Inhibition of skeletal muscle satellite cell differentiation by transforming growth factor-beta

Regulation of Satellite Cells Functions during Skeletal Muscle Regeneration: A Critical Step in Physiological and Pathological Conditions

Skeletal muscle regeneration is a complex process involving the generation of new myofibers after trauma, competitive physical activity, or disease. In this context, adult skeletal muscle stem cells, also known as satellite cells (SCs), play a crucial role in regulating muscle tissue homeostasis and activating regeneration. Alterations in their number or function have been associated with various pathological conditions. The main factors involved in the dysregulation of SCs' activity are inflammation, oxidative stress, and fibrosis. This review critically summarizes the current knowledge on the role of SCs in skeletal muscle regeneration. It examines the changes in the activity of SCs in three of the most common and severe muscle disorders: sarcopenia, muscular dystrophy, and cancer cachexia. Understanding the molecular mechanisms involved in their dysregulations is essential for improving current treatments, such as exercise, and developing personalized approaches to reactivate SCs.

A Surgical Approach to Hindlimb Suspension: A Mouse Model of Disuse-Induced Atrophy

Hindlimb suspension is a well-established rodent model of disuse-induced atrophy and is commonly used to simulate the effects of bed rest and space flight on humans. Over the decades, this method has undergone many changes to reduce the stress response on the animals and improve the reliability of the data. Here, we detail our method of performing hindlimb suspension in mice that minimizes stress, maximizes the replicability of the data, and uses space efficiently.

Physical Activity as the Best Supportive Care in Cancer: The Clinician's and the Researcher's Perspectives

Multidisciplinary supportive care, integrating the dimensions of exercise alongside oncological treatments, is now regarded as a new paradigm to improve patient survival and quality of life. Its impact is important on the factors that control tumor development, such as the immune system, inflammation, tissue perfusion, hypoxia, insulin resistance, metabolism, glucocorticoid levels, and cachexia. An increasing amount of research has been published in the last years on the effects of physical activity within the framework of oncology, marking the appearance of a new medical field, commonly known as "exercise oncology". This emerging research field is trying to determine the biological mechanisms by which, aerobic exercise affects the incidence of cancer, the progression and/or the appearance of metastases. We propose an overview of the current state of the art physical exercise interventions in the management of cancer patients, including a pragmatic perspective with tips for routine practice. We then develop the emerging mechanistic views about physical exercise and their potential clinical applications. Moving toward a more personalized, integrated, patient-centered, and multidisciplinary management, by trying to understand the different interactions between the cancer and the host, as well as the impact of the disease and the treatments on the different organs, this seems to be the most promising method to improve the care of cancer patients.

NOX4 inhibition promotes the remodeling of dystrophic muscle

The muscular dystrophies (MDs) are genetic muscle diseases that result in progressive muscle degeneration followed by the fibrotic replacement of affected muscles as regenerative processes fail. Therapeutics that specifically address the fibrosis and failed regeneration associated with MDs represent a major unmet clinical need for MD patients, particularly those with advanced-stage disease progression. The current study investigated targeting NAD(P)H oxidase 4 (NOX4) as a potential strategy to reduce fibrosis and promote regeneration in disease-burdened muscle that models Duchenne muscular dystrophy (DMD). NOX4 was elevated in the muscles of dystrophic mice and DMD patients, localizing primarily to interstitial cells located between muscle fibers. Genetic and pharmacological targeting of NOX4 significantly reduced fibrosis in dystrophic respiratory and limb muscles. Mechanistically, NOX4 targeting decreased the number of fibrosis-depositing cells (myofibroblasts) and restored the number of muscle-specific stem cells (satellite cells) localized to their physiological niche, thereby rejuvenating muscle regeneration. Furthermore, acute inhibition of NOX4 was sufficient to induce apoptotic clearing of myofibroblasts within dystrophic muscle. These data indicate that targeting NOX4 is an effective strategy to promote the beneficial remodeling of disease-burdened muscle representative of DMD and, potentially, other MDs and muscle pathologies.

Macrophages play a key role in tissue repair and regeneration

Tissue regeneration after body injury has always been a complex problem to resolve for mammals. In adult mammals, the repair process after tissue injury is often accompanied by continuous and extensive fibrosis, which leads to scars. This process has been shown to severely hinder regeneration. Macrophages, as widely distributed innate immune cells, not only play an important role in various pathological processes, but also participate in the repair process before tissue regeneration and coordinate the regeneration process after repair. This review will discuss the various forms and indispensability of macrophages involved in repair and regeneration, and how macrophages play a role in the repair and regeneration of different tissues.

Development of a Chemically Defined Medium for in vitro Expansion of Primary Bovine Satellite Cells

The use of fetal bovine serum (FBS) in animal cell culture media is widely spread since it provides a broad spectrum of molecules that are known to support cell attachment and growth. However, the harvest and collection procedures of FBS raise ethical concerns and serum is an ill-defined and expensive component. This is especially problematic when it comes to regulatory approval for food applications like cultured meat. The aim of this study is to develop a chemically defined, cost efficient serum-free and animal-free medium that supports the attachment and expansion of bovine myoblasts while maintaining their differentiation capacity. Bovine satellite cells were harvested and isolated from a fresh sample of skeletal muscle tissue and cultured in planar systems. The efficacy of the tested formulations was assessed with metabolic assays and cell counting techniques. Optical microscopy was used to observe cellular morphology and statistical analysis was applied. Based on a comprehensive literature analysis, a defined serum-free medium (SFM) composition was developed consisting of DMEM/F12 as basal medium, supplemented with L-ascorbic acid 2-phosphate, fibronectin, hydrocortisone, GlutaMAX^™, albumin, ITS-X, hIL-6, α-linolenic acid, and growth factors such as FGF-2, VEGF, IGF-1, HGF, and PDGF-BB. To our knowledge, this is the first defined serum-free and animal free medium formulation specific for bovine myoblasts to date. We conclude that the SFM formulation supported exponential cell growth up to 97% of the serum—containing golden standard growth medium. All reagents used in this study are chemically defined.

Myostatin/Activin Receptor Ligands in Muscle and the Development Status of Attenuating Drugs

Muscle wasting disease indications are among the most debilitating and often deadly noncommunicable disease states. As a comorbidity, muscle wasting is associated with different neuromuscular diseases and myopathies, cancer, heart failure, chronic pulmonary and renal diseases, peripheral neuropathies, inflammatory disorders, and, of course, musculoskeletal injuries. Current treatment strategies are relatively ineffective and can at best only limit the rate of muscle degeneration. This includes nutritional supplementation and appetite stimulants as well as immunosuppressants capable of exacerbating muscle loss. Arguably, the most promising treatments in development attempt to disrupt myostatin and activin receptor signaling because these circulating factors are potent inhibitors of muscle growth and regulators of muscle progenitor cell differentiation. Indeed, several studies demonstrated the clinical potential of "inhibiting the inhibitors," increasing muscle cell protein synthesis, decreasing degradation, enhancing mitochondrial biogenesis, and preserving muscle function. Such changes can prevent muscle wasting in various disease animal models yet many drugs targeting this pathway failed during clinical trials, some from serious treatment-related adverse events and off-target interactions. More often, however, failures resulted from the inability to improve muscle function despite preserving muscle mass. Drugs still in development include antibodies and gene therapeutics, all with different targets and thus, safety, efficacy, and proposed use profiles. Each is unique in design and, if successful, could revolutionize the treatment of both acute and chronic muscle wasting. They could also be used in combination with other developing therapeutics for related muscle pathologies or even metabolic diseases.

The Role of Satellite Cells in Skeletal Muscle Regeneration-The Effect of Exercise and Age

Simple Summary

Studies describing the effects of various forms of exercise and age on muscle regeneration were reviewed. Satellite cells are a heterogeneous group of cells that includes stem cells and skeletal muscle progenitor cells. Each skeletal muscle fiber has its own pool of satellite cells that remain inactive until the muscle is damaged. Minor damage within the cell membrane of muscle fibers is patched by fusing intracellular vesicles with the damaged sarcolemma. More severe muscle damage initiates a multistep regeneration process in which satellite cells play an essential role. The condition that initiates the cascade of reactions is the formation of inflammation at the structural discontinuity site, resulting in satellite cell activation. The multitude of reactions and pathways occurring during this process means that many different substances are involved in it and control it. Not all of them are well-understood yet. In parallel, the body’s own population of satellite cells is being rebuilt so that more fibers can be regenerated in the future. Athletes and the elderly are primarily at risk for muscle damage, and pathologies in muscle fiber regeneration cause serious diseases.

Abstract

The population of satellite cells (mSCs) is highly diversified. The cells comprising it differ in their ability to regenerate their own population and differentiate, as well as in the properties they exhibit. The heterogeneity of this group of cells is evidenced by multiple differentiating markers that enable their recognition, classification, labeling, and characterization. One of the main tasks of satellite cells is skeletal muscle regeneration. Myofibers are often damaged during vigorous exercise in people who participate in sports activities. The number of satellite cells and the speed of the regeneration processes that depend on them affect the time structure of an athlete’s training. This process depends on inflammatory cells. The multitude of reactions and pathways that occur during the regeneration process results in the participation and control of many factors that are activated and secreted during muscle fiber damage and at different stages of its regeneration. However, not all of them are well understood yet. This paper presents the current state of knowledge on satellite cell-dependent skeletal muscle regeneration. Studies describing the effects of various forms of exercise and age on this process were reviewed.

Role of the Renin-Angiotensin-Aldosterone System in Dystrophin-Deficient Cardiomyopathy

Dystrophin-deficient cardiomyopathy (DDC) is currently the leading cause of death in patients with dystrophinopathies. Targeting myocardial fibrosis (MF) has become a major therapeutic goal in order to prevent the occurrence of DDC. We aimed to review and summarize the current evidence about the role of the renin-angiotensin-aldosterone system (RAAS) in the development and perpetuation of MF in DCC. We conducted a comprehensive search of peer-reviewed English literature on PubMed about this subject. We found increasing preclinical evidence from studies in animal models during the last 20 years pointing out a central role of RAAS in the development of MF in DDC. Local tissue RAAS acts directly mainly through its main fibrotic component angiotensin II (ANG2) and its transducer receptor (AT1R) and downstream TGF-b pathway. Additionally, it modulates the actions of most of the remaining pro-fibrotic factors involved in DDC. Despite limited clinical evidence, RAAS blockade constitutes the most studied, available and promising therapeutic strategy against MF and DDC. Conclusion: Based on the evidence reviewed, it would be recommendable to start RAAS blockade therapy through angiotensin converter enzyme inhibitors (ACEI) or AT1R blockers (ARBs) alone or in combination with mineralocorticoid receptor antagonists (MRa) at the youngest age after the diagnosis of dystrophinopathies, in order to delay the occurrence or slow the progression of MF, even before the detection of any cardiovascular alteration.

Enhanced transforming growth factor-beta signaling and fibrosis in the pectoralis major muscle of broiler chickens affected by wooden breast myopathy

Fibrosis has also been recorded as a prominent pathological feature within wooden breast (WB) myopathy of broiler chickens. This study was conducted to evaluate the accumulation of fibril collagen, deposition of the extracellular matrix (ECM) components, and the underlying mechanism mediating the pathogenic fibrotic process in the pectoralis major (PM) muscle of WB-affected birds. Broiler chickens were categorized into the control and WB groups based on the evaluation of myopathic lesions. Results indicated that the total content and area of collagen in cross-sections of the PM muscle, as well as the augmented expression of collagen-I and fibronectin in the ECM, were greatly increased in birds with WB. Wooden breast myopathy upregulated expressions of transforming growth factor-beta (TGF-β) and the phosphorylation of Smad 2 and 3, thereby activating TGF-β-mediated Smad signaling pathway, which further enhanced the transcription of profibrotic mediators. In addition, regulators involved in collagen biosynthesis and cross-linking including prolyl 4-hydroxylase, lysyl oxidase, lysyl hydroxylase, and decorin were increased in the WB muscle. Finally, the expressions of both matrix metalloproteinases (MMP) and tissue inhibitor of metalloproteinases (TIMP) were increased in the WB muscle, which might be related with reduced ECM remodeling. Overall, WB myopathy induces severe fibrosis by enhancing ECM deposition and collagen cross-linking in the PM muscle of broiler chickens, possibly via the activation of TGF-β signaling and the dysregulation of the MMP and TIMP system.

Cellular senescence-mediated exacerbation of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a progressive disease characterised by chronic muscle degeneration and inflammation. Our previously established DMD model rats (DMD rats) have a more severe disease phenotype than the broadly used mouse model. We aimed to investigate the role of senescence in DMD using DMD rats and patients. Senescence was induced in satellite cells and mesenchymal progenitor cells, owing to the increased expression of CDKN2A, p16- and p19-encoding gene. Genetic ablation of p16 in DMD rats dramatically restored body weight and muscle strength. Histological analysis showed a reduction of fibrotic and adipose tissues invading skeletal muscle, with increased muscle regeneration. Senolytic drug ABT263 prevented loss of body weight and muscle strength, and increased muscle regeneration in rats even at 8 months—the late stage of DMD. Moreover, senescence markers were highly expressed in the skeletal muscle of DMD patients. In situ hybridization of CDKN2A confirmed the expression of it in satellite cells and mesenchymal progenitor cells in patients with DMD. Collectively, these data provide new insights into the integral role of senescence in DMD progression.

TRIENNIAL GROWTH SYMPOSIUM: THE NUTRITION OF MUSCLE GROWTH: Impacts of nutrition on the proliferation and differentiation of satellite cells in livestock species1,2

Nutrition and other external factors are known to have a marked effect on growth of skeletal muscle, modulated, at least in part, through effects on satellite cells. Satellite cells and their embryonic precursors play an integral role in both prenatal and postnatal skeletal muscle growth of mammals. Changes in maternal nutrition can impact embryonic muscle progenitor cells which ultimately impacts both prenatal and postnatal skeletal muscle development. Satellite cells are important in postnatal skeletal muscle growth as they support the hypertrophy of existing myofibers. Hypertrophy of existing fibers is the only mechanism of postnatal muscle growth because muscle fiber number is fixed at birth and fiber nuclei have exited the cell cycle. Because fiber nuclei do not divide, additional nuclei required for hypertrophy must be acquired from satellite cells. To date, little research has aimed at determining whether nutrition directly impacts satellite cell populations within skeletal muscle of livestock species. However, it is well established that nutrition alters circulating concentrations of various growth factors such as insulin-like growth factor 1, epidermal growth factor, hepatocyte growth factor, and fibroblast growth factor. Each of these different growth factors impacts satellite cell proliferation and/or activation, indicating that nutrition likely plays a large role in skeletal muscle growth through impacting the satellite cell pool in both prenatal and postnatal growth. The relationship among nutrition, growth factors, and satellite cells relative to skeletal muscle growth is an important area of research that warrants further consideration.

Angiotensin II receptor blocker losartan exacerbates muscle damage and exhibits weak blood pressure-lowering activity in a dysferlin-null model of Limb-Girdle muscular dystrophy type 2B

There is no cure or beneficial management option for Limb-Girdle muscular dystrophy (MD) type 2B (LGMD2B). Losartan, a blood pressure (BP) lowering angiotensin II (AngII) receptor type 1 (ATR1) blocker (ARB) with unique anti-transforming growth factor-β (TGF-β) properties, can protect muscles in various types of MD such as Duchenne MD, suggesting a potential benefit for LGMD2B patients. Herein, we show in a mild, dysferlin-null mouse model of LGMD2B that losartan increased quadriceps muscle fibrosis (142%; P<0.0001). In a severe, atherogenic diet-fed model of LGMD2B recently described by our group, losartan further exacerbated dysferlin-null mouse muscle wasting in quadriceps and triceps brachii, two muscles typically affected by LGMD2B, by 40% and 51%, respectively (P<0.05). Lower TGF-β signalling was not observed with losartan, therefore plasma levels of atherogenic lipids known to aggravate LGMD2B severity were investigated. We report that losartan increased both plasma triglycerides and cholesterol concentrations in dysferlin-null mice. Other protective properties of losartan, such as increased nitric oxide release and BP lowering, were also reduced in the absence of dysferlin expression. Our data suggest that LGMD2B patients may show some resistance to the primary BP-lowering effects of losartan along with accelerated muscle wasting and dyslipidemia. Hence, we urge caution on the use of ARBs in this population as their ATR1 pathway may be dysfunctional.

Pluripotent stem cell-derived myogenic progenitors remodel their molecular signature upon in vivo engraftment

Optimal cell-based therapies for the treatment of muscle degenerative disorders should not only regenerate fibers but provide a quiescent satellite cell pool ensuring long-term maintenance and regeneration. Conditional expression of Pax3/Pax7 in differentiating pluripotent stem cells (PSCs) allows the generation of myogenic progenitors endowed with enhanced regenerative capacity. To identify the molecular determinants underlying their regenerative potential, we performed transcriptome analyses of these cells along with primary myogenic cells from several developmental stages. Here we show that in vitro-generated PSC-derived myogenic progenitors possess a molecular signature similar to embryonic/fetal myoblasts. However, compared with fetal myoblasts, following transplantation they show superior myofiber engraftment and ability to seed the satellite cell niche, respond to multiple reinjuries, and contribute to long-term regeneration. Upon engraftment, the transcriptome of reisolated Pax3/Pax7-induced PSC-derived myogenic progenitors changes toward a postnatal molecular signature, particularly in genes involved in extracellular matrix remodeling. These findings demonstrate that Pax3/Pax7-induced myogenic progenitors remodel their molecular signature and functionally mature upon in vivo exposure to the adult muscle environment.

BAMBI shuttling between cytosol and membrane is required for skeletal muscle development and regeneration

Bone morphogenetic protein and activin membrane-bound inhibitor (BAMBI) gene encodes a transmembrane protein and is involved in multiple physiological and pathological processes, such as inflammatory response, tumor development and progression, cell proliferation and differentiation. A previous study suggested that BAMBI may interact with the Wnt/β-catenin signaling pathway via promoting β-catenin nuclear translocation associated with C2C12 myogenic myoblast differentiation. However, its biological function in skeletal muscle still remains unknown and requires further characterization. The present work sought to investigate its biological function in skeletal muscle, especially the physiological roles of BAMBI during skeletal muscle growth and regeneration. Our current work suggests that BAMBI protein is highly expressed in skeletal muscle and is only detected in cytosolic fraction in the resting muscle. Moreover, BAMBI protein is co-localized in fast-twitch (glycolytic) fibers, but not in slow-twitch (oxidative) fibers. Comparing with the cytosolic trapping in resting muscle, BAMBI protein is enriched on cellular membrane during the muscle growth and regeneration, suggesting that BAMBI-mediated a significant signaling pathway may be an essential part of muscle growth and regeneration.

Immunobiology of Inherited Muscular Dystrophies

The immune response to acute muscle damage is important for normal repair. However, in chronic diseases such as many muscular dystrophies, the immune response can amplify pathology and play a major role in determining disease severity. Muscular dystrophies are inheritable diseases that vary tremendously in severity, but share the progressive loss of muscle mass and function that can be debilitating and lethal. Mutations in diverse genes cause muscular dystrophy, including genes that encode proteins that maintain membrane strength, participate in membrane repair, or are components of the extracellular matrix or the nuclear envelope. In this article, we explore the hypothesis that an important feature of many muscular dystrophies is an immune response adapted to acute, infrequent muscle damage that is misapplied in the context of chronic injury. We discuss the involvement of the immune system in the most common muscular dystrophy, Duchenne muscular dystrophy, and show that the immune system influences muscle death and fibrosis as disease progresses. We then present information on immune cell function in other muscular dystrophies and show that for many muscular dystrophies, release of cytosolic proteins into the extracellular space may provide an initial signal, leading to an immune response that is typically dominated by macrophages, neutrophils, helper T-lymphocytes, and cytotoxic T-lymphocytes. Although those features are similar in many muscular dystrophies, each muscular dystrophy shows distinguishing features in the magnitude and type of inflammatory response. These differences indicate that there are disease-specific immunomodulatory molecules that determine response to muscle cell damage caused by diverse genetic mutations. © 2018 American Physiological Society. Compr Physiol 8:1313-1356, 2018.

The physiopathologic interplay between stem cells and tissue niche in muscle regeneration and the role of IL-6 on muscle homeostasis and diseases

Skeletal muscle is a complex, dynamic tissue characterized by an elevated plasticity. Although the adult muscle is mainly composed of multinucleated fibers with post mitotic nuclei, it retains a remarkable ability to regenerate in response to traumatic events. The regenerative potential of the adult skeletal muscle relies in the activity of satellite cells, mononucleated cells residing within the muscle in intimate association with myofibers. Satellite cells normally remain quiescent in their sublaminar position, sporadically entering the cell cycle to guarantee an efficient cellular turnover, by fusing with pre-existing myofibers, and to maintain the stem cell pool. However, after muscle injury satellite cells undergo an extensive increase of their activity in response to environmental stimuli, thereby participating to the regeneration of a functional muscle tissue. Nevertheless, regeneration is affected in several pathologic conditions and by a wide range of environmental signals that are highly variable, not only through time, but also depending on the physiological or pathological conditions of the musculature. Among these factors, the interleukin-6 (IL-6) plays a critical physiopathologic role on muscle homeostasis and diseases. The basis of muscle regeneration and the impact of IL-6 on the physiopathology of skeletal muscle will be discussed.

Activated dendritic cells modulate proliferation and differentiation of human myoblasts

Idiopathic Inflammatory Myopathies (IIMs) are a heterogeneous group of autoimmune diseases affecting skeletal muscle tissue homeostasis. They are characterized by muscle weakness and inflammatory infiltration with tissue damage. Amongst the cells in the muscle inflammatory infiltration, dendritic cells (DCs) are potent antigen-presenting and key components in autoimmunity exhibiting an increased activation in inflamed tissues. Since, the IIMs are characterized by the focal necrosis/regeneration and muscle atrophy, we hypothesized that DCs may play a role in these processes. Due to the absence of a reliable in vivo model for IIMs, we first performed co-culture experiments with immature DCs (iDC) or LPS-activated DCs (actDC) and proliferating myoblasts or differentiating myotubes. We demonstrated that both iDC or actDCs tightly interact with myoblasts and myotubes, increased myoblast proliferation and migration, but inhibited myotube differentiation. We also observed that actDCs increased HLA-ABC, HLA-DR, VLA-5, and VLA-6 expression and induced cytokine secretion on myoblasts. In an in vivo regeneration model, the co-injection of human myoblasts and DCs enhanced human myoblast migration, whereas the absolute number of human myofibres was unchanged. In conclusion, we suggest that in the early stages of myositis, DCs may play a crucial role in inducing muscle-damage through cell–cell contact and inflammatory cytokine secretion, leading to muscle regeneration impairment.

TGF-β Family Signaling in Mesenchymal Differentiation

Mesenchymal stem cells (MSCs) can differentiate into several lineages during development and also contribute to tissue homeostasis and regeneration, although the requirements for both may be distinct. MSC lineage commitment and progression in differentiation are regulated by members of the transforming growth factor-β (TGF-β) family. This review focuses on the roles of TGF-β family signaling in mesenchymal lineage commitment and differentiation into osteoblasts, chondrocytes, myoblasts, adipocytes, and tenocytes. We summarize the reported findings of cell culture studies, animal models, and interactions with other signaling pathways and highlight how aberrations in TGF-β family signaling can drive human disease by affecting mesenchymal differentiation.

miR-22 regulates C2C12 myoblast proliferation and differentiation by targeting TGFBR1

Recently, miR-22 was found to be differentially expressed in different skeletal muscle growth period, indicated that it might have function in skeletal muscle myogenesis. In this study, we found that the expression of miR-22 was the most in skeletal muscle and was gradually up-regulated during mouse myoblast cell (C2C12 myoblast cell line) differentiation. Overexpression of miR-22 repressed C2C12 myoblast proliferation and promoted myoblast differentiation into myotubes, whereas inhibition of miR-22 showed the opposite results. During myogenesis, we predicted and verified transforming growth factor beta receptor 1 (TGFBR1), a key receptor of the TGF-β/Smad signaling pathway, was a target gene of miR-22. Then, we found miR-22 could regulate the expression of TGFBR1 and down-regulate the Smad3 signaling pathway. Knockdown of TGFBR1 by siRNA suppressed the proliferation of C2C12 cells but induced its differentiation. Conversely, overexpression of TGFBR1 significantly promoted proliferation but inhibited differentiation of the myoblast. Additionally, when C2C12 cells were treated with different concentrations of transforming growth factor beta 1 (TGF-β1), the level of miR-22 in C2C12 cells was reduced. The TGFBR1 protein level was significantly elevated in C2C12 cells treated with TGF-β1. Moreover, miR-22 was able to inhibit TGF-β1-induced TGFBR1 expression in C2C12 cells. Altogether, we demonstrated that TGF-β1 inhibited miR-22 expression in C2C12 cells and miR-22 regulated C2C12 cell myogenesis by targeting TGFBR1.

Expression Profiling of Differentiating Emerin-Null Myogenic Progenitor Identifies Molecular Pathways Implicated in Their Impaired Differentiation

Mutations in the gene encoding emerin cause Emery-Dreifuss muscular dystrophy (EDMD), a disorder causing progressive skeletal muscle wasting, irregular heart rhythms and contractures of major tendons. RNA sequencing was performed on differentiating wildtype and emerin-null myogenic progenitors to identify molecular pathways implicated in EDMD, 340 genes were uniquely differentially expressed during the transition from day 0 to day 1 in wildtype cells. 1605 genes were uniquely expressed in emerin-null cells; 1706 genes were shared among both wildtype and emerin-null cells. One thousand and forty-seven transcripts showed differential expression during the transition from day 1 to day 2. Four hundred and thirty-one transcripts showed altered expression in both wildtype and emerin-null cells. Two hundred and ninety-five transcripts were differentially expressed only in emerin-null cells and 321 transcripts were differentially expressed only in wildtype cells. DAVID, STRING and Ingenuity Pathway Analysis identified pathways implicated in impaired emerin-null differentiation, including cell signaling, cell cycle checkpoints, integrin signaling, YAP/TAZ signaling, stem cell differentiation, and multiple muscle development and myogenic differentiation pathways. Functional enrichment analysis showed biological functions associated with the growth of muscle tissue and myogenesis of skeletal muscle were inhibited. The large number of differentially expressed transcripts upon differentiation induction suggests emerin functions during transcriptional reprograming of progenitors to committed myoblasts.

Outside in: The matrix as a modifier of muscular dystrophy

Muscular dystrophies are genetic conditions leading to muscle degeneration and often, impaired regeneration. Duchenne Muscular Dystrophy is a prototypical form of muscular dystrophy, and like other forms of genetically inherited muscle diseases, pathological progression is variable. Variability in muscular dystrophy can arise from differences in the manner in which the primary mutation impacts the affected protein's function; however, clinical heterogeneity also derives from secondary mutations in other genes that can enhance or reduce pathogenic features of disease. These genes, called genetic modifiers, regulate the pathophysiological context of dystrophic degeneration and regeneration. Understanding the mechanistic links between genetic modifiers and dystrophic progression sheds light on pathologic remodeling, and provides novel avenues to therapeutically intervene to reduce muscle degeneration. Based on targeted genetic approaches and unbiased genomewide screens, several modifiers have been identified for muscular dystrophy, including extracellular agonists of signaling cascades. This review will focus on identification and possible mechanisms of recently identified modifiers for muscular dystrophy, including osteopontin, latent TGFβ binding protein 4 (LTBP4) and Jagged1. Moreover, we will review the investigational approaches that aim to target modifier pathways and thereby counteract dystrophic muscle wasting.

The Use of Platelet-Rich and Platelet-Poor Plasma to Enhance Differentiation of Skeletal Myoblasts: Implications for the Use of Autologous Blood Products for Muscle Regeneration

BACKGROUND

Platelet-rich plasma (PRP) has been used to augment tissue repair and regeneration after musculoskeletal injury. However, there is increasing clinical evidence that PRP does not show a consistent clinical effect. Purpose/Hypothesis: This study aimed to compare the effects of the following non-neutrophil-containing (leukocyte-poor) plasma fractions on human skeletal muscle myoblast (HSMM) differentiation: (1) PRP, (2) modified PRP (Mod-PRP), in which transforming growth factor β1 (TGF-β1) and myostatin (MSTN) were depleted, and (3) platelet-poor plasma (PPP). The hypothesis was that leukocyte-poor PRP would lead to myoblast proliferation (not differentiation), whereas certain modifications of PRP preparations would increase myoblast differentiation, which is necessary for skeletal muscle regeneration.

STUDY DESIGN

Controlled laboratory study.

METHODS

Blood from 7 human donors was individually processed to simultaneously create leukocyte-poor fractions: PRP, Mod-PRP, PPP, and secondarily spun PRP and Mod-PRP (PRP_ss and Mod-PRP_ss, respectively). Mod-PRP was produced by removing TGF-β1 and MSTN from PRP using antibodies attached to sterile beads, while a second-stage centrifugal spin of PRP was performed to remove platelets. The biologics were individually added to cell culture groups. Analysis for induction into myoblast differentiation pathways included Western blot analysis, reverse-transcription polymerase chain reaction, and immunohistochemistry, as well as confocal microscopy to assess polynucleated myotubule formation.

RESULTS

HSMMs cultured with PRP showed an increase in proliferation but no evidence of differentiation. Western blot analysis confirmed that MSTN and TGF-β1 could be decreased in Mod-PRP using antibody-coated beads, but this modification mildly improved myoblast differentiation. However, cell culture with PPP, PRP_ss, and Mod-PRP_ss led to a decreased proliferation rate but a significant induction of myoblast differentiation verified by increased multinucleated myotubule formation and myosin heavy chain expression (mean 8-fold change in mRNA level; P < .05), which was comparable with 2% horse serum, the positive control.

CONCLUSION

PPP and leukocyte-poor PRP preparations subjected to a second spin to remove the platelets led to induction of myoblast cells into the muscle differentiation pathway, whereas unmodified leukocyte-poor PRP led to myoblast proliferation.

CLINICAL RELEVANCE

These results indicate that traditionally formulated PRP may not be appropriate to induce muscle regeneration. Laboratory evidence suggests that PPP or non-neutrophil-containing PRP_ss, subjected to an additional spin to remove platelets, should be used to stimulate myoblast differentiation, which is necessary for skeletal muscle regeneration. Clinical studies will be required to confirm the effect of these biologics on muscle regeneration.

In Silico and In Vivo Experiments Reveal M-CSF Injections Accelerate Regeneration Following Muscle Laceration

Numerous studies have pharmacologically modulated the muscle milieu in the hopes of promoting muscle regeneration; however, the timing and duration of these interventions are difficult to determine. This study utilized a combination of in silico and in vivo experiments to investigate how inflammation manipulation improves muscle recovery following injury. First, we measured macrophage populations following laceration injury in the rat tibialis anterior (TA). Then we calibrated an agent-based model (ABM) of muscle injury to mimic the observed inflammation profiles. The calibrated ABM was used to simulate macrophage and satellite stem cell (SC) dynamics, and suggested that delivering macrophage colony stimulating factor (M-CSF) prior to injury would promote SC-mediated injury recovery. Next, we performed an experiment wherein 1 day prior to injury, we injected M-CSF into the rat TA muscle. M-CSF increased the number of macrophages during the first 4 days post-injury. Furthermore, treated muscles experienced a swifter increase in the appearance of PAX7⁺ SCs and regenerating muscle fibers. Our study suggests that computational models of muscle injury provide novel insights into cellular dynamics during regeneration, and further, that pharmacologically altering inflammation dynamics prior to injury can accelerate the muscle regeneration process.

The TGF-β Signalling Network in Muscle Development, Adaptation and Disease

Skeletal muscle possesses remarkable ability to change its size and force-producing capacity in response to physiological stimuli. Impairment of the cellular processes that govern these attributes also affects muscle mass and function in pathological conditions. Myostatin, a member of the TGF-β family, has been identified as a key regulator of muscle development, and adaptation in adulthood. In muscle, myostatin binds to its type I (ALK4/5) and type II (ActRIIA/B) receptors to initiate Smad2/3 signalling and the regulation of target genes that co-ordinate the balance between protein synthesis and degradation. Interestingly, evidence is emerging that other TGF-β proteins act in concert with myostatin to regulate the growth and remodelling of skeletal muscle. Consequently, dysregulation of TGF-β proteins and their associated signalling components is increasingly being implicated in muscle wasting associated with chronic illness, ageing, and inactivity. The growing understanding of TGF-β biology in muscle, and its potential to advance the development of therapeutics for muscle-related conditions is reviewed here.

Low Oxygen Tension Enhances Expression of Myogenic Genes When Human Myoblasts Are Activated from G0 Arrest

OBJECTIVES

Most cell culture studies have been performed at atmospheric oxygen tension of 21%, however the physiological oxygen tension is much lower and is a factor that may affect skeletal muscle myoblasts. In this study we have compared activation of G0 arrested myoblasts in 21% O2 and in 1% O2 in order to see how oxygen tension affects activation and proliferation of human myoblasts.

MATERIALS AND METHODS

Human myoblasts were isolated from skeletal muscle tissue and G0 arrested in vitro followed by reactivation at 21% O2 and 1% O2. The effect was assesses by Real-time RT-PCR, immunocytochemistry and western blot.

RESULTS AND CONCLUSIONS

We found an increase in proliferation rate of myoblasts when activated at a low oxygen tension (1% O2) compared to 21% O2. In addition, the gene expression studies showed up regulation of the myogenesis related genes PAX3, PAX7, MYOD, MYOG (myogenin), MET, NCAM, DES (desmin), MEF2A, MEF2C and CDH15 (M-cadherin), however, the fraction of DES and MYOD positive cells was not increased by low oxygen tension, indicating that 1% O2 may not have a functional effect on the myogenic response. Furthermore, the expression of genes involved in the TGFβ, Notch and Wnt signaling pathways were also up regulated in low oxygen tension. The differences in gene expression were most pronounced at day one after activation from G0-arrest, thus the initial activation of myoblasts seemed most sensitive to changes in oxygen tension. Protein expression of HES1 and β-catenin indicated that notch signaling may be induced in 21% O2, while the canonical Wnt signaling may be induced in 1% O2 during activation and proliferation of myoblasts.

Effects of icing or heat stress on the induction of fibrosis and/or regeneration of injured rat soleus muscle

The effects of icing or heat stress on the regeneration of injured soleus muscle were investigated in male Wistar rats. Bupivacaine was injected into soleus muscles bilaterally to induce muscle injury. Icing (0 °C, 20 min) was carried out immediately after the injury. Heat stress (42 °C, 30 min) was applied every other day during 2-14 days after the bupivacaine injection. Injury-related increase in collagen deposition was promoted by icing. However, the level of collagen deposition in heat-stressed animals was maintained at control levels throughout the experimental period and was significantly lower than that in icing-treated animals at 15 and 28 days after bupivacaine injection. Furthermore, the recovery of muscle mass, protein content, and muscle fiber size of injured soleus toward control levels was partially facilitated by heat stress. These results suggest that, compared with icing, heat stress may be a beneficial treatment for successful muscle regeneration at least by reducing fibrosis.

Muscle wasting and aging: Experimental models, fatty infiltrations, and prevention

Identification of cost-effective interventions to maintain muscle mass, muscle strength, and physical performance during muscle wasting and aging is an important public health challenge. It requires understanding of the cellular and molecular mechanisms involved. Muscle-deconditioning processes have been deciphered by means of several experimental models, bringing together the opportunities to devise comprehensive analysis of muscle wasting. Studies have increasingly recognized the importance of fatty infiltrations or intermuscular adipose tissue for the age-mediated loss of skeletal-muscle function and emphasized that this new important factor is closely linked to inactivity. The present review aims to address three main points. We first mainly focus on available experimental models involving cell, animal, or human experiments on muscle wasting. We next point out the role of intermuscular adipose tissue in muscle wasting and aging and try to highlight new findings concerning aging and muscle-resident mesenchymal stem cells called fibro/adipogenic progenitors by linking some cellular players implicated in both FAP fate modulation and advancing age. In the last part, we review the main data on the efficiency and molecular and cellular mechanisms by which exercise, replacement hormone therapies, and β-hydroxy-β-methylbutyrate prevent muscle wasting and sarcopenia. Finally, we will discuss a potential therapeutic target of sarcopenia: glucose 6-phosphate dehydrogenase.

Transforming growth factor type beta (TGF-β) requires reactive oxygen species to induce skeletal muscle atrophy

Transforming growth factor beta 1 (TGF-β1) is a classical modulator of skeletal muscle and regulates several processes, such as myogenesis, regeneration, and muscle function in skeletal muscle diseases. Skeletal muscle atrophy, characterised by the loss of muscle strength and mass, is one of the pathological conditions regulated by TGF-β. Atrophy also results in increased myosin heavy chain (MHC) degradation and the expression of two muscle-specific E3 ubiquitin ligases, atrogin-1 and MuRF-1. Reactive oxygen species (ROS) are modulators of muscle wasting, and NAD(P)H oxidase (NOX) is one of the main sources of ROS. While it was recently found that TGF-β1 induces atrophy in skeletal muscle, the underlying mechanism is not fully understood. In this study, the role of NOX-derived ROS in skeletal muscle atrophy induced by TGF-β was assessed. TGF-β1 induced an atrophic effect in C2C12 myotubes, as evidenced by decreased myotube diameter and MHC levels, together with increased MuRF-1 levels. Concomitantly, TGF-β increased NOX-induced ROS contents. Interestingly, NOX inhibition through apocynin and the antioxidant treatment with N-acetyl cysteine (NAC) decreased increased ROS levels in myotubes. Additionally, both apocynin and NAC completely prevented the decreased MHC, decreased myotube diameter, and increased MuRF-1 induced by TGF-β. Injection of TGF-β1 into the tibialis anterior muscle induced atrophy, as observed by decreased fibre diameter and MHC levels, together with increased MuRF-1 levels. Likewise, TGF-β increased the ROS contents in the smaller fibres of skeletal muscle. Additionally, the administration of NAC to mice prevented all atrophic effects and the increase in ROS induced by TGF-β in the tibialis anterior. This is the first study to report that TGF-β has an atrophic effect dependent on NOX-induced ROS in skeletal muscle.

Protein-leucine ingestion activates a regenerative inflammo-myogenic transcriptome in skeletal muscle following intense endurance exercise

Protein-leucine supplement ingestion following strenuous endurance exercise accentuates skeletal-muscle protein synthesis and adaptive molecular responses, but the underlying transcriptome is uncharacterized. In a randomized single-blind triple-crossover design, 12 trained men completed 100 min of high-intensity cycling then ingested 70/15/180/30 g protein-leucine-carbohydrate-fat (15LEU), 23/5/180/30 g (5LEU), or 0/0/274/30 g (CON) beverages during the first 90 min of a 240 min recovery period. Vastus lateralis muscle samples (30 and 240 min postexercise) underwent transcriptome analysis by microarray followed by bioinformatic analysis. Gene expression was regulated by protein-leucine in a dose-dependent manner affecting the inflammatory response and muscle growth and development. At 30 min, 15LEU and 5LEU vs. CON activated transcriptome networks with gene-set functions involving cell-cycle arrest (Z-score 2.0-2.7, P < 0.01), leukocyte maturation (1.7, P = 0.007), cell viability (2.4, P = 0.005), promyogenic networks encompassing myocyte differentiation and myogenin (MYOD1, MYOG), and a proteinaceous extracellular matrix, adhesion, and development program correlated with plasma lysine, arginine, tyrosine, taurine, glutamic acid, and asparagine concentrations. High protein-leucine dose (15LEU-5LEU) activated an IL-1I-centered proinflammatory network and leukocyte migration, differentiation, and survival functions (2.0-2.6, <0.001). By 240 min, the protein-leucine transcriptome was anti-inflammatory and promyogenic (IL-6, NF- β, SMAD, STAT3 network inhibition), with overrepresented functions including decreased leukocyte migration and connective tissue development (-1.8-2.4, P < 0.01), increased apoptosis of myeloid and muscle cells (2.2-3.0, P < 0.002), and cell metabolism (2.0-2.4, P < 0.01). The analysis suggests protein-leucine ingestion modulates inflammatory-myogenic regenerative processes during skeletal muscle recovery from endurance exercise. Further cellular and translational research is warranted to validate amino acid-mediated myeloid and myocellular mechanisms within skeletal-muscle functional plasticity.

Validation of genetic modifiers for Duchenne muscular dystrophy: a multicentre study assessing SPP1 and LTBP4 variants

OBJECTIVE

Duchenne muscular dystrophy (DMD) is characterised by progressive muscle weakness. It has recently been reported that single nucleotide polymorphisms (SNPs) located in the SPP1 and LTBP4 loci can account for some of the inter-individual variability observed in the clinical disease course. The validation of genetic association in large independent cohorts is a key process for rare diseases in order to qualify prognostic biomarkers and stratify patients in clinical trials.

METHODS

Duchenne patients from five European neuromuscular centres were included. Information about age at wheelchair dependence and steroid use was gathered. Melting curve analysis of PCR fragments or Sanger sequencing were used to genotype SNP rs28357094 in the SPP1 gene in 336 patients. The genotype of SNPs rs2303729, rs1131620, rs1051303 and rs10880 in the LTBP4 locus was determined in 265 patients by mass spectrometry. For both loci, a multivariate analysis was performed, using genotype/haplotype, steroid use and cohort as covariates.

RESULTS

We show that corticosteroid treatment and the IAAM haplotype of the LTBP4 gene are significantly associated with prolonged ambulation in patients with DMD. There was no significant association between the SNP rs28357094 in the SPP1 gene and the age of ambulation loss.

CONCLUSIONS

This study underlines the importance of replicating genetic association studies for rare diseases in large independent cohorts to identify the most robust associations. We anticipate that genotyping of validated genetic associations will become important for the design and interpretation of clinical trials.

Non-Invasive Biomarkers for Duchenne Muscular Dystrophy and Carrier Detection

Non-invasive biological indicators of the absence/presence or progress of the disease that could be used to support diagnosis and to evaluate the effectiveness of treatment are of utmost importance in Duchenne Muscular Dystrophy (DMD). This neuromuscular disorder affects male children, causing weakness and disability, whereas female relatives are at risk of being carriers of the disease. A biomarker with both high sensitivity and specificity for accurate prediction is preferred. Until now creatine kinase (CK) levels have been used for DMD diagnosis but these fail to assess disease progression. Herein we examined the potential applicability of serum levels of matrix metalloproteinase 9 (MMP-9) and matrix metalloproteinase 2 (MMP-2), tissue inhibitor of metalloproteinases 1 (TIMP-1), myostatin (GDF-8) and follistatin (FSTN) as non-invasive biomarkers to distinguish between DMD steroid naïve patients and healthy controls of similar age and also for carrier detection. Our data suggest that serum levels of MMP-9, GDF-8 and FSTN are useful to discriminate DMD from controls (p < 0.05), to correlate with some neuromuscular assessments for DMD, and also to differentiate between Becker muscular dystrophy (BMD) and Limb-girdle muscular dystrophy (LGMD) patients. In DMD individuals under steroid treatment, GDF-8 levels increased as FSTN levels decreased, resembling the proportions of these proteins in healthy controls and also the baseline ratio of patients without steroids. GDF-8 and FSTN serum levels were also useful for carrier detection (p < 0.05). Longitudinal studies with larger cohorts are necessary to confirm that these molecules correlate with disease progression. The biomarkers presented herein could potentially outperform CK levels for carrier detection and also harbor potential for monitoring disease progression.

Cancer-associated muscle weakness: What's bone got to do with it?

Cancer-associated muscle weakness is an important paraneoplastic syndrome for which there is currently no treatment. Tumor cells commonly metastasize to bone in advanced cancer to disrupt normal bone remodeling and result in morbidity that includes muscle weakness. Tumor in bone stimulates excessive osteoclast activity, which causes the release of growth factors stored in the mineralized bone matrix. These factors fuel a feed-forward vicious cycle of tumor growth in bone and bone destruction. Recent evidence indicates that these bone-derived growth factors can act systemically to cause muscle weakness. Muscle weakness can be caused by reduced muscle mass or reduced muscle function; in advanced disease, it is likely due to a combination of both reduced quantity and quality of muscle. In this review, we discuss possible mechanisms that lead to skeletal muscle weakness due to bone metastases.

MicroRNA-21 Regulates PI3K/Akt/mTOR Signaling by Targeting TGFβI during Skeletal Muscle Development in Pigs

MicroRNAs (miRNAs), which are short (22–24 base pairs), non-coding RNAs, play critical roles in myogenesis. Using Solexa deep sequencing, we detected the expression levels of 229 and 209 miRNAs in swine skeletal muscle at 90 days post-coitus (E90) and 100 days postnatal (D100), respectively. A total of 138 miRNAs were up-regulated on E90, and 31 were up-regulated on D100. Of these, 9 miRNAs were selected for the validation of the small RNA libraries by quantitative RT-PCR (RT-qPCR). We found that miRNA-21 was down-regulated by 17-fold on D100 (P<0.001). Bioinformatics analysis suggested that the transforming growth factor beta-induced (TGFβI) gene was a potential target of miRNA-21. Both dual luciferase reporter assays and western blotting demonstrated that the TGFβI gene was regulated by miRNA-21. Co-expression analysis revealed that the mRNA expression levels of miRNA-21 and TGFβI were negatively correlated (r = -0.421, P = 0.026) in skeletal muscle during the 28 developmental stages. Our results revealed that more miRNAs are expressed in prenatal than in postnatal skeletal muscle. The miRNA-21 is a novel myogenic miRNA that is involved in skeletal muscle development and regulates PI3K/Akt/mTOR signaling by targeting the TGFβI gene.

The altered fate of aging satellite cells is determined by signaling and epigenetic changes

Skeletal muscle is a striated tissue composed of multinucleated fibers that contract under the control of the somatic nervous system to direct movement. The stem cells of skeletal muscle, known as satellite cells, are responsible for muscle fiber growth, turnover, and regeneration. Satellite cells are activated and proliferate in response to stimuli, and simplistically, have two main fates—to repopulate the satellite cell niche, or differentiate to regenerate or repair muscle fibers. However, the ability to regenerate muscle and replace lost myofibers declines with age. This loss of function may be a result of extrinsic changes in the niche, such as alterations in signaling or modifications to the extracellular matrix. However, intrinsic epigenetic changes within satellite cells may also affect cell fate and cause a decline in regenerative capacity. This review will describe the mechanisms that regulate cell fate decisions in adult skeletal muscle, and how changes during aging affect muscle fiber turnover and regeneration.

Satellite cells: regenerative mechanisms and applicability in muscular dystrophy

The satellite cells are long regarded as heterogeneous cell population, which is intimately linked to the processes of muscular recovery. The heterogeneous cell population may be classified by specific markers. In spite of the significant amount of variation amongst the satellite cell populations, it seems that their activity is tightly bound to the paired box 7 transcription factor expression, which is, therefore, used as a canonical marker for these cells. Muscular dystrophic diseases, such as Duchenne muscular dystrophy, elicit severe tissue injuries leading those patients to display a very specific pattern of muscular recovery abnormalities. There have been works on the application of precursors cells as a therapeutic alternative for Duchenne muscular dystrophy and initial attempts have proven the cells inefficient; however later endeavours have proposed solutions for the experiments improving significantly the results. The presence of a range of satellite cells populations indicates the existence of specific cells with enhanced capability of muscular recovery in afflicted muscles.

Dynamic expression of tgf-β2, tgf-β3 and inhibin βA during muscle growth resumption and satellite cell differentiation in rainbow trout (Oncorhynchus mykiss)

Members of the TGF-β superfamily are involved in numerous cell functions; however, except for myostatin, their roles in the regulation of muscle growth in fish are completely unknown. We measured tgf-β1, tgf-β2, tgf-β3, inhibin βA (inh) and follistatin (fst) gene expression during muscle growth recovery following a fasting period. We observed that tgf-β1a and tgf-β2 expression were quickly down-regulated after refeeding and that tgf-β3 reached its highest level of expression 7days post-refeeding, mirroring myogenin expression. Inh βA1 mRNA levels decreased sharply after refeeding, in contrast to fst b2 expression, which peaked at day 2. No significant modification of expression was observed for tgf-β1a, tgf-β1b, tgf-β1c and tgf-β6 during refeeding. In vitro, tgf-β2 and inh βA1 expression decreased during the differentiation of satellite cells, whereas tgf-β3 expression increased following the same pattern as myogenin. Surprisingly, fst b1 and fst b2 expression decreased during differentiation, whereas no variation was observed in fst a1 and fst a2 expression levels. In vitro analyses also indicated that IGF1 treatment up-regulated tgf-β3, inh βA1 and myogenin expression, and that MSTN treatment increased fst b1 and fst b2 expression. In conclusion, we showed that the expression of tgf-β2, tgf-β3 and inh βA1 is dynamically regulated during muscle growth resumption and satellite cell differentiation, strongly suggesting that these genes have a role in the regulation of muscle growth.

Multi-omic integrated networks connect DNA methylation and miRNA with skeletal muscle plasticity to chronic exercise in Type 2 diabetic obesity

Epigenomic regulation of the transcriptome by DNA methylation and posttranscriptional gene silencing by miRNAs are potential environmental modulators of skeletal muscle plasticity to chronic exercise in healthy and diseased populations. We utilized transcriptome networks to connect exercise-induced differential methylation and miRNA with functional skeletal muscle plasticity. Biopsies of the vastus lateralis were collected from middle-aged Polynesian men and women with morbid obesity (44 kg/m(2) ± 10) and Type 2 diabetes before and following 16 wk of resistance (n = 9) or endurance training (n = 8). Longitudinal transcriptome, methylome, and microRNA (miRNA) responses were obtained via microarray, filtered by novel effect-size based false discovery rate probe selection preceding bioinformatic interrogation. Metabolic and microvascular transcriptome topology dominated the network landscape following endurance exercise. Lipid and glucose metabolism modules were connected to: microRNA (miR)-29a; promoter region hypomethylation of nuclear receptor factor (NRF1) and fatty acid transporter (SLC27A4), and hypermethylation of fatty acid synthase, and to exon hypomethylation of 6-phosphofructo-2-kinase and Ser/Thr protein kinase. Directional change in the endurance networks was validated by lower intramyocellular lipid, increased capillarity, GLUT4, hexokinase, and mitochondrial enzyme activity and proteome. Resistance training also lowered lipid and increased enzyme activity and caused GLUT4 promoter hypomethylation; however, training was inconsequential to GLUT4, capillarity, and metabolic transcriptome. miR-195 connected to negative regulation of vascular development. To conclude, integrated molecular network modelling revealed differential DNA methylation and miRNA expression changes occur in skeletal muscle in response to chronic exercise training that are most pronounced with endurance training and topographically associated with functional metabolic and microvascular plasticity relevant to diabetes rehabilitation.

Identification of differentially expressed genes in breast muscle and skin fat of postnatal Pekin duck

Lean-type Pekin duck is a commercial breed that has been obtained through long-term selection. Investigation of the differentially expressed genes in breast muscle and skin fat at different developmental stages will contribute to a comprehensive understanding of the potential mechanisms underlying the lean-type Pekin duck phenotype. In the present study, RNA-seq was performed on breast muscle and skin fat at 2-, 4- and 6-weeks of age. More than 89% of the annotated duck genes were covered by our RNA-seq dataset. Thousands of differentially expressed genes, including many important genes involved in the regulation of muscle development and fat deposition, were detected through comparison of the expression levels in the muscle and skin fat of the same time point, or the same tissue at different time points. KEGG pathway analysis showed that the differentially expressed genes clustered significantly in many muscle development and fat deposition related pathways such as MAPK signaling pathway, PPAR signaling pathway, Calcium signaling pathway, Fat digestion and absorption, and TGF-beta signaling pathway. The results presented here could provide a basis for further investigation of the mechanisms involved in muscle development and fat deposition in Pekin duck.

Myofiber-specific inhibition of TGFβ signaling protects skeletal muscle from injury and dystrophic disease in mice

Muscular dystrophy (MD) is a disease characterized by skeletal muscle necrosis and the progressive accumulation of fibrotic tissue. While transforming growth factor (TGF)-β has emerged as central effector of MD and fibrotic disease, the cell types in diseased muscle that underlie TGFβ-dependent pathology have not been segregated. Here, we generated transgenic mice with myofiber-specific inhibition of TGFβ signaling owing to expression of a TGFβ type II receptor dominant-negative (dnTGFβRII) truncation mutant. Expression of dnTGFβRII in myofibers mitigated the dystrophic phenotype observed in δ-sarcoglycan-null (Sgcd(-/-)) mice through a mechanism involving reduced myofiber membrane fragility. The dnTGFβRII transgene also reduced muscle injury and improved muscle regeneration after cardiotoxin injury, as well as increased satellite cell numbers and activity. An unbiased global expression analysis revealed a number of potential mechanisms for dnTGFβRII-mediated protection, one of which was induction of the antioxidant protein metallothionein (Mt). Indeed, TGFβ directly inhibited Mt gene expression in vitro, the dnTGFβRII transgene conferred protection against reactive oxygen species accumulation in dystrophic muscle and treatment with Mt mimetics protected skeletal muscle upon injury in vivo and improved the membrane stability of dystrophic myofibers. Hence, our results show that the myofibers are central mediators of the deleterious effects associated with TGFβ signaling in MD.

Molecular mechanisms of bone metastasis and associated muscle weakness

Bone is a preferred site for breast cancer metastasis and leads to pathologic bone loss due to increased osteoclast-induced bone resorption. The homing of tumor cells to the bone depends on the support of the bone microenvironment in which the tumor cells prime the premetastatic niche. The colonization and growth of tumor cells then depend on adaptations in the invading tumor cells to take advantage of normal physiologic responses by mimicking bone marrow cells. This concerted effort by tumor cells leads to uncoupled bone remodeling in which the balance of osteoclast-driven bone resorption and osteoblast-driven bone deposition is lost. Breast cancer bone metastases often lead to osteolytic lesions due to hyperactive bone resorption. Release of growth factors from bone matrix during resorption then feeds a "vicious cycle" of bone destruction leading to many skeletal-related events. In addition to activity in bone, some of the factors released during bone resorption are also known to be involved in skeletal muscle regeneration and contraction. In this review, we discuss the mechanisms that lead to osteolytic breast cancer bone metastases and the potential for cancer-induced bone-muscle cross-talk leading to skeletal muscle weakness.

Myostatin stimulates, not inihibits, C2C12 myoblast proliferation

The immortal C2C12 cell line originates from dystrophic mouse thigh muscle and has been used to study the endocrine control of muscle cell growth, development, and function, including those actions regulated by myostatin. Previous studies suggest that high concentrations of recombinant myostatin generated in bacteria inhibit C2C12 proliferation and differentiation. Recombinant myostatin generated in eukaryotic systems similarly inhibits the proliferation of primary myosatellite cells, but consequently initiates, rather than inhibits, their differentiation and is bioactive at far lower concentrations. Our studies indicate that 2 different sources of recombinant myostatin made in eukaryotes stimulate, not inhibit, C2C12 proliferation. This effect occurred at different cell densities and serum concentrations and in the presence of IGF-I, a potent myoblast mitogen. This stimulatory effect was comparable to that obtained with TGFβ1, a related factor that also inhibits primary myosatellite cell proliferation. Attenuating the myostatin/activin (ie, Acvr2b) and TGFβ1 receptor signaling pathways with the Alk4/5 and Alk5 inhibitors, SB431542 and SB505142, respectively, similarly attenuated proliferation induced by serum, myostatin or TGFβ1 and in a dose-dependent manner. In serum-free medium, both myostatin and TGFβ1 stimulated Smad2 phosphorylation, but not that of Smad3, and a Smad3 inhibitor (SIS3) only inhibited proliferation in cells cultured in high serum. Thus, myostatin and TGFβ1 stimulate C2C12 proliferation primarily via Smad2. These results together question the physiological relevance of the C2C12 model and previous studies using recombinant myostatin generated in bacteria. They also support the alternative use of primary myosatellite cells and recombinant myostatin generated in eukaryotes.

Mechanisms of muscle injury, repair, and regeneration

Skeletal muscle continuously adapts to changes in its mechanical environment through modifications in gene expression and protein stability that affect its physiological function and mass. However, mechanical stresses commonly exceed the parameters that induce adaptations, producing instead acute injury. Furthermore, the relatively superficial location of many muscles in the body leaves them further vulnerable to acute injuries by exposure to extreme temperatures, contusions, lacerations or toxins. In this article, the molecular, cellular, and mechanical factors that underlie muscle injury and the capacity of muscle to repair and regenerate are presented. Evidence shows that muscle injuries that are caused by eccentric contractions result from direct mechanical damage to myofibrils. However, muscle pathology following other acute injuries is largely attributable to damage to the muscle cell membrane. Many feaures in the injury-repair-regeneration cascade relate to the unregulated influx of calcium through membrane lesions, including: (i) activation of proteases and hydrolases that contribute muscle damage, (ii) activation of enzymes that drive the production of mitogens and motogens for muscle and immune cells involved in injury and repair, and (iii) enabling protein-protein interactions that promote membrane repair. Evidence is also presented to show that the myogenic program that is activated by acute muscle injury and the inflammatory process that follows are highly coordinated, with myeloid cells playing a central role in modulating repair and regeneration. The early-invading, proinflammatory M1 macrophages remove debris caused by injury and express Th1 cytokines that play key roles in regulating the proliferation, migration, and differentiation of satellite cells. The subsequent invasion by anti-inflammatory, M2 macrophages promotes tissue repair and attenuates inflammation. Although this system provides an effective mechanism for muscle repair and regeneration following acute injury, it is dysregulated in chronic injuries. In this article, the process of muscle injury, repair and regeneration that occurs in muscular dystrophy is used as an example of chronic muscle injury, to highlight similarities and differences between the injury and repair processes that occur in acutely and chronically injured muscle.