Muscle-specific gene expression controlled by a regulatory element lacking a MyoD1-binding site

Isolation of novel cDNA encompassing the ADU balanced translocation break point in the DiGeorge critical region

DiGeorge syndrome (DGS) is a developmental field defect of the third and fourth pharyngeal pouches that are associated with congenital heart defects, hypoparathyroidism, cell-mediated immunodeficiency, velopharyngeal insufficiency, and craniofacial anomalities. Approximately 90% of patients exhibit monosomy in the 22q11 region. In order to isolate the critical gene responsible for DGS, the cDNA libraries were screened with a probe containing the ADU balanced translocation break point, that is a locus reported in one patient (ADU) caused by a balanced translocation between chromosomes 22 and 2. Out of 10(6) clones, three independent overlapping clones were isolated, which were assumed to have originated from a single transcript, DGCR7. This transcript contained a 175-aa long open reading frame (ORF), encoding an acidic (pI = 5.81) and a proline-rich peptide, which are often found in the activation domain of several transcription factors. Also, it was predicted to be a nuclear protein. Northern hybridization detected an approx 1.9 kb transcript in all fetal and adult tissues tested, with strong expression in the fetal liver and kidney. In the case of adult tissues, strong expression was also detected in areas such as the heart, skeletal muscle, liver, and kidney.

Expression of the alpha 5 integrin subunit gene promoter is positively regulated by the extracellular matrix component fibronectin through the transcription factor Sp1 in corneal epithelial cells in vitro

The accumulation of fibronectin (FN) in response to corneal epithelium injury has been postulated to turn on expression of the FN-binding integrin alpha(5)beta(1). In this work, we determined whether the activity directed by the alpha(5) gene promoter can be modulated by FN in rabbit corneal epithelial cells (RCEC). The activity driven by chloramphenicol acetyltransferase/alpha(5) promoter-bearing plasmids was drastically increased when transfected into RCEC grown on FN-coated culture dishes. The promoter sequence mediating FN responsiveness was shown to bear a perfect inverted repeat that we designated the fibronectin-responsive element (FRE). Analyses in electrophoretic mobility shift assays provided evidence that Sp1 is the predominant transcription factor binding the FRE. Its DNA binding affinity was found to be increased when RCEC are grown on FN-coated dishes. The addition of the MEK kinase inhibitor PD98059 abolished FN responsiveness suggesting that alteration in the state of phosphorylation of Sp1 likely accounts for its increased binding to the alpha(5) FRE. The FRE also proved sufficient to confer FN responsiveness to an otherwise unresponsive heterologous promoter. However, site-directed mutagenesis indicated that only the 3' half-site of the FRE was required to direct FN responsiveness. Collectively, binding of FN to its alpha(5)beta(1) integrin activates a signal transduction pathway that results in the transcriptional activation of the alpha(5) gene likely through altering the phosphorylation state of Sp1.

Murine and human cathepsin Z: cDNA-cloning, characterization of the genes and chromosomal localization

A novel murine cysteine protease from the family of papain-like cysteine proteinases was identified by dbEST-database search. A 1. 4-kb full-length cDNA encoding a predicted polypeptide of 306 amino acids was characterized. The new protease, tentatively named cathepsin Z, exhibits all features characteristics of a papain-like cysteine protease, including the highly conserved residues of the 'catalytic triad'. Cathepsin Z shares only 26-35% overall homology with previously described mammalian papain-like cysteine peptidases and has an unusually short propeptide, which may indicate that it is a member of a putative new subfamily within the family of papain-like cysteine peptidases. Genomic clones covering the murine and human cathepsin Z genes were isolated. They comprise six exons and five introns spanning a 12-kb region of genomic DNA, respectively. Murine cathepsin Z was mapped to chromosome 2, a region with synteny homology to a region of human chromosome 20 to which human cathepsin Z has been mapped previously. Northern blot analysis revealed ubiquitous expression of murine cathepsin Z. Multiple transcriptional start sites were identified for the murine cathepsin Z gene and together with the absence of a TATA box, a high G+C content, a CpG island and the presence of several Sp1-binding sites in the promoter region, murine cathepsin Z may be classified as a 'housekeeping' gene.

Tissue-selective expression of alpha-dystrobrevin is determined by multiple promoters

alpha-Dystrobrevin, the mammalian orthologue of the Torpedo 87-kDa postsynaptic protein, is a dystrophin-associated and dystrophin-related protein. Knockout of the gene in the mouse results in muscular dystrophy. The control of the alpha-dystrobrevin gene in the various tissues is therefore of interest. Multiple dystrobrevin isoforms differing in their domain content are generated by alternative splicing of a single gene. The data presented here demonstrate that expression of alpha-dystrobrevin from three promoters, that are active in a tissue-selective manner, also plays a role in the function of the protein in different tissues. The most proximal promoter A is active in brain and to a lesser extent in lung, whereas the most distal promoter B, which possesses several Sp1 binding sites, is restricted to brain. Promoter C, which contains multiple consensus myogenic binding sites, is up-regulated during in vitro myoblast differentiation. Interestingly, the organization and the activity of the alpha-dystrobrevin promoters is reminiscent of those in the dystrophin gene. Taken together we suggest that the multipromoter system, distributed over a region of 270 kilobases at the 5'-end of the alpha-dystrobrevin gene, has been developed to allow the regulation of this gene in different cell types and/or different developmental stages.

M band proteins myomesin and skelemin are encoded by the same gene: analysis of its organization and expression

The complete exon-intron organization of the murine gene encoding sarcomeric myomesin has been determined. The gene is composed of 38 exons and 37 introns, spanning approximately 105 kb of DNA. Intron positions and phases are essentially identical to those identified in M-protein. They are related to the modular structure of myomesin, which is composed almost entirely of immunoglobulin and fibronectin type III domains. Nearly all repeats follow a two exon-one domain structure. The start and end of each domain are defined by introns in phase I, while internal introns are more divergent in position and very rarely use phase I. Genomic Southern blotting and reverse transcription-polymerase chain reaction revealed that differential splicing of a single exon gives rise to two polypeptides, described in the literature as myomesin and skelemin, respectively. A single transcriptional start point was detected in both skeletal and cardiac muscle. Analysis of the presumptive promoter region revealed several potential regulatory elements. CAT expression assays using promoter deletion constructs identified three regions that seem to be important for the muscle-specific transcriptional activation of the myomesin gene. These results provide the basis for a comparative analysis of the regulation of myomesin and M-protein genes in vivo.

Structure and polymorphism of the mouse prion protein gene

Missense mutations in the prion protein (PrP) gene, overexpression of the cellular isoform of PrP (PrPC), and infection with prions containing the scrapie isoform of PrP (PrPSc) all cause neurodegenerative disease. To understand better the physiology and expression of PrPC, we retrieved mouse PrP gene (Prn-p) yeast artificial chromosome (YAC), cosmid, phage, and cDNA clones. Physical mapping positions Prn-p approximately 300 kb from ecotropic virus integration site number 4 (Evi-4), compatible with failure to detect recombination between Prn-p and Evi-4 in genetic crosses. The Prn-pa allele encompasses three exons, with exons 1 and 2 encoding the mRNA 5' untranslated region. Exon 2 has no equivalent in the Syrian hamster and human PrP genes. The Prn-pb gene shares this intron/exon structure but harbors an approximately 6-kb deletion within intron 2. While the Prn-pb open reading frame encodes two amino acid substitutions linked to prolonged scrapie incubation periods, a deletion of intron 2 sequences also characterizes inbred strains such as RIII/S and MOLF/Ei with shorter incubation periods, making a relationship between intron 2 size and scrapie pathogenesis unlikely. The promoter regions of a and b Prn-p alleles include consensus Sp1 and AP-1 sites, as well as other conserved motifs which may represent binding sites for as yet unidentified transcription factors.

Molecular cloning and functional analysis of the promoter of rat skeletal muscle voltage-sensitive sodium channel subtype 2 (rSkM2): evidence for muscle-specific nuclear protein binding to the core promoter

rSkM2 is a tetrodotoxin-resistant rat skeletal muscle voltage-sensitive sodium channel that is expressed in immature and denervated skeletal muscle and in adult heart. We have isolated a 3.7-kb gene segment that contains the first exon, multiple transcription initiation sites, the core promoter (nt -102 to +1), GC-rich elements (Sp1 recognition sites), three overlapping C-rich motifs (important for muscle-specific expression of some muscle genes), and multiple CANNTG (E-box) motifs (MyoD binding sites). A deletion analysis of the 5' upstream 2.8-kb segment, driving the rSkM2 core promoter, has localized a muscle-restrictive enhancer element (MRSE) at least 2 kb upstream from the core promoter. The core promoter is silenced by an additional cis element (-645/-506). The positive and negative cis-elements together drive transcription of the chloramphenicol acetyltransferase (CAT) reporter gene from the core promoter at about the same level as does the core promoter alone in a skeletal muscle differentiation stage-specific manner. Gel-shift assays have identified sequence- and cell-type-specific proteins that bind to a 16-bp region (-44/-29) containing C-rich motifs. Muscle-specific complexes formed from muscle cell nuclear extracts and a 16-bp element (-44/-29) are competed by unlabeled -44/-29 oligonucleotide but not by several mutant oligonucleotides that implicate nucleotides -40 to -38 and -34 to -32 in the binding of a nuclear protein (designated SkM2 transcription factor 1, SkM2-TF1). We conclude that rSkM2 gene expression depends on the interactions of positive and negative transcriptional regulators with tissue- and developmental stage-specific core promoter elements.

Functional nicotinic receptor expression in mesodermal cells transfected with MyoD cDNA

Previous studies had shown that MyoD promoted nicotinic acetylcholine subunit gene expression; the present experiments were done to determine whether this subsequently led to the development of functional nicotinic acetylcholine receptors. Transfection of C3H 10T1/2 cells with MyoD cDNA resulted in the appearance of [125I]alpha-bungarotoxin binding sites; radiolabelled alpha-toxin binding was not observed in cells transfected with a plasmid that lacked MyoD cDNA. Receptor development plateaued over a time course of several days with maximal binding seven and 11 days after exposure to fusion medium. [125I]alpha-bungarotoxin binding was of high affinity (Kd = 1 nM), saturable and was inhibited by nicotinic but not muscarinic receptor ligands, with IC50s of 1-3 nM for alpha-bungarotoxin, 1-3 microM for d-tubocurarine and 3-10 microM for nicotine. Not only did the cells exhibit a cell surface nicotinic receptor but they also expressed a nicotinic receptor mediated functional response. Carbachol resulted in uptake of 22Na into the cells at concentrations similar to those required for receptor activation at a muscle type nicotinic receptor; furthermore, the functional response was effectively blocked by nicotinic receptor ligands, including alpha-bungarotoxin (IC50 = 2 to 6 nM) and d-tubocurarine (IC50 = 0.1 to 0.4 microM); muscarinic receptor ligands had no effect. A time course study showed that alpha-bungarotoxin binding and carbachol stimulated 22Na uptake developed in parallel, suggesting that the observed functional response was mediated through an interaction at the alpha-bungarotoxin recognition site.(ABSTRACT TRUNCATED AT 250 WORDS)

An 83-nucleotide promoter of the acetylcholine receptor epsilon-subunit gene confers preferential synaptic expression in mouse muscle

The expression of the acetylcholine receptor epsilon-subunit gene is restricted to the endplate of adult muscle fibers. We have started to study the regulatory elements of the epsilon-subunit gene promoter that are important for its synaptic expression. We used, for this purpose, a rapid method of in vivo expression after DNA injection into the muscle tissue [Wolff, J. A., Malone, R. W., Williams, P., Chong, W., Acsadi, G., Jani, A. & Felgner, P. L. (1990) Science 247, 1465-1468]. Our results show that a construction containing 83 nucleotides upstream from the transcription start site is sufficient to obtain preferential endplate expression. Moreover, mutation of a MyoD binding site located around position-70 does not alter this synaptic expression. We also studied the expression of this promoter in vitro in muscle primary cultures and showed the presence of a positive element between positions -122 and -83. Comparison of in vivo and in vitro results reveals that the elements important for in vivo localization at the synapse and in vitro expression in cultured muscle cells may differ.

Desmin sequence elements regulating skeletal muscle-specific expression in transgenic mice

During the development of the mouse embryo, desmin is one of the first muscle proteins detected in both the heart and the somites. The expression of the desmin gene differs from most other muscle genes, since it is initiated in replicating myoblasts and accumulates as the muscle differentiates. We have characterized a muscle-specific enhancer which directs the expression of desmin in vitro in the myoblasts and myotubes of C2 cells but not in non-myogenic cells. We report here on the generation and characterization of transgenic mice bearing a transgene in which the 1 kb DNA 5′ regulatory sequence of the desmin gene is linked to a reporter gene coding for Escherichia coli beta-galactosidase (Des1-nlacZ). The enhancer activity of the desmin promoter is very strong and the reporter gene expression is easily detected in tissue sections. We have demonstrated that the regulatory elements present in the transgene Des1-nlacZ are sufficient to direct muscle-specific and developmentally regulated expression of nlacZ in skeletal muscles. Endogenous desmin expression and transgene activity were found to be correlated during the development of skeletal muscles. The transgene was expressed in the committed mononucleate myoblasts as well as in the myotubes. In addition, we have shown that the desmin-derived sequences direct a highly selective expression of nlacZ in cells that leave the somites and invade the limb bud, indicating that the cells that migrate from the somites are already predetermined for myogenesis. In contrast, smooth and cardiac muscle cells were beta-galactosidase negative both during embryonic and foetal development. Interestingly, the transgene was found to be expressed in the conduction system of the heart, which exhibits many features characteristic of skeletal muscles.

Regulation of the mouse desmin gene: transactivated by MyoD, myogenin, MRF4 and Myf5

Desmin, the muscle specific intermediate filament (IF) protein, is expressed at low levels in myoblasts and at the onset of differentiation its expression increases several fold. In an effort to explore the mechanism involved in the tissue-specific and developmentally regulated expression of desmin, we have isolated the mouse desmin gene. Sequence analysis of 976 bp 5' flanking region revealed several potential cis-acting elements: 1) Three E boxes (MyoD binding sites), namely, E1, E2 and E3, located at -79, -832 and -936, respectively; 2) one MEF2 binding site at -864; 3) a region with homology to M-CAT motif at -587; 4) several GC boxes. Transient transfections with various 5' flank deletion mutants into C2C12 muscle cells have revealed both positive and negative elements that seem to be involved in the expression of desmin. The first 81 bp upstream of the transcription initiation site, including E1 box, were sufficient to confer muscle specific expression of the desmin gene. The maximal level of expression was achieved by the construct containing up to -897 base pairs. The region between -578 to -976 behaves as a classical enhancer in the absence of which the region between -578 and -81 suppresses CAT activity. Gel electrophoretic mobility shift assays using both C2C12 muscle cell nuclear extracts as well as in vitro translated myoD/E12 and myogenin/E12 heterodimers, showed that both myoD and myogenin bind to the proximal E1 and the distal E2 boxes of the desmin promoter and enhancer respectively. Co-transfection of myoD, myogenin, MRF4 and Myf5, with the desmin-CAT construct into 10T-1/2 cells demonstrated that all these factors could transactivate desmin gene expression.

Different E-box regulatory sequences are functionally distinct when placed within the context of the troponin I enhancer

Basic helix-loop-helix (bHLH) regulatory proteins are known to bind to a single DNA consensus sequence referred to as an E-box. The E-box is present in the regulatory elements of many developmentally controlled genes, including most muscle-specific genes such as troponin I (TnI). Although the E-box consensus is minimally defined as CANNTG, the adjacent nucleotides of functional E-boxes are variable for genes regulated by the bHLH proteins. In order to examine how E-box regulatory regions containing different internal and flanking nucleotides function when placed within the context of a single regulatory element, the E-box region (14 bp) present within the TnI enhancer was substituted with the corresponding E-box sequences derived from the muscle-specific M-creatine kinase (MCK) and cardiac alpha-actin regulatory elements as well as from the immunoglobulin kappa (Ig kappa) enhancer. Within the TnI enhancer, the E-box sequence derived from cardiac alpha-actin was inactive whereas the corresponding sequence from the MCK right E-box efficiently restored wild-type enhancer activity in muscle cells. Intermediate levels of gene activity were observed for TnI enhancers containing E-boxes derived from the MCK left E-box site or from the Ig kappa E2 E-box. DNA binding studies of MyoD:E12 protein complexes with each substituted TnI enhancer confirmed that DNA binding activity in vitro mimics the relative strength of the enhancers in vivo. These studies demonstrate that the specific nucleotide composition of individual E-boxes, which are contained within the regulatory elements of most if not all muscle-specific genes, contributes to the complex regulatory mechanisms governing bHLH-mediated gene expression.

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

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

The 5'-flanking region of the mouse muscle nicotinic acetylcholine receptor beta subunit gene promotes expression in cultured muscle cells and is activated by MRF4, myogenin and myoD

The expression of the nicotinic acetylcholine receptor (AChR) in vertebrate striated muscle is regulated both during development by nerve-evoked muscle activity and by local factors released or associated with the nerve ending. The expression pattern of AChR is achieved by coordinate regulation of four embryonic subunit mRNAs, alpha, beta, gamma and delta. We have taken the approach of identifying the similarities and differences among cis-acting regulatory elements of AChR genes to gain a better understanding of these mechanisms. Thus, to begin to define DNA sequences necessary for the transcriptional regulation of the mouse beta AChR gene, we have analyzed its 5'-flanking region. Primer extension and RNAase protection analyses showed that transcription initiates at one major and two minor sites, all of which are close to the translational initiation site. Using plasmids in which segments of the 5'-flanking region were linked to the bacterial chloramphenicol acetyltransferase (CAT) gene, we have demonstrated that 150 bp of the 5'-flanking region is active in C2 myotubes but not C2 myoblasts or NIH3T3 fibroblasts. This region contains a putative binding site for myoD, and when linked to CAT was transactivated by the muscle regulatory factors myoD, myogenin, and MRF4. Thus, a 150 bp sequence of the beta-subunit gene contains information necessary for developmental specificity and responsiveness to myogenic factors.

Localization of mRNAs coding for CMD1, myogenin and the alpha-subunit of the acetylcholine receptor during skeletal muscle development in the chicken

Myogenin and CMD1, the chicken homologue of MyoD, transactivate the promoter of the alpha-subunit of the acetylcholine receptor (AChR) in chicken fibroblasts. The expression of these three genes was followed by in situ hybridization. In two-day-old embryos the CMD1 gene is expressed shortly before the AChR alpha-subunit and the myogenin genes. At day 19 extrajunctional AChR mRNA clusters have disappeared and myogenin mRNAs are no longer detected in PLD muscle. Moreover, both myogenin and CMD1 mRNA levels increase after muscle denervation in chicks. These data are compatible with a role for myogenic factors in the induction and maintenance of extra-junctional expression of the AChR genes during early muscle development. Using digoxygenin labelled RNA probes, we also show that the mRNAs for the AChR alpha-subunit display a punctated, probably perinuclear distribution, whereas mRNAs for myogenic genes accumulate in the sarcoplasm around subsets of nuclei in the muscle fiber.

Autonomous activity of the alternate aldolase A muscle promoter is maintained by a sequestering mechanism

The mouse aldolase A gene contains two closely-spaced alternate promoter/first exons. The more distal of the two, the M promoter, is muscle-specific while the 3' promoter, the H promoter, is expressed constitutively. Various segments from these promoter regions were linked to a reporter gene and used to transfect the myogenic cell line C2C12 and the hepatoma cell line BWTG3. A muscle-specific enhancer, MEN1, responsible for 80% of promoter M activity and containing 4 consensus MyoD binding sites was localized between -2578 to -2723 of the M promoter. Another muscle-specific enhancer and a restrictive element, MEN2/MSE, were found in the interval -1100 to -350. The MSE restrictive element was found to prohibit inappropriate up-regulation of the M promoter by selectively sequestering it from H promoter elements in both myoblasts and myotubes. Among the H promoter elements was found an enhancer, HEN, situated between -533 and -200 which did not function in myotubes. These studies also show that H promoter elements can act synergistically with a non-specific element, MAE, located between -350 and -130 of the M cap site greatly stimulating M promoter transcription in all cell types when the MSE restrictive element was absent. Through the analysis of interactions between these elements and the aldolase A and HSV-TK promoters we showed that neither the enhancers nor the promoter proximal sequences by themselves contain adequate information to reproduce the native pattern of aldolase A promoter modulation. Rather, the sequestering of the M promoter by the MSE restrictive element and the relative positioning and context of promoters M and H appear critical to the regulated expression of aldolase A.

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

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

Neural regulation of calmodulin in adult Xenopus leg muscle

Two Ca(2+)-binding proteins important in regulating muscle responses to Ca2+ flux are differentially expressed following denervation of Xenopus laevis gastrocnemius. Levels of parvalbumin (PV) RNA transcripts and proteins decrease in abundance, while calmodulin (CaM) transcript and protein levels increase. Our studies on PV kinetics in Xenopus follow a pattern observed in other species, however, our observation of a concomitant increase in CaM has not been documented in any system. Molecular analyses of the Xenopus CaM gene indicate that its structure and upstream sequences are highly conserved across several vertebrate species and implicate several transcription factors in the regulation of its expression.

Multiple CArG boxes in the human cardiac actin gene promoter required for expression in embryonic cardiac muscle cells developing in vitro from embryonal carcinoma cells

Chimeric genes composed of the human cardiac actin promoter driving the Escherichia coli lacZ reporter gene were constructed, transfected, and stably integrated into genomes of P19 embryonal carcinoma cells. The transfected constructs were expressed actively in cardiac myocytes formed following dimethyl sulfoxide (DMSO)-induced cell differentiation but poorly in undifferentiated cultures and in cultures treated with retinoic acid to develop into derivatives of the neuroectoderm. A number of deletions of the promoter were constructed and tested. Three regions required for efficient expression in P19-derived cardiac muscle were identified, each containing sequences referred to as CArG boxes (CC[AT-rich]6GG). This analysis indicated that regulatory sequences important for expression in cardiac muscle were present upstream of the core promoter identified previously by transient assays in skeletal myoblasts. Expression of the cardiac actin promoter was enhanced 10-fold in undifferentiated P19 cells in the presence of the myoD protein. The promoter regions important for expression in P19-derived cardiocytes were similar to those important for myoD-induced enhancement, a result we interpret to be consistent with the idea that cardiac muscle might contain a myoD-like activity.

Multiple CArG boxes in the human cardiac actin gene promoter required for expression in embryonic cardiac muscle cells developing in vitro from embryonal carcinoma cells

Chimeric genes composed of the human cardiac actin promoter driving the Escherichia coli lacZ reporter gene were constructed, transfected, and stably integrated into genomes of P19 embryonal carcinoma cells. The transfected constructs were expressed actively in cardiac myocytes formed following dimethyl sulfoxide (DMSO)-induced cell differentiation but poorly in undifferentiated cultures and in cultures treated with retinoic acid to develop into derivatives of the neuroectoderm. A number of deletions of the promoter were constructed and tested. Three regions required for efficient expression in P19-derived cardiac muscle were identified, each containing sequences referred to as CArG boxes (CC[AT-rich]6GG). This analysis indicated that regulatory sequences important for expression in cardiac muscle were present upstream of the core promoter identified previously by transient assays in skeletal myoblasts. Expression of the cardiac actin promoter was enhanced 10-fold in undifferentiated P19 cells in the presence of the myoD protein. The promoter regions important for expression in P19-derived cardiocytes were similar to those important for myoD-induced enhancement, a result we interpret to be consistent with the idea that cardiac muscle might contain a myoD-like activity.

Differential regulation of MyoD and myogenin mRNA levels by nerve induced muscle activity

The levels of mRNAs coding for the myogenic factors MyoD and myogenin were measured during synapse formation in developing muscle and in adult muscle, after denervation and reinnervation and after muscle paralysis induced by blocking of neuromuscular transmission by neurotoxins known to alter the density and localization of synaptic proteins such as the acetylcholine receptor (AChR). The mRNA levels of both factors depend on usage of the neuromuscular synapses, but they change to different extents. Myogenin mRNA levels decrease drastically with innervation and increase strongly following blocking of transmission whereas the level of MyoD mRNA showed only a small decrease in response to innervation, denervation or muscle paralysis by neurotoxins. Neither mRNA showed a synapse-related cellular distribution. The results suggest that nerve-induced electrical muscle activity determines the cellular ratio of MyoD and myogenin mRNAs in adult muscle.

Differences and similarities in DNA-binding preferences of MyoD and E2A protein complexes revealed by binding site selection

A technique was developed for studying protein-DNA recognition that can be applied to any purified protein, partially purified protein, or cloned gene. From oligonucleotides in which particular positions are of random sequence, that subset to which a given protein binds is amplified by the polymerase chain reaction and sequenced as a pool. These selected and amplified binding site (SAAB) "imprints" provide a characteristic set of preferred sequences for protein binding. With this technique, it was shown that homo- and heterooligomers of the helix-loop-helix proteins MyoD and E2A recognize a common consensus sequence, CA--TG, but otherwise bind to flanking and internal positions with different sequence preferences that suggest half-site recognition. These findings suggest that different combinations of dimeric proteins can have different binding sequence preferences.

Brain and muscle creatine kinase genes contain common TA-rich recognition protein-binding regulatory elements

We have previously reported that the rat brain creatine kinase (ckb) gene promoter contains an AT-rich sequence that is a binding site for a protein called TARP (TA-rich recognition protein). This AT-rich segment is a positively acting regulatory element for the ckb promoter. A similar AT-rich DNA segment is found at the 3' end of the 5' muscle-specific enhancer of the rat muscle creatine kinase (ckm) gene and has been shown to be necessary for full muscle-specific enhancer activity. In this report, we show that TARP binds not only to the ckb promoter but also to the AT-rich segment at the 3' end of the muscle-specific ckm enhancer. A second, weaker TARP-binding site was identified in the ckm enhancer and lies at the 5' end of the minimal enhancer segment. TARP was found in both muscle cells (C2 and L6 myotubes) and nonmuscle (HeLa) cells and appeared to be indistinguishable from both sources, as judged by gel retardation and footprinting assays. The TARP-binding sites in the ckm enhancer and the ckb promoter were found to be functionally interchangeable. We propose that TARP is active in both muscle and nonmuscle cells and that it is one of many potential activators that may interact with muscle-specific regulators to determine the myogenic phenotype.

Brain and muscle creatine kinase genes contain common TA-rich recognition protein-binding regulatory elements

We have previously reported that the rat brain creatine kinase (ckb) gene promoter contains an AT-rich sequence that is a binding site for a protein called TARP (TA-rich recognition protein). This AT-rich segment is a positively acting regulatory element for the ckb promoter. A similar AT-rich DNA segment is found at the 3' end of the 5' muscle-specific enhancer of the rat muscle creatine kinase (ckm) gene and has been shown to be necessary for full muscle-specific enhancer activity. In this report, we show that TARP binds not only to the ckb promoter but also to the AT-rich segment at the 3' end of the muscle-specific ckm enhancer. A second, weaker TARP-binding site was identified in the ckm enhancer and lies at the 5' end of the minimal enhancer segment. TARP was found in both muscle cells (C2 and L6 myotubes) and nonmuscle (HeLa) cells and appeared to be indistinguishable from both sources, as judged by gel retardation and footprinting assays. The TARP-binding sites in the ckm enhancer and the ckb promoter were found to be functionally interchangeable. We propose that TARP is active in both muscle and nonmuscle cells and that it is one of many potential activators that may interact with muscle-specific regulators to determine the myogenic phenotype.

Human alpha B-crystallin gene and preferential promoter function in lens

alpha B-Crystallin, first identified as a structural component of the vertebrate eye lens, is expressed at high levels in lens and at lower levels in a number of other tissues, most notably cardiac and skeletal muscle, kidney, and brain. We have cloned and sequenced the human alpha B-crystallin gene and show that it is structurally similar to its hamster homolog. We have also identified its transcription initiation site in human lens RNA. Functional analysis of a promoter fragment extending from -537 to +21 (relative to the transcription initiation site) and fused to the bacterial chloramphenicol acetyltransferase gene suggests that this fragment contains regulatory elements that function preferentially, but not exclusively, in lens. In contrast, this fragment is apparently insufficient to promote transcription in glial cells, as this construct functioned poorly in a glioblastoma-astrocytoma cell line (U-373MG) that synthesizes high levels of the endogenous alpha B-crystallin gene product.

Two adjacent MyoD1-binding sites regulate expression of the acetylcholine receptor alpha-subunit gene

Several genes encoding putative myogenic regulatory factors have been isolated on the basis of their ability to convert nonmuscle cells into myoblasts. Four of these genes code for nuclear proteins that belong to a larger family characterized by a conserved helix-loop-helix motif required for DNA-binding and dimerization. At least one protein, MyoD1, can function as a transcription factor and activate muscle-specific genes during differentiation. But the promoter of the delta-subunit gene of the mouse acetylcholine receptor (AChR) was recently reported to be functional in the absence of MyoD1 binding sites and it has been suggested that the genes coding for the AChR could be regulated independently of MyoD1 protein. Here, we identify two functional MyoD1-binding sites in the muscle-specific enhancer of the chicken AChR alpha-subunit gene that are essential for full activity in transfected myotubes.

Myogenin resides in the nucleus and acquires high affinity for a conserved enhancer element on heterodimerization

Myogenin is a member of a family of muscle-specific factors that can activate the muscle differentiation program in nonmyogenin cells. Using antibodies directed against unique domains of myogenin, we show in the present study that myogenin resides in the nucleus of differentiated muscle cells. Myogenin translated in vitro does not exhibit detectable DNA binding activity; however, when dimerized with the ubiquitous enhancer-binding factor E12, it acquires high affinity for an element in the core of the muscle creatine kinase (MCK) enhancer that is conserved among many muscle-specific genes. Antibody disruption experiments show that myogenin, synthesized during differentiation of the BC3H1 and C2 muscle cell lines, is part of a complex that binds to the same site in the MCK enhancer as myogenin-E12 translated in vitro. Mutagenesis of the myogenin-E12-binding site in the MCK enhancer abolishes binding of the heterodimer and prevents trans-activation of the enhancer by myogenin. The properties of myogenin suggest that its functions as a sequence-specific DNA-binding factor that interacts directly with muscle-specific genes during myogenesis. The dependence of myogenin on E12 for high-affinity DNA binding activity also suggests that the susceptibility of various cell types to the actions of myogenin may be influenced by the cellular factors with which it may interact.