The basic helix-loop-helix protein upstream stimulating factor regulates the cardiac ventricular myosin light-chain 2 gene via independent cis regulatory elements

Enhancing atrial-specific gene expression using a calsequestrin cis-regulatory module 4 with a sarcolipin promoter

Background

Cardiac gene therapy using the adeno‐associated virus serotype 9 vector is widely used because of its efficient transduction. However, the promoters used to drive expression often cause off‐target localization. To overcome this, studies have applied cardiac‐specific promoters, although expression is debilitated compared to that of ubiquitous promoters. To address these issues in the context of atrial‐specific gene expression, an enhancer calsequestrin cis‐regulatory module 4 (CRM4) and the highly atrial‐specific promoter sarcolipin were combined to enhance expression and minimize off tissue expression.

Methods

To observe expression and bio‐distribution, constructs were generated using two different reporter genes: luciferase and enhanced green fluorescent protein (EGFP). The ubiquitous cytomegalovirus (CMV), sarcolipin (SLN) and CRM4 combined with sarcolipin (CRM4.SLN) were compared and analyzed using the luciferase assay, western blotting, a quantitative polymerase chain reaction and fluorescence imaging.

Results

The CMV promoter containing vectors showed the strongest expression in vitro and in vivo. However, the module SLN combination showed enhanced atrial expression and a minimized off‐target effect even when compared with the individual SLN promoter.

Conclusions

For gene therapy involving atrial gene transfer, the CRM4.SLN combination is a promising alternative to the use of the CMV promoter. CRM4.SLN had significant atrial expression and minimized extra‐atrial expression.

Myocyte-specific M-CAT and MEF-1 elements regulate G-protein gamma 3 gene (gamma3) expression in cardiac myocytes

Little is known regarding the mechanisms that control the expression of G-protein alpha, beta, and gamma subtypes. We have previously shown that the G-protein gamma(3) gene is expressed in the heart, brain, lung, spleen, kidney, muscle, and testis in mice. We have also reported that the G-protein gamma(3) subunit is expressed in rat cardiac myocytes, but not in cardiac fibroblasts. Other studies have shown that the gamma(3) subunit couples to the angiotensin A1A receptor in portal vein myocytes, and has been shown to mediate beta-adrenergic desensitization in cardiac myocytes treated with atorvastatin. In the present study, we evaluated G-protein gamma(3) promoter-luciferase reporter constructs in primary myocytes to identify key regulatory promoter regions. We identified two important regions of the promoter (upstream promoter region [UPR] and downstream promoter region [DPR]), which are required for expression in cardiac myocytes. We observed that removal of 48 bp in the UPR diminished gene transcription by 75%, and that the UPR contains consensus elements for myocyte-specific M-CAT and myocyte enhancer factor 1 (MEF-1) elements. The UPR and DPR share transcription factor elements for myocyte-specific M-CAT element. We observed that cardiac myocyte proteins bind to gamma(3) oligonucleotides containing transcription factor elements for myocyte-specific M-CAT and MEF-1. Myocyte-specific M-CAT proteins were supershifted with transcriptional enhancer factor-1 (TEF-1) antibodies binding to the gamma(3) M-CAT element, which is in agreement with reports showing that the M-CAT element binds the TEF-1 family of transcription factors. The 150 bp DPR contains three M-CAT elements, an INR element, an upstream stimulatory factor 1 element, and the transcription start site. We have shown that myocyte gamma(3) gene expression is regulated by myocyte-specific M-CAT and MEF-1 elements.

Microphthalmia transcription factor isoforms in mast cells and the heart

The microphthalmia transcription factor (Mitf) is critical for the survival and differentiation of a variety of cell types. While on the transcript level it has been noted that melanocytes and cardiomyocytes express specific Mitf isoforms, mast cells express several isoforms, mainly Mitf-H and Mitf-MC, whose function has not been thoroughly investigated. We found that in mast cells the expression of the specific Mitf isoforms is dependent on physiological stimuli that cause a major shifting of promoter usage and internal splicing. For example, activation of the c-kit signaling pathway almost totally abolished one of the main splice isoforms. Since cardiomyocytes express only the Mitf-H isoform, they were an ideal system to determine this isoform's physiological role. We identified that the expression of myosin light-chain 1a (MLC-1a) is regulated by Mitf-H. Interestingly, the transactivation of MLC-1a by Mitf-H in cardiomyocytes is decreased by overexpression of the splice form with exon 6a. In conclusion, we found that there is physiological switching of Mitf isoforms and that the promoter context and the cell context have a combined influence on gene expression programs.

Characterization of cis-regulatory elements and transcription factor binding: gel mobility shift assay

To understand how cardiac gene expression is regulated, the identification and characterization of cis-regulatory elements and their trans-acting factors by gel mobility shift assay (GMSA) or gel retardation assay are essential and common steps. In addition to providing a general protocol for GMSA, this chapter describes some applications of this assay to characterize cardiac-specific and ubiquitous trans-acting factors bound to regulatory elements [novel TCTG(G/C) direct repeat and A/T-rich region] of the rat cardiac troponin T promoter. In GMSA, the specificity of the binding of trans-acting factor to labeled DNA probe should be verified by the addition of unlabeled probe in the reaction mixture. The migratory property of DNA-protein complexes formed by protein extracts prepared from different tissues can be compared to determine the tissue specificity of trans-acting factors. GMSA, coupled with specific antibody to trans-acting factor (antibody supershift assay), is used to identify proteins present in the DNA-protein complex. The gel-shift competition assay with an unlabeled probe containing a slightly different sequence is a powerful technique used to assess the sequence specificity and relative binding affinity of a DNA-protein interaction. GMSA with SDS-PAGE fractionated proteins allows for the determination of the apparent molecular mass of bound trans-acting factor.

Cross-regulatory interaction between Stra13 and USF results in functional antagonism

Transcription factors belonging to the basic helix-loop-helix (bHLH) family are critical regulators of cellular proliferation and differentiation. The functional activity of these proteins can be regulated by heterodimerization through the HLH domain, as a result of formation of functional or non-functional heterodimers. The presence of a leucine zipper in bHLH-leucine zipper (bHLHZip) proteins, however, prevents such heterodimeric interactions via the HLH domain between bHLH and bHLHZip proteins. To identify cellular proteins that directly interact with and modulate transcriptional repression mediated by the bHLH protein Stra13, we carried out a yeast two hybrid screen. The bHLHZip protein USF (Upstream Stimulatory factor) was identified as a Stra13 interacting protein. We demonstrate a direct interaction between Stra13 and USF that is dependent upon the C-terminal repression domain of Stra13 and the DNA-binding domain of USF. Stra13 and USF also colocalize and functionally interact in mammalian cells. Co-expression of USF abrogates Stra13-mediated repression of target genes and conversely, Stra13 inhibits DNA-binding and USF-mediated transactivation. Taken together, our data demonstrate that Stra13 and USF interact physically and functionally, and identify a novel mode of cross regulatory interaction between members of the bHLH and bHLHZip families that abrogates their functional activity.

Modulation of MLC-2v gene expression by AP-1: complex regulatory role of Jun in cardiac myocytes

Hypertrophic stimulation of cardiac myocytes results in rapid induction of a number of transcription factors, including members of the AP-1 family, which is followed by a programmed alteration in the pattern of gene expression. In the ventricular cardiocytes there is re-expression of the fetal atrial natriuretic factor (ANF) gene and upregulation of its myosin light chain-2 (MLC-2v). The mechanism(s) by which the induction ofAP-1 is coupled to the promoters of these target genes is largely unknown. In this report, we demonstrate that in transient co-transfection assay, c-Jun inhibited while Jun B stimulated the MLC-2v promoter activity. Mutant c-Jun recombinants, in which the activation domains were deleted, still remained inhibitory, but a specific mutation in the leucine zipper, which changes the alignment of Jun with its dimerization partner, caused a reversal of its effect on the target MLC-2v promoter. Based on these findings, we propose that in chicken cardiac myocytes, the regulation of MLC-2v promoter by Jun may occur via its interaction with other proteins, possibly of the leucine zipper family.

Transcriptional regulation of mouse delta-opioid receptor gene

Three major types of opioid receptors, mu (MOR), delta (DOR), and kappa (KOR), have been cloned and characterized. Each opioid receptor exhibits a distinct pharmacological profile as well as a distinct pattern of temporal and spatial expression in the brain, suggesting the critical role of transcription regulatory elements and their associated factors. Here, we report the identification of a minimum core promoter, in the 5'-flanking region of the mouse DOR gene, containing an E box and a GC box that are crucial for DOR promoter activity in NS20Y cells, a DOR-expressing mouse neuronal cell line. In vitro protein-DNA binding assays and in vivo transient transfection assays indicated that members of both the upstream stimulatory factor and Sp families of transcription factors bound to and trans-activated the DOR promoter via the E box and GC box, respectively. Furthermore, functional and physical interactions between these factors were critical for the basal as well as maximum promoter activity of the DOR gene. Thus, the distinct developmental emergence and brain regional distribution of the delta opioid receptor appear to be controlled, at least in part, by these two regulatory elements and their associated factors.

Transcription enhancer factor 1 interacts with a basic helix-loop-helix zipper protein, Max, for positive regulation of cardiac alpha-myosin heavy-chain gene expression

The M-CAT binding factor transcription enhancer factor 1 (TEF-1) has been implicated in the regulation of several cardiac and skeletal muscle genes. Previously, we identified an E-box-M-CAT hybrid (EM) motif that is responsible for the basal and cyclic AMP-inducible expression of the rat cardiac alpha-myosin heavy chain (alpha-MHC) gene in cardiac myocytes. In this study, we report that two factors, TEF-1 and a basic helix-loop-helix leucine zipper protein, Max, bind to the alpha-MHC EM motif. We also found that Max was a part of the cardiac troponin T M-CAT-TEF-1 complex even when the DNA template did not contain an apparent E-box binding site. In the protein-protein interaction assay, a stable association of Max with TEF-1 was observed when glutathione S-transferase (GST)-TEF-1 or GST-Max was used to pull down in vitro-translated Max or TEF-1, respectively. In addition, Max was coimmunoprecipitated with TEF-1, thus documenting an in vivo TEF-1-Max interaction. In the transient transcription assay, overexpression of either Max or TEF-1 resulted a mild activation of the alpha-MHC-chloramphenicol acetyltransferase (CAT) reporter gene at lower concentrations and repression of this gene at higher concentrations. However, when Max and TEF-1 expression plasmids were transfected together, the repression mediated by a single expression plasmid was alleviated and a three- to fourfold transactivation of the alpha-MHC-CAT reporter gene was observed. This effect was abolished once the EM motif in the promoter-reporter construct was mutated, thus suggesting that the synergistic transactivation function of the TEF-1-Max heterotypic complex is mediated through binding of the complex to the EM motif. These results demonstrate a novel association between Max and TEF-1 and indicate a positive cooperation between these two factors in alpha-MHC gene regulation.

Combinatorial control of muscle development by basic helix-loop-helix and MADS-box transcription factors

Members of the MyoD family of muscle-specific basic helix-loop-helix (bHLH) proteins function within a genetic pathway to control skeletal muscle development. Mutational analyses of these factors suggested that their DNA binding domains mediated interaction with a coregulator required for activation of muscle-specific transcription. Members of the myocyte enhancer binding factor 2 (MEF2) family of MADS-box proteins are expressed at high levels in muscle and neural cells and at lower levels in several other cell types. MEF2 factors are unable to activate muscle gene expression alone, but they potentiate the transcriptional activity of myogenic bHLH proteins. This potentiation appears to be mediated by direct interactions between the DNA binding domains of these different types of transcription factors. Biochemical and genetic evidence suggests that MEF2 factors are the coregulators for myogenic bHLH proteins. The presence of MEF2 and cell-specific bHLH proteins in other cell types raises the possibility that these proteins may also cooperate to regulate other programs of cell-specific gene expression. We present a model to account for such cooperative interactions.

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

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

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

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

Fast-muscle-specific DNA-protein interactions occurring in vivo at the human aldolase A M promoter are necessary for correct promoter activity in transgenic mice

The human aldolase A tissue-specific M promoter (pM) has served as a model system for identifying pathways that lead to fast-muscle-specialized expression. The current study has delimited the sequences necessary and sufficient for fast-muscle-specific expression in transgenic mice to a short 209-bp fragment extending from bp -164 to +45 relative to the pM transcription start site. Genomic footprinting methods showed that in this proximal region, the same elements that bind muscle nuclear proteins in vitro are involved in DNA-protein interactions in intact muscle nuclei of transgenic mice. Furthermore, these experiments provided the first evidence that different DNA-binding activities exist between slow and fast muscles in vivo. Fast-muscle-specific interactions occur at an element named M1 and at a muscle-specific DNase I-hypersensitive site that was previously detected by in vitro methods. The formation of the muscle-specific DNase I-hypersensitive site reflects binding of proteins to a close element, named M2, which contains a binding site for nuclear factors of the NF1 family. Mutational analysis performed with transgenic mice confirmed the importance of the M1 element for high-level fast-muscle-specific pM activity and suggested that the M2/NF1 element is differently required for correct pM expression in distinct fast muscles. In addition, two other protein binding sites, the MEF3 motif and the USF site, seem to act as stage-specific activators and/or as participants in the establishment of an active chromatin configuration at pM.

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

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

Inhibition of a signaling pathway in cardiac muscle cells by active mitogen-activated protein kinase kinase

Signaling via the Ras pathway involves sequential activation of Ras, Raf-1, mitogen-activated protein kinase kinase (MKK), and the extracellular signal-regulated (ERK) group of mitogen-activated protein (MAP) kinases. Expression from the c-Fos, atrial natriuretic factor (ANF), and myosin light chain-2 (MLC-2) promoters during phenylephrine-induced cardiac muscle cell hypertrophy requires activation of this pathway. Furthermore, constitutively active Ras or Raf-1 can mimic the action of phenylephrine in inducing expression from these promoters. In this study, we tested whether constitutively active MKK, the molecule immediately downstream of Raf, was sufficient to induce expression. Expression of constitutively active MKK induce ERK2 kinase activity and caused expression from the c-Fos promoter, but did not significantly activate expression of reporter genes under the control of either the ANF or MLC-2 promoters. Expression of CL100, a phosphatase that inactivates ERKs, prevented expression from all of the promoters. Taken together, these data suggest that ERK activation is required for expression from the Fos, ANF, and MLC-2 promoters but MKK and ERK activation is sufficient for expression only from the Fos promoter. Constitutively active MKK synergized with phenylephrine to increase expression from a c-Fos- or an AP1-driven reporter. However, active MKK inhibited phenylephrine- and Raf-1-induced expression from the ANF and MLC-2 promoters. A DNA sequence in the MLC-2 promoter that is a target for inhibition by active MKK, but not CL100, was mapped to a previously characterized DNA element (HF1) that is responsible for cardiac specificity. Thus, activation of cardiac gene expression during phenylephrine-induced hypertrophy requires ERK activation but constitutive activation by MKK can inhibit expression by targeting a DNA element that controls the cardiac specificity of gene expression.

An upstream stimulatory factor (USF) binding motif is critical for rat preprotachykinin-A promoter activity in PC12 cells

We demonstrate the presence of a functional E box motif in the proximal rat preprotachykinin-A (rPPT) promoter. This element (spanning nucleotides -67 to -47) exhibits the sequence 5'-CACGTG-3' which is recognized and bound by the basic helix-loop-helix family of regulatory proteins. We also show that at least one of the factors bound to this rPPT promoter element in both HeLa and PC12 nuclear extract is the ubiquitously expressed transcription factor, the upstream stimulatory factor (USF). Mutation of this element by insertion of a 10 bp linker into the E box motif, in an rPPT promoter fragment spanning -865 to +92, destroys the ability of this promoter fragment to support reporter gene expression in a PC12 cell model of rPPT promoter activity. The data indicate that this rPPT E box element is likely to function as an important cis-regulatory domain in the rPPT promoter.

EFIA/YB-1 is a component of cardiac HF-1A binding activity and positively regulates transcription of the myosin light-chain 2v gene

Transient assays in cultured ventricular muscle cells and studies in transgenic mice have identified two adjacent regulatory elements (HF-1a and HF-1b/MEF-2) as required to maintain ventricular chamber-specific expression of the myosin light-chain 2v (MLC-2v) gene. A rat neonatal heart cDNA library was screened with an HF-1a binding site, resulting in the isolation of EFIA, the rat homolog of human YB-1. Purified recombinant EFIA/YB-1 protein binds to the HF-1a site in a sequence-specific manner and contacts a subset of the HF-1a contact points made by the cardiac nuclear factor(s). The HF-1a sequence contains AGTGG, which is highly homologous to the inverted CCAAT core of the EFIA/YB-1 binding sites and is found to be essential for binding of the recombinant EFIA/YB-1. Antiserum against Xenopus YB-3 (100% identical in the DNA binding domain and 89% identical in overall amino acid sequence to rat EFIA) can specifically abolish a component of the endogenous HF-1a complex in the rat cardiac myocyte nuclear extracts. In cotransfection assays, EFIA/YB-1 increased 250-bp MLC-2v promoter activity by 3.4-fold specifically in the cardiac cell context and in an HF-1a site-dependent manner. EFIA/YB-1 complexes with an unknown protein in cardiac myocyte nuclear extracts to form the endogenous HF-1a binding activity. Immunocoprecipitation revealed that EFIA/YB-1 has a major associated protein of approximately 30 kDa (p30) in cardiac muscle cells. This study suggests that EFIA/YB-1, together with the partner p30, binds to the HF-1a site and, in conjunction with HF-1b/MEF-2, mediates ventricular chamber-specific expression of the MLC-2v gene.