Regenerating soleus and extensor digitorum longus muscles of the rat show elevated levels of TNF-alpha and its receptors, TNFR-60 and TNFR-80

Met and Cxcr4 cooperate to protect skeletal muscle stem cells against inflammation-induced damage during regeneration

Acute skeletal muscle injury is followed by an inflammatory response, removal of damaged tissue, and the generation of new muscle fibers by resident muscle stem cells, a process well characterized in murine injury models. Inflammatory cells are needed to remove the debris at the site of injury and provide signals that are beneficial for repair. However, they also release chemokines, reactive oxygen species, as well as enzymes for clearance of damaged cells and fibers, which muscle stem cells have to withstand in order to regenerate the muscle. We show here that MET and CXCR4 cooperate to protect muscle stem cells against the adverse environment encountered during muscle repair. This powerful cyto-protective role was revealed by the genetic ablation of Met and Cxcr4 in muscle stem cells of mice, which resulted in severe apoptosis during early stages of regeneration. TNFα neutralizing antibodies rescued the apoptosis, indicating that TNFα provides crucial cell-death signals during muscle repair that are counteracted by MET and CXCR4. We conclude that muscle stem cells require MET and CXCR4 to protect them against the harsh inflammatory environment encountered in an acute muscle injury.

IL-1β and TNF-α Modulation of Proliferated and Committed Myoblasts: IL-6 and COX-2-Derived Prostaglandins as Key Actors in the Mechanisms Involved

In this study, we investigated the effects and mechanisms of the pro-inflammatory cytokines IL-1β and TNF-α on the proliferation and commitment phases of myoblast differentiation. C2C12 mouse myoblast cells were cultured to reach a proliferated or committed status and were incubated with these cytokines for the evaluation of cell proliferation, cyclooxygenase 2 (COX-2) expression, release of prostaglandins (PGs) and myokines, and activation of myogenic regulatory factors (MRFs). We found that inhibition of the IL-6 receptor reduced IL-1β- and TNF-α-induced cell proliferation, and that the IL-1β effect also involved COX-2-derived PGs. Both cytokines modulated the release of the myokines myostatin, irisin, osteonectin, and IL-15. TNF-α and IL-6 reduced the activity of Pax7 in proliferated cells and reduced MyoD and myogenin activity at both proliferative and commitment stages. Otherwise, IL-1β increased myogenin activity only in committed cells. Our data reveal a key role of IL-6 and COX-2-derived PGs in IL-1β and TNF-α-induced myoblast proliferation and support the link between TNF-α and IL-6 and the activation of MRFs. We concluded that IL-1β and TNF-α induce similar effects at the initial stages of muscle regeneration but found critical differences between their effects with the progression of the process, bringing new insights into inflammatory signalling in skeletal muscle regeneration.

Epigenetic Regulation of Adult Myogenesis

Skeletal muscle regeneration is an efficient stem cell-based repair system that ensures healthy musculature. For this repair system to function continuously throughout life, muscle stem cells must contribute to the process of myofiber repair as well as repopulation of the stem cell niche. The decision made by the muscle stem cells to commit to the muscle repair or to remain a stem cell depends upon patterns of gene expression, a process regulated at the epigenetic level. Indeed, it is well accepted that dynamic changes in epigenetic landscapes to control DNA accessibility and expression is a critical component during myogenesis for the effective repair of damaged muscle. Changes in the epigenetic landscape are governed by various posttranslational histone tail modifications, nucleosome repositioning, and DNA methylation events which collectively allow the control of changes in transcription networks during transitions of satellite cells from a dormant quiescent state toward terminal differentiation. This chapter focuses upon the specific epigenetic changes that occur during muscle stem cell-mediated regeneration to ensure myofiber repair and continuity of the stem cell compartment. Furthermore, we explore open questions in the field that are expected to be important areas of exploration as we move toward a more thorough understanding of the epigenetic mechanism regulating muscle regeneration.

17-(allylamino)-17-demethoxygeldanamycin drives Hsp70 expression but fails to improve morphological or functional recovery in injured skeletal muscle

The stress inducible 70 kDa heat shock protein (Hsp70) is instrumental to efficient morphological and functional recovery following skeletal muscle injury because of its roles in protein quality control and molecular signalling. Therefore, in attempt to improve recovery, Hsp70 expression was increased with 17-(allylamino)-17-demethoxygeldanamycin (17-AAG) prior to and following an intramuscular injection of barium chloride (BaCl2) into the tibialis anterior (TA) of healthy young mice. To assess recovery, regenerating fibre cross-sectional area (CSA) of the TA and in vivo peak isometric torque produced by the anterior crural muscles (TA, extensor digitorum longus and extensor hallucis muscles) were analyzed for up to 3 weeks after the injury. Because treatment of 17-AAG and Hsp70 are known to influence inflammatory and myogenic signalling, tumor necrosis factor-α (TNF-α) and myogenin content were also assessed. This study reports that 17-AAG was effective at up-regulating Hsp70 expression, increasing content fivefold in the uninjured muscle. However, this significant increase in Hsp70 content did not enhance morphological or functional recovery following the injury, as the return of regenerating fibre CSA and in vivo peak isometric torque did not differ compared to that of the injured muscle from the vehicle treated mice. Treatment with 17-AAG also altered TNF-α and myogenin content, increasing both to a greater extent after the injury. Together, these findings demonstrate that although 17-AAG may alter molecular makers of regeneration, it does not improve recovery following BaCl2-induced skeletal muscle injury in healthy young mice.

Skeletal muscle regeneration and impact of aging and nutrition

After skeletal muscle injury a regeneration process takes place to repair muscle. Skeletal muscle recovery is a highly coordinated process involving cross-talk between immune and muscle cells. It is well known that the physiological activities of both immune cells and muscle stem cells decline with advancing age, thereby blunting the capacity of skeletal muscle to regenerate. The age-related reduction in muscle repair efficiency contributes to the development of sarcopenia, one of the most important factors of disability in elderly people. Preserving muscle regeneration capacity may slow the development of this syndrome. In this context, nutrition has drawn much attention: studies have demonstrated that nutrients such as amino acids, n-3 polyunsaturated fatty acids, polyphenols and vitamin D can improve skeletal muscle regeneration by targeting key functions of immune cells, muscle cells or both. Here we review the process of skeletal muscle regeneration with a special focus on the cross-talk between immune and muscle cells. We address the effect of aging on immune and skeletal muscle cells involved in muscle regeneration. Finally, the mechanisms of nutrient action on muscle regeneration are described, showing that quality of nutrition may help to preserve the capacity for skeletal muscle regeneration with age.

CX3CL1--a macrophage chemoattractant induced by a single bout of exercise in human skeletal muscle

Monocytes/macrophages (MOs/MΦs) are suggested to be crucial for skeletal muscle repair and remodeling. This has been attributed to their proangiogenic potential, secretion of growth factors, and clearance of tissue debris. Skeletal muscle injury increases the number of MΦs in the tissue, and their importance for muscle regeneration has been supported by studies demonstrating that depletion of MOs/MΦs greatly impairs repair after muscle injury. Whether noninjurious exercise leads to induced expression of chemoattractants for MOs/MΦs is poorly investigated. To this end, we analyzed the expression of CX3CL1 (fractalkine), CCL2 (MCP-1), and CCL22 (MDC) in human skeletal muscle after a bout of exercise, all of which are established MO/MΦ chemotactic factors that are expressed by human myoblasts. Muscle biopsies from the musculus vastus lateralis were obtained up to 24 h after 1 h of cycle exercise in healthy individuals and in age-matched nonexercised controls. CX3CL1 increased at both the mRNA and protein level in human skeletal muscle after one bout of exercise. It was not possible to distinguish changes in CCL2 or CCL22 mRNA levels between biopsy vs. exercise effects, and the expression of CCL22 was very low. CX3CL1 mainly localized to the skeletal muscle endothelium, and it increased in human umbilical vein endothelial cells stimulated with tissue fluid from exercised muscle. CX3CL1 increased the expression of proinflammatory and proangiogenic factors in THP-1 monocytes (a human acute monocytic leukemia cell line) and in human primary myoblasts and myotubes. Altogether, this suggests that CX3CL1 participates in cross-talk mechanisms between endothelium and other muscle tissue cells and may promote a shift in the microenvironment toward a more regenerative milieu.

Injury and subsequent regeneration of muscles for activation of local innate immunity to facilitate the development and relapse of autoimmune myositis in C57BL/6 mice

OBJECTIVE

To determine whether injury and regeneration of the skeletal muscles induce an inflammatory milieu that facilitates the development and relapse of autoimmune myositis.

METHODS

The quadriceps of C57BL/6 mice were injured with bupivacaine hydrochloride (BPVC) and evaluated histologically. Macrophages and regenerating myofibers in the treated muscles and differentiating C2C12 myotubes were examined for cytokine expression. Mice were immunized with C protein fragments at the base of the tail and in the right hind footpads (day 0) to evoke systemic anti-C protein immunity and to induce local myositis in the right hind limbs. The contralateral quadriceps muscles were injured with BPVC or phosphate buffered saline (PBS) on day 7 or after spontaneous regression of myositis (day 42). The quadriceps muscle in nonimmunized mice was injured with BPVC on day 7. The muscles were examined histologically 14 days after treatment.

RESULTS

The BPVC-injured muscles had macrophage infiltration most abundantly at 3 days after the injection, with emergence of regenerating fibers from day 5. The macrophages expressed inflammatory cytokines, including tumor necrosis factor α, interleukin-1β, and CCL2. Regenerating myofibers and C2C12 myotubes also expressed the cytokines. The BPVC-injected muscles from nonimmunized mice had regenerating myofibers with resolved cell infiltration 14 days after treatment. In mice preimmunized with C protein fragments, the muscles injected with BPVC on day 7 as well as on day 42, but not those injected with PBS, had myositis accompanied by CD8+ T cell infiltration.

CONCLUSION

Injury and regeneration could set up an inflammatory milieu in the muscles and facilitate the development and relapse of autoimmune myositis.

Rethinking regenerative medicine: a macrophage-centered approach

Regenerative medicine, a multi-disciplinary approach that seeks to restore form and function to damaged or diseased tissues and organs, has evolved significantly during the past decade. By adapting and integrating fundamental knowledge from cell biology, polymer science, and engineering, coupled with an increasing understanding of the mechanisms which underlie the pathogenesis of specific diseases, regenerative medicine has the potential for innovative and transformative therapies for heretofore unmet medical needs. However, the translation of novel technologies from the benchtop to animal models and clinical settings is non-trivial and requires an understanding of the mechanisms by which the host will respond to these novel therapeutic approaches. The role of the innate immune system, especially the role of macrophages, in the host response to regenerative medicine based strategies has recently received considerable attention. Macrophage phenotype and function have been suggested as critical and determinant factors in downstream outcomes. The constructive and regulatory, and in fact essential, role of macrophages in positive outcomes represents a significant departure from the classical paradigms of host-biomaterial interactions, which typically consider activation of the host immune system as a detrimental event. It appears desirable that emerging regenerative medicine approaches should not only accommodate but also promote the involvement of the immune system to facilitate positive outcomes. Herein, we describe the current understanding of macrophage phenotype as it pertains to regenerative medicine and suggest that improvement of our understanding of context-dependent macrophage polarization will lead to concurrent improvement in outcomes.

Regulation of myogenic activation of p38 MAPK by TACE-mediated TNFα release

The activation of p38 MAPK in myogenic precursor cells (MPCs) is a key signal for their exit of cell cycle and entry of the myogenic differentiation program. Therefore, identification of the signaling mechanism that activates p38 MAPK during this process is important for the understanding of the regulatory mechanism of muscle regeneration. This article reviews recent findings regarding the role of inflammatory cytokine tumor necrosis factor-α (TNFα) as a key activator of p38 MAPK during myogenesis in an autocrine/paracrine fashion, and the signaling mechanisms that converge upon TNFα converting enzyme (TACE) to release TNFα from differentiating MPCs in response to diverse regenerative stimuli.

β-agonists selectively modulate proinflammatory gene expression in skeletal muscle cells via non-canonical nuclear crosstalk mechanisms

The proinflammatory cytokine Tumour Necrosis Factor (TNF)-α is implicated in a variety of skeletal muscle pathologies. Here, we have investigated how in vitro cotreatment of skeletal muscle C2C12 cells with β-agonists modulates the TNF-α-induced inflammatory program. We observed that C2C12 myotubes express functional TNF receptor 1 (TNF-R1) and β2-adrenoreceptors (β2-ARs). TNF-α activated the canonical Nuclear Factor-κB (NF-κB) pathway and Mitogen-Activated Protein Kinases (MAPKs), culminating in potent induction of NF-κB-dependent proinflammatory genes. Cotreatment with the β-agonist isoproterenol potentiated the expression of inflammatory mediators, including Interleukin-6 (IL-6) and several chemokines. The enhanced production of chemotactic factors upon TNF-α/isoproterenol cotreatment was also suggested by the results from migrational analysis. Whereas we could not explain our observations by cytoplasmic crosstalk, we found that TNF-R1-and β2-AR-induced signalling cascades cooperate in the nucleus. Using the IL-6 promoter as a model, we demonstrated that TNF-α/isoproterenol cotreatment provoked phosphorylation of histone H3 at serine 10, concomitant with enhanced promoter accessibility and recruitment of the NF-κB p65 subunit, cAMP-response element-binding protein (CREB), CREB-binding protein (CBP) and RNA polymerase II. In summary, we show that β-agonists potentiate TNF-α action, via nuclear crosstalk, that promotes chromatin relaxation at selected gene promoters. Our data warrant further study into the mode of action of β-agonists and urge for caution in their use as therapeutic agents for muscular disorders.

Properties of regenerated mouse extensor digitorum longus muscle following notexin injury

Muscles of mdx mice are known to be more susceptible to contraction-induced damage than wild-type muscle. However, it is not clear whether this is because of dystrophin deficiency or because of the abnormal branching morphology of dystrophic muscle fibres. This distinction has an important bearing on our traditional understanding of the function of dystrophin as a mechanical stabilizer of the sarcolemma. In this study, we address the question: 'Does dystrophin-positive, regenerated muscle containing branched fibres also show an increased susceptibility to contraction-induced damage?' We produced a model of fibre branching by injecting dystrophin-positive extensor digitorum longus muscles with notexin. The regenerated muscle was examined at 21 days postinjection. Notexin-injected muscle contained 29% branched fibres and was not more susceptible to damage from mild eccentric contractions than contralateral saline-injected control muscle. Regenerated muscles also had greater mass, greater cross-sectional area and lower specific force than control muscles. We conclude that the number of branched fibres in this regenerated muscle is below the threshold needed to increase susceptibility to damage. However, it would serve as an ideal control for muscles of young mdx mice, allowing for clearer differentiation of the effects of dystrophin deficiency from the effects of fibre regeneration and morphology.

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.

Effects of a natural prolyl oligopeptidase inhibitor, rosmarinic acid, on lipopolysaccharide-induced acute lung injury in mice

Rosmarinic acid (RA), a polyphenolic phytochemical, is a natural prolyl oligopeptidase inhibitor. In the present study, we found that RA exerted potent anti-inflammatory effects in in vivo models of acute lung injury (ALI) induced by lipopolysaccharide (LPS). Mice were pretreated with RA one hour before challenge with a dose of 0.5 mg/kg LPS. Twenty-four hours after LPS was given, bronchoalveolar lavage fluid (BALF) was obtained to measure pro-inflammatory mediator and total cell counts. RA significantly decreased the production of LPS-induced TNF-a, IL-6, and IL-1β compare with the LPS group. When pretreated with RA (5, 10, or 20 mg/kg) the lung wet-to-dry weight (W/D) ratio of the lung tissue and the number of total cells, neutrophils and macrophages in the BALF were decreased significantly. Furthermore, RA may enhance oxidase dimutase (SOD) activity during the inflammatory response to LPS-induced ALI. And we further demonstrated that RA exerts anti-inflammation effect in vivo models of ALI through suppresses ERK/MAPK signaling in a dose dependent manner. These studies have important implications for RA administration as a potential treatment for ALI.

Cytokines in muscle damage

Multiple cellular and molecular processes are rapidly activated following skeletal muscle damage to restore normal muscle structure and function. These processes typically involve an inflammatory response and potentially the consequent occurrence of secondary damage before their resolution and the completion of muscle repair or regeneration. The overall outcome of the inflammatory process is potentially divergent, with the induction of prolonged inflammation and further muscle damage, or its active termination and the promotion of muscle repair and regeneration. The final, detrimental, or beneficial effect of the inflammatory response on muscle repair is influenced by specific interactions between inflammatory and muscle cell-derived cytokines that act as positive and/or negative regulators to coordinate local and systemic inflammatory-related events and modulate muscle repair process. A crucial balance between proinflammatory and anti-inflammatory cytokines appears to attenuate an excessive inflammatory reaction, prevent the development of muscle fibrosis, and adequately promote the regenerative process. In this review, we address the interactive cytokine responses following muscle damage, in the context of induction and progression, or resolution of muscle inflammation and the promotion of muscle repair.

Abcg2 labels multiple cell types in skeletal muscle and participates in muscle regeneration

Abcg2-expressing cells proliferate after muscle injury and are required for effective regeneration of multiple muscle cell lineages.

Skeletal muscle contains progenitor cells (satellite cells) that maintain and repair muscle. It also contains muscle side population (SP) cells, which express Abcg2 and may participate in muscle regeneration or may represent a source of satellite cell replenishment. In Abcg2-null mice, the SP fraction is lost in skeletal muscle, although the significance of this loss was previously unknown. We show that cells expressing Abcg2 increased upon injury and that muscle regeneration was impaired in Abcg2-null mice, resulting in fewer centrally nucleated myofibers, reduced myofiber size, and fewer satellite cells. Additionally, using genetic lineage tracing, we demonstrate that the progeny of Abcg2-expressing cells contributed to multiple cell types within the muscle interstitium, primarily endothelial cells. After injury, Abcg2 progeny made a minor contribution to regenerated myofibers. Furthermore, Abcg2-labeled cells increased significantly upon injury and appeared to traffic to muscle from peripheral blood. Together, these data suggest an important role for Abcg2 in positively regulating skeletal muscle regeneration.

TNFα increases resting potential in isolated fibres from rat peroneus longus by a PKC mediated mechanism: involvement in ICU acquired polyneuromyopathy

BACKGROUND AND AIMS

Our aim was to investigate the effect of TNFα on muscle resting potential (RP) and then in muscle excitability and to demonstrate another mechanism implicated in intensive care units (ICU) acquired polyneuromyopathy.

METHODS

Experiments were carried out on adult female Wistar rats. After isolation of muscle fibres from peroneus longus, influence of TNFα was tested on RP by using intracellular microelectrodes. Digoxin and chelerythrin were used to determine the mechanism of TNFα action.

RESULTS

First, we found that TNFα induced a concentration dependent increase of muscle RP and that this mechanism, which was blocked by digoxin, was due to an effect on the Na/K ATPase. As it was also blocked by chelerythrin it was concluded that this effect was mediated by PKC activation of the Na/K ATPase.

CONCLUSIONS

We demonstrated that TNFα leads to a PKC mediated increase in muscle RP. Depolarization needed to reach the threshold voltage for muscle action potential should then be higher and this could be involved in the decrease in muscle excitability observed in acquired polyneuromyopathy.

Muscle resident macrophages control the immune cell reaction in a mouse model of notexin-induced myoinjury

OBJECTIVE

Skeletal muscle may be the site of a variety of poorly understood immune reactions, particularly after myofiber injury, which is typically observed in inflammatory myopathies. This study was undertaken to explore both the cell dynamics and functions of resident macrophages and dendritic cells (DCs) in damaged muscle, using a mouse model of notexin-induced myoinjury to study innate immune cell reactions.

METHODS

The myeloid cell reaction to notexin-induced myoinjury was analyzed by microscopy and flow cytometry. Bone marrow (BM) transplantation studies were used to discriminate resident from exudate monocyte/macrophages. Functional tests included cytokine screening and an alloantigenic mixed leukocyte reaction to assess the antigen-presenting cell (APC) function. Selective resident macrophage depletion was obtained by injection of diphtheria toxin (DT) into CD11b-DT receptor-transgenic mice transplanted with DT-insensitive BM.

RESULTS

The connective tissue surrounding mouse muscle/fascicle tissue (the epimysium/perimysium) after deep muscle injury displayed a resident macrophage population of CD11b+F4/80+CD11c-Ly-6C-CX3CR1- cells, which concentrated first in the epimysium. These resident macrophages were being used by leukocytes as a centripetal migration pathway, and were found to selectively release 2 chemokines, cytokine-induced neutrophil chemoattractant and monocyte chemoattractant protein 1, and to crucially contribute to massive recruitment of neutrophils and monocytes from the blood. Early epimysial inflammation consisted of a predominance of Ly-6C(high)CX3CR1(low)CD11c- cells that were progressively substituted by Ly-6C(low)CX3CR1(high) cells displaying an intermediate, rather than high, level of CD11c expression. These CD11c(intermediate) cells were derived from circulating CCR2+ monocytes, functionally behaved as immature APCs in the absence of alloantigenic challenge, and migrated to draining lymph nodes while acquiring the phenotype of mature DCs (CD11c+Ia+CD80+ cells, corresponding to an inflammatory DC phenotype).

CONCLUSION

The results in this mouse model show that resident macrophages in the muscle epimysium/perimysium orchestrate the innate immune response to myoinjury, which is linked to adaptive immunity through the formation of inflammatory DCs.

Regulatory interactions between muscle and the immune system during muscle regeneration

Recent discoveries reveal complex interactions between skeletal muscle and the immune system that regulate muscle regeneration. In this review, we evaluate evidence that indicates that the response of myeloid cells to muscle injury promotes muscle regeneration and growth. Acute perturbations of muscle activate a sequence of interactions between muscle and inflammatory cells. The initial inflammatory response is a characteristic Th1 inflammatory response, first dominated by neutrophils and subsequently by CD68(+) M1 macrophages. M1 macrophages can propagate the Th1 response by releasing proinflammatory cytokines and cause further tissue damage through the release of nitric oxide. Myeloid cells in the early Th1 response stimulate the proliferative phase of myogenesis through mechanisms mediated by TNF-alpha and IL-6; experimental prolongation of their presence is associated with delayed transition to the early differentiation stage of myogenesis. Subsequent invasion by CD163(+)/CD206(+) M2 macrophages attenuates M1 populations through the release of anti-inflammatory cytokines, including IL-10. M2 macrophages play a major role in promoting growth and regeneration; their absence greatly slows muscle growth following injury or modified use and inhibits muscle differentiation and regeneration. Chronic muscle injury leads to profiles of macrophage invasion and function that differ from acute injuries. For example, mdx muscular dystrophy yields invasion of muscle by M1 macrophages, but their early invasion is accompanied by a subpopulation of M2a macrophages. M2a macrophages are IL-4 receptor(+)/CD206(+) cells that reduce cytotoxicity of M1 macrophages. Subsequent invasion of dystrophic muscle by M2c macrophages is associated with progression of the regenerative phase in pathophysiology. Together, these findings show that transitions in macrophage phenotype are an essential component of muscle regeneration in vivo following acute or chronic muscle damage.

TNF alpha receptor genotype influences smoking-induced muscle-fibre-type shift and atrophy in mice

Systemic manifestations of chronic obstructive pulmonary disease (COPD) include muscle wasting, and tumour necrosis factor alpha (TNFalpha) could represent a major inducer of these processes. We studied skeletal muscle histology in a murine model of cigarette smoke (CS)-induced COPD, comparing mice with different TNFalpha receptor genotypes. Muscles from hind limbs of wild type (WT), TNFalpha receptor 1 knockout (TNF alpha R1KO) and TNF alpha R2KO mice were prepared and weighed. The lower body weight, which was observed in CS-exposed WT and TNF alpha R1KO mice, was paralleled by reduced weights of gastrocnemius and biceps femoris muscle. The gastrocnemius muscle was evaluated for muscle fibre apoptosis and atrophy, and fibre-type distribution. CS-induced apoptosis was observed in all genotypes, while a significant reduction of cross-sectional areas of myofibres was present only in TNF alpha R2KO mice. A CS-induced fibre-type shift from the IIa to the IIb phenotype was observed in WT mice, an increase of muscle-fibre-type IIx was noticed in CS-exposed TNF alpha R2KO mice. Our data suggest that the skeletal muscle manifestations associated with this murine COPD model are under complex regulation by both TNFalpha receptors, but that TNF alpha R2 may be the most important determinant for the outcome of CS-induced myofibre apoptosis.

TNF-alpha increases protein content in C2C12 and primary myotubes by enhancing protein translation via the TNF-R1, PI3K, and MEK

Recent evidence supports that TNF-alpha, long considered a catabolic factor, may also have a physiological function in skeletal muscle. The catabolic view, mainly based on correlative studies in human and in vivo animal models, was challenged by experiments with myoblasts, in which TNF-alpha induced differentiation. The biological effects of TNF-alpha in differentiated muscle, however, remain poorly understood. In the present study, we tested whether TNF-alpha has growth-promoting effects in myotubes, and we characterized the mechanisms leading to these effects. Treatment of C(2)C(12) myotubes with TNF-alpha for 24 h increased protein synthesis (PS) and enhanced cellular dehydrogenase activity by 22 and 26%, respectively, without changing cell numbers. These effects were confirmed in myotubes differentiated from primary rat myoblasts. TNF-alpha activated two signaling cascades: 1) ERK1/2 and its target eIF4E and 2) Akt and its downstream effectors GSK-3, p70(S6K), and 4E-BP1. TNF-alpha-induced phosphorylation of Akt, and ERK1/2 was inhibited by an antibody against TNF-alpha receptor 1 (TNF-R1). PD-98059 pretreatment abolished TNF-alpha-induced phosphorylation of ERK1/2 and eIF4E, whereas PS was only partially inhibited. LY-294002 completely abolished TNF-alpha-induced stimulation of PS as well as phosphorylation of Akt and its downstream targets GSK-3, p70(S6K), and 4E-BP1. Rapamycin inhibited TNF-alpha-induced phosphorylation of the mTOR C1 target p70(S6K) without altering TNF-alpha-induced PS and 4E-BP1 phosphorylation. In conclusion, our results provide evidence that TNF-alpha enhances PS in myotubes and that this is based on enhanced protein translation mediated by the TNF-R1 and PI3K-Akt and MEK-ERK signaling cascades.

TACE release of TNF-alpha mediates mechanotransduction-induced activation of p38 MAPK and myogenesis

Skeletal muscle responds to mechanical stimulation by activating p38 MAPK, a key signal for myogenesis. However, the mechanotransduction mechanism that activates p38 is unknown. Here we show that mechanical stimulation of myoblasts activates p38 and myogenesis through stimulating TNF-alpha release by TNF-alpha converting enzyme (TACE). In C2C12 or mouse primary myoblasts cultured in growth medium, static stretch activated p38 along with ERK1/2, JNK and AKT. Disrupting TNF-alpha signaling by TNF-alpha-neutralizing antibody or knocking out TNF-alpha receptors blocked stretch activation of p38, but not ERK1/2, JNK or AKT. Stretch also activated differentiation markers MEF2C, myogenin, p21 and myosin heavy chain in a TNF-alpha- and p38-dependent manner. Stretch stimulated the cleavage activity of TACE. Conversely, TACE inhibitor TAPI or TACE siRNA abolished stretch activation of p38. In addition, conditioned medium from stretched myoblast cultures activated p38 in unstretched myoblasts, which required TACE activity in the donor myoblasts, and TNF-alpha receptors in the recipient myoblasts. These results indicate that posttranscriptional activation of TACE mediates the mechanotransduction that activates p38-dependent myogenesis via the release of TNF-alpha.

Prolonged superficial local cryotherapy attenuates microcirculatory impairment, regional inflammation, and muscle necrosis after closed soft tissue injury in rats

BACKGROUND

Closed soft tissue injury induces progressive microvascular dysfunction and regional inflammation. The authors tested the hypothesis that adverse trauma-induced effects can be reduced by local cooling. While superficial cooling reduces swelling, pain, and cellular oxygen demand, the effects of cryotherapy on posttraumatic microcirculation are incompletely understood.

STUDY DESIGN

Controlled laboratory study.

METHODS

After a standardized closed soft tissue injury to the left tibial compartment, male rats were randomly subjected to percutaneous perfusion for 6 hours with 0.9% NaCL (controls; room temperature) or cold NaCL (cryotherapy; 8 degrees C) (n = 7 per group). Uninjured rats served as shams (n = 7). Microcirculatory changes and leukocyte adherence were determined by intravital microscopy. Intramuscular pressure was measured, and invasion of granulocytes and macrophages was assessed by immunohistochemistry. Edema and tissue damage was quantified by gravimetry and decreased desmin staining.

RESULTS

Closed soft tissue injury significantly decreased functional capillary density (240 +/- 12 cm(-1)); increased microvascular permeability (0.75 +/- 0.03), endothelial leukocyte adherence (995 +/- 77/cm(2)), granulocyte (182.0 +/- 25.5/mm(2)) and macrophage infiltration, edema formation, and myonecrosis (ratio: 2.95 +/- 0.45) within the left extensor digitorum longus muscle. Cryotherapy for 6 hours significantly restored diminished functional capillary density (393 +/- 35), markedly decreased elevated intramuscular pressure, reduced the number of adhering (462 +/- 188/cm(2)) and invading granulocytes (119 +/- 28), and attenuated tissue damage (ratio: 1.7 +/- 0.17).

CONCLUSION

The hypothesis that prolonged cooling reduces posttraumatic microvascular dysfunction, inflammation, and structural impairment was confirmed.

CLINICAL RELEVANCE

These results may have therapeutic implications as cryotherapy after closed soft tissue injury is a valuable therapeutic approach to improve nutritive perfusion and attenuate leukocyte-mediated tissue destruction. The risk for evolving compartment syndrome may be reduced, thereby preventing further irreversible aggravation.

Notexin causes greater myotoxic damage and slower functional repair in mouse skeletal muscles than bupivacaine

Although the myotoxins bupivacaine and notexin are employed for studying processes that regulate muscle regeneration after injury, no studies have compared their efficacy in causing muscle damage or assessing functional regeneration in mouse skeletal muscles. Bupivacaine causes extensive injury in rat muscles but its effects on mouse muscles are variable. We compared functional and morphological properties of regenerating mouse extensor digitorum longus (EDL) muscles after notexin or bupivacaine injection and tested the hypothesis that muscle damage would be more extensive and functional repair less complete after notexin injection. Bupivacaine caused degeneration of 45% of fibers and reduced maximum force (Po) to 42% of control after 3 days. In contrast, notexin caused complete fiber breakdown and loss of functional capacity after 3 days (P < 0.05). At 7 and 10 days after bupivacaine, Po was restored to 65% and 71% of control, respectively, whereas Po of notexin-injected muscles was only 10% and 39% of control at these time-points, respectively (P < 0.05). At 7 and 10 days after bupivacaine, approximately 30% of fibers were centrally nucleated (regenerating), whereas notexin-injected muscles were comprised entirely of regenerating fibers (P < 0.05). The results demonstrate that notexin causes a more extensive and complete injury than bupivacaine, and is a useful model for studying muscle regeneration in mice.

TNF-alpha regulates myogenesis and muscle regeneration by activating p38 MAPK

Although p38 MAPK activation is essential for myogenesis, the upstream signaling mechanism that activates p38 during myogenesis remains undefined. We recently reported that p38 activation, myogenesis, and regeneration in cardiotoxin-injured soleus muscle are impaired in TNF-alpha receptor double-knockout (p55(-/-)p75(-/-)) mice. To fully evaluate the role of TNF-alpha in myogenic activation of p38, we tried to determine whether p38 activation in differentiating myoblasts requires autocrine TNF-alpha, and whether forced activation of p38 rescues impaired myogenesis and regeneration in the p55(-/-)p75(-/-) soleus. We observed an increase of TNF-alpha release from C2C12 or mouse primary myoblasts placed in low-serum differentiation medium. A TNF-alpha-neutralizing antibody added to differentiation medium blocked p38 activation and suppressed differentiation markers myocyte enhancer factor (MEF)-2C, myogenin, p21, and myosin heavy chain in C2C12 myoblasts. Conversely, recombinant TNF-alpha added to differentiation medium stimulated myogenesis at 0.05 ng/ml while inhibited it at 0.5 and 5 ng/ml. In addition, differentiation medium-induced p38 activation and myogenesis were compromised in primary myoblasts prepared from p55(-/-)p75(-/-) mice. Increased TNF-alpha release was also seen in cardiotoxin-injured soleus over the course of regeneration. Forced activation of p38 via the constitutive activator of p38, MKK6bE, rescued impaired myogenesis and regeneration in the cardiotoxin-injured p55(-/-)p75(-/-) soleus. These results indicate that TNF-alpha regulates myogenesis and muscle regeneration as a key activator of p38.

The expression of the neonatal sarcoplasmic reticulum Ca2+ pump (SERCA1b) hints to a role in muscle growth and development

The neonatal isoform of sarcoplasmic/endoplasmic reticulum Ca2+ ATPase 1 (SERCA1b) is a Ca2+ pump with a well-known developmentally regulated transcript level but an undefined protein expression and function. Specific antibodies were generated to show that SERCA1b is exclusively expressed in myoblasts and myotubes of cultured and regenerating muscle. However, the SERCA1b protein was not detectable in normal adult fast and slow muscles. Studies of the in vitro differentiating myogenic cell lines C2C12 and sol8 showed that SERCA1b is the main SERCA1 protein isoform induced during differentiation and that it is found in the myotubes. Remarkably in BC3H1 cells, which show incomplete differentiation and are reluctant to form myotubes, express the SERCA1b mRNA but not the corresponding protein. SERCA1b protein was also absent from stretched or denervated adult soleus, in spite of the fact that its mRNA level was upregulated. SERCA1b accounts for nearly the total of SERCA1 expression in the diaphragm of newborn mice, which suggests that the insufficient function and development of the diaphragm in the SERCA1 null mutant mice may be due to the lack of SERCA1b. Our studies point to an important regulation of SERCA1b expression at the protein level and hints to a role in the growth of the developing muscle.

Tumor necrosis factor receptor-mediated apoptosis in Trichinella spiralis-infected muscle cells

In order to reveal the mechanisms underlying nurse cell formation during Trichinella spiralis infection, the expression of the factors of tumor necrosis factor-alpha (TNF-alpha)/TNF receptor 1 (TNFR-1) signalling pathway mediating apoptosis was investigated. The analysed factors included TNF-alpha, TNFR-1, TNF receptor-associated death-domain (TRADD), caspase 3, caspase 8, TNF receptor associated factor-2 (TRAF2) and receptor interactive protein (RIP), all of which are involved in the TNF-alpha/TNFR-1 signalling pathway-mediated apoptosis. The quantitative RT-PCR indicated that the infected muscle tissues up-regulate the expression of pro-apoptosis genes (TNF-alpha, TNFR-1 and TRADD, caspase 3 and caspase 8), and anti-apoptosis genes (TRAF2 and RIP) at the beginning of cyst formation. The expression returned to the normal level after cyst formation. The quantitative RT-PCR analysis of mRNA from tissue samples isolated by laser capture micro-dissection confirmed that the up-regulation of these genes was restricted in infected muscle cells, was not in the inflammation cells around infected muscle cells nor in normal muscle cells. The in situ localization study of proapoptosis (TRADD, caspase 3) and anti-apoptosis gene products (TRAF2) indicated that these were expressed in the basophilic cytoplasm (infected muscle cell origin) of the nurse cells. Thus the present study suggests that the TNF-alpha/ TNFR-1 signalling pathway is involved in nurse cell formation.

Role of TNF-{alpha} signaling in regeneration of cardiotoxin-injured muscle

Recent data suggest a physiological role for the proinflammatory cytokine TNF-alpha in skeletal muscle regeneration. However, the underlying mechanism is not understood. In the present study, we analyzed TNF-alpha-activated signaling pathways involved in myogenesis in soleus muscle injured by cardiotoxin (CTX) in TNF-alpha receptor double-knockout mice (p55(-/-)p75(-/-)). We found that activation of p38MAPK, which is critical for myogenesis, was blocked in CTX-injured p55(-/-)p75(-/-) soleus on day 3 postinjury when myogenic differentiation was being initiated, while activation of ERK1/2 and JNK MAPK, as well as transcription factor NF-kappaB, was not reduced. Consequently, the phosphorylation of transcription factor myocyte enhancer factor-2C, which is catalyzed by p38 and crucial for the expression of muscle-specific genes, was blunted. Meanwhile, expression of p38-dependent differentiation marker myogenin and p21 were suppressed. In addition, expression of cyclin D1 was fivefold that in wild-type (WT) soleus. These results suggest that myogenic differentiation is blocked or delayed in the absence of TNF-alpha signaling. Histological studies revealed abnormalities in regenerating p55(-/-)p75(-/-) soleus. On day 5 postinjury, new myofiber formation was clearly observed in WT soleus but not in p55(-/-)p75(-/-) soleus. To the contrary, p55(-/-)p75(-/-) soleus displayed renewed inflammation and dystrophic calcification. On day 12 postinjury, the muscle architecture of WT soleus was largely restored. Yet, in p55(-/-)p75(-/-) soleus, multifocal areas of inflammation, myofiber death, and myofibers with smaller cross-sectional area were observed. Functional studies demonstrated an attenuated recovery of contractile force in injured p55(-/-)p75(-/-) soleus. These data suggest that TNF-alpha signaling plays a critical regulatory role in muscle regeneration.

The calcineurin activity and MCIP1.4 mRNA levels are increased by innervation in regenerating soleus muscle

The level of active subunit of calcineurin and the calcineurin (Cn) enzyme activity are increased in innervated but not in denervated slow type regenerating skeletal soleus muscle. These nerve-dependent increases were not accompanied by similar increases in the mRNA levels. The changes in the mRNA level of the modulatory calcineurin interacting protein, MCIP1.4, reflected the calcineurin activity and did not increase in denervated regenerating muscles compared to the innervated regenerating controls. The increases in Cn activity and in MCIP1.4 mRNA levels occurred before the switch from fast to slow-type myosin heavy chain isoforms, a phenomenon similarly known to be dependent on innervation. This highlights the role of mediators, acting between the nerve and calcineurin, in the formation of slow fiber identity.

TNF-alpha is a mitogen in skeletal muscle

Emerging evidence suggests that tumor necrosis factor (TNF)-alpha plays a role in muscle repair. To determine whether TNF-alpha modulates satellite cell proliferation, the current study evaluated TNF-alpha effects on DNA synthesis in primary myoblasts and on satellite cell activation in adult mouse muscle. Exposure to recombinant TNF-alpha increased total DNA content in rat primary myoblasts dose-dependently over a 24-h period and increased the number of primary myoblasts incorporating 5-bromo-2'-deoxyuridine (BrdU) during a 30-min pulse labeling. Systemic injection of TNF-alpha stimulated BrdU incorporation by satellite cells in muscles of adult mice, whereas no BrdU was incorporated by satellite cells in control mice. TNF-alpha stimulated serum response factor (SRF) binding to the serum response element (SRE) present in the c-fos gene promoter and stimulated reporter gene expression controlled by the same element. Our data suggest that TNF-alpha activates satellite cells to enter the cell cycle and accelerates G1-to-S phase transition, and these actions may involve activation of early response genes via SRF.

Physiological role of tumor necrosis factor alpha in traumatic muscle injury

degenerative and regenerative roles of tumor necrosis factor alpha (TNF-alpha), a pro-inflammatory cytokine with pleiotropic functions, were investigated by using TNF receptor 1 and 2 double knockout (TNFR-DKO) and TNF-alpha antibody neutralized mice following traumatic freeze injury to the tibialis anterior muscle. In wild-type control mice, TNF-alpha mRNA transcripts and protein increased following injury and gradually returned to control (uninjured) levels by 13 days. A reduction in MyoD mRNA expression occurred in TNF-alpha-deficient mice, although there were no visible differences in MyoD immunostaining or histological characteristics in regenerating muscles. At 5 days post-injury, the reductions in isometric strength in TNFR-DKO and TNF-alpha-depleted mice did not differ from that of wild-type mice but by 13 days after injury, the TNFR-DKO and TNF-alpha-depleted mice exhibited strength deficits twice that of wild-type mice (i.e., 27-31% vs 13%). Muscle injury was also accompanied by increased expression of interleukin-6 (IL-6), but IL-6-deficient mice demonstrated MyoD expression and recovery of isometric strength similar to that of wild-type mice. These data indicate that TNF-alpha is involved in the recovery of muscle function after traumatic muscle injury, and this effect might be associated with modulation of muscle regulatory genes, including MyoD.

Tristetraprolin and LPS-inducible CXC chemokine are rapidly induced in presumptive satellite cells in response to skeletal muscle injury

Myogenic precursor cells known as satellite cells persist in adult skeletal muscle and are responsible for its ability to regenerate after injury. Quiescent satellite cells are activated by signals emanating from damaged muscle. Here we describe the rapid activation of two genes in response to muscle injury; these transcripts encode LPS-inducible CXC chemokine (LIX), a neutrophil chemoattractant, and Tristetraprolin (TTP), an RNA-binding protein implicated in the regulation of cytokine expression. Using a synchronized cell culture model we show that C2C12 myoblasts arrested in G0 exhibit some molecular attributes of satellite cells in vivo: suppression of MyoD and Myf5 expression during G0 and their reactivation in G1. Synchronization also revealed cell cycle dependent expression of CD34, M-cadherin, HGF and PEA3, genes implicated in satellite cell biology. To identify other genes induced in synchronized C2C12 myoblasts we used differential display PCR and isolated LIX and TTP cDNAs. Both LIX and TTP mRNAs are short-lived, encode molecules implicated in inflammation and are transiently induced during growth activation in vitro. Further, LIX and TTP are rapidly induced in response to muscle damage in vivo. TTP expression precedes that of MyoD and is detected 30 minutes after injury. The spatial distribution of LIX and TTP transcripts in injured muscle suggests expression by satellite cells. Our studies suggest that in addition to generating new cells for repair, activated satellite cells may be a source of signaling molecules involved in tissue remodeling during regeneration.

Antisense inhibition of myoD expression in regenerating rat soleus muscle is followed by an increase in the mRNA levels of myoD, myf-5 and myogenin and by a retarded regeneration

It has been reported that muscles of myoD-/- mice present a lower potential to regenerate, but there are no reports on the effect of acute interference with myoD expression limited in space and time to only a particular regenerating muscle. Here we relied on antisense inhibition of this factor. Four different oligos were tested. The suppression of regeneration indices (the expression of desmin, the formation of myotubes and the initiation of endplates) was the most pronounced, with the oligomer targeting a region encompassing the translation start site of myoD. A mixed backbone phosphorothioate-phosphate diester oligo (200 microl at 20 microM) was still detectable in the muscles 1 h after its administration and reverse transcription-polymerase chain reaction (RT-PCR) analysis showed that the level of the targeted 5' end of the myoD mRNA was selectively decreased. The level of myoD protein was also lowered. Four hours after the antisense treatment, when the oligos were no longer detectable, the myoD mRNA level was restored and 24 h later it exceeded controls together with that of myf-5 and myogenin. After 4 weeks, the antisense-treated soleus muscles were similar to the control-treated and the untreated regenerated soleus with respect to fiber types and motor endplates, however, they contained smaller fibers which reflected the asynchronity of regeneration. This shows that successfully targeted simple antisense oligonucleotides can be used as selective tools for inhibition of individual factors in studying the process of muscle regeneration.

Increased MAPK signaling during in vitro muscle wounding

Regeneration of skeletal muscle upon injury is a complex process, involving activation of satellite cells, followed by migration, fusion, and regeneration of damaged myofibers. Previous work concerning the role of the mitogen activated protein (MAP) kinase signaling pathways in muscle injury comes primarily from studies using chemically induced wounding. The purpose of this study was to test the hypothesis that physical injury to skeletal muscle cells in vitro activates the MAP kinase signaling pathways. We demonstrate that extracellular signal regulated kinases (ERKs) 1, 2, and p38 are rapidly and transiently activated in response to injury in C2C12 cells, and are primarily localized to cells adjacent to the wound bed. Culture medium from wounded cells is able to stimulate activation of p38 but not ERK in unwounded cells. These results suggest that both ERK and p38 are involved in the response of muscle cells to physical injury in culture, and reflect what is seen in whole tissues in vivo.