Fast-muscle-specific expression of human aldolase A transgenes

Independence and interdependence of the three human aldolase A promoters in transgenic mice

The human aldolase A gene is transcribed from three alternative promoters, clustered in a small 1.6-kb DNA domain. In transgenic mice, the upstream pN and the downstream pH promoters are ubiquitous, whereas the pM promoter, located between pN and pH, is activated specifically in fast skeletal muscles. A strong ubiquitous enhancer, lying upstream of the pH promoter, is necessary for both pN and pH ubiquitous activities, whereas a fast-muscle-specific enhancer, located upstream of the pM promoter, is required for pM-specific activation. In the present study, we use the transgenic mice model to further investigate the contribution of these two regulatory elements to the overall control of these three promoters. We confirm that the pM and pH promoters are activated independently of each other and, in particular, we show that the activation of pM in fast muscle is not responsible for the downregulation of the downstream pH in this tissue. By contrast, the pN promoter needs the presence of both enhancers to reproduce its correct pattern of activity and is unable to function autonomously in vivo.

DNA methylation assessment from human slow- and fast-twitch skeletal muscle fibers

A new application of the reduced representation bisulfite sequencing method was developed using low-DNA input to investigate the epigenetic profile of human slow- and fast-twitch skeletal muscle fibers. Successful library construction was completed with as little as 15 ng of DNA, and high-quality sequencing data were obtained with 32 ng of DNA. Analysis identified 143,160 differentially methylated CpG sites across 14,046 genes. In both fiber types, selected genes predominantly expressed in slow or fast fibers were hypomethylated, which was supported by the RNA-sequencing analysis. These are the first fiber type-specific methylation data from human skeletal muscle and provide a unique platform for future research.NEW & NOTEWORTHY This study validates a low-DNA input reduced representation bisulfite sequencing method for human muscle biopsy samples to investigate the methylation patterns at a fiber type-specific level. These are the first fiber type-specific methylation data reported from human skeletal muscle and thus provide initial insight into basal state differences in myosin heavy chain I and IIa muscle fibers among young, healthy men.

The human desmin locus: gene organization and LCR-mediated transcriptional control

Locus control regions (LCRs) are defined by their ability to confer reproducible physiological levels of transgene expression in mice and therefore thought to possess the ability to generate dominantly a transcriptionally active chromatin structure. We report the first characterization of a muscle-cell-specific LCR, which is linked to the human desmin gene (DES). The DES LCR consists of five regions of muscle-specific DNase I hypersensitivity (HS) localized between -9 and -18 kb 5' of DES and reproducibly drives full physiological levels of expression in all muscle cell types. The DES LCR DNase I HS regions are highly conserved between humans and other mammals and can potentially bind a broad range of muscle-specific and ubiquitous transcription factors. Bioinformatics and direct molecular analysis show that the DES locus consists of three muscle-specific (DES) or muscle preferentially expressed genes (APEG1 and SPEG, the human orthologue of murine striated-muscle-specific serine/threonine protein kinase, Speg). The DES LCR may therefore regulate expression of SPEG and APEG1 as well as DES.

The Krüppel-like zinc-finger protein ZNF224 represses aldolase A gene transcription by interacting with the KAP-1 co-repressor protein

Transcription factors belonging to the Krüppel-like zinc finger family of proteins participate in the regulation of cell differentiation and development. Although many of these proteins have been identified, little is known about their structure and function. We recently purified ZNF224, a new Krüppel-like zinc finger protein, that contains a Krüppel-associated box (KRAB) domain at the NH2 terminus, and 19 Cys2-His2 zinc-finger domains at the COOH terminus. Using chromatin immunoprecipitation and transient transfection assays, we demonstrate that ZNF224 binds in vivo to the distal promoter of the aldolase A gene and represses its transcription. The results of transient co-transfection experiments show that ZNF224-mediated transcription repression requires the 45-amino acid long KRAB A domain. The ability of KRAB-containing ZNF224 protein to repress transcription depends on specific interaction with the KAP-1 co-repressor molecule. Finally, using selective treatment with the HDAC1 inhibitor trichostatin A, we demonstrate that ZNF224-mediated repression requires histone deacetylases.

Mouse muscle identity: the position-dependent and fast fiber-specific expression of a transgene in limb muscles is methylation-independent and cell-autonomous

We previously characterised transgenic mice in which fast-muscle-specific regulatory sequences from the human aldolase A pM promoter drive the chloramphenicol acetyltransferase gene expression. Mutation of a NF1/MEF2 binding site (M2 motif) in this promoter does not affect fibre-type specificity of the transgene but modifies its expression in a subset of fast-twitch fibres at the limb level, preferentially affecting distal limb muscles. We investigated the molecular and cellular bases of this peculiar expression pattern that provided an adequate model to characterise the mechanisms responsible for muscle positional information. By direct electrotransfer of mutated M2 construct in adult muscle, we demonstrate that positional differences in mutated M2 transgene expression are not observed when the transgene is not integrated into chromatin. Also, this transgene expression pattern does not seem to be correlated with the extent of CpG methylation in its promoter sequence. Finally, we show that positional values reflected by CAT levels are maintained in primary cultures established from different adult limb muscles, as well as in heterotopically transplanted muscles. Our results suggest that mutation of the M2 site contributes to reveal a molecular memory of fibre fate that would be set up on pM promoter during development and persist into adulthood possibly through a chromatin imprint maintained in satellite cells associated with various limb muscles.

Muscle electrotransfer as a tool for studying muscle fiber-specific and nerve-dependent activity of promoters

Muscle electrotransfer has recently become a promising tool for efficient delivery of plasmids and transgene expression in skeletal muscle. This technology has been mainly applied to use of muscle as a bioreactor for production of therapeutic proteins. However, it remains to be determined whether muscle electrotransfer may also be accurately used as an alternative tool to transgenesis for studying aspects of muscle-specific gene control that must be explored in fully mature muscle fibers in vivo, such as fiber specificity and nerve dependence. It was also not known to what extent the initial electrical stimulations alter muscle physiology and gene expression. Therefore, optimized conditions of skeletal muscle electroporation were first tested for their effects on muscles of transgenic mice harboring a pM310-CAT transgene in which the CAT reporter gene was under control of the fast IIB fiber-specific and nerve-dependent aldolase A pM promoter. Surprisingly, electrostimulation led to a drastic but transient shutdown of pM310-CAT transgene expression concomitant with very transient activation of MyoD and, mostly, with activation of myogenin, suggesting profound alterations in transcriptional status of the electroporated muscle. Return to a normal transcriptional state was observed 7-10 days after electroporation. Therefore, we investigated whether a reporter construct placed under control of pM could exhibit fiber-specific expression 10 days after electrotransfer in either fast tibialis anterior or slow soleus muscle. We show that not only fiber specificity, but also nerve dependence, of a pM-driven construct can be reproduced. However, after electrotransfer, pM displayed a less tight control than previously observed for the same promoter when integrated in a chromatin context.

Characterization of general transcription factor 3, a transcription factor involved in slow muscle-specific gene expression

General transcription factor 3 (GTF3) binds specifically to the bicoid-like motif of the troponin I(slow) upstream enhancer. This motif is part of a sequence that restricts enhancer activity to slow muscle fibers. GTF3 contains multiple helix-loop-helix domains and an amino-terminal leucine zipper motif. Here we show that helix-loop-helix domain 4 is necessary and sufficient for binding the bicoid-like motif. Moreover, the affinity of this interaction is enhanced upon removal of amino-terminal sequences including domains 1 and 2, suggesting that an unmasking of the DNA binding surface may be a precondition for GTF3 to bind DNA in vivo. We have also investigated the interactions of six GTF3 splice variants of the mouse, three of which were identified in this study, with the troponin enhancer. The gamma-isoform lacking exon 23, and exons 26-28 that encode domain 6, interacted most avidly with the bicoid-like motif; the alpha- and beta- isoforms that include these exons fail to bind in gel retardation assays. We also show that GTF3 polypeptides associate with each other via the leucine zipper. We speculate that cells can generate a large number of GTF3 proteins with distinct DNA binding properties by alternative splicing and combinatorial association of GTF3 polypeptides.

A novel zinc finger transcriptional repressor, ZNF224, interacts with the negative regulatory element (AldA-NRE) and inhibits gene expression

The interaction between the negative cis-element (AldA-NRE) and p97 repressor nuclear protein is a key step in modulating transcription of the human and mouse aldolase A (AldA) gene during the cell cycle and differentiation. In an attempt to clarify the role of transcriptional repression in regulating gene expression, we purified, from HeLa cells, the nuclear protein that specifically binds to the AldA negative regulatory element (NRE). Matrix-assisted laser desorption ionization-time of flight analysis and examination of protein profiles from the SwissProt database revealed that the previously defined p97 repressor is ZNF224, a zinc finger protein. We demonstrate that ZNF224, a Kruppel-like zinc finger transcription factor, is the repressor protein that specifically binds to the negative cis-element AldA-NRE and affects the AldA-NRE-mediated transcription.

Fiber-type specific and position-dependent expression of a transgene in limb muscles

We have previously shown that the proximal sequences of the human aldolase A fast-muscle-specific promoter (pM) are sufficient to target the expression of a linked CAT reporter gene to all fast, glycolytic trunk and limb muscles of transgenic mice (pM310CAT lines) in a manner mimicking the activity of the endogenous mouse promoter. When a NF1-binding site (motif M2) in this proximal regulatory region is mutated, the activity of the corresponding mM2 transgene is strongly affected but only in a some fast muscles. Here we show that the mutation of the M2 motif has only mild effects on pM activity in axial and proximal limb, while it drastically reduces this activity in both fore and hind limb distal muscles. At the cellular level, we show that both the pM310CAT and mM2 transgenes are highly expressed in fast glycolytic 2B fibers. However, by contrast to the pM310CAT transgene, whose expression is mainly restricted to fast glycolytic 2B fibers, the mM2 transgene is also active in a high proportion of 2X fibers. This result suggests that the M2 sequence could play a role in restricting the expression of pM to the 2B fibers. The variable expression of the mM2 transgene along the limb axis already exists at post-natal day 10 and seems to result from a change in the proportion of expressing fast fibers per muscle. Altogether, these results suggest that, although considered as phenotypically similar, different populations of fast glycolytic fibers exist, in which the requirement of the NF1 activity for pM expression varies according to the proximal versus distal position of the muscle along the limb axis.

Coregulation of fast contractile protein transgene and glycolytic enzyme expression in mouse skeletal muscle

Little is known of the gene regulatory mechanisms that coordinate the contractile and metabolic specializations of skeletal muscle fibers. Here we report a novel connection between fast isoform contractile protein transgene and glycolytic enzyme expression. In quantitative histochemical studies of transgenic mouse muscle fibers, we found extensive coregulation of the glycolytic enzyme glycerol-3-phosphate dehydrogenase (GPDH) and transgene constructs based on the fast skeletal muscle troponin I (TnIfast) gene. In addition to a common IIB > IIX > IIA fiber type pattern, TnIfast transgenes and GPDH showed correlated fiber-to-fiber variation within each fast fiber type, concerted emergence of high-level expression during early postnatal muscle maturation, and parallel responses to muscle under- or overloading. Regulatory information for GPDH-coregulated expression is carried by the TnIfast first-intron enhancer (IRE). These results identify an unexpected contractile/metabolic gene regulatory link that is amenable to further molecular characterization. They also raise the possibility that the equal expression in all fast fiber types observed for the endogenous TnIfast gene may be driven by different metabolically coordinated mechanisms in glycolytic (IIB) vs. oxidative (IIA) fast fibers.

PKC-dependent phosphorylation of the p97 repressor regulates the transcription of aldolase A L-type promoter

Expression of mouse aldolase A L-type mRNA is negatively modulated by a cis element (AldA-NRE), located within the aldolase A distal promoter (pL). AldA-NRE interacts with a 97-kDa repressor protein (p97), which binds DNA in a cell cycle-dependent manner. We demonstrate that the binding between AldA-NRE and p97 decreases during differentiation of human Caco-2 cells and is inversely correlated with L-type mRNA expression. Phosphorylation of the p97 repressor weakened its DNA binding activity in differentiated Caco-2 cells, while dephosphorylation enhanced the binding in proliferating cells. Stimulation of protein kinase C (PKC) in vivo decreased the binding of p97 to AldA-NRE and stimulated transcription, while inhibition of PKC stimulated p97 binding and downregulated transcription. These findings suggest that PKC is a mediator of the binding and silencing function of the p97/AldA-NRE repressor complex.

Developmental regulation of the aldolase A muscle-specific promoter during in vivo muscle maturation is controlled by a nuclear receptor binding element

During the post-natal period, skeletal muscles undergo important modifications leading to the appearance of different types of myofibers which exhibit distinct contractile and metabolic properties. This maturation process results from the activation of the expression of different sets of contractile proteins and metabolic enzymes, which are specific to the different types of myofibers. The muscle-specific promoter of the aldolase A gene (pM) is expressed mainly in fast-twitch glycolytic fibers in adult body muscles. We investigate here how pM is regulated during the post-natal development of different types of skeletal muscles (slow or fast-twitch muscles, head or body muscles). We show that pM is expressed preferentially in prospective fast-twitch muscles soon after birth; pM is up-regulated specifically in body muscles only later in development. This activation pattern is mimicked by a transgene which comprises only the 355 most proximal sequences of pM. Within this region, we identify a DNA element which is required for the up-regulation of the transgene during post-natal development in body muscles. Comparison of nuclear M1-binding proteins from young or adult body muscles show no qualitative differences. Distinct M1-binding proteins are present in both young and adult tongue nuclear extracts, compared to that present in gastrocnemius extracts.

Protein kinase C modulates aryl hydrocarbon receptor nuclear translocator protein-mediated transactivation potential in a dimer context

Protein kinase C (PKC)- and protein kinase A (PKA)-mediated modulation of the transactivation potential of human aryl hydrocarbon receptor nuclear translocator (hARNT), a basic helix-loop-helix (bHLH)-PAS transcription factor, and the bHLH-ZIP transcription factors USF-1 (for upstream regulatory factor 1) and c-Myc were examined. An 81 nM dose of the PKC activator phorbol-12-myristate-13-acetate (PMA), shown here to specifically activate PKC in COS-1 cells, or a 1 nM dose of the PKA activator 8-bromoadenosine-3',5'-cyclic monophosphate (8-Br-cAMP) results in 2. 6- and 1.9-fold enhancements, respectively, in hARNT-mediated transactivation of the class B, E-box-driven reporter pMyc3E1bLuc relative to identically transfected, carrier solvent-treated COS-1 cells. In contrast, 81 nM PMA and 1 nM 8-Br-cAMP did not enhance transactivation of pMyc3E1bLuc-driven by USF-1 and c-Myc expression relative to identically transfected, carrier-treated COS-1 cells. Co-transfection of pcDNA3/ARNT-474-Flag, expressing a hARNT carboxyl-terminal transactivation domain deletion, and pMyc3E1bLuc does not result in induction of reporter activity, suggesting PMA's effects do not involve formation of unknown hARNT-protein heterodimers. Additionally, PMA had no effect on hARNT expression relative to Me2SO-treated cells. Metabolic 32P labeling of hARNT in cells treated with carrier solvent or 81 nM PMA demonstrates that PMA does not increase the overall phosphorylation level of hARNT. These results demonstrate, for the first time, that the transactivation potential of ARNT in a dimer context can be specifically modulated by PKC or PKA stimulation and that the bHLH-PAS and bHLH-ZIP transcription factors are differentially regulated by these pathways in COS-1 cells.

Muscle-specific locus control region activity associated with the human desmin gene

We describe the reproduction of the full pattern of expression of the muscle-specific desmin gene in transgenic mice using a 240-kb genomic clone spanning the human desmin locus. Analysis of RNA from adult tissues demonstrated that this fragment possesses all the necessary genetic regulatory elements required to provide reproducible, site-of-integration-independent, physiological levels of tissue-specific expression that is directly proportional to transgene copy number in all muscle cell types. In situ hybridization revealed that in marked contrast to murine desmin which is strongly expressed in the myotome of the somites, skeletal muscles, the heart, and smooth muscle of the vasculature by 9.5 days postcoitum, human desmin transgene expression was completely absent from smooth muscles, was very weak and restricted to the atrium and outflow tract within the heart, and was expressed at only 5% of murine desmin mRNA levels within the myotome of the somites. The spatial distribution and levels of human and mouse desmin expression were not coincident until 14.5 days postcoitum. Immunohistochemical analysis of human embryos at comparable stages of development showed that this transgene faithfully reproduces the human and not the mouse developmental expression pattern for this gene in transgenic mice. These results indicate that the 240-kb desmin genomic clone is capable of establishing an independent, chromatin domain in transgenic mice and provides the first definitive data for muscle-specific locus control region activity. In addition, our results demonstrate that the behavior of human transgenes in mice should, whenever possible, be compared to expression patterns for that gene in human embryonic as well as adult tissues.

Proximal sequences of the aldolase A fast muscle-specific promoter direct nerve- and activity-dependent expression in transgenic mice

Muscle activity is known to modulate the muscle fiber phenotype. Changes in muscle activity (normal or experimentally induced) lead to modifications of the expression status of several muscle-specific genes. However, the transcription regulatory elements involved in the adaptative response are mainly unknown. The aldolase A muscle-specific promoter, pM, is expressed in adult fast twitch muscle with a preferential expression in fast glycolytic-2B fibers. Its activity is induced during postnatal muscle maturation, suggesting a role of nerve and/or muscle activity. Indeed, denervation of gastrocnemius in newborn mice prevented the activation of the promoter in this muscle, despite the nerve-independent formation of 2B fibers. Although the nerve was necessary for pM onset during development, denervating the gastrocnemius in adults had only mild effects on pM activity. By contrast, a transgene including the pM proximal regulatory sequences that are sufficient to reproduce the 2B fiber-specific expression of the endogenous promoter was shown to be highly sensitive to both neonatal and adult denervation. Transgenes containing muscle-specific pM proximal promoter elements were used to delineate the regulatory elements involved in this response to innervation and changes in the contractile activity pattern. Nerve- and activity-dependent elements could be localized in the 130-base pair-long proximal promoter region of the human aldolase A gene.

In vivo analysis of the myosin heavy chain IIB promoter region

The myosin heavy chain (MHC) IIB gene is preferentially expressed in fast-twitch muscles of the hindlimb, such as the tibialis anterior (TA). The molecular mechanism(s) for this preferential expression are unknown. The goals of the current study were 1) to determine whether the cloned region of the MHC IIB promoter contains the necessary cis-acting element(s) to drive fiber-type-specific expression of this gene in vivo, 2) to determine which region within the promoter is responsible for fiber-type-specific expression, and 3) to determine whether transcription off of the cloned region of the MHC IIB promoter accurately mimics endogenous gene expression in a muscle undergoing a fiber-type transition. To accomplish these goals, a 2.6-kilobase fragment of the promoter-enhancer region of the MHC IIB gene was cloned upstream of the firefly luciferase reporter gene and coinjected with pRL-cytomegalovirus (CMV) (CMV promoter driving the renilla luciferase reporter) into the TA and the slow soleus muscle. Firefly luciferase activity relative to renilla luciferase activity within the TA was 35-fold greater than within the soleus. Deletional analysis demonstrated that only the proximal 295 base pairs (pGL3IIB0.3) were required to maintain this muscle-fiber-type specificity. Reporter gene expression of pGL3IIB0.3 construct was significantly upregulated twofold in unweighted soleus muscles compared with normal soleus muscles. Thus the region within the proximal 295 base pairs of the MHC IIB gene contains at least one element that can drive fiber-type-specific expression of a reporter gene.

A binding site for nuclear receptors is required for the differential expression of the aldolase A fast-twitch muscle promoter in body and head muscles

In hind limb muscles, the aldolase A muscle-specific promoter is specifically expressed in glycolytic fast-twitch fibers. Here, we show that in addition, it is expressed at higher levels in trunk and limb muscles than in neck and head muscles independent of their fiber-type content. We have identified by analysis of transgenic mice a DNA element that is required for this differential expression and, to a lesser extent, for fiber-type specificity. We show that members of the nuclear receptor superfamily bind this element in skeletal muscle nuclear extracts. Interestingly, in gel mobility shift assays, different complexes were formed with this sequence in tongue nuclear extracts compared with limb or trunk muscle nuclear extracts. Therefore, binding of distinct nuclear receptors to a single regulatory sequence appears to be associated with the location-dependent expression of the aldolase A muscle-specific promoter.

Negative regulation of the mouse aldolase A gene. A cell cycle-dependent DNA binding activity functions as a silencer of gene transcription

The expression of aldolase A L-type mRNA is increased in growth-arrested mouse NIH3T3 cells and remarkably down-regulated in actively proliferating cells. Treatment of proliferating cells with cycloheximide abolished the down-regulation of L-type mRNA expression, thus indicating that a protein factor acts as repressor in proliferating cells. Transient transfection experiments in NIH3T3 cells showed that a negative regulatory cis-element (NRE) is involved in the modulation of the transcriptional activity of the distal L promoter. The repressor, which is a protein of approximately 97 kDa, binds the murine aldolase A NRE, revealing a much more intense DNA-protein complex in proliferating NIH3T3 cells than in serum-deprived cells. Mutations in the negative regulatory cis-element showed that the GA-rich motif is required for protein binding and silencer function. We conclude that the expression of L-type mRNA is modulated by the interaction between a cell cycle-dependent DNA-binding protein and the murine aldolase A NRE.

Structure of aldolase A (muscle-type) cDNA and its regulated expression in oocytes, embryos and adult tissues of Xenopus laevis

We obtained cDNA (XALDA; 1466 bp) for Xenopus laevis aldolase A gene (muscle-type), whose amino acid sequence had 88% similarity to those of mammalian aldolase A genes. XALDA mRNA occurred abundantly in skeletal muscle and at low levels also in other adult tissues, and such mRNA distribution was reflected in zymograms. In oocytes XALDA mRNA occurred at a relatively high level from stage I, and the mRNA level peaked at stage II, then decreased in later stages. XALDA mRNA in the full-grown oocyte was inherited as maternal mRNA throughout maturation and fertilization until midblastula stage, but its level became very low during gastrula and early neurula stages, and then increased greatly in later stages. While maternal XALDA mRNA was distributed uniformly in early embryos, mRNA zygotically expressed after late neurula stage occurred mainly in somites. In blastula animal caps XALDA mRNA occurred at a low level, but the expression was greatly enhanced by activin treatment. Thus, in Xenopus laevis aldolase A gene is actively transcribed in earlier phase of oogenesis, inherited as maternal mRNA in early embryos in a cell-type nonspecific way, then in later phases of embryogenesis, it is strongly expressed in somites with its concomitant ubiquitous expression at low levels in almost all the other cell types.

DNA binding and transcriptional activation by the Ski oncoprotein mediated by interaction with NFI

The Ski oncoprotein has been found to bind non-specifically to DNA in association with unindentified nuclear factors. In addition, Ski has been shown to activate transcription of muscle-specific and viral promoters/enhancers. The present study was undertaken to identify Ski's DNA binding and transcriptional activation partners by identifying specific DNA binding sites. We used nuclear extracts from a v-Ski-transduced mouse L-cell line and selected Ski-bound sequences from a pool of degenerate oligonucleotides with anti-Ski monoclonal antibodies. Two sequences were identified by this technique. The first (TGGC/ANNNNNT/GCCAA) is the previously identified binding site of the nuclear factor I (NFI) family of transcription factors. The second (TCCCNNGGGA) is the binding site of Olf-1/EBF. By electophoretic mobility shift assays we find that Ski is a component of one or more NFI complexes but we fail to detect Ski in Olf-1/EBF complexes. We show that Ski binds NFI proteins and activates transcription of NFI reporters, but only in the presence of NFI. We also find that homodimerization of Ski is essential for co-activation with NFI. However, the C-terminal dimerization domain of c-Ski, which is missing in v-Ski, can be substituted by the leucine zipper domain of GCN4.

Identification of the human aldolase A gene as the first induced target for the TR2 orphan receptor, a member of the steroid hormone receptor superfamily

The human TR2 orphan receptor (TR2) is a member of the steroid/thyroid hormone receptor superfamily that regulates the transcription of complex gene networks and subsequently controls diverse aspects of growth, development, and differentiation. In the present study, we have found that the TR2 is one of the M1 site (nucleotide numbers 2017-2034, 5'-AAAAGGGCAGGGGTCATT-3') binding proteins of the muscle-specific pM promoter in the human aldolase A gene. Electrophoretic mobility shift assay (EMSA) showed a specific binding with high affinity (dissociation constant = 4.6 nM) between the TR2 and the M1 element. Circular permutation assay revealed a localized DNA flexibility induced by the TR2 binding, and the bend angle was estimated to be 73 +/- 2 degrees. Furthermore, a dual-luciferase reporter gene assay demonstrated that the TR2 may enhance the expression of luciferase activities via the wild-type M1 site but not the mutant M1 element in human QM7 muscle myoblasts. In conclusion, our data represent the first case of demonstrating that the TR2 may serve as a transcriptional inducer in muscle-specific aldolase A gene expression.

Mouse USF1 gene cloning: comparative organization within the c-myc gene family

Upstream stimulatory factors (USF/MLTF) belong to the c-myc family of transcription factors. Through binding to target DNA as dimers, the ubiquitous USF proteins regulate a variety of genes. USF proteins are encoded by two genes, USF1 and USF2. Protein sequences of USF1 and 2 are highly homologous across species, suggesting functional conservation. To determine whether the genomic organization was conserved between USF1 and USF2, we isolated the murine USF1 gene and characterized its genomic structure. Both genes are similarly organized in 10 exons spanning over 10 kbp. By the 5'-rapid amplification of cDNA ends and S1 nuclease mapping methods, exon 1 was defined and the transcription initiation sites were mapped. The sequence of 8 kb of the gene, including 1.75 kb of 5'-flanking DNA, was determined. The promoter region is GC rich and lacks a typical TATA or CCAAT element. Strikingly, a comparison of the murine and human untranslated sequences reveals regions that exhibit greater than 73% sequence identity. A genomic alignment of the dimerization and DNA binding domains is presented for five genes of the c-myc family, suggesting a hypothetical common ancestor gene.

Structure and expression of the murine muscle adenylosuccinate synthetase gene

A muscle-specific isoform of adenylosuccinate synthetase (AdSS1, EC) is one of three enzymes that constitute the purine nucleotide cycle, a muscle-specific metabolic cycle. Previously, we showed that the muscle Adss1 gene was highly expressed in both skeletal muscle and heart of the adult mouse. Here we have shown that the Adss1 gene is initially activated early in embryonic development in skeletal muscle and heart precursors and is subsequently up-regulated perinatally. The earliest detectable gene expression corresponds with the establishment of the first myogenic and cardiac lineages. To allow identification of the genetic signals controlling this developmental pattern of expression, the Adss1 gene was cloned and its structure determined. Transgenic analysis has shown that 1.9 kilobase pairs of 5' flank can activate expression in skeletal muscle progenitors and direct enhanced expression to adult cardiac muscle. Sequence analysis of the promoter and 5' flanking region revealed the presence of numerous potential muscle-specific cis-regulatory elements.

An intron 1 regulatory region from the murine adenosine deaminase gene can activate heterologous promoters for ubiquitous expression in transgenic mice

Ubiquitously expressed genes contain regulatory features which allow expression in virtually all cell types. In an effort to understand the molecular basis for this regulatory feature, the chromatin structure of the murine adenosine deaminase gene was examined by DNase I digestion in nuclei of several tissues. The promoter contained a strong hypersensitive site in all tissues examined, including those with very high and very low levels of ADA expression. Transgenic mouse studies revealed that a 3.3 kb EcoRI (3.3EE) fragment from intron I was required to generate a strong promoter DNase I hypersensitive site, and to produce ubiquitous expression. The 3.3EE fragment also contained a thymic enhancer activity which mapped to sequences conserved with the human ADA gene T-lymphocyte enhancer. Mutational analysis indicated that ubiquitous expression was not dependent on the presence of a functional thymic enhancer. Both the thymic enhancer and the ubiquitous activator within the 3.3EE fragment functioned with heterologous promoters in transgenic mice.

Transcriptional control of muscle plasticity: differential regulation of troponin I genes by electrical activity

Plasticity of the skeletal muscle phenotype can result from the selective repression and activation of gene expression in response to innervation patterns. Motoneurons, eliciting different patterns of depolarization, regulate the contractile properties of the myofibers they innervate by selectively activating expression of genes encoding fiber-type-specific (fast vs. slow) contractile proteins. We have analyzed the regulation of the troponin I slow (TnIs) and fast (TnIf) genes as a model to study the molecular mechanisms regulating fiber-type plasticity. We found that expression of the two TnI isoforms is downregulated by denervation. Moreover, TnI expression is upregulated by specific patterns of electrical activity [10 Hz vs. 100 Hz] used to depolarize muscle. We previously isolated the rat TnIs gene and demonstrated that regulatory sequences reside in its upstream region and second intron [Banerjee-Basu S, Buonanno A (1993), Mol Cell Biol 12:5024-5032]. Using transgenic mice, we show that the upstream region of the TnIs gene extending from -949 to +50 is sufficient to confer transcription specifically in slowtwitch muscles. Serial deletions of the TnIs upstream and intronic regions were generated in a CAT reporter vector to delineate transcriptional regulatory elements in transiently transfected Sol8 myotubes. Sequences necessary to confer the highest levels of TnIs transcription mapped to the upstream region between -0.95 and -0.72 kb, and to a 56 bp sequence located in the second intron. Comparison of the at sequence between -0.95 and -0.72 to the human TnIs gene identified a highly homologous region of 128 bp that we named the TnI SURE (slow upstream regulatory element). Alignment of these two SURE sequences with the quail TnI fast intronic regulatory element identified common motifs, namely, two A/T-rich sequences (A/T1 and A/T2) with homology to homeotic protein and MEF2 binding sites, a CACC box, an E box, and a novel motif (GCAGGCA) that we denoted the CAGG box. Mutation of either the A/T2 site, E box, or CAGG box practically abolish the SURE function in transfected myotubes; mutation of the A/T1 and CACC sites has a lesser effect. Using competitive electrophoretic mobility shift assays with nuclear extracts derived from Sol8 myotubes, we demonstrate specific binding to these motifs. The A/T1 and A/T2 sites are shown to form different complexes. The A/T2 site, which bears extensive homology to a MEF2 site, forms complexes that are super shifted by MEF2A antisera and that are competed by a consensus MEF2 site present in the MCK enhancer. Our results demonstrate that the linear arrangement of DNA sequence motifs is conserved in the regulatory elements of the TnI slow and fast genes and suggest that the interaction of multiple protein-DNA complexes are necessary for enhancer function.