Myogenic conversion of NIH3T3 cells by exogenous MyoD family members: dissociation of terminal differentiation from myotube formation

Genome-wide analysis of chromatin structure changes upon MyoD binding in proliferative myoblasts during the cell cycle

MyoD, a myogenic differentiation protein, has been studied for its critical role in skeletal muscle differentiation. MyoD-expressing myoblasts have a potency to be differentiated with proliferation of ectopic cells. However, little is known about the effect on chromatin structure of MyoD binding in proliferative myoblasts. In this study, we evaluated the chromatin structure around MyoD-bound genome regions during the cell cycle by chromatin immunoprecipitation sequencing. Genome-wide analysis of histone modifications was performed in proliferative mouse C2C12 myoblasts during three phases (G1, S, G2/M) of the cell cycle. We found that MyoD-bound genome regions had elevated levels of active histone modifications, such as H3K4me1/2/3, and H3K27ac, compared with MyoD-unbound genome regions during the cell cycle. We also demonstrated that the elevated H3K4me2/3 modification level was maintained during the cell cycle, whereas the H3K27ac and H3K4me1 modification levels decreased to the same level as MyoD-unbound genome regions during the later phases. Immunoblot analysis revealed that MyoD abundance was high in the G1 phase then decreased in the S and G2/M phases. Our results suggest that MyoD binding formed selective epigenetic memories with H3K4me2/3 during the cell cycle in addition to myogenic gene induction via active chromatin formation coupled with transcription.

Long non-coding RNA ChRO1 facilitates ATRX/DAXX-dependent H3.3 deposition for transcription-associated heterochromatin reorganization

Constitutive heterochromatin undergoes a dynamic clustering and spatial reorganization during myogenic differentiation. However the detailed mechanisms and its role in cell differentiation remain largely elusive. Here, we report the identification of a muscle-specific long non-coding RNA, ChRO1, involved in constitutive heterochromatin reorganization. ChRO1 is induced during terminal differentiation of myoblasts, and is specifically localized to the chromocenters in myotubes. ChRO1 is required for efficient cell differentiation, with global impacts on gene expression. It influences DNA methylation and chromatin compaction at peri/centromeric regions. Inhibition of ChRO1 leads to defects in the spatial fusion of chromocenters, and mislocalization of H4K20 trimethylation, Suv420H2, HP1, MeCP2 and cohesin. In particular, ChRO1 specifically associates with ATRX/DAXX/H3.3 complex at chromocenters to promote H3.3 incorporation and transcriptional induction of satellite repeats, which is essential for chromocenter clustering. Thus, our results unveil a mechanism involving a lncRNA that plays a role in large-scale heterochromatin reorganization and cell differentiation.

Phosphorylation in intrinsically disordered regions regulates the activity of Neurogenin2

Background

Neuronal differentiation is largely under the control of basic Helix-Loop-Helix (bHLH) proneural transcription factors that play key roles during development of the embryonic nervous system. In addition to well-characterised regulation of their expression, increasing evidence is emerging for additional post-translational regulation of proneural protein activity. Of particular interest is the bHLH proneural factor Neurogenin2 (Ngn2), which orchestrates progression from neural progenitor to differentiated neuron in several regions of the central nervous system. Previous studies have demonstrated a key role for cell cycle-dependent multi-site phosphorylation of Ngn2 protein at Serine-Proline (SP) sites for regulation of its neuronal differentiation activity, although the potential structural and functional consequences of phosphorylation at different regions of the protein are unclear.

Results

Here we characterise the role of phosphorylation of specific regions of Ngn2 on the stability of Ngn2 protein and on its neuronal differentiation activity in vivo in the developing embryo, demonstrating clearly that the location of SP sites is less important than the number of SP sites available for control of Ngn2 activity in vivo. We also provide structural evidence that Ngn2 contains large, intrinsically disordered regions that undergo phosphorylation by cyclin-dependent kinases (cdks).

Conclusions

Phosphorylation of Ngn2 occurs in both the N- and C-terminal regions, either side of the conserved basic Helix-Loop-Helix domain. While these phosphorylation events do not change the intrinsic stability of Ngn2, phosphorylation on multiple sites acts to limit its ability to drive neuronal differentiation in vivo. Phosphorylated regions of Ngn2 are predicted to be intrinsically disordered and cdk-dependent phosphorylation of these intrinsically disordered regions contributes to Ngn2 regulation.

Mechanisms of muscle gene regulation in the electric organ of Sternopygus macrurus

Animals perform a remarkable diversity of movements through the coordinated mechanical contraction of skeletal muscle. This capacity for a wide range of movements is due to the presence of muscle cells with a very plastic phenotype that display many different biochemical, physiological and morphological properties. What factors influence the maintenance and plasticity of differentiated muscle fibers is a fundamental question in muscle biology. We have exploited the remarkable potential of skeletal muscle cells of the gymnotiform electric fish Sternopygus macrurus to trans-differentiate into electrocytes, the non-contractile electrogenic cells of the electric organ (EO), to investigate the mechanisms that regulate the skeletal muscle phenotype. In S. macrurus, mature electrocytes possess a phenotype that is intermediate between muscle and non-muscle cells. How some genes coding for muscle-specific proteins are downregulated while others are maintained, and novel genes are upregulated, is an intriguing problem in the control of skeletal muscle and EO phenotype. To date, the intracellular and extracellular factors that generate and maintain distinct patterns of gene expression in muscle and EO have not been defined. Expression studies in S. macrurus have started to shed light on the role that transcriptional and post-transcriptional events play in regulating specific muscle protein systems and the muscle phenotype of the EO. In addition, these findings also represent an important step toward identifying mechanisms that affect the maintenance and plasticity of the muscle cell phenotype for the evolution of highly specialized non-contractile tissues.

Highly efficient derivation of skeletal myotubes from human embryonic stem cells

Human embryonic stem cells (hESCs) are a promising model for the research of embryonic development and regenerative medicine. Since the first hESC line was established, many researchers have shown that pluripotent hESCs can be directed into many types of functional adult cells in culture. However, most of the reported methods have induced differentiation through the alteration of growth factors in the culture medium. These methods are time consuming; moreover, it is difficult to obtain a pure population of the desired cells because of the low efficiency of induction. In this study, we used a lentiviral-based inducible gene-expression system in hESCs to control the ectopic expression of MyoD, which is an essential transcription factor in skeletal muscle development. The induction of MyoD can efficiently direct the pluripotent hESCs into mesoderm in 24 h. The cells then become proliferated myoblasts and finally form multinucleated myotubes in vitro. The whole procedure took about 10 days, with an induction efficiency of over 90%. To our knowledge, this is the first time that hESCs have been induced into terminally differentiated cells with only one factor. In the future, these results could be a potential resource for cell therapy for diseases of muscle dysfunction.

Requirement of decreased O-GlcNAc glycosylation of Mef2D for its recruitment to the myogenin promoter

Previously, we demonstrated that the expression of myogenin, a critical transcription factor for myogenesis, is negatively regulated by O-linked β-N-acetylglucosamine (O-GlcNAc) glycosylation in mouse C2C12 cells. In this study, we found that Mef2 family proteins, especially Mef2D which is a crucial transcriptional activator of myogenin, are O-GlcNAc glycosylated. Between the two splice variants of Mef2D, Mef2D1a rather than Mef2D1b appears to drive the initiation of myogenin expression in the early stage of myogenesis. A deletion mutant analysis showed that Mef2D1a is glycosylated both in its DNA-binding and transactivation domains. A significant decrease in the glycosylation of Mef2D was observed in response to myogenic stimulus in C2C12 cells. Inhibition of the myogenesis-dependent decrease in the glycosylation of Mef2D suppressed its recruitment to the myogenin promoter. These results indicate that the expression of myogenin is regulated, at least in part, by the decreased glycosylation-dependent recruitment of Mef2D to the promoter region, and this is one of the negative regulatory mechanisms of skeletal myogenesis by O-GlcNAc glycosylation.

Basal expression of pluripotency-associated genes can contribute to stemness property and differentiation potential

Pluripotency and stemness is believed to be associated with high Oct-3/4, Nanog, and Sox-2 (ONS) expression. Similar to embryonic stem cells (ESCs), high ONS expression eventually became the measure of pluripotency in any cell. The threshold expression of ONS genes that underscores pluripotency, stemness, and differentiation potential is still unclear. Therefore, we raised a question as to whether pluripotency and stemness is a function of basal ONS gene expression. To prove this, we carried out a comparative study between basal ONS expressing NIH3T3 cells with pluripotent mouse bone marrow mesenchymal stem cells (mBMSC) and mouse ESC. Our studies on cellular, molecular, and immunological biomarkers between NIH3T3 and mBMSC demonstrated stemness property of undifferentiated NIH3T3 cells that was similar to mBMSC and somewhat close to ESC as well. In vivo teratoma formation with all three germ layer derivatives strengthen the fact that these cells in spite of basal ONS gene expression can differentiate into cells of multiple lineages without any genetic modification. Conclusively, our novel findings suggested that the phenomenon of pluripotency which imparts ability for multilineage cell differentiation is not necessarily a function of high ONS gene expression.

Cell cycle regulation during proliferation and differentiation of mammalian muscle precursor cells

Proliferation and differentiation of muscle precursor cells are intensively studied not only in the developing mouse embryo but also using models of skeletal muscle regeneration or analyzing in vitro cultured cells. These analyses allowed to show the universality of the cell cycle regulation and also uncovered tissue-specific interplay between major cell cycle regulators and factors crucial for the myogenic differentiation. Examination of the events accompanying proliferation and differentiation leading to the formation of functional skeletal muscle fibers allows understanding the molecular basis not only of myogenesis but also of skeletal muscle regeneration. This chapter presents the basis of the cell cycle regulation in proliferating and differentiating muscle precursor cells during development and after muscle injury. It focuses at major cell cycle regulators, myogenic factors, and extracellular environment impacting on the skeletal muscle.

Focal adhesion kinase signaling regulates the expression of caveolin 3 and beta1 integrin, genes essential for normal myoblast fusion

An essential phase of skeletal myogenesis is the fusion of mononucleated myoblasts to form multinucleated myotubes. Many cell adhesion proteins, including integrins, have been shown to be important for myoblast fusion in vertebrates, but the mechanisms by which these proteins regulate cell fusion remain mostly unknown. Here, we focused on the role of focal adhesion kinase (FAK), an important nonreceptor protein tyrosine kinase involved in integrin signaling, as a potential mediator by which integrins may regulate myoblast fusion. To test this hypothesis in vivo, we generated mice in which the Fak gene was disrupted specifically in muscle stem cells ("satellite cells") and we found that this resulted in impaired myotube formation during muscle regeneration after injury. To examine the role of FAK in the fusion of myogenic cells, we examined the expression of FAK and the effects of FAK deletion on the differentiation of myoblasts in vitro. Differentiation of mouse primary myoblasts was accompanied by a rapid and transient increase of phosphorylated FAK. To investigate the requirement of FAK in myoblast fusion, we used two loss-of-function approaches (a dominant-negative inhibitor of FAK and FAK small interfering RNA [siRNA]). Inhibition of FAK resulted in markedly impaired fusion but did not inhibit other biochemical measures of myogenic differentiation, suggesting a specific role of FAK in the morphological changes of cell fusion as part of the differentiation program. To examine the mechanisms by which FAK may be regulating fusion, we used microarray analysis to identify the genes that failed to be normally regulated in cells that were fusion defective due to FAK inhibition. Several genes that have been implicated in myoblast fusion were aberrantly regulated during differentiation when FAK was inhibited. Intriguingly, the normal increases in the transcript of caveolin 3 as well as an integrin subunit, the beta1D isoform, were suppressed by FAK inhibition. We confirmed this also at the protein level and show that direct inhibition of beta1D subunit expression by siRNA inhibited myotube formation with a prominent effect on secondary fusion. These data suggest that FAK regulation of profusion genes, including caveolin 3 and the beta1D integrin subunit, is essential for morphological muscle differentiation.

Genetic complementation of human muscle cells via directed stem cell fusion

Duchenne muscular dystrophy (DMD) is caused by mutations in the X chromosome-linked DMD gene, which encodes the sarcolemma-stabilizing protein-dystrophin. Initial attempts at DMD therapy deployed muscle progenitor cells from healthy donors. The utilization of these cells is, however, hampered by their immunogenicity, while those from DMD patients are scarce and display limited ex vivo replication. Nonmuscle cells with myogenic capacity may offer valuable alternatives especially if, to allow autologous transplantation, they are amenable to genetic intervention. As a paradigm for therapeutic gene transfer by heterotypic cell fusion we are investigating whether human mesenchymal stem cells (hMSCs) can serve as donors of recombinant DMD genes for recipient human muscle cells. Here, we show that forced MyoD expression in hMSCs greatly increases their tendency to participate in human myotube formation turning them into improved DNA delivery vehicles. Efficient loading of hMSCs with recombinant DMD was achieved through a new tropism-modified high-capacity adenoviral (hcAd) vector directing striated muscle-specific synthesis of full-length dystrophin. This study introduces the principle of genetic complementation of gene-defective cells via directed cell fusion and provides an initial framework to test whether transient MyoD synthesis in autologous, gene-corrected hMSCs increases their potential for treating DMD and, possibly, other muscular dystrophies.

In vivo interactions of MyoD, Id1, and E2A proteins determined by acceptor photobleaching fluorescence resonance energy transfer

MyoD, a skeletal muscle transcription factor, is rapidly degraded by the ubiquitin-proteasome system. MyoD interacts with ubiquitously expressed E2A or inhibitor of DNA binding (Id) proteins to activate or inhibit transcription, respectively. Furthermore, MyoD has been shown to modulate the ubiquitin-mediated degradation of Id1 and E2A proteins, E12 and E47. The molecular mechanisms governing these events are not clear but are hypothesized to occur via heterodimer formation. Fluorescence resonance energy transfer (FRET) is a technique for evaluation of protein-protein interactions in vivo. Using acceptor photobleaching FRET and chimeric proteins composed of MyoD, Id1, E12, E47, E12(NLS), or MyoD(NLS) and either cyan fluorescent protein or yellow fluorescent protein, we show that each of the wild-type proteins is capable of homodimerization. In addition, heterodimers form between Id1 and E2A proteins, as well as between MyoD and E2A proteins. The Id1:E2A interaction is stronger than the MyoD:E2A interaction, which is consistent with the notion that inhibition of MyoD action occurs by the sequestration of E2A proteins by Id. The stronger interaction of Id1 with E2A may also explain the decrease in the rate of ubiquitin-proteasome degradation of Id1 that is significantly greater than that of MyoD when E2A proteins are abundant. Thus, these studies extend our understanding of the molecular mechanisms of MyoD action.

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

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

MicroRNA-1 facilitates skeletal myogenic differentiation without affecting osteoblastic and adipogenic differentiation

MicroRNAs (miRNAs) are small non-coding RNAs emerging as important post-transcriptional gene regulators. In this study, we examined the role of miR-1, an miRNA specifically expressed in cardiac and skeletal muscle tissue, on the myogenic, osteoblastic, and adipogenic differentiation of C2C12 cells. Upon induction of myogenic differentiation, miR-1 was robustly expressed. Retrovirus-mediated overexpression of miR-1 markedly enhanced expression of muscle creatine kinase, sarcomeric myosin, and alpha-actinin, while the effects on myogenin and MyoD expression were modest. Formation of myotubes was significantly augmented in miR-1-overexpressing cells, indicating miR-1 expression enhanced not only myogenic differentiation but also maturation into myotubes. In contrast, osteoblastic and adipogenic differentiation was not affected by forced expression of miR-1. Thus, the muscle-specific miRNA, miR-1, plays important roles in controlling myogenic differentiation and maturation in lineage-committed cells, rather than functioning in fate determination.

Skeletal muscle specification by myogenin and Mef2D via the SWI/SNF ATPase Brg1

Myogenin is required not for the initiation of myogenesis but instead for skeletal muscle formation through poorly understood mechanisms. We demonstrate in cultured cells and, for the first time, in embryonic tissue, that myogenic late genes that specify the skeletal muscle phenotype are bound by MyoD prior to the initiation of gene expression. At the onset of muscle specification, a transition from MyoD to myogenin occurred at late gene loci, concomitant with loss of HDAC2, the appearance of both the Mef2D regulator and the Brg1 chromatin-remodeling enzyme, and the opening of chromatin structure. We further demonstrated that ectopic expression of myogenin and Mef2D, in the absence of MyoD, was sufficient to induce muscle differentiation in a manner entirely dependent on Brg1. These results indicate that myogenin specifies the muscle phenotype by cooperating with Mef2D to recruit an ATP-dependent chromatin-remodeling enzyme that alters chromatin structure at regulatory sequences to promote terminal differentiation.

Gene expression in skeletal tissues: application of laser capture microdissection

Tissue differentiation is based on the expression of transcription factors, receptors for cytokines, and nuclear receptors that regulate a specific phenotype. The purpose of this study was to select cells from various skeletal tissues in order to analyse differential gene expression of cells in the native environment in vivo. It is a difficult task to obtain cells from skeletal tissues, such as cartilage, periost, bone and muscle, that are structured together and do not exist as individual organs. We used laser capture microdissection which permits the selection and isolation of individual cells from tissue sections. The RNA isolated from these tissues was used for reverse transcriptase-polymerase chain reactions for molecular analysis. We analysed the expression of transcription factors (cFOS, cbfa1, MyoD), receptors for cytokines, nuclear receptors, alkaline phosphatase and the structural proteins osteocalcin and collagen II. The results obtained demonstrate differential patterns of gene expression according to the tissue arrangement in their native in vivo environment, with reliable interpretation of the functions of the analysed genes in the context of intact skeletal tissue physiology.

Cytotoxic necrotizing factor 1 hinders skeletal muscle differentiation in vitro by perturbing the activation/deactivation balance of Rho GTPases

The current knowledge assigns a crucial role to the Rho GTPases family (Rho, Rac, Cdc42) in the complex transductive pathway leading to skeletal muscle cell differentiation. Their exact function in myogenesis, however, remains largely undefined. The protein toxin CNF1 was herein employed as a tool to activate Rho, Rac and Cdc42 in the myogenic cell line C2C12. We demonstrated that CNF1 impaired myogenesis by affecting the muscle regulatory factors MyoD and myogenin and the structural protein MHC expressions. This was principally driven by Rac/Cdc42 activation whereas Rho apparently controlled only the fusion process. More importantly, we proved that a controlled balance between Rho and Rac/Cdc42 activation/deactivation state was crucial for the correct execution of the differentiation program, thus providing a novel view for the role of Rho GTPases in muscle cell differentiation. Also, the use of Rho hijacking toxins can represent a new strategy to pharmacologically influence the differentiative process.

Dual promoter structure of ZFP106: regulation by myogenin and nuclear respiratory factor-1

The WD40 repeats containing zinc finger protein 106 (ZFP106) is a conserved mammalian protein of unknown function. However, its cDNA shares an extended region of identity with the scr homology domain 3 binding protein 3 (Sh3bp3) cDNA encoding a protein implicated in the insulin signaling pathway. Asking, whether Zfp106 and Sh3bp3 are products of the same gene, we characterized the structures and transcriptional regulation of Zfp106 and its human homologue, ZFP106. A TATA-less, CpG island associated promoter (P1), was mapped by 5'-RACE to a region 19 kb upstream of the ZFP106 translation start site. P1 is active throughout development and at low levels in all adult tissues examined. A conserved cis-element in the proximal P1 region showed specific binding to nuclear respiratory factor-1 (NRF-1). Mutagenesis of this site and transfection of a dominant-negative NRF-1 both revealed the crucial role of NRF-1 in activation of P1. The broad tissue expression of P1 was in contrast to the high level of ZFP106 mRNA observed in striated muscle. This prompted additional 5'-RACE experiments that established a second, TATA box-containing promoter (P2) upstream of the third coding exon. P1 and P2 transcripts encode proteins with distinct N-terminal sequences, with Sh3bp3 corresponding to a rare, alternatively spliced P2 transcript. P2 initiated transcripts are specifically expressed in striated muscle and their level is strongly upregulated during myogenic, but not adipogenic differentiation. By deletion analysis, the region between nucleotides -296 to +96 was sufficient for robust P2 responsiveness to myogenic differentiation. This response is mediated by the additive effect of binding of myogenin to three critical E boxes within this region. In addition, transcriptional enhancer factor-1 family factors contribute to both basal and myogenesis induced P2 activity. In situ hybridization of mouse embryos confirmed predominant expression of Zfp106 in tissues with high developmental expression of either NRF-1 (brown fat and developing brain) or myogenin (striated muscle). Our results suggest distinct roles of tissue-specific ZFP106 isoforms in growth related metabolism and provide the foundation for further studies into the regulation and function of ZFP106.

Intraarterial injection of muscle-derived CD34(+)Sca-1(+) stem cells restores dystrophin in mdx mice

Duchenne muscular dystrophy is a lethal recessive disease characterized by widespread muscle damage throughout the body. This increases the difficulty of cell or gene therapy based on direct injections into muscles. One way to circumvent this obstacle would be to use circulating cells capable of homing to the sites of lesions. Here, we showed that stem cell antigen 1 (Sca-1), CD34 double-positive cells purified from the muscle tissues of newborn mice are multipotent in vitro and can undergo both myogenic and multimyeloid differentiation. These muscle-derived stem cells were isolated from newborn mice expressing the LacZ gene under the control of the muscle-specific desmin or troponin I promoter and injected into arterial circulation of the hindlimb of mdx mice. The ability of these cells to interact and firmly adhere to endothelium in mdx muscles microcirculation was demonstrated by intravital microscopy after an intraarterial injection. Donor Sca-1, CD34 muscle-derived stem cells were able to migrate from the circulation into host muscle tissues. Histochemical analysis showed colocalization of LacZ and dystrophin expression in all muscles of the injected hindlimb in all of five out of five 8-wk-old treated mdx mice. Their participation in the formation of muscle fibers was significantly increased by muscle damage done 48 h after their intraarterial injection, as indicated by the presence of 12% beta-galactosidase-positive fibers in muscle cross sections. Normal dystrophin transcripts detected enzymes in the muscles of the hind limb injected intraarterially by the mdx reverse transcription polymerase chain reaction method, which differentiates between normal and mdx message. Our results showed that the muscle-derived stem cells first attach to the capillaries of the muscles and then participate in regeneration after muscle damage.

Thrombin-induced inhibition of myoblast differentiation is mediated by Gbetagamma

Thrombin has been shown to inhibit skeletal muscle differentiation. However, the mechanisms by which thrombin represses myogenesis remain unknown. Since the thrombin receptor couples to G(i), G(q/11) and G(12), we examined which subunits of heterotrimeric guanine nucleotide-binding regulatory proteins (Galpha(i), Galpha(q/11), Galpha(12) or Gbetagamma) participate in the thrombin-induced inhibition of C2C12 myoblast differentiation. Galpha(i2) and Galpha(11) had no inhibitory effect on the myogenic differentiation. Galpha(12) prevented only myoblast fusion, whereas Gbetagamma inhibited both the induction of skeletal muscle-specific markers and the myotube formation. In addition, the thrombin-induced reduction of creatine kinase activity was blocked by the C-terminal peptide of beta-adrenergic receptor kinase, which is known to sequester free Gbetagamma. These results suggest that the thrombin-induced inhibition of muscle differentiation is mainly mediated by Gbetagamma.

Reduced amount of the glucose-regulated protein GRP94 in skeletal myoblasts results in loss of fusion competence

We previously showed that skeletal myocytes of the adult rabbit do not accumulate the endoplasmic reticulum glucose-regulated protein GRP94, neither constitutively nor inducibly, at variance with skeletal myocytes during perinatal development (5). Here we show that C2C12 cells up-regulate GRP94 during differentiation and, similarly to primary cultures of murine skeletal myocytes, specifically display GRP94 immunoreactivity on the cell surface. Stable transfection of C2C12 cells with grp94 antisense cDNA shows lack of myotube formation in clones displaying >40% reduction in GRP94 amount. The same result is obtained after in vivo injection of grp94-antisense myoblasts. Conversely, GRP94 overexpression is accompanied by accelerated myotube formation. Analyses of BrdU incorporation, p21 nuclear translocation, and muscle-gene expression show that muscle differentiation is not apparently affected in grp94-antisense clones. In contrast, cell-surface GRP94 is greatly reduced in grp94-antisense clones, as shown by immunocytochemistry and precipitation of cell-surface biotinylated proteins. Thus, cell-surface expression of GRP94 is necessary for maintenance of fusion competence. Furthermore, differentiating C2C12 cells grown in the presence of anti-GRP94 antibody show decreased myotube number suggesting that cell-surface GRP94 is directly involved in myoblast fusion process.

Syndecan-1 expression inhibits myoblast differentiation through a basic fibroblast growth factor-dependent mechanism

Expression of syndecan-1, a cell-surface heparan sulfate proteoglycan, is down-regulated during skeletal muscle differentiation (Larraín, J., Cizmeci-Smith, G., Troncoso, V., Stahl, R. C., Carey, D. J., and Brandan, E. (1997) J. Biol. Chem. 272, 18418-18424). We examined the role of syndecan-1 in basic fibroblast growth factor (bFGF)-dependent inhibition of myogenesis. C2C12 myoblasts were stably transfected with an expression plasmid containing the rat syndecan-1 coding region cDNA. Constitutive syndecan-1 expression resulted in a strongly diminished capacity of the transfected clones to differentiate and to express skeletal muscle-specific markers such as fusion, creatine kinase, and myosin. The expression of myogenin, a master transcription factor for muscle differentiation, was also reduced and delayed. Analysis of the induction of a myogenin promoter-driven reporter revealed that syndecan-1 expression resulted in a 6-7-fold increase in sensitivity to bFGF-dependent inhibition of myogenin expression. Transfecting the cells with a plasmid containing myogenin cDNA reversed the inhibition of myogenin transcriptional activation and myosin expression in syndecan-1-transfected cells; however, cell fusion was not observed. These results demonstrate that syndecan-1 expression enhances cell responsiveness to bFGF and inhibits myoblast fusion and suggest that muscle terminal differentiation is regulated by syndecan-1 expression.