A 5'-flanking region of the chicken acetylcholine receptor alpha-subunit gene confers tissue specificity and developmental control of expression in transfected cells

Synapse-specific Lrp4 mRNA enrichment requires Lrp4/MuSK signaling, muscle activity and Wnt non-canonical pathway

Background

The neuromuscular junction (NMJ) is a peripheral synapse critical to muscle contraction. Like acetylcholine receptors (AChRs), many essential proteins of NMJ are extremely concentrated at the postjunctional membrane. However, the mechanisms of synapse-specific concentration are not well understood; furthermore, it is unclear whether signaling molecules critical to NMJ formation and maintenance are also locally transcribed.

Results

We studied the β-gal activity encoded by a lacZ cassette driven by the promoter of the Lrp4 gene. As reported for Lrp4 mRNA, β-gal was in the central region in embryonic muscles and at the NMJ after its formation. However, β-gal was no longer in the central areas of muscle fibers in Lrp4 or MuSK mutant mice, indicating a requirement of Lrp4/MuSK signaling. This phenotype could be rescued by transgenic expression of LRP4 with a transmembrane domain but not soluble ECD in Lrp4 mutant mice. β-gal and AChR clusters were distributed in a broader region in lacZ/ECD than that of heterozygous lacZ/+ mice, indicating an important role of the transmembrane domain in Lrp4 signaling. Synaptic β-gal activity became diffused after denervation or treatment with µ-conotoxin, despite its mRNA was increased, indicating synaptic Lrp4 mRNA enrichment requires muscle activity. β-gal was also diffused in aged mice but became re-concentrated after muscle stimulation. Finally, Lrp4 mRNA was increased in C2C12 myotubes by Wnt ligands in a manner that could be inhibited by RKI-1447, an inhibitor of ROCK in Wnt non-canonical signaling. Injecting RKI-1447 into muscles of adult mice diminished Lrp4 synaptic expression.

Conclusions

This study demonstrates that synapse-specific enrichment of Lrp4 mRNA requires a coordinated interaction between Lrp4/MuSK signaling, muscle activity, and Wnt non-canonical signaling. Thus, the study provides a new mechanism for Lrp4 mRNA enrichment. It also provides a potential target for the treatment of NMJ aging and other NMJ-related diseases.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-021-00619-z.

Synergistic up-regulation of muscle LIM protein expression in C2C12 and NIH3T3 cells by myogenin and MEF2C

Although the role of muscle LIM protein (MLP, also known as CRP3), a LIM-only protein of LIM domain-containing protein family, is well-characterized, the mechanism by which the MLP gene expresses remains unclear. Herein, we demonstrate that myogenin and myocyte enhancer factor 2C (MEF2C) cooperate in activating the MLP gene in myogenesis. RT-PCR, real-time PCR and Western blotting showed that overexpression of myogenin or myogenin plus MEF2C led to induction of the MLP gene in differentiating C2C12 and NIH3T3 fibroblasts. By contrary, knocking-down of myogenin by RNA interference (RNAi) suppressed MLP expression in differentiating C2C12. Deletion and reporter enzyme assay revealed that the promoter activity was determined largely by the region extending from -260 to -173, which containing three E-box (CANNTG motif) candidates. Site-directed mutagenesis demonstrated that the E-box at position -186 to -180 was crucial for activating the promoter by myogenin. Furthermore, MEF2C could enhance myogenin-mediated activation of the promoter. In addition, chromatin immunoprecipitation (ChIP) and re-ChIP showed that myogenin and MEF2C were associated with the activated MLP promoter. Together, these results suggest that myogenin and MEF2C cooperate in the MLP gene activation. The linking of the MLP gene activation with myogenin and MEF2C may facilitate myogenin-mediated differentiation of striated muscle.

Essential roles of c-JUN and c-JUN N-terminal kinase (JNK) in neuregulin-increased expression of the acetylcholine receptor epsilon-subunit

Neuregulin is a neural factor implicated in upregulation of acetylcholine receptor (AChR) synthesis at the neuromuscular junction. Previous studies have demonstrated that the extracellular signal-regulated kinase (ERK) subgroup of MAP kinases is required for neuregulin-induced AChR gene expression. We report here that the neuregulin-mediated increase in AChR epsilon-subunit mRNA was a delayed response in C2C12 muscle cells. Neuregulin induced expression of immediate early genes c-jun and c-fos, which followed and depended on the ERK activation. Treatment of muscle cells with cycloheximide to inhibit c-JUN synthesis at the protein level and suppression of c-JUN function by a dominant-negative mutant blocked neuregulin-induced expression of the epsilon-subunit gene, indicating an essential role of c-JUN in neuregulin signaling. Furthermore, neuregulin activated c-JUN N-terminal kinase (JNK) in C2C12 muscle cells. Blockade of JNK activation by overexpressing dominant-negative MKK4 inhibited epsilon-promoter activation. Moreover, overexpression of the JNK dominant-negative mutant inhibited neuregulin-mediated expression of the epsilon-transgene and endogenous epsilon-mRNA. Taken together, our results demonstrate important roles of c-JUN and JNK in neuregulin-mediated expression of the AChR epsilon-subunit gene and suggest that neuregulin activates multiple signaling cascades that converge to regulate AChR epsilon-subunit gene expression.

ARIA/HRG regulates AChR epsilon subunit gene expression at the neuromuscular synapse via activation of phosphatidylinositol 3-kinase and Ras/MAPK pathway

AChR-inducing activity (ARIA)/heregulin, a ligand for erbB receptor tyrosine kinases (RTKs), is likely to be one nerve-supplied signal that induces expression of acetylcholine receptor (AChR) genes at the developing neuromuscular junction. Since some RTKs act through Ras and phosphatidylinositol 3-kinase (PI3K), we investigated the role of these pathways in ARIA signaling. Expression of activated Ras or Raf mimicked ARIA-induction of AChR epsilon subunit genes in muscle cells; whereas dominant negative Ras or Raf blocked the effect of ARIA. ARIA rapidly activated erk1 and erk2 and inhibition of both erks also abolished the effect of ARIA. ARIA stimulated association of PI3K with erbB3, expression of an activated PI3K led to ARIA-independent AChR epsilon subunit expression, and inhibition of PI3K abolished the action of ARIA. Thus, synaptic induction of AChR genes requires activation of both Ras/MAPK and PI3K signal transduction pathways.

Identification of a DNA element determining synaptic expression of the mouse acetylcholine receptor delta-subunit gene

mRNAs for acetylcholine receptor genes are highly concentrated in the endplate region of adult skeletal muscle largely as a result of a transcription restricted to the subneural nuclei. To identify the regulatory elements involved, we employed a DNA injection of a plasmid containing a fragment of the acetylcholine receptor delta-subunit gene promoter (positions -839 to +45) linked to the reporter gene lacZ with a nuclear localization signal. Injection of the wild-type construct into mouse leg muscles yielded preferential expression of the reporter gene in the synaptic region. Analysis of various mutant promoters resulted in the identification of a DNA element (positions -60 to -49), referred to as the N box, that plays a critical role in subneural expression. Disruption of this 12-bp element in the context of a mouse delta-subunit promoter from positions -839 to +45 gives widespread expression of the reporter gene throughout the entire muscle fiber, indicating that this element is a silencer that represses delta-subunit gene transcription in extrajunctional areas. On the other hand, this element inserted upstream of a heterologous basal promoter preferentially enhances expression in the endplate region. This element therefore regulates the restricted expression of the delta-subunit gene both as an enhancer at the endplate level and as a silencer in extrajunctional areas. Furthermore, gel-shift experiments with mouse muscle extracts reveal an activity that specifically binds the 6-bp sequence TTCCGG of this element, suggesting that a transcription factor(s) controls the expression of the delta-subunit gene via this element.

In vivo and in vitro analysis of electrical activity-dependent expression of muscle acetylcholine receptor genes using adenovirus

Acetylcholine receptor (AChR) genes are repressed in extrajunctional domains of adult muscle fiber by neurally evoked electrical activity. Denervation elicits upregulation of AChR gene transcription in extrasynaptic areas. We have used an adenovirus (Ad)-based strategy to analyze in vitro and in vivo the electrical activity-dependent transcription of the chicken AChR alpha 1 subunit gene. The luciferase gene placed under the control of wild-type and mutated fragments of the alpha 1 subunit promoter was inserted in a defective Ad vector designed for the study of transcriptional regulation. Animals were infected by intramuscular injection and in vivo luciferase levels were normalized by coinfection with an Ad vector containing the chloramphenicol acetyltransferase gene driven by an electrical activity-insensitive promoter. Our results demonstrate that although both proximal MyoD binding sites of the alpha 1 promoter are required for muscle-specific expression of the alpha 1 gene, only one is necessary, albeit insufficient, to enhance alpha 1 promoter activity after denervation. Parallel results were obtained with cultured muscle cells in vitro following tetrodotoxin blocking of spontaneous electrical activity. These results substantiate a direct contribution of MyoD factors in electrical activity-dependent regulation of AChR expression and further indicate that Ad-based vectors constitute a powerful tool in the field of transcriptional regulation.

Neural regulation of muscle acetylcholine receptor epsilon- and alpha-subunit gene promoters in transgenic mice

The effects of denervation were investigated in mice with transgenes containing promoter elements from the muscle acetylcholine receptor epsilon- and alpha-subunit genes. The promoter sequences were coupled to a nuclear localization signal-beta-galactosidase fusion gene (nlacZ) as a reporter. While many postsynaptic specializations form in the embryo, expression of the epsilon subunit is induced during the first two postnatal weeks. When muscles were denervated at birth, before the onset of epsilon expression, epsilon nlacZ still appeared at the former synaptic sites on schedule. This result suggests that the nerve leaves a localized "trace" in the muscle that can continue to regulate transcription. An additional finding was that epsilon nlacZ expression was much stronger in denervated than in intact muscles. This suggests that the epsilon promoter is similar to the other subunits in containing elements that are activated on cessation of neural activity. However, even after denervation, epsilon nlacZ expression was always confined to the synaptic region whereas alpha nlacZ expression increased in nuclei along the entire length of the fiber. This suggests that while the epsilon gene is similar in its activity dependence to other subunit genes, it is unique in that local nerve-derived signals are essential for its expression. Consequently, inactivity enhances epsilon expression only in synaptic nuclei where such signals are present, but enhances expression throughout the muscle fiber. Truncations and an internal deletion of the epsilon promoter indicate that cis-elements essential for the response to synaptic signals are contained within 280 bp of the transcription start site. In contrast to these results in young animals, denervation in older animals leads to an unexpected reduction in nlacZ activity. However, mRNA measurements indicated that transgene expression was increased in these animals. This discordance between nlacZ mRNA and enzyme activity, demonstrates a previously unknown limitation of nlacZ as a reporter gene in transgenic animals.

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.

Chicken neuronal acetylcholine receptor alpha 2-subunit gene exhibits neuron-specific expression in the brain and spinal cord of transgenic mice

Transgenic mice carrying the complete structural gene of the alpha 2 subunit of the chicken neuronal nicotinic acetylcholine receptor (nAChR) and 7 kilobase pairs (kbp) of 5' upstream and 3 kbp of 3' downstream sequences have been generated. The transgene was stably integrated in transgenic lines and transmitted to their progeny. Avian transgene expression was predominant in the central nervous system as detected by specific alpha 2-subunit cDNA amplification. Moreover, in at least two independent mouse lines, its expression appeared to be neuron-specific and reproducibly restricted to subregions in the brain and spinal cord, as revealed by in situ hybridization histochemistry. Most cranial motor nuclei were positive, and several of the alpha 2-subunit transgene-expressing structures corresponded to cholinergic areas in rodents. This study reveals that regulatory mechanisms giving rise to neuronal-specific gene expression have been conserved at least in part between birds and mammals.

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.

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

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

Transcriptional selectivity in early mouse embryos: a qualitative study

The mouse zygotic genome is activated at the 2-cell stage. At this stage, microinjected DNA can be expressed and its transcription, analysed qualitatively with LacZ reporter genes, has the following characteristics (i) Sp1-sensitive promoters are active; (ii) the SV40 early promoter does not require upstream enhancers; (iii) genes driven by the -447, +563 region of murine leukemia virus (M-MuLV) are repressed and; (iv) activation of promoters is possible as shown for the promoter of acetylcholine receptor alpha-subunit by MyoD. This transactivation can occur before the formation of the zygotic genome. The transcriptional selectivity of 2-cell embryos also characterizes oocytes and 4-cell embryos. Therefore the elements involved are present in the oocytes and they persist after fertilization. This transcriptional selectivity has numerous common characteristics with that in EC cells, and may be indicative of a genetic control program specific for multipotential cells.

Rabbit muscle creatine kinase: genomic cloning, sequencing, and analysis of upstream sequences important for expression in myocytes

Muscle creatine kinase (MCK) is a major enzyme of cellular energy metabolism that is expressed upon differentiation of myoblasts into myotubes. Previously we cloned and sequenced the entire rabbit enzyme cDNA which was used as a probe in these studies to obtain a genomic clone from a rabbit library. The transcription start site was identified by primer extension analysis and over 800 bp of 5' flanking DNA was sequenced. Comparison of this sequence with the published sequences from the upstream regions of the mouse MCK gene and the human MCK gene showed two conserved regions and a large intervening block of non-conserved sequence. The conserved regions are separated by about 800 bp in the mouse and by about 400 bp in the human, but are much closer (200 bp) in the rabbit. The upstream conserved region of the mouse gene encompasses a region possessing the properties of an enhancer and containing two MyoD binding sites; the downstream element is adjacent to the start of transcription. A set of of overlapping deletions of the 5' upstream DNA was fused to the CAT gene and transfected into mouse C2 myocytes, chick primary myocytes, and chick primary liver cells. Constructs which contained both conserved 5' regions were strongly expressed in C2 and chick myocytes, but were not expressed (above background) in primary liver cells. Surprisingly, while the upstream enhancer element was required for strong expression in C2 myocytes, it was less important for expression in chick myocytes. This suggests that there are important muscle-specific transcriptional signals in the proximal promoter region of mammalian MCK genes.

A conserved 28-base-pair element (HF-1) in the rat cardiac myosin light-chain-2 gene confers cardiac-specific and alpha-adrenergic-inducible expression in cultured neonatal rat myocardial cells

To study the transcriptional regulatory mechanisms which mediate cardiac-specific and inducible expression during myocardial cell hypertrophy, we have extensively characterized the rat cardiac myosin light-chain-2 (MLC-2) gene as a model system. The MLC-2 gene encodes a relatively abundant contractile protein in slow skeletal and cardiac muscle and is upregulated during in vivo cardiac hypertrophy and alpha-adrenergic-mediated hypertrophy of neonatal rat myocardial cells. In transient expression assays employing a series of MLC-2-luciferase constructs, recent studies have identified a 250-bp fragment which is sufficient for both cardiac-specific and alpha-adrenergic-inducible expression. Within this 250-bp fragment lie three regions (HF-1, HF-2, and HF-3), each greater than 10 bp in length, which are conserved between the chicken and rat cardiac MLC-2 genes, suggesting their potential role in the regulated expression of this contractile protein gene. As assessed by substitution mutations within each of the conserved regions, the present study demonstrates that HF-1 and HF-2 are important in both cardiac-specific and inducible expression, while HF-3 has no detectable role in the regulated expression of the MLC-2 gene in transient expression assays. HF-1 sequences confer both cardiac-specific and inducible expression to a neutral promoter-luciferase construct but have no significant effect in the skeletal muscle or nonmuscle cell contexts. Thus, these studies have identified a new cardiac-specific regulatory element (HF-1) which plays a role in both cardiac-specific and inducible expression during myocardial cell hypertrophy.

A conserved 28-base-pair element (HF-1) in the rat cardiac myosin light-chain-2 gene confers cardiac-specific and alpha-adrenergic-inducible expression in cultured neonatal rat myocardial cells

To study the transcriptional regulatory mechanisms which mediate cardiac-specific and inducible expression during myocardial cell hypertrophy, we have extensively characterized the rat cardiac myosin light-chain-2 (MLC-2) gene as a model system. The MLC-2 gene encodes a relatively abundant contractile protein in slow skeletal and cardiac muscle and is upregulated during in vivo cardiac hypertrophy and alpha-adrenergic-mediated hypertrophy of neonatal rat myocardial cells. In transient expression assays employing a series of MLC-2-luciferase constructs, recent studies have identified a 250-bp fragment which is sufficient for both cardiac-specific and alpha-adrenergic-inducible expression. Within this 250-bp fragment lie three regions (HF-1, HF-2, and HF-3), each greater than 10 bp in length, which are conserved between the chicken and rat cardiac MLC-2 genes, suggesting their potential role in the regulated expression of this contractile protein gene. As assessed by substitution mutations within each of the conserved regions, the present study demonstrates that HF-1 and HF-2 are important in both cardiac-specific and inducible expression, while HF-3 has no detectable role in the regulated expression of the MLC-2 gene in transient expression assays. HF-1 sequences confer both cardiac-specific and inducible expression to a neutral promoter-luciferase construct but have no significant effect in the skeletal muscle or nonmuscle cell contexts. Thus, these studies have identified a new cardiac-specific regulatory element (HF-1) which plays a role in both cardiac-specific and inducible expression during myocardial cell hypertrophy.

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

The cardiac troponin T (cTNT) promoter contains a highly muscle specific distal promoter element capable of conferring muscle-specific transcription from a heterologous TATA box-transcription initiation site. Three sequence motifs within this distal promoter element are conserved in the promoter and regulatory regions of many sarcomeric protein genes. Mutational analysis demonstrated that homologies to two of these conserved motifs (CArG/CBAR and MEF 1) were not required for activity of cTNT promoter-marker gene constructs in transfected embryonic skeletal muscle cells. In contrast, disruption of either or both copies of the conserved M-CAT motif (5'-CATTCCT-3') inactivated the cTNT promoter in these cells. Both M-CAT motifs were protected from DNase I cleavage in solution footprint assays by an M-CAT binding factor (MCBF) present in nuclear extracts from embryonic muscle tissue. M-CAT mutations that inactivated the cTNT promoter also disrupted MCBF binding, indicating that MCBF may be a key trans-acting factor required for muscle-specific expression of the cTNT promoter. MCBF also bound to the M-CAT motif in the distal promoter region of the skeletal alpha-actin gene, suggesting that it may play a role in the regulation of this and perhaps other muscle genes that contain M-CAT motifs.

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

The cardiac troponin T (cTNT) promoter contains a highly muscle specific distal promoter element capable of conferring muscle-specific transcription from a heterologous TATA box-transcription initiation site. Three sequence motifs within this distal promoter element are conserved in the promoter and regulatory regions of many sarcomeric protein genes. Mutational analysis demonstrated that homologies to two of these conserved motifs (CArG/CBAR and MEF 1) were not required for activity of cTNT promoter-marker gene constructs in transfected embryonic skeletal muscle cells. In contrast, disruption of either or both copies of the conserved M-CAT motif (5'-CATTCCT-3') inactivated the cTNT promoter in these cells. Both M-CAT motifs were protected from DNase I cleavage in solution footprint assays by an M-CAT binding factor (MCBF) present in nuclear extracts from embryonic muscle tissue. M-CAT mutations that inactivated the cTNT promoter also disrupted MCBF binding, indicating that MCBF may be a key trans-acting factor required for muscle-specific expression of the cTNT promoter. MCBF also bound to the M-CAT motif in the distal promoter region of the skeletal alpha-actin gene, suggesting that it may play a role in the regulation of this and perhaps other muscle genes that contain M-CAT motifs.

Induction of acetylcholine receptor alpha-subunit gene expression in chicken myotubes by blocking electrical activity requires ongoing protein synthesis

The level of acetylcholine receptor alpha-subunit mRNA in primary cultures of chicken myotubes increases when the spontaneous electrical activity of the myotube is suppressed by the sodium channel blocker tetrodotoxin. This increase was prevented by two translational inhibitors: cycloheximide and anisomycin. Neither the basal level of alpha-subunit mRNA nor that of muscle-specific creatine phosphokinase mRNA was affected by these inhibitors. In contrast, cycloheximide potentiated the limited increase of alpha-subunit mRNA levels evoked by the neuropeptide calcitonin gene-related peptide. The high level of alpha-subunit mRNA elicited by tetrodotoxin treatment did not persist after subsequent addition of cycloheximide in the presence of tetrodotoxin, indicating that the continuous synthesis of protein factor(s) is necessary for this regulation. Moreover, cycloheximide decreased the high level of alpha-subunit mRNA present at early stages of in vitro maturation of muscle primary cultures without blocking the further increase of the muscle-specific creatine phosphokinase mRNA. Implications of the requirement for constant synthesis of protein factors on the induction of muscle-specific gene expression by blocking myotube electrical activity are discussed.

Identification of a myocyte nuclear factor that binds to the muscle-specific enhancer of the mouse muscle creatine kinase gene

The muscle creatine kinase (MCK) gene is transcriptionally induced when skeletal muscle myoblasts differentiate into myocytes. The gene contains two muscle-specific enhancer elements, one located 1,100 nucleotides (nt)5' of the transcriptional start site and one located in the first intron. We have used gel mobility shift assays to characterize the trans-acting factors that interact with a region of the MCK gene containing the 5' enhancer. MM14 and C2C12 myocyte nuclear extracts contain a sequence-specific DNA-binding factor which recognizes a site within a 110-nt fragment of the MCK enhancer region shown to be sufficient for enhancer function. Preparative mobility shift gels were combined with DNase I footprinting to determine the site of binding within the 110-nt fragment. Site-directed mutagenesis within the footprinted region produced a 110-nt fragment which did not bind the myocyte factor in vitro. The mutant fragment had about 25-fold-less activity as a transcriptional enhancer in myocytes than did the wild-type fragment. Complementary oligomers containing 21 base pairs spanning the region protected from DNase degradation were also specifically bound by MM14 and C2C12 myocyte nuclear factors. The oligomer-binding activity was not found in nuclear extracts from the corresponding myoblasts, in nuclear extracts from a variety of nonmuscle cell types (including differentiation-defective MM14-DD1 cells and 10T1/2 mesodermal stem cells), or in cytoplasmic extracts. Both the 5' and intron 1 enhancer-containing fragments competed for factors that bind the oligomer probe, while total mouse genomic DNA and several DNA fragments containing viral and cellular enhancers did not. Interestingly, a 5' MCK proximal promoter fragment that also contains muscle-specific positive regulatory elements did not compete for factor binding to the oligomer. We have designated the factor which interacts with the two MCK enhancers myocyte-specific enhancer-binding nuclear factor 1 (MEF 1). A consensus for binding sites in muscle-specific regulatory regions is proposed.

The upstream muscle-specific enhancer of the rat muscle creatine kinase gene is composed of multiple elements

A series of constructs that links the rat muscle creatine kinase promoter to the bacterial chloramphenicol acetyltransferase gene was generated. These constructs were introduced into differentiating mouse C2C12 myogenic cells to localize sequences that are important for up-regulation of the creatine kinase gene during myogenic differentiation. A muscle-specific enhancer element responsible for induction of chloramphenicol acetyltransferase expression during myogenesis was localized to a 159-base-pair region from 1,031 to 1,190 base pairs upstream of the transcription start site. Analysis of transient expression experiments using promoters mutated by deletion indicated the presence of multiple functional domains within this muscle-specific regulatory element. A DNA fragment spanning this region was used in DNase I protection experiments. Nuclear extracts derived from C2 myotubes protected three regions (designated E1, E2, and E3) on this fragment from digestion, which indicated there may be three or more trans-acting factors that interact with the creatine kinase muscle enhancer. Gel retardation assays revealed that factors able to bind specifically to E1, E2, and E3 are present in a wide variety of tissues and cell types. Transient expression assays demonstrated that elements in regions E1 and E3, but not necessarily E2, are required for full enhancer activity.

An internal regulatory element controls troponin I gene expression

During skeletal myogenesis, approximately 20 contractile proteins and related gene products temporally accumulate as the cells fuse to form multinucleated muscle fibers. In most instances, the contractile protein genes are regulated transcriptionally, which suggests that a common molecular mechanism may coordinate the expression of this diverse and evolutionarily unrelated gene set. Recent studies have examined the muscle-specific cis-acting elements associated with numerous contractile protein genes. All of the identified regulatory elements are positioned in the 5'-flanking regions, usually within 1,500 base pairs of the transcription start site. Surprisingly, a DNA consensus sequence that is common to each contractile protein gene has not been identified. In contrast to the results of these earlier studies, we have found that the 5'-flanking region of the quail troponin I (TnI) gene is not sufficient to permit the normal myofiber transcriptional activation of the gene. Instead, the TnI gene utilizes a unique internal regulatory element that is responsible for the correct myofiber-specific expression pattern associated with the TnI gene. This is the first example in which a contractile protein gene has been shown to rely primarily on an internal regulatory element to elicit transcriptional activation during myogenesis. The diversity of regulatory elements associated with the contractile protein genes suggests that the temporal expression of the genes may involve individual cis-trans regulatory components specific for each gene.

The upstream muscle-specific enhancer of the rat muscle creatine kinase gene is composed of multiple elements

A series of constructs that links the rat muscle creatine kinase promoter to the bacterial chloramphenicol acetyltransferase gene was generated. These constructs were introduced into differentiating mouse C2C12 myogenic cells to localize sequences that are important for up-regulation of the creatine kinase gene during myogenic differentiation. A muscle-specific enhancer element responsible for induction of chloramphenicol acetyltransferase expression during myogenesis was localized to a 159-base-pair region from 1,031 to 1,190 base pairs upstream of the transcription start site. Analysis of transient expression experiments using promoters mutated by deletion indicated the presence of multiple functional domains within this muscle-specific regulatory element. A DNA fragment spanning this region was used in DNase I protection experiments. Nuclear extracts derived from C2 myotubes protected three regions (designated E1, E2, and E3) on this fragment from digestion, which indicated there may be three or more trans-acting factors that interact with the creatine kinase muscle enhancer. Gel retardation assays revealed that factors able to bind specifically to E1, E2, and E3 are present in a wide variety of tissues and cell types. Transient expression assays demonstrated that elements in regions E1 and E3, but not necessarily E2, are required for full enhancer activity.

Identification of a myocyte nuclear factor that binds to the muscle-specific enhancer of the mouse muscle creatine kinase gene

The muscle creatine kinase (MCK) gene is transcriptionally induced when skeletal muscle myoblasts differentiate into myocytes. The gene contains two muscle-specific enhancer elements, one located 1,100 nucleotides (nt)5' of the transcriptional start site and one located in the first intron. We have used gel mobility shift assays to characterize the trans-acting factors that interact with a region of the MCK gene containing the 5' enhancer. MM14 and C2C12 myocyte nuclear extracts contain a sequence-specific DNA-binding factor which recognizes a site within a 110-nt fragment of the MCK enhancer region shown to be sufficient for enhancer function. Preparative mobility shift gels were combined with DNase I footprinting to determine the site of binding within the 110-nt fragment. Site-directed mutagenesis within the footprinted region produced a 110-nt fragment which did not bind the myocyte factor in vitro. The mutant fragment had about 25-fold-less activity as a transcriptional enhancer in myocytes than did the wild-type fragment. Complementary oligomers containing 21 base pairs spanning the region protected from DNase degradation were also specifically bound by MM14 and C2C12 myocyte nuclear factors. The oligomer-binding activity was not found in nuclear extracts from the corresponding myoblasts, in nuclear extracts from a variety of nonmuscle cell types (including differentiation-defective MM14-DD1 cells and 10T1/2 mesodermal stem cells), or in cytoplasmic extracts. Both the 5' and intron 1 enhancer-containing fragments competed for factors that bind the oligomer probe, while total mouse genomic DNA and several DNA fragments containing viral and cellular enhancers did not. Interestingly, a 5' MCK proximal promoter fragment that also contains muscle-specific positive regulatory elements did not compete for factor binding to the oligomer. We have designated the factor which interacts with the two MCK enhancers myocyte-specific enhancer-binding nuclear factor 1 (MEF 1). A consensus for binding sites in muscle-specific regulatory regions is proposed.

A ras-dependent pathway abolishes activity of a muscle-specific enhancer upstream from the muscle creatine kinase gene

Differentiation of skeletal myoblasts is accompanied by induction of a series of tissue-specific genes whose products are required for the specialized functions of the mature muscle fiber. The program for myogenic differentiation is subject to negative control by several peptide growth factors and by the products of mutationally activated ras oncogenes, which persistently activate intracellular cascades normally triggered by specific growth factors. Previously, we reported that induction of the muscle creatine kinase (mck) gene during myogenesis was dependent on a distal upstream enhancer that cooperated with a proximal promoter to direct high levels of expression in developing muscle cells (E. A. Sternberg, G. Spizz, W. M. Perry, D. Vizard, T. Weil, and E. N. Olson, Mol. Cell. Biol. 8:2896-2909). To investigate the mechanisms whereby ras blocks the induction of muscle-specific genes, we have examined the ability of mck 5' regulatory elements to direct expression of the linked reporter gene for chloramphenicol acetyltransferase (cat) in C2 myoblasts bearing mutant N-ras and H-ras oncogenes. In this paper we report that expression of activated ras alleles abolishes activity of the mck upstream enhancer but does not affect the activity of the mck promoter. The ability of ras to repress the expression of mck-cat fusion genes that have been transfected either transiently or stably into myoblasts suggests that ras may exert its effects on muscle-specific genes through mechanisms independent of chromatin configurations or DNA methylation. These results also suggest that ras blocks establishment of the myogenic phenotype by preventing the accumulation of regulatory factors required for transcriptional induction of muscle-specific genes.

Stepwise activation of the mouse acetylcholine receptor delta- and gamma-subunit genes in clonal cell lines

We used the DNase I-hypersensitive sites around the mouse acetylcholine receptor delta-subunit gene as a guide toward the cloning and sequencing of delta and gamma transcriptional regulatory regions and as a means to assess chromatin structural activation of the delta- and gamma-subunit genes during myogenesis. Genomic cloning of hypersensitive sites downstream of the delta-subunit gene revealed the presence of the gamma-subunit gene approximately 5 kilobases away; the hypersensitive sites mapped to the 5' end of the gamma-subunit gene. Sequence comparison of restriction fragments containing hypersensitive sites in analogous locations at the 5' ends of the delta- and gamma-subunit genes uncovered little overall homology between the two genomic fragments; however, an 11- of 13-base-pair match between the two sequences was found. Homologs to this sequence were also found to be present in the upstream regions of the chicken alpha- and mouse beta-subunit genes. By RNase protection and primer extension analyses, the delta-subunit gene transcription start site was mapped to 56 base pairs upstream of the initiator ATG codon. Clonal cell lines with various potentials to differentiate to the skeletal muscle phenotype were examined for their hypersensitive site pattern within the delta-gamma locus. Only remote hypersensitive sites flanking the locus appear in pluripotential mesodermal cells. A cell line of determined but inducible myoblasts expressed only one more intergenic site, while in permissively differentiating myoblasts hypersensitive sites were already present at the 5' ends of the delta and gamma genes prior to differentiation. Terminal differentiation resulted in an identical pattern of hypersensitive sites in all muscle cell lines examined so far, with an intergenic site near the gamma-subunit gene being the only site specific to the differentiated muscle phenotype.

Differential expression of acetylcholine receptor mRNA in nuclei of cultured muscle cells

Muscle cells in vitro and in vivo are multinucleated and express acetylcholine receptors (AcChoRs). On innervated cells, the AcChoRs form clusters which lie under the nerve terminals. However, noninnervated cells in culture also express clusters of AcChoR. Both in vivo and in vitro the AcChoR clusters appear to be associated with clusters of nuclei. We have used in situ hybridization to determine whether all the nuclei in cultured chicken embryo myotubes are equally active in expressing the AcChoR alpha subunit message. Cells were hybridized with 35S-labeled probes that contained either both an exon and an intron region or only exon sequences. Control cultures were hybridized with a labeled actin DNA probe or poly(U). The hybrids were detected by emulsion autoradiography; simultaneously, the nuclei were visualized with bisbenzamide. Cells hybridized with the intron/exon probe showed a striking preferential silver grain localization in and around some of the myotube nuclei, whereas those hybridized with the exon probe gave a rather homogeneous grain distribution in the cytoplasm. These results show that myotube nuclei possess differential activation capacities for the expression of AcChoR alpha subunit mRNA and that this difference is due to differential rates of transcription.

An internal regulatory element controls troponin I gene expression

During skeletal myogenesis, approximately 20 contractile proteins and related gene products temporally accumulate as the cells fuse to form multinucleated muscle fibers. In most instances, the contractile protein genes are regulated transcriptionally, which suggests that a common molecular mechanism may coordinate the expression of this diverse and evolutionarily unrelated gene set. Recent studies have examined the muscle-specific cis-acting elements associated with numerous contractile protein genes. All of the identified regulatory elements are positioned in the 5'-flanking regions, usually within 1,500 base pairs of the transcription start site. Surprisingly, a DNA consensus sequence that is common to each contractile protein gene has not been identified. In contrast to the results of these earlier studies, we have found that the 5'-flanking region of the quail troponin I (TnI) gene is not sufficient to permit the normal myofiber transcriptional activation of the gene. Instead, the TnI gene utilizes a unique internal regulatory element that is responsible for the correct myofiber-specific expression pattern associated with the TnI gene. This is the first example in which a contractile protein gene has been shown to rely primarily on an internal regulatory element to elicit transcriptional activation during myogenesis. The diversity of regulatory elements associated with the contractile protein genes suggests that the temporal expression of the genes may involve individual cis-trans regulatory components specific for each gene.

A ras-dependent pathway abolishes activity of a muscle-specific enhancer upstream from the muscle creatine kinase gene

Differentiation of skeletal myoblasts is accompanied by induction of a series of tissue-specific genes whose products are required for the specialized functions of the mature muscle fiber. The program for myogenic differentiation is subject to negative control by several peptide growth factors and by the products of mutationally activated ras oncogenes, which persistently activate intracellular cascades normally triggered by specific growth factors. Previously, we reported that induction of the muscle creatine kinase (mck) gene during myogenesis was dependent on a distal upstream enhancer that cooperated with a proximal promoter to direct high levels of expression in developing muscle cells (E. A. Sternberg, G. Spizz, W. M. Perry, D. Vizard, T. Weil, and E. N. Olson, Mol. Cell. Biol. 8:2896-2909). To investigate the mechanisms whereby ras blocks the induction of muscle-specific genes, we have examined the ability of mck 5' regulatory elements to direct expression of the linked reporter gene for chloramphenicol acetyltransferase (cat) in C2 myoblasts bearing mutant N-ras and H-ras oncogenes. In this paper we report that expression of activated ras alleles abolishes activity of the mck upstream enhancer but does not affect the activity of the mck promoter. The ability of ras to repress the expression of mck-cat fusion genes that have been transfected either transiently or stably into myoblasts suggests that ras may exert its effects on muscle-specific genes through mechanisms independent of chromatin configurations or DNA methylation. These results also suggest that ras blocks establishment of the myogenic phenotype by preventing the accumulation of regulatory factors required for transcriptional induction of muscle-specific genes.

Stepwise activation of the mouse acetylcholine receptor delta- and gamma-subunit genes in clonal cell lines

We used the DNase I-hypersensitive sites around the mouse acetylcholine receptor delta-subunit gene as a guide toward the cloning and sequencing of delta and gamma transcriptional regulatory regions and as a means to assess chromatin structural activation of the delta- and gamma-subunit genes during myogenesis. Genomic cloning of hypersensitive sites downstream of the delta-subunit gene revealed the presence of the gamma-subunit gene approximately 5 kilobases away; the hypersensitive sites mapped to the 5' end of the gamma-subunit gene. Sequence comparison of restriction fragments containing hypersensitive sites in analogous locations at the 5' ends of the delta- and gamma-subunit genes uncovered little overall homology between the two genomic fragments; however, an 11- of 13-base-pair match between the two sequences was found. Homologs to this sequence were also found to be present in the upstream regions of the chicken alpha- and mouse beta-subunit genes. By RNase protection and primer extension analyses, the delta-subunit gene transcription start site was mapped to 56 base pairs upstream of the initiator ATG codon. Clonal cell lines with various potentials to differentiate to the skeletal muscle phenotype were examined for their hypersensitive site pattern within the delta-gamma locus. Only remote hypersensitive sites flanking the locus appear in pluripotential mesodermal cells. A cell line of determined but inducible myoblasts expressed only one more intergenic site, while in permissively differentiating myoblasts hypersensitive sites were already present at the 5' ends of the delta and gamma genes prior to differentiation. Terminal differentiation resulted in an identical pattern of hypersensitive sites in all muscle cell lines examined so far, with an intergenic site near the gamma-subunit gene being the only site specific to the differentiated muscle phenotype.

Isolation and characterization of the mouse acetylcholine receptor delta subunit gene: identification of a 148-bp cis-acting region that confers myotube-specific expression

We have isolated the gene encoding the delta subunit of the mouse skeletal muscle acetylcholine receptor (AChR) and have identified a 148-bp cis-acting region that controls cell type-specific and differentiation-dependent gene expression. The 5' flanking region of the delta subunit gene was fused to the protein-coding region of the chloramphenicol acetyltransferase (CAT) gene and gene fusions were transfected into C2 mouse skeletal muscle cells. Both transiently and stably transfected cells were assayed for CAT gene expression. Deletions from the 5' end of the mouse delta gene demonstrate that 148 bp of 5' flanking DNA is sufficient to confer cell type-specific and differentiation-dependent expression: CAT activity is present in transfected myotubes, but not in transfected 3T3 cells or 10T1/2 cells. Moreover, the level of CAT expression in myotubes transfected with constructs containing 148 bp of 5' flanking DNA from the delta subunit gene is identical to that in myotubes transfected with constructs containing 3.2 kb of 5' flanking DNA and similar to expression from the SV-40 early promoter. Increased CAT activity in myotubes is a result of an increased rate of transcription from the delta subunit promoter, since CAT RNA levels are also 35-fold more abundant in myotubes than myoblasts. In contrast, the SV-40 early promoter is similarly active in all cell types. Thus, 148 bp of 5' flanking DNA from the delta subunit gene contains all the information required for cell type-specific and differentiation-dependent expression of the AChR delta subunit.

Identification of upstream and intragenic regulatory elements that confer cell-type-restricted and differentiation-specific expression on the muscle creatine kinase gene

Terminal differentiation of skeletal myoblasts is accompanied by induction of a series of tissue-specific gene products, which includes the muscle isoenzyme of creatine kinase (MCK). To begin to define the sequences and signals involved in MCK regulation in developing muscle cells, the mouse MCK gene has been isolated. Sequence analysis of 4,147 bases of DNA surrounding the transcription initiation site revealed several interesting structural features, some of which are common to other muscle-specific genes and to cellular and viral enhancers. To test for sequences required for regulated expression, a region upstream of the MCK gene from -4800 to +1 base pairs, relative to the transcription initiation site, was linked to the coding sequences of the bacterial chloramphenicol acetyltransferase (CAT) gene. Introduction of this MCK-CAT fusion gene into C2 muscle cells resulted in high-level expression of CAT activity in differentiated myotubes and no detectable expression in proliferating undifferentiated myoblasts or in nonmyogenic cell lines. Deletion mutagenesis of sequences between -4800 and the transcription start site showed that the region between -1351 and -1050 was sufficient to confer cell type-specific and developmentally regulated expression on the MCK promoter. This upstream regulatory element functioned independently of position, orientation, or distance from the promoter and therefore exhibited the properties of a classical enhancer. This upstream enhancer also was able to confer muscle-specific regulation on the simian virus 40 promoter, although it exhibited a 3- to 5-fold preference for its own promoter. In contrast to the cell type- and differentiation-specific expression of the upstream enhancer, the MCK promoter was able to function in myoblasts and myotubes and in nonmyogenic cell lines when combined with the simian virus 40 enhancer. An additional positive regulatory element was identified within the first intron of the MCK gene. Like the upstream enhancer, this intragenic element functioned independently of position, orientation, and distance with respect to the MCK promoter and was active in differentiated myotubes but not in myoblasts. These results demonstrate that expression of the MCK gene in developing muscle cells is controlled by complex interactions among multiple upstream and intragenic regulatory elements that are functional only in the appropriate cellular context.

Identification of upstream and intragenic regulatory elements that confer cell-type-restricted and differentiation-specific expression on the muscle creatine kinase gene

Terminal differentiation of skeletal myoblasts is accompanied by induction of a series of tissue-specific gene products, which includes the muscle isoenzyme of creatine kinase (MCK). To begin to define the sequences and signals involved in MCK regulation in developing muscle cells, the mouse MCK gene has been isolated. Sequence analysis of 4,147 bases of DNA surrounding the transcription initiation site revealed several interesting structural features, some of which are common to other muscle-specific genes and to cellular and viral enhancers. To test for sequences required for regulated expression, a region upstream of the MCK gene from -4800 to +1 base pairs, relative to the transcription initiation site, was linked to the coding sequences of the bacterial chloramphenicol acetyltransferase (CAT) gene. Introduction of this MCK-CAT fusion gene into C2 muscle cells resulted in high-level expression of CAT activity in differentiated myotubes and no detectable expression in proliferating undifferentiated myoblasts or in nonmyogenic cell lines. Deletion mutagenesis of sequences between -4800 and the transcription start site showed that the region between -1351 and -1050 was sufficient to confer cell type-specific and developmentally regulated expression on the MCK promoter. This upstream regulatory element functioned independently of position, orientation, or distance from the promoter and therefore exhibited the properties of a classical enhancer. This upstream enhancer also was able to confer muscle-specific regulation on the simian virus 40 promoter, although it exhibited a 3- to 5-fold preference for its own promoter. In contrast to the cell type- and differentiation-specific expression of the upstream enhancer, the MCK promoter was able to function in myoblasts and myotubes and in nonmyogenic cell lines when combined with the simian virus 40 enhancer. An additional positive regulatory element was identified within the first intron of the MCK gene. Like the upstream enhancer, this intragenic element functioned independently of position, orientation, and distance with respect to the MCK promoter and was active in differentiated myotubes but not in myoblasts. These results demonstrate that expression of the MCK gene in developing muscle cells is controlled by complex interactions among multiple upstream and intragenic regulatory elements that are functional only in the appropriate cellular context.

Functional activity of the two promoters of the myosin alkali light chain gene in primary muscle cell cultures: comparison with other muscle gene promoters and other culture systems

Proximal upstream flanking sequences of the mouse myosin alkali light chain gene encoding MLC1F and MLC3F, the mouse alpha-cardiac actin gene and the chicken gene for the alpha-subunit of the acetylcholine receptor were linked to the bacterial chloramphenicol acetyl transferase (CAT) gene and transfected into primary cultures derived from mouse skeletal muscle or into myogenic cell lines. We demonstrate that the mouse MLC1F/MLC3F gene has two functional promoters. In primary muscle cultures, a 1200 bp sequence flanking exon 1 (MLC1F) and a 438 bp sequence flanking exon 2 (MLC3F) direct CAT activity in myotubes, but not in myoblasts or in non myogenic 3T6 and CV1 cells. Developmentally regulated expression is also seen with the alpha-cardiac actin (320 bp) and acetylcholine receptor alpha-subunit (850 bp) upstream sequences in the primary culture system. Transfection experiments with myogenic cell lines show different results with a given promoter construct, reflecting possible differences in the levels of regulatory factors between lines. Different muscle gene promoters behave differently in a given cell line, suggesting different regulatory factor requirements between these promoters.

The muscle creatine kinase gene is regulated by multiple upstream elements, including a muscle-specific enhancer

Muscle creatine kinase (MCK) is induced to high levels during skeletal muscle differentiation. We have examined the upstream regulatory elements of the mouse MCK gene which specify its activation during myogenesis in culture. Fusion genes containing up to 3,300 nucleotides (nt) of MCK 5' flanking DNA in various positions and orientations relative to the bacterial chloramphenicol acetyltransferase (CAT) structural gene were transfected into cultured cells. Transient expression of CAT was compared between proliferating and differentiated MM14 mouse myoblasts and with nonmyogenic mouse L cells. The major effector of high-level expression was found to have the properties of a transcriptional enhancer. This element, located between 1,050 and 1,256 nt upstream of the transcription start site, was also found to have a major influence on the tissue and differentiation specificity of MCK expression; it activated either the MCK promoter or heterologous promoters only in differentiated muscle cells. Comparisons of viral and cellular enhancer sequences with the MCK enhancer revealed some similarities to essential regions of the simian virus 40 enhancer as well as to a region of the immunoglobulin heavy-chain enhancer, which has been implicated in tissue-specific protein binding. Even in the absence of the enhancer, low-level expression from a 776-nt MCK promoter retained differentiation specificity. In addition to positive regulatory elements, our data provide some evidence for negative regulatory elements with activity in myoblasts. These may contribute to the cell type and differentiation specificity of MCK expression.

The muscle creatine kinase gene is regulated by multiple upstream elements, including a muscle-specific enhancer

Muscle creatine kinase (MCK) is induced to high levels during skeletal muscle differentiation. We have examined the upstream regulatory elements of the mouse MCK gene which specify its activation during myogenesis in culture. Fusion genes containing up to 3,300 nucleotides (nt) of MCK 5' flanking DNA in various positions and orientations relative to the bacterial chloramphenicol acetyltransferase (CAT) structural gene were transfected into cultured cells. Transient expression of CAT was compared between proliferating and differentiated MM14 mouse myoblasts and with nonmyogenic mouse L cells. The major effector of high-level expression was found to have the properties of a transcriptional enhancer. This element, located between 1,050 and 1,256 nt upstream of the transcription start site, was also found to have a major influence on the tissue and differentiation specificity of MCK expression; it activated either the MCK promoter or heterologous promoters only in differentiated muscle cells. Comparisons of viral and cellular enhancer sequences with the MCK enhancer revealed some similarities to essential regions of the simian virus 40 enhancer as well as to a region of the immunoglobulin heavy-chain enhancer, which has been implicated in tissue-specific protein binding. Even in the absence of the enhancer, low-level expression from a 776-nt MCK promoter retained differentiation specificity. In addition to positive regulatory elements, our data provide some evidence for negative regulatory elements with activity in myoblasts. These may contribute to the cell type and differentiation specificity of MCK expression.

Human thrombomodulin gene is intron depleted: nucleic acid sequences of the cDNA and gene predict protein structure and suggest sites of regulatory control

We have isolated a human thrombomodulin cDNA, and a human genomic clone containing the putative promoter domain, as well as the translated and untranslated regions of the endothelial cell receptor. The nucleotide sequence of the thrombomodulin cDNA allows us to provide a complete picture of the structure of this endothelial cell receptor, and to confirm its homology to the human low density lipoprotein receptor. The nucleotide sequence of the thrombomodulin gene suggests areas within the putative promoter domain that may be critical for regulating expression of the human endothelial cell receptor, indicates a potential signal peptide, and shows that no introns are present within the coding region. The overall organization of the human thrombomodulin gene is surprising because it represents an example of a gene that contains epidermal growth factor type B repeats and a membrane spanning region, which are not isolated within discrete exons.