Muscle-specific expression of the acetylcholine receptor alpha-subunit gene requires both positive and negative interactions between myogenic factors, Sp1 and GBF factors

Dual tandem promoter elements containing CCAC-like motifs from the tetrodotoxin-resistant voltage-sensitive Na+ channel (rSkM2) gene can independently drive muscle-specific transcription in L6 cells

cis-Elements in the -129/+124 promoter segment of the rat tetrodotoxin-resistant voltage-gated sodium channel (rSkM2) gene that are responsible for reporter gene expression in cultured muscle cells were identified by deletion and scanning mutations. Nested 5' deletion constructs, assayed in L6 myotubes and NIH3T3 cells, revealed that the minimum promoter allowing muscle-specific expression is contained within the -57 to +1 segment relative to the major transcription initiation site. In the context of the -129/+1 construct, however, scanning mutations in the -69/+1 segment failed to identify any critical promoter elements. In contrast, identical mutations in a minimal promoter (-57/+124) showed that all regions except -29/-20 are essential for expression, especially the -57/-40 segment, consistent with the 5' deletion analysis. Further experiments showed that the distal (-129/-58) and proximal promoter (-57/+1) elements can independently drive reporter expression in L6 myotubes, but not in NIH3T3 fibroblasts. This pair of elements is similar in sequence and contains Sp1 sites (CCGCCC), CCAC-like motifs, but no E-boxes or MEF-2 sites. The two segments form similarly migrating complexes with L6 myotube nuclear extracts in gel-shift assays. Critical elements within the distal promoter element were defined by 10 base pair scanning mutations in the -119 to -60 region in the context of the -129/+1 segment containing a mutated -59/-50 segment that inactivates the proximal promoter. Nucleotides in the -119/-90 region, especially -109/-100, were the most important regions for distal promoter function. We conclude that the -129/+1 segment contains two tandem promoter elements, each of which can independently drive muscle-specific transcription. Supershifts with antibodies to Sp1 and myocyte nuclear factor (MNF) implicate the involvement of Sp1, MNF, and other novel factors in the transcriptional regulation of rSkM2 gene expression.

Mechanisms Regulating Neuromuscular Junction Development and Function and Causes of Muscle Wasting

The neuromuscular junction is the chemical synapse between motor neurons and skeletal muscle fibers. It is designed to reliably convert the action potential from the presynaptic motor neuron into the contraction of the postsynaptic muscle fiber. Diseases that affect the neuromuscular junction may cause failure of this conversion and result in loss of ambulation and respiration. The loss of motor input also causes muscle wasting as muscle mass is constantly adapted to contractile needs by the balancing of protein synthesis and protein degradation. Finally, neuromuscular activity and muscle mass have a major impact on metabolic properties of the organisms. This review discusses the mechanisms involved in the development and maintenance of the neuromuscular junction, the consequences of and the mechanisms involved in its dysfunction, and its role in maintaining muscle mass during aging. As life expectancy is increasing, loss of muscle mass during aging, called sarcopenia, has emerged as a field of high medical need. Interestingly, aging is also accompanied by structural changes at the neuromuscular junction, suggesting that the mechanisms involved in neuromuscular junction maintenance might be disturbed during aging. In addition, there is now evidence that behavioral paradigms and signaling pathways that are involved in longevity also affect neuromuscular junction stability and sarcopenia.

TGF-beta receptors, in a Smad-independent manner, are required for terminal skeletal muscle differentiation

Skeletal muscle differentiation is strongly inhibited by transforming growth factor type beta (TGF-beta), although muscle formation as well as regeneration normally occurs in an environment rich in this growth factor. In this study, we evaluated the role of intracellular regulatory Smads proteins as well as TGF-beta-receptors (TGF-beta-Rs) during skeletal muscle differentiation. We found a decrease of TGF-beta signaling during differentiation. This phenomenon is explained by a decline in the levels of the regulatory proteins Smad-2, -3, and -4, a decrease in the phosphorylation of Smad-2 and lost of nuclear translocation of Smad-3 and -4 in response to TGF-beta. No change in the levels and inhibitory function of Smad-7 was observed. In contrast, we found that TGF-beta-R type I (TGF-beta-RI) and type II (TGF-beta-RII) increased on the cell surface during skeletal muscle differentiation. To analyze the direct role of the serine/threonine kinase activities of TGF-beta-Rs, we used the specific inhibitor SB 431542 and the dominant-negative form of TGF-beta-RII lacking the cytoplasmic domain. The TGF-beta-Rs were important for successful muscle formation, determined by the induction of myogenin, creatine kinase activity, and myosin. Silencing of Smad-2/3 expression by specific siRNA treatments accelerated myogenin, myosin expression, and myotube formation; although when SB 431542 was present inhibition in myosin induction and myotube formation was observed, suggesting that these last steps of skeletal muscle differentiation require active TGF-beta-Rs. These results suggest that both down-regulation of Smad regulatory proteins and cell signaling through the TGF-beta receptors independent of Smad proteins are essential for skeletal muscle differentiation.

Identification of Sp1 and GC-boxes as transcriptional regulators of mouse Dag1 gene promoter

Dystroglycan is a widely expressed adhesion complex that anchors cells to the basement membrane and is involved in embryonic development and differentiation. Dystroglycan expression is frequently reduced in human dystrophies and malignancies, and its molecular functions are not completely understood. Several posttranslational mechanisms have been identified that regulate dystroglycan expression and/or function, while little is known about how expression of the corresponding Dag1 gene is regulated. This study aimed to clone the Dag1 gene promoter and to characterize its regulatory elements. Analysis of the mouse Dag1 gene 5'-flanking region revealed a TATA and CAAT box-lacking promoter including a GC-rich region. Transfection studies with serially deleted promoter constructs allowed us to identify a minimal promoter region containing three Specificity protein 1 (Sp1) sites and an E-box. Sp1 binding was confirmed by chromatin immunoprecipitation assay, and Sp1 downregulation reduced dystroglycan expression in muscle cells. Treatment with 5-aza-2'-deoxycytidine and/or the histone deacetylase inhibitor trichostatin A increased Dag1 mRNA expression levels in myoblasts, and methylation decreased promoter activity in vitro. Furthermore, Dag1 gene promoter methylation was reduced while its expression increased during differentiation of C(2)C(12) myoblast cells in myotubes. In conclusion, for the first time we have characterized the activity of the mouse Dag1 gene promoter, confirming a complex regulation by Sp1 transcription factor, DNA methylation, and histone acetylation, which might be relevant for a better understanding of the physiopathology of the dystroglycan complex.

Sp1 and AP-1 regulate expression of the human gene VIL2 in esophageal carcinoma cells

Ezrin, encoded by VIL2, is a membrane-cytoskeletal linker protein that has been suggested to be involved in tumorigenesis. Ezrin expression in esophageal squamous cell carcinoma (ESCC) was described recently, but its clinical significance and the molecular mechanism underlying its regulated expression remain unclear. Thus, we retrospectively evaluated ezrin expression by immunohistochemistry in a tissue microarray representing 193 ESCCs. Ezrin overexpression in 90 of 193 tumors (46.6%) was associated with poor survival (p = 0.048). We then explored the mechanism by which ezrin expression is controlled in ESCC by assessing the transcriptional regulatory regions of human VIL2 by fusing deletions or site-directed mutants of the 5'-flanking region of the gene to a luciferase reporter. We found that the region -87/-32 containing consensus Sp1 (-75/-69) and AP-1 (-64/-58) binding sites is crucial for VIL2 promoter activity in esophageal carcinoma cells (EC109) derived from ESCC. AP-1 is comprised of c-Jun and c-Fos. Electrophoretic mobility shift and chromatin immunoprecipitation experiments demonstrated that Sp1 and c-Jun bound specifically to their respective binding sites within the VIL2 promoter. In addition, transient expression of Sp1, c-Jun, or c-Fos increased ezrin expression and VIL2 promoter activity. Use of selective inhibitors revealed that VIL2 transactivation required the MEK1/2 signal transduction pathway but not JNK or p38 MAPK. Taken together, we propose a possible signal transduction pathway whereby MEK1/2 phosphorylates ERK1/2, which phosphorylates Sp1 and AP-1 that in turn bind to their respective binding sites to regulate the expression of human VIL2 in ESCC cells.

Inferring biological functions and associated transcriptional regulators using gene set expression coherence analysis

Background

Gene clustering has been widely used to group genes with similar expression pattern in microarray data analysis. Subsequent enrichment analysis using predefined gene sets can provide clues on which functional themes or regulatory sequence motifs are associated with individual gene clusters. In spite of the potential utility, gene clustering and enrichment analysis have been used in separate platforms, thus, the development of integrative algorithm linking both methods is highly challenging.

Results

In this study, we propose an algorithm for discovery of molecular functions and elucidation of transcriptional logics using two kinds of gene information, functional and regulatory motif gene sets. The algorithm, termed gene set expression coherence analysis first selects functional gene sets with significantly high expression coherences. Those candidate gene sets are further processed into a number of functionally related themes or functional clusters according to the expression similarities. Each functional cluster is then, investigated for the enrichment of transcriptional regulatory motifs using modified gene set enrichment analysis and regulatory motif gene sets. The method was tested for two publicly available expression profiles representing murine myogenesis and erythropoiesis. For respective profiles, our algorithm identified myocyte- and erythrocyte-related molecular functions, along with the putative transcriptional regulators for the corresponding molecular functions.

Conclusion

As an integrative and comprehensive method for the analysis of large-scaled gene expression profiles, our method is able to generate a set of testable hypotheses: the transcriptional regulator X regulates function Y under cellular condition Z. GSECA algorithm is implemented into freely available software package.

Transforming growth factor beta (TGF-beta) signaling is regulated by electrical activity in skeletal muscle cells. TGF-beta type I receptor is transcriptionally regulated by myotube excitability

Transforming growth factor (TGF-beta) is involved in several cellular processes such as cell proliferation, differentiation, and apoptosis. At the cell surface, TGF-beta binds to serine-threonine kinase transmembrane receptors (type II and type I) to initiate Smad-dependent intracellular signaling cascades. During the early stages of skeletal muscle differentiation, myotubes start to evoke spontaneous electrical activity in association with contractions that arise following the maturation of the excitation-contraction apparatus. In this work, we report that TGF-beta-dependent signaling is regulated by electrical activity in developing rat primary myotubes, as determined by Smad2 phosphorylation, Smad4 nuclear translocation, and p3TPLux reporter activity. This electrical activity-dependent regulation is associated with changes in TGF-beta type I receptor (TbetaRI) levels, correlated with changes in transducing receptors at the cell membrane (measured through radiolabeling binding assays). The inhibition of electrical activity with tetrodotoxin, a voltage-dependent sodium channel blocker, increases TbetaRI levels via a transcription-dependent mechanism. In contrast, the promotion of electrical activity in myotube cultures, induced by the up-regulation of voltage-dependent sodium channels or by direct stimulation with extracellular electrodes, causes TbetaRI levels to decrease. Similar results were obtained in denervated adult muscles, suggesting that electrical activity-dependent regulation of TbetaRI also occurs in vivo. Additional results suggest that this activity-dependent regulation is mediated by myogenin. Altogether, these findings support the possibility for a novel regulatory mechanism acting on TGF-beta signaling cascade in skeletal muscle cells.

Methyl-CpG binding proteins identify novel sites of epigenetic inactivation in human cancer

Methyl-CpG binding proteins (MBDs) mediate histone deacetylase-dependent transcriptional silencing at methylated CpG islands. Using chromatin immunoprecitation (ChIP) we have found that gene-specific profiles of MBDs exist for hypermethylated promoters of breast cancer cells, whilst a common pattern of histone modifications is shared. This unique distribution of MBDs is also characterized in chromosomes by comparative genomic hybridization of immunoprecipitated DNA and immunolocalization. Most importantly, we demonstrate that MBD association to methylated DNA serves to identify novel targets of epigenetic inactivation in human cancer. We combined the ChIP assay of MBDs with a CpG island microarray (ChIP on chip). The scenario revealed shows that, while many genes are regulated by multiple MBDs, others are associated with a single MBD. These target genes displayed methylation- associated transcriptional silencing in breast cancer cells and primary tumours. The candidates include the homeobox gene PAX6, the prolactin hormone receptor, and dipeptidylpeptidase IV among others. Our results support an essential role for MBDs in gene silencing and, when combined with genomic strategies, their potential to 'catch' new hypermethylated genes in cancer.

Evidence that E-box promoter elements and MyoD transcription factors play a role in the induction of cathepsin B gene expression during human myoblast differentiation

HB13 human myoblasts express physiological and biochemical markers associated with myoblast differentiation in non-human cell culture model systems. During differentiation, HB13 myoblasts also demonstrate fusion-related increases in cathepsin B activity and protein levels. These increases are associated with an increase in levels of cathepsin B mRNA suggesting the involvement of transcriptional regulatory mechanisms. To examine these mechanisms human myoblasts were transfected with cathepsin B nested deletion promoter constructs within the 1.8 kb 5' promoter 1 region of the human catB gene. Transfected myoblasts that were maintained under differentiating conditions demonstrated higher promoter activity than those maintained in proliferating conditions. The highest activity was obtained with pSCB2-3 (-1279/+56 bp), a construct containing two putative upstream E-box elements. Co-transfection experiments demonstrated that MyoD and myogenin transactivate cathepsin B promoter activity. Electrophoretic mobility shift assays of nuclear extracts incubated with an oligonucleotide containing two upstream E-box elements found within the cathepsin B promoter demonstrated two band shifts. The band shifts were abolished using an oligonucleotide with mutations in both E-box elements. Moreover, the shifted bands were super-shifted and abolished when incubated with anti-myogenin and anti-MyoD, respectively. Collectively, these data support myogenic transcription factor-mediated activation of cathepsin B expression during myogenesis.

The role of Sp1 and AP-2 in basal and protein kinase A--induced expression of mitochondrial serine:pyruvate aminotransferase in hepatocytes

Transcription of mitochondrial serine:pyruvate aminotransferase (SPT) mRNA (SPTm-mRNA) in rat liver is unique in that it occurs from the upstream site of the two transcription start sites within the first exon of the SPT gene and is selectively enhanced by cAMP via the protein kinase A (PKA) signaling pathway. In this study, we identified the DNA elements and nuclear factors responsible for the basal and PKA-induced activities of the upstream promoter. By using a luciferase reporter assay with HepG2 cells, DNase I footprinting analysis, and gel shift experiments, we identified the binding sites for Sp1 and AP-2 within the regions -125 to -89 and -14 to +10, respectively. Mutational analyses indicated that these regions are essential for the transcription factor binding and the SPT promoter activity. Expression of AP-2 caused a marked increase in the basal promoter activity to about the same level as that achieved by PKA. On the other hand, both the basal and PKA-induced activities were elevated by overexpression of Sp1, its effect on PKA-induced activity being more pronounced with coexpression of CBP and repressed by E1A oncoprotein. These results suggest that AP-2 and Sp1 regulate basal promoter activity, and Sp1 is also involved in PKA-mediated expression of the rat SPT gene in concert with the transcriptional coactivator CBP.

Sp1- and Sp3-mediated transcriptional regulation of the fibroblast growth factor receptor 1 gene in chicken skeletal muscle cells

Expression of the fibroblast growth factor receptor 1 (FGFR1) gene in skeletal muscle is positively regulated in proliferating myoblasts and declines during differentiation. We have characterized the cis-regulatory elements in the proximal region of the FGFR1 promoter which render positive transcriptional activity. Multiple elements between -69 and -14 activate the FGFR1 promoter. Myoblast transfections revealed that potential Sp transcription factor binding sites are required for promoter activity. Electromobility shift assays indicated that myoblast nuclear proteins specifically bind to these cis-elements and that differentiated myotube nuclear extracts do not form these same complexes. In addition, Southwestern blot analysis detected binding of the most proximal Sp motif to a Sp1-like protein present in myoblast nuclear extracts but not in myotubes. In corroboration, Sp1 and Sp3 proteins were detected only in myoblasts and not in differentiated myotubes. Finally, transfection of Drosophila SL2 cells showed that Sp1 is a positive regulator of FGFR1 promoter activity and that Sp3 is a coactivator via the proximal Sp binding sites. These studies demonstrate that the FGFR1 promoter is activated by Sp transcription factors in proliferating myoblasts and demonstrate at least part of the mechanism by which FGFR1 gene expression is down-regulated in differentiated muscle fibers.

Role of Sp1 in cAMP-dependent transcriptional regulation of the bovine CYP11A gene

The pituitary peptide hormone ACTH regulates transcription of the cholesterol side chain cleavage cytochrome P450 (CYP11A) gene via cAMP and activation of cAMP-dependent protein kinase. A G-rich sequence element conferring cAMP-dependent regulation has been found to reside within region -118 to -100 of the bovine CYP11A promoter. Previous studies have suggested that it binds a protein antigenically related to the transcription factor Sp1. We now report that the -118/-100 element binds both Sp1 and Sp3, members of the Sp family of transcription factors. We have made use of Drosophila SL2 cells, which lack endogenous Sp factors, to dissect the possible functional roles of Sp1, Sp3, and Sp4. All factors stimulated the activity of cotransfected reporter constructs in which the promoter of the bovine CYP11A gene regulates luciferase expression. Sp3 did not repress Sp1-dependent activation, as has previously been shown for other G-rich promoters. Mutation of the -118/-100 element of CYP11A abolished Sp1-mediated activation of a CYP11A reporter gene in SL2 cells as well as cAMP responsiveness in human H295R cells. Furthermore, cotransfection of SL2 cells with the catalytic subunit of cAMP-dependent protein kinase together with Sp1 and a CYP11A reporter construct enhanced Sp1-dependent activation of the reporter 4.2-fold, demonstrating that Sp1 confers cAMP responsiveness in these cells. Thus, we show that introduction of Sp1 alone in an Sp-negative cell such as SL2 is sufficient to achieve the cAMP-dependent regulation observed using the -118/-100 element of CYP11A in adrenocortical cells.

Myogenic basic helix-loop-helix proteins and Sp1 interact as components of a multiprotein transcriptional complex required for activity of the human cardiac alpha-actin promoter

Activation of the human cardiac alpha-actin (HCA) promoter in skeletal muscle cells requires the integrity of DNA binding sites for the serum response factor (SRF), Sp1, and the myogenic basic helix-loop-helix (bHLH) family. In this study we report that activation of the HCA correlates with formation of a muscle-specific multiprotein complex on the promoter. We provide evidence that proteins eluted from the multiprotein complex specifically react with antibodies directed against myogenin, Sp1, and SRF and that the complex can be assembled in vitro by using the HCA promoter and purified MyoD, E12, SRF, and Sp1. In vitro and in vivo assays revealed a direct association of Sp1 and myogenin-MyoD mediated by the DNA-binding domain of Sp1 and the HLH motif of myogenin. The results obtained in this study indicate that protein-protein interactions and the cooperative DNA binding of transcriptional activators are critical steps in the formation of a transcriptionally productive multiprotein complex on the HCA promoter and suggest that the same mechanisms might be utilized to regulate the transcription of muscle-specific and other genes.

Neu differentiation factor stimulates phosphorylation and activation of the Sp1 transcription factor

Neu differentiation factors (NDFs), or neuregulins, are epidermal growth factor-like growth factors which bind to two tyrosine kinase receptors, ErbB-3 and ErbB-4. The transcription of several genes is regulated by neuregulins, including genes encoding specific subunits of the acetylcholine receptor at the neuromuscular junction. Here, we have examined the promoter of the acetylcholine receptor epsilon subunit and delineated a minimal CA-rich sequence which mediates transcriptional activation by NDF (NDF-response element [NRE]). Using gel mobility shift analysis with an NRE oligonucleotide, we detected two complexes that are induced by treatment with neuregulin and other growth factors and identified Sp1, a constitutively expressed zinc finger phosphoprotein, as a component of one of these complexes. Phosphatase treatment, two-dimensional gel electrophoresis, and an in-gel kinase assay indicated that Sp1 is phosphorylated by a 60-kDa kinase in response to NDF-induced signals. Moreover, Sp1 seems to act downstream of all members of the ErbB family and thus may funnel the signaling of the ErbB network into the nucleus.

The transcription factor Sp1 regulates the myeloid-specific expression of the human hematopoietic cell kinase (HCK) gene through binding to two adjacent GC boxes within the HCK promoter-proximal region

The human hemopoietic cell kinase (HCK) is a member of the src family of protein tyrosine kinases specifically expressed in myeloid cells and to a minor extent in B-lymphoid cells. HCK expression is up-regulated at the transcriptional level during myeloid differentiation of hematopoietic cells. To elucidate the molecular basis of the differential HCK gene expression, the genomic region containing the HCK promoter was isolated and functionally characterized. A DNA fragment containing 101 base pairs of the 5'-flanking sequence showed strong promoter activity in the macrophage cell line RAW264 but was inactive in the non-monocytic cell lines HUT-78 and NIH-3T3. Site-directed mutagenesis of the proximal promoter region showed that two GC-rich sequence elements are essential for transcriptional activity in myeloid cells. Electrophoretic mobility shift analysis using nuclear extracts obtained from RAW264 cells and from the promonocytic cell line U-937 revealed the formation of at least three distinct protein-DNA complexes at each of these sites, one of which was found to contain the transcription factor Sp1. Expression of a reporter gene linked to the -101 HCK promoter region was up-regulated by Sp1, but not by other members of the Sp1 family of transcription factors, in Drosophila Schneider cells. A synergistic effect on HCK promoter activity was observed at high concentrations of Sp1. Our results show that Sp1 plays an essential role in the regulation of the differential gene expression of the HCK gene.

Nonmyogenic factors bind nicotinic acetylcholine receptor promoter elements required for response to denervation

Nicotinic acetylcholine receptors (AChRs) belong to a class of muscle proteins whose expression is regulated by muscle electrical activity. In innervated muscle fiber, AChR genes are transcriptionally repressed outside of the synapse, while after denervation they become reexpressed throughout the fiber. The myogenic determination factors (MDFs) of the MyoD family have been shown to play a central role in this innervation-dependent regulation. In the chicken AChR alpha-subunit gene promoter, two E-boxes that bind MDFs are necessary to achieve the enhancement of transcription following muscle denervation. However, the deletion of promoter sequences located upstream to these E-boxes greatly impairs the response to denervation (Bessereau, J. L., Stratford- Perricaudet, L. D., Piette, J., Le Poupon, C. and Changeux, J. P. (1994) Proc. Natl. Acad. Sci. U. S. A. 91, 1304-1308). Here we identified two additional cis-regulatory elements of the alpha-subunit gene promoter that cooperate with the E-boxes in the denervation response. One region binds the Sp1 and Sp3 zinc finger transcription factors. The second region binds at least three distinct factors, among which we identified an upstream stimulatory factor, a b-ZIP-HLH transcription factor. We propose that among MDF-responsive muscle promoters, a specific combination between myogenic and nonmyogenic factors specify innervation-dependent versus innervation-independent promoters.

Nuclear clustering in myotubes: a proposed role in acetylcholine receptor mRNA expression

We investigated the functional relationship between nuclear topology, as expressed by degree and type of nuclear aggregation, and appearance of acetylcholine receptor (AChR) subunit mRNAs. Embryonic chick muscle cell cultures treated with the muscle activity blocking agents decamethonium (DCM), d-tubocurare (TBC), and tetrodotoxin (TTX) or co-cultured with cholinergic neurons were examined for the influence of muscle activity on nuclear aggregation and its effects on AChR alpha-, gamma-, and delta-subunit message expression. mRNA was measured by in situ hybridization and nuclei were visualized by bis-benzimide DNA staining. DCM and TBC treatments, as well as neuronal co-culture, resulted in increased nuclear clustering within myotubes and a per nucleus upregulation in mRNA expression relative to control for each subunit. The pattern of nuclear aggregation was treatment dependent, with more and larger aggregates found when myotubes were co-cultured with neurons. Moreover, as nuclear aggregates became larger: (1) nearly all nuclei within active aggregates expressed mRNA and (2) local accumulation (mRNA per unit area) was elevated relative to single nuclei, while per nucleus mRNA production decreased. To determine whether mRNA expression was transient and did not result in steady-state upregulation of AChR receptor protein, we performed a double labeling of surface AChRs with 125I-alpha-bungarotoxin (125I-alpha-BTX) concomitant to the in situ hybridization for mRNA quantification on TTX treated muscle cells. Surface receptor expression tracked mRNA expression forall types of nuclear topology observed, indicating that message levels are in fact reliable indicators of receptor population on the plasma membrane surface in myotubes. We propose that nuclear clustering is an organelle-level, accessory mechanism whereby cells concentrate relatively large amounts of AChR mRNA/protein in specific myotube regions.

The epidermis: genes on - genes off

The epidermal keratinocyte stem cell is distinguished by a relatively undifferentiated phenotype and an ability to proliferate. As part of a carefully orchestrated process, the offspring of these stem cells lose the ability to proliferate and begin a process of morphologic and biochemical transformation that results in their conversion into corneocytes. This process requires the coordinated expression of a host of cellular genes. The mechanisms responsible for regulation of these genes is an area of intense interest. In keratinocytes, as in other cell types, the expression of most genes is regulated at the transcriptional level by a class of proteins called transcription factors. Transcription factors are nuclear proteins that regulate transcription by mediating the final steps in the relay of information from the cell surface to the nucleus and the gene. These factors bind to specific DNA sequence elements located within the target gene. In this brief review we summarize evidence implicating activator protein 1 (AP1), AP2, Sp1, POU domain, CCAAT enhancer binding protein, and several other transcription factors as regulators of expression of keratinocyte genes.

Multiple nuclear proteins bind a novel cis-acting element that regulates the muscle-specific expression of the mouse nicotinic acetylcholine receptor alpha-subunit gene

Expression of the nicotinic acetylcholine receptor (AChR) is transcriptionally regulated during the development of vertebrate striated muscle. To better define regulatory elements involved in this process, site-directed mutations were made in the gene's 86 bp muscle specific enhancer. Transient expression assays in skeletal muscle C2C12 cells indicated that all three E-boxes, plus a novel sequence outside the E-boxes, are necessary for full activity of the AChR gene in myotubes. Gel mobility shift assays demonstrated that mutations in the non-E-box sequence disrupted the formation of two DNA/protein complexes while not affecting myoD binding. Methylation interference footprinting confirmed that the complexes form at nucleotides within the mutated region, and also include part of the central E-box. UV crosslinking of nuclear proteins to a DNA probe identified five proteins of 125, 81, 55, 42, and 35 kDa that bind to this region; with the 125 kDa protein being differentially bound in U.V. crosslink assays during the transition from myoblasts to myotubes. These data suggest that interactions between this DNA element and the five proteins contribute to the transcriptional control of the AChR alpha-subunit gene expression during the differentiation of skeletal muscle.

CD11c integrin gene promoter activity during myeloid differentiation

The integrin CD11c/CD18 functions as a cell surface receptor for numerous soluble factors and proteins (LPS, fibrinogen, iC3b), mediates leukocyte interactions with other cell types and is a signal transducing receptor. CD11c/CD18 is found primarily on myeloid cells, where its expression is regulated both during differentiation and during monocyte maturation into tissue macrophages. To determine the transcription factors and cis-acting elements driving the developmentally-regulated expression of CD11c/CD18 the proximal regulatory region of the CD11c gene has been structurally and functionally characterized using the U937 and HL-60 cell lines as myeloid differentiation models. The tissue-specific activity of the CD11c promoter is conferred by two Sp1-binding sites and an adjacent C/EBP-binding element, with a likely contribution from other transcription factors with a more limited tissue distribution (PU.1, Oct-2, Myb). The participation of Sp1 in the transcription of the CD11c gene strongly suggests that CD11c/CD18 expression is dependent on the proliferative state of the cell, thus establishing a first level of control for the regulated expression of CD11c/CD18 during myeloid differentiation. The differentiation responsiveness of the CD11c promoter has been mapped to an AP-1-binding site whose mutation greatly decreases the inducibility of the promoter during the PMA-triggered differentiation of U937 cells. Although AP-1 mediates the responsiveness to several other differentiating agents including GM-CSF, additional elements are required for induction of the CD11c promoter activity upon Sodium Butyrate-triggered differentiation. In fact, the Sodium Butyrate-responsiveness and the presence of both AP-1- and C/EBP-binding sites suggests that the proximal regulatory region of the CD11c promoter might include an extracellular matrix-response element. As a whole, the transcription of the CD11c gene appears to be controlled by the proliferative state of the cell and is tightly coupled to progression along the myeloid differentiation pathway. The differentiation inducibility of the CD11c promoter has been further demonstrated after stable transfection into U937 cells, where the -361/+43 fragment retains the capacity to drive luciferase expression upon PMA-, GM-CSF- or Sodium Butyrate-triggered myeloid differentiation. Thus, while the characterization of the transcription factors regulating CD11c expression is still in progress, the CD11c promoter has been shown to constitute a very useful tool for the identification of myeloid-differenting agents which might be of potential therapeutical interest.

Structural and functional analyses of DNA bending induced by Sp1 family transcription factors

DNA bending induced by eukaryotic transcription factors may play a direct role in the activation of transcription by bringing together factors bound at non-adjacent sites or facilitate binding of factors involved in the formation of an initiation complex. The ubiquitously expressed zinc-finger transcription factor Sp1 is involved in the regulation of a variety of viral and cellular genes. During the past few years proteins homologous to Sp1 have been described constituting a family of Sp1 transcription factors. We have used gel electrophoretic methods to analyse the extent, location and direction of the DNA bend induced by four different Sp1 family proteins upon binding to a consensus GC box. We found that the Sp1 family proteins induce an asymmetric bend in DNA directed towards the major groove, with a bend centre displaced towards the 3' end of the GC box. The zinc-finger domain was alone responsible for introducing this distortion. The magnitude of the induced bend varied between the different proteins. Construction of a hybrid protein and mutation of the 3' end of the GC box indicated that zinc finger 1 is important both for the magnitude of the bend angle, location of the bend centre and the binding affinity. Transactivation studies of a Sp1-dependent promoter revealed that a 5 bp insertion between the TATA box and the GC box, or inversion of the GC box significantly reduced the promoter activity, indicating that protein-induced bending could be important for promoter activity. However, no stimulatory effect could be observed in cotransfections with the DNA binding domain of Sp1 in Drosophila SL-2 cells, suggesting that the bending activity alone is not sufficient for transactivation.

Identification and characterization of a 47 base pair activity-dependent enhancer of the rat nicotinic acetylcholine receptor delta-subunit promoter

Nicotinic acetylcholine receptor (nAChR) genes are regulated by muscle electrical activity. E-box sequences found in their promoters are necessary for this regulation. However, many muscle genes contain E-boxes, yet are not regulated by muscle depolarization. This suggests that other elements are necessary, perhaps working in conjunction with E-boxes, to confer depolarization-dependent control onto promoter activity. We have used direct DNA injection into muscle as an in vivo assay to identify and characterize these additional elements. Mutagenesis and expression assays identified multiple elements within the first 81 base pairs (bp) of the nAChR delta-subunit promoter that contribute to its regulation by muscle electrical activity. Within this 81 bp sequence, two regions of DNA were identified that were capable of conferring activity-dependent regulation onto a heterologous promoter. The stronger of these two putative enhancers was characterized further. It is a 47 bp sequence that contains an E-box along with sequences similar to the SV40 core enhancer and an SP1 site. Site-directed mutagenesis identified residues within each of these sequences that were necessary for enhancer activity. Furthermore, methylation interference DNA footprinting assays showed increased nuclear protein binding to sequences within both these enhancers after muscle denervation, and this pattern of binding was very similar to that observed with nuclear protein isolated from myotube extracts. These latter results suggest that similar mechanisms may mediate increased nAChR expression during muscle development and after muscle denervation.

Cloning and characterization of a novel transcriptional repressor of the nicotinic acetylcholine receptor delta-subunit gene

We have identified a negative cis-acting regulatory element in the nicotinic acetylcholine receptor delta-subunit gene's promoter. This element resides within a previously identified 47-base pair activity-dependent enhancer. Proteins that bind this region of DNA were cloned from a lambdagt11 innervated muscle expression library. Two cDNAs (MY1 and MY1a) were isolated that encode members of the Y-box family of transcription factors. MY1/1a RNAs are expressed at relatively high levels in heart, skeletal muscle, testis, glia, and specific regions of the central nervous system. MY1/1a are nuclear proteins that bind specifically to the coding strand of the 47-base pair enhancer and suppress delta-promoter activity in a sequence-specific manner. These results suggest a novel mechanism of repression by MY1/1a, which may contribute to the low level expression of the delta-subunit gene in innervated muscle. Finally, the gene encoding MY1/1a, Yb2, maps to the mid-distal region of mouse chromosome 6.

Transcriptional regulation of murine beta1,4-galactosyltransferase in somatic cells. Analysis of a gene that serves both a housekeeping and a mammary gland-specific function

beta1,4-Galactosyltransferase (beta4-GT) is a constitutively expressed enzyme that synthesizes the beta4-N-acetyllactosamine structure in glycoconjugates. In mammals, beta4-GT has been recruited for a second biosynthetic function, the production of lactose which occurs exclusively in the lactating mammary gland. In somatic tissues, the murine beta4-GT gene specifies two mRNAs of 4. 1 and 3.9 kilobases (kb), as a consequence of initiation at two different start sites approximately 200 base pairs apart. We have proposed that the region upstream of the 4.1-kb start site functions as a housekeeping promoter, while the region adjacent to the 3.9-kb start site functions primarily as a mammary gland-specific promoter (Harduin-Lepers, A., Shaper, J. H., and Shaper, N. L. (1993) J. Biol. Chem. 268, 14348-14359). Using DNase I footprinting and electrophoretic mobility shift assays, we show that the region immediately upstream of the 4.1-kb start site is occupied mainly by the ubiquitous factor Sp1. In contrast, the region adjacent to the 3.9-kb start site is bound by multiple proteins which include the tissue-restricted factor AP2, a mammary gland-specific form of CTF/NF1, Sp1, as well as a candidate negative regulatory factor that represses transcription from the 3.9-kb start site. These data experimentally support our conclusion that the 3.9-kb start site has been introduced into the mammalian beta4-GT gene to accommodate the recruited role of beta4-GT in lactose biosynthesis.

Expression of myosin isoforms in denervated, cross-reinnervated, and electrically stimulated rabbit muscles

The expression of myosin heavy (MyHC) and light (MyLC) chain isoforms was analyzed after denervation and cross-reinnervation by a fast nerve of the slow-twitch Semimembranosus proprius (SMp) muscle, and after denervation and electrical stimulation at low frequency of the fast-twitch Semimembranous accessorius (SMa) muscle of the rabbit. The control SMp (100% type I fibers) expressed 100% type I MyHC and 100% slow-type (1S', 1S and 2S) MyLC isoforms. Five month denervation did not alter significantly the MyHC expression of the muscle, but induced the expression of a new type 1 MyLC corresponding most probably to an embryonic MyLC. Five-month cross-reinnervation of the SMp by the fast SMa nerve induced a large change of its fiber type properties. As shown by immunocytochemistry, almost all fibers were stained by fast myosin antibody, but a high proportion of them co-expressed slow myosin. This result was in agreement with biochemical data showing that fast MyHC and MyLC isoforms became predominant. The control SMa (nearly 100% type II fibers) expressed almost 100% type II MyHC (70% type IIb and 22% IIx/d) and 100% fast-type (1F, 2F and 3F) MyLC isoforms. Five month denervation of the SMa induced a shift in its MyHC, with 98% type IIx/d and 2% type IIb isoforms, and no change in the proportions of its MyLC. Three month electrical stimulation at 10 Hz of the SMa transformed its fiber type composition. All fibers reacted with the slow myosin antibody and a minor proportion of them were stained by the fast myosin antibody. These observations were in agreement with the biochemical analysis showing a large predominance of the slow-type MyHC and MyLC isoforms. Taken together, these results obtained from rabbit muscles which are normally homogeneous in either fast-twitch or slow-twitch fiber types, further support the idea that the different myosin isoforms, particularly the MyHC, are differentially regulated by motor innervation. Type I MyHC is maintained in denervated SMp muscle, but is not expressed in denervated SMa. Type IIb isoform is the most sensitive to neural influence, as it disappears rapidly in denervated and electrically stimulated fast-twitch SMa muscle, and is barely expressed in cross-reinnervated slow-twitch SMp muscle. In contrast, type IIa and type IIx/d are less dependent upon motor innervation. In addition to the previous studies of d'Albis et al. analysis of these results leads us to conclude that, in the rabbit, sensitivity to motor innervation increases from the glycolytic to the oxydative types of fibers, in the order IIB > IIX/IID > IIA > I.

Identification of Sp1-binding sites in the CD11c (p150,95 alpha) and CD11a (LFA-1 alpha) integrin subunit promoters and their involvement in the tissue-specific expression of CD11c

The leukocyte integrins LFA-1 (CD11a/CD18) and p150,95 (CD11c/CD18) mediate cell-cell and cell-extracellular matrix interactions during inflammatory responses and signal transduction into the cytoplasm. While the CD11a integrin subunit is expressed on all leukocytes, CD11c is almost exclusively expressed on cells of the myeloid lineage and on activated B lymphocytes. Its expression is regulated during cell activation and differentiation by transcriptional mechanisms. We have previously demonstrated that the proximal region of the CD11c promoter directs tissue-restricted and developmentally-regulated expression of reporter genes. Structural studies by electrophoretic mobility shift assays have demonstrated the presence of two Sp1-binding sites at -70 (Sp1-70) and -120 (Sp1-120) which mediate the Sp1 transactivation of the CD11c promoter in Sp1-defective SL2 cells, and which are involved in cell lineage-specific DNA-protein interactions, as demonstrated by footprinting in vivo. More importantly, mutation of either Sp1 site inhibited the activity of the CD11c promoter both in myeloid U937 cells and the CD11c-expressing B lymphoblastoid JY cell line, while the opposite effect was observed in the CD11c-negative epithelial HeLa cell line, demonstrating the involvement of both Sp1-binding sites in the basal and the tissue-restricted expression of the CD11c integrin subunit gene. Interestingly, the analysis of the CD11a proximal promoter also revealed the existence of an Sp1-binding site at -70, indicating a common role for these cis-acting elements in the transcription of the leukocyte integrin alpha subunit genes. The binding of Sp1 to the regulatory regions of the leukocyte integrin genes raises the possibility that the retinoblastoma susceptibility gene product is implicated in integrin expression through its functional interaction with Sp1, thus establishing a link between integrin-dependent leukocyte adhesiveness and the state of cellular differentiation/proliferation.

Promoter elements conferring neuron-specific expression of the beta 2-subunit of the neuronal nicotinic acetylcholine receptor studied in vitro and in transgenic mice

Several genes encoding subunits of the neuronal nicotinic acetylcholine receptors have been cloned and regulatory elements involved in the transcription of the alpha 2 and alpha 7-subunit genes have been described. Yet, the detailed mechanisms governing the neuron-specific transcription and the spatio-temporal expression pattern of these genes remain largely uninvestigated. The beta 2-subunit is the most widely expressed neuronal nicotinic receptor subunit in the nervous system. We have studied the structural and regulatory properties of the 5' sequence of this gene. A fragment of 1163 bp of upstream sequence is sufficient to drive the cell-specific transcription of a reporter gene in both transient transfection assays and in transgenic mice. Deletion analysis and site-directed mutagenesis of this promoter reveal two negative elements and one positive element. The positively-acting sequence includes one functional E-box. One of the repressor elements is located in the transcribed region and is the NRSE/RE1 sequence already described in promoters of neuronal genes. In this paper, we describe the neuron-specific promoter of the gene encoding the neuronal nicotinic acetylcholine receptor beta 2-subunit.

The human medium chain Acyl-CoA dehydrogenase gene promoter consists of a complex arrangement of nuclear receptor response elements and Sp1 binding sites

Expression of the gene encoding the mitochondrial fatty acid. beta-oxidation enzyme, medium-chain acyl-CoA dehydrogenase (MCAD), is regulated among tissues during development and in response to alterations in substrate availability. To identify and characterize cis-acting MCAD gene promoter regulatory elements and corresponding transcription factors, DNA-protein binding studies and mammalian cell transfection analyses were performed with hjman MCAD gene promoter fragments. DNA:protein binding studies with nuclear protein extracts prepared from hepatoma G2 cells, 3T3 fibroblasts, or Y-1 adrenal tumor cells identified three sequences (nuclear receptor response element 1 or NRRE-1, NRRE-2, and NRRE-3) that bind orphan members of the steroid/thyroid nuclear receptor superfamily including chicken ovalbumin upstream promoter transcription factor and steroidogenic factor 1. Sp1 binding sites (A-C) were identified in close proximity to each of the NRREs. NRRE-3 conferred cell line-specific transcriptional repression by interacting with chicken ovalbumin upstream promoter transcription factor or activation via steroidogenic factor 1. In contrast, the Sp1 binding site A behaved as a transcriptional activator in all cell lines examined. We propose that multiple nuclear receptor transcription factors interact with MCAD gene promoter elements to differentially regulate transcription among a variety of cell types.

Transcriptional activation of the neuronal peripherin-encoding gene depends on a G + C-rich element that binds Sp1 in vitro and in vivo

Peripherin (Prph) is a type-III intermediate filament (IF) protein principally synthesized in peripheral nervous system neurons. We have previously shown that three regulatory elements, PER1, PER2 and PER3, in the first 98 bp of the Prph gene promoter, were sufficient to direct cell-type specific expression of a reporter gene [Desmarais et al., EMBO J. 11 (1992) 2971-2980]. Of these elements, PER1 was found to be important for cell-type specificity, but required the presence of other elements for transcriptional activity. Here, we show that PER3 is a stronger activator than PER2 and that it can stimulate cell-type-specific transcription when combined with PER1. We have characterized the G + C-rich PER3 element for its ability to bind trans-acting factors. Gel retardation and methylation interference (MI) assays show that PER3 binds transcription factor Sp1. In addition, an anti-Sp1 antibody recognizes the PER3 DNA-binding protein. A 3-bp mutation abrogating the capacity of PER3 to bind Sp1 in vitro completely abolished expression of the reporter gene construct containing only PER3 and PER1, while in a construct containing the first 256 bp of the Prph promoter, it led to an 80% decrease with respect to the control wild-type construct. Finally, by co-transfection of a Sp1-expressing plasmid, we show that Sp1 can stimulate transcription from a reporter gene containing the PER3 sequence. Together, these results indicate that interactions between Sp1 and the proteins binding PER1 are involved in the control of the Prph gene.

Myoblast and myotube nuclei display similar patterns of heterogeneous acetylcholine receptor subunit mRNA expression

Muscle progenitor cells differentiate to myoblasts, and subsequently myotubes, upon expression of muscle specific genes. We and others have previously shown that myotube nuclei, even in the absence of nerve, express AChR alpha subunit RNA at varying levels, with a small subset (about ten percent) of the nuclei expressing at high levels. These findings raised two important questions: 1) is the observed heterogeneity a unique property of the alpha subunits, and 2) when does the heterogeneity begin? In particular, is it induced only at or after the time of fusion, or does it exist at the myoblast stage? We have, therefore, extended our observations to the gamma and delta subunits and we also have examined the distributions of AChR alpha, gamma, and delta subunit RNAs in both myoblasts and myotubes. We used intron and intron-exon probes to detect prespliced transcripts or mature mRNAs in the cells. Because intron-containing transcripts are not transported out of the nuclei, the distributions of these transcripts can indicate their expression patterns among nuclei in the same myotubes. Our results show that both myotubes and myoblasts have distributions of the AChR alpha, gamma, and delta subunit RNAs which differ sharply from that of the U1 RNA or Myo D. Thus, the heterogeneous expression of AChR genes is not only an intrinsic property of muscle cell nuclei (in the sense that it does not require the presence of nerves), but it also exists prior to fusion. Our results suggest that muscle nuclei attain individualized capacities for AChR subunit mRNA production early in their development. Conceptual models consistent with such individuality imply an additional level of regulation beyond the known diffusible transcriptional factors.

Multiple regulatory elements control the basal promoter activity of the human alpha 4 integrin gene

It has been suggested that expression of the genes encoding the alpha 4/beta 1 integrin increases during wound healing of the cornea. As a first step in understanding the mechanisms required to stimulate alpha 4 gene expression during this process, we defined the minimal upstream sequence required to direct basal promoter activity for this gene. Using deletion analyses of the alpha 4 gene upstream sequence, we identified two functionally important negative regulatory elements. Dimethylsulfate (DMS) methylation interference assays provided evidence for the binding of a single nuclear protein to tandemly repeated homologous cis-acting elements (designated alpha 4.1 and alpha 4.2) from the alpha 4 basal promoter that share the core sequence 5'-GTGGGT-3'. The formation of a protection only at alpha 4.1 in DNase I footprinting suggested that it is the primary target element for the binding of nuclear proteins. Three distinct nuclear proteins bound a double-stranded oligonucleotide bearing the DNA sequence of alpha 4.1 to produce specific DNA-protein complexes (R1 to R3) in gel-shift assays, from which that producing R3 was identified as the protein yielding DNase I protection at alpha 4.1. Detailed mutational analysis of alpha 4.1 and alpha 4.2 indicated that both elements positively regulate gene expression in primary cultures of corneal epithelial cells and Jurkat tissue culture cells, which is consistent with the deletion analysis. However, when transiently transfected into pituitary GH4C1, the alpha 4.2 mutants yielded increased chloramphenicol acetyl transferase activity therefore demonstrating that these elements have the ability to function either as positive or negative regulators of gene transcription in a manner that is dependent on the type of cell transfected.

Identification of human T-cell lymphotropic virus type I 21-base-pair repeat-specific and glial cell-specific DNA-protein complexes

The human T-cell lymphotropic virus type I (HTLV-I)-encoded protein, Tax, is capable of trans-activating HTLV-I transcription by interacting with specific sequences in the HTLV-I long terminal repeat (LTR) which comprise an inducible enhancer containing three imperfect tandem repeats of a 21-bp sequence. There is no evidence that purified Tax can bind to DNA in the absence of cellular factors, suggesting that Tax most likely regulates transcription via interaction with cellular factors. Since HTLV-I is a documented agent of adult T-cell leukemia and tropical spastic paraparesis, disorders of the immune and nervous systems, respectively, characterization of cellular factors of lymphoid and neuroglial origin which interact with the 21-bp repeat elements is essential to understanding of the mechanisms involved in basal and Tax-mediated transcription in cells of immune and nervous system origin. Utilizing electrophoretic mobility shift (EMS) analyses, we have detected both 21-bp repeat-specific and glial cell-specific DNA-protein complexes. Several 21-bp repeat-specific DNA-protein complexes were detected when nuclear extracts derived from cells of lymphoid (Jurkat, SupT1, and H9), neuronal (IMR-32 and SK-N-MC), and glial (U-373 MG, Hs683, and U-118) origin were used in reactions with each of the three 21-bp repeat elements. In addition, a glial cell-specific DNA-protein complex was detected when nuclear extracts derived from U-373 MG, Hs683, and U-118 glial cell lines reacted with the promoter-distal and central 21-bp repeat elements. Furthermore, EMS analyses performed with nuclear extracts derived from lymphocytic and glial cell origin and a 223-bp fragment of the HTLV-I long terminal repeat encompassing the three 21-bp repeat elements (designated Tax-responsive elements 1 and 2, TRE-1/-2) have also resulted in the detection of glial cell type-specific DNA-protein complexes. Competition EMS analyses with oligonucleotides containing transcription factor binding site sequences indicate the involvement of a cyclic AMP response element binding protein in the formation of DNA-protein complexes which form with all three 21-bp repeat elements and the glial cell-specific DNA-protein complex as well as the involvement of Sp1 or an Sp1-related factor in the formation of the 21-bp repeat III-specific DNA-protein complexes.

Acetylcholine receptors of the Drosophila brain: a 900 bp promoter fragment contains the essential information for specific expression of the ard gene in vivo

The ard gene encodes a beta-subunit of Drosophila nicotinic acetylcholine receptors specifically expressed in a subset of neurons. To identify the cis-regulatory region responsible for this cell-specific expression, various 5' fragments of the ard gene were fused to a lacZ reporter gene and introduced into the Drosophila genome. A DNA fragment spanning approximately 760 bp upstream and approximately 140 bp downstream of a cluster of putative transcription start sites produced a pattern of beta-galactosidase activity that resembles the distribution of ARD transcripts. Both in embryos and adults the levels of lacZ RNA were similar to those of endogenous ARD transcripts, suggesting that the 900 bp fragment harbors all essential elements for proper expression of the ard gene.