Skeletal muscle stem cells

Attributes of adult stem cells

While cultured embryonic stem (ES) cells can be harvested in abundance and appear to be the most versatile of cells for regenerative medicine, adult stem cells also hold promise, but the identity and subsequent isolation of these comparatively rare cells remains problematic in most tissues, perhaps with the notable exception of the bone marrow. The ability to continuously self-renew and produce the differentiated progeny of the tissue of their location are their defining properties. Identifying surface molecules (markers) that would aid in stem cell isolation is a major goal. Considerable overlap exists between different putative organ-specific stem cells in their repertoire of gene expression, often related to self-renewal, cell survival and cell adhesion. More robust tests of 'stemness' are now being employed, using lineage-specific genetic marking and tracking to show production of long-lived clones and multipotentiality in vivo. Moreover, the characterization of normal stem cells in specific tissues may provide a dividend for the treatment of cancer. The successful treatment of neoplastic disease may well require the specific targeting of neoplastic stem cells, cells that may well have many of the characteristics of their normal counterparts.

Regulation of cell proliferation by fast Myosin light chain 1 in myoblasts derived from extraocular muscle, diaphragm and gastrocnemius

The extraocular muscle (EOM) suffers much less injury from Duchenne muscular dystrophy (DMD) than other skeletal muscles such as diaphragm and gastrocnemius. The present study was undertaken to test the hypothesis that differential expression of regulatory proteins between the EOM and other skeletal muscles is responsible for the observed difference in the sensitivity to DMD-associated damage. Protein expression in the tissue samples obtained from EOM, diaphragm or gastrocnemius of C57BL/6 mice was analyzed by two-dimensional gel electrophoresis and mass spectrometry. There were 35 proteins that were identified to be differentially expressed among different skeletal muscle tissues. Among the 35 proteins, a fast skeletal muscle isoform myosin light chain 1 (MLC1f) protein was further studied in relation to muscle cell proliferation. The EOM-derived myoblasts had much lower levels of MLC1f and higher rate of cell proliferation in contrast to the myoblasts derived from diaphragm or gastrocnemius, which displayed a higher expression of MLC1f along with a slow proliferation. Deletion of MLC1f using siRNA targeting MLC1f resulted in an increased rate of cell proliferation in the myoblasts. Cell cycle analysis revealed that MLC1f inhibited the transition of the cell cycle from the G1 to the S phase. Therefore, the present study demonstrates that MLC1f may negatively regulate proliferation of myoblasts through inhibition of the transition from the G1 to the S phase of the cell cycle. Low levels of MLC1f in myoblasts of EOM may ensure cell proliferation and enhance the repair process for EOM under the DMD disease condition, thus making EOM suffer less injury from DMD.

Cells that express MyoD mRNA in the epiblast are stably committed to the skeletal muscle lineage

The epiblast of the chick embryo contains cells that express MyoD mRNA but not MyoD protein. We investigated whether MyoD-positive (MyoD^pos) epiblast cells are stably committed to the skeletal muscle lineage or whether their fate can be altered in different environments. A small number of MyoD^pos epiblast cells were tracked into the heart and nervous system. In these locations, they expressed MyoD mRNA and some synthesized MyoD protein. No MyoD^pos epiblast cells differentiated into cardiac muscle or neurons. Similar results were obtained when MyoD^pos cells were isolated from the epiblast and microinjected into the precardiac mesoderm or neural plate. In contrast, epiblast cells lacking MyoD differentiated according to their environment. These results demonstrate that the epiblast contains both multipotent cells and a subpopulation of cells that are stably committed to the skeletal muscle lineage before the onset of gastrulation. Stable programming in the epiblast may ensure that MyoD^pos cells express similar signaling molecules in a variety of environments.

FoxK1splice variants show developmental stage-specific plasticity of expression with temperature in the tiger pufferfish

FoxK1 is a member of the highly conserved forkhead/winged helix (Fox)family of transcription factors and it is known to play a key role in mammalian muscle development and myogenic stem cell function. The tiger pufferfish (Takifugu rubripes) orthologue of mammalian FoxK1(TFoxK1) has seven exons and is located in a region of conserved synteny between pufferfish and mouse. TFoxK1 is expressed as three alternative transcripts: TFoxK1-α, TFoxK1-γ and TFoxK1-δ. TFoxK1-α is the orthologue of mouse FoxK1-α, coding for a putative protein of 558 residues that contains the forkhead and forkhead-associated domains typical of Fox proteins and shares 53% global identity with its mammalian homologue. TFoxK1-γ and TFoxK1-δ arise from intron retention events and these transcripts translate into the same 344-amino acid protein with a truncated forkhead domain. Neither are orthologues of mouse FoxK1-β. In adult fish, the TFoxK1 splice variants were differentially expressed between fast and slow myotomal muscle, as well as other tissues, and the FoxK1-α protein was expressed in myogenic progenitor cells of fast myotomal muscle. During embryonic development, TFoxK1 was transiently expressed in the developing somites, heart,brain and eye. The relative expression of TFoxK1-α and the other two alternative transcripts varied with the incubation temperature regime for equivalent embryonic stages and the differences were particularly marked at later developmental stages. The developmental expression pattern of TFoxK1 and its localisation to mononuclear myogenic progenitor cells in adult fast muscle indicate that it may play an essential role in myogenesis in T. rubripes.

Human adult skeletal muscle stem cells differentiate into cardiomyocyte phenotype in vitro

Cell transplantation to repair or regenerate injured myocardium is a new frontier in the treatment of cardiovascular disease. Most studies on stem cell transplantation therapy in both experimental heart infarct and in phase-I human clinical trials have focused on the use of undifferentiated stem cells. Based on our previous observations demonstrating the presence of multipotent progenitor cells in human adult skeletal muscle, in this study we investigated the capacity of these progenitors to differentiate into cardiomyocytes. Here we show an efficient protocol for the cardiomyogenic differentiation of human adult skeletal muscle stem cells in vitro. We found that treatment with Retinoic Acid directed cardiomyogenic differentiation of skeletal muscle stem cells in vitro. After Retinoic Acid treatment, cells expressed cardiomyocyte markers and acquired spontaneous contraction. Functional assays exhibited cardiac-like response to increased extracellular calcium. When cocultured with mouse cardiomyocytes, Retinoic Acid-treated skeletal muscle stem cells expressed connexin43 and when transplanted into ischemic heart were detectable even 5 weeks after injection. Based on these results, we can conclude that human adult skeletal muscle stem cells, if opportunely treated, can transdifferentiate into cells of cardiac lineage and once injected into infarcted heart can integrate, survive in cardiac tissue and improve the cardiac function.

Differential roles of HIC-5 isoforms in the regulation of cell death and myotube formation during myogenesis

Hic-5 is a LIM-Only member of the paxillin superfamily of focal adhesion proteins. It has been shown to regulate a range of biological processes including: senescence, tumorigenesis, steroid hormone action, integrin signaling, differentiation, and apoptosis. To better understand the roles of Hic-5 during development, we initiated a detailed analysis of Hic-5 expression and function in C(2)C(12) myoblasts, a well-established model for myogenesis. We have found that: (1) myoblasts express at least 6 distinct Hic-5 isoforms; (2) the two predominant isoforms, Hic-5alpha and Hic-5beta, are differentially expressed during myogenesis; (3) any experimentally induced change in Hic-5 expression results in a substantial increase in apoptosis during differentiation; (4) ectopic expression of Hic-5alpha is permissive to differentiation while expression of either Hic-5beta or antisense Hic-5 blocks myoblast fusion but not chemodifferentiation; (5) Hic-5 localizes to focal adhesions in C(2)C(12) myoblasts and perturbation of Hic-5 leads to defects in cell spreading; (6) alterations in Hic-5 expression interfere with the normal dynamics of laminin expression; and (7) ectopic laminin, but not fibronectin, can rescue the Hic-5-induced blockade of myoblast survival and differentiation. Our data demonstrate differential roles for individual Hic-5 isoforms during myogenesis and support the hypothesis that Hic-5 mediates these effects via integrin signaling.

Stra13 regulates satellite cell activation by antagonizing Notch signaling

Satellite cells play a critical role in skeletal muscle regeneration in response to injury. Notch signaling is vital for satellite cell activation and myogenic precursor cell expansion but inhibits myogenic differentiation. Thus, precise spatial and temporal regulation of Notch activity is necessary for efficient muscle regeneration. We report that the basic helix-loop-helix transcription factor Stra13 modulates Notch signaling in regenerating muscle. Upon injury, Stra13(-/-) mice exhibit increased cellular proliferation, elevated Notch signaling, a striking regeneration defect characterized by degenerated myotubes, increased mononuclear cells, and fibrosis. Stra13(-/-) primary myoblasts also exhibit enhanced Notch activity, increased proliferation, and defective differentiation. Inhibition of Notch signaling ex vivo and in vivo ameliorates the phenotype of Stra13(-/-) mutants. We demonstrate in vitro that Stra13 antagonizes Notch activity and reverses the Notch-imposed inhibition of myogenesis. Thus, Stra13 plays an important role in postnatal myogenesis by attenuating Notch signaling to reduce myoblast proliferation and promote myogenic differentiation.

Simvastatin reduces insulin-like growth factor-1 signaling in differentiating C2C12 mouse myoblast cells in an HMG-CoA reductase inhibition-independent manner

Inhibitors of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase occasionally cause myopathy characterized by weakness, pain, and elevated serum creatine phosphokinase (CK). In this study, we investigated the effects of simvastatin, an HMG-CoA reductase inhibitor, on the viability and insulin-like growth factor-1 (IGF-1) signaling in differentiating C2C12 mouse myoblast cells. Simvastatin decreased cell viability and CK activity, a marker of myogenesis, in differentiating cells in a dose-dependent manner. Although the simvastatin-induced decrease in viability in proliferating and differentiated cells was completely abolished by mevalonate or geranylgeranyl-pyrophosphate, the inhibitory effects of simvastatin in differentiating cells were not abolished by mevalonate or isoprenoid derivatives of mevalonate. Moreover, the sensitivity of differentiating cells to simvastatin regarding cell viability was about 7 times higher than that of proliferating cells. After induction of differentiation in the presence of 1 microM simvastatin for 2 days, IGF-1-induced activation of ERK1/2 and Akt was significantly decreased. Although mRNA expression of the IGF-1 receptor beta-chain (IGF-1R beta) did not change, protein level of the 200 kDa IGF-1Rbeta precursor was significantly increased by simvastatin in a dose-dependent manner. Mevalonate did not abolish the effect of simvastatin on IGF-1Rbeta expression. These results suggest that simvastatin decreases IGF-1 signaling via a regulation of the post-translational modification of IGF-1Rbeta in an HMG-CoA reductase inhibition-independent manner.

Control of fatty acid metabolism by leptin in L6 rat myoblasts is regulated by hyperinsulinemia

The development of hypothalamic leptin resistance plays a role in the development of obesity, yet whether peripheral leptin resistance occurs in obesity and diabetes is controversial. Here we investigate whether hyperinsulinemia, as observed during the development of Type 2 diabetes, modifies the effects of leptin on long chain fatty acid metabolism in skeletal muscle cells. We used boron dipyrromethene difluoride (BODIPY)-labeled palmitate to show that leptin (60 nM) caused a time-dependent (0-60 min) increase in fatty acid uptake in L6 myoblasts. Quantitative analysis using 3H-palmitate showed that pre-incubation with insulin (100 nM, 24 h) prevented stimulation of fatty acid uptake by leptin. Insulin pre-treatment also attenuated the ability of leptin to phosphorylate acetyl Co-A carboxylase and increase palmitate oxidation. Suppressor of cytokine-3 (SOCS-3) has been proposed as a possible mediator of insulin-induced leptin resistance. Here we show that treatment of L6 cells with insulin elicited a time-dependent increase in both SOCS-3 mRNA and protein content. In summary, hyperinsulinemia can induce leptin resistance in L6 myoblasts and this may be mediated via a SOCS-3-dependent mechanism.

Bone marrow-derived mesenchymal stem cells for regenerative medicine in craniofacial region

The craniofacial region contains many specified tissues including bone, cartilage, muscle, blood vessels and neurons. Defect or dysfunction of the craniofacial tissue after post-cancer ablative surgery, trauma, congenital malformations and progressive deforming skeletal diseases has a huge influence on the patient's life. Therefore, functional reconstruction of damaged tissues is highly expected. Bone marrow-derived mesenchymal stem cells (BMMSCs) are one of the most well characterized postnatal stem cell populations, and considered to be utilized for cell-based clinical therapies. Here, the current understanding and the potential applications in craniofacial tissue regeneration of BMMSCs are reviewed, and the current limitations and drawbacks are also discussed.

Transdifferentiation of porcine satellite cells to adipoblasts with ciglitizone

Ciglitizone, a class of thiazolidinediones, acts as a potent activator of the adipose differentiation program in established preadipose cell lines. Thiazolidinediones have also been investigated in diabetic patients and have been reported to act as peroxisome proliferator-activated receptor-gamma ligands. Intramuscular adipogenesis or marbling through transdifferentiation of satellite cells in cattle was successfully conducted earlier. In this report, the effects of ciglitizone on the differentiation pathway of porcine myogenic satellite cells was investigated. Semitendinosus muscle was aseptically taken from 10-d-old piglets under general anesthesia, and porcine satellite cells were obtained and grown to near confluence. Postconfluent cells (d 0) were further cultured in differentiation medium containing an adipogenic mixture plus ciglitizone (10 microM) for 48 h. From d 2 onward, the cells were cultured only in the presence of ciglitizone until d 10. Controls were cultured in differentiation medium only. Exposure of porcine satellite cells to the adipogenic mixture plus ciglitizone generated lipid droplets on d 2, and subsequently, exposure of cells to ciglitizone alone helped in cytoplasmic lipid filling, providing them with the acquisition of adipocyte morphology. An increase (P < 0.05) in the fusion (structures containing 2 to 3 nuclei) of satellite cells was observed, and myosin heavy chain appeared with greater intensity (immunohistochemistry) in the control group from d 2 onward. Adipocyte-specific transcriptional factors (i.e., CCAAT/enhancer binding protein-alpha and peroxisome proliferator-activated receptor-gamma) were predominant during transdifferentiation and were observed with immunohistochemistry, Western blot (approximately 47.2 and approximately 60.4 kDa, respectively), and real-time PCR. Ciglitizone appeared to convert the differentiation pathway of satellite cells into that of adipoblasts.

Muscling in on stem cells

Skeletal muscle is one of the few adult tissues that possesses the capacity for regeneration (restoration of lost functional tissue) as opposed to repair. This capacity is due to the presence of 'muscle stem cells' known as satellite cells. Detailed investigation of these cells over the past 50 years has revealed that both these and other cells within the skeletal muscle complex are capable of regenerating both muscle and other cell types as well. Here, we review this information, and suggest that skeletal muscle is an exciting reservoir of cells for regenerating skeletal muscle itself, as well as other cell types.

Integrin α3 subunit participates in myoblast adhesion and fusion in vitro

Satellite cells are myogenic precursor cells, participating in growth, and regeneration of skeletal muscles. The proteins that play a role in myogenesis are integrins. In this report, we show that the integrin alpha3 subunit is expressed in quiescent satellite cells and activated myoblasts. We also find that in myoblasts the integrin alpha3 subunit is localized at cell-cell and cell-extracellular matrix contacts. We notice that increase in protein and mRNA encoding the integrin alpha3 subunit accompanies myoblast differentiation. Using double immunofluorescence and immunoprecipitation experiments, we demonstrate that the integrin alpha3 subunit co-localizes with actin, and binds the integrin beta1 subunit and ADAM12, suggesting that the complex alpha3beta1/ADAM12 is probably involved in myoblast fusion. Importantly, overexpression of the full-length integrin alpha3 subunit increases myoblast fusion whereas an antibody against its extracellular domain inhibits fusion. These data demonstrate that the integrin alpha3 subunit may contribute to satellite cell activation and then myoblast adhesion and fusion.

Transition of Homer isoforms during skeletal muscle regeneration

Homer represents a new and diversified family of proteins that includes several isoforms, Homer 1, 2, and 3; some of these isoforms have been reported to be present in striated muscles. In this study, the presence of Homer isoforms 1a, 1b/c/d, 2b, and 3 was thoroughly investigated in rat skeletal muscles under resting conditions. Transition in Homer isoforms compositon was studied under experimental conditions of short-term and long-term adaptation, e.g., fatigue and regeneration, respectively. First, we show that Homer 1a was constitutively expressed and was transiently upregulated during regeneration. In C2C12 cell cultures, Homer 1a was also upregulated during formation of myotubes. No change of Homer 1a was observed in fatigue. Second, Homer 1b/c/d and Homer 2b were positively and linearly related to muscle mass change during regeneration, and third, Homer 3 was not detectable under resting conditions but was transiently expressed during regeneration although with a temporal pattern distinct from that of Homer 1a. Thus a switch in Homer isoforms is associated to muscle differentiation and regeneration. Homers may play a role not only in signal transduction of skeletal muscle, in particular regulation of Ca2+ release from sarcoplasmic reticulum (Ward CW, Feng W, Tu J, Pessah IN, Worley PF, and Schneider MF. Homer protein increases activation of Ca2+ sparks in permeabilized skeletal muscle. J Biol Chem 279: 5781–5787, 2004), but also in adaptation.

Regulating activation of transplanted cells controls tissue regeneration

Current approaches to tissue regeneration are limited by the death of most transplanted cells and/or resultant poor integration of transplanted cells with host tissue. We hypothesized that transplanting progenitor cells within synthetic microenvironments that maintain viability, prevent terminal differentiation, and promote outward migration would significantly enhance their repopulation and regeneration of damaged host tissue. This hypothesis was addressed in the context of muscle regeneration by transplanting satellite cells to muscle laceration sites on a delivery vehicle releasing factors that induce cell activation and migration (hepatocyte growth factor and fibroblast growth factor 2) or transplantation on materials lacking factor release. Controls included direct cell injection into muscle, the implantation of blank scaffolds, and scaffolds releasing factors without cells. Injected cells demonstrated a limited repopulation of damaged muscle and led to a slight improvement in muscle regeneration, as expected. Delivery of cells on scaffolds that did not promote migration resulted in no improvement in muscle regeneration. Strikingly, delivery of cells on scaffolds that promoted myoblast activation and migration led to extensive repopulation of host muscle tissue and increased the regeneration of muscle fibers at the wound and the mass of the injured muscle. This previously undescribed strategy for cell transplantation significantly enhances muscle regeneration from transplanted cells and may be broadly applicable to the various tissues and organ systems in which provision and instruction of a cell population competent to participate in regeneration may be clinically useful.

Autologous Bone Marrow-Derived Cells Enhance Muscle Strength Following Skeletal Muscle Crush Injury in Rats

Insufficient post-traumatic skeletal muscle regeneration with consecutive functional deficiency continues to be a serious problem in orthopedic and trauma surgery. Transplantation of autologous muscle precursor cells has shown encouraging results in muscle trauma treatment but is associated with significant donor site morbidity. In contrast to this, bone marrow-derived (BMD) cells can be obtained without any functional deficit by puncture. The goal of this study was to examine whether regular muscle regeneration can be improved by local application of autologous BMD cells in a rat model of blunt skeletal muscle trauma. One week after standardized open blunt crush injury to the left soleus muscle, 10(6) autologous BMD cells were injected into the traumatized muscle of male Sprague Dawley rats. Rats of the control group received saline solution as treatment. Three weeks after application, the fast twitch and tetanic contraction capacity of the soleus muscles was measured bilaterally by stimulating the sciatic nerves. Contraction forces of injured soleus muscles in control animals recovered to 39 +/- 10% (tetanic) and 59 +/- 12% (fast twitch) of the contralateral noninjured soleus muscles (p < 0.001). In contrast, autologous BMD cell injection significantly restored contractile forces to 53 +/- 8% (tetanic) and 72 +/- 13% (fast twitch) compared to those observed in contralateral noninjured soleus muscles. Thus, muscle function was significantly increased by BMD cell treatment (tetanic, p = 0.014; fast twitch, p = 0.05). In conclusion, autologous BMD cell grafting leads to an increase in contraction force, 14% in tetanic and 13% in fast twitch stimulation, demonstrating its potential to improve functional outcome after skeletal muscle crush injury.

Modulation of muscle regeneration, myogenesis, and adipogenesis by the Rho family guanine nucleotide exchange factor GEFT

Rho family guanine nucleotide exchange factors (GEFs) regulate diverse cellular processes including cytoskeletal reorganization, cell adhesion, and differentiation via activation of the Rho GTPases. However, no studies have yet implicated Rho-GEFs as molecular regulators of the mesenchymal cell fate decisions which occur during development and repair of tissue damage. In this study, we demonstrate that the steady-state protein level of the Rho-specific GEF GEFT is modulated during skeletal muscle regeneration and that gene transfer of GEFT into cardiotoxin-injured mouse tibialis anterior muscle exerts a powerful promotion of skeletal muscle regeneration in vivo. In order to molecularly characterize this regenerative effect, we extrapolate the mechanism of action by examining the consequence of GEFT expression in multipotent cell lines capable of differentiating into a number of cell types, including muscle and adipocyte lineages. Our data demonstrate that endogenous GEFT is transcriptionally upregulated during myogenic differentiation and downregulated during adipogenic differentiation. Exogenous expression of GEFT promotes myogenesis of C2C12 cells via activation of RhoA, Rac1, and Cdc42 and their downstream effector proteins, while a dominant-negative mutant of GEFT inhibits this process. Moreover, we show that GEFT inhibits insulin-induced adipogenesis in 3T3L1 preadipocytes. In summary, we provide the first evidence that the Rho family signaling pathways act as potential regulators of skeletal muscle regeneration and provide the first reported molecular mechanism illustrating how a mammalian Rho family GEF controls this process by modulating mesenchymal cell fate decisions.

MyoD-cre transgenic mice: a model for conditional mutagenesis and lineage tracing of skeletal muscle

The Cre-loxP recombination system has been used to great advantage in vivo for conditional gene targeting, lineage tracing, and other applications. To express cre in skeletal myoblasts and muscle fibers, we utilized the well-characterized transcriptional regulatory regions of the muscle determination gene, MyoD. Transgenic mouse lines were produced (F3/-2.5cre) in which the cre gene is driven by the MyoD promoter and core enhancer, which directs the early activation of MyoD. Specificity of cre expression and efficiency of recombination was determined by monitoring reporter gene expression after crossing to the Cre-dependent reporter lines, R26R and Z/AP. Efficient labeling of embryonic and fetal myoblasts and muscle fibers was observed, with timing that was similar (branchial arches and limb buds) or slightly delayed (myotomes) relative to the endogenous MyoD gene. In satellite cell cultures, a strict concordance between MyoD protein and reporter gene expression was observed, demonstrating the muscle specificity and efficiency of Cre-mediated recombination. Nascent muscle fibers were labeled following injury of adult muscle, indicating recombination in satellite cells or their daughters in vivo.

Homologous muscle acellular matrix seeded with autologous myoblasts as a tissue-engineering approach to abdominal wall-defect repair

Myoblasts were obtained by culturing in vitro, single muscle fibers, isolated by enzymatic digestion from rat flexor digitorum brevis, and their phenotype was confirmed by myogenic differentiation factor, myogenic factor-5, myogenin and desmin. Cultured myoblasts were harvested and seeded on patches of homologous acellular matrix, obtained by detergent-enzymatic treatment of abdominal muscle fragments. Myoblast-seeded patches were inserted between obliqui abdominis muscles on the right side of 1-month-old rats, while non-seeded patches were implanted on the left side. Thirty days after surgery, non-seeded patches were completely replaced by fibrous tissue, while the structure of myoblast-seeded patches was well preserved until the 2nd month. Seeded patches displayed abundant blood vessels and myoblasts, and electromyography evidenced in them single motor-unit potentials, sometimes grouped into arithmic discharges. Ninty days after implantation, the thickness of myoblast-seeded patches and their electric activity decreased, suggesting a loss of contractile muscle fibers. In conclusion, the present results indicate that autologous myoblast-homologous acellular muscle matrix constructs are a promising tool for body-wall defect repair, and studies are under way to identify strategies able to improve and maintain the structural and functional integrity of implants for longer periods.

Cyclic mechanical strain inhibits skeletal myogenesis through activation of focal adhesion kinase, Rac-1 GTPase, and NF-kappaB transcription factor

Myogenesis is a multistep developmental program that generates and regenerates skeletal muscles. Several extracellular factors have been identified that participate in the regulation of myogenesis. Although skeletal muscles are always subjected to mechanical stress in vivo, the role of mechanical forces in the regulation of myogenesis remains unknown. We have investigated the molecular mechanisms by which cyclic mechanical strain modulates myogenesis. Application of cyclic mechanical strain using the computer-controlled Flexcell Strain Unit increased the proliferation of C2C12 cells and inhibited their differentiation into myotubes. Cyclic strain increased the activity of cyclin-dependent kinase 2 (cdk2) and the cellular level of cyclin A, and inhibited the expression of myosin heavy chain and formation of myotubes in C2C12 cultures. The activity of nuclear factor-kappa B (NF-kappaB) transcription factor and the expression of NF-kappaB-regulated genes, cyclin D1 and IL-6, were augmented in response to mechanical strain. Cyclic strain also increased the activity of Rho GTPases, especially Rac-1. The inhibition of Rho GTPases activity, by overexpression of Rho GDP dissociation inhibitor (Rho-GDI), inhibited the strain-induced activation of NF-kappaB in C2C12 cells. Overexpression of either NF-kappaB inhibitory protein IkappaBalphaDeltaN (a degradation resistant mutant IkappaBalpha) or Rho-GDI blocked the strain-induced proliferation of C2C12 cells. Furthermore, overexpression of FRNK, a dominant negative mutant of focal adhesion kinase (FAK), inhibited the strain-induced proliferation of C2C12 cells. Our study demonstrates that cyclic mechanical strain inhibits myogenesis through the activation of FAK, Rac-1, and NF-kappaB.

Endothelial cells within embryonic skeletal muscles: a potential source of myogenic progenitors

We investigated whether the vessel-associated or endothelial cells within mouse embryo muscles can be a source of myogenic progenitors. Immunodetection of the stem cell surface markers, CD34 and Flk1, which are known to characterize the endothelial lineage, was done throughout the course of embryo muscle development. Both markers appeared to be restricted to the vessel-associated cells. On the basis of CD34 labeling, the reactive cells were purified by magnetic-bead selection from the limb muscles of 17-dpc desmin+/-LacZ mouse embryos and characterized by fluorescence-activated cell sorting. The cells in the selected CD34(+) population appeared to be approximately 95% positive for Flk1, but usually negative for CD45. We demonstrated that in vitro the CD34(+)/Flk1(+) population differentiated into endothelial cells and skeletal myofibers. When transplanted into mdx mouse muscle, this population displayed a high propensity to disperse within the recipient muscle, fuse with the host myofibers, and restore dystrophin expression. The marked ability of the embryonic muscle endothelial cells to activate myogenic program could be related to their somitic origin.

Skeletal Stem Cells in Regenerative Medicine

Postnatal stem cells have been isolated from a variety of tissues and they are highly expected to have potentiality to be utilized for cell-based clinical therapies. Bone marrow stromal stem cells (BMSSCs) derived from bone marrow stromal tissue have been identified as a population of multipotent mesenchymal stem cells that are capable of differentiating into osteoblasts, adipocytes, chondrocytes, muscle cells, and neural cells. The most significant tissue regeneration trait of BMSSCs is their in vivo bone regeneration capability, which has been widely studied for understanding molecular and cellular mechanisms of osteogenesis, and, more importantly, developing into a stem-cell-based therapy. Recent studies further demonstrated that BMSSC-mediated bone regeneration is a promising approach for regenerative medicine in clinical trials. However, there are some fundamental questions that remain to be answered prior to successful utilization of BMSSCs in clinical therapy. For instance, how to maintain stemness of BMSSCs will be a critical issue for developing methodologies to propagate multi-potential stem cells in vitro, in order to allow the development of effective clinical therapies.

Membrane traffic in skeletal muscle

Skeletal muscle tissue is made up of highly organized multinuclear cells. The internal organization of the muscle cell is dictated by the necessary regular arrangement of repeated units within the protein myofibrils that mediate muscle contraction. Skeletal muscle cells have the usual membrane traffic pathways for partitioning newly synthesized proteins, internalizing cell surface receptors for hormones and nutrients, and mediating membrane repair. However, in muscle, these pathways must be further specialized to deal with targeting to and organizing muscle-specific membrane structures, satisfying the unique metabolic requirements of muscle and meeting the high demand for membrane repair in a tissue that is constantly under mechanical stress. Specialized membrane traffic pathways in muscle also play a role in the formation of muscle through fusion of myoblast membranes and the development of internal muscle-specific membrane structures during myogenesis and regeneration. It has recently become apparent that muscle-specific isoforms of proteins that are known to mediate ubiquitous membrane traffic pathways, as well as novel muscle-specific proteins, are involved in tissue-specific aspects of muscle membrane traffic. Here we describe the specialized membrane structures of skeletal muscle, how these are developed, maintained and repaired by specialized and generic membrane traffic pathways, and how defects in these pathways result in muscle disease.

Ca2+ channel regulation by transforming growth factor-beta 1 and bone morphogenetic protein-2 in developing mice myotubes

In skeletal muscle myogenesis, precursor cells or myoblasts fuse to form multinucleated cells (myotubes), which then further develop into functional muscle. We investigated if the inhibition of myogenesis by transforming growth factor-beta1 (TGF-beta1) and bone morphogenetic protein-2 (BMP-2) involve regulation of voltage-dependent Ca(2+) channels. Primary cultured myoblasts were kept in fusion medium (0-6 days) in either the absence (control conditions) or the presence of 40 pm TGF-beta1 or 5 nm BMP-2. Subsequently, the developing myotubes were transferred to a growth factor-free recording solution, and subjected to whole cell patch-clamp experiments. At day 0, 14% of non-fusing myoblasts exhibited T-current, whereas the L-current was practically absent. Under control conditions, however, the percentage of T- and L-channel-expressing myotubes increased sharply, from 25% at day 1 to approximately 100% at days 2-6. In addition, parallel increases were determined for Ca(2+)-currents density and cell membrane capacitance (C(m)), which is proportional to the size of myotubes. Interestingly, at days 1-2 TGF-beta1 and BMP-2 eliminated the T-current on initial 14% of T-channel-expressing myoblasts. Moreover, at day 6 the growth factors significantly reduced the maximal values of both T-current density (80%) and C(m) (60%). The effect of BMP-2 was selective on T-channels, whereas TGF-beta1 decreased also the L-current density (90%). A similar reduction in maximal conductance of the Ca(2+) channels was determined, in the absence of significant alterations in other essential properties of the channels, including the time course and voltage dependence of activation and inactivation. The results suggest these growth factors markedly reduce the number of functional T- (both TGF-beta1 and BMP-2) and L-channels (only TGF-beta1) in the surface of the plasma membrane, and contribute to explaining the associated effects on myogenesis.

Satellite cell activation on fibers: modeling events in vivo — an invited review

Knowledge of the events underlying satellite cell activation and the counterpart maintenance of quiescence is essential for planning therapies that will promote the growth and regeneration of skeletal muscle in healthy, disease and aging. By modeling those events of satellite cell activation in studies of single muscle fibers or muscles in culture, the roles of mechanical stretching and nitric oxide are becoming understood. Recent studies demonstrated that stretch-induced activation is very rapid and exhibits some features of satellite cell heterogeneity. As well, gene expression studies showed that expression of the c-met receptor gene rises rapidly after stretching muscles in culture compared to those without stretch. This change in gene expression during activation, and the maintenance of quiescence in both normal and dystrophic muscles are dependent on NO, as they are blocked by inhibition of nitric oxide synthase (NOS). Mechanical, contractile activity is the defining feature of muscle function. Therefore, ongoing studies of stretch effects in satellite cell activation and quiescence in quiescent fiber and muscle cultures provides appropriate models by which to explore the regulatory steps in muscle in vivo under many conditions related to disease, repair, rehabilitation, growth and the prevention or treatment of atrophy.Key words: regeneration, stretch, myofiber culture, muscular dystrophy, quiescence.