The satellite cell as a companion in skeletal muscle plasticity: currency, conveyance, clue, connector and colander

Possible implication of satellite cells in regenerative motoneuritogenesis: HGF upregulates neural chemorepellent Sema3A during myogenic differentiation

Regenerative coordination and remodeling of the intramuscular motoneuron network and neuromuscular connections are critical for restoring skeletal muscle function and physiological properties. The regulatory mechanisms of such coordination remain unclear, although both attractive and repulsive axon guidance molecules may be involved in the signaling pathway. Here we show that expression of a neural secreted chemorepellent semaphorin 3A (Sema3A) is remarkably upregulated in satellite cells of resident myogenic stem cells that are positioned beneath the basal lamina of mature muscle fibers, when treated with hepatocyte growth factor (HGF), established as an essential cue in muscle fiber growth and regeneration. When satellite cells were treated with HGF in primary cultures of cells or muscle fibers, Sema3A message and protein were upregulated as revealed by reverse transcription-polymerase chain reaction and immunochemical studies. Other growth factors had no inductive effect except for a slight effect of epidermal growth factor treatment. Sema3A upregulation was HGF dose dependent with a maximum (about 7- to 8-fold units relative to the control) at 10–25 ng/ml and occurred exclusively at the early-differentiation stage, as characterized by the level of myogenin expression and proliferation (bromodeoxyuridine incorporation) of the cells. Neutralizing antibody to the HGF-specific receptor, c-met, did not abolish the HGF response, indicating that c-met may not mediate the Sema3A expression signaling. Finally, in vivo Sema3A was upregulated in the differentiation phase of satellite cells isolated from muscle regenerating following crush injury. Overall, the data highlight a heretofore unexplored and active role for satellite cells as a key source of Sema3A expression triggered by HGF, hence suggesting that regenerative activity toward motor innervation may importantly reside in satellite cells and could be a crucial contributor during postnatal myogenesis.

A heart full of stem cells: the spectrum of myocardial progenitor cells in the postnatal heart

Influencing cellular regeneration processes in the heart has been a long-standing goal in cardiovascular medicine. To some extent, this has been successful in terms of vascular regeneration as well as intercellular connective tissue remodeling processes. Several components of today's routine heart failure medication influence endothelial progenitor cell behavior and support collateral vessel growth in the heart, or have been shown to prevent or reverse fibrosis processes. Cardiomyocyte regeneration, however, has so far escaped therapeutic manipulation strategies. Delivery of exogenous cells of bone marrow origin to the human myocardium may improve heart function, but is not associated with relevant neomyogenesis. However, accumulating evidence indicates that the myocardium contains resident cardiac progenitor cells (CPC) that may be therapeutically useful. This notion indeed represents a paradigm shift but is still controversial. The purpose of this review is to summarize the rapidly expanding current knowledge on CPC, and to assess whether it may be translated into solid therapeutic concepts.

Extrinsic regulation of domestic animal-derived myogenic satellite cells II

The existence of myogenic satellite cells was reported some 47 years ago, and, since that time, satellite cell research has flourished. So much new information is generated (daily) on these cells that it can be difficult for individuals to keep abreast of important issues related to their activation and proliferation, the modulation of the activity of other cell types, the differentiation of the cells to facilitate normal skeletal muscle growth and development, or to the repair of damaged myofibers. The intent of this review is to summarize new information about the extrinsic regulation of myogenic satellite cells and to provide specific mechanisms involved in altering satellite cell physiology. Where possible, examples from agriculturally important animals are used for illustrative purposes.

Myogenic stem cells

Both skeletal muscle and bone marrow tissue contain myogenic stem cells. The population residing in muscles is heterogenic. Predominant in number are "typical" satellite cells - muscle progenitors migrating from somites during embryonic life. Another population is group of multipotent muscle stem cells which, at least in part, are derived from bone marrow. These cells are tracked by gradient of growth factors releasing from muscle during injury or exercise. Recruited bone marrow-derived cells gradually change their phenotype becoming muscle stem cells and eventually can attain satellite cell position and express Pax7 protein. Mesenchymal stem cells (MSC) isolated directly from bone marrow also display myogenic potential, although methods of induction of myogenic differentiation in vitro have not been optimized yet. Concerning efforts of exploiting myogenic stem cells in cell-mediated therapies it is important to understand the cause of impaired regenerative potential of aged muscle. Up to now, most of research data suggest that majority of age related changes in skeletal muscles are reversible, thus depending on extrinsic factors. However, irreversible intrinsic features of muscle stem cells are also taken into consideration.

Muscle Research and Gene Ontology: New standards for improved data integration

BACKGROUND

The Gene Ontology Project provides structured controlled vocabularies for molecular biology that can be used for the functional annotation of genes and gene products. In a collaboration between the Gene Ontology (GO) Consortium and the muscle biology community, we have made large-scale additions to the GO biological process and cellular component ontologies. The main focus of this ontology development work concerns skeletal muscle, with specific consideration given to the processes of muscle contraction, plasticity, development, and regeneration, and to the sarcomere and membrane-delimited compartments. Our aims were to update the existing structure to reflect current knowledge, and to resolve, in an accommodating manner, the ambiguity in the language used by the community.

RESULTS

The updated muscle terminologies have been incorporated into the GO. There are now 159 new terms covering critical research areas, and 57 existing terms have been improved and reorganized to follow their usage in muscle literature.

CONCLUSION

The revised GO structure should improve the interpretation of data from high-throughput (e.g. microarray and proteomic) experiments in the area of muscle science and muscle disease. We actively encourage community feedback on, and gene product annotation with these new terms. Please visit the Muscle Community Annotation Wiki http://wiki.geneontology.org/index.php/Muscle_Biology.

A role for calcium-calmodulin in regulating nitric oxide production during skeletal muscle satellite cell activation

When skeletal muscle is stretched or injured, myogenic satellite cells are activated to enter the cell cycle. This process depends on nitric oxide (NO) production by NO synthase (NOS), matrix metalloproteinase activation, release of hepatocyte growth factor (HGF) from the extracellular matrix, and presentation of HGF to the c-met receptor as demonstrated by a primary culture and in vivo assays. We now add evidence that calcium-calmodulin is involved in the satellite cell activation cascade in vitro. Conditioned medium from cultures that were treated with a calcium ionophore (A23187, ionomycin) for 2 h activated cultured satellite cells and contained active HGF, similar to the effect of mechanical stretch or NO donor treatments. The response was abolished by addition of calmodulin inhibitors (calmidazolium, W-13, W-12) or a NOS inhibitor N(G)-nitro-l-arginine methyl ester hydrochloride but not by its less inactive enantiomer N(G)-nitro-d-arginine methyl ester hydrochloride. Satellite cells were also shown to express functional calmodulin protein having a calcium-binding activity at 12 h postplating, which is the time at which the calcium ionophore was added in this study and the stretch treatment was applied in our previous experiments. Therefore, results from these experiments provide an additional insight that calcium-calmodulin mediates HGF release from the matrix and that this step in the activation pathway is upstream from NO synthesis.

Erythropoietin improves functional and histological recovery of traumatized skeletal muscle tissue

Apart from its hematopoietic effect, erythropoietin (EPO) is known as pleiotropic cytokine with anti-inflammatory and anti-apoptotic properties. Here, we evaluated for the first time the EPO-dependent regeneration capacity in an in vivo rat model of skeletal muscle trauma. A myoblast cell line was used to study the effect of EPO on serum deprivation-induced cell apoptosis in vitro. A crush injury was performed to the left soleus muscle in 80 rats treated with either EPO or saline. Muscle recovery was assessed by analysis of contraction capacities. Intravital microscopy, BrdU/laminin double immunohistochemistry and cleaved caspase-3 immunohistochemistry of muscle tissue on days 1, 7, 14, and 42 posttrauma served for assessment of local microcirculation, tissue integrity, and cell proliferation. Serum deprivation-induced myoblast apoptosis of 23.9 +/- 1.5% was reduced by EPO to 17.2 +/- 0.8%. Contraction force analysis in the EPO-treated animals revealed significantly improved muscle strength with 10-20% higher values of twitch and tetanic forces over the 42-day observation period. EPO-treated muscle tissue displayed improved functional capillary density as well as reduced leukocytic response and consecutively macromolecular leakage over day 14. Concomitantly, muscle histology showed significantly increased numbers of BrdU-positive satellite cells and interstitial cells as well as slightly lower counts of cleaved caspase-3-positive interstitial cells. EPO results in faster and better regeneration of skeletal muscle tissue after severe trauma and goes along with improved microcirculation. Thus, EPO, a compound established as clinically safe, may represent a promising therapeutic option to optimize the posttraumatic course of muscle tissue healing.

Pax7 shows higher satellite cell frequencies and concentrations within intrafusal fibers of muscle spindles

Intrafusal fibers within muscle spindles make up a small subpopulation of muscle fibers. These proprioceptive fibers differ from most extrafusal fibers because, even in maturity, their diameters remain small, and they retain expression of developmental myosins. Although both extrafusal and intrafusal fibers contain satellite cells (SCs), comparatively little is known about intrafusal SCs. Analyzing chicken fast-phasic posterior (PLD) and slow-tonic anterior (ALD) latissimus dorsi muscles, we show that SCs of both intrafusal and extrafusal fibers express Pax7. We further test the hypotheses that intrafusal fibers display parameters reflective of extrafusal immaturity. These hypotheses are that intrafusal fibers contain (a) higher SC frequencies (number of SC nuclei/all nuclei within basal lamina) and concentrations (closer together) and (b) smaller myonuclear domains than do adjacent extrafusal fibers. IHC techniques were applied to PLD and ALD muscles excised at 30 and 138 days posthatch. The hypotheses were validated, suggesting that intrafusal fibers have greater capacities for growth, regeneration, and repair than do adjacent extrafusal fibers. During maturation, extrafusal and intrafusal fibers show similar trends of decreasing SC frequencies and concentrations and increases in myonuclear domains. Thus, extrafusal and intrafusal fibers alike should exhibit reduced capacities for growth, regeneration, and repair during maturation.

Cellular adaptations in soleus muscle during recovery after hindlimb unloading

AIM

We used a model of chronic unloading followed by reloading to examine the apoptotic responses associated with soleus muscle atrophy and subsequent recovery.

METHODS

Male Wistar rats were subjected to hindlimb unloading (HU) for 2 weeks and subsequent reloading for 0, 3, 7 and 14 days. One-half of the HU-reloaded rats were administered cyclosporine A (CsA), a calcineurin (CaN) inhibitor.

RESULTS

There was fibre atrophy (73%) and a decrease in slow type I fibre/myosin heavy chain (MyHC) composition in the soleus muscle after 2 weeks of HU. Fibre size and type I MyHC composition recovered to near the age-matched control levels by recovery day 14 in non-treated, but not in CsA-treated, rats. Myonuclear number was lower and the number of apoptotic nuclei higher in 2-week HU than control rats. These values returned to control levels after 7 and 14 days of recovery, respectively, in both HU-recovery groups. After 2 weeks of HU, the levels of heat shock proteins (Hsp) 60 and 72, mitochondrial cytochrome c oxidase subunit IV (Cox IV), and peroxisome proliferator-activated receptor gamma coactivator 1 (PGC-1) proteins were lower than control. The levels of all of these proteins gradually increased to or above the control levels during cage recovery in both groups.

CONCLUSION

Our results indicate that apoptotic mechanisms are involved in the modulation of myonuclear number during chronic unloading and subsequent reloading. Furthermore, it appears that CaN is related to fibre size and phenotype adaptations, but not to apoptotic responses.

Granulocyte-colony stimulating factor enhances muscle proliferation and strength following skeletal muscle injury in rats

Insufficiency of skeletal muscle regeneration often impedes the healing process with functional deficiencies and scar formation. We tested the hematopoietic growth factor granulocyte-colony stimulating factor (G-CSF) with respect to its efficacy to improve functional muscle regeneration following skeletal muscle injury in Wistar rats. After crush injury to the left soleus muscle, animals received daily G-CSF (20 μg/kg ip) or vehicle solution ( n = 30 per group each). Sham-operated animals without muscle injury served as controls ( n = 15). After in vivo assessment of the fast-twitch and tetanic contraction capacity of the soleus muscles at days 4, 7, and 14 post-injury, sampling of muscle tissue served for analysis of satellite cell proliferation [bromodeoxyuridine (BrdU)/laminin and BrdU/desmin double immunohistochemistry] and cell apoptosis (transferase nick-end labeling analysis). Muscle strength analysis revealed recovery of contraction forces to 26 ± 2, 35 ± 3, and 53 ± 3% (twitch force) and to 20 ± 3, 24 ± 2, and 37 ± 2% (tetanic force) within the 14-day observation period in vehicle-treated animals. In contrast, G-CSF increased contractile forces with markedly higher values at day 7 (twitch force: 42 ± 2%; tetanic force: 34 ± 2%) and day 14 (twitch force: 62 ± 3%; tetanic force: 43 ± 3%). This enhancement of muscle function was preceded by a significant increase of satellite cell proliferation (BrdU-positive cells/mm2: 27 ± 6 vs. vehicle: 12 ± 3) and a moderate decrease of cell apoptosis (transferase nick-end labeling-positive cells/mm2: 11 ± 2 vs. vehicle: 16 ± 3) at day 4. In conclusion, G-CSF histologically promoted viability and proliferation of muscle cells and functionally enhanced recovery of muscle strength. Thus G-CSF might represent a therapeutic option to optimize the posttraumatic course of muscle tissue healing.

Defining the transcriptional signature of skeletal muscle stem cells

Satellite cells, the main source of myoblasts in postnatal muscle, are located beneath the myofiber basal lamina. The myogenic potential of satellite cells was initially documented based on their capacity to produce progeny that fused into myotubes. More recently, molecular markers of resident satellite cells were identified, further contributing to defining these cells as myogenic stem cells that produce differentiating progeny and self-renew. Herein, we discuss aspects of the satellite cell transcriptional milieu that have been intensively investigated in our research. We elaborate on the expression patterns of the paired box (Pax) transcription factors Pax3 and Pax7, and on the myogenic regulatory factors myogenic factor 5 (Myf5), myogenic determination factor 1 (MyoD), and myogenin. We also introduce original data on MyoD upregulation in newly activated satellite cells, which precedes the first round of cell proliferation. Such MyoD upregulation occurred even when parent myofibers with their associated satellite cells were exposed to pharmacological inhibitors of hepatocyte growth factor and fibroblast growth factor receptors, which are typically involved in promoting satellite cell proliferation. These observations support the hypothesis that most satellite cells in adult muscle are committed to rapidly entering myogenesis. We also detected expression of serum response factor in resident satellite cells prior to MyoD expression, which may facilitate the rapid upregulation of MyoD. Aspects of satellite cell self-renewal based on the reemergence of cells expressing Pax7, but not MyoD, in myogenic cultures are discussed further herein. We conclude by describing our recent studies using transgenic mice in which satellite cells are traced and isolated based on their expression of green fluorescence protein driven by regulatory elements of the nestin promoter (nestin-green fluorescence protein). This feature provides us with a novel means of studying satellite cell transcriptional signatures, heterogeneity among muscle groups, and the role of the myogenic niche in directing satellite cell self-renewal.

Nitric oxide-dependence of satellite stem cell activation and quiescence on normal skeletal muscle fibers

Satellite cells (quiescent precursors in normal adult skeletal muscle) are activated for growth and regeneration. Signaling by nitric oxide (NO) and hepatocyte growth factor (HGF) during activation has not been examined in a model that can distinguish quiescent from activated satellite cells. We tested the hypothesis that NO and HGF are required to regulate activation using the single-fiber culture model. In normal fibers, HGF and inhibition of NO synthase (NOS) each increased activation without stretching, and NOS inhibition reduced stretch-activation. Activation in unstretched mdx and NOS-I(-/-) fibers was three- to fourfold higher than normal, and was reduced by stretching. Distinctions were not due to different pax7-expressing populations on normal and mdx fibers. The population of c-met-expressing satellite cells on normal fibers was increased by stretch, demonstrating functional heterogeneity among normal satellite cells. Cycloheximide did not prevent the stretch-related increase in c-met expression, suggesting c-met may be an immediate-early gene in satellite cell activation. Results have important implications for designing new therapies that target the role of exercise in health, aging, and disease.

Regulation of skeletal muscle differentiation in fibroblasts by exogenous MyoD gene in vitro and in vivo

MyoD of the myogenic regulatory factors (MRFs) family regulates the skeletal muscle differentiation program. In this study, stably transfected NIH3T3-derived cell lines were established, in which exogenous MyoD was expressed at high levels. Transcriptional activation of endogenous muscle regulatory gene and induction towards the skeletal muscle lineages were observed with phase-contrast microscopy when continuously cultured in vitro. Moreover, to determine their ability of myogenic formation in vivo, the transfected cells were implanted in nude mice subcutaneously for up to 10 weeks. The morphological characterization of inductive cells was observed using transmission electron microscope and histological staining. Myogenesis of fibroblasts incubated in the medium was activated by overexpression of MyoD, and the cells were accumulated and fused into multinucleated myotubes. Correlatively, RT-PCR and immunohistochemistry confirmed the increased expression of characteristic downstream molecule myogenin and mysion heavy chains during myogenic differentiation. Ecoptic myogenesis was found and remained stable phenotype when the transfected cells were seeded in vivo. Our results suggest that MyoD can be considered to be a determining factor of myogenic lineages, and it may play an important role in the cell therapy and cell-mediated gene therapy of the skeletal muscle.

Satellite cells express distinct patterns of myogenic proteins in immature skeletal muscle

Satellite cells are the myogenic cells lying between the myofiber sarcolemma and basal lamina. The objective of this study was to determine the expression patterns of MyoD, myogenin, and Pax7 within the satellite cell population in the growing rat soleus and extensor digitorum longus (EDL) muscles. Secondly, the expression of the myogenic markers was also studied within the interstitial cell compartment and myonuclei. It was discovered that the soleus contained a higher number of Pax7, MyoD, or myogenin-positive nuclei compared with the EDL. Similarly, myogenin was expressed at a lower level in the myonuclei of the soleus compared with the EDL, and myogenin was expressed at a higher level in the interstitial compartment of the soleus compared with the EDL. When interstitial nuclei, myonuclei, and double-labeled nuclei were used in the estimate of the satellite cell population, it was discovered that approximately of 13% of the myofibers in a transverse section of the soleus muscle and 4.1% of EDL myofibers exhibit a labeled satellite cell nucleus. Overall, results from this study suggest that expression patterns of these markers vary predictably among muscles with different growth dynamics and phenotypic characteristics.

A musculoskeletal model of low grade connective tissue inflammation in patients with thyroid associated ophthalmopathy (TAO): the WOMED concept of lateral tension and its general implications in disease

Background

Low level connective tissue inflammation has been proposed to play a role in thyroid associated ophthalmopathy (TAO). The aim of this study was to investigate this postulate by a musculoskeletal approach together with biochemical parameters.

Methods

13 patients with TAO and 16 controls were examined. Erythrocyte levels of Zn, Cu, Ca²⁺, Mg, and Fe were determined. The musculoskeletal evaluation included observational data on body posture with emphasis on the orbit-head region. The angular foot position in the frontal plane was quantified following gait observation. The axial orientation of the legs and feet was evaluated in an unloaded supine position. Functional propioceptive tests based on stretch stimuli were done by using foot inversion and foot rotation.

Results

Alterations in the control group included neck tilt in 3 cases, asymmetrical foot angle during gait in 2, and a reaction to foot inversion in 5 cases. TAO patients presented facial asymmetry with displaced eye fissure inclination (mean 9.1°) as well as tilted head-on-neck position (mean 5.7°). A further asymmetry feature was external rotation of the legs and feet (mean 27°). Both foot inversion as well as foot rotation induced a condition of neuromuscular deficit. This condition could be regulated by gentle acupressure either on the lateral abdomen or the lateral ankle at the acupuncture points gall bladder 26 or bladder 62, respectively. In 5 patients, foot rotation produced a phenomenon of moving toes in the contra lateral foot. In addition foot rotation was accompanied by an audible tendon snapping. Lower erythrocyte Zn levels and altered correlations between Ca²⁺, Mg, and Fe were found in TAO.

Conclusion

This whole body observational study has revealed axial deviations and body asymmetry as well as the phenomenon of moving toes in TAO. The most common finding was an arch-like displacement of the body, i.e. eccentric position, with foot inversion and head tilt to the contra lateral side and tendon snapping. We propose that eccentric muscle action over time can be the basis for a low grade inflammatory condition. The general implications of this model and its relations to Zn and Se will be 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.