M-CAT binding factor, a novel trans-acting factor governing muscle-specific transcription

DNA-binding mechanism of the Hippo pathway transcription factor TEAD4

TEA domain (TEAD) family transcription factors are key regulators in development, tissue homeostasis and cancer progression. TEAD4 acts as a critical downstream effector of the evolutionarily conserved Hippo signaling pathway. The well-studied oncogenic protein YAP forms a complex with TEAD4 to regulate gene transcription; so does the tumor suppressor VGLL4. Although it is known that TEAD proteins can bind promoter regions of target genes through the TEA domain, the specific and detailed mechanism of DNA recognition by the TEA domain remains partially understood. Here, we report the crystal structure of TEAD4 TEA domain in complex with a muscle-CAT DNA element. The structure revealed extensive interactions between the TEA domain and the DNA duplex involving both the major and minor grooves of DNA helix. The DNA recognition helix, α3 helix, determines the specificity of the TEA domain binding to DNA sequence. Structure-guided biochemical analysis identified two major binding sites on the interface of the TEA domain-DNA complex. Mutation of TEAD4 at either site substantially decreases its occupancy on the promoter region of target genes, and largely impaired YAP-induced TEAD4 transactivation and target gene transcription, leading to inhibition of growth and colony formation of gastric cancer cell HGC-27. Collectively, our work provides a structural basis for understanding the regulatory mechanism of TEAD-mediated gene transcription.

A Potential Structural Switch for Regulating DNA-Binding by TEAD Transcription Factors

TEA domain (TEAD) transcription factors are essential for the normal development of eukaryotes and are the downstream effectors of the Hippo tumor suppressor pathway. Whereas our earlier work established the three-dimensional structure of the highly conserved DNA-binding domain using solution NMR spectroscopy, the structural basis for regulating the DNA-binding activity remains unknown. Here, we present the X-ray crystallographic structure and activity of a TEAD mutant containing a truncated L1 loop, ΔL1 TEAD DBD. Unexpectedly, the three-dimensional structure of the ΔL1 TEAD DBD reveals a helix-swapped homodimer wherein helix 1 is swapped between monomers. Furthermore, each three-helix bundle in the domain-swapped dimer is a structural homolog of MYB-like domains. Our investigations of the DNA-binding activity reveal that although the formation of the three-helix bundle by the ΔL1 TEAD DBD is sufficient for binding to an isolated M-CAT-like DNA element, multimeric forms are deficient for cooperative binding to tandemly duplicated elements, indicating that the L1 loop contributes to the DNA-binding activity of TEAD. These results suggest that switching between monomeric and domain-swapped forms may regulate DNA selectivity of TEAD proteins.

Members of the TEAD family of transcription factors regulate the expression of Myf5 in ventral somitic compartments

The transcriptional regulation of the Mrf4/Myf5 locus depends on a multitude of enhancers that, in equilibria with transcription balancing sequences and the promoters, regulate the expression of the two genes throughout embryonic development and in the adult. Transcription in a particular set of muscle progenitors can be driven by the combined outputs of several enhancers that are not able to recapitulate the entire expression pattern in isolation, or by the action of a single enhancer the activity of which in isolation is equivalent to that within the context of the locus. We identified a new enhancer element of this second class, ECR111, which is highly conserved in all vertebrate species and is necessary and sufficient to drive Myf5 expression in ventro-caudal and ventro-rostral somitic compartments in the mouse embryo. EMSA analyses and data obtained from binding-site mutations in transgenic embryos show that a binding site for a TEA Domain (TEAD) transcription factor is essential for the function of this new enhancer, while ChIP assays show that at least two members of the family of transcription factors bind to it in vivo.

Vascular Endothelial Growth Factor Increases Endothelial Nitric Oxide Synthase Transcription In Huvec Cells

Although it is known that VEGF increases eNOS protein, the mechanisms responsible remain unclear. To determine if VEGF alters eNOS transcription, human umbilical vein endothelial cells were transfected with reporters under the control of the eNOS promoter and stimulated with VEGF₁₆₅. VEGF significantly increased eNOS-mRNA after 2 hours exposure. VEGF significantly increased eNOS reporter activity as early as one hour (268±32%), but this increase returned to baseline after 6 hours. Using deletion constructs, the VEGF response region was initially localized to within the -722/-494 region. GMSA indicated that VEGF increased DNA binding to both a cAMP-like and AP1-like response elements. Site-specific mutations and heterologous constructs indicated that the site centered at AP1-like site was both necessary and sufficient to meditate VEGF transcriptional activation. These results indicate that VEGF rapidly activates eNOS transcription prior to a rise eNOS-mRNA, an effect mediated by a cis-trans interaction localized to an AP1-like site within the eNOS promoter.

Differential expression of novH and CTGF in human glioma cell lines

Aims-(1) To investigate the expression in human derived glioblastoma cell lines of two structurally related genes, novH (nephroblastoma overexpressed gene) and CTGF (connective tissue growth factor), which encode putative insulin-like growth factor binding proteins of a novel type. (2) To investigate whether the same transcription factors regulate CTGF and novH expression.Methods-Expression of novH and CTGF was analysed in 24 glioblastoma derived cell lines by northern blotting. The CTGF promoter region was characterised by nucleotide sequencing, RNase protection experiments, by transient transfections, and CAT assays.Results-CTGF and novH mRNA levels differed in the glioma cell lines studied. NovH and CTGF genes were not co-expressed in all cell lines. The CTGF promoter region was highly conserved compared with the corresponding region in the mouse (FISP12) and exhibited in vitro transcriptional activity.Conclusions-Although the coding regions of novH and CTGF are highly homologous, their promoter regions are substantially different, suggesting that these two genes may be regulated by different mechanisms. Considering that novH and CTGF are likely to be, respectively, negative and positive regulators of growth and that some glioma cell lines expressing novH are not tumorigenic, expression of these two genes might represent a key element in determining the stage of differentiation or the malignant potential, or both, of some tumour cell lines.

Transcription enhancer factor 3 (TEF3) mediates the expression of Down syndrome candidate region 1 isoform 1 (DSCR1-1L) in endothelial cells

The Down syndrome candidate region 1 gene (DSCR1) can be expressed as four isoforms, one of which is the well-studied isoform 4 (DSCR1-4) that is induced by VEGF-A(165) to provide a negative feedback loop in the VEGF-A(165)-induced angiogenesis. We reported previously that another DSCR1 isoform, DSCR1-1L, was also up-regulated by VEGF-A(165) in cultured endothelial cells and in several in vivo models of pathological angiogenesis and that different from DSCR1-4, DSCR1-1L overexpression alone induced cultured endothelial cell proliferation and promoted angiogenesis in Matrigel assays. It was reported recently that tumor growth was greatly repressed in DSCR1 knock-out mice. Although DSCR1-4 transcription was primarily regulated by NFAT, the mechanism regulating DSCR1-1L expression was still unknown. We developed human DSCR1-1L promoter-driven luciferase system and found that deletion of a putative conserved M-CAT site located 1426-bp upstream of the translation start site blunted promoter activity. We further showed that knockdown of TEF3, not other members of TEF family inhibited VEGF-A(165)-induced DSCR1-1L expression. We also demonstrated that TEF3 directly interacted with the putative M-CAT site in the DSCR1-1L promoter in vitro and in vivo. Finally, overexpression of TEF3 isoform 1, not isoform 3, in HUVEC was sufficient to induce DSCR1-1L expression even in the absence of VEGF-A(165) stimulation. Taken together, we elucidated a novel function of transcriptional factor TEF3. TEF3 was required for DSCR1-1L expression through binding to the M-CAT site in its promoter and could be an attractive target for anti-angiogenesis therapy.

Characterization of cis-regulatory elements and transcription factor binding: gel mobility shift assay

To understand how cardiac gene expression is regulated, the identification and characterization of cis-regulatory elements and their trans-acting factors by gel mobility shift assay (GMSA) or gel retardation assay are essential and common steps. In addition to providing a general protocol for GMSA, this chapter describes some applications of this assay to characterize cardiac-specific and ubiquitous trans-acting factors bound to regulatory elements [novel TCTG(G/C) direct repeat and A/T-rich region] of the rat cardiac troponin T promoter. In GMSA, the specificity of the binding of trans-acting factor to labeled DNA probe should be verified by the addition of unlabeled probe in the reaction mixture. The migratory property of DNA-protein complexes formed by protein extracts prepared from different tissues can be compared to determine the tissue specificity of trans-acting factors. GMSA, coupled with specific antibody to trans-acting factor (antibody supershift assay), is used to identify proteins present in the DNA-protein complex. The gel-shift competition assay with an unlabeled probe containing a slightly different sequence is a powerful technique used to assess the sequence specificity and relative binding affinity of a DNA-protein interaction. GMSA with SDS-PAGE fractionated proteins allows for the determination of the apparent molecular mass of bound trans-acting factor.

Insights into transcription enhancer factor 1 (TEF-1) activity from the solution structure of the TEA domain

Transcription enhancer factor 1 is essential for cardiac, skeletal, and smooth muscle development and uses its N-terminal TEA domain (TEAD) to bind M-CAT elements. Here, we present the first structure of TEAD and show that it is a three-helix bundle with a homeodomain fold. Structural data reveal how TEAD binds DNA. Using structure-function correlations, we find that the L1 loop is essential for cooperative loading of TEAD molecules on to tandemly duplicated M-CAT sites. Furthermore, using a microarray chip-based assay, we establish that known binding sites of the full-length protein are only a subset of DNA elements recognized by TEAD. Our results provide a model for understanding the regulation of genome-wide gene expression during development by TEA/ATTS family of transcription factors.

In Vivo beta-adrenergic activation of atrial natriuretic factor (ANF) reporter expression

Isoproterenol (ISO) infusion increases ANF-mRNA levels and control of ANF expression lies at the level of transcription. In neonatal cardiomyocytes, previous investigations determined that the -125 to -100 region of the rat ANF 5' flanking region contained cis-elements critical for control of ISO induced ANF transcription. However, it is unclear if these same cis-elements regulate ANF transcription in vivo. To examine this question, reporter plasmids containing the ANF 5' flanking/promoter region were injected directly into the left ventricle. Following a recovery period, osmotic pumps were implanted to infuse vehicle or ISO (0.2 or 2.0 mg/kg/d). ISO significantly (p < .05) increased the LV/BW ratio in a dose dependent, but not a time dependent manner. ISO significantly (p < .05) increased ANF reporter expression in both a dose-dependent and time dependent manner. Injections into the midwall of the LV or into the apex did not lead to significant differences in ISO-induced ANF reporter expression. Using site-specific mutations of ANF reporter constructs, comparisons were made of ISO induced ANF transcription in vitro in neonatal cardiomyocytes and in vivo in the adult heart. Cis-elements critical for ISO activation in cultured cardiomyocytes were not essential for the increased expression of the ANF reporters in vivo. The results indicate that distinct differences in ANF transcriptional regulation exist in vivo in the adult heart as compared with neonatal cardiomyocytes, and suggest the recruitment of other signaling pathways beyond adrenergic-receptor mediated pathways.

Fgfr4 is required for effective muscle regeneration in vivo. Delineation of a MyoD-Tead2-Fgfr4 transcriptional pathway

Fgfr4 has been shown to be important for appropriate muscle development in chick limb buds; however, Fgfr4 null mice show no phenotype. Here, we show that staged induction of muscle regeneration in Fgfr4 null mice becomes highly abnormal at the time point when Fgfr4 is normally expressed. By 7 days of regeneration, differentiation of myotubes became poorly coordinated and delayed by both histology and embryonic myosin heavy chain staining. By 14 days much of the muscle was replaced by fat and calcifications. To begin to dissect the molecular pathways involving Fgfr4, we queried the promoter sequences for transcriptional factor binding sites and tested candidate regulators in a 27-time point regeneration series. The Fgfr4 promoter region contained a Tead protein binding site (M-CAT 5'-CATTCCT-3'), and Tead2 showed induction during regeneration commensurate with Fgfr4 regulation. Co-transfection of Tead2 and Fgfr4 promoter reporter constructs into C2C12 myotubes showed Tead2 to activate Fgfr4, and mutation of the M-CAT motif in the Fgfr4 promoter abolished these effects. Immunostaining for Tead2 showed timed expression in myotube nuclei consistent with the mRNA data. Query of the expression timing and genomic sequences of Tead2 suggested direct regulation by MyoD, and consistent with this, MyoD directly bound to two strong E-boxes in the first intron of Tead2 by chromatin immunoprecipitation assay. Moreover, co-transfection of MyoD and Tead2 intron reporter constructs into 10T1/2 cells activated reporter activity in a dose-dependent manner. This activation was greatly reduced when the two E-boxes were mutated. Our data suggest a novel MyoD-Tead2-Fgfr4 pathway important for effective muscle regeneration.

Poly(ADP-ribose) polymerase binds with transcription enhancer factor 1 to MCAT1 elements to regulate muscle-specific transcription

Striated muscle-specific expression of the cardiac troponin T (cTNT) gene is mediated through two MCAT elements that act via binding of transcription enhancer factor 1 (TEF-1) to the MCAT core motifs and binding of an auxiliary protein to nucleotides flanking the 5' side of the core motif. Using DNA-protein and protein-protein binding experiments, we identified a 140-kDa polypeptide that bound both the muscle-specific flanking sequences of the most distal MCAT1 element and TEF-1. Screening of an expression library with the MCAT1 element yielded a cDNA encoding a truncated form of poly(ADP-ribose) polymerase (PARP). Endogenous PARP from embryonic tissue nuclear extracts migrated as a 140-kDa protein. Recombinant full-length PARP preferentially bound the wild-type MCAT1 element and was shown to physically interact with TEF-1. In addition, endogenous TEF-1 could be coimmunoprecipitated with PARP from extracts of primary skeletal muscle cells. Recombinant PARP was able to ADP-ribosylate TEF-1 in vitro. Inhibition of the enzymatic activity of PARP repressed expression of an MCAT1-dependent reporter in transiently transfected primary muscle cells. Together, these data implicate PARP as the auxiliary protein that binds with TEF-1 to the MCAT1 element to provide muscle-specific gene transcription.

Troponin T: genetics, properties and function

Troponin T (TnT) is present in striated muscle of vertebrates and invertebrates as a group of homologous proteins with molecular weights usually in the 31-36 kDa range. It occupies a unique role in the regulatory protein system in that it interacts with TnC and TnI of the troponin complex and the proteins of the myofibrillar thin filament, tropomyosin and actin. In the myofibril the molecule is about 18 nm long and for much its length interacts with tropomyosin. The ability of TnT to form a complex with tropomyosin is responsible for locating the troponin complex with a periodicity of 38.5 nm along the thin filament of the myofibril. In addition to it structural role, TnT has the important function of transforming the TnI-TnC complex into a system, the inhibitory activity of which, on the tropomyosin-actomyosin MgATPase of the myofibril, becomes sensitive to calcium ions. Different genes control the expression of TnT in fast skeletal, slow skeletal and cardiac muscles. In all muscles, and particularly in fast skeletal, alternative splicing of mRNA produces a series of isoforms in a developmentally regulated manner. In consequence TnT exists in many more isoforms than any of the other thin filament proteins, the TnT superfamily. Despite the general homology of TnT isoforms, this alternative splicing leads to variable regions close to the N- and C-termini. As the isoforms have slightly different effects on the calcium sensitivity of the actomyosin MgATPase, modulation of the contractile response to calcium can occur during development and in different muscle types. TnT has recently aroused clinical interest in its potential for detecting myocardial damage and the association of mutations in the cardiac isoform with hypertrophic cardiomyopathy.

Transcription enhancer factor 1 interacts with a basic helix-loop-helix zipper protein, Max, for positive regulation of cardiac alpha-myosin heavy-chain gene expression

The M-CAT binding factor transcription enhancer factor 1 (TEF-1) has been implicated in the regulation of several cardiac and skeletal muscle genes. Previously, we identified an E-box-M-CAT hybrid (EM) motif that is responsible for the basal and cyclic AMP-inducible expression of the rat cardiac alpha-myosin heavy chain (alpha-MHC) gene in cardiac myocytes. In this study, we report that two factors, TEF-1 and a basic helix-loop-helix leucine zipper protein, Max, bind to the alpha-MHC EM motif. We also found that Max was a part of the cardiac troponin T M-CAT-TEF-1 complex even when the DNA template did not contain an apparent E-box binding site. In the protein-protein interaction assay, a stable association of Max with TEF-1 was observed when glutathione S-transferase (GST)-TEF-1 or GST-Max was used to pull down in vitro-translated Max or TEF-1, respectively. In addition, Max was coimmunoprecipitated with TEF-1, thus documenting an in vivo TEF-1-Max interaction. In the transient transcription assay, overexpression of either Max or TEF-1 resulted a mild activation of the alpha-MHC-chloramphenicol acetyltransferase (CAT) reporter gene at lower concentrations and repression of this gene at higher concentrations. However, when Max and TEF-1 expression plasmids were transfected together, the repression mediated by a single expression plasmid was alleviated and a three- to fourfold transactivation of the alpha-MHC-CAT reporter gene was observed. This effect was abolished once the EM motif in the promoter-reporter construct was mutated, thus suggesting that the synergistic transactivation function of the TEF-1-Max heterotypic complex is mediated through binding of the complex to the EM motif. These results demonstrate a novel association between Max and TEF-1 and indicate a positive cooperation between these two factors in alpha-MHC gene regulation.

A muscle-specific enhancer within intron 1 of the human dystrophin gene is functionally dependent on single MEF-1/E box and MEF-2/AT-rich sequence motifs

In previous studies we have described a 5.0 kb Hin dIII fragment downstream of muscle exon 1 that exhibits properties consistent with a muscle-specific transcriptional enhancer. The goal of this study has been to identify the sequence elements responsible for muscle-specific enhancer activity. Functional studies indicated that this enhancer is active in pre- and post-differentiated H9C2(2-1) myoblasts but functions poorly in L6 and C2C12 myotubes. The core enhancer region was delimited to a 195 bp Spe I- Acc I fragment and sequence analysis identified three MEF-1/E box and two MEF-2/AT-rich motifs as potential muscle-specific regulatory domains. EMSA competition and DNase footprinting indicated that sequences within a 30 bp region containing single adjoining MEF-1/E box and MEF-2/AT-rich motifs are target binding sites for trans -acting factors expressed in H9C2(2-1) myotubes but not in L6 or C2C12 myotubes. Site-specific mutations within these motifs resulted in a significant reduction in enhancer activity in H9C2(2-1) myotubes. These results suggest that the mechanisms governing DMD gene expression in muscle are similar to those identified in other muscle-specific genes. However, the myogenic profile of enhancer activity and trans -acting factor binding suggests a more specialized role for this enhancer that is consistent with its potential involvement in dystrophin gene regulation in cardiac muscle.

Regulation of the rat cardiac troponin I gene by the transcription factor GATA-4

Troponin I is a thin-filament contractile protein expressed in striated muscle. There are three known troponin I genes which are expressed in a muscle-fibre-type-specific manner in mature animals. Although the slow skeletal troponin I isoform is expressed in fetal and neonatal heart, the cardiac isoform is restricted in its expression to the myocardium at all developmental stages. To study the regulation of this cardiac-specific and developmentally regulated gene in vitro, the rat cardiac troponin I gene was cloned. Transient transfection assays were performed with troponin I-luciferase fusion plasmids to characterize the regulatory regions of the gene. Proximal regions of the upstream sequence were sufficient to support high levels of expression of the reporter gene in cardiocytes and relatively low levels in other cell types. The highest luciferase activity in the cardiocytes was noted with a plasmid that included the region spanning -896 to +45 of the troponin I genomic sequence. Co-transfection of GATA-4, a recently identified cardiac transcription factor, with troponin I-luciferase constructs permitted high levels of luciferase expression in non-cardiac cells. Electrophoretic mobility-shift assays demonstrated specific binding of GATA-4 to oligonucleotides representative of multiple sites of the troponin I sequence. Mutation of a proximal GATA-4 DNA-binding site decreased transcriptional activation in transfected cardiocytes. These results indicate that the proximal cardiac troponin I sequence is sufficient to support high levels of cardiac-specific gene expression and that the GATA-4 transcription factor regulates troponin I-luciferase expression in vitro.

Flanking sequences modulate the cell specificity of M-CAT elements

M-CAT elements mediate both muscle-specific and non-muscle-specific transcription. We used artificial promoters to dissect M-CAT elements derived from the cardiac troponin T promoter, whose regulation is highly striated muscle specific. We show that muscle-specific M-CAT-dependent expression requires two distinct components: the core heptameric M-CAT motif (5'-CATTCCT-3'), which constitutes the canonical binding site for TEF-1-related proteins, and specific sequences immediately flanking the core motif that bind an additional factor(s). These factors are found in higher-order M-CAT DNA-protein complexes with TEF-1 proteins. Non-muscle-specific promoters are produced when the sequences flanking the M-CAT motif are removed or modified to match those of non-muscle-specific promoters such as the simian virus 40 promoter. Moreover, a mutation of the 5'-flanking region of the cardiac troponin T M-CAT-1 element upregulated expression in nonmuscle cells. That mutation also disrupts a potential E box that apparently does not bind myogenic basic helix-loop-helix proteins. We propose a model in which M-CAT motifs are potentially active in many cell types but are modulated through protein binding to specific flanking sequences. In nonmuscle cells, these flanking sequences bind a factor(s) that represses M-CAT-dependent activity. In muscle cells, on the other hand, the factor(s) binding to these flanking sequences contributes to both the cell specificity and the overall transcriptional strength of M-CAT-dependent promoters.

Structure and tissue-specific expression of the Drosophila melanogaster organellar-type Ca(2+)-ATPase gene

A 14 kb genomic clone covering the organellar-type Ca(2+)-ATPase gene of Drosophila melanogaster has been isolated and characterized. The sequence of a 7132 bp region extending from 1.1 kb 5' upstream of the initiation ATG codon over the polyadenylation signal at the 3' end has been determined. The gene consists of nine exons including one with an exceptional size of 2172 bp representing 72% of the protein coding region. Introns are relatively small (< 100 bp) except for the 3' intron which has a size of 2239 bp, an exceptionally large size among Drosophila introns. Five of the introns are in the same positions in Drosophila, Artemia and rabbit SERCA1 Ca(2+)-ATPase genes. There is only one organellar-type Ca(2+)-ATPase gene in the Drosophila genome, as was shown by Southern-blot analysis [Váradi, Gilmore-Hebert and Benz (1989) FEBS Lett. 258, 203-207] and by chromosomal localization [Magyar and Váradi (1990) Biochem. Biophys. Res. Commun. 173, 872-877]. Primer extension and S1-nuclease assays revealed a potential transcription initiation site 876 bp upstream of the translation initiation ATG with a TATA-box 23 bp upstream of this site. Analysis of the 5' region of the Drosophila organellar-type Ca(2+)-ATPase gene suggests the presence of potential recognition sequences of various muscle-specific transcription factors and shows a region with remarkable similarity to that in the rabbit SERCA2 gene. The tissue distribution of expression of the organellar-type Ca(2+)-ATPase gene has been studied by in situ RNA-RNA hybridization on microscopic sections. A low mRNA abundance can be detected in each tissue of adult flies, suggesting a housekeeping function for the gene. On the other hand a pronounced tissue specificity of expression has also been found as the organellar-type Ca(2+)-ATPase is expressed at a very high level in cell bodies of the central nervous system and in various muscles.

Cloning and analysis of the promoter region of the rat SM22 alpha gene

We have cloned and sequenced a 1.9 kb fragment of the 5'-upstream sequence of the smooth-muscle-specific gene SM22 alpha. The region cloned consisted of the SM22 alpha promoter, a 65 bp exon containing most of the 5'-untranslated region and 307 bp of the first intron. A 1.5 kb fragment at the 5' end of this sequence was able to drive the expression of a reporter chloramphenicol acetyltransferase (CAT) gene in both vascular smooth-muscle cells and Rat-1 fibroblasts. This promoter region did not contain a consensus TATAA box but contained the sequence TTTAAA 25 bp from the major start site identified by primer extension. Deletion analysis showed that a fragment of the promoter from +65 to -303 was more active in both cell types than the 1.5 kb fragment suggesting that there are silencer sequences in the region 5' to the core promoter. CAT activity was also observed with fragments containing bases +65 to -193 and +65 to -117 in smooth-muscle cells. In contrast with the smooth-muscle cells, no CAT activity was detected in Rat-1 fibroblasts with the smallest two fragments. The residual promoter activity in the smallest fragment of the SM22 alpha promoter tested suggested that, unlike the smooth-muscle alpha-actin promoter, transcription from the SM22 alpha promoter can occur in smooth-muscle cells in the absence of factors binding to CC(A/Trich)6GG (CArG box) or CANNTG (E box) motifs.

EFIA/YB-1 is a component of cardiac HF-1A binding activity and positively regulates transcription of the myosin light-chain 2v gene

Transient assays in cultured ventricular muscle cells and studies in transgenic mice have identified two adjacent regulatory elements (HF-1a and HF-1b/MEF-2) as required to maintain ventricular chamber-specific expression of the myosin light-chain 2v (MLC-2v) gene. A rat neonatal heart cDNA library was screened with an HF-1a binding site, resulting in the isolation of EFIA, the rat homolog of human YB-1. Purified recombinant EFIA/YB-1 protein binds to the HF-1a site in a sequence-specific manner and contacts a subset of the HF-1a contact points made by the cardiac nuclear factor(s). The HF-1a sequence contains AGTGG, which is highly homologous to the inverted CCAAT core of the EFIA/YB-1 binding sites and is found to be essential for binding of the recombinant EFIA/YB-1. Antiserum against Xenopus YB-3 (100% identical in the DNA binding domain and 89% identical in overall amino acid sequence to rat EFIA) can specifically abolish a component of the endogenous HF-1a complex in the rat cardiac myocyte nuclear extracts. In cotransfection assays, EFIA/YB-1 increased 250-bp MLC-2v promoter activity by 3.4-fold specifically in the cardiac cell context and in an HF-1a site-dependent manner. EFIA/YB-1 complexes with an unknown protein in cardiac myocyte nuclear extracts to form the endogenous HF-1a binding activity. Immunocoprecipitation revealed that EFIA/YB-1 has a major associated protein of approximately 30 kDa (p30) in cardiac muscle cells. This study suggests that EFIA/YB-1, together with the partner p30, binds to the HF-1a site and, in conjunction with HF-1b/MEF-2, mediates ventricular chamber-specific expression of the MLC-2v gene.

Negative regulation of the vascular smooth muscle alpha-actin gene in fibroblasts and myoblasts: disruption of enhancer function by sequence-specific single-stranded-DNA-binding proteins

Transcriptional activation and repression of the vascular smooth muscle (VSM) alpha-actin gene in myoblasts and fibroblasts is mediated, in part, by positive and negative elements contained within an approximately 30-bp polypurine-polypyrimidine tract. This region contains binding sites for an essential transcription-activating protein, identified as transcriptional enhancer factor I (TEF-1), and two tissue-restrictive, sequence-specific, single-stranded-DNA-binding activities termed VACssBF1 and VACssBF2. TEF-1 has no detectable single-stranded-DNA-binding activity, while VACssBF1 and VACssBF2 have little, if any, affinity for double-stranded DNA. Site-specific mutagenesis experiments demonstrate that the determinants of VACssBF1 and VACssBF2 binding lie on opposite strands of the DNA helix and include the TEF-1 recognition sequence. Functional analysis of this region reveals that the CCAAT box-binding protein nuclear factor Y (NF-Y) can substitute for TEF-1 in activating VSM alpha-actin transcription but that the TEF-1-binding site is essential for the maintenance of full transcriptional repression. Importantly, replacement of the TEF-1-binding site with that for NF-Y diminishes the ability of VACssBF1 and VACssBF2 to bind to separated single strands. Additional activating mutations have been identified which lie outside of the TEF-1-binding site but which also impair single-stranded-DNA-binding activity. These data support a model in which VACssBF1 and VACssBF2 function as repressors of VSM alpha-actin transcription by stabilizing a local single-stranded-DNA conformation, thus precluding double-stranded-DNA binding by the essential transcriptional activator TEF-1.

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.

An M-CAT binding factor and an RSRF-related A-rich binding factor positively regulate expression of the alpha-cardiac myosin heavy-chain gene in vivo

Cardiac muscle-restricted expression of the alpha-myosin heavy-chain (alpha-MHC) gene is regulated by multiple elements in the proximal enhancer/promoter. Within this region, an M-CAT site and an A-rich site were identified as potential regulatory elements. Site-specific mutations in each site, individually, reduced activity from the wild-type promoter by approximately 85% in the adult rat heart, demonstrating that these sites were positive regulatory elements. alpha-MHC, beta-MHC, and chicken cardiac troponin T (cTnT) M-CAT sites interacted with an M-CAT-binding factor (MCBF) from rat heart nuclear extracts that was immunologically related to transcriptional enhancer factor 1, a factor that binds within the simian virus 40 enhancer. The factor that bound the A-rich region (ARF) was antigenically related to the RSRF family of proteins, ARF was distinct from myocyte-specific enhancer factor 2 (MEF-2) on the basis of DNA-binding specificity and developmental expression. Like MEF-2, ARF DNA-binding activity was present in the heart and brain; however, no ARF activity was detected in extracts from skeletal muscle or C2C12 myotubes. MCBF and ARF DNA-binding activities were developmentally regulated with peak levels in the 1- to 2-day neonatal heart. The activity of both factors increased nearly fivefold in adult rat hearts subjected to a pressure overload. By comparison, the levels of alpha-MHC binding factor 2 did not change during hypertrophy. Binding sites for MCBF and ARF are present in several genes that are upregulated during cardiac hypertrophy. Our results suggest that these factors participate in the alterations in gene expression that occur during cardiac development and hypertrophy.

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.

An M-CAT binding factor and an RSRF-related A-rich binding factor positively regulate expression of the alpha-cardiac myosin heavy-chain gene in vivo

Cardiac muscle-restricted expression of the alpha-myosin heavy-chain (alpha-MHC) gene is regulated by multiple elements in the proximal enhancer/promoter. Within this region, an M-CAT site and an A-rich site were identified as potential regulatory elements. Site-specific mutations in each site, individually, reduced activity from the wild-type promoter by approximately 85% in the adult rat heart, demonstrating that these sites were positive regulatory elements. alpha-MHC, beta-MHC, and chicken cardiac troponin T (cTnT) M-CAT sites interacted with an M-CAT-binding factor (MCBF) from rat heart nuclear extracts that was immunologically related to transcriptional enhancer factor 1, a factor that binds within the simian virus 40 enhancer. The factor that bound the A-rich region (ARF) was antigenically related to the RSRF family of proteins, ARF was distinct from myocyte-specific enhancer factor 2 (MEF-2) on the basis of DNA-binding specificity and developmental expression. Like MEF-2, ARF DNA-binding activity was present in the heart and brain; however, no ARF activity was detected in extracts from skeletal muscle or C2C12 myotubes. MCBF and ARF DNA-binding activities were developmentally regulated with peak levels in the 1- to 2-day neonatal heart. The activity of both factors increased nearly fivefold in adult rat hearts subjected to a pressure overload. By comparison, the levels of alpha-MHC binding factor 2 did not change during hypertrophy. Binding sites for MCBF and ARF are present in several genes that are upregulated during cardiac hypertrophy. Our results suggest that these factors participate in the alterations in gene expression that occur during cardiac development and hypertrophy.

The Aspergillus nidulans abaA gene encodes a transcriptional activator that acts as a genetic switch to control development

The Aspergillus nidulans abaA gene encodes a protein containing an ATTS DNA-binding motif and is required for the terminal stages of conidiophore development. Results from gel mobility shift and protection, missing-contact, and interference footprint assays showed that AbaA binds to the sequence 5'-CATTCY-3', where Y is a pyrimidine, making both major- and minor-groove contacts. Multiple AbaA binding sites are present in the cis-acting regulatory regions of several developmentally controlled structural genes as well as those of the upstream regulatory gene brlA, the downstream regulatory gene wetA, and abaA itself. These cis-acting regulatory regions confer AbaA-dependent transcriptional activation in a heterologous Saccharomyces cerevisiae gene expression system. From these observations, we propose that the AbaA transcription factor establishes a novel set of feedback regulatory loops responsible for determination of conidiophore development.

The Aspergillus nidulans abaA gene encodes a transcriptional activator that acts as a genetic switch to control development

The Aspergillus nidulans abaA gene encodes a protein containing an ATTS DNA-binding motif and is required for the terminal stages of conidiophore development. Results from gel mobility shift and protection, missing-contact, and interference footprint assays showed that AbaA binds to the sequence 5'-CATTCY-3', where Y is a pyrimidine, making both major- and minor-groove contacts. Multiple AbaA binding sites are present in the cis-acting regulatory regions of several developmentally controlled structural genes as well as those of the upstream regulatory gene brlA, the downstream regulatory gene wetA, and abaA itself. These cis-acting regulatory regions confer AbaA-dependent transcriptional activation in a heterologous Saccharomyces cerevisiae gene expression system. From these observations, we propose that the AbaA transcription factor establishes a novel set of feedback regulatory loops responsible for determination of conidiophore development.

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 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.

The MEF-3 motif is required for MEF-2-mediated skeletal muscle-specific induction of the rat aldolase A gene

The rat aldolase A gene contains two alternative promoters and two alternative first exons. The distal promoter M is expressed at a high level only in skeletal muscle. Previous in vitro transfection studies identified the region from -202 to -85 as an enhancer that is responsible for dramatic activation during the differentiation of chicken primary myoblasts. This enhancer contains an A/T-rich sequence resembling the MEF-2 motif, which is an important element of muscle enhancers and promoters. In this study, we demonstrate that the MEF-2 sequence is essential but not sufficient for the activity of the enhancer. Another region required for the activity was recognized by a nuclear factor, tentatively named MAF1. MAF1 was found in both muscle cells and nonmuscle cells, and MAF1 from both cell types was indistinguishable by gel retardation and DNase I footprint experiments. The sequence required for MAF1 binding is very similar to the MEF-3 motif, which is an element of the skeletal muscle-specific enhancer of the cardiac troponin C gene. Because MAF1 and MEF-3 are closely related in both recognition sequence and distribution, MAF1 and MEF-3 probably represent the same nuclear factor which may play an important role in muscle gene transcription. Thus, the muscle-specific induction of the aldolase A gene is governed by muscle-specific MEF-2 and existing MEF-3 (MAF1).

The MEF-3 motif is required for MEF-2-mediated skeletal muscle-specific induction of the rat aldolase A gene

The rat aldolase A gene contains two alternative promoters and two alternative first exons. The distal promoter M is expressed at a high level only in skeletal muscle. Previous in vitro transfection studies identified the region from -202 to -85 as an enhancer that is responsible for dramatic activation during the differentiation of chicken primary myoblasts. This enhancer contains an A/T-rich sequence resembling the MEF-2 motif, which is an important element of muscle enhancers and promoters. In this study, we demonstrate that the MEF-2 sequence is essential but not sufficient for the activity of the enhancer. Another region required for the activity was recognized by a nuclear factor, tentatively named MAF1. MAF1 was found in both muscle cells and nonmuscle cells, and MAF1 from both cell types was indistinguishable by gel retardation and DNase I footprint experiments. The sequence required for MAF1 binding is very similar to the MEF-3 motif, which is an element of the skeletal muscle-specific enhancer of the cardiac troponin C gene. Because MAF1 and MEF-3 are closely related in both recognition sequence and distribution, MAF1 and MEF-3 probably represent the same nuclear factor which may play an important role in muscle gene transcription. Thus, the muscle-specific induction of the aldolase A gene is governed by muscle-specific MEF-2 and existing MEF-3 (MAF1).

Both a ubiquitous factor mTEF-1 and a distinct muscle-specific factor bind to the M-CAT motif of the myosin heavy chain beta gene

The A element, a fourteen base pair sequence in the rabbit myosin heavy chain (HC) beta promoter (-276/-263), contains the M-CAT motif, a cis-acting element found in several muscle-specific genes. The A element is essential for muscle-specific transcription of the myosin HC beta gene. Recently, we have identified both muscle-specific and ubiquitous factors (A1 and A2 factors, respectively) that bind to the A element. Since the sequence of the A element is very similar to the GTIIC motif in the SV40 enhancer, we examined the relationship between A-element-binding factors and a GTIIC binding factor TEF-1, recently isolated from HeLa cells. The GTIIC motif was bound by the A1 and A2 factors in muscle nuclear extracts and competed with the A element for DNA-protein complex formation. Antibody against human TEF-1 'supershifted' the ubiquitous A2 factor-DNA complex, but did not alter the mobility of the muscle-specific A1 factor-DNA complex. We isolated a murine cDNA clone (mTEF-1) from a cardiac cDNA library. The clone is highly homologous to Hela cell TEF-1. The in vitro transcription/translation product of mTEF-1 cDNA bound to the A element, and the DNA binding property of mTEF-1 was identical to that of the A2 factor. Transfection of mTEF-1 cDNA into muscle and non-muscle cells confirmed that mTEF-1 corresponds to A2, but not to A1 factors. The mTEF-1 mRNA is expressed abundantly in skeletal and cardiac muscles, kidney and lung, but it is also expressed at lower levels in other tissues. These results suggest that the M-CAT binding factors consist of two different factors; the ubiquitous A2 is encoded by mTEF-1, but the muscle-specific A1 factor is distinct from mTEF-1.

Analysis of the rabbit cardiac/slow twitch muscle sarcoplasmic reticulum calcium ATPase (SERCA2) gene promoter

The rabbit cardiac/slow twitch muscle sarcoplasmic reticulum (SR) Ca2+ ATPase (SERCA2) gene encodes a Ca2+ transport pump whose expression is regulated during skeletal/cardiac muscle development and by different pathophysiological states of the heart. This study was designed to delineate cis-acting regulatory elements involved in SERCA2 gene expression. A series of unidirectionally deleted fragments of the upstream 1,460 bp SERCA2 promoter were linked to the chloramphenicol acetyltransferase (CAT) reporter gene. Transient DNA transfection experiments performed with these constructs in C2C12 muscle cells and NIH3T3 fibroblasts revealed a 17 bp upstream promoter element (UPE) important for transcription of the SERCA2 gene in skeletal muscle cells. These studies have also identified a strong (muscle specific) negative regulatory region located upstream of nucleotide -658. Gel mobility shift and southwestern analyses using the 17 bp UPE have revealed a specific DNA binding complex referred to as Ca2+ ATPase promoter factor -1 (CaPF1). The binding factor has an approximate M(r) of 43 kDa. Comparison of CaPF1 with known transcription factors suggests that the CaPF1 complex may be a novel DNA-binding transcription factor which plays a role in SERCA2 gene regulation in vivo.

Multiple regulatory elements contribute differentially to muscle creatine kinase enhancer activity in skeletal and cardiac muscle

We have used transient transfections in MM14 skeletal muscle cells, newborn rat primary ventricular myocardiocytes, and nonmuscle cells to characterize regulatory elements of the mouse muscle creatine kinase (MCK) gene. Deletion analysis of MCK 5'-flanking sequence reveals a striated muscle-specific, positive regulatory region between -1256 and -1020. A 206-bp fragment from this region acts as a skeletal muscle enhancer and confers orientation-dependent activity in myocardiocytes. A 110-bp enhancer subfragment confers high-level expression in skeletal myocytes but is inactive in myocardiocytes, indicating that skeletal and cardiac muscle MCK regulatory sites are distinguishable. To further delineate muscle regulatory sequences, we tested six sites within the MCK enhancer for their functional importance. Mutations at five sites decrease expression in skeletal muscle, cardiac muscle, and nonmuscle cells. Mutations at two of these sites, Left E box and MEF2, cause similar decreases in all three cell types. Mutations at three sites have larger effects in muscle than nonmuscle cells; an A/T-rich site mutation has a pronounced effect in both striated muscle types, mutations at the MEF1 (Right E-box) site are relatively specific to expression in skeletal muscle, and mutations at the CArG site are relatively specific to expression in cardiac muscle. Changes at the AP2 site tend to increase expression in muscle cells but decrease it in nonmuscle cells. In contrast to reports involving cotransfection of 10T1/2 cells with plasmids expressing the myogenic determination factor MyoD, we show that the skeletal myocyte activity of multimerized MEF1 sites is 30-fold lower than that of the 206-bp enhancer. Thus, MyoD binding sites alone are not sufficient for high-level expression in skeletal myocytes containing endogenous levels of MyoD and other myogenic determination factors.

Multiple regulatory elements contribute differentially to muscle creatine kinase enhancer activity in skeletal and cardiac muscle

We have used transient transfections in MM14 skeletal muscle cells, newborn rat primary ventricular myocardiocytes, and nonmuscle cells to characterize regulatory elements of the mouse muscle creatine kinase (MCK) gene. Deletion analysis of MCK 5'-flanking sequence reveals a striated muscle-specific, positive regulatory region between -1256 and -1020. A 206-bp fragment from this region acts as a skeletal muscle enhancer and confers orientation-dependent activity in myocardiocytes. A 110-bp enhancer subfragment confers high-level expression in skeletal myocytes but is inactive in myocardiocytes, indicating that skeletal and cardiac muscle MCK regulatory sites are distinguishable. To further delineate muscle regulatory sequences, we tested six sites within the MCK enhancer for their functional importance. Mutations at five sites decrease expression in skeletal muscle, cardiac muscle, and nonmuscle cells. Mutations at two of these sites, Left E box and MEF2, cause similar decreases in all three cell types. Mutations at three sites have larger effects in muscle than nonmuscle cells; an A/T-rich site mutation has a pronounced effect in both striated muscle types, mutations at the MEF1 (Right E-box) site are relatively specific to expression in skeletal muscle, and mutations at the CArG site are relatively specific to expression in cardiac muscle. Changes at the AP2 site tend to increase expression in muscle cells but decrease it in nonmuscle cells. In contrast to reports involving cotransfection of 10T1/2 cells with plasmids expressing the myogenic determination factor MyoD, we show that the skeletal myocyte activity of multimerized MEF1 sites is 30-fold lower than that of the 206-bp enhancer. Thus, MyoD binding sites alone are not sufficient for high-level expression in skeletal myocytes containing endogenous levels of MyoD and other myogenic determination factors.

hMEF2C gene encodes skeletal muscle- and brain-specific transcription factors

The myocyte enhancer-binding factor 2 (MEF2) site is an essential element of many muscle-specific enhancers and promoters that binds nuclear proteins from muscle and brain. Recently, we have cloned a family of MEF2 transcription factors produced by two genes that, at the mRNA level, are broadly expressed and produce tissue-specific isoforms by posttranscriptional processes (Y.-T. Yu, R. E. Breitbart, L. B. Smoot, Y. Lee, V. Mahdavi, and B. Nadal-Ginard, Genes Dev. 6:1783-1798, 1992). Here, we report the isolation and functional characterization of cDNA clones encoding four MEF2 factors derived from a separate gene that we have named hMEF2C. In contrast to those of the previously reported genes, the transcripts of the hMEF2C gene are restricted to skeletal muscle and brain. One of the alternate exons is exclusively present in brain transcripts. The products of this gene have DNA-binding and trans-activating activities indistinguishable from those of the previously reported MEF2 factors. The hMEF2C gene is induced late during myogenic differentiation, and its expression is limited to a subset of cortical neurons. The potential targets for this transcription factor in a subset of neurons are not known at this time. The strict tissue-specific pattern of expression of hMEF2C in comparison with the more ubiquitous expression of other MEF2 genes suggests a different mode of regulation and a potentially important role of hMEF2C factors in myogenesis and neurogenesis.

hMEF2C gene encodes skeletal muscle- and brain-specific transcription factors

The myocyte enhancer-binding factor 2 (MEF2) site is an essential element of many muscle-specific enhancers and promoters that binds nuclear proteins from muscle and brain. Recently, we have cloned a family of MEF2 transcription factors produced by two genes that, at the mRNA level, are broadly expressed and produce tissue-specific isoforms by posttranscriptional processes (Y.-T. Yu, R. E. Breitbart, L. B. Smoot, Y. Lee, V. Mahdavi, and B. Nadal-Ginard, Genes Dev. 6:1783-1798, 1992). Here, we report the isolation and functional characterization of cDNA clones encoding four MEF2 factors derived from a separate gene that we have named hMEF2C. In contrast to those of the previously reported genes, the transcripts of the hMEF2C gene are restricted to skeletal muscle and brain. One of the alternate exons is exclusively present in brain transcripts. The products of this gene have DNA-binding and trans-activating activities indistinguishable from those of the previously reported MEF2 factors. The hMEF2C gene is induced late during myogenic differentiation, and its expression is limited to a subset of cortical neurons. The potential targets for this transcription factor in a subset of neurons are not known at this time. The strict tissue-specific pattern of expression of hMEF2C in comparison with the more ubiquitous expression of other MEF2 genes suggests a different mode of regulation and a potentially important role of hMEF2C factors in myogenesis and neurogenesis.

A 40-kilodalton protein binds specifically to an upstream sequence element essential for muscle-specific transcription of the human myoglobin promoter

To define transcriptional control elements responsible for muscle-specific expression of the human myoglobin gene, we performed mutational analysis of upstream sequences (nucleotide positions -373 to +7 relative to the transcriptional start site) linked to a firefly luciferase gene. Transient expression assays in avian and mammalian cells indicated that a CCCACCCCC (CCAC box) sequence (-223 to -204) is necessary for muscle-specific transcription directed either by the native myoglobin promoter or by a heterologous minimal promoter linked to the myoglobin upstream enhancer region. A putative MEF2-like site (-160 to -169) was likewise necessary for full transcriptional activity in myotubes. Mutations within either of two CANNTG (E-box) motifs (-176 to -148) had only minimal effects on promoter function. We identified and partially purified from nuclear extracts a 40-kDa protein (CBF40) that binds specifically to oligonucleotides containing the CCAC box sequence. A mutation of the CCAC box that disrupted promoter function in vivo also impaired binding of CBF40 in vitro. These data suggest that cooperative interactions between CBF40 and other factors including MEF-2 are required for expression of the human myoglobin gene in skeletal muscle.

A 40-kilodalton protein binds specifically to an upstream sequence element essential for muscle-specific transcription of the human myoglobin promoter

To define transcriptional control elements responsible for muscle-specific expression of the human myoglobin gene, we performed mutational analysis of upstream sequences (nucleotide positions -373 to +7 relative to the transcriptional start site) linked to a firefly luciferase gene. Transient expression assays in avian and mammalian cells indicated that a CCCACCCCC (CCAC box) sequence (-223 to -204) is necessary for muscle-specific transcription directed either by the native myoglobin promoter or by a heterologous minimal promoter linked to the myoglobin upstream enhancer region. A putative MEF2-like site (-160 to -169) was likewise necessary for full transcriptional activity in myotubes. Mutations within either of two CANNTG (E-box) motifs (-176 to -148) had only minimal effects on promoter function. We identified and partially purified from nuclear extracts a 40-kDa protein (CBF40) that binds specifically to oligonucleotides containing the CCAC box sequence. A mutation of the CCAC box that disrupted promoter function in vivo also impaired binding of CBF40 in vitro. These data suggest that cooperative interactions between CBF40 and other factors including MEF-2 are required for expression of the human myoglobin gene in skeletal muscle.

A single MEF-2 site is a major positive regulatory element required for transcription of the muscle-specific subunit of the human phosphoglycerate mutase gene in skeletal and cardiac muscle cells

In order to analyze the transcriptional regulation of the muscle-specific subunit of the human phosphoglycerate mutase (PGAM-M) gene, chimeric genes composed of the upstream region of the PGAM-M gene and the bacterial chloramphenicol acetyltransferase (CAT) gene were constructed and transfected into C2C12 skeletal myocytes, primary cultured cardiac muscle cells, and C3H10T1/2 fibroblasts. The expression of chimeric reporter genes was restricted in skeletal and cardiac muscle cells. In C2C12 myotubes and primary cultured cardiac muscle cells, the segment between nucleotides -165 and +41 relative to the transcription initiation site was sufficient to confer maximal CAT activity. This region contains two E boxes and one MEF-2 motif. Deletion and substitution mutation analysis showed that a single MEF-2 motif but not the E boxes had a substantial effect on skeletal and cardiac muscle-specific enhancer activity and that the cardiac muscle-specific negative regulatory region was located between nucleotides -505 and -165. When the PGAM-M gene constructs were cotransfected with MyoD into C3H10T1/2, the profile of CAT activity was similar to that observed in C2C12 myotubes. Gel mobility shift analysis revealed that when the nuclear extracts from skeletal and cardiac muscle cells were used, the PGAM-M MEF-2 site generated the specific band that was inhibited by unlabeled PGAM-M MEF-2 and muscle creatine kinase MEF-2 oligomers but not by a mutant PGAM-M MEF-2 oligomer. These observations define the PGAM-M enhancer as the only cardiac- and skeletal-muscle-specific enhancer characterized thus far that is mainly activated through MEF-2.

A single MEF-2 site is a major positive regulatory element required for transcription of the muscle-specific subunit of the human phosphoglycerate mutase gene in skeletal and cardiac muscle cells

In order to analyze the transcriptional regulation of the muscle-specific subunit of the human phosphoglycerate mutase (PGAM-M) gene, chimeric genes composed of the upstream region of the PGAM-M gene and the bacterial chloramphenicol acetyltransferase (CAT) gene were constructed and transfected into C2C12 skeletal myocytes, primary cultured cardiac muscle cells, and C3H10T1/2 fibroblasts. The expression of chimeric reporter genes was restricted in skeletal and cardiac muscle cells. In C2C12 myotubes and primary cultured cardiac muscle cells, the segment between nucleotides -165 and +41 relative to the transcription initiation site was sufficient to confer maximal CAT activity. This region contains two E boxes and one MEF-2 motif. Deletion and substitution mutation analysis showed that a single MEF-2 motif but not the E boxes had a substantial effect on skeletal and cardiac muscle-specific enhancer activity and that the cardiac muscle-specific negative regulatory region was located between nucleotides -505 and -165. When the PGAM-M gene constructs were cotransfected with MyoD into C3H10T1/2, the profile of CAT activity was similar to that observed in C2C12 myotubes. Gel mobility shift analysis revealed that when the nuclear extracts from skeletal and cardiac muscle cells were used, the PGAM-M MEF-2 site generated the specific band that was inhibited by unlabeled PGAM-M MEF-2 and muscle creatine kinase MEF-2 oligomers but not by a mutant PGAM-M MEF-2 oligomer. These observations define the PGAM-M enhancer as the only cardiac- and skeletal-muscle-specific enhancer characterized thus far that is mainly activated through MEF-2.

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

Transcriptional cascades that specify cell fate have been well described in invertebrates. In mammalian development, however, gene hierarchies involved in determination of cell lineage are not understood. With the recent cloning of the MyoD family of myogenic regulatory factors, a model system has become available with which to study the dynamics of muscle determination in mammalian development. Myogenin, along with other members of the MyoD gene family, possesses the apparent ability to redirect nonmuscle cells into the myogenic lineage. This ability appears to be due to the direct activation of an array of subordinate or downstream genes which are responsible for formation and function of the muscle contractile apparatus. Myogenin-directed transcription has been shown to occur through interaction with a DNA consensus sequence known as an E box (CANNTG) present in the control regions of numerous downstream genes. In addition to activating the transcription of subordinate genes, members of the MyoD family positively regulate their own expression and cross-activate one another's expression. These autoregulatory interactions have been suggested as a mechanism for induction and maintenance of the myogenic phenotype, but the molecular details of the autoregulatory circuits are undefined. Here we show that the myogenin promoter contains a binding site for the myocyte-specific enhancer-binding factor, MEF-2, which can function as an intermediary of myogenin autoactivation. Since MEF-2 can be induced by myogenin, these results suggest that myogenin and MEF-2 participate in a transcriptional cascade in which MEF-2, once induced by myogenin, acts to amplify and maintain the myogenic phenotype by acting as a positive regulator of myogenin expression.

Serum response factor p67SRF is expressed and required during myogenic differentiation of both mouse C2 and rat L6 muscle cell lines

The 67-kD serum response factor (p67SRF) is a ubiquitous nuclear transcription factor that acts by direct binding to a consensus DNA sequence, the serum response element (SRE), present in the promoter region of numerous genes. Although p67SRF was initially implicated in the activation of mitogen-stimulated genes, the identification of a sequence similar to SRE, the CArG box motif, competent to interact with SRE binding factors in many muscle-specific genes, has led to speculation that, in addition to its function in cell proliferation, p67SRF may play a role in muscle differentiation. Indirect immunofluorescence using affinity-purified antibodies specifically directed against p67SRF reveals that this factor is constitutively expressed and localized in the nucleus of two skeletal muscle cell lines: rat L6 and mouse C2 myogenic cells during myogenic differentiation. This result was further confirmed through immunoblotting and Northern blot analysis. Furthermore, specific inhibition of p67SRF in vivo through microinjection of purified p67SRF antibodies prevented the myoblast-myotube transition and the expression of muscle-specific genes such as the protein troponin T. We further showed that anti-p67SRF injection also inhibited the expression of the myogenic factor myogenin, implying an early requirement for p67SRF in muscle differentiation. These results demonstrate that p67SRF is involved in the process of skeletal muscle differentiation. The potential action of p67SRF via CArG sequences is discussed.

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

Transcriptional cascades that specify cell fate have been well described in invertebrates. In mammalian development, however, gene hierarchies involved in determination of cell lineage are not understood. With the recent cloning of the MyoD family of myogenic regulatory factors, a model system has become available with which to study the dynamics of muscle determination in mammalian development. Myogenin, along with other members of the MyoD gene family, possesses the apparent ability to redirect nonmuscle cells into the myogenic lineage. This ability appears to be due to the direct activation of an array of subordinate or downstream genes which are responsible for formation and function of the muscle contractile apparatus. Myogenin-directed transcription has been shown to occur through interaction with a DNA consensus sequence known as an E box (CANNTG) present in the control regions of numerous downstream genes. In addition to activating the transcription of subordinate genes, members of the MyoD family positively regulate their own expression and cross-activate one another's expression. These autoregulatory interactions have been suggested as a mechanism for induction and maintenance of the myogenic phenotype, but the molecular details of the autoregulatory circuits are undefined. Here we show that the myogenin promoter contains a binding site for the myocyte-specific enhancer-binding factor, MEF-2, which can function as an intermediary of myogenin autoactivation. Since MEF-2 can be induced by myogenin, these results suggest that myogenin and MEF-2 participate in a transcriptional cascade in which MEF-2, once induced by myogenin, acts to amplify and maintain the myogenic phenotype by acting as a positive regulator of myogenin expression.

Analysis of binding and activating functions of the chick muscle acetylcholine receptor gamma-subunit upstream sequence

1. The skeletal muscle acetylcholine receptor comprises several subunits whose coordinated expression during myogenesis is probably controlled by cis elements in the individual subunit genes. We have previously analyzed promoter regions of the alpha and delta genes (Wang et al., 1988, 1990); to gain further insight into receptor regulation, we have now studied the promoter of the chick muscle gamma-subunit gene. 2. This analysis was faciliated by the close upstream proximity of the coding region of the delta-subunit gene and the consequent brevity (740 bp) of the untranslated linker connecting the two genes (Nef et al., 1984). Nuclease protection and primer extension analysis revealed that transcription of the gamma-subunit gene starts at position 56 upstream of the translational initiation site. 3. Nested deletions of the promoter region were employed to identify functionally important elements. A 360-bp sequence (-324 to +36) was found to activate transcription, in a position- and orientation-independent manner, during myotube formation. This sequence comprises 5 M-CAT (Nikovits et al., 1986) similarities and contains, at positions -52/-47 and -33/-28, two CANNTG (Lassar et al., 1989) motifs. 4. Binding experiments were performed by means of gel retardation, gel shift competition, and footprint analysis. The CANNTG motifs were found to bind MyoD and myogenin fusion proteins and to interact with proteins in nuclear extracts from cultured myotubes. 5. Point mutations in the CANNTG motifs revealed that these elements are crucial for full promoter activity in myotubes and essential in fibroblasts cotransfected with a myogenin expression vector. 6. We conclude that the activity of the gamma-subunit gene is determined largely by E boxes, which in vivo are likely to be activated by MyoD family proteins; in addition, other transactivators such as the M-CAT binding protein presumably play a role. Both CANNTG elements and M-CAT motifs are also present in the alpha- and delta-subunit enhancer and may therefore account for the coordinate expression of the three subunits during muscle differentiation.

The human M creatine kinase gene enhancer contains multiple functional interacting domains

Cis-elements (-933 to -641) upstream of the human M creatine kinase gene cap site contain an enhancer that confers developmental and tissue-specific expression to the chloramphenicol acetyltransferase gene in C2C12 myogenic cells transfected in culture. Division of the enhancer at -770 into a 5' fragment that includes the MyoD binding sites (-933 to -770) and a 3' fragment that includes the MEF-2 binding site (-770 to -641) resulted in two subfragments that showed minimal activity but in combination interacted in a position- and orientation-independent fashion to enhance activity of the SV40 promoter in transient transfection experiments. A 5' enhancer construct (-877 to -832) including only one (the low affinity) MyoD binding site was active when present in multiple copies. In contrast, a 3' enhancer construct (-749 to -732) including the MEF-2 binding site was inactive even when present in multiple copies. However, if the 5' construct was extended to include the high-affinity MyoD binding site (-877 to -803) the 5' and 3' constructs interacted in a position- and orientation-independent fashion to activate the SV40 promoter. Thus, the human M creatine kinase enhancer comprises multiple functional interacting domains.

A ubiquitous factor (HF-1a) and a distinct muscle factor (HF-1b/MEF-2) form an E-box-independent pathway for cardiac muscle gene expression

Recent studies have identified a conserved 28-bp element (HF-1) within the rat cardiac MLC-2 gene which confers cardiac muscle-specific and inducible expression during myocardial cell hypertrophy. Utilizing a combination of independent experimental approaches, this study characterizes two cardiac nuclear factors which bind to HF-1, a ubiquitous factor (HF-1a), and an A + T-rich binding factor (HF-1b) which is preferentially expressed in differentiated cardiac and skeletal muscle cells. The HF-1a binding site is located in a core region of the 28-bp conserved element, immediately upstream from the A + T-rich HF-1b site, which is homologous to the MEF-2 site found in a number of muscle genes. By a number of separate criteria (gel mobility shift, competition, and mutagenesis studies), HF-1b and MEF-2 appear to be indistinguishable and thus are either identical or closely related muscle factors. Transient assays of luciferase reporter genes containing point mutations throughout the 28-bp HF-1 regulatory element document the importance of both the HF-1a and HF-1b sites in transient assays in ventricular muscle cells. In the native 250-bp MLC-2 promoter fragment, mutations in the single E box had little effect on cardiac muscle specificity, while point mutations in either the HF-1a or HF-1b binding site significantly reduced promoter activity, underscoring the importance of both the HF-1a and HF-1b sites in the transcriptional activation of this cardiac muscle gene. Thus, this study provides evidence that a novel, ubiquitous factor (HF-1a) and a muscle factor (HF-1b/MEF-2) can form a novel, E-box-independent pathway for muscle-specific expression in ventricular cardiac muscle cells.

DNA binding and transcriptional regulatory activity of mammalian achaete-scute homologous (MASH) proteins revealed by interaction with a muscle-specific enhancer

The MASH genes are vertebrate homologues of achaete-scute, genes required for neuronal determination in Drosophila. The sequence of MASH1 and MASH2 contains a basic helix-loop-helix (bHLH) motif that is present in other transcriptional regulators such as MyoD and E12. In the absence of an authentic target for the MASH proteins, we examined their DNA binding and transcriptional regulatory activity by using a binding site (the E box) from the muscle creatine kinase (MCK) gene, a target of MyoD. Like myogenic bHLH proteins, the MASH proteins form heterooligomers with E12 that bind the MCK E box with high affinity in vitro. Unexpectedly, however, MASH1 and MASH2 also activate transcription of both exogenous and endogenous MCK in transfected C3H/10T1/2 fibroblasts. However, they do not induce myogenesis. Myogenic activity is not exclusively a property of the MyoD basic region, as substitution of this domain fails to confer myogenic activity on MASH1. These data suggest that different bHLH proteins may activate overlapping but distinct sets of target genes in the same cell type.

A single transcription factor binds to two divergent sequence elements with a common function in cardiac myosin light chain-2 promoter

The cardiac myosin light chain-2 (MLC-2) gene promoter contains several positive and negative cis-acting sequences that are involved in the regulation of its expression. We describe here the properties of two activator sequences, elements A and P, and their DNA-binding factors (ABFs). Element A (CCAAAAGTGG), located at -61, has homology with the evolutionarily conserved sequence CC(A/T)6GG, present in the genes of many contractile proteins. Element P (TAACCTTGAAAGC), located 114 bp upstream of element A, is conserved in both chicken and rat cardiac MLC-2 gene promoters. Deletion mutagenesis demonstrated that these two elements are involved in the positive regulation of MLC-2 gene transcription. At least two sequence-specific element A-binding proteins, ABF-1 and ABF-2, were identified by gel shift analysis of the fractionated cardiac nuclear proteins. ABF-1 binds to element A with strict dependence on the internal element A sequence AAAAGT. In contrast, ABF-2 exhibits a relaxed sequence requirement, as it recognizes the consensus CArG and CCAAT box sequences as well. ABF-2 also recognizes the distal element P despite the fact that the sequences of elements A and P are divergent. DNase I footprinting, methylation interference, and gel shift analyses demonstrated unequivocally that the element A-DNA affinity-purified protein ABF-2 binds to element P with sequence specificity. Since both elements A and P play a positive regulatory role in MLC-2 gene transcription and bind to a single protein (ABF-2), it would appear that ABF-2 is a key transcription factor with the ability to recognize divergent sequence elements involved in a common regulatory pathway during myogenesis.