Identification of skeletal muscle precursor cells in vivo by use of MyoD1 and myogenin probes

Clonogenic analysis reveals reserve stem cells in postnatal mammals: I. Pluripotent mesenchymal stem cells

Clonal populations of lineage-uncommitted pluripotent mesenchymal stem cells have been identified in prenatal avians and rodents. These cells reside in the connective tissue matrices of many organs and tissues. They demonstrate extended capabilities for self-renewal and the ability to differentiate into multiple separate tissues within the mesodermal germ line. This study was designed to determine whether such cells are present in the connective tissues of postnatal mammals. This report describes a cell clone derived by isolation from postnatal rat connective tissues, cryopreservation, extended propagation, and serial dilution clonogenic analysis. In the undifferentiated state, this clone demonstrates a high nuclear-to-cytoplasmic ratio and extended capacity for self-renewal. Subsequent morphological, histochemical, and immunochemical analysis after the induction of differentiation revealed phenotypic markers characteristic of multiple cell types of mesodermal origin, such as skeletal muscle, smooth muscle, fat cells, cartilage, and bone. These results indicate that this clone consists of pluripotent mesenchymal stem cells. This report demonstrates that clonal populations of reserve stem cells are present in mammals after birth. Potential roles for such cells in the maintenance, repair, and regeneration of mesodermal tissues are discussed.

Neutrophils injure cultured skeletal myotubes

The purpose of the study was to test the hypothesis that neutrophils can injure cultured skeletal myotubes. Human myotubes were grown and then cultured with human blood neutrophils. Myotube injury was quantitatively and qualitatively determined using a cytotoxicity (51Cr) assay and electron microscopy, respectively. For the 51Cr assay, neutrophils, under non-in vitro-stimulated and N-formylmethionyl-leucyl-phenylalanine (FMLP)-stimulated conditions, were cultured with myotubes at effector-to-target cell (E:T) ratios of 10, 30, and 50 for 6 h. Statistical analyses revealed that myotube injury was proportional to the E:T ratio and was greater in FMLP-stimulated conditions relative to non-in vitro-stimulated conditions. Transmission electron microscopy, using lanthanum as an extracellular tracer, revealed in cocultures a diffuse appearance of lanthanum in the cytoplasm of myotubes and a localized appearance within cytoplasmic vacuoles of myotubes. These observations and their absence in control cultures (myotubes only) suggest that neutrophils caused membrane rupture and increased myotube endocytosis, respectively. Myotube membrane blebs were prevalent in scanning and transmission electron micrographs of cultures consisting of neutrophils and myotubes (E:T ratio of 5) and were absent in control cultures. These data support the hypothesis that neutrophils can injure skeletal myotubes in vitro and may indicate that neutrophils exacerbate muscle injury and/or delay muscle regeneration in vivo.

Expression and neural control of myogenic regulatory factor genes during regeneration of mouse soleus

Given the importance of the myogenic regulatory factors (MRFs) for myoblast differentiation during development, the aims of this work were to clarify the spatial and temporal expression pattern of the four MRF mRNAs during soleus regeneration in mouse after cardiotoxin injury, using in situ hybridization, and to investigate the influence of innervation on the expression of each MRF during a complete degeneration/regeneration process. For this, we performed cardiotoxin injury-induced regeneration experiments on denervated soleus muscle. Myf-5, MyoD, and MRF4 mRNAs were detected in satellite cell-derived myoblasts in the first stages of muscle regeneration analyzed (2--3 days P-I). The Myf-5 transcript level dramatically decreased in young multinucleated myotubes, whereas MyoD and MRF4 transcripts were expressed persistently throughout the regeneration process. Myogenin mRNA was transiently expressed in forming myotubes. These results are discussed with regard to the potential relationships between MyoD and MRF4 in the satellite cell differentiation pathway. Muscle denervation precociously (at 8 days P-I) upregulated both the Myf-5 and the MRF4 mRNA levels, whereas the increase of both MyoD and myogenin mRNA levels was observed later, in the late stages of regeneration (30 days P-I). This significant accumulation of each differentially upregulated MRF during soleus regeneration after denervation suggests that each myogenic factor might have a distinct role in the regulatory control of muscle gene expression. This role is discussed in relation to the expression of the nerve-regulated genes, such as the nAChR subunit gene family. (J Histochem Cytochem 49:887-899, 2001)

Two upstream enhancers collaborate to regulate the spatial patterning and timing of MyoD transcription during mouse development

MyoD is a member of the basic-helix-loop-helix (bHLH) transcription factor family, which regulates muscle determination and differentiation in vertebrates. While it is now well established that the MyoD gene is regulated by Sonic hedgehog, Wnts, and other signals, it is not known how MyoD transcription is initiated and maintained in response to these signals. We have investigated the cis control of MyoD expression to identify and characterize the DNA targets that mediate MyoD transcription in embryos. By monitoring lacZ reporter gene expression in transgenic mice, we show that regulatory information contained in 24 kb of human MyoD 5' flanking sequence is sufficient to accurately control MyoD expression in embryos. Previous studies have identified two muscle-specific regulatory regions upstream of MyoD, a 4-kb region centered at -20 kb (designated fragment 3) that contains a highly conserved 258-bp core enhancer sequence, and a more proximal enhancer at -5 kb, termed the distal regulatory region (DRR), that heretofore has been identified only in mice. Here, we identify DRR-related sequences in humans and show that DRR function is conserved in humans and mice. In addition, transcriptional activity of MyoD 5' flanking sequences in somites and limb buds is largely a composite of the individual specificities of the two enhancers. Deletion of fragment 3 resulted in dramatic but temporary expression defects in the hypaxial myotome and limb buds, suggesting that this regulatory region is essential for proper temporal and spatial patterning of MyoD expression. These data indicate that regulatory sequences in fragment 3 are important targets of embryonic signaling required for the initiation of MyoD expression.

Early-age heat exposure affects skeletal muscle satellite cell proliferation and differentiation in chicks

Exposure of young chicks to thermal conditioning (TC; i.e., 37 degrees C for 24 h) resulted in significantly improved body and muscle growth at a later age. We hypothesized that TC causes an increase in satellite cell proliferation, necessary for further muscle hypertrophy. An immediate increase was observed in satellite cell DNA synthesis in culture and in vivo in response to TC of 3-day-old chicks to levels that were significantly higher than those of control chicks. This was accompanied by a marked induction of insulin-like growth factor-I (IFG-I), but not hepatocyte growth factor in the breast muscle. No significant difference between treatments in plasma IGF-I levels was observed. A marked elevation in muscle regulatory factors on day 5, followed by a decline in cell proliferation on day 6 together with continuous high levels of IGF-I in the TC chick muscle may indicate accelerated cell differentiation. These data suggest a central role for IGF-I in the immediate stimulation of satellite cell myogenic processes in response to heat exposure.

Stem cell plasticity in muscle and bone marrow

Recent discoveries have demonstrated the extraordinary plasticity of tissue-derived stem cells, raising fundamental questions about cell lineage relationships and suggesting the potential for novel cell-based therapies. We have examined this phenomenon in a potential reciprocal relationship between stem cells derived from the skeletal muscle and from the bone marrow. We have discovered that cells derived from the skeletal muscle of adult mice contain a remarkable capacity for hematopoietic differentiation. Cells prepared from muscle by enzymatic digestion and 5 day in vitro culture were harvested and introduced into each of six lethally irradiated recipients together with distinguishable whole bone marrow cells. Six and twelve weeks later, all recipients showed high-level engraftment of muscle-derived cells representing all major adult blood lineages. The mean total contribution of muscle cell progeny to peripheral blood was 56%, indicating that the cultured muscle cells generated approximately 10- to 14-fold more hematopoietic activity than whole bone marrow. Although the identity of the muscle-derived hematopoietic stem cells is still unknown, they may be identical to muscle satellite cells, some of which lack myogenic regulators and could respond to hematopoietic signals. We have also found that stem cells in the bone marrow can contribute to cardiac muscle repair and neovascularization after ischemic injury. We transplanted highly purified bone marrow stem cells into lethally irradiated mice that subsequently were rendered ischemic by coronary artery occlusion and reperfusion. The engrafted stem cells or their progeny differentiated into cardiomyocytes and endothelial cells and contributed to the formation of functional tissue.

Multilineage cells from human adipose tissue: implications for cell-based therapies

Future cell-based therapies such as tissue engineering will benefit from a source of autologous pluripotent stem cells. For mesodermal tissue engineering, one such source of cells is the bone marrow stroma. The bone marrow compartment contains several cell populations, including mesenchymal stem cells (MSCs) that are capable of differentiating into adipogenic, osteogenic, chondrogenic, and myogenic cells. However, autologous bone marrow procurement has potential limitations. An alternate source of autologous adult stem cells that is obtainable in large quantities, under local anesthesia, with minimal discomfort would be advantageous. In this study, we determined if a population of stem cells could be isolated from human adipose tissue. Human adipose tissue, obtained by suction-assisted lipectomy (i.e., liposuction), was processed to obtain a fibroblast-like population of cells or a processed lipoaspirate (PLA). These PLA cells can be maintained in vitro for extended periods with stable population doubling and low levels of senescence. Immunofluorescence and flow cytometry show that the majority of PLA cells are of mesodermal or mesenchymal origin with low levels of contaminating pericytes, endothelial cells, and smooth muscle cells. Finally, PLA cells differentiate in vitro into adipogenic, chondrogenic, myogenic, and osteogenic cells in the presence of lineage-specific induction factors. In conclusion, the data support the hypothesis that a human lipoaspirate contains multipotent cells and may represent an alternative stem cell source to bone marrow-derived MSCs.

Expression of CD34 and Myf5 defines the majority of quiescent adult skeletal muscle satellite cells

Skeletal muscle is one of a several adult post-mitotic tissues that retain the capacity to regenerate. This relies on a population of quiescent precursors, termed satellite cells. Here we describe two novel markers of quiescent satellite cells: CD34, an established marker of hematopoietic stem cells, and Myf5, the earliest marker of myogenic commitment. CD34(+ve) myoblasts can be detected in proliferating C2C12 cultures. In differentiating cultures, CD34(+ve) cells do not fuse into myotubes, nor express MyoD. Using isolated myofibers as a model of synchronous precursor cell activation, we show that quiescent satellite cells express CD34. An early feature of their activation is alternate splicing followed by complete transcriptional shutdown of CD34. This data implicates CD34 in the maintenance of satellite cell quiescence. In heterozygous Myf5(nlacZ/+) mice, all CD34(+ve) satellite cells also express beta-galactosidase, a marker of activation of Myf5, showing that quiescent satellite cells are committed to myogenesis. All such cells are positive for the accepted satellite cell marker, M-cadherin. We also show that satellite cells can be identified on isolated myofibers of the myosin light chain 3F-nlacZ-2E mouse as those that do not express the transgene. The numbers of satellite cells detected in this way are significantly greater than those identified by the other three markers. We conclude that the expression of CD34, Myf5, and M-cadherin defines quiescent, committed precursors and speculate that the CD34(-ve), Myf5(-ve) minority may be involved in maintaining the lineage-committed majority.

Myotube formation is delayed but not prevented in MyoD-deficient skeletal muscle: studies in regenerating whole muscle grafts of adult mice

We compared the time course of myogenic events in vivo in regenerating whole muscle grafts in MyoD(-/-) and control BALB/c adult mice using immunohistochemistry and electron microscopy. Immunohistochemistry with antibodies to desmin and myosin revealed a striking delay by about 3 days in the formation of myotubes in MyoD(-/-) autografts compared with BALB/c mice. However, myotube formation was not prevented, and autografts in both strains appeared similar by 8 days. Electron microscopy confirmed myotube formation in 8- but not 5-day MyoD(-/-) grafts. This pattern was not influenced by cross-transplantation experiments between strains examined at 5 days. Antibodies to proliferating cell nuclear antigen demonstrated an elevated level of replication by MyoD(-/-) myoblasts in autografts, and replication was sustained for about 3 days compared with controls. These data indicate that the delay in the onset of differentiation and hence fusion is related to extended proliferation of the MyoD(-/-) myoblasts. Overall, although muscle regeneration was delayed it was not impaired in MyoD(-/-) mice in this model.

Gene expression of receptors for IL-6, LIF, and CNTF in regenerating skeletal muscles

The biological actions of interleukin-6 (IL-6), leukemia inhibitory factor (LIF), and ciliary neurotrophic factor (CNTF) are mediated via respective functional receptor complexes consisting of a common signal-transducing component, gp130, and other specific receptor components, IL-6 receptor alpha (IL-6R), LIF receptor beta (LIFR), and CNTF receptor alpha (CNTFR). IL-6, LIF, and CNTF are implicated in skeletal muscle regeneration. However, the cell populations that express these receptor components in regenerating muscles are unknown. Using in situ hybridization histochemistry, we examined spatiotemporal expression patterns of gp130, IL-6R, LIFR, and CNTFR mRNAs in regenerating muscles after muscle contusion. At the early stages of regeneration (from 3 hr to Day 2 post contusion), significant signals for gp130 and LIFR mRNAs were detected in myonuclei and/or nuclei of muscle precursor cells (mpcs) and in mononuclear cells located in extracellular spaces between myofibers after muscle contusion, but IL-6R mRNA was expressed only in mononuclear cells. At Day 7 post contusion, signals for gp130, LIFR, and IL-6R mRNAs were not detected in newly formed myotubes, whereas the CNTFR mRNA level was upregulated in myotubes. These findings suggest that the upregulation of receptor subunits in distinct cell populations plays an important role in the effective regeneration of both myofibers and motor neurons. (J Histochem Cytochem 48:1203-1213, 2000)

MyoD-dependent induction during myoblast differentiation of p204, a protein also inducible by interferon

p204, an interferon-inducible p200 family protein, inhibits rRNA synthesis in fibroblasts by blocking the binding of the upstream binding factor transcription factor to DNA. Here we report that among 10 adult mouse tissues tested, the level of p204 was highest in heart and skeletal muscles. In cultured C2C12 skeletal muscle myoblasts, p204 was nucleoplasmic and its level was low. During myoblast fusion this level strongly increased, p204 became phosphorylated, and the bulk of p204 appeared in the cytoplasm of the myotubes. Leptomycin B, an inhibitor of nuclear export that blocked myoblast fusion, inhibited the nuclear export signal-dependent translocation of p204 to the cytoplasm. The increase in the p204 level during myoblast fusion was a consequence of MyoD transcription factor binding to several MyoD-specific sequences in the gene encoding p204, followed by transcription. Overexpression of p204 (in C2C12 myoblasts carrying an inducible p204 expression plasmid) accelerated the fusion of myoblasts to myotubes in differentiation medium and induced the fusion even in growth medium. The level of p204 in mouse heart muscle strongly increased during differentiation; it was barely detectable in 10. 5-day-old embryos, reached the peak level in 16.5-day-old embryos, and remained high thereafter. p204 is the second p200 family protein (after p202a) found to be involved in muscle differentiation. (p202a was formerly designated p202. The new designation is due to the identification of a highly similar protein-p202b [H. Wang, G. Chatterjee, J. J. Meyer, C. J. Liu, N. A. Manjunath, P. Bray-Ward, and P. Lengyel, Genomics 60:281-294, 1999].) These results reveal that p204 and p202a function in both muscle differentiation and interferon action.

Gene expression patterns of the fibroblast growth factors and their receptors during myogenesis of rat satellite cells

Satellite cells are the myogenic precursors in postnatal muscle and are situated beneath the myofiber basement membrane. We previously showed that fibroblast growth factor 2 (FGF2, basic FGF) stimulates a greater number of satellite cells to enter the cell cycle but does not modify the overall schedule of a short proliferative phase and a rapid transition to the differentiated state as the satellite cells undergo myogenesis in isolated myofibers. In this study we investigated whether other members of the FGF family can maintain the proliferative state of the satellite cells in rat myofiber cultures. We show that FGF1, FGF4, and FGF6 (as well as hepatocyte growth factor, HGF) enhance satellite cell proliferation to a similar degree as that seen with FGF2, whereas FGF5 and FGF7 are ineffective. None of the growth factors prolongs the proliferative phase or delays the transition of the satellite cells to the differentiating, myogenin(+) state. However, FGF6 retards the rapid exit of the cells from the myogenin(+) state that routinely occurs in myofiber cultures. To determine which of the above growth factors might be involved in regulating satellite cells in vivo, we examined their mRNA expression patterns in cultured rat myofibers using RT-PCR. The expression of all growth factors, excluding FGF4, was confirmed. Only FGF6 was expressed at a higher level in the isolated myofibers and not in the connective tissue cells surrounding the myofibers or in satellite cells dissociated away from the muscle. By Western blot analysis, we also demonstrated the presence of FGF6 protein in the skeletal musle tissue. Our studies therefore suggest that the myofibers serve as the main source for the muscle FGF6 in vivo. We also used RT-PCR to analyze the expression patterns of the four tyrosine kinase FGF receptors (FGFR1-FGFR4) and of the HGF receptor (c-met) in the myofiber cultures. Depending on the time in culture, expression of all receptors was detected, with FGFR2 and FGFR3 expressed only at a low level. Only FGFR4 was expressed at a higher level in the myofibers but not the connective tissue cell cultures. FGFR4 was also expressed at a higher level in satellite cells compared to the nonmyogenic cells when the two cell populations were released from the muscle tissue and fractionated by Percoll density centrifugation. The unique localization patterns of FGF6 and FGFR4 may reflect specific roles for these members of the FGF signaling complex during myogenesis in adult skeletal muscle.

Denervation induces a rapid nuclear accumulation of MRF4 in mature myofibers

Muscle regulatory factor 4 (MRF4) is a member of the family of myogenic transcription factors, including MyoD, myogenin, and myf-5, that are necessary for the commitment and differentiation of mesoderm to skeletal muscle. Although the function of these transcription factors during embryonic development has been demonstrated, their role in adult muscle has remained elusive. Regulation of the MRF4 gene differs from the genes encoding the other myogenic factors in that its transcripts accumulate in neonatal muscle during maturation and continue to be expressed at relatively high levels in the adult. On the basis of its mRNA expression pattern, MRF4 has been suggested to regulate genes encoding adult contractile proteins and acetylcholine receptor subunits. To test this hypothesis, a specific antiserum was developed to study MRF4 protein expression in adult innervated and denervated muscle, because MRF4 mRNA levels increase by approximately threefold 1 day after nerve resection. By using three different immunohistochemical methods that vary widely in sensitivity, we were unable to detect MRF4 immunoreactivity in adult innervated muscles. The same results were obtained with another MRF4 antiserum generated independently. In contrast, any of these three immunologic techniques readily detected MRF4 immunoreactivity in myofiber and satellite cell nuclei of muscles denervated for 24 hours. The highest proportion of immunopositive nuclei (80%) was found 2-3 days after denervation. Immunoreactivity was no longer detectable by 14 days. There was no differential accumulation of MRF4 protein in the nuclei of satellite cells nor in sole plate (synaptic) nuclei at any time after denervation. No differences were found in the temporal accumulation of MRF4 in nuclei of type I and type II denervated myofibers, consistent with the similar distribution of MRF4 mRNAs in slow- and fast-twitch muscles. Our results are consistent with the lack of phenotype observed in the adult muscles of MRF4-null mutant mice observed by others and suggest that MRF4 may have important roles in the gene programs activated after denervation and during muscle regeneration.

Regulation of myogenin protein expression in denervated muscles from young and old rats

Myogenin is a muscle-specific transcription factor participating in denervation-induced increases in nicotinic ACh receptor (nAChR) gene expression. Although myogenin RNA expression in denervated muscle is well documented, surprisingly little is known about myogenin protein expression. Therefore, we assayed myogenin protein and RNA in innervated and denervated muscles from young (4 mo) and old (24-32 mo) rats and compared this expression to that of the nAChR alpha-subunit RNA. These assays revealed increased myogenin protein expression within 1 day of denervation, preceding detectable increases in nAChR RNA. By 3 days of denervation, myogenin and nAChR alpha-subunit RNA were increased 500- and 130-fold, respectively, whereas myogenin protein increased 14-fold. Interestingly, old rats (32 mo) had 6-fold higher myogenin protein and approximately 80-fold higher mRNA levels than young rats. However, after denervation, expression levels were similar for young and old animals. The increased myogenin expression during aging, which tends to localize to small fibers, likely reflects spontaneous denervation and/or regeneration. Our results show that increased myogenin protein in denervated muscles correlates with the upregulation of its mRNA.

Glucocorticoid inhibition of C2C12 proliferation rate and differentiation capacity in relation to mRNA levels of the MRF gene family

The muscle regulatory factors (MRF) gene family regulate muscle fibre development. Several hormones and drugs also affect muscle development. Glucocorticoids are the only drugs reported to have a beneficial effect on muscle degenerative disorders. We investigated the glucocorticoid-related effects on C2C12 myoblast proliferation rate, morphological differentiation, and subsequent mRNA expression patterns of the MRF genes. C2C12 cells were incubated with the glucocorticoids dexamethasone or alpha-methyl-prednisolone. Both glucocorticoids showed comparable effects. Glucocorticoid treatment of C2C12 cells during the proliferative phase reduced the proliferation rate of the cells dose dependently, especially during the third and fourth day of culture, increased MyoD1, myf-5, and MRF4 mRNA levels, and reduced myogenin mRNA level, compared to untreated control cells. Thus, the mRNA level of proliferation-specific MyoD1 and myf-5 expression does not seem to associate with C2C12 myoblast proliferation rate. Glucocorticoid treatment of C2C12 cells during differentiation reduced the differentiation capacity dose dependently, which is accompanied by a dose dependent reduction of myogenin mRNA level, and increased MyoD1, myf-5, and MRF4 mRNA levels compared to untreated control cells. Therefore, we conclude that glucocorticoid treatment reduces differentiation of C2C12 myoblasts probably through reduction of differentiation-specific myogenin mRNA level, while inducing higher mRNA levels of proliferation-associated MRF genes.

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."

Neural and hormonal control of expression of myogenic regulatory factor genes during regeneration of Xenopus fast muscles: myogenin and MRF4 mRNA accumulation are neurally regulated oppositely

With the aim to investigate the influence of both innervation and thyroid hormone, on the expression of the MRFs during muscle regeneration, we performed cardiotoxin injury-induced regeneration experiments on fast muscles of adult Xenopus laevis subjected to different experimental conditions, including denervation and T3 treatment, and analyzed the accumulation of the four myogenic regulatory factors (MRFs) using RT-PCR and in situ hybridization. We show here that manipulation of hormone levels or innervation resulted in differential alterations of MRF expression. Denervation and T3 treatment transiently down-regulated Myf-5 mRNA levels at the beginning of the regeneration process. Myf-5 was the only myogenic factor subject to thyroid hormone influence. Muscle denervation persistently reduces the levels of MRF4 transcripts as early as the first stages of regeneration, whereas the levels of myogenin mRNA were increased in the late stages of regeneration. This suggests that MRF4 expression may be induced by innervation and hence may be involved in mediating transcriptional responses to innervation and that myogenin expression may compensate for the down-regulation of MRF4 gene. This switch in MRF gene expression following denervation could have important consequences for the ability of Xenopus regenerating muscles to recover function after denervation.

Hematopoietic potential of stem cells isolated from murine skeletal muscle

We have discovered that cells derived from the skeletal muscle of adult mice contain a remarkable capacity for hematopoietic differentiation. Cells prepared from muscle by enzymatic digestion and 5-day in vitro culture were harvested, and 18 x 10(3) cells were introduced into each of six lethally irradiated recipients together with 200 x 10(3) distinguishable whole bone marrow cells. After 6 or 12 weeks, all recipients showed high-level engraftment of muscle-derived cells representing all major adult blood lineages. The mean total contribution of muscle cell progeny to peripheral blood was 56 +/- 20% (SD), indicating that the cultured muscle cells generated approximately 10- to 14-fold more hematopoietic activity than whole bone marrow. When bone marrow from one mouse was harvested and transplanted into secondary recipients, all recipients showed high-level multilineage engraftment (mean 40%), establishing the extremely primitive nature of these stem cells. We also show that muscle contains a population of cells with several characteristics of bone marrow-derived hematopoietic stem cells, including high efflux of the fluorescent dye Hoechst 33342 and expression of the stem cell antigens Sca-1 and c-Kit, although the cells lack the hematopoietic marker CD45. We propose that this population accounts for the hematopoietic activity generated by cultured skeletal muscle. These putative stem cells may be identical to muscle satellite cells, some of which lack myogenic regulators and could be expected to respond to hematopoietic signals.

Gender related and dexamethasone induced differences in the mRNA levels of the MRF genes in rat anterior tibial skeletal muscle

Muscle formation and postnatal growth is under the control of the muscle regulatory factors (MRF) gene family, consisting of four genes: MyoD1, myogenin, myf-5, and myf-6. Muscle mass is also known to be affected by specific drugs, like glucocorticoids. Glucocorticoids have also been characterized as muscle atrophying agents. However, glucocorticoids are also the only drugs reported to have a beneficial effect on the treatment of muscle degenerative disorders. Since muscle mass relates to gender, this may be partially caused by gender. The aim of this study is to investigate gender-related basal and dexamethasone-induced expression of the MRF genes. Gender-specific MRF mRNA levels were investigated in anterior tibial muscles of the rat. Myogenin, myf-5, and myf-6 mRNA level was significantly higher in female rats than in male rats. Since muscle mass is usually higher in males, we conclude that the development of gender-related differences in muscle mass is not primarily under the control of the mRNA levels of the MRF genes. Male rats treated with dexamethasone for 14 days (1 mg per kg body weight) showed increased levels of MyoD1, myogenin and myf-5 compared to control male rats. Female rats treated with dexamethasone showed decreased expression of myf-6 compared to control female rats. These results suggest that dexamethasone increase satellite cell-specific MRF activity in male muscle tissue, which is suggested to be associated with muscle hypertrophy, while maintenance of muscle tissue is affected in female muscle tissue. Therefore, we conclude that both basal and dexamethasone-induced MRF gene mRNA levels are regulated gender-specific.

Satellite cells on isolated myofibers from normal and denervated adult rat muscle

Satellite cells (SCs) in normal adult muscle are quiescent. They can enter the mitotic program when stimulated with growth factors such as basic FGF. Short-term denervation stimulates SC to enter the mitotic cycle in vivo, whereas long-term denervation depletes the SC pool. The molecular basis for the neural influence on SCs has not been established. We studied the phenotype and the proliferative capacity of SCs from muscle that had been denervated before being cultured in vitro. The expression of PCNA, myogenin, and muscle (M)-cadherin in SCs of normal and denervated muscle fibers was examined at the single-cell level by immunolabeling in a culture system of isolated rat muscle fibers with attached SCs. Immediately after plating (Day 0), neither PCNA nor myogenin was present on normal muscle fibers, but we detected an average of 0.5 M-cadherin(+) SCs per muscle fiber. The number of these M-cadherin(+) cells (which are negative for PCNA and myogenin) increased over the time course examined. A larger fraction of cells negative for M-cadherin underwent mitosis and expressed PCNA, followed by myogenin. The kinetics of SCs from muscle fibers denervated for 4 days before culturing were similar to those of normal controls. Denervation from 1 to 32 weeks before plating, however, suppressed PCNA and myogenin expression almost completely. The fraction of M-cadherin(+) (PCNA(-)/myogenin(-)) SCs was decreased after 1 week of denervation, increased above normal after denervation for 4 or 8 weeks, and decreased again after denervation for 16 or 32 weeks. We suggest that the M-cadherin(+) cells are nondividing SCs because they co-express neither PCNA or myogenin, whereas the cells positive for PCNA or myogenin (and negative for M-cadherin) have entered the mitotic cycle. SCs from denervated muscle were different from normal controls when denervated for 1 week or longer. The effect of denervation on the phenotypic modulation of SCs includes resistance to recruitment into the mitotic cycle under the conditions studied here and a robust extension of the nonproliferative compartment. These characteristics of SCs deprived of neural influence may account for the failure of denervated muscle to fully regenerate. (J Histochem Cytochem 47:1375-1383, 1999)

Time course of changes in markers of myogenesis in overloaded rat skeletal muscles

During the process of compensatory muscle hypertrophy, satellite cells are thought to proliferate, differentiate, and then fuse with existing myofibers. We hypothesized that early in this process changes occur in the expression of cellular markers indicative of the onset of myogenic processes. The plantaris muscles of rats were overloaded via the unilateral ablation of synergists. Groups of rats were killed at time points from 6 h to 12 days. Changes in muscle gene expression (mRNA) of cyclin D1, p21, myogenin, MyoD, and insulin-like growth factor I (IGF-I, mRNA and peptide) were measured. Cyclin D1 (a cell cycle marker) was increased after 24 h of overloading and corresponded with changes in muscle DNA content. In contrast, p21 and myogenin, markers of cellular differentiation, were increased after just 12 h. Muscle IGF-I peptide levels were also increased at early time points. The results of this study indicate that myogenic processes are activated in response to increased loading at very early time points (e.g., 12 h) and that IGF-I may be modulating this response. Furthermore, these findings suggest that some cells may have been differentiating very early in the adaptation process before events leading to cellular proliferation have been initiated.

Activated satellite cells fail to restore myonuclear number in spinal cord transected and exercised rats

In this study, possible mechanisms underlying soleus muscle atrophy after spinal cord transection and attenuation of atrophy with cycling exercise were studied. Adult female Sprague-Dawley rats were divided into three groups; in two groups the spinal cord was transected by a lesion at T10. One group was transected and killed 10 days later, and another group was transected and exercised for 5 days starting 5 days after transection. The third group served as an uninjured control. All animals received a continuous-release 5'-bromo-2'-deoxyuridine pellet 10 days before they were killed. Transection alone and transection with exercise lead to activation of satellite cells, but only the exercise group showed a trend toward an increase in the number of proliferating satellite cells. In all cases the number of activated satellite cells was significantly higher than the number that divided. Although the number of cells undergoing proliferation increased with exercise, no increase in fusion of satellite cells into muscle fibers was apparent. Spinal cord transection resulted in a 25% decrease in myonuclear number, and exercise was not associated with a restoration of myonuclear number. The number of apoptotic nuclei was increased after transection, and exercise attenuated this increase. However, the decrease in apoptotic nuclei with exercise did not significantly affect myonuclear number. We conclude that apoptotic nuclear loss likely contributes to loss of nuclei during muscle atrophy associated with spinal cord transection and that exercise can maintain muscle mass, at least in the short term, without restoration of myonuclear number.

In vivo satellite cell activation via Myf5 and MyoD in regenerating mouse skeletal muscle

Regeneration of adult skeletal muscle is an asynchronous process requiring the activation, proliferation and fusion of satellite cells, to form new muscle fibres. This study was designed to determine the pattern of expression in vivo of the two myogenic regulatory factors, Myf5 and MyoD during this process. Cardiotoxin was used to induce regeneration in the gastrocnemius and soleus muscles of heterozygous Myf5-nlacZ mice, and the muscles were assayed for the presence of (beta)-galactosidase (Myf5) and MyoD. Adult satellite cells identified by M-cadherin labelling, when activated, initially express either MyoD or Myf5 or both myogenic factors. Subsequently all proliferating myoblasts express MyoD and part of the population is (beta)-galactosidase (Myf5) positive. Furthermore, we demonstrate that activated satellite cells, which express either Myf5 or MyoD, do not accumulate selectively on fast or slow muscle fibres.

Muscle growth and development in normal-sex-ratio and all-female diploid and triploid Atlantic salmon

Muscle development and growth were investigated in diploid populations of normal-sex-ratio and all-female Atlantic salmon (Salmo salar L.) and their triploid counterparts produced by high-pressure treatment. Somites were formed at the rate of 6 h-1 in both diploids and triploids at 6 degrees C. The rostral-to-caudal development of myotubes, myofibrils and acetylcholinesterase staining at the myosepta was slightly more advanced in triploid than in diploid fish, although the differences were smaller than among individual families. The c-met receptor tyrosine kinase was used as a molecular marker for the satellite cells involved in postembryonic muscle growth. Satellite cell nuclei comprised 17.5 % of total myonuclei in smolts and they were 24 % more abundant in diploid than in triploid fish. Cells expressing the myogenic regulatory factor myf-6, a marker of satellite cells committed to differentiation, represented 14.8 % of total myonuclei in diploids and 12.5 % in triploids. At ambient temperatures, the number of white muscle fibres in normal-sex-ratio fish increased more than 30-fold between the alevin and smolt stages, and approximately 3.5-fold further during the first year of seawater growth. The rate of muscle fibre recruitment in seawater stages was significantly greater in diploid than in triploid fish, reaching 1162 fibres day-1 and 608 fibres day-1, respectively, in all-female groups 800 days post-hatching. For 42 cm fork-length fish, there were approximately one-third more muscle fibres per myotome in diploid than in triploid groups, 649 878 and 413 619, respectively, for all-female fish. The probability density function of muscle fibre diameters in each fish was estimated using non-parametric smoothing techniques, and the mean densities for diploids (fD) and triploids (fT) were calculated. The peak fibre diameter was approximately 20 (micro)m in all age classes, irrespective of ploidy. Distinct bimodal distributions of muscle fibre diameter were evident in all groups 775 days and 839 days post-hatching, reflecting seasonal cycles of fibre recruitment. fD and fT were compared using a non-parametric bootstrap technique and the reference band representing the null-hypothesis indicated that there was no difference with ploidy. Reference bands for normal-sex-ratio fish at 315 days and 470 days indicated that diploids had a higher percentage of smaller-diameter fibres and that triploid distributions had a thicker right-hand tail. Similar differences in fD and fT of muscle fibre diameters were found for all-female fish, although the statistical evidence was less strong. Reference bands indicated differences in the middle range of the distributions of muscle fibre diameter in fish 620–775 days post-hatch, with triploids having a thicker right-hand tail. Thus, a lower density of satellite cells was associated with reduced rates of fibre recruitment but a compensatory increase in muscle fibre hypertrophy in triploid compared with diploid fish.

Quantitative study of the effects of denervation and castration on the levator ani muscle of the rat

The levator ani muscle (LA) of the rat is highly androgen-sensitive and, like all skeletal muscles, deteriorates structurally and functionally when denervated. In order to elucidate the interplay of neural and endocrine influences, the separate and combined effects of denervation and castration on myofiber cross-sectional area and nuclear populations were quantitatively studied. In one group of 4-month-old male rats (A), the LA was denervated. Another group (B) was surgically castrated and a third group (C) was both denervated and castrated. The control rats (D) remained both gonad- and nerve-intact. After two months, the LA was obtained for myofiber and nuclear enumeration, cross-sectional area and satellite cell frequency determination. In the denervated muscle of gonad-intact rats (Group A), myofiber cross-sectional area was markedly diminished (265.84+/-11.38 microm2; compared with controls [Group D]: 1519.98+/-79.41 microm2; P < 0.05). Satellite cell nuclei, as a percentage of total sublaminar nuclei (i.e., satellite cell ratio), increased significantly (4.26%, from a control value of 1.91%). Castration alone (Group B) resulted in pronounced myofiber atrophy (mean cross-sectional area: 754.03+/-89.63 microm2) but had no significant effect on satellite cell ratio (2.36%). The combination of castration and denervation (Group C) elicited the same degree of myofiber atrophy as denervation alone (Group A) but had no significant impact on satellite cell ratio. Instead, the nuclear count per myofiber declined to about a third of the control level (300.5+/-38.49 compared with 861.7+/-24.8; P < 0.05). The results indicate that the atrophic effects of denervation and castration on the LA are non-synergistic and mechanistically similar. They also show that the inability of satellite cells to respond mitotically to the withdrawal of neural input under disandrogenized conditions is a factor in the myonuclear depletion of the denervated muscle of castrated rats.

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.

Cloning of novel injury-regulated genes. Implications for an important role of the muscle-specific protein skNAC in muscle repair

To gain insight into the molecular mechanisms underlying the wound repair process, we searched for genes that are regulated by skin injury. Using the differential display reverse transcription-polymerase chain reaction technique, we identified a gene that was strongly induced as early as 12 h after wounding. Sequence analysis revealed the identity of the corresponding protein with skeletal muscle nascent polypeptide-associated complex (skNAC), a recently identified muscle-specific transcription factor. By in situ hybridization and immunohistochemistry, we demonstrated the specific expression of skNAC in skeletal muscle cells of the panniculus carnosus at the wound edge. Furthermore, in vitro studies with cultured myoblasts revealed expression of skNAC in differentiating and differentiated, but not in proliferating, nondifferentiated cells. Differentiation of cultured myoblasts was accompanied by simultaneous expression of skNAC and the muscle-specific transcription factor myogenin. Our results provide the first evidence for a role of skNAC in muscle repair processes. Furthermore, they demonstrate the usefulness of our approach in identifying new players in wound repair.

Prolonged passive stretch of rat soleus muscle provokes an increase in the mRNA levels of the muscle regulatory factors distributed along the entire length of the fibers

The mRNA levels of the adult and the neonatal sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPases (SERCA1a and SERCA1b, respectively) and those of the muscle regulatory factors (MRFs: myoD, myf-5, myogenin, MRF4) have been assessed by RT PCR in rat soleus muscles immobilized for 3 days in an extended position (passive stretch). The transcript level of the fast type SERCA1a Ca(2+)-transport ATPase decreased to half of its normal value, whereas that of neonatal SERCA1b isoform increased 5-fold above control in stretched muscles. Immunostaining of muscle cross sections showed that the fraction of fibers expressing the SERCA1a protein was decreased evenly along the length of the stretched muscles indicating that a transformation occurred of fast fibers to slow ones. The mRNA levels of MRFs were elevated 3- to 6-fold above the normal level and were distributed evenly along the length of the stretched muscles. However in the controls these transcripts were more abundant at both ends of the muscle. The stretch increased the level of myoD and immunocytochemistry showed the expression of myoD protein in a number of nuclei of the stretched muscles whereas it was practically undetectable by this method in the control muscles. Western blotting did not indicate a significant stretch-induced increase in the level of the myogenin protein, in spite of the fact that immunocytochemistry tended to show more myogenin-positive nuclei in stretched muscles as compared to the controls. These data indicate that after 3 days of passive stretch the central and the terminal parts of the soleus muscle adapt similarly by increasing the levels of the MRFs, by decreasing the overall levels of the fast SERCA1-type of ATPase and by partially re-establishing a neonatal mode of alternative SERCA1 transcript splicing resulting in an increased SERCA1b/1a ratio.

Myoid cell density in the thymus is reduced during mdx dystrophy and after muscle crush

Thymic myoid cells share structural and behavioural features with cells of the skeletal muscle lineage: they express regulatory genes and contractile proteins, and they can form myofibers in culture. Historically, those features suggested that myoid cells could be precursors for muscle repair in addition to the satellite cells in muscle that are typically designated as the only muscle precursors. Muscles of the mutant mdx dystrophic mouse strain have a large demand for precursors, which is greatest at a young age. In the present study, immunostaining for troponin T was used to localize myoid cells. We tested the hypothesis that the myoid cell population changes when there is a demand for muscle precursors and that these changes would be anticipated if myoid cells have a role as myogenic precursors or stem cells in muscle. Chronic demands for muscle precursors in mdx dystrophic mice were accompanied by lower myoid cell density in comparison with density in two normal strains (C57BL10/ScSn and Swiss Webster). Acute demand for precursors was accompanied by a sharp decline in thymic myoid cell density within 2 days after a crush injury to one tibialis anterior muscle in normal but not dystrophic animals. To standardize the developmental age of the thymus, density was determined in all animals at 28 days of age. Given the current interest in nonmuscle sources of myogenic stem cells, these data suggest that changes in the density of thymic myoid cells may accompany acute and chronic demands for muscle precursors. Further experiments are required to determine whether thymic myoid cells are participants in distant muscle cell proliferation, new fiber formation, or the establishment of new stem cells in regenerated muscle.Key words: thymus, myoid cells, troponin T, MyoD, tissue repair, myoblasts, mdx dystrophy.

Association of IGF-I and IGF-II with myofiber regeneration in vivo

This study examined expression of insulinlike growth factor (IGF) in the myofibers and nonmyofibrillar structures of murine soleus muscle following contraction-induced damage. Identifying the cellular sources of this myogenic growth factor could improve muscle rehabilitation strategies. Immunohistochemical analysis of muscle sections indicated that the number of myofibers expressing both IGF-I and IGF-II increased significantly at 4, 7, and 10 days following injury, compared with control. Muscle spindles and vascular tissue expressed only IGF-II, and staining intensity did not change following injury. The number of fibers expressing developmental myosin heavy chain increased significantly at 7 and 10 days postinjury, and these usually coexpressed IGF. No IGF-specific staining of interstitial/inflammatory cells was observed. Therefore, expression of IGF after mechanically induced fiber damage occurs exclusively within regenerating fibers without supplemental delivery of IGF to the tissue by inflammatory cells or changes in constitutive expression of IGF-II in vascular tissue.

Expression of matrix metalloproteinases 2 and 9 in regenerating skeletal muscle: a study in experimentally injured and mdx muscles

Matrix metalloproteinases (MMPs) cooperatively degrade all components of the extracellular matrix (ECM). Remodeling of ECM during skeletal muscle degeneration and regeneration suggests a tight regulation of matrix-degrading activity during muscle regeneration. In this study, we investigated the expression of MMP-2 and MMP-9, in normal muscles and their regulation during regeneration process. We further investigated their secretion by C2C12 myogenic cell line. Two models of muscle degeneration-regeneration were used: (1) normal muscles in which necrosis was experimentally induced by cardiotoxin injection; (2) mdx muscles which exhibit recurrent signs of focal myofiber necrosis followed by successful regeneration. MMPs were studied by zymography; their free activity was quantified using 3H-labeled gelatin substrate and mRNA expression was followed by Northern hybridization. Muscle degeneration-regeneration was analyzed by conventional morphological methods and in situ hybridization was performed on muscle sections to identify the cells expressing these MMPs. Results show that MMP-2, but not MMP-9 expression, is constitutive in normal muscles. Upon injury, the active form of MMP-2 is transiently increased, whereas MMP-9 is induced within 24 h and remains present for several days. Quantitative assays of free gelatinolytic activity show a progressive and steady increase that culminates at 7 days postinjury and slowly returns to normal levels. In adult mdx mice, both pro and active forms of MMP-2 and MMP-9 are expressed. Northern blot results support these findings. Zymography of C2C12-conditioned medium shows that myogenic cells produce MMP-2. By in situ hybridization we localized MMP-9 mRNA in inflammatory cells and putative activated satellite cells in injured muscles. Our data allow the correlation of the differential expression of pro and/or active forms of MMP-2 and MMP-9 with different stages of the degeneration-regeneration process: MMP-9 expression is related to the inflammatory response and probably to the activation of satellite cells, whereas MMP-2 activation is concomitant with the regeneration of new myofibers.

Fibroblast growth factor promotes recruitment of skeletal muscle satellite cells in young and old rats

Although the role of satellite cells in muscle growth and repair is well recognized, understanding of the molecular events that accompany their activation and proliferation is limited. In this study, we used the single myofiber culture model for comparing the proliferative dynamics of satellite cells from growing (3-week-old), young adult (8- to 10-week-old), and old (9- to 11-month-old) rats. In these fiber cultures, the satellite cells are maintained in their in situ position underneath the fiber basement membrane. We first demonstrate that the cytoplasm of fiber-associated satellite cells can be monitored with an antibody against the extracellular signal regulated kinases 1 and 2 (ERK1 and ERK2), which belong to the mitogen-activated protein kinase (MAPK) superfamily. With this immunocytological marker, we show that the satellite cells from all three age groups first proliferate and express PCNA and MyoD, and subsequently, about 24 hr later, exit the PCNA+/MyoD+ state and become positive for myogenin. For all three age groups, fibroblast growth factor 2 (FGF2) enhances by about twofold the number of satellite cells that are capable of proliferation, as determined by monitoring the number of cells that transit from the MAPK+ phenotype to the PCNA+/MAPK+ or MyoD+/MAPK+ phenotype. Furthermore, contrary to the commonly accepted convention, we show that in the fiber cultures FGF2 does not suppress the subsequent transition of the proliferating cells into the myogenin+ compartment. Although myogenesis of satellite cells from growing, young adult, and old rats follows a similar program, two distinctive features were identified for satellite cells in fiber cultures from the old rats. First, a large number of MAPK+ cells do not appear to enter the MyoD-myogenin expression program. Second, the maximal number of proliferating satellite cells is attained a day later than in cultures from the young adults. This apparent "lag" in proliferation was not affected by hepatocyte growth factor (HGF), which has been implicated in accelerating the first round of satellite cell proliferation. HGF and FGF2 were equally efficient in promoting proliferation of satellite cells in fibers from old rats. Collectively, the investigation suggests that FGF plays a critical role in the recruitment of satellite cells into proliferation.

mRNA levels of myogenic regulatory factors in rat slow and fast muscles regenerating from notexin-induced necrosis

The transcript levels of the myogenic regulatory factors (myoD, myf5, myogenin and MRF4) were measured by RT PCR in rat soleus (slow) and EDL (fast) muscles which were regenerating from notexin-induced necrosis. Some muscle fibers in the EDL were more resistant to the toxin, therefore the necrosis and the dominance of myoblasts were delayed for two days in EDL compared to soleus. In spite of this shift in time-course of necrosis, both types of muscle presented roughly similar, although variable, changes in the expression pattern of MRF mRNA levels. For both muscles, the myoD mRNA was upregulated on the first day after administration of the toxin, whereas concomitantly myf-5 mRNA disappeared but showed a substantial increase in later stages of regeneration. In contrast, the mRNA levels of the late MRFs myogenin and MRF4 decreased on day one only in the soleus, then increased on day three in both types of muscle. Meanwhile in EDL the level of MRF4 mRNA remained relatively normal. Four weeks after administration of the toxin the mRNA levels for each of the MRFs returned to nearly control levels. This shows that in spite of the different time course of the necrosis and regeneration, also documented by the microscopical morphology and the skeletal actin mRNA levels of the muscles, the level of MRF transcripts changed according to a quite predictable pattern; the upregulation corresponded to myoblast activation and the downregulation to the reinnervation.

Early changes in muscle fiber size and gene expression in response to spinal cord transection and exercise

Muscles of spinal cord-transected rats exhibit severe atrophy and a shift toward a faster phenotype. Exercise can partially prevent these changes. The goal of this study was to investigate early events involved in regulating the muscle response to spinal transection and passive hindlimb exercise. Adult female Sprague-Dawley rats were anesthetized, and a complete spinal cord transection lesion (T10) was created in all rats except controls. Rats were killed 5 or 10 days after transection or they were exercised daily on motor-driven bicycles starting at 5 days after transection and were killed 0.5, 1, or 5 days after the first bout of exercise. Structural and biochemical features of soleus and extensor digitorum longus (EDL) muscles were studied. Atrophy was decreased in all fiber types of soleus and in type 2a and type 2x fibers of EDL after 5 days of exercise. However, exercise did not appear to affect fiber type that was altered within 5 days of spinal cord transection: fibers expressing myosin heavy chain 2x increased in soleus and EDL, and extensive coexpression of myosin heavy chain in soleus was apparent. Activation of satellite cells was observed in both muscles of transected rats regardless of exercise status, evidenced by increased accumulation of MyoD and myogenin. Increased expression was transient, except for MyoD, which remained elevated in soleus. MyoD and myogenin were detected both in myofiber and in satellite cell nuclei in both muscles, but in soleus, MyoD was preferentially expressed in satellite cell nuclei, and in EDL, MyoD was more readily detectable in myofiber nuclei, suggesting that MyoD and myogenin have different functions in different muscles. Exercise did not affect the level or localization of MyoD and myogenin expression. Similarly, Id-1 expression was transiently increased in soleus and EDL upon spinal cord transection, and no effect of exercise was observed. These results indicate that passive exercise can ameliorate muscle atrophy after spinal cord transection and that satellite cell activation may play a role in muscle plasticity in response to spinal cord transection and exercise. Finally, the mechanisms underlying maintenance of muscle mass are likely distinct from those controlling myosin heavy chain expression.

MyoD, myogenin, and desmin-nls-lacZ transgene emphasize the distinct patterns of satellite cell activation in growth and regeneration

Although satellite cell differentiation is involved in postnatal myogenesis from growth to posttrauma regeneration, the early stages of this process remain unclear. This study investigated pHuDes-nls-lacZ transgene activity, as revealed by X-gal staining and the accumulation of MyoD, myogenin, endogenous desmin, and myosin, in order to determine whether satellite cells share the same activation program during growth and regeneration. After birth, skeletal myonuclei in which myogenin expression was limited were briefly characterized by transgene activity. Satellite cells were only evidenced by MyoD and slow myosin accumulation, but failed to initiate transgene expression. After freeze trauma, satellite cell activation led to MyoD, myogenin, and desmin expression. Subsequently, when myosin expression occurred, transgene activation was apparent in regenerating structures, with more intense X-gal staining in mononucleated cells than regenerating myotubes. After the second week posttrauma, only desmin and myogenin expression were maintained in regenerating structures. In culture, the behavior of satellite cells showed that desmin expression was committed before transgene activation occurred, i.e., concurrently with MyoD, myogenin, myosin expression, and the first fusion events. Quantitative analysis confirmed the discrepancy between endogenous desmin and transgene expression and demonstrated the close correlation between transgene activation and the fusion index. Our results strongly suggest that satellite cells promote distinct pathways of myogenic response during growth and regeneration.

Dystrophy and myogenesis in mdx diaphragm muscle

In order to determine why the diaphragm is more severely affected by progressive dystrophy than limb muscles in the mdx mouse, we examined how regional variations in diaphragm dystrophy, the measures of disease and repair, proliferation by committed myogenic cells, and the expression of mitogenic basic fibroblast growth factor (bFGF) could contribute to muscle-specific disease phenotypes. There were regional variations in new myotube formation in the diaphragm, with disease more severe in crural than costal leaflets. New repair increased in hyperthyroidism without changes in accumulated repair, probably due to fiber loss. General proliferation was nearly twofold higher in limb than diaphragm mononuclear cells. Since only 2.5-8.4% of committed muscle precursors were proliferating, the higher proliferation by myf5+ myogenic cells in diaphragm did not account for muscle-specific differences. Proliferation by bFGF+ mononuclear cells and an immunogold labeling index for bFGF protein in diaphragm myoblasts were lower in diaphragm than limb muscle. In culture, mixed limb myoblast and fibroblasts contained more S phase cells than diaphragm cells, although myoblasts cycled similarly between muscles. Therefore while muscle architecture and the formation and number of new myotubes certainly affect disease phenotype, the differential outcome of regeneration in mdx diaphragm and limb muscle appears to be contributed by both nonmyogenic and myogenic cells.

Skeletal muscle regeneration mimicking rhabdomyosarcoma: a potential diagnostic pitfall

AIMS

We report three cases of skeletal muscle regeneration, of which two mimicked a small round cell tumour, especially a rhabdomyosarcoma.

METHODS AND RESULTS

One case presented as an intramuscular mass, located in the right quadriceps of a 12-year-old male; the second patient was a 25-year-old football player who complained of painful left peroneus muscles; the third patient was a 22-year-old male who underwent an amputation of the right thigh 5 days after right leg amputation due to limb crush. Histologically, muscle biopsy specimens showed a proliferation of small round cells, either infiltrating the striated muscle in a diffuse manner or growing within and around necrotic myofibres. Immunohistochemically and ultrastructurally, the cellular population was composed of two types of cells: phagocytic cells the nuclei of which occasionally showed a wreathlike arrangement around necrotic myofibres resulting in structures resembling Langhans-type multinucleated giant cells, and proliferating satellite cells showing enlarged nuclei, prominent nucleoli, mitotic figures, myogenic differentiation and fusion features in order to form regenerating myotubes.

CONCLUSIONS

Muscle regeneration is a benign process that may occasionally mimic a small round cell proliferation resembling a lymphoma or an alveolar rhabdomyosarcoma with which it should not be confused.

Hypertrophy-stimulated myogenic regulatory factor mRNA increases are attenuated in fast muscle of aged quails

Myogenic regulatory factors (MRFs) are a family of skeletal muscle-specific transcription factors that regulate the expression of several muscle genes. This study was designed to determine whether MRF transcripts were increased in hypertrophy-stimulated muscle of adult quails and whether equivalent increases occurred in muscles of older quails. Slow-tonic anterior latissimus dorsi and fast-twitch patagialis muscles of adult, middle-aged, aged, and senescent quails were stretch overloaded for 6, 24, or 72 h, with contralateral muscles serving as controls. RNase protection assays showed that MRF4 and MyoD transcript levels were increased and myogenin and Myf5 transcripts were induced in stretch-overloaded muscles. However, MRF4 and MyoD increases were significantly attenuated in patagialis muscles of older quails. RT-PCR analyses of three MRF-regulated genes showed that increases in the transcription of these genes occurred with stretch overload, but the increases were less in muscles of older quails. In summary, attenuated MRF responses in muscles from aged animals may partially explain why muscles from older animals do not hypertrophy to the same extent as muscles from younger animals.

Localization of leukemia inhibitory factor and interleukin-6 messenger ribonucleic acids in regenerating rat skeletal muscle

In the present study, we characterized both temporal and spatial expression patterns of leukemia inhibitory factor (LIF) and interleukin-6 (IL-6) messenger ribonucleic acids (mRNAs) in injured skeletal muscle using in situ hybridization. LIF and IL-6 mRNAs were expressed in mononucleated cells and damaged muscle cells. Further, signals for LIF mRNA were also detected in Schwann cell-like cells of intramuscular nerves. These results suggest that the earliest events involved in the repair of injured muscles and nerves may be triggered by these cytokines.

Localization of Myf-5, MRF4 and alpha cardiac actin mRNAs in regenerating Xenopus skeletal muscle

We have analysed the spatial and temporal expression patterns of Myf-5, MRF4 and alpha cardiac actin mRNAs during muscle regeneration following cardiotoxin injury in adult Xenopus laevis using in situ hybridization. Myf-5 transcripts began to be detected in the activated satellite cells as early as the beginning of the regeneration process, then dramatically decreased in young plurinucleated myotubes. MRF4 mRNA was detected later, just before the young myotube stage, and was strongly expressed during the different stages of the maturation of myotubes. Like Myf-5, alpha cardiac actin mRNA began to accumulate early in activated satellite cells. These results, which contribute to an overview of the expression of the genes coding for myogenic bHLH proteins during muscle regeneration, are discussed in relation to the expression of these factors during development.

Muscle regeneration by bone marrow-derived myogenic progenitors

Growth and repair of skeletal muscle are normally mediated by the satellite cells that surround muscle fibers. In regenerating muscle, however, the number of myogenic precursors exceeds that of resident satellite cells, implying migration or recruitment of undifferentiated progenitors from other sources. Transplantation of genetically marked bone marrow into immunodeficient mice revealed that marrow-derived cells migrate into areas of induced muscle degeneration, undergo myogenic differentiation, and participate in the regeneration of the damaged fibers. Genetically modified, marrow-derived myogenic progenitors could potentially be used to target therapeutic genes to muscle tissue, providing an alternative strategy for treatment of muscular dystrophies.

Studies of the dynamics of skeletal muscle regeneration: the mouse came back!

Regeneration of skeletal muscle tissue includes sequential processes of muscle cell proliferation and commitment, cell fusion, muscle fiber differentiation, and communication between cells of various tissues of origin. Central to the process is the myosatellite cell, a quiescent precursor cell located between the mature muscle fiber and its sheath of external lamina. To form new fibers in a muscle damaged by disease or direct injury, satellite cells must be activated, proliferate, and subsequently fuse into an elongated multinucleated cell. Current investigations in the field concern modulation of the effectiveness of skeletal muscle regeneration, the regeneration-specific role of myogenic regulatory gene expression distinct from expression during development, the impact of growth and scatter factors and their respective receptors in amplifying precursor numbers, and promoting fusion and maturation of new fibers and the ultimate clinical therapeutic applications of such information to alleviate disease. One approach to muscle regeneration integrates observations of muscle gene expression, proliferation, myoblast fusion, and fiber growth in vivo with parallel studies of cell cycling behaviour, endocrine perturbation, and potential biochemical markers of steps in the disease-repair process detected by magnetic resonance spectroscopy techniques. Experiments on muscles from limb, diaphragm, and heart of the mdx dystrophic mouse, made to parallel clinical trials on human Duchenne muscular dystrophy, help to elucidate mechanisms underlying the positive treatment effects of the glucocorticoid drug deflazacort. This review illustrates an effective combination of in vivo and in vitro experiments to integrate the distinctive complexities of post-natal myogenesis in regeneration of skeletal muscle tissue.Key words: satellite cell, cell cycling, HGF/SF, c-met receptor, MyoD, myogenin, magnetic resonance spectroscopy, mdx dystrophic mouse, deflazacort.

Increase in p202 expression during skeletal muscle differentiation: inhibition of MyoD protein expression and activity by p202

p202 is a primarily nuclear, interferon-inducible murine protein that is encoded by the Ifi 202 gene. Overexpression of p202 in transfected cells retards cell proliferation. p202 modulates the pattern of gene expression by inhibiting the activity of various transcription factors including NF-kappaB, c-Fos, c-Jun, E2F-1, and p53. Here we report that p202 was constitutively expressed in mouse skeletal muscle and that the levels of 202 RNA and p202 greatly increased during the differentiation of cultured C2C12 myoblasts to myotubes. When overexpressed in transfected myoblasts, p202 inhibited the expression of one muscle protein (MyoD) without affecting the expression of a second one (myogenin). Thus, the decrease in the level of MyoD (but not of myogenin) during muscle differentiation may be the consequence of the increase in p202 level. Overexpressed p202 also inhibited the transcriptional activity of both MyoD and myogenin. This inhibition was correlated with an interaction of p202 with both proteins, as well as the inhibition by p202 of the sequence-specific binding of both proteins to DNA. This inhibition of the expression of MyoD and of the transcriptional activity of MyoD and myogenin may account for the inhibition of the induction of myoblast differentiation by premature overexpression of p202.

The identification of myogenic cells in skeletal muscle, with emphasis on the use of tritiated thymidine autoradiography and desmin antibodies

The identification of myogenic precursor cells (mpc) is a key factor in determining the early events in the myogenesis and regeneration of skeletal muscle. Although satellite cells have long been established as the providers of myoblastic cells, very little is really known (apart from their anatomical location in relation to muscle fibres and their ability to migrate) about the precise role of satellite cells in myogenesis. Numerous techniques for labelling mpc have been devised, but none of these has proven to be completely reliable in firmly establishing the origin of myogenic cells. The use of tritiated thymidine to label DNA in proliferating mpc (which are not specifically distinguishable at the time) and the subsequent location of their labelled progeny in myotube nuclei has revealed a great deal of data on the timing of myogenesis, but not about the nature of mpc themselves. DNA synthesis can also be detected by antibodies to the thymidine analogue, bromodeoxyuridine, and also by antibody staining for proliferating nuclear cell antigen. Like tritiated thymidine, these other markers are not specific for muscle but are general markers for DNA synthesis. In situ hybridisation of various muscle-specific genetic markers and their products has been informative, as has immunolabelling of myogenin, MyoD1 and desmin. Desmin labelling has been particularly instructive in identifying mpc because it is one of the first muscle-specific proteins to be produced in mpc. This review covers some of the techniques mentioned above and their usefulness in determining the early events in myogenesis.

Levels of MyoD protein expression following injury of mdx and normal limb muscle are modified by thyroid hormone

Thyroid hormone (T3) affects muscle development and muscle regeneration. It also interacts with the muscle regulatory gene MyoD in culture and affects myoblast proliferation. We studied the localization of MyoD protein using a well-characterized polyclonal antibody for immunohistochemistry. Relative numbers of myogenic precursor cells per field were identified by their MyoD expression during muscle regeneration in normal and mdx dystrophic mice, with particular reference to the expression in mononuclear cells and myotubes at various T3 levels. In regeneration by normal muscles, relatively few MyoD+ nuclei per field were present in mononuclear cells of euthyroid and hypothyroid mice. MyoD staining of mononuclear cell nuclei was approximately doubled in fields of regenerating muscles of normal hyperthyroid compared to euthyroid mice, and was observed in precursors that appeared to be aligned before fusion into myotubes. In mdx regenerating muscle, twofold more mononuclear cells positive for MyoD were present in all three treatment groups compared to normal muscles regenerating under the same conditions. Localization was similar to the pattern in normal euthyroid mice. However, in muscles regenerating in hyperthyroid mdx mice, both mononuclear cell nuclei and centrally located nuclei in a subpopulation (about 15%) of new myotubes formed after the crush injury were intensely stained for MyoD protein. The changes observed are consistent with reports on T3-induced alteration of muscle repair, and propose a link between MyoD regulation and the accelerated differentiation during regeneration under high T3 conditions. (J Histochem Cytochem 46:59-67, 1998)

Expression of myogenic regulatory factors (MRFs) in human neuromuscular disorders

Immunohistochemical studies using antibodies to myogenic regulatory factors (MRFs) Myo D, myogenin, myf-5, and myf-6, and transcription factors c-Fos and c-Jun, were performed on muscle biopsies from patients suffering from Duchenne and Becker muscular dystrophies, polymyositis, and denervation atrophy, to investigate whether expression of these factors occurs during degeneration and regeneration of adult muscle fibres. Strong Myo D, myogenin, myf-5 and myf-6 immunoreactivity was observed in the nuclei of small regenerating fibres and satellite cells, as revealed by double-labelling immunohistochemistry with N-CAM antibodies, in Duchenne and Becker muscular dystrophies and in polymyositis. This suggests that the myogenic programme is activated during regeneration of adult human muscle fibres. In addition, strong myf-6 and c-Jun immunoreactivity was found in the cytoplasm of some necrotic muscle fibres in patients with Duchenne and Becker muscular dystrophies and in patients with polymyositis. The latter findings suggest that strong cytoplasmic expression of myf-6 and c-Jun is related to the process of muscle fibre degeneration that occurs in these conditions. Increased Myo D, myogenin, myf-5 and myf-6 immunoreactivity was not observed in the nuclei of denervated muscle fibres, although strong c-Fos and c-Jun immunoreactivity was seen in the nuclei of denervated muscle fibres; this suggests that denervation triggers the expression of these transcription factors. Taken together, these observations demonstrate that MRFs and c-Fos and c-Jun are selectively expressed in different human muscular disorders.

Single-cell analysis of regulatory gene expression in quiescent and activated mouse skeletal muscle satellite cells

Repair and regeneration of adult skeletal muscle are mediated by satellite cells. In healthy muscle these rare mononucleate muscle precursor cells are mitotically quiescent. Upon muscle injury or degeneration, members of this self-renewing pool are activated to proliferate and then differentiate. Here we analyzed in single satellite cells the expression of a set of regulatory genes that are candidates for causal roles in satellite cell activation, maturation, and differentiation. Individual cells were identified as satellite cells and selected for analysis based on their physical association with single explanted myofibers or their position beneath the basal lamina in unperturbed muscle tissue. Using a multiplex single-cell RT-PCR assay we simultaneously monitored expression of all four MyoD family regulators of muscle determination and differentiation (MRFs) together with two candidate markers of satellite cell identity, c-met and m-cadherin. By making these measurements on large numbers of individual cells during the time course of satellite cell activation, we were able to define which expression states (possible combinations of the six genes) were represented and to specify how the representation of each state changed with time. Activated satellite cells began to express either MyoD or myf5 first among the MRFs; most cells then expressed both myf-5 and MyoD simultaneously; myogenin came on later in cells expressing both MyoD and myf5; and many cells ultimately expressed all four MRFs simultaneously. The results for fiber-associated satellite cells from either predominantly fast or slow muscles were indistinguishable from each other. The c-met receptor tyrosine kinase was also monitored because it is a candidate for mediating activation of quiescent satellite cells (Allen et al., 1995) and because it might also be a candidate molecular marker for satellite cells. A significant difficulty in studying mouse satellite cells has been the absence of molecular markers that could identify them in the quiescent state before expression of MRFs or desmin and distinguish them from fibroblasts. We show here that c-met receptor is present beneath the basal lamina on presumptive satellite cells in intact muscle and that c-met mRNA and protein are expressed by all myofiber-associated satellite cells from the time of explant through the course of activation, proliferation, and differentiation. c-met was not detected in muscle-derived fibroblasts or in other mononucleate cells from healthy muscle explants. When compared directly with m-cadherin, which has previously been suggested as a marker for quiescent satellite cells, m-cadherin mRNA was detected only in a small subset of satellite cells at early times after myofiber explant. However, at late times following activation (by 96 hr in this fiber culture system), c-met and m-cadherin were uniformly coexpressed. From the individual satellite cell expression types observed, a model of the satellite cell population at rest and during the time course of activation was generated.