Effect of weak static magnetic fields on the development of cultured skeletal muscle cells

We studied the effect produced on the development and functional activity of skeletal muscle cells from newborn Wistar rats in primary culture by weak static magnetic fields (WSMF; 60-400 µT) with a high capacity of penetrating the biological media. To reduce the impact of external magnetic fields, cells were cultured at 37 °C in a multilayered shielding chamber with the attenuation coefficient equal to 160. WSMF inside the chamber was created by a circular permanent magnet. We found that the application of WSMF with the magnetic field strength only a few times that of the geomagnetic field can accelerate the development of skeletal muscle cells, resulting in the formation of multinuclear hypertrophied myotubes. WSMF was shown to induce 1.5- to 3.5-fold rise in the concentration of intracellular calcium [Ca(2+)]i due to the release of Ca(2+) from the sarcoplasmic reticulum (SR) through ryanodine receptors (RyR), which increases in the maturation of myotubes. We also found that fully differentiated myotubes at late stages of development were less sensitive to WSMF, manifesting a gradual decrease in the frequency of contractions. However, myotubes at the stage when electromechanical coupling was forming dramatically reduced the frequency of contractions during the first minutes of their exposure to WSMF.

Cardiac and respiratory dysfunction in Duchenne muscular dystrophy and the role of second messengers

Duchenne muscular dystrophy (DMD) affects young boys and is characterized by the absence of dystrophin, a large cytoskeletal protein present in skeletal and cardiac muscle cells and neurons. The heart and diaphragm become necrotic in DMD patients and animal models of DMD, resulting in cardiorespiratory failure as the leading cause of death. The major consequences of the absence of dystrophin are high levels of intracellular Ca(2+) and the unbalanced production of NO that can finally trigger protein degradation and cell death. Cytoplasmic increase in Ca(2+) concentration directly and indirectly triggers different processes such as necrosis, fibrosis, and activation of macrophages. The absence of the neuronal isoform of nitric oxide synthase (nNOS) and the overproduction of NO by the inducible isoform (iNOS) further increase the intracellular Ca(2+) via a hypernitrosylation of the ryanodine receptor. NO overproduction, which further induces the expression of iNOS but decreases the expression of the endothelial isoform (eNOS), deregulates the muscle tissue blood flow creating an ischemic situation. The high levels of Ca(2+) in dystrophic muscles and the ischemic state of the muscle tissue would culminate in a positive feedback loop. While efforts continue toward optimizing cardiac and respiratory care of DMD patients, both Ca(2+) and NO in cardiac and respiratory muscle pathways have been shown to be important to the etiology of the disease. Understanding the mechanisms behind the fine regulation of Ca(2+) -NO may be important for a noninterventional and noninvasive supportive approach to treat DMD patients, improving the quality of life and natural history of DMD patients.

MiR-351 transiently increases during muscle regeneration and promotes progenitor cell proliferation and survival upon differentiation

MicroRNAs (miRNAs) regulate many biological processes including muscle development. However, little is known regarding miRNA regulation of muscle regeneration. Murine tibialis anterior muscle was evaluated after cardiotoxin-induced injury and used for global miRNA expression analysis. From day 1 through day 21 following injury, 298 miRNAs were significantly changed at least at one time point, including 86 miRNAs that were altered >10-fold compared with uninjured skeletal muscle. Temporal miRNA expression patterns included inflammation-related miRNAs (miR-223 and -147) that increased immediately after injury; this pattern contrasted to that of mature muscle-specific miRNAs (miR-1, -133a, and -499) that abruptly decreased following injury followed by upregulation in later regenerative events. Another cluster of miRNAs were transiently increased in the early days of muscle regeneration including miR-351, a miRNA that was also transiently expressed during myogenic progenitor cell (MPC) differentiation in vitro. Based on computational predictions, further studies demonstrated that E2f3 was a target of miR-351 in myoblasts. Moreover, knockdown of miR-351 expression inhibited MPC proliferation and promoted apoptosis during MPC differentiation, whereas miR-351 overexpression protected MPC from apoptosis during differentiation. Collectively, these observations suggest that miR-351 is involved in both the maintenance of MPC proliferation and the transition into differentiated myotubes. Thus, a novel, time-dependent sequence of molecular events during muscle regeneration has been identified; miR-351 inhibits E2f3 expression, a key regulator of cell cycle progression and proliferation, and promotes MPC proliferation and protects early differentiating MPC from apoptosis, important events in the hostile tissue environment after acute muscle injury.

Lack of Smad3 signaling leads to impaired skeletal muscle regeneration

Smad3 is a key intracellular signaling mediator for both transforming growth factor-β and myostatin, two major regulators of skeletal muscle growth. Previous published work has revealed pronounced muscle atrophy together with impaired satellite cell functionality in Smad3-null muscles. In the present study, we have further validated a role for Smad3 signaling in skeletal muscle regeneration. Here, we show that Smad3-null mice had incomplete recovery of muscle weight and myofiber size after muscle injury. Histological/immunohistochemical analysis suggested impaired inflammatory response and reduced number of activated myoblasts during the early stages of muscle regeneration in the tibialis anterior muscle of Smad3-null mice. Nascent myofibers formed after muscle injury were also reduced in number. Moreover, Smad3-null regenerated muscle had decreased oxidative enzyme activity and impaired mitochondrial biogenesis, evident by the downregulation of the gene encoding mitochondrial transcription factor A, a master regulator of mitochondrial biogenesis. Consistent with known Smad3 function, reduced fibrotic tissue formation was also seen in regenerated Smad3-null muscle. In conclusion, Smad3 deficiency leads to impaired muscle regeneration, which underscores an essential role of Smad3 in postnatal myogenesis. Given the negative role of myostatin during muscle regeneration, the increased expression of myostatin observed in Smad3-null muscle may contribute to the regeneration defects.

Single muscle-fiber isolation and culture for cellular, molecular, pharmacological, and evolutionary studies

The technique of single muscle-fiber cultures has already proven valuable in extending knowledge of myogenesis, stem cell heterogeneity, the stem cell niche in skeletal muscle, and satellite cell activation. This report reviews the background of the model and applications, and details the procedures of muscle dissection, fiber digestion and isolation, cleaning the fiber preparation, plating fibers, and extensions of the technique for studying activation from stable quiescence of satellite cells, mRNA expression by in situ hybridization and regulation of satellite cell activation in zebrafish muscle by nitric oxide, hepatocyte growth factor.

Transcriptional profile of GTP-mediated differentiation of C2C12 skeletal muscle cells

Several purine receptors have been localised on skeletal muscle membranes. Previous data support the hypothesis that extracellular guanosine 5'-triphosphate (GTP) is an important regulatory factor in the development and function of muscle tissue. We have previously described specific extracellular binding sites for GTP on the plasma membrane of mouse skeletal muscle (C2C12) cells. Extracellular GTP induces an increase in intracellular Ca(2+) concentrations that results in membrane hyperpolarisation through Ca(2+)-activated K(+) channels, as has been demonstrated by patch-clamp experiments. This GTP-evoked increase in intracellular Ca(2+) is due to release of Ca(2+) from intracellular inositol-1,4,5-trisphosphate-sensitive stores. This enhances the expression of the myosin heavy chain in these C2C12 myoblasts and commits them to fuse into multinucleated myotubes, probably via a phosphoinositide-3-kinase-dependent signal-transduction mechanism. To define the signalling of extracellular GTP as an enhancer or modulator of myogenesis, we investigated whether the gene-expression profile of differentiated C2C12 cells (4 and 24 h in culture) is affected by extracellular GTP. To investigate the nuclear activity and target genes modulated by GTP, transcriptional profile analysis and real-time PCR were used. We demonstrate that in the early stages of differentiation, GTP up-regulates genes involved in different pathways associated with myogenic processes, including cytoskeleton structure, the respiratory chain, myogenesis, chromatin reorganisation, cell adhesion, and the Jak/Stat pathway, and down-regulates the mitogen-activated protein kinase pathway. GTP also increases the expression of three genes involved in myogenesis, Pp3ca, Gsk3b, and Pax7. Our data suggests that in the myogenic C2C12 cell line, extracellular GTP acts as a differentiative factor in the induction and sustaining of myogenesis.

Teashirt-3, a novel regulator of muscle differentiation, associates with BRG1-associated factor 57 (BAF57) to inhibit myogenin gene expression

In adult muscles and under normal physiological conditions, satellite cells are found in a quiescent state but can be induced to enter the cell cycle by signals resulting from exercise, injury-induced muscle regeneration, or specific disease states. Once activated, satellite cells proliferate, self-renew, and differentiate to form myofibers. In the present study, we found that the zinc finger-containing factor Teashirt-3 (TSHZ3) was expressed in quiescent satellite cells of adult mouse skeletal muscles. We showed that following treatment with cardiotoxin TSHZ3 was strongly expressed in satellite cells of regenerating muscles. Moreover, immunohistochemical analysis indicated that TSHZ3 was expressed in both quiescent and activated satellite cells on intact myofibers in culture. TSHZ3 expression was maintained in myoblasts but disappeared with myotube formation. In C2C12 myoblasts, we showed that overexpression of Tshz3 impaired myogenic differentiation and promoted the down-regulation of myogenin (Myog) and up-regulation of paired-box factor 7 (Pax7). Moreover, knockdown experiments revealed a selective effect of Tshz3 on Myog regulation, and transcriptional reporter experiments indicated that TSHZ3 repressed Myog promoter. We identified the BRG1-associated factor 57 (BAF57), a subunit of the SWI/SNF complex, as a partner of TSHZ3. We showed that TSHZ3 cooperated with BAF57 to repress MYOD-dependent Myog expression. These results suggest a novel mechanism for transcriptional repression by TSHZ3 in which TSHZ3 and BAF57 cooperate to modulate MyoD activity on the Myog promoter to regulate skeletal muscle differentiation.

Uncovering the transcriptional circuitry in skeletal muscle regeneration

Skeletal muscle has a remarkable ability to regenerate after repeated and complete destruction of the tissue. The healing phases for an injured muscle undergo an activation program controlled by a dynamically inducible transcriptional regulatory network. Mapping a complex mammalian transcriptional network is confronted by significant challenges and requires the integration of multiple experimental data types. In this work we present a system approach to describe the transcriptional circuitry during skeletal muscle regeneration using time-course expression data and motif scanning information. Time-lagged correlation analysis was utilized to evaluate the transcription factor (TF) → target associations. Our analysis identified six TFs that potentially play a central role throughout the regeneration process. Four of them have previously been described to be important for muscle regeneration and differentiation. The remaining two TFs are identified as novel regulators that may have a role in the regeneration process. We hope that our work may provide useful clues to help accelerate the recovery process in injured skeletal muscle.

Bupivacaine-induced Regeneration of Rat Soleus Muscle: Ultrastructural and Immunohistochemical Aspects

The regeneration of soleus muscle injury induced by the bupivacaine model was studied ultrastructurally and immunohistochemically. Twenty-one young (age range 3-3.5 months) male Wistar rats were subjected to a single intramuscular injection of 1 mL of 0.5% Marcaine. The muscles were examined on biopsy days 1, 2, 3, 5, 7, 14, and 21. By day 1, mononuclear inflammatory cells had invaded the necrotic sarcoplasm. Degenerative morphological findings counted mainly for the hypercontracted fibers, dilation of sarcoplasmic reticulum, plasma membrane defects, mitochondrial alterations, and myofibril discontinuities. By day 2 proliferating myoblasts were seen with variety in shape, which fused on the day 3. Myotubes with multiple central nuclei and euchromatic nucleoli were formed by day 5. Asynchronous repair events were seen with bundles of myofilaments toward the core of the fibers, in contrast to the least mature distal growth cones, which had free myoblasts in proximity and formatted pseudopods. Chronologically asynchronous regeneration stages possibly suggested successive satellite cell activation profiles or heterogeneity in satellite cell population. In parallel with the electron microscopy, in light microscope immunocytochemistry, desmin- and vimentin-positive mononuclear cells were observed within the first 3 biopsy days, but as regeneration proceeded, desmin predominated over vimentin. Merosin immunoreactivity revealed preservation of the basal lamina, which is crucial for the stability and survival of myotubes. By day 21, fibers restored the overall control architecture.

Regulation of connexin gene expression during skeletal muscle regeneration in the adult rat

In the adult skeletal muscle, various kinds of trauma promote proliferation of satellite cells that differentiate into myoblasts forming new myofibers or to repair the damaged one. The aim of present work was to perform a comparative spatial and temporal analysis of connexin (Cx) 37, Cx39, Cx40, Cx43, and Cx45 expression in the adult regenerating skeletal muscle in response to crush injury. Within 24 h from injury, Cx37 expression was upregulated in the endothelial cells of blood vessels, and, 5 days after injury, Cx37-expressing cells were found inside the area of lesion and formed clusters generating new blood vessels with endothelial cells expressing Cx37. Three days after injury, Cx39 mRNA was selectively expressed in myogenin-positive cells, forming rows of closely apposed cell nuclei fusing in myotubes. Cx40 mRNA-labeled cells were observed within 24 h from injury in the endothelium of blood vessels, and, 5 days after lesion, Cx40-labeled cells were found inside the area of lesion-forming rows of myogenin-positive, closely apposed cells coexpressing Cx39. Within 24 h from lesion, both Cx43 and Cx45 mRNAs were upregulated in individual cells, and some of them were positive for M-cadherin. Three days after injury, a large number of both Cx43 and Cx45 mRNA-labeled and myogenin-positive cells were found inside the area of lesion. Taken together, these results show that at least four Cxs, out of five expressed in regenerating skeletal muscle, can be differentially involved in communication of myogenic cells during the process of cell proliferation, aggregation, and fusion to form new myotubes or to repair damaged myofibers.

Upregulation of the creatine synthetic pathway in skeletal muscles of mature mdx mice

Duchenne muscular dystrophy (DMD) is a fatal neuromuscular human disease caused by dystrophin deficiency. The mdx mouse lacks dystrophin protein, yet does not exhibit the debilitating DMD phenotype. Investigating compensatory mechanisms in the mdx mouse may shed new insights into modifying DMD pathogenesis. This study targets two metabolic genes, guanidinoacetate methyltransferase (GAMT) and arginine:glycine amidinotransferase (AGAT) which are required for creatine synthesis. We show that GAMT and AGAT mRNA are up-regulated 5.4- and 1.9-fold respectively in adult mdx muscle compared to C57. In addition, GAMT protein expression is up-regulated at least 2.5-fold in five different muscles of mdx vs. control. Furthermore, we find GAMT immunoreactivity in up to 80% of mature mdx muscle fibers in addition to small regenerating fibers and rare revertants; while GAMT immunoreactivity is equal to background levels in all muscle fibers of mature C57 mice. The up-regulation of the creatine synthetic pathway may help maintain muscle creatine levels and limit cellular energy failure in leaky mdx skeletal muscles. These results may help better understand the mild phenotype of the mdx mouse and may offer new treatment horizons for DMD.

Identification of a new hybrid serum response factor and myocyte enhancer factor 2-binding element in MyoD enhancer required for MyoD expression during myogenesis

MyoD is a critical myogenic factor induced rapidly upon activation of quiescent satellite cells, and required for their differentiation during muscle regeneration. One of the two enhancers of MyoD, the distal regulatory region, is essential for MyoD expression in postnatal muscle. This enhancer contains a functional divergent serum response factor (SRF)-binding CArG element required for MyoD expression during myoblast growth and muscle regeneration in vivo. Electrophoretic mobility shift assay, chromatin immunoprecipitation, and microinjection analyses show this element is a hybrid SRF- and MEF2 Binding (SMB) sequence where myocyte enhancer factor 2 (MEF2) complexes can compete out binding of SRF at the onset of differentiation. As cells differentiate into postmitotic myotubes, MyoD expression no longer requires SRF but instead MEF2 binding to this dual-specificity element. As such, the MyoD enhancer SMB element is the site for a molecular relay where MyoD expression is first initiated in activated satellite cells in an SRF-dependent manner and then increased and maintained by MEF2 binding in differentiated myotubes. Therefore, SMB is a DNA element with dual and stage-specific binding activity, which modulates the effects of regulatory proteins critical in controlling the balance between proliferation and differentiation.

Satellite cells from dystrophic (mdx) mice display accelerated differentiation in primary cultures and in isolated myofibers

In the dystrophic (mdx) mouse, skeletal muscle undergoes cycles of degeneration and regeneration, and myogenic progenitors (satellite cells) show ongoing proliferation and differentiation at a time when counterpart cells in normal healthy muscle enter quiescence. However, it remains unclear whether this enhanced satellite cell activity is triggered solely by the muscle environment or is also governed by factors inherent in satellite cells. To obtain a better picture of myogenesis in dystrophic muscle, a direct cell-by-cell analysis was performed to compare satellite cell dynamics from mdx and normal (C57Bl/10) mice in two cell culture models. In one model, the kinetics of satellite cell differentiation was quantified in primary cell cultures from diaphragm and limb muscles by immunodetection of MyoD, myogenin, and MEF2. In mdx cell cultures, myogenin protein was expressed earlier than normal and was followed more rapidly by dual myogenin/MEF2A expression and myotube formation. In the second model, the dynamics of satellite cell myogenesis were investigated in cultured myofibers isolated from flexor digitorum brevis (FDB) muscle, which retain satellite cells in the native position. Consistent with primary cultures, satellite cells in mdx myofibers displayed earlier myogenin expression, as well as an enhanced number of myogenin-expressing satellite cells per myofiber compared to normal. The addition of fibroblast growth factor 2 (FGF2) led to an increase in the number of satellite cells expressing myogenin in normal and mdx myofibers. However, the extent of the FGF effect was more robust in mdx myofibers. Notably, many myonuclei in mdx myofibers were centralized, evidence of segmental regeneration; all central nuclei and many peripheral nuclei in mdx myofibers were positive for MEF2A. Results indicated that myogenic cells in dystrophic muscle display accelerated differentiation. Furthermore, the study demonstrated that FDB myofibers are an excellent model of the in vivo state of muscle, as they accurately represented the dystrophic phenotype.

Expression of the rat connexin 39 (rCx39) gene in myoblasts and myotubes in developing and regenerating skeletal muscles: an in situ hybridization study

We report a detailed analysis of the expression pattern of the recently identified rat connexin gene, named rat connexin 39 (rCx39), both during embryonic development and in adult life. Qualitative and quantitative reverse transcription/polymerase chain reaction analysis showed intense expression of rCx39 restricted to differentiating skeletal muscles, with a peak of expression detected at 18 days of embryonic life, followed by a rapid decline to undetectable levels within the first week of postnatal life. A combination of the in situ hybridization technique for the detection of rCx39 mRNA and immunohistochemistry for myogenin, a myoblast-specific marker, allowed us to establish that the mRNA for this connexin was expressed in myogenin-positive myoblasts and early myotubes but disappeared in mature myotubes. Moreover, in adult animals, rCx39 mRNA was expressed in myogenic cells involved in skeletal myofiber regeneration following a crush injury. This is the first case of a connexin being mainly expressed in the myogenic cell lineage. The information presented should pave the way to novel molecular approaches in studies on the role of connexin-based gap-junctional communication in skeletal muscle differentiation and regeneration.

Signaling satellite-cell activation in skeletal muscle: markers, models, stretch, and potential alternate pathways

Activation of skeletal muscle satellite cells, defined as entry to the cell cycle from a quiescent state, is essential for normal growth and for regeneration of tissue damaged by injury or disease. This review focuses on early events of activation by signaling through nitric oxide and hepatocyte growth factor, and by mechanical stimuli. The impact of various model systems used to study activation and the regulation of satellite-cell quiescence are placed in the context of activation events in other tissues, concluding with a speculative model of alternate pathways signaling satellite-cell activation.

m-Calpain implication in cell cycle during muscle precursor cell activation

Milli-calpain, a member of the ubiquitous cysteine protease family, is known to control late events of cell-cell fusion in skeletal muscle tissue through its involvement in cell membrane and cytoskeleton component reorganization. In this report, we describe the characterization of m-calpain compartmentalization and activation during the initial steps of muscle precursor cell recruitment and differentiation. By immunofluorescence analysis, we show that m-calpain is present throughout the cell cycle in the nucleus of proliferating myoblast C2 cells. However, when myoblasts enter a quiescent/G0 stage, m-calpain staining is detected only in the cytoplasm. Moreover, comparison of healthy and injured muscle shows distinct m-calpain localization in satellite stem cells. Indeed, m-calpain is not found in quiescent satellite cells, but following muscle injury, when satellite cells start to proliferate, m-calpain appears in the nucleus. To determine the implication of m-calpain during the cell cycle progression, quiescent myoblasts were forced to re-enter the cell cycle in the presence or not of the specific calpain inhibitor MDL 28170. We demonstrate that this calpain inhibitor blocks the cell cycle, prevents accumulation of MyoD in the G1 phase and enhances Myf5 expression. These data support an important new role for m-calpain in the control of muscle precursor cell activation and thus suggest its possible implication during the initial events of muscle regeneration.

Degeneration and regeneration of murine skeletal neuromuscular junctions after intramuscular injection with a sublethal dose of Clostridium sordellii lethal toxin

Clostridium sordellii lethal toxin (LT), a 250-kDa protein which is the bacteria's major virulence factor, belongs to a family of large clostridial cytotoxins which glucosylate small GTP-binding proteins. Here, we report the results of our ex vivo analysis of the structure and function of skeletal neuromuscular tissue obtained from mice at various times after intramuscular injection of a sublethal dose of LT (0.25 ng/g of body wt). The toxin caused, within 24 h, pronounced localized edema, inflammation, myofibril disassembly, and degeneration of skeletal muscle fibers in the injected area, and it glucosylated the muscle tissue's small GTPases. Regeneration of the damaged fibers was evident 6 to 9 days postinjury and was completed by 60 days. The expression of dystrophin, laminin, and fast and neonatal myosin in regenerating fibers, detected by immunofluorescence microscopy, confirmed that LT does not impair the high regenerative capacity of murine skeletal muscle fibers. Functional studies revealed that LT affects muscle contractility and neuromuscular transmission. However, partial recovery of nerve-evoked muscle twitches and tetanic contractions was observed by day 15 postinjection, and extensive remodeling of the neuromuscular junction's nerve terminals and clusters of muscle acetylcholine receptors was still evident 30 days postinjection. In conclusion, to the best of our knowledge, this is the first report to characterize the degeneration and regeneration of skeletal neuromuscular tissue after in vivo exposure to a large clostridial cytotoxin. In addition, our data may provide an explanation for the severe neuromuscular alterations accompanying wound infections caused by C. sordellii.

C-Met expression and mechanical activation of satellite cells on cultured muscle fibers

Single-fiber cultures can be used to model satellite cell activation in vivo. Although technical deficiencies previously prevented study of stretch-induced events, here we describe a method developed to study satellite cell gene expression by in situ hybridization (ISH) using protocol modifications for fiber adhesion and fixation. The hypothesis that mechanical stretching activates satellite cells was tested. Fiber cultures were established from normal flexor digitorum brevis muscles and plated on FlexCell dishes with a layer of Vitrogen. After 2 hr of stretch in the presence of BrdU, satellite cells on fibers attached to Vitrogen were activated above control levels. In the absence of activating treatments or mechanical stretch, ISH studies showed 0-6 c-Met+ satellite cells per fiber. Time course experiments demonstrated stable quiescence in the absence of stretch and significant peaks in activation after 30 min and 2 hr of stretch. Frequency distributions for unstretched fiber cultures showed a significantly greater number of quiescent c-Met+ satellite cells than were activated by stretching, suggesting that typical activation stimuli did not trigger cycling in the entire c-Met+ population of satellite cells. These methods have a strong potential to further dissect the nature of stretch-induced activation and gene expression among characterized populations of individual quiescent and activated satellite cells.

Expression profiling and identification of novel genes involved in myogenic differentiation

Skeletal muscle differentiation is a complex, highly coordinated process that relies on precise temporal gene expression patterns. To better understand this cascade of transcriptional events, we used expression profiling to analyze gene expression in a 12-day time course of differentiating C2C12 myoblasts. Cluster analysis specific for time-ordered microarray experiments classified 2895 genes and ESTs with variable expression levels between proliferating and differentiating cells into 22 clusters with distinct expression patterns during myogenesis. Expression patterns for several known and novel genes were independently confirmed by real-time quantitative RT-PCR and/or Western blotting and immunofluorescence. MyoD and MEF family members exhibited unique expression kinetics that were highly coordinated with cell-cycle withdrawal regulators. Among genes with peak expression levels during cell cycle withdrawal were Vcam1, Itgb3, Itga5, Vcl, as well as Ptger4, a gene not previously associated with the process of myogenesis. One interesting uncharacterized transcript that is highly induced during myogenesis encodes several immunoglobulin repeats with sequence similarity to titin, a large sarcomeric protein. These data sets identify many additional uncharacterized transcripts that may play important functions in muscle cell proliferation and differentiation and provide a baseline for comparison with C2C12 cells expressing various mutant genes involved in myopathic disorders.

Calcineurin initiates skeletal muscle differentiation by activating MEF2 and MyoD

Skeletal muscle differentiation is characterized by withdrawal from the cell cycle, expression of muscle specific genes, fusion into multinucleated cells, and assembly of the contractile apparatus. Although many of the key regulatory elements have been identified, the factors that initiate the differentiation process are not well understood. The calcium-dependent phosphatase calcineurin plays an important regulatory role early in myogenesis, but the downstream effectors of calcineurin in differentiation are not known. Here, we show that calcium and calcineurin regulate expression of the myogenin gene at the level of transcription. The myogenin promoter contains two essential elements; an E-box and an A/T rich element that bind MRF and MEF2 transcription factors, respectively. Both of these elements are responsive to calcium and calcineurin. In differentiating myoblasts, MyoD is the major MRF protein that binds to the myogenin promoter E-box. Calcineurin activates MyoD indirectly by decreasing the expression of the Id inhibitory proteins, probably by down-regulating Egr-1 expression, an upstream activator of Id transcription. These results demonstrate that calcineurin regulates skeletal muscle differentiation by activating MEF2 and MyoD transcription factors leading to the induction of myogenin expression.

Highly coordinated gene regulation in mouse skeletal muscle regeneration

Mammalian skeletal muscles are capable of regeneration after injury. Quiescent satellite cells are activated to reenter the cell cycle and to differentiate for repair, recapitulating features of myogenesis during embryonic development. To understand better the molecular mechanism involved in this process in vivo, we employed high density cDNA microarrays for gene expression profiling in mouse tibialis anterior muscles after a cardiotoxin injection. Among 16,267 gene elements surveyed, 3,532 elements showed at least a 2.5-fold change at one or more time points during a 14-day time course. Hierarchical cluster analysis and semiquantitative reverse transcription-PCR showed induction of genes important for cell cycle control and DNA replication during the early phase of muscle regeneration. Subsequently, genes for myogenic regulatory factors, a group of imprinted genes and genes with functions to inhibit cell cycle progression and promote myogenic differentiation, were induced when myogenic stem cells started to differentiate. Induction of a majority of these genes, including E2f1 and E2f2, was abolished in muscles lacking satellite cell activity after gamma radiation. Regeneration was severely compromised in E2f1 null mice but not affected in E2f2 null mice. This study identifies novel genes potentially important for muscle regeneration and reveals highly coordinated myogenic cell proliferation and differentiation programs in adult skeletal muscle regeneration in vivo.

Gene therapy and tissue engineering for sports medicine

Sports injuries usually involve tissues that display a limited capacity for healing. The treatment of sports injuries has improved over the past 10 to 20 years through sophisticated rehabilitation programs, novel operative techniques, and advances in the field of biomechanical research. Despite this considerable progress, no optimal solution has been found for treatment of various sports-related injuries, including muscle injuries, ligament and tendon ruptures, central meniscal tears, cartilage lesions, and delayed bone fracture healing. New biological approaches focus on the treatment of these injuries with growth factors to stimulate and hasten the healing process. Gene therapy using the transfer of defined genes encoding therapeutic proteins represents a promising way to efficiently deliver suitable growth factors into the injured tissue. Tissue engineering, which may eventually be combined with gene therapy, may potentially result in the creation of tissues or scaffolds for regeneration of tissue defects following trauma. In this article we will discuss why gene therapy and tissue engineering are becoming increasingly important in modern orthopaedic sports medicine practice. We then will review recent research achievements in the area of gene therapy and tissue engineering for sports-related injuries, and highlight the potential clinical applications of this technology in the treatment of patients with musculoskeletal problems following sports-related injuries.

Down-regulation of an ankyrin repeat-containing protein, V-1, during skeletal muscle differentiation and its re-expression in the regenerative process of muscular dystrophy

Using Western blot analysis and immunohistochemical methods, we examined the expression of V-1, a member of the ankyrin repeat-containing protein family, during differentiation and regeneration of skeletal muscle. The expression of V-1 was high in cultured myoblasts and decreased during their differentiation into myotubes, while high expression was maintained when muscle differentiation was inhibited by treatment with basic fibroblast growth factor. Down-regulation of V-1 also occurred during in vivo muscle differentiation from embryonic to postnatal stages, reaching an undetectable level in mature skeletal muscle. In contrast, strong V-1 immunoreactivity was detected again in myoblasts and regenerating muscle fibers with a small diameter, which were observed in Duchenne muscular dystrophy and its animal model, mdx mouse. Thus, it seems that V-1 is a good marker for early stage of muscle regeneration and changes of its expression suggest that V-1 plays a role in prenatal muscle differentiation and postnatal muscle regeneration.

Early posthatch starvation induces myonuclear apoptosis in chickens

The effect of early posthatch starvation on myonuclear apoptosis was examined in chickens. Male broiler chickens were or were not provided feed for the first 3-d posthatch. Subsequently, all chickens were provided feed for an additional 4-d posthatch. Chickens were killed at 3- and 7-d posthatch, and the pectoralis thoracicus was harvested, fixed and embedded in paraffin. Muscle sections were labeled with the terminal deoxynucleotidyl transferase histochemical staining technique to identify apoptotic nuclei. At 3- and 7-d posthatch, there was a significantly (P < 0.05) smaller myofiber cross-sectional area for the starved compared with the fed chickens. A larger proportion (P < 0.05) of apoptotic nuclei relative to total nuclei was observed in the starved compared to the fed chickens killed at 3-d posthatch, but the proportion of apoptotic nuclei relative to total nuclei did not differ (P > 0.05) between the starved and fed chickens killed at 7-d posthatch. It appears that apoptosis is a mechanism contributing to the smaller myofiber size observed when feed is not provided early posthatch.

Transient expression of phosphatidylserine at cell-cell contact areas is required for myotube formation

Cell surface exposure of phosphatidylserine (PS) is shown to be part of normal physiology of skeletal muscle development and to mediate myotube formation. A transient exposure of PS was observed on mouse embryonic myotubes at E13, at a stage of development when primary myotubes are formed. The study of this process in cell cultures of differentiating C2C12 and H9C2 myoblasts also reveals a transient expression of PS at the cell surface. This exposure of PS locates mainly at cell-cell contact areas and takes place at a stage when the structural organization of the sarcomeric protein titin is initiated, prior to actual fusion of individual myoblast into multinucleated myotubes. Myotube formation in vitro can be inhibited by the PS binding protein annexin V, in contrast to its mutant M1234, which lacks the ability to bind to PS. Although apoptotic myoblasts also expose PS, differentiating muscle cells show neither loss of mitochondrial membrane potential nor detectable levels of active caspase-3 protein. Moreover, myotube formation and exposure of PS cannot be blocked by the caspase inhibitor zVAD(OMe)-fmk. Our findings indicate that different mechanisms regulate PS exposure during apoptosis and muscle cell differentiation, and that surface exposed PS plays a crucial role in the process of myotube formation.

Muscle regeneration during hindlimb unloading results in a reduction in muscle size after reloading

The hindlimb-unloading model was used to study the ability of muscle injured in a weightless environment to recover after reloading. Satellite cell mitotic activity and DNA unit size were determined in injured and intact soleus muscles from hindlimb-unloaded and age-matched weight-bearing rats at the conclusion of 28 days of hindlimb unloading, 2 wk after reloading, and 9 wk after reloading. The body weights of hindlimb-unloaded rats were significantly (P < 0.05) less than those of weight-bearing rats at the conclusion of hindlimb unloading, but they were the same (P > 0.05) as those of weight-bearing rats 2 and 9 wk after reloading. The soleus muscle weight, soleus muscle weight-to-body weight ratio, myofiber diameter, number of nuclei per millimeter, and DNA unit size were significantly (P < 0.05) smaller for the injured soleus muscles from hindlimb-unloaded rats than for the soleus muscles from weight-bearing rats at each recovery time. Satellite cell mitotic activity was significantly (P < 0.05) higher in the injured soleus muscles from hindlimb-unloaded rats than from weight-bearing rats 2 wk after reloading, but it was the same (P > 0.05) as in the injured soleus muscles from weight-bearing rats 9 wk after reloading. The injured soleus muscles from hindlimb-unloaded rats failed to achieve weight-bearing muscle size 9 wk after reloading, because incomplete compensation for the decrease in myonuclear accretion and DNA unit size expansion occurred during the unloading period.

Calpain 3 mRNA expression in mice after denervation and during muscle regeneration

Lack of functional calpain 3 in humans is a cause of limb girdle muscular dystrophy, but the function(s) of calpain 3 remain(s) unknown. Special muscle conditions in which calpain 3 is downregulated could yield valuable clues to the understanding of its function(s). We monitored calpain 3 mRNA amounts by quantitative RT-PCR and compared them with those of α-skeletal actin mRNA in mouse leg muscles for different types of denervation and muscle injury. Intact muscle denervation reduced calpain 3 mRNA expression by a factor of 5 to 10, while α-skeletal actin mRNA was reduced in a slower and less extensive manner. Muscle injury (denervation-devascularization), which leads to muscle degeneration and regeneration, induced a 20-fold decrease in the mRNA level of both calpain 3 and α-skeletal actin. Furthermore, whereas in normal muscle and intact denervated muscle, the full-length transcript is the major calpain 3 mRNA, in injured muscle, isoforms lacking exon 6 are predominant during the early regeneration process. These data suggest that muscle condition determines the specific calpain 3 isoform pattern of expression and that calpain 3 expression is downregulated by denervation.

Dynamic restoration of dystrophin to dystrophin-deficient myotubes

Dystrophin domains are observed in myoblast transplantation experiments and in muscle fibers after somatic reversion in human Duchenne and mdx mouse muscular dystrophy. However, the formation and evolution of dystrophin-positive domains are not well established. Using a muscle satellite cell coculture system, we examined the dynamic restoration of dystrophin expression in dystrophin-deficient myotubes. The dystrophin-positive domains around source nuclei were clearly identified in hybrid myotubes. The occurrence of dystrophin domains was higher in myotubes differentiated from cocultures with a low concentration of normal wild-type satellite cells in relation to dystrophin-deficient satellite cells. At higher seeding ratios, the domain feature of dystrophin expression was more transitory and decreased as myotubes differentiated over time in culture. The average domain size initially increased with the addition of new nuclei by fusion early after differentiation of cocultures. However, separating dystrophin-positive domains according to their number of dystrophin-expressing contributory nuclei showed that diffusion of dystrophin contributed to domain elongation, even in early myotubes and later without fusion of additional nuclei. Diffusion occurred for all domains of one to six wild-type nuclei, and the diffusion rate was higher in domains with larger numbers of nuclei. This dynamic domain feature of dystrophin expression was also related to restoring the organization of dystrophin-associated proteins and acetylcholine receptors to hybrid myotubes. Factors regulating domain formation and diffusion therefore are important considerations in the design of strategies for both myoblast transplantation and gene therapy of Duchenne muscular dystrophy.

Hoechst fluorescence intensity can be used to separate viable bromodeoxyuridine-labeled cells from viable non-bromodeoxyuridine-labeled cells

BACKGROUND

5-Bromo-2'-deoxyuridine (BrdU) is a powerful compound to study the mitotic activity of a cell. Most techniques that identify BrdU-labeled cells require conditions that kill the cells. However, the fluorescence intensity of the membrane-permeable Hoechst dyes is reduced by the incorporation of BrdU into DNA, allowing the separation of viable BrdU positive (BrdU+) cells from viable BrdU negative (BrdU-) cells.

METHODS

Cultures of proliferating cells were supplemented with BrdU for 48 h and other cultures of proliferating cells were maintained without BrdU. Mixtures of viable BrdU+ and viable BrdU- cells from the two proliferating cultures were stained with Hoechst 33342. The viable BrdU+ and BrdU- cells were sorted into different fractions from a mixture of BrdU+ and BrdU- cells based on Hoechst fluorescence intensity and the ability to exclude the vital dye, propidium iodide. Subsequently, samples from the original mixture, the sorted BrdU+ cell population, and the sorted BrdU- cell population were immunostained using an anti-BrdU monoclonal antibody and evaluated using flow cytometry.

RESULTS

Two mixtures consisting of approximately 55% and 69% BrdU+ cells were sorted into fractions consisting of greater than 93% BrdU+ cells and 92% BrdU- cells. The separated cell populations were maintained in vitro after sorting to demonstrate their viability.

CONCLUSIONS

Hoechst fluorescence intensity in combination with cell sorting is an effective tool to separate viable BrdU+ from viable BrdU- cells for further study. The separated cell populations were maintained in vitro after sorting to demonstrate their viability.

p38 and extracellular signal-regulated kinases regulate the myogenic program at multiple steps

The extracellular signals which regulate the myogenic program are transduced to the nucleus by mitogen-activated protein kinases (MAPKs). We have investigated the role of two MAPKs, p38 and extracellular signal-regulated kinase (ERK), whose activities undergo significant changes during muscle differentiation. p38 is rapidly activated in myocytes induced to differentiate. This activation differs from those triggered by stress and cytokines, because it is not linked to Jun-N-terminal kinase stimulation and is maintained during the whole process of myotube formation. Moreover, p38 activation is independent of a parallel promyogenic pathway stimulated by insulin-like growth factor 1. Inhibition of p38 prevents the differentiation program in myogenic cell lines and human primary myocytes. Conversely, deliberate activation of endogenous p38 stimulates muscle differentiation even in the presence of antimyogenic cues. Much evidence indicates that p38 is an activator of MyoD: (i) p38 kinase activity is required for the expression of MyoD-responsive genes, (ii) enforced induction of p38 stimulates the transcriptional activity of a Gal4-MyoD fusion protein and allows efficient activation of chromatin-integrated reporters by MyoD, and (iii) MyoD-dependent myogenic conversion is reduced in mouse embryonic fibroblasts derived from p38alpha(-/-) embryos. Activation of p38 also enhances the transcriptional activities of myocyte enhancer binding factor 2A (MEF2A) and MEF2C by direct phosphorylation. With MEF2C, selective phosphorylation of one residue (Thr293) is a tissue-specific activating signal in differentiating myocytes. Finally, ERK shows a biphasic activation profile, with peaks of activity in undifferentiated myoblasts and postmitotic myotubes. Importantly, activation of ERK is inhibitory toward myogenic transcription in myoblasts but contributes to the activation of myogenic transcription and regulates postmitotic responses (i.e., hypertrophic growth) in myotubes.

Skeletal muscle regeneration is not impaired in Fgf6 -/- mutant mice

FGF6 is a member of the fibroblast growth factor family. The Fgf6 gene is almost exclusively expressed in adult and developing skeletal muscle. We have obtained mice deficient in FGF6 by targeting the Fgf6 gene by homologous recombination. We studied regeneration of adult skeletal muscle in Fgf6 -/- mice derived on a standard inbred background. Muscle degeneration was induced by notexin drug or crush injury. The defect in FGF6 did not modify the kinetics of muscle regeneration. We bred Fgf6 -/- mice with mdx dystrophin deficient mice; Fgf6 -/-:mdx and mdx muscles were similar. Our study suggests that FGF6 does not play a role in muscle regeneration, i.e., in satellite cell proliferation and fusion, or that this role is strictly compensated by other factors, possibly other FGFs.

A role for nitric oxide in muscle repair: nitric oxide-mediated activation of muscle satellite cells

Muscle satellite cells are quiescent precursors interposed between myofibers and a sheath of external lamina. Although their activation and recruitment to cycle enable muscle repair and adaptation, the activation signal is not known. Evidence is presented that nitric oxide (NO) mediates satellite cell activation, including morphological hypertrophy and decreased adhesion in the fiber-lamina complex. Activation in vivo occurred within 1 min after injury. Cell isolation and histology showed that pharmacological inhibition of nitric oxide synthase (NOS) activity prevented the immediate injury-induced myogenic cell release and delayed the hypertrophy of satellite cells in that muscle. Transient activation of satellite cells in contralateral muscles 10 min later suggested that a circulating factor may interact with NO-mediated signaling. Interestingly, satellite cell activation in muscles of mdx dystrophic mice and NOS-I knockout mice quantitatively resembled NOS-inhibited release of normal cells, in agreement with reports of displaced and reduced NOS expression in dystrophin-deficient mdx muscle and the complete loss of NOS-I expression in knockout mice. Brief NOS inhibition in normal and mdx mice during injury produced subtle alterations in subsequent repair, including apoptosis in myotube nuclei and myotube formation inside laminar sheaths. Longer NOS inhibition delayed and restricted the extent of repair and resulted in fiber branching. A model proposes the hypothesis that NO release mediates satellite cell activation, possibly via shear-induced rapid increases in NOS activity that produce "NO transients."

Retroviral labeling is an appropriate marker for dividing cells

5-Bromo-2'-deoxyuridne (BrdU) and 3H-thymidine label mitotically active cells, but they do not adequately mark the progeny of dividing cells for long term study. An alternative method is to label cells using the replication-defective CXL retroviral vector, which carries the lacZ gene encoding beta-galactosidase; however, the ability of the CXL retroviral vector to pulse-label mitotically active cells selectively is not known. Cultures of proliferating muscle cells were simultaneously incubated with the CXL retrovirus and BrdU (10 microM) for 2 hr. After removing the retrovirus containing medium, the cells were maintained for an additional 24 hr in vitro before they were stained to detect beta-galactosidase and BrdU simultaneously. More than 95% of beta-galactosidase positive cells were also BrdU positive suggesting that the majority of beta-galactosidase positive cells were in the S-phase of the cell cycle at the time of CXL retroviral administration. Therefore, the CXL retroviral vector is an appropriate pulse marker for dividing cells, and it is useful when it is desirable to know the fate of the progeny of a particular cell following a mitotic event.

Unloading of juvenile muscle results in a reduced muscle size 9 wk after reloading

The role of satellite cells and DNA unit size in determining muscle size was examined by inhibiting postnatal skeletal muscle development by using hindlimb suspension. Satellite cell mitotic activity and DNA unit size were determined in the soleus muscles from hindlimb-suspended and age-matched weight-bearing rats before the initiation of hindlimb suspension, at the conclusion of a 28-day hindlimb-suspension period, 2 wk after reloading, and 9 wk after reloading. The body weights of hindlimb-suspended rats were significantly (P < 0.05) less than those of weight-bearing rats at the conclusion of hindlimb suspension, but they were the same (P > 0. 05) as those of weight-bearing rats 9 wk after reloading. The soleus muscle weight, soleus muscle weight-to-body weight ratio, myofiber diameter, nuclei per millimeter, and DNA unit size for the hindlimb-suspended rats were significantly (P < 0.05) smaller than for the weight-bearing rats at all recovery times. Satellite cell mitotic activity was significantly (P < 0.05) higher in the soleus muscles from hindlimb-suspended rats 2 wk after reloading, but it was the same (P > 0.05) as in weight-bearing rats 9 wk after reloading. Juvenile soleus muscles failed to achieve normal muscle size 9 wk after reloading because there was incomplete compensation for the hindlimb-suspension-induced interruptions in myonuclear accretion and DNA unit size expansion.

The transition from proliferation to differentiation is delayed in satellite cells from mice lacking MyoD

Satellite cells from adult rat muscle coexpress proliferating cell nuclear antigen and MyoD upon entry into the cell cycle, suggesting that MyoD plays a role during the recruitment of satellite cells. Moreover, the finding that muscle regeneration is compromised in MyoD-/- mice, has provided evidence for the role of MyoD during myogenesis in adult muscle. In order to gain further insight into the role of MyoD during myogenesis in the adult, we compared satellite cells from MyoD-/- and wildtype mice as they progress through myogenesis in single-myofiber cultures and in tissue-dissociated cell cultures (primary cultures). Satellite cells undergoing proliferation and differentiation were traced immunohistochemically using antibodies against various regulatory proteins. In addition, an antibody against the mitogen-activated protein kinases ERK1 and ERK2 was used to localize the cytoplasm of the fiber-associated satellite cells regardless of their ability to express specific myogenic regulatory factor proteins. We show that during the initial days in culture the myofibers isolated from both the MyoD-/- and the wildtype mice contain the same number of proliferating, ERK+ satellite cells. However, the MyoD-/- satellite cells continue to proliferate and only a very small number of cells transit into the myogenin+ state, whereas the wildtype cells exit the proliferative compartment and enter the myogenin+ stage. Analyzing tissue-dissociated cultures of MyoD-/- satellite cells, we identified numerous cells whose nuclei were positive for the Myf5 protein. In contrast, quantification of Myf5+ cells in the wildtype cultures was difficult due to the low level of Myf5 protein present. The Myf5+ cells in the MyoD-/- cultures were often positive for desmin, similar to the MyoD+ cells in the wildtype cultures. Myogenin+ cells were identified in the MyoD-/- primary cultures, but their appearance was delayed compared to the wildtype cells. These "delayed" myogenin+ cells can express other differentiation markers such as MEF2A and cyclin D3 and fuse into myotubes. Taken together, our studies suggest that the presence of MyoD is critical for the normal progression of satellite cells into the myogenin+, differentiative state. It is further proposed that the Myf5+/MyoD- phenotype may represent the myogenic stem cell compartment which is capable of maintaining the myogenic precursor pool in the adult muscle.