Distinct regulatory elements control muscle-specific, fiber-type-selective, and axially graded expression of a myosin light-chain gene in transgenic mice

Developmental, physiologic and phylogenetic perspectives on the expression and regulation of myosin heavy chains in mammalian skeletal muscles

The kinetics of myosin controls the speed and power of muscle contraction. Mammalian skeletal muscles express twelve kinetically different myosin heavy chain (MyHC) genes which provides a wide range of muscle speeds to meet different functional demands. Myogenic progenitors from diverse craniofacial and somitic mesoderm specify muscle allotypes with different repertoires for MyHC expression. This review provides a brief synopsis on the historical and current views on how cell lineage, neural impulse patterns, and thyroid hormone influence MyHC gene expression in muscles of the limb allotype during development and in adult life and the molecular mechanisms thereof. During somitic myogenesis, embryonic and foetal myoblast lineages form slow and fast primary and secondary myotube ontotypes which respond differently to postnatal neural and thyroidal influences to generate fully differentiated fibre phenotypes. Fibres of a given phenotype may arise from myotubes of different ontotypes which retain their capacity to respond differently to neural and thyroidal influences during postnatal life. This gives muscles physiological plasticity to adapt to fluctuations in thyroid hormone levels and patterns of use. The kinetics of MyHC isoforms vary inversely with animal body mass. Fast 2b fibres are specifically absent in muscles involved in elastic energy saving in hopping marsupials and generally absent in large eutherian mammals. Changes in MyHC expression are viewed in the context of the physiology of the whole animal. The roles of myoblast lineage and thyroid hormone in regulating MyHC gene expression are phylogenetically the most ancient while that of neural impulse patterns the most recent.

Skeletal Muscle-Restricted Expression of Human SOD1 in Transgenic Mice Causes a Fatal ALS-Like Syndrome

Amyotrophic lateral sclerosis (ALS) is a fatal heterogeneous neurodegenerative disease that causes motor neuron (MN) loss and skeletal muscle paralysis. It is uncertain whether this degeneration of MNs is triggered intrinsically and is autonomous, or if the disease initiating mechanisms are extrinsic to MNs. We hypothesized that skeletal muscle is a primary site of pathogenesis in ALS that triggers MN degeneration. Some inherited forms of ALS are caused by mutations in the superoxide dismutase-1 (SOD1) gene, that encodes an antioxidant protein, so we created transgenic (tg) mice expressing wild-type-, G37R-, and G93A-human SOD1 gene variants only in skeletal muscle. Presence of human SOD1 (hSOD1) protein in skeletal muscle was verified by western blotting, enzyme activity gels, and immunofluorescence in myofibers and satellite cells. These tg mice developed limb weakness and paresis with motor deficits, limb and chest muscle wasting, diaphragm atrophy, and age-related fatal disease with a lifespan shortening of 10–16%. Brown and white adipose tissue also became wasted. Myofibers of tg mice developed crystalline-like inclusions, individualized sarcomere destruction, mitochondriopathy with vesiculation, DNA damage, and activated p53. Satellite cells became apoptotic. The diaphragm developed severe loss of neuromuscular junction presynaptic and postsynaptic integrity, including decreased innervation, loss of synaptophysin, nitration of synaptophysin, and loss of nicotinic acetylcholine receptor and scaffold protein rapsyn. Co-immunoprecipitation identified hSOD1 interaction with rapsyn. Spinal cords of tg mice developed gross atrophy. Spinal MNs formed cytoplasmic and nuclear inclusions, axonopathy, mitochondriopathy, accumulated DNA damage, activated p53 and cleaved caspase-3, and died. Tg mice had a 40–50% loss of MNs. This work shows that hSOD1 in skeletal muscle is a driver of pathogenesis in ALS, that involves myofiber and satellite cell toxicity, and apparent muscle-adipose tissue disease relationships. It also identifies a non-autonomous mechanism for MN degeneration explaining their selective vulnerability as likely a form of target-deprivation retrograde neurodegeneration.

Mapping Transgene Insertion Sites Reveals Complex Interactions Between Mouse Transgenes and Neighboring Endogenous Genes

Transgenic mouse lines are routinely employed to label and manipulate distinct cell types. The transgene generally comprises cell-type specific regulatory elements linked to a cDNA encoding a reporter or other protein. However, off-target expression seemingly unrelated to the regulatory elements in the transgene is often observed, it is sometimes suspected to reflect influences related to the site of transgene integration in the genome. To test this hypothesis, we used a proximity ligation-based method, Targeted Locus Amplification (TLA), to map the insertion sites of three well-characterized transgenes that appeared to exhibit insertion site-dependent expression in retina. The nearest endogenous genes to transgenes HB9-GFP, Mito-P, and TYW3 are Cdh6, Fat4 and Khdrbs2, respectively. For two lines, we demonstrate that expression reflects that of the closest endogenous gene (Fat4 and Cdh6), even though the distance between transgene and endogenous gene is 550 and 680 kb, respectively. In all three lines, the transgenes decrease expression of the neighboring endogenous genes. In each case, the affected endogenous gene was expressed in at least some of the cell types that the transgenic line has been used to mark and study. These results provide insights into the effects of transgenes and endogenous genes on each other's expression, demonstrate that mapping insertion site is valuable for interpreting results obtained with transgenic lines, and indicate that TLA is a reliable method for integration site discovery.

Functional Segregation within the Muscles of Aquatic Propulsion in the Asiatic Water Monitor (Varanus salvator)

Water monitor lizards (Varanus salvator) swim using sinusoidal oscillations generated at the base of their long (50% of total body length) tail. In an effort to determine which level of the structural/organizational hierarchy of muscle is associated with functional segregation between the muscles of the tail base, an array of muscle features-myosin heavy chain profiles, enzymatic fiber types, twitch and tetanic force production, rates of fatigue, muscle compliance, and electrical activity patterns-were quantitated. The two examined axial muscles, longissimus, and iliocaudalis, were generally similar at the molecular, biochemical, and physiological levels, but differed at the biomechanics level and in their activation pattern. The appendicular muscle examined, caudofemoralis, differed from the axial muscles particularly at the molecular and physiological levels, and it exhibited a unique compliance profile and pattern of electrical activation. There were some apparent contradictions between the different structural/organizational levels examined. These contradictions, coupled with a unique myosin heavy chain profile, lead to the hypothesis that there are previously un-described molecular/biochemical specializations within varanid skeletal muscles.

Molecular characterization of Myostatin gene from Zhikong scallop Chlamys farreri (Jones et Preston 1904)

The scallop is an economically important sea food prized for its large and delicious adductor muscle. Studying the molecular basis of scallop muscle growth is important for both scallop breeding and our understanding of muscle mass regulation in bivalve. Myostatin (MSTN) is a conserved negative regulator of muscle growth and development. Here we report the MSTN gene from Zhikong scallop (Chlamys farreri Jones et Preston 1904). The C. farreri MSTN consists of 11651 nucleotides encoding 457 amino acids. The gene has a 3-exon/2-intron structure that is conserved with vertebrate homologs. The exons are 586, 380 and 408 bp in length, respectively, and separated by introns of 5086 and 1518 bp. The protein sequence contains characteristic conserved residues including a cleavage motif of proteolysis (RXXR) and nine cysteines. Three transcription initiation sites were found at 62, 146, and 296 bp upstream of the translation start codon ATG. In silico analysis of the promoter region identified a TATA-box and several muscle-specific regulatory elements including COMP, MEF2s, MTBFs and E-boxes. Minisatellite DNA was found in intron 1. By fluorescence in situ hybridization (FISH), the gene was mapped to the long arm of a pair of middle subtelocentric chromosome. Quantitative analysis of MSTN transcripts in embryos/larvae indicated high expression level in gastrulae and limited expression at other stages. In adult scallops, MSTN is predominantly expressed in striated muscle, with different expression levels in other tissues. Our data provide valuable genomic and expression information which will aid the further study on scallop MSTN function and MSTN evolution.

Regulation of jaw-specific isoforms of myosin-binding protein-C and tropomyosin in regenerating cat temporalis muscle innervated by limb fast and slow motor nerves

Cat jaw-closing muscles are a distinct muscle allotype characterized by the expression of masticatory-specific myofibrillar proteins. Transplantation studies showed that expression of masticatory myosin heavy chain (m-MyHC) is promoted by fast motor nerves, but suppressed by slow motor nerves. We investigated whether masticatory myosin-binding protein-C (m-MBP-C) and masticatory tropomyosin (m-Tm) are similarly regulated. Temporalis muscle strips were transplanted into limb muscle beds to allow innervation by fast or slow muscle nerve during regeneration. Regenerated muscles were examined postoperatively up to 168 days by peroxidase IHC using monoclonal antibodies to m-MyHC, m-MBP-C, and m-Tm. Regenerates in both muscle beds expressed fetal and slow MyHCs, m-MyHC, m-MBP-C, and m-Tm during the first 4 weeks. Longer-term regenerates innervated by fast nerve suppressed fetal and slow MyHCs, retaining m-MyHC, m-MBP-C, and m-Tm, whereas fibers innervated by slow nerve suppressed fetal MyHCs and the three masticatory-specific proteins, induced slow MyHC, and showed immunohistochemical characteristics of jaw-slow fibers. We concluded that expression of m-MBP-C and m-Tm is coregulated by m-MyHC and that neural impulses to limb slow muscle are capable of suppressing masticatory-specific proteins and to channel gene expression along the jaw-slow phenotype unique to jaw-closing muscle.

Identification of novel transcripts from the porcine MYL1 gene and initial characterization of its promoters

The fast skeletal alkali myosin light polypeptide 1 (MYL1) gene is one of three mammalian alkali MLC genes and encodes two isoforms, 1f and 3f, which play a vital role in embryonic, fetal, and adult skeletal muscle development. We isolated the MYL1 gene from a pig BAC library with the goal of characterizing its promoter and identifying its transcripts. Genes and isoforms were identified by reverse transcriptase-PCR, northern blot and RACE (Rapid Amplification of cDNA Ends). Potential MYL1 gene promoters were characterized using a luciferase reporter assay and electrophoretic mobility shift assays (EMSA). MLC1f, MLC3f, and three additional isoforms of porcine MYL1, MLC5f-A, -B, and -C were identified. Up to now, the three novel isoforms had not been reported in human or mouse. Northern blot analysis indicated that MLC1f, MLC3f, and MLC5fs were expressed only in longissimus dorsi muscles. Two transcription initiation and termination sites were identified by RACE. Promoter analysis and EMSA demonstrated the presence of a MEF3 (skeletal muscle-specific transcriptional enhancer) binding site (+384 to +481), which might be essential for porcine MYL1 transcription. Our results suggested that five transcript variants were generated using alternative promoters, two transcription start sites, and polyA sites, as well as variable splicing of the pig MYL1 exon 5. The identification of alternative promoters and splice variants, the expression of the splice variants in different muscle tissues, and the definition of regulatory elements provide important molecular genetic knowledge concerning the MYL1 gene.

Identification and characterization of alternative promoters of zebrafish Rtn-4/Nogo genes in cultured cells and zebrafish embryos

In mammals, the Nogo family consists of Nogo-A, Nogo-B and Nogo-C. However, there are three Rtn-4/Nogo-related transcripts were identified in zebrafish. In addition to the common C-terminal region, the N-terminal regions of Rtn4-n/Nogo-C1, Rtn4-m/Nogo-C2 and Rtn4-l/Nogo-B, respectively, contain 9, 25 and 132 amino acid residues. In this study, we isolated the 5'-upstream region of each gene from a BAC clone and demonstrated that the putative promoter regions, P1-P3, are functional in cultured cells and zebrafish embryos. A transgenic zebrafish Tg(Nogo-B:GFP) line was generated using P1 promoter region to drive green fluorescent protein (GFP) expression through Tol2-mediated transgenesis. This line recapitulates the endogenous expression pattern of Rtn4-l/Nogo-B mRNA in the brain, brachial arches, eyes, muscle, liver and intestines. In contrast, GFP expressions by P2 and P3 promoters were localized to skeletal muscles of zebrafish embryos. Several GATA and E-box motifs are found in these promoter regions. Using morpholino knockdown experiments, GATA4 and GATA6 were involved in the control of P1 promoter activity in the liver and intestine, while Myf5 and MyoD for the control of P1 and P3 promoter activities in muscles. These data demonstrate that zebrafish Rtn4/Nogo transcripts might be generated by coupling mechanisms of alternative first exons and alternative promoter usage.

Polymorphism of chicken myocyte-specific enhancer-binding factor 2A gene and its association with chicken carcass traits

Myocyte-specific enhancer-binding factor 2A (MEF2A) gene is a member of the myocyte-specific enhancer-binding factor 2 (MEF2) protein family which involved in vertebrate skeletal muscle development and differentiation. The aim of the current study is to investigate the potential associations between MEF2A gene SNPs (single nucleotide polymorphisms) and the carcass traits in 471 chicken samples from four populations. Three new SNPs (T46023C, A72626G, and T89232G) were detected in the chicken MEF2A gene. The T46023C genotypes were associated with live body weight (BW), carcass weight (CW), eviscerated weight, semi-eviscerated weight (SEW), and leg muscle weight (LMW) (P < 0.05); the A72626G genotypes were associated with BW, CW, LMW (P < 0.01) and breast muscle weight (BMW), leg muscle percentage (LMP) (P < 0.05); whereas the T89232G genotypes were associated with carcass percentage (CP) and semi-eviscerated percentage (SEP) (P < 0.05). The haplotypes constructed on the three SNPs were associated with BW, CW, LMW (P < 0.01), SEW, BMW, CP (P < 0.05). Significantly and suggestive dominant effects of diplotype H1H2 were observed for BW, CW, SEW, BMW and CP, whereas diplotype H5H5 had a negative effect on BW, CW, SEW, BMW and LMW. Our results suggest that the MEF2A gene may be a potential marker affecting the muscle trait of chickens.

Effects of hypothyroidism on myosin heavy chain composition and fibre types of fast skeletal muscles in a small marsupial, Antechinus flavipes

Effects of drug-induced hypothyroidism on myosin heavy chain (MyHC) content and fibre types of fast skeletal muscles were studied in a small marsupial, Antechinus flavipes. SDS-PAGE of MyHCs from the tibialis anterior and gastrocnemius revealed four isoforms, 2B, 2X, 2A and slow, in that order of decreasing abundance. After 5 weeks treatment with methimazole, the functionally fastest 2B MyHC significantly decreased, while 2X, 2A and slow MyHCs increased. Immunohistochemistry using monospecific antibodies to each of the four MyHCs revealed decreased 2b and 2x fibres, and increased 2a and hybrid fibres co-expressing two or three MyHCs. In the normally homogeneously fast superficial regions of these muscles, evenly distributed slow-staining fibres appeared, resembling the distribution of slow primary myotubes in fast muscles during development. Hybrid fibres containing 2A and slow MyHCs were virtually absent. These results are more detailed but broadly similar to the earlier studies on eutherians. We hypothesize that hypothyroidism essentially reverses the effects of thyroid hormone on MyHC gene expression of muscle fibres during myogenesis, which differ according to the developmental origin of the fibre: it induces slow MyHC expression in 2b fibres derived from fast primary myotubes, and shifts fast MyHC expression in fibres of secondary origin towards 2A, but not slow, MyHC.

Cloning and sequence analysis of myostatin promoter in sheep

To better understand the structure and function of the myostatin's gene promoter region in sheep, we cloned and sequenced a 1.517 kb fragment containing the 5'-regulatory region of the sheep myostatin gene (GenBank accession number is AY918121). The promoter sequence consists of three TATA boxes, one CAAT box, and eight putative E-boxes. Some putative muscle growth response elements for Octamer-binding factor 1(Octamer), Activator protein 1(AP1), Growth factor independence 1 zinc finger protein (Gfi-1B), Myocyte enhancer factor 2 (MEF2), Muscle-specific Mt binding site (MTBF), Glucocorticoid response elements (GRE) and Progesterone receptor binding site (PRE) were detected. Some of the motifs are conserved as compared to with that in the goat, bovine and porcine myostatin promoters. However, some differences were also found.

Myostatin regulates fiber-type composition of skeletal muscle by regulating MEF2 and MyoD gene expression

Myostatin (Mstn) is a secreted growth factor belonging to the tranforming growth factor (TGF)-β superfamily. Inactivation of murine Mstn by gene targeting, or natural mutation of bovine or human Mstn, induces the double muscling (DM) phenotype. In DM cattle, Mstn deficiency increases fast glycolytic (type IIB) fiber formation in the biceps femoris (BF) muscle. Using Mstn null (−/−) mice, we suggest a possible mechanism behind Mstn-mediated fiber-type diversity. Histological analysis revealed increased type IIB fibers with a concomitant decrease in type IIA and type I fibers in the Mstn−/−tibialis anterior and BF muscle. Functional electrical stimulation of Mstn−/−BF revealed increased fatigue susceptibility, supporting increased type IIB fiber content. Given the role of myocyte enhancer factor 2 (MEF2) in oxidative type I fiber formation, MEF2 levels in Mstn−/−tissue were quantified. Results revealed reduced MEF2C protein in Mstn−/−muscle and myoblast nuclear extracts. Reduced MEF2-DNA complex was also observed in electrophoretic mobility-shift assay using Mstn−/−nuclear extracts. Furthermore, reduced expression of MEF2 downstream target genes MLC1F and calcineurin were found in Mstn−/−muscle. Conversely, Mstn addition was sufficient to directly upregulate MLC promoter-enhancer activity in cultured myoblasts. Since high MyoD levels are seen in fast fibers, we analyzed MyoD levels in the muscle. In contrast to MEF2C, MyoD levels were increased in Mstn−/−muscle. Together, these results suggest that while Mstn positively regulates MEF2C levels, it negatively regulates MyoD expression in muscle. We propose that Mstn could regulate fiber-type composition by regulating the expression of MEF2C and MyoD during myogenesis.

Hox specificity unique roles for cofactors and collaborators

Hox proteins are well known for executing highly specific functions in vivo, but our understanding of the molecular mechanisms underlying gene regulation by these fascinating proteins has lagged behind. The premise of this review is that an understanding of gene regulation-by any transcription factor-requires the dissection of the cis-regulatory elements that they act upon. With this goal in mind, we review the concepts and ideas regarding gene regulation by Hox proteins and apply them to a curated list of directly regulated Hox cis-regulatory elements that have been validated in the literature. Our analysis of the Hox-binding sites within these elements suggests several emerging generalizations. We distinguish between Hox cofactors, proteins that bind DNA cooperatively with Hox proteins and thereby help with DNA-binding site selection, and Hox collaborators, proteins that bind in parallel to Hox-targeted cis-regulatory elements and dictate the sign and strength of gene regulation. Finally, we summarize insights that come from examining five X-ray crystal structures of Hox-cofactor-DNA complexes. Together, these analyses reveal an enormous amount of flexibility into how Hox proteins function to regulate gene expression, perhaps providing an explanation for why these factors have been central players in the evolution of morphological diversity in the animal kingdom.

Integration of embryonic and fetal skeletal myogenic programs at the myosin light chain 1f/3f locus

The genetic control of skeletal muscle differentiation at the onset of myogenesis in the embryo is relatively well understood compared to the formation of muscle during the fetal period giving rise to the bulk of skeletal muscle fibers at birth. The Mlc1f/3f (Myl1) locus encodes two alkali myosin light chains, Mlc1f and Mlc3f, from two promoters that are differentially regulated during development. The Mlc1f promoter is active in embryonic, fetal and adult fast skeletal muscle whereas the Mlc3f promoter is upregulated during fetal development and remains on in adult fast skeletal muscle. Two enhancer elements have been identified at the mammalian Mlc1f/3f locus, a 3' element active at all developmental stages and an intronic enhancer activated during fetal development. Here, using transgenesis, we demonstrate that these enhancers act combinatorially to confer the spatial, temporal and quantitative expression profile of the endogenous Mlc3f promoter. Using double reporter transgenes we demonstrate that each enhancer can activate both Mlc1f and Mlc3f promoters in vivo, revealing enhancer sharing rather than exclusive enhancer-promoter interactions. Finally, we demonstrate that the fetal activated enhancer contains critical E-box myogenic regulatory factor binding sites and that enhancer activation is impaired in vivo in the absence of myogenin but not in the absence of innervation. Together our observations provide insights into the regulation of fetal myogenesis and the mechanisms by which temporally distinct genetic programs are integrated at a single locus.

Distinctive morphological and gene/protein expression signatures during myogenesis in novel cell lines from extraocular and hindlimb muscle

Skeletal muscles are not created equal. The underutilized concept of muscle allotypes defines distinct muscle groups that differ in their intrinsic capacity to express novel traits when exposed to a facilitating extrinsic environment. Allotype-specific traits may have significance as determinants of the preferential involvement or sparing of muscle groups that is observed in a variety of neuromuscular diseases. Little is known, however, of the developmental mechanisms underlying the distinctive skeletal muscle allotypes. The lack of appropriate in vitro models, to dissociate the cell-autonomous and non-cell-autonomous mechanisms behind allotype diversity, has been a barrier to such studies. Here, we derived novel cell lines from the extraocular and hindlimb muscle allotypes and assessed their similarities and differences during early myogenesis using morphological and gene/protein expression profiling tools. Our data establish that there are fundamental differences in the transcriptional and cellular signaling pathways used by the two myoblast lineages. Taken together, these data show that myoblast lineage plays a significant role in the divergence of the distinctive muscle groups or allotypes.

Transgenic mice that express Cre recombinase under control of a skeletal muscle-specific promoter from mef2c

Genes expressed in skeletal muscle are often required in other tissues. This is particularly the case for cardiac and smooth muscle, both contractile tissues that share numerous characteristics with skeletal muscle, such that targeted inactivation can lead to embryonic lethality prior to a requirement for gene function in skeletal muscle. Thus, it is essential that conditional inactivation approaches are developed to disrupt genes specifically in skeletal muscle. In this report, we describe a transgenic mouse that expresses Cre recombinase under the control of a skeletal muscle-specific promoter from the mef2c gene. Cre expression in this transgenic line is completely restricted to skeletal muscle from early in development and is present in all skeletal muscles, including those of epaxial and hypaxial origins and in fast and slow fibers. This early skeletal muscle-specific Cre line will be a useful tool to define the function of genes specifically in skeletal muscle.

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

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

Identification of 5'-upstream region of pufferfish ribosomal protein L29 gene as a strong constitutive promoter to drive GFP expression in zebrafish

The genomic structure of Tetraodon fluviatilis L29 gene was determined and its promoter activity was analyzed in COS-1 cells and zebrafish embryos. The TfL29 gene comprises four exons and three introns, spanning approximately 1.7kb. The 5(')-upstream 2.2-kb of the first exon contains 10 E-boxes and many putative binding motifs for transcription factors GATA-1, AML-1a, c-Myb, Oct-1, CdxA, and NRF-2. Promoter activity assay showed that the distal 2.2-kb fragment not only had high luciferase activity in COS-1 cells, but also strong and ubiquitous GFP expression in a variety of tissues in zebrafish embryos. On the other hand, there are no TATA or CAAT boxes within a 300-bp region upstream from the transcription initiation site. Although this region has high luciferase activity in COS-1 cells, it is not sufficient to drive GFP expression in zebrafish embryos. In this proximal 300-bp region, there are two E-boxes, two CdxA sites, and one NRF-2 site that is immediately downstream of the transcription start site.

Gene transfer into intact fetal skeletal muscle grown in vitro

The development of an organ culture system for growing prenatal intercostal muscle in vitro and its use to study gene function is described. Fetal skeletal muscle is relatively inaccessible during the key stages of its development, and this method enables DNA transfections and other manipulations to be carried out. The system allows cell proliferation and differentiation to continue and also maintains the morphology and fiber types of developing muscle. Gene transfer into cultured embryonic intercostal muscle was achieved by square-pulse electroporation of intact pieces of tissue. Expression of a marker gene (GFP) was found within 5 h and maintained for 2 days in muscle fibers and cells. The technique should enable the function of genes implicated in muscle development and disease to be studied at stages when access is difficult and in a controlled environment.

Up-regulation of muscle-specific transcription factors during embryonic somitogenesis of zebrafish (Danio rerio) by knock-down of myostatin-1

Myostatin, a secreted growth and differentiation factor (GDF-8) belongs to transforming growth factor (TGF-beta) superfamily that plays as a negative regulator of skeletal muscle development and growth. Recently, myostatin has been isolated from fish; however, its role in muscle development and growth remains unknown. Here, we present the expression of myostatin during development and the effects of its knock-down on various genes such as muscle regulatory transcription factors (MRFs), muscle-specific proteins (MSP), and insulin-like growth factors (IGFs). The myostatin expression was found to be maternal as it starts in one-cell stage onward. The reverse transcription-polymerase chain reaction (RT-PCR), in situ hybridization, and Southern and Northern blots demonstrated that the myostatin expression is not only restricted to skeletal muscle, but it expressed all the tested tissues. Expression of myostatin was effected by using antisense morpholinos resulted in significant phenotypic difference in stages 18 and 20 hours postfertilization (hpf). To confirm the specificity of myostatin morpholino, furthermore, a rescue experiment was conducted. The length as well as width of somites was increased with almost no gap in between the somites. In addition, it deserves to mention that this is a first animal model that shows changes in the size of the somites. Moreover, analyses of MRFs, MSP, and IGFs in the knock-down embryos by RT-PCR revealed the up-regulation of MyoD, Myogenin, and Mck transcription, whereas IGF-2 transcription showed mild response with no effect on IGF-1, Desmin, and Myf5. In situ hybridization showed that there was an increase in the number of somites from 3 to 4 at 13 and 22 hpf. Taken together, these data suggest that myostatin plays a major role during myogenesis, apart from inhibition of proliferation as well as differentiation.

Molecular regulation of individual skeletal muscle fibre types

The purpose of this review is to present current understanding of cellular and molecular regulation of fibre type expression in skeletal muscle. Published literature seems to conclusively suggest that muscle fibre type expression is regulated by multiple signalling pathways and transcription factors rather than a single 'master' switch or signalling pathway. While the current nomenclature for fibre types is convenient for communication, based upon the evolution of this nomenclature, the prediction that fibre type classifications may change in the future to incorporate post-genomic information is made. It is predicted that future fibre type classifications could be based upon the contractile-activity-induced changes in a common regulatory factor(s) within a subpopulation of genes whose expressions are altered to modify and maintain the new muscle fibre phenotype.

Expression of mutant Ets protein at the neuromuscular synapse causes alterations in morphology and gene expression

The localized transcription of several muscle genes at the motor endplate is controlled by the Ets transcription factor GABP. To evaluate directly its contribution to the formation of the neuromuscular junction, we generated transgenic mice expressing a general Ets dominant-negative mutant specifically in skeletal muscle. Quantitative RT-PCR analysis demonstrated that the expression of genes containing an Ets-binding site was severely affected in the mutant mice. Conversely, the expression of other synaptic genes, including MuSK and Rapsyn, was unchanged. In these animals, muscles expressing the mutant transcription factor developed normally, but examination of the post-synaptic morphology revealed marked alterations of both the primary gutters and secondary folds of the neuromuscular junction. Our results demonstrate that Ets transcription factors are crucial for the normal formation of the neuromuscular junction. They further show that Ets-independent mechanisms control the synaptic expression of a distinct set of synaptic genes.

The myostatin gene is a downstream target gene of basic helix-loop-helix transcription factor MyoD

Myostatin is a negative regulator of myogenesis, and inactivation of myostatin leads to heavy muscle growth. Here we have cloned and characterized the bovine myostatin gene promoter. Alignment of the upstream sequences shows that the myostatin promoter is highly conserved during evolution. Sequence analysis of 1.6 kb of the bovine myostatin gene upstream region revealed that it contains 10 E-box motifs (E1 to E10), arranged in three clusters, and a single MEF2 site. Deletion and mutation analysis of the myostatin gene promoter showed that out of three important E boxes (E3, E4, and E6) of the proximal cluster, E6 plays a significant role in the regulation of a reporter gene in C(2)C(12) cells. We also demonstrate by band shift and chromatin immunoprecipitation assay that the E6 E-box motif binds to MyoD in vitro and in vivo. Furthermore, cotransfection experiments indicate that among the myogenic regulatory factors, MyoD preferentially up-regulates myostatin promoter activity. Since MyoD expression varies during the myoblast cell cycle, we analyzed the myostatin promoter activity in synchronized myoblasts and quiescent "reserve" cells. Our results suggest that myostatin promoter activity is relatively higher during the G(1) phase of the cell cycle, when MyoD expression levels are maximal. However, in the reserve cells, which lack MyoD expression, a significant reduction in the myostatin promoter activity is observed. Taken together, these results suggest that the myostatin gene is a downstream target gene of MyoD. Since the myostatin gene is implicated in controlling G(1)-to-S progression of myoblasts, MyoD could be triggering myoblast withdrawal from the cell cycle by regulating myostatin gene expression.

Fiber-type specific and position-dependent expression of a transgene in limb muscles

We have previously shown that the proximal sequences of the human aldolase A fast-muscle-specific promoter (pM) are sufficient to target the expression of a linked CAT reporter gene to all fast, glycolytic trunk and limb muscles of transgenic mice (pM310CAT lines) in a manner mimicking the activity of the endogenous mouse promoter. When a NF1-binding site (motif M2) in this proximal regulatory region is mutated, the activity of the corresponding mM2 transgene is strongly affected but only in a some fast muscles. Here we show that the mutation of the M2 motif has only mild effects on pM activity in axial and proximal limb, while it drastically reduces this activity in both fore and hind limb distal muscles. At the cellular level, we show that both the pM310CAT and mM2 transgenes are highly expressed in fast glycolytic 2B fibers. However, by contrast to the pM310CAT transgene, whose expression is mainly restricted to fast glycolytic 2B fibers, the mM2 transgene is also active in a high proportion of 2X fibers. This result suggests that the M2 sequence could play a role in restricting the expression of pM to the 2B fibers. The variable expression of the mM2 transgene along the limb axis already exists at post-natal day 10 and seems to result from a change in the proportion of expressing fast fibers per muscle. Altogether, these results suggest that, although considered as phenotypically similar, different populations of fast glycolytic fibers exist, in which the requirement of the NF1 activity for pM expression varies according to the proximal versus distal position of the muscle along the limb axis.

Different regulatory sequences are required for parvalbumin gene expression in skeletal muscles and neuronal cells of transgenic mice

Parvalbumin (PV) is expressed in fast-twitch fibers in skeletal muscles and a subpopulation of inhibitory neurons in the CNS. We generated transgenic mice that expressed the human interleukin-2 receptor alpha-subunit-green fluorescent fusion protein (hIL-2R-GFP) using two types of PV transgene. One contained the hIL-2R-GFP gene downstream of a 16.5-kb 5'-upstream PV genomic sequence (PV line). The other comprised the hIL-2R-GFP gene in bacterial artificial chromosome (BAC) with either a 180-kb (PA line) or 155-kb (PB line) insert encompassing the PV gene. Independent lines of all transgenic mice showed a faithful hIL-2R-GFP expression in fast-twitch muscle fibers. However, appreciable hIL-2R-GFP expression in the CNS occurred only in the PA transgenic lines. In one line of PA transgenic mice, hIL-2R-GFP was properly expressed in PV-containing neurons in the cerebellum, thalamic reticular nucleus, globus pallidus and cerebral cortex, though ectopic expression was observed in a particular subset of cerebellar astrocytes. Another line of PA transgenic mice showed a selective and mosaic expression of hIL-2R-GFP in PV-containing Purkinje, basket and stellate cells in the cerebellum. These results indicate that the 16.5-kb PV genomic sequence is sufficient for fiber-type-selective transcription but additional regulatory sequences comprised in BAC DNA are required for proper expression in PV-containing neurons.

Molecular regulation of tongue and craniofacial muscle differentiation

The molecular regulation of muscle development is tightly controlled at three distinct stages of the process: determination, differentiation, and maturation. Developmentally, specific populations of myoblasts exhibit distinct molecular phenotypes that begin to limit the ultimate characteristics of the muscle fibers. The expression of the myogenic regulatory factor family of the transcription process plays a key role in muscle development and, ultimately, in the subset of contractile genes expressed in a specific muscle. Craniofacial muscles have distinct functional requirements and associated molecular phenotypes that distinguish them from other skeletal muscles. The general principles of muscle molecular differentiation with specific reference to craniofacial muscles, such as the tongue, are discussed in this review.

Myofibroblasts: molecular crossdressers

Myofibroblasts are unique mesenchymal cells with properties inherent to both muscle and nonmuscle cells. They are widely distributed in embryos, are essential for the formation of functional adult tissues, and are intimately involved in tissue homeostasis and wound healing. Cytoskeletal protein expression and contractile properties distinguish them from other cell types. Myofibroblasts also express skeletal muscle structural and regulatory proteins, including sarcomeric myosin heavy chain and MyoD. Despite the presence of such myogenic regulatory proteins, these cells do not terminally differentiate into skeletal muscle. This article focuses on the interesting biology of myofibroblasts, their origin, and the molecular mechanisms that allow these cells to maintain a state intermediate between muscle and nonmuscle cells.

Postnatal regulation of limb proprioception by muscle-derived neurotrophin-3

To investigate the effects of neurotrophin-3 (NT-3) on postnatal proprioceptive neurons and their targets, transgenic mice were generated that use the myosin light chain 1 (mlc) promoter to overexpress NT-3 in skeletal muscle. Ribonuclease protection assays revealed that NT-3 overexpression in hindlimb skeletal muscle began at embryonic day 14 (E14) and continued throughout adulthood. Overexpression of NT-3 during late embryogenesis resulted in increased numbers of large sensory and small fusimotor axons. Within a week of birth, mlc/NT-3 mice retract their limbs to the torso when lifted by the tail. Footprint analysis revealed that mlc/NT-3 mice had significant abnormalities in their gait compared with wild-types. Beam walking and rotorod analysis confirmed the poor limb control by mlc/NT-3 mice. These locomotive deficits progressively worsened with age and were likely related to the formation of morphologically abnormal muscle spindles. The most common spindle anomaly was the presence of excessive intrafusal bag fibers within individual muscle spindles. To assess the role of NT-3 in recovery from nerve injury, sciatic nerve crushes were performed in young adult mice. Two days after injury, mlc/NT-3 mice displayed significantly improved sciatic functional indexes and a significant increase in muscle spindles that remained associated with axons. The latter finding suggests that excess NT-3 in muscle may retard the degeneration of proprioceptive axons after nerve crush. Long-term survival after nerve injury in mlc/NT-3 mice did not induce further changes in spindle number or morphology. These findings demonstrate that, in addition to promoting embryonic proprioceptive neuron survival, postnatal overexpression of NT-3 in muscle leads to abnormal spindle formation and deficits in locomotive control. However, our results also show that NT-3 may be therapeutic for proprioceptive axons immediately after nerve injury by delaying axon degeneration.

A role for Engrailed-2 in determination of skeletal muscle physiologic properties

The molecular basis underlying the establishment of the myogenic lineage, subsequent differentiation, and the establishment of specific fiber types (i.e., fast versus slow) is becoming well understood. In contrast, the regulation of the general properties of a specific anatomical muscle group (e.g., leg versus jaw muscles) and the regulation of muscle-fiber properties within a particular group are less well characterized. We have investigated the potential role of the homeobox-containing gene, Engrailed-2 (En-2), in the mouse, which is specifically expressed in myoblasts in the first arch and maintained in the muscles of mastication in the adult. We have generated mice that ectopically express En-2 in all muscles during early development and primarily in fast muscles in the adult. Ectopic En-2 in nonjaw muscles leads to a decrease in fiber size, whereas overexpression in the jaw muscles leads to a shift in fiber metabolic properties as well as a decrease in fiber size. In contrast, loss of En-2 in the jaw leads to a shift in fiber metabolic properties in the jaw of female mice only. Jaw muscles are sexually dimorphic, and we propose that the function of En-2 and mechanisms guiding sexual dimorphism of the jaw muscles are integrated. We conclude that the specific expression of En-2 in the jaw therefore plays a role in specifying muscle-fiber characteristics that contribute to the physiologic properties of specific muscle groups.

Cloning of human acetyl-CoA carboxylase beta promoter and its regulation by muscle regulatory factors

The 280-kDa beta-isoform of acetyl-CoA carboxylase (ACCbeta) is predominantly expressed in heart and skeletal muscle, whereas the 265-kDa alpha-isoform (ACCalpha) is the major ACC in lipogenic tissues. The ACCbeta promoter showed myoblast-specific promoter activity and was strongly induced by MyoD in NIH3T3 cells. Serial deletions of the promoter revealed that MyoD acts on the E-boxes located at positions -498 to -403 and on the proximal region including the 5'-untranslated region. Destruction of the E-boxes at positions -498 to -403 by site-directed mutagenesis resulted in a significant decrease of MyoD responsiveness. The "TGAAA" at -32 to -28 and the region around the transcription start site play important roles in basal transcription, probably as a TATA box and an Inr element, respectively. Mutations of another E-box at -14 to -9 and a "GCCTGTCA" sequence at +17 to +24 drastically decreased the MyoD responsiveness. The novel cis-element GCCTGTCA was preferentially bound by MyoD homodimer in EMSA and conferred MyoD responsiveness to a luciferase reporter, which was repressed by the overexpression of E12. This finding is unique since activation via E-boxes is mediated by heterodimers of MyoD and E-proteins. We screened a human skeletal muscle cDNA library to isolate clones expressing proteins that bind to the region around the GCCTGTCA (+8 to +27) sequence, and isolated Myf4 and Myf6 cDNAs. Electrophoretic mobility shift assay showed that recombinant Myf4 and Myf6 bind to this novel cis-element. Moreover, transient expression of Myf6 induced significant activation on the ACCbeta promoter or an artificial promoter harboring this novel cis-element. These findings suggest that muscle regulatory factors, such as MyoD, Myf4, and Myf6, contribute to the muscle-specific expression of ACCbeta via E-boxes and the novel cis-element GCCTGTCA.

Modular regulation of the MLC1F/3F gene and striated muscle diversity

Isoform diversity in striated muscle is largely controlled at the level of transcription. In this review we will concentrate on studies concerning transcriptional regulation of the alkali myosin light chain 1F/3F gene. Uncoupled activity of the MLC1F and 3F promoters, together with complex patterns of transcription in developing skeletal and cardiac muscle, combine to make analysis of this gene particularly intriguing. In vitro and transgenic studies of MLC1F/3F regulatory elements have revealed an array of cis-acting modules that each drive a subset of the expression pattern of the two promoters. These cis-acting regulatory modules, including the MLC1F and 3F promoter regions and two skeletal muscle enhancers, control tissue-specificity, cell or fibre-type specificity, and the spatiotemporal regulation of gene expression, including positional information. How each of these regulatory modules acts and how their individual activites are integrated to coordinate transcription at this locus are discussed.

Roles for ephrins in positionally selective synaptogenesis between motor neurons and muscle fibers

Motor axons form topographic maps on muscles: rostral motor pools innervate rostral muscles, and rostral portions of motor pools innervate rostral fibers within their targets. Here, we implicate A subfamily ephrins in this topographic mapping. First, developing muscles express all five of the ephrin-A genes. Second, rostrally and caudally derived motor axons differ in sensitivity to outgrowth inhibition by ephrin-A5. Third, the topographic map of motor axons on the gluteus muscle is degraded in transgenic mice that overexpress ephrin-A5 in muscles. Fourth, topographic mapping is impaired in muscles of mutant mice lacking ephrin-A2 plus ephrin-A5. Thus, ephrins mediate or modulate positionally selective synapse formation. In addition, the rostrocaudal position of at least one motor pool is altered in ephrin-A5 mutant mice, indicating that ephrins affect nerve-muscle matching by intraspinal as well as intramuscular mechanisms.

Regulation of a muscle-specific transgene by persistent expression of Hox genes in postnatal murine limb muscle

Homeobox genes are necessary for the generation of the embryonic body plan in both invertebrate and vertebrate organisms. To investigate the potential function of homeodomain proteins in normal and regenerating skeletal muscle, we analyzed patterns of clustered homeobox gene expression in neonatal and adult muscle tissue. Transcripts encoding 5' genes in the HoxA cluster were detected in muscles from both the fore- and hindlimbs of neonatal and adult mice, whereas expression of HoxC gene transcripts was generally restricted to the muscles of the hindlimb. In contrast, transcripts encoding genes of the HoxB or HoxD clusters were not detected in muscles from either fore- or hindlimbs. Although ectopic expression of select HOX proteins in muscle cell cultures had modest effects upon the activity of a co-transfected myosin light chain (MLC) enhancer, mutation of a Hox binding site in this enhancer elicited increased linked reporter gene expression. Induction of muscle damage and regeneration was accompanied by the down-regulation of at least one Hox gene, concurrent with the activation of the regenerative program. Moreover, targeted ablation of the Hoxc-8 gene, normally expressed in mature fore- and hindlimb muscles, resulted in reduced expression of an MLC enhancer-driven transgene only in specific leg muscles. These results indicate that members of the HoxA and C clusters may, in combination, mediate various aspects of differentiation and patterning in adult musculature.

An E box comprises a positional sensor for regional differences in skeletal muscle gene expression and methylation

To dissect the molecular mechanisms conferring positional information in skeletal muscles, we characterized the control elements responsible for the positionally restricted expression patterns of a muscle-specific transgene reporter, driven by regulatory sequences from the MLC1/3 locus. These sequences have previously been shown to generate graded transgene expression in the segmented axial muscles and their myotomal precursors, fortuitously marking their positional address. An evolutionarily conserved E box in the MLC enhancer core, not recognized by MyoD, is a target for a nuclear protein complex, present in a variety of tissues, which includes Hox proteins and Zbu1, a DNA-binding member of the SW12/SNF2 gene family. Mutation of this E box in the MLC enhancer has only a modest positive effect on linked CAT gene expression in transfected muscle cells, but when introduced into transgenic mice the same mutation elevates CAT transgene expression in skeletal muscles, specifically releasing the rostral restriction on MLC-CAT transgene expression in the segmented axial musculature. Increased transgene activity resulting from the E box mutation in the MLC enhancer correlates with reduced DNA methylation of the distal transgenic MLC1 promoter as well as in the enhancer itself. These results identify an E box and the proteins that bind to it as a positional sensor responsible for regional differences in axial skeletal muscle gene expression and accessibility.

Developmental regulation of the aldolase A muscle-specific promoter during in vivo muscle maturation is controlled by a nuclear receptor binding element

During the post-natal period, skeletal muscles undergo important modifications leading to the appearance of different types of myofibers which exhibit distinct contractile and metabolic properties. This maturation process results from the activation of the expression of different sets of contractile proteins and metabolic enzymes, which are specific to the different types of myofibers. The muscle-specific promoter of the aldolase A gene (pM) is expressed mainly in fast-twitch glycolytic fibers in adult body muscles. We investigate here how pM is regulated during the post-natal development of different types of skeletal muscles (slow or fast-twitch muscles, head or body muscles). We show that pM is expressed preferentially in prospective fast-twitch muscles soon after birth; pM is up-regulated specifically in body muscles only later in development. This activation pattern is mimicked by a transgene which comprises only the 355 most proximal sequences of pM. Within this region, we identify a DNA element which is required for the up-regulation of the transgene during post-natal development in body muscles. Comparison of nuclear M1-binding proteins from young or adult body muscles show no qualitative differences. Distinct M1-binding proteins are present in both young and adult tongue nuclear extracts, compared to that present in gastrocnemius extracts.

What does chronic electrical stimulation teach us about muscle plasticity?

The model of chronic low-frequency stimulation for the study of muscle plasticity was developed over 30 years ago. This protocol leads to a transformation of fast, fatigable muscles toward slower, fatigue-resistant ones. It involves qualitative and quantitative changes of all elements of the muscle fiber studied so far. The multitude of stimulation-induced changes makes it possible to establish the full adaptive potential of skeletal muscle. Both functional and structural alterations are caused by orchestrated exchanges of fast protein isoforms with their slow counterparts, as well as by altered levels of expression. This remodeling of the muscle fiber encompasses the major, myofibrillar proteins, membrane-bound and soluble proteins involved in Ca2+ dynamics, and mitochondrial and cytosolic enzymes of energy metabolism. Most transitions occur in a coordinated, time-dependent manner and result from altered gene expression, including transcriptional and posttranscriptional processes. This review summarizes the advantages of chronic low-frequency stimulation for studying activity-induced changes in phenotype, and its potential for investigating regulatory mechanisms of gene expression. The potential clinical relevance or utility of the technique is also considered.

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

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

Seeking muscle stem cells

Skeletal muscle development requires the formation of myoblasts that can fuse with each other to form multinucleate myofibers. Distinct primary and secondary, slow and fast, populations of myofibers form by the time of birth. At embryonic, fetal, and perinatal stages of development, temporally distinct lineages of myogenic cells arise and contribute to the formation of these multiple types of myofibers. In addition, spatially distinct lineages of myogenic cells arise and form the anterior head muscles, limb (hypaxial) muscles, and dorsal (epaxial) muscles. There is strong evidence that myoblasts are produced from muscle stem cells, which are self-renewing cells that do not themselves terminally differentiate but produce progeny that are capable of becoming myoblasts and myofibers. Muscle stem cells, which may be multipotent, appear to be distinguishable from myoblasts by a number of indirect and direct criteria. Muscle stem cells arise either in unsegmented paraxial mesoderm (anterior head muscle progenitors) or in segmented mesoderm of the somites (epaxial and hypaxial muscle progenitors). These initial stages of myogenesis are regulated by positive and negative signals, including Wnt, BMP, and Shh family members, from nearby notochord, neural tube, ectoderm, and lateral mesoderm tissues. The formation of skeletal muscles, therefore, depends on the generation of spatially and temporally distinct lineages of myogenic cells. Myogenic cell lineages begin with muscle stem cells which produce the myoblasts that fuse to form myofibers.

Fiber-type-specific transcription of the troponin I slow gene is regulated by multiple elements

The regulatory elements that restrict transcription of genes encoding contractile proteins specifically to either slow- or fast-twitch skeletal muscles are unknown. As an initial step towards understanding the mechanisms that generate muscle diversity during development, we have identified a 128-bp troponin I slow upstream element (SURE) and a 144-bp troponin I fast intronic element (FIRE) that confer fiber type specificity in transgenic mice (M. Nakayama et al., Mol. Cell. Biol. 16:2408-2417, 1996). SURE and FIRE have maintained the spatial organization of four conserved motifs (3' to 5'): an E box, an AT-rich site (A/T2) that binds MEF-2, a CACC site, and a novel CAGG motif. Troponin I slow (TnIs) constructs harboring mutations in these motifs were analyzed in transiently and stably transfected Sol8 myocytes and in transgenic mice to assess their function. Mutations of the E-box, A/T2, and CAGG motifs completely abolish transcription from the TnI SURE. In contrast, mutation of the CACC motif had no significant effect in transfected myocytes or on the slow-specific transcription of the TnI SURE in transgenic mice. To assess the role of E boxes in fiber type specificity, a chimeric enhancer was constructed in which the E box of SURE was replaced with the E box from FIRE. This TnI E box chimera, which lacks the SURE NFAT site, confers essentially the same levels of transcription in transgenic mice as those conferred by wild-type SURE and is specifically expressed in slow-twitch muscles, indicating that the E box on its own cannot determine the fiber-type-specific expression of the TnI promoter. The importance of the 5' half of SURE, which bears little homology to the TnI FIRE, in muscle-specific expression was analyzed by deletion and linker scanning analyses. Removal of the 5' half of SURE (-846 to -811) results in the loss of expression in stably transfected but not in transiently expressing myocytes. Linker scanning mutations identified sequences in this region that are necessary for the function of SURE when integrated into chromatin. One of these sites (GTTAATCCG), which is highly homologous to a bicoid consensus site, binds to nuclear proteins from several mesodermal cells. These results show that multiple elements are involved in the muscle-specific activity of the TnIs promoter and that interactions between upstream and downstream regions of SURE are important for transcription in the context of native chromatin.

Identification of a novel slow-muscle-fiber enhancer binding protein, MusTRD1

The molecular mechanisms which are responsible for restricting skeletal muscle gene expression to specific fiber types, either slow or fast twitch, are unknown. As a first step toward defining the components which direct slow-fiber-specific gene expression, we identified the sequence elements of the human troponin I slow upstream enhancer (USE) that bind muscle nuclear proteins. These include an E-box, a MEF2 element, and two other elements, USE B1 and USE C1. In vivo analysis of a mutation that disrupts USE B1 binding activity suggested that the USE B1 element is essential for high-level expression in slow-twitch muscles. This mutation does not, however, abolish slow-fiber specificity. A similar analysis indicated that the USE C1 element may play only a minor role. We report the cloning of a novel human USE B1 binding protein, MusTRD1 (muscle TFII-I repeat domain-containing protein 1), which is expressed predominantly in skeletal muscle. Significantly, MusTRD1 contains two repeat domains which show remarkable homology to the six repeat domains of the recently cloned transcription factor TFII-I. Furthermore, both TFII-I and MusTRD1 bind to similar but distinct sequences, which happen to conform with the initiator (Inr) consensus sequence. Given the roles of MEF2 and basic helix-loop-helix (bHLH) proteins in muscle gene expression, the similarity of TFII-I and MusTRD1 is intriguing, as TFII-I is believed to coordinate the interaction of MADS-box proteins, bHLH proteins, and the general transcription machinery.

In vivo analysis of the myosin heavy chain IIB promoter region

The myosin heavy chain (MHC) IIB gene is preferentially expressed in fast-twitch muscles of the hindlimb, such as the tibialis anterior (TA). The molecular mechanism(s) for this preferential expression are unknown. The goals of the current study were 1) to determine whether the cloned region of the MHC IIB promoter contains the necessary cis-acting element(s) to drive fiber-type-specific expression of this gene in vivo, 2) to determine which region within the promoter is responsible for fiber-type-specific expression, and 3) to determine whether transcription off of the cloned region of the MHC IIB promoter accurately mimics endogenous gene expression in a muscle undergoing a fiber-type transition. To accomplish these goals, a 2.6-kilobase fragment of the promoter-enhancer region of the MHC IIB gene was cloned upstream of the firefly luciferase reporter gene and coinjected with pRL-cytomegalovirus (CMV) (CMV promoter driving the renilla luciferase reporter) into the TA and the slow soleus muscle. Firefly luciferase activity relative to renilla luciferase activity within the TA was 35-fold greater than within the soleus. Deletional analysis demonstrated that only the proximal 295 base pairs (pGL3IIB0.3) were required to maintain this muscle-fiber-type specificity. Reporter gene expression of pGL3IIB0.3 construct was significantly upregulated twofold in unweighted soleus muscles compared with normal soleus muscles. Thus the region within the proximal 295 base pairs of the MHC IIB gene contains at least one element that can drive fiber-type-specific expression of a reporter gene.

A binding site for nuclear receptors is required for the differential expression of the aldolase A fast-twitch muscle promoter in body and head muscles

In hind limb muscles, the aldolase A muscle-specific promoter is specifically expressed in glycolytic fast-twitch fibers. Here, we show that in addition, it is expressed at higher levels in trunk and limb muscles than in neck and head muscles independent of their fiber-type content. We have identified by analysis of transgenic mice a DNA element that is required for this differential expression and, to a lesser extent, for fiber-type specificity. We show that members of the nuclear receptor superfamily bind this element in skeletal muscle nuclear extracts. Interestingly, in gel mobility shift assays, different complexes were formed with this sequence in tongue nuclear extracts compared with limb or trunk muscle nuclear extracts. Therefore, binding of distinct nuclear receptors to a single regulatory sequence appears to be associated with the location-dependent expression of the aldolase A muscle-specific promoter.

Embryonic and fetal myogenic programs act through separate enhancers at the MLC1F/3F locus

Embryonic and fetal stages of skeletal muscle development are characterized by the differential expression of a number of muscle-specific genes. These include the products of independent promoters at the fast myosin light chain 1F/3F locus. In the mouse embryo MLC1F transcripts accumulate in embryonic skeletal muscle from E9, 4-5 days before high-level accumulation of MLC3F transcripts. A 3' enhancer can activate MLC1F and MLC3F promoters in differentiated muscle cells in vitro and in transgenic mice; both promoters, however, are activated at the time of MLC1F transcript accumulation. We now demonstrate the presence of a second muscle-specific enhancer at this locus, located in the intron separating the MLC1F and MLC3F promoters. Transgenic mice containing the intronic, but lacking the 3' enhancer, express high levels of an nlacZ reporter gene from the MLC3F promoter in adult fast skeletal muscle fibers. In contrast to the 3' enhancer, the intronic element is inactive both in embryonic muscle cells in vivo and in embryonic myocyte cultures. The intronic enhancer is activated at the onset of fetal development in both primary and secondary muscle fibers, at the time of endogenous MLC3F transcript accumulation. Late-activated MLC3F transgenes thus provide a novel in toto marker of fetal myogenesis. These results suggest that temporal regulation of transcription at the MLC1F/3F locus is controlled by separate enhancers which are differentially activated during embryonic and fetal development.

The hardwiring of development: organization and function of genomic regulatory systems

The gene regulatory apparatus that directs development is encoded in the DNA, in the form of organized arrays of transcription factor target sites. Genes are regulated by interactions with multiple transcription factors and the target sites for the transcription factors required for the control of each gene constitute its cis-regulatory system. These systems are remarkably complex. Their hardwired internal organization enables them to behave as genomic information processing systems. Developmental gene regulatory networks consist of the cis-regulatory systems of all the relevant genes and the regulatory linkages amongst them. Though there is yet little explicit information, some general properties of genomic regulatory networks have become apparent. The key to understanding how genomic regulatory networks are organized, and how they work, lies in experimental analysis of cis-regulatory systems at all levels of the regulatory network.

A combination of MEF3 and NFI proteins activates transcription in a subset of fast-twitch muscles

The human aldolase A pM promoter is active in fast-twitch muscles. To understand the role of the different transcription factors which bind to this promoter and determine which ones are responsible for its restricted pattern of expression, we analyzed several transgenic lines harboring different combinations of pM regulatory elements. We show that muscle-specific expression can be achieved without any binding sites for the myogenic factors MyoD and MEF2 and that a 64-bp fragment comprising a MEF3 motif and an NFI binding site is sufficient to drive reporter gene expression in some but, interestingly, not all fast-twitch muscles. A result related to this pattern of expression is that some isoforms of NFI proteins accumulate differentially in fast- and slow-twitch muscles and in distinct fast-twitch muscles. We propose that these isoforms of NFI proteins might provide a molecular basis for skeletal muscle diversity.

MyoD protein is differentially accumulated in fast and slow skeletal muscle fibres and required for normal fibre type balance in rodents

MyoD is a muscle-specific transcription factor involved in commitment of cells to myogenesis. MyoD mRNA levels differ between fast and slow muscles, suggesting that MyoD may regulate aspects of fibre type. Here we show that detectable MyoD protein becomes restricted during development to the nuclei of the fastest classes of fibres in fast muscles. myoDm1 mice, in which the myoD gene has been disrupted, show subtle shifts in fibre type of fast muscles toward a slower character, suggesting that MyoD is involved in the maintenance of the fast IIB/IIX fibre type. In contrast, slow muscle shifts to a faster phenotype in myoDm1. Moreover, MD6.0-lacZ transgenic mice with the myoD promoter driving lacZ, show highest beta-galactosidase activity in the fastest fibres of fast muscles, but also express low levels in slow fibres of slow, but not fast, muscles, suggesting distinct regulation of gene expression in slow fibres of fast and slow muscles.