The Role of the CANNTG Promoter Element (E box) and the Myocyte-enhancer-binding-factor-2 (MEF-2) Site in the Transcriptional Regulation of the Chick Myogenin Gene

Antidiabetic Activity and Chemical Composition of Sanbai Melon Seed Oil

Objectives

Many fruits and herbs had been used in Traditional Chinese Medicines for treating diabetes mellitus (DM); however, scientific and accurate evidences regarding their efficacy and possible mechanisms were largely unknown. Sanbai melon seed oil (SMSO) was used in folk medicine in treating DM, but there is no literature about these effects. The present study was aimed at confirming the treatment effects of SMSO in type 1 DM.

Methods

Diabetes was induced by single intraperitoneal injection of streptozotocin (STZ) at a dose of 65 mg/kg body weight. After diabetes induction, mice were treated with SMSO at dose of 1 g/kg, 2 g/kg, and 4 g/kg. Drugs were given by gavage administration once a day continuously for 28 days. At the end of treatment, several biochemical parameters and molecular mechanisms were determined by biochemical assays, ELISA, and Western blotting. The chemical compositions of SMSO were also tested.

Results

SMSO treatment significantly improved the symptoms of weight loss, polydipsia, reduced FBG level, increased plasma insulin levels, reduced plasma lipids levels, and protected islet injury. The results also showed that SMSO mitigated oxidative stress and alleviated the liver and renal injury in diabetes mice. SMSO also protected islet cells from apoptotic damage by suppressing ER mediated and mitochondrial dependent apoptotic pathways. Further constituent analysis results showed that SMSO had rich natural resources which had beneficial effects on DM.

Conclusions

This study showed that SMSO had excellent antidiabetes effect and provided scientific basis for the use of SMSO as the functional ingredients production and dietary supplements production in the food and pharmaceutical industries.

Loss of microRNA-22 prevents high-fat diet induced dyslipidemia and increases energy expenditure without affecting cardiac hypertrophy

Obesity is associated with development of diverse diseases, including cardiovascular diseases and dyslipidemia. MiRNA-22 (miR-22) is a critical regulator of cardiac function and targets genes involved in metabolic processes. Previously, we generated miR-22 null mice and we showed that loss of miR-22 blunted cardiac hypertrophy induced by mechanohormornal stress. In the present study, we examined the role of miR-22 in the cardiac and metabolic alterations promoted by high-fat (HF) diet. We found that loss of miR-22 attenuated the gain of fat mass and prevented dyslipidemia induced by HF diet, although the body weight gain, or glucose intolerance and insulin resistance did not seem to be affected. Mechanistically, loss of miR-22 attenuated the increased expression of genes involved in lipogenesis and inflammation mediated by HF diet. Similarly, we found that miR-22 mediates metabolic alterations and inflammation induced by obesity in the liver. However, loss of miR-22 did not appear to alter HF diet induced cardiac hypertrophy or fibrosis in the heart. Our study therefore establishes miR-22 as an important regulator of dyslipidemia and suggests it may serve as a potential candidate in the treatment of dyslipidemia associated with obesity.

TFE3 inhibits myoblast differentiation in C2C12 cells via down-regulating gene expression of myogenin

Transcription factor E3 (TFE3) belongs to a basic helix-loop-helix family, and is involved in the biology of osteoclasts, melanocytes and their malignancies. We previously reported the metabolic effects of TFE3 on insulin in the liver and skeletal muscles in animal models. In the present study, we explored a novel role for TFE3 in a skeletal muscle cell line. When TFE3 was overexpressed in C2C12 myoblasts by adenovirus before induction of differentiation, myogenic differentiation of C2C12 cells was significantly inhibited. Adenovirus-mediated TFE3 overexpression also suppressed the gene expression of muscle regulatory factors (MRFs), such as MyoD and myogenin, during C2C12 differentiation. In contrast, knockdown of TFE3 using adenovirus encoding short-hairpin RNAi specific for TFE3 dramatically promoted myoblast differentiation associated with significantly increased expression of MRFs. Consistent with these findings, promoter analyses via luciferase reporter assay and electrophoretic mobility shift assay suggested that TFE3 negatively regulated myogenin promoter activity by direct binding to an E-box, E2, in the myogenin promoter. These findings indicated that TFE3 has a regulatory role in myoblast differentiation, and that transcriptional suppression of myogenin expression may be part of the mechanism of action.

Discovery and characterization of nutritionally regulated genes associated with muscle growth in Atlantic salmon

A genomics approach was used to identify nutritionally regulated genes involved in growth of fast skeletal muscle in Atlantic salmon (Salmo salar L.). Forward and reverse subtractive cDNA libraries were prepared comparing fish with zero growth rates to fish growing rapidly. We produced 7,420 ESTs and assembled them into nonredundant clusters prior to annotation. Contigs representing 40 potentially unrecognized nutritionally responsive candidate genes were identified. Twenty-three of the subtractive library candidates were also differentially regulated by nutritional state in an independent fasting-refeeding experiment and their expression placed in the context of 26 genes with established roles in muscle growth regulation. The expression of these genes was also determined during the maturation of a primary myocyte culture, identifying 13 candidates from the subtractive cDNA libraries with putative roles in the myogenic program. During early stages of refeeding DNAJA4, HSPA1B, HSP90A, and CHAC1 expression increased, indicating activation of unfolded protein response pathways. Four genes were considered inhibitory to myogenesis based on their in vivo and in vitro expression profiles (CEBPD, ASB2, HSP30, novel transcript GE623928). Other genes showed increased expression with feeding and highest in vitro expression during the proliferative phase of the culture (FOXD1, DRG1) or as cells differentiated (SMYD1, RTN1, MID1IP1, HSP90A, novel transcript GE617747). The genes identified were associated with chromatin modification (SMYD1, RTN1), microtubule stabilization (MID1IP1), cell cycle regulation (FOXD1, CEBPD, DRG1), and negative regulation of signaling (ASB2) and may play a role in the stimulation of myogenesis during the transition from a catabolic to anabolic state in skeletal muscle.

Identification of gene co-regulatory modules and associated cis-elements involved in degenerative heart disease

BACKGROUND

Cardiomyopathies, degenerative diseases of cardiac muscle, are among the leading causes of death in the developed world. Microarray studies of cardiomyopathies have identified up to several hundred genes that significantly alter their expression patterns as the disease progresses. However, the regulatory mechanisms driving these changes, in particular the networks of transcription factors involved, remain poorly understood. Our goals are (A) to identify modules of co-regulated genes that undergo similar changes in expression in various types of cardiomyopathies, and (B) to reveal the specific pattern of transcription factor binding sites, cis-elements, in the proximal promoter region of genes comprising such modules.

METHODS

We analyzed 149 microarray samples from human hypertrophic and dilated cardiomyopathies of various etiologies. Hierarchical clustering and Gene Ontology annotations were applied to identify modules enriched in genes with highly correlated expression and a similar physiological function. To discover motifs that may underly changes in expression, we used the promoter regions for genes in three of the most interesting modules as input to motif discovery algorithms. The resulting motifs were used to construct a probabilistic model predictive of changes in expression across different cardiomyopathies.

RESULTS

We found that three modules with the highest degree of functional enrichment contain genes involved in myocardial contraction (n = 9), energy generation (n = 20), or protein translation (n = 20). Using motif discovery tools revealed that genes in the contractile module were found to contain a TATA-box followed by a CACC-box, and are depleted in other GC-rich motifs; whereas genes in the translation module contain a pyrimidine-rich initiator, Elk-1, SP-1, and a novel motif with a GCGC core. Using a naïve Bayes classifier revealed that patterns of motifs are statistically predictive of expression patterns, with odds ratios of 2.7 (contractile), 1.9 (energy generation), and 5.5 (protein translation).

CONCLUSION

We identified patterns comprised of putative cis-regulatory motifs enriched in the upstream promoter sequence of genes that undergo similar changes in expression secondary to cardiomyopathies of various etiologies. Our analysis is a first step towards understanding transcription factor networks that are active in regulating gene expression during degenerative heart disease.

p38 MAPK signaling regulates recruitment of Ash2L-containing methyltransferase complexes to specific genes during differentiation

Cell-specific patterns of gene expression are established through the antagonistic functions of trithorax group (TrxG) and Polycomb group (PcG) proteins. Several muscle-specific genes have previously been shown to be epigenetically marked for repression by PcG proteins in muscle progenitor cells. Here we demonstrate that these developmentally regulated genes become epigenetically marked for gene expression (trimethylated on histone H3 Lys4, H3K4me3) during muscle differentiation through specific recruitment of Ash2L-containing methyltransferase complexes. Targeting of Ash2L to specific genes is mediated by the transcriptional regulator Mef2d. Furthermore, this interaction is modulated during differentiation through activation of the p38 MAPK signaling pathway via phosphorylation of Mef2d. Thus, we provide evidence that signaling pathways regulate the targeting of TrxG-mediated epigenetic modifications at specific promoters during cellular differentiation.

MEF2: a central regulator of diverse developmental programs

The myocyte enhancer factor 2 (MEF2) transcription factor acts as a lynchpin in the transcriptional circuits that control cell differentiation and organogenesis. The spectrum of genes activated by MEF2 in different cell types depends on extracellular signaling and on co-factor interactions that modulate MEF2 activity. Recent studies have revealed MEF2 to form an intimate partnership with class IIa histone deacetylases, which together function as a point of convergence of multiple epigenetic regulatory mechanisms. We review the myriad roles of MEF2 in development and the mechanisms through which it couples developmental, physiological and pathological signals with programs of cell-specific transcription.

Identification of a new class of PAX3-FKHR target promoters: a role of the Pax3 paired box DNA binding domain

Alveolar rhabdomyosarcoma (aRMS), an aggressive skeletal muscle cancer, carries a unique t(2;13) chromosomal translocation resulting in the formation of a chimeric transcription factor PAX3-FKHR. This fusion protein contains the intact DNA-binding domains (PD: paired box binding domain; HD: paired-type homeodomain) of Pax3 fused to the activation domain of FKHR. Cells expressing Pax3 and PAX3-FKHR show vastly different gene expression patterns, despite that they share the same DNA-binding domains. We present evidence of a gain of function mechanism that allows the fusion protein to recognize and transcriptionally activate response elements containing a PD-specific binding site. This DNA recognition specificity is in contrast to the requirement for Pax3-specific target sequences that must contain a composite of PD-and HD-binding sites. Domain swapping studies suggest that an increased structural flexibility could account for the relaxed DNA targeting specificity in PAX3-FKHR. Here, we identify myogenin gene as a direct target of PD-dependent PAX3-FKHR activation pathway in vitro and in vivo. We demonstrate that PAX3-FKHR could induce myogenin expression in undifferentiated myoblasts by a MyoD independent pathway, and that PAX3-FKHR is directly involved in myogenin expression in aRMS cells.

Avian MyoD and c-Jun coordinately induce transcriptional activity of the 3,5,3'-triiodothyronine nuclear receptor c-ErbAalpha1 in proliferating myoblasts

Although physical interactions with other receptors have been reported, heterodimeric complexes of T(3) nuclear receptors (TR) with retinoid X receptors (RXRs) are considered as major regulators of T(3) target gene expression. However, despite the potent T(3) influence in proliferating myoblasts, RXR isoforms are not expressed during proliferation, raising the question of the nature of the complex involved in TRalpha transcriptional activity. We have previously established that c-Jun induces TRalpha1 transcriptional activity in proliferating myoblasts not expressing RXR. This regulation is specific to the muscle lineage, suggesting the involvement of a muscle-specific factor. In this study, we found that MyoD expression in HeLa cells stimulates TRalpha1 activity, an influence potentiated by c-Jun coexpression. Similarly, in the absence of RXR, MyoD or c-Jun overexpression in myoblasts induces TRalpha1 transcriptional activity through a direct repeat 4 or an inverted palindrome 6 thyroid hormone response element. The highest rate of activity was recorded when c-Jun and MyoD were coexpressed. Using c-Jun-negative dominants, we established that MyoD influence on TRalpha1 activity needs c-Jun functionality. Furthermore, we demonstrated that TRalpha1 and MyoD physically interact in the hinge region of the receptor and the transactivation and basic helix loop helix domains of MyoD. RXR expression (spontaneously occurring at the onset of myoblast differentiation) in proliferating myoblasts abrogates these interactions. These data suggest that in the absence of RXR, TRalpha1 transcriptional activity in myoblasts is mediated through a complex including MyoD and c-Jun.

Coactivation of nuclear receptors and myogenic factors induces the major BTG1 influence on muscle differentiation

The btg1 (B-cell translocation gene 1) gene coding sequence was isolated from a translocation break point in a case of B-cell chronic lymphocytic leukaemia. We have already shown that BTG1, considered as an antiproliferative protein, strongly stimulates myoblast differentiation. However, the mechanisms involved in this influence remained unknown. In cultured myoblasts, we found that BTG1 stimulates the transcriptional activity of nuclear receptors (T3 and all-trans retinoic acid receptors but not RXRalpha and PPARgamma), c-Jun and myogenic factors (CMD1, Myf5, myogenin). Immunoprecipitation experiments performed in cells or using in vitro-synthesized proteins and GST pull-down assays established that BTG1 directly interacts with T3 and all-trans retinoic acid receptors and with avian MyoD (CMD1). These interactions are mediated by the transactivation domain of each transcription factor and the A box and C-terminal part of BTG1. NCoR presence induces the ligand dependency of the interaction with nuclear receptors. Lastly, deletion of BTG1 interacting domains abrogates its ability to stimulate nuclear receptors and CMD1 activity, and its myogenic influence. In conclusion, BTG1 is a novel important coactivator involved in the regulation of myoblast differentiation. It not only stimulates the activity of myogenic factors, but also of nuclear receptors already known as positive myogenic regulators.

Mirk/dyrk1B decreases the nuclear accumulation of class II histone deacetylases during skeletal muscle differentiation

Mirk/dyrk1B is a member of the dyrk/minibrain family of serine/threonine kinases that mediate the transition from growth to differentiation in lower eukaryotes and mammals. Depletion of endogenous Mirk from C2C12 myoblasts by RNA interference blocks skeletal muscle differentiation (Deng, X., Ewton, D., Pawlikowski, B., Maimone, M., and Friedman, E. (2003) J. Biol. Chem. 278, 41347-41354). We now demonstrate that knockdown of Mirk blocks transcription of the muscle regulatory factor myogenin. Co-expression of Mirk with MEF2C, but not MyoD or Myf5, enhanced activation of the myogenin promoter in a Mirk kinase-dependent manner. Mirk activated MEF2 not through direct phosphorylation of MEF2 but by phosphorylation of its inhibitors, the class II histone deacetylases (HDACs). MEF2 is sequestered by class II HDACs such as HDAC5 and MEF2-interacting transcriptional repressor (MITR). Mirk antagonized the inhibition of MEF2C by MITR, whereas kinase-inactive Mirk was ineffective. Mirk phosphorylates class II HDACs at a conserved site within the nuclear localization region, reducing their nuclear accumulation in a dose-dependent and kinase-dependent manner. Moreover, less mutant MITR phosphomimetic at the Mirk phosphorylation site localized in the nucleus than wild-type MITR. Regulation of class II HDACs occurs by multiple mechanisms. Others have shown that calcium signaling leads to phosphorylation of HDACs at 14-3-3-binding sites, blocking their association with MEF2 within the nucleus. Mirk provides another level of regulation. Mirk is induced within the initial 24 h of myogenic differentiation and enables MEF2 to transcribe the myogenin gene by decreasing the nuclear accumulation of class II HDACs.

Overexpression of chemokine-like factor 2 promotes the proliferation and survival of C2C12 skeletal muscle cells

Chemokine-like factor 1 (CKLF1) is a novel cytokine first cloned from U937 cells. It contains different splicing forms and has chemotactic effects on a wide spectrum of cells both in vitro and in vivo; it can also stimulate the regeneration of skeletal muscle cells in vivo, but the mechanism remains unclear. To probe the myogenesis function of CKLF2, which is the largest isoform of CKLFs, C2C12 murine myoblasts were stably transfected with human CKLF2 eukaryotic expression vector. Compared with control vector transfected C2C12 cells, CKLF2 overexpression causes accelerated myoblast proliferation as determined by cell counting and [(3)H]TdR incorporation assays. In addition, CKLF2 overexpression also promotes cell differentiation, which was determined by higher expression levels of myogenin, creatine kinase, myosin and the accelerated myoblast fusion. Further analysis also indicates that CKLF2 could activate the transcription activity of the bHLH/MyoD and MEF2 families. Finally, DNA synthesis and myotube formation could also be promoted by growing C2C12 cells in conditioned media from CKLF2-transfected cells. These findings strongly suggest a role for human CKLF2 in regulation of skeletal muscle myogenesis.

Inhibition of myogenin expression by activated Raf is not responsible for the block to avian myogenesis

Activated Raf is a potent inhibitor of skeletal muscle gene transcription and myocyte formation through stimulation of downstream MAPK. However, the molecular targets of elevated MAPK with regard to myogenic repression remain elusive. We examined the effects of activated Raf on myogenin gene expression in avian myoblasts. Overexpression of activated Raf in embryonic chick myoblasts prevented myogenin gene transcription and myocyte differentiation. Treatment with PD98059, an inhibitor of MAPK kinase (MEK), restored myogenin expression but did not reinstate the myogenic program. Using a panel of myogenin promoter deletion mutants, we were unable to identify a region within the proximal 829-bp promoter that confers responsiveness to MEK. Interestingly, our experiments identified MEF2A as a target of Raf-mediated inhibition in mouse myoblasts but not in avian myogenic cells. Embryonic myoblasts overexpressing activated Raf were unable to drive transcription from a minimal myogenin promoter reporter, containing a single E-box and MEF2 site, to levels comparable with controls. Unlike mouse myoblasts, forced expression of MEF2A did not synergistically enhance transcription from the myogenin promoter in chick myoblasts, indicating that additional molecular determinants of the block to myogenesis exist. Results of these experiments further exemplify specie differences in the mode of Raf-mediated inhibition of muscle differentiation.

Selective repression of myoD transcription by v-Myc prevents terminal differentiation of quail embryo myoblasts transformed by the MC29 strain of avian myelocytomatosis virus

We have investigated the mechanism by which expression of the v-myc oncogene interferes with the competence of primary quail myoblasts to undergo terminal differentiation. Previous studies have established that quail myoblasts transformed by myc oncogenes are severely impaired in the accumulation of mRNAs encoding the myogenic transcription factors Myf-5, MyoD and Myogenin. However, the mechanism responsible for such a repression remains largely unknown. Here we present evidence that v-Myc selectively interferes with quail myoD expression at the transcriptional level. Cis-regulatory elements involved in the auto-activation of qmyoD are specifically targeted in this unique example of transrepression by v-Myc, without the apparent participation of Myc-specific E-boxes or InR sequences. Transiently expressed v-Myc efficiently interfered with MyoD-dependent transactivation of the qmyoD regulatory elements, while the myogenin promoter was unaffected. Finally, we show that forced expression of MyoD in v-myc-transformed quail myoblasts restored myogenin expression and promoted extensive terminal differentiation. These data suggest that transcriptional repression of qmyoD is a major and rate-limiting step in the molecular pathway by which v-Myc severely inhibits terminal differentiation in myogenic cells.

Interaction of the myogenic determination factor myogenin with E12 and a DNA target: mechanism and kinetics

The myogenic determination factors MyoD, myogenin, myf5, and MRF4 are members of the basic helix-loop-helix (bHLH) family of transcription factors and crucial agents of myogenesis. The bHLH regions of these proteins enable them to dimerize with E proteins, another class of the bHLH family, and to bind a specific DNA element known as an E box (CANNTG consensus sequence), which results in the activation of muscle-specific gene expression. As a model for such assembly of the myogenic determination factor/E protein-DNA ternary complex, we have studied the physiologically relevant association of myogenin, E12, and the 3' E box of the acetylcholine receptor (AChR) alpha-subunit gene enhancer. Using the technique of electrophoretic mobility shift assay combined with order-of-addition and time-course experiments, we find that heterodimerization of myogenin with E12 occurs prior to DNA-binding. In addition, we deduce the dissociation (Kd) and rate (k) constants for each step in the formation of the myogenin/E12-DNA ternary complex. Kinetic simulations indicate that at 37 degrees C myogenin and E12 heterodimerize with a Kd of 36 microM (k(on) of 573 M(-1) x s(-1) and k(off )of 0.0205 x s(-1)), and that subsequently the heterodimer binds the AChR alpha-subunit gene enhancer 3' E box with a Kd of 8.8 nM (with possible k(on) and k(off) values ranging from 1.0x10(8) to 14.1x10(8) M(-1) x s(-1), and 0.875 to 12.3 s(-1), respectively).

Opposing functions of ATF2 and Fos-like transcription factors in c-Jun-mediated myogenin expression and terminal differentiation of avian myoblasts

With the aim to identify the oncoprotein partners implicated in the c-Jun myogenic influence, we carried out stable transfection experiments of c-Jun and/or ATF2, Fra2, c-Fos overexpression in avian myoblasts. Before induction of differentiation, c-Jun repressed myoblast withdrawal from the cell cycle, as did a TPA treatment. However, after serum removal, unlike TPA, c-Jun significantly stimulated myoblast differentiation. In search for specific partners involved in this dual influence, we found that a reduction in the amounts of c-Fos and Fra2 and an increase in c-Jun proteins occurred at cell confluence, a situation likely to favor cooperation between c-Jun and ATF2 during terminal differentiation. Whereas c-Fos and Fra2 cooperated with c-Jun to abrogate myoblast withdrawal from the cell cycle and terminal differentiation, ATF2 co-expression potentiated the positive myogenic c-Jun influence. In addition, myogenin expression was a positive target of this cooperation and this regulation occurred through a stimulation of myogenin promoter activity: (1) whereas c-Fos or Fra2 co-expression abrogated c-Jun stimulatory activity on this promoter, ATF2 co-expression potentiated this influence; (2) using a dominant negative ATF2 mutant, we established that c-Jun transcriptional activity required functionality of endogenous ATF2. These data suggest that through this dual myogenic influence due to cooperations with different partners, c-Jun is involved in the control of duration of myoblast proliferation and thereafter of fusion efficiency.

The triiodothyronine nuclear receptor c-ErbAalpha1 inhibits avian MyoD transcriptional activity in myoblasts

Thyroid hormone stimulates myoblast differentiation, through an inhibition of AP-1 activity occurring at the onset of differentiation. In this study we found that the T3 nuclear receptor c-ErbAalpha1 (T3Ralpha1) is involved in a mechanism preserving the duration of myoblast proliferation. Independently of the hormone presence, T3Ralpha1 represses avian MyoD transcriptional activity. Using several mutants of T3Ralpha1, we found that the hinge region plays a crucial role in the inhibition of MyoD activity. In particular, mutations of two small basic sequences included in alpha helices abrogate the T3Ralpha1/MyoD functional interaction. Similarly, the T3 receptor also represses myogenin transcriptional activity. Therefore, despite stimulating avian myoblast differentiation by a T3-dependent pathway not involving myogenic factors, T3Ralpha1 contributes to maintain an optimal myoblast proliferation period by inhibiting MyoD and myogenin activity.

The insulin-like growth factor-phosphatidylinositol 3-kinase-Akt signaling pathway regulates myogenin expression in normal myogenic cells but not in rhabdomyosarcoma-derived RD cells

Insulin-like growth factors (IGFs) can stimulate skeletal muscle differentiation. One of the molecular mechanisms underlying IGF-stimulated myogenesis is transcriptional induction of myogenin. The current work is aimed to elucidate the signaling pathways mediating the IGF effect on myogenin promoter in mouse C2C12 myogenic cells. We show that phosphatidylinositol 3-kinase (PI3K)/Akt and p70(S6K) are crucial signaling molecules mediating the stimulatory effect of IGFs on myogenin expression. We have identified three cis-elements, namely the E box, MEF2, and MEF3 sites, within the 133-base pair mouse proximal myogenin promoter that are under the control of the IGF/PI3K/Akt pathway. Simultaneous mutation of all three elements completely abolishes activation of the myogenin promoter by PI3K/Akt. We demonstrate that PI3K/Akt can increase both the MyoD and the MEF2-dependent reporter activity by enhancing the transcriptional activity of MyoD and MEF2. Interestingly, IGF1 does not enhance myogenin expression in Rhabdomyosarcoma-derived RD cells. Consistently, the constitutively active PI3K/Akt fail to activate the myogenic reporters, suggesting the IGF/PI3K/Akt pathway is defective in RD cells and the defect(s) is downstream to PI3K/Akt. This is the first time that a defect in the IGF/PI3K/Akt pathway has been revealed in RD cells which provides another clue to future therapeutic treatment of Rhabdomyosarcoma.

Muscle regulatory factor gene: zebrafish (Danio rerio) myogenin cDNA

Myogenin is one of the basic helix-loop-helix proteins that regulate muscle-specific gene expression. Using reverse transciption-polymerase chain reaction (RT-PCR), 5'- and 3'-rapid amplification of cDNA ends (RACE), zebrafish myogenin cDNA was cloned from mRNA of embryos at 10-96 h post-fertilization. The cDNA, at 1384 base pairs (bp), contained a 771-bp open reading frame with 113- and 500-bp flanking regions at the 5'- and 3'-ends, respectively. The deduced amino acid sequences of zebrafish myogenin encoded a 256-amino-acid polypeptide. In a comparison with myogenin of carp, trout, Xenopus, chicken and human, zebrafish myogenin shared 90.9, 77.6, 70.3, 62.9 and 51.5% amino acid identity, respectively. The basic helix-loop-helix domains in myogenin are all conserved. The molecular phylogenic tree demonstrated that myogenin of zebrafish is more closely related to that of fish than to the myogenin of other vertebrates.

Characterization of a gene encoding a developmentally regulated winged helix transcription factor of the sea urchin Strongylocentrotus purpuratus

Spfkh1 is a Strongylocentrotus purpuratus transcription factor that contains a winged helix DNA binding domain. Both the gene and overlapping cDNAs encoding this factor have been cloned and completely sequenced. We have mapped the start of transcription by primer extension to a site 600 base pairs 5' to the start of translation. Spfkh1 is transcribed in one open reading frame that contains the DNA binding domain, nuclear localization signal and transactivation domain. The deduced amino acid sequence encodes a 40. 7kDa protein with a pI of 9.96. Alignments of the DNA binding domain with other forkhead domains reveal that this gene falls into Class II of the winged helix transcription factors. We have identified a unique carboxyl-terminal motif of unknown function that is present in all winged helix Class II transcription factors. A phylogenetic analysis of the DNA binding domains shows that, within the Class II, Spfkh1 groups with the deuterostomes as opposed to the protostomes. Analysis of the sequence 5' to the start of translation revealed binding sites for a large number of different transcription factors, many of which are present in multiple copies. The constellation of binding sites in the cis-regulatory region indicates that Spfkh1 is regulated by a complex set of factors, some of which are known to be endoderm specific. Included among these are binding sites for factors downstream of the Wnt/beta-catenin and hedgehog signaling pathways, implicating these pathways in both regulation of Spfkh1 and specification of endoderm.

Transcriptional control of muscle development by myocyte enhancer factor-2 (MEF2) proteins

Metazoans contain multiple types of muscle cells that share several common properties, including contractility, excitability, and expression of overlapping sets of muscle structural genes that mediate these functions. Recent biochemical and genetic studies have demonstrated that members of the myocyte enhancer factor-2 (MEF2) family of MADS (MCM1, agamous, deficiens, serum response factor)-box transcription factors play multiple roles in muscle cells to control myogenesis and morphogenesis. Like other MADS-box proteins, MEF2 proteins act combinatorially through protein-protein interactions with other transcription factors to control specific sets of target genes. Genetic studies in Drosophila have also begun to reveal the upstream elements of myogenic regulatory hierarchies that control MEF2 expression during development of skeletal, cardiac, and visceral muscle lineages. Paradoxically, MEF2 factors also regulate cell proliferation by functioning as endpoints for a variety of growth factor-regulated intracellular signaling pathways that are antagonistic to muscle differentiation. We discuss the diverse functions of this family of transcription factors, the ways in which they are regulated, and their mechanisms of action.

The E protein CTF4 and acetylcholine receptor expression in development and denervation supersensitivity

Motor activity blocks the extrasynaptic expression of many genes in skeletal muscle, including those encoding ion channels, receptors, and adhesion molecules. Denervation reinduces transcription throughout the multinucleated myofiber, restoring the developmental pattern of expression, especially of the genes coding for the acetylcholine receptor. A screen for trans-acting factors binding to the enhancer region of the alpha-subunit gene of the acetylcholine receptor identified CTF4, a ubiquitously expressed and alternatively spliced chicken homologue of the human E protein transcription factor HTF4/HEB. Expression of the CTF4 locus closely parallels that of myogenin and acetylcholine receptor during development and maturation of skeletal muscle, but transcription is not similarly regulated by neuronal cues. Alternative splicing within the region encoding the transactivation domain generates two CTF4 isoforms with different tissue distributions, but similar binding affinities for the acetylcholine receptor alpha-subunit enhancer and similar transcriptional potential when complexed to myogenin. Direct injection of a myogenin, but not a MyoD, antisense expression vector into denervated skeletal muscle caused a significant decrease in the transcriptional activation of a depolarization-sensitive reporter gene. Similarly, injection of a CTF4, but less so of an E12, antisense expression vector impaired the denervation response, further implicating the involvement of a myogenin/CTF4 heterodimer in the expression of AChR genes in vivo.

The Rho family G proteins play a critical role in muscle differentiation

The Rho family GTP-binding proteins play a critical role in a variety of cytoskeleton-dependent cell functions. In this study, we examined the role of Rho family G proteins in muscle differentiation. Dominant negative forms of Rho family proteins and RhoGDI, a GDP dissociation inhibitor, suppressed transcription of muscle-specific genes, while mutationally activated forms of Rho family proteins strongly activated their transcription. C2C12 cells overexpressing RhoGDI (C2C12RhoGDI cells) did not differentiate into myotubes, and expression levels of myogenin, MRF4, and contractile protein genes but not MyoD and myf5 genes were markedly reduced in C2C12RhoGDI cells. The promoter activity of the myogenin gene was suppressed by dominant negative mutants of Rho family proteins and was reduced in C2C12RhoGDI cells. Expression of myocyte enhancer binding factor 2 (MEF2), which has been reported to be required for the expression of the myogenin gene, was reduced at the mRNA and protein levels in C2C12RhoGDI cells. These results suggest that the Rho family proteins play a critical role in muscle differentiation, possibly by regulating the expression of the myogenin and MEF2 genes.

Two domains of MyoD mediate transcriptional activation of genes in repressive chromatin: a mechanism for lineage determination in myogenesis

Genetic studies have demonstrated that MyoD and Myf5 establish the skeletal muscle lineage, whereas myogenin mediates terminal differentiation, yet the molecular basis for this distinction is not understood. We show that MyoD can remodel chromatin at binding sites in muscle gene enhancers and activate transcription at previously silent loci. TGF-beta, basic-FGF, and sodium butyrate blocked MyoD-mediated chromatin reorganization and the initiation of transcription. In contrast, TGF-beta and sodium butyrate did not block transcription when added after chromatin remodeling had occurred. MyoD and Myf-5 were 10-fold more efficient than myogenin at activating genes in regions of transcriptionally silent chromatin. Deletion mutagenesis of the MyoD protein demonstrated that the ability to activate endogenous genes depended on two regions: a region rich in cysteine and histidine residues between the acidic activation domain and the bHLH domain, and a second region in the carboxyl terminus of the protein. Neither region has been shown previously to regulate gene transcription and both have domains that are conserved in the Myf5 protein. Our results establish a mechanism for chromatin modeling in the skeletal muscle lineage and define domains of MyoD, independent of the activation domain, that participate in chromatin reorganization.