Regulation of muscle transcription by the MyoD family. The heart of the matter

Cardiac commitment driven by MyoD expression in pericardial stem cells

Cellular therapy holds immense promise to remuscularize the damaged myocardium but is practically hindered by limited allogeneic sources of cardiac-committed cells that engraft stably in the recipient heart after transplantation. Here, we demonstrate that the pericardial tissue harbors myogenic stem cells (pSCs) that are activated in response to inflammatory signaling after myocardial infarction (MI). The pSCs derived from the MI rats (MI-pSCs) show in vivo and in vitro cardiac commitment characterized by cardiac-specific Tnnt2 expression and formation of rhythmic contraction in culture. Bulk RNA-seq analysis reveals significant upregulation of a panel of genes related to cardiac/myogenic differentiation, paracrine factors, and extracellular matrix in the activated pSCs compared to the control pSCs (Sham-pSCs). Notably, we define MyoD as a key factor that governs the process of cardiac commitment, as siRNA-mediated MyoD gene silencing results in a significant reduction of myogenic potential. Injection of the cardiac-committed cells into the infarcted rat heart leads to long-term survival and stable engraftment in the recipient myocardium. Therefore, these findings point to pericardial myogenic progenitors as an attractive candidate for cardiac cell-based therapy to remuscularize the damaged myocardium.

A CRISPR-Cas9-mediated versatile method for targeted integration of a fluorescent protein gene to visualize endogenous gene expression in Xenopus laevis

Xenopus laevis is a widely used model organism in developmental and regeneration studies. Despite several reports regarding targeted integration techniques in Xenopus, there is still room for improvement of them, especially in creating reporter lines that rely on endogenous regulatory enhancers/promoters. We developed a CRISPR-Cas9-based simple method to efficiently introduce a fluorescent protein gene into 5' untranslated regions (5'UTRs) of target genes in Xenopus laevis. A donor plasmid DNA encoding an enhanced green fluorescent protein (eGFP) flanked by a genomic fragment ranging from 66 bp to 878 bp including target 5'UTR was co-injected into fertilized eggs with a single guide RNA and Cas9 protein. Injections for krt12.2.L, myod1.S, sox2.L or brevican.S resulted in embryos expressing eGFP fluorescence in a tissue-specific manner, recapitulating endogenous expression of target genes. Integrations of the donor DNA into the target regions were examined by genotyping PCR for the eGFP-expressing embryos. The rate of embryos expressing the specific eGFP varied from 2.1% to 13.2% depending on the target locus and length of the genomic fragment in the donor plasmids. Germline transmission of an integrated DNA was observed. This simple method provides a powerful tool for exploring gene expression and function in developmental and regeneration research in X. laevis.

The long noncoding RNA Charme supervises cardiomyocyte maturation by controlling cell differentiation programs in the developing heart

Long noncoding RNAs (lncRNAs) are emerging as critical regulators of heart physiology and disease, although the studies unveiling their modes of action are still limited to few examples. We recently identified pCharme, a chromatin-associated lncRNA whose functional knockout in mice results in defective myogenesis and morphological remodeling of the cardiac muscle. Here, we combined Cap-Analysis of Gene Expression (CAGE), single-cell (sc)RNA sequencing, and whole-mount in situ hybridization analyses to study pCharme cardiac expression. Since the early steps of cardiomyogenesis, we found the lncRNA being specifically restricted to cardiomyocytes, where it assists the formation of specific nuclear condensates containing MATR3, as well as important RNAs for cardiac development. In line with the functional significance of these activities, pCharme ablation in mice results in a delayed maturation of cardiomyocytes, which ultimately leads to morphological alterations of the ventricular myocardium. Since congenital anomalies in myocardium are clinically relevant in humans and predispose patients to major complications, the identification of novel genes controlling cardiac morphology becomes crucial. Our study offers unique insights into a novel lncRNA-mediated regulatory mechanism promoting cardiomyocyte maturation and bears relevance to Charme locus for future theranostic applications.

Irreversibility of Pulmonary Fibrosis

Pulmonary fibrosis, a kind of terminal pathological changes in the lung, is caused by aberrant wound healing, deposition of extracellular matrix (ECM), and eventually replacement of lung parenchyma by ECM. Pulmonary fibrosis induced by acute lung injury and some diseases is reversible under treatment. While idiopathic pulmonary fibrosis is persistent and irreversible even after treatment. Currently, the pathogenesis of irreversible pulmonary fibrosis is not fully elucidated. The known factors associated with the development of irreversible fibrosis include apoptosis resistance of (myo)fibroblasts, dysfunction of pulmonary vessel, cell mitochondria and autophagy, aberrant epithelia hyperplasia and lipid metabolism disorder. In this review, other than a brief introduction of reversible pulmonary fibrosis, we focus on the underlying pathogenesis of irreversible pulmonary fibrosis from the above aspects as well as preclinical disease models, and also suggest directions for future studies.

Polymorphisms in coding and non-coding regions of rabbit (Oryctolagus cuniculus) myogenin (MyoG) gene

In animal breeding, selection based on growth is very often used, as this trait affects the profitability of animal production. Identification of  polymorphisms within the genes affecting the growth process seems to be very important. Therefore, we decided to analyse rabbit myogenin (<em>MyoG</em> gene) for potential polymorphic sites and their association with growth and carcass traits in Termond White (TER), Belgian Giant Grey (BGG) and crossbred New Zealand White×Belgian Giant Grey (NZW×BGG) rabbits. We found three single nucleotide polymorphisms (SNPs) – in 5’ upstream sequence g.68679476 C&gt;T, in exon 1 – silent mutation g.68680096 T&gt;C and g.68680097 G&gt;A resulting in change of GTG triplet (valine) into ATG triplet (methionine). Association analysis showed that GG genotype weaning weight was statistically higher compared to GA in TER population (<em>P</em>=0.005), and that the hind parts for GG genotypes were heavier compared to those of GA (<em>P</em>=0.024), but association analysis of dissectible parts showed this was caused by higher bone weight (<em>P</em>=0.015). For g.68679476 C&gt;T in NZW×BGG population, the CC genotypes for fore (678±35) and hind part (615±29) weights were heavier compared to CT (588±16 and 549±13, respectively); moreover, association analysis of dissectible parts showed that weight of dissectible meat in hind part. Unfortunately, we did not find similar associations for other analysed breeds. For g.68679476 C&gt;T in NZWxBGG musculus longissimus lumborum pH leg after 24 h chilling (pH24L) were statistically lower for CC genotypes compared to CT (<em>P</em>=0.027). For g.68680097 G&gt;A in Termond White population L* value on the hind leg after 24 h chilling (L*24H) was higher for GA genotypes compared to GG (<em>P</em>=0.03), while for g.68679476 C&gt;T for musculus longissimus lumborum L* value after 24 h (L*24L) CC genotypes had higher value compared to CT (<em>P</em>=0.016) in BGG population. Moreover, in BGG population CT genotypes had higher weaning weight compared to CC (<em>P</em>=0.018). Our results show that SNPs within the <em>MyoG </em>gene may influence growth traits in some rabbit breeds, but the evolutionary conserved sequence may not be favourable for changes within coding sequences. For a better understanding thereof, additional analysis is required.

Andrographolide promotes skeletal muscle regeneration after acute injury through epigenetic modulation

Myopathy is a muscle disease in which muscle fibers do not function properly, and eventually cause severe diseases, such as muscular dystrophy. The properly regeneration of skeletal muscle plays a pivotal role to maintain the muscle function after muscle injury. The aim of this study is to determine whether andrographolide plays an effect role on regulating skeletal muscle regeneration. Mouse satellite cells, C2C12 cells and Cardiotoxin (CTX) intramuscular injection induced acute skeletal muscle injury model were used to evaluate whether andrographolide is essential for skeletal muscle regeneration. The underling mechanism detected using immunohistochemistry stain, western blot, real time PCR. Andrographolide promotes mouse skeletal muscle regeneration. In cardiotoxin induced skeletal muscle injury model, andrographolide treatment enhanced myotube generation and promoted myotube fusion. Andrographolide treatment dramatically increased expression of myotube differentiation related genes, including Desmin, MyoD, MyoG, Myomaker, Tnni2, Dmd, Myoz1 and Myoz3. For the mechanism studies, we observed that andrographolide treatment significantly promoted histone modification, such as H3K4Me2, H3K4Me3 and H3K36Me2, both in vivo and in vitro. Treatment with DZNep, a Lysine methyltransferase EZH2 inhibitor, significantly attenuated andrographolide-induced expression of Myf5, Myomaker, Skeletal muscle α-actin, MyoD and MyoG. Taken together, our data in this study demonstrate andrographolide epigenetically drives differentiation and fusion of myotube, eventually promotes skeletal muscle regeneration. This should be a therapeutic treatment for skeletal muscle regeneration after muscle damage.

Role of Zebrafish fhl1A in Satellite Cell and Skeletal Muscle Development

Background:

Four-and-a-half LIM domains protein 1 (FHL1) mutations are associated with human myopathies. However, the function of this protein in skeletal development remains unclear.

Methods:

Whole-mount in situ hybridization and embryo immunostaining were performed.

Results:

Zebrafish Fhl1A is the homologue of human FHL1. We showed that fhl1A knockdown causes defective skeletal muscle development, while injection with fhl1A mRNA largely recovered the muscle development in these fhl1A morphants. We also demonstrated that fhl1A knockdown decreases the number of satellite cells. This decrease in satellite cells and the emergence of skeletal muscle abnormalities were associated with alterations in the gene expression of myoD, pax7, mef2ca and skMLCK. We also demonstrated that fhl1A expression and retinoic acid (RA) signalling caused similar skeletal muscle development phenotypes. Moreover, when treated with exogenous RA, endogenous fhl1A expression in skeletal muscles was robust. When treated with DEAB, an RA signalling inhibitor which inhibits the activity of retinaldehyde dehydrogenase, fhl1A was downregulated.

Conclusion:

fhl1A functions as an activator in regulating the number of satellite cells and in skeletal muscle development. The role of fhl1A in skeletal myogenesis is regulated by RA signaling.

Prevention of chemotherapy-induced cachexia by ACVR2B ligand blocking has different effects on heart and skeletal muscle

BACKGROUND

Toxicity of chemotherapy on skeletal muscles and the heart may significantly contribute to cancer cachexia, mortality, and decreased quality of life. Doxorubicin (DOX) is an effective cytostatic agent, which unfortunately has toxic effects on many healthy tissues. Blocking of activin receptor type IIB (ACVR2B) ligands is an often used strategy to prevent skeletal muscle loss, but its effects on the heart are relatively unknown.

METHODS

The effects of DOX treatment with or without pre-treatment with soluble ACVR2B-Fc (sACVR2B-Fc) were investigated. The mice were randomly assigned into one of the three groups: (1) vehicle (PBS)-treated controls, (2) DOX-treated mice (DOX), and (3) DOX-treated mice administered with sACVR2B-Fc during the experiment (DOX + sACVR2B-Fc). DOX was administered with a cumulative dose of 24 mg/kg during 2 weeks to investigate cachexia outcome in the heart and skeletal muscle. To understand similarities and differences between skeletal and cardiac muscles in their responses to chemotherapy, the tissues were collected 20 h after a single DOX (15 mg/kg) injection and analysed with genome-wide transcriptomics and mRNA and protein analyses. The combination group was pre-treated with sACVR2B-Fc 48 h before DOX administration. Major findings were also studied in mice receiving only sACVR2B-Fc.

RESULTS

The DOX treatment induced similar (~10%) wasting in skeletal muscle and the heart. However, transcriptional changes in response to DOX were much greater in skeletal muscle. Pathway analysis and unbiased transcription factor analysis showed that p53-p21-REDD1 is the main common pathway activated by DOX in both skeletal and cardiac muscles. These changes were attenuated by blocking ACVR2B ligands especially in skeletal muscle. Tceal7 (3-fold to 5-fold increase), transferrin receptor (1.5-fold increase), and Ccl21 (0.6-fold to 0.9-fold decrease) were identified as novel genes responsive to blocking ACVR2B ligands. Overall, at the transcriptome level, ACVR2B ligand blocking had only minor influence in the heart while it had marked effects in skeletal muscle. The same was also true for the effects on tissue wasting. This may be explained in part by about 18-fold higher gene expression of myostatin in skeletal muscle compared with the heart.

CONCLUSIONS

Cardiac and skeletal muscles display similar atrophy after DOX treatment, but the mechanisms for this may differ between the tissues. The present results suggest that p53-p21-REDD1 signalling is the main common DOX-activated pathway in these tissues and that blocking activin receptor ligands attenuates this response, especially in skeletal muscle supporting the overall stronger effects of this treatment in skeletal muscles.

Analysis of LncRNA expression in cell differentiation

Lineage-specific cell differentiation is a precise and coordinated biological process. To explore the roles of long noncoding RNA (lncRNA) in this process, the expression of polyA-minus RNAs was comparatively studied during the course of myocyte and adipocyte differentiation. In addition to identifying thousands of novel lncRNAs, distinct lncRNA profiles were revealed during lineage-specific differentiation, showing their active involvement in this process. This study further found that lncRNAs were organized in clusters and are co-regulated, constituting transcription open domains (TODs). In myocyte differentiation of C2C12 cells, loss-of-function screening identified three myogenic lncRNAs. Knockdown of their expression compromised not only the differentiation process, but also the essential signaling pathway. In addition to showing that lncRNAs are actively involved in cell differentiation, our results start to reveal a comprehensive signaling pathway, involving both protein and lncRNA factors.

Muscle-relevant genes marked by stable H3K4me2/3 profiles and enriched MyoD binding during myogenic differentiation

Post-translational modifications of histones play a key role in the regulation of gene expression during development and differentiation. Numerous studies have shown the dynamics of combinatorial regulation by transcription factors and histone modifications, in the sense that different combinations lead to distinct expression outcomes. Here, we investigated gene regulation by stable enrichment patterns of histone marks H3K4me2 and H3K4me3 in combination with the chromatin binding of the muscle tissue-specific transcription factor MyoD during myogenic differentiation of C2C12 cells. Using k-means clustering, we found that specific combinations of H3K4me2/3 profiles over and towards the gene body impact on gene expression and marks a subset of genes important for muscle development and differentiation. By further analysis, we found that the muscle key regulator MyoD was significantly enriched on this subset of genes and played a repressive role during myogenic differentiation. Among these genes, we identified the pluripotency gene Patz1, which is repressed during myogenic differentiation through direct binding of MyoD to promoter elements. These results point to the importance of integrating histone modifications and MyoD chromatin binding for coordinated gene activation and repression during myogenic differentiation.

Resistance training with excessive training load and insufficient recovery alters skeletal muscle mass-related protein expression

The aim of this study was to investigate the effects of a resistance training program with excessive training load and insufficient recovery time between bouts on muscle hypertrophy- and atrophy-related protein expression. Male Wistar rats were randomly assigned to either a trained (TR, N = 9) or a sedentary (SE, N = 9) group. The TR group was subjected to a 12-week resistance training program with excessive training load and insufficient recovery between bouts that was designed to induce plantaris muscle atrophy. After the 12-week experiment, the plantaris muscle was collected to analyze the cross-sectional area (CSA) of the muscle fibers, and MAFbx, MyoD, myogenin, and IGF-I protein expression (Western blot). The CSA was reduced significantly (-17%, p ≤ 0.05) in the TR group compared with the SE group. Reciprocally, there was a significant (p ≤ 0.05) 20% increase in MAFbx protein expression, whereas the MyoD (-27%), myogenin (-29%), and IGF-I (-43%) protein levels decreased significantly (p ≤ 0.05) in the TR group compared with the SE group. In conclusion, our data indicated that muscle atrophy induced by resistance training with excessive training load and insufficient recovery was associated with upregulation of the MAFbx catabolic protein and downregulation of the MyoD, myogenin, and IGF-I anabolic proteins. These findings suggest that quantitative analysis of these proteins can be important and complementary with other biochemical markers to confirm a possible overtraining diagnosis.

Transcriptome analysis of post-hatch breast muscle in legacy and modern broiler chickens reveals enrichment of several regulators of myogenic growth

Agriculture provides excellent model systems for understanding how selective pressure, as applied by humans, can affect the genomes of plants and animals. One such system is modern poultry breeding in which intensive genetic selection has been applied for meat production in the domesticated chicken. As a result, modern meat-type chickens (broilers) exhibit enhanced growth, especially of the skeletal muscle, relative to their legacy counterparts. Comparative studies of modern and legacy broiler chickens provide an opportunity to identify genes and pathways affected by this human-directed evolution. This study used RNA-seq to compare the transcriptomes of a modern and a legacy broiler line to identify differentially enriched genes in the breast muscle at days 6 and 21 post-hatch. Among the 15,945 genes analyzed, 10,841 were expressed at greater than 0.1 RPKM. At day 6 post-hatch 189 genes, including several regulators of myogenic growth and development, were differentially enriched between the two lines. The transcriptional profiles between lines at day 21 post-hatch identify 193 genes differentially enriched and still include genes associated with myogenic growth. This study identified differentially enriched genes that regulate myogenic growth and differentiation between the modern and legacy broiler lines. Specifically, differences in the ratios of several positive (IGF1, IGF1R, WFIKKN2) and negative (MSTN, ACE) myogenic growth regulators may help explain the differences underlying the enhanced growth characteristics of the modern broilers.

The CO-Regulation Database (CORD): a tool to identify coordinately expressed genes

Background

Meta-analysis of gene expression array databases has the potential to reveal information about gene function. The identification of gene-gene interactions may be inferred from gene expression information but such meta-analysis is often limited to a single microarray platform. To address this limitation, we developed a gene-centered approach to analyze differential expression across thousands of gene expression experiments and created the CO-Regulation Database (CORD) to determine which genes are correlated with a queried gene.

Results

Using the GEO and ArrayExpress database, we analyzed over 120,000 group by group experiments from gene microarrays to determine the correlating genes for over 30,000 different genes or hypothesized genes. CORD output data is presented for sample queries with focus on genes with well-known interaction networks including p16 (CDKN2A), vimentin (VIM), MyoD (MYOD1). CDKN2A, VIM, and MYOD1 all displayed gene correlations consistent with known interacting genes.

Conclusions

We developed a facile, web-enabled program to determine gene-gene correlations across different gene expression microarray platforms. Using well-characterized genes, we illustrate how CORD's identification of co-expressed genes contributes to a better understanding a gene's potential function. The website is found at http://cord-db.org.

Engineered Human Muscle Tissue from Skeletal Muscle Derived Stem Cells and Induced Pluripotent Stem Cell Derived Cardiac Cells

During development, cardiac and skeletal muscle share major transcription factors and sarcomere proteins which were generally regarded as specific to either cardiac or skeletal muscle but not both in terminally differentiated adult cardiac or skeletal muscle. Here, we investigated whether artificial muscle constructed from human skeletal muscle derived stem cells (MDSCs) recapitulates developmental similarities between cardiac and skeletal muscle. We constructed 3-dimensional collagen-based engineered muscle tissue (EMT) using MDSCs (MDSC-EMT) and compared the biochemical and contractile properties with EMT using induced pluripotent stem (iPS) cell-derived cardiac cells (iPS-EMT). Both MDSC-EMT and iPS-EMT expressed cardiac specific troponins, fast skeletal muscle myosin heavy chain, and connexin-43 mimicking developing cardiac or skeletal muscle. At the transcriptional level, MDSC-EMT and iPS-EMT upregulated both cardiac and skeletal muscle-specific genes and expressed Nkx2.5 and Myo-D proteins. MDSC-EMT displayed intracellular calcium ion transients and responses to isoproterenol. Contractile force measurements of MDSC-EMT demonstrated functional properties of immature cardiac and skeletal muscle in both tissues. Results suggest that the EMT from MDSCs mimics developing cardiac and skeletal muscle and can serve as a useful in vitro functioning striated muscle model for investigation of stem cell differentiation and therapeutic options of MDSCs for cardiac repair.

Differential effects of hypoxic and hyperoxic stress-induced hypertrophy in cultured chick fetal cardiac myocytes

The adult heart responds to contraction demands by hypertrophy, or enlargement, of cardiac myocytes. Adaptive hypertrophy can occur in response to hyperoxic conditions such as exercise, while pathological factors that result in hypoxia ultimately result in heart failure. The difference in the outcomes produced by pathologically versus physiologically induced hypertrophy suggests that the cellular signaling pathways or conditions of myocytes may be different at the cellular level. The structural and functional changes in myocytes resulting from hyperoxia (simulated using hydrogen peroxide) and hypoxia (using oxygen deprivation) were tested on fetal chick cardiac myocytes grown in vitro. Structural changes were measured using immunostaining for α-sarcomeric actin or MyoD, while functional changes were assessed using immunostaining for calcium/calmodulin-dependent kinase (CaMKII) and by measuring intracellular calcium fluxes using live cell fluorescence imaging. Both hypoxic and hyperoxic stress resulted in an upregulation of actin and MyoD expression. Similarly, voltage-gated channels governing myocyte depolarization and the regulation of CaMK were unchanged by hyperoxic or hypoxic conditions. However, the dynamic features of calcium fluxes elicited by caffeine or epinephrine were different in cells subjected to hypoxia versus hyperoxia, suggesting that these different conditions differentially affect components of ligand-activated signaling pathways that regulate calcium. Our results suggest that changes in signaling pathways, rather than structural organization, may mediate the different outcomes associated with hyperoxia-induced versus hypoxia-induced hypertrophy, and these changes are likely initiated at the cellular level.

Predicting tissue specific cis-regulatory modules in the human genome using pairs of co-occurring motifs

Background

Researchers seeking to unlock the genetic basis of human physiology and diseases have been studying gene transcription regulation. The temporal and spatial patterns of gene expression are controlled by mainly non-coding elements known as cis-regulatory modules (CRMs) and epigenetic factors. CRMs modulating related genes share the regulatory signature which consists of transcription factor (TF) binding sites (TFBSs). Identifying such CRMs is a challenging problem due to the prohibitive number of sequence sets that need to be analyzed.

Results

We formulated the challenge as a supervised classification problem even though experimentally validated CRMs were not required. Our efforts resulted in a software system named CrmMiner. The system mines for CRMs in the vicinity of related genes. CrmMiner requires two sets of sequences: a mixed set and a control set. Sequences in the vicinity of the related genes comprise the mixed set, whereas the control set includes random genomic sequences. CrmMiner assumes that a large percentage of the mixed set is made of background sequences that do not include CRMs. The system identifies pairs of closely located motifs representing vertebrate TFBSs that are enriched in the training mixed set consisting of 50% of the gene loci. In addition, CrmMiner selects a group of the enriched pairs to represent the tissue-specific regulatory signature. The mixed and the control sets are searched for candidate sequences that include any of the selected pairs. Next, an optimal Bayesian classifier is used to distinguish candidates found in the mixed set from their control counterparts. Our study proposes 62 tissue-specific regulatory signatures and putative CRMs for different human tissues and cell types. These signatures consist of assortments of ubiquitously expressed TFs and tissue-specific TFs. Under controlled settings, CrmMiner identified known CRMs in noisy sets up to 1:25 signal-to-noise ratio. CrmMiner was 21-75% more precise than a related CRM predictor. The sensitivity of the system to locate known human heart enhancers reached up to 83%. CrmMiner precision reached 82% while mining for CRMs specific to the human CD4⁺ T cells. On several data sets, the system achieved 99% specificity.

Conclusion

These results suggest that CrmMiner predictions are accurate and likely to be tissue-specific CRMs. We expect that the predicted tissue-specific CRMs and the regulatory signatures broaden our knowledge of gene transcription regulation.

Reversible differentiation of myofibroblasts by MyoD

Myofibroblasts participate in tissue repair processes in diverse mammalian organ systems. The deactivation of myofibroblasts is critical for termination of the reparative response and restoration of tissue structure and function. The current paradigm on normal tissue repair is the apoptotic clearance of terminally differentiated myofibroblasts; while, the accumulation of activated myofibroblasts is associated with progressive human fibrotic disorders. The capacity of myofibroblasts to undergo de-differentiation as a potential mechanism for myofibroblast deactivation has not been examined. In this report, we have uncovered a role for MyoD in the induction of myofibroblast differentiation by transforming growth factor-β1 (TGF-β1). Myofibroblasts demonstrate the capacity for de-differentiation and proliferation by modulation of endogenous levels of MyoD. We propose a model of reciprocal signaling between TGF-β1/ALK5/MyoD and mitogen(s)/ERK-MAPK/CDKs that regulate myofibroblast differentiation and de-differentiation, respectively. Our studies provide the first evidence for MyoD in controlling myofibroblast activation and deactivation. Restricted capacity for de-differentiation of myofibroblasts may underlie the progressive nature of recalcitrant human fibrotic disorders.

Stat3 directly controls the expression of Tbx5, Nkx2.5, and GATA4 and is essential for cardiomyocyte differentiation of P19CL6 cells

The transcription factor Stat3 (signal transducer and activator of transcription 3) mediates many physiological processes, including embryogenesis, stem cell self-renewal, and postnatal survival. In response to gp130 receptor activation, Stat3 becomes phosphorylated by the receptor-associated Janus kinase, forms dimers, and enters the nucleus where it binds to Stat3 target genes and regulates their expression. In this report, we demonstrate that Stat3 binds directly to the promoters and regulates the expression of three genes that are essential for cardiac differentiation: Tbx5, Nkx2.5, and GATA4. We further demonstrate that Tbx5, Nkx2.5, and GATA4 expression is dependent on Stat3 in response to ligand treatment and during ligand-independent differentiation of P19CL6 cells into cardiomyocytes. Finally, we show that Stat3 is necessary for the differentiation of P19CL6 cells into beating cardiomyocytes. All together, these results demonstrate that Stat3 is required for the differentiation of cardiomyocytes through direct transcriptional regulation of Tbx5, Nkx2.5, and GATA4.

A bHLH code for cardiac morphogenesis

Cell specification and differentiation of cardiomyocytes from mesodermal precursors is orchestrated by epigenetic and transcriptional inputs throughout heart formation. Of the many transcription factor super families that play a role in this process, the basic Helix-loop Helix (bHLH) family of proteins is well represented. The bHLH protein by design allows for dimerization-both as homodimers and heterodimers with other proteins within the family. Although DNA binding is mediated via a short variable cis-element termed an E-box, it is clear that DNA-affinity for these elements as well as the transcriptional input conveyed is dictated largely by the transcriptional partners within the dimer complex. Dimer partner choice has a number of inputs requiring co-expression within a given cell nucleus and dimerization modulation by the level of protein present, and post-translational modifications that can both enhance or reduce protein-protein interactions. Due to these complex interrelationships, it has been difficult to identity bona-fide downstream transcriptional targets and define the molecular pathways regulated of bHLH factors within cardiogenesis, despite the clear roles suggested via loss-of-function animals models. This review focuses on the Hand bHLH proteins-key members of the Twist-family of bHLH factors. Despite over a decade of investigation, questions regarding functional redundancy, downstream targets, and biological role during heart specification and differentiation have still not been fully addressed. Our goal is to review what is currently known and address strategies for gaining further understanding of Hand/Twist gene dosage and functional redundancy relationships within the developing heart that may underlie congenital heart defect pathogenesis.

Integrative genomics identifies DSCR1 (RCAN1) as a novel NFAT-dependent mediator of phenotypic modulation in vascular smooth muscle cells

Vascular smooth muscle cells (SMCs) display remarkable phenotypic plasticity in response to environmental cues. The nuclear factor of activated T-cells (NFAT) family of transcription factors plays a critical role in vascular pathology. However, known functional NFAT gene targets in vascular SMCs are currently limited. Publicly available whole-genome expression array data sets were analyzed to identify differentially expressed genes in human, mouse and rat SMCs. Comparison between vehicle and phenotypic modulatory stimuli identified 63 species-conserved, upregulated genes. Integration of the 63 upregulated genes with an in silico NFAT-ome (a species-conserved list of gene promoters containing at least one NFAT binding site) identified 18 putative NFAT-dependent genes. Further intersection of these 18 potential NFAT target genes with a mouse in vivo vascular injury microarray identified four putative NFAT-dependent, injury-responsive genes. In vitro validations substantiated the NFAT-dependent role of Cyclooxygenase 2 (COX2/PTGS2) in SMC phenotypic modulation and uncovered Down Syndrome Candidate Region 1 (DSCR1/RCAN1) as a novel NFAT target gene in SMCs. We show that induction of DSCR1 inhibits calcineurin/NFAT signaling through a negative feedback mechanism; DSCR1 overexpression attenuates NFAT transcriptional activity and COX2 protein expression, whereas knockdown of endogenous DSCR1 enhances NFAT transcriptional activity. Our integrative genomics approach illustrates how the combination of publicly available gene expression arrays, computational databases and empirical research methods can answer specific questions in any cell type for a transcriptional network of interest. Herein, we report DSCR1 as a novel NFAT-dependent, injury-inducible, early gene that may serve to negatively regulate SMC phenotypic switching.

Differentiation of human adipose tissue SVF cells into cardiomyocytes

Progenitor cells have been extensively studied and therapeutically applied in tissue reconstructive therapy. Stromal vascular fraction (SVF) cells, which are derived from adipose tissue, may represent a potential source of the cells which undergo phenotypical differentiation into many lineages both in vitro as well as in vivo. The goal of this study was to check whether human SVF cells may differentiate into cardiomyocyte-like entities. Human SVF cells were induced to differentiate by their incubation in Methocult medium in the presence of SCF, IL-3 and IL-6. Morphological transformation of the cells was monitored using optical light microscope, whereas changes in expression of the genes typical for cardiac phenotype were measured by qRT-PCR. Incubation of the human SVF cells in the medium that promotes cardiomyocyte differentiation in vitro resulted in formation of myotubule-like structures accompanied by up-regulation of the myocardium-characteristic genes, such as GATA, MEF2C, MYOD1, but not ANP. Human SVF cells differentiate into cardiomyocyte-like cells in the presence of the certain set of myogenesis promoting cytokines.

In vitro evidence suggests that miR-133a-mediated regulation of uncoupling protein 2 (UCP2) is an indispensable step in myogenic differentiation

UCP2 and UCP3, two novel uncoupling proteins, are important regulators of energy expenditure and thermogenesis in various organisms. The striking disparity between UCP2 mRNA and protein levels in muscle tissues prompted initial speculation that microRNAs are implicated in the regulatory pathway of UCP2. We found, for the first time, that the repression of UCP2 expression in cardiac and skeletal muscle resulted from its targeting by a muscle-specific microRNA, miR-133a. Moreover, our findings illustrate a novel function of UCP2 as a brake for muscle development. We also show that MyoD can remove the braking role of UCP2 via direct up-regulation of miR-133a during myogenic differentiation. Taken together, our current work delineates a novel regulatory network employing MyoD, microRNA, and uncoupling proteins to fine-tune the balance between muscle differentiation and proliferation during myogenesis.

Regulation of myocardin factor protein stability by the LIM-only protein FHL2

Extensive evidence indicates that serum response factor (SRF) regulates muscle-specific gene expression and that myocardin family SRF cofactors are critical for smooth muscle cell differentiation. In a yeast two hybrid screen for novel SRF binding partners expressed in aortic SMC, we identified four and a half LIM domain protein 2 (FHL2) and confirmed this interaction by GST pull-down and coimmunoprecipitation assays. FHL2 also interacted with all three myocardin factors and enhanced myocardin and myocardin-related transcription factor (MRTF)-A-dependent transactivation of smooth muscle alpha-actin, SM22, and cardiac atrial natriuretic factor promoters in 10T1/2 cells. The expression of FHL2 increased myocardin and MRTF-A protein levels, and, importantly, this effect was due to an increase in protein stability not due to an increase in myocardin factor mRNA expression. Treatment of cells with proteasome inhibitors MG-132 and lactacystin strongly upregulated endogenous MRTF-A protein levels and resulted in a substantial increase in ubiquitin immunoreactivity in MRTF-A immunoprecipitants. Interestingly, the expression of FHL2 attenuated the effects of RhoA and MRTF-B on promoter activity, perhaps through decreased MRTF-B nuclear localization or decreased SRF-CArG binding. Taken together, these data indicate that myocardin factors are regulated by proteasome-mediated degradation and that FHL2 regulates SRF-dependent transcription by multiple mechanisms, including stabilization of myocardin and MRTF-A.

Motif discovery in tissue-specific regulatory sequences using directed information

Motif discovery for the identification of functional regulatory elements underlying gene expression is a challenging problem. Sequence inspection often leads to discovery of novel motifs (including transcription factor sites) with previously uncharacterized function in gene expression. Coupled with the complexity underlying tissue-specific gene expression, there are several motifs that are putatively responsible for expression in a certain cell type. This has important implications in understanding fundamental biological processes such as development and disease progression. In this work, we present an approach to the identification of motifs (not necessarily transcription factor sites) and examine its application to some questions in current bioinformatics research. These motifs are seen to discriminate tissue-specific gene promoter or regulatory regions from those that are not tissue-specific. There are two main contributions of this work. Firstly, we propose the use of directed information for such classification constrained motif discovery, and then use the selected features with a support vector machine (SVM) classifier to find the tissue specificity of any sequence of interest. Such analysis yields several novel interesting motifs that merit further experimental characterization. Furthermore, this approach leads to a principled framework for the prospective examination of any chosen motif to be discriminatory motif for a group of coexpressed/coregulated genes, thereby integrating sequence and expression perspectives. We hypothesize that the discovery of these motifs would enable the large-scale investigation for the tissue-specific regulatory role of any conserved sequence element identified from genome-wide studies.

RhoA/Rho kinase blocks muscle differentiation via serine phosphorylation of insulin receptor substrate-1 and -2

Although the RhoA/Rho kinase (RhoA/ROK) pathway has been extensively investigated, its roles and downstream signaling pathways are still not well understood in myogenic processes. Therefore, we examined the effects of RhoA/ROK on myogenic processes and their signaling molecules using H9c2 and C2C12 cells. Increases in RhoA/ROK activities and serine phosphorylation levels of insulin receptor substrate (IRS)-1 (Ser307 and Ser636/639) and IRS-2 were found in proliferating myoblasts, whereas IRS-1/2 tyrosine phosphorylation and phosphatidylinositol (PI) 3-kinase activity increased during the differentiation process. ROK strongly bound to IRS-1/2 in proliferation medium but dissociated from them in differentiation medium (DM). ROK inactivation by a ROK inhibitor, Y27632, or a dominant-negative ROK, decreased IRS-1/2 serine phosphorylation with increases in IRS-1/2 tyrosine phosphorylation and PI 3-kinase activity, which led to muscle differentiation even in proliferation medium. Inhibition of ROK also enhanced differentiation in DM. ROK activation by a constitutive active ROK blocked muscle differentiation with the increased IRS-1/2 serine phosphorylation, followed by decreases in IRS-1/2 tyrosine phosphorylation and PI 3-kinase activity in DM. Interestingly, fibroblast growth factor-2 added to DM also blocked muscle differentiation through RhoA/ROK activation. Fibroblast growth factor-2 blockage of muscle differentiation was reversed by Y27632. Collectively, these results suggest that the RhoA/ROK pathway blocks muscle differentiation by phosphorylating IRS proteins at serine residues, resulting in the decreased IRS-1/2 tyrosine phosphorylation and PI 3-kinase activity. The absence of the inhibitory effects of RhoA/ROK in DM due to low concentrations of myogenic inhibitory growth factors seems to allow IRS-1/2 tyrosine phosphorylation, which stimulates muscle differentiation via transducing normal myogenic signaling.

Suppression of c-Src activity stimulates muscle differentiation via p38 MAPK activation

Role of c-Src in muscle differentiation has been controversial. Here, we investigated if c-Src positively or negatively regulates muscle differentiation, using H9c2 and C2C12 cell lines. Inhibition of c-Src by treatment with PP1 and SU6656, pharmacologic inhibitors of Src family kinases, or by expression of a dominant negative c-Src, all induced muscle differentiation in proliferation medium (PM). In differentiating cells in differentiation medium (DM), c-Src activity gradually decreased and reached basal level 3 days after induction of differentiation. Inhibition of c-Src suppressed Raf/MEK/ERK pathway but activated p38 MAPK. Inhibition of p38 MAPK did not affect c-Src activity in PM. However, it reactivated Raf/MEK/ERK pathway in c-Src-inhibited cells regardless of PM or DM. Concomitant inhibition of c-Src and p38 MAPK activities blocked muscle differentiation in both media. In conclusion, suppression of c-Src activity stimulates muscle differentiation by activating p38 MAPK uni-directionally.

Expression of the rat connexin 39 (rCx39) gene in myoblasts and myotubes in developing and regenerating skeletal muscles: an in situ hybridization study

We report a detailed analysis of the expression pattern of the recently identified rat connexin gene, named rat connexin 39 (rCx39), both during embryonic development and in adult life. Qualitative and quantitative reverse transcription/polymerase chain reaction analysis showed intense expression of rCx39 restricted to differentiating skeletal muscles, with a peak of expression detected at 18 days of embryonic life, followed by a rapid decline to undetectable levels within the first week of postnatal life. A combination of the in situ hybridization technique for the detection of rCx39 mRNA and immunohistochemistry for myogenin, a myoblast-specific marker, allowed us to establish that the mRNA for this connexin was expressed in myogenin-positive myoblasts and early myotubes but disappeared in mature myotubes. Moreover, in adult animals, rCx39 mRNA was expressed in myogenic cells involved in skeletal myofiber regeneration following a crush injury. This is the first case of a connexin being mainly expressed in the myogenic cell lineage. The information presented should pave the way to novel molecular approaches in studies on the role of connexin-based gap-junctional communication in skeletal muscle differentiation and regeneration.

Altered Twist1 and Hand2 dimerization is associated with Saethre-Chotzen syndrome and limb abnormalities

Autosomal dominant mutations in the gene encoding the basic helix-loop-helix transcription factor Twist1 are associated with limb and craniofacial defects in humans with Saethre-Chotzen syndrome. The molecular mechanism underlying these phenotypes is poorly understood. We show that ectopic expression of the related basic helix-loop-helix factor Hand2 phenocopies Twist1 loss of function in the limb and that the two factors have a gene dosage-dependent antagonistic interaction. Dimerization partner choice by Twist1 and Hand2 can be modulated by protein kinase A- and protein phosphatase 2A-regulated phosphorylation of conserved helix I residues. Notably, multiple Twist1 mutations associated with Saethre-Chotzen syndrome alter protein kinase A-mediated phosphorylation of Twist1, suggesting that misregulation of Twist1 dimerization through either stoichiometric or post-translational mechanisms underlies phenotypes of individuals with Saethre-Chotzen syndrome.

Combinatorial transcriptional interaction within the cardiac neural crest: a pair of HANDs in heart formation

The cardiac neural crest cells migrate from the rostral dorsal neural folds and populate the branchial arches, which contribute directly to the cardiac-outflow structures. Although neural crest cell specification is associated with a number of morphogenic factors, little is understood about the mechanisms by which transcription factors actually implement the transcriptional programs that dictate cell migration and later the differentiation into the proper cell types within the great vessels and the heart. It is clear from genetic evidence that members of the paired box family and basic helix-loop-helix (bHLH) transcription factors from the twist family of proteins are expressed in and play an important function in cardiac neural crest specification and differentiation. Interestingly, both paired box and bHLH factors can function as dimers and, in the case of twist family bHLH factors, partner choice can clearly dictate a change in transcriptional program. The focus of this review is to consider what role the protein-protein interactions of these transcription factors may play in determining cardiac neural crest specification and differentiation, and how genetic alteration of transcription factor stoichiometry within the cell may reflect more than a simple null event.

Ectopic Expression of p73α, but Not p73β, Suppresses Myogenic Differentiation

The TRP73 gene, a member of the p53 family, encodes several variants through differential splicing and use of alternative promoters. At the N terminus, two different promoters generate the full-length and the DeltaN isoforms, with or without the transactivating domain. At the C terminus, seven isoforms generated through alternative splicing have been cloned. Previous studies have demonstrated that DeltaN-p73 interferes with p73-induced apoptosis. However, there has been no evidence for functional diversity of the C-terminal p73 variants. In this study, we found that p73alpha and p73beta exerted differential effect on the differentiation of C2C12 myoblasts. Although p73beta lacked any detectable effect on differentiation, p73alpha caused a substantial delay in the expression of muscle-specific genes. In co-transfection experiments p73alpha, but not p73beta, attenuated the transcriptional activity of MyoD. Microarray-based gene profiling confirmed the protraction of MyoD-dependent gene expression in C2C12 cells stably expressing p73alpha. Notwithstanding the differential effect on differentiation, p73alpha and p73beta showed similar activity in sensitizing C2C12 myoblasts to cisplatin-induced cell death. These results demonstrated a functional diversity between the two C-terminal variants of p73 and suggested that p73alpha can regulate cellular differentiation in addition to its role in stimulating cell death.

Transcriptional regulation of cardiac conduction system development: 2004 FASEB cardiac conduction system minimeeting, Washington, DC

The development of the complex network of specialized cells that form the atrioventricular conduction system (AVCS) during cardiac morphogenesis occurs by progressive recruitment within a multipotent cardiomyogenic lineage. Understanding the molecular control of this developmental process has been the focus of recent research. Transcription factors representative of multiple subfamilies have been identified and include members of zinc-finger subfamilies (GATA4, GATA6 HF-1b), skeletal muscle transcription factors (MyoD), T-box genes (Tbx5), and also homeodomain transcription factors (Msx2 and Nkx2.5). Mutations in some of these transcription factors cause congenital heart disease and are associated with cardiac abnormalities, including deficits within the AVCS. Mouse models that closely phenocopy known human heart disease provide powerful tools for the study of molecular effectors of AVCS development. Indeed, investigations of the Nkx2.5 haploinsufficient mouse have shown that peripheral Purkinje fibers are significantly underrepresented. This piece of data corroborates our previous work showing in chick, mouse, and humans that Nkx2.5 is elevated in the differentiating AVCS relative to adjacent working ventricular myocardial tissues. Using the chick embryo as a model, we show that this elevation of Nkx2.5 is transient in the network of conduction cells comprising the peripheral Purkinje fiber system. Functional studies using defective adenoviral constructs, which disrupt the normal variation in level of this gene, result in perturbations of Purkinje fiber phenotype. Thus, the precise spatiotemporal regulation of Nkx2.5 levels during development may be required for the progressive emergence of gene expression patterns specific to differentiated Purkinje fiber cells.

Resistance exercise alters MRF and IGF-I mRNA content in human skeletal muscle

Increasing evidence suggests that the myogenic regulatory factors (MRFs) and IGF-I have important roles in the hypertrophy response observed after mechanical loading. We, therefore, hypothesized that a bout of heavy-resistance training would affect the MRF and IGF-I mRNA levels in human skeletal muscle. Six male subjects completed four sets of 6-12 repetitions on a leg press and knee extensor machine separated by 3 min. Myogenin, MRF4, MyoD, IGF-IEabc (isoforms a, b, and c) and IGF-IEbc (isoform b and c) mRNA levels were determined in the vastus lateralis muscle by RT-PCR before exercise, immediately after, and 1, 2, 6, 24, and 48 h postexercise. Myogenin, MyoD, and MRF4 mRNA levels were elevated (P < 0.005) by 100-400% 0-24 h postexercise. IGF-IEabc mRNA content decreased (P < 0.005) by approximately 44% after 1 and 6 h of recovery. The IGF-IEbc mRNA level was unaffected. The present study shows that myogenin, MyoD, and MRF4 mRNA levels are transiently elevated in human skeletal muscle after a single bout of heavy-resistance training, supporting the idea that the MRFs may be involved in regulating hypertrophy and/or fiber-type transitions. The results also suggest that IGF-IEa expression may be downregulated at the mRNA level during the initial part of recovery from resistance exercise.

Activation of p38 MAPK induces cell cycle arrest via inhibition of Raf/ERK pathway during muscle differentiation

Cell cycle arrest is essential for initiation of muscle differentiation in myoblasts. Given the previously described essential role for p38 MAPK in myogenesis, we undertook the present study to investigate the role of p38 MAPK in the cell cycle arrest that initiates muscle differentiation. p38 MAPK activity increased during, and was required for, muscle differentiation. Inhibition of p38 MAPK stimulated Raf and ERK activities, and induced cell proliferation in differentiation medium. The concomitant inhibition of p38 MAPK and ERK, however, failed to induce differentiation or proliferation. In conclusion, inhibition of the Raf/ERK pathway and the consequent cell cycle arrest is one of the major functions of p38 MAPK during muscle differentiation.

The basic-helix-loop-helix transcription factor HAND2 directly regulates transcription of the atrial naturetic peptide gene

The HAND basic Helix-Loop-Helix (bHLH) transcription factors are essential for the development of heart and extra embryonic structures. Although essential for embryonic development, the molecular pathways in which HAND factors participate are poorly understood. In efforts to identify downstream transcriptional targets, we have determined that HAND2 regulates the transcription of the Atrial Naturetic Peptide (ANP) gene. Results show that ANP expression is reduced in HAND2 null mice. Transactivation assays show significant transcriptional upregulation of ANP by HAND2 and cotransfection experiments using HAND2 and E12 suggest that an E-protein/HAND heterodimer is the likely trans -acting complex. The required cis -elements reside within a 258bp proximal region that contains three evolutionarily conserved Ebox consensus sites. Surprisingly, mutations in these three sites suggest HAND2 activity is DNA-binding independent. In addition, HAND2 and the homeobox factor Nkx2.5 exhibit transcriptional synergy in the regulation of ANP. Taken together, this data shows that HAND2 is an upstream transcriptional regulator of ANP expression, and furthermore HAND2 can synergistically interact with Nkx2.5, showing a functional relationship between HAND2 and Nkx2.5 supporting the genetic observation, that mice null for both HAND2 and Nkx2.5 lack ventricle specification.

New frontiers in molecular pediatric cardiology

Numerous advances in understanding the molecular basis of congenital heart disease have been published in the past year. Highlights are reviewed, focusing on two major topics: genetic syndromes and cardiac organogenesis. Genetic syndromes are discussed in the context of complementary data from targeted mutations in animals and genetic mapping studies in humans. These include the DiGeorge, Holt-Oram, Alagille, familial primary pulmonary hypertension, and Noonan syndromes. Novel concepts in cardiac organogenesis are discussed, including the existence and contribution of an anterior heart field to the developing cardiac outflow tract, novel cell-cell signaling involving migrating neural crest, the origins of the conduction system and initial embryonic heartbeat, and the possibility of a population of cardiac stem cells in the adult heart. The studies reviewed have potential clinical relevance in the near future and will be of interest to the clinician interested in congenital heart disease.

Precocious expression of cardiac troponin T in early chick embryos is independent of bone morphogenetic protein signaling

Cardiac troponin T (cTNT) is a component of the troponin complex, which confers calcium sensitivity to contraction in skeletal and cardiac muscle. Although it is thought that most components of the contractile myofibril are expressed exclusively in differentiated muscle cells, we observed that mRNAs coding for cTNT were detectable in explanted late gastrula mesoderm at least 12 hr before cardiac myocyte differentiation. We therefore conducted a detailed analysis of cTNT gene expression in the early chick embryo. Whole-mount in situ hybridization studies showed that by Hamburger and Hamilton stage 5, cTNT mRNAs are detectable in lateral mesoderm and, by stage 6, are observed throughout the lateral embryonic and extraembryonic mesoderm in a distribution that is much broader than the recognized heart field. As myocardial cell differentiation commences, cTNT transcripts become progressively localized to the forming heart and, by stage 14, are completely restricted to heart muscle cells. Western blot analyses demonstrated that cTNT protein expression is under translational control, as cTNT protein is not detectable until stage 9, concomitant with myocardial cell differentiation. Removal of endoderm at stage 5 had no effect on cTNT mRNA levels, and the bone morphogenetic protein (BMP) inhibitor noggin failed to block cTNT expression, even in the heart-forming region and in cases where heart formation was inhibited. Implantation of noggin-expressing CHO cells at the anterior midline of stage 7 embryos resulted in cardia bifida. These findings demonstrate the precocious, BMP-independent expression of a gene coding for a myofibrillar protein and suggest that an additional regulatory pathway exists for activation of some cardiogenic genes.

Diaphragm contractile dysfunction in MyoD gene-inactivated mice

MyoD is one of four myogenic regulatory factors found exclusively in skeletal muscle. In an effort to better understand the role that MyoD plays in determining muscle contractile properties, we examined the effects of MyoD deletion on both diaphragmatic contractile properties and myosin heavy chain (MHC) phenotype. Regions of the costal diaphragm from wild-type and MyoD knockout [MyoD (-/-)] adult male BALB/c mice (n = 8/group) were removed, and in vitro diaphragmatic contractile properties were measured. Diaphragmatic contractile measurements revealed that MyoD (-/-) animals exhibited a significant (P < 0.05) downward shift in the force-frequency relationship, a decrement in maximal specific tension (P(o); -33%), a decline in maximal shortening velocity (V(max); -37%), and concomitant decrease in peak power output (-47%). Determination of MHC isoforms in the diaphragm via gel electrophoresis revealed that MyoD elimination resulted in a fast-to-slow shift (P < 0.05) in the MHC phenotype toward MHC types IIA and IIX in MyoD (-/-) animals. These data indicate that MyoD deletion results in a decrease in diaphragmatic submaximal force generation and P(o), along with decrements in both V(max) and peak power output. Hence, MyoD plays an important role in determining diaphragmatic contractile properties.

TFEC can function as a transcriptional activator of the nonmuscle myosin II heavy chain-A gene in transfected cells

Transcription of the human nonmuscle myosin II heavy chain-A (NMHC-A) gene is regulated via multiple elements located in intron 1, including element F which contains an E-box. In this study we have identified and characterized the factors that are capable of binding to element F. Yeast one-hybrid screening using element F allowed isolation of cDNAs encoding transcriptional factors TFEC, TFE3, and USF2, each of which contains basic helix-loop-helix and leucine zipper motifs. Furthermore, cDNA cloning by polymerase chain reaction yielded cDNAs for two TFEC isoforms, designated TFEC-l and TFEC-s, which are generated by alternative pre-mRNA splicing. In addition to these four factors, USF1, which is known to share the same DNA binding elements with USF2, was isolated for comparison. Electrophoretic mobility shift assays and cotransfection studies of the expression constructs with reporter gene constructs revealed that the above five factors have different binding activities for element F with different transactivation potencies. USF1 and USF2 demonstrate the highest binding activity to element F, yet show the lowest element F-dependent transactivation. TFE3 has a high transactivation potency but the lowest binding activity. TFEC-l demonstrates a high binding activity with the highest transactivation potency, whereas TFEC-s has the same binding activity as TFEC-l with intermediate transactivation. We also demonstrate that an N-terminal activation domain exists only in TFEC-l, whereas a C-terminal activation domain is common to both the l and s isoforms. This study provides the first evidence of TFEC being an activator of transcription, with two separate activation domains.

Upstream stimulating factor-1 (USF1) and USF2 bind to and activate the promoter of the adenomatous polyposis coli (APC) tumor suppressor gene

The adenomatous polyposis coli (APC) gene product is involved in cell cycle arrest and apoptosis, and loss of function is associated with the development of colorectal carcinogenesis. Although it has been demonstrated that the APC gene is inducible, its transcriptional regulation has not been elucidated. Therefore, we characterized the promoter region of the APC gene and transcription factors required for basal expression. The APC gene has a TATA-less promoter and contains consensus binding sites for Octamer, AP2, Sp1, a CAAT-box, and three nucleotide sequences for E-box A, B, and M. The E-boxes are functional in several cancer cell lines and upstream stimulating factor-1 (USF1) and USF2 interact with these sites, with a preferred sequence-specificity for the B site. Analysis of activation of the cloned APC promoter by USF1 and USF2 in transient transfection assays in HCT-116 cells demonstrated that mutation of the E-box B site completely abolished the basal promoter activity. Further, the ectopic USF1 and USF2 expression in HCT-116 cells with deletion mutations of E-box A, B, and M sites showed that these E-boxes contribute to USF1- and USF2-mediated transcriptional activation of the APC promoter, with maximum promoter activity being associated with the E-box B site. Thus, USF1 and USF2 transcription factors are critical for APC gene expression.

Genes that control the development of migrating muscle precursor cells

Skeletal muscles in vertebrates, despite their functional and biochemical similarities, are generated via diverse developmental mechanisms. A major subclass of hypaxial muscle groups is derived from long-range migrating progenitor cells that delaminate from the dermomyotome. The development of this lineage is controlled by Pax3, the c-Met tyrosine kinase receptor, its ligand SF/HGF (scatter factor/hepatocyte growth factor) and the homeobox factor Lbx1. These molecules are essential for establishment of the precursor pool, delamination, migration and target finding. Progress has been made in understanding patterning of the muscles, which requires a precise control of proliferation and differentiation of myogenic precursor cells.

Molecular cloning and functional analysis of the promoter region of rat nonmuscle myosin heavy chain-B gene

Rat nonmuscle myosin heavy chain-B (r-nmMHC-B) mRNA was previously found downregulated in Rat 6 fibroblasts transformed by mutant p53(val135) [J. W. P. Yam, J. Y. Zheng, and W. L. W. Hsiao (1987) Biochem. Biophys. Res. Commun. 266, 472-480]. Overexpression of exogenous r-nmMHC-B could partially reverse the transforming phenotypes both in vitro and in vivo. The downregulation of r-nmMHC-B was also observed in Rat 6 transformed by c-H-ras and v-myc oncogenes. We cloned a 5.2-kb r-nmMHC-B promoter region. Sequence analysis of -1248 to +1 revealed no TATA box, but did show that it contained CAAT boxes, E12/E47, MyoD, MEF, E2F, CREB, and SP1 binding sites. Based on transient reporter assays, the promoter/enhancer activities were unusually extended to the entire 5.2 kb region in normal Rat 6 cultures, but markedly suppressed in p53(val135)-, and c-H-ras-transformed cells. The activity detected by the reporter assay corresponded to levels of mRNA as analyzed previously by Northern blots in each respective cell line. Thus, the switch-off of the r-nmMHC-B in the transformed cells is very likely controlled by upstream transcriptional factors, which might have been altered in the course of neoplastic transformation.

Molecular characterization of the TrkA/NGF receptor minimal enhancer reveals regulation by multiple cis elements to drive embryonic neuron expression

Neural development relies on stringent regulation of key genes that mediate specialized function. TrkA is primarily expressed in neural crest-derived sensory and sympathetic neurons where it transmits critical survival information. We have identified a 457 base pair sequence upstream of the murine first TrkA coding exon that is conserved in human and in chick, and is sufficient for expression in the correct cells with appropriate timing. Mutation analysis of consensus transcription factor binding domains within the minimal enhancer reveals a complex positive regulation that includes sites required for global expression and sites that are specifically required for DRG, trigeminal or sympathetic expression. These results provide a foundation for identification of the transcriptional machinery that specifies neurotrophin receptor expression.

A fluorescent reporter gene as a marker for ventricular specification in ES-derived cardiac cells

We have established a CGR8 embryonic stem (ES) cell clone (MLC2ECFP) which expresses the enhanced cyan variant of Aequorea victoria green fluorescent protein (ECFP) under the transcriptional control of the ventricular myosin light chain 2 (MLC2v) promoter. Using epifluorescence imaging of vital embryoid bodies (EB) and reverse transcription-polymerase chain reaction (RT-PCR), we found that the MLC2v promoter is switched on as early as day 7 and is accompanied by formation of cell clusters featuring a bright ECFP blue fluorescence. The fluorescent areas within the EBs were all beating on day 8. MLC2ECFP ES cells showed the same time course of cardiac differentiation as mock ES cells as assessed by RT-PCR of genes encoding cardiac-specific transcription factors and contractile proteins. The MLC2v promoter conferred ventricular specificity to ECFP expression within the EB as revealed by MLC2v co-staining of ECFP fluorescent cells. MLC2ECFP-derived cardiac cells still undergo cell division on day 12 after isolation from EBs but withdraw from the cell cycle on day 16. This ES cell clone provides a powerful cell model to study the signalling roads of factors regulating cardiac cell proliferation and terminal differentiation with a view to using them for experimental cell therapy.