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

The LIM domain protein nTRIP6 acts as a co-repressor for the transcription factor MEF2C in myoblasts

The transcription factor Myocyte enhancer factor 2C (MEF2C) plays a key role in the late differentiation of skeletal muscle progenitor cells, the so-called myoblasts. During myoblast differentiation, both MEF2C expression and transcriptional activity are regulated. We have reported that nTRIP6, the nuclear isoform of the focal adhesion LIM domain protein TRIP6, acts as an adaptor transcriptional co-activator for several transcription factors. It interacts with the promoter-bound transcription factors and consequently mediates the recruitment of other co-activators. Based on a described interaction between MEF2C and TRIP6 in a yeast-two-hybrid screen, we hypothesised a co-regulatory function of nTRIP6 for MEF2C. In proliferating myoblasts, nTRIP6 interacted with MEF2C and was recruited together with MEF2C to the MEF2-binding regions of the MEF2C target genes Myom2, Mb, Tnni2 and Des. Silencing nTRIP6 or preventing its interaction with MEF2C increased MEF2C transcriptional activity and increased the expression of these MEF2C target genes. Thus, nTRIP6 acts as a co-repressor for MEF2C. Mechanistically, nTRIP6 mediated the recruitment of the class IIa histone deacetylase HDAC5 to the MEF2C-bound promoters. In conclusion, our results unravel a transcriptional co-repressor function for nTRIP6. This adaptor co-regulator can thus exert either co-activator or co-repressor functions, depending on the transcription factor it interacts with.

Molecular Analysis of a Recurrent Sarcoma Identifies a Mutation in FAF1

A patient presented with a recurrent sarcoma (diagnosed as leiomyosarcoma) 12 years after the removal of an initial cancer (diagnosed as extracompartmental osteosarcoma) distally on the same limb. Following surgery, the sarcoma and unaffected muscle and bone were subjected to measurements of DNA exome sequence, RNA and protein expression, and transcription factor binding. The investigation provided corroboration of the diagnosis leiomyosarcoma, as the major upregulations in this tumor comprise muscle-specific gene products and calcium-regulating molecules (calcium is an important second messenger in smooth muscle cells). A likely culprit for the disease is the point mutation S181G in FAF1, which may cause a loss of apoptotic function consecutive to transforming DNA damage. The RNA levels of genes for drug transport and metabolism were extensively skewed in the tumor tissue as compared to muscle and bone. The results suggest that the tumor represents a recurrence of a dormant metastasis from an originally misdiagnosed neoplasm. A loss of FAF1 function could cause constitutive WNT pathway activity (consistent with the downstream inductions of IGF2BP1 and E2F1 in this cancer). While the study has informed on drug transport and drug metabolism pharmacogenetics, it has fallen short of identifying a suitable target for molecular therapy.

Disruption of MEF2C signaling and loss of sarcomeric and mitochondrial integrity in cancer-induced skeletal muscle wasting

Cancer cachexia is a highly debilitating paraneoplastic disease observed in more than 50% of patients with advanced cancers and directly contributes to 20% of cancer deaths. Loss of skeletal muscle is a defining characteristic of patients with cancer cachexia and is associated with poor survival. The present study reveals the involvement of a myogenic transcription factor Myocyte Enhancer Factor (MEF) 2C in cancer-induced skeletal muscle wasting. Increased skeletal muscle mRNA expression of Suppressor of Cytokine Signaling (Socs) 3 and the IL-6 receptor indicative of active IL-6 signaling was seen in skeletal muscle of mice bearing the Colon 26 (C26) carcinoma. Loss of skeletal muscle structural integrity and distorted mitochondria were also observed using electron microscopy. Gene and protein expression of MEF2C was significantly downregulated in skeletal muscle from C26-bearing mice. MEF2C gene targets myozenin and myoglobin as well as myokinase were also altered during cachexia, suggesting dysregulated oxygen transport capacity and ATP regeneration in addition to distorted structural integrity. In addition, reduced expression of calcineurin was observed which suggested a potential pathway of MEF2C dysregulation. Together, these effects may limit sarcomeric contractile ability and also predispose skeletal muscle to structural instability; associated with muscle wasting and fatigue in cachexia.

Hypoxia reprograms calcium signaling and regulates myoglobin expression

Myoglobin is an oxygen storage molecule that is selectively expressed in cardiac and slow-twitch skeletal muscles that have a high oxygen demand. Numerous studies have implicated hypoxia in the regulation of myoglobin expression as an adaptive response to hypoxic stress. However, the details of this relationship remain undefined. In the present study, adult mice exposed to 10% oxygen for periods up to 3 wk exhibited increased myoglobin expression only in the working heart, whereas myoglobin was either diminished or unchanged in skeletal muscle groups. In vitro and in vivo studies revealed that hypoxia in the presence or absence of exercise-induced stimuli reprograms calcium signaling and modulates myoglobin gene expression. Hypoxia alone significantly altered calcium influx in response to cell depolarization or depletion of endoplasmic reticulum calcium stores, which inhibited the expression of myoglobin. In contrast, our whole animal and transcriptional studies indicate that hypoxia in combination with exercise enhanced the release of calcium from the sarcoplasmic reticulum via the ryanodine receptors triggered by caffeine, which increased the translocation of nuclear factor of activated T-cells into the nucleus to transcriptionally activate myoglobin expression. The present study unveils a previously unrecognized mechanism where the hypoxia-mediated regulation of calcium transients from different intracellular pools modulates myoglobin gene expression. In addition, we observed that changes in myoglobin expression, in response to hypoxia, are not dependent on hypoxia-inducible factor-1 or changes in skeletal muscle fiber type. These studies enhance our understanding of hypoxia-mediated gene regulation and will have broad applications for the treatment of myopathic diseases.

Multiple non-collinear TF-map alignments of promoter regions

Background

The analysis of the promoter sequence of genes with similar expression patterns is a basic tool to annotate common regulatory elements. Multiple sequence alignments are on the basis of most comparative approaches. The characterization of regulatory regions from co-expressed genes at the sequence level, however, does not yield satisfactory results in many occasions as promoter regions of genes sharing similar expression programs often do not show nucleotide sequence conservation.

Results

In a recent approach to circumvent this limitation, we proposed to align the maps of predicted transcription factors (referred as TF-maps) instead of the nucleotide sequence of two related promoters, taking into account the label of the corresponding factor and the position in the primary sequence. We have now extended the basic algorithm to permit multiple promoter comparisons using the progressive alignment paradigm. In addition, non-collinear conservation blocks might now be identified in the resulting alignments. We have optimized the parameters of the algorithm in a small, but well-characterized collection of human-mouse-chicken-zebrafish orthologous gene promoters.

Conclusion

Results in this dataset indicate that TF-map alignments are able to detect high-level regulatory conservation at the promoter and the 3'UTR gene regions, which cannot be detected by the typical sequence alignments. Three particular examples are introduced here to illustrate the power of the multiple TF-map alignments to characterize conserved regulatory elements in absence of sequence similarity. We consider this kind of approach can be extremely useful in the future to annotate potential transcription factor binding sites on sets of co-regulated genes from high-throughput expression experiments.

Transcription enhancer factor 1 binds multiple muscle MEF2 and A/T-rich elements during fast-to-slow skeletal muscle fiber type transitions

In adult mouse skeletal muscle, beta-myosin heavy chain (betaMyHC) gene expression is primarily restricted to slow type I fibers; however, its expression can be induced in fast type II fibers in response to a sustained increase in load-bearing work (mechanical overload [MOV]). Our previous betaMyHC transgenic and protein-DNA interaction studies have identified an A/T-rich element (betaA/T-rich -269/-258) that is required for slow muscle expression and which potentiates MOV responsiveness of a 293-bp betaMyHC promoter (beta293wt). Despite the GATA/MEF2-like homology of this element, we found binding of two unknown proteins that were antigenically distinct from GATA and MEF2 isoforms. By using the betaA/T-rich element as bait in a yeast one-hybrid screen of an MOV-plantaris cDNA library, we identified nominal transcription enhancer factor 1 (NTEF-1) as the specific betaA/T-rich binding factor. Electrophoretic mobility shift assay analysis confirmed that NTEF-1 represents the enriched binding activity obtained only when the betaA/T-rich element is reacted with MOV-plantaris nuclear extract. Moreover, we show that TEF proteins bind MEF2 elements located in the control region of a select set of muscle genes. In transient-coexpression assays using mouse C2C12 myotubes, TEF proteins transcriptionally activated a 293-bp betaMyHC promoter devoid of any muscle CAT (MCAT) sites, as well as a minimal thymidine kinase promoter-luciferase reporter gene driven by three tandem copies of the desmin MEF2 or palindromic Mt elements or four tandem betaA/T-rich elements. These novel findings suggest that in addition to exerting a regulatory effect by binding MCAT elements, TEF proteins likely contribute to regulation of skeletal, cardiac, and smooth muscle gene networks by binding select A/T-rich and MEF2 elements under basal and hypertrophic conditions.

Functional analysis of the human annexin A5 gene promoter: a downstream DNA element and an upstream long terminal repeat regulate transcription

Human annexin A5 is a ubiquitous protein implicated in diverse signal transduction processes associated with cell growth and differentiation, and its gene regulation is an important component of this function. Promoter transcriptional activity was determined for a wide 5' portion of the human annexin A5 gene, from bp -1275 to +79 relative to the most 5' of several discrete transcription start points. Transfection experiments carried out in HeLa cells identified the segment from bp -202 to +79 as the minimal promoter conferring optimal transcriptional activity. Two canonical Sp1 sites in the immediate 5' flanking region of a CpG island were required for significant transcription. Strong repressive activity in the distal promoter region between bp -717 to -1153 was attributed to the presence of an endogenous retroviral long terminal repeat, homologous with long terminal repeat 47B. The downstream sequence from bp position +31 to +79 in untranslated exon 1 was also essential for transcription, as its deletion from any of the plasmid constructs abolished activity in transfection assays. Electrophoretic mobility-shift assays, Southwestern-blot analysis and affinity chromatography were used to identify a protein doublet of relative molecular mass 35 kDa that bound an octanucleotide palindromic sequence in exon 1. The DNA cis-element resembled an E-box, but did not bind higher molecular mass transcription factors, such as upstream stimulatory factor or activator protein 4. The discovery of a downstream element crucial for annexin A5 gene transcription, and its interaction with a potentially novel transcription factor or complex, may provide a clue to understanding the initiation of transcription by TATA-less, multiple start site promoters.

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.

Identification of a novel slow-muscle-fiber enhancer binding protein, MusTRD1

The molecular mechanisms which are responsible for restricting skeletal muscle gene expression to specific fiber types, either slow or fast twitch, are unknown. As a first step toward defining the components which direct slow-fiber-specific gene expression, we identified the sequence elements of the human troponin I slow upstream enhancer (USE) that bind muscle nuclear proteins. These include an E-box, a MEF2 element, and two other elements, USE B1 and USE C1. In vivo analysis of a mutation that disrupts USE B1 binding activity suggested that the USE B1 element is essential for high-level expression in slow-twitch muscles. This mutation does not, however, abolish slow-fiber specificity. A similar analysis indicated that the USE C1 element may play only a minor role. We report the cloning of a novel human USE B1 binding protein, MusTRD1 (muscle TFII-I repeat domain-containing protein 1), which is expressed predominantly in skeletal muscle. Significantly, MusTRD1 contains two repeat domains which show remarkable homology to the six repeat domains of the recently cloned transcription factor TFII-I. Furthermore, both TFII-I and MusTRD1 bind to similar but distinct sequences, which happen to conform with the initiator (Inr) consensus sequence. Given the roles of MEF2 and basic helix-loop-helix (bHLH) proteins in muscle gene expression, the similarity of TFII-I and MusTRD1 is intriguing, as TFII-I is believed to coordinate the interaction of MADS-box proteins, bHLH proteins, and the general transcription machinery.

Transcriptional regulation of CLN3 expression by glucose in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, the transition from the G1 phase of the mitotic cycle into S phase is controlled by a set of G1 cyclins that regulate the activity of the protein kinase encoded by CDC28. Yeast cells regulate progress through the G1/S boundary in response to nutrients, moving quickly through G1 in glucose medium and more slowly in poorer medium. We have examined connections between glucose and the level of the message encoding Cln3, a G1 cyclin. We found that glucose positively regulates CLN3 mRNA levels through a set of repeated AAGAAAAA (A2GA5) elements within the CLN3 promoter. Mutations in these sequences reduce both transcriptional activation and specific interaction between CLN3 promoter elements and proteins in yeast extracts. Creation of five point mutations, replacing the G's within these repeats with T's, in the CLN3 promoter substantially reduces CLN3 expression in glucose medium and inhibits the ability of the cells to maintain a constant size when shifted into glucose.

The human pH aldolase A promoter directs widespread but muscle-predominant expression in transgenic mice

In order to identify regulatory elements that direct widespread in vivo expression of a linked gene, we have examined one of the human aldolase A alternative promoters, the ubiquitous pH promoter, which is active in most foetal and adult tissues. We have used the pH promoter region to drive expression of an heterologous CAT reporter gene in transgenic mice. We show that a short 820 bp pH promoter fragment is able to confer a ubiquitous and reproducible activity pattern on the CAT reporter gene in most of the transgenic lines analysed, with a particularly high level of expression in adult skeletal muscle. Activity of this transgene was detected from early embryonic stages. Therefore, this pH promoter region appears to be a powerful tool to direct ubiquitous and early expression of a transgene in vivo. Deletion analysis revealed that: (i) the region between -651 and -369 bp relative to the pH promoter transcription start site includes DNA elements capable of overriding effects of the surrounding chromatin at the integration site, (ii) the region between -285 and -211 bp is involved in pH promoter tissue-specific expression pattern in skeletal muscle and/or nervous tissues, (iii) the region located between -211 and -108 bp is necessary for its ubiquitous and muscle-predominant activity and (iv) the most proximal region downstream from -108 bp is still sufficient to confer an activity in brain and lung.

A 27 kDa protein binds to a positive and a negative regulatory sequence in the promoter of the ICL1 gene from Saccharomyces cerevisiae

IsocitrateICL1, is one of the key enzymes of the glyoxylate pathway, which operates as an anaplerotic route for replenishing the tricarboxylic acid cycle; it is required for growth of Saccharomyces cerevisiae on carbon sources such as ethanol, but is dispensable when fermentable carbon sources are available. The positive regulation of the ICL1 gene by an upstream activating sequence (UAS) element located between -397 and -388 has been previously reported. In this paper we show that the ICL1 promoter sequence 5'-AGTCCGGACTAGCATCCCAG-3' located between -261 and -242 contains an upstream repressing sequence (URS) element. We have identified and partially purified a 27 kDa protein that binds specifically to both the UAS and URS sequences of the ICL1 promoter. For both UAS and URS, binding requires the protein Snf1 (Cat1), a protein kinase essential for the derepression of genes repressed by glucose. Binding does not take place with extracts from glucose-grown strains, unless they lack Mig1, a negative regulatory protein involved in glucose repression.

Transient expression of a winged-helix protein, MNF-beta, during myogenesis

A novel winged-helix transcription factor, MNF-beta, is expressed coincidentally with cell cycle withdrawal and differentiation of skeletal myogenic cells. MNF-beta is closely related to the myocyte nuclear factor (MNF) protein previously described (now termed MNF-alpha), but expression of the two isoforms is differentially regulated, and they exhibit distinctive functional properties with respect to DNA binding in vitro and transcriptional regulatory activity in transient-transfection assays. A DNA sequence motif binding MNF-beta with high affinity was selected from a library of random oligonucleotides and was found to be similar to but distinct from the cognate binding site for HNF-3beta, a more distantly related winged-helix protein. The temporal pattern of MNF-beta expression and the presence of MNF binding motifs within conserved promoter elements of several genes that modulate cell cycle progression support a working hypothesis that MNF proteins may modulate proliferation of myogenic precursor cells during development and muscle regeneration.

ZBP-89, a Krüppel-like zinc finger protein, inhibits epidermal growth factor induction of the gastrin promoter

We have shown previously that a GC-rich element (GGGGCGGGGTGGGGGG) conferring epidermal growth factor (EGF) responsiveness to the human gastrin promoter binds Sp1 and additional undefined complexes. A rat GH4 cell line expression library was screened by using a multimer of the gastrin EGF response element, and three overlapping cDNA clones were identified. The full-length rat cDNA encoded an 89-kDa zinc finger protein (ZBP-89) that was 89% identical to a 49-kDa human factor, ht(beta), that binds a GTGGG/CACCC element in T-cell receptor promoters. The conservation of amino acids between the zinc fingers indicates that ZBP-89 is a member of the C2H2 zinc finger family subclass typified by the Drosophila Krüppel protein. ZBP-89 is ubiquitously expressed in normal adult tissues. It binds specifically to the gastrin EGF response element and inhibits EGF induction of the gastrin promoter. Collectively, these results demonstrate that ZBP-89 functions as a repressor of basal and inducible expression of the gastrin gene.

Identification and characterisation of two transcriptional repressor elements within the coding sequence of the Saccharomyces cerevisiae HXK2 gene

A well-defined set of isogenic yeast strains has been constructed whereby each strain contains a different HXK2::lacZ gene fusion integrated at the URA3 locus. These HXK2::lacZ fusions differ in the amount of the HXK2 gene (encoding hexokinase 2 isoenzyme) that is fused to the lacZ reporter gene. Comparison of the beta-galactosidase activities of each strain during growth on glucose or ethanol revealed that some part of the coding region between +39 and +404 bp is involved in repressing gene expression in a carbon source dependent manner. A series of deletions of this HXK2 coding region were constructed and fused upstream of a minimal CYC1::lacZ promoter. beta-Galactosidase activities on glucose or ethanol growth yeast calls revealed that two different regulatory elements are present in this DNA region. Gel mobility shift analysis and in vitro DNase I footprinting have shown that proteins bind specifically to two downstream repressor sequences (DRS1 located from +140 to +163 and DRS2 located between +231 and +251) that influence the rate of HXK2 transcription when ethanol is used as carbon source by Saccharomyces cerevisiae. We identified and partially purified a 18 kDa protein that binds specifically to synthetic double-stranded oligonucleotides containing the (A/C)(A/G)GAAAT box sequence. Our data suggest that p18 synthesis is under the control of genes involved in glucose repression (MIG1 = CAT4) and glucose derepression (SNF1 = CAT1).

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.

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.

Synergistic interactions between heterologous upstream activation elements and specific TATA sequences in a muscle-specific promoter

Previous investigations have defined three upstream activation elements--CCAC, A/T, and TATA sequences--necessary for muscle-specific transcription of the myoglobin gene. In the present study, we demonstrate that these three sequences elements, prepared as synthetic oligonucleotide cassettes, function synergistically to constitute a cell-type-specific transcription unit. Previously, cognate binding factors that recognize the CCAC and TATA elements were identified. In this study we determine that the A/T element binds two nuclear factors, including myocyte enhancer factor-2 (MEF-2) and an apparently unknown factor we provisionally termed ATF35 (A/T-binding factor, 35 kDa). Mutations that alter in vitro binding of either MEF-2 or ATF35 to this site diminish promoter function in vivo. Functional synergism between factors binding the CCAC and A/T elements is sensitive to subtle mutations in the TATA sequence, recapitulating the unusual preference for specific TATA variants exhibited by the native myoglobin promoter. These results provide new insights into mechanisms that underlie the distinctive pattern of myoglobin gene regulation in mammalian muscle development and lay a foundation for further studies to elucidate general principles of transcriptional control of complex mammalian promoters through combinatorial actions of heterologous transcriptional factors.

Myocyte nuclear factor, a novel winged-helix transcription factor under both developmental and neural regulation in striated myocytes

A sequence motif (CCAC box) within an upstream enhancer region of the human myoglobin gene is essential for transcriptional activity in both cardiac and skeletal muscle. A cDNA clone, myocyte nuclear factor (MNF), was isolated from a murine expression library on the basis of sequence-specific binding to the myoglobin CCAC box motif and was found to encode a novel member of the winged-helix or HNF-3/fork head family of transcription factors. Probes based on this sequence identify two mRNA species that are upregulated during myocyte differentiation, and antibodies raised against recombinant MNF identify proteins of approximately 90, 68, and 65 kDa whose expression is regulated following differentiation of myogenic cells in culture. In addition, the 90-kDa form of MNF is phosphorylated and is upregulated in intact muscles subjected to chronic motor nerve stimulation, a potent stimulus to myoglobin gene regulation. Amino acid residues 280 to 389 of MNF demonstrate 35 to 89% sequence identity to the winged-helix domain from other known members of this family, but MNF is otherwise divergent. A proline-rich amino-terminal region (residues 1 to 206) of MNF functions as a transcriptional activation domain. These studies provide the first evidence that members of the winged-helix family of transcription factors have a role in myogenic differentiation and in remodeling processes of adult muscles that occur in response to physiological stimuli.

Myocyte nuclear factor, a novel winged-helix transcription factor under both developmental and neural regulation in striated myocytes

A sequence motif (CCAC box) within an upstream enhancer region of the human myoglobin gene is essential for transcriptional activity in both cardiac and skeletal muscle. A cDNA clone, myocyte nuclear factor (MNF), was isolated from a murine expression library on the basis of sequence-specific binding to the myoglobin CCAC box motif and was found to encode a novel member of the winged-helix or HNF-3/fork head family of transcription factors. Probes based on this sequence identify two mRNA species that are upregulated during myocyte differentiation, and antibodies raised against recombinant MNF identify proteins of approximately 90, 68, and 65 kDa whose expression is regulated following differentiation of myogenic cells in culture. In addition, the 90-kDa form of MNF is phosphorylated and is upregulated in intact muscles subjected to chronic motor nerve stimulation, a potent stimulus to myoglobin gene regulation. Amino acid residues 280 to 389 of MNF demonstrate 35 to 89% sequence identity to the winged-helix domain from other known members of this family, but MNF is otherwise divergent. A proline-rich amino-terminal region (residues 1 to 206) of MNF functions as a transcriptional activation domain. These studies provide the first evidence that members of the winged-helix family of transcription factors have a role in myogenic differentiation and in remodeling processes of adult muscles that occur in response to physiological stimuli.

Regulation of the human cardiac/slow-twitch troponin C gene by multiple, cooperative, cell-type-specific, and MyoD-responsive elements

The cardiac troponin C (cTnC) gene produces identical transcripts in slow-twitch skeletal muscle and in heart muscle (R. Gahlmann, R. Wade, P. Gunning, and L. Kedes, J. Mol. Biol. 201:379-391, 1988). A separate gene encodes the fast-twitch skeletal muscle troponin C and is not expressed in heart muscle. We have used transient transfection to characterize the regulatory elements responsible for skeletal and cardiac cell-type-specific expression of the human cTnC (HcTnC) gene. At least four separate elements cooperate to confer tissue-specific expression of this gene in differentiated myotubes; a basal promoter (between -61 and -13) augments transcription 9-fold, upstream major regulatory sequences (between -68 and -142 and between -1319 and -4500) augment transcription as much as 39-fold, and at least two enhancer-like elements in the first intron (between +58 and +1028 and between +1029 and +1523) independently augment transcription 4- to 5-fold. These enhancers in the first intron increase myotube-specific chloramphenicol acetyltransferase activity when linked to their own promoter elements or to the heterologous simian virus 40 promoter, and the effects are multiplicative rather than additive. Each of the major myotube regulatory regions is capable of responding directly or indirectly to the myogenic determination factor, MyoD.A MyoD expression vector in 10T1/2 cells induced constructs carrying either the upstream HcTnC promoter elements or the first intron of the gene 300- to 500-fold. Expression was inhibited by cotransfection with Id, a negative regulator of basic helix-loop-helix transcription factors. The basal promoter contains five tandem TGGGC repeats that interact with Sp1 or an Sp1-like factor in nuclear extracts. Mutational analysis of this element demonstrated that two of the five repeat sequences were sufficient to support basal level muscle cell-specific transcription. Whereas the basal promoter is also critical for expression in cardiac myocytes, the elements upstream of -67 appear to play little or no role. Major augmentation of expression in cardiomyocytes is also provided by sequences in the first intron, but these are upstream (between +58 and +1028). The downstream segment of the first intron has no enhancer activity in cardiomyocytes. A specific DNA-protein complex is formed by this C2 cell enhancer with extracts from C2 cells but not cardiomyocytes. These observations suggest that tissue-specific expression of the HcTnC gene is cooperatively regulated by the complex interactions of multiple regulatory elements and that different elements are used to regulate expression in myogenic and cardiac cells.

Regulation of the human cardiac/slow-twitch troponin C gene by multiple, cooperative, cell-type-specific, and MyoD-responsive elements

The cardiac troponin C (cTnC) gene produces identical transcripts in slow-twitch skeletal muscle and in heart muscle (R. Gahlmann, R. Wade, P. Gunning, and L. Kedes, J. Mol. Biol. 201:379-391, 1988). A separate gene encodes the fast-twitch skeletal muscle troponin C and is not expressed in heart muscle. We have used transient transfection to characterize the regulatory elements responsible for skeletal and cardiac cell-type-specific expression of the human cTnC (HcTnC) gene. At least four separate elements cooperate to confer tissue-specific expression of this gene in differentiated myotubes; a basal promoter (between -61 and -13) augments transcription 9-fold, upstream major regulatory sequences (between -68 and -142 and between -1319 and -4500) augment transcription as much as 39-fold, and at least two enhancer-like elements in the first intron (between +58 and +1028 and between +1029 and +1523) independently augment transcription 4- to 5-fold. These enhancers in the first intron increase myotube-specific chloramphenicol acetyltransferase activity when linked to their own promoter elements or to the heterologous simian virus 40 promoter, and the effects are multiplicative rather than additive. Each of the major myotube regulatory regions is capable of responding directly or indirectly to the myogenic determination factor, MyoD.A MyoD expression vector in 10T1/2 cells induced constructs carrying either the upstream HcTnC promoter elements or the first intron of the gene 300- to 500-fold. Expression was inhibited by cotransfection with Id, a negative regulator of basic helix-loop-helix transcription factors. The basal promoter contains five tandem TGGGC repeats that interact with Sp1 or an Sp1-like factor in nuclear extracts. Mutational analysis of this element demonstrated that two of the five repeat sequences were sufficient to support basal level muscle cell-specific transcription. Whereas the basal promoter is also critical for expression in cardiac myocytes, the elements upstream of -67 appear to play little or no role. Major augmentation of expression in cardiomyocytes is also provided by sequences in the first intron, but these are upstream (between +58 and +1028). The downstream segment of the first intron has no enhancer activity in cardiomyocytes. A specific DNA-protein complex is formed by this C2 cell enhancer with extracts from C2 cells but not cardiomyocytes. These observations suggest that tissue-specific expression of the HcTnC gene is cooperatively regulated by the complex interactions of multiple regulatory elements and that different elements are used to regulate expression in myogenic and cardiac cells.

An E box mediates activation and repression of the acetylcholine receptor delta-subunit gene during myogenesis

The genes encoding the skeletal muscle acetylcholine receptor (AChR) are induced during muscle development and are regulated subsequently by innervation. Because both the initiation and the subsequent regulation of AChR expression are controlled by transcriptional mechanisms, an understanding of the steps that regulate AChR expression following innervation is likely to require knowledge of the pathway that activates AChR genes during myogenesis. Thus, we sought to identify the cis-acting sequences that regulate expression of the AChR delta-subunit gene during muscle differentiation. We transfected muscle and nonmuscle cell lines with gene fusions between 5'-flanking DNA from the AChR delta-subunit gene and the human growth hormone gene, and we show here that 148 bp of 5'-flanking DNA from the AChR delta-subunit gene contains two regulatory elements that control muscle-specific gene expression. One element is an E box, which is important both for activation of the delta-subunit gene in myotubes and for its repression in myoblasts and nonmuscle cells. Mutation of this E box, which prevents binding of MyoD-E2A and myogenin-E2A heterodimers, decreases expression in myotubes and increases expression in myoblasts and nonmuscle cells. An E-box binding activity, which does not contain MyoD, myogenin, or E2A proteins, is present in muscle and nonmuscle cells and may be responsible for repressing the delta-subunit gene in myoblasts and nonmuscle cells. An enhancer, which lacks E boxes, is also required for expression of the delta-subunit gene but does not confer muscle-specific expression.

An E box mediates activation and repression of the acetylcholine receptor delta-subunit gene during myogenesis

The genes encoding the skeletal muscle acetylcholine receptor (AChR) are induced during muscle development and are regulated subsequently by innervation. Because both the initiation and the subsequent regulation of AChR expression are controlled by transcriptional mechanisms, an understanding of the steps that regulate AChR expression following innervation is likely to require knowledge of the pathway that activates AChR genes during myogenesis. Thus, we sought to identify the cis-acting sequences that regulate expression of the AChR delta-subunit gene during muscle differentiation. We transfected muscle and nonmuscle cell lines with gene fusions between 5'-flanking DNA from the AChR delta-subunit gene and the human growth hormone gene, and we show here that 148 bp of 5'-flanking DNA from the AChR delta-subunit gene contains two regulatory elements that control muscle-specific gene expression. One element is an E box, which is important both for activation of the delta-subunit gene in myotubes and for its repression in myoblasts and nonmuscle cells. Mutation of this E box, which prevents binding of MyoD-E2A and myogenin-E2A heterodimers, decreases expression in myotubes and increases expression in myoblasts and nonmuscle cells. An E-box binding activity, which does not contain MyoD, myogenin, or E2A proteins, is present in muscle and nonmuscle cells and may be responsible for repressing the delta-subunit gene in myoblasts and nonmuscle cells. An enhancer, which lacks E boxes, is also required for expression of the delta-subunit gene but does not confer muscle-specific expression.

Gradients of transgene expression directed by the human myoglobin promoter in the developing mouse heart

Prior studies using transient transfection assays in cultured avian and murine skeletal myotubes indicate that the proximal 2-kb segment of the 5' flanking region of the human myoglobin gene contains transcriptional control elements sufficient to direct muscle-specific and developmentally regulated expression of reporter genes. To examine the function of the human myoglobin gene promoter during development of skeletal and cardiac myocytes in the intact animal, a 2.0-kb myoglobin gene upstream fragment was fused to an Escherichia coli lacZ reporter gene and injected into fertilized mouse oocytes. beta-Galactosidase (beta-gal) activity was detected selectively in cardiac and skeletal myocytes of fetal and adult transgenic mice. A distinctive spatial pattern of myoglobin promoter activity was observed in fetal hearts: beta-gal staining was more pronounced within the left ventricular subendocardium than within the subepicardium and was essentially undetectable in the ventricular trabeculae or atria. Expression of endogenous myoglobin mRNA and protein, assessed by in situ hybridization and immunohistochemistry, demonstrated a similar spatial pattern. In contrast, hearts from adult transgenic mice demonstrated essentially homogeneous expression of beta-gal and of endogenous myoglobin mRNA and protein throughout the myocardium, including the trabeculae and atria. These data indicate that the 2.0-kb upstream region of the human myoglobin gene includes cis-acting regulatory elements sufficient to direct transgene expression during murine cardiac development that is myocyte-specific and responsive to positional cues in a similar manner to the endogenous myoglobin gene.