In vivo activation of STAT3 signaling in satellite cells and myofibers in regenerating rat skeletal muscles

Beyond muscles: Investigating immunoregulatory myokines in acute resistance exercise - A systematic review and meta-analysis

Myokines, released from the muscle, enable communication between the working muscles and other tissues. Their release during physical exercise is assumed to depend on immune-hormonal-metabolic interactions concerning mode (endurance or resistance exercise), duration, and intensity. This meta-analysis aims to examine the acute changes of circulating myokines inducing immunoregulatory effects caused by a bout of resistance exercise and to consider potential moderators of the results. Based on this selection strategy, a systematic literature search was conducted for resistance exercise intervention studies measuring interleukin (IL-) 6, IL-10, IL-1ra, tumor necrosis factor (TNF-) α, IL-15, IL-7, transforming growth factor (TGF-) β1, and fractalkines (FKN) before and immediately after resistance exercise in healthy individuals. Random-effects meta-analysis was performed for each myokine. We identified a moderate positive effect of resistance exercise for IL-6 and IL-1ra. Regarding IL-15 and TNF-α, small to moderate effects were found. For IL-10, no significant effect was observed. Due to no data, meta-analyses for IL-7, TGF-β1, and FKN could not be performed. No moderators (training status, type of exercise, risk of bias, age, sex, time of day, exercise volume, exercise intensity, exercise dose) of the results were detected for all tested myokines. Taken together, this systematic review and meta-analysis showed immediate positive effects of an acute resistance exercise session on IL-6, IL-1ra, TNF-α, and IL-15 levels.

Effects of the immune system on muscle regeneration

Muscle regeneration is a complex process orchestrated by multiple steps. Recent findings indicate that inflammatory responses could play central roles in bridging initial muscle injury responses and timely muscle injury reparation. The various types of immune cells and cytokines have crucial roles in muscle regeneration process. In this review, we provide an overview of the functions of acute inflammation in muscle regeneration.

Cyclin D3 Colocalizes with Myogenin and p21 in Skeletal Muscle Satellite Cells during Early-Stage Functional Overload

Myogenic cell differentiation is modulated by multiple regulatory factors, such as myogenin, p21, and cyclin D3 during myogenesis in vitro. It is also recognized that myogenin and p21 play important roles in regulating muscle satellite cell (SC) differentiation during overload-induced muscle hypertrophy in vivo. However, the expression patterns and functional role of cyclin D3 in the progress of muscle hypertrophy remain unclear. Thus, the present study investigated cyclin D3 expression in skeletal muscles during early-stage functional overload. Plantaris muscles were exposed to functional overload due to ablation of the gastrocnemius and soleus muscles. As a result, cyclin D3 expression was detected in the nuclei of SCs but not in myonuclei on day 1 after surgery. Cyclin D3 expression, after functional overload, gradually increased, reaching a maximum on day 7 along with myogenin expression. Moreover, in response to the functional overload, cyclin D3 was expressed simultaneously with myogenin and p21 in SC nuclei. Therefore, the present study suggests that cyclin D3 with myogenin and p21 may interactively regulate SC differentiation during early-stage functional overload.

Effects of Tofacitinib on Muscle Remodeling in Experimental Rheumatoid Sarcopenia

Sarcopenia is a frequent comorbidity of rheumatoid arthritis (RA). Clinical trials have shown that JAK inhibitors (JAKi) produce an asymptomatic increase in serum creatine kinase (CK) in RA, suggesting an impact on muscle. We evaluated the effect of JAKi in muscle remodeling in an experimental RA model. Antigen-induced arthritis (experimental RA, e-RA) was performed in 14 rabbits. Seven rabbits received tofacitinib (TOFA, orally 10 mg/kg/day). Animals were euthanized one day after the last ovalbumin injection, and muscles were prepared for histology, RT-PCR, and WB. C-reactive protein (CRP) and Myostatin (MSTN) serum concentration were determined by ELISA. Creatine and creatine kinase (CK) were analyzed. An increase in body weight as well as tibialis anterior cross-sectional area and diameter was observed in e-RA+TOFA vs. e-RA. e-RA decreased type II fibers and increased the myonuclei number, with all reverted by TOFA. TOFA did not modify CRP levels, neither did MSTN. TOFA significantly reduced IL-6, atrogin-1, and MuRF-1 compared with e-RA. e-RA+TOFA showed higher CK and lower creatine levels compared with e-RA. No differences in PAX-7 were found, while TOFA prevented the increase in MyoD1 in e-RA. Our model reflects the features of rheumatoid sarcopenia in RA. JAKi increased muscle mass through attenuating IL-6/JAK/STAT activation, decreasing atrogenes, and restoring muscle differentiation markers. These data together with an increase in CK support the role of CK as a valuable marker of muscle gain following JAKi treatment.

Cancer cachexia: molecular mechanisms and treatment strategies

Muscle wasting is a consequence of physiological changes or a pathology characterized by increased catabolic activity that leads to progressive loss of skeletal muscle mass and strength. Numerous diseases, including cancer, organ failure, infection, and aging-associated diseases, are associated with muscle wasting. Cancer cachexia is a multifactorial syndrome characterized by loss of skeletal muscle mass, with or without the loss of fat mass, resulting in functional impairment and reduced quality of life. It is caused by the upregulation of systemic inflammation and catabolic stimuli, leading to inhibition of protein synthesis and enhancement of muscle catabolism. Here, we summarize the complex molecular networks that regulate muscle mass and function. Moreover, we describe complex multi-organ roles in cancer cachexia. Although cachexia is one of the main causes of cancer-related deaths, there are still no approved drugs for cancer cachexia. Thus, we compiled recent ongoing pre-clinical and clinical trials and further discussed potential therapeutic approaches for cancer cachexia.

Regenerating myofiber directs Tregs and Th17 responses in inflamed muscle through the intrinsic TGF-β signaling-mediated IL-6 production

Transforming growth factor-β (TGF-β) is considered to be an important immune regulatory cytokine. However, it remains unknown whether and how the muscle fiber specific-TGF-β signaling is directly involved in intramuscular inflammatory regulation by affecting T cells. Here, we addressed these in a mouse tibialis anterior muscle Cardiotoxin injection-induced injury repair model in muscle creatine kinase (MCK)-Cre control or transgenic mice with TGF-β receptor II (TGF-βr2) being specifically deleted in muscle cells (SM TGF-βr2^-/-). In control mice, TGF-β2 and TGF-βr2 were found significantly upregulated in muscle after the acute injury. In mutant mice, deficiency of TGF-β signaling in muscle cells caused more serious muscle inflammation, with the increased infiltration of macrophages and CD4⁺ T cells at the degeneration stage (D4) and the early stage of regeneration (D7) after myoinjury. Notably, the loss of TGF-β signaling in myofibers dramatically affected CD4⁺ T cell function and delayed T cells withdrawal at the later stage of muscle regeneration (D10 and D15), marked by the elevated Th17, but the impaired Tregs response. Furthermore, in vivo and in vitro, the intrinsic TGF-β signaling affected immune behaviors of muscle cells and directed CD4⁺ T cells differentiation by impairing IL-6 production and release. It suggests that local muscle inflammation can be inhibited potentially by directly activating the TGF-β signaling pathway in muscle cells to suppress Th17, but induce Tregs responses. Thus, according to the results of this study, we found a new idea for the control of local acute inflammation in skeletal muscle.NEW & NOTEWORTHY Myofiber mediates muscle inflammatory response through activating the intrinsic TGF-β signaling. The specific TGF-β signaling activation contributes to myofiber IL-6 production and directs muscle-specific Th17 and Treg cell responses.

Effective therapy of polymyositis in mice via selective inhibition of the immunoproteasome

Polymyositis (PM) is a chronic autoimmune inflammatory myopathy resulting in muscle weakness. The limited approved therapies and their poor efficacy contribute to its comorbidity. We investigated the therapeutic use of ONX 0914 and KZR-616, selective inhibitors of the immunoproteasome, in C protein-induced myositis (CIM), a mouse model of PM that closely resembles the human disease. Diseased mice (day 13 post-immunization) were treated with 10 mg/kg ONX 0914 or KZR-616 or vehicle on alternate days until day 28. Endpoints included muscle strength assessed by a grip strength meter, serum creatine kinase activity, histology, and immunohistochemistry analysis. Treatment with ONX 0914 or KZR-616 prevented the loss of grip strength in mice after CIM induction, while vehicle-treated animals displayed progressive muscle weakness. Immunoproteasome inhibition lowered PM-associated leukocyte infiltration of the muscle and prevented increased serum creatine kinase levels. LMP7-deficient mice were resistant to CIM induction as they depicted no alteration in the grip strength, creatine kinase (CK) levels, nor showed muscular alterations. In conclusion, selective inhibition of the immunoproteasome displays therapeutic efficacy in a pre-clinical mouse model of PM with suppression of muscle inflammation and preservation of muscle strength. Positive results from this study support the rationale for using KZR-616 in clinical studies. This article is protected by copyright. All rights reserved.

Muscle allele-specific expression QTLs may affect meat quality traits in Bos indicus

Single nucleotide polymorphisms (SNPs) located in transcript sequences showing allele-specific expression (ASE SNPs) were previously identified in the Longissimus thoracis muscle of a Nelore (Bos indicus) population consisting of 190 steers. Given that the allele-specific expression pattern may result from cis-regulatory SNPs, called allele-specific expression quantitative trait loci (aseQTLs), in this study, we searched for aseQTLs in a window of 1 Mb upstream and downstream from each ASE SNP. After this initial analysis, aiming to investigate variants with a potential regulatory role, we further screened our aseQTL data for sequence similarity with transcription factor binding sites and microRNA (miRNA) binding sites. These aseQTLs were overlapped with methylation data from reduced representation bisulfite sequencing (RRBS) obtained from 12 animals of the same population. We identified 1134 aseQTLs associated with 126 different ASE SNPs. For 215 aseQTLs, one allele potentially affected the affinity of a muscle-expressed transcription factor to its binding site. 162 aseQTLs were predicted to affect 149 miRNA binding sites, from which 114 miRNAs were expressed in muscle. Also, 16 aseQTLs were methylated in our population. Integration of aseQTL with GWAS data revealed enrichment for traits such as meat tenderness, ribeye area, and intramuscular fat . To our knowledge, this is the first report of aseQTLs identification in bovine muscle. Our findings indicate that various cis-regulatory and epigenetic mechanisms can affect multiple variants to modulate the allelic expression. Some of the potential regulatory variants described here were associated with the expression pattern of genes related to interesting phenotypes for livestock. Thus, these variants might be useful for the comprehension of the genetic control of these phenotypes.

Sunitinib promotes myogenic regeneration and mitigates disease progression in the mdx mouse model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a lethal, muscle degenerative disease causing premature death of affected children. DMD is characterized by mutations in the dystrophin gene that result in a loss of the dystrophin protein. Loss of dystrophin causes an associated reduction in proteins of the dystrophin glycoprotein complex, leading to contraction-induced sarcolemmal weakening, muscle tearing, fibrotic infiltration and rounds of degeneration and failed regeneration affecting satellite cell populations. The α7β1 integrin has been implicated in increasing myogenic capacity of satellite cells, therefore restoring muscle viability, increasing muscle force and preserving muscle function in dystrophic mouse models. In this study, we show that a Food and Drug Administration (FDA)-approved small molecule, Sunitinib, is a potent α7 integrin enhancer capable of promoting myogenic regeneration by stimulating satellite cell activation and increasing myofiber fusion. Sunitinib exerts its regenerative effects via transient inhibition of SHP-2 and subsequent activation of the STAT3 pathway. Treatment of mdx mice with Sunitinib demonstrated decreased membrane leakiness and damage owing to myofiber regeneration and enhanced support at the extracellular matrix. The decreased myofiber damage translated into a significant increase in muscle force production. This study identifies an already FDA-approved compound, Sunitinib, as a possible DMD therapeutic with the potential to treat other muscular dystrophies in which there is defective muscle repair.

Exercise as an anti-inflammatory therapy for cancer cachexia: a focus on interleukin-6 regulation

Cancer cachexia is a complicated disorder of extreme, progressive skeletal muscle wasting. It is directed by metabolic alterations and systemic inflammation dysregulation. Numerous studies have demonstrated that increased systemic inflammation promotes this type of cachexia and have suggested that cytokines are implicated in the skeletal muscle loss. Exercise is firmly established as an anti-inflammatory therapy that can attenuate or even reverse the process of muscle wasting in cancer cachexia. The interleukin IL-6 is generally considered to be a key player in the development of the microenvironment of malignancy; it promotes tumor growth and metastasis by acting as a bridge between chronic inflammation and cancerous tissue and it also induces skeletal muscle atrophy and protein breakdown. Paradoxically, a beneficial role for IL-6 has also been identified recently, and that is its status as a “founding member” of the myokine class of proteins. Skeletal muscle is an important source of circulating IL-6 in people who participate in exercise training. IL-6 acts as an anti-inflammatory myokine by inhibiting TNFα and improving glucose uptake through the stimulation of AMPK signaling. This review discusses the action of IL-6 in skeletal muscle tissue dysfunction and the role of IL-6 as an “exercise factor” that modulates the immune system. This review also sheds light on the main considerations related to the treatment of muscle wasting in cancer cachexia.

Interleukin-6 Induces Myogenic Differentiation via JAK2-STAT3 Signaling in Mouse C2C12 Myoblast Cell Line and Primary Human Myoblasts

Postnatal muscle growth and exercise- or injury-induced regeneration are facilitated by myoblasts. Myoblasts respond to a variety of proteins such as cytokines that activate various signaling cascades. Cytokines belonging to the interleukin 6 superfamily (IL-6) influence myoblasts' proliferation but their effect on differentiation is still being researched. The Janus kinase (JAK)-signal transducer and activator of transcription (STAT) pathway is one of the key signaling pathways identified to be activated by IL-6. The aim of this study was to investigate myoblast fate as well as activation of JAK-STAT pathway at different physiologically relevant IL-6 concentrations (10 pg/mL; 100 pg/mL; 10 ng/mL) in the C2C12 mouse myoblast cell line and primary human myoblasts, isolated from eight young healthy male volunteers. Myoblasts' cell cycle progression, proliferation and differentiation in vitro were assessed. Low IL-6 concentrations facilitated cell cycle transition from the quiescence/Gap1 (G0/G1) to the synthesis (S-) phases. Low and medium IL-6 concentrations decreased the expression of myoblast determination protein 1 (MyoD) and myogenin and increased proliferating cell nuclear antigen (PCNA) expression. In contrast, high IL-6 concentration shifted a larger proportion of cells to the pro-differentiation G0/G1 phase of the cell cycle, substantiated by significant increases of both MyoD and myogenin expression and decreased PCNA expression. Low IL-6 concentration was responsible for prolonged JAK1 activation and increased suppressor of cytokine signaling 1 (SOCS1) protein expression. JAK-STAT inhibition abrogated IL-6-mediated C2C12 cell proliferation. In contrast, high IL-6 initially increased JAK1 activation but resulted in prolonged JAK2 activation and elevated SOCS3 protein expression. High IL-6 concentration decreased interleukin-6 receptor (IL-6R) expression 24 h after treatment whilst low IL-6 concentration increased IL-6 receptor (IL-6R) expression at the same time point. In conclusion, this study demonstrated that IL-6 has concentration- and time-dependent effects on both C2C12 mouse myoblasts and primary human myoblasts. Low IL-6 concentration induces proliferation whilst high IL-6 concentration induces differentiation. These effects are mediated by specific components of the JAK/STAT/SOCS pathway.

The influence of capillarization on satellite cell pool expansion and activation following exercise-induced muscle damage in healthy young men

KEY POINTS

Skeletal muscle stem cells (satellite cells) play a crucial role in repair and remodelling of muscle in response to exercise. Satellite cells are in close spatial proximity to muscle capillaries and therefore may be influenced by them. In this study, we describe the activation and expansion of the satellite cell pool in response to eccentric contraction-induced muscle damage in individuals with significantly different levels of muscle capillarization. Individuals with greater capillarization and capacity for muscle perfusion demonstrated enhanced activation and/or expansion of the satellite cell pool allowing for an accelerated recovery of muscle function. These results provide insight into the critical relationship between muscle capillarization and satellite cells during skeletal muscle repair.

ABSTRACT

Factors that determine the skeletal muscle satellite cell (SC) response remain incompletely understood. It is known, however, that SC activation status is closely related to the anatomical relationship between SCs and muscle capillaries. We investigated the impact of muscle fibre capillarization on the expansion and activation status of SCs following a muscle-damaging exercise protocol in healthy young men. Twenty-nine young men (21 ± 0.5 years) performed 300 unilateral eccentric contractions (180 deg s^-1 ) of the knee extensors. Percutaneous muscle biopsies from the vastus lateralis and blood samples from the antecubital vein were taken prior to (Pre) exercise and at 6, 24, 72 and 96 h of post-exercise recovery. A comparison was made between subjects who had a relative low mixed muscle capillary-to-fibre perimeter exchange index (CFPE; Low group) and high mixed muscle CFPE index (High group) at baseline. Type I and type II muscle fibre size, myonuclear content, capillarization, and SC response were determined via immunohistochemistry. Overall, there was a significant correlation (r = 0.39; P < 0.05) between the expansion of SC content (change in total Pax7⁺ cells/100 myofibres) 24 h following eccentric exercise and mixed muscle CFPE index. There was a greater increase in activated SCs (MyoD⁺ /Pax7⁺ cells) in the High as compared to the Low CFPE group 72 h following eccentric exercise (P < 0.05). The current study provides further evidence that muscle fibre capillarization may play an important role in the activation and expansion of the SC pool during the process of skeletal muscle repair.

Role of 72-kDa Heat Shock Protein in Heat-stimulated Regeneration of Injured Muscle in Rat

The regeneration of injured muscles is facilitated by intermittent heat stress. The 72-kDa heat shock protein (HSP72), the level of which is increased by heat stress, is likely involved in this effect, but the precise mechanism remains unclear. This study was conducted to investigate the localization and role(s) of HSP72 in the regenerating muscles in heat-stressed rats using immunohistochemistry. Heat stress was applied by immersion of the rat lower body into hot water (42C, 30 min, every other day) following injection of bupivacaine into the soleus muscles. After 1 week, we found that HSP72 was expressed at high levels not only in the surviving myofibers but also in the blood vessels of the regenerating muscles in heated rats. In addition, leukocytes, possibly granulocytes, expressing cluster of differentiation 43 within the blood capillaries surrounding the regenerating myofibers also highly expressed HSP72. In contrast, marked expression of HSP72 was not observed in the intact or regenerating muscles without heat stress. These results suggest that heat-stress-induced HSP72 within the myofibers, blood vessels, and circulating leukocytes may play important roles in enhancing regeneration of injured muscles by heat stress. Our findings would be useful to investigate cell-specific role(s) of HSP72 during skeletal muscle regeneration.

Aging-associated genes and let-7 microRNAs: a contribution to myogenic program dysregulation in oculopharyngeal muscular dystrophy

Oculopharyngeal muscular dystrophy (OPMD) is a late-onset muscle disease caused by an abnormal (GCN) triplet expansion within the polyadenylate-binding protein nuclear 1 gene and consequent mRNA processing impairment and myogenic defects. Because a reduced cell proliferation potential and the consequent regeneration failure of aging muscle have been shown to be governed by lethal-7 (let-7) microRNA-mediated mechanisms, in the present study, we evaluated the role of let-7 in the pathogenesis of OPMD. By a multidisciplinary approach, including confocal microscopy, Western blot, and quantitative PCR analyses on muscle biopsies from patients and unaffected individuals, we found a significant increase in let-7 expression in OPMD muscles associated with an unusual high percentage of paired box 7-positive satellite cells. Furthermore, IL-6, a cytokine involved in the regulation of satellite cell proliferation and differentiation and a potential target of let-7, was found strongly down-regulated in OPMD compared with control muscles. The decrease in IL-6 transcript levels and protein content was also confirmed in vitro during differentiation of patients' and controls' muscle cells. Overall, our data suggest a key role of let-7 in the regeneration and degeneration process in OPMD muscle and pointed to IL-6 as a potential target molecule for new therapeutic approaches for this disorder.-Cappelletti, C., Galbardi, B., Bruttini, M., Salerno, F., Canioni, E., Pasanisi, M. B., Rodolico, C., Brizzi, T., Mora, M., Renieri, A., Maggi, L., Bernasconi, P., Mantegazza, R. Aging-associated genes and let-7 microRNAs: a contribution to myogenic program dysregulation in oculopharyngeal muscular dystrophy.

Co-Expression Network Approach to Studying the Effects of Botulinum Neurotoxin-A

Botulinum Neurotoxin A (BoNT-A) is a potent neurotoxin with several clinical applications. The goal of this study was to utilize co-expression network theory to analyze temporal transcriptional data from skeletal muscle after BoNT-A treatment. Expression data for 2000 genes (extracted using a ranking heuristic) served as the basis for this analysis. Using weighted gene co-expression network analysis (WGCNA), we identified 19 co-expressed modules, further hierarchically clustered into five groups. Quantifying average expression and co-expression patterns across these groups revealed temporal aspects of muscle's response to BoNT-A. Functional analysis revealed enrichment of group 1 with metabolism; group 5 with contradictory functions of atrophy and cellular recovery; and groups 2 and 3 with extracellular matrix (ECM) and non-fast fiber isoforms. Topological positioning of two highly ranked, significantly expressed genes-Dclk1 and Ostalpha-within group 5 suggested possible mechanistic roles in recovery from BoNT-A induced atrophy. Phenotypic correlations of groups with titin and myosin protein content further emphasized the effect of BoNT-A on the sarcomeric contraction machinery in early phase of chemodenervation. In summary, our approach revealed a hierarchical functional response to BoNT-A induced paralysis with early metabolic and later ECM responses and identified putative biomarkers associated with chemodenervation. Additionally, our results provide an unbiased validation of the response documented in our previous work.

Mediators and Patterns of Muscle Loss in Chronic Systemic Inflammation

Besides its primary function in locomotion, skeletal muscle (SKM), which represents up to half of human's weight, also plays a fundamental homeostatic role. Through the secretion of soluble peptides, or myokines, SKM interacts with major organs involved in metabolic processes. In turn, metabolic cues from these organs are received by muscle cells, which adapt their response accordingly. This is done through an intricate intracellular signaling network characterized by the cross-talking between anabolic and catabolic pathways. A fine regulation of the network is required to protect the organism from an excessive energy expenditure. Systemic inflammation evokes a catabolic reaction in SKM known as sarcopenia. In turn this response comprises several mechanisms, which vary depending on the nature of the insult and its magnitude. In this regard, aging, chronic inflammatory systemic diseases, osteoarthritis and idiopathic inflammatory myopathies can lead to muscle loss. Interestingly, sarcopenia may persist despite remission of chronic inflammation, an issue which warrants further research. The Janus kinase/signal transducer and activator of transcription (JAK/STAT) system stands as a major participant in muscle loss during systemic inflammation, while it is also a well-recognized orchestrator of muscle cell turnover. Herein we summarize current knowledge about models of sarcopenia, their triggers and major mediators and their effect on both protein and cell growth yields. Also, the dual action of the JAK/STAT pathway in muscle mass changes is discussed. We highlight the need to unravel the precise contribution of this system to sarcopenia in order to design targeted therapeutic strategies.

Skeletal muscle regeneration is modulated by inflammation

Skeletal muscle regeneration is a complex process orchestrated by multiple steps. Recent findings indicate that inflammatory responses could play central roles in bridging initial muscle injury responses and timely muscle injury reparation. The various types of immune cells and cytokines have crucial roles in muscle regeneration process. In this review, we briefly summarise the functions of acute inflammation in muscle regeneration.

THE TRANSLATIONAL POTENTIAL OF THIS ARTICLE

Immune system is closely relevant to the muscle regeneration. Understanding the mechanisms of inflammation in muscle regeneration is therefore critical for the development of effective regenerative, and therapeutic strategies in muscular disorders. This review provides information for muscle regeneration research regarding the effects of inflammation on muscle regeneration.

Granulocyte Colony-Stimulating Factor and Its Potential Application for Skeletal Muscle Repair and Regeneration

Granulocyte colony-stimulating factor (G-CSF) was originally discovered in the context of hematopoiesis. However, the identification of the G-CSF receptor (G-CSFR) being expressed outside the hematopoietic system has revealed wider roles for G-CSF, particularly in tissue repair and regeneration. Skeletal muscle damage, including that following strenuous exercise, induces an elevation in plasma G-CSF, implicating it as a potential mediator of skeletal muscle repair. This has been supported by preclinical studies and clinical trials investigating G-CSF as a potential therapeutic agent in relevant disease states. This review focuses on the growing literature associated with G-CSF and G-CSFR in skeletal muscle under healthy and disease conditions and highlights the current controversies.

Muscle-specific deletion of SOCS3 does not reduce the anabolic response to leucine in a mouse model of acute inflammation

Excessive inflammation reduces skeletal muscle protein synthesis leading to wasting and weakness. The janus kinase/signal transducers and activators of transcription-3 (JAK/STAT3) pathway is important for the regulation of inflammatory signaling. As such, suppressor of cytokine signaling-3 (SOCS3), the negative regulator of JAK/STAT signaling, is thought to be important in the control of muscle homeostasis. We hypothesized that muscle-specific deletion of SOCS3 would impair the anabolic response to leucine during an inflammatory insult. Twelve week old (n=8 per group) SOCS3 muscle-specific knockout mice (SOCS3-MKO) and littermate controls (WT) were injected with lipopolysaccharide (LPS, 1mg/kg) or saline and were studied during fasted conditions or after receiving 0.5g/kg leucine 3h after the injection of LPS. Markers of inflammation, anabolic signaling, and protein synthesis were measured 4h after LPS injection. LPS injection robustly increased mRNA expression of inflammatory molecules (Socs3, Socs1, Il-6, Ccl2, Tnfα and Cd68). In muscles from SOCS3-MKO mice, the Socs3 mRNA response to LPS was significantly blunted (∼6-fold) while STAT3 Tyr705 phosphorylation was exacerbated (18-fold). Leucine administration increased protein synthesis in both WT (∼1.6-fold) and SOCS3-MKO mice (∼1.5-fold) compared to basal levels. LPS administration blunted this effect, but there were no differences between WT and SOCS3-MKO mice. Muscle-specific SOCS3 deletion did not alter the response of AKT, mTOR, S6 or 4EBP1 under any treatment conditions. Therefore, SOCS3 does not appear to mediate the early inflammatory or leucine-induced changes in protein synthesis in skeletal muscle.

Therapeutic Pulsed Ultrasound Promotes Revascularization and Functional Recovery of Rat Skeletal Muscle after Contusion Injury

The mechanism by which therapeutic pulsed ultrasound (TPU) promotes the repair of damaged gastrocnemius muscle was investigated. Male Wistar rats were divided into uninjured, sham-treated injured and TPU-treated injured (TPU) groups. Injury was induced by mass-drop technique. TPU was applied to the injured muscle for 5 min, daily, started at day 1 post-injury and continuing for 3, 7 and 14 d. For 3 d post-injury, a significant reduction in muscle force was observed in both the sham-treated injured and TPU groups. TPU treatment significantly increased recovery force of the injured muscle after day 7 post-injury. This effect of TPU is associated with increased centronucleated fibers and cross-sectional area, mRNA expression of the vascular endothelial growth factor and capillary density of the regenerated fibers, but not with mRNA expression of nitric oxide synthase. We conclude that TPU hastens muscle recovery, at least in part, by upregulating angiogenesis.

Combined administration of anisodamine and neostigmine rescued acute lethal crush syndrome through α7nAChR-dependent JAK2-STAT3 signaling

Previously we showed that Ani (anisodamine)/Neo (neostigmine) combination produced anti-shock effect via activating α7 nicotinic acetylcholine receptor (α7nAChR). In this study, we aim to investigate the therapeutic effect and underlying mechanisms of Ani/Neo combination in acute lethal crush syndrome (CS). In rat and rabbit CS models, Ani/Neo combination increased the 24 h survival rates, improved hemodynamics and decreased the levels of creatine kinase, MB isoenzyme of creatine kinase, blood urea nitrogen, creatinine, K⁺ in serum. It also decreased the levels of H₂O₂, myeloperoxidase (MPO) and nitric oxide (NO) in serum and compressed muscle in rat CS model. In wild-type (WT) mice with CS, Ani/Neo combination increased 24 h survival rate and decreased the levels of H₂O₂, MPO, NO, TNFα, IL-6 and IL-10 in compressed muscle. These effects were attenuated by α7nAChR knockout (KO). Moreover, Ani/Neo combination prevented the decrease of phosphorylation of Janus kinase 2 (JAK2) and phosphorylation of signal transducer and activator of transcription 3 (STAT3) induced by CS. These effects of Ani/Neo in CS mice were cancelled by methyllycaconitine (α7nAChR antagonist) and α7nAChR KO. Collectively, our results demonstrate that Ani/Neo combination could produce therapeutic effects in CS. The underlying mechanism involves the activation of α7nAChR-dependent JAK2-STAT3 signaling pathway.

Raf kinase inhibitor protein1 is a myogenic inhibitor with conserved function in avians and mammals

BACKGROUND

Raf Kinase Inhibitor Protein1 (RKIP) is a tumor suppressor that is present in several adult tissues. It functions as an inhibitor of both Raf/Mek/Erk and NFĸB signaling when unphosphorylated, but following phosphorylation the ability to inhibit Raf/Mek/Erk signaling is lost and RKIP becomes an activator of G-protein coupled receptor signaling. In neonates and adults, RKIP is known to be expressed in muscle; however, its physiological function is currently unknown.

RESULTS

In this study, we show by in situ hybridization and immunofluorescence that RKIP is also expressed in developing chick embryonic muscle, and mouse C2C12 myoblasts. Furthermore, we demonstrate that, in these systems, it functions as an inhibitor of myogenesis: increased levels of RKIP suppress myotube differentiation whereas decreasing RKIP promotes differentiation. Additionally, we show that the ability of RKIP to inhibit myogenesis is dependent upon its phosphorylation state as only the nonphosphorylated form of RKIP suppresses myogenesis.

CONCLUSIONS

This study, therefore, clearly demonstrates that RKIP has conserved functions as a myogenic inhibitor in both mammalian and avian muscle. Developmental Dynamics 245:902-912, 2016. © 2016 Wiley Periodicals, Inc.

Improvements in impaired GABA and GAD65/67 production in the spinal dorsal horn contribute to exercise-induced hypoalgesia in a mouse model of neuropathic pain

Background

Physical exercise effectively attenuates neuropathic pain, and multiple events including the inhibition of activated glial cells in the spinal dorsal horn, activation of the descending pain inhibitory system, and reductions in pro-inflammatory cytokines in injured peripheral nerves may contribute to exercise-induced hypoalgesia. Since fewer GABAergic hypoalgesic interneurons exist in the dorsal horn in neuropathic pain model animals, the recovery of impaired GABAergic inhibition in the dorsal horn may improve pain behavior. We herein determined whether the production of gamma-aminobutyric acid (GABA) and glutamic acid decarboxylase (GAD) in the dorsal horn is restored by treadmill running and contributes to exercise-induced hypoalgesia in neuropathic pain model mice. C57BL/6 J mice underwent partial sciatic nerve ligation (PSL). PSL-Runner mice ran on a treadmill at 7 m/min for 60 min/day, 5 days/week, from two days after PSL.

Results

Mechanical allodynia and heat hyperalgesia developed in PSL-Sedentary mice but were significantly attenuated in PSL-Runner mice. PSL markedly decreased GABA and GAD65/67 levels in neuropils in the ipsilateral dorsal horn, while treadmill running inhibited these reductions. GABA+ neuronal nuclei+ interneuron numbers in the ipsilateral dorsal horn were significantly decreased in PSL-Sedentary mice but not in PSL-Runner mice. Pain behavior thresholds positively correlated with GABA and GAD65/67 levels and GABAergic interneuron numbers in the ipsilateral dorsal horns of PSL-Sedentary and -Runner mice.

Conclusions

Treadmill running prevented PSL-induced reductions in GAD65/67 production, and, thus, GABA levels may be retained in interneurons and neuropils in the superficial dorsal horn. Therefore, improvements in impaired GABAergic inhibition may be involved in exercise-induced hypoalgesia.

Histone Acetylation in Microglia Contributes to Exercise-Induced Hypoalgesia in Neuropathic Pain Model Mice

Physical exercise can attenuate neuropathic pain (NPP), but the exact mechanism underlying exercise-induced hypoalgesia (EIH) remains unclear. Recent studies have shown that histone hyperacetylation via pharmacological inhibition of histone deacetylases in the spinal cord attenuates NPP, and that histone acetylation may lead to the production of analgesic factors including interleukin 10. We intended to clarify whether histone acetylation in microglia in the spinal dorsal horn contributes to EIH in NPP model mice. C57BL/6J mice underwent partial sciatic nerve ligation (PSL) and PSL- and sham-runner mice ran on a treadmill at a speed of 7 m/min for 60 min/d, 5 days per week, from 2 days after the surgery. PSL-sedentary mice developed mechanical allodynia and heat hyperalgesia, but such behaviors were significantly attenuated in PSL-runner mice. In immunofluorescence analysis, PSL surgery markedly increased the number of histone deacetylase 1-positive/CD11b-positive microglia in the ipsilateral superficial dorsal horn, and they were significantly decreased by treadmill-running. Moreover, the number of microglia with nuclear expression of acetylated H3K9 in the ipsilateral superficial dorsal horn was maintained at low levels in PSL-sedentary mice, but running exercise significantly increased them. Therefore, we conclude that the epigenetic modification that causes hyperacetylation of H3K9 in activated microglia may play a role in producing EIH.

PERSPECTIVE

This article presents the importance of epigenetic modification in microglia in producing EIH. The current research is not only helpful for developing novel nonpharmacological therapy for NPP, but will also enhance our understanding of the mechanisms and availability of exercise in our daily life.

Effect of low-energy laser irradiation and antioxidant supplementation on cell apoptosis during skeletal muscle post-injury regeneration in pigs

The aim of this study was to evaluate the effect of low-energy laser irradiation, coenzyme Q10 and vitamin E supplementation on the apoptosis of macrophages and muscle precursor cells during skeletal muscle regeneration after bupivacaine-induced injury. The experiment was conducted on 75 gilts, divided into 5 experimental groups: I--control, II--low-energy laser irradiation, III--coenzyme Q10, IV--coenzyme Q10 and vitamin E, V--vitamin E. Muscle necrosis was induced by injection of 0.5% bupivacaine hydrochloride. The animals were euthanized on subsequent days after injury. Samples were formalin fixed and processed routinely for histopathology. Apoptosis was detected using the TUNEL method. The obtained results indicate that low-energy laser irradiation has a beneficial effect on macrophages and muscle precursor cell activity during muscle post-injury regeneration and protects these cells against apoptosis. Vitamin E has a slightly lower protective effect, limited mainly to the macrophages. Coenzyme Q10 co-supplemented with vitamin E increases the activity of macrophages and muscle precursor cells, myotube and young muscle formation. Importantly, muscle precursor cells seem to be more sensitive to apoptosis than macrophages in the environment of regenerating damaged muscle.

Acute inflammation stimulates a regenerative response in the neonatal mouse heart

Cardiac injury in neonatal 1-day-old mice stimulates a regenerative response characterized by reactive cardiomyocyte proliferation, which is distinguished from the fibrotic repair process in adults. Acute inflammation occurs immediately after heart injury and has generally been believed to exert a negative effect on heart regeneration by promoting scar formation in adults; however, little is known about the role of acute inflammation in the cardiac regenerative response in neonatal mice. Here, we show that acute inflammation induced cardiomyocyte proliferation after apical intramyocardial microinjection of immunogenic zymosan A particles into the neonatal mouse heart. We also found that cardiac injury-induced regenerative response was suspended after immunosuppression in neonatal mice, and that cardiomyocytes could not be reactivated to proliferate after neonatal heart injury in the absence of interleukin-6 (IL-6). Furthermore, cardiomyocyte-specific deletion of signal transducer and activator of transcription 3 (STAT3), the major downstream effector of IL-6 signaling, decreased reactive cardiomyocyte proliferation after apical resection. Our results indicate that acute inflammation stimulates the regenerative response in neonatal mouse heart, and suggest that modulation of inflammatory signals might have important implications in cardiac regenerative medicine.

Characteristics of the Localization of Connexin 43 in Satellite Cells during Skeletal Muscle Regeneration In Vivo

For myogenesis, new myotubes are formed by the fusion of differentiated myoblasts. In the sequence of events for myotube formation, intercellular communication through gap junctions composed of connexin 43 (Cx43) plays critical roles in regulating the alignment and fusion of myoblasts in advances of myotube formation in vitro. On the other hand, the relationship between the expression patterns of Cx43 and the process of myotube formation in satellite cells during muscle regeneration in vivo remains poorly understood. The present study investigated the relationship between Cx43 and satellite cells in muscle regeneration in vivo. The expression of Cx43 was detected in skeletal muscles on day 1 post-muscle injury, but not in control muscles. Interestingly, the expression of Cx43 was not localized on the inside of the basement membrane of myofibers in the regenerating muscles. Moreover, although the clusters of differentiated satellite cells, which represent a more advanced stage of myotube formation, were observed on the inside of the basement membrane of myofibers in regenerating muscles, the expression of Cx43 was not localized in the clusters of these satellite cells. Therefore, in the present study, it was suggested that Cx43 may not directly contribute to muscle regeneration via satellite cells.

Calcitonin gene-related peptide regulation of glial cell-line derived neurotrophic factor in differentiated rat myotubes

Glial cell-line derived neurotrophic factor (GDNF) is the most potent trophic factor for motoneuron survival and neuromuscular junction formation. GDNF is upregulated in injured or denervated skeletal muscle and returns to normal levels following reinnervation. However, the mechanism by which GDNF is regulated in denervated muscle is not well understood. The nerve-derived neurotransmitter calcitonin gene-related peptide (CGRP) is upregulated following neuromuscular injury and is subsequently released from motoneurons at the neuromuscular junction. CGRP also promotes nerve regeneration, but the mechanism is not well understood. The current study investigates whether this increase in CGRP regulates GDNF, thus playing a key role in promoting regeneration of injured nerves. This study demonstrates that CGRP increases GDNF secretion without affecting its transcription or translation. Rat L6 myoblasts were differentiated into myotubes and subsequently treated with CGRP. GDNF mRNA expression levels were quantified by quantitative real-time reverse transcription-polymerase chain reaction, and secreted GDNF was quantified in the conditioned medium by ELISA. CGRP treatment increased secreted GDNF protein without altering GDNF mRNA levels. The translational inhibitor cycloheximide did not affect CGRP-induced GDNF secreted protein levels, whereas the secretional inhibitor brefeldin A blocked the CGRP-induced increase in GDNF. This study highlights the importance of injury-induced upregulation of CGRP by exposing its ability to increase GDNF levels and demonstrates a secretional mechanism for regulation of this key regeneration-promoting neurotrophic factor.

Anti-interleukin-6 receptor antibody (MR16-1) promotes muscle regeneration via modulation of gene expressions in infiltrated macrophages

BACKGROUND

Although rat anti-mouse IL-6 receptor (IL-6R) antibody (MR16-1) has been reported to effectively ameliorate various tissue damages, its effect on skeletal muscle regeneration has not been determined. Moreover, the localization, persistence and duration of action of this reagent in damaged tissues after systemic administration have not been assessed.

METHODS

The MR16-1 was administered i.p. immediately after cardiotoxin (CTX)-induced muscle damage on mice.

RESULTS

MR16-1 administered i.p. was observed only to the damaged muscle. This delivered MR16-1 was dramatically decreased from 3 to 7days post-injury concomitantly with a reduction of IL-6R expression. This reduction of the MR16-1 level in the damaged muscle was not rescued by additional administration of MR16-1, suggesting the short half-life of MR16-1 was not the factor for the remaining levels. In addition, a significant inhibitory effect of MR16-1 on phosphorylation of the signal transducer and activator of transcription 3 was observed in the macrophage-enriched area of damaged muscle 3days after injury. Finally, the acceleration of muscle regeneration observed at day 7 post-injury following MR16-1 treatment was associated with reduced expression of fibrosis-related genes, such as interleukin-10 and arginase, in the infiltrated macrophages.

CONCLUSIONS

These results suggest that MR16-1 which was found primarily localized in infiltrated macrophages in the damaged muscle might facilitate muscle regeneration via immune modulation.

GENERAL SIGNIFICANCE

These findings are deemed to provide further insight into the understanding not only of MR16-1 treatment on muscle regeneration, but also of the other anti-cytokine treatment on the cytokine-related disease.

Isolation, characterization, and molecular regulation of muscle stem cells

Skeletal muscle has great regenerative capacity which is dependent on muscle stem cells, also known as satellite cells. A loss of satellite cells and/or their function impairs skeletal muscle regeneration and leads to a loss of skeletal muscle power; therefore, the molecular mechanisms for maintaining satellite cells in a quiescent and undifferentiated state are of great interest in skeletal muscle biology. Many studies have demonstrated proteins expressed by satellite cells, including Pax7, M-cadherin, Cxcr4, syndecan3/4, and c-met. To further characterize satellite cells, we established a method to directly isolate satellite cells using a monoclonal antibody, SM/C-2.6. Using SM/C-2.6 and microarrays, we measured the genes expressed in quiescent satellite cells and demonstrated that Hesr3 may complement Hesr1 in generating quiescent satellite cells. Although Hesr1- or Hesr3-single knockout mice show a normal skeletal muscle phenotype, including satellite cells, Hesr1/Hesr3-double knockout mice show a gradual decrease in the number of satellite cells and increase in regenerative defects dependent on satellite cell numbers. We also observed that a mouse's genetic background affects the regenerative capacity of its skeletal muscle and have established a line of DBA/2-background mdx mice that has a much more severe phenotype than the frequently used C57BL/10-mdx mice. The phenotype of DBA/2-mdx mice also seems to depend on the function of satellite cells. In this review, we summarize the methodology of direct isolation, characterization, and molecular regulation of satellite cells based on our results. The relationship between the regenerative capacity of satellite cells and progression of muscular disorders is also summarized. In the last part, we discuss application of the accumulating scientific information on satellite cells to treatment of patients with muscular disorders.

Muscle injury: review of experimental models

Skeletal muscle is the most abundant tissue in the human body. Its main characteristic is the capacity to regenerate after injury independent of the cause of injury through a process called inflammatory response. Mechanical injuries are the most common type of the skeletal muscle injuries and are classified into one of three areas strain, contusion, and laceration. First, this review aims to describe and compare the main experimental methods that replicate the mechanical muscle injuries. There are several ways to replicate each kind of mechanical injury; there are, however, specific characteristics that must be taken into account when choosing the most appropriate model for the experiment. Finally, this review discusses the context of mechanical injury considering types, variability of methods, and the ability to reproduce injury models.

Interleukin-6 myokine signaling in skeletal muscle: a double-edged sword?

Interleukin (IL)-6 is a cytokine with pleiotropic functions in different tissues and organs. Skeletal muscle produces and releases significant levels of IL-6 after prolonged exercise and is therefore considered as a myokine. Muscle is also an important target of the cytokine. IL-6 signaling has been associated with stimulation of hypertrophic muscle growth and myogenesis through regulation of the proliferative capacity of muscle stem cells. Additional beneficial effects of IL-6 include regulation of energy metabolism, which is related to the capacity of actively contracting muscle to synthesize and release IL-6. Paradoxically, deleterious actions for IL-6 have also been proposed, such as promotion of atrophy and muscle wasting. We review the current evidence for these apparently contradictory effects, the mechanisms involved and discuss their possible biological implications.

Characteristics of myogenic response and ankle torque recovery after lengthening contraction-induced rat gastrocnemius injury

Background

Although muscle dysfunction caused by unfamiliar lengthening contraction is one of most important issues in sports medicine, there is little known about the molecular events on regeneration process. The purpose of this study was to investigate the temporal and spatial expression patterns of myogenin, myoD, pax7, and myostatin after acute lengthening contraction (LC)-induced injury in the rat hindlimb.

Methods

We employed our originally developed device with LC in rat gastrocnemius muscle (n = 24). Male Wistar rats were anesthetized with isoflurane (aspiration rate, 450 ml/min, concentration, 2.0%). The triceps surae muscle of the right hindlimb was then electrically stimulated with forced isokinetic dorsi-flexion (180°/sec and from 0 to 45°). Tissue contents of myoD, myogenin, pax7, myostatin were measured by western blotting and localizations of myoD and pax7 was measured by immunohistochemistry. After measuring isometric tetanic torque, a single bout of LC was performed in vivo.

Results

The torque was significantly decreased on days 2 and 5 as compared to the pre-treatment value, and recovered by day 7. The content of myoD and pax7 showed significant increases on day 2. Myogenin showed an increase from day 2 to 5. Myostatin on days 5 and 7 were significantly increased. Immunohistochemical analysis showed that myoD-positive/pax7-positive cells increased on day 2, suggesting that activated satellite cells play a role in the destruction and the early recovery phases.

Conclusion

We, thus, conclude that myogenic events associate with torque recovery after LC-induced injury.

In Vivo Real-Time Imaging of Exogenous HGF-Triggered Cell Migration in Rat Intact Soleus Muscles

The transplantation of myogenic cells is a potentially effective therapy for muscular dystrophy. However, this therapy has achieved little success because the diffusion of transplanted myogenic cells is limited. Hepatocyte growth factor (HGF) is one of the primary triggers to induce myogenic cell migration in vitro. However, to our knowledge, whether exogenous HGF can trigger the migration of myogenic cells (i.e. satellite cells) in intact skeletal muscles in vivo has not been reported. We previously reported a novel in vivo real-time imaging method in rat skeletal muscles. Therefore, the present study examined the relationship between exogenous HGF treatment and cell migration in rat intact soleus muscles using this imaging method. As a result, it was indicated that the cell migration velocity was enhanced in response to increasing exogenous HGF concentration in skeletal muscles. Furthermore, the expression of MyoD was induced in satellite cells in response to HGF treatment. We first demonstrated in vivo real-time imaging of cell migration triggered by exogenous HGF in intact soleus muscles. The experimental method used in the present study will be a useful tool to understand further the regulatory mechanism of HGF-induced satellite cell migration in skeletal muscles in vivo.

Loss of STAT1 in bone marrow-derived cells accelerates skeletal muscle regeneration

Background

Skeletal muscle regeneration is a complex process which is not yet completely understood. Evidence suggested that the Janus kinase (JAK)–signal transducer and activator of transcription (STAT) pathway may have a role in myogenesis. In this study, we aim to explore the possible role of STAT1 in muscle regeneration.

Methods

Wild-type and STAT1 knockout mice were used in this study. Tibialis anterior muscle injury was conducted by cardiotoxin (CTX) injection. Bone marrow transplantation and glucocorticoid treatment were performed to manipulate the immune system of the mice.

Results

Muscle regeneration was accelerated in STAT1−/− mice after CTX injury. Bone marrow transplantation experiments showed that the regeneration process relied on the type of donor mice rather than on recipient mice. Levels of pro-inflammatory cytokines, TNFα and IL-1β, were significantly higher in STAT1−/− mice at 1 day and/or 2 days post-injury, while levels of anti-inflammatory cytokine, IL-10, were lower in STAT1−/− mice at 2 days and 3 days post-injury. Levels of IGF-1 were significantly higher in the STAT1−/− mice at 1 day and 2 days post-injury. Furthermore, the muscle regeneration process was inhibited in glucocorticoid-treated mice.

Conclusions

Loss of STAT1 in bone marrow–derived cells accelerates skeletal muscle regeneration.

Sphingosine-1-phosphate enhances satellite cell activation in dystrophic muscles through a S1PR2/STAT3 signaling pathway

Sphingosine-1-phosphate (S1P) activates a widely expressed family of G protein-coupled receptors, serves as a muscle trophic factor and activates muscle stem cells called satellite cells (SCs) through unknown mechanisms. Here we show that muscle injury induces dynamic changes in S1P signaling and metabolism in vivo. These changes include early and profound induction of the gene encoding the S1P biosynthetic enzyme SphK1, followed by induction of the catabolic enzyme sphingosine phosphate lyase (SPL) 3 days later. These changes correlate with a transient increase in circulating S1P levels after muscle injury. We show a specific requirement for SphK1 to support efficient muscle regeneration and SC proliferation and differentiation. Mdx mice, which serve as a model for muscular dystrophy (MD), were found to be S1P-deficient and exhibited muscle SPL upregulation, suggesting that S1P catabolism is enhanced in dystrophic muscle. Pharmacological SPL inhibition increased muscle S1P levels, improved mdx muscle regeneration and enhanced SC proliferation via S1P receptor 2 (S1PR2)-dependent inhibition of Rac1, thereby activating Signal Transducer and Activator of Transcription 3 (STAT3), a central player in inflammatory signaling. STAT3 activation resulted in p21 and p27 downregulation in a S1PR2-dependent fashion in myoblasts. Our findings suggest that S1P promotes SC progression through the cell cycle by repression of cell cycle inhibitors via S1PR2/STAT3-dependent signaling and that SPL inhibition may provide a therapeutic strategy for MD.

Leukaemia inhibitory factor is expressed in rat gastrocnemius muscle after contusion and increases proliferation of rat L6 myoblasts via c-Myc signalling

1. Leukaemia inhibitory factor (LIF) has been shown to have an important role during muscle regeneration. The regenerative capacity of muscles after contusion injury in LIF-knockout mice is impaired compared with that of wild-type mice. 2. To clarify whether LIF modulates muscle regeneration by regulating myogenic precursor cell activity, we studied LIF expression and myogenic precursor cell activity in gastrocnemius muscles from Wistar rats at various times after contusion injury using immunohistochemistry and the direct effect of LIF on a rat myoblast cell line (L6). 3. After contusion injury, transient upregulation of the mRNA expression of LIF, LIF receptors and signal transducer and activator of transcription (STAT) 3, downstream of LIF and involved in enhanced cell proliferation, was observed. A marked increase in LIF protein in the cytosol of damaged myofibres was strongly correlated with a significant increase in the number of myogenic precursor cells (MyoD-positive cells) by 12 h after contusion. In addition, coexpression of LIF and MyoD protein in control and injured muscles after contusion injury from 3 h to 7 days was evident. 4. Treatment of L6 cells with LIF (1 ng/mL) in serum-free medium enhanced proliferation (bromodeoxyuridine incorporation) by 24 h. This was accompanied by increased expression of c-Myc protein within 12 h and was abolished by short interference RNA against c-Myc mRNA. 5. Together, the results of the present study suggest that LIF acts via paracrine and autocrine actions to regulate myogenic precursor cell activity during muscle regeneration after contusion injury and that the proliferative effect of LIF on L6 cells occurs via c-Myc signalling.

Sensitivity of skeletal muscle to pro-apoptotic factors

In mononuclear cells, apoptosis leads to DNA fragmentation and cell destruction, regardless of the activated pathway. As regards multinuclear cells, e.g. skeletal muscle fibers, apoptosis rarely induces the death of the entire cell, and it generally affects single nuclei. This process, referred to as nuclear apoptosis, has a negative effect on the expression of genes in the myonuclear domain. Apoptosis may be initiated in muscle cells by external stimuli which activate cell membrane death receptors as well as by internal stimuli which stimulate the mitochondrial release of pro-apoptotic proteins. Reactive oxygen species also play an important role in the initiation of apoptosis. In muscle cells, ROS are produced in response to extracellular reactions or by cell mitochondria. It is, therefore, believed that mitochondria play a central role in apoptosis within skeletal muscle. Skeletal muscles have a well-developed system that protects them against oxidative damage. Myogenic stem cells are an integral part of multinucleated myofibers, and they are critically important for the maintenance of normal muscle mass, muscle growth, regeneration and hypertrophy. The latest research results indicate that myogenic cells are more sensitive to oxidative stress and pro-apoptotic factors than well-differentiated cells, such as myotubes. The complex structure and activity of skeletal muscle prompted research into the role of apoptosis and its intensity under various physiological and pathological conditions. This review summarizes the results of research investigating control mechanisms and the apoptosis process in skeletal muscle fibers, and indicates unresearched areas where further work is required.

A Rodent Model to Advance the Field Treatment of Crush Muscle Injury During Earthquakes and Other Natural Disasters

Approximately 170 earthquakes of 6.0 or higher magnitude occur annually worldwide. Victims often suffer crush muscle injuries involving impaired blood flow to the affected muscle and damage to the muscle fiber membrane. Current rescue efforts are directed toward preventing acute kidney injury (AKI), which develops upon extrication and muscle reperfusion. But field-usable, muscle-specific interventions may promote muscle regeneration and prevent or minimize the pathologic changes of reperfusion. Although current rodent crush injury models involve reperfusion upon removal of the crush stimulus, an analysis of their methodological aspects is needed to ensure adequate simulation of the earthquake-related crush injury. The objectives of this systematic review are to (a) describe rodent crush muscle injury models, (b) discuss the benefits and limitations of these models, and (c) offer a recommendation for animal models that would increase our understanding of muscle recovery processes after an earthquake-induced crush muscle injury. The most commonly used rodent model uses a clamping or pressing crush stimulus directly applied to murine hindlimb muscle. This model has increased our understanding of muscle regeneration but its open approach does not adequately represent the earthquake-related crush injury. The model we recommend for developing field-usable, muscle-specific interventions is a closed approach that involves a nonclamping crush stimulus. Findings from studies employing this recommended model may have greater relevance for developing interventions that lessen the earthquake’s devastating impact on individual and community health and quality of life, especially in developing countries.

Slow-adhering stem cells derived from injured skeletal muscle have improved regenerative capacity

A wide variety of myogenic cell sources have been used for repair of injured and diseased muscle including muscle stem cells, which can be isolated from skeletal muscle as a group of slow-adhering cells on a collagen-coated surface. The therapeutic use of muscle stem cells for improving muscle regeneration is promising; however, the effect of injury on their characteristics and engraftment potential has yet to be described. In the present study, slow-adhering stem cells (SASCs) from both laceration-injured and control noninjured skeletal muscles in mice were isolated and studied. Migration and proliferation rates, multidifferentiation potentials, and differences in gene expression in both groups of cells were compared in vitro. Results demonstrated that a larger population of SASCs could be isolated from injured muscle than from control noninjured muscle. In addition, SASCs derived from injured muscle demonstrated improved migration, a higher rate of proliferation and multidifferentiation, and increased expression of Notch1, STAT3, Msx1, and MMP2. Moreover, when transplanted into dystrophic muscle in MDX/SCID mice, SASCs from injured muscle generated greater engraftments with a higher capillary density than did SASCs from control noninjured muscle. These data suggest that traumatic injury may modify stem cell characteristics through trophic factors and improve the transplantation potential of SASCs in alleviating skeletal muscle injuries and diseases.

Impact of SOCS3 overexpression on human skeletal muscle development in vitro

The Janus kinase/signal transducers and activators of transcription (JAK/STAT) signaling cascade has been identified as a crucial factor for myogenesis. The STAT3 isoform is essential for satellite cell migration and myogenic differentiation as it mediates the expression of muscle specific myogenic factors. The SOCS (suppressors of cytokine signaling) family of proteins down-regulates STAT activation. Primary human skeletal muscle cells were isolated and cultured to investigate the effect of SOCS3 adenoviral overexpression on myotube maturation. It was demonstrated that STAT3 inhibition did not influence myotube development or survival. Moreover, SOCS3 overexpression enhances the mRNA expression of downstream targets of STAT3, c-FOS and VEGF. These increases were correlated with enhanced mRNA expression of genes associated with muscle maturation and hypertrophy. Thus SOCS3 influences myoblast differentiation and SOCS3 may be significant in regulating the activity of genes previously identified as transcriptionally regulated by STAT3.

IL-6 induced STAT3 signalling is associated with the proliferation of human muscle satellite cells following acute muscle damage

Background

Although the satellite cell (SC) is a key regulator of muscle growth during development and muscle adaptation following exercise, the regulation of human muscle SC function remains largely unexplored. STAT3 signalling mediated via interleukin-6 (IL-6) has recently come to the forefront as a potential regulator of SC proliferation. The early response of the SC population in human muscle to muscle-lengthening contractions (MLC) as mediated by STAT3 has not been studied.

Methodology/Principal Findings

Twelve male subjects (21±2 y; 83±12 kg) performed 300 maximal MLC of the quadriceps femoris at 180°•s⁻¹ over a 55° range of motion with muscle samples (vastus lateralis) and blood samples (antecubital vein) taken prior to exercise (PRE), 1 hour (T1), 3 hours (T3) and 24 hours (T24) post-exercise. Cytoplasmic and nuclear fractions of muscle biopsies were purified and analyzed for total and phosphorylated STAT3 (p-STAT3) by western blot. p-STAT3 was detected in cytoplasmic fractions across the time course peaking at T24 (p<0.01 vs. PRE). Nuclear total and p-STAT3 were not detected at appreciable levels. However, immunohistochemical analysis revealed a progressive increase in the proportion of SCs expressing p-STAT3 with ∼60% of all SCs positive for p-STAT3 at T24 (p<0.001 vs. PRE). Additionally, cMyc, a STAT3 downstream gene, was significantly up-regulated in SCs at T24 versus PRE (p<0.05). Whole muscle mRNA analysis revealed induction of the STAT3 target genes IL-6, SOCS3, cMyc (peaking at T3, p<0.05), IL-6Rα and GP130 (peaking at T24, p<0.05). In addition, Myf5 mRNA was up-regulated at T24 (p<0.05) with no appreciable change in MRF4 mRNA.

Conclusions/Significant Findings

We demonstrate that IL-6 induction of STAT3 signaling occurred exclusively in the nuclei of SCs in response to MLC. An increase in the number of cMyc+ SCs indicated that human SCs were induced to proliferate under the control of STAT3 signaling.

JAK/STAT signaling and human in vitro myogenesis

BACKGROUND

A population of satellite cells exists in skeletal muscle. These cells are thought to be primarily responsible for postnatal muscle growth and injury-induced muscle regeneration. The Janus kinase/signal transducers and activators of transcription (JAK/STAT) signaling cascade has a crucial role in regulating myogenesis. In rodent skeletal muscle, STAT3 is essential for satellite cell migration and myogenic differentiation, regulating the expression of myogenic factors. The aim of the present study was to investigate and compare the expression profile of JAK/STAT family members, using cultured primary human skeletal muscle cells.

RESULTS

Near confluent proliferating myoblasts were induced to differentiate for 1, 5 or 10 days. During these developmental stages, members of the JAK/STAT family were examined, along with factors known to regulate myogenesis. We demonstrate the phosphorylation of JAK1 and STAT1 only during myoblast proliferation, while JAK2 and STAT3 phosphorylation increases during differentiation. These increases were correlated with the upregulation of genes associated with muscle maturation and hypertrophy.

CONCLUSIONS

Taken together, these results provide insight into JAK/STAT signaling in human skeletal muscle development, and confirm recent observations in rodents.

Satellite cell activity in muscle regeneration after contusion in rats

1. The role of satellite cells in muscle growth during development is well documented, but the involvement of these cells in muscle repair after contusion is less well known. In the present study, we investigated the time-course of satellite cell activity (from 3h to 7days) after contusion of rat gastrocnemius muscle using specific molecular markers for immunofluorescence and real-time polymerase chain reaction (PCR). 2. Inflammation of the injured muscle occurred within 6h, followed by disintegration of the damaged myofibres within 12h. Newly formed myofibres appeared by Day 7. 3. The number of MyoD-positive nuclei (activated satellite cells) in the injured muscle was significantly increased by 6h, reaching a maximum by 12h after contusion. However, the number of MyoD-positive nuclei decreased towards control levels by Day 7. Changes in the number of bromodeoxyuridine-labelled nuclei (proliferating satellite cells) paralleled the changes seen in the number of MyoD-positive nuclei. Conversely, expression of myogenin protein was not apparent until Day 3 and increased further by Day 7. Colabelling of MyoD and myogenin was seen in only a few cells. 4. The time-course of MyoD mRNA expression corresponded with MyoD protein expression. However, there were two peaks in myogenin mRNA expression: 6h and Day 7 after contusion. The second peak coincided with upregulation of myostatin mRNA levels. 5. The results of the present study suggest that contusion activates a homogeneous population of satellite cells to proliferate within 3days, followed by differentiation to form new myofibres. The latter may be regulated, in part, by myostatin.

Nitric oxide donors improve prednisone effects on muscular dystrophy in the mdx mouse diaphragm

In Duchenne muscular dystrophy (DMD), palliative glucocorticoid therapy can produce myopathy or calcification. Since increased nitric oxide synthase activity in dystrophic mice promotes regeneration, the outcome of two nitric oxide (NO) donor drugs, MyoNovin (M) and isosorbide dinitrate (I), on the effectiveness of the anti-inflammatory drug prednisone (P) in alleviating progression of dystrophy was tested. Dystrophic mdx mice were treated (18 days) as controls or with an NO donor ± P. Fiber permeability and DNA synthesis were labeled by Evans blue dye (EBD) and bromodeoxyuridine uptake, respectively. P decreased body weight gain, M increased quadriceps mass, and I increased heart mass. P increased fiber permeability (%EBD+ fibers) and calcification in diaphragm. Treatment with NO donors + P (M+P, I+P) reduced %EBD+ fibers and calcification vs. P alone. %EBD+ fibers in M+P diaphragm did not differ from control. NO donor treatment reduced proliferation and the population of c-met+ cells and accelerated fiber regeneration. Concurrent with P, NO donor treatment suppressed two important detrimental effects of P in mice, possibly by accelerating regeneration, rebalancing satellite cell quiescence and activation in dystrophy, and/or increasing perfusion. Results suggest that NO donors could improve current therapy for DMD.

Stat3 directly controls the expression of Tbx5, Nkx2.5, and GATA4 and is essential for cardiomyocyte differentiation of P19CL6 cells

The transcription factor Stat3 (signal transducer and activator of transcription 3) mediates many physiological processes, including embryogenesis, stem cell self-renewal, and postnatal survival. In response to gp130 receptor activation, Stat3 becomes phosphorylated by the receptor-associated Janus kinase, forms dimers, and enters the nucleus where it binds to Stat3 target genes and regulates their expression. In this report, we demonstrate that Stat3 binds directly to the promoters and regulates the expression of three genes that are essential for cardiac differentiation: Tbx5, Nkx2.5, and GATA4. We further demonstrate that Tbx5, Nkx2.5, and GATA4 expression is dependent on Stat3 in response to ligand treatment and during ligand-independent differentiation of P19CL6 cells into cardiomyocytes. Finally, we show that Stat3 is necessary for the differentiation of P19CL6 cells into beating cardiomyocytes. All together, these results demonstrate that Stat3 is required for the differentiation of cardiomyocytes through direct transcriptional regulation of Tbx5, Nkx2.5, and GATA4.

Characteristics of locomotion, muscle strength, and muscle tissue in regenerating rat skeletal muscles

Although numerous studies have aimed to elucidate the mechanisms used to repair the structure and function of injured skeletal muscles, it remains unclear how and when movement recovers following damage. We performed a temporal analysis to characterize the changes in movement, muscle function, and muscle structure after muscle injury induced by the drop-mass technique. At each time-point, movement recovery was determined by ankle kinematic analysis of locomotion, and functional recovery was represented by isometric force. As a histological analysis, the cross-sectional area of myotubes was measured to examine structural regeneration. The dorsiflexion angle of the ankle, as assessed by kinematic analysis of locomotion, increased after injury and then returned to control levels by day 14 post-injury. The isometric force returned to normal levels by day 21 post-injury. However, the size of the myotubes did not reach normal levels, even at day 21 post-injury. These results indicate that recovery of locomotion occurs prior to recovery of isometric force and that functional recovery occurs earlier than structural regeneration. Thus, it is suggested that recovery of the movement and function of injured skeletal muscles might be insufficient as markers for estimating the degree of neuromuscular system reconstitution.

Heat stress increases myonuclear number and fiber size via satellite cell activation in rat regenerating soleus fibers

To investigate the effects of heat stress (hyperthermia) on muscle degeneration-regeneration, the soleus muscles of adult male Wistar rats were injected bilaterally with a single injection of bupivacaine. The rats were assigned to a sedentary control (Con), heat stress (Heat), bupivacaine-injected (BPVC), or bupivacaine-injected plus heat stress (BPVC+Heat) group. Heat stress was induced in the Heat and BPVC+Heat groups by immersion of the lower half of the body into water maintained at 42 ± 1°C for 30 min 48 h after the injection of bupivacaine and every other day during the following 1 or 2 wk. The soleus muscles in all groups were excised 24 h after the final bout of heat stress. Mean muscle weight, fiber cross-sectional area, myonuclear number, and heat shock protein 72 (Hsp72) and calcineurin protein levels were lower in the BPVC than in the Con or Heat groups at both time points. In contrast, several of these parameters in the BPVC+Heat group were not different or higher than in the Con or Heat groups at the 1- and/or 2-wk time points. The number of total and activated satellite cells, estimated by analyses of Pax7-negative, M-cadherin-negative, and MyoD-positive nuclei, was greater in BPVC+Heat than in all other groups. Combined, the results indicate that heat stress-related activation of satellite cells and upregulation of Hsp72 and calcineurin expression played important roles in the regeneration of the soleus fibers after bupivacaine injection.