Expression of myogenic factors in denervated chicken breast muscle: isolation of the chicken Myf5 gene

Homer 1 genotype AA variant relates to congenital splay leg syndrome in piglets by repressing Pax7 in myogenic progenitors

Introduction

Porcine congenital splay leg syndrome (PCS) is a major birth defect in piglets, resulting in lameness and high mortality rates. The multifactorial pathogenesis of PSC is not well understood but includes a polygenic inheritance.

Methods

Here, in addition to morphological investigations, we characterized the expression of myogenic genes and functional (proliferation and differentiation) properties of myogenic precursor/satellite cells (SATCs) in 1 day-old PCS piglets, non-affected littermates (LCs), and piglets from PCS-free healthy litters (HCs). In addition, PCS phenotypes were related to the SNP Homer1_rs325197091 within the Homer1 locus, which has been identified as a potential hereditary cause of PCS.

Results and discussion

Samples from musculus semitendinosus (ST) of PCS piglets had a higher proportion of type II fibers, reflecting myofiber immaturity. In addition, myofiber atrophy, a lower number of myonuclei per fiber (ST), and a higher apoptotic activity (in ST and longissimus dorsi muscle; LD) were found in the PCS group. A higher proportion of cycling committed myoblasts (Pax7+/Ki67+ cells) occurred in samples from PCS-affected piglets, and on the other hand, the mRNA expression of genes involved in differentiation (muscle differentiation 1; MyoD, myogenin; MyoG) was repressed compared with HCs. Cultured SATCs from PCS-affected animals showed a temporal shift in peak expression of Pax7, MyoD, and MyoG toward days 3 and 4 of their 7 days differentiation regime. In vitro experiments with isolated SATCs confirmed the lower differentiation potential and the delayed progression of the myogenic processes in cells from piglets with PCS phenotype. In addition, Pax7 and desmin were differently expressed in Homer1_rs325197091 genotype variants (GG, GA, and AA). Both genes showed the lowest expression in the homozygous AA-variant, which was most frequently found in PCS-affected animals. The homozygous AA-variant was also associated with lower expression of the truncated Homer1-subtype 205. Thus, we hypothesize that in PCS, the balance between Homer1 proteins and its signaling functions is changed in a way detrimental to the myogenic differentiation program. Our results demonstrated direct negative effects of the Homer1 AA genotype on Pax7 expression, but the exact mode of action still needs to be elucidated.

Temporal Expression of Myogenic Regulatory Genes in Different Chicken Breeds during Embryonic Development

The basic units of skeletal muscle in all vertebrates are multinucleate myofibers, which are formed from the fusion of mononuclear myoblasts during the embryonic period. In order to understand the regulation of embryonic muscle development, we selected four chicken breeds, namely, Cornish (CN), White Plymouth Rock (WPR), White Leghorn (WL), and Beijing-You Chicken (BYC), for evaluation of their temporal expression patterns of known key regulatory genes (Myomaker, MYOD, and MSTN) during pectoral muscle (PM) and thigh muscle (TM) development. The highest expression level of Myomaker occurred from embryonic days E13 to E15 for all breeds, indicating that it was the crucial stage of myoblast fusion. Interestingly, the fast-growing CN showed the highest gene expression level of Myomaker during the crucial stage. The MYOD gene expression at D1 was much higher, implying that MYOD might have an important role after hatching. Histomorphology of PM and TM suggested that the myofibers was largely complete at E17, which was speculated to have occurred because of the expression increase in MSTN and the expression decrease in Myomaker. Our research contributes to lay a foundation for the study of myofiber development during the embryonic period in different chicken breeds.

Silver Nanoparticles as Modulators of Myogenesis-Related Gene Expression in Chicken Embryos

The present study was conducted to investigate the effects of colloidal nanoparticles of silver (Nano-Ag) on the expression of myogenesis-related genes in chicken embryos. The investigated genes included the members of the myogenic regulatory factors family (MRFs) and myocyte enhancer factor 2A (MEF2A) genes. A total of 200 fertilized broiler eggs (Indian River) were randomly distributed into four groups; non-injected control, injected control with placebo, treatment I in ovo injected with 20 ppm Nano-Ag, and treatment II in ovo injected with 40 ppm Nano-Ag. The eggs were then incubated for 21 days at the optimum temperature and humidity conditions. Breast muscle tissues were collected at the 5th, 8th, and 18th days of the incubation period. The mRNA expression of myogenic determination factor 1 (MYOD1), myogenic factor 5 (MYF5), myogenic factor 6 (MYF6), myogenin (MYOG), and MEF2A was measured at the three sampling points using real-time quantitative PCR, while MYOD1 protein expression was evaluated on day 18 using western blot. Breast muscle tissues were histologically examined on day 18 to detect the changes at the cellular level. Our results indicate that myogenesis was enhanced with the low concentration (20 ppm) of Nano-Ag due to the higher expression of MYOD1, MYF5, and MYF6 at the transcriptional level and MYOD1 at the translational level. Moreover, histological analysis revealed the presence of hyperplasia (31.4% more muscle fibers) in treatment I (injected with 20 ppm). Our findings indicate that in ovo injection of 20 ppm Nano-Ag enhances the development of muscles in chicken embryos compared with the 40-ppm dosage and provide crucial information for the use of silver nanoparticles in poultry production.

Skeletal myogenesis and Myf5 activation

Myogenic regulatory factors (MRFs) are the master regulators of skeletal myogenesis. Among the MRFs, Myf5 is the earliest to be expressed and is regulated by a complex set of enhancers. The expression of Myf5 defines different muscle populations in the somite. Furthermore, Myf5 expression is also found in non-muscle tissues, such as preadipocytes and neurons. Here, we present a current view on the regulation of skeletal myogenesis by transcription factors and cellular signals, with an emphasis on the complexity of transcriptional activation of Myf5. We also discuss Myf5 expression in different populations of myoblasts, preadipocytes and neuronal tissue.

Molecular structure of the largemouth bass (Micropterus salmoides) Myf5 gene and its effect on skeletal muscle growth

Myogenic Regulatory Factors (MRFs), a family of basic helix-loop-helix (bHLH) transcription factors, play important roles in regulating skeletal muscle development and growth. Myf5, the primary factor of MRFs, initiates myogenesis. Its expression pattern during somitomyogenesis in some fish has been revealed. To further study its effect on fish muscle during postembryonic growth, characterization and function analysis of myf5 cDNA were carried out in largemouth bass. The 1,093 bp cDNA sequence was identified by RT-PCR and 3'RACE, then the ORF of Myf5 cDNA was cloned into the expression vector pcDNA3.1(-)/mycHisB. The recombinant plasmid pcDNA3.1(-)/mycHisB-Myf5 was injected into the dorsal muscle of tilapias. RT-PCR and histochemical results showed that the exogenous gene was transcribed and translated in vivo. Its effect on muscle growth focused on myofiber hypertrophy in white muscle 60 days post injection. This indicated that overexpression of Myf5 can promote myogenesis during the fish muscle postembryonic growth period.

The dissociation of the Fgf-feedback loop controls the limbless state of the neck

In tetrapods, limbs develop at two specific positions along the anteroposterior axis of the embryo, whereas other regions of the embryo, most prominently the neck and the flank, are limbless. However, the flank can generate an ectopic limb when the Fgf-feedback loop crucial for the initiation of limb budding is activated. Thus, despite its limblessness, the flank is a limb-competent area. Using the chick embryo as model, we investigated whether the neck, as the flank, has the competence to form a limb, and what mechanism may regulate its limblessness. We show that forelimb lateral mesoderm plus ectoderm grafted into the neck can continue limb development, suggesting that the neck does not actively inhibit this process. However, neck tissues themselves do not support or take part in limb formation. Hence, the neck is limb-incompetent. This is due to the dismantling of Fgf signalling at distinct points of the MAPK signalling cascade in the neck lateral mesoderm and ectoderm.

The molecular setup of the avian head mesoderm and its implication for craniofacial myogenesis

The head mesoderm is the mesodermal tissue on either side of the brain, from forebrain to hindbrain levels, and gives rise to the genuine head muscles. Its relatedness to the more posterior paraxial mesoderm, the somites, which generate the muscles of the trunk, is conversely debated. To gain insight into the molecular setup of the head mesoderm, its similarity or dissimilarity to the somitic mesoderm, and the implications of its setup for the progress of muscle formation, we investigated the expression of markers (1) for mesoderm segmentation and boundary formation, (2) for regional specification and somitogenesis and (3) for the positive and negative control of myogenic differentiation. We show that the head mesoderm is molecularly distinct from somites. It is not segmented; even the boundary to the first somite is ill-defined. Importantly, the head mesoderm lacks the transcription factors driving muscle differentiation while genes suppressing differentiation and promoting cell proliferation are expressed. These factors show anteroposteriorly and dorsoventrally regionalised but overlapping expression. Notably, expression extends into the areas that actively contribute to the heart, overlapping with the expression of cardiac markers.

Neural tube derived signals and Fgf8 act antagonistically to specify eye versus mandibular arch muscles

Recent knockout experiments in the mouse generated amazing craniofacial skeletal muscle phenotypes. Yet none of the genes could be placed into a molecular network, because the programme to control the development of muscles in the head is not known. Here we show that antagonistic signals from the neural tube and the branchial arches specify extraocular versus branchiomeric muscles. Moreover, we identified Fgf8 as the branchial arch derived signal. However, this molecule has an additional function in supporting the proliferative state of myoblasts, suppressing their differentiation, while a further branchial arch derived signal, namely Bmp7, is an overall negative regulator of head myogenesis.

Foxd3 mediates zebrafish myf5 expression during early somitogenesis

Myf5, one of the basic helix-loop-helix transcription factors, controls muscle differentiation and is expressed in somites during early embryogenesis. However, the transcription factors bound to the cis-elements of myf5 are poorly understood. In this study, we used the yeast one-hybrid assay and found that Forkhead box d3 (Foxd3) interacted specifically with the -82/-62 cassette, a key element directing somite-specific expression of myf5. The dual-luciferase assay revealed that the expression of Foxd3 potently transactivated the myf5 promoter. Knocking down foxd3 with morpholino oligonucleotide (MO) resulted in a dramatic down-regulation of myf5 in somites and adaxial cells but not in the presomitic mesoderm. On the other hand, myod expression remained unchanged in foxd3 morphants. Foxd3 mediation of myf5 expression is stage-dependent, maintaining myf5 expression in the somites and adaxial cells during the 7- to 18-somite stage. Furthermore, in the pax3 morphant, the expression of foxd3 was down-regulated greatly and the expression of myf5 was similar to that of the foxd3 morphant. Co-injection of foxd3 mRNA and pax3-MO1 greatly restored the expression of myf5 in the somites and adaxial cells, suggesting that pax3 induces foxd3 expression, which then induces the expression of myf5. This report is the first study to show that Foxd3, a well-known regulator in neural crest development, is also involved in myf5 regulation.

Sequence, conservation, and quantitative expression of rainbow trout Myf5

The success of rainbow trout as an aquaculture species is dependent on the ability to produce fish with large amounts of high-quality lean muscle. It is therefore important to understand not only the best conditions under which to raise the fish but also the molecular control of muscle growth. Vertebrate muscle growth is initiated by the specification of myogenic precursor cells into myoblasts. The myoblasts proliferate and fuse to form multinucleated myotubes, which mature into myofibers. A family of basic helix-loop-helix (bHLH) transcription factors, the Myogenic Regulatory Factors (MRFs), controls these events. In trout, two MRF-encoding genes, TMyoD (of which there are two) and Tmyogenin, have been identified. However, the primary MRF-encoding Myf5 is not yet sequenced. Here, using degenerate PCR and 5' and 3' RACE, the cDNA sequence of trout Myf5 (TMyf5) is identified. Translation of the cDNA reveals that TMyf5 is a bHLH protein with homology to Myf5 and MRFs in other organisms. It is expressed mainly in red and white muscle, suggesting that it shares functional homology to Myf5 in other species. The molecular control of muscle growth has been well-characterized in mammals, but there are differences in the growth of fish muscle, highlighting the need for characterization of MRFs in fish species, particularly those in which understanding muscle growth will have a positive impact on the economic potential of the species.

Identification of a critical control element directing expression of the muscle-specific transcription factor MRF4 in the mouse embryo

Skeletal muscle development in the vertebrate embryo critically depends on the myogenic regulatory factors (MRFs) including MRF4 and Myf5. Both genes exhibit distinct expression patterns during mouse embryogenesis, although they are genetically closely linked with multiple regulatory elements dispersed throughout the common gene locus. MRF4 has a biphasic expression profile, first in somites and later in foetal skeletal muscles. Here, we demonstrate by transgenic analysis that elements within a 7.5-kb promoter fragment of the MRF4 gene are sufficient to drive the embryonic wave of expression very similar to the endogenous gene in somites of mouse embryos. In contrast, a 3-kb fragment of the proximal promoter fails to support expression in the myotome, suggesting that essential cis-acting elements are located between -7.5 and -3 kb upstream of MRF4. Further analysis of this sequence delimits an essential region between -6.6 and -5.6 kb that together with the 3-kb promoter fragment directs transgene expression in the epaxial myotome of all somites during the appropriate developmental period. These data provide evidence that the partly overlapping expression patterns of Mrf4 and Myf5 in somites are controlled by distinct regulatory elements. We also show that 11.4 kb sequence upstream of MRF4, including the promoter and the somitic control region identified in this study, is not sufficient to elicit target specificity towards the strong Myf5 (-58/-48 kb) enhancer, suggesting that additional yet unidentified elements are necessary to convey promoter selectivity and protect the MRF4 gene from this enhancer.

Standard-curve competitive RT-PCR quantification of myogenic regulatory factors in chicken embryos

The reverse transcription-polymerase chain reaction (RT-PCR) is the most sensitive method used to evaluate gene expression. Although many advances have been made since quantitative RT-PCR was first described, few reports deal with the mathematical bases of this technique. The aim of the present study was to develop and standardize a competitive PCR method using standard-curves to quantify transcripts of the myogenic regulatory factors MyoD, Myf-5, Myogenin and MRF4 in chicken embryos. Competitor cDNA molecules were constructed for each gene under study using deletion primers, which were designed to maintain the anchorage sites for the primers used to amplify target cDNAs. Standard-curves were prepared by co-amplification of different amounts of target cDNA with a constant amount of competitor. The content of specific mRNAs in embryo cDNAs was determined after PCR with a known amount of competitor and comparison to standard-curves. Transcripts of the housekeeping -actin gene were measured to normalize the results. As predicted by the model, most of the standard-curves showed a slope close to 1, while intercepts varied depending on the relative efficiency of competitor amplification. The sensitivity of the RT-PCR method permitted the detection of as few as 60 MyoD/Myf-5 molecules per reaction but approximately 600 molecules of MRF4/Myogenin mRNAS were necessary to produce a measurable signal. A coefficient of variation of 6 to 19% was estimated for the different genes analyzed (6 to 9 repetitions). The competitive RT-PCR assay described here is sensitive, precise and allows quantification of up to 9 transcripts from a single cDNA sample.

Novel regulatory sequence -82/-62 functions as a key element to drive the somite-specificity of zebrafish myf-5

Myf-5, a transcription factor that controls muscle differentiation, is expressed in somites during early embryogenesis. However, gene regulation of myf-5 is poorly understood and detailed functional analysis of the regulatory cis-elements is needed. In zebrafish, the myf-5 upstream sequence from -82 to -62 (-82/-62) was fused with a basal promoter and transferred to fertilized zebrafish eggs. The -82/-62 cassette drove green fluorescent protein (GFP) reporter gene expression specifically in the somites. Moreover, GFP signals were detected exclusively in the somites of 28-hpf embryos derived from eggs injected with pCMV-5x(-82/-62), which contained five copies of the -82/-62 cassette inserted within cytomegalovirus promoter/enhancer. Thus, the -82/-62 cassette, conserved in mouse myf-5, functions to drive somite-specific expression and to repress nonspecific expression during the early development of zebrafish embryos. Mutated sequence analysis of -82/-62 cassette showed that the -70/-62 sequence was the key element for controlling myf-5 specificity. The putative CCAAT-like box, located at -66/-62, could not direct somite-specific expression. A DNA-protein complex was specifically formed between the -70/-62 probe and embryonic nuclear extracts. We conclude that the -70/-62 motif is essential for controlling somite-specific expression and the CCAAT-like box is essential for activating gene transcription.

Myf5 expression in somites and limb buds of mouse embryos is controlled by two distinct distal enhancer activities

The initiation of skeletal muscle development in the mouse embryo is strictly associated with the expression of the muscle-specific transcription factor Myf5, the first of four myogenic regulatory factors (MRFs) to be expressed in muscle progenitors, and ablation of the Myf5 gene prevents myogenesis. The complex spatiotemporal expression pattern of Myf5 depends on many discrete regulatory elements that are dispersed over long distances throughout the gene locus. These multiple control modules act differently in the various muscle precursor populations, presumably in response to diverse signals that control myogenesis. A potent enhancer region regulating Myf5 expression in limb muscles and somites has been identified previously at -58/-48 kb upstream of the transcriptional start site (Hadchouel et al., 2000). Here, we focus on the physical and functional dissection of this control region. We demonstrate that a conserved sequence of 270 bp located around -57 kb is required and sufficient to drive Myf5 expression in limbs and to maintain it in somites. A second enhancer nearby is responsible for Myf5 transcription in occipital/cranial somites. This enhancer activity also directs expression accurately to the myotome, preventing ectopic expression in the dermomyotome during the second phase of Myf5 gene activation in somites. Our data suggest that the enhancer identified here collaborates with other somitic enhancers to ensure correct myotomal Myf5 expression. Moreover, it constitutes an important element that mediates somitic expression after the initial and transient Myf5 activation through a previously described sonic hedgehog-dependent early epaxial enhancer.

Myogenic regulatory factors and the specification of muscle progenitors in vertebrate embryos

Embryological and genetic studies of mouse, bird, zebrafish, and frog embryos are providing new insights into the regulatory functions of the myogenic regulatory factors, MyoD, Myf5, Myogenin, and MRF4, and the transcriptional and signaling mechanisms that control their expression during the specification and differentiation of muscle progenitors. Myf5 and MyoD genes have genetically redundant, but developmentally distinct regulatory functions in the specification and the differentiation of somite and head muscle progenitor lineages. Myogenin and MRF4 have later functions in muscle differentiation, and Pax and Hox genes coordinate the migration and specification of somite progenitors at sites of hypaxial and limb muscle formation in the embryo body. Transcription enhancers that control Myf5 and MyoD activation in muscle progenitors and maintain their expression during muscle differentiation have been identified by transgenic analysis. In epaxial, hypaxial, limb, and head muscle progenitors, Myf5 is controlled by lineage-specific transcription enhancers, providing evidence that multiple mechanisms control progenitor specification at different sites of myogenesis in the embryo. Developmental signaling ligands and their signal transduction effectors function both interactively and independently to control Myf5 and MyoD activation in muscle progenitor lineages, likely through direct regulation of their transcription enhancers. Future investigations of the signaling and transcriptional mechanisms that control Myf5 and MyoD in the muscle progenitor lineages of different vertebrate embryos can be expected to provide a detailed understanding of the developmental and evolutionary mechanisms for anatomical muscles formation in vertebrates. This knowledge will be a foundation for development of stem cell therapies to repair diseased and damaged muscles.

Differential regulation of the chick dorsal thoracic dermal progenitors from the medial dermomyotome

The chick dorsal feather-forming dermis originates from the dorsomedial somite and its formation depends primarily on Wnt1 from the dorsal neural tube. We investigate further the origin and specification of dermal progenitors from the medial dermomyotome. This comprises two distinct domains: the dorsomedial lip and a more central region (or intervening zone) that derives from it. We confirm that Wnt1 induces Wnt11 expression in the dorsomedial lip as previously shown, and show using DiI injections that some of these cells, which continue to express Wnt11 migrate under the ectoderm, towards the midline, to form most of the dorsal dermis. Transplantation of left somites to the right side to reverse the mediolateral axis confirms this finding and moreover suggests the presence of an attractive or permissive environment produced by the midline tissues or/and a repellent or inadequate environment by the lateral tissues. By contrast, the dorsolateral dermal cells just delaminate from the surface of the intervening space, which expresses En1. Excision of the axial organs or the ectoderm, and grafting of Wnt1-secreting cells, shows that, although the two populations of dermal progenitors both requires Wnt1 for their survival, the signalling required for their specification differs. Indeed Wnt11 expression relies on dorsal neural tube-derived Wnt1, while En1 expression depends on the presence of the ectoderm. The dorsal feather-forming dermal progenitors thus appear to be differentially regulated by dorsal signals from the neural tube and the ectoderm, and derive directly and indirectly from the dorsomedial lip. As these two dermomyotomal populations are well known to also give rise to epaxial muscles, an isolated domain of the dermomyotome that contains only dermal precursors does not exist and none of the dermomyotomal domains can be considered uniquely as a dermatome.

Treatment with Myf5-morpholino results in somite patterning and brain formation defects in zebrafish

Myf-5 is a stage-dependent transcription factor associated with somitogenesis. To study its biological functions in zebrafish, we injected the Myf5-morpholinos ZMF-MO (antisense nucleotides 28 to 52) and ZMF-OTHER (antisense nucleotides 3 to 27) into zebrafish embryos to establish a myf-5 gene knockdown. No phenotypic abnormalities were observed following injection with 0.2 ng of ZMF-MO, but defects were displayed in 2 of 118 (1.7%) surviving embryos injected with 1 ng ZMF-MO. Morphological defects became more severe with increased dosages: 105 of 270 (38.9%) surviving embryos injected with 4.5 ng of ZMF-MO displayed such abnormalities as the absence of eyes or brains in addition to the following low-dosage defects in 24 hpf embryos: longitudinal yolk sacs, incomplete epiboly coverage, abnormal and suspended tail buds, diffused somite boundaries, and head shrinkage. Similar results were observed in the 4.5 ng ZMF-OTHER injection group. However, when fish were co-injected with 4.5 ng ZMF-MO and 4.5 ng myf-5 mRNA, abnormality rates decreased from 49.6% to 5.5%. Our results show that the brain krox20 gene was down-regulated at rhombomere 3; the pax2.1 gene was completely down-regulated; myoD was expressed normally; myogenin was substantially down-regulated in whole somites; and desmin was partly inhibited in newly forming somites. Our conclusion is that zebrafish Myf-5 may play important roles in brain formation and in the convergence and extension of shield epiblasts and tail buds during early embryogenesis, in addition to its well-understood role as a muscle regulatory factor in somites.

Distinct regulatory cascades for head and trunk myogenesis

Most head muscles arise from the pre-otic axial and paraxial head mesoderm. This tissue does not form somites, yet expresses the somitic markers Lbx1, Pax7 and Paraxis in a regionalised fashion. The domain set aside by these markers provides the lateral rectus muscle, the most caudal of the extrinsic eye muscles. In contrast to somitic cells that express Lbx1, lateral rectus precursors are non-migratory. Moreover, the set of markers characteristic for the lateral rectus precursors differs from the marker sets indicative of somitic muscle precursors. This suggests distinct roles for Lbx1/Pax7/Paraxis in the development of head and trunk muscles. When grafted to the trunk, the pre-otic head mesoderm fails to activate Lbx1, Pax7 or Paraxis. Likewise, somites grafted into the region of the lateral rectus precursors fail to activate the lateral rectus marker set. This suggests that distinct regulatory cascades act in the development of trunk and head muscles, possibly reflecting their distinct function and evolution.

Purkinje fibers of the avian heart express a myogenic transcription factor program distinct from cardiac and skeletal muscle

A rhythmic heart beat is coordinated by conduction of pacemaking impulses through the cardiac conduction system. Cells of the conduction system, including Purkinje fibers, terminally differentiate from a subset of cardiac muscle cells that respond to signals from endocardial and coronary arterial cells. A vessel-associated paracrine factor, endothelin, can induce embryonic heart muscle cells to differentiate into Purkinje fibers both in vivo and in vitro. During this phenotypic conversion, the conduction cells down-regulate genes characteristic of cardiac muscle and up-regulate subsets of genes typical of both skeletal muscle and neuronal cells. In the present study, we examined the expression of myogenic transcription factors associated with the switch of the gene expression program during terminal differentiation of heart muscle cells into Purkinje fibers. In situ hybridization analyses and immunohistochemistry of embryonic and adult hearts revealed that Purkinje fibers up-regulate skeletal and atrial muscle myosin heavy chains, connexin-42, and neurofilament protein. Concurrently, a cardiac muscle-specific myofibrillar protein, myosin-binding protein-C (cMyBP-C), is down-regulated. During this change in transcription, however, Purkinje fibers continue to express cardiac muscle transcription factors, such as Nkx2.5, GATA4, and MEF2C. Importantly, significantly higher levels of Nkx2.5 and GATA4 mRNAs were detected in Purkinje fibers as compared to ordinary heart muscle cells. No detectable difference was observed in MEF2C expression. In culture, endothelin-induced Purkinje fibers from embryonic cardiac muscle cells dramatically down-regulated cMyBP-C transcription, whereas expression of Nkx2.5 and GATA4 persisted. In addition, myoD, a skeletal muscle transcription factor, was up-regulated in endothelin-induced Purkinje cells, while Myf5 and MRF4 transcripts were undetectable in these cells. These results show that during and after conversion from heart muscle cells, Purkinje fibers express a unique myogenic transcription factor program. The mechanism underlying down-regulation of cardiac muscle genes and up-regulation of skeletal muscle genes during conduction cell differentiation may be independent from the transcriptional control seen in ordinary cardiac and skeletal muscle cells.

Isolation of the avian homologue of the homeobox gene Mox2 and analysis of its expression pattern in developing somites and limbs

We have isolated the cDNA of avian Mox2 and analyzed its expression pattern during somitogenesis and limb bud formation. Mox2 plays an important role in limb muscle differentiation in the mouse. Mox2 is expressed in the somites of developing chick embryos and in presumptive migrating myoblasts from the dermomyotome to the limb buds. It is also expressed in the ventral and dorsal part of limb buds and is associated with non-proliferating myoblasts. Significant differences were observed in chick and mouse expression patterns, namely in the chick dermomyotome and limb.

A BAC transgenic analysis of the Mrf4/Myf5 locus reveals interdigitated elements that control activation and maintenance of gene expression during muscle development

The muscle-specific transcription factors Myf5 and Mrf4 are two of the four myogenic regulatory factors involved in the transcriptional cascade responsible for skeletal myogenesis in the vertebrate embryo. Myf5 is the first of these four genes to be expressed in the mouse. We have previously described discrete enhancers that drive Myf5 expression in epaxial and hypaxial somites, branchial arches and central nervous system, and argued that additional elements are required for proper expression (Summerbell, D., Ashby, P. R., Coutelle, O., Cox, D., Yee, S. P. and Rigby, P. W. J. (2000) Development 127, 3745–3757). We have now investigated the transcriptional regulation of both Myf5 and Mrf4 using bacterial artificial chromosome transgenesis. We show that a clone containing Myf5 and 140 kb of upstream sequences is sufficient to recapitulate the known expression patterns of both genes. Our results confirm and reinforce the conclusion of our earlier studies, that Myf5 expression is regulated differently in each of a considerable number of populations of muscle progenitors, and they begin to illuminate the evolutionary origins of this complex regulation. We further show that separate elements are involved in the activation and maintenance of expression in the various precursor populations, reflecting the diversity of the signals that control myogenesis. Mrf4 expression requires at least four elements, one of which may be shared with Myf5, providing a possible explanation for the linkage of these genes throughout vertebrate phylogeny. Further complexity is revealed by the demonstration that elements which control Mrf4 and Myf5 are embedded in an unrelated neighbouring gene.

Inductive signals from the somatopleure mediated by bone morphogenetic proteins are essential for the formation of the sternal component of avian ribs

The posterior five pairs of avian ribs are composed of vertebral and sternal components, both derived from the somitic mesoderm. For the patterning of the rib cartilage, inductive signals from neighboring tissues on the somitic mesoderm have been suggested to play critical roles. The notochord and surface ectoderm overlying the somitic mesoderm are essentially required for the development of proximal and distal regions of the ribs, respectively. Involvement of the somatopleure in rib development has already been suggested but is less understood than those of the notochord and surface ectoderm. In this study, we reinvestigated the role of the somatopleure during rib development. We first identified the chicken homologue of the mouse Mesenchymal forkhead-1 (cMfh-1) gene based on sequence similarities. cMfh-1 was observed to be expressed in the nonaxial mesoderm, including the somitic mesoderm, and, subsequently, in cartilage forming the ribs, vertebrae, and appendicular skeletal system. In the interlimb region, corresponding to somites 21-25 (or 26), cMfh-1-positive somitic mesoderm was seen penetrating the somatopleure of E4 embryos, and cMfh-1 was used as a molecular marker demarcating prospective rib cartilage. A series of experiments affecting the penetration of the somitic mesoderm into the somatopleure was performed in the present study, resulting in defects in sternal rib formation. The inductive signals emanating from the somatopleure mediated by BMP family proteins were observed to be essentially involved in the ingrowth of the somitic mesoderm. BMP4 alone, however, could not completely replace inductive signals from the somatopleure, suggesting the involvement of additional signals for rib formation.

MyoD and myogenin expression patterns in cultures of fetal and adult chicken myoblasts

Isolated chicken myoblasts had previously been utilized in many studies aiming at understanding the emergence and regulation of the adult myogenic precursors (satellite cells). However, in recent years only a small number of chicken satellite cell studies have been published compared to the increasing number of studies with rodent satellite cells. In large part this is due to the lack of markers for tracing avian myogenic cells before they become terminally differentiated and express muscle-specific structural proteins. We previously demonstrated that myoblasts isolated from fetal and adult chicken muscle display distinct schedules of myosin heavy-chain isoform expression in culture. We further showed that myoblasts isolated from newly hatched and young chickens already possess the adult myoblast phenotype. In this article, we report on the use of polyclonal antibodies against the chicken myogenic regulatory factor proteins MyoD and myogenin for monitoring fetal and adult chicken myoblasts as they progress from proliferation to differentiation in culture. Fetal-type myoblasts were isolated from 11-day-old embryos and adult-type myoblasts were isolated from 3-week-old chickens. We conclude that fetal myoblasts express both MyoD and myogenin within the first day in culture and rapidly transit into the differentiated myosin-expressing state. In contrast, adult myoblasts are essentially negative for MyoD and myogenin by culture Day 1 and subsequently express first MyoD and then myogenin before expressing sarcomeric myosin. The delayed MyoD-to-myogenin transition in adult myoblasts is accompanied by a lag in the fusion into myotubes, compared to fetal myoblasts. We also report on the use of a commercial antibody against the myocyte enhancer factor 2A (MEF2A) to detect terminally differentiated chicken myoblasts by their MEF2+ nuclei. Collectively, the results support the hypothesis that fetal and adult myoblasts represent different phenotypic populations. The fetal myoblasts may already be destined for terminal differentiation at the time of their isolation, and the adult myoblasts may represent progenitors that reside in an earlier compartment of the myogenic lineage.

Notch signalling acts in postmitotic avian myogenic cells to control MyoD activation

During Drosophila myogenesis, Notch signalling acts at multiple steps of the muscle differentiation process. In vertebrates, Notch activation has been shown to block MyoD activation and muscle differentiation in vitro, suggesting that this pathway may act to maintain the cells in an undifferentiated proliferative state. In this paper, we address the role of Notch signalling in vivo during chick myogenesis. We first demonstrate that the Notch1 receptor is expressed in postmitotic cells of the myotome and that the Notch ligands Delta1 and Serrate2 are detected in subsets of differentiating myogenic cells and are thus in position to signal to Notch1 during myogenic differentiation. We also reinvestigate the expression of MyoD and Myf5 during avian myogenesis, and observe that Myf5 is expressed earlier than MyoD, consistent with previous results in the mouse. We then show that forced expression of the Notch ligand, Delta1, during early myogenesis, using a retroviral system, has no effect on the expression of the early myogenic markers Pax3 and Myf5, but causes strong down-regulation of MyoD in infected somites. Although Delta1 overexpression results in the complete lack of differentiated muscles, detailed examination of the infected embryos shows that initial formation of a myotome is not prevented, indicating that exit from the cell cycle has not been blocked. These results suggest that Notch signalling acts in postmitotic myogenic cells to control a critical step of muscle differentiation.

Molecular structure, dynamic expression, and promoter analysis of zebrafish (Danio rerio) myf-5 gene

We isolated a 1,438 bp cDNA fragment that encoded Myf-5 myogenic factor of zebrafish. The deduced amino acid contained 237 residues, including the basic helix-loop-helix domain that is conserved in all known Myf-5. The zebrafish myf-5 transcripts were first detectable at 7.5 hpf, increased substantially until 16 hpf, and then declined gradually to an undetectable level by 26 hpf. During somitogenesis, zebrafish myf-5 transcripts were distributed mainly in the somites and segmental plates. Prominent signals occurred transiently in adaxial cells in two parallel rows but did not extend beyond the positive-signal somites. Various lengths of upstream region of zebrafish myf-5 fused with EGFP gene were used to carry out transgenic analysis. Results showed that a small, 82 bp (nucleotide positions from -82 to -1), regulatory cassette is sufficient to control the somite- and stage-specific expression of zebrafish myf-5 during early development.

Delta 1-activated notch inhibits muscle differentiation without affecting Myf5 and Pax3 expression in chick limb myogenesis

The myogenic basic helix-loop-helix (bHLH) transcription factors, Myf5, MyoD, myogenin and MRF4, are unique in their ability to direct a program of specific gene transcription leading to skeletal muscle phenotype. The observation that Myf5 and MyoD can force myogenic conversion in non-muscle cells in vitro does not imply that they are equivalent. In this paper, we show that Myf5 transcripts are detected before those of MyoD during chick limb development. The Myf5 expression domain resembles that of Pax3 and is larger than that of MyoD. Moreover, Myf5 and Pax3 expression is correlated with myoblast proliferation, while MyoD is detected in post-mitotic myoblasts. These data indicate that Myf5 and MyoD are involved in different steps during chick limb bud myogenesis, Myf5 acting upstream of MyoD. The progression of myoblasts through the differentiation steps must be carefully controlled to ensure myogenesis at the right place and time during wing development. Because Notch signalling is known to prevent differentiation in different systems and species, we sought to determine whether these molecules regulate the steps occurring during chick limb myogenesis. Notch1 transcripts are associated with immature myoblasts, while cells expressing the ligands Delta1 and Serrate2 are more advanced in myogenesis. Misexpression of Delta1 using a replication-competent retrovirus activates the Notch pathway. After activation of this pathway, myoblasts still express Myf5 and Pax3 but have downregulated MyoD, resulting in inhibition of terminal muscle differentiation. We conclude that activation of Notch signalling during chick limb myogenesis prevents Myf5-expressing myoblasts from progressing to the MyoD-expressing stage.

Neural and hormonal control of expression of myogenic regulatory factor genes during regeneration of Xenopus fast muscles: myogenin and MRF4 mRNA accumulation are neurally regulated oppositely

With the aim to investigate the influence of both innervation and thyroid hormone, on the expression of the MRFs during muscle regeneration, we performed cardiotoxin injury-induced regeneration experiments on fast muscles of adult Xenopus laevis subjected to different experimental conditions, including denervation and T3 treatment, and analyzed the accumulation of the four myogenic regulatory factors (MRFs) using RT-PCR and in situ hybridization. We show here that manipulation of hormone levels or innervation resulted in differential alterations of MRF expression. Denervation and T3 treatment transiently down-regulated Myf-5 mRNA levels at the beginning of the regeneration process. Myf-5 was the only myogenic factor subject to thyroid hormone influence. Muscle denervation persistently reduces the levels of MRF4 transcripts as early as the first stages of regeneration, whereas the levels of myogenin mRNA were increased in the late stages of regeneration. This suggests that MRF4 expression may be induced by innervation and hence may be involved in mediating transcriptional responses to innervation and that myogenin expression may compensate for the down-regulation of MRF4 gene. This switch in MRF gene expression following denervation could have important consequences for the ability of Xenopus regenerating muscles to recover function after denervation.

Glyceraldehyde-3-phosphate dehydrogenase varies with age in glycolytic muscles of rats

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA, protein, and enzyme activity levels in hindlimb muscles of adult and senescent Fischer 344 x Brown Norway rats were investigated. Soleus muscles from adult and senescent rats had similar levels of GAPDH. In contrast, muscles containing a large proportion of glycolytic fibers had lower GAPDH levels in senescent rats relative to these muscles in adult rats; this was observed at both the mRNA and protein levels. These data indicate that skeletal muscle glycolytic capacity of fast muscles is diminished with age and that it may be caused by changes at the level of transcription. Also, because GAPDH mRNA levels change with age in several rat muscles, GAPDH mRNA is not always a proper internal control for mRNA analyses of aging skeletal muscle.

Bone morphogenetic proteins are required in vivo for the generation of sympathetic neurons

Bone morphogenetic proteins (BMPs) induce autonomic neurogenesis in neural crest cultures and stimulate sympathetic neuron development when overexpressed in vivo. We demonstrate that inhibition of BMPs in the chick embryo bythe BMP antagonist Noggin prevents sympathetic neuron generation. In Noggin-treated embryos, the noradrenergic marker genes tyrosine hydroxylase (TH) and dopamine-beta-hydroxylase (DBH), panneuronal neurofilament 160 (NF160) and SCG10 genes, and the transcriptional regulators Phox2b and Phox2a are not expressed in sympathetic ganglia. Whereas initial ganglion development is not affected, the expression of the basic helix-loop-helix transcription factor Cash-1 is strongly reduced. These results demonstrate that BMPs are essential for sympathetic neuron development and establish Cash-1 and Phox2 genes as downstream effectors of BMPs in this lineage.

Macrophages enhance muscle satellite cell proliferation and delay their differentiation

This study investigated the effect of macrophages on in vitro satellite cell myogenesis in the turkey and mouse. Macrophages are considered to act as scavengers of tissue debris during the muscle degeneration-regeneration process. The number of dividing cells and of myoblasts expressing the myogenic regulatory factor MyoD indicated that macrophages enhanced satellite cell proliferation in both species. This was confirmed by observations with cultures treated for bromodeoxyuridine (BrdU) incorporation. In mouse and turkey macrophage-satellite cell cocultures, the number of differentiated myoblasts, the frequency of myogenin-positive cells, and the expression of developmental myosin isoforms were reduced as compared with control cultures, indicating that macrophages delayed satellite cell differentiation. The possibility that macrophages facilitate muscle fiber reconstitution by enhancing satellite cell proliferation should be taken into consideration in designing future strategies of satellite cell transplantation as a treatment for muscular dystrophies.

A distinct developmental programme for the cranial paraxial mesoderm in the chick embryo

Cells of the cranial paraxial mesoderm give rise to parts of the skull and muscles of the head. Some mesoderm cells migrate from locations close to the hindbrain into the branchial arches where they undergo muscle differentiation. We have characterised these migratory pathways in chick embryos either by DiI-labelling cells before migration or by grafting quail cranial paraxial mesoderm orthotopically. These experiments demonstrate that depending on their initial rostrocaudal position, cranial paraxial mesoderm cells migrate to fill the core of specific branchial arches. A survey of the expression of myogenic genes showed that the myogenic markers Myf5, MyoD and myogenin were expressed in branchial arch muscle, but at comparatively late stages compared with their expression in the somites. Pax3 was not expressed by myogenic cells that migrate into the branchial arches despite its expression in migrating precursors of limb muscles. In order to test whether segmental plate or somitic mesoderm has the ability to migrate in a cranial location, we grafted quail trunk mesoderm into the cranial paraxial mesoderm region. While segmental plate mesoderm cells did not migrate into the branchial arches, somitic cells were capable of migrating and were incorporated into the branchial arch muscle mass. Grafted somitic cells in the vicinity of the neural tube maintained expression of the somitic markers Pax3, MyoD and Pax1. By contrast, ectopic somitic cells located distal to the neural tube and in the branchial arches did not express Pax3. These data imply that signals in the vicinity of the hindbrain and branchial arches act on migrating myogenic cells to influence their gene expression and developmental pathways.

Drosophila myogenesis and insights into the role of nautilus

Several aspects of muscle development appear to be conserved between Drosophila and vertebrate organisms. Among these is the conservation of genes that are critical to the myogenic process, including transcription factors such as nautilus. From a simplistic point of view, Drosophila therefore seems to be a useful organism for the identification of molecules that are essential for myogenesis in both Drosophila and in other species. nautilus, the focal point of this review, appears to be involved in the specification and/or differentiation of a specific subset of muscle founder cells. As with several of its vertebrate and invertebrate counterparts, it is capable of inducing a myogenic program of differentiation reminiscent of that of somatic muscle precursors when expressed in other cell types. We therefore favor the model that nautilus implements the specific differentiation program of these founder cells, rather than their specification. Further analyses are necessary to establish the validity of this working hypothesis. Studies have revealed a critical role for Pax-3 in specifying a particular subset of myogenic cells, the progenitors of the limb muscles. These myogenic cells migrate from the somite into the periphery of the organism, where they differentiate. These myoblasts do not express MyoD or myf5 until they have arrived at their destination and begin the morphologic process of myogenesis (Bober et al., 1994; Goulding et al., 1994; Williams and Ordahl, 1994). They then begin to express these genes, possibly to put the myogenic plan into action. Thus, as with nautilus, MyoD and myf5 may be necessary for the manifestation of a muscle-specific commitment that has already occurred. By comparison with vertebrates, it was anticipated that the single Drosophila gene would serve the purpose of all four vertebrate genes. However, its restricted pattern of expression and apparent loss-of-function phenotype are inconsistent with this expectation. It remains to be determined whether nautilus functions in a manner similar to just one of the vertebrate genes. Since the myf5- and MyoD-expressing myoblasts are proliferative, the loss of one cell type appears to be compensated by proliferation of the remaining cell type. This apparent plasticity may obscure differences in mutant phenotype resulting from the loss of particular cells that express each of these genes. In Drosophila, by comparison, nautilus-expressing cells committed to the myogenic program undergo few, if any, additional cell divisions, and thus no other cells are available to compensate for the loss of nautilus. Therefore, the apparent differences between the Drosophila nautilus gene and its vertebrate counterparts may reflect, at least in part, differences in the developmental systems rather than differences in the function of the genes themselves.

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.

The myogenic regulatory factor MRF4 represses the cardiac alpha-actin promoter through a negative-acting N-terminal protein domain

Cardiac alpha-actin is activated early during the development of embryonic skeletal muscle and cardiac myocytes. The gene product remains highly expressed in adult striated cardiac muscle yet is dramatically reduced in skeletal muscle. Activation and repression of cardiac alpha-actin gene activity in developing skeletal muscle correlates with changes in the relative content of the four myogenic regulatory factors. Cardiac alpha-actin promoter activity, assessed in primary chick myogenic cultures, was activated by endogenous myogenic regulatory factors but was inhibited in the presence of co-expressed MRF4. By exchanging N- and C-terminal domains of MRF4 and MyoD, the N terminus of MRF4 was identified as the mediator of repressive activity, revealing a novel negative regulatory role for MRF4. The relative ratios of myogenic regulatory factors may have fundamental roles in selecting specific muscle genes for activation and/or repression.

Expression of myosin isoforms in denervated, cross-reinnervated, and electrically stimulated rabbit muscles

The expression of myosin heavy (MyHC) and light (MyLC) chain isoforms was analyzed after denervation and cross-reinnervation by a fast nerve of the slow-twitch Semimembranosus proprius (SMp) muscle, and after denervation and electrical stimulation at low frequency of the fast-twitch Semimembranous accessorius (SMa) muscle of the rabbit. The control SMp (100% type I fibers) expressed 100% type I MyHC and 100% slow-type (1S', 1S and 2S) MyLC isoforms. Five month denervation did not alter significantly the MyHC expression of the muscle, but induced the expression of a new type 1 MyLC corresponding most probably to an embryonic MyLC. Five-month cross-reinnervation of the SMp by the fast SMa nerve induced a large change of its fiber type properties. As shown by immunocytochemistry, almost all fibers were stained by fast myosin antibody, but a high proportion of them co-expressed slow myosin. This result was in agreement with biochemical data showing that fast MyHC and MyLC isoforms became predominant. The control SMa (nearly 100% type II fibers) expressed almost 100% type II MyHC (70% type IIb and 22% IIx/d) and 100% fast-type (1F, 2F and 3F) MyLC isoforms. Five month denervation of the SMa induced a shift in its MyHC, with 98% type IIx/d and 2% type IIb isoforms, and no change in the proportions of its MyLC. Three month electrical stimulation at 10 Hz of the SMa transformed its fiber type composition. All fibers reacted with the slow myosin antibody and a minor proportion of them were stained by the fast myosin antibody. These observations were in agreement with the biochemical analysis showing a large predominance of the slow-type MyHC and MyLC isoforms. Taken together, these results obtained from rabbit muscles which are normally homogeneous in either fast-twitch or slow-twitch fiber types, further support the idea that the different myosin isoforms, particularly the MyHC, are differentially regulated by motor innervation. Type I MyHC is maintained in denervated SMp muscle, but is not expressed in denervated SMa. Type IIb isoform is the most sensitive to neural influence, as it disappears rapidly in denervated and electrically stimulated fast-twitch SMa muscle, and is barely expressed in cross-reinnervated slow-twitch SMp muscle. In contrast, type IIa and type IIx/d are less dependent upon motor innervation. In addition to the previous studies of d'Albis et al. analysis of these results leads us to conclude that, in the rabbit, sensitivity to motor innervation increases from the glycolytic to the oxydative types of fibers, in the order IIB > IIX/IID > IIA > I.

Lateral and axial signals involved in avian somite patterning: a role for BMP4

In vertebrates, muscles of the limbs and body wall derive from the lateral compartment of the embryonic somites, and axial muscles derive from the medial compartment. Whereas the mechanisms that direct patterning of somites along the dorsoventral axis are beginning to be understood, little is known about the tissue interactions and signaling molecules that direct somite patterning along the mediolateral axis. We report the identification of a specific marker for the lateral somitic compartment and its early derivatives, cSim1, an avian homolog of the Drosophila single minded gene. Using this marker, we provide evidence that specification of the lateral somitic lineage results from the antagonistic actions of a diffusible medializing signal from the neural tube and a diffusible lateralizing signal from the lateral plate mesoderm, and we implicate bone morphogenetic protein 4(BMP4) in directing this lateralization.

Combinatorial signals from the neural tube, floor plate and notochord induce myogenic bHLH gene expression in the somite

The neural tube, floor plate and notochord are axial tissues in the vertebrate embryo which have been demonstrated to play a role in somite morphogenesis. Using in vitro coculture of tissue explants, we have monitored inductive interactions of these axial tissues with the adjacent somitic mesoderm in chick embryos. We have found that signals from the neural tube and floor plate/notochord are necessary for expression of the myogenic bHLH regulators MyoD, Myf5 and myogenin in the somite. Eventually somitic expression of the myogenic bHLH genes is maintained in the absence of the axial tissues. In organ culture, at early developmental stages (HH 11-), induction of myogenesis in the three most recently formed somites can be mediated by the neural tube together with the floor plate/notochord, while in more rostral somites (stages IV-IX) the neural tube without the floor plate/notochord is sufficient. By recombining somites and neural tubes from different axial levels of the embryo, we have found that a second signal is necessary to promote competence of the somite to respond to inducing signals from the neural tube. Thus, we propose that at least two signals from axial tissues work in combination to induce myogenic bHLH gene expression; one signal derives from the floor plate/notochord and the other signal derives from regions of the neural tube other than the floor plate.

MyoD protein accumulates in satellite cells and is neurally regulated in regenerating myotubes and skeletal muscle fibers

MyoD belongs to a family of helix-loop-helix proteins that control myogenic differentiation. Transfection of various non-myogenic cell lines with MyoD transforms them into myogenic cells. In normal embryonic development MyoD is upregulated at the time when the hypaxial musculature begins to form, but its role in the function of adult muscle remains to be elucidated. In this study we examined the cellular locations of MyoD protein in normal and abnormal muscles to see whether the presence of MyoD protein is correlated with a particular cellular behaviour and to assess the usefulness of MyoD as a marker for satellite cells. Adult rats were anaesthetised and their tibialis anterior or soleus muscles either denervated, tenotomised, freeze lesioned, lesioned and denervated, or lesioned and tenotomised. At various intervals after the operations the rats were killed and their muscles removed, snap frozen, and sectioned with a cryostat along with muscles from unoperated neonatal and adult rats. The sections were processed for immunohistochemistry using a rabbit affinity-purified antibody to recombinant MyoD. MyoD proved to be an excellent marker for active satellite cells; satellite cells in neonatal and regenerating muscles contained high levels of MyoD protein. MyoD positive cells were not observed in the muscles of old adults, in which the satellite cells are fully quiescent. MyoD immunoreactivity was rapidly lost from satellite cell nuclei after they fused into myotubes and was not detected in either sub-synaptic or non-synaptic nuclei of mature fibers. Denervation, and to a lesser extent tenotomy, of lesioned muscles induced expression of MyoD in myotubal nuclei. Denervation of normal muscles also upregulated MyoD in muscle fiber nuclei, an effect which was maximal after 3 days. We conclude that MyoD protein is neurally regulated in both myotubes and muscle fibers.

MyoD and myogenin act on the chicken myosin light-chain 1 gene as distinct transcriptional factors

Expression of MyoD, myogenin, MRF4, and Myf-5 converts nonmuscle cells to muscle cells. In an attempt to analyze the roles of these factors, we have investigated their effects on transcription driven by the promoter of the chicken myosin alkaline light-chain (MLC1) gene. The activation by CMD1 or c-myogenin (chicken MyoD or myogenin, respectively) was dependent on the existence of a muscle-specific regulatory region located from positions -2096 to -1743. Its distal half, containing a pair of E boxes (CANNTG), had been previously characterized as an enhancer responsive to CMD1 but not to c-myogenin. In this study, we report the identification of another enhancer in the muscle-specific regulatory region which is preferentially responsive to c-myogenin. Deletion and mutation analyses indicated that this enhancer requires a single E box and its flanking sequences. Furthermore, analysis of chimeric proteins of CMD1 and c-myogenin indicated that regions outside the basic helix-loop-helix domain of c-myogenin are involved in the specificity of the enhancer. These results show that CMD1 and c-myogenin act on the MLC1 gene by recognizing different upstream DNA sequences and that direct or indirect interactions between the regions outside the basic helix-loop-helix domain and flanking sequences of E boxes are involved in the target sequence specificity.

MyoD and myogenin act on the chicken myosin light-chain 1 gene as distinct transcriptional factors

Expression of MyoD, myogenin, MRF4, and Myf-5 converts nonmuscle cells to muscle cells. In an attempt to analyze the roles of these factors, we have investigated their effects on transcription driven by the promoter of the chicken myosin alkaline light-chain (MLC1) gene. The activation by CMD1 or c-myogenin (chicken MyoD or myogenin, respectively) was dependent on the existence of a muscle-specific regulatory region located from positions -2096 to -1743. Its distal half, containing a pair of E boxes (CANNTG), had been previously characterized as an enhancer responsive to CMD1 but not to c-myogenin. In this study, we report the identification of another enhancer in the muscle-specific regulatory region which is preferentially responsive to c-myogenin. Deletion and mutation analyses indicated that this enhancer requires a single E box and its flanking sequences. Furthermore, analysis of chimeric proteins of CMD1 and c-myogenin indicated that regions outside the basic helix-loop-helix domain of c-myogenin are involved in the specificity of the enhancer. These results show that CMD1 and c-myogenin act on the MLC1 gene by recognizing different upstream DNA sequences and that direct or indirect interactions between the regions outside the basic helix-loop-helix domain and flanking sequences of E boxes are involved in the target sequence specificity.