[Isolation, culture and identification of satellite cells in rat multifidus muscle]

Objective To establish an efficient protocol for isolation, culture and identification of satellite cells in rat multifidus muscle. Methods After deep anesthesia, lumber 4 and 5 multifidus muscle were isolated from 4-week postnatal female SD rats and digested with collagenase 1. Then, the satellite cells were purified via differential adhesion velocity. They were characterized by the detection of marker proteins such as paired box 7 (Pax7), myogenic differentiation (MyoD) and myosin heavy chain (MyHC) with immunofluorescence staining. Results After isolation and purification, satellite cells were found to be circular in morphology and with high optical refraction. They became spindle-shaped after cultured for 5 days. The isolated satellite cells had similar growth style and morphological characteristics. They expressed Pax7 at the interphase of mitosis, MyoD at the proliferating phase and MyHC at the early differentiation stage. Conclusion This study successfully established a simple and efficient method of isolation, culture and identification of satellite cells in rat multifidus muscle.

Suppression of Myogenic Differentiation of Mammalian Cells Caused by Fluidity of a Liquid-Liquid Interface

There is growing evidence to suggest that the prevailing physical microenvironment and mechanical stress regulate cellular functions, including adhesion, proliferation, and differentiation. Moreover, the physical microenvironment determines the stem-cell lineage depending on stiffness of the substrate relative to biological tissues as well as the stress relaxation properties of the viscoelastic substrates used for cell culture. However, there is little known regarding the biological effects of a fluid substrate, where viscoelastic stress is essentially absent. Here, we demonstrate the regulation of myogenic differentiation on fluid substrates by using a liquid-liquid interface as a scaffold. C2C12 myoblast cells were cultured using water-perfluorocarbon (PFC) interfaces as the fluid microenvironment. We found that, for controlled in vitro culture at water-PFC interfaces, expression of myogenin, myogenic regulatory factors (MRF) family gene, is remarkably attenuated even when myogenic differentiation was induced by reducing levels of growth factors, although MyoD was expressed at the usual level (MyoD up-regulates myogenin under an elastic and/or viscoelastic environment). These results strongly suggest that this unique regulation of myogenic differentiation can be attributed to the fluid microenvironment of the interfacial culture medium. This interfacial culture system represents a powerful tool for investigation of the mechanisms by which physical properties regulate cellular adhesion and proliferation as well as their differentiation. Furthermore, we successfully transferred the cells cultured at such interfaces using Langmuir-Blodgett (LB) techniques. The combination of the interfacial culture system with the LB approach enables investigation of the effects of mechanical compression on cell functions.

The Hippo transducer YAP1 transforms activated satellite cells and is a potent effector of embryonal rhabdomyosarcoma formation

The role of the Hippo pathway effector YAP1 in soft tissue sarcomas is poorly defined. Here we report that YAP1 activity is elevated in human embryonal rhabdomyosarcoma (ERMS). In mice, sustained YAP1 hyperactivity in activated, but not quiescent, satellite cells induces ERMS with high penetrance and short latency. Via its transcriptional program with TEAD1, YAP1 directly regulates several major hallmarks of ERMS. YAP1-TEAD1 upregulate pro-proliferative and oncogenic genes and maintain the ERMS differentiation block by interfering with MYOD1 and MEF2 pro-differentiation activities. Normalization of YAP1 expression reduces tumor burden in human ERMS xenografts and allows YAP1-driven ERMS to differentiate in situ. Collectively, our results identify YAP1 as a potent ERMS oncogenic driver and a promising target for differentiation therapy.

Markers of inflammation and disuse in vastus lateralis of chronic obstructive pulmonary disease patients

BACKGROUND

Disuse and/or local inflammation in the muscle cannot be excluded as potential influences for the decreased muscle force in patients hospitalised due to an acute chronic obstructive pulmonary disease (COPD) exacerbation. This study aims to compare expression levels of markers of disuse (insulin-like growth factor-1 (IGF-I), MyoD and myogenin) and inflammation [interleukin-6 (IL-6), IL-8 and tumour necrosis factor-alpha (TNF-alpha)] in the muscle of hospitalised and stable COPD patients and healthy elderly.

MATERIAL AND METHODS

Muscle biopsies (m. vastus lateralis) were taken in 14 hospitalised COPD patients (aged 68 +/- 8), 11 clinically stable COPD patients (aged 68 +/- 9) and seven healthy subjects (aged 70 +/- 7) to analyse local mRNA expression levels of IL-6, IL-8, TNF-alpha, IGF-I and protein expression levels of IGF-I, MyoD and myogenin. Relationships of these expression levels with lung and skeletal muscle function were investigated.

RESULTS

IGF-I mRNA and MyoD protein levels were significantly lower in hospitalised patients compared to healthy subjects. MyoD protein levels were positively related to quadriceps force. Muscle IL-6 and IL-8 expression in hospitalised patients was similar compared to stable patients and healthy subjects and was not related to expression levels of muscle markers of disuse or quadriceps force. Muscle TNF-alpha and myogenin were not detected.

CONCLUSION

Decreased expression levels of muscle IGF-I and MyoD in hospitalised patients suggest a potential influence of disuse in the increased muscle weakness during an acute COPD exacerbation. This study did not find any evidence supporting local inflammation via IL-6, IL-8 and/or TNF-alpha in the vastus lateralis muscle of COPD patients.

Laser microdissection-based expression analysis of key genes involved in muscle regeneration in mdx mice

We have used the mdx mice strain (C57BL/10ScSn-mdx) as an experimental subject for the study of reiterative skeletal muscle necrosis-regeneration with basement membrane preservation. In young mdx muscle, by means of Hematoxylin-Eosin staining, different types of degenerative-regenerative groups (DRG) can be recognized and assigned to a defined muscle regeneration phase. To evaluate the expression of known key-regulatory genes in muscle regeneration, we have applied Laser Capture Microdissection technique to obtain tissue from different DRGs encompassing the complete skeletal muscle regenerative process. The expression of MyoD, Myf-5 and Myogenin showed a rapid increase in the first two days post-necrosis, which were followed by MRF4 expression, when newly regenerating fibers started to appear (3-5days post-necrosis). MHCd mRNA levels, undetectable in mature non-injured fibers, increased progressively from the first day post-necrosis and reached its maximum level of expression in DRGs showing basophilic regenerating fibers. TGFbeta-1 mRNA expression showed a prompt and strong increase following fiber necrosis that persisted during the inflammatory phase, and progressively decreased when new regenerating fibers began to appear. In contrast, IGF-2 mRNA expression decreased during the first days post-necrosis but was followed by a progressive rise in its expression coinciding with the appearance of the newly formed myofibers, reaching the maximum expression levels in DRGs composed of medium caliber basophilic regenerating myofibers (5-7 days post-necrosis). mdx degenerative-regenerative group typing, in conjunction with laser microdissection-based gene expression analysis, opens up a new approach to the molecular study of skeletal muscle regeneration.

Mechanisms in TGF-beta action

The various isoforms of TGF-beta are multifunctional. We are exploring pathways of cellular regulation by TGF-beta that lead to suppression of cell proliferation, modulation of cell adhesion and control of cell differentiation. These cellular responses appear to be activated by binding of TGF-beta to a similar set of receptor glycoproteins in all cell types. TGF-beta receptor types I and II are specifically lost in cell mutants that are resistant to TGF-beta. The concomitant loss of these two receptors in certain mutants suggests that they are components of the TGF-beta signal-transducing receptor complex. Inhibition of epithelial cell proliferation by TGF-beta is linked to retention of the retinoblastoma growth suppressor gene product in an underphosphorylated state that is presumed to have growth suppressive activity. Inhibition of myogenic differentiation by TGF-beta involves a block in the expression of the master myogenic differentiation genes, such as myogenin, but appears also to involve up-regulation of extracellular matrix production. Expression of components of the cell adhesion apparatus--cell adhesion receptors and extracellular matrix proteins--is controlled by TGF-beta in an array of cell types. This response could have a great impact on the ability of cells to migrate, home to specific tissue locations and differentiate during development, invasion and metastasis.

Myogenin and the SWI/SNF ATPase Brg1 maintain myogenic gene expression at different stages of skeletal myogenesis

Many studies have examined transcriptional regulation during the initiation of skeletal muscle differentiation; however, there is less information regarding transcriptional control during adult myogenesis and during the maintenance of the differentiated state. MyoD and the mammalian SWI/SNF chromatin-remodeling enzymes containing the Brg1 ATPase are necessary to induce myogenesis in cell culture models and in developing embryonic tissue, whereas myogenin and Brg1 are critical for the expression of the late genes that induce terminal muscle differentiation. Here, we demonstrate that myogenin also binds to its own promoter during the late stages of embryonic muscle development. As is the case during embryonic myogenesis, MyoD and Brg1 co-localize to the myogenin promoter in primary adult muscle satellite cells. However, in mature myofibers, myogenin and Brg1 are preferentially co-localized to the myogenin promoter. Thus, the myogenin promoter is occupied by different myogenic factors at different times of myogenesis. The relevance of myogenin in the continued expression from its own promoter is demonstrated in culture, where we show that myogenin, in the absence of MyoD, is capable of maintaining its own expression by recruiting the Brg1 ATPase to modify promoter chromatin structure and facilitate myogenin expression. Finally, we utilized in vivo electroporation to demonstrate that Brg1 is required for the continued production of the myogenin protein in newborn skeletal muscle tissue. These findings strongly suggest that the skeletal muscle phenotype is maintained by myogenin and the continuous activity of Brg1-based SWI/SNF chromatin-remodeling enzymes.

Cooperative activation of atrial naturetic peptide promoter by dHAND and MEF2C

An intricate array of cell-specific multiprotein complexes participate in programs of cell-specific gene expression through combinatorial interaction with different transcription factors and cofactors. The dHAND basic helix-loop-helix (bHLH) transcription factor, which is essential for heart development and extra embryonic structures, is thought to regulate cardiomyocyte-specific gene expression through combinatorial interactions with other cardiac-restricted transcription factors such as GATA4 and NKX2.5. Here, we determine that dHAND also interacts with the myocyte enhancer binding factor-2c (MEF2C) protein, which belongs to MADS-box transcription factors and is essential for heart development. dHAND and MEF2C synergistically activated expression of the atrial naturetic peptide gene (ANP) in transfected HeLa cells. GST-pulldown and immunoprecipitation assay demonstrate that full-length MEF2C protein is able to interact with dHAND in vitro and in vivo, just like MEF2A and bHLH transcription factors MyoD in skeletal muscle cells. In addition, electrophoretic mobility shift assays (EMSAs) demonstrate that MEF2C and dHAND do not influence each other's DNA binding activity. Using chromatin immunoprecipitation (ChIP) analysis in H9c2 cells we show that dHAND interact with MEF2C to form protein complex and bind A/T sequence in promoter of ANP. Taken together with previous observations, these results suggest the existence of large multiprotein transcriptional complex with core DNA binding proteins that physically interact with other transcriptional factors to form favorable conformation to potentiate transcription.

Myogenic specification of side population cells in skeletal muscle

Skeletal muscle contains myogenic progenitors called satellite cells and muscle-derived stem cells that have been suggested to be pluripotent. We further investigated the differentiation potential of muscle-derived stem cells and satellite cells to elucidate relationships between these two populations of cells. FACS(R) analysis of muscle side population (SP) cells, a fraction of muscle-derived stem cells, revealed expression of hematopoietic stem cell marker Sca-1 but did not reveal expression of any satellite cell markers. Muscle SP cells were greatly enriched for cells competent to form hematopoietic colonies. Moreover, muscle SP cells with hematopoietic potential were CD45 positive. However, muscle SP cells did not differentiate into myocytes in vitro. By contrast, satellite cells gave rise to myocytes but did not express Sca-1 or CD45 and never formed hematopoietic colonies. Importantly, muscle SP cells exhibited the potential to give rise to both myocytes and satellite cells after intramuscular transplantation. In addition, muscle SP cells underwent myogenic specification after co-culture with myoblasts. Co-culture with myoblasts or forced expression of MyoD also induced muscle differentiation of muscle SP cells prepared from mice lacking Pax7 gene, an essential gene for satellite cell development. Therefore, these data document that satellite cells and muscle-derived stem cells represent distinct populations and demonstrate that muscle-derived stem cells have the potential to give rise to myogenic cells via a myocyte-mediated inductive interaction.

Immunohistochemical evaluation of chemically induced rhabdomyosarcomas in rats: diagnostic utility of MyoD1

Monoclonal antibodies (mAbs) to selected muscle proteins were assessed as potential immunohistochemical markers to assist in the definitive diagnosis of poorly differentiated soft tissue sarcomas in rats. A series of 7 rat rhabdomyosarcomas (RMS) induced with nickel subsulfide were studied by light microscopy and were evaluated for immunoreactivity to desmin, vimentin, fast (type II isoform) skeletal myosin, alpha-actin (smooth muscle isoform), or MyoD1 (myogenic regulatory protein) mAbs using an avidin-biotin-chromogen technique. Consecutive RMS slices were fixed in 10% neutral buffered formalin (the fixative routinely used in carcinogenicity bioassays) for periods of 3 days or 2 mo prior to paraffin embedding to determine the effect of fixation time on immunoreactivity. Desmin and vimentin mAbs bound to many cells of all tumors, but fixation for 2 mo resulted in irretrievable loss of desmin and vimentin binding. Fast myosin and alpha-actin mAbs bound to many cells in 1 RMS but to < 1% of the cells in the remainder. MyoD1 mAb bound to tumor cell nuclei in 5/7 RMS with no loss of staining in tissue fixed for 2 mo. Results indicate that MyoD1 immunostaining, in contrast to desmin, maintains its sensitivity following prolonged formalin fixation and may be of value to distinguish RMS from other soft tissue sarcomas in the rat.

Binding of myc proteins to canonical and noncanonical DNA sequences

Using an in vitro binding-site selection assay, we have demonstrated that c-Myc-Max complexes bind not only to canonical CACGTG or CATGTG motifs that are flanked by variable sequences but also to noncanonical sites that consist of an internal CG or TG dinucleotide in the context of particular variations in the CA--TG consensus. None of the selected sites contain an internal TA dinucleotide, suggesting that Myc proteins necessarily bind asymmetrically in the context of a CAT half-site. The noncanonical sites can all be bound by proteins of the Myc-Max family but not necessarily by the related CACGTG- and CATGTG-binding proteins USF and TFE3. Substitution of an arginine that is conserved in these proteins into MyoD (MyoD-R) changes its binding specificity so that it recognizes CACGTG instead of the MyoD cognate sequence (CAGCTG). However, like USF and TFE3, MyoD-R does not bind to all of the noncanonical c-Myc-Max sites. Although this R substitution changes the internal dinucleotide specificity of MyoD, it does not significantly alter its wild-type binding sequence preferences at positions outside of the CA--TG motif, suggesting that it does not dramatically change other important amino acid-DNA contacts; this observation has important implications for models of basic-helix-loop-helix protein-DNA binding.

Rhabdomyosarcoma-derived cell lines exhibit aberrant expression of the cell-cell adhesion molecules N-CAM, N-cadherin, and cadherin-associated proteins

Many cancer cells show aberrant adhesion properties that likely contribute to tumorigenesis, invasion, and metastasis. Here, we examine three MyoD-expressing, rhabdomyosarcoma-derived human cell lines (RD, A-204, and HS 729) for their expression of the neural cell adhesion molecule (N-CAM), N-cadherin, and the cadherin-associated proteins, alpha-catenin, beta-catenin, and plakoglobin, using specific antibodies and immunoblotting and immunocytochemical methods. Normally, during the formation of skeletal muscle, both N-CAM and N-cadherin are expressed and participate in mediating myoblast adhesion accompanying cell fusion. RD cells express N-CAM, N-cadherin, and the cadherin-associated proteins; however, N-CAM is expressed as a highly sialylated isoform that functions poorly in promoting Ca(2+)-independent cell aggregation. HS 729 cells express N-cadherin and its associated intracellular proteins but have no detectable N-CAM. A-204 cells express no detectable N-CAM or N-cadherin but do express cadherin-associated proteins. Thus, all three rhabdomyosarcoma cell lines exhibit some abnormality in the expression of adhesion molecules known to participate in skeletal myogenesis; however, no common defect was observed that might be considered as a characteristic marker for rhabdomyosarcomas.

Use of a conditional MyoD transcription factor in studies of MyoD trans-activation and muscle determination

DNA sequences encoding the hormone-binding domains of several steroid hormone receptors were fused in frame to the MyoD gene. When the gene for this chimeric protein was expressed in NIH 3T3 or 10T1/2 fibroblasts, these cells displayed hormone-dependent induction of myogenesis. Our experiments focused on cell lines expressing estrogen receptor-MyoD chimeras. Induction of these lines in the presence of estradiol and an inhibitor of protein synthesis, cycloheximide, resulted in the activation of the endogenous myogenin gene but did not activate the muscle-specific creatine kinase or cardiac alpha-actin gene. This result suggests that MyoD is not a "direct" activator of these downstream myogenic genes but must first activate myogenin as an intermediary. Once muscle is induced by estrogen receptor-MyoD the muscle phenotype is very stable and does not need the continued presence of estradiol for its maintenance.

Potential role of helix-loop-helix proteins in cardiac gene expression

Because helix-loop-helix (HLH) transcription factors appear to play an important role in mesodermal development, we have investigated the potential role of these factors in cardiac gene expression. HLH proteins interact with DNA at consensus "E-box" sites and may be tissue specific or more widely expressed. We have examined cardiac cells for expression and regulation of widely expressed factors Pan1/Pan2 and the inhibitor of differentiation (Id) by RNase protection analysis. The effect of MyoD, Id, and Pan1/Pan2 expression on skeletal and cardiac promoters in cardiac cells was examined by transient cotransfection studies. Our results indicate that neonatal ventricular cells are a functional HLH environment, because MyoD can activate a skeletal muscle-specific promoter in these cells. MyoD, however, has no effect on the expression of several genes that are expressed in cardiac cells. In addition, Id may be an early response gene for signal transduction in cardiac cells, because increases in Id mRNA occurred within 30 minutes of stimulation with serum or phenylephrine. Activities of three cardiac promoter elements in primary ventricular myocytes were not downregulated by Id. Surprisingly, expression of Pan1 and Pan2 exhibited a strong negative effect on cardiac expression of the myosin light chain-2 promoter.

TPA-induced differentiation of human rhabdomyosarcoma cells: expression of the myogenic regulatory factors

RD cells (a cell line derived from a human rhabdomyosarcoma) undergo a very limited myogenic differentiation despite the fact that they express several myogenic determination genes. Since we have previously shown (Aguanno et al., Cancer Res. 50, 3377, 1990) that the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) induces myogenic differentiation in these cells, in this paper we investigate the mechanism by which TPA interferes with the expression and/or function of the myogenic determination genes. Northern blot analysis revealed that RD cells express the myf3 (the human analog of MyoD) and myf4 (the human analog of myogenin) transcripts, but not myf5 or myf6 transcripts. The myf3 and the myf4 gene products are correctly translated and accumulated in the nuclei as shown by immunofluorescence analysis. The tumor promoter (TPA) does not modify the pattern of expression of the myf factors while it induces the accumulation of muscle-specific transcripts, such as alpha-actin and fast myosin light chain 1, and their corresponding proteins. On the other hand, within 1 day of treatment, TPA inhibits the expression of the Id gene, which is a negative regulator of MyoD activity. However, while the TPA-induced inhibition of Id message accumulation correlates with differentiation, cell confluence also causes a reduction in Id message accumulation, without inducing differentiation. Under our experimental conditions, overexpression of any of the myf cDNAs in RD cells does induce spontaneous differentiation but enhances the effect of TPA treatment independently from the level of the expressed message. These data suggest that differentiation of RD cells is likely to depend upon the activity of complexes containing the various members of the MyoD family, which can be regulated by proteins affecting MyoD dimerization such as Id, but also by other mechanisms induced by TPA, such as phosphorylation.

Muscle deficiency and neonatal death in mice with a targeted mutation in the myogenin gene

Myogenin is a muscle-specific transcription factor that can induce myogenesis in a variety of cell types in tissue culture. To test myogenin's role in vivo, mice homozygous for a targeted mutation in the myogenin gene were generated. These mice survive fetal development but die immediately after birth and show a severe reduction of all skeletal muscle. Myogenin-mutant mice differ from mice carrying mutations in genes for the related myogenic factors Myf5 and MyoD, which have no muscle defects. Myogenin is therefore essential for the development of functional skeletal muscle.

Modulation of myogenic determination factor 1 expression by cell-cell contact

Myogenic determination factor 1 (MyoD1) expression is modulated by a variety of agents including growth factors and activated cellular proto-oncogenes. However, little is known about the effect of cell-cell contact, which increases as myoblasts terminally differentiate, on the control of MyoD1 expression. Steady-state levels of MyoD1 transcripts decline over a 6-12 hour time period when myoblasts plated at a single cell density are incubated in media supplemented with 0.2% serum; by 48 hours MyoD1 mRNA levels have returned to the initial basal level. The decline in MyoD1 transcripts is diminished, but not prevented in myoblasts which maintain cell-cell contacts (at least 50% of cells with two or more sites of contact). MyoD1 transcript levels do not change if single cell cultures are maintained in 10% serum or are cocultured with fibroblasts. Analysis of conditioned media revealed that myoblasts plated at the single cell density or at a density which allowed multiple sites of cell-cell contact are not producing an activity(s) responsible for modulating MyoD1 mRNA levels. The changes in MyoD1 expression are mediated at the transcriptional level. Thus changes in the degree of cell-cell contact in cultures of myogenically determined cell lines effect changes in MyoD1 gene expression. Consequently when the influence of cytokines or other pharmacological agents on commitment to terminal myogenic differentiation is examined, the degree of cell-cell contact within the culture system may affect the response elicited.

MyoD and regulation of prostaglandin H synthase

MM14 myoblasts, in contrast to their differentiation defective variant (DD-1) cells, do not synthesize detectable levels of prostaglandins or of the initial enzyme in the pathway of prostaglandin synthesis, prostaglandin H synthase (PGHS) but do exhibit readily detectable level of PGHS mRNA (Steiner, S., et al., 1991, Exp. Cell Res. 192, 643). These findings suggest a possible relationship between the myogenic phenotype and the synthesis of prostaglandins. This relationship was examined in the current study by analysis of the effect of transfection of DD-1 cells with a MyoD expression vector (termed MyoDD-1 cells) on expression of MyoD and synthesis of prostaglandins. Proliferating MyoDD-1 cells express readily detectable levels of MyoD protein and mRNA and exhibit markedly diminished levels of PGHS protein and prostaglandins. In contrast, serum-deprived MyoDD-1 cells express little MyoD mRNA or protein and exhibit a readily detectable level of PGHS protein despite having only a slightly higher PGHS mRNA abundance compared to growing MyoDD-1 cells. These studies are consistent with the hypothesis that MyoD expression contributes to the inhibition of prostaglandin synthesis.

A myogenic regulatory gene, qmf1, is expressed by adult myonuclei after injury

Myogenic regulatory factors (MRFs) induce differentiation in developing muscle. We examined the role of MRFs in the repair of adult muscle using a model of stretch-induced injury in 5-wk-old chickens. The anterior latissimus dorsi muscle was stretched by loading the wing with 10% of body weight, while the contralateral muscle served as a control. At various intervals (0.5-72 h), chickens were killed by CO2 asphyxiation and the muscles were frozen. Slot hybridizations showed that the onset of high qmf1 expression occurred as early as 0.5 h, which was before regenerative processes involving satellite cell proliferation were observed. Maximal qmf1 expression varied among animals from 3 to 16 h and returned to control levels by 72 h. Within a muscle, in situ hybridization showed that maximal qmf1 expression varied spatially with > 60% of the nuclei within active fascicles being positive. We interpret this high percentage to mean that many of the nuclei of preexisting muscle fibers must be expressing qmf1. The expression of the protooncogene c-myc (presumably by proliferating cells such as satellite cells, fibroblasts, and capillary epithelial cells) and the MRF qmf1 (by myoblasts and adult muscle nuclei) are among the early molecular responses of injured muscle. We conclude that myogenic regulatory factors are not permanently repressed after embryonic development and that derepression plays a role in the repair of terminally differentiated myofibers.

A cellular factor stimulates the DNA-binding activity of MyoD and E47

We show that mixing purified MyoD and E47 proteins results in heterodimers that fail to bind DNA, even though MyoD and E47 homodimers can bind DNA efficiently. Addition of cell extracts or a specific fraction from a cell extract enables the heterodimer to bind DNA, but components in this fraction fail to enter the DNA complex. The activity is sensitive to heat and protease and is not ATP-dependent. The activity functions on E47 and MyoD homodimers and can stimulate DNA binding of the basic-helix-loop-helix region of MyoD. The effectiveness of the activity, for MyoD homodimers, depends on the exact DNA sequence of the binding site. Our results suggest that specific factors in the cell might control the DNA-binding properties of helix-loop-helix proteins.

Clenbuterol mimics effects of innervation on myogenic regulatory factor expression

The amelioration of denervation atrophy by the beta-adrenoceptor agonist clenbuterol has led to the suggestion that the drug mimics or stimulates production of neurotrophic factors. Neurotrophic factors have profound effects on muscle growth, but the precise mechanisms through which this influence is exerted are unknown. The expression of myoD and myogenin, proteins that in turn regulate the transcription of tissue-specific genes during skeletal muscle differentiation, is controlled by innervation. In muscle undergoing denervation-induced atrophy, myoD and myogenin mRNAs increase. However, this is only partially reversed by electrical activity, thus implicating neurotrophic factors in regulation of these genes. Here we demonstrate that clenbuterol represses myoD and myogenin expression and decreases the levels of acetylcholine receptors in denervated muscles. The data provide the first evidence that the action of clenbuterol is directed through the neural axis.

Basic fibroblast growth factor has a differential effect on MyoD conversion of cultured aortic smooth muscle cells from newborn and adult rats

MyoD is a master regulatory gene for myogenesis that also converts many mesoderm-derived cells into the skeletal muscle phenotype. Rat aortic smooth muscle cells do not contain MyoD homologous mRNA. However, expression of an exogenously supplied MyoD gene in aortic smooth muscle cells cultured from newborn and adult animals converts these cells to elongated myoblasts and myotubes expressing the skeletal muscle genes for titin, nebulin, myosin, and skeletal alpha-actin. The presence of basic fibroblast growth factor during growth and serum starvation completely inhibits MyoD-mediated conversion in cultures of newborn smooth muscle cells. However, in smooth muscle cell cultures derived from adult rats the presence of fibroblast growth factor increases the conversion frequency. The differential response of exogenous MyoD suggests that the two morphological types of aortic smooth muscle cells, one typical for the newborn rat, the other for the adult rat, represent two distinctive states of differentiation.

Possible role of phosphorylation in the function of chicken MyoD1

Chicken MyoD1 (CMD1), an equivalent to the mouse MyoD1, expressed in chicken skeletal muscle (Lin, Z.-Y., Dechesne, C. A., Eldridge, J., and Paterson, B. M. (1989) Genes & Dev. 3, 986-996), was produced in Spodoptera frugiperda (Sf9) cells by Baculovirus expression vector and purified to almost homogeneity. This CMD1 was directly demonstrated to be a phosphoprotein by a 32P-labeling experiment. Phosphoamino acid analysis revealed that only serine residue was phosphorylated. Phosphoamino acid of CMD1 from chick primary culture of 11-day embryonic breast muscle was also serine. Electrophoretic mobility on SDS-polyacrylamide gel electrophoresis of the phosphorylated CMD1 produced in Sf9 cells and that obtained from primary culture of muscle were indistinguishable. Gel retardation and methylation interference assays showed that purified CMD1 bound specifically to the mouse muscle creatine kinase enhancer in combination with the in vitro translated E12. CMD1 alone had almost no affinity to the target DNA. When purified CMD1 was treated with calf intestinal phosphatase, its affinity to the target DNA in combination with E12 was reduced by approximately 5-fold. The affinity recovered at least partially by possible rephosphorylation of CMD1 by kinase(s) contained in wheat germ extract. These results suggested that phosphorylation of CMD1 could be involved in the regulation of muscle differentiation.

Nucleolar localization of myc transcripts

In situ hybridization has revealed a striking subnuclear distribution of c-myc RNA transcripts. A major fraction of the sense-strand nuclear c-myc transcripts was localized to the nucleoli. myc intron 1-containing RNAs were noticeably absent from nucleoli, accumulating instead in the nucleoplasm. The localization of myc RNA to nucleoli was shown to be common to a number of diverse cell types, including primary Sertoli cells and several cell lines. Furthermore, nucleolar localization was not restricted to c-myc and N-myc and myoD transcripts also displayed this phenomenon. In contrast, gamma-actin or lactate dehydrogenase transcripts did not display nucleolar localization. These observations suggest a new role for the nucleolus in transport and/or turnover of potential mRNAs.

Expression of myogenic factors in denervated chicken breast muscle: isolation of the chicken Myf5 gene

In this study, we have isolated and characterized the chicken Myf5 gene, and cDNA clones encoding chicken MyoD1 and myogenin. The chicken Myf5 and MRF4 genes are tandemly located on a single genomic DNA fragment, and the chicken Myf5 gene is organized into at least three exons. Using genomic and cDNA probes, we further analyzed the mRNA levels of four myogenic factors during chicken breast muscle development. This analysis revealed that myogenin expression is restricted to in ovo stages in breast muscle, and is not detectable in neonatal and adult stages. On the other hand, Myf5 expression is detectable until day 7 post-hatching, and is not found in adult muscle, whereas high levels of MyoD1 and MRF4 are detectable at all stages. To further understand the roles of innervation on muscle maturation, we analyzed the expression of the four myogenic factors in denervated adult breast muscle. We found that MyoD1, myogenin, and MRF4 are induced at high levels in denervated muscle, whereas no change occurs in the level of Myf5. These studies suggest that innervation controls the relative abundance and type of myogenic factors that are expressed in adult muscle, and that when nerve control is removed, the muscle reverts to a neonatal phenotype, with the enhanced expression of three myogenic factors (MyoD1, myogenin, and MRF4).

Different factors interact with myoblast-specific and myotube-specific enhancer regions of the human desmin gene

We have previously reported that high level human desmin expression depends on a 280-base pair muscle-specific enhancer which can function not only in myotubes, but can also activate gene expression in myoblasts. We report here that this enhancer contains two different regions, one active in myotubes and the other in myoblasts. In the myotube-specific region, one MyoD1 site and one MEF2 site are necessary for full enhancer activity. Site-directed mutation of the MyoD1 binding site revealed that the intact site is essential for gene expression in myotubes and for transactivation by MyoD1 or myogenin in co-transfected fibroblasts. In the myoblast-specific region, four regions are protected by nuclear factors from the myogenic cell line C2, 7; three regions contain a GC-rich sequence sharing homology with the Krox binding site. Deletion and site-directed mutation experiments demonstrated that at least two Krox-like sequences are required for enhancer activity in myoblasts. In addition, another GC-rich sequence, designated Mb, is also required for full enhancer activity in myoblasts.

Repression of c-fos promoter by MyoD on muscle cell differentiation

Terminal differentiation and cell proliferation are in many cases, as in muscle cells, mutually exclusive processes. While differentiating myoblasts are withdrawn from the cell cycle, myogenesis is inhibited by some mitogens and overexpression of some oncogenes, including proto-oncogene c-fos (which expresses a growth-associated protein constituting the regulatory factor AP-1 in conjunction with c-Jun). MyoD, a muscle-specific transcription factor of the basic helix-loop-helix family, acts at both levels because it triggers a muscle differentiation programme in non-muscle cells, and induces a complete block of cell proliferation. Antagonistic interaction between MyoD and c-Jun has been demonstrated. We here show that c-fos expression greatly decreases upon muscle cell differentiation, concomitant with MyoD-induced activity. We have identified a MyoD-binding site overlapping with the serum-responsive element in the c-fos promoter. We demonstrate that MyoD can act as a negative regulator for c-fos transcription by blocking serum responsiveness through this binding site. These data suggest that the MyoD negative effect on cell growth could be partly mediated by transcriptional inactivation of growth-responsive genes.

cAMP-dependent protein kinase represses myogenic differentiation and the activity of the muscle-specific helix-loop-helix transcription factors Myf-5 and MyoD

Myf-5 and MyoD are members of a family of muscle-specific basic helix-loop-helix (bHLH) proteins that are fundamental for myogenic cell differentiation and transcriptional activation of muscle-specific genes. Here we report that elevated levels of the intracellular signaling molecule cAMP and overexpression of cAMP-dependent protein kinase (PKA) inhibit myogenic differentiation. PKA represses the transcriptional activation of muscle-specific genes by the myogenic regulators Myf-5 and MyoD. The repression is directed at the basic HLH domain and is mediated through the E-box DNA consensus motif to which these proteins bind. However, phosphorylation of Myf-5 and MyoD by PKA in vitro does not affect their ability to bind to DNA. PKA specifically inhibits the activity of myogenic bHLH proteins, but not of other HLH proteins, such as the ubiquitously expressed E2A gene products E12 and E47 (E2-5). Our results demonstrate that PKA mediates the cAMP-induced inhibition of muscle cell differentiation by repressing the activity of Myf-5 and MyoD. The inhibition by PKA occurs post-translationally and presumably affects the transactivation process at a step following DNA-binding. The regulation of Myf-5 and MyoD function by a cAMP-dependent pathway may partly explain how external signals generated by serum and certain peptide growth factors can be transduced to the nucleus and inhibit dominant-acting factors that are responsible for myoblast differentiation.

The TFIIIB-assembling subunit of yeast transcription factor TFIIIC has both tetratricopeptide repeats and basic helix-loop-helix motifs

The multisubunit yeast transcription factor IIIC (TFIIIC; also called tau) can undergo considerable conformational changes upon binding to the A and B blocks of tRNA genes. After binding to DNA encoding tRNA (tDNA), TFIIIC acts as an assembly factor to recruit an initiation factor, TFIIIB, via its tau 131 subunit. We have cloned the gene encoding the tau 131 subunit and named it TFC4. This gene is unique, essential for cell viability, and encodes a M(r) 120,153 protein. Epitope-tagging and mobility-shift assays indicated the presence of a single tau 131 subunit in TFIIIC-tDNA complexes. tau 131 contains two sequence motifs, accounting for nearly one-half of the protein mass, that may provide a molecular explanation for the properties of TFIIIC-tDNA complex. A series of 11 copies of the tetratricopeptide repeat motif may account for the flexibility and interaction properties of TFIIIC. A motif akin to the basic helix-loop-helix motif of MyoD suggests the direct involvement of tau 131 in promoting DNA binding of TFIIIB.

5-Aza-2'-deoxycytidine: cell differentiation and DNA methylation

5-Azacytidine (5-aza-CR) and 5-aza-2'-deoxycytidine (5-aza-CdR), analogs of cytidine modified in position 5, were originally developed as antitumor agents, and have been useful in the treatment of both childhood and adult leukemias. These agents are cytotoxic per se, but also induce differentiation in several experimental systems, most notably the induction of muscle, adipocytes, and chondrocytes in cultures of drug-treated mouse embryo fibroblasts. The mechanisms underlying this drug-induced differentiation have been difficult to unravel, a fact which limits the rational design of differentiation therapy as a modulator of cancer using these agents. Induction of new developmental pathways in cultured cells involves stable, heritable changes, presumably of an epigenetic nature. Our early studies demonstrated that changes in methylation of cytosine in DNA occurred concurrently with changes in developmental potential, and that the presence of 5-azacytosine in DNA interfered with the action of DNA methyltransferase. Since DNA methylation is believed to be involved at some level in the regulation of gene expression, the hypothesis was developed that changes in methylation allowed the expression of new genes whose activity initiated new pathways of differentiation. The characterization of this drug-induced system of differentiation has therefore opened the way to identifying genes directly involved in the initiation or modification of pathways of differentiation. The first of these was MyoD, a member of a family of myogenic determination genes. Expression of MyoD in myogenic cell lines has been correlated with loss of methylation at specific sites in the genome, but the critical events leading to expression of MyoD and muscle differentiation are poorly understood. Recent developments in understanding this mechanism are discussed.

Skeletal myogenesis: genetics and embryology to the fore

The MyoD family of transcription factors are expressed in the skeletal muscles of vertebrate and invertebrate embryos and have dominant regulatory activities that indicate their important developmental functions in myogenic lineage determination and muscle differentiation. Genetic studies, however, reveal that individually, myoD-related genes are not essential for myogenic lineage determination in mouse and Caenorhabditis elegans embryos, but have differentiation functions and perhaps redundant functions in lineage determination that remain to be defined by further genetic studies.

Deficiency in rhabdomyosarcomas of a factor required for MyoD activity and myogenesis

Rhabdomyosarcoma cells express the myogenic helix-loop-helix proteins of the MyoD family but do not differentiate into skeletal muscle cells. Gel shift and transient transfection assays revealed that MyoD in the rhabdomyosarcoma cells was capable of binding DNA but was relatively nonfunctional as a transcriptional activator. Heterokaryon formation with fibroblasts resulted in the restoration of transcriptional activation by MyoD and the differentiation of the rhabdomyosarcoma cells into skeletal muscle cells. These results suggest that rhabdomyosarcomas are deficient in a factor required for MyoD activity.

Interactions among vertebrate helix-loop-helix proteins in yeast using the two-hybrid system

The helix-loop-helix (HLH) motif is contained in a rapidly growing family of transcription factors and has been shown to mediate dimerization among heterologous HLH-containing proteins. E12 is a widely expressed HLH protein that preferentially forms heterodimers with cell type-specific HLH proteins such as MyoD, myogenin, and the achaete-scute gene products. As a first step toward screening for novel cell type-specific partners of E12, we used a modification of the two-hybrid assay for detection of protein-protein interactions in vivo to determine whether dimerization of HLH proteins with E12 can occur in yeast. Using the GAL4 DNA-binding domain fused to the E12 HLH motif and the GAL4 transcription activation domain fused to MyoD, we show that E12 and MyoD can efficiently dimerize in yeast and reconstruct a hybrid transcription factor that activates reporter genes linked to the GAL4 DNA-binding site. The GAL4 DNA-binding domain fused to E12 was used to screen a mouse T-cell cDNA library in which the cDNA was fused to the GAL4 activation domain. Several cDNA clones encoding proteins that interact with E12 were isolated, one of which corresponded to the HLH protein Id-2. Given the ability of E12 to dimerize preferentially with cell type-specific HLH proteins, this strategy should be useful for cloning novel partners for E12 from a variety of cell types.

Electrical activity-dependent regulation of the acetylcholine receptor delta-subunit gene, MyoD, and myogenin in primary myotubes

Expression of the skeletal muscle acetylcholine receptor (AChR) is regulated by nerve-evoked muscle activity. Studies using transgenic mice have shown that this regulation is controlled largely by transcriptional mechanisms because responsiveness to electrical activity can be conferred by transgenes containing cis-acting sequences from the AChR subunit genes. The lack of a convenient muscle cell culture system for studying electrical activity-dependent gene regulation, however, has made it difficult to identify the important cis-acting sequences and to characterize an electrical activity-dependent signaling pathway. We developed a muscle culture system to study the mechanisms of electrical activity-dependent gene expression. Gene fusions between the murine AChR delta-subunit gene and the human growth hormone gene were transfected into primary myoblasts, and the amount of growth hormone secreted into the culture medium from either spontaneously electrically active or inactive myotube cultures was measured. We show that 181 bp of 5'-flanking DNA from the AChR delta-subunit gene are sufficient to confer electrical activity-dependent gene expression. In addition, we show that the rate of AChR delta-subunit gene expression differs among individual nuclei in a single myotube but that highly expressing nuclei are not necessarily colocalized with AChR clusters. We also show that expression of MyoD and myogenin are regulated by electrical activity in primary myotube cultures and that all nuclei within a myotube express similar levels of MyoD and similar levels of myogenin.

Cellular and molecular diversity in skeletal muscle development: news from in vitro and in vivo

Skeletal muscle formation is studied in vitro with myogenic cell lines and primary muscle cell cultures, and in vivo with embryos of several species. We review several of the notable advances obtained from studies of cultured cells, including the recognition of myoblast diversity, isolation of the MyoD family of muscle regulatory factors, and identification of promoter elements required for muscle-specific gene expression. These studies have led to the ideas that myoblast diversity underlies the formation of the multiple types of fast and slow muscle fibers, and that myogenesis is controlled by a combination of ubiquitous and muscle-specific transcriptional regulators that may be different for each gene. We further review some unexpected results that have been obtained when ideas from work in culture have been tested in developing animals. The studies in vivo point to additional molecular and cellular mechanisms that regulate muscle formation in the animal.

Serum-induced inhibition of myogenesis is differentially relieved by retinoic acid and triiodothyronine in C2 murine muscle cells

We recently reported that triiodothyronine (T3) enhances MyoD gene expression and accelerates terminal differentiation in murine C2 myoblasts. In this paper, we are interested in the effects of other hormones acting through related nuclear receptors. Retinoic acid (RA), but not estradiol or dexamethasone, is also able to enhance MyoD gene expression (about threefold). However, the effects of RA and T3 on myogenesis are quite distinct, with a much more potent RA action. Indeed, although T3 and RA positively regulate myogenesis with similar efficiency in poorly mitogenic conditions, in presence of high serum concentrations T3 can no longer trigger terminal differentiation whereas RA still remains efficient. Thus, serum concentration is a crucial parameter in discriminating between the effects of T3 and RA on myogenesis. The differential effects between these two hormone are likely to be related to the ability of RA-activated endogenous retinoic acid receptors (RARs) to induce C2 myoblasts growth-arrest and to extinguish AP1 activity (thought to act as an inhibitor of myogenesis) whereas T3-activated endogenous thyroid hormones receptors (THRs) are relatively inefficient. We propose that the much higher level of RARs in C2 cells versus THRs could to some extent account for the differential ability of T3 and RA to antagonize serum-regulated mitogenic pathways in myogenic cells. This study provides clear evidence for an important role of RA on MyoD gene expression and myogenesis and suggests that T3 and RA could play overlapping, but distinct, roles on muscle development.

A unique pattern of expression of the four muscle regulatory factor proteins distinguishes somitic from embryonic, fetal and newborn mouse myogenic cells

A unique pattern of expression of the four muscle regulatory factor (MRF) proteins was found to distinguish early somitic from embryonic, fetal and newborn limb myogenic cells in vitro. Expression of the myosin heavy chain (MHC), MyoD, myogenin, Myf-5, and MRF4 proteins was examined by immunocytochemistry in cultures of four distinct types of mouse myogenic cells: somitic (E8.5), embryonic (E11.5), fetal (E16.5) and newborn limb. In embryonic, fetal and newborn cultures, the MRF proteins were expressed in generally similar patterns: MyoD was the first MRF expressed; MyoD and myogenin were expressed by more cells than Myf-5 or MRF4; and each of the four MRFs was found both in cells that expressed MHC and in cells that did not express MHC. In cultures of somitic cells, in contrast, Myf-5 was expressed first and by more cells than MyoD or myogenin; MRF4 was not detected; and the MRFs were never found to be coexpressed with MHC in the same cell. Thus, some somitic cells had the unexpected ability to maintain MHC expression in the absence of detectable MRF protein expression. The different myogenic programs of embryonic, fetal and newborn myogenic cells are not, therefore, a simple result of qualitatively different MRF expression patterns, whereas myogenesis by somitic cells does include a unique pattern of MRF expression.

Studies on the evolution and function of different forms of the mouse myogenic gene Myo-D1 and upstream flanking region

The product of the murine Myo-D1 gene is able to initiate the complete sequence of genetic events required for formation of skeletal muscle. Because efficiency of regeneration of skeletal muscle is more pronounced in SJL/J mice, as compared to other strains, differences in the structure of Myo-D1 and the upstream regulatory region were sought to determine whether efficiency of tissue repair was influenced by the structure of the gene itself. Analysis of the restriction-fragment length polymorphism (RFLP) of genomic DNA from SJL/J and different sub-strains of mouse indicated that there are at least three different structural forms of Myo-D1, one of which is unique to SJL/J mice and may have been derived from a double recombinational event involving founder forms of Myo-D1. The unique form of Myo-D1 in SJL/J mice also exhibits a PvuII RFLP upstream from the gene, which may reflect some form of rearrangement or variation in methylation of a potential Myo-D1-binding region. Reference to the size of fragments hybridising with the Myo-D1 probe, following digestion of genomic DNA with TaqI, suggests that in most tissues, adenine residues within Myo-D1 may be extensively methylated. Segregation of Myo-D1 allotypes with response to mechanical injury to skeletal muscle in F2 offspring derived from SJL/J and BALB/c parental strains reveals that increased efficiency of tissue repair is associated with the SJL/J type of Myo-D1 gene. These observations provide new approaches to investigation of genetic control of tissue regeneration and cellular differentiation and proliferation in general.

Single-cell transplantation determines the time when Xenopus muscle precursor cells acquire a capacity for autonomous differentiation

We have used a single-cell transplantation technique to find out whether there is a stage in development when a single cell can reach and maintain its differentiated state in the absence of its neighbors. Muscle precursor cells from early, mid-, and late gastrula stages of Xenopus laevis embryos were isolated and transplanted singly into the ventral region of late gastrula hosts. Single cells from late gastrulae differentiated into muscle when surrounded by nonmuscle cells. Similar cells from early or mid-gastrulae did not, unless they were transplanted as a group of adjacent cells taken from the same region of an embryo. These results show that single embryonic cells in a tissue can complete their differentiation without interacting with their normal neighbors and that, in the case of Xenopus muscle precursor cells, they acquire this capacity at the late gastrula stage. Our results also suggest that, in addition to mesoderm induction, further cell interactions during gastrulation are required for Xenopus muscle cell differentiation.

Interaction of myogenic factors and the retinoblastoma protein mediates muscle cell commitment and differentiation

The experiments reported here document that the tumor suppressor retinoblastoma protein (pRB) plays an important role in the production and maintenance of the terminally differentiated phenotype of muscle cells. We show that pRB inactivation, through either phosphorylation, binding to T antigen, or genetic alteration, inhibits myogenesis. Moreover, inactivation of pRB in terminally differentiated cells allows them to reenter the cell cycle. In addition to its involvement in the myogenic activities of MyoD, pRB is also required for the cell growth-inhibitory activity of this myogenic factor. We also show that pRB and MyoD directly bind to each other, both in vivo and in vitro, through a region that involves the pocket and the basic-helix-loop-helix domains, respectively. All the results obtained are consistent with the proposal that the effects of MyoD on the cell cycle and of pRB on the myogenic pathway result from the direct binding of the two molecules.

Muscle-specific expression of the acetylcholine receptor alpha-subunit gene requires both positive and negative interactions between myogenic factors, Sp1 and GBF factors

The dependence of the muscle-specific enhancer of the acetylcholine receptor alpha-subunit gene on other domains of the promoter has been analysed by performing point mutagenesis and modular reconstitution of the enhancer--promoter sequences. The enhancer is inactive in the absence of the proximal region containing an Sp1 binding site and an overlapping G-C homopolymer binding factor site (referred to as GBF). The proximal region can be replaced by an Sp1 binding site from SV40 or an MEF-2 binding site from the muscle creatine kinase gene. Specific mutation of the Sp1 site markedly affects transactivation by CMD1 or myogenin. Mutation of the GBF binding site leads to higher promoter activity in primary cultures of chick myotubes or in quail fibroblasts. In addition, binding of a purified Sp1 protein prevents the binding of GBF in vitro. It is proposed that in the case of the alpha-subunit promoter, the myogenic factors activate transcription in cooperation with Sp1, and that GBF contributes to muscle-specific expression of the promoter by interfering with Sp1 binding in nonmuscle muscle cells or myoblasts.

Regulation of MyoD gene transcription and protein function by the transforming domains of the adenovirus E1A oncoprotein

It has been demonstrated that the adenovirus E1A gene products inhibit myogenic differentiation in the mouse C2 muscle cell line. During myogenic differentiation, cell growth and tissue-specific gene expression are mutually exclusive. Since E1A exerts multiple effects on different cellular pathways through alteration of cell growth control and transcriptional regulation, we investigated in more detail the molecular mechanisms underlying the inhibitory effect of E1A on myogenic differentiation. To this end, we used mutant derivatives of E1A that lack the 'conserved domain' sequences to which the functional domains of E1A have been mapped, and we observed the effect of constitutive expression of these E1A mutants on myogenesis in the murine C2 muscle cell line. Our results demonstrate that E1A interferes with myogenesis through at least two mechanisms: (i) the inhibition of MyoD expression; (ii) the repression of MyoD-dependent transcriptional activation. In addition, we demonstrate also that the repression of MyoD transcription depends upon sequences located in the N-terminus of E1A and correlates well with the site of E1A/p300 association. Further, the inhibition of transcriptional activation by MyoD depends both on conserved region 1 and on conserved region 2, the two transforming domains of E1A. We demonstrate also that a similar inhibitory effect on the MyoD transactivating function is provided by the polyomavirus and SV40 large T oncoproteins.

Regulation of the mouse desmin gene: transactivated by MyoD, myogenin, MRF4 and Myf5

Desmin, the muscle specific intermediate filament (IF) protein, is expressed at low levels in myoblasts and at the onset of differentiation its expression increases several fold. In an effort to explore the mechanism involved in the tissue-specific and developmentally regulated expression of desmin, we have isolated the mouse desmin gene. Sequence analysis of 976 bp 5' flanking region revealed several potential cis-acting elements: 1) Three E boxes (MyoD binding sites), namely, E1, E2 and E3, located at -79, -832 and -936, respectively; 2) one MEF2 binding site at -864; 3) a region with homology to M-CAT motif at -587; 4) several GC boxes. Transient transfections with various 5' flank deletion mutants into C2C12 muscle cells have revealed both positive and negative elements that seem to be involved in the expression of desmin. The first 81 bp upstream of the transcription initiation site, including E1 box, were sufficient to confer muscle specific expression of the desmin gene. The maximal level of expression was achieved by the construct containing up to -897 base pairs. The region between -578 to -976 behaves as a classical enhancer in the absence of which the region between -578 and -81 suppresses CAT activity. Gel electrophoretic mobility shift assays using both C2C12 muscle cell nuclear extracts as well as in vitro translated myoD/E12 and myogenin/E12 heterodimers, showed that both myoD and myogenin bind to the proximal E1 and the distal E2 boxes of the desmin promoter and enhancer respectively. Co-transfection of myoD, myogenin, MRF4 and Myf5, with the desmin-CAT construct into 10T-1/2 cells demonstrated that all these factors could transactivate desmin gene expression.

Detection and modulation in vivo of helix-loop-helix protein-protein interactions

Studies are described that allow for the in vivo detection of helix-loop-helix (HLH) protein-protein interaction. The assay used requires HLH protein-protein interaction to reconstitute a functional GAL4 transcriptional activator, which in turn activates a reporter gene placed downstream of GAL4 DNA binding sequences. Using this assay, we are able to detect intracellular heterodimerization but not homodimerization of the MyoD, E12, and Id gene products. In addition, using this system we are unable to detect stable heterodimerization between MyoD and c-Jun. We also show that expression of activated rasH gene product does not inhibit and may stabilize HLH protein-protein interaction. This system may be of general utility in studying the modulation of transcription factor interactions.

Cloning of alpha 2 chain of type VI collagen and expression during mouse development

We have previously described the molecular cloning of a cDNA probe which detects a 6 kb mRNA termed pOb24. pOb24 mRNA appeared to be a marker of the preadipose state both in vitro and in vivo. A pOb24 genomic fragment was isolated and used to screen cDNA libraries in order to isolate the full-length pOb24 cDNA and to identify the corresponding protein. The screening yielded a new cDNA clone which detected a 3.7 kb mRNA species in addition to the 6 kb mRNA species. Sequences at the 3' end of the 6 kb and 3.7 kb mRNAs indicate that both mRNAs are generated from the same gene through the use of two different polyadenylation sites. The protein encoded by the 3.7 kb mRNA appeared to be homologous to the human alpha 2 chain of type VI collagen (A2COL6). The expression of the A2COL6 gene was not confined to adipose tissue; mRNA species can be detected in ovaries, adrenal glands and lungs but not in liver and skeletal muscle. The expression appeared specific for initial phase(s) of cell differentiation since it is parallel to that of the MyoD1 gene during muscle embryogenesis in vivo. In the myogenic C2C12 cell line, the A2COL6 gene exhibited the same regulation as MyoD1 and myogenin genes. These results indicate that A2COL6 gene expression is a marker of the preadipose state, but may also be a marker of other differentiation programmes such as that of muscle.