TAK1 modulates satellite stem cell homeostasis and skeletal muscle repair

Mechanisms of gelofusine protection in an in vitro model of polymyxin B-associated renal injury

Polymyxins are a last-resort treatment option for multidrug-resistant gram-negative bacterial infections, but they are associated with nephrotoxicity. Gelofusine was previously shown to reduce polymyxin-associated kidney injury in an animal model. However, the mechanism(s) of renal protection has not been fully elucidated. Here, we report the use of a cell culture model to provide insights into the mechanisms of renal protection. Murine epithelial proximal tubular cells were exposed to polymyxin B. Cell viability, lactate dehydrogenase (LDH) release, polymyxin B uptake, mitochondrial superoxide production, nuclear morphology, and apoptosis activation were evaluated with or without concomitant gelofusine. A megalin knockout cell line was used as an uptake inhibition control. Methionine was included in selected experiments as an antioxidant control. A polymyxin B concentration-dependent reduction in cell viability was observed. Increased viability was observed in megalin knockout cells following comparable polymyxin B exposures. Compared with polymyxin B exposure alone, concomitant gelofusine significantly increased cell viability as well as reduced LDH release, polymyxin B uptake, mitochondrial superoxide, and apoptosis. Gelofusine and methionine were more effective at reducing renal cell injury in combination than either agent alone. In conclusion, the mechanisms of renal protection by gelofusine involve decreasing cellular drug uptake, reducing subsequent oxidative stress and apoptosis activation. These findings would be valuable for translational research into clinical strategies to attenuate drug-associated acute kidney injury.NEW & NOTEWORTHY Gelofusine is a gelatinous saline solution with the potential to attenuate polymyxin-associated nephrotoxicity. We demonstrated that the mechanisms of gelofusine renal protection involve reducing polymyxin B uptake by proximal tubule cells, limiting subsequent oxidative stress and apoptosis activation. In addition, gelofusine was more effective at reducing cellular injury than a known antioxidant control, methionine, and a megalin knockout cell line, indicating that gelofusine likely has additional pharmacological properties besides only megalin inhibition.

Inflammatory Cytokine-Induced Muscle Atrophy and Weakness Can Be Ameliorated by an Inhibition of TGF-β-Activated Kinase-1

Chronic inflammation causes muscle wasting. Because most inflammatory cytokine signals are mediated via TGF-β-activated kinase-1 (TAK1) activation, inflammatory cytokine-induced muscle wasting may be ameliorated by the inhibition of TAK1 activity. The present study was undertaken to clarify whether TAK1 inhibition can ameliorate inflammation-induced muscle wasting. SKG/Jcl mice as an autoimmune arthritis animal model were treated with a small amount of mannan as an adjuvant to enhance the production of TNF-α and IL-1β. The increase in these inflammatory cytokines caused a reduction in muscle mass and strength along with an induction of arthritis in SKG/Jcl mice. Those changes in muscle fibers were mediated via the phosphorylation of TAK1, which activated the downstream signaling cascade via NF-κB, p38 MAPK, and ERK pathways, resulting in an increase in myostatin expression. Myostatin then reduced the expression of muscle proteins not only via a reduction in MyoD1 expression but also via an enhancement of Atrogin-1 and Murf1 expression. TAK1 inhibitor, LL-Z1640-2, prevented all the cytokine-induced changes in muscle wasting. Thus, TAK1 inhibition can be a new therapeutic target of not only joint destruction but also muscle wasting induced by inflammatory cytokines.

Hypoxia promotes proliferation and inhibits myogenesis in broiler satellite cells

Breast muscle myopathies in broilers compromise meat quality and continue to plague the poultry industry. Broiler breast muscle myopathies are characterized by impaired satellite cell (SC)-mediated repair, and localized tissue hypoxia and dysregulation of oxygen homeostasis have been implicated as contributing factors. The present study was designed to test the hypothesis that hypoxia disrupts the ability of SC to differentiate and form myotubes, both of which are key components of myofiber repair, and to determine the extent to which effects are reversed by restoration of oxygen tension. Primary SC were isolated from pectoralis major of young (5 d) Cobb 700 chicks and maintained in growth conditions or induced to differentiate under normoxic (20% O2) or hypoxic (1% O2) conditions for up to 48 h. Hypoxia enhanced SC proliferation while inhibiting myogenic potential, with decreased fusion index and suppressed myotube formation. Reoxygenation after hypoxia partially reversed effects on both proliferation and myogenesis. Western blotting showed that hypoxia diminished myogenin expression, activated AMPK, upregulated proliferation markers, and increased molecular signaling of cellular stress. Hypoxia also promoted accumulation of lipid droplets in myotubes. Targeted RNAseq identified numerous differentially expressed genes across differentiation under hypoxia, including several genes that have been associated with myopathies in vivo. Altogether, these data demonstrate localized hypoxia may influence SC behavior in ways that disrupt muscle repair and promote the formation of myopathies in broilers.

Fn14 promotes myoblast fusion during regenerative myogenesis

Skeletal muscle regeneration involves coordinated activation of an array of signaling pathways. Fibroblast growth factor-inducible 14 (Fn14) is a bona fide receptor for the TWEAK cytokine. Levels of Fn14 are increased in the skeletal muscle of mice after injury. However, the cell-autonomous role of Fn14 in muscle regeneration remains unknown. Here, we demonstrate that global deletion of the Fn14 receptor in mice attenuates muscle regeneration. Conditional ablation of Fn14 in myoblasts but not in differentiated myofibers of mice inhibits skeletal muscle regeneration. Fn14 promotes myoblast fusion without affecting the levels of myogenic regulatory factors in the regenerating muscle. Fn14 deletion in myoblasts hastens initial differentiation but impairs their fusion. The overexpression of Fn14 in myoblasts results in the formation of myotubes having an increased diameter after induction of differentiation. Ablation of Fn14 also reduces the levels of various components of canonical Wnt and calcium signaling both in vitro and in vivo. Forced activation of Wnt signaling rescues fusion defects in Fn14-deficient myoblast cultures. Collectively, our results demonstrate that Fn14-mediated signaling positively regulates myoblast fusion and skeletal muscle regeneration.

PAX7, a Key for Myogenesis Modulation in Muscular Dystrophies through Multiple Signaling Pathways: A Systematic Review

Muscular dystrophy is a heterogenous group of hereditary muscle disorders caused by mutations in the genes responsible for muscle development, and is generally defined by a disastrous progression of muscle wasting and massive loss in muscle regeneration. Pax7 is closely associated with myogenesis, which is governed by various signaling pathways throughout a lifetime and is frequently used as an indicator in muscle research. In this review, an extensive literature search adhering to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines was performed to identify research that examined signaling pathways in living models, while quantifying Pax7 expression in myogenesis. A total of 247 articles were retrieved from the Web of Science (WoS), PubMed and Scopus databases and were thoroughly examined and evaluated, resulting in 19 articles which met the inclusion criteria. Admittedly, we were only able to discuss the quantification of Pax7 carried out in research affecting various type of genes and signaling pathways, rather than the expression of Pax7 itself, due to the massive differences in approach, factor molecules and signaling pathways analyzed across the research. However, we highlighted the thorough evidence for the alteration of the muscle stem cell precursor Pax7 in multiple signaling pathways described in different living models, with an emphasis on the novel approach that could be taken in manipulating Pax7 expression itself in dystrophic muscle, towards the discovery of an effective treatment for muscular dystrophy. Therefore, we believe that this could be applied to the potential gap in muscle research that could be filled by tuning the well-established marker expression to improve dystrophic muscle.

Targeted regulation of TAK1 counteracts dystrophinopathy in a DMD mouse model

Muscular dystrophies make up a group of genetic neuromuscular disorders that involve severe muscle wasting. TGF-β-activated kinase 1 (TAK1) is an important signaling protein that regulates cell survival, growth, and inflammation. TAK1 has been recently found to promote myofiber growth in the skeletal muscle of adult mice. However, the role of TAK1 in muscle diseases remains poorly understood. In the present study, we have investigated how TAK1 affects the progression of dystrophic phenotype in the mdx mouse model of Duchenne muscular dystrophy (DMD). TAK1 is highly activated in the dystrophic muscle of mdx mice during the peak necrotic phase. While targeted inducible inactivation of TAK1 inhibits myofiber injury in young mdx mice, it results in reduced muscle mass and contractile function. TAK1 inactivation also causes loss of muscle mass in adult mdx mice. By contrast, forced activation of TAK1 through overexpression of TAK1 and TAB1 induces myofiber growth without having any deleterious effect on muscle histopathology. Collectively, our results suggest that TAK1 is a positive regulator of skeletal muscle mass and that targeted regulation of TAK1 can suppress myonecrosis and ameliorate disease progression in DMD.

Inactivating IL34 promotes regenerating muscle stem cell expansion and attenuates Duchenne muscular dystrophy in mouse models

Background: The balance between the differentiation and self-renewal of satellite cells (SCs) is essential for skeletal muscle homeostasis and regeneration. Our knowledge of this regulatory process is incomplete. Methods: Using global and conditional knockout mice as in vivo models and isolated satellite cells as in vitro system, we investigated the regulatory mechanisms of IL34 in the process of skeletal muscle regeneration in vivo and in vitro. Results: Myocytes and regenerating fibers are major source of IL34. Deletion of interleukin 34 (IL34) sustains expansion by sacrificing the differentiation of SCs and leads to significant muscle regeneration defects. We further found that inactivating IL34 in SCs leads to hyperactivation of NFKB1 signaling; NFKB1 translocates to the nucleus and binds to the promoter region of Igfbp5 to synergistically disturb protein kinase B (Akt) activity. Notably, augmented Igfbp5 function in SCs led to deficient differentiation and Akt activity. Furthermore, disrupting Akt activity both in vivo and in vitro mimicked the phenotype of IL34 knockout. Finally, deleting IL34 or interfering Akt in mdx mice ameliorates dystrophic muscles. Conclusion: We comprehensively characterized regenerating myofibers-expressed IL34 plays a pivotal role in controlling myonuclear domain. The results also indicate that impairing IL34 function by promoting SC maintenance can lead to improved muscular performance in mdx mice in which the stem cell pool is compromised.

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.

Long noncoding RNA lncMREF promotes myogenic differentiation and muscle regeneration by interacting with the Smarca5/p300 complex

Long noncoding RNAs (lncRNAs) play important roles in the spatial and temporal regulation of muscle development and regeneration. Nevertheless, the determination of their biological functions and mechanisms underlying muscle regeneration remains challenging. Here, we identified a lncRNA named lncMREF (lncRNA muscle regeneration enhancement factor) as a conserved positive regulator of muscle regeneration among mice, pigs and humans. Functional studies demonstrated that lncMREF, which is mainly expressed in differentiated muscle satellite cells, promotes myogenic differentiation and muscle regeneration. Mechanistically, lncMREF interacts with Smarca5 to promote chromatin accessibility when muscle satellite cells are activated and start to differentiate, thereby facilitating genomic binding of p300/CBP/H3K27ac to upregulate the expression of myogenic regulators, such as MyoD and cell differentiation. Our results unravel a novel temporal-specific epigenetic regulation during muscle regeneration and reveal that lncMREF/Smarca5-mediated epigenetic programming is responsible for muscle cell differentiation, which provides new insights into the regulatory mechanism of muscle regeneration.

p-TAK1 acts as a switch between myoblast proliferation phase and differentiation phase in mdx mice via regulating HO-1 expression

Skeletal muscle transforming growth factor-β-activated kinase 1 (TAK1) continuous excessive phosphorylation was observed in Duchenne muscular dystrophy (DMD) patients and mdx mice. Inhibiting TAK1 phosphorylation ameliorated fibrosis and muscular atrophy, while TAK1 knockout also impaired muscle regeneration. The definite effect and mechanism of p-TAK1 in muscle regeneration disorder is still obscure. In this study, BaCl₂-induced acute muscle injury model was used to investigate the role of p-TAK1 in myoblast proliferation and differentiation phase. The results showed that TAK1 phosphorylation was significantly up-regulated in proliferation phase along with Keap1/Nrf2/HO-1 signaling pathway activation, which was down-regulated in differentiation phase yet. In C2C12 cells, inhibiting TAK1 phosphorylation markedly suppressed the expression of heme oxygenase-1 (HO-1), and both myoblast proliferation and differentiation were inhibited. As for activation, p-TAK1 promoted myoblast proliferation via up-regulating HO-1 level. However, excessive TAK1 phosphorylation (induced by 20 ng·mL^-1 TGF-β1) notably up-regulated HO-1 expression, inhibiting myogenic differentiation antigen (MyOD) and myogenic differentiation. A mild p-TAK1 level (induced by 5 or 10 ng·mL^-1 TGF-β1) was beneficial for myoblast differentiation. In mdx mice, robust myoblast proliferation and differentiation arrest were observed with high p-TAK1 level in skeletal muscle. HO-1 expression was significantly up-regulated. TAK1 phosphorylation inhibitor NG25 (N-[4-[(4-ethylpiperazin-1-yl)methyl]-3-(trifluoromethyl)phenyl]-4-methyl-3-(1H-pyrrolo[2,3-b]pyridin-4-yloxy)benzamide) significantly inhibited HO-1 expression, relieved excessive myoblast proliferation and differentiation arrest, promoted new myofiber formation, and eventually improved muscle function. In conclusion, p-TAK1 acted as "a switch" between proliferation and differentiation phase. Mitigating p-TAK1 level transformed myoblast excessive proliferation phase into differentiation phase in mdx mouse via regulating HO-1 expression.

Electroacupuncture of Weizhong (BL-40) Acupoint Inspires Muscular Satellite Cell Regeneration and Promotes Muscle Repair Capacity after Back Muscle Injury in Sprague-Dawley Rat Model

Background

Back muscle injury is the most common illness involved in aged people. Muscular satellite cells, playing a key role in the muscle repairing process, are gradually losing their regenerative ability with aging, which attenuates the injured muscle repairing process. Electroacupuncture at Weizhong acupoint has been widely used in the treatment of young and aged patients with back muscle damage. Its efficacy has been proven by a randomized double-blind placebo clinical trial. However, the rehabilitation mechanisms are largely unknown. This study will explore the possible mechanisms associated with electroacupuncture at the Weizhong acupoint (BL 40) promoting muscle repairing ability.

Method

A total of 58 male and female Sprague-Dawley rats were divided into a younger group (4-month-old) and an aged group (16-month-old), younger and aged rats were further divided as a sham, injured, injured rats treated with electroacupuncture at Weizhong point or treated with Non-Weizhong point groups. The back muscle injury model was produced in rats as a previously described method with modification. Furthermore, Weizhong acupoints underwent electroacupuncture treatment with 15 V magnitude, 2 Hz/10 Hz frequency density, 1.0 mA current intensity, and 10 min each day for 10 consecutive days using HANS's electroacupuncture apparatus. After the last treatment, the paravertebral muscles and serum of all animals were undergone histological, immunohistochemistry, and flow cytometry analysis. Serum levels of Creatine Kinase (CK) and proinflammatory cytokine, interleukin 6 (IL-6), were measured separately by using ELISA kit.

Results

Electroacupuncture of Weizhong (BL 40) acupoints significantly attenuated back muscle damage in both young and aged rats, increasing PAX7 (a marker of muscle satellite cells) and MYOD (major marker of myoblasts) cells, simultaneously, reducing serum proinflammatory cytokines, IL-6, and downregulation of p38 MAPK signaling in aged muscular satellite cells.

Conclusion

Our studies suggest that electroacupuncture of Weizhong (BL 40) acupoints can restore aged back muscular satellite cells and their regeneration capacity. These suggested electroacupuncture may be a potential means of promoting rehabilitation for muscular injury in aged patients.

Redox Signaling and Stress in Inherited Myopathies

Significance: Reactive oxygen species (ROS) are highly reactive compounds that behave like a double-edged sword; they damage cellular structures and act as second messengers in signal transduction. Mitochondria and endoplasmic reticulum (ER) are interconnected organelles with a central role in ROS production, detoxification, and oxidative stress response. Skeletal muscle is the most abundant tissue in mammals and one of the most metabolically active ones and thus relies mainly on oxidative phosphorylation (OxPhos) to synthesize adenosine triphosphate. The impairment of OxPhos leads to myopathy and increased ROS production, thus affecting both redox poise and signaling. In addition, ROS enter the ER and trigger ER stress and its maladaptive response, which also lead to a myopathic phenotype with mitochondrial involvement. Here, we review the role of ROS signaling in myopathies due to either mitochondrial or ER dysfunction. Recent Advances: Relevant advances have been evolving over the last 10 years on the intricate ROS-dependent pathways that act as modifiers of the disease course in several myopathies. To this end, pathways related to mitochondrial biogenesis, satellite cell differentiation, and ER stress have been studied extensively in myopathies. Critical Issues: The analysis of the chemistry and the exact quantitation, as well as the localization of ROS, are still challenging due to the intrinsic labile nature of ROS and the technical limitations of their sensors. Future Directions: The mechanistic studies of the pathogenesis of mitochondrial and ER-related myopathies offer a unique possibility to discover novel ROS-dependent pathways. Antioxid. Redox Signal. 37, 301-323.

Supraphysiological activation of TAK1 promotes skeletal muscle growth and mitigates neurogenic atrophy

Skeletal muscle mass is regulated through coordinated activation of multiple signaling pathways. TAK1 signalosome has been found to be activated in various conditions of muscle atrophy and hypertrophy. However, the role and mechanisms by which TAK1 regulates skeletal muscle mass remain less understood. Here, we demonstrate that supraphysiological activation of TAK1 in skeletal muscle of adult mice stimulates translational machinery, protein synthesis, and myofiber growth. TAK1 causes phosphorylation of elongation initiation factor 4E (eIF4E) independent of mTOR. Inactivation of TAK1 disrupts neuromuscular junction morphology and causes deregulation of Smad signaling. Using genetic approaches, we demonstrate that TAK1 prevents excessive loss of muscle mass during denervation. TAK1 favors the nuclear translocation of Smad4 and cytoplasmic retention of Smad6. TAK1 is also required for the phosphorylation of eIF4E in denervated skeletal muscle. Collectively, our results demonstrate that TAK1 supports skeletal muscle growth and prevents neurogenic muscle atrophy in adult mice.

TGF-β-activated kinase 1 (TAK1) is essential for the maintenance of skeletal muscle mass through incompletely understood mechanisms. Here the authors show that supraphysiological activation of TAK1 leads to muscle hypertrophy through the elongation initiation factor 4E (eIF4E) involved in protein synthesis, and prevents denervation-induced atrophy in mice.

Effects of type II collagen hydrolysates on osteoarthritis through the NF-κB, Wnt/β-catenin and MAPK pathways

Osteoarthritis (OA), a degenerative disease, has attracted extensive attention all over the world. In this study, a rat model involving medial meniscus resection (MMx) and anterior to medial collateral ligament (ACL) operation was successfully established to study the effects of bovine cartilage hydrolysates rich in type II collagen peptides (BIIP) on cartilage protection. The results of histological analysis indicated that oral administration of BIIP at doses of 200 and 500 mg kg^-1 d^-1 ameliorated cartilage degeneration. Moreover, the potential targets of BIIP affecting OA in vivo were studied by proteomics, and the effects of BIIP on OA through signaling pathways, such as NF-κB, Wnt/β-catenin and MAPK, were further explored at mRNA and protein levels. BIIP downregulated the expression of IL-6, RUNX2, NF-κB p65, HIF-2α, β-catenin and p-JNK, which may be the main factor leading to the prevention of OA. These results suggest that BIIP can be used as a novel potential substance of functional foods to exert chondroprotective action.

The thymus regulates skeletal muscle regeneration by directly promoting satellite cell expansion

The thymus is the central immune organ, but it is known to progressively degenerate with age. As thymus degeneration is paralleled by the wasting of aging skeletal muscle, we speculated that the thymus may play a role in muscle wasting. Here, using thymectomized mice, we show that the thymus is necessary for skeletal muscle regeneration, a process tightly associated with muscle aging. Compared to control mice, the thymectomized mice displayed comparable growth of muscle mass, but decreased muscle regeneration in response to injury, as evidenced by small and sparse regenerative myofibers along with inhibited expression of regeneration-associated genes myh3, myod and myogenin. Using Pax7 immunofluorescence staining and BrdU incorporation assay, we determined that the decreased regeneration capacity was caused by a limited satellite cell pool. Interestingly, the conditioned culture medium of isolated thymocytes (TCMs) had a potent capacity to directly stimulate satellite cell expansion in vitro. These expanded cells were enriched in subpopulations of quiescent satellite cells (Pax7^highMyoD^lowEdU^pos) and activated satellite cells (Pax7^highMyoD^highEdU^pos), which were efficiently incorporated into the regenerative myofibers. We thus propose that the thymus plays an essential role in muscle regeneration by directly promoting satellite cell expansion and may function profoundly in the muscle aging process.

The IRE1/XBP1 signaling axis promotes skeletal muscle regeneration through a cell non-autonomous mechanism

Skeletal muscle regeneration is regulated by coordinated activation of multiple signaling pathways. The unfolded protein response (UPR) is a major mechanism that detects and alleviates protein-folding stresses in the endoplasmic reticulum. However, the role of individual arms of the UPR in skeletal muscle regeneration remain less understood. In the present study, we demonstrate that IRE1α (also known as ERN1) and its downstream target, XBP1, are activated in skeletal muscle of mice upon injury. Myofiber-specific ablation of IRE1α or XBP1 in mice diminishes skeletal muscle regeneration that is accompanied with reduced number of satellite cells. Ex vivo cultures of myofiber explants demonstrate that ablation of IRE1α reduces the proliferative capacity of myofiber-associated satellite cells. Myofiber-specific ablation of IRE1α dampens Notch signaling and canonical NF-κB pathway in skeletal muscle of adult mice. Finally, targeted ablation of IRE1α also reduces Notch signaling, abundance of satellite cells, and skeletal muscle regeneration in the mdx mice, a model of Duchenne muscular dystrophy. Collectively, our experiments suggest that the IRE1α-mediated signaling promotes muscle regeneration through augmenting the proliferation of satellite cells in a cell non-autonomous manner.

TAK1: A Molecular Link Between Liver Inflammation, Fibrosis, Steatosis, and Carcinogenesis

Chronic insult and persistent injury can cause liver inflammation, fibrosis, and carcinogenesis; it can also be associated with metabolic disorders. Identification of critical molecules that link the process of inflammation and carcinogenesis will provide prospective therapeutic targets for liver diseases. Rapid advancements in gene engineering technology have allowed the elucidation of the underlying mechanism of transformation, from inflammation and metabolic disorders to carcinogenesis. Transforming growth factor-β-activated kinase 1 (TAK1) is an upstream intracellular protein kinase of nuclear factor kappa-B (NF-κB) and c-Jun N-terminal kinases, which are activated by numerous cytokines, growth factors, and microbial products. In this study, we highlighted the functional roles of TAK1 and its interaction with transforming growth factor-β, WNT, AMP-activated protein kinase, and NF-κB signaling pathways in liver inflammation, steatosis, fibrosis, and carcinogenesis based on previously published articles.

Catalpol counteracts the pathology in a mouse model of Duchenne muscular dystrophy by inhibiting the TGF-β1/TAK1 signaling pathway

Duchenne muscular dystrophy (DMD) is a progressive neuromuscular disease caused by a mutation in the gene encoding the dystrophin protein. Catalpol is an iridoid glycoside found in Chinese herbs with anti-inflammatory, anti-oxidant, anti-apoptotic, and hypoglycemic activities that can protect against muscle wasting. In the present study we investigated the effects of catalpol on DMD. Aged Dystrophin-deficient (mdx) mice (12 months old) were treated with catalpol (100, 200 mg·kg-1·d-1, ig) for 6 weeks. At the end of the experiment, the mice were sacrificed, and gastrocnemius (GAS), tibialis anterior (TA), extensor digitorum longus (EDL), soleus (SOL) muscles were collected. We found that catalpol administration dose-dependently increased stride length and decreased stride width in Gait test. Wire grip test showed that the time of wire grip and grip strength were increased. We found that catalpol administration dose-dependently alleviated skeletal muscle damage, evidenced by reduced plasma CK and LDH activity as well as increased the weight of skeletal muscles. Catalpol administration had no effect on dystrophin expression, but exerted anti-inflammatory effects. Furthermore, catalpol administration dose-dependently decreased tibialis anterior (TA) muscle fibrosis, and inhibited the expression of TGF-β1, TAK1 and α-SMA. In primary myoblasts from mdx mice, knockdown of TAK1 abolished the inhibitory effects of catalpol on the expression levels of TGF-β1 and α-SMA. In conclusion, catalpol can restore skeletal muscle strength and alleviate skeletal muscle damage in aged mdx mice, thus may provide a novel therapy for DMD. Catalpol attenuates muscle fibrosis by inhibiting the TGF-β1/TAK1 signaling pathway.

The stabilization of yes-associated protein by TGFβ-activated kinase 1 regulates the self-renewal and oncogenesis of gastric cancer stem cells

Gastric cancer (GC) is the most frequent digestive system malignant tumour and the second most common cause of cancer death globally. Cancer stem cell (CSC) is a small percentage of cancer cells in solid tumours that have differentiation, self‐renewal and tumorigenic capabilities. They have an active participation in the initiation, development, metastasis, recurrence and resistance of tumours to chemotherapy and radiotherapy. Gastric cancer stem cells (GCSCs) have been shown to be correlated with GC initiation and metastasis. In this study, we found that TAK1 expression level in GC tissues was significantly increased compared to the adjacent non‐cancerous tissues by RT‐qPCR, Western blot and immunohistochemistry. TAK1 has been identified as a critical molecule that promoted a variety of malignant GC phenotypes both in vivo and in vitro and promoted the self‐renewal of GCSCs. Mechanistically, TAK1 was up‐regulated by IL‐6 and prevented the degradation of yes‐associated protein (YAP) in the cytoplasm by binding to YAP. Thus, TAK1 promoted the SOX2 and SOX9 transcription and the self‐renewal and oncogenesis of GCSCs. Our findings provide insights into the mechanism of self‐renewal and tumorigenesis of TAK1 in GCSCs and have broad implications for clinical therapies.

Transcription factor signal transducer and activator of transcription 6 (STAT6) is an inhibitory factor for adult myogenesis

Background

The signal transducer and activator of transcription 6 (STAT6) transcription factor plays a vitally important role in immune cells, where it is activated mainly by interleukin-4 (IL-4). Because IL-4 is an essential cytokine for myotube formation, STAT6 might also be involved in myogenesis as part of IL-4 signaling. This study was conducted to elucidate the role of STAT6 in adult myogenesis in vitro and in vivo.

Methods

Myoblasts were isolated from male mice and were differentiated on a culture dish to evaluate the change in STAT6 during myotube formation. Then, the effects of STAT6 overexpression and inhibition on proliferation, differentiation, and fusion in those cells were studied. Additionally, to elucidate the myogenic role of STAT6 in vivo, muscle regeneration after injury was evaluated in STAT6 knockout mice.

Results

IL-4 can increase STAT6 phosphorylation, but STAT6 phosphorylation decreased during myotube formation in culture. STAT6 overexpression decreased, but STAT6 knockdown increased the differentiation index and the fusion index. Results indicate that STAT6 inhibited myogenin protein expression. Results of in vivo experiments show that STAT6 knockout mice exhibited better regeneration than wild-type mice 5 days after cardiotoxin-induced injury. It is particularly interesting that results obtained using cells from STAT6 knockout mice suggest that this STAT6 inhibitory action for myogenesis was not mediated by IL-4 but might instead be associated with p38 mitogen-activated protein kinase phosphorylation. However, STAT6 was not involved in the proliferation of myogenic cells in vitro and in vivo.

Conclusion

Results suggest that STAT6 functions as an inhibitor of adult myogenesis. Moreover, results suggest that the IL-4-STAT6 signaling axis is unlikely to be responsible for myotube formation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13395-021-00271-8.

In vitro assessment of anti-fibrotic drug activity does not predict in vivo efficacy in murine models of Duchenne muscular dystrophy

AIM

Fibrosis is the most common complication from chronic diseases, and yet no therapy capable of mitigating its effects is available. Our goal is to unveil specific signaling regulating the fibrogenic process and to identify potential small molecule candidates that block fibrogenic differentiation of fibro/adipogenic progenitors.

METHOD

We performed a large-scale drug screen using muscle-resident fibro/adipogenic progenitors from a mouse model expressing EGFP under the Collagen1a1 promotor. We first confirmed that the EGFP was expressed in response to TGFβ1 stimulation in vitro. Then we treated cells with TGFβ1 alone or with drugs from two libraries of known compounds. The drugs ability to block the fibrogenic differentiation was quantified by imaging and flow cytometry. From a two-rounds screening, positive hits were tested in vivo in the mice model for the Duchenne Muscular Dystrophy (mdx mice). The histopathology of the muscles was assessed with picrosirius red (fibrosis) and laminin staining (myofiber size).

KEY FINDINGS

From the in vitro drug screening, we identified 21 drugs and tested 3 in vivo on the mdx mice. None of the three drugs significantly improved muscle histopathology.

SIGNIFICANCE

The in vitro drug screen identified various efficient compounds, none of them strongly inhibited fibrosis in skeletal muscle of mdx mice. To explain these observations, we hypothesize that in Duchenne Muscular Dystrophy, in which fibrosis is a secondary event due to chronic degeneration and inflammation, the drugs tested could have adverse effect on regeneration or inflammation, balancing off any positive effects and leading to the absence of significant results.

TAK1 inhibition improves myoblast differentiation and alleviates fibrosis in a mouse model of Duchenne muscular dystrophy

BACKGROUND

Transforming growth factor-β-activated kinase 1 (TAK1) plays a key role in regulating fibroblast and myoblast proliferation and differentiation. However, the TAK1 changes associated with Duchenne muscular dystrophy (DMD) are poorly understood, and it remains unclear how TAK1 regulation could be exploited to aid the treatment of this disease.

METHODS

Muscle biopsies were obtained from control donors or DMD patients for diagnosis (n = 6 per group, male, 2-3 years, respectively). Protein expression of phosphorylated TAK1 was measured by western blot and immunofluorescence analysis. In vivo overexpression of TAK1 was performed in skeletal muscle to assess whether TAK1 is sufficient to induce or aggravate atrophy and fibrosis. To explore whether TAK1 inhibition protects against muscle damage, mdx (loss of dystrophin) mice were treated with adeno-associated virus (AAV)-short hairpin TAK1 (shTAK1) or NG25 (a TAK1 inhibitor). Serum analysis, skeletal muscle performance and histology, muscle contractile function, and gene and protein expression were performed.

RESULTS

We found that TAK1 was activated in the dystrophic muscles of DMD patients (n = 6, +72.2%, P < 0.001), resulting in fibrosis ( +65.9% for fibronectin expression, P < 0.001) and loss of muscle fibres (-32.5%, P < 0.01). Moreover, TAK1 was activated by interleukin-1β, tumour necrosis factor-α, and transforming growth factor-β1 (P < 0.01). Overexpression of TAK1 by AAV vectors further aggravated fibrosis (n = 8, +39.6% for hydroxyproline content, P < 0.01) and exacerbated muscle wasting (-31.6%, P < 0.01) in mdx mice; however, these effects were reversed in mdx mice by treatment with AAV-short hairpin TAK1 (shTAK1) or NG25 (a TAK1 inhibitor). The molecular mechanism underlying these effects may be related to the prevention of TAK1-mediated transdifferentiation of myoblasts into fibroblasts, thereby reducing fibrosis and increasing myoblast differentiation.

CONCLUSIONS

Our findings show that TAK1 activation exacerbated fibrosis and muscle degeneration and that TAK1 inhibition can improve whole-body muscle quality and the function of dystrophic skeletal muscle. Thus, TAK1 inhibition may constitute a novel therapy for DMD.

A potential therapeutic effect of catalpol in Duchenne muscular dystrophy revealed by binding with TAK1

BACKGROUND

Duchenne muscular dystrophy (DMD) is a progressive muscle disease caused by the loss of dystrophin, which results in inflammation, fibrosis, and the inhibition of myoblast differentiation in skeletal muscle. Catalpol, an iridoid glycoside, improves skeletal muscle function by enhancing myogenesis; it has potential to treat DMD. We demonstrate the positive effects of catalpol in dystrophic skeletal muscle.

METHODS

mdx (loss of dystrophin) mice (n = 18 per group) were treated with catalpol (200 mg/kg) for six consecutive weeks. Serum analysis, skeletal muscle performance and histology, muscle contractile function, and gene and protein expression were performed. Molecular docking and ligand-target interactions, RNA interference, immunofluorescence, and plasmids transfection were utilized to explore the protective mechanism in DMD by which catalpol binding with transforming growth factor-β-activated kinase 1 (TAK1) in skeletal muscle.

RESULTS

Six weeks of catalpol treatment improved whole-body muscle health in mdx mice, which was characterized by reduced plasma creatine kinase (n = 18, -35.1%, P < 0.05) and lactic dehydrogenase (n = 18, -10.3%, P < 0.05) activity. These effects were accompanied by enhanced grip strength (n = 18, +25.4%, P < 0.05) and reduced fibrosis (n = 18, -29.0% for hydroxyproline content, P < 0.05). Moreover, catalpol treatment protected against muscle fatigue and promoted muscle recovery in the tibialis anterior (TA) and diaphragm (DIA) muscles (n = 6, +69.8%, P < 0.05 and + 74.8%, P < 0.001, respectively), which was accompanied by enhanced differentiation in primary myoblasts from DMD patients (n = 6, male, mean age: 4.7 ± 1.9 years) and mdx mice. In addition, catalpol eliminated p-TAK1 overexpression in mdx mice (n = 12, -21.3%, P < 0.05) and primary myoblasts. The catalpol-induced reduction in fibrosis and increased myoblast differentiation resulted from the inhibition of TAK1 phosphorylation, leading to reduced myoblast trans-differentiation into myofibroblasts. Catalpol inhibited the phosphorylation of TAK1 by binding to TAK1, possibly at Asp-206, Thr-208, Asn-211, Glu-297, Lys-294, and Tyr-293.

CONCLUSIONS

Our findings show that catalpol and TAK1 inhibitors substantially improve whole-body muscle health and the function of dystrophic skeletal muscles and may provide a novel therapy for DMD.

Micronized sacchachitin promotes satellite cell proliferation through TAK1-JNK-AP-1 signaling pathway predominantly by TLR2 activation

BACKGROUND

Ganoderma sp., such as Ganoderma tsugae (GT), play an important role in traditional Chinese medicine. Ganoderma sp. contains several constituents, including Sacacchin, which has recently drawn attention because it can not only enhance the repair of muscle damage but also strengthen the muscle enforcement. Although Ganoderma sp. have a therapeutic effect for neuromuscular disorders, the underlying mechanism remains unclear. This study investigated the effect and underlying molecular mechanism of micronized sacchachitin (mSC) on satellite cells (SCs), which are known as the muscle stem cells.

METHODS

The myogenic cells, included SCs (Pax7⁺) were isolated from tibialis anterior muscles of a healthy rat and were cultured in growth media with different mSC concentrations. For the evaluation of SC proliferation, these cultivated cells were immunostained with Pax7 and bromodeoxyuridine assessed simultaneously. The molecular signal pathway was further investigated by using Western blotting and signal pathway inhibitors.

RESULTS

Our data revealed that 200 µg/mL mSC had an optimal capability to significantly enhance the SC proliferation. Furthermore, this enhancement of SC proliferation was verified to be involved with activation of TAK1-JNK-AP-1 signaling pathway through TLR2, whose expression on SC surface was confirmed for the first time here.

CONCLUSION

Micronized sacchachitin extracted from GT was capable of promoting the proliferation of SC under a correct concentration.

TAK1 preserves skeletal muscle mass and mitochondrial function through redox homeostasis

Skeletal muscle atrophy is debilitating consequence of a large number of chronic disease states, aging, and disuse conditions. Skeletal muscle mass is regulated through coordinated activation of a number of signaling cascades. Transforming growth factor-β activated kinase 1 (TAK1) is a central kinase that mediates the activation of multiple signaling pathways in response to various growth factors, cytokines, and microbial products. Accumulating evidence suggests that TAK1 promotes skeletal muscle growth and essential for the maintenance of muscle mass in adults. Targeted inactivation of TAK1 leads to severe muscle wasting and kyphosis in mice. However, the mechanisms by which TAK1 prevents loss of muscle mass remain poorly understood. Through generation of inducible skeletal muscle-specific Tak1-knockout mice, we demonstrate that targeted ablation of TAK1 disrupts redox signaling leading to the accumulation of reactive oxygen species and loss of skeletal muscle mass and contractile function. Suppression of oxidative stress using Trolox improves muscle contractile function and inhibits the activation of catabolic signaling pathways in Tak1-deficient muscle. Moreover, Trolox inhibits the activation of ubiquitin-proteasome system and autophagy markers in skeletal muscle of Tak1-deficient mice. Furthermore, inhibition of oxidative stress using Trolox prevents the slow-to-fast type fiber transition and improves mitochondrial respiration in skeletal muscle of Tak1-deficient mice. Overall, our results demonstrate that TAK1 maintains skeletal muscle mass and health through redox homeostasis.

Myofiber necroptosis promotes muscle stem cell proliferation via releasing Tenascin-C during regeneration

Necroptosis, a form of programmed cell death, is characterized by the loss of membrane integrity and release of intracellular contents, the execution of which depends on the membrane-disrupting activity of the Mixed Lineage Kinase Domain-Like protein (MLKL) upon its phosphorylation. Here we found myofibers committed MLKL-dependent necroptosis after muscle injury. Either pharmacological inhibition of the necroptosis upstream kinase Receptor Interacting Protein Kinases 1 (RIPK1) or genetic ablation of MLKL expression in myofibers led to significant muscle regeneration defects. By releasing factors into the muscle stem cell (MuSC) microenvironment, necroptotic myofibers facilitated muscle regeneration. Tenascin-C (TNC), released by necroptotic myofibers, was found to be critical for MuSC proliferation. The temporary expression of TNC in myofibers is tightly controlled by necroptosis; the extracellular release of TNC depends on necroptotic membrane rupture. TNC directly activated EGF receptor (EGFR) signaling pathway in MuSCs through its N-terminus assembly domain together with the EGF-like domain. These findings indicate that necroptosis plays a key role in promoting MuSC proliferation to facilitate muscle regeneration.

Canonical NF-κB signaling regulates satellite stem cell homeostasis and function during regenerative myogenesis

Skeletal muscle regeneration in adults is attributed to the presence of satellite stem cells that proliferate, differentiate, and eventually fuse with injured myofibers. However, the signaling mechanisms that regulate satellite cell homeostasis and function remain less understood. While IKKβ-mediated canonical NF-κB signaling has been implicated in the regulation of myogenesis and skeletal muscle mass, its role in the regulation of satellite cell function during muscle regeneration has not been fully elucidated. Here, we report that canonical NF-κB signaling is induced in skeletal muscle upon injury. Satellite cell-specific inducible ablation of IKKβ attenuates skeletal muscle regeneration in adult mice. Targeted ablation of IKKβ also reduces the number of satellite cells in injured skeletal muscle of adult mice, potentially through inhibiting their proliferation and survival. We also demonstrate that the inhibition of specific components of the canonical NF-κB pathway causes precocious differentiation of cultured satellite cells both ex vivo and in vitro. Finally, our results highlight that the constitutive activation of canonical NF-κB signaling in satellite cells also attenuates skeletal muscle regeneration following injury in adult mice. Collectively, our study demonstrates that the proper regulation of canonical NF-κB signaling is important for the regeneration of adult skeletal muscle.

The noncanonical BMP signaling pathway plays an important role in club cell regeneration

The bronchiole is a major site for the development of several life-threatening disorders, including chronic obstructive pulmonary disease and lung adenocarcinomas. The bronchiolar epithelium is composed of club cells and ciliated epithelial cells, with club cells serving as progenitor cells. Presently, the identity of the cells involved in regeneration of bronchiolar epithelium and the underlying mechanisms remain incompletely understood. Here, we show that Prrx1, a homeobox transcription factor, can mark club cells in adult mice during homeostasis and regeneration. We further show that the noncanonical signaling pathway of BMPs, BMPR1A-Tak1-p38MAPK, plays a critical role in club cell regeneration. Ablation of Bmpr1a, Tak1, or Mapk14 (encoding p38α) in Prrx1⁺ club cells caused minimal effect on bronchiolar epithelium homeostasis, yet it resulted in severe defects in club cell regeneration and bronchiole repair in adult mice. We further show that this pathway supports proliferation and expansion of the regenerating club cells. Our findings thus identify a marker for club cells and reveal a critical role for the BMP noncanonical pathway in club cell regeneration.

Transforming Growth Factor β-Activated Kinase 1 Regulates Mesenchymal Stem Cell Proliferation Through Stabilization of Yap1/Taz Proteins

Bone marrow-derived mesenchymal stem cells (BMMSCs) are multipotent stem cells capable of differentiation into a variety of cell types, proliferation, and production of clinically useful secretory factors. These advantages make BMMSCs highly useful for cell transplantation therapy. However, the molecular network underlying BMMSC proliferation remains poorly understood. Here, we showed that TGFβ-activated kinase 1 (Tak1) is a critical molecule that regulates the activation of cell cycling and that Tak1 inhibition leads to quiescence in BMMSCs both in vivo and in vitro. Mechanistically, Tak1 was phosphorylated by growth factor stimulations, allowing it to bind and stabilize Yap1/Taz, which could then be localized to the nucleus. We also demonstrated that the quiescence induction by inhibiting Tak1 increased oxidized stress tolerance and improved BMMSC engraftment in intramuscular and intrabone marrow cell transplantation models. This study reveals a novel pathway controlling BMMSC proliferation and suggests a useful method to improve the therapeutic effect of BMMSC transplantation. Stem Cells 2019;37:1595-1605.

Age-related decrease in muscle satellite cells is accompanied with diminished expression of early growth response 3 in mice

Skeletal muscle regeneration is mostly dependent on muscle satellite cells. Proper muscle regeneration requires enough number of satellite cells. Recent studies have suggested that the number of satellite cells in skeletal muscle declines as we age, leading to the impairment of muscle regeneration in older population. Our earlier study demonstrated that zinc finger transcription factor early growth response 3 (Egr3) plays an important role for maintaining the number of myoblasts, suggesting that age-related decrease in muscle satellite cell should be associated with the expression levels of Egr3. The aim of this study was to investigate whether aging would alter the Egr3 expression in satellite cells. A couple groups of male C57BL/6J mice were examined in this study: young (3 Mo) and old (17 Mo). Immunohistochemical staining showed that the satellite cell number decreased in normal and injured muscles of old mice. In fluorescence-activated cell sorting-isolated muscle satellite cells from normal and injured muscles, the mRNA expression of Egr3 was significantly decreased with age regardless of injury. In harmony with these results, Pax7 mRNA levels also decreased in the satellite cells from old mice. Alternatively, inhibition of Egr3 expression by shRNA decreased Pax7 protein expression in cultured myoblasts. These results suggest that Egr3 is associated with the age-related decline of muscle satellite cells in older population. Also, Egr3 might be implicated in the regulation of Pax7. Therefore, the loss of Egr3 expression may elucidate attenuated MSCs function and muscle regeneration in older age.

Multifaceted roles of TAK1 signaling in cancer

Context-specific signaling is a prevalent theme in cancer biology wherein individual molecules and pathways can have multiple or even opposite effects depending on the tumor type. TAK1 represents a particularly notable example of such signaling diversity in cancer progression. Originally discovered as a TGF-β-activated kinase, over the years it has been shown to respond to numerous other stimuli to phosphorylate a wide range of downstream targets and elicit distinct cellular responses across cell and tissue types. Here we present a comprehensive review of TAK1 signaling and provide important therapeutic perspectives related to its function in different cancers.

Pim1 kinase positively regulates myoblast behaviors and skeletal muscle regeneration

Adult skeletal muscle regeneration after injury depends on normal myoblast function. However, the intrinsic mechanisms for the control of myoblast behaviors are not well defined. Herein, we identified Pim1 kinase as a novel positive regulator of myoblast behaviors in vitro and muscle regeneration in vivo. Specifically, knockdown of Pim1 significantly restrains the proliferation and accelerates the apoptosis of myoblasts in vitro, indicating that Pim1 is critical for myoblast survival and amplification. Meanwhile, we found that Pim1 kinase is increased and translocated from cytoplasm into nucleus during myogenic differentiation. By using Pim1 kinase inhibitor, we proved that inhibition of Pim1 activity prevents myoblast differentiation and fusion, suggesting the necessity of Pim1 kinase activity for proper myogenesis. Mechanistic studies demonstrated that Pim1 kinase interacts with myogenic regulator MyoD and controls its transcriptional activity, inducing the expression of muscle-specific genes, which consequently promotes myogenic differentiation. Additionally, in skeletal muscle injury mouse model, deletion of Pim1 hinders the regeneration of muscle fibers and the recovery of muscle strength. Taken together, our study provides a potential target for the manipulation of myoblast behaviors in vitro and the myoblast-based therapeutics of skeletal muscle injury.

Fate decision of satellite cell differentiation and self-renewal by miR-31-IL34 axis

Quiescent satellite cells (SCs) that are activated to produce numerous myoblasts underpin the complete healing of damaged skeletal muscle. How cell-autonomous regulatory mechanisms modulate the balance among cells committed to differentiation and those committed to self-renewal to maintain the stem cell pool remains poorly explored. Here, we show that miR-31 inactivation compromises muscle regeneration in adult mice by impairing the expansion of myoblasts. miR-31 is pivotal for SC proliferation, and its deletion promotes asymmetric cell fate segregation of proliferating cells, resulting in enhanced myogenic commitment and re-entry into quiescence. Further analysis revealed that miR-31 posttranscriptionally suppresses interleukin 34 (IL34) mRNA, the protein product of which activates JAK-STAT3 signaling required for myogenic progression. IL34 inhibition rescues the regenerative deficiency of miR-31 knockout mice. Our results provide evidence that targeting miR-31 or IL34 activities in SCs could be used to counteract the functional exhaustion of SCs in pathological conditions.

Growth/differentiation factor 15 causes TGFβ-activated kinase 1-dependent muscle atrophy in pulmonary arterial hypertension

INTRODUCTION

Skeletal muscle dysfunction is a clinically important complication of pulmonary arterial hypertension (PAH). Growth/differentiation factor 15 (GDF-15), a prognostic marker in PAH, has been associated with muscle loss in other conditions. We aimed to define the associations of GDF-15 and muscle wasting in PAH, to assess its utility as a biomarker of muscle loss and to investigate its downstream signalling pathway as a therapeutic target.

METHODS

GDF-15 levels and measures of muscle size and strength were analysed in the monocrotaline (MCT) rat, Sugen/hypoxia mouse and in 30 patients with PAH. In C2C12 myotubes the downstream targets of GDF-15 were identified. The pathway elucidated was then antagonised in vivo.

RESULTS

Circulating GDF-15 levels correlated with tibialis anterior (TA) muscle fibre diameter in the MCT rat (Pearson r=-0.61, p=0.003). In patients with PAH, plasma GDF-15 levels of <564 pg/L predicted those with preserved muscle strength with a sensitivity and specificity of ≥80%. In vitro GDF-15 stimulated an increase in phosphorylation of TGFβ-activated kinase 1 (TAK1). Antagonising TAK1, with 5(Z)-7-oxozeaenol, in vitro and in vivo led to an increase in fibre diameter and a reduction in mRNA expression of atrogin-1 in both C2C12 cells and in the TA of animals who continued to grow. Circulating GDF-15 levels were also reduced in those animals which responded to treatment.

CONCLUSIONS

Circulating GDF-15 is a biomarker of muscle loss in PAH that is responsive to treatment. TAK1 inhibition shows promise as a method by which muscle atrophy may be directly prevented in PAH.

TRIAL REGISTRATION NUMBER

NCT01847716; Results.

Necroptosis mediates myofibre death in dystrophin-deficient mice

Duchenne muscular dystrophy (DMD) is a severe degenerative disorder caused by mutations in the dystrophin gene. Dystrophin-deficient muscles are characterised by progressive myofibre necrosis in which inflammation plays a deleterious role. However, the molecular mechanisms underlying inflammation-induced necrosis in muscle cells are unknown. Here we show that necroptosis is a mechanism underlying myofibre death in dystrophin-deficient muscle. RIPK1, RIPK3 and MLKL are upregulated in dystrophic mouse myofibres. In human DMD samples, there is strong immunoreactivity to RIPK3 and phospho-MLKL in myofibres. In vitro, TNFα can elicit necroptosis in C2C12 myoblasts, and RIPK3 overexpression sensitises myoblasts to undergo TNF-induced death. Furthermore, genetic ablation of Ripk3 in mdx mice reduces myofibre degeneration, inflammatory infiltrate, and muscle fibrosis, and eventually improves muscle function. These findings provide the first evidence of necroptotic cell death in a disease affecting skeletal muscle and identify RIPK3 as a key player in the degenerative process in dystrophin-deficient muscles.

RNA helicase, DDX27 regulates skeletal muscle growth and regeneration by modulation of translational processes

Gene expression in a tissue-specific context depends on the combined efforts of epigenetic, transcriptional and post-transcriptional processes that lead to the production of specific proteins that are important determinants of cellular identity. Ribosomes are a central component of the protein biosynthesis machinery in cells; however, their regulatory roles in the translational control of gene expression in skeletal muscle remain to be defined. In a genetic screen to identify critical regulators of myogenesis, we identified a DEAD-Box RNA helicase, DDX27, that is required for skeletal muscle growth and regeneration. We demonstrate that DDX27 regulates ribosomal RNA (rRNA) maturation, and thereby the ribosome biogenesis and the translation of specific transcripts during myogenesis. These findings provide insight into the translational regulation of gene expression in myogenesis and suggest novel functions for ribosomes in regulating gene expression in skeletal muscles.

TAK1 regulates skeletal muscle mass and mitochondrial function

Skeletal muscle mass is regulated by a complex array of signaling pathways. TGF-β-activated kinase 1 (TAK1) is an important signaling protein, which regulates context-dependent activation of multiple intracellular pathways. However, the role of TAK1 in the regulation of skeletal muscle mass remains unknown. Here, we report that inducible inactivation of TAK1 causes severe muscle wasting, leading to kyphosis, in both young and adult mice.. Inactivation of TAK1 inhibits protein synthesis and induces proteolysis, potentially through upregulating the activity of the ubiquitin-proteasome system and autophagy. Phosphorylation and enzymatic activity of AMPK are increased, whereas levels of phosphorylated mTOR and p38 MAPK are diminished upon inducible inactivation of TAK1 in skeletal muscle. In addition, targeted inactivation of TAK1 leads to the accumulation of dysfunctional mitochondria and oxidative stress in skeletal muscle of adult mice. Inhibition of TAK1 does not attenuate denervation-induced muscle wasting in adult mice. Finally, TAK1 activity is highly upregulated during overload-induced skeletal muscle growth, and inactivation of TAK1 prevents myofiber hypertrophy in response to functional overload. Overall, our study demonstrates that TAK1 is a key regulator of skeletal muscle mass and oxidative metabolism.

MyD88 promotes myoblast fusion in a cell-autonomous manner

Myoblast fusion is an indispensable step for skeletal muscle development, postnatal growth, and regeneration. Myeloid differentiation primary response gene 88 (MyD88) is an adaptor protein that mediates Toll-like receptors and interleukin-1 receptor signaling. Here we report a cell-autonomous role of MyD88 in the regulation of myoblast fusion. MyD88 protein levels are increased during in vitro myogenesis and in conditions that promote skeletal muscle growth in vivo. Deletion of MyD88 impairs fusion of myoblasts without affecting their survival, proliferation, or differentiation. MyD88 regulates non-canonical NF-κB and canonical Wnt signaling during myogenesis and promotes skeletal muscle growth and overload-induced myofiber hypertrophy in mice. Ablation of MyD88 reduces myofiber size during muscle regeneration, whereas its overexpression promotes fusion of exogenous myoblasts to injured myofibers. Our study shows that MyD88 modulates myoblast fusion and suggests that augmenting its levels may be a therapeutic approach to improve skeletal muscle formation in degenerative muscle disorders.

Data-Modeling Identifies Conflicting Signaling Axes Governing Myoblast Proliferation and Differentiation Responses to Diverse Ligand Stimuli

Introduction

Skeletal muscle tissue development and regeneration relies on the proliferation, maturation and fusion of muscle progenitor cells (myoblasts), which arise transiently from muscle stem cells (satellite cells). Following muscle damage, myoblasts proliferate and differentiate in response to temporally-varying inflammatory cytokines, growth factors, and extracellular matrix cues, which stimulate a shared network of intracellular signaling pathways. Here we present an integrated data-modeling approach to elucidate synergies and antagonisms among proliferation and differentiation signaling axes in myoblasts stimulated by regeneration-associated ligands.

Methods

We treated mouse primary myoblasts in culture with combinations of eight regeneration-associated growth factors and cytokines in mixtures that induced additive, synergistic, and antagonistic effects on myoblast proliferation and differentiation responses. For these combinatorial stimuli, we measured the activation dynamics of seven signal transduction pathways using multiplexed phosphoprotein assays and scored proliferation and differentiation responses based on expression of myogenic commitment factors to assemble a cue-signaling-response data compendium. We interrogated the relationship between these signals and responses by partial least-squares (PLS) regression modeling.

Results

Partial least-squares data-modeling accurately predicted response outcomes in cross-validation on the training compendium (cumulative R ² = 0.96). The PLS model highlighted signaling axes that distinctly govern myoblast proliferation (MEK-ERK, Stat3) and differentiation (JNK) in response to these combinatorial cues, and we confirmed these signal-response associations with small molecule perturbations. Unexpectedly, we observed that a negative feedback circuit involving the phosphatase DUSP6/MKP-3 auto-regulates MEK-ERK signaling in myoblasts.

Conclusion

This data-modeling approach identified conflicting signaling axes that underlie muscle progenitor cell proliferation and differentiation.

Rapid Activation of Transforming Growth Factor β-Activated Kinase 1 in Chondrocytes by Phosphorylation and K63 -Linked Polyubiquitination Upon Injury to Animal Articular Cartilage

Objective

Mechanical injury to cartilage predisposes to osteoarthritis (OA). Wounding of the articular cartilage surface causes rapid activation of MAP kinases and NF‐κB, mimicking the response to inflammatory cytokines. This study was undertaken to identify the upstream signaling mechanisms involved.

Methods

Cartilage was injured by dissecting it from the articular surface of porcine metacarpophalangeal (MCP) joints or by avulsing murine proximal femoral epiphyses. Protein phosphorylation was assayed by Western blotting of cartilage lysates. Immunolocalization of phosphorylated activating transcription factor 2 (ATF‐2) and NF‐κB/p65 was detected by confocal microscopy. Messenger RNA (mRNA) was measured by quantitative reverse transcriptase–polymerase chain reaction (qRT‐PCR). Receptor associated protein 80 (RAP‐80) ubiquitin interacting motif agarose was used in a pull‐down assay to obtain K⁶³‐polyubiquitinated proteins. Ubiquitin linkages on immunoprecipitated transforming growth factor β–activated kinase 1 (TAK‐1) were analyzed with deubiquitinases.

Results

Sharp injury to porcine cartilage caused rapid activation of JNK and NF‐κB pathways and the upstream kinases MKK‐4, IKK, and TAK‐1. Pharmacologic inhibition of TAK‐1 in porcine cartilage abolished JNK and NF‐κB activation and reduced the injury‐dependent inflammatory gene response. High molecular weight species of phosphorylated TAK‐1 were induced by injury, indicating its ubiquitination. An overall increase in K⁶³‐linked polyubiquitination was detected upon injury, and TAK‐1 was specifically linked to K⁶³‐ but not K⁴⁸‐polyubiquitin chains. In mice, avulsion of wild‐type femoral epiphyses caused similar intracellular signaling that was reduced in cartilage‐specific TAK‐1–null mice. Epiphyseal cartilage of MyD88‐null and TRAF‐6–null mice responded to injury, suggesting the involvement of a ubiquitin E3 ligase other than TRAF‐6.

Conclusion

Activation of TAK‐1 by phosphorylation and K⁶³‐linked polyubiquitination by injury indicates its role in driving cell activation. Further studies are needed to identify the upstream ubiquitination mechanisms, including the E3 ligase involved.

Noncanonical cell death program independent of caspase activation cascade and necroptotic modules is elicited by loss of TGFβ-activated kinase 1

Programmed cell death (PCD) occurs in several forms including apoptosis and necroptosis. Apoptosis is executed by the activation of caspases, while necroptosis is dependent on the receptor interacting protein kinase 3 (RIPK3). Precise control of cell death is crucial for tissue homeostasis. Indeed, necroptosis is triggered by caspase inhibition to ensure cell death. Here we identified a previously uncharacterized cell death pathway regulated by TAK1, which is unexpectedly provoked by inhibition of caspase activity and necroptosis cascades. Ablation of TAK1 triggers spontaneous death in macrophages. Simultaneous inhibition of caspases and RIPK3 did not completely restore cell viability. Previous studies demonstrated that loss of TAK1 in fibroblasts causes TNF-induced apoptosis and that additional inhibition of caspase leads to necroptotic cell death. However, we surprisingly found that caspase and RIPK3 inhibitions do not completely suppress cell death in Tak1-deficient cells. Mechanistically, the execution of the third cell death pathway in Tak1-deficient macrophages and fibroblasts were mediated by RIPK1-dependent rapid accumulation of reactive oxygen species (ROS). Conversely, activation of RIPK1 was sufficient to induce cell death. Therefore, loss of TAK1 elicits noncanonical cell death which is mediated by RIPK1-induced oxidative stress upon caspase and necroptosis inhibition to further ensure induction of cell death.

The PERK arm of the unfolded protein response regulates satellite cell-mediated skeletal muscle regeneration

Regeneration of skeletal muscle in adults is mediated by satellite stem cells. Accumulation of misfolded proteins triggers endoplasmic reticulum stress that leads to unfolded protein response (UPR). The UPR is relayed to the cell through the activation of PERK, IRE1/XBP1, and ATF6. Here, we demonstrate that levels of PERK and IRE1 are increased in satellite cells upon muscle injury. Inhibition of PERK, but not the IRE1 arm of the UPR in satellite cells inhibits myofiber regeneration in adult mice. PERK is essential for the survival and differentiation of activated satellite cells into the myogenic lineage. Deletion of PERK causes hyper-activation of p38 MAPK during myogenesis. Blocking p38 MAPK activity improves the survival and differentiation of PERK-deficient satellite cells in vitro and muscle formation in vivo. Collectively, our results suggest that the PERK arm of the UPR plays a pivotal role in the regulation of satellite cell homeostasis during regenerative myogenesis.

The MEK-Inhibitor Selumetinib Attenuates Tumor Growth and Reduces IL-6 Expression but Does Not Protect against Muscle Wasting in Lewis Lung Cancer Cachexia

Cachexia, or wasting of skeletal muscle and fat, afflicts many patients with chronic diseases including cancer, organ failure, and AIDS. Muscle wasting reduces quality of life and decreases response to therapy. Cachexia is caused partly by elevated inflammatory cytokines, including interleukin-6 (IL-6). Others and we have shown that IL-6 alone is sufficient to induce cachexia both in vitro and in vivo. The mitogen-activated protein/extracellular signal-regulated kinase kinase (MEK) inhibitor Selumetinib has been tested in clinical trials for various cancers. Moreover, Selumetinib has also been shown to inhibit the production of IL-6. In a retrospective analysis of a phase II clinical trial in advanced cholangiocarcinoma, patients treated with Selumetinib experienced significant gains in skeletal muscle vs. patients receiving standard therapy. However, the use of Selumetinib as a treatment for cachexia has yet to be investigated mechanistically. We sought to determine whether MEK inhibition could protect against cancer-induced cachexia in mice. In vitro, Selumetinib induced C2C12 myotube hypertrophy and nuclear accretion. Next we tested Selumetinib in the Lewis lung carcinoma (LLC) model of cancer cachexia. Treatment with Selumetinib reduced tumor mass and reduced circulating and tumor IL-6; however MEK inhibition did not preserve muscle mass. Similar wasting was seen in limb muscles of Selumetinib and vehicle-treated LLC mice, while greater fat and carcass weight loss was observed with Selumetinib treatment. As well, Selumetinib did not block wasting in C2C12 myotubes treated with LLC serum. Taken together, out results suggest that this MEK inhibitor is not protective in LLC cancer cachexia despite lowering IL-6 levels, and further that it might exacerbate tumor-induced weight loss. Differences from other studies might be disease, species or model-specific.

A Gingiva-Derived Mesenchymal Stem Cell-Laden Porcine Small Intestinal Submucosa Extracellular Matrix Construct Promotes Myomucosal Regeneration of the Tongue

In the oral cavity, the tongue is the anatomic subsite most commonly involved by invasive squamous cell carcinoma. Current treatment protocols often require significant tissue resection to achieve adequate negative margins and optimal local tumor control. Reconstruction of the tongue while preserving and/or restoring its critical vocal, chewing, and swallowing functions remains one of the major challenges in head and neck oncologic surgery. We investigated the in vitro feasibility of fabricating a novel combinatorial construct using porcine small intestinal submucosa extracellular matrix (SIS-ECM) and human gingiva-derived mesenchymal stem cells (GMSCs) as a GMSC/SIS-ECM tissue graft for the tongue reconstruction. We developed a rat model of critical-sized myomucosal defect of the tongue that allowed the testing of therapeutic effects of an acellular SIS-ECM construct versus a GMSC/SIS-ECM construct on repair and regeneration of the tongue defect. We showed that the GMSC/SIS-ECM construct engrafted at the host recipient site, promoted soft tissue healing, and regenerated the muscular layer, compared to the SIS-ECM alone or nontreated defect controls. Furthermore, our results revealed that transplantation of the GMSC/SIS-ECM construct significantly increased the expression of several myogenic transcriptional factors and simultaneously suppressed the expression of type I collagen at the wounded area of the tongue. These compelling findings suggest that, unlike the tongue contracture and fibrosis of the nontreated defect group, transplantation of the combinatorial GMSC/SIS-ECM constructs accelerates wound healing and muscle regeneration and maintains the overall tongue shape, possibly by both enhancing the function of endogenous skeletal progenitor cells and suppressing fibrosis. Together, our findings indicate that GMSC/SIS-ECM potentially served as a myomucosal graft for tongue reconstruction postsurgery of head and neck cancer.

Isolation, Culturing, and Differentiation of Primary Myoblasts from Skeletal Muscle of Adult Mice

Myogenesis is a multi-step process that leads to the formation of skeletal muscle during embryonic development and repair of injured myofibers. In this process, myoblasts are the main effector cell type which fuse with each other or to injured myofibers leading to the formation of new myofibers or regeneration of skeletal muscle in adults. Many steps of myogenesis can be recapitulated through in vitro differentiation of myoblasts into myotubes. Most laboratories use immortalized myogenic cells lines that also differentiate into myotubes. Although these cell lines have been found quite useful to delineating the regulatory mechanisms of myogenesis, they often show a great degree of variability depending on the origin of the cells and culture conditions. Primary myoblasts have been suggested as the most physiologically relevant model for studying myogenesis in vitro. However, due to their low abundance in adult skeletal muscle, isolation of primary myoblasts is technically challenging. In this article, we describe an improved protocol for the isolation of primary myoblasts from adult skeletal muscle of mice. We also describe methods for their culturing and differentiation into myotubes.

Early Growth Response 3 (Egr3) Contributes a Maintenance of C2C12 Myoblast Proliferation

Satellite cell proliferation is a crucially important process for adult myogenesis. However, its regulatory mechanisms remain unknown. Early growth response 3 (Egr3) is a zinc-finger transcription factor that regulates different cellular functions. Reportedly, Egr3 interacts with multiple signaling molecules that are also known to regulate satellite cell proliferation. Therefore, it is possible that Egr3 is involved in satellite cell proliferation. Results of this study have demonstrated that Egr3 transcript levels are upregulated in regenerating mouse skeletal muscle after cardiotoxin injury. Using C2C12 myoblast culture (a model of activated satellite cells), results show that inhibition of Egr3 by shRNA impairs the myoblast proliferation rate. Results also show reduction of NF-кB transcriptional activity in Egr3-inhibited cells. Inhibition of Egr3 is associated with an increase in annexin V⁺ cell fraction and apoptotic protein activity including caspase-3 and caspase-7, and Poly-ADP ribose polymerase. By contrast, the reduction of cellular proliferation by inhibition of Egr3 was partially recovered by treatment of pan-caspase inhibitor Z-VAD-FMK. Collectively, these results suggest that Egr3 is involved in myoblast proliferation by interaction with survival signaling. J. Cell. Physiol. 232: 1114-1122, 2017. © 2016 Wiley Periodicals, Inc.

Modification of TAK1 by O-linked N-acetylglucosamine facilitates TAK1 activation and promotes M1 macrophage polarization

Macrophages play many different roles in tissue inflammation and immunity, and the plasticity of macrophage polarization is closely associated with acute inflammatory responses. O-GlcNAcylation is an important type of post-translational modification, which subtly modulates inflammation responses. Transforming growth factor (TGF)-β-activated kinase 1 (TAK1) is a key serine/threonine protein kinase that mediates signals transduced by pro-inflammatory cytokines such as TGF-β, tumor necrosis factor (TNF), and interleukin-1 (IL-1). It is here reported that TGFβ-activated kinase (TAK1) is modified with N-acetylglucosamine (O-GlcNAc) on S427. Both IL-1 and osmotic stress, which are known as the TAK1-signaling inducers, significantly trigger the O-GlcNAcylation of TAK1 in macrophages. By overexpressing wild-type (WT) or S427A TAK1 mutant into macrophages, it was determined that O-GlcNAcylation of TAK1 on S427 is required for T187/S192 phosphorylation and full activation of TAK1 upon stimulation with IL-1α and NaCl. Aborting O-GlcNAcylation of TAK1 on S427 was found to inhibit the downstream JNK and nuclear factor-κB activation and reduce the final amount of cytokines produced in activated macrophages to a great extent. Results also showed that overexpression of the O-GlcNAcylation-deficient mutant of TAK1 promotes LPS-mediated apoptosis in macrophages. Importantly, TAK1 O-GlcNAcylation was found to promote M1 macrophage polarization in activated macrophages. Taken together, these data demonstrate that O-GlcNAcylation of TAK1 on S427 critically regulates the pro-inflammatory activation and M1 polarization of macrophages via modulation of the TAK1/JNK/NF-κB signaling pathway.

Muscle redox disturbances and oxidative stress as pathomechanisms and therapeutic targets in early-onset myopathies

Because of their contractile activity and their high oxygen consumption and metabolic rate, skeletal muscles continually produce moderate levels of reactive oxygen and nitrogen species (ROS/RNS), which increase during exercise and are buffered by multiple antioxidant systems to maintain redox homeostasis. Imbalance between ROS/RNS production and elimination results in oxidative stress (OxS), which has been implicated in ageing and in numerous human diseases, including cancer, diabetes or age-related muscle loss (sarcopenia). The study of redox homeostasis in muscle was hindered by its lability, by the many factors influencing technical OxS measures and by ROS/RNS important roles in signaling pathways and adaptative responses to muscle contraction and effort, which make it difficult to define a threshold between physiological signaling and pathological conditions. In the last years, new tools have been developed that facilitate the study of these key mechanisms, and deregulation of redox homeostasis has emerged as a key pathogenic mechanism and potential therapeutic target in muscle conditions. This is in particular the case for early-onset myopathies, genetic muscle diseases which present from birth or early childhood with muscle weakness interfering with ambulation and often with cardiac or respiratory failure leading to premature death. Inherited defects of the reductase selenoprotein N in SEPN1-related myopathy leads to chronic OxS of monogenic origin as a primary disease pathomechanism. In myopathies associated with mutations of the genes encoding the calcium channel RyR1, the extracellular matrix protein collagen VI or the sarcolemmal protein dystrophin (Duchenne Muscular Dystrophy), OxS has been identified as a relevant secondary pathophysiological mechanism. OxS being drug-targetable, it represents an interesting therapeutic target for these incurable conditions, and following preclinical correction of the cell or animal model phenotype, the first clinical trials with the antioxidants N-acetylcysteine (SEPN1- and RYR1-related myopathies) or epigallocatechin-gallate (DMD) have been launched recently. In this review, we provide an overview of the mechanisms involved in redox regulation in skeletal muscle, the technical tools available to measure redox homeostasis in muscle cells, the bases of OxS as a primary or secondary pathomechanism in early-onset myopathies and the innovative clinical trials with antioxidants which are currently in progress for these so-far untreatable infantile muscle diseases. Progress in our knowledge of redox homeostasis defects in these rare muscle conditions may be useful as a model paradigm to understand and treat other conditions in which OxS is involved, including prevalent conditions with major socioeconomic impact such as insulin resistance, cachexia, obesity, sarcopenia or ageing.