Proliferation and differentiation of human fetal myoblasts is regulated by PDGF-BB

Rabdomyosarcoma of the Mandible: An Uncommon Clinical Presentation

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in children and adolescents. Most patients present with a mass in the head and neck region, urogenital region, or with distal extremity involvement. The authors describe a challenging case of a 6-year-old male patient presenting with mandibular RMS. The clinical/radiographic/tomographic evaluations classified the tumor as an advanced stage (stage IV), with a mass of 6.0 cm involving the left side of the mandible and parotid region. The biopsy revealed round, spindled, and pleomorphic cells with hyperchromatic nuclei and rare larger rhabdomyoblasts with eosinophilic cytoplasm. The diagnosis was of embryonal RMS. The patient was referred for treatment with cycles of chemotherapy; however, pulmonary and bone marrow metastasis were identified. Radiotherapy and local surgery with microvascular reconstruction were performed later; however, the patient died after a few months. Early diagnosis is critical for a good prognosis and cure of patients with RMS. Correct diagnosis considering also the histological subtype is important for adequate treatment, which according to the literature is not uniform probably because of the rarity of this neoplasm.

Involvement of vessels and PDGFB in muscle splitting during chick limb development

Muscle formation and vascular assembly during embryonic development are usually considered separately. In this paper, we investigate the relationship between the vasculature and muscles during limb bud development. We show that endothelial cells are detected in limb regions before muscle cells and can organize themselves in space in the absence of muscles. In chick limbs, endothelial cells are detected in the future zones of muscle cleavage, delineating the cleavage pattern of muscle masses. We therefore perturbed vascular assembly in chick limbs by overexpressing VEGFA and demonstrated that ectopic blood vessels inhibit muscle formation, while promoting connective tissue. Conversely, local inhibition of vessel formation using a soluble form of VEGFR1 leads to muscle fusion. The endogenous location of endothelial cells in the future muscle cleavage zones and the inverse correlation between blood vessels and muscle suggests that vessels are involved in the muscle splitting process. We also identify the secreted factor PDGFB (expressed in endothelial cells) as a putative molecular candidate mediating the muscle-inhibiting and connective tissue-promoting functions of blood vessels. Finally, we propose that PDGFB promotes the production of extracellular matrix and attracts connective tissue cells to the future splitting site, allowing separation of the muscle masses during the splitting process.

Inactivation of platelet-derived growth factor-BB following modification by ADP-ribosyltransferase

1. Arginine-specific ADP-ribosyltransferase (ART1) is expressed on the surface of a number of cell types, and catalyses the transfer of ADP-ribose from NAD(+) to target proteins. We investigated whether extracellular proteins such as growth factors may serve as substrates for this enzyme, with subsequent alteration in their biological activity. Experiments were performed with rat skeletal muscle membranes and V79 Chinese hamster lung fibroblasts with doxycycline-inducible expression of human ART. 2. From a panel of growth factors, platelet-derived growth factor-BB (PDGF-BB) was found to be the best substrate for ART1, whereas the structural homologue PDGF-AA was not a substrate. Under conditions of maximum labelling 5 mol ADP-ribose was incorporated per mol of PDGF-BB. 3. Purified (ADP-ribosyl)-PDGF-BB did not stimulate a mitogenic or chemotactic response in human pulmonary smooth muscle cells, and showed a reduced capacity to bind to PDGF receptors in competition binding experiments, when compared to unmodified PDGF-BB. 4. PDGF-dependent [(3)H-methyl]-thymidine incorporation was measured in the ART1-transfected fibroblast cell line at physiological concentrations of PDGF-BB, and without addition of extracellular NAD(+). Fibroblasts expressing human ART1 at the cell surface showed reduced mitogenic responses to PDGF-BB, but not to PDGF-AA. This loss of mitogenic response in cells expressing ART1 activity was reversed by the addition of agmatine (an ART1 substrate). 5. In conclusion, we propose that PDGF-BB-dependent signalling may be regulated by post-translational modification of the growth factor by ART1 at the cell surface. This has been demonstrated in membranes of rat skeletal muscle, and the reaction confirmed in ART1-transfected fibroblasts.

cDNA cloning and chromosomal localization of human alpha(11) integrin. A collagen-binding, I domain-containing, beta(1)-associated integrin alpha-chain present in muscle tissues

We previously identified a novel integrin alpha-chain in human fetal muscle cells (Gullberg, D., Velling, T., Sjöberg, G., and Sejersen, T. (1995) Dev. Dyn. 204, 57-65). We have now isolated the full-length cDNA for this integrin subunit, alpha(11). The open reading frame of the cDNA encodes a precursor of 1188 amino acids. The predicted mature protein of 1166 amino acids contains seven conserved FG-GAP repeats, an I domain with a metal ion-dependent adhesion site motif, a short transmembrane region, and a unique cytoplasmic domain of 24 amino acids containing the sequence GFFRS. alpha(11), like other I domain integrins, lacks a dibasic cleavage site for generation of a heavy chain and a light chain, and it contains three potential divalent cation binding sites in repeats 5-7. The presence of 22 inserted amino acids in the extracellular stalk portion (amino acids 804-826) distinguishes the alpha(11) integrin sequence from other integrin alpha-chains. Amino acid sequence comparisons reveal the highest identity of 42% with the alpha(10) integrin chain. Immunoprecipitation with antibodies to alpha(11) integrin captures a 145-kDa protein distinctly larger than the 140-kDa alpha(2) integrin chain when analyzed by SDS-polyacrylamide gel electrophoresis under nonreducing conditions. Fluorescence in situ hybridization maps the integrin alpha(11) gene to chromosome 15q23, in the vicinity of an identified locus for Bardet-Biedl syndrome. Based on Northern blotting, integrin alpha(11) mRNA levels are high in the adult human uterus and in the heart and intermediate in skeletal muscle and some other tissues tested. During in vitro myogenic differentiation, alpha(11) mRNA and protein are up-regulated. Studies of ligand binding properties show that alpha(11)beta(1) binds collagen type I-Sepharose, and cultured muscle cells localize alpha(11)beta(1) into focal contacts on collagen type I. Future studies will reveal the importance of alpha(11)beta(1) for muscle development and integrity in adult muscle and other tissues.

Tumor-specific PAX3-FKHR transcription factor, but not PAX3, activates the platelet-derived growth factor alpha receptor

The t(2;13) chromosomal translocation occurs at a high frequency in alveolar rhabdomyosarcoma, a common pediatric tumor of muscle. This translocation results in the production of a chimeric fusion protein derived from two developmentally regulated transcription factors, PAX3 and FKHR. The two DNA binding modules, the paired domain and the homeodomain, of PAX3 are fused in frame to the transactivation domain of FKHR. Previously, tumor-specific PAX3-FKHR has been shown to bind to DNA sequences normally recognized by wild-type PAX3 and to exhibit relatively enhanced transcriptional activity. The DNA binding sites used to demonstrate that PAX3-FKHR is a more potent transcriptional activator than PAX3 have included recognition sequences for the paired domain of PAX3. In this report, we demonstrate the ability of PAX3-FKHR to activate the product of a growth control gene, platelet-derived growth factor alpha receptor (PDGFalphaR), by recognizing a paired-type homeodomain binding site located in the PDGFalphaR promoter. PAX3 alone cannot mediate transcriptional activation of this promoter under the conditions tested. This provides the first evidence that chromosomal translocation results in altered target gene specificity of PAX3-FKHR and suggests a transcriptional target that may play a significant role in oncogenic activity and rhabdomyosarcoma development.

Chimaeric analysis reveals role of Pdgf receptors in all muscle lineages

Blood vessels originate as simple endothelial cell tubes. It has been proposed that platelet-derived growth factor B polypeptide (Pdgfb) secreted by these endothelial cells drives the formation of the surrounding muscular wall by recruiting nearby mesenchymal cells. However, targetted inactivation of the Pdgfb gene or the Pdgf receptor beta (Pdgfrb) gene, by homologous recombination, does not prevent the development of apparently normal large arteries and connective tissue. We have used an in vivo competition assay in which we prepared chimaeric blastocysts, composed of a mixture of wild-type (Pdgfrb[+/+]) and Pdgfrb(+/-) or wild-type and Pdgfrb(-/-) cells, and quantified the relative success of cells of the two component genotypes in competing for representation in different cell lineages as the chimaeric embryos developed. This study revealed that the participation of Pdgfrb(-/-) cells in all muscle lineages (smooth, cardiac, skeletal and pericyte) was reduced by eightfold compared with Pdgfrb(+/+) cells, and that participation of Pdgfrb(+/-) cells was reduced by twofold (eightfold for pericytes). Pdgfrb inactivation did not affect cell contribution to non-muscle mesodermal lineages, including fibroblasts and endothelial cells. Chimaera competition is therefore a sensitive, quantitative method for determining developmental roles of specific genes, even when those roles are not apparent from analysis of purebred mutants; most likely because they are masked by homeostatic mechanisms.

Presence of laminin alpha5 chain and lack of laminin alpha1 chain during human muscle development and in muscular dystrophies

There is currently a great interest in identifying laminin isoforms expressed in developing and regenerating skeletal muscle. Laminin alpha1 has been reported to localize to human fetal muscle and to be induced in muscular dystrophies based on immunohistochemistry with the monoclonal antibody 4C7, suggested to recognize the human laminin alpha1 chain. Nevertheless, there seems to be no expression of laminin alpha1 protein or mRNA in developing or dystrophic mouse skeletal muscle fibers. To address the discrepancy between the results obtained in developing and dystrophic human and mouse muscle we expressed the E3 domain of human laminin alpha1 chain as a recombinant protein and made antibodies specific for human laminin alpha1 chain (anti-hLN-alpha1G4/G5). We also made antibodies to the human laminin alpha5 chain purified from placenta. In the present report we show that hLN-alpha1G4/G5 antibodies react with a 400-kDa laminin alpha1 chain and that 4C7 reacts with a 380-kDa laminin alpha5 chain. Immunohistochemistry with the hLN-alpha1G4/G5 antibody and 4C7 revealed that the two antibodies stained human kidney, developing and dystrophic muscle in distinct patterns. Our data indicate that the previously reported expression patterns in developing, adult, and dystrophic human muscle tissues with 4C7 should be re-interpreted as an expression of laminin alpha5 chain. Our data are also consistent with earlier work in mouse, indicating that laminin alpha1 is largely an epithelial laminin chain not present in developing or dystrophic muscle fibers.

Effects of divalent cations on M-cadherin expression and distribution during primary rat myogenesis in vitro

In the process of myogenesis, cadherins are thought to be involved in the initial cell-cell recognition and possible initiation of myoblast fusion to form multinucleated myotubes. Of the cadherins, M-cadherin, but not N-cadherin, is down-regulated upon inhibition of myogenesis, suggesting that M-cadherin may be a key receptor involved in myogenesis. M-cadherin binds in a calcium-dependent manner, and depletion of divalent cations inhibits myoblast fusion. We analyzed the regulation of M-cadherin protein and mRNA levels in primary rat myogenic cultures in the presence and absence of divalent cations. In untreated cultures M-cadherin was localized to various myogenic cell-cell contacts. M-cadherin protein and mRNA levels showed a peak at day 2 after the initiation of growth. When divalent cations were removed from the cell culture medium, myoblast fusion was inhibited and immunocytochemical analysis revealed a failure of M-cadherin to localize to cell-cell contacts. Analysis of M-cadherin protein and mRNA in fusion-inhibited cultures still revealed a peak at day 2. However, by day 3, M-cadherin protein levels in the fusion-inhibited cultures were reduced in both the detergent-soluble and -insoluble fractions in comparison with the untreated cultures. Interestingly, beta-catenin, a protein associated with cadherins, was frequently observed at intercellular contacts in the fusion-inhibited cultures. We could also show that the intracellular levels of beta-catenin protein remained constant regardless of the presence or absence of divalent cations. In summary, the dynamic regulation of M-cadherin in muscle-fusion-related events is an indication of the importance of M-cadherin for myoblast fusion and myogenic differentiation.

Expression of insulin-like growth factor II (IGF-II), IGF binding protein-2 and myogenin during differentiation of myogenic satellite cells derived from the turkey

Myogenic satellite cells are essential for the development of postnatal skeletal muscle. The proliferation and differentiation of these cells are, in part, regulated by the insulin-like growth factors (IGFs), and it has been shown that the IGF binding proteins (IGFBPs) are capable of modulating the actions of IGFs. We have examined the endogenous expression of IGF-II, IGFBP-2 and the myogenic regulatory factor, myogenin, during differentiation of clonally derived turkey muscle satellite cells. Cells were harvested at approximately 80% of confluent density. Additional cultures were rinsed, fed differentiation medium and harvested when approximately 20%, 60% and 80% differentiated (fused). Northern blot analyses were performed using total cellular RNA and labeled rat cDNAs specific for IGF-II, IGFBP-2 and myogenin. A single IGF-II mRNA transcript of approximately 4.0 kb was observed. The relative mRNA abundance was highest in proliferating cultures and decreased with the onset of differentiation, to approximately 60% of initial levels where it remained throughout differentiation. Use of the IGFBP-2 cDNA probe indicated a single mRNA transcript of approximately 2.0 kb. The level of expression of IGFBP-2 mRNA was highest in proliferating cells and decreased to 25%, 16% and 11% of initial levels as differentiation progressed. A single 1.8 kb mRNA transcript was detected with the myogenin probe. Expression of myogenin was undetectable in proliferating cultures and increased significantly as differentiation progressed. Serum-free medium was conditioned for 24 h (CM) at each time point and collected from similar cultures. An IGFBP species of M(r) approximately 30,000 was detected in CM by probing western blots with [125I] IGF-I (ligand blot analysis). The intensity of this band decreased with differentiation to 35%, 24% and 18% of the level for proliferating cultures. Western blots were also probed with an antibody raised against the M(r)-34,000 bovine IGFBP-2. This antibody specifically bound to the M(r)-30,000 IGFBP, and the level of antibody binding decreased as differentiation progressed. It therefore appears that IGF-II, IGFBP-2 and myogenin are expressed in a differentiation-dependent manner by turkey myogenic satellite cells and may thus be involved in the process of differentiation of avian muscle cells.

Molecular basis of skeletal muscle regeneration

Skeletal muscle regeneration is a vital process with important implications for various muscle myopathies and adaptations to physiological overload. Few of the molecular regulatory proteins controlling this process have so far been identified. Several growth factors have defined effects on myogenic precursor cells and appear to also be involved during regeneration. In addition, factors that may be released by cells of the immune system may activate satellite cells during regeneration. Many of these growth factors are associated with signalling cascades which transmit information to the nucleus. The nuclear "receptors" that receive the incoming signals are transcription factors that interact with DNA regulatory sequences in order to modulate gene expression. Of the nuclear factors isolated so far, the immediate-early genes are associated with muscle precursor cell proliferation. This review aims to synthesize the extensive research on myogenic differentiation and relate this to research concerning the molecular regulation of skeletal muscle regeneration.

Transient expression of the intermediate filament nestin during skeletal muscle development

It has previously been established that skeletal muscle development is accompanied by changes in the composition of intermediate filaments: vimentin is expressed predominantly in myoblasts and desmin in adult myotubes. We show that the intermediate filament transitions during muscle development are more complex, and involve a transient expression of the recently discovered intermediate filament nestin. Nestin RNA is expressed predominantly early, in a biphasic pattern, and is markedly downregulated in adult rat muscle, whereas desmin RNA becomes more abundant throughout development. Nestin protein was found up to the postnatal myotube stage, where it colocalized with desmin in Z bands. The intracellular distribution of nestin, vimentin and desmin was analysed in the human myogenic cell line G6 before and after in vitro differentiation. Despite its more distant evolutionary and structural relationship to the other two intermediate filaments, nestin formed a cytoplasmic filamentous network indistinguishable from that of desmin and vimentin, both in undifferentiated myoblasts and after differentiation to multinuclear myotubes. In conclusion, our data suggest that nestin is an integrated component of the dynamic intermediate filament network during muscle development and that nestin copolymerizes with desmin and vimentin at stages of coexpression.