Transcription enhancer factor 1 binds multiple muscle MEF2 and A/T-rich elements during fast-to-slow skeletal muscle fiber type transitions

Crosstalk among canonical Wnt and Hippo pathway members in skeletal muscle and at the neuromuscular junction

Skeletal muscles are essential for locomotion, posture, and metabolic regulation. To understand physiological processes, exercise adaptation, and muscle-related disorders, it is critical to understand the molecular pathways that underlie skeletal muscle function. The process of muscle contraction, orchestrated by a complex interplay of molecular events, is at the core of skeletal muscle function. Muscle contraction is initiated by an action potential and neuromuscular transmission requiring a neuromuscular junction. Within muscle fibers, calcium ions play a critical role in mediating the interaction between actin and myosin filaments that generate force. Regulation of calcium release from the sarcoplasmic reticulum plays a key role in excitation-contraction coupling. The development and growth of skeletal muscle are regulated by a network of molecular pathways collectively known as myogenesis. Myogenic regulators coordinate the differentiation of myoblasts into mature muscle fibers. Signaling pathways regulate muscle protein synthesis and hypertrophy in response to mechanical stimuli and nutrient availability. Several muscle-related diseases, including congenital myasthenic disorders, sarcopenia, muscular dystrophies, and metabolic myopathies, are underpinned by dysregulated molecular pathways in skeletal muscle. Therapeutic interventions aimed at preserving muscle mass and function, enhancing regeneration, and improving metabolic health hold promise by targeting specific molecular pathways. Other molecular signaling pathways in skeletal muscle include the canonical Wnt signaling pathway, a critical regulator of myogenesis, muscle regeneration, and metabolic function, and the Hippo signaling pathway. In recent years, more details have been uncovered about the role of these two pathways during myogenesis and in developing and adult skeletal muscle fibers, and at the neuromuscular junction. In fact, research in the last few years now suggests that these two signaling pathways are interconnected and that they jointly control physiological and pathophysiological processes in muscle fibers. In this review, we will summarize and discuss the data on these two pathways, focusing on their concerted action next to their contribution to skeletal muscle biology. However, an in-depth discussion of the non-canonical Wnt pathway, the fibro/adipogenic precursors, or the mechanosensory aspects of these pathways is not the focus of this review.

Myosin 7b is a regulatory long noncoding RNA (lncMYH7b) in the human heart

Myosin heavy chain 7b (MYH7b) is an ancient member of the myosin heavy chain motor protein family that is expressed in striated muscles. In mammalian cardiac muscle, MYH7b RNA is expressed along with two other myosin heavy chains, β-myosin heavy chain (β-MyHC) and α-myosin heavy chain (α-MyHC). However, unlike β-MyHC and α-MyHC, which are maintained in a careful balance at the protein level, the MYH7b locus does not produce a full-length protein in the heart due to a posttranscriptional exon-skipping mechanism that occurs in a tissue-specific manner. Whether this locus has a role in the heart beyond producing its intronic microRNA, miR-499, was unclear. Using cardiomyocytes derived from human induced pluripotent stem cells as a model system, we found that the noncoding exon-skipped RNA (lncMYH7b) affects the transcriptional landscape of human cardiomyocytes, independent of miR-499. Specifically, lncMYH7b regulates the ratio of β-MyHC to α-MyHC, which is crucial for cardiac contractility. We also found that lncMYH7b regulates beat rate and sarcomere formation in cardiomyocytes. This regulation is likely achieved through control of a member of the TEA domain transcription factor family (TEAD3, which is known to regulate β-MyHC). Therefore, we conclude that this ancient gene has been repurposed by alternative splicing to produce a regulatory long-noncoding RNA in the human heart that affects cardiac myosin composition.

Transcriptional and posttranscriptional regulation of the SMC-selective blood pressure-associated gene, ARHGAP42

We previously showed that ARHGAP42 is a smooth muscle cell (SMC)-selective, RhoA-specific GTPase activating protein that regulates blood pressure and that a minor allele single nucleotide variation within a DNAse hypersensitive regulatory element in intron1 (Int1DHS) increased ARHGAP42 expression by promoting serum response factor binding. The goal of the current study was to identify additional transcriptional and posttranscriptional mechanisms that control ARHGAP42 expression. Using deletion/mutation, gel shift, and chromatin immunoprecipitation experiments, we showed that recombination signal binding protein for immunoglobulin κ-J region (RBPJ) and TEA domain family member 1 (TEAD1) binding to a conserved core region was required for full IntDHS transcriptional activity. Importantly, overexpression of the notch intracellular domain (NICD) or plating SMCs on recombinant jagged-1 increased IntDHS activity and endogenous ARHGAP42 expression while siRNA-mediated knockdown of TEAD1 inhibited ARHGAP42 mRNA levels. Re-chromatin immunoprecipitation experiments indicated that RBPJ and TEAD1 were bound to the Int1DHS enhancer at the same time, and coimmunoprecipitation assays indicated that these factors interacted physically. Our results also suggest TEAD1 and RBPJ bound cooperatively to the Int1DHS and that the presence of TEAD1 promoted the recruitment of NICD by RBPJ. Finally, we showed that ARHGAP42 expression was inhibited by micro-RNA 505 (miR505) which interacted with the ARHGAP42 3'-untranslated region (UTR) to facilitate its degradation and by AK124326, a long noncoding RNA that overlaps with the ARHGAP42 transcription start site on the opposite DNA strand. Since siRNA-mediated depletion of AK124326 was associated with increased H3K9 acetylation and RNA Pol-II binding at the ARHGAP42 gene, it is likely that AK124326 inhibits ARHGAP42 transcription.NEW & NOTEWORTHY First, RBPJ and TEAD1 converge at an intronic enhancer to regulate ARHGAP42 expression in SMCs. Second, TEAD1 and RBPJ interact physically and bind cooperatively to the ARHGAP42 enhancer. Third, miR505 interacts with the ARHGAP42 3'-UTR to facilitate its degradation. Finally, LncRNA, AK124326, inhibits ARHGAP42 transcription.

The NOTCH1-dependent HIF1α/VGLL4/IRF2BP2 oxygen sensing pathway triggers erythropoiesis terminal differentiation

Hypoxia is widely considered as a limiting factor in vertebrate embryonic development, which requires adequate oxygen delivery for efficient energy metabolism, while nowadays some researches have revealed that hypoxia can induce stem cells so as to improve embryonic development. Erythroid differentiation is the oxygen delivery method employed by vertebrates at the very early step of embryo development, however, the mechanism how erythroid progenitor cell was triggered into mature erythrocyte is still not clear. In this study, after detecting the upregulation of vgll4b in response to oxygen levels, we generated vgll4b mutant zebrafish using CRISPR/Cas9, and verified the resulting impaired heme and dysfunctional erythroid terminal differentiation phenotype. Neither the vgll4b-deficient nor the γ-secretase inhibitor IX (DAPT)-adapted zebrafish were able to mediate HIF1α-induced heme generation. In addition, we showed that vgll4b mutant zebrafish were associated with an impaired erythroid phenotype, induced by the downregulation of alas2, which could be rescued by irf2bp2 depletion. Further mechanistic studies revealed that zebrafish VGLL4 sequesters IRF2BP2, thereby inhibiting its repression of alas2 expression and heme biosynthesis. These processes occur primarily via the VGLL4 TDU1 and IRF2BP2 ring finger domains. Our study also indicates that VGLL4 is a key player in the mediation of NOTCH1-dependent HIF1α-regulated erythropoiesis and can be sensitively regulated by oxygen concentrations. On the other hand, VGLL4 is a pivotal regulator of heme biosynthesis and erythroid terminal differentiation, which collectively improve oxygen metabolism.

Transcriptional cofactor Vgll2 is required for functional adaptations of skeletal muscle induced by chronic overload

Skeletal muscle is composed of heterogeneous populations of myofibers classified as slow- and fast-twitch fibers. Myofiber size and composition are drastically changed in response to physiological demands. We previously showed that transcriptional cofactor vestigial-like (Vgll) 2 is a pivotal regulator of slow muscle gene programming under sedentary conditions. However, whether Vgll2 is required for skeletal muscle adaptations after chronic overload is unclear. Therefore, we investigated the role of Vgll2 in chronic overload-inducing skeletal muscle adaptations using synergist ablation (SA) on plantaris. We found that Vgll2 is an essential regulator of the switch towards a slow-contractile phenotype and oxidative metabolism during chronic overload. Mice lacking Vgll2 exhibited limited fiber type transition and downregulation of genes related to lactate metabolism and their regulator peroxisome proliferator-activated receptor gamma coactivator 1α1, after SA, was augmented in Vgll2-deficient mice compared with in wild-type mice. Mechanistically, increased muscle usage elevated Vgll2 levels and promoted the interaction between Vgll2 and its transcription partners such as TEA domain1 (TEAD1), MEF2c, and NFATc1. Calcium ionophore treatment promoted nuclear translocation of Vgll2 and increased TEAD-dependent MYH7 promotor activity in a Vgll2-dependent manner. Taken together, these data demonstrate that Vgll2 plays an important role for functional adaptation of skeletal muscle to chronic overload.

Vestigial-like 2 contributes to normal muscle fiber type distribution in mice

Skeletal muscle is composed of heterogeneous populations of myofibers that are classified as slow- and fast-twitch fibers. The muscle fiber-type is regulated in a coordinated fashion by multiple genes, including transcriptional factors and microRNAs (miRNAs). However, players involved in this regulation are not fully elucidated. One of the members of the Vestigial-like factors, Vgll2, is thought to play a pivotal role in TEA domain (TEAD) transcription factor-mediated muscle-specific gene expression because of its restricted expression in skeletal muscles of adult mice. Here, we generated Vgll2 null mice and investigated Vgll2 function in adult skeletal muscles. These mice presented an increased number of fast-twitch type IIb fibers and exhibited a down-regulation of slow type I myosin heavy chain (MyHC) gene, Myh7, which resulted in exercise intolerance. In accordance with the decrease in Myh7, down-regulation of miR-208b, encoded within Myh7 gene and up-regulation of targets of miR-208b, Sox6, Sp3, and Purβ, were observed in Vgll2 deficient mice. Moreover, we detected the physical interaction between Vgll2 and TEAD1/4 in neonatal skeletal muscles. These results suggest that Vgll2 may be both directly and indirectly involved in the programing of slow muscle fibers through the formation of the Vgll2-TEAD complex.

Muscle plasticity related to changes in tubulin and αB-crystallin levels induced by eccentric contraction in rat skeletal muscles

We used the model of eccentric contraction of the hindlimb muscle by Ochi et al. to examine the role of eccentric contraction in muscle plasticity. This model aims to focus on stimulated skeletal muscle responses by measuring tissue weights and tracing the quantities of αB-crystallin and tubulin. The medial gastrocnemius muscle (GCM) responded to electrically induced eccentric contraction (EIEC) with significant increases in tissue weight (p < 0.01) and the ratio of tissue weight to body weight (p < 0.05); however, there was a decrease in soleus muscle weight after EIEC. EIEC in the GCM caused contractile-induced sustenance of the traced proteins, but the soleus muscle exhibited a remarkable decrease in α-tubulin and a 19% decrease in αB-crystallin. EIEC caused fast-to-slow myosin heavy chain (MHC) isoform type-oriented shift within both the GCM and soleus muscle. These results have shown that different MHC isoform type-expressing slow and fast muscles commonly undergo fast-to-slow type MHC isoform transformation. This suggests that different levels of EIEC affected each of the slow and fast muscles to induce different quantitative changes in the expression of αB-crystallin and α-tubulin.

The Hippo signal transduction network for exercise physiologists

The ubiquitous transcriptional coactivators Yap (gene symbol Yap1) and Taz (gene symbol Wwtr1) regulate gene expression mainly by coactivating the Tead transcription factors. Being at the center of the Hippo signaling network, Yap and Taz are regulated by the Hippo kinase cassette and additionally by a plethora of exercise-associated signals and signaling modules. These include mechanotransduction, the AKT-mTORC1 network, the SMAD transcription factors, hypoxia, glucose homeostasis, AMPK, adrenaline/epinephrine and angiotensin II through G protein-coupled receptors, and IL-6. Consequently, exercise should alter Hippo signaling in several organs to mediate at least some aspects of the organ-specific adaptations to exercise. Indeed, Tead1 overexpression in muscle fibers has been shown to promote a fast-to-slow fiber type switch, whereas Yap in muscle fibers and cardiomyocytes promotes skeletal muscle hypertrophy and cardiomyocyte adaptations, respectively. Finally, genome-wide association studies in humans have linked the Hippo pathway members LATS2, TEAD1, YAP1, VGLL2, VGLL3, and VGLL4 to body height, which is a key factor in sports.

HDAC4-myogenin axis as an important marker of HD-related skeletal muscle atrophy

Skeletal muscle remodelling and contractile dysfunction occur through both acute and chronic disease processes. These include the accumulation of insoluble aggregates of misfolded amyloid proteins that is a pathological feature of Huntington’s disease (HD). While HD has been described primarily as a neurological disease, HD patients’ exhibit pronounced skeletal muscle atrophy. Given that huntingtin is a ubiquitously expressed protein, skeletal muscle fibres may be at risk of a cell autonomous HD-related dysfunction. However the mechanism leading to skeletal muscle abnormalities in the clinical and pre-clinical HD settings remains unknown. To unravel this mechanism, we employed the R6/2 transgenic and HdhQ150 knock-in mouse models of HD. We found that symptomatic animals developed a progressive impairment of the contractile characteristics of the hind limb muscles tibialis anterior (TA) and extensor digitorum longus (EDL), accompanied by a significant loss of motor units in the EDL. In symptomatic animals, these pronounced functional changes were accompanied by an aberrant deregulation of contractile protein transcripts and their up-stream transcriptional regulators. In addition, HD mouse models develop a significant reduction in muscle force, possibly as a result of a deterioration in energy metabolism and decreased oxidation that is accompanied by the re-expression of the HDAC4-DACH2-myogenin axis. These results show that muscle dysfunction is a key pathological feature of HD.

Huntington’s disease (HD) is a neurodegenerative disorder in which the mutation results in an extra-long tract of glutamines that causes the huntingtin protein to aggregate. It is characterized by neurological symptoms and brain pathology, which is associated with nuclear and cytoplasmic protein aggregates and with transcriptional deregulation. Despite the fact that HD has been recognized principally as a neurological disease, there are multiple studies indicating that peripheral pathologies including cardiac dysfunction and skeletal muscle atrophy, contribute to the overall progression of HD. To unravel the cause of the skeletal muscle dysfunction, we applied a wide range of molecular and physiological methods to the analysis of two well established genetic mouse models of this disease. We found that symptomatic animals developed muscle dysfunction characterised by a change in the contractile characteristics of fast twitch muscles and a decrease in twitch and tetanic force of hindlimb muscles. In addition, there is a significant decrease in the number of motor units innervating the EDL muscle, and this motor unit loss progresses during the course of the disease. These changes were accompanied by the re-expression of contractile transcripts and markers of muscle denervation such as the HDAC4-Dach2-myogenin axis, as well as the apparent deterioration in energy metabolism and decreased oxidation. Therefore, we conclude, that the HD-related skeletal muscle atrophy is accompanied by progressive loss of functional motor units.

Thyroid hormone regulates muscle fiber type conversion via miR-133a1

Thyroid hormone promotes slow-to-fast muscle fiber type conversion by inducing miR-133a1 and thereby repressing the expression of the slow muscle determinant TEAD1.

It is known that thyroid hormone (TH) is a major determinant of muscle fiber composition, but the molecular mechanism by which it does so remains unclear. Here, we demonstrated that miR-133a1 is a direct target gene of TH in muscle. Intriguingly, miR-133a, which is enriched in fast-twitch muscle, regulates slow-to-fast muscle fiber type conversion by targeting TEA domain family member 1 (TEAD1), a key regulator of slow muscle gene expression. Inhibition of miR-133a in vivo abrogated TH action on muscle fiber type conversion. Moreover, TEAD1 overexpression antagonized the effect of miR-133a as well as TH on muscle fiber type switch. Additionally, we demonstrate that TH negatively regulates the transcription of myosin heavy chain I indirectly via miR-133a/TEAD1. Collectively, we propose that TH inhibits the slow muscle phenotype through a novel epigenetic mechanism involving repression of TEAD1 expression via targeting by miR-133a1. This identification of a TH-regulated microRNA therefore sheds new light on how TH achieves its diverse biological activities.

Essential role of TEA domain transcription factors in the negative regulation of the MYH 7 gene by thyroid hormone and its receptors

MYH7 (also referred to as cardiac myosin heavy chain β) gene expression is known to be repressed by thyroid hormone (T3). However, the molecular mechanism by which T3 inhibits the transcription of its target genes (negative regulation) remains to be clarified, whereas those of transcriptional activation by T3 (positive regulation) have been elucidated in detail. Two MCAT (muscle C, A, and T) sites and an A/T-rich region in the MYH7 gene have been shown to play a critical role in the expression of this gene and are known to be recognized by the TEAD/TEF family of transcription factors (TEADs). Using a reconstitution system with CV-1 cells, which has been utilized in the analysis of positive as well as negative regulation, we demonstrate that both T3 receptor (TR) β1 and α1 inhibit TEAD-dependent activation of the MYH7 promoter in a T3 dose-dependent manner. TRβ1 bound with GC-1, a TRβ-selective T3 analog, also repressed TEAD-induced activity. Although T3-dependent inhibition required the DNA-binding domain (DBD) of TRβ1, it remained after the putative negative T3-responsive elements were mutated. A co-immunoprecipitation study demonstrated the in vivo association of TRβ1 with TEAD-1, and the interaction surfaces were mapped to the DBD of the TRβ1 and TEA domains of TEAD-1, both of which are highly conserved among TRs and TEADs, respectively. The importance of TEADs in MYH7 expression was also validated with RNA interference using rat embryonic cardiomyocyte H9c2 cells. These results indicate that T3-bound TRs interfere with transactivation by TEADs via protein-protein interactions, resulting in the negative regulation of MYH7 promoter activity.

Prognostic value of a nine-gene signature in glioma patients based on mRNA expression profiling

INTRODUCTION

Gliomas are the most common primary brain tumors in adults and a significant cause of cancer-related mortality. A 9-gene signature was identified as a novel prognostic model reflecting survival situation obviously in gliomas.

AIMS

To identify an mRNA expression signature to improve outcome prediction for patients with different glioma grades.

RESULTS

We used whole-genome mRNA expression microarray data of 220 glioma samples of all grades from the Chinese Glioma Genome Atlas (CGGA) database (http://www.cgga.org.cn) as a discovery set and data from Rembrandt and GSE16011 for validation sets. Data from every single grade were analyzed by the Kaplan-Meier method with a two-sided log-rank test. Univariate Cox regression and linear risk score formula were applied to derive a gene signature with better prognostic performance. We found that patients who had high risk score according to the signature had poor overall survival compared with patients who had low risk score. Highly expressed genes in the high-risk group were analyzed by gene ontology (GO) and gene set variation analysis (GSVA). As a result, the reason for the divisibility of gliomas was likely due to cell life processes and adhesion.

CONCLUSION

This 9-gene-signature prediction model provided a more accurate predictor of prognosis that denoted patients with high risk score have poor outcome. Moreover, these risk models based on defined molecular profiles showed the considerable prospect in personalized cancer management.

MicroRNAs involved in skeletal muscle differentiation

MicroRNAs (miRNAs) negatively regulate gene expression by promoting degradation of target mRNAs or inhibiting their translation. Previous studies have expanded our understanding that miRNAs play an important role in myogenesis and have a big impact on muscle mass, muscle fiber type and muscle-related diseases. The muscle-specific miRNAs, miR-206, miR-1 and miR-133, are among the most studied and best characterized miRNAs in skeletal muscle differentiation. They have a profound influence on multiple muscle differentiation processes, such as alternative splicing, DNA synthesis, and cell apoptosis. Many non-muscle-specific miRNAs are also required for the differentiation of muscle through interaction with myogenic factors. Studying the regulatory mechanisms of these miRNAs in muscle differentiation will extend our knowledge of miRNAs in muscle biology and will improve our understanding of the myogenesis regulation.

Myocyte enhancer factor 2 (MEF2) tethering to muscle selective A-kinase anchoring protein (mAKAP) is necessary for myogenic differentiation

Differentiation of skeletal myoblast cells to functional myotubes involves highly regulated transcriptional dynamics. The myocyte enhancer factor 2 (MEF2) transcription factors are critical to this process, synergizing with the master regulator MyoD to promote muscle specific gene transcription. MEF2 is extensively regulated by myogenic stimuli, both transcriptionally and post-translationally, but to date there has been little progress in understanding how signals upstream of MEF2 are coordinated to produce a coherent response. In this study, we define a novel interaction between the muscle A-kinase anchoring protein (mAKAP) and MEF2 in skeletal muscle. Discrete domains of MEF2 and mAKAP bind directly. Their interaction was exploited to probe the function of mAKAP-tethered MEF2 during myogenic differentiation. Dominant interference of MEF2/mAKAP binding was sufficient to block MEF2 activation during the early stages of differentiation. Furthermore, extended expression of this disrupting domain effectively blocked myogenic differentiation, halting the formation of myotubes and decreasing expression of several differentiation markers. This study expands our understanding of the regulation of MEF2 in skeletal muscle and identifies the mAKAP scaffold as a facilitator of MEF2 transcription and myogenic differentiation.

CREB is activated by muscle injury and promotes muscle regeneration

The cAMP response element binding protein (CREB) plays key roles in differentiation of embryonic skeletal muscle progenitors and survival of adult skeletal muscle. However, little is known about the physiologic signals that activate CREB in normal muscle. Here we show that CREB phosphorylation and target genes are induced after acute muscle injury and during regeneration due to genetic mutation. Activated CREB localizes to both myogenic precursor cells and newly regenerating myofibers within regenerating areas. Moreover, we found that signals from damaged skeletal muscle tissue induce CREB phosphorylation and target gene expression in primary mouse myoblasts. An activated CREB mutant (CREBY134F) potentiates myoblast proliferation as well as expression of early myogenic transcription factors in cultured primary myocytes. Consistently, activated CREB-YF promotes myoblast proliferation after acute muscle injury in vivo and enhances muscle regeneration in dystrophic mdx mice. Our findings reveal a new physiologic function for CREB in contributing to skeletal muscle regeneration.

Differentiation and fiber type-specific activity of a muscle creatine kinase intronic enhancer

BACKGROUND

Hundreds of genes, including muscle creatine kinase (MCK), are differentially expressed in fast- and slow-twitch muscle fibers, but the fiber type-specific regulatory mechanisms are not well understood.

RESULTS

Modulatory region 1 (MR1) is a 1-kb regulatory region within MCK intron 1 that is highly active in terminally differentiating skeletal myocytes in vitro. A MCK small intronic enhancer (MCK-SIE) containing a paired E-box/myocyte enhancer factor 2 (MEF2) regulatory motif resides within MR1. The SIE's transcriptional activity equals that of the extensively characterized 206-bp MCK 5'-enhancer, but the MCK-SIE is flanked by regions that can repress its activity via the individual and combined effects of about 15 different but highly conserved 9- to 24-bp sequences. ChIP and ChIP-Seq analyses indicate that the SIE and the MCK 5'-enhancer are occupied by MyoD, myogenin and MEF2. Many other E-boxes located within or immediately adjacent to intron 1 are not occupied by MyoD or myogenin. Transgenic analysis of a 6.5-kb MCK genomic fragment containing the 5'-enhancer and proximal promoter plus the 3.2-kb intron 1, with and without MR1, indicates that MR1 is critical for MCK expression in slow- and intermediate-twitch muscle fibers (types I and IIa, respectively), but is not required for expression in fast-twitch muscle fibers (types IIb and IId).

CONCLUSIONS

In this study, we discovered that MR1 is critical for MCK expression in slow- and intermediate-twitch muscle fibers and that MR1's positive transcriptional activity depends on a paired E-box MEF2 site motif within a SIE. This is the first study to delineate the DNA controls for MCK expression in different skeletal muscle fiber types.

Design and testing of regulatory cassettes for optimal activity in skeletal and cardiac muscles

Gene therapy for muscular dystrophies requires efficient gene delivery to the striated musculature and specific, high-level expression of the therapeutic gene in a physiologically diverse array of muscles. This can be achieved by the use of recombinant adeno-associated virus vectors in conjunction with muscle-specific regulatory cassettes. We have constructed several generations of regulatory cassettes based on the enhancer and promoter of the muscle creatine kinase gene, some of which include heterologous enhancers and individual elements from other muscle genes. Since the relative importance of many control elements varies among different anatomical muscles, we are aiming to tailor these cassettes for high-level expression in cardiac muscle, and in fast and slow skeletal muscles. With the achievement of efficient intravascular gene delivery to isolated limbs, selected muscle groups, and heart in large animal models, the design of cassettes optimized for activity in different muscle types is now a practical goal. In this protocol, we outline the key steps involved in the design of regulatory cassettes for optimal activity in skeletal and cardiac muscle, and testing in mature muscle fiber cultures. The basic principles described here can also be applied to engineering tissue-specific regulatory cassettes for other cell types.

TEAD-1 overexpression in the mouse heart promotes an age-dependent heart dysfunction

TEA domain transcription factor-1 (TEAD-1) is essential for proper heart development and is implicated in cardiac specific gene expression and the hypertrophic response of primary cardiomyocytes to hormonal and mechanical stimuli, and its activity increases in the pressure-overloaded hypertrophied rat heart. To investigate whether TEAD-1 is an in vivo modulator of cardiac specific gene expression and hypertrophy, we developed transgenic mice expressing hemagglutinin-tagged TEAD-1 under the control of the muscle creatine kinase promoter. We show that a sustained increase in TEAD-1 protein leads to an age-dependent dysfunction. Magnetic resonance imaging revealed decreases in cardiac output, stroke volume, ejection fraction, and fractional shortening. Isolated TEAD-1 hearts revealed decreased left ventricular power output that correlated with increased betaMyHC protein. Histological analysis showed altered alignment of cardiomyocytes, septal wall thickening, and fibrosis, although electrocardiography displayed a left axis shift of mean electrical axis. Transcripts representing most members of the fetal heart gene program remained elevated from fetal to adult life. Western blot analyses revealed decreases in p-phospholamban, SERCA2a, p-CX43, p-GSK-3alpha/beta, nuclear beta-catenin, GATA4, NFATc3/c4, and increased NCX1, nuclear DYKR1A, and Pur alpha/beta protein. TEAD-1 mice did not display cardiac hypertrophy. TEAD-1 mice do not tolerate stress as they die over a 4-day period after surgical induction of pressure overload. These data provide the first in vivo evidence that increased TEAD-1 can induce characteristics of cardiac remodeling associated with cardiomyopathy and heart failure.

Human tyrosine hydroxylase natural genetic variation: delineation of functional transcriptional control motifs disrupted in the proximal promoter

BACKGROUND

Tyrosine hydroxylase (TH) is the rate-limiting enzyme in catecholamine biosynthesis. Common genetic variation at the human TH promoter predicts alterations in autonomic activity and blood pressure, but how such variation influences human traits and, specifically, whether such variation affects transcription are not yet known.

METHODS AND RESULTS

Pairwise linkage disequilibrium across the TH locus indicated that common promoter variants (C-824T, G-801C, A-581G, and G-494A) were located in a single 5' linkage disequilibrium block in white, black, Hispanic, and Asian populations. Polymorphisms C-824T and A-581G were located in highly conserved regions and were predicted to disrupt known transcriptional control motifs myocyte enhancer factor-2 (MEF2), sex-determining region Y (SRY), and forkhead box D1 (FOXD1) at C-824T and G/C-rich binding factors specificity protein 1 (SP1), activating enhancer-binding protein 2 (AP2)], early growth response protein 1 (EGR1) at A-581G. At C-824T and A-581G, promoter and luciferase reporter plasmids indicated differential allele strength (T>C at C-824T; G>A at A-581G) under both basal circumstances and secretory stimulation. C-824T and A-581G displayed the most pronounced effects on both transcription in cella and catecholamine secretion in vivo. We further probed the functional significance of C-824T and A-581G by cotransfection of trans-activating factors in cella; MEF2, SRY, and FOXD1 differentially activated C-824T, whereas the G/C-rich binding factors SP1, AP2, and EGR1 differentially activated A-581G. At C-824T, factor MEF2 acted in a directionally coordinate fashion (at T>C) to explain the in vivo trait associations, whereas at A-581G, factors SP1, AP2, and EGR1 displayed similar differential actions (at G>A). Finally, chromatin immunoprecipitation demonstrated that the endogenous factors bound to the motifs in cella.

CONCLUSIONS

We conclude that common genetic variants in the proximal TH promoter, especially at C-824T and A-581G, are functional in cella and alter transcription so as to explain promoter marker-on-trait associations in vivo. MEF2, FOXD1, and SRY contribute to functional differences in C-824T expression, whereas SP1, AP2, and EGR1 mediate those of A-581G. The SRY effect on TH transcription suggests a mechanism whereby male and female sex may differ in sympathetic activity and hence blood pressure. These results point to new strategies for diagnostic and therapeutic intervention into disorders of human autonomic function and their cardiovascular consequences.

Vestigial-like 2 acts downstream of MyoD activation and is associated with skeletal muscle differentiation in chick myogenesis

The co-factor Vestigial-like 2 (Vgl-2), in association with the Scalloped/Tef/Tead transcription factors, has been identified as a component of the myogenic program in the C2C12 cell line. In order to understand Vgl-2 function in embryonic muscle formation, we analysed Vgl-2 expression and regulation during chick embryonic development. Vgl-2 expression was associated with all known sites of skeletal muscle formation, including those in the head, trunk and limb. Vgl-2 was expressed after the myogenic factor MyoD, regardless of the site of myogenesis. Analysis of Vgl-2 regulation by Notch signalling showed that Vgl-2 expression was down-regulated by Delta1-activated Notch, similarly to the muscle differentiation genes MyoD, Myogenin,Desmin, and Mef2c, while the expression of the muscle progenitor markers such as Myf5, Six1 and FgfR4 was not modified. Moreover, we established that the Myogenic Regulatory Factors (MRFs) associated with skeletal muscle differentiation (MyoD, Myogenin and Mrf4) were sufficient to activate Vgl-2 expression, while Myf5 was not able to do so. The Vgl-2 endogenous expression, the similar regulation of Vgl-2 and that of MyoD and Myogenin by Notch signalling, and the positive regulation of Vgl-2 by these MRFs suggest that Vgl-2 acts downstream of MyoD activation and is associated with the differentiation step in embryonic skeletal myogenesis.

The transcriptional co-activator TAZ interacts differentially with transcriptional enhancer factor-1 (TEF-1) family members

Members of the highly related TEF-1 (transcriptional enhancer factor-1) family (also known as TEAD, for TEF-1, TEC1, ABAA domain) bind to MCAT (muscle C, A and T sites) and A/T-rich sites in promoters active in cardiac, skeletal and smooth muscle, placenta, and neural crest. TEF-1 activity is regulated by interactions with transcriptional co-factors [p160, TONDU (Vgl-1, Vestigial-like protein-1), Vgl-2 and YAP65 (Yes-associated protein 65 kDa)]. The strong transcriptional co-activator YAP65 interacts with all TEF-1 family members, and, since YAP65 is related to TAZ (transcriptional co-activator with PDZ-binding motif), we wanted to determine if TAZ also interacts with members of the TEF-1 family. In the present study, we show by GST (glutathione S-transferase) pull-down assays, by co-immunoprecipitation and by modified mammalian two-hybrid assays that TEF-1 interacts with TAZ in vitro and in vivo. Electrophoretic mobility-shift assays with purified TEF-1 and GST-TAZ fusion protein showed that TAZ interacts with TEF-1 bound to MCAT DNA. TAZ can interact with endogenous TEF-1 proteins, since exogenous TAZ activated MCAT-dependent reporter promoters. Like YAP65, TAZ interacted with all four TEF-1 family members. GST pull-down assays with increasing amounts of [35S]TEF-1 and [35S]RTEF-1 (related TEF-1) showed that TAZ interacts more efficiently with TEF-1 than with RTEF-1. This differential interaction also extended to the interaction of TEF-1 and RTEF-1 with TAZ in vivo, as assayed by a modified mammalian two-hybrid experiment. These data show that differential association of TEF-1 proteins with transcriptional co-activators may regulate the activity of TEF-1 family members.

Evidence of MyomiR network regulation of beta-myosin heavy chain gene expression during skeletal muscle atrophy

There is a growing recognition that noncoding RNAs (ncRNA) play an important role in the regulation of gene expression. A class of small (19-22 nt) ncRNAs, known as microRNAs (miRs), have received a great deal of attention lately because of their ability to repress gene expression through a unique posttranscriptional 3'-untranslated region (UTR) mechanism. The objectives of the current study were to identify miRs expressed in the rat soleus muscle and determine if their expression was changed in response to hindlimb suspension. Comprehensive profiling revealed 151 miRs were expressed in the soleus muscle and expression of 18 miRs were significantly (P < 0.01) changed after 2 and/or 7 days of hindlimb suspension. The significant decrease (16%) in expression of muscle-specific miR-499 in response to hindlimb suspension was confirmed by RT-PCR and suggested activation of the recently proposed miR encoded by myosin gene (MyomiR) network during atrophy. Further analysis of soleus muscle subjected to hindlimb suspension for 28 days provided evidence consistent with MyomiR network repression of beta-myosin heavy chain gene (beta-MHC) expression. The significant downregulation of network components miR-499 and miR-208b by 40 and 60%, respectively, was associated with increased expression of Sox6 (2.2-fold) and Purbeta (23%), predicted target genes of miR-499 and known repressors of beta-MHC expression. A Sox6 3'-UTR reporter gene confirmed Sox6 is a target gene of miR-499. These results further expand the role of miRs in adult skeletal muscle and are consistent with a model in which the MyomiR network regulates slow myosin expression during muscle atrophy.

IGF-I activates the mouse type IIb myosin heavy chain gene

IGF-I increases skeletal muscle mass, but whether IGF-I increases type IIb myosin heavy chain (MyHC) transcriptional activity is not known. C2C12 myotubes were cultured with or without IGF-I to determine whether IGF-I increases type IIb MyHC promoter activity, and if so, what region of the promoter might IGF-I signaling regulate. At differentiation days 3 and 4, IGF-I increased type IIb MyHC mRNA and mouse 3.0-kb type IIb MyHC promoter activity. Deletion construct studies identified a potential IGF-I-responsive region between 1.25 and 1.2 kb of the type IIb MyHC promoter, which contained an exact 6-bp T-cell factor/lymphoid enhancer factor (Tcf/Lef) binding site at position -1206 to -1201. Site-specific mutation of the putative Tcf/Lef binding site reduced IGF-I-induced 1.3-kb type IIb MyHC promoter activity. To identify potential IGF-I signaling molecules, the phosphatidylinositol 3-kinase (PI3K) inhibitors wortmannin and LY-294002 were both found to markedly attenuate IGF-I activation of the 1.3-kb type IIb MyHC promoter. Downstream signaling of IGF-I can phosphorylate and inactivate GSK-3beta, thereby enhancing beta-catenin protein. The GSK-3beta inhibitor, LiCl, dramatically enhanced IGF-I induction of the 1.3-kb type IIb MyHC promoter, and constitutively active GSK-3beta attenuated IGF-I-induced 1.3-kb type IIb MyHC promoter activity. Finally, IGF-I increased nuclear beta-catenin protein, and small interfering RNA knockdown of beta-catenin attenuated IGF-I-induced 1.3-kb type IIb MyHC promoter activity and type IIb MyHC mRNA. In summary, IGF-I stimulation of C2C12 myotubes increases mouse type IIb MyHC promoter activity, likely through signaling of PI3K, GSK-3beta, beta-catenin, and a Tcf/Lef binding site at -1,206 to -1,201 bp in the promoter.

Overexpression of TEAD-1 in transgenic mouse striated muscles produces a slower skeletal muscle contractile phenotype

TEA domain (TEAD) transcription factors serve important functional roles during embryonic development and in striated muscle gene expression. Our previous work has implicated a role for TEAD-1 in the fast-to-slow fiber-type transition in response to mechanical overload. To investigate whether TEAD-1 is a modulator of slow muscle gene expression in vivo, we developed transgenic mice expressing hemagglutinin (HA)-tagged TEAD-1 under the control of the muscle creatine kinase promoter. We show that striated muscle-restricted HA-TEAD-1 expression induced a transition toward a slow muscle contractile protein phenotype, slower shortening velocity (Vmax), and longer contraction and relaxation times in adult fast twitch extensor digitalis longus muscle. Notably, HA-TEAD-1 overexpression resulted in an unexpected activation of GSK-3alpha/beta and decreased nuclear beta-catenin and NFATc1/c3 protein. These effects could be reversed in vivo by mechanical overload, which decreased muscle creatine kinase-driven TEAD-1 transgene expression, and in cultured satellite cells by TEAD-1-specific small interfering RNA. These novel in vivo data support a role for TEAD-1 in modulating slow muscle gene expression.

Redundant roles of Tead1 and Tead2 in notochord development and the regulation of cell proliferation and survival

Four members of the TEAD/TEF family of transcription factors are expressed widely in mouse embryos and adult tissues. Although in vitro studies have suggested various roles for TEAD proteins, their in vivo functions remain poorly understood. Here we examined the role of Tead genes by generating mouse mutants for Tead1 and Tead2. Tead2(-/-) mice appeared normal, but Tead1(-/-); Tead2(-/-) embryos died at embryonic day 9.5 (E9.5) with severe growth defects and morphological abnormalities. At E8.5, Tead1(-/-); Tead2(-/-) embryos were already small and lacked characteristic structures such as a closed neural tube, a notochord, and somites. Despite these overt abnormalities, differentiation and patterning of the neural plate and endoderm were relatively normal. In contrast, the paraxial mesoderm and lateral plate mesoderm were displaced laterally, and a differentiated notochord was not maintained. These abnormalities and defects in yolk sac vasculature organization resemble those of mutants for Yap, which encodes a coactivator of TEAD proteins. Moreover, we demonstrated genetic interactions between Tead1 and Tead2 and Yap. Finally, Tead1(-/-); Tead2(-/-) embryos showed reduced cell proliferation and increased apoptosis. These results suggest that Tead1 and Tead2 are functionally redundant, use YAP as a major coactivator, and support notochord maintenance as well as cell proliferation and survival in mouse development.

MCAT elements and the TEF-1 family of transcription factors in muscle development and disease

MCAT elements are located in the promoter-enhancer regions of cardiac, smooth, and skeletal muscle-specific genes including cardiac troponin T, beta-myosin heavy chain, smooth muscle alpha-actin, and skeletal alpha-actin, and play a key role in the regulation of these genes during muscle development and disease. The binding factors of MCAT elements are members of the transcriptional enhancer factor-1 (TEF-1) family. However, it has not been fully understood how these transcription factors confer cell-specific expression in muscle, because their expression patterns are relatively broad. Results of recent studies revealed multiple mechanisms whereby TEF-1 family members control MCAT element-dependent muscle-specific gene expression, including posttranslational modifications of TEF-1 family members, the presence of muscle-selective TEF-1 cofactors, and cell-selective control of TEF-1 accessibility to MCAT elements. In addition, of particular interest, recent studies regarding MCAT element-dependent transcription of the myocardin gene and the smooth muscle alpha-actin gene in muscle provide evidence for the transcriptional diversity among distinct cell types and subtypes. This article summarizes the role of MCAT elements and the TEF-1 family of transcription factors in muscle development and disease, and reviews recent progress in our understanding of the transcriptional regulatory mechanisms involved in MCAT element-dependent muscle-specific gene expression.

Target genes of myostatin loss-of-function in muscles of late bovine fetuses

BACKGROUND

Myostatin, a muscle-specific member of the Transforming Growth Factor beta family, negatively regulates muscle development. Double-muscled (DM) cattle have a loss-of-function mutation in their myostatin gene responsible for the hypermuscular phenotype. Thus, these animals are a good model for understanding the mechanisms underpinning muscular hypertrophy. In order to identify individual genes or networks that may be myostatin targets, we looked for genes that were differentially expressed between DM and normal (NM) animals (n = 3 per group) in the semitendinosus muscle (hypertrophied in DM animals) at 260 days of fetal development (when the biochemical differentiation of muscle is intensive). A heterologous microarray (human and murine oligonucleotide sequences) of around 6,000 genes expressed in muscle was used.

RESULTS

Many genes were found to be differentially expressed according to genetic type (some with a more than 5-fold change), and according to the presence of one or two functional myostatin allele(s). They belonged to various functional categories. The genes down-regulated in DM fetuses were mainly those encoding extracellular matrix proteins, slow contractile proteins and ribosomal proteins. The genes up-regulated in DM fetuses were mainly involved in the regulation of transcription, cell cycle/apoptosis, translation or DNA metabolism. These data highlight features indicating that DM muscle is shifted towards a more glycolytic metabolism, and has an altered extracellular matrix composition (e.g. down-regulation of COL1A1 and COL1A2, and up-regulation of COL4A2) and decreased adipocyte differentiation (down-regulation of C1QTNF3). The altered gene expression in the three major muscle compartments (fibers, connective tissue and intramuscular adipose tissue) is consistent with the well-known characteristics of DM cattle. In addition, novel potential targets of the myostatin gene were identified (MB, PLN, troponins, ZFHX1B).

CONCLUSION

Thus, the myostatin loss-of-function mutation affected several physiological processes involved in the development and determination of the functional characteristics of muscle tissue.

Puralpha and Purbeta collaborate with Sp3 to negatively regulate beta-myosin heavy chain gene expression during skeletal muscle inactivity

Adult skeletal muscle retains the capability of transcriptional reprogramming. This attribute is readily observable in the non-weight-bearing (NWB) soleus muscle, which undergoes a slow-to-fast fiber type transition concurrent with decreased beta-myosin heavy chain (betaMyHC) gene expression. Our previous work showed that Sp3 contributes to decreased betaMyHC gene expression under NWB conditions. In this study, we demonstrate that physical and functional interactions between Sp3, Puralpha, and Purbeta proteins mediate repression of betaMyHC expression under NWB conditions. Binding of Puralpha or Purbeta to the single-stranded betaMyHC distal negative regulatory element-sense strand (dbetaNRE-S) element is markedly increased under NWB conditions. Ectopic expression of Puralpha and Purbeta decreased betaMyHC reporter gene expression, while mutation of the dbetaNRE-S element increased expression in C2C12 myotubes. The dbetaNRE-S element conferred Pur-dependent decreased expression on a minimal thymidine kinase promoter. Short interfering RNA sequences specific for Sp3 or for Puralpha and Purbeta decreased endogenous Sp3 and Pur protein levels and increased betaMyHC reporter gene expression in C2C12 myotubes. Immunoprecipitation assays revealed an association between endogenous Puralpha, Purbeta, and Sp3, while chromatin immunoprecipitation assays demonstrated Puralpha, Purbeta, and Sp3 binding to the betaMyHC proximal promoter region harboring the dbetaNRE-S and C-rich elements in vivo. These data demonstrate that Pur proteins collaborate with Sp3 to regulate a transcriptional program that enables muscle cells to remodel their phenotype.

Re-employment of developmental transcription factors in adult heart disease

A finite number of transcription factors constitute a combinatorial code that orchestrates cardiac development and the specification and differentiation of myocytes. Many, if not all of these same transcription factors are re-employed in the adult heart in response to disease stimuli that promote hypertrophic enlargement and/or dilated cardiomyopathy, as part of the so-called "fetal gene program". This review will discuss the transcription factors that regulate the hypertrophic growth response of the adult heart, with a special emphasis on those regulators that participate in cardiac development.

Adaptive changes in TEF-1 gene expression during cold acclimation in the medaka

How animals adaptively respond to a cold or hot environment has been questioned for a long time. Recently, with the aid of microarray analysis, various temperature-sensitive genes have been identified in several species. However, a definitive hypothesis regarding the mechanism of adaptation has not been proposed. In the present study, we surveyed, in medaka (Oryzias latipes), genes for which the level of expression changes depending on the surrounding temperature. A messenger RNA differential display of medaka muscle total RNA revealed one such gene encoding transcription enhancer factor-1 (TEF-1). In medaka muscle, the TEF-1 gene produces two splicing variants, TEF-1A and TEF-1B mRNAs. During cold acclimation, the mRNA level of TEF-1A decreased, whereas that of TEF-1B increased. We also found that three putative downstream genes of TEF-1, two for myosin heavy chain (MyHC) and one for troponin T (TnT), a specific group of muscle proteins, were transcribed in a temperature-dependent manner. These results suggest that the transcription of MyHC and/or TnT is regulated by TEF-1 and that these molecules participate in muscle reconstruction during temperature adaptation in fish.

TRPC6 fulfills a calcineurin signaling circuit during pathologic cardiac remodeling

The heart responds to injury and chronic pressure overload by pathologic growth and remodeling, which frequently result in heart failure and sudden death. Calcium-dependent signaling pathways promote cardiac growth and associated changes in gene expression in response to stress. The calcium/calmodulin-dependent phosphatase calcineurin, which signals to nuclear factor of activated T cells (NFAT) transcription factors, serves as a transducer of calcium signals and is sufficient and necessary for pathologic cardiac hypertrophy and remodeling. Transient receptor potential (TRP) proteins regulate cation entry into cells in response to a variety of signals, and in skeletal muscle, expression of TRP cation channel, subfamily C, member 3 (TRPC3) is increased in response to neurostimulation and calcineurin signaling. Here we show that TRPC6 was upregulated in mouse hearts in response to activated calcineurin and pressure overload, as well as in failing human hearts. Two conserved NFAT consensus sites in the promoter of the TRPC6 gene conferred responsiveness to cardiac stress. Cardiac-specific overexpression of TRPC6 in transgenic mice resulted in heightened sensitivity to stress, a propensity for lethal cardiac growth and heart failure, and an increase in NFAT-dependent expression of beta-myosin heavy chain, a sensitive marker for pathologic hypertrophy. These findings implicate TRPC6 as a positive regulator of calcineurin-NFAT signaling and a key component of a calcium-dependent regulatory loop that drives pathologic cardiac remodeling.

MusTRD can regulate postnatal fiber-specific expression

Human MusTRD1alpha1 was isolated as a result of its ability to bind a critical element within the Troponin I slow upstream enhancer (TnIslow USE) and was predicted to be a regulator of slow fiber-specific genes. To test this hypothesis in vivo, we generated transgenic mice expressing hMusTRD1alpha1 in skeletal muscle. Adult transgenic mice show a complete loss of slow fibers and a concomitant replacement by fast IIA fibers, resulting in postural muscle weakness. However, developmental analysis demonstrates that transgene expression has no impact on embryonic patterning of slow fibers but causes a gradual postnatal slow to fast fiber conversion. This conversion was underpinned by a demonstrable repression of many slow fiber-specific genes, whereas fast fiber-specific gene expression was either unchanged or enhanced. These data are consistent with our initial predictions for hMusTRD1alpha1 and suggest that slow fiber genes contain a specific common regulatory element that can be targeted by MusTRD proteins.

Myocyte enhancer factors 2A and 2C induce dilated cardiomyopathy in transgenic mice

Cardiac hypertrophy and dilation are mediated by neuroendocrine factors and/or mitogens as well as through internal stretch- and stress-sensitive signaling pathways, which in turn transduce alterations in cardiac gene expression through specific signaling pathways. The transcription factor family known as myocyte enhancer factor 2 (MEF2) has been implicated as a signal-responsive mediator of the cardiac transcriptional program. For example, known hypertrophic signaling pathways that utilize calcineurin, calmodulin-dependent protein kinase, and MAPKs can each affect MEF2 activity. Here we demonstrate that MEF2 transcription factors induced dilated cardiomyopathy and lengthening of myocytes. Specifically, multiple transgenic mouse lines with cardiac-specific overexpression of MEF2A or MEF2C presented with cardiomyopathy at base line or were predisposed to more fulminant disease following pressure overload stimulation. The cardiomyopathic response associated with MEF2A and MEF2C was not further altered by activated calcineurin, suggesting that MEF2 functions independently of calcineurin in this response. In cultured cardiomyocytes, MEF2A, MEF2C, and MEF2-VP16 overexpression induced sarcomeric disorganization and focal elongation. Mechanistically, MEF2A and MEF2C each programmed similar profiles of altered gene expression in the heart that included extracellular matrix remodeling, ion handling, and metabolic genes. Indeed, adenoviral transfection of cultured cardiomyocytes with MEF2A or of myocytes from the hearts of MEF2A transgenic adult mice showed reduced transient outward K(+) currents, consistent with the alterations in gene expression observed in transgenic mice and partially suggesting a proximal mechanism underlying MEF2-dependent cardiomyopathy.

Transcriptional regulation of tissue-specific genes by the ERK5 mitogen-activated protein kinase

The ERK5 mitogen-activated protein kinase (MAPK) differs from other MAPKs in possessing a potent transcriptional activation domain. ERK5-/- embryos die from angiogenic defects, but the precise physiological role of ERK5 remains poorly understood. To elucidate molecular functions of ERK5 in the development of vasculature and other tissues, we performed gene profile analyses of erk5-/- mouse embryos and erk5-/- fibroblast cells reconstituted with ERK5 or ERK5(1-740), which lacks the transactivation domain. These experiments revealed several potential ERK5 target genes, including a proapoptotic gene bnip3, known angiogenic genes flt1 and lklf (lung Krüppel-like factor), and genes that regulate cardiovascular development. Among these, LKLF, known for its roles in angiogenesis, T-cell quiescence, and survival, was found to be absolutely dependent on ERK5 for expression in endothelial and T cells. We show that ERK5 drives lklf transcription by activating MEF2 transcription factors. Expression of erk5 short hairpin or a dominant-negative form of the ERK5 upstream activator, MEK5, in T cells led to downregulation of LKLF, increased cell size and upregulation of activation markers. Thus, through its kinase and transcriptional activation domains, ERK5 regulates transcriptional responses of cell survival and quiescence critical for angiogenesis and T-cell function.

Fgfr4 is required for effective muscle regeneration in vivo. Delineation of a MyoD-Tead2-Fgfr4 transcriptional pathway

Fgfr4 has been shown to be important for appropriate muscle development in chick limb buds; however, Fgfr4 null mice show no phenotype. Here, we show that staged induction of muscle regeneration in Fgfr4 null mice becomes highly abnormal at the time point when Fgfr4 is normally expressed. By 7 days of regeneration, differentiation of myotubes became poorly coordinated and delayed by both histology and embryonic myosin heavy chain staining. By 14 days much of the muscle was replaced by fat and calcifications. To begin to dissect the molecular pathways involving Fgfr4, we queried the promoter sequences for transcriptional factor binding sites and tested candidate regulators in a 27-time point regeneration series. The Fgfr4 promoter region contained a Tead protein binding site (M-CAT 5'-CATTCCT-3'), and Tead2 showed induction during regeneration commensurate with Fgfr4 regulation. Co-transfection of Tead2 and Fgfr4 promoter reporter constructs into C2C12 myotubes showed Tead2 to activate Fgfr4, and mutation of the M-CAT motif in the Fgfr4 promoter abolished these effects. Immunostaining for Tead2 showed timed expression in myotube nuclei consistent with the mRNA data. Query of the expression timing and genomic sequences of Tead2 suggested direct regulation by MyoD, and consistent with this, MyoD directly bound to two strong E-boxes in the first intron of Tead2 by chromatin immunoprecipitation assay. Moreover, co-transfection of MyoD and Tead2 intron reporter constructs into 10T1/2 cells activated reporter activity in a dose-dependent manner. This activation was greatly reduced when the two E-boxes were mutated. Our data suggest a novel MyoD-Tead2-Fgfr4 pathway important for effective muscle regeneration.

Adult myogenesis in Drosophila melanogaster can proceed independently of myocyte enhancer factor-2

Myocyte enhancer factor-2 (MEF2) is a transcription factor that is necessary for embryonic muscle development in Drosophila and vertebrates; however, whether this factor is required during later muscle development remains largely unknown. Using heteroallelic combinations of different Mef2 mutant alleles, we isolated and characterized a temperature-sensitive combination. Through temperature-shift experiments, we obtained adult animals that were lacking proper MEF2 function. Many of these individuals died as mature pupae, and those that eclosed showed poor locomotion and an inability to fly. Histological analysis of these animals revealed a requirement for MEF2 in skeletal muscle patterning, although these animals had strikingly normal amounts of muscle tissue. Using quantitative polymerase chain reaction, we determined that expression of the MEF2-regulated actin gene Act57B was severely reduced in these animals. By contrast myofibrillar actin genes unique to the adult stage were only mildly affected. Since MEF2 mutant adults were still capable of forming muscle tissue, we conclude that MEF2 is required for the expression of only a subset of muscle structural genes in the adult. These results indicate that additional muscle-specific factors function to control the myogenesis of complex and diverse muscle in the adult.

The Ankrd2, Cdkn1c and Calcyclin Genes are Under the Control of MyoD During Myogenic Differentiation

Skeletal muscle development requires the coordinated expression of numerous transcription factors to control the specification of the muscle fate in mesodermal cells and the differentiation of the committed myoblasts into functional contractile fibers. The bHLH transcription factor MyoD plays a key role in these processes, since its forced expression is sufficient to induce the myogenesis in a variety of non-muscle cells in culture. Consistent with this observation, the majority of skeletal muscle genes require MyoD to activate their own transcription. In order to identify novel MyoD-target genes we generated C2C12 MyoD-silenced clones, and used a muscle-specific cDNA microarray to study the induced modifications of the transcriptional profile. Gene expression was analyzed at three different stages in differentiating MyoD(-)C2C12 myoblasts. These microarray data sets identified many additional uncharacterized downstream MyoD transcripts that may play important functions in muscle cell differentiation. Among these genes, we concentrated our study on the cell cycle regulators Cdkn1c and calcyclin and on the muscle-specific putative myogenic regulator Ankrd2. Bioinformatic and functional studies on the promoters of these genes clarified their dependence on MyoD activity. Clues of other regulatory mechanisms that might interact with the principal bHLH transcription factor have been revealed by the unexpected up-regulation in MyoD(-) cells of these novel (and other) target transcripts, at the differentiation stage in which MyoD became normally down-regulated.

An initial blueprint for myogenic differentiation

We have combined genome-wide transcription factor binding and expression profiling to assemble a regulatory network controlling the myogenic differentiation program in mammalian cells. We identified a cadre of overlapping and distinct targets of the key myogenic regulatory factors (MRFs)--MyoD and myogenin--and Myocyte Enhancer Factor 2 (MEF2). We discovered that MRFs and MEF2 regulate a remarkably extensive array of transcription factor genes that propagate and amplify the signals initiated by MRFs. We found that MRFs play an unexpectedly wide-ranging role in directing the assembly and usage of the neuromuscular junction. Interestingly, these factors also prepare myoblasts to respond to diverse types of stress. Computational analyses identified novel combinations of factors that, depending on the differentiation state, might collaborate with MRFs. Our studies suggest unanticipated biological insights into muscle development and highlight new directions for further studies of genes involved in muscle repair and responses to stress and damage.

Sp3 proteins negatively regulate beta myosin heavy chain gene expression during skeletal muscle inactivity

In adult skeletal muscle, beta myosin heavy chain (betaMyHC) gene expression is primarily restricted to slow type I fibers; however, its expression is down-regulated in response to muscle inactivity. Little is known about the signaling pathways and transcription factors that mediate this important functional response. This study demonstrates that increased binding of Sp3 to GC-rich elements in the betaMyHC promoter is a critical event in down-regulation of betaMyHC gene expression under non-weight-bearing conditions. Conversely, binding of Sp3 to these elements decreased while Sp1 binding increased with nuclear extracts from plantaris muscle exposed to mechanical overload, a stimulus that increases betaMyHC gene expression. In addition, these experiments revealed the existence of an Sp4-DNA binding complex when using adult skeletal muscle nuclear extract was used but not when nuclear extracts from cultured myotubes were used. Sp3 proteins are competitive inhibitors of Sp1-mediated betaMyHC reporter gene transactivation in both Drosophila SL-2 and mouse C2C12 myotubes. Sp4 is a weak activator of betaMyHC gene expression in SL-2 cells, which lack endogenous Sp1 activity, but does not activate betaMyHC gene expression in C2C12 myotubes, which have high levels of Sp1. These results suggest that competitive binding of Sp family proteins regulate betaMyHC gene transcription in response to altered neuromuscular activity.

Effect of unloading on type I myosin heavy chain gene regulation in rat soleus muscle

Slow-twitch soleus, a weight-bearing hindlimb muscle, predominantly expresses the type I myosin heavy chain (MHC) isoform. However, under unloading conditions, a transition in MHC expression occurs from slow type I toward the fast-type isoforms. Transcriptional processes are believed to be involved in this adaptation. To test the hypothesis that the downregulation of MHC1 in soleus muscle following unloading is controlled through cis element(s) in the proximal region of the promoter, the MHC1 promoter was injected into soleus muscles of control rats and those subjected to 7 days of hindlimb suspension. Mutation analyses of six putative regulatory elements within the -408-bp region demonstrated that three elements, an A/T-rich, the proximal muscle-type CAT (betae3), and an E-box (-63 bp), play an important role in the basal level of MHC1 gene activity in the control soleus and function as unloading-responsive elements. Gel mobility shift assays revealed a diminished level of complex formation of the betae3 and E-box probes with nuclear extract from hindlimb suspension soleus compared with control soleus. Supershift assays indicated that transcriptional enhancer factor 1 and myogenin factors bind the betae3 and E-box elements, respectively, in the control soleus. Western blots showed that the relative concentrations of the transcriptional enhancer factor 1 and myogenin factors were significantly attenuated in the unloaded soleus compared with the control muscle. We conclude that the downregulation of MHC1 in response to unloading is due, in part, to a significant decrease in the concentration of these transcription factors available for binding the positive regulatory elements.

Skeletal muscle FOXO1 (FKHR) transgenic mice have less skeletal muscle mass, down-regulated Type I (slow twitch/red muscle) fiber genes, and impaired glycemic control

FOXO1, a member of the FOXO forkhead type transcription factors, is markedly up-regulated in skeletal muscle in energy-deprived states such as fasting and severe diabetes, but its functions in skeletal muscle have remained poorly understood. In this study, we created transgenic mice specifically overexpressing FOXO1 in skeletal muscle. These mice weighed less than the wild-type control mice, had a reduced skeletal muscle mass, and the muscle was paler in color. Microarray analysis revealed that the expression of many genes related to the structural proteins of type I muscles (slow twitch, red muscle) was decreased. Histological analyses showed a marked decrease in size of both type I and type II fibers and a significant decrease in the number of type I fibers in the skeletal muscle of FOXO1 mice. Enhanced gene expression of a lysosomal proteinase, cathepsin L, which is known to be up-regulated during skeletal muscle atrophy, suggested increased protein degradation in the skeletal muscle of FOXO1 mice. Running wheel activity (spontaneous locomotive activity) was significantly reduced in FOXO1 mice compared with control mice. Moreover, the FOXO1 mice showed impaired glycemic control after oral glucose and intraperitoneal insulin administration. These results suggest that FOXO1 negatively regulates skeletal muscle mass and type I fiber gene expression and leads to impaired skeletal muscle function. Activation of FOXO1 may be involved in the pathogenesis of sarcopenia, the age-related decline in muscle mass in humans, which leads to obesity and diabetes.

Quantitative proteomic identification of six4 as the trex-binding factor in the muscle creatine kinase enhancer

Transcriptional regulatory element X (Trex) is a positive control site within the Muscle creatine kinase (MCK) enhancer. Cell culture and transgenic studies indicate that the Trex site is important for MCK expression in skeletal and cardiac muscle. After selectively enriching for the Trex-binding factor (TrexBF) using magnetic beads coupled to oligonucleotides containing either wild-type or mutant Trex sites, quantitative proteomics was used to identify TrexBF as Six4, a homeodomain transcription factor of the Six/sine oculis family, from a background of approximately 900 copurifying proteins. Using gel shift assays and Six-specific antisera, we demonstrated that Six4 is TrexBF in mouse skeletal myocytes and embryonic day 10 chick skeletal and cardiac muscle, while Six5 is the major TrexBF in adult mouse heart. In cotransfection studies, Six4 transactivates the MCK enhancer as well as muscle-specific regulatory regions of Aldolase A and Cardiac troponin C via Trex/MEF3 sites. Our results are consistent with Six4 being a key regulator of muscle gene expression in adult skeletal muscle and in developing striated muscle. The Trex/MEF3 composite sequence ([C/A]ACC[C/T]GA) allowed us to identify novel putative Six-binding sites in six other muscle genes. Our proteomics strategy will be useful for identifying transcription factors from complex mixtures using only defined DNA fragments for purification.

Innervation-dependent and fiber type-specific transcriptional regulation of the slow myosin heavy chain 2 promoter in avian skeletal muscle fibers

Skeletal muscle fiber type is regulated, in part, by innervation leading to transcriptional regulation of fiber type-specific genes. Here, we report the initial characterization of the transcriptional regulation of the slow myosin heavy chain 2 (MyHC2) promoter in innervated and noninnervated slow medial adductor (MA) and fast pectoralis major (PM) muscle fibers in cell culture. The proximal 1358 bp of slow MyHC2 upstream DNA contains a functional E-box and binding sites for myocyte enhancer factor 2 (MEF2) and nuclear factor of activated T cells (NFAT). Mutagenesis studies indicated that both MEF2 and NFAT binding sites are required for innervation-induced slow MyHC2 promoter activity in MA muscle fibers. However, MEF2 transcription factor activity was unaffected by innervation and did not demonstrate fiber type-specific interactions with the slow MyHC2 MEF2 binding site. NFAT transcription factor activity did increase in innervated MA muscle fibers and not in PM muscle fibers, indicating innervation and muscle fiber type-specific regulation. However, transfection of constitutively active NFAT indicated that NFAT is insufficient to induce slow MyHC2 gene expression in either fast PM or slow MA muscle fibers without innervation. These results indicate the requirement for MEF2 and NFAT in innervation-induced slow MyHC2 gene expression and suggest that additional innervation-dependent and fiber type-specific control of slow MyHC2 gene expression resides in MA and PM muscle fibers, respectively.