cis-acting elements responsible for muscle-specific expression of the myosin heavy chain beta gene

A triple stranded G-quadruplex formation in the promoter region of human myosin β(Myh7) gene

Regulatory regions in human genome, enriched in guanine-rich DNA sequences have the propensity to fold into G-quadruplex structures. On exploring the genome for search of G-tracts, it was interesting to find that promoter of Human Myosin Gene (MYH7) contains a conserved 23-mer G-rich sequence (HM-23). Mutations in this gene are associated with familial cardiomyopathy. Enrichment of MYH7 gene in G-rich sequences could possibly play a critical role in its regulation. We used polyacrylamide gel electrophoresis (PAGE), UV-Thermal denaturation (UV-Tm) and Circular Dichroism (CD), to demonstrate the formation of a G-quadruplex by 23-mer G-rich sequence HM23 in promoter location of MYH7 gene. We observed that the wild G-rich sequence HM23 containing consecutive G₅ stretch in two stacks adopt G-quadruplexes of diverse molecularity by involvement of four-strand, three-strand and two-strands with same parallel topology. Interestingly, the mutated sequence in the absence of continuous G₅ stretch obstructs the formation of three-stranded G-quadruplex. We demonstrated that continuous G₅ stretch is mandatory for the formation of a unique three-stranded G-quadruplex. Presence of various transcription factors (TF) in vicinity of the sequence HM23 leave fair possibility of recognition by TF binding sites, and so modulate gene expression. These findings may add on our understanding about the effect of base change in the formation of varied structural species in similar solution condition. This study may give insight about structural polymorphism arising due to recognition of non-Watson-Crick G-quadruplex structures by cellular proteins and designing structure specific molecules.

Differential regulation of the myosin heavy chain genes alpha and beta in rat atria and ventricles: role of antisense RNA

BACKGROUND

The myosin heavy chain (MHC) genes are regulated by triiodothyronine (T3) in a reciprocal and chamber-specific manner. To further our understanding of the potential mechanisms involved, we determined the T3 responsiveness of the MHC genes, alpha and beta, and the beta-MHC antisense (AS) gene in the rat ventricles and atria.

METHODS

Hypothyroid rats were administered a single physiologic (1 microg) or pharmacologic (20 microg) dose of T3, and sequential measurements of beta-MHC hn- and AS RNA and alpha-MHC heterogeneous nuclear RNA from rat ventricular and atrial myocardium were performed with reverse transcription PCR.

RESULTS

We have demonstrated that T3 treatment increases the myocyte content of an AS beta-MHC RNA in atria and ventricles that includes sequences complementary to both the first 5' and last 3' introns of the beta-MHC sense transcript. In the hypothyroid rat ventricle, beta-MHC sense RNA expression is maximal, while in the euthyroid rat ventricle, beta-MHC AS RNA is maximal. beta-MHC AS expression increased by 52 +/- 9.8% at the peak, 24 hours after injection of a physiologic dose of T3 (1 microg/animal), while beta-MHC sense RNA decreased by 41 +/- 2.2% at 36 hours, the nadir. In hypothyroid atria, beta-MHC AS RNA was induced by threefold within 6 hours of administration of 1 microg T3, demonstrating that in the atria, beta-MHC AS expression is regulated by T3, while alpha-MHC expression is not.

CONCLUSIONS

In the hypothyroid rat heart ventricle, beta-MHC AS RNA expression increases in response to T3 similar to that of alpha-MHC. Simultaneous measures of beta-MHC sense RNA are decreased, suggesting a possible mechanism for AS to regulate sense expression. In atria, while alpha-MHC is not influenced by thyroid state, beta-MHC sense and AS RNA were simultaneously and inversely altered in response to T3. This confirms a close positive relationship between T3 and beta-MHC AS RNA in both the atria and ventricles, while demonstrating for the first time that alpha- and beta-MHC expression is not coupled in the atria.

Sox6 is required for normal fiber type differentiation of fetal skeletal muscle in mice

Sox6, a member of the Sox family of transcription factors, is highly expressed in skeletal muscle. Despite its abundant expression, the role of Sox6 in muscle development is not well understood. We hypothesize that, in fetal muscle, Sox6 functions as a repressor of slow fiber type-specific genes. In the wild-type mouse, differentiation of fast and slow fibers becomes apparent during late fetal stages (after approximately embryonic day 16). However, in the Sox6 null-p(100H) mutant mouse, all fetal muscle fibers maintain slow fiber characteristics, as evidenced by expression of the slow myosin heavy chain MyHC-beta. Knockdown of Sox6 expression in wild-type myotubes results in a significant increase in MyHC-beta expression, supporting our hypothesis. Analysis of the MyHC-beta promoter revealed a Sox consensus sequence that likely functions as a negative cis-regulatory element. Together, our results suggest that Sox6 plays a critical role in the fiber type differentiation of fetal skeletal muscle.

Characterization of cardiac gene promoter activity: reporter constructs and heterologous promoter studies

Cardiac gene promoter analysis remains an integral method in molecular cardiology and continues to provide novel insights into the transcriptional mechanisms that regulate gene expression in the myocardium. Initial studies focused on the regulated expression of contractile genes, since their transcripts are abundant and their cDNAs were among the first to be cloned. More recent studies have focused on the promoters of genes expressed at much lower levels, including those that encode ion channels, signaling proteins, and the cardiac transcription factors. The standard approach to analyze myocardial gene promoters has been to transfect reporter plasmids into cultured neonatal rat cardiac myocytes. This approach has the unique advantage of allowing the exploration of different signaling mechanisms by supplementing culture media with different agonists and inhibitors. In addition, cis-elements that control gene expression under different physiological stresses have been further characterized in the context of heterologous promoters to demonstrate their "stand-alone" functional properties in the absence of confounding influences from other cis-elements and their cognate transcription factors. Here we illustrate the characterization of cardiac gene promoter activity using reporter constructs and heterologous promoter studies in cultured cardiac myocytes.

Transcription Enhancer Factor-1-dependent Expression of the α-Tropomyosin Gene in the Three Muscle Cell Types

In vertebrates, the actin-binding proteins tropomyosins are encoded by four distinct genes that are expressed in a complex pattern during development and muscle differentiation. In this study, we have characterized the transcriptional machinery of the alpha-tropomyosin (alpha-Tm) gene in muscle cells. Promoter analysis revealed that a 284-bp proximal promoter region of the Xenopus laevis alpha-Tm gene is sufficient for maximal activity in the three muscle cell types. The transcriptional activity of this promoter in the three muscle cell types depends on both distinct and common cis-regulatory sequences. We have identified a 30-bp conserved sequence unique to all vertebrate alpha-Tm genes that contains an MCAT site that is critical for expression of the gene in all muscle cell types. This site can bind transcription enhancer factor-1 (TEF-1) present in muscle cells both in vitro and in vivo. In serum-deprived differentiated smooth muscle cells, TEF-1 was redistributed to the nucleus, and this correlated with increased activity of the alpha-Tm promoter. Overexpression of TEF-1 mRNA in Xenopus embryonic cells led to activation of both the endogenous alpha-Tm gene and the exogenous 284-bp promoter. Finally, we show that, in transgenic embryos and juveniles, an intact MCAT sequence is required for correct temporal and spatial expression of the 284-bp gene promoter. This study represents the first analysis of the transcriptional regulation of the alpha-Tm gene in vivo and highlights a common TEF-1-dependent regulatory mechanism necessary for expression of the gene in the three muscle lineages.

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.

Posttranscriptional regulation of myosin heavy chain expression in the heart by triiodothyronine

Triiodothyronine (T3) regulates cardiac contractility in part by regulating the expression of several important cardiac myocyte genes. In the rat, the T3-mediated induction of alpha-myosin heavy chain (MHC) transcription in hypothyroid hearts is rapid, exhibiting zero-order kinetics, whereas the repression of beta-MHC in these same hearts is much slower. To elucidate the mechanism for T3 transcriptional as well as posttranscriptional regulation of both MHC gene isoforms, we used an RT-PCR-based transcription assay and the RNA polymerase II inhibitor actinomycin D in an in vivo model to simultaneously measure specific alpha- and beta-MHC heterogeneous nuclear RNA (hnRNA), mRNA kinetics, and MHC antisense RNA. In vivo actinomycin D treatment blocked alpha-MHC transcription in euthyroid rats by >80% at 2 h and suggested a half-life of alpha-MHC hnRNA of approximately 1 h, whereas actinomycin D inhibited beta-MHC transcription in hypothyroid rats by >75% at 6 h, suggesting a significantly longer hnRNA half-life of approximately 4 h. The effect of actinomycin D on beta-MHC transcription was independent of T3. T3 treatment in hypothyroid animals caused beta-MHC mRNA to decline more rapidly than beta-MHC hnRNA, demonstrating, for the first time, a posttranscriptional mechanism(s). The measured change in beta-MHC mRNA half-life indicates a T3-mediated destabilization of beta-MHC mRNA. To understand the mechanism by which T3 destabilizes beta-MHC mRNA, we measured beta-MHC antisense RNA. beta-MHC antisense RNA is present in euthyroid myocytes, but levels are not significant in hypothyroid myocytes. This differential expression may explain some of the effects of T3 on MHC posttranscriptional regulation.

Transcriptional regulation of the type I myosin heavy chain gene in denervated rat soleus

Denervation (DEN) of rat soleus is associated with a decreased expression of slow type I myosin heavy chain (MHC) and an increased expression of the faster MHC isoforms. The molecular mechanisms behind these shifts remain unclear. We first investigated endogenous transcriptional activity of the type I MHC gene in normal and denervated soleus muscles via pre-mRNA analysis. Our results suggest that the type I MHC gene is regulated via transcriptional processes in the denervated soleus. Deletion and mutational analysis of the rat type I MHC promoter was then used to identify cis elements or regions of the promoter involved in this response. DEN significantly decreased in vivo activity of the -3,500, -2,500, -914, -408, -299, and -215 bp type I MHC promoters, relative to the alpha-skeletal actin promoter. In contrast, normalized -171 promoter activity was unchanged. Mutation of the betae3 element (-214/-190) in the -215 promoter and deletion of this element (-171 promoter) blunted type I downregulation with DEN. In contrast, betae3 mutation in the -408 promoters was not effective in attenuating the DEN response, suggesting the existence of additional DEN-responsive sites between -408 and -215. Western blotting and gel mobility supershift assays demonstrated decreased expression and DNA binding of transcription enhancer factor 1 (TEF-1) with DEN, suggesting that this decrease may contribute to type I MHC downregulation in denervated muscle.

Human MYO18B, a novel unconventional myosin heavy chain expressed in striated muscles moves into the myonuclei upon differentiation

We have characterized a novel unconventional myosin heavy chain, named MYO18B, that appears to be expressed mainly in human cardiac and skeletal muscles and, at lower levels, in testis. MYO18B transcript is detected in all types of striated muscles but at much lower levels compared to class II sarcomeric myosins, and it is up regulated after in vitro differentiation of myoblasts into myotubes. Phylogenetic analysis shows that this myosin belongs to the recently identified class XVIII, however, unlike the other member of this class, it seems to be unique to Vertebrate since it contains two large amino acid domains of unknown function at the N and C-termini. Immunolocalization of MYO18B protein in skeletal muscle cells shows that this myosin heavy chain is located in the cytoplasm of undifferentiated myoblasts. After in vitro differentiation into myotubes, a fraction of this protein is accumulated in a subset of myonuclei. This nuclear localization was confirmed by immunofluorescence experiments on primary cardiomyocytes and adult muscle sections. In the cytoplasm MYO18B shows a punctate staining, both in cardiac and skeletal fibers. In some cases, cardiomyocytes show a partial sarcomeric pattern of MYO18B alternating that of alpha-actinin-2. In skeletal muscle the cytoplasmic MYO18B results much more evident in the fast type fibers.

Phylogenetic implications of the superfast myosin in extraocular muscles

Extraocular muscle exhibits higher-velocity and lower-tension contractions than other vertebrate striated muscles. These distinctive physiological properties are associated with the expression of a novel extraocular myosin heavy chain (MYH). Encoded by the MYH13 gene, the extraocular myosin heavy chain is a member of the fast/developmental MYH gene cluster on human chromosome 17 and the syntenic MYH cluster on mouse chromosome 11. Comparison of cDNA sequences reveals that MYH13 also encodes the atypical MYH identified in laryngeal muscles, which have similar fast contractile properties. Comparing the MYH13 sequence with the other members of the fast/developmental cluster, the slow/cardiac MYH genes and two orphan skeletal MYH genes in the human genome provides insights into the origins of specialization in striated muscle myosins. Specifically, these studies indicate (i) that the extraocular myosin is not derived from the adult fast skeletal muscle myosins, but was the first member of the fast/developmental MYH gene cluster to diverge and specialize, (ii) that the motor and rod domains of the MYH13 have evolved under different selective pressures and (iii) that the MYH13 gene has been largely insulated from genomic events that have shaped other members of the fast/developmental cluster. In addition, phylogenetic footprinting suggests that regulation of the extraocular MYH gene is not governed primarily by myogenic factors, but by a hierarchical network of regulatory factors that relate its expression to the development of extraocular muscles.

Transcriptional regulation of the type I myosin heavy chain promoter in inactive rat soleus

Chronic muscle inactivity with spinal cord isolation (SI) decreases expression of slow type I myosin heavy chain (MHC) while increasing expression of the faster MHC isoforms, primarily IIx. The purpose of this study was to determine whether type I MHC downregulation in the soleus muscle of SI rats is regulated transcriptionally and to identify cis-acting elements or regions of the rat type I MHC gene promoter involved in this response. One week of SI significantly decreased in vivo activity of the -3500-, -408-, -299-, -215-, and -171-bp type I MHC promoters. The activity of all tested deletions of the type I MHC promoter, relative to the human skeletal alpha-actin promoter, were significantly reduced in the SI soleus, except activity of the -171-bp promoter, which increased. Mutation of the betae3 element (-214/-190 bp) in the -215- and -408-bp promoters and deletion of this element (-171-bp promoter) attenuated type I downregulation with SI. Gel mobility shift assays demonstrated a decrease in transcription enhancer factor-1 binding to the betae3 element with SI, despite an increase in total binding to this region. These results demonstrate that type I MHC downregulation with SI is transcriptionally regulated and suggest that interactions between transcription enhancer factor-1 and the betae3 element are likely involved in this response.

Control of cardiac myosin heavy chain gene expression

The alpha- and beta-myosin genes extend over 51 kb on chromosome 14 in human and 11 in mouse separated by about 4.5 kb of intergenic sequence. They are located in tandem in the order of their expression during development. Transcription of each gene is independently controlled but coordinately regulated. During each embryogenesis, the beta-MHC gene is expressed as part of the cardiac myogenic program under the control of NKX-2.5, MEF-2C, and GATA-4/5/6. After birth, thyroid hormone induces expression of alpha-MHC mRNA and inhibits expression of the beta-MHC gene. While a large number of physiological stimuli are capable of modifying this basic paradigm, thyroid hormone is required for expression of alpha-MHC in ventricular muscle. The positive TRE for T(3)-stimulation of alpha-MHC is an imperfect direct repeat located in the proximal promoter of the gene. The negative TRE for the beta-MHC gene is probably a binding half-site that is located adjacent to the TATA box. Binding of TEF-1 to a strong positive element in the proximal promoter is important in basal expression of beta-MHC gene and in the response to alpha(1)-adrenergic stimulation. The beta-MHC gene also is induced together with several other "fetal" genes during cardiac hypertrophy by a mechanism involving Ca(2+)-mediated activation of calcineurin and NF-AT3. Upon activation, NF-AT3 translocates to the nucleus and interacts with GATA-4 to stimulate beta-MHC expression. Changes in chromatin structure mediated by the association of histone acetylases and deacetylases with transcription factors are essential in regulating cell-specific expression of MHC genes.

Early specialization of the superfast myosin in extraocular and laryngeal muscles

Extraocular muscle (EOM) exhibits high-velocity, low-tension contractions compared with other vertebrate striated muscles. These distinctive properties have been associated with a novel myosin heavy chain (MyHC) isoform, MyHC-EO. An atypical MyHC, MyHC IIL, has also been identified in laryngeal muscles that have similarly fast contractile properties. It co-migrates with MyHC-EO on high-resolution SDS gels, but appeared to be encoded by a different mRNA. We combined CNBr peptide maps and full-length cDNA sequences to show that rabbit muscle EO and IIL MyHCs are identical. Analysis of the 5; untranslated region (5;UTR) of the mRNAs identified three variants that result from a combination of alternative splicing and multiple transcription initiation sites. This complex pattern of 5;UTRs has not been reported previously for MyHC genes. We identified the human homologue of the MyHC-EO gene in GenBank, and analyzed the 5; upstream region, which revealed a paucity of muscle-specific transcription factor binding sites compared with the other MyHC genes. These features are likely to be critical to the unique regulation and tissue-specific expression of the MyHC-EO/IIL gene.Phylogenetic analysis indicates that MyHC-EO/IIL diverged from an ancestral MyHC gene to generate the first specialized fast myosin. The catalytic S1 head domain is more closely related to the fast MyHCs, while the rod is more closely related to the slow/cardiac MyHCs. The exon boundaries of the MyHC-EO are identical to those of the embryonic MyHC gene and virtually identical to those of the α and (β) cardiac genes. This implies that most of the current exon boundaries were present in the ancestral gene, predating the duplications that generated the family of skeletal and cardiac myosin genes.

Genetic manipulation of the rabbit heart via transgenesis

BACKGROUND

Transgenesis using cardiac-specific expression has been valuable in exploring cardiac structure-function relationships. To date, cardiac-selective studies have been confined to the mouse. However, the utility of the mouse is limited in certain, possibly critical, aspects with respect to cardiovascular function.

METHODS AND RESULTS

To establish the potential validity of transgenic methodology for remodeling a larger mammalian heart, we explored cardiac-selective expression in transgenic rabbits. The murine alpha- and beta-cardiac myosin heavy chain gene promoters were used to express a reporter gene, and transgene expression was quantified in cardiac, skeletal, and smooth muscles as well as in nonmuscle tissues. Although neither promoter exactly mimics endogenous patterns of myosin heavy chain expression, both are able to drive high levels of transgene expression in the cardiac compartment. Neither promoter is active in smooth muscle or nonmuscle tissues.

CONCLUSIONS

Directed organ-specific expression is feasible in a larger animal with existing reagents, and cardiac-selective transgenic manipulation is possible in the rabbit.

Segregated regulatory elements direct beta-myosin heavy chain expression in response to altered muscle activity

Our previous transgenic analyses revealed that a 600-base pair beta-myosin heavy chain (betaMyHC) promoter conferred mechanical overload (MOV) and non-weight-bearing (NWB) responsiveness to a chloramphenicol acetyltransferase reporter gene. Whether the same DNA regulatory element(s) direct betaMyHC expression following MOV or NWB activity in vivo remains unknown. We now show that a 293-base pair betaMyHC promoter fused to chloramphenicol acetyltransferase (beta293) responds to MOV, but not NWB activity, indicating a segregation of these two diverse elements. Inclusion of the betaMyHC negative regulatory element (-332 to -300; betaNRE) within transgene beta350 repressed expression in all transgenic lines. Electrophoretic mobility shift assays showed highly enriched binding activity only in NWB soleus nuclear extracts that was specific to the distal region of the betaNRE sense strand (dbetaNRE-S; -332 to -311). Supershift electrophoretic mobility shift assay revealed that the binding at the distal region of the betaNRE sense strand was antigenically distinct from cellular nucleic acid-binding protein and Y-box-binding factor 1, two proteins shown to bind this element. Two-dimensional UV cross-linking and shift Southwestern blotting analyses detected two proteins (50 and 52 kDa) that bind to this element. These in vivo results demonstrate that segregated betaMyHC promoter elements transcriptionally regulate betaMyHC transgene expression in response to two diverse modes of neuromuscular activity.

GATA-5 is involved in leukemia inhibitory factor-responsive transcription of the beta-myosin heavy chain gene in cardiac myocytes

Leukemia inhibitory factor is a member of a family of structurally related cytokines sharing the receptor component gp130. Activation of gp130 by leukemia inhibitory factor is sufficient to induce myocardial cell hypertrophy accompanied by specific changes in the pattern of gene expression. However, the molecular mechanisms that link gp130 activation to these changes have not been clarified. The present study investigated the transcriptional pathways by which leukemia inhibitory factor activates beta-myosin heavy chain expression during myocardial cell hypertrophy. Mutation of the GATA motif in the beta-myosin heavy chain promoter totally abolished leukemia inhibitory factor-responsive transcription without changing basal transcriptional activity. In contrast, endothelin-1 responsiveness was unaffected by the GATA mutation. Among members of the cardiac GATA transcription factor subfamily (GATA-4, -5, and -6), GATA-5 was the sole and potent transactivator for the beta-myosin heavy chain promoter. This transactivation was dependent on sequence-specific binding of GATA-5 to the beta-myosin heavy chain GATA element. Cardiac nuclear factors that bind to to the beta-myosin heavy chain GATA element were induced by leukemia inhibitory factor stimulation. Last, leukemia inhibitory factor stimulation markedly increased transcripts of cardiac GATA-5, the expression of which is normally restricted to the early embryo. Thus, GATA-5 may be involved in gp130 signaling in cardiac myocytes.

Organization of human and mouse skeletal myosin heavy chain gene clusters is highly conserved

Myosin heavy chains (MyHCs) are highly conserved ubiquitous actin-based motor proteins that drive a wide range of motile processes in eukaryotic cells. MyHC isoforms expressed in skeletal muscles are encoded by a multigene family that is clustered on syntenic regions of human and mouse chromosomes 17 and 11, respectively. In an effort to gain a better understanding of the genomic organization of the skeletal MyHC genes and its effects on the regulation, function, and molecular genetics of this multigene family, we have constructed high-resolution physical maps of both human and mouse loci using PCR-based marker content mapping of P1-artificial chromosome clones. Genes encoding six MyHC isoforms have been mapped with respect to their linear order and transcriptional orientations within a 350-kb region in both human and mouse. These maps reveal that the order, transcriptional orientation, and relative intergenic distances of these genes are remarkably conserved between these species. Unlike many clustered gene families, this order does not reflect the known temporal expression patterns of these genes. However, the conservation of gene organization since the estimated divergence of these species (approximately 75-110 million years ago) suggests that the physical organization of these genes may be significant for their regulation and function.

Tissue-restricted expression of the cardiac alpha-myosin heavy chain gene is controlled by a downstream repressor element containing a palindrome of two ets-binding sites

The expression of the alpha-myosin heavy chain (MHC) gene is restricted primarily to cardiac myocytes. To date, several positive regulatory elements and their binding factors involved in alpha-MHC gene regulation have been identified; however, the mechanism restricting the expression of this gene to cardiac myocytes has yet to be elucidated. In this study, we have identified by using sequential deletion mutants of the rat cardiac alpha-MHC gene a 30-bp purine-rich negative regulatory (PNR) element located in the first intronic region that appeared to be essential for the tissue-specific expression of the alpha-MHC gene. Removal of this element alone elevated (20- to 30-fold) the expression of the alpha-MHC gene in cardiac myocyte cultures and in heart muscle directly injected with plasmid DNA. Surprisingly, this deletion also allowed a significant expression of the alpha-MHC gene in HeLa and other nonmuscle cells, where it is normally inactive. The PNR element required upstream sequences of the alpha-MHC gene for negative gene regulation. By DNase I footprint analysis of the PNR element, a palindrome of two high-affinity Ets-binding sites (CTTCCCTGGAAG) was identified. Furthermore, by analyses of site-specific base-pair mutation, mobility gel shift competition, and UV cross-linking, two different Ets-like proteins from cardiac and HeLa cell nuclear extracts were found to bind to the PNR motif. Moreover, the activity of the PNR-binding factor was found to be increased two- to threefold in adult rat hearts subjected to pressure overload hypertrophy, where the alpha-MHC gene is usually suppressed. These data demonstrate that the PNR element plays a dual role, both downregulating the expression of the alpha-MHC gene in cardiac myocytes and silencing the muscle gene activity in nonmuscle cells. Similar palindromic Ets-binding motifs are found conserved in the alpha-MHC genes from different species and in other cardiac myocyte-restricted genes. These results are the first to reveal a role of the Ets class of proteins in controlling the tissue-specific expression of a cardiac muscle gene.

Identification of regulatory regions which confer muscle-specific gene expression

For many newly sequenced genes, sequence analysis of the putative protein yields no clue on function. It would be beneficial to be able to identify in the genome the regulatory regions that confer temporal and spatial expression patterns for the uncharacterized genes. Additionally, it would be advantageous to identify regulatory regions within genes of known expression pattern without performing the costly and time consuming laboratory studies now required. To achieve these goals, the wealth of case studies performed over the past 15 years will have to be collected into predictive models of expression. Extensive studies of genes expressed in skeletal muscle have identified specific transcription factors which bind to regulatory elements to control gene expression. However, potential binding sites for these factors occur with sufficient frequency that it is rare for a gene to be found without one. Analysis of experimentally determined muscle regulatory sequences indicates that muscle expression requires multiple elements in close proximity. A model is generated with predictive capability for identifying these muscle-specific regulatory modules. Phylogenetic footprinting, the identification of sequences conserved between distantly related species, complements the statistical predictions. Through the use of logistic regression analysis, the model promises to be easily modified to take advantage of the elucidation of additional factors, cooperation rules, and spacing constraints.

Chromatin remodelling of the cardiac beta-myosin heavy chain gene

To investigate the role of chromatin structure in cardiac gene expression, we used the DNase I and micrococcal nuclease to probe the chromatin structure of the hamster cardiac beta-MyHC gene. Two cardiac-specific DNase I hypersensitive sites (DHS) were identified, one of which was mapped to the -2.3 kb (beta-2.3 kb) region and the other to the proximal promoter region of the beta-MyHC gene. The two sites were readily detectable using nuclei from neonatal hamster heart; however, the proximal promoter site disappeared when adult hamster heart nuclei were used, and the -2.3 kb site decreased in intensity. We were able to demonstrate the gradual disappearance of this proximal promoter DHS by comparing heart nuclei isolated from animals at late-gestation and 1-day-old stages. Furthermore, injecting thyroid hormone caused the disappearance of the proximal promoter DHS in late gestational fetal ventricular nuclei. Digestion of nuclei from various tissues by micrococcal nuclease revealed that the beta-MyHC gene proximal promoter exists in an array of three specifically-positioned nucleosomes only in fetal heart chromatin. The beta-MyHC gene proximal promoter is DNase I hypersensitive within one of the nucleosomal particles. Our data suggest that chromatin structure may participate actively in cardiac gene expression.

cis-Acting sequences that mediate induction of beta-myosin heavy chain gene expression during left ventricular hypertrophy due to aortic constriction

BACKGROUND

Marked alterations in the expression of specific genes occur during the development of cardiac hypertrophy in vivo. Little is known, however, about the cis-acting elements that mediate these changes in response to clinically relevant hypertrophic stimuli, such as hemodynamic overload, in intact adult animals.

METHODS AND RESULTS

The left ventricular expression of a directly injected reporter gene driven by 3542 bp of rat beta-myosin heavy chain (beta-MHC) promoter was increased 3.0-fold by aortic constriction (P<.005), an increment similar to the 3.2-fold increase in the level of the endogenous beta-MHC mRNA in the same left ventricles. Subsequent analysis identified a 107-bp beta-MHC promoter sequence (-303/-197) sufficient to convert a heterologous neutral promoter to one that is activated by aortic constriction. These sequences contain two M-CAT elements, which have previously been demonstrated to mediate inducible expression during alpha1-adrenergic-stimulated hypertrophy in cultured neonatal cardiac myocytes, and a GATA element. Although simultaneous mutation of both M-CAT elements markedly decreased the basal transcriptional activity of an injected 333-bp beta-MHC promoter, it had no effect on aortic constriction-stimulated transcription (3.5-fold increase, P<.005 for both wild type and mutant). In contrast, mutation of the GATA motif markedly attenuated aortic constriction-stimulated transcription (1.6-fold, P=NS) without affecting the basal transcriptional activity. This GATA site can interact with in vitro translated GATA-4 and compete with an established GATA site for GATA-4 binding activity in nuclear extracts from aortic constricted hearts.

CONCLUSIONS

Basal and aortic constriction-stimulated transcription of the beta-MHC gene is mediated, at least in part, through different mechanisms. A GATA element within beta-MHC sequences -303/-197 plays a role in the transcriptional activation of this gene by aortic constriction.

Multiple muscle-specific regulatory elements are associated with a DNase I hypersensitive site of the cardiac beta-myosin heavy-chain gene

Using nuclei isolated from neonatal cardiomyocytes, we have mapped the DNase I hypersensitive sites (DHSs) residing within the 5'-upstream regions of the hamster cardiac myosin heavy-chain (MyHC) gene. Two cardiac-specific DHSs within the 5 kb upstream region of the cardiac MyHC gene were identified. One of the DHSs was mapped to the -2.3 kb (beta-2.3 kb) region and the other to the proximal promoter region. We further localized the beta-2.3 kb site to a range of 250 bp. Multiple, conserved, muscle regulatory motifs were found within the beta-2.3 kb site, consisting of three E-boxes, one AP-2 site, one CArG motif, one CT/ACCC box and one myocyte-specific enhancer factor-2 site. This cluster of regulatory elements is strikingly similar to a cluster found in the enhancer of the mouse muscle creatine kinase gene (-1256 to -1050). The specific interaction of the motifs within the beta-2.3 kb site and the cardiac nuclear proteins was demonstrated using gel mobility-shift assays and footprinting analysis. In addition, transfection analysis revealed a significant increase in chloramphenicol acetyltransferase activity when the beta-2.3 kb site was linked to a heterologous promoter. These results suggest that previously undefined regulatory elements of the beta-MyHC gene may be associated with the beta-2.3 kb site.

Transcriptional coactivator p300 stimulates cell type-specific gene expression in cardiac myocytes

Terminal differentiation is characterized by cell cycle arrest and the expression of cell type-specific genes. Previous work has suggested that the p300 family of transcriptional coactivators plays an important role in preventing the re-initiation of DNA synthesis in terminally differentiated cardiac myocytes. In this study, we investigated whether p300 proteins are also involved in the transcriptional activation of cell type-specific genes in these cells. Since p300 function can be abrogated through direct binding by the adenovirus E1A protein, we overexpressed E1A in cardiac myocytes using recombinant adenoviral vectors. The expression of transfected reporter genes driven by alpha- or beta-myosin heavy chain promoters was markedly diminished by expression of the 12 S E1A protein. In contrast, the activity of a promoter derived from the ubiquitously expressed beta-actin gene was affected only modestly. While an E1A mutant unable to bind members of the retinoblastoma family of pocket proteins decreased the activity of alpha- and beta-myosin heavy chain promoters to nearly the same extent as wild type 12 S E1A, transcriptional repression by a mutant defective for p300 binding was severely impaired. Furthermore, overexpression of p300 and, to an even greater extent, p300del33, a mutant lacking residues required for binding by E1A, relieved E1A's repression of beta-myosin heavy chain promoter activity while having no effect on the activity of the beta-actin promoter. Thus, E1A's transcriptional repression of cell type-specific genes in cardiac myocytes is mediated through its binding of p300 proteins, and these proteins appear to be involved in maintaining both cell type-specific gene expression and cell cycle arrest in cardiac myocytes.

Divergence of beta-myosin heavy chain (betaMHC) expression in fetal rat cardiomyocytes in vitro and adult rat heart in vivo

The myosin heavy chain gene products are an important determinant of myocardial functional properties. Although a strong positive element (beta f1) within the betaMHC promoter region has previously been identified, to date no species comparisons in promoter strength have been made. To examine this question, we have used betaMHC deletion constructs, containing the rat or human beta f1 enhancer region, to determine expression both in vitro using rat fetal cardiomyocytes and in vivo by direct injection into adult rat heart. When reporter constructs were transfected into cultured fetal rat cardiomyocytes, the human beta reporter was expressed more than 3 fold above the equivalent rat construct. Exchange of the beta f1 enhancer indicated that the human sequence of the beta f1 enhancer was largely responsible. However, these findings were not replicated when the reporters were injected into the adult rat heart. In the adult myocardium the levels of reporter expression were similar for the betaMHC promoter reporters studied. These findings demonstrate a divergence between primary cardiomyocytes maintained in culture and the cardiomyocytes found within the intact adult heart.

Induction of beta-MHC transgene in overloaded skeletal muscle is not eliminated by mutation of conserved elements

Mechanical overload leads to hypertrophy, increased type I fiber composition, and beta-myosin heavy chain (beta-MHC) induction in the fast-twitch plantaris muscle. To better understand the mechanism(s) involved in beta-MHC induction, we have examined inducible expression of transgenes carrying the simultaneous mutation of three DNA regulatory subregions [muscle CAT (MCAT), C-rich, and beta e3] in the context of either 5,600-base pair (bp; beta 5.6mut3) or 600-bp (beta 0.6mut3) beta-MHC promoter in overloaded plantaris muscles of transgenic mice. Protein extract from mechanically overloaded plantaris muscle of mice, harboring either mutant transgene beta 5.6mut3 or beta 0.6mut3, showed an unexpected 2.8- to 4.5-fold increase in chloramphenicol acetyltransferase (CAT) specific activity relative to their respective controls. Similar results were obtained with wild-type (wt) beta-MHC transgenes (beta 5.6wt, beta 0.6wt). Histochemical staining for both myofibrillar ATPase and CAT activity and CAT immunohistochemistry revealed a striking increase in type I fibers and that CAT expression was restricted to these fibers in overloaded plantaris muscle of beta 5.6mut3 transgenic mice. Our transgenic data suggest that beta-MHC transgenes, and perhaps the endogenous beta-MHC gene, are induced by mechanical overload via a mechanism(s) that does not exclusively require the MCAT, C-rich, or beta e3 subregions.

The role of transcription enhancer factor-1 (TEF-1) related proteins in the formation of M-CAT binding complexes in muscle and non-muscle tissues

M-CAT sites are required for the activity of many promoters in cardiac and skeletal muscle. M-CAT binding activity is muscle-enriched, but is found in many tissues and is immunologically related to the HeLa transcription enhancer factor-1 (TEF-1). TEF-1-related cDNAs (RTEF-1) have been cloned from chick heart. RTEF-1 mRNA is muscle-enriched, consistent with a role for RTEF-1 in the regulation of muscle-specific gene expression. Here, we have examined the tissue distribution of TEF-1-related proteins and of M-CAT binding activity by Western analysis and mobility shift polyacrylamide gel electrophoresis. TEF-1-related proteins of 57, 54 and 52 kDa were found in most tissues with the highest levels in muscle tissues. All of these TEF-1-related proteins bound M-CAT DNA and the 57- and 54-kDa TEF-1-related polypeptides were phosphorylated. Proteolytic digestion mapping showed that the 54-kDa TEF-1-related polypeptide is encoded by a different gene than the 52- and 57-kDa TEF-1-related polypeptides. A comparison of the migration and proteolytic digestion of the 54-kDa TEF-1-related polypeptide with proteins encoded by the cloned RTEF-1 cDNAs showed that the 54-kDa TEF-1-related polypeptide is encoded by RTEF-1A. High resolution mobility shift polyacrylamide gel electrophoresis showed multiple M-CAT binding activities in tissues. All of these activities contained TEF-1-related proteins. One protein-M-CAT DNA complex was muscle-enriched and was up-regulated upon differentiation of a skeletal muscle cell line. This complex contained the 54-kDa TEF-1-related polypeptide. Therefore, RTEF1-A protein is a component of a muscle-enriched transcription complex that forms on M-CAT sites and may play a key role in the regulation of transcription in muscle.

A novel site in the muscle creatine kinase enhancer is required for expression in skeletal but not cardiac muscle

Expression of the muscle creatine kinase (MCK) gene in skeletal and heart muscle is controlled in part by a 5' tissue-specific enhancer. In order to identify new regulatory elements, we designed mutations in a previously untested conserved portion of this enhancer. Transfection analysis of these mutations delineated a new control element, named Trex (Transcriptional regulatory element x), which is required for full transcriptional activity of the MCK enhancer in skeletal but not cardiac muscle cells. Gel mobility shift assays demonstrate that myocyte, myoblast, and fibroblast nuclear extracts but not primary cardiomyocyte nuclear extracts contain a trans-acting factor that binds specifically to Trex. The Trex sequence is similar (7/8 bases) to the TEF-1 consensus DNA-binding site involved in regulating other muscle genes. To determine if TEF-1 interacts with Trex, selected TEF-1 binding sites such as GTIIc and M-CAT and two anti-TEF-1 antisera were used in gel shift assays. These experiments strongly suggest that a factor distinct from TEF-1 binds specifically to Trex. Thus it appears that MCK transcription is regulated in skeletal muscles through a Trex-dependent pathway while Trex is not required for MCK expression in heart. This distinction could account partially for the difference in levels of muscle creatine kinase in these tissues.

cDNA cloning and characterization of murine transcriptional enhancer factor-1-related protein 1, a transcription factor that binds to the M-CAT motif

The M-CAT motif is a cis-regulatory DNA sequence that is essential for muscle-specific transcription of several genes. Previously, we had shown that both muscle-specific (A1) and ubiquitous (A2) factors bind to an essential M-CAT motif in the myosin heavy chain beta gene and that the ubiquitous factor is transcriptional enhancer factor (TEF)-1. Here we report the isolation of mouse cDNAs encoding two forms (a and b) of a TEF-1-related protein, TEFR1. The TEFR1a cDNA encodes a 427-amino acid protein. The coding region of TEFR1b is identical to 1a in both nucleotide and predicted amino acid sequence except for the absence of 43 amino acids downstream of the TEA DNA-binding domain. Three TEFR1 transcripts (approximately 7, approximately 3.5, and approximately 2 kilobase pairs) are enriched in differentiated skeletal muscle (myotubes) relative to undifferentiated skeletal muscle (myoblasts) and non-muscle cells in culture. In situ hybridization analysis indicated that TEFR1 transcripts are enriched in the skeletal muscle lineage during mouse embryogenesis. Transient expression of fusion proteins of TEFR1 and the yeast GAL4 DNA-binding domain in cell lines activated the expression of chloramphenicol acetyltransferase (CAT) reporter constructs containing GAL4 binding sites, indicating that TEFR1 contains an activation domain. An anti-TEFR1 polyclonal antibody supershifted the muscle-specific M-CAT.A1 factor complex in gel mobility shift assays, suggesting that TEFR1 is a major component of this complex. Our results suggest that TEFR1 might play a role in the embryonic development of skeletal muscle in the mouse.

Control of beta-myosin heavy chain expression in systemic hypertension and caloric restriction in the rat heart

In the rat left ventricle, both pressure overload induced by abdominal aortic constriction (Abcon) and caloric restriction (CR) induce an increase in the steady-state level of the beta-myosin heavy chain (MHC) protein and mRNA. Both models also induce a concomitant decrease in the alpha-MHC protein and mRNA. The goals of this study were to 1) determine if the changes in MHC expression in the models are due to altered transcription and 2) identify the relative levels of some key factors interacting with the regulatory regions of these genes. Female Sprague-Dawley rats were randomly assigned to the following groups: 1) normal control (NC), 2) Abcon, and 3) CR. After 5 wk of experimental manipulations, myocardial nuclei were isolated. These nuclei were used for 1) nuclear run-on assays or 2) nuclear extract, which was prepared and used for gel mobility shift assays (GMSAs). Nuclear run-on assays demonstrated that the increase in beta-MHC mRNA and protein expression in both Abcon and CR can be at least partially attributed to increased transcription. The concomitant decrease in alpha-MHC content can similarly be attributed to a decrease in transcription of this gene. Furthermore, GMSAs demonstrate that nuclear extract from each group interact differently with certain elements known to be important for expression in vitro. CR nuclear extracts have a 25.6 +/- 7.2% decrease (P < 0.05 vs. NC) in interaction with a thyroid-responsive element, a potential repressor of beta-MHC transcription.(ABSTRACT TRUNCATED AT 250 WORDS)

Position independent expression and developmental regulation is directed by the beta myosin heavy chain gene's 5' upstream region in transgenic mice

Transgenic mice generated with constructs containing 5.6 kb of the beta myosin heavy chain (MyHC) gene's 5' flanking region linked to the cat reporter gene express the transgene at high levels. In all 47 lines analyzed, tissue-specific accumulation of chloramphenicol acetyltransferase was found at levels proportional to the number of integrated transgene copies. Deletion constructs containing only 0.6 kb of 5' upstream region showed position effects in transgenic mice and did not demonstrate copy number dependence although transgene expression remained muscle-specific. The 5.6 kb 5' upstream region conferred appropriate developmental control of the transgene to the cardiac compartment and directs copy number dependent and position independent expression. Lines generated with a construct in which three proximal cis-acting elements were mutated showed reduced levels of transgene expression, but all maintained their position independence and copy number dependence, suggesting the presence of distinct regulatory mechanisms.

Plasticity of vascular smooth muscle alpha-actin gene transcription. Characterization of multiple, single-, and double-strand specific DNA-binding proteins in myoblasts and fibroblasts

Transcriptional activity of the mouse vascular smooth muscle (VSM) alpha-actin promoter was governed by both cell type and developmental stage-specific mechanisms. A purine-rich motif (PrM) located as -181 to -176 in the promoter was absolutely required for activation in mouse AKR-2B embryonic fibroblasts and partially contributed to activation in undifferentiated mouse BC3H1 myoblasts. Transcriptional enhancer factor 1 recognized the PrM and cooperated with other promoter-binding proteins to regulate serum growth factor-dependent transcription in both myoblasts and fibroblasts. Two distinct protein factors (VAC-ssBF1 and VAC-ssBF2) also were identified that bound sequence-specifically to single-stranded oligonucleotide probes that spanned both the PrM and a closely positioned negative regulatory element. VAC-ssBF1 and BF2 binding activity was detected in undifferentiated myoblasts, embryonic fibroblasts, and several smooth muscle tissues in the mouse and human. A myoblast-specific protein (VAC-RF1) also was detected that bound double-stranded probes containing a CArG-like sequence that previously was shown to impart strong, cell type specific repression. The binding activity of transcription enhancer factor 1, VAC-RF1, and VAC-ssBF1 was significantly diminished when confluent BC3H1 myoblasts differentiated into myocytes and expressed VSM alpha-actin mRNA after exposure to serum-free medium. The results indicated that cell type-specific control of the VSM alpha-actin gene promoter required the participation of multiple DNA-binding proteins, including two that were enriched in smooth muscle and had preferential affinity for single-stranded DNA.

Segregation of cardiac and skeletal muscle-specific regulatory elements of the beta-myosin heavy chain gene

The beta-myosin heavy chain (beta-MyHC) gene is expressed in cardiac and slow skeletal muscles. To examine the regulatory sequences that are required for the gene's expression in the two compartments in vivo, we analyzed the expression pattern of a transgene consisting of the beta-MyHC gene 5' upstream region linked to the chloramphenicol acetyltransferase reporter gene. By using 5600 bp of 5' upstream region, the transgene was expressed at high levels in the slow skeletal muscles. Decreased levels of thyroid hormone led to the up-regulation of the transgene in both cardiac and skeletal muscles, mimicking the behavior of the endogenous beta-MyHC gene. After deleting the distal 5000 bp, the level of reporter gene expression was strongly reduced. However, decreased levels of thyroid hormone led to an 80-fold skeletal muscle-specific increase in transgene expression, even upon the ablation of a conserved cis-regulatory element termed MCAT, which under normal (euthyroid) conditions abolishes muscle-specific expression. In contrast, cardiac-specific induction was not detected with the deletion construct. These observations indicate that the cardiac and skeletal muscle regulatory elements can be functionally segregated on the beta-MyHC gene promoter.

In vivo expression and molecular characterization of the porcine slow-myosin heavy chain

We report on the molecular characterization of the porcine slow-myosin heavy chain (HC) beta gene and the isolation of its 5′ end cDNA. In vivo expression study, by in situ hybridization and histochemistry, revealed a highly regular rosette pattern of fiber arrangement, with a slow fiber occupying the central core, in all the skeletal muscles examined. This feature can be advantageous in the distinction of primary and secondary fibers in myogenic lineage studies. In the neonatal heart, beta isoform expression is diffuse, with higher expression occurring in the ventricle than in the atrium. Transient transfection assays showed the porcine promoter functions in a muscle- and differentiation stage-specific manner. In the 5′ regulatory region are several putative positive and negative regulatory elements, including a positive and a negative element in close proximity to each other in intron 1.

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.

Regulation of myosin heavy chain genes in the heart

Advances in our knowledge of the regulation of cardiac myosin isoforms made possible by molecular cloning of the alpha- and beta-MHCs genes are reviewed. Expression of these genes in heart does not seem to require MyoD or related proteins of the skeletal muscle myogenic program. Cardiac MHC genes are under the control of T3, which stimulates transcription of the alpha-MHC gene and inhibits beta-MHC mRNA production both in vivo and in cultured heart cells. The responsiveness of the genes to T3 varies in different mammals, however. The genes are most responsive in rat and rabbit, intermediate in sensitivity in calf and subhuman primate (baboon), and very resistant in the dog. The human alpha-MHC gene is T3-inducible in ventricle, but the degree of response has not been quantified. Introduction of chimeric plasmids containing 5' flanking sequences of cardiac MHC genes fused to the CAT gene into cultured heart cells and transgenic animals has permitted identification of regulatory elements. Although the genes are closely linked in genomic DNA, they are controlled independently. The element within the alpha-MHC promoter responsible for induction by T3 is located approximately 160 base pairs from the transcription initiation site. Additional transcriptional activators located 5' upstream amplify the response to T3, probably by looping out intervening DNA sequences. The proximal region of the beta-MHC gene contains important regulatory elements, including those required for repression by T3, muscle-specific expression, a MyoD-independent positive element, and a hormone-independent repressor.(ABSTRACT TRUNCATED AT 250 WORDS)