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.

Differential gene expressions in atrial and ventricular myocytes: insights into the road of applying embryonic stem cell-derived cardiomyocytes for future therapies

Myocardial infarction has been the leading cause of morbidity and mortality in developed countries over the past few decades. The transplantation of cardiomyocytes offers a potential method of treatment. However, cardiomyocytes are in high demand and their supply is extremely limited. Embryonic stem cells (ESCs), which have been isolated from the inner cell mass of blastocysts, can self-renew and are pluripotent, meaning they have the ability to develop into any type of cell, including cardiomyocytes. This suggests that ESCs could be a good source of genuine cardiomyocytes for future therapeutic purposes. However, problems with the yield and purity of ESC-derived cardiomyocytes, among other hurdles for the therapeutic application of ESC-derived cardiomyocytes (e.g., potential immunorejection and tumor formation problems), need to be overcome before these cells can be used effectively for cell replacement therapy. ESC-derived cardiomyocytes consist of nodal, atrial, and ventricular cardiomyocytes. Specifically, for treatment of myocardial infarction, transplantation of a sufficient quantity of ventricular cardiomyocytes, rather than nodal or atrial cardiomyocytes, is preferred. Hence, it is important to find ways of increasing the yield and purity of specific types of cardiomyocytes. Atrial and ventricular cardiomyocytes have differential expression of genes (transcription factors, structural proteins, ion channels, etc.) and are functionally distinct. This paper presents a thorough review of differential gene expression in atrial and ventricular myocytes, their expression throughout development, and their regulation. An understanding of the molecular and functional differences between atrial and ventricular myocytes allows discussion of potential strategies for preferentially directing ESCs to differentiate into chamber-specific cells, or for fine tuning the ESC-derived cardiomyocytes into specific electrical and contractile phenotypes resembling chamber-specific cells.

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.

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.

Human follicle-stimulating hormone receptor (FSH-R) promoter/enhancer activity is inhibited by transcriptional factors, from the upstream stimulating factors family, via E-box and newly identified initiator element (Inr) in FSH-R non-expressing cells

To localize the regulatory elements in the human follicle-stimulating hormone receptor (FSH-R) promoter/enhancer and to determine the role of upstream stimulatingfactors (USFs) in these elements, we transiently transfected constructs of FSH-R promoter/enhancer in pGL3 luciferase reporter plasmids into Chinese hamster ovary cells and the activities were determined by measuring luciferase luminescence of the cell lysates. The 5'-flanking regions of the human FSH-R gene from nt -1485 to -1 with respect to the gene translation start site were amplified by polymerase chain reaction (PCR) and subcloned in pGL3. Deletion mutants were created using PCR or restriction enzyme digestion. Mutation in the E-box sequence from nt -124 to -119 (E-box 3), in the construct from -224 to nt -1 or in the Inr element, which encompasses the transcriptional start site at nt -99, resulted in a substantial reduction in the human FSH-R promoter/enhancer activity. Overexpression of upstream stimulating factor-1 (USF1) suppresses the activity of the human FSH-R promoter/enhancer via Inr and E-box elements. Upstream stimulating factor-2 (USF2) decreases FSH-R promoter/enhancer activity by acting on E-box 3. The results indicate that E-box 3 and the Inr element are important elements of the human FSH-R promoter/ enhancer. USF family members inhibit FSH-R gene activity by acting via these elements. USF1 and USF2 suppress human FSH-R promoter/enhancer activity by acting on E-box 3. USF1 also decreases activity by interacting with the Inr element.

An enhancer directs differential expression of the linked Mrf4 and Myf5 myogenic regulatory genes in the mouse

The myogenic regulatory factors, Mrf4 and Myf5, play a key role in skeletal muscle formation. An enhancer trap approach, devised to isolate positive-acting elements from a 200-kb YAC covering the mouse Mrf4-Myf5 locus in a C2 myoblast assay, yielded an enhancer, A17, which mapped at -8 kb 5' of Mrf4 and -17 kb 5' of Myf5. An E-box bound by complexes containing the USF transcription factor is critical for enhancer activity. In transgenic mice, A17 gave two distinct and mutually exclusive expression profiles before birth, which correspond to two phases of Mrf4 transcription. Linked to the Tk or Mrf4 minimal promoters, the nlacZ reporter was expressed either in embryonic myotomes, or later in fetal muscle, with the majority of Mrf4 lines showing embryonic expression. When linked to the Myf5 minimal promoter, only fetal muscle expression was detected. These observations identify A17 as a sequence that targets sites of myogenesis in vivo and raise questions about the mutually exclusive modes of expression and possible promoter/enhancer interactions at the Mrf4-Myf5 locus.

Upstream stimulatory factor represses the induction of carnitine palmitoyltransferase-Ibeta expression by PGC-1

Transcriptional regulation of carnitine palmitoyltransferase-1beta (CPT-1beta) is coordinated with contractile gene expression through cardiac-enriched transcription factors, GATA4 and SRF. Metabolic modulation of CPT-1beta promoter activity has been described with the stimulation of gene expression by oleate that is mediated through the peroxisome proliferator-activated receptor (PPAR) pathway. The coactivator, peroxisomal proliferator-activated receptor gamma coactivator (PGC-1), enhances gene expression through interactions with nuclear hormone receptors and the myocyte enhancer factor 2 (MEF2) family. PGC-1 and MEF2A synergistically activate CPT-1beta promoter activity. This stimulation is enhanced by mutation of the E-box sequences that flank the MEF2A binding site. These elements bind the upstream stimulatory factors (USF1 and USF2), which activate transcription in CV-1 fibroblasts. However, overexpression of the USF proteins in myocytes depresses CPT-1beta activity and significantly reduces MEF2A and PGC-1 synergy. Co-immunoprecipitation studies demonstrate that PGC-1 and USF2 proteins can physically interact. Our studies demonstrate that PGC-1 stimulates CPT-1beta gene expression through MEF2A. USF proteins have a novel role in repressing the expression of the CPT-1beta gene and modulating the induction by the coactivator, PGC-1.

The circadian E-box: when perfect is not good enough

Life on earth has evolved on a photic carousel, spinning through alternating periods of light and darkness. This playful image belies the fact that only those organisms that learned how to benefit from the recurring features in their environment were allowed to ride on. This selection process has engendered many daily rhythms in our biosphere, most of which rely on the anticipatory power of an endogenously generated marker of phase: the biological clock. The basic mechanisms driving this remarkable device have been really tough to decode but are finally beginning to unravel as chronobiologists probe deeper and wider in and around the recently discovered gears of the clock. Like its chemical predecessors, biological circadian oscillators are characterized by interlaced positive and negative feedback loops, but with constants and variables carefully balanced to achieve an approximately 24h period. The loops at the heart of these biological oscillators are sustained by specific patterns of gene expression and precisely tuned posttranscriptional modifications. It follows that a molecular understanding of the biological clock hinges, in no small measure, on a better understanding of the cis-acting elements that bestow a given gene with its circadian properties. The present review summarizes what is known about these elements and what remains to be elucidated.

Transcription factors in cardiogenesis: the combinations that unlock the mysteries of the heart

Heart formation is one of the first signs of organogenesis within the developing embryo and this process is conserved from flies to man. Completing the genetic roadmap of the molecular mechanisms that control the cell specification and differentiation of cells that form the developing heart has been an exciting and fast-moving area of research in the fields of molecular and developmental biology. At the core of these studies is an interest in the transcription factors that are responsible for initiation of a pluripotent cell to become programmed to the cardiac lineage and the subsequent transcription factors that implement the instructions set up by the cells commitment decision. To gain a better understanding of these pathways, cardiac-expressed transcription factors have been identified, cloned, overexpressed, and mutated to try to determine function. Although results vary depending on the gene in question, it is clear that there is a striking evolutionary conservation of the cardiogenic program among species. As we move up the evolutionary ladder toward man, we encounter cases of functional redundancy and combinatorial interactions that reflect the complex networks of gene expression that orchestrate heart development. This review focuses on what is known about the transcription factors implicated in heart formation and the role they play in this intricate genetic program.

Circadian Transcription. Thinking outside the E-Box

The E-Box is a widely used DNA control element. Despite its brevity and broad distribution the E-Box is a remarkably versatile sequence that affects many different genetic programs, including proliferation, differentiation, tissue-specific responses, and cell death. The circadian clock is one of the latest pathways shown to employ this element. In this context, E-Boxes are likely to play a key role in establishing the robust waves of gene expression characteristic of circadian transcription. The regulatory flexibility of the E-Box hinges on the sequence ambiguity allowed at its core, the strong influence of the surrounding sequences, and the recruitment of spatially and temporally regulated E-Box-binding factors. Therefore, understanding how a particular E-Box can accomplish a specific task entails the identification and systematic analysis of these cis- and trans-acting E-Box modifiers. In the present study we compared the E-Box-containing minimal promoters of vasopressin and cyclin B1, two genes that can respond to the transcriptional oscillators driving the circadian clock and cell cycle, respectively. Results of this comparison will help elucidate the manner in which discreet DNA modules associate to either augment or restrain the activation of potential circadian E-Boxes in response to competing regulatory signals.

Transcriptional regulation of the human high affinity IgE receptor alpha-chain gene

Transcriptional regulation of the gene encoding human high affinity IgE receptor (Fc epsilon RI) alpha-chain was analyzed. Previously, we reported that GATA-1 and Elf-1 recognition sites were necessary for cell type-specific activation of the alpha-chain gene promoter. More detailed analysis revealed that other transcription factors bound the regions close to the Elf-1 recognition site and there was a more complex mechanism for the regulation of the promoter activity. On the other hand, during a course of studies to find cis-elements over this gene, CAGCTG sequence in the first intron was revealed to serve as an enhancer. A complex composed of USF1 and USF2 activated the human alpha-chain gene expression via this intronic element. Furthermore, we found two novel exons at 18.4 and 12.6kb upstream from the reported first exon and discovered an additional distal promoter.

Myocilin and glaucoma: facts and ideas

Mutations in the MYOC gene that encodes for myocilin are causative for some forms of juvenile and adult-onset primary open-angle glaucoma (POAG). Myocilin is a secreted 55-57kDa glycoprotein that forms dimers and multimers. Characteristic structural motifs include a myosin-like domain, a leucine zipper region and an olfactomedin domain. Most of the mutations that have been identified in patients with POAG are localized in the olfactomedin domain, which is highly conserved among species. In the eye, myocilin is expressed in high amounts in the trabecular meshwork (TM), sclera, ciliary body and iris, and at considerable lower amounts in retina and optic nerve head. Secreted myocilin is present in the aqueous humor. In the TM, myocilin is found within the cytoplasm of TM cells and in the juxtacanalicular region in association with fibrillar extracellular matrix components. Since patients with mutations in myocilin may have high intraocular pressures, the role of myocilin for aqueous humor outflow has been investigated and conflicting results have been obtained. Recombinant myocilin increases outflow resistance in perfused anterior segment organ cultures, while overexpression of myocilin after viral gene transfer appears to reduce outflow resistance. In TM cells, the expression of myocilin is induced upon treatment with dexamethasone at a time course similar to that observed in steroid-induced glaucoma. Other factors that induce myocilin expression are transforming growth factor-beta and mechanical stretch. Promoter elements that are important for the glucocorticoid induction have not been identified, but it has been shown that upstream stimulatory factor is critical for the basal promoter activity of MYOC. Mice with a targeted disruption of the myocilin gene do not express a phenotype, indicating that the glaucomatous phenotype in humans is not because of a loss-of-function effect. Experimental studies show that mutated myocilin is not secreted, but appears to accumulate in the cells. Such an accumulation might interfere with TM function and lead to impaired outflow resistance, but, so far, experimental evidence for such a scenario is lacking. In addition, the normal function(s) of myocilin is (are) still elusive.

Role of USF1 phosphorylation on cardiac alpha-myosin heavy chain promoter activity

Contractile activity of the cardiac myocyte is required for maintaining cell mass and phenotype, including expression of the cardiac-specific alpha-myosin heavy chain (alpha-MHC) gene. An E-box hemodynamic response element (HME) located at position -47 within the alpha-MHC promoter is both necessary and sufficient to confer contractile responsiveness to the gene and has been shown to bind upstream stimulatory factor-1 (USF1). When studied in spontaneously contracting cardiac myocytes, there is enhanced binding of USF1 to the HME compared with quiescent cells, which correlates with a threefold increase in alpha-MHC promoter activity. A molecular mechanism by which contractile function modulates alpha-MHC transcriptional activity may involve signaling via phosphorylation of USF1. The present studies showed that purified rat USF1 was phosphorylated in vitro by protein kinase C (PKC) and cAMP-dependent protein kinase (PKA) but not casein kinase II. Phosphorylated USF1 by either PKC or PKA had increased DNA binding activity to the HME. PKC-mediated phosphorylation also leads to the formation of USF1 multimers as assessed by gel shift assay. Analysis of in vivo phosphorylated nuclear proteins from cultured ventricular myocytes showed that USF1 was phosphorylated, and resolution by two-dimensional gel electrophoresis identified at least two distinct phosphorylated USF1 molecules. These results suggest that endogenous kinases can covalently modify USF1 and provide a potential molecular mechanism by which the contractile stimulus mediates changes in myocyte gene transcription.

Activity of the nicotinic acetylcholine receptor alpha5 and alpha7 subunit promoters in muscle cells

The acetylcholine receptor alpha5 and alpha7 subunits are components of different nicotinic receptor subtypes expressed in the nervous system. However, they are also present in non-neuronal tissues. We have detected alpha5 and alpha7 transcripts in mouse C2C12 muscle cells. Moreover, on differentiation of myoblasts into myotubes, the amount of alpha7 transcripts increased significantly, whereas alpha5 remained unchanged. In order to explore how the expression of these neuronal genes is regulated in muscle, we have characterized their promoter activities. Deletion and mutagenesis analysis with transfected reporter genes showed that transcriptional activity was controlled by regulatory elements also operative in neuronal-like cells. Thus, the activity of the alpha5 subunit core promoter decreased to approximately 50% on alteration of one, two, or three of the five Sp1 binding sites present in this region and was almost abolished when four or five sites were mutated simultaneously. In the case of the alpha7 subunit promoter, the upstream stimulatory factor and the early growth response gene transcription factor were involved in regulating its transcriptional activity. In addition, the alpha7 promoter was activated during the differentiation process, in a mechanism partially dependent on the mentioned factors.

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.

The role of myocilin/TIGR in glaucoma: results of the Glaucoma Research Foundation catalyst meeting in Berkeley, California, March 2000

Approximately 3 years ago, the first major (biochemical, molecular biologic, and biologic) insight into primary open-angle glaucoma was the finding that mutations in the myocilin (MYOC/TIGR) gene were related to certain forms of juvenile onset of this disease. Since then, a great deal of work has been done to determine the possible mechanisms by which MYOC/TIGR might cause not only juvenile but also adult-onset primary open-angle glaucoma. To assess the current knowledge and those areas in which more research is necessary, a meeting of scientists was held by the Glaucoma Research Foundation of San Francisco, California in the spring of 2000. This meeting attempted to concentrate on the MYOC/TIGR protein rather than the genetics of this gene. Possible functions and roles of this protein intracellularly and extracellularly were critically examined and discussed. Normal transcriptional and translational events and the effect of mutations on these events were explored. The discussions yielded insight not only in those areas in which important information is known but also in vital areas in which little is currently understood. This review attempts to summarize the current knowledge regarding MYOC/TIGR and to elucidate the points that the people attending the meeting thought needed further study.

GATA-4 and serum response factor regulate transcription of the muscle-specific carnitine palmitoyltransferase I beta in rat heart

Transcriptional regulation of nuclear encoded mitochondrial proteins is dependent on nuclear transcription factors that act on genes encoding key components of mitochondrial transcription, replication, and heme biosynthetic machinery. Cellular factors that target expression of proteins to the heart have been well characterized with respect to excitation-contraction coupling. No information currently exists that examines whether parallel transcriptional mechanisms regulate nuclear encoded expression of heart-specific mitochondrial isoforms. The muscle CPT-Ibeta isoform in heart is a TATA-less gene that uses Sp-1 proteins to support basal expression. The rat cardiac fatty acid response element (-301/-289), previously characterized in the human gene, is responsive to oleic acid following serum deprivation. Deletion and mutational analysis of the 5'-flanking sequence of the carnitine palmitoyltransferase Ibeta (CPT-Ibeta) gene defines regulatory regions in the -391/+80 promoter luciferase construct. When deleted or mutated constructs were individually transfected into cardiac myocytes, CPT-I/luciferase reporter gene expression was significantly depressed at sites involving a putative MEF2 sequence downstream from the fatty acid response element and a cluster of heart-specific regulatory regions flanked by two Sp1 elements. Each site demonstrated binding to cardiac nuclear proteins and competition specificity (or supershifts) with oligonucleotides and antibodies. Individual expression vectors for Nkx2.5, serum response factor (SRF), and GATA4 enhanced CPT-I reporter gene expression 4-36-fold in CV-1 cells. Although cotransfection of Nkx and SRF produced additive luciferase expression, the combination of SRF and GATA-4 cotransfection resulted in synergistic activation of CPT-Ibeta. The results demonstrate that SRF and the tissue-restricted isoform, GATA-4, drive robust gene transcription of a mitochondrial protein highly expressed in heart.

Molecular cloning of rat USF2 cDNA and characterization of splicing variants

The complete nucleotide sequence of rat USF2 cDNA was determined. In addition to the full length clone (USF2FL), four isoforms (delta1, delta2, delta3, and delta4) suggested to be generated by alternative splicing were isolated. USF2delta1 and delta2 lacked 27 and 67 internal amino acid residues, respectively. USF2delta3 and delta4 lacked most of the entire sequence but encoded short peptides of an N-terminal portion of USF2FL. Overexpression of USF2FL increased the transcription of the human high affinity IgE receptor (FcepsilonRI) alpha chain gene through specific binding to the CAGCTG motif in the first intron. On the other hand, overexpression of USF2delta1 or delta2 reduced the transcription of the human FcepsilonRI alpha chain gene. Both USF2FL and USF2delta1 bound to CACGTG as well as CAGCTG, while USF2delta2 bound to CACGTG but not to CAGCTG. These results suggested the presence of a different and definitive role of each variant in the expression of the alpha chain gene.

Regulation of human myocilin/TIGR gene transcription in trabecular meshwork cells and astrocytes: role of upstream stimulatory factor

BACKGROUND

Mutations in the myocilin (MYOC)/TIGR gene are responsible for autosomal-dominant juvenile primary open-angle glaucoma (POAG). In patients with non-autosomal-dominant POAG, such mutations are rare, but the expression of MYOC/TIGR in the trabecular meshwork (TM) of the eye is considerably higher than in normals. We performed transfection, DNAse I footprinting, mutagenesis and electrophoretic mobility shift assays (EMSA) to identify elements responsible for the basal transcription of MYOC/TIGR in TM cells and astrocytes.

RESULTS

DNAse I footprinting experiments of the human MYOC/TIGR promoter showed a major protected area between nt -106 to -77, which was not conserved in the homologous region of the mouse myoc/tigr promoter. In addition, the TATA-box was protected, as well as at least three downstream sites, including an AP-1-like sequence. Deletion of the -106 to -77 region caused a substantial loss of functional promotor activity in all cell types. Site-directed mutagenesis and EMSA experiments revealed the presence of two regulatory elements in the -106 to -77 region. Each of these cis-elements is essential for minimal promoter activity. The 5'-half of the region contains a sequence with similarities to NF-kappaB-related sites, however, binding of NF-kappaB could not be confirmed by EMSA. The 3'-half contains a canonical E-box sequence. EMSA experiments showed that the upstream regulatory factor (USF) was binding to the E-box sequence and that the binding can be supershifted by specific antibodies.

CONCLUSIONS

Several DNA-protein binding elements contribute to a transcription of MYOC/TIGR, and USF is critically required for its basal transcription in trabecular meshwork cells and astrocytes.

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.

Upstream stimulatory factor 1 regulates osteopontin expression in smooth muscle cells

Vascular smooth muscle cells (SMCs) undergo a dramatic phenotypic transition in response to injury and ex vivo culture that includes enhanced proliferation, migration, matrix deposition, and alterations in gene expression. Osteopontin is a good marker for the injury-induced SMC phenotypic state in vivo and in vitro. To identify transcription factors that might control the regulation of osteopontin expression, we investigated cultured vascular SMCs that express high and low levels of osteopontin. Using nuclear run-on assays, mRNA stability studies, and deletion analysis, we demonstrate that regulation of osteopontin steady-state mRNA levels in SMCs occurs at the transcriptional level. Transient transfection and gel-shift analyses of osteopontin promoter indicated that a region between -123 and +66 was involved in the expression of osteopontin. Supershift EMSAs identified the bHLH-leucine zipper transcription factor upstream stimulatory factor-1 (USF1) as the protein binding to this sequence. Finally, we show that USF1 protein is induced in vivo within 24 h of balloon angioplasty of rat carotids coordinately with osteopontin induction. These data suggest that USF1 governs expression of osteopontin in cultured vascular SMCs and might contribute to initial osteopontin expression observed post carotid injury and in vascular pathologies in vivo.

Differential activation of the SMalphaA promoter in smooth vs. skeletal muscle cells by bHLH factors

E-box/basic helix-loop-helix (bHLH)-dependent regulation of promoters for skeletal muscle-specific genes is well established, but similar regulation of smooth muscle-selective promoters has not been reported. Using transient transfection assays of smooth muscle alpha-actin (SMalphaA) promoter-chloramphenicol acetyltransferase (CAT) reporter constructs in rat vascular smooth muscle cells (SMCs) and L6 skeletal myotubes, we identified two activator elements, smE1 and smE2, with sequences corresponding to E-box (5'-CAnnTG-3') motifs. In L6 myotubes, 4-bp mutations of smE1 or smE2 E-box motif alone completely abolished promoter activity. In contrast, mutation of smE1 and smE2 was required to reduce promoter activity in SMCs. Supershift analyses identified a myogenin-containing complex as the predominant smE1 and smE2 binding activity in skeletal muscle, and myogenin overexpression transactivated the promoter. Supershift analyses with SMC extracts demonstrated that the bHLH protein upstream stimulatory factor (USF) bound smE1, and USF overexpression transactivated the promoter in an smE1-dependent manner. In summary, our results provide novel evidence implicating E-box elements in directing expression of the SMalphaA promoter through distinct bHLH factor complexes in skeletal vs. smooth muscle.

B cell-specific activator protein prevents two activator factors from binding to the immunoglobulin J chain promoter until the antigen-driven stages of B cell development

The immunoglobulin J chain gene is inducibly transcribed in mature B cells upon antigen recognition and a signal from interleukin-2 (IL-2). B cell-specific activator protein (BSAP), a transcription factor that silences J chain transcription, has been identified as a nuclear target of the IL-2 signal. The levels of BSAP progressively decrease in response to IL-2 and this change correlates with the differentiation of B cells into antibody secreting plasma cells. Here we report the binding of the upstream stimulatory factor (USF) to an E-box motif immediately upstream from the BSAP site on the J chain promoter. Mutations in the USF binding motif significantly decrease J chain promoter activity in J chain expressing B cell lines. We also show that a functional relationship exists between USF and a second J chain positive-regulating factor, B-MEF2, using co-immunoprecipitation assays and transfections. Finally, we provide evidence that the binding of BSAP prevents USF and B-MEF2 from interacting with the J chain promoter during the antigen-independent stages of B cell development. It is not until the levels of BSAP decrease during the antigen-driven stages of B cell development that both USF and B-MEF2 are able to bind to their respective promoter elements and activate J chain transcription.

Transcriptional activity of MEF2 during mouse embryogenesis monitored with a MEF2-dependent transgene

The four members of the MEF2 family of MADS-box transcription factors, MEF2-A, MEF2-B, MEF2-C and MEF2-D, are expressed in overlapping patterns in developing muscle and neural cell lineages during embryogenesis. However, during late fetal development and postnatally, MEF2 transcripts are also expressed in a wide range of cell types. Because MEF2 expression is controlled by translational and post-translational mechanisms, it has been unclear whether the presence of MEF2 transcripts in the embryo reflects transcriptionally active MEF2 proteins. To define the temporospatial expression pattern of transcriptionally active MEF2 proteins during mouse embryogenesis, we generated transgenic mice harboring a lacZ reporter gene controlled by three tandem copies of the MEF2 site and flanking sequences from the desmin enhancer, which is active in cardiac, skeletal and smooth muscle cells. Expression of this MEF2-dependent transgene paralleled expression of MEF2 mRNAs in developing myogenic lineages and regions of the adult brain. However, it was not expressed in other cell types that express MEF2 transcripts. Tandem copies of the MEF2 site from the c-jun promoter directed expression in a similar pattern to the desmin MEF2 site, suggesting that transgene expression reflects the presence of transcriptionally active MEF2 proteins, rather than other factors specific for DNA sequences flanking the MEF2 site. These results demonstrate the presence of transcriptionally active MEF2 proteins in the early muscle and neural cell lineages during embryogenesis and argue against the existence of lineage-restricted MEF2 cofactors that discriminate between MEF2 sites with different immediate flanking sequences. The discordance between MEF2 mRNA expression and MEF2 transcriptional activity in nonmuscle cell types of embryos and adults also supports the notion that post-transcriptional mechanisms regulate the expression of MEF2 proteins.

The role of GATA, CArG, E-box, and a novel element in the regulation of cardiac expression of the Na+-Ca2+ exchanger gene

The cardiac Na+-Ca2+ exchanger (NCX1) is the principal Ca2+ efflux mechanism in cardiocytes. The exchanger is up-regulated in both cardiac hypertrophy and failure. In this report, we identify the cis-acting elements that control cardiac expression and alpha-adrenergic up-regulation of the exchanger gene. Deletion analysis revealed that a minimal cardiac promoter fragment from -184 to +172 is sufficient for cardiac expression and alpha-adrenergic stimulation. Mutational analysis revealed that both the CArG element at -80 and the GATA element at -50 were required for cardiac expression. Gel mobility shift assay supershift analysis demonstrated that the serum response factor binds to the CArG element and GATA-4 binds to the GATA element. Point mutations in the -172 E-box demonstrated that it was required for alpha-adrenergic induction. In addition, deletion analysis revealed one or more enhancer elements in the first intron (+103 to +134) that are essential for phenylephrine up-regulation but bear no homology to any known transcription element. Therefore, this work demonstrates that SRF and GATA-4 are critical for NCX1 expression in neonatal cardiomyocytes and that the -172 E-box in addition to a novel enhancer element(s) are required for phenylephrine up-regulation of NCX1 and may mediate its hypertrophic up-regulation.

Characterization of the 5'-flanking region of the murine polymeric IgA receptor gene

The regulatory elements that control basal and activated transcriptional expression of the polymeric IgA receptor gene (pIgR) have not been defined. In this study, we performed functional analysis of the murine pIgR 5'-upstream region. Transient transfection studies identified the gene's minimal promoter to reside within 110 nucleotides upstream from the start of transcription. Substitution mutations of this region identified both a putative activator (-78 to -70) and a repressor (-66 to -52) element. DNase I footprint analysis confirmed an area of protection that spans from nucleotides -85 to -62. Mobility shift assays of the putative region confirmed binding of upstream stimulatory factor 1 (USF1) to an E box element at positions -75 and -70, representing the putative enhancer. Overexpression studies using various forms of USF suggest that both USF1 and USF2 enhance activity of the pIgR minimal promoter. We report the identification and characterization of the murine pIgR minimal promoter, as well as the critical role of USF in enhancing its basal level of transcription in Caco-2 cells.

Nutritional and hormonal regulation of enzymes in fat synthesis: studies of fatty acid synthase and mitochondrial glycerol-3-phosphate acyltransferase gene transcription

The activities of critical enzymes in fatty acid and triacylglycerol biosynthesis are tightly controlled by different nutritional, hormonal, and developmental conditions. Feeding previously fasted animals high-carbohydrate, low-fat diets causes a dramatic induction of enzymes-such as fatty acid synthase (FAS) and mitochondrial glycerol-3-phosphate acyltransferase (GPAT)-involved in fatty acid and triacylglycerol synthesis. During fasting and refeeding, transcription of these two enzymes is coordinately regulated by nutrients and hormones, such as glucose, insulin, glucagon, glucocorticoids, and thyroid hormone. Insulin stimulates transcription of the FAS and mitochondrial GPAT genes, and glucagon antagonizes the insulin effect through the cis-acting elements within the promoters and their bound trans-acting factors. This review discusses advances made in the understanding of the transcriptional regulation of FAS and mitochondrial GPAT genes, with emphasis on elucidation of the mechanisms by which multiple nutrients and hormones achieve their effects.

MEF-2 and Oct-1 bind to two homologous promoter sequence elements and participate in the expression of a skeletal muscle-specific gene

The murine adult IIB myosin heavy chain (IIB MyHC) gene is expressed only in certain skeletal muscle fibers. Within the proximal promoter are two A + T-rich motifs, mAT1 and mAT2, which greatly enhance muscle-specific transcription; myogenic cells contain proteins that bind to these sequences. MEF-2 binds to both mAT1 and mAT2; a mutation abolishing its binding to mAT1 greatly diminishes the activity of the promoter. Both mAT motifs also form complexes with a protein requiring a target sequence typical of POU domain proteins, which migrate in electrophoretic mobility shift assays to the same position as a complex containing purified Oct-1 and which are supershifted by an antibody specific to Oct-1; this protein is therefore probably Oct-1. Footprinting experiments demonstrate that mAT1 is preferentially occupied by MEF-2 and mAT2 by Oct-1 and that these two proteins appear to bind cooperatively to their respective sites. Although the two mAT motifs have sequences that are very similar, they nonetheless exhibit distinct behaviors and perform differently in the activation of the promoter. The contribution of the IIB MyHC gene to specification of the myogenic phenotype is thus at least in part regulated by MEF-2 and Oct-1.

A composite DNA element in the promoter of the polymeric immunoglobulin receptor regulates its constitutive expression

The polymeric immunoglobulin receptor (pIgR), which is constitutively expressed on the basolateral surface of secretory epithelial cells, mediates external translocation of polymeric IgA and pentameric IgM (collectively called pIg) to exocrine secretions. A high level of synthesis must be maintained because the receptor is continuously cleaved to release bound secretory component (SC) in secretory IgA and secretory IgM, as well as free SC from unoccupied receptor. We have isolated the promoter of the pIgR gene and identified a short activating region that is required for the expression of pIgR promoter-driven reporter genes. This region contained an E-box and an inverted repeat sequence (IRS). Gel electrophoresis mobility shift assays with nuclear extracts from different pIgR-expressing epithelial cell lines demonstrated proteins that bind independently to both the E-box and the IRS sequence of the pIgR promoter. In addition, a DNA probe that contained both the E-box and the IRS gave rise to a larger complex that could not be competed by either element on its own. Binding was confirmed by DNase I footprinting of the E-box and IRS sequences with nuclear extracts, and by dimethyl sulfide footprinting in living HT-29 epithelial cells. Finally, a mutation in the pIgR promoter that inhibited protein binding to the E-box and the formation of the larger complex, abolished activated transcription from the reporter gene.

Molecular variation of the human angiotensinogen core promoter element located between the TATA box and transcription initiation site affects its transcriptional activity

Recent genetic studies indicate that several molecular variants discovered in angiotensinogen (AG), the precursor of vasoactive octapeptide angiotensin II, could potentially be responsible for inherited predisposition to human blood pressure variation. We have previously shown that a ubiquitously expressed nuclear factor, AGCF1, bound to AGCE1 (AG core promoter element 1 including the core nucleotides, CTCGTG, CTC-type) located between the TATA box and transcription initiation site (positions -25 to -1) is an authentic regulator of human AG transcription. In the present study, we showed that AGCF1 has biologically and immunologically similar properties to those of a helix-loop-helix nuclear factor USF1 and examined the effects of two other naturally occurring molecular variants (ATCGTG, ATC-type and ATTGTG, ATT-type) found in the AGCE1 position on the human AG transcriptional activity. Competitive gel-shift and transfection experiments demonstrated that the transcriptional activity for the CTC- and ATC-type promoters was 2.5 times higher than that for the ATT-type through the alteration of AGCF1-binding affinity. These results suggest the possible involvement of USF1 as a component in AGCF1 formation and the potential importance of AGCE1 variation in blood pressure regulation through human AG expression.

The basic helix-loop-helix-zipper transcription factor USF1 regulates expression of the surfactant protein-A gene

Expression of the rabbit pulmonary surfactant protein A (SP-A) gene is lung-specific, occurs primarily in type II cells, and is developmentally regulated. We previously identified two E-box-like enhancers, termed the distal binding element (DBE) and proximal binding element (PBE), in the 5'-flanking region of the rabbit SP-A gene. In the present study, the PBE was used to screen a rabbit fetal lung cDNA expression library; a cDNA insert was isolated which is highly similar in sequence to human upstream stimulatory factor 1 (hUSF1). By use of reverse transcription polymerase chain reaction, two isoforms of rabbit USF1 (rUSF1) mRNAs were identified in fetal rabbit lung and other tissues. The levels of rUSF1 mRNAs reach a peak in fetal rabbit lung at 23 days gestation, in concert with the time of initiation of SP-A gene transcription. Binding complexes of nuclear proteins obtained from fetal rabbit lung tissue and isolated type II cells with the DBE and PBE were supershifted by the addition of anti-rUSF1 IgG. Binding activity was enriched in type II cells compared with lung fibroblasts. Overexpression of rUSF1s in A549 adenocarcinoma cells positively regulated SP-A promoter activity of cotransfected reporter gene constructs. It is suggested that rUSF1s, which bind to two E-box elements in the SP-A gene 5'-flanking region, may serve a key role in the regulation of SP-A gene expression in pulmonary type II cells.

Binding of upstream stimulatory factor and a cell-specific activator to the calcitonin/calcitonin gene-related peptide enhancer

The calcitonin/calcitonin gene-related peptide (CT/CGRP) gene is selectively transcribed in thyroid C cells and neurons. We have previously shown that the rat CT/CGRP cell-specific enhancer is synergistically regulated by a helix-loop-helix (HLH) protein and the OB2 octamer-binding protein. In this report, we show that the HLH-OB2 enhancer is required for full promoter activity, even in the context of other HLH elements. Since this enhancer appears to be a major controlling element, we have characterized the HLH and OB2 DNA binding proteins. We have identified the major HLH complex as a heterodimer of the ubiquitous upstream stimulatory factor (USF)-1 and USF-2 proteins. USF bound the enhancer with a reasonably high affinity (KD 1.6 nM), comparable to other genes. Characterization of a series of mutations revealed that a portion of the HLH motif is also recognized by OB2 and confirmed that HLH activity requires OB2. We have shown that OB2 is a single DNA binding protein based on UV cross-linking studies. The 68-kDa protein-DNA complex was detected only in C cell lines, including a human C cell line that has robust HLH-OB2 enhancer activity. These results suggest that the calcitonin/CGRP gene is controlled by the combinatorial activity of a ubiquitous USF HLH heterodimer and an associated cell-specific activator.

Fashioning the vertebrate heart: earliest embryonic decisions

Our goal here is to set out the types of unitary decisions made by heart progenitor cells, from their appearance in the heart field until they form the simple heart tube. This provides a context to evaluate cell fate, lineage and, finally, morphogenetic decisions that configure global heart form and function. Some paradigms for cellular differentiation and for pattern generation may be borrowed from invertebrates, but neither Drosophila nor Caenorhabditis elegans suffice to unravel higher order decisions. Genetic analyses in mouse and zebrafish may provide one entrance to these pathways.

CARP, a cardiac ankyrin repeat protein, is downstream in the Nkx2-5 homeobox gene pathway

To identify the molecular pathways that guide cardiac ventricular chamber specification, maturation and morphogenesis, we have sought to characterize factors that regulate the expression of the ventricular myosin light chain-2 gene, one of the earliest markers of ventricular regionalization during mammalian cardiogenesis. Previously, our laboratory identified a 28 bp HF-la/MEF-2 element in the MLC-2v promoter region, which confers cardiac ventricular chamber-specific gene expression during murine cardiogenesis, and showed that the ubiquitous transcription factor YB-1 binds to the HF-la site in conjunction with a co-factor. In a search for interacting co-factors, a nuclear ankyrin-like repeat protein CARP (cardiac ankyrin repeat protein) was isolated from a rat neonatal heart cDNA library by yeast two-hybrid screening, using YB-1 as the bait. Co-immunoprecipitation and GST-CARP pulldown studies reveal that CARP forms a physical complex with YB-1 in cardiac myocytes and immunostaining shows that endogenous CARP is localized in the cardiac myocyte nucleus. Co-transfection assays indicate that CARP can negatively regulate an HF-1-TK minimal promoter in an HF-1 sequence-dependent manner in cardiac myocytes, and CARP displays a transcriptional inhibitory activity when fused to a GAL4 DNA-binding domain in both cardiac and noncardiac cell context. Northern analysis revealed that carp mRNA is highly enriched in the adult heart, with only trace levels in skeletal muscle. During murine embryogenesis, endogenous carp expression was first clearly detected as early as E8.5 specifically in heart and is regulated temporally and spatially in the myocardium. Nkx2-5, the murine homologue of Drosophila gene tinman was previously shown to be required for heart tube looping morphogenesis and ventricular chamber-specific myosin light chain-2 expression during mammalian heart development. In Nkx2-5(−/−)embryos, carp expression was found to be significantly and selectively reduced as assessed by both whole-mount in situ hybridizations and RNase protection assays, suggesting that carp is downstream of the homeobox gene Nkx2-5 in the cardiac regulatory network. Co-transfection assays using a dominant negative mutant Nkx2-5 construct with CARP promoter-luciferase reporter constructs in cardiac myocytes confirms that Nkx2-5 either directly or indirectly regulates carp at the transcriptional level. Finally, a carp promoter-lacZ transgene, which displays cardiac-specific expression in wild-type and Nkx2-5(+/−) background, was also significantly reduced in Nkx2-5(−/−) embryos, indicating that Nkx2-5 either directly or indirectly regulates carp promoter activity during in vivo cardiogenesis as well as in cultured cardiac myocytes. Thus, CARP is a YB-1 associated factor and represents the first identified cardiac-restricted downstream regulatory gene in the homeobox gene Nkx2-5 pathway and may serve as a negative regulator of HF-1-dependent pathways for ventricular muscle gene expression.

Binding of upstream stimulatory factor to an E-box in the 3'-flanking region stimulates alpha1(I) collagen gene transcription

Since several lines of evidence implicate the 3'-flanking region in regulating alpha1(I) collagen gene transcription, we analyzed 12. 4-kilobase pairs of 3'-flanking sequence of the murine alpha1(I) collagen gene for transcriptional elements. A region of the 3'-flanking region stimulated expression of the heterologous beta-globin gene promoter in an enhancer trap plasmid and of the alpha1(I) collagen gene promoter in a collagen-luciferase reporter gene construct when located 3' to the luciferase reporter gene. DNase I footprinting analysis demonstrated the presence of three regions where DNA binding proteins specifically interact within this 3'-stimulatory region. Inspection of the DNA sequence revealed a consensus E-box, a binding site for basic helix-loop-helix proteins, in one of the protein binding sites. Mobility shift assays demonstrated that upstream stimulatory factors (USF) USF-1 and USF-2 bind to this E-box. Mutating the E-box in the context of the 3'-flanking region confirmed that it contributes to the enhancement of transcriptional activity of the alpha1(I) collagen gene promoter. Mutations in all three protein binding sites abolished transcriptional activation by the 3'-flanking region, suggesting a complex interaction among the trans-acting factors in enhancing transcriptional activity. Thus, a region of the 3'-flanking region of the alpha1(I) collagen gene stimulates transcription of the alpha1(I) collagen gene promoter, and USF-1 and USF-2 contribute to this transcriptional stimulation.

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.

Spatial and temporal activity of the alpha B-crystallin/small heat shock protein gene promoter in transgenic mice

In order to study the spatial and temporal activity of the mouse alpha B-crystallin/small heat shock gene promoter during embryogenesis, we generated mice harboring a transgene consisting of approximately 4 kbp of alpha B-crystallin promoter sequence fused to the Escherichia coli lacZ reporter gene. beta-galactosidase activity was first observed in the heart rudiment of 8.5 days post coitum (d.p.c.) embryos. An identical expression pattern was obtained for the endogenous alpha B-crystallin gene by whole mount in situ hybridization. At 9.5 d.p.c., beta-galactosidase activity was detected in the lens placode, in the myotome of the somites, in Rathke's pouch (future anterior pituitary), and in some regions of oral ectoderm. We also examined the stress inducibility of the alpha B-crystallin promoter in vivo. Injection of sodium arsenite into mice resulted in increased endogenous alpha B-crystallin expression in the adrenal gland and possibly the liver. Our results indicate that visualization of beta-galactosidase activity provides an accurate reflection of endogenous alpha B-crystallin expression and demonstrate that the complex developmental pattern of mouse alpha B-crystallin gene expression is regulated at the transcriptional level. This expression pattern, coupled with the present literature which addresses functions of the protein, suggests a role for the alpha B-crystallin/small heat shock protein in intermediate filament turnover and cellular remodeling which occur during normal development and differentiation.

An E-box motif conveys inhibitory activity on the atrial natriuretic peptide gene

Atrial natriuretic peptide (ANP) is a potent diuretic, natriuretic, and vasorelaxant hormone that is expressed early in ventricular hypertrophy. Expression of human ANP is controlled by a series of regulatory elements located in the 5' flanking sequence of its gene. We generated a series of 5' deletion mutations extending from -2600 to -1150 relative to the transcription start site and linked them to a chloramphenicol acetyltransferase reporter gene. Using transient transfection analysis, we have identified a negative regulatory element between -1206 and -1152 relative to the start site. Each of a series of 5' deletion mutants, when introduced into fibroblast cultures, expressed the reporter function at a level that was significantly less (< 20%) than that seen with the -1152 reporter construct, whereas comparably transfected atrial cardiocytes demonstrated no change in reporter activity, implying that the repressor function is specific to cell type. The critical region (from -1206 to -1152) associates with a soluble protein present in cardiac fibroblast extracts in a sequence-specific fashion. Deoxyribonuclease I footprint analysis demonstrated the presence of several protected regions, including one that overlies an E-box motif (CAACTG), an element that in other systems has been implicated in promoting differentiation in the myocyte lineage. Site-directed mutagenesis of the E-box motif suppressed both the protein-binding and inhibitory activities of the 54-bp fragment. In summary, we have found a region in the 5' flanking sequence of the human ANP gene that represses transcriptional activity in nonmyocardial cells. This element may play an important role in the restriction of ANP gene expression to cardiac myocytes.

Factors involved in cardiogenesis and the regulation of cardiac-specific gene expression

The delineation of the mechanisms that regulate cardiac gene expression is central to our understanding of cardiac growth and development. Much progress has been made toward the identification of factors involved in tissue-restricted gene expression, especially in skeletal muscle cells. However, the mechanisms regulating the expression of cardiac-specific genes remain less well understood. Certain homeodomain proteins have been implicated in commitment to the cardiac phenotype. Among the best characterized are the murine proteins Csx, Nkx-2.5, and Nkx-2.6, related to the protein tinman, which is essential for heart formation in Drosophila. The expression of these genes precedes that of cardiac-specific genes and is therefore believed to play a critical role in the development of the heart. The GATA proteins are a family of zinc finger proteins that are also expressed early in cardiac development and may act separately from, or in concert with, the homeodomain proteins as crucial regulators of heart development. The myosin heavy and light chain genes, the actin genes, the troponin genes, and the atrial natriuretic factor and muscle creatine kinase genes have served as excellent paradigms for the study of cardiac gene expression. Although differences in cis-acting elements and their behavior in binding assays have been observed between different genes, there exist similarities that are noteworthy. In this review, we will discuss the factors involved in the regulation of cardiac-specific gene expression in an attempt to provide a better understanding of the process of cardiogenesis.

An HF-1a/HF-1b/MEF-2 combinatorial element confers cardiac ventricular specificity and established an anterior-posterior gradient of expression

The molecular determinants that direct gene expression to the ventricles of the heart are for the most part unknown. Additionally, little data is available on how the anterior/posterior axis of the heart tube is determined and whether the left and right atrial and ventricular chambers are assigned as part of this process. Utilizing myosin light chain-2 ventricular promoter/beta-galactosidase reporter transgenes, we have determined the minimal cis-acting sequences required for ventricular-specific gene expression. In multiple independent transgenic mouse lines, we found that both a 250 base pair myosin light chain-2 ventricular promoter fragment, as well as a dimerized 28 bp sub-element (HF-1) containing binding sites for HF1a and HF1b/MEF2 factors, directed ventricular-specific reporter expression from as early as the endogenous gene, at day 7.5-8.0 post coitum. While the endogenous gene is expressed uniformly throughout both ventricles, the transgenes were expressed in a right ventricular/conotruncal dominant fashion, suggesting that they contain only a subset of the elements which respond to positional information in the developing heart tube. Expression of the transgene was cell autonomous and its temporospatial characteristics not affected by mouse strain/methylation state of the genome. To determine whether ventricular-specific expression of the transgene was dependent upon regulatory genes required for correct ventricular differentiation, the 250 base pair transgene was bred into both retinoid X receptoralpha and Nkx2-5 null backgrounds. The transgene was expressed in both mutant backgrounds, despite the absence of endogenous myosin light chain-2 ventricular transcript in Nkx2-5 null embryos. Ventricular specification, as judged by transgene expression, appeared to occur normally in both mutants. Thus, the HF-1 element, directs chamber-specific transcription of a transgene reporter independently of retinoid X receptoralpha and Nkx2-5, and defines a minimal combinatorial pathway for ventricular chamber gene expression. The patterned expression of this transgene may provide a model system in which to investigate the cues that dictate anterior-posterior (right ventricle/left ventricle) gradients during mammalian heart development.

Acetylation of histone H4 plays a primary role in enhancing transcription factor binding to nucleosomal DNA in vitro

Core histones isolated from normal and butyrate-treated HeLa cells have been reconstituted into nucleosome cores in order to analyze the role of histone acetylation in enhancing transcription factor binding to recognition sites in nucleosomal DNA. Moderate stimulation of nucleosome binding was observed for the basic helix-loop-helix factor USF and the Zn cluster DNA binding domain factor GAL4-AH using heterogeneously acetylated histones. However, by coupling novel immunoblotting techniques to a gel retardation assay, we observed that nucleosome cores containing the most highly acetylated forms of histone H4 have the highest affinity for these two transcription factors. Western analysis of gel-purified USF-nucleosome and GAL4-AH-nucleosome complexes demonstrated the predominant presence of acetylated histone H4 relative to acetylated histone H3. Immunoprecipitation of USF-nucleosome complexes with anti-USF antibodies also demonstrated that these complexes were enriched preferentially in acetylated histone H4. These data show that USF and GAL4-AH preferentially interact with nucleosome cores containing highly acetylated histone H4. Acetylation of histone H4 thus appears to play a primary role in the structural changes that mediate enhanced binding of transcription factors to their recognition sites within nucleosomes.

The role of transcription enhancer factor-1 (TEF-1) related proteins in the formation of M-CAT binding complexes in muscle and non-muscle tissues

M-CAT sites are required for the activity of many promoters in cardiac and skeletal muscle. M-CAT binding activity is muscle-enriched, but is found in many tissues and is immunologically related to the HeLa transcription enhancer factor-1 (TEF-1). TEF-1-related cDNAs (RTEF-1) have been cloned from chick heart. RTEF-1 mRNA is muscle-enriched, consistent with a role for RTEF-1 in the regulation of muscle-specific gene expression. Here, we have examined the tissue distribution of TEF-1-related proteins and of M-CAT binding activity by Western analysis and mobility shift polyacrylamide gel electrophoresis. TEF-1-related proteins of 57, 54 and 52 kDa were found in most tissues with the highest levels in muscle tissues. All of these TEF-1-related proteins bound M-CAT DNA and the 57- and 54-kDa TEF-1-related polypeptides were phosphorylated. Proteolytic digestion mapping showed that the 54-kDa TEF-1-related polypeptide is encoded by a different gene than the 52- and 57-kDa TEF-1-related polypeptides. A comparison of the migration and proteolytic digestion of the 54-kDa TEF-1-related polypeptide with proteins encoded by the cloned RTEF-1 cDNAs showed that the 54-kDa TEF-1-related polypeptide is encoded by RTEF-1A. High resolution mobility shift polyacrylamide gel electrophoresis showed multiple M-CAT binding activities in tissues. All of these activities contained TEF-1-related proteins. One protein-M-CAT DNA complex was muscle-enriched and was up-regulated upon differentiation of a skeletal muscle cell line. This complex contained the 54-kDa TEF-1-related polypeptide. Therefore, RTEF1-A protein is a component of a muscle-enriched transcription complex that forms on M-CAT sites and may play a key role in the regulation of transcription in muscle.

Transcription factors and the cardiac gene programme

During the past decade, major advances have been made in uncovering the mechanisms that switch genes on and off. Gene methylation and histones play an important role in gene (in)activation. Following gene activation, the initiation of transcription by RNA polymerase requires the assembly of multiple protein complexes on the promoter region of a gene. How a cell type-specific gene expression pattern can be induced is a key question in cardiovascular biology today. Members of the helix-loop-helix-family of the transcription factors play a dominant role in skeletal muscle formation. In cardiac muscle the situation is less obvious. Recent studies identified muscle transcription factors like MEF-2, TEF-1 and MNF, which are common to both the skeletal and cardiac muscle lineages. A few transcription factors, among which Nkx 2.5 and GATA-4, are expressed predominantly in the heart. The absence of master regulators in the heart points to the importance of interaction between ubiquitous factors and tissue restricted factors to initiate the cardiac gene programme and to lock these cells in their differentiated state. The recent development of murine transgenic and gene-targeting technology provides tools to study the role of mammalian transcription factors in vivo. Interesting cardiac phenotypes are found in gene targeted mice, indicating a crucial role for retinoic acid and homeobox genes in murine cardiogenesis.

A novel site in the muscle creatine kinase enhancer is required for expression in skeletal but not cardiac muscle

Expression of the muscle creatine kinase (MCK) gene in skeletal and heart muscle is controlled in part by a 5' tissue-specific enhancer. In order to identify new regulatory elements, we designed mutations in a previously untested conserved portion of this enhancer. Transfection analysis of these mutations delineated a new control element, named Trex (Transcriptional regulatory element x), which is required for full transcriptional activity of the MCK enhancer in skeletal but not cardiac muscle cells. Gel mobility shift assays demonstrate that myocyte, myoblast, and fibroblast nuclear extracts but not primary cardiomyocyte nuclear extracts contain a trans-acting factor that binds specifically to Trex. The Trex sequence is similar (7/8 bases) to the TEF-1 consensus DNA-binding site involved in regulating other muscle genes. To determine if TEF-1 interacts with Trex, selected TEF-1 binding sites such as GTIIc and M-CAT and two anti-TEF-1 antisera were used in gel shift assays. These experiments strongly suggest that a factor distinct from TEF-1 binds specifically to Trex. Thus it appears that MCK transcription is regulated in skeletal muscles through a Trex-dependent pathway while Trex is not required for MCK expression in heart. This distinction could account partially for the difference in levels of muscle creatine kinase in these tissues.

Immunochemical characterization and transacting properties of upstream stimulatory factor isoforms

The ubiquitous upstream stimulatory factor (USF) transcription factors encoded by two distinct genes (USF1 and USF2) exist under the form of various dimers able to bind E-boxes. We report the molecular cloning and functional characterization of USF2 isoforms, corresponding to a 44-kDa subunit, USF2a, and a new 38-kDa subunit, USF2b, generated by differential splicing. Using specific anti-USF antibodies, we define the different binding complexes in various nuclear extracts. In vivo, the USF1/USF2a heterodimer represents over 66% of the USF binding activity whereas the USF1 and USF2a homodimers represent less than 10%, which strongly suggests an in vivo preferential association in heterodimers. In particular, an USF1/USF2b heterodimer accounted for almost 15% of the USF species in some cells. The preferential heterodimerization of USF subunits was reproduced ex vivo, while the in vitro association of cotranslated subunits, or recombinant USF proteins, appeared to be random. In transiently transfected HeLa or hepatoma cells, USF2a and USF1 homodimers transactivated a minimal promoter with similar efficiency, whereas USF2b, which lacks an internal 67-amino acid domain, was a poor transactivator. Additionally, USF2b was an efficient as USF1 and USF2a homodimers in transactivating the liver-specific pyruvate kinase gene promoter.

Identification of a contractile-responsive element in the cardiac alpha-myosin heavy chain gene

The mechanisms by which the cardiac-specific alpha-myosin heavy chain (alpha-MHC) gene responds to contractile activity was studied in cultured cardiomyocytes and in vivo. Deletion analysis of the alpha-MHC promoter transiently transfected into neonatal rat cardiomyocytes localized the contractile-responsive element within -80 to -40 base pairs of the transcriptional start site. Mutational analysis of an E-box motif at position -47 showed that it was necessary for the contractile response both in cultured cardiomyocytes and in the intact heart. Competition gel mobility shift experiments indicated that the protein-DNA complex formed within the -39 to -59 base pair region could be competed by the E-box element at -309 of the alpha-MHC gene and that base substitutions within an E-box motif at -47 eliminated the protein-DNA complex. To identify the contractile-responsive nuclear protein, antibodies specific for E12/E47, an E-box binding basic-helix-loop-helix (bHLH) protein, and antibodies recognizing upstream stimulatory factor (USF), a widely expressed bHLH-leucine zipper transcription factor, were studied for their ability to inhibit cardiomyocyte nuclear protein binding to the E-box motif at -47. Anti-USF antibody abolished formation of the protein-DNA complex, thus identifying the protein as antigenically related to USF and demonstrating that bHLH-leucine zipper proteins are involved in the contractile-induced expression of the cardiac alpha-MHC gene.

Upstream stimulatory factors bind to insulin response sequence of the fatty acid synthase promoter. USF1 is regulated

Fatty acid synthase (FAS) plays a central role in de novo lipogenesis in mammals. The concentration or activity of FAS in liver and adipose tissue changes dramatically when animals are subjected to nutritional and hormonal manipulations. We previously reported that due to changes in transcription, FAS synthesis declines and increases in an insulin-dependent manner during fasting and refeeding, respectively, and that insulin administration of streptozotocin-diabetic mice stimulates FAS transcription. We previously mapped the FAS insulin response sequence (IRS) to the proximal promoter region from position -71 to position -50, which contains an E-box DNA binding motif. Here, using competition gel shift assays and specific upstream stimulatory factor (USF) antibodies, we identified USF1 and USF2 as major components of complexes that bind to the FAS IRS. UV-cross-linking experiments further supported that USFs bind the FAS IRS. We also found that the amount of the 43-kDa USF1 was dramatically increased in liver of refed rats. In contrast, the amount of USF2 remained the same in liver of fasted or refed rats. Moreover, a 17-kDa protein in both fasted and refed rat liver was recognized by anti-USF1 antibodies, and this 17-kDa USF1-related protein was expressed in a manner opposite to that of the 43-kDa USF1, i.e. high in liver of fasted rats and decreased in liver of refed rats. These data suggest that the regulation of USF expression may play an important role in the regulation of FAS transcription.