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

The role of the LRPPRC (leucine-rich pentatricopeptide repeat cassette) gene in cytochrome oxidase assembly: mutation causes lowered levels of COX (cytochrome c oxidase) I and COX III mRNA

Leigh syndrome French Canadian (LSFC) is a variant of cytochrome oxidase deficiency found in Québec and caused by mutations in the LRPPRC (leucine-rich pentatricopeptide repeat cassette) gene. Northern blots showed that the LRPPRC mRNA levels seen in skeletal muscle>heart>placenta>kidney>liver>lung=brain were proportionally almost opposite in strength to the severity of the enzymic cytochrome oxidase defect. The levels of COX (cytochrome c oxidase) I and COX III mRNA visible on Northern blots were reduced in LSFC patients due to the common (A354V, Ala354-->Val) founder mutation. The amount of LRPPRC protein found in both fibroblast and liver mitochondria from LSFC patients was consistently reduced to <30% of control levels. Import of [(35)S]methionine LRPPRC into rat liver mitochondria was slower for the mutant (A354V) protein. A titre of LRPPRC protein was also found in nuclear fractions that could not be easily accounted for by mitochondrial contamination. [35S]Methionine labelling of mitochondrial translation products showed that the translation of COX I, and perhaps COX III, was specifically reduced in the presence of the mutation. These results suggest that the gene product of LRPPRC, like PET 309p, has a role in the translation or stability of the mRNA for mitochondrially encoded COX subunits. A more diffuse distribution of LRPPRC in LSFC cells compared with controls was evident when viewed by immunofluorescence microscopy, with less LRPPRC present in peripheral mitochondria.

Lens connexins alpha3Cx46 and alpha8Cx50 interact with zonula occludens protein-1 (ZO-1)

Connexin alpha1Cx43 has previously been shown to bind to the PDZ domain-containing protein ZO-1. The similarity of the carboxyl termini of this connexin and the lens fiber connexins alpha3Cx46 and alpha8Cx50 suggested that these connexins may also interact with ZO-1. ZO-1 was shown to be highly expressed in mouse lenses. Colocalization of ZO-1 with alpha3Cx46 and alpha8Cx50 connexins in fiber cells was demonstrated by immunofluorescence and by fracture-labeling electron microscopy but showed regional variations throughout the lens. ZO-1 was found to coimmunoprecipitate with alpha3Cx46 and alpha8Cx50, and pull-down experiments showed that the second PDZ domain of ZO-1 was involved in this interaction. Transiently expressed alpha3Cx46 and alpha8Cx50 connexins lacking the COOH-terminal residues did not bind to the second PDZ domain but still formed structures resembling gap junctions by immunofluorescence. These results indicate that ZO-1 interacts with lens fiber connexins alpha3Cx46 and alpha8Cx50 in a manner similar to that previously described for alpha1Cx43. The spatial variation in the interaction of ZO-1 with lens gap junctions is intriguing and is suggestive of multiple dynamic roles for this association.

A novel heterogeneous nuclear ribonucleoprotein-like protein interacts with NS1 of the minute virus of mice

NS1, the major nonstructural parvovirus protein of the minute virus of mice, is a multifunctional protein responsible for several aspects of viral replication. NS1 transactivates the P38 promoter (used to express the structural proteins), as well as its own strong promoter, P4. To study the mechanism of activation and to map regions of NS1 responsible for transactivation, NS1 and various deletions of NS1 were cloned in frame with the GAL4DB and cotransfected into COS-7 and LA9 cells with a synthetic GAL4-responsive reporter plasmid. These studies showed NS1 can directly activate transcription through its 129 carboxyl-terminal amino acid residues. Any deletion from this region of the C terminus, even as few as 8 amino acids, completely abolishes transactivation. A yeast two-hybrid system used to identify protein-protein interactions demonstrated that NS1 is able to dimerize when expressed in yeast cells. However, only an almost complete NS11-638 bait was able to interact with the full-length NS1. A two-hybrid screen identified a HeLa cell cDNA clone (NS1-associated protein 1 [NSAP1]) that interacts with NS11-276 and NS11-638. An additional sequence was predicted from human EST (expressed sequence tag) data, and the cDNA was estimated to be at least 2,221 bp long, potentially encoding a 562-amino-acid protein product. A polyclonal antibody raised to a synthetic peptide within NSAP1 recognizes an approximately 65-kDa cellular protein. This NSAP1 cDNA has not previously been characterized, but the predicted protein sequence is 80% identical to the recently identified heterogeneous nuclear ribonucleoprotein (hnRNP) R (W. Hassfeld et al., Nucleic Acids Res. 26:439-445, 1998). NSAP1 contains four ribonucleoprotein domains, as well as a highly repetitive C-terminal region. A closely related mouse cDNA (deduced from murine EST data) encodes a protein with only a single amino acid residue change from the human protein. NSAP1 is predicted to be a 65-kDa polynucleotide binding protein, and it likely functions in the regulation of splicing and/or transport of mRNAs from the nucleus.

Functional characteristics of ES cell-derived cardiac precursor cells identified by tissue-specific expression of the green fluorescent protein

In contrast to terminally differentiated cardiomyocytes, relatively little is known about the characteristics of mammalian cardiac cells before the initiation of spontaneous contractions (precursor cells). Functional studies on these cells have so far been impossible because murine embryos of the corresponding stage are very small, and cardiac precursor cells cannot be identified because of the lack of cross striation and spontaneous contractions. In the present study, we have used the murine embryonic stem (ES, D3 cell line) cell system for the in vitro differentiation of cardiomyocytes. To identify the cardiac precursor cells, we have generated stably transfected ES cells with a vector containing the gene of the green fluorescent protein (GFP) under control of the cardiac alpha-actin promoter. First, fluorescent areas in ES cell-derived cell aggregates (embryoid bodies [EBs]) were detected 2 d before the initiation of contractions. Since Ca2+ homeostasis plays a key role in cardiac function, we investigated how Ca2+ channels and Ca2+ release sites were built up in these GFP-labeled cardiac precursor cells and early stage cardiomyocytes. Patch clamp and Ca2+ imaging experiments proved the functional expression of the L-type Ca2+ current (ICa) starting from day 7 of EB development. On day 7, using 10 mM Ca2+ as charge carrier, ICa was expressed at very low densities 4 pA/pF. The biophysical and pharmacological properties of ICa proved similar to terminally differentiated cardiomyocytes. In cardiac precursor cells, ICa was found to be already under control of cAMP-dependent phosphorylation since intracellular infusion of the catalytic subunit of protein kinase A resulted in a 1.7-fold stimulation. The adenylyl cyclase activator forskolin was without effect. IP3-sensitive intracellular Ca2+ stores and Ca2+-ATPases are present during all stages of differentiation in both GFP-positive and GFP-negative cells. Functional ryanodine-sensitive Ca2+ stores, detected by caffeine-induced Ca2+ release, appeared in most GFP-positive cells 1-2 d after ICa. Coexpression of both ICa and ryanodine-sensitive Ca2+ stores at day 10 of development coincided with the beginning of spontaneous contractions in most EBs. Thus, the functional expression of voltage-dependent L-type Ca2+ channel (VDCC) is a hallmark of early cardiomyogenesis, whereas IP3 receptors and sarcoplasmic Ca2+-ATPases are expressed before the initiation of cardiomyogenesis. Interestingly, the functional expression of ryanodine receptors/sensitive stores is delayed as compared with VDCC.

Human mitochondrial phosphoenolpyruvate carboxykinase 2 gene. Structure, chromosomal localization and tissue-specific expression

The mitochodrial (mt) phosphoenolpyruvate carboxykinase 2 (PCK2) gene was isolated by screening a human genomic library with a rat cytosolic (cy) PCK1 cDNA probe comprising sequences from exons 2-9 and by PCR amplification of human genomic DNA spanning consecutive exons with known primer pairs from mtPCK2 cDNA containing sequences from two putative neighbouring exons. The mtPCK2 gene spans approx. 10 kb and consists of ten exons and nine introns. All exon-intron junction sequences match the classical GT/AG rule. Northern blot analysis of poly(A)+ and total RNA from various tissues revealed one mRNA species of approx. 2.4 kb. The gene is expressed in a variety of human tissues, mainly in liver, kidney, pancreas, intestine and fibroblasts. In contrast with the cytosolic isoenzyme, the mitochondrial form might not have a purely gluconeogenic function. The mtPCK2 gene maps to chromosome 14q11.2-q12, in contrast with the cyPCK1 gene located on 20q13.2-q13.31.

Cloning and characterization of the promoter for the liver isoform of the rat carnitine palmitoyltransferase I (L-CPT I) gene

Carnitine palmitoyltransferase I (CPTI) catalyses the transfer of long chain fatty acids to carnitine for translocation across the mitochondrial inner membrane. The cDNAs of two isoforms of CPT I, termed the hepatic and muscle isoforms, have been cloned. Expression of the hepatic CPT I gene (L-CPT I) is subject to developmental, hormonal and tissue specific regulation. We have cloned the promoter of the L-CPTI gene from a rat genomic library. In the L-CPTI gene, there are two exons 5' to the exon containing the ATG that initiates translation. Exon 1 and the 5' end of exon 2 contain sequences that were not previously described in the rat L-CPTI cDNA. There is an alternatively spliced form of the L-CPTI mRNA in which exon 2 is skipped. The proximal promoter of the L-CPTI gene is extremely GC rich and does not contain a TATA box. There are several putative Sp1 binding sites near the transcriptional start site. A 190 base pair fragment of the promoter can efficiently drive transcription of luciferase and CAT (chloramphenicol acetyltransferase) reporter genes transiently transfected into HepG2 cells. Sequences in both the first intron and the promoter contribute to basal expression. Our results provide the foundation for further studies into the regulation of L-CPTI gene expression.

E-box sites and a proximal regulatory region of the muscle creatine kinase gene differentially regulate expression in diverse skeletal muscles and cardiac muscle of transgenic mice

Previous analysis of the muscle creatine kinase (MCK) gene indicated that control elements required for transcription in adult mouse muscle differed from those required in cell culture, suggesting that distinct modes of muscle gene regulation occur in vivo. To examine this further, we measured the activity of MCK transgenes containing E-box and promoter deletions in a variety of striated muscles. Simultaneous mutation of three E boxes in the 1,256-bp MCK 5' region, which abolished transcription in muscle cultures, had strikingly different effects in mice. The mutations abolished transgene expression in cardiac and tongue muscle and caused a reduction in expression in the soleus muscle (a muscle with many slow fibers) but did not affect expression in predominantly fast muscles: quadriceps, abdominals, and extensor digitorum longus. Other regulatory sequences with muscle-type-specific activities were found within the 358-bp 5'-flanking region. This proximal region conferred relatively strong expression in limb and abdominal skeletal muscles but was inactive in cardiac and tongue muscles. However, when the 206-bp 5' enhancer was ligated to the 358-bp region, high levels of tissue-specific expression were restored in all muscle types. These results indicate that E boxes and a proximal regulatory region are differentially required for maximal MCK transgene expression in different striated muscles. The overall results also imply that within skeletal muscles, the steady-state expression of the MCK gene and possibly other muscle genes depends on transcriptional mechanisms that differ between fast and slow fibers as well as between the anatomical and physiological attributes of each specific muscle.

Analysis of muscle creatine kinase gene regulatory elements in skeletal and cardiac muscles of transgenic mice

Regulatory regions of the mouse muscle creatine kinase (MCK) gene, previously discovered by analysis in cultured muscle cells, were analyzed in transgenic mice. The 206-bp MCK enhancer at nt-1256 was required for high-level expression of MCK-chloramphenicol acetyltransferase fusion genes in skeletal and cardiac muscle; however, unlike its behavior in cell culture, inclusion of the 1-kb region of DNA between the enhancer and the basal promoter produced a 100-fold increase in skeletal muscle activity. Analysis of enhancer control elements also indicated major differences between their properties in transgenic muscles and in cultured muscle cells. Transgenes in which the enhancer right E box or CArG element were mutated exhibited expression levels that were indistinguishable from the wild-type transgene. Mutation of three conserved E boxes in the MCK 1,256-bp 5' region also had no effect on transgene expression in thigh skeletal muscle expression. All these mutations significantly reduced activity in cultured skeletal myocytes. However, the enhancer AT-rich element at nt - 1195 was critical for expression in transgenic skeletal muscle. Mutation of this site reduced skeletal muscle expression to the same level as transgenes lacking the 206-bp enhancer, although mutation of the AT-rich site did not affect cardiac muscle expression. These results demonstrate clear differences between the activity of MCK regulatory regions in cultured muscles cells and in whole adult transgenic muscle. This suggests that there are alternative mechanism of regulating the MCK gene in skeletal and cardiac muscle under different physiological states.

Regulation of the murine alpha B-crystallin/small heat shock protein gene in cardiac muscle

The murine alpha B-crystallin/small heat shock protein gene is expressed at high levels in the lens and at lower levels in the heart, skeletal muscle, and numerous other tissues. Previously we have found a skeletal-muscle-preferred enhancer at positions -427 to -259 of the alpha B-crystallin gene containing at least four cis-acting regulatory elements (alpha BE-1, alpha BE-2, alpha BE-3, and MRF, which has an E box). Here we show that in transgenic mice, the alpha B-crystallin enhancer directs the chloramphenicol acetyltransferase reporter gene driven by the alpha B-crystallin promoter specifically to myocardiocytes of the heart. The alpha B-crystallin enhancer was active in conjugation with the herpes simplex virus thymidine kinase promoter/human growth hormone reporter gene in transfected rat myocardiocytes. DNase I footprinting and site-specific mutagenesis experiments showed that alpha BE-1, alpha BE-2, alpha BE-3, MRF, and a novel, heart-specific element called alpha BE-4 are required for alpha B-crystallin enhancer activity in transfected myocardiocytes. By contrast, alpha BE-4 is not utilized for enhancer activity in transfected lens or skeletal muscle cell lines. Alpha BE-4 contains an overlapping heat shock sequence and a reverse CArG box [5'-GG(A/T)6CC-3']. Electrophoretic mobility shift assays with an antibody to serum response factor and a CArG-box-competing sequence from the c-fos promoter indicated that a cardiac-specific protein with DNA-binding and antigenic similarities to serum response factor binds to alpha BE-4 via the reverse CArG box; electrophoretic mobility shift assays and antibody experiments with anti-USF antiserum and heart nuclear extract also raised the possibility that the MRF E box utilizes USF or an antigenically related protein. We conclude that the activity of the alpha B-crystallin enhancer in the heart utilizes a reverse CArG box and an E-box-dependent pathway.

Cellular aggregation enhances MyoD-directed skeletal myogenesis in embryonal carcinoma cells

When introduced into P19 embryonal carcinoma cells, recombinant genes encoding MyoD converted only a small percentage (< 3%) of the transfected cells into skeletal muscle. We isolated stably transfected cells that expressed the MyoD transcript. These P19[MyoD] cells continued to express markers characteristic of undifferentiated stem cells but also expressed myf-5 and the myotonic dystrophy kinase, transcripts normally present in myoblasts but absent from P19 cells. Aggregation of P19[MyoD] cells induced the expression of myogenin, desmin, and the retinoblastoma protein and resulted in the rapid and abundant development of skeletal muscle. Both the embryonic and the slow isoforms of myosin heavy chain were present in this muscle, indicating that it resembled skeletal muscle formed from primary myoblasts. Since aggregation of P19 cells normally results in inefficient differentiation and the development of only low levels of cardiac muscle but no skeletal muscle, we conclude that MyoD imposes the skeletal muscle program on P19 cells and that the differentiation of these cells requires inductive events provided by cell aggregation.

Isolation and characterization of cDNA for human 120 kDa mitochondrial 2,4-dienoyl-coenzyme A reductase

2,4-Dienoyl-CoA reductase (EC 1.3.1.34) participates in beta-oxidation of (poly)unsaturated enoyl-CoAs and it appears in mammalian mitochondria as two isoforms with molecular masses of 120 and 60 kDa [Hakkola and Hiltunen (1993) Eur. J. Biochem. 215, 199-204]. The 120 kDa isomer is a homotetrameric enzyme, and here we report cDNA cloning of its subunit from human. cDNA clones were isolated by reverse transcriptase-PCR from a fibrosarcoma cell line and by screening from a human liver lambda gt11 cDNA library. The 1128 bp clone contained an open reading frame of 1008 bp encoding a polypeptide of 335 amino acid residues with a predicted molecular mass of 36066 Da. This polypeptide represents the immature monomer of the 120 kDa enzyme, and it contains a predicted N-terminal mitochondrial targeting signal. The amino acid (nucleotide) sequence of human 2,4-dienoyl-CoA reductase shows 82.7% (81.7%) similarity (identity) to the corresponding sequence from the rat. Northern-blot analysis gave a single mRNA species of 1.2 kb in several human tissues, the amounts present in the tissues tested ranking as follows: heart approximately liver approximately pancreas > kidney >> skeletal muscle approximately lung. Immunoblotting of human and rat liver samples with an antibody to the subunit of the rat 120 kDa isoform indicates that the mature human enzyme is larger than its counterpart in the rat. The comparison of amino acid sequences for rat and human enzymes proposes that the difference in the size is 10 amino acid residues. The results show that the rat and human reductases are similar in many characteristics and that the reductase is expressed in human tissues capable of beta-oxidation of fatty acids.

Cellular aggregation enhances MyoD-directed skeletal myogenesis in embryonal carcinoma cells

When introduced into P19 embryonal carcinoma cells, recombinant genes encoding MyoD converted only a small percentage (< 3%) of the transfected cells into skeletal muscle. We isolated stably transfected cells that expressed the MyoD transcript. These P19[MyoD] cells continued to express markers characteristic of undifferentiated stem cells but also expressed myf-5 and the myotonic dystrophy kinase, transcripts normally present in myoblasts but absent from P19 cells. Aggregation of P19[MyoD] cells induced the expression of myogenin, desmin, and the retinoblastoma protein and resulted in the rapid and abundant development of skeletal muscle. Both the embryonic and the slow isoforms of myosin heavy chain were present in this muscle, indicating that it resembled skeletal muscle formed from primary myoblasts. Since aggregation of P19 cells normally results in inefficient differentiation and the development of only low levels of cardiac muscle but no skeletal muscle, we conclude that MyoD imposes the skeletal muscle program on P19 cells and that the differentiation of these cells requires inductive events provided by cell aggregation.

Heterogeneity of vacuolar H(+)-ATPase: differential expression of two human subunit B isoforms

The catalytic domain of the vacuolar proton ATPase is composed of a hexamer of three A subunits and three B subunits. Here we describe the cloning and characterization of a cDNA isoform of subunit B, HO57, from an osteoclastoma cDNA library. HO57 is represented by three species of mRNA of 1.6, 2.6 and 2.8 kb and is expressed at low levels in a range of human tissues, but at significantly higher levels in brain, kidney and osteoclastoma, and is probably an ubiquitously expressed isoform. In contrast, the kidney-specific isoform has an mRNA of 2 kb and is specifically expressed at high levels only in kidney and, at a lower level, in placenta. Thus the HO57 isoform is integral to the vacuolar ATPase found in the general secretory system of all cells as well as in vacuolar-ATPase-rich sources such as neurones and osteoclasts, whereas both the kidney-specific isoform and HO57 are highly expressed in the kidney. Furthermore, we show by in situ hybridization that HO57 is the only isoform that is exclusively and highly expressed by osteoclasts.

Inducible and cell type-specific expression of VL30 U3 subgroups correlate with their enhancer design

The murine VL30 elements constitute one family of retrotransposons represented in 100 to 200 copies that are dispersed among the mouse chromosomes. On the basis of sequence homology, we have subdivided mouse VL30 members into four distinct U3 subgroups. The use of subgroup-specific probes in Northern (RNA) blot analyses shows that individual VL30 U3 subgroups are expressed in a tissue-specific manner. We show by in situ hybridization of mouse skin treated with 12-O-tetradecanoylphorbol-13-acetate (TPA) that VL30 expression is induced in epidermal keratinocytes but not in dermal fibroblasts. Transient transfections of reporter gene plasmids together with in vitro binding analysis indicate that TPA-induced VL30 transcription specific for keratinocytes is mediated by two cooperating sequence motifs in juxtaposed position. One sequence motif is shown to constitutively bind CREB- and Jun-related proteins in both keratinocytes and fibroblasts, whereas the other is a target for TPA-induced c-Rel/p65(NF-kappa B)-binding activity specifically in keratinocytes. These binding sites are found to be conserved within U3 subgroups and individual U3 regions showing induced expression in TPA-treated mouse epidermis. These results together with a sequence comparison between different U3 subgroups indicate that cell type-specific activity of transcription factors known to regulate VL30 transcription and the presence or absence of their cognate binding sites within individual U3 regions determine inducible and cell type-specific VL30 expression. The variable VL30 U3 regions might thus be useful tools to study inducible and cell type-specific transcription in many different cell systems.

Islet cell autoantigen 69 kD (ICA69). Molecular cloning and characterization of a novel diabetes-associated autoantigen

We have identified a novel 69-kD peptide autoantigen (ICA69) associated with insulin-dependent diabetes mellitus (IDDM) by screening a human islet lambda gt11 cDNA expression library with cytoplasmic islet cell antibody positive sera from relatives of IDDM patients who progressed to the overt disease. The deduced open reading frame of the ICA69 cDNA predicts a 483-amino acid protein. ICA69 shows no nucleotide or amino acid sequence relation to any known sequence in GenBank, except for two short regions of similarity with BSA. The ICA69 cDNA probe hybridizes with a 2-kb mRNA in poly(A+) RNA from human pancreas, brain, heart, thyroid, and kidney, but not with skeletal muscle, placenta, spleen, or ovary. Expression of ICA69 was also detected in beta cells and cell lines, as well as in tumoral tissue of islet cell origin. The native ICA69 molecule migrates to 69 kD in SDS-PAGE as detected with specific antibodies. Serum samples from relatives of IDDM patients specifically reacted with affinity-purified recombinant ICA69 on Western blotting. The structural gene for ICA69 was designated ICA1. A homologue in the mouse, designated Ica-1 was mapped to the proximal end of chromosome 6 (within 6 cM of the Met protooncogene). ICA69 adds a novel autoantigen to the family of identified islet target molecules, and by the manner of its identification and characterization large amounts of antigen are available for development of quantitative, convenient predictive assays for autoantibodies and analysis of the role of this molecule in diabetes autoimmunity, as well as its physiologic function.

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

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

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

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

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

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

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

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