Selective accumulation of MyoD and myogenin mRNAs in fast and slow adult skeletal muscle is controlled by innervation and hormones

Analysis of MyoD, myogenin, and muscle-specific gene mRNAs in regenerating Xenopus skeletal muscle

We have analyzed in adult Xenopus laevis, using in situ hybridization, the spatial and temporal expression patterns of MyoD, myogenin, and alpha-skeletal actin and fast myosin heavy chain mRNAs during muscle regeneration following cardiotoxin injury. MyoD transcripts could be detected in the satellite cells as early as the first stage of regeneration and were expressed persistently throughout the regeneration process. Myogenin mRNAs were transiently expressed in forming myotubes. alpha-Skeletal actin and fast myosin heavy chain mRNAs were detected precociously, before the young myotube stage. This work has shown, for the first time, the presence of myogenin transcripts during Xenopus myogenesis.

Induction of beta-MHC transgene in overloaded skeletal muscle is not eliminated by mutation of conserved elements

Mechanical overload leads to hypertrophy, increased type I fiber composition, and beta-myosin heavy chain (beta-MHC) induction in the fast-twitch plantaris muscle. To better understand the mechanism(s) involved in beta-MHC induction, we have examined inducible expression of transgenes carrying the simultaneous mutation of three DNA regulatory subregions [muscle CAT (MCAT), C-rich, and beta e3] in the context of either 5,600-base pair (bp; beta 5.6mut3) or 600-bp (beta 0.6mut3) beta-MHC promoter in overloaded plantaris muscles of transgenic mice. Protein extract from mechanically overloaded plantaris muscle of mice, harboring either mutant transgene beta 5.6mut3 or beta 0.6mut3, showed an unexpected 2.8- to 4.5-fold increase in chloramphenicol acetyltransferase (CAT) specific activity relative to their respective controls. Similar results were obtained with wild-type (wt) beta-MHC transgenes (beta 5.6wt, beta 0.6wt). Histochemical staining for both myofibrillar ATPase and CAT activity and CAT immunohistochemistry revealed a striking increase in type I fibers and that CAT expression was restricted to these fibers in overloaded plantaris muscle of beta 5.6mut3 transgenic mice. Our transgenic data suggest that beta-MHC transgenes, and perhaps the endogenous beta-MHC gene, are induced by mechanical overload via a mechanism(s) that does not exclusively require the MCAT, C-rich, or beta e3 subregions.

Two myogenic regulatory factor transcripts exhibit muscle-specific responses to disuse and passive stretch in adult rats

Levels of myogenic regulatory factor (MRF) transcripts are altered in a muscle-specific manner in response to hind limb immobilisation of adult male rats, for a 2 day period, in either a lengthened or shortened position which result in passive stretch or disuse atrophy respectively. Myogenin transcript levels were dramatically elevated in the stretched plantaris but not soleus, whereas the MRF4 transcript was significantly elevated in soleus but not plantaris. Levels of myogenin mRNA were unaffected by disuse in either muscle and MRF4 was markedly lower in plantaris in response to disuse.

Beta-MHC and SMLC1 transgene induction in overloaded skeletal muscle of transgenic mice

The hypertrophic responses of white fast-twitch muscle to mechanical overload has been investigated using transgenic mice. After 7 wk of overload, endogenous beta-myosin heavy chain (MHC) and slow myosin light chain 1 and 2 (SMLC1, SMLC2) protein were increased in the overloaded plantaris (OP) muscle compared with sham-operated control plantaris (CP)muscle. Concurrently, the levels of endogenous beta-MHC, SMLC1, SMLC2, and cardiac/slow troponin C (CTnC) mRNA transcripts were significantly increased in OP muscles, whereas skeletal troponin C (sTnC) mRNA transcript levels decreased. As an initial attempt to locate DNA sequence(s) that governs beta-MHC induction in response to mechanical overload, multiple independent transgenic lines harboring four different human beta-MHC transgenes (beta 1286, beta 988, beta 450, beta 141) were generated. Except for transgene beta 141, muscle-specific expression and induction (3- to 22-fold) in OP muscles were observed by measuring chloramphenicol acetyltransferase activity (CAT assay). Induction of a SMLC1 transgene (3920SMLC1) in OP muscles was also observed. Collectively, these in vivo data provide evidence that 1) a mechanical overload inducible element(s) is located between nucleotides -450 and +120 of the human beta-MHC transgene, 2) 3,900 bp of 5' sequence is sufficient to confer mechanical overload induction of a SMLC1 transgene, and 3) the increased expression of slow/type I isomyosin (beta-MHC, SMLC1, SMLC2) in response to mechanical overload is regulated, in part, transcriptionally.

The characterisation of the 5' regulatory region of a temperature-induced myosin-heavy-chain gene associated with myotomal muscle growth in the carp

We have isolated and characterised the 5' region of a member of the carp myosin heavy chain gene family. Expression of this gene has previously been shown to be induced by an increase in environmental temperature and is restricted to the small-diameter white myotomal muscle fibres which are associated with growth. The whole isoform gene, including potential regulatory sequence 5' to the transcription start site and the 3' untranslated region was cloned in a lambda2001 bacteriophage vector. Studies of the structure of the 5'-end of the gene revealed high amino acid sequence similarity with translated exons 3-7 of mammalian myosin heavy chain genes indicating identical exon/intron boundaries. The overall length of the gene was however only about one half of that in mammals and birds due to shorter introns. The region 5' to the transcription unit was sequenced and revealed the presence of putative TATA and CCAAAT boxes. In order to study the regulation of expression, a series of endonuclease-generated fragments from the 5' flanking sequence were spliced to chloramphenicol acetyltransferase reporter vectors and used in cell transfection assays or direct gene injection into carp skeletal muscle. The 5' flanking region, which contains a consensus sequence known as an E-box (CANNTG) and a MEF2 binding site, was shown to improve the expression of the reporter gene in fish acclimated at 18 degrees C or 28 degrees C. Unlike the coding region, there was little similarity between the 5'-upstream sequence (promoter region) when compared with sequences flanking the 5'-end of the other myosin heavy chain genes in mammals or chicken.

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.

Cardiomyopathy in transgenic myf5 mice

To explore the compatibility of skeletal and cardiac programs of gene expression, transgenic mice that express a skeletal muscle myogenic regulator, bmyf5, in the heart were analyzed. These mice develop a severe cardiomyopathy and exhibit a significantly shorter life span than do their nontransgenic littermates. The transgene was expressed from day 7.5 post coitum forward, resulting in activation of skeletal muscle genes not normally seen in the myocardium. Cardiac pathology was not apparent at midgestation but was evident by day 2 of postnatal life, and by 42 days, hearts exhibited multifocal interstitial inflammation, fibrosis, cellular hypertrophy, and occasional myocyte degeneration. All four chambers of the heart were enlarged to varying degrees, with the atria demonstrating the most significant hypertrophy (>100% in 42-day-old mice). The transgene and several skeletal muscle-specific genes were expressed only in patchy areas of the heart in heterozygous mice. However, molecular markers of hypertrophy (such as alpha-skeletal actin and atrial myosin light chain- 1) were expressed with a wider distribution, suggesting that their induction was secondary to the expression of the transgene, In older (28-week-old) mice, lung weights were also significantly increased, consistent with congestive heart failure. The life span of bmyf5 mice was significantly shortened, with an average life span of 109 days, compared with at least a twofold longer life expectancy for nontransgenic littermates. Expression of the transgene was associated with an increase in Ca2+-stimulated myofibrillar ATPase in myofibrils obtained from the left ventricles of 42-day-old bmyf5 mice. Myocardial bmyf5 expression therefore induces a program of skeletal muscle gene expression that results in progressive cardiomyopathy that may be due to incompatibility of heart and skeletal muscle structural proteins.

Modular elements of the MLC 1f/3f locus confer fiber-specific transcription regulation in transgenic mice

The two proteins encoded by the fast alkali myosin light chain (MLC) 1f/3f locus are developmentally regulated, muscle specific, and expressed exclusively in fast-twitch fibers. Their expression is independently regulated by two separate promoters and a downstream enhancer. Previous studies showed a reporter gene directed by the rat MLC If promoter and MLC enhancer to exhibit correct skeletal muscle-specific expression in transgenic mice during development and to be preferentially expressed in fast-twitch Type IIB fibers [Donoghue et al., (1991) J. Cell B.ol. 115:423-434]. The MLC 3f promoter also directed muscle-specific expression of a CAT reporter gene in adult transgenic mice and showed little dependence upon the enhancer. Here, we show that the MLC 3f promoter also directs transgene expression in the fast-twitch fibers of adult skeletal muscle, but almost exclusively to fiber Types IIA and IIX. MLC 3f transgene expression occurs in only a subset of the fiber types that express the endogenous locus, indicating modular elements included in the transgene confer fiber-specific transcription regulation. MyoD protein was also found to be restricted to fiber Types IIA and IIX, providing evidence for its possible role in mediating fiber-specific gene expression.

Postnatal development and plasticity of specialized muscle fiber characteristics in the hindlimb

Recent progress in defining molecular components of pathways controlling early stages of myogenesis has been substantial, but regulatory factors that govern the striking functional specialization of adult skeletal muscle fibers in vertebrate organisms have not yet been identified. A more detailed understanding of the temporal and spatial patterns by which specialized fiber characteristics arise may provide clues to the identity of the relevant regulatory factors. In this study, we used immunohistochemical, in situ hybridization, and Northern blot analyses to examine the time course and spatial characteristics of expression of myoglobin protein and mRNA during development of the distal hindlimb in the mouse. In adult animals, myoglobin is expressed selectively in oxidative, mitochondria-rich, fatigue-resistant myofibers, and it provides a convenient marker for this particular subset of specialized fibers. We observed only minimal expression of myoglobin in the hindlimb prior to the second day after birth, but a rapid and large (50-fold) induction of this gene in the ensuing neonatal period. Myoglobin expression was limited, however, to fibers located centrally within the limb which coexpress myosin isoforms characteristic of type I, IIA, and IIX fibers. This induction of myoglobin expression within the early postnatal period was accompanied by increased expression of nuclear genes encoding mitochondrial proteins, and exhibited a time course similar to the upregulation of myoglobin and mitochondrial proteins, and exhibited a time course similar to the upregulation of myoglobin and mitochondrial protein expression that can be induced in adult muscle fibers by continuous motor nerve stimulation. This comparison suggests that progressive locomotor activity of neonatal animals may provide signals which trigger the development of the specialized features of oxidative, fatigue-resistant skeletal muscle fibers.

Transcriptional control of muscle plasticity: differential regulation of troponin I genes by electrical activity

Plasticity of the skeletal muscle phenotype can result from the selective repression and activation of gene expression in response to innervation patterns. Motoneurons, eliciting different patterns of depolarization, regulate the contractile properties of the myofibers they innervate by selectively activating expression of genes encoding fiber-type-specific (fast vs. slow) contractile proteins. We have analyzed the regulation of the troponin I slow (TnIs) and fast (TnIf) genes as a model to study the molecular mechanisms regulating fiber-type plasticity. We found that expression of the two TnI isoforms is downregulated by denervation. Moreover, TnI expression is upregulated by specific patterns of electrical activity [10 Hz vs. 100 Hz] used to depolarize muscle. We previously isolated the rat TnIs gene and demonstrated that regulatory sequences reside in its upstream region and second intron [Banerjee-Basu S, Buonanno A (1993), Mol Cell Biol 12:5024-5032]. Using transgenic mice, we show that the upstream region of the TnIs gene extending from -949 to +50 is sufficient to confer transcription specifically in slowtwitch muscles. Serial deletions of the TnIs upstream and intronic regions were generated in a CAT reporter vector to delineate transcriptional regulatory elements in transiently transfected Sol8 myotubes. Sequences necessary to confer the highest levels of TnIs transcription mapped to the upstream region between -0.95 and -0.72 kb, and to a 56 bp sequence located in the second intron. Comparison of the at sequence between -0.95 and -0.72 to the human TnIs gene identified a highly homologous region of 128 bp that we named the TnI SURE (slow upstream regulatory element). Alignment of these two SURE sequences with the quail TnI fast intronic regulatory element identified common motifs, namely, two A/T-rich sequences (A/T1 and A/T2) with homology to homeotic protein and MEF2 binding sites, a CACC box, an E box, and a novel motif (GCAGGCA) that we denoted the CAGG box. Mutation of either the A/T2 site, E box, or CAGG box practically abolish the SURE function in transfected myotubes; mutation of the A/T1 and CACC sites has a lesser effect. Using competitive electrophoretic mobility shift assays with nuclear extracts derived from Sol8 myotubes, we demonstrate specific binding to these motifs. The A/T1 and A/T2 sites are shown to form different complexes. The A/T2 site, which bears extensive homology to a MEF2 site, forms complexes that are super shifted by MEF2A antisera and that are competed by a consensus MEF2 site present in the MCK enhancer. Our results demonstrate that the linear arrangement of DNA sequence motifs is conserved in the regulatory elements of the TnI slow and fast genes and suggest that the interaction of multiple protein-DNA complexes are necessary for enhancer function.

Determination versus differentiation and the MyoD family of transcription factors

The myogenic regulatory factors (MRFs) form a family of basic helix-loop-helix transcription factors consisting of Myf-5, MyoD, myogenin, and MRF4. The MRFs play key regulatory roles in the development of skeletal muscle during embryogenesis. Sequence homology, expression patterns, and gene-targeting experiments have revealed a two-tiered subclassification within the MRF family. Myf-5 and MyoD are more homologous to one another than to the others, are expressed in myoblasts before differentiation, and are required for the determination or survival of muscle progenitor cells. By contrast, myogenin and MRF4 are more homologous to one another than to the others and are expressed upon differentiation, and myogenin is required in vivo as a differentiation factor while the role of MRF4 remains unclear. On this basis, MyoD and Myf-5 are classified as primary MRFs, as they are required for the determination of myoblasts, and myogenin and MRF4 are classified as secondary MRFs, as they likely function during terminal differentiation.

Differential expression patterns of five acetylcholine receptor subunit genes in rat muscle during development

The spatial and temporal expression patterns of five genes which encode the alpha-, beta-, gamma-, delta- and epsilon-subunits of the nicotinic acetylcholine receptor in skeletal muscle were followed during development in the rat by in situ hybridization analysis. Three major developmental phases, characterized by specific expression patterns, could be distinguished. (i) During myogenic differentiation alpha-, beta-, gamma- and delta-subunit genes are activated and transcripts are expressed in muscle precursor cells at embryonic day 12 (E12) and during subsequent cell fusion. (ii) Following innervation of myotubes at approximately E15-E17 the mRNA of the alpha-, beta-, gamma- and delta-subunit genes accumulate in synaptic and decrease in extrasynaptic fibre regions during early synaptogenesis. The mRNA of the epsilon-subunit gene becomes detectable first in subsynaptic nuclei 2-3 days after innervation has occurred. (iii) During postnatal development alpha-, beta- and delta- subunit transcript levels are reduced predominantly in extrasynaptic fibre segments and show significant differences in distribution depending on the muscle subtype whereas the gamma-subunit mRNA disappears completely within the first postnatal week in all muscles. In contrast, the epsilon-subunit gene is transcribed only in subsynaptic myonuclei throughout development and in the adult muscle.

Inactivation of the myogenic bHLH gene MRF4 results in up-regulation of myogenin and rib anomalies

The myogenic basic helix-loop-helix (bHLH) proteins MyoD, myf5, myogenin, and MRF4 can initiate myogenesis when expressed in nonmuscle cells. During embryogenesis, each of the myogenic bHLH genes is expressed in a unique temporospatial pattern within the skeletal muscle lineage, suggesting that they play distinct roles in muscle development. Gene targeting has shown that MyoD and myf5 play partially redundant roles in the genesis of myoblasts, whereas myogenin is required for terminal differentiation. MRF4 is expressed transiently in the somite myotome during embryogenesis and then becomes up-regulated during late fetal development to eventually become the predominant myogenic bHLH factor expressed in adult skeletal muscle. On the basis of its expression pattern, it has been proposed that MRF4 may regulate skeletal muscle maturation and aspects of adult myogenesis. To determine the function of MRF4, we generated mice carrying a homozygous germ-line mutation in the MRF4 gene. These mice showed only a subtle reduction in expression of a subset of muscle-specific genes but showed a dramatic increase in expression of myogenin, suggesting that it may compensate for the absence of MRF4 and demonstrating that MRF4 is required for the down-regulation of myogenin expression that normally occurs in postnatal skeletal muscle. Paradoxically, MRF4-null mice exhibited multiple rib anomalies, including extensive bifurcations, fusions, and supernumerary processes. These results demonstrate an unanticipated regulatory relationship between myogenin and MRF4 and suggest that MRF4 influences rib outgrowth through an indirect mechanism.

MRF4, Myf-5, and myogenin mRNAs in the adaptive responses of mature rat muscle

We studied the possible role of specific muscle regulatory factors (MRF) in the adaptive response to changes in contractile activity in mature skeletal muscle. The tibialis anterior muscle of anesthetized female rats was subjected to low-frequency stimulation, static stretch, or a combination of both. Message levels of MRF were observed after 2 h of activity, and the subsequent 20-h recovery period by slot blot and in situ hybridizations for MRF4, Myf-5, and myogenin. A combination of stimulation and stretch for 2 h increased MRF4 (11.6 +/- 5.3-fold) and Myf-5 (6.6 +/- 1.4-fold). In situ hybridization showed abundance in some regions of the muscle with positive staining near peripheral nuclei of both large and small fibers. Message levels remained high for 30 min and declined to near control levels by 20 h of recovery. Myogenin mRNA levels were unaffected by any manipulations. Neither stretch alone nor 10 Hz of electrical stimulation alone induced a significant increase in MRF. We conclude that myonuclei, and possibly activated myoblasts, increase expression of Myf-5 and MRF4 after a combination of both stimulation and stretch for 2 h.

A gene with homology to myogenin is expressed in developing myotomal musculature of the rainbow trout and in vitro during the conversion of myosatellite cells to myotubes

We report the cloning of a new trout myogenic cDNA which encodes helix-loop-helix protein homologous to the myogenic factor myogenin. Northern analyses indicate that trout myogenin (Tmyogenin) transcripts accumulate in large amounts in the myotomal musculature of embryos and frys. In adults, transcripts concentrate within the thin lateral layer of red (slow oxydative) muscle fibres. They are present only in low amounts in white (fast glycolytic) muscle fibres which constitute the major part of the trunk musculature. Using an in vitro myogenesis system, we observed that the trout myogenin encoding gene is not activated until myosatellite cells fuse to generate multinucleated myotubes, indicating that Tmyogenin lies downstream of muscle determination factors. All these observations show that in a major taxinomic group like teleosts, a gene with homology to myogenin exists. Its activation during myogenesis suggests that it acts as a major developmental regulator of muscle differentiation.

Embryonic activation of the myoD gene is regulated by a highly conserved distal control element

MyoD belongs to a small family of basic helix-loop-helix transcription factors implicated in skeletal muscle lineage determination and differentiation. Previously, we identified a transcriptional enhancer that regulates the embryonic expression of the human myoD gene. This enhancer had been localized to a 4 kb fragment located 18 to 22 kb upstream of the myoD transcriptional start site. We now present a molecular characterization of this enhancer. Transgenic and transfection analyses localize the myoD enhancer to a core sequence of 258 bp. In transgenic mice, this enhancer directs expression of a lacZ reporter gene to skeletal muscle compartments in a spatiotemporal pattern indistinguishable from the normal myoD expression domain, and distinct from expression patterns reported for the other myogenic factors. In contrast to the myoD promoter, the myoD enhancer shows striking conservation between humans and mice both in its sequence and its distal position. Furthermore, a myoD enhancer/heterologous promoter construct exhibits muscle-specific expression in transgenic mice, demonstrating that the myoD promoter is dispensable for myoD activation. With the exception of E-boxes, the myoD enhancer has no apparent sequence similarity with regulatory regions of other characterized muscle-specific structural or regulatory genes. Mutation of these E-boxes, however, does not affect the pattern of lacZ transgene expression, suggesting that myoD activation in the embryo is E-box-independent. DNase I protection assays reveal multiple nuclear protein binding sites in the core enhancer, although none are strictly muscle-specific. Interestingly, extracts from myoblasts and 10T1/2 fibroblasts yield identical protection profiles, indicating a similar complement of enhancer-binding factors in muscle and this non-muscle cell type. However, a clear difference exists between myoblasts and 10T1/2 cells (and other non-muscle cell types) in the chromatin structure of the chromosomal myoD core enhancer, suggesting that the myoD enhancer is repressed by epigenetic mechanisms in 10T1/2 cells. These data indicate that myoD activation is regulated at multiple levels by mechanisms that are distinct from those controlling other characterized muscle-specific genes.

Regulation of vertebrate muscle differentiation by thyroid hormone: the role of the myoD gene family

Skeletal myoblasts have their origin early in embryogenesis within specific somites. Determined myoblasts are committed to a myogenic fate; however, they only differentiate and express a muscle-specific phenotype after they have received the appropriate environmental signals. Once proliferating myoblasts enter the differentiation programme they withdraw from the cell cycle and form post-mitotic multinucleated myofibres (myogenesis); this transformation is accompanied by muscle-specific gene expression. Muscle development is associated with complex and diverse protein isoform transitions, generated by differential gene expression and mRNA splicing. The myofibres are in a state of dynamic adaptation in response to hormones, mechanical activity and motor innervation, which modulate differential gene expression and splicing during this functional acclimatisation. This review will focus on the profound effects of thyroid hormone on skeletal muscle, which produce alterations in gene and isoform expression, biochemical properties and morphological features that precipitate in modified contractile/mechanical characteristics. Insight into the molecular events that control these events was provided by the recent characterisation of the MyoD gene family, which encodes helix-loop-helix proteins; these activate muscle-specific transcription and serve as targets for a variety of physiological stimuli. The current hypothesis on hormonal regulation of myogenesis is that thyroid hormones (1) directly regulate the myoD and contractile protein gene families, and (2) induce thyroid hormone receptor-transcription factor interactions critical to gene expression.

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.

The MyoD family of transcription factors and skeletal myogenesis

Gene targeting has allowed the dissection of complex biological processes at the genetic level. Our understanding of the nuances of skeletal muscle development has been greatly increased by the analysis of mice carrying targeted null mutations in the Myf-5, MyoD and myogenin genes, encoding members of the myogenic regulatory factor (MRF) family. These experiments have elucidated the hierarchical relationships existing between the MRFs, and established that functional redundancy is a feature of the MRF regulatory network. Either MyoD or Myf-5 is sufficient for the formation or survival of skeletal myoblasts. Myogenin acts later in development and plays an essential in vivo role in the terminal differentiation of myotubes.

New fiber formation in rat soleus muscle following administration of denervated muscle extract

A study was made of Wistar rat soleus muscle following intraperitoneal administration of denervated muscle extract over 1 and 2 days. Light microscopy revealed the appearance on fiber surfaces of basophilic satellite structures whose histochemical behaviour differed from that of the parent fiber. Small fibers showing regenerative characteristics were also detected, mainly in the extrafascicular spaces. At ultrastructural examination, activated satellite cells were visible, and there was evidence of splitting in subsarcolemmal regions of apparently hypertrophic muscle fibers. Interstitial cells were occasionally observed, containing structures like myofilaments. The hypothesis is advanced that denervated muscle extract contains substances able to stimulate new fiber formation in adult skeletal muscle.

MyoD expression marks the onset of skeletal myogenesis in Myf-5 mutant mice

The expression pattern of myogenic regulatory factors and myotome-specific contractile proteins was studied during embryonic development of Myf-5 mutant mice by in situ hybridization and immunohistochemistry. In contrast to somites in wild-type embryos, no expression of myogenin and Myf-6 (MRF4), or any other myotomal markers was detected in mutant animals at E9.0 and E10.0 indicating that Myf-5 plays a crucial role during this developmental period. Significantly, the onset of MyoD expression in rostral somites of E10.5 embryos was unaffected by the Myf-5 mutation suggesting that the activation of the MyoD gene occurs independently of Myf-5 at the correct developmental time. Immediately after the activation of MyoD myogenin transcripts and protein accumulated within the myotome. The first contractile proteins of the sarcomeric apparatus appeared slightly later. By E11.5 the expression of muscle markers were indistinguishable between wild-type and Myf-5 mutant mice. The migration of muscle precursor cells that leave the somites to form limb musculature was monitored in Myf-5-mutant mice by Pax-3 expression. Pax-3-positive cells were equally found in somites and limbs of E10.0 wild-type and mutant mice indicating that myogenic factor expression at the level of somites is not a prerequisite for determination and subsequent migration of limb precursor cells.

Muscle: the regulation of myogenesis

The study of myogenesis in the embryo is a rapidly expanding field. In this context, the consequences of mutating different members of the MyoD family, together with an increasing number of observations that point to the importance of the MEF2 or RSRF family as myogenic regulators, and the identification of Pax-3 as a marker of early myogenic cells, have advanced our understanding of the molecular embryology of skeletal muscle. Novel cardiac regulatory factors such as Nkx-2.5 and GATA-4, in addition to MEF2 isoforms, are also beginning to be identified. At the molecular level, crystallographic studies have led to a structural model of the actinomyosin complex and also to information about how MyoD contacts DNA.

Dystrophin deficiency, altered cell signalling and fibre hypertrophy

Dystrophin is a subsarcolemmal protein which is defective in Duchenne and Becker muscular dystrophy (DMD/BMD), and in three animal models. Clinical manifestations of dystrophin deficiency in humans range from a mild calf muscle hypertrophy with cramps to the classical progressive degenerative hypertrophic myopathy of Duchenne. A common feature in the clinical presentation of dystrophin deficiency in humans and in the three documented animal models is the presence of muscle fibre hypertrophy. This paper explores the hypothesis that membrane-bound signalling processes are disrupted in the absence of dystrophin, and suggests that these abnormalities may contribute to both the hypertrophic and degenerative changes of dystrophin deficiency.

Wiring diagrams: regulatory circuits and the control of skeletal myogenesis

During the past year, targeted mutagenesis in mice has begun to clarify the roles of individual members of the MyoD family of myogenic regulators in vertebrate development. In this review, we discuss these studies both in the context of tissue interactions necessary to induce skeletal muscle precursor cells during embryogenesis and the molecular circuitry that regulates the terminal differentiation of these cells.

New insights into the pathogenesis of congenital myopathies

Congenital myopathies are developmental disorders of muscle that are best understood in the context of ontogenesis. Segmental amyoplasia results from a defective somite, usually because of lack of induction by the notochord and neural tube; the connective tissue matrix of the muscle is derived from lateral mesoderm and is present, but the myocytes are derived from somitic mesoderm and are replaced by adipose cells. Generalized amyoplasia is due to defective myogenic regulatory genes. X-linked recessive myotubular myopathy is associated with overexpression of vimentin and desmin, fetal intermediate filaments that attach to nuclear, mitochondrial, and inner sarcolemmal membranes and Z-bands of sarcomeres to preserve the morphologic organization of the myotube. Neonatal myotonic dystrophy is a true maturational delay in muscle development. Congenital muscle fiber-type disproportion is a syndrome of multiple etiologies but in some cases is associated with cerebellar hypoplasia and may be the result of abnormal suprasegmental stimulation of the developing motor unit at 20 to 28 weeks' gestation, mediated through bulbospinal pathways but not the corticospinal tract. Maturational delay of muscle in late developmental stages is less specific than in stages before midgestation. The Proteus syndrome is a muscular dysgenesis; abnormal paracrine growth factors and perhaps altered genes that regulate muscle differentiation and growth, such as myoD and myogenin, are the suspected cause. Focal proliferative myositis may be another example of a "paracrine myopathy."

bHLH factors in muscle development: dead lines and commitments, what to leave in and what to leave out

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Differential expression of muscle regulatory factor genes in normal and denervated adult rat hindlimb muscles

Skeletal muscle represents an excellent model system in which to examine regulatory mechanisms that modulate gene expression in the mature adult organism. Individual muscle fibers can be categorized as fast- or slow-twitch based upon several physiological and molecular criteria, including metabolic enzyme activity and contractile protein isoforms. Each property can be influenced by a variety of factors such as changes in motor neuron activity or alterations in hormone levels, although the molecular pathways by which environmental factors affect gene expression remain largely unknown. As a first step in identifying potential regulators of fiber-type diversity, the expression patterns of four basic/helix-loop-helix muscle regulatory factors (MRFs), referred to as MyoD, myogenin, Myf-5, and MRF4, were examined in normal adult rat muscles which differed in their phenotypic properties. As expected, all four MRFs were expressed at detectable levels in the muscles studied. However, different muscles accumulated different proportions and combinations of MRF transcripts. For example, myogenin expression was maximally detected in slow-twitch muscles whereas MyoD transcripts were found predominantly in muscles exhibiting a fast-twitch phenotype. Induced phenotypic changes in two fast-twitch muscles via denervation lead to a large and rapid increase in transcript levels of all four MRFs as early as 24 hr following denervation, with myogenin transcripts approaching 150-200-fold higher levels than innervated contralateral muscles within 7 days. These results suggest that myogenin, as well as the other three MRFs, may be involved in both the initial establishment as well as maintenance of fiber-type diversity in the developing organism.