanaxadiol (PPD)-type ginsenoside group, which produces multiple pharmacological effects. Herein, we examined the effects of CK on muscle atrophy under hyperlipidemic conditions along with its pro-myogenic effects. Further, the molecular pathways underlying the effects of CK on skeletal muscle have been justified.

Methods

C2C12 myotubes were treated with palmitate and CK. C2C12 myoblasts were differentiated using CK for 4-5 days. For the in vivo experiments, CK was administered to mice fed on a high-fat diet for 8 weeks. The protein expression levels were analyzed using western blotting analysis. Target protein suppression was performed using small interfering (si) RNA transfection. Histological examination was performed using Jenner-Giemsa and H&E staining techniques.

Results

CK treatment attenuated ER stress markers, such as eIF2α phosphorylation and CHOP expression and impaired myotube formation in palmitate-treated C2C12 myotubes and skeletal muscle of mice fed on HFD. CK treatment augmented AMPK along with autophagy markers in skeletal muscle cells in vitro and in vivo experiments. AMPK siRNA or 3-MA, an autophagy inhibitor, abrogated the impacts of CK in C2C12 myotubes. CK treatment augmented p38 and Akt phosphorylation, leading to an enhancement of C2C12 myogenesis. However, AMPK siRNA abolished the effects of CK in C2C12 myoblasts.

Conclusion

These findings denote that CK prevents lipid-induced skeletal muscle apoptosis via AMPK/autophagy-mediated attenuation of ER stress and induction of myoblast differentiation. Therefore, we may suggest the use of CK as a potential therapeutic approach for treating muscle-wasting conditions associated with obesity.

Ginsenoside Rg3 protects glucocorticoid‑induced muscle atrophy in vitro through improving mitochondrial biogenesis and myotube growth

Ginsenoside Rg3 (Rg3), amplified by iterative heating processing with fresh ginseng, has a broad range of pharmacological activities and improves mitochondrial biogenesis in skeletal muscle. However, thus far no study has examined how Rg3 affects myotube growth or muscle atrophy, to the best of the authors' knowledge. The present study was conducted to examine the myogenic effect of Rg3 on dexamethasone (DEX)‑induced myotube atrophy and the underlying molecular mechanisms. Rg3 activated Akt/mammalian target of rapamycin signaling to prevent DEX‑induced myotube atrophy thereby stimulating the expression of muscle‑specific genes, including myosin heavy chain and myogenin, and suppressing muscle‑specific ubiquitin ligases as demonstrated by immunoblotting and immunostaining assays. Furthermore, Rg3 efficiently prevented DEX‑triggered mitochondrial dysfunction of myotubes through peroxisome proliferator‑activated receptor‑γ coactivator1α activities and its mitochondrial biogenetic transcription factors, nuclear respiratory factor‑1 and mitochondrial transcription factor A. These were confirmed by immunoblotting, luciferase assays, RT‑qPCR and mitochondrial analysis measuring the levels of ROS, ATP and membrane potential. By providing a mechanistic insight into the effect of Rg3 on myotube atrophy, the present study suggested that Rg3 has potential as a therapeutic or nutraceutical remedy to intervene in muscle aging or diseases including cancer cachexia.

Aronia Upregulates Myogenic Differentiation and Augments Muscle Mass and Function Through Muscle Metabolism

Black chokeberry or aronia (the fruit of Aronia melanocarpa) has been reported to having pharmacological activities against metabolic syndrome, such as hypertension, obesity, diabetes, and pro-inflammatory conditions. However, the effects of aronia on myogenic differentiation and muscle homoeostasis are uncharacterized. In this study, we investigated the effects of aronia (black chokeberry) on myogenic differentiation and muscle metabolic functions in young mice. Aronia extract (AR) promotes myogenic differentiation and elevates the formation of multinucleated myotubes through Akt activation. AR protects dexamethasone (DEX)-induced myotube atrophy through inhibition of muscle-specific ubiquitin ligases mediated by Akt activation. The treatment with AR increases muscle mass and strength in mice without cardiac hypertrophy. AR treatment enhances both oxidative and glycolytic myofibers and muscle metabolism with elevated mitochondrial genes and glucose metabolism-related genes. Furthermore, AR-fed muscle fibers display increased levels of total OxPHOS and myoglobin proteins. Taken together, AR enhances myogenic differentiation and improves muscle mass and function, suggesting that AR has a promising potential as a nutraceutical remedy to intervene in muscle weakness and atrophy.

Cell adhesion an important determinant of myogenesis and satellite cell activity

Skeletal muscles represent a complex and highly organised tissue responsible for all voluntary body movements. Developed through an intricate and tightly controlled process known as myogenesis, muscles form early in development and are maintained throughout life. Due to the constant stresses that muscles are subjected to, skeletal muscles maintain a complex course of regeneration to both replace and repair damaged myofibers and to form new functional myofibers. This process, made possible by a pool of resident muscle stem cells, termed satellite cells, and controlled by an array of transcription factors, is additionally reliant on a diverse range of cell adhesion molecules and the numerous signaling cascades that they initiate. This article will review the literature surrounding adhesion molecules and their roles in skeletal muscle myogenesis and repair.

Akt scaffold proteins: the key to controlling specificity of Akt signaling

The phosphatidylinositol 3-kinase-Akt signaling pathway plays an essential role in regulating cell proliferation and apoptosis. Akt kinase is at the center of this signaling pathway and interacts with a variety of proteins. Akt is overexpressed in almost 80% of tumors. However, inhibiting Akt has serious clinical side effects so is not a suitable treatment for cancer. During recent years, Akt scaffold proteins have received increasing attention for their ability to regulate Akt signaling and have emerged as potential targets for cancer therapy. In this paper, we categorize Akt kinase scaffold proteins into four groups based on their cellular location: membrane-bound activator and inhibitor, cytoplasm, and endosome. We describe how these scaffolds interact with Akt kinase, how they affect Akt activity, and how they regulate the specificity of Akt signaling. We also discuss the clinical application of Akt scaffold proteins as targets for cancer therapy.

BST204 Protects Dexamethasone-Induced Myotube Atrophy through the Upregulation of Myotube Formation and Mitochondrial Function

BST204 is a purified ginseng dry extract that has an inhibitory effect on lipopolysaccharide-induced inflammatory responses, but its effect on muscle atrophy is yet to be investigated. In this study, C2C12 myoblasts were induced to differentiate for three days followed by the treatment of dexamethasone (DEX), a corticosteroid drug, with vehicle or BST204 for one day and subjected to immunoblotting, immunocytochemistry, qRT-PCR and biochemical analysis for mitochondrial function. BST204 alleviates the myotube atrophic effect mediated by DEX via the activation of protein kinase B/mammalian target of rapamycin (Akt/mTOR) signaling. Through this pathway, BST204 suppresses the expression of muscle-specific E3 ubiquitin ligases contributing to the enhanced myotube formation and enlarged myotube diameter in DEX-treated myotubes. In addition, BST204 treatment significantly decreases the mitochondrial reactive oxygen species production in DEX-treated myotubes. Furthermore, BST204 improves mitochondrial function by upregulating the expression of peroxisome proliferator-activated receptor-γ coactivator-1α (PGC1α) in DEX-induced myotube atrophy. This study provides a mechanistic insight into the effect of BST204 on DEX-induced myotube atrophy, suggesting that BST204 has protective effects against the toxicity of a corticosteroid drug in muscle and promising potential as a nutraceutical remedy for the treatment of muscle weakness and atrophy.

Indoprofen prevents muscle wasting in aged mice through activation of PDK1/AKT pathway

BACKGROUND

Muscle wasting, resulting from aging or pathological conditions, leads to reduced quality of life, increased morbidity, and increased mortality. Much research effort has been focused on the development of exercise mimetics to prevent muscle atrophy and weakness. In this study, we identified indoprofen from a screen for peroxisome proliferator-activated receptor γ coactivator α (PGC-1α) inducers and report its potential as a drug for muscle wasting.

METHODS

The effects of indoprofen treatment on dexamethasone-induced atrophy in mice and in 3-phosphoinositide-dependent protein kinase-1 (PDK1)-deleted C2C12 myotubes were evaluated by immunoblotting to determine the expression levels of myosin heavy chain and anabolic-related and oxidative metabolism-related proteins. Young, old, and disuse-induced muscle atrophic mice were administered indoprofen (2 mg/kg body weight) by gavage. Body weight, muscle weight, grip strength, isometric force, and muscle histology were assessed. The expression levels of muscle mass-related and function-related proteins were analysed by immunoblotting or immunostaining.

RESULTS

In young (3-month-old) and aged (22-month-old) mice, indoprofen treatment activated oxidative metabolism-related enzymes and led to increased muscle mass. Mechanistic analysis using animal models and muscle cells revealed that indoprofen treatment induced the sequential activation of AKT/p70S6 kinase (S6K) and AMP-activated protein kinase (AMPK), which in turn can augment protein synthesis and PGC-1α induction, respectively. Structural prediction analysis identified PDK1 as a target of indoprofen and, indeed, short-term treatment with indoprofen activated the PDK1/AKT/S6K pathway in muscle cells. Consistent with this finding, PDK1 inhibition abrogated indoprofen-induced AKT/S6K activation and hypertrophic response.

CONCLUSIONS

Our findings demonstrate the effects of indoprofen in boosting skeletal muscle mass through the sequential activation of PDK1/AKT/S6K and AMPK/PGC-1α. Taken together, our results suggest that indoprofen represents a potential drug to prevent muscle wasting and weakness related to aging or muscle diseases.

Ginsenoside Rb1 and Rb2 upregulate Akt/mTOR signaling-mediated muscular hypertrophy and myoblast differentiation

BACKGROUND

As a process of aging, skeletal muscle mass and function gradually decrease. It is reported that ginsenoside Rb1 and Rb2 play a role as AMP-activated protein kinase activator, resulting in regulating glucose homeostasis, and Rb1 reduces oxidative stress in aged skeletal muscles through activating the phosphatidylinositol 3-kinase/Akt/Nrf2 pathway. We examined the effects of Rb1 and Rb2 on differentiation of the muscle stem cells and myotube formation.

METHODS

C2C12 myoblasts treated with Rb1 and/or Rb2 were differentiated and induced to myotube formation, followed by immunoblotting for myogenic marker proteins, such as myosin heavy chain, MyoD, and myogenin, or immunostaining for myosin heavy chain or immunoprecipitation analysis for heterodimerization of MyoD/E-proteins.

RESULTS

Rb1 and Rb2 enhanced myoblast differentiation through accelerating MyoD/E-protein heterodimerization and increased myotube hypertrophy, accompanied by activation of Akt/mammalian target of rapamycin signaling. In addition, Rb1 and Rb2 induced the MyoD-mediated transdifferentiation of the rhabdomyosarcoma cells into myoblasts. Furthermore, co-treatment with Rb1 and Rb2 had synergistically enhanced myoblast differentiation through Akt activation.

CONCLUSION

Rb1 and Rb2 upregulate myotube growth and myogenic differentiation through activating Akt/mammalian target of rapamycin signaling and inducing myogenic conversion of fibroblasts. Thus, our first finding indicates that Rb1 and Rb2 have strong potential as a helpful remedy to prevent and treat muscle atrophy, such as age-related muscular dystrophy.

BST204, a Rg3 and Rh2 Enriched Ginseng Extract, Upregulates Myotube Formation and Mitochondrial Function in TNF-α-Induced Atrophic Myotubes

The loss of skeletal muscle mass and function is a serious consequence of chronic diseases and aging. BST204 is a purified ginseng (the root of Panax ginseng) extract that has been processed using ginsenoside-β-glucosidase and acid hydrolysis to enrich ginsenosides Rg3 and Rh2 from the crude ginseng. BST204 has a broad range of health benefits, but its effects and mechanism on muscle atrophy are currently unknown. In this study, we have examined the effects and underlying mechanisms of BST204 on myotube formation and myotube atrophy induced by tumor necrosis factor-α (TNF-α). BST204 promotes myogenic differentiation and multinucleated myotube formation through Akt activation. BST204 prevents myotube atrophy induced by TNF-α through the activation of Akt/mTOR signaling and down-regulation of muscle-specific ubiquitin ligases, MuRF1, and Atrogin-1. Furthermore, BST204 treatment in atrophic myotubes suppresses mitochondrial reactive oxygen species (ROS) production and regulates mitochondrial transcription factors such as NRF1 and Tfam, through enhancing the activity and expression of peroxisome proliferator-activated receptor-γ coactivator1α (PGC1α). Collectively, our findings indicate that BST204 improves myotube formation and PGC1α-mediated mitochondrial function, suggesting that BST204 is a potential therapeutic or neutraceutical remedy to intervene muscle weakness and atrophy.

Ginsenoside Rg3 upregulates myotube formation and mitochondrial function, thereby protecting myotube atrophy induced by tumor necrosis factor-alpha

ETHNOPHARMACOLOGICAL RELEVANCE

Ginsenoside Rg3 from Panax ginseng has reported to have multiple pharmacological activities including anti-diabetics, anti-inflammation and anti-cancer. However, the effect of ginsenoside Rg3 on myogenic differentiation and muscle atrophy is unknown.

AIM TO THE STUDY

In this study, we investigated the myogenic effect and underlying molecular mechanisms of ginsenoside Rg3 on myotube atrophy induced by tumor necrosis factor-α (TNF-α).

MATERIALS AND METHODS

C2C12 myoblasts were induced to differentiate for one day followed by the treatment of TNF-α along with vehicle or ginsenoside Rg3 for additional 2 days and subjected to immunoblotting, immunocytochemistry, quantitative RT-PCR and biochemical analysis for mitochondrial function.

RESULTS

Ginsenoside Rg3 promotes myogenic differentiation and multinucleated myotube formation through Akt activation in a dose-dependent manner, without any cytotoxicity. Ginsenoside Rg3 treatment restores myotube formation and increases myotube diameters under TNF-α-treated conditions. Ginsenoside Rg3 enhances Akt/mTOR (mammalian target of rapamycin) signaling that in turn stimulates muscle-specific gene expression such as myosin heavy chain (MHC) and Myogenin, and suppresses the expression of muscle-specific ubiquitin ligases. In addition, ginsenoside Rg3 in TNF-α-treated myotubes significantly inhibits the production of mitochondrial ROS and restores mitochondrial membrane potential (MMP) and ATP contents. Furthermore, ginsenoside Rg3 upregulates the activities and expression of peroxisome proliferator-activated receptor-γ coactivator-1α (PGC1α) and the mitochondrial biogenetic transcription factors, nuclear respiratory factor-1 (NRF1) and mitochondrial transcription factor A (Tfam) in TNF-α-induced myotube atrophy.

CONCLUSIONS

This study provides a mechanistic insight into the effect of ginsenoside Rg3 on myogenic differentiation and myotube atrophy, suggesting that ginsenoside Rg3 has a promising potential as a therapeutic or neutraceutical remedy to intervene muscle weakness and atrophy.

Disease-specific and glucocorticoid-responsive serum biomarkers for Duchenne Muscular Dystrophy

Extensive biomarker discoveries for DMD have occurred in the past 7 years, and a vast array of these biomarkers were confirmed in independent cohorts and across different laboratories. In these previous studies, glucocorticoids and age were two major confounding variables. In this new study, using SomaScan technology and focusing on a subset of young DMD patients who were not yet treated with glucocorticoids, we identified 108 elevated and 70 decreased proteins in DMD relative to age matched healthy controls (p value < 0.05 after adjusting for multiple testing). The majority of the elevated proteins were muscle centric followed by cell adhesion, extracellular matrix proteins and a few pro-inflammatory proteins. The majority of decreased proteins were of cell adhesion, however, some had to do with cell differentiation and growth factors. Subsequent treatment of this group of DMD patients with glucocorticoids affected two major groups of pharmacodynamic biomarkers. The first group consisted of 80 serum proteins that were not associated with DMD and either decreased or increased following treatment with glucocorticoids, and therefore were reflective of a broader effect of glucocorticoids. The second group consisted of 17 serum proteins that were associated with DMD and these tended to normalize under treatment, thus reflecting physiologic effects of glucocorticoid treatment in DMD. In summary, we have identified a variety of circulating protein biomarkers that reflect the complex nature of DMD pathogenesis and response to glucocorticoids.

Ginsenoside Rg1 augments oxidative metabolism and anabolic response of skeletal muscle in mice

Background

The ginsenoside Rg1 has been shown to exert various pharmacological activities with health benefits. Previously, we have reported that Rg1 promoted myogenic differentiation and myotube growth in C2C12 myoblasts. In this study, the in vivo effect of Rg1 on fiber-type composition and oxidative metabolism in skeletal muscle was examined.

Methods

To examine the effect of Rg1 on skeletal muscle, 3-month-old mice were treated with Rg1 for 5 weeks. To assess muscle strength, grip strength tests were performed, and the lower hind limb muscles were harvested, followed by various detailed analysis, such as histological staining, immunoblotting, immunostaining, and real-time quantitative reverse transcription polymerase chain reaction. In addition, to verify the in vivo data, primary myoblasts isolated from mice were treated with Rg1, and the Rg1 effect on myotube growth was examined by immunoblotting and immunostaining analysis.

Results

Rg1 treatment increased the expression of myosin heavy chain isoforms characteristic for both oxidative and glycolytic muscle fibers; increased myofiber sizes were accompanied by enhanced muscle strength. Rg1 treatment also enhanced oxidative muscle metabolism with elevated oxidative phosphorylation proteins. Furthermore, Rg1-treated muscles exhibited increased levels of anabolic S6 kinase signaling.

Conclusion

Rg1 improves muscle functionality via enhancing muscle gene expression and oxidative muscle metabolism in mice.

Canadine from Corydalis turtschaninovii Stimulates Myoblast Differentiation and Protects against Myotube Atrophy

Cachexia and sarcopenia are the main causes of muscle atrophy. These result in a reduction in the muscle fiber area, myo-protein content, and muscle strength, with various molecular modulators being involved. Although several reports have proposed potential therapeutic agents, no effective treatments have been found for muscle atrophy. We searched for myogenic modulators from medicinal plants to treat muscle diseases. We isolated six alkaloids from Corydalis turtschaninovii and evaluated their myogenic potential by using the MyoD reporter gene assay in C2C12 cells. Among the tested compounds, canadine showed the strongest transactivation of MyoD and increased MHC expression during myogenesis. The activation of p38 MAP kinase and Akt are major mechanisms that contribute to the myogenesis by canadine. Canadine increased the number of multinucleated and cylinder-shaped myotubes during myogenesis of C2C12 myoblasts. To determine the preventive effect of canadine in cancer-induced muscle wasting, differentiated C2C12 myotubes were treated with conditioned media from CT26 colon carcinoma culture (CT26 CM) in the presence of canadine. Canadine ameliorated the muscle protein degradation caused by CT26-CM by down-regulating the muscle specific-E3 ligases, MAFbx/atrogin-1 and MuRF1. In this study, we found that canadine from C. turtschaninovii stimulates myogenesis and also inhibits muscle protein degradation. Therefore, we suggest canadine as a protective agent against muscle atrophy.

MicroRNA-135a is involved in podocyte injury in a transient receptor potential channel 1-dependent manner

Transient receptor potential (TRP) cation channels are essential for normal cellular physiology, and their abnormal expression may lead to a number of disorders, including podocytopathy. Therefore, it is crucial to understand the mechanisms underlying the regulation of TRP channels. In the present study, microRNA (miR)-135a was found to be upregulated in patients with focal segmental glomerulosclerosis and mice treated with adriamycin (ADR). In cultured podocytes, transforming growth factor (TGF)-β and ADR were found to promote miR-135a expression. Conversely, TRP channel 1 (TRPC1) protein levels were markedly downregulated in podocytes from mice treated with ADR, as well as in cultured podocytes treated with ADR and TGF-β. Ectopic expression of miR-135a led to severe podocyte injury and disarray of the podocyte cytoskeleton, whereas podocyte-specific expression of TRPC1 was able to reverse the pathological effects of miR-135a in cultured podocytes. Moreover, using Luciferase reporter assays and western blot analysis, TRPC1 was identified as a target gene of miR-135a. To the best of our knowledge, this is the first study to demonstrate the role of TRPC1 in the development of podocyte injury and disorders of the podocyte cytoskeleton, which may contribute to the development of novel therapeutics for podocyte injury-associated kidney diseases.

Black ginseng activates Akt signaling, thereby enhancing myoblast differentiation and myotube growth

Background

Black ginseng (BG) has greatly enhanced pharmacological activities relative to white or red ginseng. However, the effect and molecular mechanism of BG on muscle growth has not yet been examined. In this study, we investigated whether BG could regulate myoblast differentiation and myotube hypertrophy.

Methods

BG-treated C2C12 myoblasts were differentiated, followed by immunoblotting for myogenic regulators, immunostaining for a muscle marker, myosin heavy chain or immunoprecipitation analysis for myogenic transcription factors.

Results

BG treatment of C2C12 cells resulted in the activation of Akt, thereby enhancing heterodimerization of MyoD and E proteins, which in turn promoted muscle-specific gene expression and myoblast differentiation. BG-treated myoblasts formed larger multinucleated myotubes with increased diameter and thickness, accompanied by enhanced Akt/mTOR/p70S6K activation. Furthermore, the BG treatment of human rhabdomyosarcoma cells restored myogenic differentiation.

Conclusion

BG enhances myoblast differentiation and myotube hypertrophy by activating Akt/mTOR/p70S6k axis. Thus, our study demonstrates that BG has promising potential to treat or prevent muscle loss related to aging or other pathological conditions, such as diabetes.

APPL1 is a multifunctional endosomal signaling adaptor protein

Endosomal adaptor proteins are important regulators of signaling pathways underlying many biological processes. These adaptors can integrate signals from multiple pathways via localization to specific endosomal compartments, as well as through multiple protein-protein interactions. One such adaptor protein that has been implicated in regulating signaling pathways is the adaptor protein containing a pleckstrin homology (PH) domain, phosphotyrosine-binding (PTB) domain, and leucine zipper motif 1 (APPL1). APPL1 localizes to a subset of Rab5-positive endosomes through its Bin-Amphiphysin-Rvs and PH domains, and it coordinates signaling pathways through its interaction with many signaling receptors and proteins through its PTB domain. This review discusses our current understanding of the role of APPL1 in signaling and trafficking, as well as highlights recent work into the function of APPL1 in cell migration and adhesion.

Ginsenoside Rg1 from Panax ginseng enhances myoblast differentiation and myotube growth

BACKGROUND

Ginsenoside Rg1 belongs to protopanaxatriol-type ginsenosides and has diverse pharmacological activities. In this report, we investigated whether Rg1 could upregulate muscular stem cell differentiation and muscle growth.

METHODS

C2C12 myoblasts, MyoD-transfected 10T1/2 embryonic fibroblasts, and HEK293T cells were treated with Rg1 and differentiated for 2 d, subjected to immunoblotting, immunocytochemistry, or immunoprecipitation.

RESULTS

Rg1 activated promyogenic kinases, p38MAPK (mitogen-activated protein kinase) and Akt signaling, that in turn promote the heterodimerization with MyoD and E proteins, resulting in enhancing myogenic differentiation. Through the activation of Akt/mammalian target of rapamycin pathway, Rg1 induced myotube growth and prevented dexamethasone-induced myotube atrophy. Furthermore, Rg1 increased MyoD-dependent myogenic conversion of fibroblast.

CONCLUSION

Rg1 upregulates promyogenic kinases, especially Akt, resulting in improvement of myoblast differentiation and myotube growth.

Epicatechin elicits MyoD-dependent myoblast differentiation and myogenic conversion of fibroblasts

Prevention of age-associated reduction in muscle mass and function is required to manage a healthy life. Supplemental (-)-Epicatechin (EC) appears to act as a potential regulator for muscle growth and strength. However, its cellular and molecular mechanisms as a potential muscle growth agent have not been studied well. In the current study, we investigated a role of EC in differentiation of muscle progenitors to gain the molecular insight into how EC regulates muscle growth. EC enhanced myogenic differentiation in a dose-dependent manner through stimulation of promyogenic signaling pathways, p38MAPK and Akt. EC treatment elevated MyoD activity by enhancing its heterodimerization with E protein. Consistently, EC also positively regulated myogenic conversion and differentiation of fibroblasts. In conclusion, EC has a potential as a therapeutic or nutraceutical remedy to treat degenerative muscle diseases or age-related muscle weakness.

A novel function of TBK1 as a target of Cdon in oligodendrocyte differentiation and myelination

During central nervous system development, oligodendrocyte progenitors elaborate multiple branched processes to contact axons and initiate myelination. Using cultured primary rat oligodendrocytes (OLGs), we have recently demonstrated that a cell surface protein belonging to the immunoglobulin superfamily, cell adhesion molecule-related, down-regulated by oncogenes (Cdon), is important in initiating OLG differentiation and axon myelination by promoting the formation of branched cellular processes; however, the molecular mechanism by which Cdon regulates OLG differentiation is not known. Here, using Cdon immunoprecipitation (IP) and liquid chromatography-tandem mass spectrometry analysis, we identified serine/threonine kinase TANK-binding kinase 1 (TBK1) as a candidate novel target of Cdon. We confirmed this interaction using co-IP and immunofluorescence with TBK1 antibodies, showing that TBK1 partly co-localizes with Cdon along cellular processes in puncta-like structures. We show that TBK1 is expressed throughout OLG differentiation, and surprisingly, that levels of phosphorylated TBK1 (ser172) increase during OLG maturation, while total levels of TBK1 protein decrease. To investigate function, TBK1 expression was knocked down using siRNA in OLG primary cultures, reducing protein levels by 69%. Two myelin-specific proteins, myelin basic protein and myelin-associated glycoprotein, were similarly reduced when examined at day 2 and day 4 of OLG differentiation. Reduced Cdon or TBK1 expression also decreased Akt phosphorylation at Threonine 308 in OLG. Our findings provide evidence that a Cdon-TBK1 complex is associated with Akt phosphorylation and early OLG differentiation.

PKN2 and Cdo interact to activate AKT and promote myoblast differentiation

Skeletal myogenesis is coordinated by multiple signaling pathways that control cell adhesion/migration, survival and differentiation accompanied by muscle-specific gene expression. A cell surface protein Cdo is involved in cell contact-mediated promyogenic signals through activation of p38MAPK and AKT. Protein kinase C-related kinase 2 (PKN2/PRK2) is implicated in regulation of various biological processes, including cell migration, adhesion and death. It has been shown to interact with and inhibit AKT thereby inducing cell death. This led us to investigate the role of PKN2 in skeletal myogenesis and the crosstalk between PKN2 and Cdo. Like Cdo, PKN2 was upregulated in C2C12 myoblasts during differentiation and decreased in cells with Cdo depletion caused by shRNA or cultured on integrin-independent substratum. This decline of PKN2 levels resulted in diminished AKT activation during myoblast differentiation. Consistently, PKN2 overexpression-enhanced C2C12 myoblast differentiation, whereas PKN2-depletion impaired it, without affecting cell survival. PKN2 formed complexes with Cdo, APPL1 and AKT via its C-terminal region and this interaction appeared to be important for induction of AKT activity as well as myoblast differentiation. Furthermore, PKN2-enhanced MyoD-responsive reporter activities by mediating the recruitment of BAF60c and MyoD to the myogenin promoter. Taken together, PKN2 has a critical role in cell adhesion-mediated AKT activation during myoblast differentiation.

Cdo Regulates Surface Expression of Kir2.1 K+ Channel in Myoblast Differentiation

A potassium channel Kir2.1-associated membrane hyperpolarization is required for myogenic differentiation. However the molecular regulatory mechanisms modulating Kir2.1 channel activities in early stage of myogenesis are largely unknown. A cell surface protein, Cdo functions as a component of multiprotein cell surface complexes to promote myogenesis. In this study, we report that Cdo forms a complex with Kir2.1 during myogenic differentiation, and is required for the channel activity by enhancing the surface expression of Kir2.1 in the early stage of differentiation. The expression of a constitutively active form of the upstream kinase for p38MAPK, MKK6(EE) can restore Kir2.1 activities in Cdo-depleted C2C12 cells, while the treatment with a p38MAPK inhibitor, SB203580 exhibits a similar effect of Cdo depletion on Kir2.1 surface expression. Furthermore, Cdo^-/- primary myoblasts, which display a defective differentiation program, exhibit a defective Kir2.1 activity. Taken together, our results suggest that a promyogenic Cdo signaling is critical for Kir2.1 activities in the induction of myogenic differentiation.

Prmt7 Deficiency Causes Reduced Skeletal Muscle Oxidative Metabolism and Age-Related Obesity

Maintenance of skeletal muscle function is critical for metabolic health and the disruption of which exacerbates many chronic diseases such as obesity and diabetes. Skeletal muscle responds to exercise or metabolic demands by a fiber-type switch regulated by signaling-transcription networks that remains to be fully defined. Here, we report that protein arginine methyltransferase 7 (Prmt7) is a key regulator for skeletal muscle oxidative metabolism. Prmt7 is expressed at the highest levels in skeletal muscle and decreased in skeletal muscles with age or obesity. Prmt7(-/-) muscles exhibit decreased oxidative metabolism with decreased expression of genes involved in muscle oxidative metabolism, including PGC-1α. Consistently, Prmt7(-/-) mice exhibited significantly reduced endurance exercise capacities. Furthermore, Prmt7(-/-) mice exhibit decreased energy expenditure, which might contribute to the exacerbated age-related obesity of Prmt7(-/-) mice. Similarly to Prmt7(-/-) muscles, Prmt7 depletion in myoblasts also reduces PGC-1α expression and PGC-1α-promoter driven reporter activities. Prmt7 regulates PGC-1α expression through interaction with and activation of p38 mitogen-activated protein kinase (p38MAPK), which in turn activates ATF2, an upstream transcriptional activator for PGC-1α. Taken together, Prmt7 is a novel regulator for muscle oxidative metabolism via activation of p38MAPK/ATF2/PGC-1α.

Bakuchiol augments MyoD activation leading to enhanced myoblast differentiation

Myoblast differentiation is fundamental to skeletal muscle development and regeneration after injury and defects in this process are implicated in muscle atrophy associated with aging or pathological conditions. MyoD family transcription factors function as mater regulators in induction of muscle-specific genes during myoblast differentiation. We have identified bakuchiol, a prenylated phenolic monoterpene, as an inducer of MyoD-mediated transcription and myogenic differentiation. C2C12 myoblasts treated with bakuchiol exhibit enhanced muscle-specific gene expression and myotube formation. A key promyogenic kinase p38MAPK is activated dramatically by bakuchiol which in turn induced the formation of MyoD/E protein active transcription complexes. Consistently, the recruitment of MyoD and Baf60c to the Myogenin promoter is enhanced in bakuchiol-treated myoblasts. Furthermore, bakuchiol rescues defective p38MAPK activation and myogenic differentiation caused by Cdo-depletion or in RD rhabdomyosarcoma cells. Taken together, these results indicate that bakuchiol enhances myogenic differentiation through p38MAPK and MyoD activation. Thus bakuchiol can be developed into a potential agent to improve muscular regeneration and repair to treat muscular atrophy.

Invited Review: Polycomb group genes in the regeneration of the healthy and pathological skeletal muscle

The polycomb group (PcG) proteins are epigenetic repressors required during key developmental processes, such as maintenance of cell identity and stem cell differentiation. To exert their repressive function, PcG proteins assemble on chromatin into multiprotein complexes, known as polycomb repressive complex 1 and 2. In this review, we will focus on the role and mode of function of PcG proteins in the development and regeneration of the skeletal muscle, both in normal and pathological conditions and we will discuss the emerging concept of modulation of their expression to enhance the muscle-specific regenerative process for patient benefit.

Syntaxin 4 regulates the surface localization of a promyogenic receptor Cdo thereby promoting myogenic differentiation

Background

Syntaxins are a family of membrane proteins involved in vesicle trafficking, such as synaptic vesicle exocytosis. Syntaxin 4 (Stx4) is expressed highly in skeletal muscle and plays a critical role in insulin-stimulated glucose uptake by promoting translocation of glucose transporter 4 (GLUT4) to the cell surface. A cell surface receptor cell adhesion molecule-related, down-regulated by oncogenes (Cdo) is a component of cell adhesion complexes and promotes myoblast differentiation via activation of key signalings, including p38MAPK and AKT. In this study, we investigate the function of Stx4 in myoblast differentiation and the crosstalk between Stx4 and Cdo in myoblast differentiation.

Methods

The effects of overexpression or shRNA-based depletion of Stx4 and Cdo genes on C2C12 myoblast differentiation are assessed by Western blotting and immunofluorescence approaches. The interaction between Cdo and Stx4 and the responsible domain mapping are assessed by coimmunoprecipitation or pulldown assays. The effect of Stx4 depletion on cell surface localization of Cdo and GLUT4 in C2C12 myoblasts is assessed by surface biotinylation and Western blotting.

Results

Overexpression or knockdown of Stx4 enhances or inhibits myogenic differentiation, respectively. Stx4 binds to the cytoplasmic tail of Cdo, and this interaction seems to be critical for induction of p38MAPK activation and myotube formation. Stx4 depletion decreases specifically the cell surface localization of Cdo without changes in surface N-Cadherin levels. Interestingly, Cdo depletion reduces the level of GLUT4 and Stx4 at cell surface. Consistently, overexpression of Cdo in C2C12 myoblasts generally increases glucose uptake, while Cdo depletion reduces it.

Conclusions

Stx4 promotes myoblast differentiation through interaction with Cdo and stimulation of its surface translocation. Both Cdo and Stx4 are required for GLUT4 translocation to cell surface and glucose uptake in myoblast differentiation.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-015-0052-8) contains supplementary material, which is available to authorized users.

Long-Term Endurance Exercise in Humans Stimulates Cell Fusion of Myoblasts along with Fusogenic Endogenous Retroviral Genes In Vivo

Myogenesis is defined as growth, differentiation and repair of muscles where cell fusion of myoblasts to multinucleated myofibers is one major characteristic. Other cell fusion events in humans are found with bone resorbing osteoclasts and placental syncytiotrophoblasts. No unifying gene regulation for natural cell fusions has been found. We analyzed skeletal muscle biopsies of competitive cyclists for muscle-specific attributes and expression of human endogenous retrovirus (ERV) envelope genes due to their involvement in cell fusion of osteoclasts and syncytiotrophoblasts. Comparing muscle biopsies from post- with the pre-competitive seasons a significant 2.25-fold increase of myonuclei/mm fiber, a 2.38-fold decrease of fiber area/nucleus and a 3.1-fold decrease of satellite cells (SCs) occurred. We propose that during the pre-competitive season SC proliferation occurred following with increased cell fusion during the competitive season. Expression of twenty-two envelope genes of muscle biopsies demonstrated a significant increase of putative muscle-cell fusogenic genes Syncytin-1 and Syncytin-3, but also for the non-fusogenic erv3. Immunohistochemistry analyses showed that Syncytin-1 mainly localized to the sarcolemma of myofibers positive for myosin heavy-chain isotypes. Cellular receptors SLC1A4 and SLC1A5 of Syncytin-1 showed significant decrease of expression in post-competitive muscles compared with the pre-competitive season, but only SLC1A4 protein expression localized throughout the myofiber. Erv3 protein was strongly expressed throughout the myofiber, whereas envK1-7 localized to SC nuclei and myonuclei. Syncytin-1 transcription factors, PPARγ and RXRα, showed no protein expression in the myofiber, whereas the pCREB-Ser133 activator of Syncytin-1 was enriched to SC nuclei and myonuclei. Syncytin-1, Syncytin-3, SLC1A4 and PAX7 gene regulations along with MyoD1 and myogenin were verified during proliferating or actively-fusing human primary myoblast cell cultures, resembling muscle biopsies of cyclists. Myoblast treatment with anti-Synycytin-1 abrogated cell fusion in vitro. Our findings support functional roles for ERV envelope proteins, especially Syncytin-1, contributing to cell fusion of myotubes.

miR-135 family members mediate podocyte injury through the activation of Wnt/β-catenin signaling

The upregulation of Wnt/β-catenin signaling occurs in virtually all types of kidney disease and is associated with podocyte injury. However, the precise mechanisms involved in the development of kidney disease remain to be elucidated. MicroRNAs (miRNAs or miRs) are a class of short non-coding RNAs and they have been shown to be regulators of gene expression, mainly by binding to the untranslated region (UTR) of mRNAs. The aim of the present study was to determine the role of the 2 members of the miR-135 family (miR-135a and miR-135b) in podocyte injury and to elucidate the mechanisms responsible for the damage to podocytes. The results revealed that miR-135a and miR-135b were upregulated in models of podocyte injury and in glomeruli isolated from patients with focal segmental glomerulosclerosis (FSGS). The ectopic expression of miR-135a and miR-135b led to severe podocyte injury and the disorder of the podocyte cytoskeleton. Our findings demonstrated that miR-135a and miR-135b activated Wnt/β-catenin signaling and induced the nuclear translocation of β-catenin. Using luciferase reporter assays, reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and western blot analysis, glycogen synthase kinase 3β (GSK3β) was identified as a target gene of miR-135a and miR-135b. To the best of our knowledge, this is the first study to demonstrate that members of the miR-135 family (specifically miR-135a and miR-135b) regulate the expression of GSK3β, thus playing a role in the development of podocyte injury and the disorder of the podocyte cytoskeleton. This is an important finding as it may contribute to the development of novel therapeutics for podocyte injury-associated glomerulopathies.

Kazinol-P from Broussonetia kazinoki enhances skeletal muscle differentiation via p38MAPK and MyoD

The activation of MyoD family transcription factors is critical for myogenic differentiation, which is fundamental to the regeneration of skeletal muscle after injury. Kazinol-P (KP) from Broussonetia kazinoki (B. kazinoki), a natural compound, has been reported to possess an anti-oxidant function. In a screen of natural compounds for agonists of the MyoD activity, we identified KP and examined its effect on myoblast differentiation. Consistently, KP enhanced the myotube formation, accompanied with upregulation of myogenic markers such as MHC, Myogenin and Troponin-T. KP treatment in C2C12 myoblasts led to strong activation of a key promyogenic kinase p38MAPK in a dose, and time-dependent manner. Furthermore, KP treatment enhanced the MyoD-mediated trans-differentiation of 10T1/2 fibroblasts into myoblasts. Taken together, KP promotes myogenic differentiation through activation of p38MAPK and MyoD transcription activities. Thus KP may be a potential therapeutic candidate to prevent fibrosis and improve muscle regeneration and repair.

Up-regulation of adiponectin expression in antigravitational soleus muscle in response to unloading followed by reloading, and functional overloading in mice

The purpose of this study was to investigate the expression level of adiponectin and its related molecules in hypertrophied and atrophied skeletal muscle in mice. The expression was also evaluated in C2C12 myoblasts and myotubes. Both mRNA and protein expression of adiponectin, mRNA expression of adiponectin receptor (AdipoR) 1 and AdipoR2, and protein expression of adaptor protein containing pleckstrin homology domain, phosphotyrosine binding domain, and leucine zipper motif 1 (APPL1) were observed in C2C12 myoblasts. The expression levels of these molecules in myotubes were higher than those in myoblasts. The expression of adiponectin-related molecules in soleus muscle was observed at mRNA (adiponectin, AdipoR1, AdipoR2) and protein (adiponectin, APPL1) levels. The protein expression levels of adiponectin and APPL1 were up-regulated by 3 weeks of functional overloading. Down-regulation of AdipoR1 mRNA, but not AdipoR2 mRNA, was observed in atrophied soleus muscle. The expression of adiponectin protein, AdipoR1 mRNA, and APPL1 protein was up-regulated during regrowth of unloading-associated atrophied soleus muscle. Mechanical loading, which could increase skeletal muscle mass, might be a useful stimulus for the up-regulations of adiponectin and its related molecules in skeletal muscle.

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.

Phosphorylation of Stim1 at serine 575 via netrin-2/Cdo-activated ERK1/2 is critical for the promyogenic function of Stim1

The promyogenic cell surface molecule Cdo is required for activation of extracellular signal-regulated kinase (ERK) and nuclear factor of activated T cells c3 (NFATc3) induced by netrin-2 in myogenic differentiation. However, the molecular mechanism leading to NFATc3 activation is unknown. Stromal interaction molecule 1 (Stim1), an internal calcium sensor of the endoplasmic reticulum store, promotes myogenesis via activation of NFATc3. In this study we investigated the functional interaction between Cdo and Stim1 in myogenic differentiation. Overexpression and depletion of Stim1 enhanced or decreased myotube formation, respectively. Of interest, Stim1 protein levels were decreased in Cdo-deficient perinatal hindlimb muscles or primary myoblasts; this correlates with defective NFATc3 activation in Cdo(-/-) myoblasts upon differentiation. Forced activation of NFATc3 by overexpression of calcineurin restored differentiation of Cdo-depleted C2C12 myoblasts. Furthermore, Cdo and Stim1 formed a complex in 293T cells or in differentiating C2C12 myoblasts. The netrin-2-mediated NFATc3 activation was coincident with robust interactions between Cdo and Stim1 in myoblasts and the ERK-mediated Stim1 phosphorylation at serine 575. The serine 575 phosphorylation was enhanced in C2C12 cells upon differentiation, and the alanine substitution of serine 575 failed to restore differentiation of Stim1-depleted myoblasts. Taken together, the results indicate that cell adhesion signaling triggered by netrin-2/Cdo induces Stim1 phosphorylation at serine 575 by ERK, which promotes myoblast differentiation.

Adaptor protein containing PH domain, PTB domain and leucine zipper (APPL1) regulates the protein level of EGFR by modulating its trafficking

The EGFR-mediated signaling pathway regulates multiple biological processes such as cell proliferation, survival and differentiation. Previously APPL1 (adaptor protein containing PH domain, PTB domain and leucine zipper 1) has been reported to function as a downstream effector of EGF-initiated signaling. Here we demonstrate that APPL1 regulates EGFR protein levels in response to EGF stimulation. Overexpression of APPL1 enhances EGFR stabilization while APPL1 depletion by siRNA reduces EGFR protein levels. APPL1 depletion accelerates EGFR internalization and movement of EGF/EGFR from cell surface to the perinuclear region in response to EGF treatment. Conversely, overexpression of APPL1 decelerates EGFR internalization and translocation of EGF/EGFR to the perinuclear region. Furthermore, APPL1 depletion enhances the activity of Rab5 which is involved in internalization and trafficking of EGFR and inhibition of Rab5 in APPL1-depleted cells restored EGFR levels. Consistently, APPL1 depletion reduced activation of Akt, the downstream signaling effector of EGFR and this is restored by inhibition of Rab5. These findings suggest that APPL1 is required for EGFR signaling by regulation of EGFR stabilities through inhibition of Rab5.

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.