Analysis of the myogenin promoter reveals an indirect pathway for positive autoregulation mediated by the muscle-specific enhancer factor MEF-2

Centronuclear myopathy in mice lacking a novel muscle-specific protein kinase transcriptionally regulated by MEF2

Myocyte enhancer factor 2 (MEF2) plays essential roles in transcriptional control of muscle development. However, signaling pathways acting downstream of MEF2 are largely unknown. Here, we performed a microarray analysis using Mef2c-null mouse embryos and identified a novel MEF2-regulated gene encoding a muscle-specific protein kinase, Srpk3, belonging to the serine arginine protein kinase (SRPK) family, which phosphorylates serine/arginine repeat-containing proteins. The Srpk3 gene is specifically expressed in the heart and skeletal muscle from embryogenesis to adulthood and is controlled by a muscle-specific enhancer directly regulated by MEF2. Srpk3-null mice display a new entity of type 2 fiber-specific myopathy with a marked increase in centrally placed nuclei; while transgenic mice overexpressing Srpk3 in skeletal muscle show severe myofiber degeneration and early lethality. We conclude that normal muscle growth and homeostasis require MEF2-dependent signaling by Srpk3.

Vasopressin-dependent myogenic cell differentiation is mediated by both Ca2+/calmodulin-dependent kinase and calcineurin pathways

Arg8-vasopressin (AVP) promotes the differentiation of myogenic cell lines and mouse primary satellite cells by mechanisms involving the transcriptional activation of myogenic bHLH regulatory factors and myocyte enhancer factor 2 (MEF2). We here report that AVP treatment of L6 cells results in the activation of calcineurin-dependent differentiation, increased expression of MEF2 and GATA2, and nuclear translocation of the calcineurin target NFATc1. Interaction of these three factors occurs at MEF2 sites of muscle specific genes. The different kinetics of AVP-dependent expression of early (myogenin) and late (MCK) muscle-specific genes correlate with different acetylation levels of histones at their MEF2 sites. The cooperative role of calcineurin and Ca2+/calmodulin-dependent kinase (CaMK) in AVP-dependent differentiation is demonstrated by the effect of inhibitors of the two pathways. We show here, for the first time, that AVP, a "novel" myogenesis promoting factor, activates both the calcineurin and the CaMK pathways, whose combined activation leads to the formation of multifactor complexes and is required for the full expression of the differentiated phenotype. Although MEF2-NFATc1 complexes appear to regulate the expression of an early muscle-specific gene product (myogenin), the activation of late muscle-specific gene expression (MCK) involves the formation of complexes including GATA2.

PC4 coactivates MyoD by relieving the histone deacetylase 4-mediated inhibition of myocyte enhancer factor 2C

Histone deacetylase 4 (HDAC4) negatively regulates skeletal myogenesis by associating with the myocyte enhancer factor 2 (MEF2) transcription factors. Our data indicate that the gene PC4 (interferon-related developmental regulator 1 [IFRD1], Tis7), which we have previously shown to be required for myoblast differentiation, is both induced by MyoD and potentiates the transcriptional activity of MyoD, thus revealing a positive regulatory loop between these molecules. Enhancement by PC4 of MyoD-dependent activation of muscle gene promoters occurs selectively through MEF2 binding sites. Furthermore, PC4 localizes in the nucleus of differentiating myoblasts, associates with MEF2C, and is able to counteract the HDAC4-mediated inhibition of MEF2C. This latter action can be explained by the observed ability of PC4 to dose dependently displace HDAC4 from MEF2C. Consistently, we have observed that (i) the region of PC4 that binds MEF2C is sufficient to counteract the inhibition by HDAC4; (ii) PC4, although able to bind HDAC4, does not inhibit the enzymatic activity of HDAC4; and (iii) PC4 overcomes the inhibition mediated by the amino-terminal domain of HDAC4, which associates with MEF2C but not with PC4. Together, our findings strongly suggest that PC4 acts as a coactivator of MyoD and MEF2C by removing the inhibitory effect of HDAC4, thus exerting a pivotal function during myogenesis.

HRC is a direct transcriptional target of MEF2 during cardiac, skeletal, and arterial smooth muscle development in vivo

The HRC gene encodes the histidine-rich calcium-binding protein, which is found in the lumen of the junctional sarcoplasmic reticulum (SR) of cardiac and skeletal muscle and within calciosomes of arterial smooth muscle. The expression of HRC in cardiac, skeletal, and smooth muscle raises the possibility of a common transcriptional mechanism governing its expression in all three muscle cell types. In this study, we identified a transcriptional enhancer from the HRC gene that is sufficient to direct the expression of lacZ in the expression pattern of endogenous HRC in transgenic mice. The HRC enhancer contains a small, highly conserved sequence that is required for expression in all three muscle lineages. Within this conserved region is a consensus site for myocyte enhancer factor 2 (MEF2) proteins that we show is bound efficiently by MEF2 and is required for transgene expression in all three muscle lineages in vivo. Furthermore, the entire HRC enhancer sequence lacks any discernible CArG motifs, the binding site for serum response factor (SRF), and we show that the enhancer is not activated by SRF. Thus, these studies identify the HRC enhancer as the first MEF2-dependent, CArG-independent transcriptional target in smooth muscle and represent the first analysis of the transcriptional regulation of an SR gene in vivo.

TGF-beta-activated Smad3 represses MEF2-dependent transcription in myogenic differentiation

Transforming growth factor beta (TGF-beta) inhibits myogenesis and associated gene expression. We previously reported that the TGF-beta signaling effector Smad3 mediates this inhibition, by interfering with the assembly of myogenic bHLH transcription factor heterodimers on E-box sequences in the regulatory regions of muscle-specific genes. We now show that TGF-beta-activated Smad3 suppresses the function of MEF2, a second class of essential myogenic factors. TGF-beta signaling through Smad3 represses myogenin expression independently of E-boxes, and prevents a tethered MyoD-E47 dimer to activate transcription indirectly through MEF2-binding sites. In addition, Smad3 interacts with MEF2C, which requires its MADS domain, and disrupts its association with the SRC-family coactivator GRIP-1, thus diminishing the transcription activity of MEF2C. Consistent with this physical displacement, TGF-beta signaling blocks the GRIP-1-induced redistribution of MEF2C to discrete nuclear subdomains in 10T1/2 cells, and the recruitment of GRIP-1 to the myogenin promoter in differentiating myoblasts. These findings indicate that the TGF-beta/Smad3 pathway targets two critical components of the myogenic transcription machinery to inhibit terminal differentiation.

Accelerated response of the myogenin gene to denervation in mutant mice lacking phosphorylation of myogenin at threonine 87

Gene expression in skeletal muscle is regulated by a family of myogenic basic helix-loop-helix (bHLH) proteins. The binding of these bHLH proteins, notably MyoD and myogenin, to E-boxes in their own regulatory regions is blocked by protein kinase C (PKC)-mediated phosphorylation of a single threonine residue in their basic region. Because electrical stimulation increases PKC activity in skeletal muscle, these data have led to an attractive model suggesting that electrical activity suppresses gene expression by stimulating phosphorylation of this critical threonine residue in myogenic bHLH proteins. We show that electrical activity stimulates phosphorylation of myogenin at threonine 87 (T87) in vivo and that calmodulin-dependent kinase II (CaMKII), as well as PKC, catalyzes this reaction in vitro. We find that phosphorylation of myogenin at T87 is dispensable for skeletal muscle development. We show, however, that the decrease in myogenin (myg) expression following innervation is delayed and that the increase in expression following denervation is accelerated in mutant mice lacking phosphorylation of myogenin at T87. These data indicate that two distinct innervation-dependent mechanisms restrain myogenin activity: an inactivation mechanism mediated by phosphorylation of myogenin at T87, and a second, novel regulatory mechanism that regulates myg gene activity independently of T87 phosphorylation.

Repositioning of muscle-specific genes relative to the periphery of SC-35 domains during skeletal myogenesis

Previous studies have shown that in a given cell type, certain active genes associate with SC-35 domains, nuclear regions rich in RNA metabolic factors and excluded from heterochromatin. This organization is not seen for all active genes; therefore, it is important to determine whether and when this locus-specific organization arises during development and differentiation of specific cell types. Here, we investigate whether gene organization relative to SC-35 domains is cell type specific by following several muscle and nonmuscle genes in human fibroblasts, committed but proliferative myoblasts, and terminally differentiated muscle. Although no change was seen for other loci, two muscle genes (Human beta-cardiac myosin heavy chain and myogenin) became localized to the periphery of an SC-35 domain in terminally differentiated muscle nuclei, but not in proliferative myoblasts or in fibroblasts. There was no apparent change in gene localization relative to either the chromosome territory or the heterochromatic compartment; thus, the gene repositioning seemed to occur specifically with respect to SC-35 domains. This gene relocation adjacent to a prominent SC-35 domain was recapitulated in mouse 3T3 cells induced into myogenesis by introduction of MyoD. Results demonstrate a cell type-specific reorganization of specific developmentally regulated loci relative to large domains of RNA metabolic factors, which may facilitate developmental regulation of genome expression.

Permissive roles of phosphatidyl inositol 3-kinase and Akt in skeletal myocyte maturation

Skeletal muscle differentiation, maturation, and regeneration are regulated by interactions between signaling pathways activated by hormones and growth factors, and intrinsic genetic programs controlled by myogenic transcription factors, including members of the MyoD and myocyte enhancer factor 2 (MEF2) families. Insulin-like growth factors (IGFs) play key roles in muscle development in the embryo, and in the maintenance and hypertrophy of mature muscle in the adult, but the precise signaling pathways responsible for these effects remain incompletely defined. To study mechanisms of IGF action in muscle, we have developed a mouse myoblast cell line termed C2BP5 that is dependent on activation of the IGF-I receptor and the phosphatidyl inositol 3-kinase (PI3-kinase)-Akt pathway for initiation of differentiation. Here, we show that differentiation of C2BP5 myoblasts could be induced in the absence of IGF action by recombinant adenoviruses expressing MyoD or myogenin, but it was reversibly impaired by the PI3-kinase inhibitor LY294002. Similar results were observed using a dominant-negative version of Akt, a key downstream component of PI3-kinase signaling, and also were seen in C3H 10T1/2 fibroblasts. Inhibition of PI3-kinase did not prevent accumulation of muscle differentiation-specific proteins (myogenin, troponin T, or myosin heavy chain), did not block transcriptional activation of E-box containing muscle reporter genes by MyoD or myogenin, and did not inhibit the expression or function of endogenous MEF2C or MEF2D. An adenovirus encoding active Akt could partially restore terminal differentiation of MyoD-expressing and LY294002-treated myoblasts, but the resultant myofibers contained fewer nuclei and were smaller and thinner than normal, indicating that another PI3-kinase-stimulated pathway in addition to Akt is required for full myocyte maturation. Our results support the idea that an IGF-regulated PI3-kinase pathway functions downstream of or in parallel with MyoD, myogenin, and MEF2 in muscle development to govern the late steps of differentiation that lead to multinucleated myotubes.

Requirement for down-regulation of the CCAAT-binding activity of the NF-Y transcription factor during skeletal muscle differentiation

NF-Y is composed of three subunits, NF-YA, NF-YB, and NF-YC, all required for DNA binding. All subunits are expressed in proliferating skeletal muscle cells, whereas NF-YA alone is undetectable in terminally differentiated cells in vitro. By immunohistochemistry, we show that the NF-YA protein is not expressed in the nuclei of skeletal and cardiac muscle cells in vivo. By chromatin immunoprecipitation experiments, we demonstrate herein that NF-Y does not bind to the CCAAT boxes of target promoters in differentiated muscle cells. Consistent with this, the activity of these promoters is down-regulated in differentiated muscle cells. Finally, forced expression of the NF-YA protein in cells committed to differentiate leads to an impairment in the down-regulation of cyclin A, cyclin B1, and cdk1 expression and is accompanied by a delay in myogenin expression. Thus, our results indicate that the suppression of NF-Y function is of crucial importance for the inhibition of several cell cycle genes and the induction of the early muscle-specific program in postmitotic muscle cells.

Alteration of mesodermal cell differentiation by EWS/FLI-1, the oncogene implicated in Ewing's sarcoma

The chimeric fusion gene EWS/FLI-1 is detected in most cases of Ewing's sarcoma (ES), the second most common malignant bone tumor of childhood. Although 80% of ES tumors develop in skeletal sites, the remainder can arise in almost any soft tissue location. The lineage of the cell developing the EWS/FLI-1 gene fusion has not been fully characterized but is generally considered to be of either mesenchymal or neural crest origin. To study this oncogene in a conceptually relevant target cell, EWS/FLI-1 was introduced into the murine cell line C2C12, a myoblast cell line capable of differentiation into muscle, bone, or fat. In this cellular context, EWS/FLI-1 profoundly inhibited the myogenic differentiation program. The block in C2C12 myogenic differentiation required the nuclear localization and DNA-binding functions of EWS/FLI-1 and was mediated by transcriptional and posttranscriptional suppression of the myogenic transcription factors MyoD and myogenin. Interestingly, C2C12-EWS/FLI-1 cells constitutively expressed alkaline phosphatase, a bone lineage marker, and were alkaline phosphatase positive by histochemistry but showed no other evidence of bone lineage commitment. Consistent with recent findings in human ES tumor cell lines, C2C12-EWS/FLI-1 cells constitutively expressed cyclin D1 and demonstrated decreased expression of the cell cycle regulator p21(cip1), even under differentiation conditions and at confluent density. This C2C12-EWS/FLI-1 cell model may assist in the identification of novel differentially expressed genes relevant to ES and provide further insight into the cell(s) of origin developing ES-associated genetic fusions.

Smad proteins function as co-modulators for MEF2 transcriptional regulatory proteins

An emerging theme in transforming growth factor-ss (TGF-ss) signalling is the association of the Smad proteins with diverse groups of transcriptional regulatory proteins. Several Smad cofactors have been identified to date but the diversity of TGF-ss effects on gene transcription suggests that interactions with other co-regulators must occur. In these studies we addressed the possible interaction of Smad proteins with the myocyte enhancer-binding factor 2 (MEF2) transcriptional regulators. Our studies indicate that Smad2 and 4 (Smad2/4) complexes cooperate with MEF2 regulatory proteins in a GAL4-based one-hybrid reporter gene assay. We have also observed in vivo interactions between Smad2 and MEF2A using co-immunoprecipitation assays. This interaction is confirmed by glutathione S:-transferase pull-down analysis. Immunofluorescence studies in C2C12 myotubes show that Smad2 and MEF2A co-localise in the nucleus of multinuclear myotubes during differentiation. Interestingly, phospho-acceptor site mutations of MEF2 that render it unresponsive to p38 MAP kinase signalling abrogate the cooperativity with the Smads suggesting that p38 MAP Kinase-catalysed phosphorylation of MEF2 is a prerequisite for the Smad-MEF2 interaction. Thus, the association between Smad2 and MEF2A may subserve a physical link between TGF-ss signalling and a diverse array of genes controlled by the MEF2 cis element.

mRNA 5'-leader trans-splicing in the chordates

We report the discovery of mRNA 5'-leader trans-splicing (SL trans-splicing) in the chordates. In the ascidian protochordate Ciona intestinalis, the mRNAs of at least seven genes undergo trans-splicing of a 16-nucleotide 5'-leader apparently derived from a 46-nucleotide RNA that shares features with previously characterized splice donor SL RNAs. SL trans-splicing was known previously to occur in several protist and metazoan phyla, however, this is the first report of SL trans-splicing within the deuterostome division of the metazoa. SL trans-splicing is not known to occur in the vertebrates. However, because ascidians are primitive chordates related to vertebrate ancestors, our findings raise the possibility of ancestral SL trans-splicing in the vertebrate lineage.

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

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

Independent repressor domains in ZEB regulate muscle and T-cell differentiation

ZEB is a zinc finger-homeodomain protein that represses transcription by binding to a subset of E-box sequences. ZEB inhibits muscle differentiation in mammalian systems, and its Drosophila orthologue, zfh-1, inhibits somatic and cardiac muscle differentiation during Drosophila embryogenesis. ZEB also binds to the promoter of pivotal hematopoietic genes (including those encoding interleukin-2, CD4, GATA-3, and alpha(4)-integrin), and mice in which ZEB has been genetically targeted show thymic atrophy, severe defects in lymphocyte differentiation, and increased expression of the alpha(4)-integrin and CD4. Here, we demonstrate that ZEB contains separate repressor domains which function in T lymphocytes and muscle, respectively. The most C-terminal domain inhibits muscle differentiation in mammalian cells by specifically blocking the transcriptional activity of the myogenic factor MEF2C. The more N-terminal domain blocks activity of hematopoietic transcription factors such as c-myb, members of the ets family, and TFE-III. Our results demonstrate that ZEB has evolved with two independent repressor domains which target distinct sets of transcription factors and function in different tissues.

Involvement of type 4 cAMP-phosphodiesterase in the myogenic differentiation of L6 cells

Myogenic cell differentiation is induced by Arg(8)-vasopressin, whereas high cAMP levels and protein kinase A (PKA) activity inhibit myogenesis. We investigated the role of type 4 phosphodiesterase (PDE4) during L6-C5 myoblast differentiation. Selective PDE4 inhibition resulted in suppression of differentiation induced by vasopressin. PDE4 inhibition prevented vasopressin-induced nuclear translocation of the muscle-specific transcription factor myogenin without affecting its overall expression level. The effects of PDE4 inhibition could be attributed to an increase of cAMP levels and PKA activity. RNase protection, reverse transcriptase PCR, immunoprecipitation, Western blot, and enzyme activity assays demonstrated that the PDE4D3 isoform is the major PDE4 expressed in L6-C5 myoblasts and myotubes, accounting for 75% of total cAMP-hydrolyzing activity. Vasopressin cell stimulation caused a biphasic increase of PDE4 activity, which peaked at 2 and 15 min and remained elevated for 48 h. In the continuous presence of vasopressin, cAMP levels and PKA activity were lowered. PDE4D3 overexpression increased spontaneous and vasopressin-dependent differentiation of L6-C5 cells. These results show that PDE4D3 plays a key role in the control of cAMP levels and differentiation of L6-C5 cells. Through the modulation of PDE4 activity, vasopressin inhibits the cAMP signal transduction pathway, which regulates myogenesis possibly by controlling the subcellular localization of myogenin.

Expression and localisation of annexin VII (synexin) isoforms in differentiating myoblasts

Annexin VII exists in a 47 kDa and a 51 kDa isoform with the 51 kDa protein being the only isoform present in skeletal muscle. Expression of the 51 kDa isoform during myogenesis and localization was studied in cells after conversion into myogenic cells by transduction with MyoD and in mouse and human myogenic cell lines. MyoD expression in NIH3T3 and C3H10T1/2 fibroblasts led to disappearance of the mRNA specific for the 47 kDa isoform and appearance of the 51 kDa isoform-specific mRNA. The overall amount of annexin VII protein was reduced in myogenic converted cells. Both in undifferentiated and differentiated cells annexin VII was localized by immunofluorescence microscopy to punctate structures which were distributed all over the cell. A GFP annexin VII fusion protein showed a similar distribution. Cell fractionation studies indicated that annexin VII is equally distributed between cytosol and membrane fractions in undifferentiated cells, while in differentiated cells it is exclusively present in the membrane fraction. By sucrose gradient centrifugation of postnuclear supernatants we identified two distinct annexin VII-containing membrane populations that cofractionated with caveolin 3- and sorcin-containing membranes.

Fiber-type-specific transcription of the troponin I slow gene is regulated by multiple elements

The regulatory elements that restrict transcription of genes encoding contractile proteins specifically to either slow- or fast-twitch skeletal muscles are unknown. As an initial step towards understanding the mechanisms that generate muscle diversity during development, we have identified a 128-bp troponin I slow upstream element (SURE) and a 144-bp troponin I fast intronic element (FIRE) that confer fiber type specificity in transgenic mice (M. Nakayama et al., Mol. Cell. Biol. 16:2408-2417, 1996). SURE and FIRE have maintained the spatial organization of four conserved motifs (3' to 5'): an E box, an AT-rich site (A/T2) that binds MEF-2, a CACC site, and a novel CAGG motif. Troponin I slow (TnIs) constructs harboring mutations in these motifs were analyzed in transiently and stably transfected Sol8 myocytes and in transgenic mice to assess their function. Mutations of the E-box, A/T2, and CAGG motifs completely abolish transcription from the TnI SURE. In contrast, mutation of the CACC motif had no significant effect in transfected myocytes or on the slow-specific transcription of the TnI SURE in transgenic mice. To assess the role of E boxes in fiber type specificity, a chimeric enhancer was constructed in which the E box of SURE was replaced with the E box from FIRE. This TnI E box chimera, which lacks the SURE NFAT site, confers essentially the same levels of transcription in transgenic mice as those conferred by wild-type SURE and is specifically expressed in slow-twitch muscles, indicating that the E box on its own cannot determine the fiber-type-specific expression of the TnI promoter. The importance of the 5' half of SURE, which bears little homology to the TnI FIRE, in muscle-specific expression was analyzed by deletion and linker scanning analyses. Removal of the 5' half of SURE (-846 to -811) results in the loss of expression in stably transfected but not in transiently expressing myocytes. Linker scanning mutations identified sequences in this region that are necessary for the function of SURE when integrated into chromatin. One of these sites (GTTAATCCG), which is highly homologous to a bicoid consensus site, binds to nuclear proteins from several mesodermal cells. These results show that multiple elements are involved in the muscle-specific activity of the TnIs promoter and that interactions between upstream and downstream regions of SURE are important for transcription in the context of native chromatin.

Expression of myogenin during embryogenesis is controlled by Six/sine oculis homeoproteins through a conserved MEF3 binding site

Myogenin, one of the MyoD family of proteins, is expressed early during somitogenesis and is required for myoblast fusion in vivo. Previous studies in transgenic mice have shown that a 184-bp myogenin promoter fragment is sufficient to correctly drive expression of a beta-galactosidase transgene during embryogenesis. We show here that mutation of one of the DNA motifs present in this region, the MEF3 motif, abolished correct expression of this beta-galactosidase transgene. We have found that the proteins that bind to the MEF3 site are homeoproteins of the Six/sine oculis family. Antibodies directed specifically against Six1 or Six4 proteins reveal that each of these proteins is present in the embryo when myogenin is activated and constitutes a muscle-specific MEF3-binding activity in adult muscle nuclear extracts. Both of these proteins accumulate in the nucleus of C2C12 myogenic cells, and transient transfection experiments confirm that Six1 and Six4 are able to transactivate a reporter gene containing MEF3 sites. Altogether these results establish Six homeoproteins as a family of transcription factors controlling muscle formation through activation of one of its key regulators, myogenin.

The Rho family G proteins play a critical role in muscle differentiation

The Rho family GTP-binding proteins play a critical role in a variety of cytoskeleton-dependent cell functions. In this study, we examined the role of Rho family G proteins in muscle differentiation. Dominant negative forms of Rho family proteins and RhoGDI, a GDP dissociation inhibitor, suppressed transcription of muscle-specific genes, while mutationally activated forms of Rho family proteins strongly activated their transcription. C2C12 cells overexpressing RhoGDI (C2C12RhoGDI cells) did not differentiate into myotubes, and expression levels of myogenin, MRF4, and contractile protein genes but not MyoD and myf5 genes were markedly reduced in C2C12RhoGDI cells. The promoter activity of the myogenin gene was suppressed by dominant negative mutants of Rho family proteins and was reduced in C2C12RhoGDI cells. Expression of myocyte enhancer binding factor 2 (MEF2), which has been reported to be required for the expression of the myogenin gene, was reduced at the mRNA and protein levels in C2C12RhoGDI cells. These results suggest that the Rho family proteins play a critical role in muscle differentiation, possibly by regulating the expression of the myogenin and MEF2 genes.

Trans-regulation of myogenin promoter/enhancer activity by c-ski during skeletal-muscle differentiation: the C-terminus of the c-Ski protein is essential for transcriptional regulatory activity in myotubes

c-ski gene product is a nuclear protein with myogenesis-promoting and transforming activities. We have analysed the effects of c-ski transfection on the promoter/enhancer activity of the upstream region of the myogenin gene during in vitro myogenesis using CAT reporter assay. When co-transfected with c-ski into myogenic C2C12 cells, promoter/enhancer activity was efficiently suppressed in proliferating cells, but the myogenesis-induced increase in activity was potentiated approximately ten times more (150-fold in the ski-transfected cells) than the ordinary increase (12-fold in the mock) 48 h after induction of differentiation. In non-myogenic 10T1/2 cells, c-ski transfection caused persistent suppression of promoter/enhancer activity in both proliferating and growth-arrested (i.e. myogenesis-inducing) conditions. Thus the ski-dependent potentiation of myogenin gene transcriptional activity appears to be specific for myogenesis. The C-terminal region (amino acids 595-663) of the c-Ski protein was essential for the potentiating activity in myotubes. Other members of the ski-gene family, snoN and snoA, were ineffective in transactivation, possibly because of the defect in the corresponding C-terminal region. c-Ski protein underwent a mobility shift on SDS/PAGE after in vitro myogenesis which may explain the conversion of the activity from suppressive in myoblasts to potentiating in myotubes. Deletion analysis of the upstream region of the myogenin gene revealed that a responsive element to c-ski in myotubes is located at a distinct site upstream of the basal promoter/enhancer region.

Muscle LIM protein promotes myogenesis by enhancing the activity of MyoD

The muscle LIM protein (MLP) is a muscle-specific LIM-only factor that exhibits a dual subcellular localization, being present in both the nucleus and in the cytoplasm. Overexpression of MLP in C2C12 myoblasts enhances skeletal myogenesis, whereas inhibition of MLP activity blocks terminal differentiation. Thus, MLP functions as a positive developmental regulator, although the mechanism through which MLP promotes terminal differentiation events remains unknown. While examining the distinct roles associated with the nuclear and cytoplasmic forms of MLP, we found that nuclear MLP functions through a physical interaction with the muscle basic helix-loop-helix (bHLH) transcription factors MyoD, MRF4, and myogenin. This interaction is highly specific since MLP does not associate with nonmuscle bHLH proteins E12 or E47 or with the myocyte enhancer factor-2 (MEF2) protein, which acts cooperatively with the myogenic bHLH proteins to promote myogenesis. The first LIM motif in MLP and the highly conserved bHLH region of MyoD are responsible for mediating the association between these muscle-specific factors. MLP also interacts with MyoD-E47 heterodimers, leading to an increase in the DNA-binding activity associated with this active bHLH complex. Although MLP lacks a functional transcription activation domain, we propose that it serves as a cofactor for the myogenic bHLH proteins by increasing their interaction with specific DNA regulatory elements. Thus, the functional complex of MLP-MyoD-E protein reveals a novel mechanism for both initiating and maintaining the myogenic program and suggests a global strategy for how LIM-only proteins may control a variety of developmental pathways.

ZEB, a vertebrate homolog of Drosophila Zfh-1, is a negative regulator of muscle differentiation

A number of genes, spanning the evolutionary scale from yeast to mammals, that are involved in spatial and temporal patterning during development contain zinc finger and homeodomain motifs. One such zinc finger/homeodomain protein is Drosophila Zfh-1, a member of the zfh family of Drosophila genes, which is expressed in muscle precursors and is critical for the proper development of muscle. Here we demonstrate that a vertebrate homolog of Zfh-1 (ZEB) is a negative regulator of muscle differentiation. We show that ZEB binds to a subset of E boxes in muscle genes and functions by actively repressing transcription. One target of this repression is the members of the MEF-2 family, which synergize with proteins of the myogenic basic helix-loop-helix family (bHLH) (myoD, myf-5, myogenin and MRF-4) to induce myogenic differentiation. As muscle differentiation proceeds, myogenic bHLH proteins accumulate to levels sufficient to displace ZEB from the E boxes, releasing the repression and allowing bHLH proteins to further activate transcription. This mechanism of active transcriptional repression distinguishes ZEB from other negative regulators of myogenesis (Id, Twist and I-mfa) that inhibit muscle differentiation by simply binding and inactivating myogenic factors. The relative affinity of ZEB versus myogenic bHLH proteins varies for E boxes in different genes such that ZEB would be displaced from different genes at distinct times as myogenic bHLH proteins accumulate during myogenesis, thus providing a mechanism to regulate temporal order of gene expression.

The anti-cancer agent distamycin A displaces essential transcription factors and selectively inhibits myogenic differentiation

The anticancer drug, distamycin A, alters DNA conformation by binding to A/T-rich domains. We propose that binding of the drug to DNA alters transcription factor interactions and that this may alter genetic regulation. We have analyzed the effects of distamycin A upon expression of the muscle-specific cardiac and skeletal alpha-actin genes which have A/T-rich regulatory elements in their promoters. Distamycin A specifically inhibited endogenous muscle genes in the myogenic C2 cell line and effectively eliminated the myogenic program. Conversely, when 10T1/2C18 derived pleuripotential TA1 cells were induced to differentiate in the presence of distamycin A, adipocyte differentiation was enhanced whereas the numbers of cells committing to the myogenic program decreased dramatically. Using the mobility shift assay distamycin A selectively inhibited binding of two important transcription factors, SRF and MEF2, to their respective A/T-rich elements. The binding of factors Sp1 and MyoD were not affected. The inhibition of factor binding correlated with a repression of muscle-specific promoter activity as assayed by transient transfection assays. Co-expression of the myoD gene, driven by a distamycin A-insensitive promoter, failed to relieve the inhibition of these muscle-specific promoters by distamycin A. Additionally, SRF and MEF2 dependent promoters were selectively down regulated by distamycin A. These results suggest that distamycin A may inhibit muscle-specific gene expression by selectively interfering with transcription factor interactions and demonstrate the importance of these A/T-rich elements in regulating differentiation of this specific cell type.

Induction of cyclins E and A in response to mitogen removal: a basic alteration associated with the arrest of differentiation of C2 myoblasts transformed by simian virus 40 large T antigen

We previously showed that C2 myoblasts transformed by simian virus 40 large T antigen (SVLT) stop the myogenic process after the induction of myogenin and of high Rb levels; the induced Rb, however, becomes notably phosphorylated. We have analyzed the protein levels and activities of cyclin-dependent kinases (cdks) in untransformed C2 cells and in transformants of either SVLT or the cytoplasmic mutant NKT1 (which permits differentiation) upon a shift from growth medium (GM) to mitogen-poor differentiation medium (DM). After the shift, cdk4 levels remained constant and cdk6 levels decreased in all cell types; cdk2 minimally increased only in SVLT cells. Cyclin D1 was downregulated in DM in all cell types, and cyclin D3 was upregulated (albeit less strongly in SVLT cells than in the others). In contrast, a dramatic difference between SVLT cells and the other cells was observed for cyclins E and A, which essentially disappeared (as protein and RNA) in normal C2 and NKT1 cells upon the shift from GM to DM, whereas they increased in SVLT cells. Concurrently, cdk2 activity ceased in C2 and NKT1 cells in DM, whereas it persisted at 20% of the GM level in SVLT cells. cdk4 activity was detectable in all cells only in GM. Cyclin E and A induction thus appeared to sustain enough Rb phosphorylation to interfere with tissue-specific expression, with cdk activity not high enough to activate cyclin self-regulation. In DM, cdk2 complexed to D3 was underphosphorylated in all cells, and SVLT allowed strong inductions of p21 and p27 without affecting their complexes with cdks.

Molecular mechanisms of myogenic coactivation by p300: direct interaction with the activation domain of MyoD and with the MADS box of MEF2C

By searching for molecules that assist MyoD in converting fibroblasts to muscle cells, we have found that p300 and CBP, two related molecules that act as transcriptional adapters, coactivate the myogenic basic-helix-loop-helix (bHLH) proteins. Coactivation by p300 involves novel physical interactions between p300 and the amino-terminal activation domain of MyoD. In particular, disruption of the FYD domain, a group of three amino acids conserved in the activation domains of other myogenic bHLH proteins, drastically diminishes the transactivation potential of MyoD and abolishes both p300-mediated coactivation and the physical interaction between MyoD and p300. Two domains of p300, at its amino and carboxy terminals, independently function to both mediate coactivation and physically interact with MyoD. A truncated segment of p300, unable to bind MyoD, acts as a dominant negative mutation and abrogates both myogenic conversion and transactivation by MyoD, suggesting that endogenous p300 is a required coactivator for MyoD function. The p300 dominant negative peptide forms multimers with intact p300. p300 and CBP serve as coactivators of another class of transcriptional activators critical for myogenesis, myocyte enhancer factor 2 (MEF2). In fact, transactivation mediated by the MEF2C protein is potentiated by the two coactivators, and this phenomenon is associated with the ability of p300 to interact with the MADS domain of MEF2C. Our results suggest that p300 and CBP may positively influence myogenesis by reinforcing the transcriptional autoregulatory loop established between the myogenic bHLH and the MEF2 factors.

Combinatorial control of muscle development by basic helix-loop-helix and MADS-box transcription factors

Members of the MyoD family of muscle-specific basic helix-loop-helix (bHLH) proteins function within a genetic pathway to control skeletal muscle development. Mutational analyses of these factors suggested that their DNA binding domains mediated interaction with a coregulator required for activation of muscle-specific transcription. Members of the myocyte enhancer binding factor 2 (MEF2) family of MADS-box proteins are expressed at high levels in muscle and neural cells and at lower levels in several other cell types. MEF2 factors are unable to activate muscle gene expression alone, but they potentiate the transcriptional activity of myogenic bHLH proteins. This potentiation appears to be mediated by direct interactions between the DNA binding domains of these different types of transcription factors. Biochemical and genetic evidence suggests that MEF2 factors are the coregulators for myogenic bHLH proteins. The presence of MEF2 and cell-specific bHLH proteins in other cell types raises the possibility that these proteins may also cooperate to regulate other programs of cell-specific gene expression. We present a model to account for such cooperative interactions.

Multiple steps in the regulation of transcription-factor level and activity

This review focuses on the regulation of transcription factors, many of which are DNA-binding proteins that recognize cis-regulatory elements of target genes and are the most direct regulators of gene transcription. Transcription factors serve as integration centres of the different signal-transduction pathways affecting a given gene. It is obvious that the regulation of these regulators themselves is of crucial importance for differential gene expression during development and in terminally differentiated cells. Transcription factors can be regulated at two, principally different, levels, namely concentration and activity, each of which can be modulated in a variety of ways. The concentrations of transcription factors, as of intracellular proteins in general, may be regulated at any of the steps leading from DNA to protein, including transcription, RNA processing, mRNA degradation and translation. The activity of a transcription factor is often regulated by (de) phosphorylation, which may affect different functions, e.g. nuclear localization DNA binding and trans-activation. Ligand binding is another mode of transcription-factor activation. It is typical for the large super-family of nuclear hormone receptors. Heterodimerization between transcription factors adds another dimension to the regulatory diversity and signal integration. Finally, non-DNA-binding (accessory) factors may mediate a diverse range of functions, e.g. serving as a bridge between the transcription factor and the basal transcription machinery, stabilizing the DNA-binding complex or changing the specificity of the target sequence recognition. The present review presents an overview of different modes of transcription-factor regulation, each illustrated by typical examples.

Mutational analysis of the DNA binding, dimerization, and transcriptional activation domains of MEF2C

There are four members of the myocyte enhancer factor 2 (MEF2) family of transcription factors in vertebrates, MEF2A, -B, -C, and -D, which have homology within a MADS box at their amino termini and an adjacent motif known as the MEF2 domain. These factors activate muscle gene expression by binding as homo- and heterodimers to an A/T-rich DNA sequence in the control regions of muscle-specific genes. To understand the mechanisms of muscle gene activation of MEF2 factors, we generated a series of deletion and site-directed mutants of MEF2C. These mutants demonstrated that the MADS and MEF2 domains mediate DNA binding and dimerization, whereas the carboxyl terminus is required for transcriptional activation. Amino acids that are essential for MEF2 site-dependent transcription but which do not affect DNA binding were also identified in the MEF2 domain. This type of positive-control mutant demonstrates that the transcription activation domain of MEF2C, although separate from the MEF2 domain, is dependent on this domain for transcriptional activation through the MEF2 site. MEF2 mutants that are defective for DNA binding act as dominant negative mutants and can inhibit activation of MEF2-dependent genes by wild-type MEF2C.

Common core sequences are found in skeletal muscle slow- and fast-fiber-type-specific regulatory elements

The molecular mechanisms generating muscle diversity during development are unknown. The phenotypic properties of slow- and fast-twitch myofibers are determined by the selective transcription of genes coding for contractile proteins and metabolic enzymes in these muscles, properties that fail to develop in cultured muscle. Using transgenic mice, we have identified regulatory elements in the evolutionarily related troponin slow (TnIs) and fast (TnIf) genes that confer specific transcription in either slow or fast muscles. Analysis of serial deletions of the rat TnIs upstream region revealed that sequences between kb -0.95 and -0.5 are necessary to confer slow-fiber-specific transcription; the -0.5-kb fragment containing the basal promoter was inactive in five transgenic mouse lines tested. We identified a 128-bp regulatory element residing at kb -0.8 that, when linked to the -0.5-kb TnIs promoter, specifically confers transcription to slow-twitch muscles. To identify sequences directing fast-fiber-specific transcription, we generated transgenic mice harboring a construct containing the TnIs kb -0.5 promoter fused to a 144-bp enhancer derived from the quail TnIf gene. Mice harboring the TnIf/TnIs chimera construct expressed the transgene in fast but not in slow muscles, indicating that these regulatory elements are sufficient to confer fiber-type-specific transcription. Alignment of rat TnIs and quail TnIf regulatory sequences indicates that there is a conserved spatial organization of core elements, namely, an E box, a CCAC box, a MEF-2-like sequence, and a previously uncharacterized motif. The core elements were shown to bind their cognate factors by electrophoretic mobility shift assays, and their mutation demonstrated that the TnIs CCAC and E boxes are necessary for transgene expression. Our results suggest that the interaction of closely related transcriptional protein-DNA complexes is utilized to specify fiber type diversity.

Quantitative discrimination of MEF2 sites

Myocyte-specific enhancer factor 2 (MEF2) is a family of closely related transcription factors that play a key role in the differentiation of muscle tissues and are important in the muscle-specific expression of a number of genes. Given the centrality of MEF2 in muscle differentiation, regulatory regions newly determined to be muscle specific are often studied for potential MEF2 binding sites. Possible sites are often located by comparison to a homologous gene or by matching to the consensus MEF2 sequence. Enough data have accumulated that a richer description of the MEF2 binding site, a position weight matrix, can be reliably constructed and its usefulness can be assessed. It was shown that scores from such a matrix approximate MEF2 binding energy and enable recognition of naturally occurring MEF2 sites with high sensitivity and specificity. Regulation of genes via MEF2-like sites is complicated by the fact that a number of transcription factors are involved. Not only is MEF2 itself a family of proteins, but several other, nonhomologous, transcription factors overlap MEF2 in DNA-binding specificity. Thus, more quantitative methods for recognizing potential sites may help with the lengthy process of disentangling the complex regulatory circuits of muscle-specific expression.

Transcription of the human beta enolase gene (ENO-3) is regulated by an intronic muscle-specific enhancer that binds myocyte-specific enhancer factor 2 proteins and ubiquitous G-rich-box binding factors

To provide evidence for the cis-regulatory DNA sequences and trans-acting factors involved in the complex pattern of tissue- and stage-specific expression of the beta enolase gene, constructs containing fragments of the gene fused to the chloramphenicol acetyltransferase gene were used in transient-transfection assays of C2C12 myogenic cells. Deletion analysis revealed the presence of four major regions: two negative regions in the 5'-flanking sequence, a basal promoter region which directs expression at low levels in proliferating and differentiated muscle cells, and a positive region within the first intron that confers cell-type-specific and differentiation-induced expression. This positive regulatory element is located in the 3'-proximal portion of the first intron (nucleotides +504 to +637) and acts as an enhancer irrespective of orientation and position from the homologous beta enolase promoter or the heterologous thymidine kinase promoter, conferring in both cases muscle-specific expression to the linked reporter gene. Deletion of a putative myocyte-specific enhancer factor 1 (MEF-1) binding site, containing a canonical E-box motif, had no effects on muscle-specific transcription, indicating that this site is not required for the activity of the enhancer. Gel mobility shift assays, competition analysis, DNase I footprinting, and mutagenesis studies indicated that this element interacts through an A/T-rich box with a MEF-2 protein(s) and through a G-rich box with a novel ubiquitous factor(s). Mutation of either the G-rich box or the A/T-rich box resulted in a significantly reduced activity of the enhancer in transient-transfection assays. These data indicate that MEF-2 and G-rich-box binding factors are each necessary for tissue-specific expression of the beta enolase gene in skeletal muscle cells.

A skeletal muscle-specific enhancer regulated by factors binding to E and CArG boxes is present in the promoter of the mouse myosin light-chain 1A gene

The mouse myosin light-chain 1A (MLC1A) gene, expressed in the atria of the adult heart, is one of the first muscle genes to be activated when skeletal as well as cardiac muscles form in the embryo. It is also transcribed in skeletal muscle cell lines at the onset of differentiation. Transient transfection assays of mouse skeletal muscle cell lines with DNA constructs containing MLC1A promoter fragments fused to the chloramphenicol acetyltransferase (CAT) gene show that the first 630 bp of the promoter is sufficient to direct expression of the reporter gene during myotube formation. Two E boxes located at bp -76 and -519 are necessary for this regulation. MyoD and myogenin proteins bind to them as heterodimers with E12 protein and, moreover, transactivate them in cotransfection experiments with the MLC1A promoter in nonmuscle cells. Interestingly, the effect of mutating each E box is less striking in primary cultures than in the C2 or Sol8 muscle cell line. A DNA fragment from bp -36 to -597 confers tissue- and stage-specific activity to the herpes simplex virus thymidine kinase promoter in both orientations, showing that the skeletal muscle-specific regulation of the MLC1A gene is under the control of a muscle-specific enhancer which extends into the proximal promoter region. At bp -89 is a diverged CArG box, CC(A/T)6AG, which binds the serum response factor (SRF) in myotube nuclear extracts, as does the wild-type sequence, CC(A/T)6GG. Both types of CArG box also bind a novel myotube-enriched complex which has contact points with the AT-rich part of the CArG box and adjacent 3' nucleotides. Mutations within the CArG box distinguish between the binding of this complex and binding of SRF; only SRF binding is directly involved in the specific regulation of the MLC1A gene in skeletal muscle cell lines.

Regulation of Gax homeobox gene transcription by a combination of positive factors including myocyte-specific enhancer factor 2

Homeobox-containing genes play an essential role in basic processes during embryogenesis and development, but little is known about the regulation of their expression. To elucidate regulatory networks that govern homeobox gene expression, we defined the core promoter of the mouse Gax homeobox gene and characterized its interactions with cellular proteins. Transient transfection experiments revealed Gax promoter activity in several cell types. Deletion analysis defined a 138-bp minimal promoter fragment between positions -125 and +13 relative to the transcription initiation site. Mutagenesis and protein-DNA binding assays suggested that at least three positive factors interact with this fragment and are required for transcriptional activity. One of these factors, HRF-1, recognizes a cis element consisting of an inverted palindromic motif. A second factor is Sp1, that binds to a G/C-rich element. The third is the MADS box factor referred to as MEF2 or RSRF. Mutations in the MEF2/RSRF site had the greatest effect on transcription in cell types that expressed the highest levels of endogenous MEF2 activity. Conversely, overexpression of MEF2A transactivated the Gax promoter more efficiently in cells lacking endogenous MEF2. These data provide evidence for a direct transcriptional link between members of the MADS and homeobox families of transcription factors.

A mutation in the dystrophin gene selectively affecting dystrophin expression in the heart

We have previously shown in a large X-linked pedigree that a deletion removing the dystrophin muscle promoter, the first muscle exon and part of intron 1 caused a severe dilated cardiomyopathy with no associated muscle weakness. Dystrophin expression was present in the muscle of affected males and transcription studies indicated that this dystrophin originated from the brain and Purkinje cell isoforms, upregulated in this skeletal muscle. We have now studied dystrophin transcription and expression in the heart of one member of this family. In contrast to the skeletal muscle, dystrophin transcription and expression were absent in the heart, with the exception of the distal Dp71 dystrophin isoform, normally present in the heart. The 43- and 50-kD dystrophin-associated proteins were severely reduced in the heart, despite the presence of Dp71, but not in skeletal muscle. The absence of dystrophin and the down-regulation of the dystrophin-associated proteins in the heart accounted for the severe cardiomyopathy in this family. The mutation present in these males selectively affects dystrophin expression in the heart; this could be secondary to the removal of cardiac-specific regulatory sequences. This family may represent the first example of a mutation specifically affecting the cardiac expression of a gene, present physiologically in both the skeletal and cardiac muscles.

Serum induction of MEF2/RSRF expression in vascular myocytes is mediated at the level of translation

Vascular smooth muscle cells (VSMCs) reversibly coordinate the expression of VSMC-specific genes and the genes required for cell cycle progression. Here we demonstrate that isoforms of the MEF2/RSRF transcription factor are expressed in VSMCs and in vascular tissue. The MEF2A DNA-binding activity was upregulated when quiescent VSMCs were stimulated to proliferate with serum mitogens. The serum-induction of MEF2A DNA-binding activity occurred approximately 4 h following serum activation, and this correlated with an increase in the level of MEF2A protein without changes in the level of MEF2A mRNA or protein stability. These results indicate that MEF2A induction by serum is regulated at the level of translation.

A novel initiator regulates expression of the nontissue-specific helix-loop-helix gene ME1

The mouse ME1 gene (HEB, REB and GE1, homologues in human, rat and chick, respectively) is a member of the nontissue-specific helix-loop-helix (HLH) gene family that includes E2A, E2-2 and Drosophila daughterless. We have examined the factors that control ME1 gene expression. ME1 is a single copy gene that spans > or = 150 kb of DNA and contains > 10 exons. Transcription was directed by an unusual initiator element that contained a 13 bp poly d(A) tract flanked by palindromic and inverted repeat sequences. Both RNase protection and primer extension analyses mapped the ME1 transcriptional start site to the center of the 13 bp poly d(A) tract. The ME1 initiator and its proximal sequences were required for promoter activity, supported basal levels of transcription, and contributed to cell type-specific gene expression. Other cis-elements utilized by the TATA-less ME1 promoter included a cluster of Sp1 response elements, E-boxes and a strong repressor. Collectively, our results suggest that the ME1 initiator and other cis-elements in the proximal promoter play an important role in regulating ME1 gene expression.

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.

Fast-muscle-specific expression of human aldolase A transgenes

The expression of the human aldolase A gene is controlled by three alternative promoters. In transgenic mice, pN and pH are active in all tissues whereas pM is activated specifically in adult muscles composed mainly of fast, glycolytic fibers. To detect potential regulatory regions involved in the fast-muscle-specific activation of pM, we analyzed DNase I hypersensitivity in a 4.3-kbp fragment from the 5' end of the human aldolase A gene. Five hypersensitive sites were located near the transcription initiation site of each promoter in those transgenic-mouse tissues in which the corresponding promoter was active. Only one muscle-specific hypersensitive site was detected, mapping near pM. To functionally delimit the elements required for muscle-specific activity of pM, we performed a deletion analysis of the aldolase A 5' region in transgenic mice. Our results show that a 280-bp fragment containing 235 bp of pM proximal upstream sequences together with the noncoding M exon is sufficient for tissue-specific expression of pM. When a putative MEF-2-binding site residing in this proximal pM region is mutated, pM is still active and no change in its tissue specificity is detected. Furthermore, we observed a modulation of pM activity by elements lying further upstream and downstream from pM. Interestingly, pM was expressed in a tissue-specific way in all transgenic mice in which the 280-bp region was present (32 lines and six founder animals). This observation led us to suggest that the proximal pM region contains elements that are able to override to some extent the effects of the surrounding chromatin.

Fast-muscle-specific expression of human aldolase A transgenes

The expression of the human aldolase A gene is controlled by three alternative promoters. In transgenic mice, pN and pH are active in all tissues whereas pM is activated specifically in adult muscles composed mainly of fast, glycolytic fibers. To detect potential regulatory regions involved in the fast-muscle-specific activation of pM, we analyzed DNase I hypersensitivity in a 4.3-kbp fragment from the 5' end of the human aldolase A gene. Five hypersensitive sites were located near the transcription initiation site of each promoter in those transgenic-mouse tissues in which the corresponding promoter was active. Only one muscle-specific hypersensitive site was detected, mapping near pM. To functionally delimit the elements required for muscle-specific activity of pM, we performed a deletion analysis of the aldolase A 5' region in transgenic mice. Our results show that a 280-bp fragment containing 235 bp of pM proximal upstream sequences together with the noncoding M exon is sufficient for tissue-specific expression of pM. When a putative MEF-2-binding site residing in this proximal pM region is mutated, pM is still active and no change in its tissue specificity is detected. Furthermore, we observed a modulation of pM activity by elements lying further upstream and downstream from pM. Interestingly, pM was expressed in a tissue-specific way in all transgenic mice in which the 280-bp region was present (32 lines and six founder animals). This observation led us to suggest that the proximal pM region contains elements that are able to override to some extent the effects of the surrounding chromatin.

E-box- and MEF-2-independent muscle-specific expression, positive autoregulation, and cross-activation of the chicken MyoD (CMD1) promoter reveal an indirect regulatory pathway

Members of the MyoD family of gene-regulatory proteins (MyoD, myogenin, myf5, and MRF4) have all been shown not only to regulate the transcription of numerous muscle-specific genes but also to positively autoregulate and cross activate each other's transcription. In the case of muscle-specific genes, this transcriptional regulation can often be correlated with the presence of a DNA consensus in the regulatory region CANNTG, known as an E box. Little is known about the regulatory interactions of the myogenic factors themselves; however, these interactions are thought to be important for the activation and maintenance of the muscle phenotype. We have identified the minimal region in the chicken MyoD (CMD1) promoter necessary for muscle-specific transcription in primary cultures of embryonic chicken skeletal muscle. The CMD1 promoter is silent in primary chick fibroblast cultures and in muscle cell cultures treated with the thymidine analog bromodeoxyuridine. However, CMD1 and chicken myogenin, as well as, to a lesser degree, chicken Myf5 and MRF4, expressed in trans can activate transcription from the minimal CMD1 promoter in these primary fibroblast cultures. Here we show that the CMD1 promoter contains numerous E-box binding sites for CMD1 and the other myogenic factors, as well as a MEF-2 binding site. Surprisingly, neither muscle-specific and the other myogenic factors, as well as a MEF-2 binding site. Surprisingly, neither muscle-specific expression, autoregulation, or cross activation depends upon the presence of of these E-box or MEF-2 binding sites in the CMD1 promoter. These results demonstrate that the autoregulation and cross activation of the chicken MyoD promoter through the putative direct binding of the myogenic basic helix-loop-helix regulatory factors is mediated through an indirect pathway that involves unidentified regulatory elements and/or ancillary factors.

E-box- and MEF-2-independent muscle-specific expression, positive autoregulation, and cross-activation of the chicken MyoD (CMD1) promoter reveal an indirect regulatory pathway

Members of the MyoD family of gene-regulatory proteins (MyoD, myogenin, myf5, and MRF4) have all been shown not only to regulate the transcription of numerous muscle-specific genes but also to positively autoregulate and cross activate each other's transcription. In the case of muscle-specific genes, this transcriptional regulation can often be correlated with the presence of a DNA consensus in the regulatory region CANNTG, known as an E box. Little is known about the regulatory interactions of the myogenic factors themselves; however, these interactions are thought to be important for the activation and maintenance of the muscle phenotype. We have identified the minimal region in the chicken MyoD (CMD1) promoter necessary for muscle-specific transcription in primary cultures of embryonic chicken skeletal muscle. The CMD1 promoter is silent in primary chick fibroblast cultures and in muscle cell cultures treated with the thymidine analog bromodeoxyuridine. However, CMD1 and chicken myogenin, as well as, to a lesser degree, chicken Myf5 and MRF4, expressed in trans can activate transcription from the minimal CMD1 promoter in these primary fibroblast cultures. Here we show that the CMD1 promoter contains numerous E-box binding sites for CMD1 and the other myogenic factors, as well as a MEF-2 binding site. Surprisingly, neither muscle-specific and the other myogenic factors, as well as a MEF-2 binding site. Surprisingly, neither muscle-specific expression, autoregulation, or cross activation depends upon the presence of of these E-box or MEF-2 binding sites in the CMD1 promoter. These results demonstrate that the autoregulation and cross activation of the chicken MyoD promoter through the putative direct binding of the myogenic basic helix-loop-helix regulatory factors is mediated through an indirect pathway that involves unidentified regulatory elements and/or ancillary factors.

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.

D-MEF2: a MADS box transcription factor expressed in differentiating mesoderm and muscle cell lineages during Drosophila embryogenesis

The myocyte enhancer factor (MEF) 2 family of transcription factors has been implicated in the regulation of muscle transcription in vertebrates. We have cloned a protein from Drosophila, termed D-MEF2, that shares extensive amino acid homology with the MADS (MCM1, Agamous, Deficiens, and serum-response factor) domains of the vertebrate MEF2 proteins. D-mef2 gene expression is first detected during Drosophila embryogenesis within mesodermal precursor cells prior to specification of the somatic and visceral muscle lineages. Expression of D-mef2 is dependent on the mesodermal determinants twist and snail but independent of the homeobox-containing gene tinman, which is required for visceral muscle and heart development. D-mef2 expression precedes that of the MyoD homologue, nautilus, and, in contrast to nautilus, D-mef2 appears to be expressed in all somatic and visceral muscle cell precursors. Its temporal and spatial expression patterns suggest that D-mef2 may play an important role in commitment of mesoderm to myogenic lineages.

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.

A novel myogenic regulatory circuit controls slow/cardiac troponin C gene transcription in skeletal muscle

The slow/cardiac troponin C (cTnC) gene is expressed in three distinct striated muscle lineages: cardiac myocytes, embryonic fast skeletal myotubes, and adult slow skeletal myocytes. We have reported previously that cTnC gene expression in cardiac muscle is regulated by a cardiac-specific promoter/enhancer located in the 5' flanking region of the gene (bp -124 to +1). In this report, we demonstrate that the cTnC gene contains a second distinct and independent transcriptional enhancer which is located in the first intron. This second enhancer is skeletal myotube specific and is developmentally up-regulated during the differentiation of myoblasts to myotubes. This enhancer contains three functionally important nuclear protein binding sites: a CACCC box, a MEF-2 binding site, and a previously undescribed nuclear protein binding site, designated MEF-3, which is also present in a large number of skeletal muscle-specific transcriptional enhancers. Unlike most skeletal muscle-specific transcriptional regulatory elements, the cTnC enhancer does not contain a consensus binding site (CANNTG) for the basic helix-loop-helix (bHLH) family of transcription factors and does not directly bind MyoD-E12 protein complexes. Despite these findings, the cTnC enhancer can be transactivated by overexpression of the myogenic bHLH proteins, MyoD and myogenin, in C3H10T1/2 (10T1/2) cells. Electrophoretic mobility shift assays demonstrated changes in the patterns of MEF-2, CACCC, and MEF-3 DNA binding activities following the conversion of 10T1/2 cells into myoblasts and myotubes by stable transfection with a MyoD expression vector. In particular, MEF-2 binding activity was up-regulated in 10T1/2 cells stably transfected with a MyoD expression vector only after these cells fused and differentiated into skeletal myotubes. Taken together, these results demonstrated that distinct lineage-specific transcriptional regulatory elements control the expression of a single myofibrillar protein gene in fast skeletal and cardiac muscle. In addition, they show that bHLH transcription factors can indirectly transactivate the expression of some muscle-specific genes.

A Mef2 gene that generates a muscle-specific isoform via alternative mRNA splicing

Members of the myocyte-specific enhancer-binding factor 2 (MEF2) family of transcription factors bind a conserved A/T-rich sequence in the control regions of numerous muscle-specific genes. Mammalian MEF2 proteins have been shown previously to be encoded by three genes, Mef2, xMef2, and Mef2c, each of which gives rise to multiple alternatively spliced transcripts. We describe the cloning of a new member of the MEF2 family from mice, termed MEF2D, which shares extensive homology with other MEF2 proteins but is the product of a separate gene. MEF2D binds to and activates transcription through the MEF2 site and forms heterodimers with other members of the MEF2 family. Deletion mutations show that the carboxyl terminus of MEF2D is required for efficient transactivation. MEF2D transcripts are widely expressed, but alternative splicing of MEF2D transcripts gives rise to a muscle-specific isoform which is induced during myoblast differentiation. The mouse Mef2, Mef2c, and Mef2d genes map to chromosomes 7, 13, and 3, respectively. The complexity of the MEF2 family of regulatory proteins provides the potential for fine-tuning of transcriptional responses as a consequence of combinatorial interactions among multiple MEF2 isoforms encoded by the four Mef2 genes.

A novel myogenic regulatory circuit controls slow/cardiac troponin C gene transcription in skeletal muscle

The slow/cardiac troponin C (cTnC) gene is expressed in three distinct striated muscle lineages: cardiac myocytes, embryonic fast skeletal myotubes, and adult slow skeletal myocytes. We have reported previously that cTnC gene expression in cardiac muscle is regulated by a cardiac-specific promoter/enhancer located in the 5' flanking region of the gene (bp -124 to +1). In this report, we demonstrate that the cTnC gene contains a second distinct and independent transcriptional enhancer which is located in the first intron. This second enhancer is skeletal myotube specific and is developmentally up-regulated during the differentiation of myoblasts to myotubes. This enhancer contains three functionally important nuclear protein binding sites: a CACCC box, a MEF-2 binding site, and a previously undescribed nuclear protein binding site, designated MEF-3, which is also present in a large number of skeletal muscle-specific transcriptional enhancers. Unlike most skeletal muscle-specific transcriptional regulatory elements, the cTnC enhancer does not contain a consensus binding site (CANNTG) for the basic helix-loop-helix (bHLH) family of transcription factors and does not directly bind MyoD-E12 protein complexes. Despite these findings, the cTnC enhancer can be transactivated by overexpression of the myogenic bHLH proteins, MyoD and myogenin, in C3H10T1/2 (10T1/2) cells. Electrophoretic mobility shift assays demonstrated changes in the patterns of MEF-2, CACCC, and MEF-3 DNA binding activities following the conversion of 10T1/2 cells into myoblasts and myotubes by stable transfection with a MyoD expression vector. In particular, MEF-2 binding activity was up-regulated in 10T1/2 cells stably transfected with a MyoD expression vector only after these cells fused and differentiated into skeletal myotubes. Taken together, these results demonstrated that distinct lineage-specific transcriptional regulatory elements control the expression of a single myofibrillar protein gene in fast skeletal and cardiac muscle. In addition, they show that bHLH transcription factors can indirectly transactivate the expression of some muscle-specific genes.

A Mef2 gene that generates a muscle-specific isoform via alternative mRNA splicing

Members of the myocyte-specific enhancer-binding factor 2 (MEF2) family of transcription factors bind a conserved A/T-rich sequence in the control regions of numerous muscle-specific genes. Mammalian MEF2 proteins have been shown previously to be encoded by three genes, Mef2, xMef2, and Mef2c, each of which gives rise to multiple alternatively spliced transcripts. We describe the cloning of a new member of the MEF2 family from mice, termed MEF2D, which shares extensive homology with other MEF2 proteins but is the product of a separate gene. MEF2D binds to and activates transcription through the MEF2 site and forms heterodimers with other members of the MEF2 family. Deletion mutations show that the carboxyl terminus of MEF2D is required for efficient transactivation. MEF2D transcripts are widely expressed, but alternative splicing of MEF2D transcripts gives rise to a muscle-specific isoform which is induced during myoblast differentiation. The mouse Mef2, Mef2c, and Mef2d genes map to chromosomes 7, 13, and 3, respectively. The complexity of the MEF2 family of regulatory proteins provides the potential for fine-tuning of transcriptional responses as a consequence of combinatorial interactions among multiple MEF2 isoforms encoded by the four Mef2 genes.

Positive regulatory elements (HF-1a and HF-1b) and a novel negative regulatory element (HF-3) mediate ventricular muscle-specific expression of myosin light-chain 2-luciferase fusion genes in transgenic mice

The cardiac myosin light-chain 2v (MLC-2v) gene has served as a model system to identify the pathways which restrict the expression of cardiac muscle genes to particular chambers of the heart during cardiogenesis. To identify the critical cis regulatory elements which mediate ventricular chamber-specific expression of the MLC-2v gene in the in vivo context, a series of transgenic mice which harbor mutations in putative MLC-2 cis regulatory elements in a 250-bp MLC-2-luciferase fusion gene which is expressed in a ventricular chamber-specific fashion in transgenic mice were generated. These studies demonstrate that both components of HF-1 (HF-1a and HF-1b/MEF-2) are required to maintain ventricular chamber-specific expression and function as positive regulatory elements. Mutations in another conserved element (HF-2) are without statistically significant effect on ventricular chamber expression. Transgenics harboring mutations in the E-box site also displayed significant upregulation of reporter activity in the soleus, gastrocnemius, and uterus, with a borderline effect on expression in liver. Mutations in another conserved element (HF-3) result in a marked (> 75-fold) upregulation of the luciferase reporter activity in the soleus muscle of multiple independent or transgenic founders. Since the HF-3 mutations appeared to have only a marginal effect on luciferase reporter activity in liver tissue, HF-3 appears to function as a novel negative regulatory element to primarily suppress expression in muscle tissues. Thus, a combination of positive (HF-1a/HF-1b) and negative (E-box and HF-3) regulatory elements appear to be required to maintain ventricular chamber-specific expression in the in vivo context.

Positive regulatory elements (HF-1a and HF-1b) and a novel negative regulatory element (HF-3) mediate ventricular muscle-specific expression of myosin light-chain 2-luciferase fusion genes in transgenic mice

The cardiac myosin light-chain 2v (MLC-2v) gene has served as a model system to identify the pathways which restrict the expression of cardiac muscle genes to particular chambers of the heart during cardiogenesis. To identify the critical cis regulatory elements which mediate ventricular chamber-specific expression of the MLC-2v gene in the in vivo context, a series of transgenic mice which harbor mutations in putative MLC-2 cis regulatory elements in a 250-bp MLC-2-luciferase fusion gene which is expressed in a ventricular chamber-specific fashion in transgenic mice were generated. These studies demonstrate that both components of HF-1 (HF-1a and HF-1b/MEF-2) are required to maintain ventricular chamber-specific expression and function as positive regulatory elements. Mutations in another conserved element (HF-2) are without statistically significant effect on ventricular chamber expression. Transgenics harboring mutations in the E-box site also displayed significant upregulation of reporter activity in the soleus, gastrocnemius, and uterus, with a borderline effect on expression in liver. Mutations in another conserved element (HF-3) result in a marked (> 75-fold) upregulation of the luciferase reporter activity in the soleus muscle of multiple independent or transgenic founders. Since the HF-3 mutations appeared to have only a marginal effect on luciferase reporter activity in liver tissue, HF-3 appears to function as a novel negative regulatory element to primarily suppress expression in muscle tissues. Thus, a combination of positive (HF-1a/HF-1b) and negative (E-box and HF-3) regulatory elements appear to be required to maintain ventricular chamber-specific expression in the in vivo context.