The role of semaphorin3A in myogenic regeneration and the formation of functional neuromuscular junctions on new fibres

Current research on skeletal muscle injury and regeneration highlights the crucial role of nerve–muscle interaction in the restoration of innervation during that process. Activities of muscle satellite or stem cells, recognized as the ‘currency’ of myogenic repair, have a pivotal role in these events, as shown by ongoing research. More recent investigation of myogenic signalling events reveals intriguing roles for semaphorin3A (Sema3A), secreted by activated satellite cells, in the muscle environment during development and regeneration. For example, Sema3A makes important contributions to regulating the formation of blood vessels, balancing bone formation and bone remodelling, and inflammation, and was recently implicated in the establishment of fibre‐type distribution through effects on myosin heavy chain gene expression. This review highlights the active or potential contributions of satellite‐cell‐derived Sema3A to regulation of the processes of motor neurite ingrowth into a regenerating muscle bed. Successful restoration of functional innervation during muscle repair is essential; this review emphasizes the integrative role of satellite‐cell biology in the progressive coordination of adaptive cellular and tissue responses during the injury‐repair process in voluntary muscle.     

The satellite cell in male and female, developing and adult mouse muscle: distinct stem cells for growth and regeneration

Satellite cells are myogenic cells found between the basal lamina and the sarcolemma of the muscle fibre. Satellite cells are the source of new myofibres; as such, satellite cell transplantation holds promise as a treatment for muscular dystrophies. We have investigated age and sex differences between mouse satellite cells in vitro and assessed the importance of these factors as mediators of donor cell engraftment in an in vivo model of satellite cell transplantation. We found that satellite cell numbers are increased in growing compared to adult and in male compared to female adult mice. We saw no difference in the expression of the myogenic regulatory factors between male and female mice, but distinct profiles were observed according to developmental stage. We show that, in contrast to adult mice, the majority of satellite cells from two week old mice are proliferating to facilitate myofibre growth; however a small proportion of these cells are quiescent and not contributing to this growth programme. Despite observed changes in satellite cell populations, there is no difference in engraftment efficiency either between satellite cells derived from adult or pre-weaned donor mice, male or female donor cells, or between male and female host muscle environments. We suggest there exist two distinct satellite cell populations: one for muscle growth and maintenance and one for muscle regeneration.     

Cellular adaptation of the trapezius muscle in strength-trained athletes

 The aim of this study was to elucidate the cellular events that occur in the trapezius muscle following several years of strength training. In muscle biopsies from ten elite power lifters (PL) and six control subjects (C), several parameters were studied: cross-sectional area of muscle fibres, myosin heavy chain composition (MHC) and capillary supply [capillaries around fibres (CAF) and CAF/fibre area]. A method was also developed for counting the number of myonuclei and satellite cell nuclei. The proportion of fibres expressing MHC IIA, the cross-sectional area of each fibre type and the number of myonuclei, satellite cells and fibres expressing markers for early myogenesis were significantly higher in PL than in C (P<0.05). A significant correlation between the myonuclear number and the cross-sectional area was observed. Since myonuclei in mature muscle fibres are not able to divide, we suggest that the incorporation of satellite cell nuclei into muscle fibres resulted in the maintenance of a constant nuclear to cytoplasmic ratio. The presence of small diameter fibres expressing markers for early myogenesis indicates the formation of new muscle fibres. Accepted: 17 November 1998     

The number of satellite cells in slow and fast fibres from human vastus lateralis muscle

The aim of this investigation was to study the distribution of satellite cells in slow (type I fibres) and fast (type II fibres) fibres from human vastus lateralis muscle. This muscle is characterised by a mixed fibre type composition and is considered as the site of choice for biopsies in research work and for clinical diagnosis. Biopsy samples were obtained from five healthy young volunteers and a total of 1,747 type I fibres and 1,760 type II fibres were assessed. Satellite cells and fibre type composition were studied on serial muscle cross-sections stained with specific monoclonal antibodies. From a total of 218 satellite cells, 116 satellite cells were found in contact with type I fibres (53.6±8% of the satellite cells associated to type I fibres) and 102 satellite cells in contact with type II fibres (46.4±8% of the satellite cells associated to type II fibres). There was no significant difference (P=0.4) between the percentages of satellite cells in contact with type I and with type II fibres. Additionally, there was no relationship between the mean number of satellite cells per fibre and the mean cross-sectional area of muscle fibres. In conclusion, our results show that there is no fibre type-specific distribution of satellite cells in a human skeletal muscle with mixed fibre type composition.     

Satellite cells in the tail muscles of the urodelan larvae during development

The incidence and ultrastructure of satellite cells in the tail muscles of urodelan larvae were examined during development during which the number of satellite cells is gradually reduced. They are found more frequently in red than in the white fibres in all four stages examined (stage 53, 64, 66+ and juvenile). As development proceeds, intercellular space between satellite cell and muscle fibre is in general gradually extended and is mostly filled with basal lamina. Small muscle cells, satellite fibres, which are situated under the basal lamina of the parent fibre, are morphologically similar to satellite cells but contain a small amount of myofibrils. Three types of satellite fibres are distinguishable on the basis of differences in K2-EDTA-treated ATPase activity, width of Z line, and parent fibre type. Neuromuscular junctions are visible in satellite fibres.     

Concomitant increases in myonuclear and satellite cell content in female trapezius muscle following strength training

A skeletal muscle fibre maintains its cytoplasmic volume by means of hundreds of myonuclei distributed along its entire length. Therefore it is hypothesised that changes in fibre size would involve modifications in myonuclear number. In this study, we have examined whether 10 weeks of strength training can induce changes in the number of myonuclei and satellite cells in female trapezius muscles. Biopsies were taken pre- and posttraining from the upper part of the descending trapezius muscle of nine subjects. Muscle samples were analysed for fibre area and myonuclear and satellite cell number using immunohistochemistry. There was a 36% increase in the cross-sectional area of muscle fibres. The hypertrophy of muscle fibres was accompanied by an approximately 70% increase in myonuclear number and a 46% increase in the number of satellite cells. Myonuclei number was positively correlated to satellite cell number indicating that a muscle with an increased concentration of myonuclei will contain a correspondingly higher number of satellite cells. The acquisition of additional myonuclei appears to be required to support the enlargement of multinucleated muscle cells following 10 weeks of strength training. Increased satellite cell content suggests that mitotic divisions of satellite cells produced daughter cells that became satellite cells. Accepted: 30 November 1999     

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

Matrix metalloproteinases (MMPs) are thought to play an important role in skeletal muscle cell growth and differentiation. In view of the MMP inducing function of EMMPRIN/CD147, its role in myogenic cell differentiation was investigated. EMMPRIN level increased during differentiation of both rat primary myoblasts derived from satellite cells and mouse C2.7 myogenic cells and was associated with an alteration in its molecular forms. In parallel, expression of pro‐MMP‐9 gradually decreased and that of pro‐MMP‐2 and active MMP‐2 increased. While small interfering RNA (siRNA) inhibition of EMMPRIN expression accelerated cell differentiation, exogenously added recombinant EMMPRIN inhibited differentiation by an MMP‐mediated mechanism, as the MMP inhibitor marimastat abrogated EMMPRIN's effect. Our results further suggest that EMMPRIN regulates differentiation through an MMP activation of transforming growth factor beta (TGFβ), a known inhibitor of myoblast's differentiation, as the increased activation and signaling of TGFβ by EMMPRIN was attenuated in the presence of marimastat. EMMPRIN inhibition may thus represent a novel strategy in the treatment of muscular degenerative disorders. J. Cell. Physiol. 226: 141–149, 2010. © 2010 Wiley‐Liss, Inc.     

Differential expression of FGF receptors and of myogenic regulatory factors in primary cultures of satellite cells originating from fast (EDL) and slow (Soleus) twitch rat muscles.

In the rat, the fast and slow twitch muscles respectively Extensor digitorum longus (EDL) and Soleus present differential characteristics during regeneration. This suggests that their satellite cells responsible for muscle growth and repair represent distinct cellular populations. We have previously shown that satellite cells dissociated from Soleus and grown in vitro proliferate more readily than those isolated from EDL muscle. Fibroblast growth factors (FGFs) are known as regulators of myoblast proliferation and several studies have revealed a relationship between the response of myoblasts to FGF and the expression of myogenic regulatory factors (MRF) of the MyoD family by myoblasts. Therefore, we presently examined the possibility that the satellite cells isolated from EDL and Soleus muscles differ in the expression of FGF receptors (FGF-R) and of MRF expression. FGF-R1 and -R4 were strongly expressed in proliferating cultures whereas FGF-R2 and R3 were not detected in these cultures. In differentiating cultures, only -R1 was present in EDL satellite cells while FGF-R4 was also still expressed in Soleus cells. Interestingly, the unconventional receptor for FGF called cystein rich FGF receptor (CFR), of yet unknown function, was mainly detected in EDL satellite cell cultures. Soleus and EDL satellite cell cultures also differed in the expression MRFs. These results are consistent with the notion that satellite cells from fast and slow twitch muscles belong to different types of myogenic cells and suggest that satellite cells might play distinct roles in the formation and diversification of fast and slow fibres.     

Regulation of myogenic stem cell behaviour by vessel cells: The "ménage à trois" of satellite cells,periendothelial cells and endothelial cells

In skeletal muscle, satellite cells, that are responsible of muscle repair, are localized close to capillaries. Although angiogenesis is known for a long time to be crucial for muscle repair and satellite cell survival, cellular interplays between vessel cells and satellite/myogenic cells have been poorly explored. We analyzed the interrelationships between myogenic cells, endothelial cells, and periendothelial cells that includes smooth muscle cells and endomysial fibroblasts. We found that endothelial cells strongly stimulate myogenic cell growth and, inversely, myogenic cells increase angiogenesis. VEGF plays a essential role in this bidirectional interaction. On the contrary, periendothelial cells promote the return to quiescence of a subset of muscle precursor cells to quiescence that ensures self-renewal of adult muscle stem cells. We have shown that Angiopoietin-1/Tie-2 signalling controls the entry into quiescence. We propose that during muscle regeneration, i.e. while vessels are not stabilized, endothelial cells and myogenic cells interact with each other to promote both myogenesis and angiogenesis, that have been shown to be concomitant processes in several models. On the other hand, once homeostasis of muscle is reached, the proximity of satellite cells and periendothelial cells allows the responsiveness of satellite cells, that bear Tie-2 receptor, to the secretion of Angiopoietin-1 by periendothelial cells, that, in the same time, stabilize vessels by promoting quiescence of endothelial cells.     

Differential effects of muscle fibre length and insulin on muscle-specific mRNA content in isolated mature muscle fibres during long-term culture

The aims of this study were (1) to determine the relationship between muscle fibre cross-sectional area and cytoplasmic density of myonuclei in high- and low-oxidative Xenopus muscle fibres and (2) to test whether insulin and long-term high fibre length caused an increase in the number of myonuclei and in the expression of α-skeletal actin and of myogenic regulatory factors (myogenin and MyoD) in these muscle fibres. In high- and low-oxidative muscle fibres from freshly frozen iliofibularis muscles, the number of myonuclei per millimetre fibre length was proportional to muscle fibre cross-sectional area. The in vivo myonuclear density thus seemed to be strictly regulated, suggesting that the induction of hypertrophy required the activation of satellite cells. The effects of muscle fibre length and insulin on myonuclear density and myonuclear mRNA content were investigated on high-oxidative single muscle fibres cultured for 4–5 days. Muscle fibres were kept at a low length (~15% below passive slack length) in culture medium with a high insulin concentration (~6 nmol/l: “high insulin medium”) or without insulin, and at a high length (~5% above passive slack length) in high insulin medium. High fibre length and high insulin medium did not change the myonuclear density of isolated muscle fibres during culture. High insulin increased the myonuclear α-skeletal actin mRNA content, whereas fibre length had no effect on α-skeletal actin mRNA content. After culture at high fibre length in high insulin medium, the myonuclear myogenin mRNA content was 2.5-fold higher than that of fibres cultured at low length in high insulin medium or in medium without insulin. Myonuclear MyoD mRNA content was not affected by fibre length or insulin. These in vitro experiments indicate that high muscle fibre length and insulin enhance muscle gene expression but that other critical factors are required to induce adaptation of muscle fibre size and performance.This work was partially supported by a research grant from the Haak Bastiaanse Kuneman Stichting.     

Pericytes of human skeletal muscle are myogenic precursors distinct from satellite cells

Cells derived from blood vessels of human skeletal muscle can regenerate skeletal muscle, similarly to embryonic mesoangioblasts. However, adult cells do not express endothelial markers, but instead express markers of pericytes, such as NG2 proteoglycan and alkaline phosphatase (ALP), and can be prospectively isolated from freshly dissociated ALP(+) cells. Unlike canonical myogenic precursors (satellite cells), pericyte-derived cells express myogenic markers only in differentiated myotubes, which they form spontaneously with high efficiency. When transplanted into severe combined immune deficient-X-linked, mouse muscular dystrophy (scid-mdx) mice, pericyte-derived cells colonize host muscle and generate numerous fibres expressing human dystrophin. Similar cells isolated from Duchenne patients, and engineered to express human mini-dystrophin, also give rise to many dystrophin-positive fibres in vivo. These data show that myogenic precursors, distinct from satellite cells, are associated with microvascular walls in the human skeletal muscle, may represent a correlate of embryonic 'mesoangioblasts' present after birth and may be a promising candidate for future cell-therapy protocols in patients.     

Visualization of satellite cells in living muscle fibres of the frog

Satellite cells were visualized in living muscle fibres of the frog. Single fibres or bundles consisting of a few fibres were isolated after treatment with collagenase, and viewed under the light microscope. Subsequent electron microscopy of identified cells confirmed that they were satellite muscle cells. Under the light microscope, satellite cells appear as fusiform cells, tapering into long fine processes usually orientated parallel to the muscle fibre axis. Horseradish peroxidase injected into the muscle fibre was not transferred to the satellite cells.     

Differentiation of activated satellite cells in denervated muscle following single fusions in situ and in cell culture

Satellite cells represent a cellular source of regeneration in adult skeletal muscle. It remains unclear why a large pool of stem myoblasts in denervated muscle does not compensate for the loss of muscle mass during post-denervation atrophy. In this study, we present evidence that satellite cells in long-term denervated rat muscle are able to activate synthesis of contractile proteins after single fusions in situ. This process of early differentiation leads to formation of abnormally diminutive myotubes. The localization of such dwarf myotubes beneath the intact basal lamina on the surface of differentiated muscle fibers shows that they form by fusion of neighboring satellites or by the progeny of a single satellite cell following one or two mitotic divisions. We demonstrated single fusions of myoblasts using electron microscopy, immunocytochemical labeling and high resolution confocal digital imaging. Sequestration of nascent myotubes by the rapidly forming basal laminae creates a barrier that limits further fusions. The recruitment of satellite cells in the formation of new muscle fibers results in a progressive decrease in their local densities, spatial separation and ultimate exhaustion of the myogenic cell pool. To determine whether the accumulation of aberrant dwarf myotubes is explained by the intrinsic decline of myogenic properties of satellite cells, or depends on their spatial separation and the environment in the tissue, we studied the fusion of myoblasts isolated from normal and denervated muscle in cell culture. The experiments with a culture system demonstrated that the capacity of myoblasts to synthesize contractile proteins without serial fusions depended on cell density and the availability of partners for fusion. Satellite cells isolated from denervated muscle and plated at fusion-permissive densities progressed through the myogenic program and actively formed myotubes, which shows that their myogenic potential is not considerably impaired. The results of this study suggest that under conditions of denervation, progressive spatial separation and confinement of many satellite cells within the endomysial tubes of atrophic muscle fibers and progressive interstitial fibrosis are the important factors that prevent their normal differentiation. Our findings also provide an explanation of why denervated muscle partially and temporarily is able to restore its functional capacity following injury and regeneration: the release of satellite cells from their sublaminal location provides the necessary space for a more active regenerative process.     

Expression of the paired box domain Pax7 protein in myogenic cells isolated from the porcine semitendinosus muscle after birth

The paired box domain gene Pax7 plays a pivotal role in satellite cell physiology and may represent one of the candidate genes influencing the dynamic stages of early post-natal growth observed in pig. Quiescent satellite cells express Pax7 and, when activated, they co-express the myogenic bHLH protein MyoD. The aims of this study were to investigate, by immunohistochemistry, the putative differential expression of Pax7 and to ascertain the amount of activated satellite cells (Pax7(+)/MyoD(+)) in myogenic cells isolated at different post-natal time points and in adults. Our results indicate that Pax7(+) cells represent between 10 and 15% of the whole myogenic cell population found at birth indicating that these cells provide a modest contribution to the development of new fibres. The number of activated satellite cells (Pax7(+)/MyoD(+)) was scarce after birth but it was higher respect to adults. An interesting result was that at 1 month after birth the number of Pax7(+) cells had increased within the pool of myogenic cells with respect to myogenic cells extracted at birth. We speculate that Pax7 might be one of the molecules involved in controlling the proliferation/differentiation ratio in the pool of satellite cells present in post-natal porcine skeletal muscles.     

Mechano growth factor (MGF) promotes proliferation and inhibits differentiation of porcine satellite cells (PSCs) by down-regulation of key myogenic transcriptional factors

Porcine satellite cells represent an ideal model system for studying the cellular and molecular basis regulating myogenic stem cell proliferation and differentiation and for exploring the experimental conditions for myoblast transplantation. Here, we investigated the effects of mechano growth factor (MGF), a spliced variant of the IGF-1 gene, on porcine satellite cells. We show that MGF potently stimulated proliferation while inhibited differentiation of porcine satellite cells. MGF-treatment acutely down-regulates the expression of myogenic determination factor (MyoD) and the cyclin-dependent kinase inhibitor p21. MGF-treatment also markedly reduced the overall expression of cyclin B1 and key factors of the myogenic regulatory and myocyte enhancer families, including Myogenein and MEF2A. Taken together, the gene expression data from MGF-treated porcine satellite cells are in favor of a molecular model in which MGF inhibits porcine satellite cell differentiation by down-regulating either the activity or expression of MyoD, which, in turn, suppresses the expression of key genes required for cell cycle progression and differentiation, such as p21, Myogenin, and MEF2. Overall, our findings are in support of the previous suggestion that MGF may be used in vivo and in vitro to promote proliferation of myogenic stem cells to prevent and treat age-related muscle degenerative diseases.