Matrix metalloproteinases and skeletal muscle: A brief review

The response of matrix metalloproteinase-9 and -2 to exercise

BACKGROUND

Matrix metalloproteinases (MMPs) are a major group of enzymes that play essential roles in normal functioning of diverse tissues during growth, development, and aging. However, among the MMPs little is known regarding the role of exercise in MMP-9 and MMP-2 function in humans.

OBJECTIVE

The aim of this study was to provide a systematic comprehensive review of the literature examining the effect of different exercise interventions on MMP-9 and MMP-2 in human investigations.

DATA SOURCES

A comprehensive systematic database search was performed, including PubMed/MEDLINE, Scopus, ScienceDirect, and Web of Science.

STUDY SELECTION

Both the acute and chronic effects of exercise were included for evaluation in this systematic review. Inclusion criteria included the use of any type of planned, structured, and repetitive movement and its effects on the MMP-2 and MMP-9 response (obtained from plasma samples), participants (humans only) of any age with or without diseases, sedentary participants and those involved in light, moderate, and vigorous activity, randomized controlled trials (RCTs) and clinical trials (CTs), full text article citations with no restrictions in terms of language, and scored at least 5/11 on the Physiotherapy Evidence Database (PEDro) quality scale.

STUDY APPRAISAL AND SYNTHESIS METHODS

The PEDro scale was used to appraise study quality of RCTs and CTs. Two reviewers independently reviewed the full texts of all potentially relevant articles for eligibility and disagreements were discussed and resolved.

RESULTS

Seven studies met the previously determined quality indicators and were reviewed; three were RCTs and four were CTs. In general, the quality of the studies ranged from 5 to 9 out of a maximum of 11 on the PEDro quality criteria scale. Results revealed that chronic aerobic training induces a decrease in MMP-9 and MMP-2 levels, possibly indicating a cardioprotective effect, while resistance exercise training displayed conflicting results.

CONCLUSION

Alterations in MMP-9 and MMP-2 plasma concentrations may be valuable biomarkers to reflect the influence of exercise on the inflammatory state. Nevertheless, the limited evidence available regarding the effects of exercise on the MMP-9 and MMP-2 response in human participants suggests that further studies are needed to fully define the connection between the role of exercise on the MMP-9 and MMP-2 response.

Alcohol Differentially Alters Extracellular Matrix and Adhesion Molecule Expression in Skeletal Muscle and Heart

BACKGROUND

The production of fibrosis in response to chronic alcohol abuse is well recognized in liver but has not been fully characterized in striated muscle and may contribute to functional impairment. Therefore, the purpose of this study was to use an unbiased discovery-based approach to determine the effect of chronic alcohol consumption on the expression profile of genes important for cell-cell and cell-extracellular matrix (ECM) interactions in both skeletal and cardiac muscle.

METHODS

Adult male rats were pair-fed an alcohol-containing liquid diet or control diet for 24 weeks, and skeletal muscle (gastrocnemius) and heart were collected in the freely fed state. A pathway-focused gene expression polymerase chain reaction array was performed on these tissues to assess mRNA content for 84 ECM proteins, and selected proteins were confirmed by Western blot analysis.

RESULTS

In gastrocnemius, alcohol feeding up-regulated the expression of 11 genes and down-regulated the expression of 1 gene. Alcohol increased fibrosis as indicated by increased mRNA and/or protein for collagens α1(I), α2(I), α1(III), and α2(IV) as well as hydroxyproline. Alcohol also increased α-smooth muscle actin protein, an index of myofibroblast activation, but no concomitant change in transforming growth factor-β was detected. The mRNA and protein content for other ECM components, such as integrin-α5, L-selectin, PECAM, SPARC, and ADAMTS2, were also increased by alcohol. Only laminin-α3 mRNA was decreased in gastrocnemius from alcohol-fed rats, while 66 ECM- or cell adhesion-related mRNAs were unchanged by alcohol. For heart, expression of 16 genes was up-regulated, expression of 3 genes was down-regulated, and 65 mRNAs were unchanged by alcohol; there were no common alcohol-induced gene expression changes between heart and skeletal muscle. Finally, alcohol increased tumor necrosis factor-α and interleukin (IL)-12 mRNA in both skeletal and cardiac muscle, but IL-6 mRNA was increased and IL-10 mRNA decreased only in skeletal muscle.

CONCLUSIONS

These data demonstrate a fibrotic response in striated muscle from chronic alcohol-fed rats which is tissue specific in nature, suggesting different regulatory mechanisms.

Messenger RNA sequencing and pathway analysis provide novel insights into the biological basis of chickens' feed efficiency

Background

Advanced selection technologies have been developed and continually optimized to improve traits of agricultural importance; however, these methods have been primarily applied without knowledge of underlying biological changes that may be induced by selection. This study aims to characterize the biological basis of differences between chickens with low and high feed efficiency (FE) with a long-term goal of improving the ability to select for FE.

Results

High-throughput RNA sequencing was performed on 23 breast muscle samples from commercial broiler chickens with extremely high (n = 10) and low (n = 13) FE. An average of 34 million paired-end reads (75 bp) were produced for each sample, 80% of which were properly mapped to the chicken reference genome (Ensembl Galgal4). Differential expression analysis identified 1,059 genes (FDR < 0.05) that significantly divergently expressed in breast muscle between the high- and low-FE chickens. Gene function analysis revealed that genes involved in muscle remodeling, inflammatory response and free radical scavenging were mostly up-regulated in the high-FE birds. Additionally, growth hormone and IGFs/PI3K/Akt signaling pathways were enriched in differentially expressed genes, which might contribute to the high breast muscle yield in high-FE birds and partly explain the FE advantage of high-FE chickens.

Conclusions

This study provides novel insights into transcriptional differences in breast muscle between high- and low-FE broiler chickens. Our results show that feed efficiency is associated with breast muscle growth in these birds; furthermore, some physiological changes, e.g., inflammatory response and oxidative stress, may occur in the breast muscle of the high-FE chickens, which may be of concern for continued selection for both of these traits together in modern broiler chickens.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1364-0) contains supplementary material, which is available to authorized users.

Effects of low-level laser therapy on skeletal muscle repair: a systematic review

A review of the literature was performed to demonstrate the most current applicability of low-level laser therapy (LLLT) for the treatment of skeletal muscle injuries, addressing different lasers, irradiation parameters, and treatment results in animal models. Searches were performed in the PubMed/MEDLINE, SCOPUS, and SPIE Digital Library databases for studies published from January 2006 to August 2013 on the use of LLLT for the repair of skeletal muscle in any animal model. All selected articles were critically appraised by two independent raters. Seventeen of the 36 original articles on LLLT and muscle injuries met the inclusion criteria and were critically evaluated. The main effects of LLLT were a reduction in the inflammatory process, the modulation of growth factors and myogenic regulatory factors, and increased angiogenesis. The studies analyzed demonstrate the positive effects of LLLT on the muscle repair process, which are dependent on irradiation and treatment parameters. The findings suggest that LLLT is an excellent therapeutic resource for the treatment of skeletal muscle injuries in the short-term.

Exercise-induced capillary growth in human skeletal muscle and the dynamics of VEGF

In skeletal muscle, growth of capillaries is an important adaptation to exercise training that secures adequate diffusion capacity for oxygen and nutrients even at high-intensity exercise when increases in muscle blood flow are profound. Mechanical forces present during muscle activity, such as shear stress and passive stretch, lead to cellular signaling, enhanced expression of angiogenic factors, and initiation of capillary growth. The most central angiogenic factor in skeletal muscle capillary growth is VEGF. During muscle contraction, VEGF increases in the muscle interstitium, acts on VEGF receptors on the capillary endothelium, and thereby stimulates angiogenic processes. A primary source of muscle interstitial VEGF during exercise is the skeletal muscle fibers which contain large stores of VEGF within vesicles. We propose that, during muscle activity, these VEGF-containing vesicles are redistributed toward the sarcolemma where the contents are secreted into the extracellular fluid. VEGF mRNA expression is increased primarily after exercise, which allows for a more rapid replenishment of VEGF stores lost through secretion during exercise. Future studies should focus on elucidating mechanisms and regulation of VEGF secretion.

Stem cell activation in skeletal muscle regeneration

Muscle stem cell (satellite cell) activation post muscle injury is a transient and critical step in muscle regeneration. It is regulated by physiological cues, signaling molecules, and epigenetic regulatory factors. The mechanisms that coherently turn on the complex activation process shortly after trauma are just beginning to be illuminated. In this review, we will discuss the current knowledge of satellite cell activation regulation.

Acute eccentric resistance exercise decreases matrix metalloproteinase activity in obese elderly women

The association of ageing with obesity commits elderly women and has been correlated with multiple degenerative processes, which could be occasioned by an enhancing in levels of matrix metalloproteinase-2 and metalloproteinase-9 (MMPs) as well by an cytokine unbalance that included an enhancing on interleukin-6 (IL-6). Furthermore, other factors could be also related to degenerative process, as they could be reduced by eccentric resistance exercise (ERE), which seems particularly important to initiate resistance training in obese older adults. In this view, this study aims to determinate the effects of an acute ERE session on serum MMP-2, MMP-9 and IL-6 in elderly obese women. Ten elderly obese women participated in this study and completed a 10 repetitions maximum test (10 RM) utilizing leg extension exercise. Subjects then completed an acute ERE session consisting of seven sets of 10 repetitions at 110% of 10 RM with a rest of 3 min between sets. Blood samples were collected before, immediately after, 3, 24 and 48 h following the ERE session. Zymograms were utilized to measure the MMP-2 and MMP-9 enzymes from all individuals. Moreover, IL-6 concentration was also determinated. After ERE session, MMP-2 and MMP-9 decreased, remaining significantly below baseline values after 48 h (P<0·05). Although not statistically significant, there was a tendency for IL-6 to decrease 48 h after the ERE when compared with 3 h (P = 0·06). An acute ERE session decreases MMP-9, MMP-2 and IL-6 in elderly obese women, possibly indicating a transient protection against the low grade inflammation present in this specific population.

Resistance exercise increases active MMP and β1-integrin protein expression in skeletal muscle

Recent studies indicate that matrix metalloproteinases (MMPs) and critical linkage proteins in the extracellular matrix (ECM) regulate skeletal muscle mass, although the effects of resistance training (RT) on protein expression and activity are unclear. Thus, the purpose of the present study was to investigate the effects of RT on MMP activity and expression of ECM-related proteins. Ten male Sprague-Dawley rats were randomly assigned to 1 bout (1B) or 18 bouts (18B) of electrical stimulation. The right gastrocnemius muscle was isometrically contracted via percutaneous electrical stimulation (five sets of 5 sec stimulation × five contractions/set with 5 sec interval between contractions and 3 min rest between sets) once (1B) or every other day for 5 weeks (18B). The left leg served as a control. Activity of MMP-2 and MMP-9, determined via gelatin zymography, was increased (P < 0.05) immediately after 1B. However, MMP activation was not evident following 18B. No changes in collagen IV, laminin α2, α7-integrin, or ILK protein expression were detected immediately following 1B or 18B. However, β1-integrin protein expression was significantly increased (P < 0.05) with 18B. Our results suggest that resistance exercise activates MMPs during the initial phase of RT but this response is attenuated with continuation of RT.

Morphological aspects and Cox-2 expression after exposure to 780-nm laser therapy in injured skeletal muscle: an in vivo study

Background:

The effectiveness of low-level laser therapy in muscle regeneration is still not well known. Objective: To investigate the effects of laser irradiation during muscle healing.

Method:

For this purpose, 63 rats were distributed to 3 groups: non-irradiated control group (CG); group irradiated at 10 J/cm² (G10); and group irradiated at 50 J/cm² (G50). Each group was divided into 3 different subgroups (n=7), and on days 7, 14 and 21 post-injury the rats were sacrificed.

Results:

Seven days post-surgery, the CG showed destroyed zones and extensive myofibrillar degeneration. For both treated groups, the necrosis area was smaller compared to the CG. On day 14 post-injury, treated groups demonstrated better tissue organization, with newly formed muscle fibers compared to the CG. On the 21st day, the irradiated groups showed similar patterns of tissue repair, with improved muscle structure at the site of the injury, resembling uninjured muscle tissue organization. Regarding collagen deposition, the G10 showed an increase in collagen synthesis. In the last period evaluated, both treated groups showed statistically higher values in comparison with the CG. Furthermore, laser irradiation at 10 J/cm2 produced a down-regulation of cyclooxygenase 2 (Cox-2) immunoexpression on day 7 post-injury. Moreover, Cox-2 immunoexpression was decreased in both treated groups on day 14.

Conclusions:

Laser therapy at both fluencies stimulated muscle repair through the formation of new muscle fiber, increase in collagen synthesis, and down-regulation of Cox-2 expression.

MMP-14 is necessary but not sufficient for invasion of three-dimensional collagen by human muscle satellite cells

The twenty-five known matrix metalloproteases (MMPs) and their endogenous inhibitors, tissue inhibitors of metalloproteases (TIMPs), mediate cell invasion through the extracellular matrix (ECM). In a comparative three-dimensional assay, we analyzed human and mouse satellite cells' competence to invade an artificial ECM (collagen I). We identified a single MMP that 1) is expressed by human muscle satellite cells; 2) is induced at the mRNA/protein level by adhesion to collagen I; and 3) is necessary for invasion into a collagen I matrix. Interestingly, murine satellite cells neither express this MMP, nor invade the collagen matrix. However, exogenous human MMP-14 is not sufficient to induce invasion of a collagen matrix by murine cells, emphasizing species differences.

Adiponectin as a tissue regenerating hormone: more than a metabolic function

The great interest that scientists have for adiponectin is primarily due to its central metabolic role. Indeed, the major function of this adipokine is the control of glucose homeostasis that it exerts regulating liver and muscle metabolism. Adiponectin has insulin-sensitizing action and leads to down-regulation of hepatic gluconeogenesis and an increase of fatty acid oxidation. In addition, adiponectin is reported to play an important role in the inhibition of inflammation. The hormone is secreted in full-length form, which can either assemble into complexes or be converted into globular form by proteolytic cleavage. Over the past few years, emerging publications reveal a more varied and pleiotropic action of this hormone. Many studies emphasize a key role of adiponectin during tissue regeneration and show that adiponectin deficiency greatly inhibits the mechanisms underlying tissue renewal. This review deals with the role of adiponectin in tissue regeneration, mainly referring to skeletal muscle regeneration, a process in which adiponectin is deeply involved. In this tissue, globular adiponectin increases proliferation, migration and myogenic properties of both resident stem cells (namely satellite cells) and non-resident muscle precursors (namely mesoangioblasts). Furthermore, skeletal muscle could be a site for the local production of the globular form that occurs in an inflamed environment. Overall, these recent findings contribute to highlight an intriguing function of adiponectin in addition to its well-recognized metabolic action.

Cytoskeleton and Adhesion in Myogenesis

The function of muscle is to contract, which means to exert force on a substrate. The adaptations required for skeletal muscle differentiation, from a prototypic cell, involve specialization of housekeeping cytoskeletal contracting and supporting systems into crystalline arrays of proteins. Here I discuss the changes that all three cytoskeletal systems (microfilaments, intermediate filaments, and microtubules) undergo through myogenesis. I also discuss their interaction, through the membrane, to extracellular matrix and to other cells, where force will be exerted during contraction. The three cytoskeletal systems are necessary for the muscle cell and must exert complementary roles in the cell. Muscle is a responsive system, where structure and function are integrated: the structural adaptations it undergoes depend on force production. In this way, the muscle cytoskeleton is a portrait of its physiology. I review the cytoskeletal proteins and structures involved in muscle function and focus particularly on their role in myogenesis, the process by which this incredible muscle machine is made. Although the focus is on skeletal muscle, some of the discussion is applicable to cardiac and smooth muscle.

Masseter muscle myofibrillar protein synthesis and degradation in an experimental critical illness myopathy model

Critical illness myopathy (CIM) is a debilitating common consequence of modern intensive care, characterized by severe muscle wasting, weakness and a decreased myosin/actin (M/A) ratio. Limb/trunk muscles are primarily affected by this myopathy while cranial nerve innervated muscles are spared or less affected, but the mechanisms underlying these muscle-specific differences remain unknown. In this time-resolved study, the cranial nerve innervated masseter muscle was studied in a unique experimental rat intensive care unit (ICU) model, where animals were exposed to sedation, neuromuscular blockade (NMB), mechanical ventilation, and immobilization for durations varying between 6 h and 14d. Gel electrophoresis, immunoblotting, RT-PCR and morphological staining techniques were used to analyze M/A ratios, myofiber size, synthesis and degradation of myofibrillar proteins, and levels of heat shock proteins (HSPs). Results obtained in the masseter muscle were compared with previous observations in experimental and clinical studies of limb muscles. Significant muscle-specific differences were observed, i.e., in the masseter, the decline in M/A ratio and muscle fiber size was small and delayed. Furthermore, transcriptional regulation of myosin and actin synthesis was maintained, and Akt phosphorylation was only briefly reduced. In studied degradation pathways, only mRNA, but not protein levels of MuRF1, atrogin-1 and the autophagy marker LC3b were activated by the ICU condition. The matrix metalloproteinase MMP-2 was inhibited and protective HSPs were up-regulated early. These results confirm that the cranial nerve innervated masticatory muscles is less affected by the ICU-stress response than limb muscles, in accordance with clinical observation in ICU patients with CIM, supporting the model' credibility as a valid CIM model.

Effect of exercise training of different intensities on anti-inflammatory reaction in streptozotocin-induced diabetic rats

The study investigated the effect of high- and low-intensity exercise training on inflammatory reaction of blood and skeletal muscle in streptozotocin (STZ)-induced diabetic male Sprague-Dawley rats (243 ± 7 g, 8 weeks). The rats completed treadmill running in either high-intensity exercise (6 weeks of exercise training, acute bouts of exercise) or low-intensity exercise (6 weeks of exercise training). Non-running, sedentary rats served as controls. To induce diabetes mellitus, rats received a peritoneal injection of STZ (50 mg · kg⁻¹). Rats were sacrificed immediately after an acute bout of exercise and 6 weeks of exercise training. Inflammatory factors were analyzed by ELISA and by immune blotting from the soleus and extensor digitorum longus muscles. In the serum, inflammatory cytokines (IL-1β, TNF-α, IL-6, IL-4) and reactive oxygen species (ROS) (nitric oxide and malondialdehyde) increased in diabetic rats. However, all exercise training groups displayed reduced inflammatory cytokines and reactive oxygen species. In skeletal muscles, low-intensity exercise training, but not high intensity exercise, reduced the levels of COX-2, iNOS, and MMP-2, which were otherwise markedly elevated in the presence of STZ. Moreover, the levels of GLUT-4 and MyoD were effectively increased by different exercise intensity and exercise duration. Low-intensity exercise training appeared most effective to reduce diabetes-related inflammation. However, high-intensity training also reduced inflammatory factors in tissue-specific muscles. The data implicate regular exercise in protecting against chronic inflammatory diseases, such as diabetes.

Resistance training improves body composition and increases matrix metalloproteinase 2 activity in biceps and gastrocnemius muscles of diet-induced obese rats

OBJECTIVE

We investigated the influence of resistance training on body composition and matrix metalloproteinase 2 activity in skeletal muscles of rats fed a high-fat diet.

METHODS

Thirty-two Wistar rats were divided into four experimental groups (n = 8/each) according to diet and exercise status: Control (standard diet), Obese Control (high-fat diet), Resistance Training (standard diet) and Obese Resistance Training (high-fat diet) groups. Animals were fed a high-fat diet for 12 weeks to promote excessive weight gain. Resistance Training groups performed 12 weeks of training periods after this period in a vertical ladder three times/week. Fat percentage, fat-free mass and fat mass were assessed using dual-energy X-ray absorptiometry, and matrix metalloproteinase 2 activity in biceps and gastrocnemius muscles was analyzed using zymography.

RESULTS

Resistance training significantly reduced body and fat masses and fat percentages in both trained groups (p<0.05). The maximal carrying load between trained groups was not different, but relative force was higher in the Resistance Training group (p<0.05). Of note, increased matrix metalloproteinase 2 activity was noted in the tested muscles of both trained groups (p<0.05).

CONCLUSION

In conclusion, altered body composition and muscle matrix metalloproteinase 2 activity promoted by excessive weight gain were positively modified by resistance training.

The involvement of MMP-2 and MMP-9 in heart exercise-related angiogenesis

BACKGROUND

Little is known about the involvement of matrix metalloproteinases (MMPs) in cardiac vascular remodelling induced by exercise. Our aim was to evaluate and localize MMP-2 and MMP-9's activities in relation to capillary proliferation in mouse hearts trained for 15, 30 and 45 days.

METHODS

Sixty-three mice were randomly assigned to 7 groups: four control sedentary groups (C0, C15, C30 and C45) and three groups trained by an endurance protocol (T15, T30 and T45). MMP-2 and MMP-9 were examined with zymography and immunostaining analyses. Capillary proliferation was evaluated counting the number of CD31-positive cells.

RESULTS

Different activity patterns of the latent form of both MMPs were found. Pro-MMP-9 increased after 15 days of training; whereas pro-MMP-2 gradually decreased after 30 and 45 days of training below the control groups. The latter was inversely correlated with capillary growth. MMP-9 was mainly localized in myocardiocytes and less evident in capillaries. Conversely, MMP-2 was more intense in capillary endothelial cells and slightly in myocardiocytes.

CONCLUSIONS

A different spatiotemporal modulation of pro-MMP-2 and pro-MMP-9 activities has been detected in the myocardium during angiogenesis related to the aerobic training. These results can be useful to draw up training protocols for improving the performance of healthy and diseased human hearts.

Matrix metalloproteinase-9 inhibition improves proliferation and engraftment of myogenic cells in dystrophic muscle of mdx mice

Duchenne muscular dystrophy (DMD) caused by loss of cytoskeletal protein dystrophin is a devastating disorder of skeletal muscle. Primary deficiency of dystrophin leads to several secondary pathological changes including fiber degeneration and regeneration, extracellular matrix breakdown, inflammation, and fibrosis. Matrix metalloproteinases (MMPs) are a group of extracellular proteases that are involved in tissue remodeling, inflammation, and development of interstitial fibrosis in many disease states. We have recently reported that the inhibition of MMP-9 improves myopathy and augments myofiber regeneration in mdx mice (a mouse model of DMD). However, the mechanisms by which MMP-9 regulates disease progression in mdx mice remain less understood. In this report, we demonstrate that the inhibition of MMP-9 augments the proliferation of satellite cells in dystrophic muscle. MMP-9 inhibition also causes significant reduction in percentage of M1 macrophages with concomitant increase in the proportion of promyogenic M2 macrophages in mdx mice. Moreover, inhibition of MMP-9 increases the expression of Notch ligands and receptors, and Notch target genes in skeletal muscle of mdx mice. Furthermore, our results show that while MMP-9 inhibition augments the expression of components of canonical Wnt signaling, it reduces the expression of genes whose products are involved in activation of non-canonical Wnt signaling in mdx mice. Finally, the inhibition of MMP-9 was found to dramatically improve the engraftment of transplanted myoblasts in skeletal muscle of mdx mice. Collectively, our study suggests that the inhibition of MMP-9 is a promising approach to stimulate myofiber regeneration and improving engraftment of muscle progenitor cells in dystrophic muscle.

Serological muscle loss biomarkers: an overview of current concepts and future possibilities

BACKGROUND

The skeletal muscle mass is the largest organ in the healthy body, comprising 30-40 % of the body weight of an adult man. It confers protection from trauma, locomotion, ventilation, and it represents a "sink" in glucose metabolism and a reservoir of amino acids to other tissues such as the brain and blood cells. Naturally, loss of muscle has dire consequences for health as well as functionality. Muscle loss is a natural consequence of especially aging, inactivity, and their associated metabolic dysfunction, but it is strongly accelerated in critical illness such as organ failure, sepsis, or cancer. Whether this muscle loss is considered a primary or secondary condition, it is known that muscle loss is a symptom that predicts morbidity and mortality and one that is known to impact quality of life and independence. Therefore, monitoring of muscle mass is relevant in a number of pathologies as well as in clinical trials as measures of efficacy as well as safety.

METHODS AND RESULTS

Existing biomarkers of muscle mass or muscle loss have shown to be either too unreliable or too impractical in relation to the perceived clinical benefit to reach regular clinical research or use. We suggest serological neoepitope biomarkers as a possible technology to address some of these problems. Blood biomarkers of this kind have previously been shown to respond with high sensitivity and shorter time to minimum significant change than available biomarkers of muscle mass. We provide brief reviews of existing muscle mass or function biomarker technologies, muscle protein biology, and existing neoepitope biomarkers and proceed to present tentative recommendations on how to select and detect neoepitope biomarkers.

CONCLUSION

We suggest that serological peptide biomarkers whose tissue and pathology specificity are derived from post-translational modification of proteins in tissues of interest, presenting so-called neoepitopes, represents an exciting candidate technology to fill out an empty niche in biomarker technology.

Satellite cells and the muscle stem cell niche

Adult skeletal muscle in mammals is a stable tissue under normal circumstances but has remarkable ability to repair after injury. Skeletal muscle regeneration is a highly orchestrated process involving the activation of various cellular and molecular responses. As skeletal muscle stem cells, satellite cells play an indispensible role in this process. The self-renewing proliferation of satellite cells not only maintains the stem cell population but also provides numerous myogenic cells, which proliferate, differentiate, fuse, and lead to new myofiber formation and reconstitution of a functional contractile apparatus. The complex behavior of satellite cells during skeletal muscle regeneration is tightly regulated through the dynamic interplay between intrinsic factors within satellite cells and extrinsic factors constituting the muscle stem cell niche/microenvironment. For the last half century, the advance of molecular biology, cell biology, and genetics has greatly improved our understanding of skeletal muscle biology. Here, we review some recent advances, with focuses on functions of satellite cells and their niche during the process of skeletal muscle regeneration.

Systems analysis of biological networks in skeletal muscle function

Skeletal muscle function depends on the efficient coordination among subcellular systems. These systems are composed of proteins encoded by a subset of genes, all of which are tightly regulated. In the cases where regulation is altered because of disease or injury, dysfunction occurs. To enable objective analysis of muscle gene expression profiles, we have defined nine biological networks whose coordination is critical to muscle function. We begin by describing the expression of proteins necessary for optimal neuromuscular junction function that results in the muscle cell action potential. That action potential is transmitted to proteins involved in excitation-contraction coupling enabling Ca(2+) release. Ca(2+) then activates contractile proteins supporting actin and myosin cross-bridge cycling. Force generated by cross-bridges is transmitted via cytoskeletal proteins through the sarcolemma and out to critical proteins that support the muscle extracellular matrix. Muscle contraction is fueled through many proteins that regulate energy metabolism. Inflammation is a common response to injury that can result in alteration of many pathways within muscle. Muscle also has multiple pathways that regulate size through atrophy or hypertrophy. Finally, the isoforms associated with fast muscle fibers and their corresponding isoforms in slow muscle fibers are delineated. These nine networks represent important biological systems that affect skeletal muscle function. Combining high-throughput systems analysis with advanced networking software will allow researchers to use these networks to objectively study skeletal muscle systems.

Interleukin-17 modulates myoblast cell migration by inhibiting urokinase type plasminogen activator expression through p38 mitogen-activated protein kinase

Interleukin-17 belongs to a family of pro-inflammatory cytokines with pleiotropic effects, which can be associated with several inflammatory diseases of the muscle tissue. Although elevated levels of interleukin-17 have been described in inflammatory myopathies, its role in muscle homeostasis remains to be elucidated. The requirement of the urokinase type plasminogen activator in skeletal myogenesis was recently demonstrated in vivo and in vitro, suggesting its involvement in the regulation of extracellular matrix remodeling, cell migration and myoblast fusion. Our previous results have demonstrated that interleukin-17 inhibits myogenic differentiation of C2C12 myoblasts in vitro concomitantly with the inhibition of cell migration. However, the involvement of urokinase type plasminogen activator in interleukin-17-inhibited myogenesis and migration remained to be analyzed. Therefore, the effect of interleukin-17 on the production of urokinase type plasminogen activator by C2C12 myoblasts was determined in the present study. Our results demonstrated that interleukin-17 strongly inhibits urokinase type plasminogen activator expression during myogenic differentiation. This reduction of urokinase type plasminogen activator production corresponded with the inhibition of cell migration by interleukin-17. Activation of p38 signaling pathway elicited by interleukin-17 mediated the inhibition of both urokinase type plasminogen activator expression and cell migration. Additionally, IL-17 inhibited C2C12 cells migration by causing the cells to reorganize their cytoskeleton and lose polarity. Therefore, our results suggest a novel mechanism by which interleukin-17 regulates myogenic differentiation through the inhibition of urokinase type plasminogen activator expression and cell migration. Accordingly, interleukin-17 may represent a potential clinical target worth investigating for the treatment of inflammatory muscle diseases.

Hyaluronic acid, HAS1, and HAS2 are significantly upregulated during muscle hypertrophy

Hyaluronic acid (HA) is a component of the extracellular matrix (ECM) in most vertebrate tissues and is thought to play a significant role during development, wound healing, and regeneration. In vitro studies have shown that HA enhances muscle progenitor cell recruitment and inhibits premature myotube fusion, implicating a role for this glycosaminoglycan in functional repair. However, the spatiotemporal distribution of HA during muscle growth and repair was unknown. We hypothesized that inducing hypertrophy via synergist ablation would increase the expression of HA and the HA synthases (HAS1-HAS3). We found that HA and HAS1-HAS3 were significantly upregulated within the plantaris muscle in response to Achilles tenectomy. HA concentration significantly increased 2.8-fold after 2 days but decreased towards levels comparable to age-matched controls by 14 days. Using immunohistochemistry, we found the colocalization of HAS1-HAS3 with macrophages, blood vessel epithelia, and fibroblasts varied in response to time and/or tenectomy. At the level of gene expression, only HAS1 and HAS2 significantly increased with respect to both time and tenectomy. The profiles of additional genes that influence ECM composition during muscle repair, tenascin-C, type I collagen, the HA-degrading hyaluronidases (Hyal) and matrix metalloproteinases (MMP) were also investigated. Hyal1 and Hyal2 were highly expressed in skeletal muscle but did not change after tenectomy; however, indicators of hypertrophy, MMP-2 and MMP-14, were significantly upregulated from 2 to 14 days. These results indicate that HA levels dynamically change in response to a hypertrophic stimulus and various cells may participate in this mechanism of skeletal muscle adaptation.

Experimental muscle injury: correlation between ultrasound and histological findings

INTRODUCTION

In this study we correlated ultrasound findings with histological changes taking place during experimentally induced degeneration-regeneration in rat skeletal muscle.

METHODS

Gastrocnemius muscles were injected with mepivacaine, and the progress of the muscle injury was monitored by ultrasound from day 1 to day 20. Muscles were extracted on the same days for histological examination.

RESULTS

The degenerative phase was characterized by increased echogenicity in the injured area; thereafter, echogenicity gradually diminished during the regenerative phase, attaining normal levels by 20 days postinjection. By this stage, histological examination revealed that regeneration was complete. The heteroechoic texture observed from day 4 to day 10 appeared to reflect the coexistence of degenerative and regenerative processes.

CONCLUSIONS

The results suggest that the degenerative and regenerative phases of muscle injury may be distinguished sonographically through differences in echogenicity and echotexture and, using Doppler ultrasound, differences in the degree of vascularization.

Globular adiponectin activates motility and regenerative traits of muscle satellite cells

Regeneration of adult injured skeletal muscle is due to activation of satellite cells, a population of stem cells resident beneath the basal lamina. Thus, information on soluble factors affecting satellite cell activation, as well as migration towards injury and fusion into new myofibers are essential. Here, we show that globular adiponectin (gAd), positively affects several features of muscle satellite cells. gAd activates satellite cells to exit quiescence and increases their recruitment towards myotubes. gAd elicits in satellite cells a specific motility program, involving activation of the small GTPase Rac1, as well as expression of Snail and Twist transcription factors driving a proteolytic motility, useful to reach the site of injury. We show that satellite cells produce autocrine full length adiponectin (fAd), which is converted to gAd by activated macrophages. In turns, gAd concurs to attract to the site of injury both satellite cells and macrophages and induces myogenesis in muscle satellite cells. Thus, these findings add a further role for gAd in skeletal muscle, including the hormone among factors participating in muscle regeneration.

A new class of highly potent matrix metalloproteinase inhibitors based on triazole-substituted hydroxamates: (radio)synthesis and in vitro and first in vivo evaluation

In vivo imaging of MMPs is of great (pre)clinical interest and can potentially be realized with modern three-dimensional and noninvasive in vivo molecular imaging techniques such as positron emission tomography (PET). Consequently, MMP inhibitors (MMPIs) radiolabeled with positron emitting nuclides (e.g., (18)F) represent a suitable tool for the visualization of activated MMPs with PET. On the basis of our previous work and results regarding radiolabeled and unlabeled derivatives of the nonselective MMPIs, we discovered a new class of fluorinated MMPIs with a triazole-substituted hydroxamate substructure. These novel MMPIs are characterized by an increased hydrophilicity compared with the lead structures and excellent MMP inhibition potencies for MMP-2, MMP-8, MMP-9, and MMP-13 (IC(50) = 0.006-107 nM). Therefore, one promising fluorinated triazole-substituted hydroxamate (30b) was selected and resynthesised as its (18)F-labeled version to yield the potential PET radioligand [(18)F]30b. The biodistribution behavior of this novel compound was investigated with small animal PET.

Skeletal muscle fibrosis develops in response to desmin deletion

Skeletal muscle is a dynamic composite of proteins that responds to both internal and external cues to facilitate muscle adaptation. In cases of disease or altered use, these messages can be distorted resulting in myopathic conditions such as fibrosis. In this work, we describe a mild and progressive fibrotic adaptation in skeletal muscle lacking the cytoskeletal intermediate filament protein desmin. Muscles lacking desmin become progressively stiffer, accumulate increased collagen, and increase expression of genes involved in extracellular matrix turnover. Additionally, in the absence of desmin, skeletal muscle is in an increased state of inflammation and regeneration as indicated by increased centrally nucleated fibers, elevated inflammation and regeneration related gene expression, and increased numbers of inflammatory cells. These data suggest a potential link between increased cellular damage and the development of fibrosis in muscles lacking the cytoskeletal support of the desmin filament network.

Fine-structural distribution of MMP-2 and MMP-9 activities in the rat skeletal muscle upon training: a study by high-resolution in situ zymography

Matrix metalloproteinases (MMPs) are key regulators of extracellular matrix remodeling, but have also important intracellular targets. The purpose of this study was to examine the activity and subcellular localization of the gelatinases MMP-2 and MMP-9 in skeletal muscle of control and physically trained rats. In control hind limb muscle, the activity of the gelatinases was barely detectable. In contrast, after 5 days of intense exercise, in Soleus (Sol), but not Extensor digitorum longus (EDL) muscle, significant upregulation of gelatinolytic activity in myofibers was observed mainly in the nuclei, as assessed by high resolution in situ zymography. The nuclei of quiescent satellite cells did not contain the activity. Within the myonuclei, the gelatinolytic activity colocalized with an activated RNA Polymerase II. Also in Sol, but not in EDL, there were few foci of mononuclear cells with strongly positive cytoplasm, associated with apparent necrotic myofibers. These cells were identified as activated satellite cells/myoblasts. No extracellular gelatinase activity was observed. Gel zymography combined with subcellular fractionation revealed training-related upregulation of active MMP-2 in the nuclear fraction, and increase of active MMP-9 in the cytoplasmic fraction of Sol. Using RT-PCR, selective increase in MMP-9 mRNA was observed. We conclude that training activates nuclear MMP-2, and increases expression and activity of cytoplasmic MMP-9 in Sol, but not in EDL. Our results suggest that the gelatinases are involved in muscle adaptation to training, and that MMP-2 may play a novel role in myonuclear functions.

Transforming growth factor-beta induces skeletal muscle atrophy and fibrosis through the induction of atrogin-1 and scleraxis

INTRODUCTION

Transforming growth factor-beta (TGF-β) is a well-known regulator of fibrosis and inflammation in many tissues. During embryonic development, TGF-β signaling induces expression of the transcription factor scleraxis, which promotes fibroblast proliferation and collagen synthesis in tendons. In skeletal muscle, TGF-β has been shown to induce atrophy and fibrosis, but the effect of TGF-β on muscle contractility and the expression of scleraxis and atrogin-1, an important regulator of muscle atrophy, were not known.

METHODS

We treated muscles from mice with TGF-β and measured force production, scleraxis, procollagen Iα2, and atrogin-1 protein levels.

RESULTS

TGF-β decreased muscle fiber size and dramatically reduced maximum isometric force production. TGF-β also induced scleraxis expression in muscle fibroblasts, and increased procollagen Iα2 and atrogin-1 levels in muscles.

CONCLUSION

These results provide new insight into the effect of TGF-β on muscle contractility and the molecular mechanisms behind TGF-β-mediated muscle atrophy and fibrosis.

Immune response and mitochondrial metabolism are commonly deregulated in DMD and aging skeletal muscle

Duchenne Muscular Dystrophy (DMD) is a complex process involving multiple pathways downstream of the primary genetic insult leading to fatal muscle degeneration. Aging muscle is a multifactorial neuromuscular process characterized by impaired muscle regeneration leading to progressive atrophy. We hypothesized that these chronic atrophying situations may share specific myogenic adaptative responses at transcriptional level according to tissue remodeling. Muscle biopsies from four young DMD and four AGED subjects were referred to a group of seven muscle biopsies from young subjects without any neuromuscular disorder and explored through a dedicated expression microarray. We identified 528 differentially expressed genes (out of 2,745 analyzed), of which 328 could be validated by an exhaustive meta-analysis of public microarray datasets referring to DMD and Aging in skeletal muscle. Among the 328 validated co-expressed genes, 50% had the same expression profile in both groups and corresponded to immune/fibrosis responses and mitochondrial metabolism. Generalizing these observed meta-signatures with large compendia of public datasets reinforced our results as they could be also identified in other pathological processes and in diverse physiological conditions. Focusing on the common gene signatures in these two atrophying conditions, we observed enrichment in motifs for candidate transcription factors that may coordinate either the immune/fibrosis responses (ETS1, IRF1, NF1) or the mitochondrial metabolism (ESRRA). Deregulation in their expression could be responsible, at least in part, for the same transcriptome changes initiating the chronic muscle atrophy. This study suggests that distinct pathophysiological processes may share common gene responses and pathways related to specific transcription factors.

Mechanisms underlying the sparing of masticatory versus limb muscle function in an experimental critical illness model

Acute quadriplegic myopathy (AQM) is a common debilitating acquired disorder in critically ill intensive care unit (ICU) patients that is characterized by tetraplegia/generalized weakness of limb and trunk muscles. Masticatory muscles, on the other hand, are typically spared or less affected, yet the mechanisms underlying this striking muscle-specific difference remain unknown. This study aims to evaluate physiological parameters and the gene expression profiles of masticatory and limb muscles exposed to factors suggested to trigger AQM, such as mechanical ventilation, immobilization, neuromuscular blocking agents, corticosteroids (CS), and sepsis for 5 days by using a unique porcine model mimicking the ICU conditions. Single muscle fiber cross-sectional area and force-generating capacity, i.e., maximum force normalized to fiber cross-sectional area (specific force), revealed maintained masseter single muscle fiber cross-sectional area and specific-force after 5 days' exposure to all triggering factors. This is in sharp contrast to observations in limb and trunk muscles, showing a dramatic decline in specific force in response to 5 days' exposure to the triggering factors. Significant differences in gene expression were observed between craniofacial and limb muscles, indicating a highly complex and muscle-specific response involving transcription and growth factors, heat shock proteins, matrix metalloproteinase inhibitor, oxidative stress responsive elements, and sarcomeric proteins underlying the relative sparing of cranial vs. spinal nerve innervated muscles during exposure to the ICU intervention.

Mechanisms of action of intravenous immunoglobulin in inflammatory muscle disease

Intravenous immunoglobulin (IVIG) is a unique immune-modulating therapy that has a wide range of effects on the immune system at multiple levels. This allows it to be used successfully in a variety of immune-mediated, systemic, and neurological disorders, including the inflammatory myopathies. It is likely that the specific action of IVIG varies depending on the underlying pathogenesis of a given disease. In dermatomyositis (DM), IVIG has been shown to diminish the activity of complement and deposition of membrane attack complex on capillaries and muscle fibers, the expression of adhesion molecules, and cytokine production. IVIG also appears to modify gene expression in the muscle of DM patients. The mechanism by which IVIG affects muscle in polymyositis and inclusion body myositis has not been well-studied. However, it may work via suppression of T-cell activation (including cytotoxic T cells) and migration into muscle tissue and alterations in cytokine production. IVIG generally yields the greatest therapeutic benefit in DM and is often of marginal utility in inclusion body myositis. It is generally considered as second-line or adjunctive therapy in the inflammatory myopathies.

Elevated levels of active matrix metalloproteinase-9 cause hypertrophy in skeletal muscle of normal and dystrophin-deficient mdx mice

Matrix metalloproteinases (MMPs) are a group of extracellular proteases involved in tissue remodeling in several physiological and pathophysiological conditions. While increased expression of MMPs (especially MMP-9) has been observed in skeletal muscle in numerous conditions, their physiological significance remains less-well understood. By generating novel skeletal muscle-specific transgenic (Tg) mice expressing constitutively active mutant of MMP-9 (i.e. MMP-9G100L), in this study, we have investigated the effects of elevated levels of MMP-9 on skeletal muscle structure and function in vivo. Tg expression of enzymatically active MMP-9 protein significantly increased skeletal muscle fiber cross-section area, levels of contractile proteins and force production in isometric contractions. MMP-9 stimulated the activation of the Akt signaling pathway in Tg mice. Moreover, expression of active MMP-9 increased the proportion of fast-type fiber in soleus muscle of mice. Overexpression of MMP-9 also considerably reduced the deposition of collagens I and IV in skeletal muscle in vivo. In one-year-old mdx mice (a model for Duchenne muscular dystrophy, DMD), deletion of the Mmp9 gene reduced fiber hypertrophy and phosphorylation of Akt and p38 mitogen-activated protein kinase. Collectively, our study suggests that elevated levels of active MMP-9 protein cause hypertrophy in skeletal muscle and that the modulation of MMP-9 levels may have therapeutic value in various muscular disorders including DMD.

Phototherapy with low-level laser affects the remodeling of types I and III collagen in skeletal muscle repair

The purpose of this article was to analyze the photobiomodulator role of low-level laser therapy (LLLT) on the skeletal muscle remodeling following cryoinjury in rats, focusing the types I and III collagen proteins. Laser phototherapy has been employed to stimulate repair in different tissues. However, its role in skeletal muscle remodeling is not yet well clarified, especially its effect on the collagen component of the extracellular matrix. Fifty adult Wistar rats were divided into four groups: control, sham, cryoinjury, and laser-treated cryoinjury. Laser irradiation was performed three times a week on the injured region using the InGaAlP (indium-gallium-aluminum-phosphorous) laser (660 nm; beam spot of 0.04 cm(2), output power of 20 mW, power density of 0.5 mW/cm(2), energy density of 5 J/cm(2), 10-s exposure time, with a total energy dose of 0.2 J). Five animals were killed after short-term (days 1 and 7) and long-term (14 and 21) durations following injury. The muscles were processed and submitted to hematoxylin and eosin (H&E) and immunohistochemical staining. The histological slices were analyzed qualitatively, semi-quantitatively, and quantitatively. The data were submitted to statistical analysis using the Kruskal-Wallis test. The qualitative analysis of morphological aspects revealed that the muscle repair were very similar in cryoinjury and laser groups on days 1, 14 and 21. However, at 7 days, differences could be observed because there was a reduction in myonecrosis associated to formation of new vessels (angiogenesis) in the laser-treated group. The analysis of the distribution of types I and III collagen, on day 7, revealed a significant increase in the depositing of these proteins in the laser-treated group when compared to the cryoinjury group. InGaAlP diode laser within the power parameters and conditions tested had a biostimulatory effect at the regenerative and fibrotic phases of the skeletal muscle repairs, by promoting angiogenesis, reducing myonecrosis, and inducing types I and III collagen synthesis, following cryoinjury in rat.

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.

Effects of 660 and 780 nm low-level laser therapy on neuromuscular recovery after crush injury in rat sciatic nerve

BACKGROUND AND OBJECTIVE

Post-traumatic nerve repair is still a challenge for rehabilitation. It is particularly important to develop clinical protocols to enhance nerve regeneration. The present study investigated the effects of 660 and 780 nm low-level laser therapy (LLLT) using different energy densities (10, 60, and 120 J/cm²) on neuromuscular and functional recovery as well as on matrix metalloproteinase (MMP) activity after crush injury in rat sciatic nerve.

MATERIALS AND METHODS

Rats received transcutaneous LLLT irradiation at the lesion site for 10 consecutive days post-injury and were sacrificed 28 days after injury. Both the sciatic nerve and tibialis anterior muscles were analyzed. Nerve analyses consisted of histology (light microscopy) and measurements of myelin, axon, and nerve fiber cross-sectional area (CSA). S-100 labeling was used to identify myelin sheath and Schwann cells. Muscle fiber CSA and zymography were carried out to assess the degree of muscle atrophy and MMP activity, respectively. Statistical significance was set at 5% (P≤0.05).

RESULTS

Six hundred sixty nanometer LLLT either using 10 or 60 J/cm² restored muscle fiber, myelin and nerve fiber CSA compared to the normal group (N). Furthermore, it increased MMP-2 activity in nerve and decreased MMP-2 activity in muscle and MMP-9 activity in nerve. In contrast, 780 nm LLLT using 10 J/cm² decreased MMP-9 activity in nerve compared to the crush group (CR) and N; it also restored normal levels of myelin and nerve fiber CSA. Both 60 and 120 J/cm² decreased MMP-2 activity in muscle compared to CR and N. 780 nm did not prevent muscle fiber atrophy. Functional recovery in the irradiated groups did not differ from the non-irradiated CR.

CONCLUSION

Data suggest that 660 nm LLLT with low (10 J/cm²) or moderate (60 J/cm²) energy densities is able to accelerate neuromuscular recovery after nerve crush injury in rats.

The Ras antagonist, farnesylthiosalicylic acid (FTS), decreases fibrosis and improves muscle strength in dy/dy mouse model of muscular dystrophy

The Ras superfamily of guanosine-triphosphate (GTP)-binding proteins regulates a diverse spectrum of intracellular processes involved in inflammation and fibrosis. Farnesythiosalicylic acid (FTS) is a unique and potent Ras inhibitor which decreased inflammation and fibrosis in experimentally induced liver cirrhosis and ameliorated inflammatory processes in systemic lupus erythematosus, neuritis and nephritis animal models. FTS effect on Ras expression and activity, muscle strength and fibrosis was evaluated in the dy^2J/dy^2J mouse model of merosin deficient congenital muscular dystrophy. The dy^2J/dy^2J mice had significantly increased RAS expression and activity compared with the wild type mice. FTS treatment significantly decreased RAS expression and activity. In addition, phosphorylation of ERK, a Ras downstream protein, was significantly decreased following FTS treatment in the dy^2J/dy^2J mice. Clinically, FTS treated mice showed significant improvement in hind limb muscle strength measured by electronic grip strength meter. Significant reduction of fibrosis was demonstrated in the treated group by quantitative Sirius Red staining and lower muscle collagen content. FTS effect was associated with significantly inhibition of both MMP-2 and MMP-9 activities. We conclude that active RAS inhibition by FTS was associated with attenuated fibrosis and improved muscle strength in the dy^2J/dy^2J mouse model of congenital muscular dystrophy.

Identification of differentially regulated secretome components during skeletal myogenesis

Myogenesis is a well-characterized program of cellular differentiation that is exquisitely sensitive to the extracellular milieu. Systematic characterization of the myogenic secretome (i.e. the ensemble of secreted proteins) is, therefore, warranted for the identification of novel secretome components that regulate both the pluripotency of these progenitor mesenchymal cells, and also their commitment and passage through the differentiation program. Previously, we have successfully identified 26 secreted proteins in the mouse skeletal muscle cell line C2C12 (1). In an effort to attain a more comprehensive picture of the regulation of myogenesis by its extracellular milieu, quantitative profiling employing stable isotope labeling by amino acids in cell culture was implemented in conjunction with two parallel high throughput online reverse phase liquid chromatography-tandem mass spectrometry systems. In summary, 34 secreted proteins were quantified, 30 of which were shown to be differentially expressed during muscle development. Intriguingly, our analysis has revealed several novel up- and down-regulated secretome components that may have critical biological relevance for both the maintenance of pluripotency and the passage of cells through the differentiation program. In particular, the altered regulation of secretome components, including follistatin-like protein-1, osteoglycin, spondin-2, and cytokine-induced apoptosis inhibitor-1, along with constitutively expressed factors, such as fibulin-2, illustrate dynamic changes in the secretome that take place when differentiation to a specific lineage occurs.

Extracellular matrix metalloproteinase inducer (EMMPRIN/CD147) as a novel regulator of myogenic cell differentiation

Matrix metalloproteinases (MMPs) are thought to play an important role in skeletal muscle cell growth and differentiation. In view of the MMP inducing function of EMMPRIN/CD147, its role in myogenic cell differentiation was investigated. EMMPRIN level increased during differentiation of both rat primary myoblasts derived from satellite cells and mouse C2.7 myogenic cells and was associated with an alteration in its molecular forms. In parallel, expression of pro-MMP-9 gradually decreased and that of pro-MMP-2 and active MMP-2 increased. While small interfering RNA (siRNA) inhibition of EMMPRIN expression accelerated cell differentiation, exogenously added recombinant EMMPRIN inhibited differentiation by an MMP-mediated mechanism, as the MMP inhibitor marimastat abrogated EMMPRIN's effect. Our results further suggest that EMMPRIN regulates differentiation through an MMP activation of transforming growth factor beta (TGFβ), a known inhibitor of myoblast's differentiation, as the increased activation and signaling of TGFβ by EMMPRIN was attenuated in the presence of marimastat. EMMPRIN inhibition may thus represent a novel strategy in the treatment of muscular degenerative disorders.

Characteristic pattern of skeletal muscle remodelling in different mouse strains

Muscular injury associated with local inflammatory reaction frequently occurs in sports medicine, but the individual response and capacity of regeneration vary among subjects. Inflammatory cytokines are probably implicated in activation of repair mechanisms by specifically influencing tissue microenvironment. This work aimed to compare muscle tissue repair in different mouse lineages. We used C57BL/6 and BALB/c mice genetically predisposed to either Type1 or Type2 cytokine production. The role of Type1 cytokines was also investigated in C57IFN-γ (IFNγ-KO) and C57IL-12 (IL12-KO) knockout mice. Participation of T lymphocytes was assessed in athymic BALB/c nude (nu/nu) mice. Muscular lesion was induced with bupivacaine injection in the Triceps brachii muscle. BALB/c mice showed marked collagen deposition and increased TGF-β mRNA content, contrasting with mild fibrosis observed in C57BL/6 mice. C57-IFNγ-KO mice, exhibited pronounced fibrosis, but IL12-KO collagen deposition was similar to that of C57. Twenty-four hours after lesion, C57BL/6 and BALB/c(nu/nu) presented numerous regenerating myofibres and marked increase of metalloprotease-9 activity compared with BALB/c. These data support that skeletal muscle remodelling is greatly influenced by the genetic backgrounds, shedding light on the molecular mechanisms influencing differential muscular remodelling and tissue regeneration among individuals.