Negative control of the helix-loop-helix family of myogenic regulators in the NFB mutant

The effects of age upon the expression of three miRNAs in muscle stem cells isolated from two different porcine skeletal muscles

Aging is associated with a gradual loss of skeletal muscle mass and an impaired ability of this tissue to compensate for trauma. Studies in rodents and humans have also shown that resident stem cells within muscle have a reduced ability to proliferate and differentiate. In this study muscle stem cells have been isolated from two muscles, the diaphragm (DIA) and the semimembranosus (SM), from young and old pigs. The levels of three micro-RNAs (miRNAs) were measured when cells were in a proliferative phase and after 24 and 72h in differentiation medium. All three miRNAs are abundant in skeletal muscle with miR-1 and miR-206 known to regulate myogenic differentiation and miR-24 is involved in cell cycle regulation. The levels of expression of Pax7 and the myogenic regulatory factors MyoD and myogenin were also measured. There were marked differences in expression of all three miRNAs between the two age groups. Both miR-1 and miR-206 were reduced in the cells from the older animals. In contrast miR-24 expression was significantly higher in cells from older animals under differentiation conditions. There were also significant differences in the relative expression of all three miRNAs between cells from the SM and DIA in both young and old animals. The changes in miRNA expression described in this study that relate to age, may play a role in the impaired differentiation capacity of older muscle stem cells.

Plasticity of cell fate: insights from heterokaryons

Experiments with somatic cell hybrids and stable heterokaryons have demonstrated that differentiated cells exhibit a remarkable capacity to change. Heterokaryons have been particularly useful in determining the extent to which the differentiated state of a cell is plastic. Cell fate can be altered by a change in the balance of positive and negative trans-acting regulators. Although a single regulator may be sufficient in certain environments to trigger a change in cell fate, that regulator may be ineffective in other cell contexts where it encounters a different composition of regulators.

Properties and expression of Ca2+-activated K+ channels in H9c2 cells derived from rat ventricle

H9c2 is a clonal myogenic cell line derived from embryonic rat ventricle that can serve as a surrogate for cardiac or skeletal muscle in vitro. Using whole cell clamp with H9c2 myotubes, we observed that depolarizing pulses activated slow outward K+ currents and then slow tail currents. The K+ currents were abolished in a Ca2+-free external solution, indicating that they were Ca2+-activated K+ currents. They were blocked by apamin, a small-conductance Ca2+-activated K+ (SK) channel antagonist (IC50 = 6.2 nM), and by d-tubocurarine (IC50 = 49.4 microM). Activation of SK channels exhibited a bell-shaped voltage dependence that paralleled the current-voltage relation for L-type Ca2+ currents (ICa,L). ICa,L exhibited a slow time course similar to skeletal ICa, L, were unaffected by apamin, and were only slightly depressed by d-tubocurarine. RT-PCR analysis of the mRNAs revealed that rSK3, but not rSK1 or rSK2, was expressed in H9c2 myotubes but not in myoblasts. These results suggest that rSK3 channels are expressed in H9c2 myotubes and are primarily activated by ICa,L directly or indirectly via Ca2+-induced Ca2+ release from sarcoplasmic reticulum.

Insulin receptor substrate-1 and phosphatidylinositol 3-kinase regulate extracellular signal-regulated kinase-dependent and -independent signaling pathways during myogenic differentiation

Activation of the insulin-like growth factor (IGF) autocrine loop is required for myogenic differentiation and results in sustained activation of extracellular signal-regulated kinases-1 and -2 (ERK-1 and -2). We show here that insulin receptor substrate-1 (IRS-1) phosphorylation on tyrosine and serine residues and association with phosphatidylinositol 3-kinase (PI 3-kinase) are also associated with IGF-dependent myogenic differentiation. Down-regulation of IRS-1 is linked to its serine phosphorylation dependent on PI 3-kinase activity and appears required for differentiation to occur, as IRS-1 is not modified and continues to accumulate in a nondifferentiating myoblast cell line. Furthermore, inhibition of PI 3-kinase activity with LY294002 blocks differentiation, as demonstrated by inhibition of myogenin and myosin heavy chain expression and ERK activation. Blocking the Raf/MEK/ERK cascade with PD98059 does not block myogenic differentiation; however, myotubes do not survive. Thus, PI 3-kinase, in association with IRS-1, is involved in an ERK-independent signaling pathway in myoblasts required for IGF-dependent myogenic differentiation and in inducing sustained activation of ERKs necessary for later stages of differentiation.

Differential regulation of the retinoblastoma family of proteins during cell proliferation and differentiation

In the present study we have analysed the regulation of pocket protein expression and post-transcriptional modifications on cell proliferation and differentiation, both in vivo and in vitro. There are marked changes in pocket protein levels during these transitions, the most striking differences being observed between p130 and p107. The mechanisms responsible for regulating pocket protein levels seem to be dependent on both cell type and pocket protein, in addition to their dependence on the cell growth status. Changes in retinoblastoma protein and p107 levels are independent of their state of phosphorylation. However, whereas p130 phosphorylation to forms characteristic of quiescent/differentiated cells results in the accumulation of p130 protein, phosphorylation of p130 to one or more forms characteristic of cycling cells is accompanied by down-regulation of its protein levels. We also show here that the phosphorylation status and protein levels of p130 and p107 are regulated in vivo as in cultured cells. In vivo, changes in p130 forms are correlated with changes in E2F complexes. Moreover, the modulation of p130 and p107 status during cell differentiation in vitro is consistent with the patterns of protein expression and phosphorylation status found in mouse tissues. Thus in addition to the direct disruption of pocket protein/E2F complexes induced by cyclin/cyclin-dependent kinase, the results we report here indicate that the differential modulation of pocket protein levels constitutes a major mechanism that regulates the pool of each pocket protein that is accessible to E2F and/or other transcription factors.

Duplication of ATR inhibits MyoD, induces aneuploidy and eliminates radiation-induced G1 arrest

Chromosome 3q alterations occur frequently in many types of tumours. In a genetic screen for loci present in rhabdomyosarcomas, we identified an isochromosome 3q [i(3q)], which inhibits muscle differentiation when transferred into myoblasts. The i(3q) inhibits MyoD function, resulting in a non-differentiating phenotype. Furthermore, the i(3q) induces a 'cut' phenotype, abnormal centrosome amplification, aneuploidy and loss of G1 arrest following gamma-irradiation. Testing candidate genes within this region reveals that forced expression of ataxia-telangiectasia and rad3-related (ATR) results in a phenocopy of the i(3q). Thus, genetic alteration of ATR leads to loss of differentiation as well as cell-cycle abnormalities.

A role for the ETS domain transcription factor PEA3 in myogenic differentiation

Activation of adult myoblasts called satellite cells during muscle degeneration is an important aspect of muscle regeneration. Satellite cells are believed to be the only myogenic stem cells in adult skeletal muscle and the source of regenerating muscle fibers. Upon activation, satellite cells proliferate, migrate to the site of degeneration, and become competent to fuse and differentiate. We show here that the transcription factor polyomavirus enhancer activator 3 (PEA3) is expressed in adult myoblasts in vitro when they are proliferative and during the early stages of differentiation. Overexpression of PEA3 accelerates differentiation, whereas blocking of PEA3 function delays myoblast fusion. PEA3 activates gene expression following binding to the ets motif most efficiently in conjunction with the transcription factor myocyte enhancer factor 2 (MEF2). In vivo, PEA3 is expressed in satellite cells only after muscle degeneration. Taken together, these results suggest that PEA3 is an important regulator of activated satellite cell function.

Isolation of a differentiation-defective myoblastic cell line, INC-2, expressing muscle LIM protein under differentiation-inducing conditions

A non-differentiating myoblastic cell line, INC2, and a differentiating cell line, COM3, were established from the mouse myoblastic cell line C2C12. Under differentiation conditions, both COM3 and INC2 cells stopped proliferation in a similar manner. The COM3 cells then differentiated into myotubes during the 4-day differentiation culture. In contrast, almost none of the INC2 cells differentiated into myotubes even in differentiation medium. Reverse transcriptase-polymerase chain reaction (RT-PCR) and immunoblot analyses showed that the levels of myogenin and MyoD proteins were significantly decreased in INC2 cells. The differentiation marker sarcomeric myosin heavy chain (MHC) was expressed in COM3 but not in INC2 cells. In contrast, both INC2 and COM3 cells expressed another myogenic regulatory factor, muscle LIM protein (MLP), in a differentiation condition-dependent manner. These results suggest that MLP gene expression is regulated in a myogenin/MyoD-independent manner. Enforced expression of the myogenin gene induced MHC expression in INC2 cells. Thus, the signaling pathway situated downstream is assumed to be intact in INC2 cells and suppression of myogenin, gene expression may be a primary defect in INC2 cells.

HBP1: a HMG box transcriptional repressor that is targeted by the retinoblastoma family

A prominent feature of cell differentiation is the initiation and maintenance of an irreversible cell cycle arrest with the complex involvement of the retinoblastoma (RB) family (RB, p130, p107). We have isolated the HBP1 transcriptional repressor as a potential target of the RB family in differentiated cells. By homology, HBP1 is a sequence-specific HMG transcription factor, of which LEF-1 is the best-characterized family member. Several features of HBP1 suggest an intriguing role as a transcriptional and cell cycle regulator in differentiated cells. First, inspection of the HBP1 protein sequence revealed two consensus RB interaction motifs (LXCXE and IXCXE). Second, HBP1 interaction was selective for RB and p130, but not p107. HBP1, RB, and p130 levels are all up-regulated with differentiation; in contrast, p107 levels decline. Third, HBP1 can function as a transcriptional repressor of the promoter for N-MYC, which is a critical cell cycle and developmental gene. Fourth, because the activation of the N-MYC promoter in cycling cells required the E2F transcription factor, we show that E2F-1 and HBP1 represent opposite transcriptional signals that can be integrated within the N-MYC promoter. Fifth, the expression of HBP1 lead to efficient cell cycle arrest. The arrest phenotype was manifested in the presence of optimal proliferation signals, suggesting that HBP1 exerted a dominant regulatory role. Taken together, the results suggest that HBP1 may represent a unique transcriptional repressor with a role in initiation and establishment of cell cycle arrest during differentiation.

Paediatric rhabdomyosarcoma: MyoD1 demonstration in routinely processed tissue sections using wet heat pretreatment (pressure cooking) for antigen retrieval

AIMS

To investigate wet heat pretreatment (pressure cooking) as a means of antigen retrieval for demonstration of MyoD1 in paraffin wax embedded tissue.

METHODS

Routinely processed tissue sections of transmission electron microscope confirmed cases of rhabdomyosarcoma were stained immunohistochemically with the MyoD1 antibody. Antigen retrieval was achieved by wet heat pretreatment of the tissue sections.

RESULTS

MyoD1 was stained successfully in all seven cases. The protein was localised to nuclei and cytoplasm depending on the type of tumour cell.

CONCLUSIONS

Wet heat pretreatment for antigen retrieval from routinely processed tissue sections permits excellent subsequent immunostaining for MyoD1 in rhabdomyoblasts.

Defective myogenesis in NFB-s mutant associated with a saturable suppression of MYF5 activity

Myogenic cell lines have proved to be useful tools for investigating the molecular mechanisms that control cellular differentiation. NFB-s is a mutant myogenic cell line which fails to differentiate in vitro, and can repress differentiation in normal myogenic cells when fused to form heterokaryons. The NFB-s cell line was used here to study the molecular mechanisms underlying such myogenic repression. Using muscle-specific reporter genes, we show that NFB-s cells fail to activate fully the muscle differentiation program at a transcriptional level, although muscle-specific transcription can be enhanced by regulators of differentiation such as pertussis toxin. Paradoxically we find that the myogenic regulator myf5 is expressed at constitutively high levels in NFB-s cells, and retains DNA binding activity. Expression plasmids encoding NFB-derived myf5 cDNA can rescue the myogenic phenotype in NFB-s cells, demonstrating that a threshold level of positive regulators must be reached before the myogenic program is activated. Thus, the dominant negative phenotype does not appear to result from defective myf5, but is due to a dosage-dependent saturable mechanism that interferes with myf5 function. These studies demonstrate that the stoichiometric ratio of positive and negative regulators is critical for determining the myogenic differentiation state.

Amplification of MDM2 inhibits MyoD-mediated myogenesis

One obvious phenotype of tumor cells is the lack of terminal differentiation. We previously classified rhabdomyosarcoma cell lines as having either a recessive or a dominant nondifferentiating phenotype. To study the genetic basis of the dominant nondifferentiating phenotype, we utilized microcell fusion to transfer chromosomes from rhabdomyosarcoma cells into C2C12 myoblasts. Transfer of a derivative chromosome 14 inhibits differentiation. The derivative chromosome 14 contains a DNA amplification. MDM2 is amplified and overexpressed in these nondifferentiating hybrids and in the parental rhabdomyosarcoma. Forced expression of MDM2 inhibits MyoD-dependent transcription. Expression of antisense MDM2 restores MyoD-dependent transcriptional activity. We conclude that amplification and overexpression of MDM2 inhibit MyoD function, resulting in a dominant nondifferentiating phenotype.

Autonomous differentiation in the mouse myogenic cell line, C2, involves a mutual positive control between insulin-like growth factor II and MyoD, operating as early as at the myoblast stage

We have studied the contribution of the endogenous production of insulin-like growth factor II (IGFII) and of the muscle regulatory factor, MyoD, to the autonomy of differentiation in isolated skeletal myoblasts. Inhibition of MyoD and IGFII gene expression in myoblasts of the mouse myogenic cell line, C2, was achieved by transfection and selection of stably transfected cells (anti-MyoD and anti-IGFII cells) with vectors producing MyoD or IGFII antisense RNA. We observed that inhibiting either MyoD or IGFII has multiple and similar consequences. In addition to the inhibition of the target gene, expression of MyoD transcripts in anti-IGFII myoblasts and expression of IGFII in anti-MyoD myoblasts were also abolished, whereas accumulation of transcripts for the muscle regulatory factor, Myf5, was markedly increased in both cell types. However, despite this Myf5 up-regulation, both anti-IGFII and anti-MyoD myoblasts lost the ability to undergo autonomous differentiation (differentiation in the absence of added IGF), further indicating that Myf5 and MyoD are not strictly interchangeable. Additional evidence of a link between MyoD and IGFII was obtained: (1) forced expression of the MyoD cDNA stimulated IGFII gene expression, and (2) treatment of C2 myoblasts with fibroblast growth factor, not only diminished MyoD expression and compromised differentiation as previously shown by others, but also abolished IGFII expression. These experiments showing loss or gain of function argue in favor of a mutual positive control between IGFII and MyoD operating as early as the myoblast stage.

Role of insulin-like growth factors and myogenin in the altered program of proliferation and differentiation in the NFB4 mutant muscle cell line

In the present study we used the mutant muscle cell line NFB4 to study the balance between proliferation and myogenic differentiation. We show that removal of serum, which induced the parental C2C12 cells to withdraw from the cell cycle and differentiate, had little effect on NFB4 cells. Gene products characteristic of the proliferation state, such as c-Jun, continued to accumulate in the mutant cells in low serum, whereas those involved in differentiation, like myogenin, insulin-like growth factor II (IGF-II), and IGF-binding protein 5 (IGFBP-5) were undetectable. Moreover, NFB4 cells displayed a unique pattern of tyrosine phosphorylated proteins, especially in low serum, suggesting that the signal transduction pathway(s) that controls differentiation is not properly regulated in these cells. Treatment of NFB4 cells with exogenous IGF-I or IGF-II at concentrations shown to promote myogenic differentiation in wild-type cells resulted in activation of myogenin but not MyoD gene expression, secretion of IG-FBP-5, changes in tyrosine phosphorylation, and enhanced myogenic differentiation. Similarly, transfection of myogenin expression constructs also enhanced differentiation and resulted in activation of IGF-II expression, showing that myogenin and IGF-II cross-activate each other's expression. However, in both cases, the expression of Jun mRNA remained elevated, suggesting that IGFs and myogenin cannot overcome all aspects of the block to differentiation in NFB4 cells.

Inhibition of an erythroid differentiation switch by the helix-loop-helix protein Id1

The Id proteins function as negative regulators of basic-helix-loop-helix transcription factors, which play important roles in determination of cell lineage and in tissue-specific differentiation. Down-regulation of Id1 mRNA is associated with dimethyl sulfoxide-induced terminal differentiation of mouse erythroleukemia cells. To examine the significance of Id1 down-regulation in erythroid differentiation, we generated stable mouse erythroleukemia cell lines that constitutively express a "marked" form of the murine Id1 gene. Terminal erythroid differentiation was inhibited in these lines, as indicated by a block in activation of the erythroid-specific genes alpha-globin, beta-globin, and band 3 and continued proliferation in the presence of dimethyl sulfoxide. Interestingly, this block occurred even in the presence of normal levels of the lineage-specific transcription factors GATA-1, NF-E2, and EKLF. Constitutive expression of Id1 did not interfere with DNase I hypersensitivity at site HS2 of the locus control region, expression of the erythropoietin receptor gene, or down-regulation of the endogenous Id1 or c-myc genes. The differentiation block is reversible in these lines and can be rescued by fusion with human erythroleukemia cells. These findings suggest that in vivo, Id1 functions as an antagonist of terminal erythroid differentiation.

Multiple change in E2F function and regulation occur upon muscle differentiation

We have examined regulation of the E2F transcription factor during differentiation of muscle cells. E2F regulates many genes involved in growth control and is also the target of regulation by diverse cellular signals, including the RB family of growth suppressors (e.g., the retinoblastoma protein [RB], p107, and p130). The following aspects of E2F function and regulation during muscle differentiation were investigated: (i) protein-protein interactions, (ii) protein levels, (iii) phosphorylation of the E2F protein, and (iv) transcriptional activity. A distinct E2F complex was present in differentiated cells but not in undifferentiated cells. The p130 protein was a prominent component of the E2F complex associated with differentiation. In contrast, in undifferentiated cells, the p107 protein was the prominent component in one of three E2F complexes. In addition, use of a differentiation-defective muscle line provided genetic and biochemical evidence that quiescence and differentiation are separable events. Exclusive formation of the E2F-p130 complex did not occur in this differentiation-defective line; however, E2F complexes diagnostic of quiescence were readily apparent. Thus, sole formation of the E2F-p130 complex is a necessary event in terminal differentiation. Other changes in E2F function and regulation upon differentiation include decreased phosphorylation and increased repression by E2F. These observations suggest that the regulation of E2F function during terminal differentiation may proceed through differential interaction within the RB family and/or phosphorylation.

Activation of the myogenin promoter during mouse embryogenesis in the absence of positive autoregulation

Myogenin, a member of the MyoD family of helix-loop-helix proteins, can induce myogenesis in a wide range of cell types. In addition to activating muscle structural genes, members of the MyoD family can autoactivate their own and cross-activate one another's expression in transfected cells. This has led to the hypothesis that autoregulatory loops among these factors provide a mechanism for amplifying and maintaining the muscle-specific gene expression program in vivo. Here, we make use of myogenin-null mice to directly test this hypothesis. To investigate whether the myogenin protein autoregulates the myogenin gene during embryogenesis, we introduced a myogenin-lacZ transgene into mice harboring a null mutation at the myogenin locus. Despite a severe deficiency of skeletal muscle in myogenin-null neonates, the myogenin-lacZ transgene was expressed normally in myogenic cells throughout embryogenesis. These results show that myogenin is not required for regulation of the myogenin gene and argue against the existence of a myogenin autoregulatory loop in the embryo.

Innervation of MyoD-converted human amniocytes and fibroblasts by fetal rodent spinal cord neurons

MyoD is one member of a gene family involved in the regulation of myogenesis. MyoD transfection induces myogenesis in a variety of non-muscle cells. Human amniocytes and fibroblasts were infected with a MyoD-retrovirus vector, to determine whether the converted cells can mature normally to form functional muscle fibers. MyoD-converted cells were cocultured with fetal rat spinal cord. After 2-3 weeks of co-culture cross-striated, contracting muscle fibers were observed. Combined acetylcholinesterase cytochemistry and acetylcholine receptor labeling showed prominent staining at nerve-muscle contacts. Approximately half of the total creatine kinase activity was due to the muscle-specific isozyme. Innervated MyoD-converted cells might represent a new source of muscle cells for studying the molecular events leading toward the formation of functional muscle. This system also appears suitable for studying the pathogenesis of hereditary, often rare, myopathies affecting muscle-specific proteins, for which muscle tissue is frequently unavailable for in vitro analysis.

Heterokaryons of cardiac myocytes and fibroblasts reveal the lack of dominance of the cardiac muscle phenotype

The molecular characterization of a cardiac determination gene has been an elusive goal for the past several years. Prior to cloning of the skeletal muscle determination factor MyoD, the presence of a dominantly acting skeletal muscle determination factor had been inferred from the observation that the skeletal muscle phenotype was dominant in skeletal muscle-fibroblast heterokaryons (H. M. Blau, G. K. Pavlath, E. C. Hardeman, C.-P. Chiu, L. Siberstein, S. G. Webster, S. C. Miller, and D. Webster, Science 230:758-766, 1985). In these experiments, we have examined cardiac-fibroblast heterokaryons to investigate the existence of a dominantly acting cardiac determination factor. We have employed a novel experimental approach using primary embryonic fibroblasts from transgenic mice as a means of assaying for the activation of a cardiac promoter-luciferase reporter transgene within fibroblast nuclei. This approach provides a potential means of genetic selection for a dominantly acting positive factor and can be generalized to other systems. We have examined the expression of three markers of the cardiac lineage: a myofibrillar protein promoter (MLC2), a secreted protein (ANF), and a transcription factor (MEF2). MEF2 is specific to both cardiac and skeletal muscle cells. Our results indicate that in a majority of heterokaryons with an equal ratio of cardiac to fibroblast nuclei, none of these cardiac markers are expressed, indicating that the cardiac phenotype is not dominant over the embryonic fibroblast phenotype. The distinction from previous results with skeletal muscle is emphasized by our results with MEF2, which is dominantly expressed in skeletal muscle-fibroblast but not cardiac-fibroblast heterokaryons, supporting its divergent regulation in the two cell types.

Wiring diagrams: regulatory circuits and the control of skeletal myogenesis

During the past year, targeted mutagenesis in mice has begun to clarify the roles of individual members of the MyoD family of myogenic regulators in vertebrate development. In this review, we discuss these studies both in the context of tissue interactions necessary to induce skeletal muscle precursor cells during embryogenesis and the molecular circuitry that regulates the terminal differentiation of these cells.

Heterokaryons of cardiac myocytes and fibroblasts reveal the lack of dominance of the cardiac muscle phenotype

The molecular characterization of a cardiac determination gene has been an elusive goal for the past several years. Prior to cloning of the skeletal muscle determination factor MyoD, the presence of a dominantly acting skeletal muscle determination factor had been inferred from the observation that the skeletal muscle phenotype was dominant in skeletal muscle-fibroblast heterokaryons (H. M. Blau, G. K. Pavlath, E. C. Hardeman, C.-P. Chiu, L. Siberstein, S. G. Webster, S. C. Miller, and D. Webster, Science 230:758-766, 1985). In these experiments, we have examined cardiac-fibroblast heterokaryons to investigate the existence of a dominantly acting cardiac determination factor. We have employed a novel experimental approach using primary embryonic fibroblasts from transgenic mice as a means of assaying for the activation of a cardiac promoter-luciferase reporter transgene within fibroblast nuclei. This approach provides a potential means of genetic selection for a dominantly acting positive factor and can be generalized to other systems. We have examined the expression of three markers of the cardiac lineage: a myofibrillar protein promoter (MLC2), a secreted protein (ANF), and a transcription factor (MEF2). MEF2 is specific to both cardiac and skeletal muscle cells. Our results indicate that in a majority of heterokaryons with an equal ratio of cardiac to fibroblast nuclei, none of these cardiac markers are expressed, indicating that the cardiac phenotype is not dominant over the embryonic fibroblast phenotype. The distinction from previous results with skeletal muscle is emphasized by our results with MEF2, which is dominantly expressed in skeletal muscle-fibroblast but not cardiac-fibroblast heterokaryons, supporting its divergent regulation in the two cell types.

Positive regulators of the lineage-specific transcription factor GATA-1 in differentiating erythroid cells

The zinc finger transcription factor GATA-1 is a major regulator of gene expression in erythroid, megakaryocyte, and mast cell lineages. GATA-1 binds to WGATAR consensus motifs in the regulatory regions of virtually all erythroid cell-specific genes. Analyses with cultured cells and cell-free systems have provided strong evidence that GATA-1 is involved in control of globin gene expression during erythroid differentiation. Targeted mutagenesis of the GATA-1 gene in embryonic stem cells has demonstrated its requirement in normal erythroid development. Efficient rescue of the defect requires an intact GATA element in the distal promoter, suggesting autoregulatory control of GATA-1 transcription. To examine whether GATA-1 expression involves additional regulatory factors or is maintained entirely by an autoregulatory loop, we have used a transient heterokaryon system to test the ability of erythroid factors to activate the GATA-1 gene in nonerythroid nuclei. We show here that proerythroblasts and mature erythroid cells contain a diffusible activity (TAG) capable of transcriptional activation of GATA-1 and that this activity decreases during the terminal differentiation of erythroid cells. Nuclei from GATA-1- mutant embryonic stem cells can still be reprogrammed to express their globin genes in erythroid heterokaryons, indicating that de novo induction of GATA-1 is not required for globin gene activation following cell fusion.

Inhibition of myogenic differentiation in myoblasts expressing a truncated type II TGF-beta receptor

Transforming growth factor-beta (TGF-beta) is thought to play a role in mesenchymal cell development and, specifically, in muscle differentiation, yet its precise role in the latter process remains unclear. TGF-beta has been shown to both inhibit and induce myoblast maturation in vitro, depending on the culture conditions. Whether the type I or type II TGF-beta receptor mediates the various TGF-beta effects on myogenesis is not known. In the present study, C2C12 myoblasts were transfected with an expression vector for a truncated type II TGF-beta receptor, which has been shown to act as a dominant negative inhibitor of type II receptor signaling. In contrast to the parental cells, the transfected clones did not efficiently form myotubes or induce expression of MyoD, myogenin and several other differentiation markers following incubation in low serum media. However, some muscle differentiation markers continued to be expressed in the transfected cells suggesting that at least two pathways are involved in muscle cell differentiation. These cells could still growth arrest in low serum media, showing that decreased proliferation can be dissociated from differentiation. Unlike several oncogenes known to block myogenic differentiation, expression of the truncated TGF-beta receptor did not result in myoblast transformation. Injection of the parental or the transfected C2C12 cells into the limb muscle of nude mice revealed quantitative and qualitative differences in their behavior, and suggested that myoblasts expressing the truncated TGF-beta receptor cannot fuse in vivo. Finally, retrovirus-mediated expression of MyoD in the transfected cells restored their ability to form myotubes in vitro, indicating that inhibition of myoblast differentiation by the truncated TGF-beta receptor may depend on decreased MyoD expression. We propose that TGF-beta signaling through the type II receptor is required for several distinct aspects of myogenic differentiation and that TGF-beta acts as a competence factor in this multistep process.

Positive regulators of the lineage-specific transcription factor GATA-1 in differentiating erythroid cells

The zinc finger transcription factor GATA-1 is a major regulator of gene expression in erythroid, megakaryocyte, and mast cell lineages. GATA-1 binds to WGATAR consensus motifs in the regulatory regions of virtually all erythroid cell-specific genes. Analyses with cultured cells and cell-free systems have provided strong evidence that GATA-1 is involved in control of globin gene expression during erythroid differentiation. Targeted mutagenesis of the GATA-1 gene in embryonic stem cells has demonstrated its requirement in normal erythroid development. Efficient rescue of the defect requires an intact GATA element in the distal promoter, suggesting autoregulatory control of GATA-1 transcription. To examine whether GATA-1 expression involves additional regulatory factors or is maintained entirely by an autoregulatory loop, we have used a transient heterokaryon system to test the ability of erythroid factors to activate the GATA-1 gene in nonerythroid nuclei. We show here that proerythroblasts and mature erythroid cells contain a diffusible activity (TAG) capable of transcriptional activation of GATA-1 and that this activity decreases during the terminal differentiation of erythroid cells. Nuclei from GATA-1- mutant embryonic stem cells can still be reprogrammed to express their globin genes in erythroid heterokaryons, indicating that de novo induction of GATA-1 is not required for globin gene activation following cell fusion.

Myf5, MyoD, myogenin and MRF4 myogenic derivatives of the embryonic mesenchymal cell line C3H10T1/2 exhibit the same adult muscle phenotype

Cells of the embryonic mesenchymal cell line C3H10T1/2 have revealed the potential that the four regulatory factors belonging to the MyoD family have to activate myogenesis. In the present study we have further investigated the myogenic phenotype of C3H10T1/2 cells stably transfected with either Myf5, MyoD, myogenin or MRF4 cDNAs. We have studied the influence of each transfected cDNA on expression of the four endogenous muscle regulatory genes and on the ability of these embryonic myogenic derivatives to express adult muscle genes. No trace of endogenous transcripts distinct from the exogenous one was found in any of the four converted populations at the myoblast stage. This indicates that cross-activation within the MyoD family does not occur at the myoblast stage in these cells. Similarly, evidence was obtained that auto- or cross-activation of the Myf5 gene occurs neither at the myoblast stage nor at the myotube stage and that no autoactivation of the MRF4 gene occurs. Our results together with previous observations indicate that in C3H10T1/2 myogenic derivatives: (1) Autoactivation at the myoblast stage is restricted to MyoD (2) Expression from each cDNA alone is sufficient to establish and maintain the myoblast phenotype (3) The endogenous Myf5 gene is not mobilized. We have also observed that endogenous transcripts for MyoD and myogenin begin to accumulate at the onset of differentiation in the four myogenic derivatives, whereas accumulation of endogenous MRF4 transcripts starts after myotubes have formed and occurs at a much lower level (100- to 500-fold lower) than in differentiated cultures of myosatellite cells.(ABSTRACT TRUNCATED AT 250 WORDS)

bHLH factors in muscle development: dead lines and commitments, what to leave in and what to leave out

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Tumor suppression by RNA from the 3' untranslated region of alpha-tropomyosin

NMU2, a nondifferentiating mutant myogenic cell line, gives rise to rhabdomyosarcomas in mice. We show that constitutive expression of RNA from 0.2 kb of the alpha-tropomyosin (Tm) 3' untranslated region (UTR), but not control 3'UTRs, suppresses anchorage-independent growth and tumor formation by NMU2 cells. When beta-galactosidase (beta-gal)-labeled cells were implanted into muscles of adult mouse hindlimbs, Tm 3'UTR expression suppressed the proliferation, invasion, and destruction of muscle tissues characteristic of NMU2. In the rare tumors that developed from Tm 3'UTR transfectants, RNA expression was extinguished. These results suggest that suppression of tumorigenicity is dependent on the continued expression of Tm transcripts lacking a coding region. We conclude that untranslated RNAs can function as regulators (riboregulators) that suppress tumor formation.

Reversibility of the differentiated state in somatic cells

Analysis of de novo gene activation in multinucleated heterokaryons has shown that the differentiated state, although stable, is not irreversible, and can be reprogrammed in the presence of appropriate combinations of trans-acting regulatory molecules. These properties have been exploited to design strategies for identifying novel regulators of cellular differentiation.

Use of p112-deficient myoblasts to determine the temporal order of the in vitro expression of myogenic components

The present investigation examines the function and site(s) of involvement of an ecto-protein kinase and its substrate protein (a cell surface 112 kDa protein) in the in vitro myogenic pathway. The phosphorylated 112 kDa protein (p112) has recently been shown to be involved in myogenesis. Not much information is currently available on the role of the ecto-protein kinase and the 112 kDa protein in modulating the expression of the myogenic factors and various muscle-specific proteins. Five different p112-deficient rat myoblasts were used to examine the temporal order of the in vitro expression of the myogenic components; namely, L6 myoblasts treated with BrdUrd or phloretin, a conditional p112-defective mutant (clone D1), an ecto-protein kinase-deficient mutant (clone F72), and a mutant defective in the 112 kDa protein (clone D1/S4). All these p112-deficient myoblasts were also impaired in myogenesis. The absence of p112, ecto-protein kinase, and/or the 112 kDa protein was found to have no effect on the Myf-5 mRNA level. However, the expected increase in NCAM and Myf-4 mRNAs was not observed in any of the p112-deficient myoblasts examined. This suggests that the p112 site of action is probably located upstream of the Myf-4 and NCAM sites in the myogenic pathway. While 7-28 fold increases in the MLC, MHC, and TnT transcripts were observed during myogenesis, such increases were not observed in the p112-deficient myoblasts. However, when mutant D1/S4 was transfected with the myf-4 cDNA, expression of Myf-4 in the transfectant resulted in increased level of the MLC, MHC, and TnT mRNAs, and in myotube formation, even though the Myf-5 and NCAM mRNA levels and p112 were not altered. This suggests that p112 may function by activating transcription of Myf-4, which will subsequently promote the expression of muscle-specific proteins and myotube formation. In the absence of p112, Myf-5 cannot activate the expression of Myf-4, NCAM, MLC, MHC, TnT, and myotube formation. If all these components are involved in the same myogenic pathway, then p112 may be acting downstream from Myf-5, and upstream from NCAM and Myf-4.

Selective accumulation of MyoD and myogenin mRNAs in fast and slow adult skeletal muscle is controlled by innervation and hormones

Each of the myogenic helix-loop-helix transcription factors (MyoD, Myogenin, Myf-5, and MRF4) is capable of activating muscle-specific gene expression, yet distinct functions have not been ascribed to the individual proteins. We report here that MyoD and Myogenin mRNAs selectively accumulate in hindlimb muscles of the adult rat that differ in contractile properties: MyoD is prevalent in fast twitch and Myogenin in slow twitch muscles. The distribution of MyoD and Myogenin transcripts also differ within a single muscle and correlate with the proportions of fast glycolytic and slow oxidative muscle fibres, respectively. Furthermore, the expression of a transgene consisting of a muscle-specific cis-regulatory region from the myoD gene controlling lacZ was primarily associated with the fast glycolytic fibres. Alteration of the fast/slow fibre type distribution by thyroid hormone treatment or by cross-reinnervation resulted in a corresponding alteration in the MyoD/Myogenin mRNA expression pattern. These findings show that the expression of specific myogenic helix-loop-helix regulators is under the control of innervation and humoral factors and may mediate differential control of contractile protein gene expression in adult muscle.

A fourth human MEF2 transcription factor, hMEF2D, is an early marker of the myogenic lineage

The transition from multipotent mesodermal precursor to committed myoblast and its differentiation into a mature myocyte involve molecular events that enable the cell to activate muscle-specific genes. Among the participants in this process is the myocyte-specific enhancer factor 2 (MEF2) family of tissue-restricted transcription factors. These factors, which share a highly conserved DNA-binding domain including a MADS box, are essential for the expression of multiple muscle genes with cognate target MEF2 sites in cis. We report here a new human MEF2 factor, hMEF2D, which is unique among the members of this family in that it is present not only in myotubes but also in undifferentiated myoblasts, even before the appearance of myogenin. hMEF2D comprises several alternatively spliced products of a single gene, one of which is the human homolog of the Xenopus SRF-related factor SL-1. Like its relatives, cloned hMEF2D is capable of activating transcription via sequence-specific binding to the MEF2 site, recapitulating endogenous tissue-specific MEF2 activity. Indeed, while MEF2D mRNAs are ubiquitous, the protein is highly restricted to those cell types that contain this activity, implicating posttranscriptional mechanisms in the regulation of MEF2D expression. Alternative splicing may be important in this process: two alternative MEF2D domains, at least one of which is specifically included during myogenic differentiation, also correlate precisely with endogenous MEF2 activity. These findings provide compelling evidence that MEF2D is an integral link in the regulatory network for muscle gene expression. Its presence in undifferentiated myoblasts further suggests that it may be a mediator of commitment in the myogenic lineage.

Expression of c-jun/AP-1 during myogenic differentiation in mouse C2C12 myoblasts

Mitogen withdrawal triggers myogenic differentiation in skeletal myoblasts in culture. We have examined the expression of the proto-oncogene c-jun during this process in mouse C2C12 myoblasts. c-jun belongs to a family of immediate early genes whose expression is activated in cultured cells in response to the addition of serum growth factors. Interestingly, expression of c-jun was maintained in mouse C2C12 and rat L6 myoblasts undergoing myogenic differentiation under low-serum conditions. Previously it has been reported that expression of c-jun is downregulated during differentiation of C2 cells. However, our results using C2C12 cells, a subclone of the C2 line, show that c-jun mRNA, protein and the activator-protein 1 (AP-1) DNA-binding activity were easily detected in proliferating myoblasts and differentiated myotubes. Although overexpression of c-jun has been shown to block myogenic differentiation in C2 cells, results presented here suggest that expression of c-jun at physiological levels may not interfere with skeletal myogenesis.

Expression of wild-type and nuclear localization-deficient human lamin A in chick myogenic cells

Previous analysis of chick embryonic muscle (CEM) differentiation in vivo and in ovo demonstrated that lamin A accumulation to steady-state levels preceded the accumulation of muscle-specific proteins. These observations have suggested the appearance of A-type lamins may be important for differentiation. To test this hypothesis, we have temporally and quantitatively altered the expression of A-type lamins in CEM cells by transient transfection of wild-type (wt; pHLA) or nuclear localization-deficient (NLd; pHLA-del) human lamin A expression plasmids. Transfected CEM cells synthesized the wt and NLd human lamin As to high levels, both of which were resistant to high-salt extraction. The wt human lamin A localized to the nucleus, whereas the NLd protein showed cytoplasmic staining patterns, as well as time-dependent nuclear localization. The presence of endogenous chicken lamins A and B2 in NLd human lamin A cytoplasmic structures suggested the interspecies lamin copolymerization. Thus, this approach may provide a possible method for analysis of lamin-lamin or lamin-lamina component interactions in vivo. With regard to muscle differentiation, CEM cells transfected with either pHLA or pHLA-del demonstrated moderate and transient increased levels of the muscle-specific myosin heavy chain and creatine kinase activity. These increases appeared temporally and quantitatively to reflect the transient accumulation of the human lamin As. In contrast, beta-tubulin and skeletal tropomyosin protein accumulations appeared unaffected. On the basis of these results, we suggest that nuclear lamina content and structure may play a limited, permissive and indirect role in the temporally regulated expression of the myogenic phenotype.

Inactivation of MyoD in mice leads to up-regulation of the myogenic HLH gene Myf-5 and results in apparently normal muscle development

The myogenic basic HLH transcription factor family of genes, composed of MyoD, myogenin, Myf-5, and Myf-6, are thought to regulate skeletal muscle differentiation. To understand the role of MyoD in myogenesis, we have introduced a null mutation of MyoD into the germline of mice. Surprisingly, mice lacking MyoD are viable and fertile. Histological examination of skeletal muscle failed to reveal any morphological abnormalities in these mice. Furthermore, Northern analysis revealed normal levels of skeletal muscle-specific mRNAs. Significantly, Myf-5 mRNA levels are elevated in postnatal mutant mice. Normally, Myf-5 expression becomes markedly reduced at day 12 of gestation when MyoD mRNA first appears. This suggests that Myf-5 expression is repressed by MyoD. Our results indicate that MyoD is dispensable for skeletal muscle development in mice, revealing some degree of functional redundancy in the control of the skeletal myogenic developmental program.

Human myocyte-specific enhancer factor 2 comprises a group of tissue-restricted MADS box transcription factors

The MEF2 site is an essential element of muscle enhancers and promoters that is bound by a nuclear activity found, so far, only in muscle and required for tissue-specific transcription. We have cloned a group of transcription factors from human muscle that are responsible for this activity: They are present in muscle-specific DNA-binding complexes, have a target sequence specificity identical to that of the endogenous activity, and are MEF2 site-dependent transcriptional activators. These MEF2 proteins comprise several alternatively spliced isoforms from one gene and a related factor encoded by a second gene. All share a conserved amino-terminal DNA-binding domain that includes the MADS homology. MEF2 transcripts are ubiquitous but accumulate preferentially in skeletal muscle, heart, and brain. Specific alternatively spliced isoforms are restricted to these tissues, correlating exactly with the presence of endogenous MEF2 activity. Furthermore, MEF2 protein is detected only in skeletal and cardiac muscle nuclei and not in myoblast and nonmuscle cells. Thus, post-transcriptional regulation is important in the generation of tissue-specific MEF2 activity. Cardiac and smooth, as well as skeletal, muscles contain functionally saturating levels of MEF2 trans-activating factors that are absent in nonmuscle cells. Moreover, MEF2 is induced in nonmuscle cells by MyoD; however, MEF2 alone is insufficient to produce the full muscle phenotype. Implications for the molecular mechanisms of myogenesis are considered.

Isolation and structural analysis of the rat MyoD gene

We have cloned and determined the nucleotide (nt) sequence of a 6.5-kb genomic DNA fragment containing the rat MyoD gene (encoding a muscle regulatory factor, MyoD). Mouse fibroblasts transfected with this DNA display a high degree of conversion to a muscle phenotype, suggesting that this genomic clone contains sufficient sequence information to allow the production of the rat MyoD protein in these cells. The 6.5-kb genomic fragment contains the complete coding region of MyoD, distributed over three exons, plus 2.3 kb of 5'-noncoding sequence and 1.4 kb of 3'-noncoding sequence. Based on RNase protection assays, the major transcription start point of MyoD is located 210 nt 5' to a methionine start codon and 26 nt 3' to a TAAATA motif which bears similarity to a consensus recognition sequence (TATA) utilized by eukaryotic RNA polymerase II transcription complexes. The high degree of identity between the amino acid sequence of rat MyoD and the MyoD proteins isolated from other vertebrates indicates that this muscle regulatory protein has been evolutionarily conserved.

Expression of MRF4, a myogenic helix-loop-helix protein, produces multiple changes in the myogenic program of BC3H-1 cells

Expression of MRF4, a myogenic regulatory factor of the basic helix-loop-helix type, produced multiple changes in the myogenic program of the BC3H-1 cell line. BC3H-1 cells that stably expressed exogenous MRF4 were prepared and termed BR cell lines. Upon differentiation, the BR cells were found to have three muscle-specific properties (endogenous MyoD expression, myoblast fusion, and fast myosin light-chain 1 expression) that the parent BC3H-1 cells did not have. Of the four known myogenic regulatory factors (MyoD, myogenin, Myf-5, and MRF4), only MRF4 was capable of activating expression of the endogenous BC3H-1 myoD gene. In addition, the pattern of Myf-5 expression in BR cells was the opposite of that in BC3H-1 cells. Myf-5 expression was low in BR myoblasts and showed a small increase upon myotube formation, whereas Myf-5 expression was high in BC3H-1 myoblasts and decreased upon differentiation. Though the MRF4-transfected BR cells fused to form large myotubes and expressed fast myosin light-chain 1, the pattern of myosin heavy-chain isoform expression was the same in the BR and the nonfusing parent BC3H-1 cells, suggesting that factors in addition to the MyoD family members regulate myosin heavy-chain isoform expression patterns in BC3H-1 cells. In contrast to the changes produced by MRF4 expression, overexpression of Myf-5 did not alter BC3H-1 myogenesis. The results suggest that differential expression of the myogenic regulatory factors of the MyoD family may be one mechanism for generating cells with diverse myogenic phenotypes.

Beta-enolase is a marker of human myoblast heterogeneity prior to differentiation

In this report, we define a muscle-specific marker, beta-enolase, that distinguishes proliferating myoblasts from different stages of development. Enolase exists as multiple isoforms and in the course of cardiac and skeletal muscle development the beta isoform progressively replaces the alpha isoform. In skeletal muscle, this change in gene expression, unlike most developmental changes in myogenic gene expression, is evident in undifferentiated myoblasts. Whereas myoblasts from fetal tissues express alpha-enolase mRNA, beta-enolase is the predominant mRNA expressed by myoblasts from postnatal tissues. Our results are consistent with the idea that distinct precursor myoblasts contribute to the diversity of fiber types characteristic of muscle tissue at different stages of development.

Expression of MRF4, a myogenic helix-loop-helix protein, produces multiple changes in the myogenic program of BC3H-1 cells

Expression of MRF4, a myogenic regulatory factor of the basic helix-loop-helix type, produced multiple changes in the myogenic program of the BC3H-1 cell line. BC3H-1 cells that stably expressed exogenous MRF4 were prepared and termed BR cell lines. Upon differentiation, the BR cells were found to have three muscle-specific properties (endogenous MyoD expression, myoblast fusion, and fast myosin light-chain 1 expression) that the parent BC3H-1 cells did not have. Of the four known myogenic regulatory factors (MyoD, myogenin, Myf-5, and MRF4), only MRF4 was capable of activating expression of the endogenous BC3H-1 myoD gene. In addition, the pattern of Myf-5 expression in BR cells was the opposite of that in BC3H-1 cells. Myf-5 expression was low in BR myoblasts and showed a small increase upon myotube formation, whereas Myf-5 expression was high in BC3H-1 myoblasts and decreased upon differentiation. Though the MRF4-transfected BR cells fused to form large myotubes and expressed fast myosin light-chain 1, the pattern of myosin heavy-chain isoform expression was the same in the BR and the nonfusing parent BC3H-1 cells, suggesting that factors in addition to the MyoD family members regulate myosin heavy-chain isoform expression patterns in BC3H-1 cells. In contrast to the changes produced by MRF4 expression, overexpression of Myf-5 did not alter BC3H-1 myogenesis. The results suggest that differential expression of the myogenic regulatory factors of the MyoD family may be one mechanism for generating cells with diverse myogenic phenotypes.

Inhibition of muscle differentiation by the adenovirus E1a protein: repression of the transcriptional activating function of the HLH protein Myf-5

Myogenic differentiation can be inhibited by the adenovirus E1a protein in the rat L6 muscle cell line. The present investigation provides evidence that E1a interferes with the expression of myogenin and the activity of Myf-5, the two myogenic helix-loop-helix (HLH) proteins that are expressed in L6 muscle cells. In nuclei of E1a-expressing L6 cells, Myf-5 protein accumulates to normal or even elevated levels and shows no alterations of its ability to bind to the DNA-binding site (CANNTG). However, trans-activation of muscle-specific reporter genes by Myf-5 is strongly inhibited. The same inhibition by E1a can be shown for the other myogenic HLH proteins, MyoD, myogenin, and MRF4/Myf-6, that have been expressed in 10T1/2 fibroblasts. In contrast to the normal level of Myf-5 expression, synthesis of myogenin is entirely abolished in the differentiation-defective L6-E1a cells. Here, we demonstrate that the carboxy-terminal trans-activator domain and probably the basic-HLH (bHLH) region of Myf-5 constitute targets for the inhibition by E1a. The effect of E1a depends on its intact transforming regions but not on the transcriptional activator domain. Our data suggest that activation of myogenin gene expression and the establishment of the differentiated phenotype may require functional Myf-5. Expression of the Myf-5 gene, however, is apparently independent of auto- or cross-regulation by the myogenic HLH proteins.

Regulation of muscle cell growth and differentiation by the MyoD family of helix-loop-helix proteins

The skeletal muscle cell system provides a powerful model for exploring the mechanistic basis for the antagonism between cell growth and differentiation. The recent identification of the MyoD family of muscle-specific transcription factors now offers opportunities to dissect at the molecular level of the mechanisms through which defined cell type-specific transcription factors can activate an entire differentiation program as well as to unravel the mechanisms through which growth factor and oncogenic signals can disrupt cellular differentiation. Because the mechanisms for growth factor signaling and induction of cell proliferation are conserved in diverse cell types, it is tempting to speculate that the molecular mechanisms responsible for the antagonism between cell proliferation and differentiation in muscle cells are also operative in other cell types. Resolution of this question, however, must await identification of the regulatory factors that specify cell fate in other lineages.

Characterisation of a genomic clone covering the structural mouse MyoD1 gene and its promoter region

We have isolated the mouse MyoD1 gene flanked by its promoter region by screening a genomic library with synthetic oligonucleotides. The structural gene is interrupted by two G + C rich introns. Transfection of the cloned gene inserted into an expression vector converts fibroblasts to myoblasts. Sequence analysis of about 650 bp of the 5' upstream region revealed the presence of several potential regulatory elements such as a TATA-box, an AP2-box, two SP1-boxes and a CAAT-box. In addition, there are three half palindromic estrogen response elements, a potential cAMP response element and various muscle specific elements such as a muscle-specific CAAT-box (MCAT) and four potential binding sites for MyoD1. Using S1 protection analysis the major start site of transcription in muscle and myoblast cells was mapped 3 bp upstream of the published cDNA 5' end. Promoter activity of the 650 bp upstream fragment was tested by in vitro transcription and by transfection analysis of myoblasts and fibroblasts. In all promoter test systems used, MyoD1 promoter activity was detected in myoblasts as well as in fibroblasts. Furthermore, DNA methylation was found to turn off MyoD1 promoter activity both in myoblasts and in fibroblasts.

Systemic delivery of human growth hormone by injection of genetically engineered myoblasts

A recombinant gene encoding human growth hormone (hGH) was stably introduced into cultured myoblasts with a retroviral vector. After injection of genetically engineered myoblasts into mouse muscle, hGH could be detected in serum for 3 months. The fate of injected myoblasts was assessed by coinfecting the cells with two retroviral vectors, one encoding hGH and the other encoding beta-galactosidase from Escherichia coli. These results provide evidence that myoblasts, which can fuse into preexisting multinucleated myofibers that are vascularized and innervated, may be advantageous as vehicles for systemic delivery of recombinant proteins.

Mutagenesis of the myogenin basic region identifies an ancient protein motif critical for activation of myogenesis

Myogenin is a muscle-specific nuclear factor that acts as a genetic switch to activate myogenesis. Myogenin, MyoD, and a growing number of proteins implicated in transcriptional control share sequence homology within a basic region and an adjacent helix-loop-helix motif. Here we identify by site-directed mutagenesis a 12-amino acid subdomain of the myogenin basic region essential for binding of DNA and activation of myogenesis. The basic region of the widely expressed helix-loop-helix protein E12 is conserved at 8 of these 12 residues and can mediate DNA binding when placed in myogenin, but it cannot activate myogenesis. Replacement of each of the four nonconserved residues of the myogenin basic region with the corresponding residues of E12 reveals two adjacent amino acids (Ala86-Thr) that can impart muscle specificity to the basic region. These residues are specific to, and conserved in, the basic regions of all known myogenic helix-loop-helix proteins from Drosophila to man, suggesting that they constitute part of an ancient protein motif required for activation of the myogenic program.

Insulin gene expression in nonexpressing cells appears to be regulated by multiple distinct negative-acting control elements

Selective transcription of the insulin gene in pancreatic beta cells is regulated by its enhancer, located between nucleotides -340 and -91 relative to the transcription start site. Transcription from the enhancer is controlled by both positive- and negative-acting cellular factors. Cell-type-specific expression is mediated principally by a single cis-acting enhancer element located between -100 and -91 in the rat insulin II gene (referred to as the insulin control element [ICE]), which is acted upon by both of these cellular activities. Analysis of the effect of 5' deletions within the insulin enhancer has identified a region between nucleotides -217 and -197 that is also a site of negative control. Deletion of these sequences from the 5' end of the enhancer leads to transcription of the enhancer in non-insulin-producing cells, even though the ICE is intact. Derepression of this ICE-mediated effect was shown to be due to the binding of a ubiquitously distributed cellular factor to a sequence element which resides just upstream of the ICE (i.e., between nucleotides -110 and -100). We discuss the possible relationship of these results to cell-type-specific regulation of the insulin gene.