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

Lactate induces C2C12 myoblasts differentiation by mediating ROS/p38 MAPK signalling pathway

Lactate serves not merely as an energy substrate for skeletal muscle but also regulates myogenic differentiation, leading to an elevation of reactive oxygen species (ROS) levels. The present study was focused on exploring the effects of lactate and ROS/p38 MAPK in promoting C2C12 myoblasts differentiation. Our results demonstrated that lactate increased C2C12 myoblasts differentiation at a range of physiological concentrations, accompanied by enhanced ROS contents. We used n-acetylcysteine (NAC, a ROS scavenger) pretreatment and found that it delayed lactate-induced C2C12 myoblast differentiation by upregulating Myf5 expression on days 5 and 7 and lowering MyoD and MyoG expression. The finding implies that lactate accompanies ROS-dependent manner to promote C2C12 myoblast differentiation. Additionally, lactate significantly increased p38 MAPK phosphorylation to promote C2C12 cell differentiation, but pretreatment with SB203580 (p38 MAPK inhibitor) reduced lactate-induced C2C12 myoblasts differentiation. whereas lactate pretreatment with NAC inhibited p38 MAPK phosphorylation in C2C12 cells, demonstrating that lactate mediated ROS and regulated the p38 MAPK signalling pathway to promote C2C12 cell differentiation. In conclusion, our results suggest that the promotion of C2C12 myoblasts differentiation by lactate is dependent on ROS and the p38 MAPK signalling pathway. These observations reveal a beneficial role for lactate in increasing myogenesis through ROS-sensitive mechanisms as well as providing new ideas regarding the positive impact of ROS in improving the function of skeletal muscle.

Promising therapeutic mechanism for Chinese herbal medicine in ameliorating renal fibrosis in diabetic nephropathy

Diabetic nephropathy (DN) is one of the most serious chronic microvascular abnormalities of diabetes mellitus and the major cause of uremia. Accumulating evidence has confirmed that fibrosis is a significant pathological feature that contributes to the development of chronic kidney disease in DN. However, the exact mechanism of renal fibrosis in DN is still unclear, which greatly hinders the treatment of DN. Chinese herbal medicine (CHM) has shown efficacy and safety in ameliorating inflammation and albuminuria in diabetic patients. In this review, we outline the underlying mechanisms of renal fibrosis in DN, including oxidative stress (OS) generation and OS-elicited ASK1-p38/JNK activation. Also, we briefly summarize the current status of CHM treating DN by improving renal fibrosis. The treatment of DN by inhibiting ASK1 activation to alleviate renal fibrosis in DN with CHM will promote the discovery of novel therapeutic targets for DN and provide a beneficial therapeutic method for DN.

Emerging role of TAK1 in the regulation of skeletal muscle mass

Maintenance of skeletal muscle mass and strength throughout life is crucial for heathy living and longevity. Several signaling pathways have been implicated in the regulation of skeletal muscle mass in adults. TGF-β-activated kinase 1 (TAK1) is a key protein, which coordinates the activation of multiple signaling pathways. Recently, it was discovered that TAK1 is essential for the maintenance of skeletal muscle mass and myofiber hypertrophy following mechanical overload. Forced activation of TAK1 in skeletal muscle causes hypertrophy and attenuates denervation-induced muscle atrophy. TAK1-mediated signaling in skeletal muscle promotes protein synthesis, redox homeostasis, mitochondrial health, and integrity of neuromuscular junctions. In this article, we have reviewed the role and potential mechanisms through which TAK1 regulates skeletal muscle mass and growth. We have also proposed future areas of research that could be instrumental in exploring TAK1 as therapeutic target for improving muscle mass in various catabolic conditions and diseases.

TAK1 of blunt snout bream promotes NF-κB activation via interaction with TAB1 in response to pathogenic bacteria

Transforming growth factor-β activated kinase-1 (TAK1) is an important upstream signaling molecules involved in the NF-κB signaling pathway. TAK1 interacts with TAB1 to form the TAK1-TAB1 complex, which elicits NF-κB activation through a series of cascade reactions in mammals. However, the function of TAK1 in blunt snout bream (Megalobrama amblycephala ( maTak1) and the effects of their interaction between TAK1 and TAB1 on the NF-κB activation still remains largely unknown. In the present study, maTak1 was cloned and characterized successfully based on transcriptome data. Its open reading frame is composed of 1626 nucleotides and the predicted maTAK1 protein contains 541 amino acids, which includes an N-terminal Serine/Threonine protein kinases (S/TKc) and a C-terminal coiled-coil region. Phylogenetic analysis showed that maTAK1 were clustered with those of other teleosts. MaTak1 displayed ubiquitous transcriptional expression in all the examined tissues of healthy blunt snout bream but with varied expression levels. And maTrak1 expression was dramatically enhanced in different tissues and MAF cells after LPS stimulation and A. hydrophila challenge. The result from subcellular localization analysis indicated that both maTAK1 and maTAB1 were cytoplasmic protein. The activity of NF-κB promoter could not be induced by overexpression of maTak1 or maTab1 alone, however, it could be enhanced by co-expression of maTak1 and maTab1. Co-immunoprecipitation and subcellular co-localization assay revealed that maTAK1 can combine with maTAB1 directly. The transcriptional expression level of pro-inflammatory cytokines (IL-1β, IL-6 and IL-8) increased distinctly after the overexpression of maTak1 and maTab1. Taken together, the data obtained in this study demonstrated that the direct interaction between maTAK1 and maTAB1 might play a pivotal role in mediating host innate immune response to pathogen invasion by the production of pro-inflammatory cytokines via NF-κB signaling pathway, which might lay a solid foundation for the establishment of novel therapeutic approach to combat bacterial infection in fish.

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.

Muscle-specific deletion of SLK/Stk2 enhances p38 activity and myogenesis in mdx mice

Duchenne's muscular dystrophy (DMD) is a severe muscle wasting disorder characterized by the loss of dystrophin expression, muscle necrosis, inflammation and fibrosis. Ongoing muscle regeneration is impaired by persistent cytokine stress, further decreasing muscle function. Patients with DMD rarely survive beyond their early 20s, with cardiac and respiratory dysfunction being the primary cause of death. Despite an increase in our understanding of disease progression as well as promising preclinical animal models for therapeutic intervention, treatment options for muscular dystrophy remain limited and novel therapeutic targets are required. Many reports suggest that the TGFβ signalling pathway is activated in dystrophic muscle and contributes to the pathology of DMD in part by impairing the differentiation of myoblasts into mature myofibers. Here, we show that in vitro knockdown of the Ste20-like kinase, SLK, can partially restore myoblast differentiation downstream of TGFβ in a Smad2/3 independent manner. In an mdx model, we demonstrate that SLK is expressed at high levels in regenerating myofibers. Muscle-specific deletion of SLK reduced leukocyte infiltration, increased myogenin and utrophin expression and enhanced differentiation. This was accompanied by resistance to eccentric contraction-induced injury in slow fiber type-enriched soleus muscles. Finally, we found that these effects were partially dependent on the upregulation of p38 signalling. Collectively, these results demonstrate that SLK downregulation can restore some aspects of disease progression in DMD.

PRMT7 deficiency causes dysregulation of the HCN channels in the CA1 pyramidal cells and impairment of social behaviors

HCN channels regulate excitability and rhythmicity in the hippocampal CA1 pyramidal cells. Perturbation in the HCN channel current (I_h) is associated with neuropsychiatric disorders, such as autism spectrum disorders. Recently, protein arginine methyltransferase 7 (PRMT7) was shown to be highly expressed in the hippocampus, including the CA1 region. However, the physiological function of PRMT7 in the CA1 neurons and the relationship to psychiatric disorders are unclear. Here we showed that PRMT7 knockout (KO) mice exhibit hyperactivity and deficits in social interaction. The firing frequency of the CA1 neurons in the PRMT7 KO mice was significantly higher than that in the wild-type (WT) mice. Compared with the WT CA1 neurons, the PRMT7 KO CA1 neurons showed a more hyperpolarized resting potential and a higher input resistance, which were occluded by the I_h-current inhibitor ZD7288; these findings were consistent with the decreased I_h and suggested the contribution of I_h-channel dysfunction to the PRMT7 KO phenotypes. The HCN1 protein level was decreased in the CA1 region of the PRMT7 KO mice in conjunction with a decrease in the expression of Shank3, which encodes a core scaffolding protein for HCN channel proteins. A brief application of the PRMT7 inhibitor DS437 did not reproduce the phenotype of the PRMT7 KO neurons, further indicating that PRMT7 regulates I_h by controlling the channel number rather than the open probability. Moreover, shRNA-mediated PRMT7 suppression reduced both the mRNA and protein levels of SHANK3, implying that PRMT7 deficiency might be responsible for the decrease in the HCN protein levels by altering Shank3 expression. These findings reveal a key role for PRMT7 in the regulation of HCN channel density in the CA1 pyramidal cells that may be amenable to pharmacological intervention for neuropsychiatric disorders.

Disrupted expression of an ion channel that helps stabilize brain cell activity contributes to behavioral symptoms in mice resembling those seen in autism spectrum disorders (ASDs). Nerve cell firing depends on the right balance of ions inside and outside cells, and a channel protein called HCN helps establish ionic conditions that prevent excessive activity. Researchers led by Hana Cho and Ilmin Kwon of the Sungkyunkwan University School of Medicine, Suwon, South Korea have demonstrated that mice lacking another protein called PRMT7 exhibit reduced numbers of HCN channels in brain structures known to be affected in animal models of ASDs. These mice exhibit hyperactivity and social anxiety, presumably as a consequence of poor regulation of nerve cell firing. The authors propose that this PRMT7-HCN pathway may offer a fruitful target for the development of neuropsychiatric therapies.

Differences in the fast muscle methylome provide insight into sex-specific epigenetic regulation of growth in Nile tilapia during early stages of domestication

Growth is a complex trait whose variability within a population cannot be explained solely by genetic variation. Epigenetic regulation is often suggested as an important factor shaping the phenotype, but its association with growth can be highly context- and species-dependent. Nevertheless, the mechanisms involved in epigenetic regulation of growth in fish are poorly understood. We have used reduced representation bisulphite sequencing to determine the genome-wide CpG methylation patterns in male and female Nile tilapia of different sizes but at the same early stage of domestication. The average CpG methylation level in the reduced genome representation was 63% across groups but many sites displayed group-specific methylation patterns. The number of differentially methylated (DM) CpGs was much higher when the interaction between sex and weight was included rather than when these factors were considered separately. There were 1128 DM CpGs between large and small females and 970 DM CpGs between large and small males. We have found many growth-related genes associated with DM CpGs, namely map3k5 and akt3 in females and gadd45g and ppargc1a in males. Only 5% of CpG locations associated with growth were common to both sexes. In particular, the autophagy-related gene atg14 displayed a high association of methylation with growth exclusively in males. The sexually dimorphic association between atg14 methylation and growth may uncover novel metabolic mechanisms at play during mouth brooding in Nile tilapia females. Taken together, our data suggest that epigenetic regulation of growth in Nile tilapia involves different gene networks in males and females.

Prmt7 promotes myoblast differentiation via methylation of p38MAPK on arginine residue 70

MyoD functions as a master regulator to induce muscle-specific gene expression and myogenic differentiation. Here, we demonstrate a positive role of Protein arginine methyltransferase 7 (Prmt7) in MyoD-mediated myoblast differentiation through p38MAPK activation. Prmt7 depletion in primary or C2C12 myoblasts impairs cell cycle withdrawal and myogenic differentiation. Furthermore, Prmt7 depletion decreases the MyoD-reporter activities and the MyoD-mediated myogenic conversion of fibroblasts. Together with MyoD, Prmt7 is recruited to the Myogenin promoter region and Prmt7 depletion attenuates the recruitment of MyoD and its coactivators. The mechanistic study reveals that Prmt7 methylates p38MAPKα at the arginine residue 70, thereby promoting its activation which in turn enhances MyoD activities. The arginine residue 70 to alanine mutation in p38MAPKα impedes MyoD/E47 heterodimerization and the recruitment of Prmt7, MyoD and Baf60c to the Myogenin promoter resulting in blunted Myogenin expression. In conclusion, Prmt7 promotes MyoD-mediated myoblast differentiation through methylation of p38MAPKα at arginine residue 70.

PRMT7 methylates and suppresses GLI2 binding to SUFU thereby promoting its activation

Cellular senescence is implicated in aging or age-related diseases. Sonic hedgehog (Shh) signaling, an inducer of embryonic development, has recently been demonstrated to inhibit cellular senescence. However, the detailed mechanisms to activate Shh signaling to prevent senescence is not well understood. Here, we demonstrate that Protein arginine methyltransferase 7 (PRMT7) promotes Shh signaling via GLI2 methylation which is critical for suppression of cellular senescence. PRMT7-deficient mouse embryonic fibroblasts (MEFs) exhibited a premature cellular senescence with accompanied increase in the cell cycle inhibitors p16 and p21. PRMT7 depletion results in reduced Shh signaling activity in MEFs while PRMT7 overexpression enhances GLI2-reporter activities that are sensitive to methylation inhibition. PRMT7 interacts with and methylates GLI2 on arginine residues 225 and 227 nearby a binding region of SUFU, a negative regulator of GLI2. This methylation interferes with GLI2-SUFU binding, leading to facilitation of GLI2 nuclear accumulation and Shh signaling. Taken together, these data suggest that PRMT7 induces GLI2 methylation, reducing its binding to SUFU and increasing Shh signaling, ultimately leading to prevention of cellular senescence.

Regulation of Redox Homeostasis by Nonthermal Biocompatible Plasma Discharge in Stem Cell Differentiation

Recently, a growing body of evidence has shown the role of reactive species as secondary messengers in cell proliferation and differentiation, as opposed to the harmful metabolism byproducts that they were previously solely recognized as. Thus, the balance of intracellular reduction-oxidation (redox) homeostasis plays a vital role in the regulation of stem cell self-renewal and differentiation. Nonthermal biocompatible plasma (NBP) has emerged as a novel tool in biomedical applications. Recently, NBP has also emerged as a powerful tool in the tissue engineering field for the surface modification of biomaterial and the promotion of stem cell differentiation by the regulation of intracellular redox biology. NBP can generate various kinds of reactive oxygen species (ROS) and reactive nitrogen species (RNS), which may play the role of the second passenger in the cell signaling network and active antioxidant system in cells. Herein, we review the current knowledge on mechanisms by which NBP regulates cell proliferation and differentiation through redox modification. Considering the importance of redox homeostasis in the regulation of stem cell differentiation, understanding the underlying molecular mechanisms involved will provide important new insights into NBP-induced stem cell differentiation for tissue engineering.

Continuous exposure of isoprenaline inhibits myoblast differentiation and fusion through PKA/ERK1/2-FOXO1 signaling pathway

Aim

The objective of this study is to determine if exuberant sympathetic nerve activity is involved in muscle satellite cell differentiation and myoblast fusion.

Methods and results

By using immunoassaying and western blot analyses, we found that β1 and β2-adrenergic receptors (AdR) were expressed in C2C12 cells. The differentiated satellite cells exhibited an increased expression of β2-AdR, as compared with the proliferating cells. Continuous exposure of isoprenaline (ISO), a β-AdR agonist, delayed C2C12 cell differentiation, and myoblast fusion in time- and dose-dependent manner. ISO also increased short myotube numbers while decreasing long myotube numbers, consistent with the greater reduction in MyHC1, MyHC2a, and MyHC2x expression. Moreover, continuous exposure of ISO gradually decreased the ratio of PKA RI/RII, and PKA RI activator efficiently reversed the ISO effect on C2C12 cell differentiation and myoblast fusion while PKA inhibitor H-89 deteriorated the effects. Continuous single-dose ISO increased β1-AdR expression in C2C12 cells. More importantly, the cells showed enhanced phospho-ERK1/2 levels, resulting in increasing phospho-β2-AdR levels while decreasing β2-AdR levels, and the specific effects could be abolished by ERK1/2 inhibitor. Furthermore, continuous exposure of ISO induced FOXO1 nuclear translocation and increased the levels of FOXO1 in nuclear extracts while reducing pAKT, p-p38MAPK, and pFOXO1 levels. Conversely, blockade of ERK1/2 signaling partially abrogated ISO effects on AKT, p38MAPK, and FOXO1signaling, which partially restored C2C12 cell differentiation and myoblast fusion, leading to an increase in the numbers of medium myotube along with the increased expression of MyHC1 and MyHC2a.

Conclusion

Continuous exposure of ISO impedes satellite cell differentiation and myoblast fusion, at least in part, through PKA-ERK1/2-FOXO1 signaling pathways, which were associated with the reduced β2-AdR and increased β1-AdR levels.

Electronic supplementary material

The online version of this article (10.1186/s13287-019-1160-x) contains supplementary material, which is available to authorized users.

SGTb regulates a surface localization of a guidance receptor BOC to promote neurite outgrowth

Neuritogenesis is a critical event for neuronal differentiation and neuronal circuitry formation during neuronal development and regeneration. Our previous study revealed a critical role of a guidance receptor BOC in a neuronal differentiation and neurite outgrowth. However, regulatory mechanisms for BOC signaling pathway remain largely unexplored. In the current study, we have identified Small glutamine-rich tetratricopeptide repeat (TPR)-containing b (SGTb) as a BOC interacting protein through yeast two-hybrid screening. Like BOC, SGTb is highly expressed in brain and P19 embryonal carcinoma (EC) cells differentiated into neuronal cells. BOC and SGTb proteins co-precipitate in mouse brain and differentiated P19 EC cells. Furthermore, BOC and SGTb co-localize in neurites and especially are concentrated at the tip of neurites in various neuronal cells. SGTb depletion attenuates neuronal differentiation of P19 cells through reduction of the surface level of BOC. Additionally, SGTb depletion causes BOC localization at neurite tip, coinciding with decreased p-JNK levels critical for actin cytoskeleton remodeling. The overexpression of SGTb or BOC restores JNK activation in BOC or SGTb-depleted cells, respectively. Finally, SGTb elevates the level of surface-resident BOC in BOC-depleted cells, restoring JNK activation. Taken together, our data suggest that SGTb interacts with BOC and regulates its surface level and consequent JNK activation, thereby promoting neuronal differentiation and neurite outgrowth.

Hsp70 Interacts with Mitogen-Activated Protein Kinase (MAPK)-Activated Protein Kinase 2 To Regulate p38MAPK Stability and Myoblast Differentiation during Skeletal Muscle Regeneration

The regenerative process of injured muscle is dependent on the fusion and differentiation of myoblasts derived from muscle stem cells. Hsp70 is important for maintaining skeletal muscle homeostasis and regeneration, but the precise cellular mechanism remains elusive. In this study, we found that Hsp70 was upregulated during myoblast differentiation. Depletion or inhibition of Hsp70/Hsc70 impaired myoblast differentiation. Importantly, overexpression of p38 mitogen-activated protein kinase α (p38MAPKα) but not AKT1 rescued the impairment of myogenic differentiation in Hsp70- or Hsc70-depleted myoblasts. Moreover, Hsp70 interacted with MK2, a substrate of p38MAPK, to regulate the stability of p38MAPK. Knockdown of Hsp70 also led to downregulation of both MK2 and p38MAPK in intact muscles and during cardiotoxin-induced muscle regeneration. Hsp70 bound MK2 to regulate MK2-p38MAPK interaction in myoblasts. We subsequently identified the essential regions required for Hsp70-MK2 interaction. Functional analyses showed that MK2 is essential for both myoblast differentiation and skeletal muscle regeneration. Taken together, our findings reveal a novel role of Hsp70 in regulating myoblast differentiation by interacting with MK2 to stabilize p38MAPK.

Apoptosis signal-regulating kinase 1 mediates the inhibitory effect of hepatocyte nuclear factor-4α on hepatocellular carcinoma

Previous studies provided substantial evidence of a striking suppressive effect of hepatocyte nuclear factor 4α (HNF4α) on hepatocellular carcinoma (HCC). Apoptosis signal-regulating kinase 1 (ASK1) is involved in death receptor-mediated apoptosis and may acts as a tumor suppressor in hepatocarcinogenesis. However, the status and function of ASK1 during HCC progression are unclear. In this study, we found that HNF4α increased ASK1 expression by directly binding to its promoter. ASK1 expression was dramatically suppressed and correlated with HNF4α levels in HCC tissues. Reduced ASK1 expression was associated with aggressive tumors and poor prognosis for human HCC. Moreover, ASK1 inhibited the malignant phenotype of HCC cells in vitro. Intratumoral ASK1 injection significantly suppressed the growth of subcutaneous HCC xenografts in nude mice. More interestingly, systemic ASK1 delivery strikingly inhibited the growth of orthotopic HCC nodules in NOD/SCID mice. In addition, inhibition of endogenous ASK1 partially reversed the suppressive effects of HNF4α on HCC. Collectively, this study highlights the suppressive effect of ASK1 on HCC and its biological significance in HCC development. These outcomes broaden the knowledge of ASK1 function in HCC progression, and provide a novel potential prognostic biomarker and therapeutic target for advanced HCC.

The regulatory and signaling mechanisms of the ASK family

Apoptosis signal-regulating kinase 1 (ASK1) was identified as a MAP3K that activates the JNK and p38 pathways, and subsequent studies have reported ASK2 and ASK3 as members of the ASK family. The ASK family is activated by various intrinsic and extrinsic stresses, including oxidative stress, ER stress and osmotic stress. Numerous lines of evidence have revealed that members of the ASK family are critical for signal transduction systems to control a wide range of stress responses such as cell death, differentiation and cytokine induction. In this review, we focus on the precise signaling mechanisms of the ASK family in response to diverse stressors.

Cdon deficiency causes cardiac remodeling through hyperactivation of WNT/β-catenin signaling

On pathological stress, Wnt signaling is reactivated and induces genes associated with cardiac remodeling and fibrosis. We have previously shown that a cell surface receptor Cdon (cell-adhesion associated, oncogene regulated) suppresses Wnt signaling to promote neuronal differentiation however its role in heart is unknown. Here, we demonstrate a critical role of Cdon in cardiac function and remodeling. Cdon is expressed and predominantly localized at intercalated disk in both mouse and human hearts. Cdon-deficient mice develop cardiac dysfunction including reduced ejection fraction and ECG abnormalities. Cdon^-/- hearts exhibit increased fibrosis and up-regulation of genes associated with cardiac remodeling and fibrosis. Electrical remodeling was demonstrated by up-regulation and mislocalization of the gap junction protein, Connexin 43 (Cx43) in Cdon^-/- hearts. In agreement with altered Cx43 expression, functional analysis both using Cdon^-/- cardiomyocytes and shRNA-mediated knockdown in rat cardiomyocytes shows aberrant gap junction activities. Analysis of the underlying mechanism reveals that Cdon^-/- hearts exhibit hyperactive Wnt signaling as evident by β-catenin accumulation and Axin2 up-regulation. On the other hand, the treatment of rat cardiomyocytes with a Wnt activator TWS119 reduces Cdon levels and aberrant Cx43 activities, similarly to Cdon-deficient cardiomyocytes, suggesting a negative feedback between Cdon and Wnt signaling. Finally, inhibition of Wnt/β-catenin signaling by XAV939, IWP2 or dickkopf (DKK)1 prevented Cdon depletion-induced up-regulation of collagen 1a and Cx43. Taken together, these results demonstrate that Cdon deficiency causes hyperactive Wnt signaling leading to aberrant intercellular coupling and cardiac fibrosis. Cdon exhibits great potential as a target for the treatment of cardiac fibrosis and cardiomyopathy.

Group I Paks Promote Skeletal Myoblast Differentiation In Vivo and In Vitro

Skeletal myogenesis is regulated by signal transduction, but the factors and mechanisms involved are not well understood. The group I Paks Pak1 and Pak2 are related protein kinases and direct effectors of Cdc42 and Rac1. Group I Paks are ubiquitously expressed and specifically required for myoblast fusion in Drosophila We report that both Pak1 and Pak2 are activated during mammalian myoblast differentiation. One pathway of activation is initiated by N-cadherin ligation and involves the cadherin coreceptor Cdo with its downstream effector, Cdc42. Individual genetic deletion of Pak1 and Pak2 in mice has no overt effect on skeletal muscle development or regeneration. However, combined muscle-specific deletion of Pak1 and Pak2 results in reduced muscle mass and a higher proportion of myofibers with a smaller cross-sectional area. This phenotype is exacerbated after repair to acute injury. Furthermore, primary myoblasts lacking Pak1 and Pak2 display delayed expression of myogenic differentiation markers and myotube formation. These results identify Pak1 and Pak2 as redundant regulators of myoblast differentiation in vitro and in vivo and as components of the promyogenic Ncad/Cdo/Cdc42 signaling pathway.

Keep Your Friends Close: Cell-Cell Contact and Skeletal Myogenesis

Development of skeletal muscle is a multistage process that includes lineage commitment of multipotent progenitor cells, differentiation and fusion of myoblasts into multinucleated myofibers, and maturation of myofibers into distinct types. Lineage-specific transcriptional regulation lies at the core of this process, but myogenesis is also regulated by extracellular cues. Some of these cues are initiated by direct cell-cell contact between muscle precursor cells themselves or between muscle precursors and cells of other lineages. Examples of the latter include interaction of migrating neural crest cells with multipotent muscle progenitor cells, muscle interstitial cells with myoblasts, and neurons with myofibers. Among the signaling factors involved are Notch ligands and receptors, cadherins, Ig superfamily members, and Ephrins and Eph receptors. In this article we describe recent progress in this area and highlight open questions raised by the findings.

A Sonic hedgehog coreceptor, BOC regulates neuronal differentiation and neurite outgrowth via interaction with ABL and JNK activation

Neurite outgrowth is a critical step for neurogenesis and remodeling synaptic circuitry during neuronal development and regeneration. An immunoglobulin superfamily member, BOC functions as Sonic hedgehog (Shh) coreceptor in canonical and noncanonical Shh signaling in neuronal development and axon outgrowth/guidance. However signaling mechanisms responsible for BOC action during these processes remain unknown. In our previous studies, a multiprotein complex containing BOC and a closely related protein CDO promotes myogenic differentiation through activation of multiple signaling pathways, including non-receptor tyrosine kinase ABL. Given that ABL and Jun. N-terminal kinase (JNK) are implicated in actin cytoskeletal dynamics required for neurogenesis, we investigated the relationship between BOC, ABL and JNK during neuronal differentiation. Here, we demonstrate that BOC and ABL are induced in P19 embryonal carcinoma (EC) cells and cortical neural progenitor cells (NPCs) during neuronal differentiation. BOC-depleted EC cells or Boc^-/- NPCs exhibit impaired neuronal differentiation with shorter neurite formation. BOC interacts with ABL through its putative SH2 binding domain and seems to be phosphorylated in an ABL activity-dependent manner. Unlike wildtype BOC, ABL-binding defective BOC mutants exhibit impaired JNK activation and neuronal differentiation. Finally, Shh treatment enhances JNK activation which is diminished by BOC depletion. These data suggest that BOC interacts with ABL and activates JNK thereby promoting neuronal differentiation and neurite outgrowth.

Dehydrocorydaline promotes myogenic differentiation via p38 MAPK activation

Muscle regeneration is a coordinated process that involves proliferation and differentiation of muscle progenitor cells. Activation of MyoD is a key event in myogenic differentiation, which is regulated by p38 mitogen‑activated protein kinases (MAPK). In a screen of natural compounds for the enhancement of MyoD activity, dehydrocorydaline (DHC) from the Corydalis tuber was identified. Treatment of C2C12 myoblasts with DHC increased the expression levels of muscle‑specific proteins, including MyoD, myogenin and myosin heavy chain. In addition, C2C12 myoblasts exhibited enhanced multinucleated myotube formation without any cytotoxicity. Treatment with DHC elevated p38 MAPK activation and the interaction of MyoD with an E protein, which is likely to result in activation of MyoD and promotion of myoblast differentiation. Furthermore, defects in differentiation‑induced p38 MAPK activation and myoblast differentiation induced by depletion of the promyogenic receptor protein Cdo in C2C12 myoblasts were restored by DHC treatment. In conclusion, these results indicated that DHC stimulates p38 MAPK activation, which can enhance heterodimerization of MyoD and E proteins, thus resulting in MyoD activation and myoblast differentiation. These findings suggested that DHC may be considered a potential therapeutic compound for the improvement of muscle stem cell regenerative capacity in injured muscle.

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.

TAK1 modulates satellite stem cell homeostasis and skeletal muscle repair

Satellite cells are resident adult stem cells that are required for regeneration of skeletal muscle. However, signalling mechanisms that regulate satellite cell function are less understood. Here we demonstrate that transforming growth factor-β-activated kinase 1 (TAK1) is important in satellite stem cell homeostasis and function. Inactivation of TAK1 in satellite cells inhibits muscle regeneration in adult mice. TAK1 is essential for satellite cell proliferation and its inactivation causes precocious differentiation. Moreover, TAK1-deficient satellite cells exhibit increased oxidative stress and undergo spontaneous cell death, primarily through necroptosis. TAK1 is required for the activation of NF-κB and JNK in satellite cells. Forced activation of NF-κB improves survival and proliferation of TAK1-deficient satellite cells. Furthermore, TAK1-mediated activation of JNK is essential to prevent oxidative stress and precocious differentiation of satellite cells. Collectively, our study suggests that TAK1 is required for maintaining the pool of satellite stem cells and for regenerative myogenesis.

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.

Separating myoblast differentiation from muscle cell fusion using IGF-I and the p38 MAP kinase inhibitor SB202190

The p38 MAP kinases play critical roles in skeletal muscle biology, but the specific processes regulated by these kinases remain poorly defined. Here we find that activity of p38α/β is important not only in early phases of myoblast differentiation, but also in later stages of myocyte fusion and myofibrillogenesis. By treatment of C2 myoblasts with the promyogenic growth factor insulin-like growth factor (IGF)-I, the early block in differentiation imposed by the p38 chemical inhibitor SB202190 could be overcome. Yet, under these conditions, IGF-I could not prevent the later impairment of muscle cell fusion, as marked by the nearly complete absence of multinucleated myofibers. Removal of SB202190 from the medium of differentiating myoblasts reversed the fusion block, as multinucleated myofibers were detected several hours later and reached ∼90% of the culture within 30 h. Analysis by quantitative mass spectroscopy of proteins that changed in abundance following removal of the inhibitor revealed a cohort of upregulated muscle-enriched molecules that may be important for both myofibrillogenesis and fusion. We have thus developed a model system that allows separation of myoblast differentiation from muscle cell fusion and should be useful in identifying specific steps regulated by p38 MAP kinase-mediated signaling in myogenesis.

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.

Mouse models of heart failure: cell signaling and cell survival

Heart failure is one of the paramount global causes of morbidity and mortality. Despite this pandemic need, the available clinical counter-measures have not altered substantially in recent decades, most notably in the context of pharmacological interventions. Cell death plays a causal role in heart failure, and its inhibition poses a promising approach that has not been thoroughly explored. In previous approaches to target discovery, clinical failures have reflected a deficiency in mechanistic understanding, and in some instances, failure to systematically translate laboratory findings toward the clinic. Here, we review diverse mouse models of heart failure, with an emphasis on those that identify potential targets for pharmacological inhibition of cell death, and on how their translation into effective therapies might be improved in the future.

Skeletal cell differentiation is enhanced by atmospheric dielectric barrier discharge plasma treatment

Enhancing chondrogenic and osteogenic differentiation is of paramount importance in providing effective regenerative therapies and improving the rate of fracture healing. This study investigated the potential of non-thermal atmospheric dielectric barrier discharge plasma (NT-plasma) to enhance chondrocyte and osteoblast proliferation and differentiation. Although the exact mechanism by which NT-plasma interacts with cells is undefined, it is known that during treatment the atmosphere is ionized generating extracellular reactive oxygen and nitrogen species (ROS and RNS) and an electric field. Appropriate NT-plasma conditions were determined using lactate-dehydrogenase release, flow cytometric live/dead assay, flow cytometric cell cycle analysis, and Western blots to evaluate DNA damage and mitochondrial integrity. We observed that specific NT-plasma conditions were required to prevent cell death, and that loss of pre-osteoblastic cell viability was dependent on intracellular ROS and RNS production. To further investigate the involvement of intracellular ROS, fluorescent intracellular dyes Mitosox (superoxide) and dihydrorhodamine (peroxide) were used to assess onset and duration after NT-plasma treatment. Both intracellular superoxide and peroxide were found to increase immediately post NT-plasma treatment. These increases were sustained for one hour but returned to control levels by 24 hr. Using the same treatment conditions, osteogenic differentiation by NT-plasma was assessed and compared to peroxide or osteogenic media containing β-glycerolphosphate. Although both NT-plasma and peroxide induced differentiation-specific gene expression, neither was as effective as the osteogenic media. However, treatment of cells with NT-plasma after 24 hr in osteogenic or chondrogenic media significantly enhanced differentiation as compared to differentiation media alone. The results of this study show that NT-plasma can selectively initiate and amplify ROS signaling to enhance differentiation, and suggest this technology could be used to enhance bone fusion and improve healing after skeletal injury.