Mutagenesis of the myogenin basic region identifies an ancient protein motif critical for activation of myogenesis

Optimisation of cell fate determination for cultivated muscle differentiation

Production of cultivated meat requires defined medium formulations for the robust differentiation of myogenic cells into mature skeletal muscle fibres in vitro. Although these formulations can drive myogenic differentiation levels comparable to serum-starvation-based protocols, the resulting cultures are often heterogeneous, with a significant proportion of cells not participating in myofusion, limiting maturation of the muscle. To address this problem, we employed RNA sequencing to analyse heterogeneity in differentiating bovine satellite cells at single-nucleus resolution, identifying distinct cellular subpopulations including proliferative cells that fail to exit the cell cycle and quiescent 'reserve cells' that do not commit to myogenic differentiation. Our findings indicate that the MEK/ERK, NOTCH, and RXR pathways are active during the initial stages of myogenic cell fate determination, and by targeting these pathways, we can promote cell cycle exit while reducing reserve cell formation. This optimised medium formulation consistently yields fusion indices close to 100% in 2D culture. Furthermore, we demonstrate that these conditions enhance myotube formation and actomyosin accumulation in 3D bovine skeletal muscle constructs, providing proof of principle for the generation of highly differentiated cultivated muscle with excellent mimicry to traditional muscle.

Characterization of myogenic regulatory factors, myod and myf5 from Megalobrama amblycephala and the effect of lipopolysaccharide on satellite cells in skeletal muscle

Myod and Myf5 are muscle-specific basic helix-loop-helix (bHLH) transcription factors that play essential roles in regulating skeletal muscle development and growth. In order to investigate potential function of myod and myf5 of Megalobrama amblycephala, an economically important freshwater fish species, in the present study, we characterized the sequences and expression profiles of M. amblycephala myod and myf5. The open reading frame (ORF) sequences of myod and myf5 encoded 275 and 240 amino acids, respectively, possessing analogous structure with the highly conserved domains, bHLH and C-terminal helix III domains. Spatio-temporal expression patterns revealed that myod and myf5 were predominant in skeletal muscle with the highest expression in white muscle, and the highest at 10 days post-hatching (dph) and the segmentation period, respectively. Furthermore, we evaluated the effects of lipopolysaccharide (LPS) on the expression of muscle-related genes in white and red muscle, and proliferation and differentiation of satellite cells. The myod, myf5 and pax-7 expression generally increased and then decreased with increase of LPS concentration and treatment time in red muscle, while these genes showed inconsistent expression patterns in white muscle. In addition, LPS administration caused the frequency increase of satellite cells in red and white muscle especially at 3 and 7 days after LPS-injection.

Functional Characterization of an In-Frame Deletion in the Basic Domain of the Retinal Transcription Factor ATOH7

Basic helix–loop–helix (bHLH) transcription factors are evolutionarily conserved and structurally similar proteins important in development. The temporospatial expression of atonal bHLH transcription factor 7 (ATOH7) directs the differentiation of retinal ganglion cells and mutations in the human gene lead to vitreoretinal and/or optic nerve abnormalities. Characterization of pathogenic ATOH7 mutations is needed to understand the functions of the conserved bHLH motif. The published ATOH7 in-frame deletion p.(Arg41_Arg48del) removes eight highly conserved amino acids in the basic domain. We functionally characterized the mutant protein by expressing V5-tagged ATOH7 constructs in human embryonic kidney 293T (HEK293T) cells for subsequent protein analyses, including Western blot, cycloheximide chase assays, Förster resonance energy transfer fluorescence lifetime imaging, enzyme-linked immunosorbent assays and dual-luciferase assays. Our results indicate that the in-frame deletion in the basic domain causes mislocalization of the protein, which can be rescued by a putative dimerization partner transcription factor 3 isoform E47 (E47), suggesting synergistic nuclear import. Furthermore, we observed (i) increased proteasomal degradation of the mutant protein, (ii) reduced protein heterodimerization, (iii) decreased DNA-binding and transcriptional activation of a reporter gene, as well as (iv) inhibited E47 activity. Altogether our observations suggest that the DNA-binding basic domain of ATOH7 has additional roles in regulating the nuclear import, dimerization, and protein stability.

Mechanisms of Binding Specificity among bHLH Transcription Factors

The transcriptome of every cell is orchestrated by the complex network of interaction between transcription factors (TFs) and their binding sites on DNA. Disruption of this network can result in many forms of organism malfunction but also can be the substrate of positive natural selection. However, understanding the specific determinants of each of these individual TF-DNA interactions is a challenging task as it requires integrating the multiple possible mechanisms by which a given TF ends up interacting with a specific genomic region. These mechanisms include DNA motif preferences, which can be determined by nucleotide sequence but also by DNA’s shape; post-translational modifications of the TF, such as phosphorylation; and dimerization partners and co-factors, which can mediate multiple forms of direct or indirect cooperative binding. Binding can also be affected by epigenetic modifications of putative target regions, including DNA methylation and nucleosome occupancy. In this review, we describe how all these mechanisms have a role and crosstalk in one specific family of TFs, the basic helix-loop-helix (bHLH), with a very conserved DNA binding domain and a similar DNA preferred motif, the E-box. Here, we compile and discuss a rich catalog of strategies used by bHLH to acquire TF-specific genome-wide landscapes of binding sites.

Combination of cell signaling molecules can facilitate MYOD1-mediated myogenic transdifferentiation of pig fibroblasts

Background

Myogenic transdifferentiation can be accomplished through ectopic MYOD1 expression, which is facilitated by various signaling pathways associated with myogenesis. In this study, we attempted to transdifferentiate pig embryonic fibroblasts (PEFs) myogenically into skeletal muscle through overexpression of the pig MYOD1 gene and modulation of the FGF, TGF-β, WNT, and cAMP signaling pathways.

Results

The MYOD1 overexpression vector was constructed based on comparative sequence analysis, demonstrating that pig MYOD1 has evolutionarily conserved domains across various species. Although forced MYOD1 expression through these vectors triggered the expression of endogenous muscle markers, transdifferentiated muscle cells from fibroblasts were not observed. Therefore, various signaling molecules, including FGF2, SB431542, CHIR99021, and forskolin, along with MYOD1 overexpression were applied to enhance the myogenic reprogramming. The modified conditions led to the derivation of myotubes and activation of muscle markers in PEFs, as determined by qPCR and immunostaining. Notably, a sarcomere-like structure was observed, indicating that terminally differentiated skeletal muscle could be obtained from transdifferentiated cells.

Conclusions

In summary, we established a protocol for reprogramming MYOD1-overexpressing PEFs into the mature skeletal muscle using signaling molecules. Our myogenic reprogramming can be used as a cell source for muscle disease models in regenerative medicine and the production of cultured meat in cellular agriculture.

The Ciona myogenic regulatory factor functions as a typical MRF but possesses a novel N-terminus that is essential for activity

Electroporation-based assays were used to test whether the myogenic regulatory factor (MRF) of Ciona intestinalis (CiMRF) interferes with endogenous developmental programs, and to evaluate the importance of its unusual N-terminus for muscle development. We found that CiMRF suppresses both notochord and endoderm development when it is expressed in these tissues by a mechanism that may involve activation of muscle-specific microRNAs. Because these results add to a large body of evidence demonstrating the exceptionally high degree of functional conservation among MRFs, we were surprised to discover that non-ascidian MRFs were not myogenic in Ciona unless they formed part of a chimeric protein containing the CiMRF N-terminus. Equally surprising, we found that despite their widely differing primary sequences, the N-termini of MRFs of other ascidian species could form chimeric MRFs that were also myogenic in Ciona. This domain did not rescue the activity of a Brachyury protein whose transcriptional activation domain had been deleted, and so does not appear to constitute such a domain. Our results indicate that ascidians have previously unrecognized and potentially novel requirements for MRF-directed myogenesis. Moreover, they provide the first example of a domain that is essential to the core function of an important family of gene regulatory proteins, one that, to date, has been found in only a single branch of the family.

The myogenic regulatory circuit that controls cardiac/slow twitch troponin C gene transcription in skeletal muscle involves E-box, MEF-2, and MEF-3 motifs

We have characterized the specific DNA regulatory elements responsible for the function of the human cardiac troponin C gene (cTnC) muscle-specific enhancer in myogenic cells. We used functional transient transfection assays with deletional and site-specific mutagenesis to evaluate the role of the conserved sequence elements. Gel electrophoresis mobility shift assays (EMSA) demonstrated the ability of the functional sites to interact with nuclear proteins. We demonstrate that three distinct transcription activator binding sites commonly found in muscle-specific enhancers (a MEF-2 site, a MEF-3 site, and at least four redundant E-box sites) all contribute to full enhancer activity but a CArG box does not. Mutation of either the MEF-2 or MEF-3 sites or deletion of the E-boxes reduces expression by 70% or more. Furthermore, the MEF-2 site and the E-boxes specifically bind, respectively, to MEF-2 and myogenic determination factors derived from nuclear extracts. EMSA assays using a MEF-3 containing oligonucleotide revealed indistinguishable separation patterns with extracts from myogenic cells and nonmyogenic cells. These data suggest that expression of the cTnC gene in slow-twitch skeletal muscle is sustained through complex interactions at the 3'Ile enhancer between muscle-specific and nontissue-specific transcription factors: either a myogenic bHLH complex or MEF-2 can activate transcription but only in the presence of a third transcriptional activator that appears not to be muscle specific. We conclude from these observations that the cTnC 3'Ile element is a composite enhancer that functions through the combined interactions of at least five regulatory elements and their cognate binding factors: three or four E-boxes, a MEF-2 site, and a MEF-3 site. The data support the notion that all of these sites contribute to enhancer function in cell systems in an additive way but that none are absolutely required for enhancer activity. The data imply that the levels of transcription of cTnC in myogenic tissues in which the activities of one of the transcriptional factors is lacking would be partially but not wholly suppressed. Our data support the critical role of E-box sites in conjunction with the adjacent elements. Hence, we assign CTnC gene regulation to the "ordinary" rather than to the "novel" category of transcriptional regulation during skeletal myogenesis.

Mef2 and the skeletal muscle differentiation program

Mef2 is a conserved and significant transcription factor in the control of muscle gene expression. In cell culture Mef2 synergises with MyoD-family members in the activation of gene expression and in the conversion of fibroblasts into myoblasts. Amongst its in vivo roles, Mef2 is required for both Drosophila muscle development and mammalian muscle regeneration. Mef2 has functions in other cell-types too, but this review focuses on skeletal muscle and surveys key findings on Mef2 from its discovery, shortly after that of MyoD, up to the present day. In particular, in vivo functions, underpinning mechanisms and areas of uncertainty are highlighted. We describe how Mef2 sits at a nexus in the gene expression network that controls the muscle differentiation program, and how Mef2 activity must be regulated in time and space to orchestrate specific outputs within the different aspects of muscle development. A theme that emerges is that there is much to be learnt about the different Mef2 proteins (from different paralogous genes, spliced transcripts and species) and how the activity of these proteins is controlled.

Ascl2 inhibits myogenesis by antagonizing the transcriptional activity of myogenic regulatory factors

Myogenic regulatory factors (MRFs), including Myf5, MyoD (Myod1) and Myog, are muscle-specific transcription factors that orchestrate myogenesis. Although MRFs are essential for myogenic commitment and differentiation, timely repression of their activity is necessary for the self-renewal and maintenance of muscle stem cells (satellite cells). Here, we define Ascl2 as a novel inhibitor of MRFs. During mouse development, Ascl2 is transiently detected in a subpopulation of Pax7⁺ MyoD⁺ progenitors (myoblasts) that become Pax7⁺ MyoD^- satellite cells prior to birth, but is not detectable in postnatal satellite cells. Ascl2 knockout in embryonic myoblasts decreases both the number of Pax7⁺ cells and the proportion of Pax7⁺ MyoD^- cells. Conversely, overexpression of Ascl2 inhibits the proliferation and differentiation of cultured myoblasts and impairs the regeneration of injured muscles. Ascl2 competes with MRFs for binding to E-boxes in the promoters of muscle genes, without activating gene transcription. Ascl2 also forms heterodimers with classical E-proteins to sequester their transcriptional activity on MRF genes. Accordingly, MyoD or Myog expression rescues myogenic differentiation despite Ascl2 overexpression. Ascl2 expression is regulated by Notch signaling, a key governor of satellite cell self-renewal. These data demonstrate that Ascl2 inhibits myogenic differentiation by targeting MRFs and facilitates the generation of postnatal satellite cells.

Molecular characterization and expression patterns of myogenin in compensatory growth of Megalobrama amblycephala

Myogenin (myog) is a muscle-specific basic helix-loop-helix (bHLH) transcription factor that plays an essential role in regulating skeletal muscle development and growth. To investigate molecular characterization of myog and the effect of starvation/refeeding on the gene expression, we isolated the myog cDNA sequence and analyzed the expression patterns using quantitative real-time polymerase chain reaction in Megalobrama amblycephala. Sequence analysis indicated that M. amblycephala myog shared an analogous structure with the highly conserved His/Cys-rich, bHLH and C-terminal helix III domains with other vertebrates. Sequence alignment and phylogenetic tree showed that M. amblycephala myog had the highest identity with the homologues of Ctenopharyngodon idella and Cyprinus carpio. Spatio-temporal expression patterns revealed that myog mRNA levels at the segmentation period and 12 h post-hatching (hph) were significantly higher than at other development stages (P<0.05). Furthermore, the highest myog expression level was predominantly observed in white muscle compared with the other types of muscle. Fish body weight continuously decreased during 21-day starvation and then significantly increased after 7days of refeeding and reached the similar level to the control at 21days of refeeding, indicating that the pattern of complete compensatory growth possibly occurred in M. amblycephala; meanwhile, the relative somatic growth rate after refeeding was also dramatically higher than the control group. In addition, the myog expression decreased during 21days of starvation and then exhibited a strong rebound effect after 7days of refeeding and subsequently declined gradually to the control level by 21days of refeeding.

Molecular and cellular regulation of skeletal myogenesis

Since the seminal discovery of the cell-fate regulator Myod, studies in skeletal myogenesis have inspired the search for cell-fate regulators of similar potential in other tissues and organs. It was perplexing that a similar transcription factor for other tissues was not found; however, it was later discovered that combinations of molecular regulators can divert somatic cell fates to other cell types. With the new era of reprogramming to induce pluripotent cells, the myogenesis paradigm can now be viewed under a different light. Here, we provide a short historical perspective and focus on how the regulation of skeletal myogenesis occurs distinctly in different scenarios and anatomical locations. In addition, some interesting features of this tissue underscore the importance of reconsidering the simple-minded view that a single stem cell population emerges after gastrulation to assure tissuegenesis. Notably, a self-renewing long-term Pax7+ myogenic stem cell population emerges during development only after a first wave of terminal differentiation occurs to establish a tissue anlagen in the mouse. How the future stem cell population is selected in this unusual scenario will be discussed. Recently, a wealth of information has emerged from epigenetic and genome-wide studies in myogenic cells. Although key transcription factors such as Pax3, Pax7, and Myod regulate only a small subset of genes, in some cases their genomic distribution and binding are considerably more promiscuous. This apparent nonspecificity can be reconciled in part by the permissivity of the cell for myogenic commitment, and also by new roles for some of these regulators as pioneer transcription factors acting on chromatin state.

Functional studies of the Ciona intestinalis myogenic regulatory factor reveal conserved features of chordate myogenesis

Ci-MRF is the sole myogenic regulatory factor (MRF) of the ascidian Ciona intestinalis, an invertebrate chordate. In order to investigate its properties we developed a simple in vivo assay based on misexpressing Ci-MRF in the notochord of Ciona embryos. We used this assay to examine the roles of three structural motifs that are conserved among MRFs: an alanine-threonine (Ala-Thr) dipeptide of the basic domain that is known in vertebrates as the myogenic code, a cysteine/histidine-rich (C/H) domain found just N-terminal to the basic domain, and a carboxy-terminal amphipathic α-helix referred to as Helix III. We show that the Ala-Thr dipeptide is necessary for normal Ci-MRF function, and that while eliminating the C/H domain or Helix III individually has no demonstrable effect on Ci-MRF, simultaneous loss of both motifs significantly reduces its activity. Our studies also indicate that direct interaction between CiMRF and an essential E-box of Ciona Troponin I is required for the expression of this muscle-specific gene and that multiple classes of MRF-regulated genes exist in Ciona. These findings are consistent with substantial conservation of MRF-directed myogenesis in chordates and demonstrate for the first time that the Ala/Thr dipeptide of the basic domain of an invertebrate MRF behaves as a myogenic code.

Structure and functions of powerful transactivators: VP16, MyoD and FoxA

Induced pluripotent stem cell (iPSC) technology is a promising approach for converting one type of a differentiated cell into another type of differentiated cell through a pluripotent state as an intermediate step. Recent studies, however, indicate the possibility of directly converting one cell type to another without going through a pluripotent state. This direct reprogramming approach is dependent on a combination of highly potent transcription factors for cell-type conversion, presumably skipping more physiological and multi-step differentiation processes. A trial-and-error strategy is commonly used to screen many candidate transcription factors to identify the correct combination of factors. We speculate, however, that a better understanding of the functional mechanisms of exemplary transcriptional activators will facilitate the identification of novel factor combinations capable of direct reprogramming. The purpose of this review is to critically examine the literature on three highly potent transcriptional activators: the herpes virus protein, VP16; the master regulator of skeletal muscle differentiation, MyoD and the "pioneer" factor for hepatogenesis, FoxA. We discuss the roles of their functional protein domains, interacting partners and chromatin remodeling mechanisms during gene activation to understand how these factors open the chromatin of inactive genes and reset the transcriptional pattern during cell type conversion.

Chromatin immunoprecipitation assay for tissue-specific genes using early-stage mouse embryos

Chromatin immunoprecipitation (ChIP) is a powerful tool to identify protein:chromatin interactions that occur in the context of living cells. This technique has been widely exploited in tissue culture cells, and to a lesser extent, in primary tissue. The application of ChIP to rodent embryonic tissue, especially at early times of development, is complicated by the limited amount of tissue and the heterogeneity of cell and tissue types in the embryo. Here we present a method to perform ChIP using a dissociated embryonic day 8.5 (E8.5) embryo. Sheared chromatin from a single E8.5 embryo can be divided into up to five aliquots, which allows the investigator sufficient material for controls and for investigation of specific protein:chromatin interactions. We have utilized this technique to begin to document protein:chromatin interactions during the specification of tissue-specific gene expression programs. The heterogeneity of cell types in an embryo necessarily restricts the application of this technique because the result is the detection of protein:chromatin interactions without distinguishing whether the interactions occur in all, a subset of, or a single cell type(s). However, examination of tissue-specific genes during or following the onset of tissue-specific gene expression is feasible for two reasons. First, immunoprecipitation of tissue specific factors necessarily isolates chromatin from the cell type where the factor is expressed. Second, immunoprecipitation of coactivators and histones containing post-translational modifications that are associated with gene activation should only be found at genes and gene regulatory sequences in the cell type where the gene is being or has been activated. The technique should be applicable to the study of most tissue-specific gene activation events. In the example described below, we utilized E8.5 and E9.5 mouse embryos to examine factor binding at a skeletal muscle specific gene promoter. Somites, which are the precursor tissues from which the skeletal muscles of the trunk and limbs will form, are present at E8.5-9.5. Myogenin is a regulatory factor required for skeletal muscle differentiation. The data demonstrate that myogenin is associated with its own promoter in E8.5 and E9.5 embryos. Because myogenin is only expressed in somites at this stage of development, the data indicate that myogenin interactions with its own promoter have already occurred in skeletal muscle precursor cells in E8.5 embryos.

MyoD directly up-regulates premyogenic mesoderm factors during induction of skeletal myogenesis in stem cells

Gain- and loss-of-function experiments have illustrated that the family of myogenic regulatory factors is necessary and sufficient for the formation of skeletal muscle. Furthermore, MyoD required cellular aggregation to induce myogenesis in P19 embryonal carcinoma stem cells. To determine the mechanism by which stem cells can be directed into skeletal muscle, a time course of P19 cell differentiation was examined in the presence and absence of exogenous MyoD. By quantitative PCR, the first MyoD up-regulated transcripts were the premyogenic mesoderm factors Meox1, Pax7, Six1, and Eya2 on day 4 of differentiation. Subsequently, the myoblast markers myogenin, MEF2C, and Myf5 were up-regulated, leading to skeletal myogenesis. These results were corroborated by Western blot analysis, showing up-regulation of Pax3, Six1, and MEF2C proteins, prior to myogenin protein expression. To determine at what stage a dominant-negative MyoD/EnR mutant could inhibit myogenesis, stable cell lines were created and examined. Interestingly, the premyogenic mesoderm factors, Meox1, Pax3/7, Six1, Eya2, and Foxc1, were down-regulated, and as expected, skeletal myogenesis was abolished. Finally, to identify direct targets of MyoD in this system, chromatin immunoprecipitation experiments were performed. MyoD was observed associated with regulatory regions of Meox1, Pax3/7, Six1, Eya2, and myogenin genes. Taken together, MyoD directs stem cells into the skeletal muscle lineage by binding and activating the expression of premyogenic mesoderm genes, prior to activating myoblast genes.

Quadruplex structures of muscle gene promoter sequences enhance in vivo MyoD-dependent gene expression

Gene promoters are enriched in guanine clusters that potentially fold into quadruplex structures. Such quadruplexes were implicated in the regulation of gene expression, plausibly by interacting with transcription factors. We showed previously that homodimers of the myogenic transcription factor MyoD bound in vitro most tightly bimolecular quadruplexes of promoter sequences of muscle-specific genes. By contrast, MyoD-E47 heterodimers formed tighter complexes with d(CANNTG) E-box motifs that govern muscle gene expression. Here, we show that DNA quadruplexes enhance in vivo MyoD and E-box-driven expression of a firefly luciferase (FL) reporter gene. HEK293 cells were transfected with FL expressing p4RTK-FL vector alone or together with MyoD expressing pEMSV-MyoD plasmid, with quadruplexes of α7 integrin or sarcomeric mitochondrial creatine kinase (sMtCK) muscle gene promoters or with a combination thereof. Whereas MyoD elevated by ∼10-fold the levels of FL mRNA and protein, the DNA quadruplexes by themselves did not affect FL expression. However, together with MyoD, quadruplex DNA increased by ∼35-fold the amounts of FL mRNA and protein. Without affecting its expression, DNA quadruplexes bound MyoD in the cells. Based on these results, we propose models for the regulation of muscle gene transcription by direct interaction of MyoD with promoter quadruplex structures.

Myogenesis and molecules - insights from zebrafish Danio rerio

Myogenesis is a fundamental process governing the formation of muscle in multicellular organisms. Recent studies in zebrafish Danio rerio have described the molecular events occurring during embryonic morphogenesis and have thus greatly clarified this process, helping to distinguish between the events that give rise to fast v. slow muscle. Coupled with the well-known Hedgehog signalling cascade and a wide variety of cellular processes during early development, the continual research on D. rerio slow muscle precursors has provided novel insights into their cellular behaviours in this organism. Similarly, analyses on fast muscle precursors have provided knowledge of the behaviour of a sub-set of epitheloid cells residing in the anterior domain of somites. Additionally, the findings by various groups on the roles of several molecules in somitic myogenesis have been clarified in the past year. In this study, the authors briefly review the current trends in the field of research of D. rerio trunk myogenesis.

DNA binding-dependent and -independent functions of the Hand2 transcription factor during mouse embryogenesis

The basic helix-loop-helix (bHLH) transcription factor Hand2 is required for growth and development of the heart, branchial arches and limb buds. To determine whether DNA binding is required for Hand2 to regulate the growth and development of these different embryonic tissues, we generated mutant mice in which the Hand2 locus was modified by a mutation (referred to as Hand2(EDE)) that abolished the DNA-binding activity of Hand2, leaving the remainder of the protein intact. In contrast to Hand2 null embryos, which display right ventricular hypoplasia and vascular abnormalities, causing severe growth retardation by E9.5 and death by E10.5, early development of the heart appeared remarkably normal in homozygous Hand2(EDE) mutant embryos. These mutant embryos also lacked the early defects in growth of the branchial arches seen in Hand2 null embryos and survived up to 2 to 3 days longer than did Hand2 null embryos. However, Hand2(EDE) mutant embryos exhibited growth defects in the limb buds similar to those of Hand2 null embryos. These findings suggest that Hand2 regulates tissue growth and development in vivo through DNA binding-dependent and -independent mechanisms.

Differential binding of quadruplex structures of muscle-specific genes regulatory sequences by MyoD, MRF4 and myogenin

Four myogenic regulatory factors (MRFs); MyoD, Myf-5, MRF4 and Myogenin direct muscle tissue differentiation. Heterodimers of MRFs with E-proteins activate muscle-specific gene expression by binding to E-box motifs d(CANNTG) in their promoters or enhancers. We showed previously that in contrast to the favored binding of E-box by MyoD-E47 heterodimers, homodimeric MyoD associated preferentially with quadruplex structures of regulatory sequences of muscle-specific genes. To inquire whether other MRFs shared the DNA binding preferences of MyoD, the DNA affinities of hetero- and homo-dimeric MyoD, MRF4 and Myogenin were compared. Similarly to MyoD, heterodimers with E47 of MRF4 or Myogenin bound E-box more tightly than quadruplex DNA. However, unlike homodimeric MyoD or MRF4, Myogenin homodimers associated weakly and nonpreferentially with quadruplex DNA. By reciprocally switching basic regions between MyoD and Myogenin we demonstrated dominance of MyoD in determining the quadruplex DNA-binding affinity. Thus, Myogenin with an implanted MyoD basic region bound quadruplex DNA nearly as tightly as MyoD. However, a grafted Myogenin basic region did not diminish the high affinity of homodimeric MyoD for quadruplex DNA. We speculate that the dissimilar interaction of MyoD and Myogenin with tetrahelical domains in muscle gene promoters may differently regulate their myogenic activities.

MyoD uses overlapping but distinct elements to bind E-box and tetraplex structures of regulatory sequences of muscle-specific genes

Muscle differentiation and expression of muscle-specific proteins are initiated by the binding of heterodimers of the transcription factor MyoD with E2A proteins to E-box motif d(CANNTG) in promoters or enhancers of muscle-specific genes. MyoD homodimers, however, form tighter complexes with tetraplex structures of guanine-rich regulatory sequences of some muscle genes. In this work, we identified elements in MyoD that bind E-box or tetraplex structures of promoter sequences of the muscle-specific genes α7 integrin and sarcomeric Mitochondrial Creatine Kinase (sMtCK). Deletions of large domains of the 315 amino acids long recombinant MyoD indicated that the binding site for both E-box and tetraplex DNA is its basic region KRKTTNADRRKAATMRERRR that encompasses the three underlined clusters of basic residues designated R₁, R₂ and R₃. Deletion of a single or pairs of R triads or R111C substitution completely abolished the E-box-binding capacity of MyoD. By contrast, the MyoD deletion mutants Δ102–114, ΔR₃, ΔR₁R₃ or ΔR₂R₃ maintained comparable tetraplex DNA-binding capacity as reflected by the similar dissociation constants of their protein–DNA complexes. Only deletion of all three basic clusters abolished the binding of tetraplex DNA. Implications of the binding of E-box and tetraplex DNA by non-identical MyoD elements are considered.

Determinants of myogenic specificity within MyoD are required for noncanonical E box binding

The MyoD family of basic helix-loop-helix (bHLH) transcription factors has the remarkable ability to induce myogenesis in vitro and in vivo. This myogenic specificity has been mapped to two amino acids in the basic domain, an alanine and threonine, referred to as the myogenic code. These essential determinants of myogenic specificity are conserved in all MyoD family members from worms to humans, yet their function in myogenesis is unclear. Induction of the muscle transcriptional program requires that MyoD be able to locate and stably bind to sequences present in the promoter regions of critical muscle genes. Recent studies have shown that MyoD binds to noncanonical E boxes in the myogenin gene, a critical locus required for myogenesis, through interactions with resident heterodimers of the HOX-TALE transcription factors Pbx1A and Meis1. In the present study, we show that the myogenic code is required for MyoD to bind to noncanonical E boxes in the myogenin promoter and for the formation of a tetrameric complex with Pbx/Meis. We also show that these essential determinants of myogenesis are sufficient to confer noncanonical E box binding to the E12 basic domain. Thus, these data show that noncanonical E box binding correlates with myogenic potential, and we speculate that the myogenic code residues in MyoD function as myogenic determinants via their role in noncanonical E box binding and recognition.

Molecular cloning and expression of grass carp MyoD in yeast Pichia pastoris

MyoD, expressed in skeletal muscle lineages of vertebrate embryo, is one of muscle-specific basic helix-loop-helix (bHLH) transcription factors, which plays a key role in the determination and differentiation of all skeletal muscle lineages. In this study, a cDNA of grass carp MyoD was cloned and characterized from total RNA of grass carp embryos by RT-PCR. The full-length cDNA of grass carp MyoD is 1597 bp. The cDNA sequence analysis reveals an open reading frame of 825 bp coding for a protein of 275 amino acids, which includes a bHLH domain composed of basic domain (1-84(th) amino acids) and HLH domain (98-142(th) amino acids), without signal peptide. Then the MyoD cDNA of grass carp was cloned to yeast expression vector pPICZalphaA and transformed into P. pastoris GS115 strain, the recombinant MyoD protein with a molecular weight of about 31KD was obtained after inducing for 2d with 0.5% methanol in pH 8.0 BMGY medium, and the maximum yield was about 250 mg/L in shaking-flask fermentation. The results were expected to benefit for further studies on the crystal structure and physiological function of fish MyoD.

Myogenin in model pufferfish species: Comparative genomic analysis and thermal plasticity of expression during early development

Myogenin (Myog) is a muscle-specific basic helix-loop-helix transcription factor that plays an essential role in the specification and differentiation of myoblasts. The myogenin genes from the tiger pufferfish, Takifugu rubripes, and green-spotted pufferfish, Tetraodon nigroviridis, were cloned and a comparative genomic analysis performed. The gene encoding myogenin is composed of three exons and has a relatively similar genomic structure in T. rubripes, T. nigroviridis and human. Introns 1 and 2 were approximately 2-fold and 8-fold longer respectively in human than pufferfish. Myogenin is located in a 100 kb region of conserved synteny between these organisms, corresponding to chromosome 1 in human, chromosome 11 in T. nigroviridis and scaffold 208 in T. rubripes. Pufferfish myogenin contained a serine-rich region at the carboxyl terminus that is highly conserved amongst teleosts. During embryonic development of T. rubripes, myogenin was expressed in a rostral-caudal gradient in the developing somites and subsequently during the pharyngula period in the pectoral fin bud primordia, jaw muscles and extraocular muscle precursors. In T. rubripes, the time required to form a somite pair during the linear phase of somitogenesis ( identical withsomite-interval) was 122 min, 97 min and 50 min in embryos incubated at 15, 18 and 21 degrees C, respectively. Myogenin mRNA transcripts were quantified using qPCR and normalised to the highest level of expression. Peak myogenin expression occurred later with respect to developmental stage (standardised using somite-intervals) and was over 2-fold higher at 21 degrees C than at either 18 or 15 degrees C. Changes in the relative timing and intensity of myogenin expression are a potential mechanism for explaining thermal plasticity of muscle phenotype in larvae via effects on the differentiation programme.

The basic domain of ATH5 mediates neuron-specific promoter activity during retina development

In the developing retina, the gene encoding the beta3 subunit of the neuronal nicotinic receptor, a specific marker of retinal ganglion cells, is under the direct control of the atonal homolog 5 (ATH5) basic helix-loop-helix (bHLH) transcription factor. Although quite short (143 bp in length), the beta3 promoter has the remarkable capacity to discriminate between ATH5 and the other neuronal bHLH proteins expressed in the developing nervous system. We have identified three amino acids within the basic domain that confer specificity to the ATH5 protein. These residues do not mediate direct DNA binding but are required for interaction between ATH5 and chromatin-associated proteins during retina development. When misexpressed in neurons, the myogenic bHLH factor MyoD is also able to activate the beta3 gene. This, however, is achieved not by binding of the protein to the promoter but by dimerization of MyoD with a partner, a process that depends not on the basic domain but on the HLH domain. By sequestering an E-box-binding protein, MyoD relieves the active repression that blocks the beta3 promoter in most neurons. The mechanisms used by bHLH proteins to activate beta3 thus highlight how ATH5 is selected by the beta3 promoter and coordinates the derepression and transcriptional activation of the beta3 gene during the specification of retinal ganglion cells.

The circuitry of a master switch: Myod and the regulation of skeletal muscle gene transcription

The expression of Myod is sufficient to convert a fibroblast to a skeletal muscle cell, and, as such, is a model system in developmental biology for studying how a single initiating event can orchestrate a highly complex and predictable response. Recent findings indicate that Myod functions in an instructive chromatin context and directly regulates genes that are expressed throughout the myogenic program, achieving promoter-specific regulation of its own binding and activity through a feed-forward mechanism. These studies are beginning to merge our understanding of how lineage-specific information is encoded in chromatin with how master regulatory factors drive programs of cell differentiation.

MyoD and the transcriptional control of myogenesis

The basic helix-loop-helix myogenic regulatory factors MyoD, Myf5, myogenin and MRF4 have critical roles in skeletal muscle development. Together with the Mef2 proteins and E proteins, these transcription factors are responsible for coordinating muscle-specific gene expression in the developing embryo. This review highlights recent studies regarding the molecular mechanisms by which the muscle-specific myogenic bHLH proteins interact with other regulatory factors to coordinate gene expression in a controlled and ordered manner.

Porcine MYF6 gene: sequence, homology analysis, and variation in the promoter region

MYF6 gene codes for the bHLH transcription factor belonging to MyoD family. Its expression accompanies the processes of differentiation and maturation of myotubes during embriogenesis and continues on a relatively high level after birth, affecting the muscle phenotype. The porcine MYF6 gene was amplified and sequenced and compared with MYF6 gene sequences of other species. The amino acid sequence was deduced and an interspecies homology analysis was performed. Myf-6 protein shows a high conservation among species of 99 and 97% identity when comparing pig with cow and human, respectively, and of 93% when comparing pig with mouse and rat. The single nucleotide polymorphism (SNP) was revealed within the promoter region, which appeared to be T --> C transition recognized by a MspI restriction enzyme.

Paraxis is a basic helix-loop-helix protein that positively regulates transcription through binding to specific E-box elements

Members of the Twist subfamily of basic helix-loop-helix transcription factors are important for the specification of mesodermal derivatives during vertebrate embryogenesis. This subfamily includes both transcriptional activators such as scleraxis, Hand2, and Dermo-1 and repressors such as Twist and Hand1. Paraxis is a member of this subfamily, and it has been shown to regulate morphogenetic events during somitogenesis, including the transition of cells from mesenchyme to epithelium and maintaining anterior/posterior polarity. Mice deficient in paraxis exhibit a caudal truncation of the axial skeleton and fusion of the vertebrae. Considering the developmental importance of paraxis, it is important for future studies to understand the molecular basis of its activity. Here we demonstrate that paraxis can function as a transcriptional activator when it forms a heterodimer with E12. Paraxis is able to bind to a set of E-boxes that overlaps with the closely related scleraxis. Paraxis expression precedes that of scleraxis in the region of the somite fated to form the axial skeleton and tendons and is able to direct transcription from an E-box found in the scleraxis promoter. Further, in the absence of paraxis, Pax-1 is no longer expressed in the somites and presomitic mesoderm. These results suggest that paraxis may regulate early events during chondrogenesis by positively directing transcription of sclerotome-specific genes.

Cloning and phylogenetic analysis of an amphioxus myogenic bHLH gene AmphiMDF

In this study, a member of the MyoD gene family, AmphiMDF, was isolated from the embryos of amphioxus by degenerate PCR, followed by rapid amplification of cDNA ends (RACE). Southern blot analysis confirmed that only a single myogenic bHLH gene was present in the genome of amphioxus Branchiostoma belcheri tsingtauense. Sequence and phylogenetic analyses indicated that AmphiMDF falls at the base of its vertebrate homologs. The amino acid sequence of AmphiMDF was almost equally similar to those of the four clusters of the vertebrate MyoD family. This suggests that AmphiMDF is not only the sister but also the archetype of the vertebrate myogenic bHLH genes. The scenarios to explain the origin of the vertebrate MyoD gene family from the ancestral myogenic bHLH gene like AmphiMDF are also discussed.

Misexpression of dHAND induces ectopic digits in the developing limb bud in the absence of direct DNA binding

Basic helix-loop-helix (bHLH) transcription factors control developmental decisions in a wide range of embryonic cell types. The HLH motif mediates homo- and heterodimerization, which juxtaposes the basic regions within the dimeric complex to form a bipartite DNA binding domain that recognizes a DNA consensus sequence known as an E-box. eHAND and dHAND (also known as HAND1 and HAND2) are closely related bHLH proteins that control cardiac, craniofacial and limb development. Within the developing limb, dHAND expression encompasses the zone of polarizing activity in the posterior region, where it has been shown to be necessary and sufficient to induce the expression of the morphogen sonic hedgehog. Misexpression of dHAND in the anterior compartment of the limb bud induces ectopic expression of sonic hedgehog, with resulting preaxial polydactyly and mirror image duplications of posterior digits. To investigate the potential transcriptional mechanisms involved in limb patterning by dHAND, we have performed a structure-function analysis of the protein in cultured cells and ectopically expressed dHAND mutant proteins in the developing limbs of transgenic mice. We show that an N-terminal transcriptional activation domain, and the bHLH region, are required for E-box-dependent transcription in vitro. Remarkably, however, digit duplication by dHAND requires neither the transcriptional activation domain nor the basic region, but only the HLH motif. eHAND has a similar limb patterning activity to dHAND in these misexpression experiments, indicating a conserved function of the HLH regions of these proteins. These findings suggest that dHAND may act via novel transcriptional mechanisms mediated by protein-protein interactions independent of direct DNA binding.

The functional specificity of NeuroD is defined by a single amino acid residue (N11) in the basic domain

In zebrafish, the basic helix-loop-helix (bHLH) gene neuroD specifies distinct neurons in the spinal cord. A preliminary experiment indicated that a related bHLH gene, ndr1a, normally expressed only in the olfactory organ in late embryos, also functions as neuroD to induce ectopic formation of spinal cord neurons in early embryos after introduction of its mRNA into early embryos. To define the functional specificity of these bHLH proteins, several mutant forms with selected point mutations in the basic domain were constructed and tested for inducing sensory neurons in the spinal cord. Our data indicate that the functional specificity of NeuroD to define sensory neurons is mainly due to a single residue (asparagine 11) in its basic domain.

The DNA binding activity of TAL-1 is not required to induce leukemia/lymphoma in mice

Activation of the basic helix-loop-helix (bHLH) gene TAL-1 (or SCL) is the most frequent gain-of-function mutation in pediatric T cell acute lymphoblastic leukemia (T-ALL). Similarly, mis-expression of tal-1 in the thymus of transgenic mice results in the development of clonal T cell lymphoblastic leukemia. To determine the mechanism(s) of tal-1-induced leukemogenesis, we created transgenic mice expressing a DNA binding mutant of tal-1. Surprisingly, these mice develop disease, demonstrating that the DNA binding properties of tal-1 are not required to induce leukemia/lymphoma in mice. However, wild type tal-1 and the DNA binding mutant both form stable complexes with E2A proteins. In addition, tal-1 stimulates differentiation of CD8-single positive thymocytes but inhibits the development of CD4-single positive cells: effects also observed in E2A-deficient mice. Our study suggests that the bHLH protein tal-1 contributes to leukemia by interfering with E2A protein function(s).

Sarcomere number regulation maintained after immobilization in desmin-null mouse skeletal muscle

The serial sarcomere number of skeletal muscle changes in response to chronic length perturbation. The role of the intermediate filament desmin in regulating these changes was investigated by comparing the architectural adaptations of the tibialis anterior, extensor digitorum longus (EDL) and soleus from wild-type mice with those of homozygous desmin knockout mice after hindlimb immobilization. After 28 days, serial sarcomere number increased significantly in the lengthened wild-type tibialis anterior (by approximately 9 %) and EDL (by approximately 17 %). Surprisingly, muscles from desmin knockout mice also experienced significant serial remodeling, with the serial sarcomere number of the tibialis anterior increasing by approximately 10 % and that of the EDL by approximately 27 %. A consistent result was observed in the shortened soleus: a significant decrease in sarcomere number was observed in the muscles from both wild-type (approximately 26 %) and knockout (approximately 12 %) mice. Thus, although desmin is not essential for sarcomerogenesis or sarcomere subtraction in mouse hindlimb muscles, the results do suggest subtle differences in the nature of sarcomere number adaptation. We speculate that desmin may play a role in regulating the optimal arrangement of sarcomeres within the muscle or in sensing the magnitude of the immobilization effect itself.

Establishment of distinct MyoD, E2A, and twist DNA binding specificities by different basic region-DNA conformations

Basic helix-loop-helix (bHLH) proteins perform a wide variety of biological functions. Most bHLH proteins recognize the consensus DNA sequence CAN NTG (the E-box consensus sequence is underlined) but acquire further functional specificity by preferring distinct internal and flanking bases. In addition, induction of myogenesis by MyoD-related bHLH proteins depends on myogenic basic region (BR) and BR-HLH junction residues that are not essential for binding to a muscle-specific site, implying that their BRs may be involved in other critical interactions. We have investigated whether the myogenic residues influence DNA sequence recognition and how MyoD, Twist, and their E2A partner proteins prefer distinct CAN NTG sites. In MyoD, the myogenic BR residues establish specificity for particular CAN NTG sites indirectly, by influencing the conformation through which the BR helix binds DNA. An analysis of DNA binding by BR and junction mutants suggests that an appropriate BR-DNA conformation is necessary but not sufficient for myogenesis, supporting the model that additional interactions with this region are important. The sequence specificities of E2A and Twist proteins require the corresponding BR residues. In addition, mechanisms that position the BR allow E2A to prefer distinct half-sites as a heterodimer with MyoD or Twist, indicating that the E2A BR can be directed toward different targets by dimerization with different partners. Our findings indicate that E2A and its partner bHLH proteins bind to CAN NTG sites by adopting particular preferred BR-DNA conformations, from which they derive differences in sequence recognition that can be important for functional specificity.

Human bHLH transcription factor gene myogenin (MYOG): genomic sequence and negative mutation analysis in patients with severe congenital myopathies

The myogenin gene encodes an evolutionarily conserved basic helix-loop-helix transcription (bHLH) factor that is required for differentiation of skeletal muscle, and its homozygous deletion in mice results in perinatal death from respiratory failure due to the lack of muscle fibers. Since the histology of skeletal muscle in myogenin null mice is reminiscent of that found in severe congenital myopathy patients, many of whom also die of respiratory complications, we sought to test the hypothesis that an aberrant human myogenin (myf4) coding region could be associated with some congenital myopathy conditions. With PCR amplification, we found similarly sized PCR products for the three exons of the myogenin gene in DNA from 37 patient and 40 control individuals. In contrast to previously reported sequencing of human myogenin (myf4), we describe with automated sequencing several base differences in flanking and coding regions plus an additional 659 and 498 bp in the first and second introns, respectively, in all 37 patient and 40 control samples. We also find a variable length (CA)-dinucleotide repeat in the second intron, which may have utility as a marker for future linkage studies. In summary, no causative mutations were detected in the myogenin coding locus of genomic DNA from 37 patients with severe congenital myopathy.

Activated notch inhibits myogenic activity of the MADS-Box transcription factor myocyte enhancer factor 2C

Skeletal muscle gene expression is dependent on combinatorial associations between members of the MyoD family of basic helix-loop-helix (bHLH) transcription factors and the myocyte enhancer factor 2 (MEF2) family of MADS-box transcription factors. The transmembrane receptor Notch interferes with the muscle-inducing activity of myogenic bHLH proteins, and it has been suggested that this inhibitory activity of Notch is directed at an essential cofactor that recognizes the DNA binding domains of the myogenic bHLH proteins. Given that MEF2 proteins interact with the DNA binding domains of myogenic bHLH factors to cooperatively regulate myogenesis, we investigated whether members of the MEF2 family might serve as targets for the inhibitory effects of Notch on myogenesis. We show that a constitutively activated form of Notch specifically blocks DNA binding by MEF2C, as well as its ability to cooperate with MyoD and myogenin to activate myogenesis. Responsiveness to Notch requires a 12-amino-acid region of MEF2C immediately adjacent to the DNA binding domain that is unique to this MEF2 isoform. Two-hybrid assays and coimmunoprecipitations show that this region of MEF2C interacts directly with the ankyrin repeat region of Notch. These findings reveal a novel mechanism for Notch-mediated inhibition of myogenesis and demonstrate that the Notch signaling pathway can discriminate between different members of the MEF2 family.

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

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

MyoR: a muscle-restricted basic helix-loop-helix transcription factor that antagonizes the actions of MyoD

Skeletal muscle development is controlled by a family of muscle-specific basic helix-loop-helix (bHLH) transcription factors that activate muscle genes by binding E-boxes (CANNTG) as heterodimers with ubiquitous bHLH proteins, called E proteins. Myogenic bHLH factors are expressed in proliferating undifferentiated myoblasts, but they do not initiate myogenesis until myoblasts exit the cell cycle. We describe a bHLH protein, MyoR (for myogenic repressor), that is expressed in undifferentiated myoblasts in culture and is down-regulated during differentiation. MyoR is also expressed specifically in the skeletal muscle lineage between days 10.5 and 16.5 of mouse embryogenesis and down-regulated thereafter during the period of secondary myogenesis. MyoR forms heterodimers with E proteins that bind the same DNA sequence as myogenic bHLH/E protein heterodimers, but MyoR acts as a potent transcriptional repressor that blocks myogenesis and activation of E-box-dependent muscle genes. These results suggest a role for MyoR as a lineage-restricted transcriptional repressor of the muscle differentiation program.

TAL1 and LIM-only proteins synergistically induce retinaldehyde dehydrogenase 2 expression in T-cell acute lymphoblastic leukemia by acting as cofactors for GATA3

Previously, we have shown that TAL1 and the LIM-only protein gene (LMO) are regularly coactivated in T-cell acute lymphoblastic leukemia (T-ALL). This observation is likely to relate to the findings that TAL1 and LMO are highly synergistic in T-cell tumorigenesis in double-transgenic mice. To understand the molecular mechanisms of functional synergy between TAL1 and LMO in tumorigenesis and transcriptional regulation, we tried to identify downstream target genes regulated by TAL1 and LMO by a subtractive PCR method. One of the isolated genes, that for retinaldehyde dehydrogenase 2 (RALDH2), was regularly expressed in most of the T-ALL cell lines that coexpressed TAL1 and LMO. Exogenously transfected TAL1 and LMO, but not either alone, induced RALDH2 expression in a T-ALL cell line, HPB-ALL, not expressing endogeneous TAL1 or LMO. The RALDH2 transcripts in T-ALL were, however, mostly initiated within the second intron. Promoter analysis revealed that a GATA site in a cryptic promoter in the second intron was essential and sufficient for the TAL1- and LMO-dependent transcriptional activation, and GATA3 binds to this site. In addition, forced expression of GATA3 potentiated the induction of RALDH2 by TAL1 and LMO, and these three factors formed a complex in vivo. Furthermore, a TAL1 mutant not binding to DNA also activated the transcription of RALDH2 in the presence of LMO and GATA3. Collectively, we have identified the RALDH2 gene as a first example of direct transcriptional target genes regulated by TAL1 and LMO in T-ALL. In this case, TAL1 and LMO act as cofactors for GATA3 to activate the transcription of RALDH2.

Overexpression of Mos(rat) proto-oncogene product enhances the positive autoregulatory loop of MyoD

The myogenic b-HLH transcription factor MyoD activates expression of muscle-specific genes and autoregulates positively its own expression. Various factors such as growth factors and oncogene products repress transcriptional activity of MyoD. The c-mos proto-oncogene product, Mos, is a serine/threonine kinase that can activate myogenic differentiation by specific phosphorylation of MyoD which favors heterodimerization of MyoD and E12 proteins. Here we show that overexpression of Mos enhances the expression level of MyoD protein in myoblasts although phosphorylation of MyoD by Mos does not modify its stability but promotes transcriptional transactivation of the MyoD promoter linked to the luciferase reporter gene. Moreover, co-expression of MyoD with Mos(wt) but not with the kinase-inactive Mos(KM) greatly enhances expression of endogenous MyoD protein and the DNA binding activity of MyoD/E12 heterodimers in 10T1/2 cells. Our data suggest that Mos increases the ability of MyoD to transactivate both muscle-specific genes and its own promoter and could therefore participate in the positive autoregulation loop of MyoD and muscle differentiation.

Phosphorylation and spatiotemporal distribution of KW8 (NDRF/NeuroD2), a NeuroD family basic helix-loop-helix protein

KW8, a NeuroD family basic helix-loop-helix protein, was initially cloned during the course of screening for the genes related to long term potentiation in rat hippocampal slice. Its homologue NDRF/NeuroD was also reported. In this report its phosphorylation and spatiotemporal distribution was studied. KW8 was expressed not only during embryonic and neonatal periods but also in adults. In adult, KW8 was expressed only in brain tissues, such as the cerebral cortex, hippocampus and cerebellum. Immunohistological studies revealed that KW8 was localized in the nuclei of neurons. On immunoblotting of rat brain tissue, COS-1 cells and Neuro2A cells overexpressing KW8, this protein was detected as several diffuse bands. Alkaline phosphatase treatment reduced the molecular weights of these bands. Metabolic labeling with 32Pi in COS-1 cells confirmed that the KW8 protein was phosphorylated in vivo. Some of the physiological functions of KW8 might be regulated by this phosphorylation. In yeast, the GAL4 fusion protein containing the C-terminal region of KW8 activated transcription of the reporter gene, suggesting that KW8 had transcriptional activity.

Intramolecular regulation of MyoD activation domain conformation and function

The MyoD family of basic helix-loop-helix (bHLH) proteins is required for myogenic determination and differentiation. The basic region carries the myogenic code and DNA binding specificity, while the N terminus contains a potent transcriptional activation domain. Myogenic activation is abolished when the basic region, bound to a myogenic E box, carries a mutation of Ala-114. It has been proposed that DNA binding of the MyoD basic region leads to recruitment of a recognition factor that unmasks the activation domain. Here we demonstrate that an A114N mutant exhibits an altered conformation in the basic region and that this local conformational difference can lead to a more global change affecting the conformation of the activation domain. This suggests that the deleterious effects of this class of mutations may result directly from defective conformation. Thus, the activation domain is unmasked only upon DNA binding by the correct basic region. Such a coupled conformational relationship may have evolved to restrict myogenic specificity to a small number of bHLH proteins among many with diverse functions yet with DNA binding specificities known to be similar.

Multiple roles for the MyoD basic region in transmission of transcriptional activation signals and interaction with MEF2

Establishment of skeletal muscle lineages is controlled by the MyoD family of basic helix-loop-helix (bHLH) transcription factors. The ability of these factors to initiate myogenesis is dependent on two conserved amino acid residues, alanine and threonine, in the basic domains of these factors. It has been postulated that these two residues may be responsible for the initiation of myogenesis via interaction with an essential myogenic cofactor. The myogenic bHLH proteins cooperatively activate transcription and myogenesis through protein-protein interactions with members of the myocyte enhancer factor 2 (MEF2) family of MADS domain transcription factors. MyoD-E12 heterodimers interact with MEF2 proteins to synergistically activate myogenesis, while homodimers of E12, which lack the conserved alanine and threonine residues in the basic domain, do not interact with MEF2. We have examined whether the myogenic alanine and threonine in the MyoD basic region are required for interaction with MEF2. Here, we show that substitution of the MyoD basic domain with that of E12 does not prevent interaction with MEF2. Instead, the inability of alanine-threonine mutants of MyoD to initiate myogenesis is due to a failure to transmit transcriptional activation signals provided either from the MyoD or the MEF2 activation domain. This defect in transcriptional transmission can be overcome by substitution of the MyoD or the MEF2 activation domain with the VP16 activation domain. These results demonstrate that myogenic bHLH-MEF2 interaction can be uncoupled from transcriptional activation and support the idea that the myogenic residues in myogenic bHLH proteins are essential for transmission of a transcriptional activation signal.

Muscle LIM protein promotes myogenesis by enhancing the activity of MyoD

The muscle LIM protein (MLP) is a muscle-specific LIM-only factor that exhibits a dual subcellular localization, being present in both the nucleus and in the cytoplasm. Overexpression of MLP in C2C12 myoblasts enhances skeletal myogenesis, whereas inhibition of MLP activity blocks terminal differentiation. Thus, MLP functions as a positive developmental regulator, although the mechanism through which MLP promotes terminal differentiation events remains unknown. While examining the distinct roles associated with the nuclear and cytoplasmic forms of MLP, we found that nuclear MLP functions through a physical interaction with the muscle basic helix-loop-helix (bHLH) transcription factors MyoD, MRF4, and myogenin. This interaction is highly specific since MLP does not associate with nonmuscle bHLH proteins E12 or E47 or with the myocyte enhancer factor-2 (MEF2) protein, which acts cooperatively with the myogenic bHLH proteins to promote myogenesis. The first LIM motif in MLP and the highly conserved bHLH region of MyoD are responsible for mediating the association between these muscle-specific factors. MLP also interacts with MyoD-E47 heterodimers, leading to an increase in the DNA-binding activity associated with this active bHLH complex. Although MLP lacks a functional transcription activation domain, we propose that it serves as a cofactor for the myogenic bHLH proteins by increasing their interaction with specific DNA regulatory elements. Thus, the functional complex of MLP-MyoD-E protein reveals a novel mechanism for both initiating and maintaining the myogenic program and suggests a global strategy for how LIM-only proteins may control a variety of developmental pathways.

The single MyoD family gene of Ciona intestinalis encodes two differentially expressed proteins: implications for the evolution of chordate muscle gene regulation

A MyoD family gene was identified in the ascidian Ciona intestinalis and designated CiMDF (Ciona intestinalis Muscle Determination Factor). Expression of CiMDF was restricted to the muscle cells of the developing embryo and the body-wall muscle of adults. Northern blots showed that two differentially regulated CiMDF transcripts were expressed during development. A 1.8 kb transcript (CiMDFa) appeared first and was gradually replaced by a 2.7 kb transcript (CiMDFb). These transcripts encoded essentially identical MyoD family proteins with the exception of a 68 amino acid C-terminal sequence present in CiMDFb that was absent from CiMDFa. Although both CiMDFa and CiMDFb contained the cysteine-rich/basic-helix loop helix domain (Cys-rich/bHLH) present in all MyoD family proteins, only CiMDFb contained the region near the C terminus (Domain III) characteristic of this gene family. Genomic Southern blots showed that C. intestinalis has only one MyoD family gene, suggesting that CiMDFa and CiMDFb result from differential processing of primary transcripts. The existence of two MyoD family proteins that are differentially expressed during ascidian embryogenesis has novel parallels to vertebrate muscle development and may reflect conserved myogenic regulatory mechanisms among chordates.

Molecular mechanisms of myogenic coactivation by p300: direct interaction with the activation domain of MyoD and with the MADS box of MEF2C

By searching for molecules that assist MyoD in converting fibroblasts to muscle cells, we have found that p300 and CBP, two related molecules that act as transcriptional adapters, coactivate the myogenic basic-helix-loop-helix (bHLH) proteins. Coactivation by p300 involves novel physical interactions between p300 and the amino-terminal activation domain of MyoD. In particular, disruption of the FYD domain, a group of three amino acids conserved in the activation domains of other myogenic bHLH proteins, drastically diminishes the transactivation potential of MyoD and abolishes both p300-mediated coactivation and the physical interaction between MyoD and p300. Two domains of p300, at its amino and carboxy terminals, independently function to both mediate coactivation and physically interact with MyoD. A truncated segment of p300, unable to bind MyoD, acts as a dominant negative mutation and abrogates both myogenic conversion and transactivation by MyoD, suggesting that endogenous p300 is a required coactivator for MyoD function. The p300 dominant negative peptide forms multimers with intact p300. p300 and CBP serve as coactivators of another class of transcriptional activators critical for myogenesis, myocyte enhancer factor 2 (MEF2). In fact, transactivation mediated by the MEF2C protein is potentiated by the two coactivators, and this phenomenon is associated with the ability of p300 to interact with the MADS domain of MEF2C. Our results suggest that p300 and CBP may positively influence myogenesis by reinforcing the transcriptional autoregulatory loop established between the myogenic bHLH and the MEF2 factors.