Muscle: the regulation of myogenesis

Skeletal muscle hypertrophy and regeneration: interplay between the myogenic regulatory factors (MRFs) and insulin-like growth factors (IGFs) pathways

Adult skeletal muscle can regenerate in response to muscle damage. This ability is conferred by the presence of myogenic stem cells called satellite cells. In response to stimuli such as injury or exercise, these cells become activated and express myogenic regulatory factors (MRFs), i.e., transcription factors of the myogenic lineage including Myf5, MyoD, myogenin, and Mrf4 to proliferate and differentiate into myofibers. The MRF family of proteins controls the transcription of important muscle-specific proteins such as myosin heavy chain and muscle creatine kinase. Different growth factors are secreted during muscle repair among which insulin-like growth factors (IGFs) are the only ones that promote both muscle cell proliferation and differentiation and that play a key role in muscle regeneration and hypertrophy. Different isoforms of IGFs are expressed during muscle repair: IGF-IEa, IGF-IEb, or IGF-IEc (also known as mechano growth factor, MGF) and IGF-II. MGF is expressed first and is observed in satellite cells and in proliferating myoblasts whereas IGF-Ia and IGF-II expression occurs at the state of muscle fiber formation. Interestingly, several studies report the induction of MRFs in response to IGFs stimulation. Inversely, IGFs expression may also be regulated by MRFs. Various mechanisms are proposed to support these interactions. In this review, we describe the general process of muscle hypertrophy and regeneration and decipher the interactions between the two groups of factors involved in the process.

Distinct roles for Ste20-like kinase SLK in muscle function and regeneration

BACKGROUND

Cell growth and terminal differentiation are controlled by complex signaling systems that regulate the tissue-specific expression of genes controlling cell fate and morphogenesis. We have previously reported that the Ste20-like kinase SLK is expressed in muscle tissue and is required for cell motility. However, the specific function of SLK in muscle tissue is still poorly understood.

METHODS

To gain further insights into the role of SLK in differentiated muscles, we expressed a kinase-inactive SLK from the human skeletal muscle actin promoter. Transgenic muscles were surveyed for potential defects. Standard histological procedures and cardiotoxin-induced regeneration assays we used to investigate the role of SLK in myogenesis and muscle repair.

RESULTS

High levels of kinase-inactive SLK in muscle tissue produced an overall decrease in SLK activity in muscle tissue, resulting in altered muscle organization, reduced litter sizes, and reduced breeding capacity. The transgenic mice did not show any differences in fiber-type distribution but displayed enhanced regeneration capacity in vivo and more robust differentiation in vitro.

CONCLUSIONS

Our results show that SLK activity is required for optimal muscle development in the embryo and muscle physiology in the adult. However, reduced kinase activity during muscle repair enhances regeneration and differentiation. Together, these results suggest complex and distinct roles for SLK in muscle development and function.

Identification of atrogin-1-targeted proteins during the myostatin-induced skeletal muscle wasting

Atrogin-1, a muscle-specific E3 ligase, targets MyoD for degradation through the ubiquitin-proteasome-mediated system. Myostatin, a member of the transforming growth factor-β superfamily, potently inhibits myogenesis by lowering MyoD levels. While atrogin-1 is upregulated by myostatin, it is currently unknown whether atrogin-1 plays a role in mediating myostatin signaling to regulate myogenesis. In this report, we have confirmed that atrogin-1 increasingly interacts with MyoD upon recombinant human myostatin (hMstn) treatment. The absence of atrogin-1, however, led to elevated MyoD levels and permitted the differentiation of atrogin-1(-/-) primary myoblast cultures despite the presence of exogenous myostatin. Furthermore, inactivation of atrogin-1 rescued myoblasts from growth inhibition by hMstn. Therefore, these results highlight the central role of atrogin-1 in regulating myostatin signaling during myogenesis. Currently, there are only two known targets of atrogin-1. Thus, we next characterized the associated proteins of atrogin-1 in control and hMstn-treated C2C12 cell cultures by stably expressing tagged atrogin-1 in myoblasts and myotubes, and sequencing the coimmunoprecipitated proteome. We found that atrogin-1 putatively interacts with sarcomeric proteins, transcriptional factors, metabolic enzymes, components of translation, and spliceosome formation. In addition, we also identified that desmin and vimentin, two components of the intermediate filament in muscle, directly interacted with and were degraded by atrogin-1 in response to hMstn. In summary, the muscle wasting effects of the myostatin-atrogin-1 axis are not only limited to the degradation of MyoD and eukaryotic translation initiation factor 3 subunit f, but also encompass several proteins that are involved in a wide variety of cellular activities in the muscle.

The over-expression of cell migratory genes in alveolar rhabdomyosarcoma could contribute to metastatic spread

Alveolar (ARMS) and Embryonal (ERMS) rhabdomyosarcoma differ in their response to current treatments. The ARMS subtype has a less favourable prognosis and often presents with widespread metastases, while the less metastatic ERMS has a 5 year survival rate of more than 80 %. In this study we investigate gene expression differences that could contribute to the high frequency of metastasis in ARMS. Microarray analysis identified significant differences in DNA repair, cell cycle and cell migration between the two RMS subtypes. Two genes up regulated in ARMS and involved in cell migration; the engulfment and cell motility gene 1 (ELMO1) and NEL-like 1 gene (NELL1) were selected for further investigation. Over-expression of ELMO1 significantly increased cell invasion from 24.70 ± 7% to 93 ± 5.4% in primary myoblasts and from 29.43 ± 2.1% to 87.33 ± 4.1% in the ERMS cell line RD. siRNA knockout of ELMO1 in the ARMS cell line RH30 significantly reduced cell invasion from 88.2 ± 3.8% to 35.2 ± 2.5%. Over-expression of NELL1 significantly increased myoblast invasion from 23.6 ± 6.9% to 100 ± 0.1%, but had no effect on invasion of the ERMS cell line RD. These findings suggest that ELMO1 may play a key role in ARMS metastasis. NELL1 increased invasion in primary myoblasts, but other factors required for it to enhance motility were not present in the RD ERMS cell line. Impairing ELMO1 function by pharmacological or siRNA knockdown could be a highly effective approach to reduce the metastatic spread of RMS.

Immunostaining of dissected zebrafish embryonic heart

Zebrafish embryo becomes a popular in vivo vertebrate model for studying cardiac development and human heart diseases due to its advantageous embryology and genetics. About 100-200 embryos are readily available every week from a single pair of adult fish. The transparent embryos that develop ex utero make them ideal for assessing cardiac defects. The expression of any gene can be manipulated via morpholino technology or RNA injection. Moreover, forward genetic screens have already generated a list of mutants that affect different perspectives of cardiogenesis. Whole mount immunostaining is an important technique in this animal model to reveal the expression pattern of the targeted protein to a particular tissue. However, high resolution images that can reveal cellular or subcellular structures have been difficult, mainly due to the physical location of the heart and the poor penetration of the antibodies. Here, we present a method to address these bottlenecks by dissecting heart first and then conducting the staining process on the surface of a microscope slide. To prevent the loss of small heart samples and to facilitate solution handling, we restricted the heart samples within a circle on the surface of the microscope slides drawn by an immEdge pen. After the staining, the fluorescence signals can be directly observed by a compound microscope. Our new method significantly improves the penetration for antibodies, since a heart from an embryonic fish only consists of few cell layers. High quality images from intact hearts can be obtained within a much reduced procession time for zebrafish embryos aged from day 2 to day 6. Our method can be potentially extended to stain other organs dissected from either zebrafish or other small animals.

Stem cells for tendon tissue engineering and regeneration

IMPORTANCE OF THE FIELD

Tendon injuries are common especially in sports activities, but tendon is a unique connective tissue with poor self-repair capability. With advances in stem cell biology, tissue engineering is becoming increasingly powerful for tissue regeneration. Stem cells with capacity of multipotency and self-renewal are an ideal cell source for tissue engineering.

AREAS COVERED IN THIS REVIEW

This review focus on discussing the potential strategies including inductive growth factors, bio-scaffolds, mechanical stimulation, genetic modification and co-culture techniques to direct tendon-lineage differentiation of stem cells for complete tendon regeneration. Attempting to use embryonic stem cells as seed cells for tendon tissue engineering have achieved encouraging results. The combination of chemical and physical signals in stem cell microenvironment could be regulated to induce differentiation of the embryonic stem cells into tendon.

WHAT THE READER WILL GAIN

We summarize fundamental questions, as well as future directions in tendon biology and tissue engineering.

TAKE HOME MESSAGE

Multifaceted technologies are increasingly required to control stem cell differentiation, to develop novel stem cell-based therapy, and, ultimately, to achieve more effective repair or regeneration of injured tendons.

The myostatin-induced E3 ubiquitin ligase RNF13 negatively regulates the proliferation of chicken myoblasts

The ubiquitin ligase RING finger protein 13 gene (RNF13) was first identified in a screen for genes whose expression is regulated by myostatin in chicken fetal myoblasts. In this study, we demonstrate that the RNF13 gene is broadly expressed in many chicken tissues. The expression of RNF13 gradually decreases during skeletal myogenesis, and myostatin up-regulates RNF13 expression at both the transcriptional and translational levels. Interestingly, ectopic expression of RNF13 inhibits cell proliferation and suppresses the expression of the myogenic genes MyoD and Caveolin-3 in muscle cells. Moreover, recently, we have reported that RNF13 is a RING-type E3 ubiquitin ligase. In this report, we provide experimental evidence to show that mutations disrupting the RING finger abolish the growth-suppressive activity of RNF13, indicating that its E3 ligase activity is required for the inhibition of cell proliferation. Taken together, our findings show that RNF13 functions as an E3 ubiquitin ligase to negatively regulate cell proliferation.

Expression of myogenic regulatory factors in the muscle-derived electric organ of Sternopygus macrurus

In most groups of electric fish, the current-producing cells of electric organs (EOs) derive from striated muscle fibers but retain some phenotypic characteristics of their precursor muscle cells. Given the role of the MyoD family of myogenic regulatory factors (MRFs) in the transcriptional activation of the muscle program in vertebrates, we examined their expression in the electrocytes of the gymnotiform Sternopygus macrurus. We estimated the number of MRF genes in the S. macrurus genome and our Southern blot analyses revealed a single MyoD, myogenin, myf5 and MRF4 gene. Quantitative RT-PCR showed that muscle and EO transcribe all MRF genes. With the exception of MyoD, the endogenous levels of myogenin, myf5 and MRF4 transcripts in electrocytes were greater than those detected in muscle fibers. These data indicate that MRF expression levels are not sufficient to predict the level to which the muscle program is manifested. Qualitative expression analysis of MRF co-regulators MEF2C, Id1 and Id2 also revealed these genes not to be unique to either muscle or EO, and detected similar expression patterns in the two tissues. Therefore, the partial muscle program of the EO is not associated with a partial expression of MRFs or with apparent distinct levels of some MRF co-factors. In addition, electrical inactivation by spinal cord transection (ST) resulted in the up-regulation of some muscle proteins in electrocytes without an accompanying increase in MRF transcript levels or notable changes in the co-factors MEF2C, Id1 and Id2. These findings suggest that the neural regulation of the skeletal muscle program via MRFs in S. macrurus might differ from that of their mammalian counterparts. Together, these data further our understanding of the molecular processes involved in the plasticity of the vertebrate skeletal muscle program that brings about the muscle-like phenotype of the non-contractile electrogenic cells in S. macrurus.

CF2 activity and enhancer integration are required for proper muscle gene expression in Drosophila

The creation of the contractile apparatus in muscle involves the co-activation of a group of genes encoding muscle-specific proteins and the production of high levels of protein in a short period of time. We have studied the transcriptional control of six Drosophila muscle genes that have similar expression profiles and we have compared these mechanisms with those employed to control the distinct expression profiles of other Drosophila genes. The regulatory elements controlling the transcription of co-expressed muscle genes share an Upstream Regulatory Element and an Intronic Regulatory Element. Moreover, similar clusters of MEF2 and CF2 binding sites are present in these elements. Here, we demonstrate that CF2 depletion alters the relative expression of thin and thick filament components. We propose that the appropriate rapid gene expression responses during muscle formation and the maintenance of each muscle type is guaranteed in Drosophila by equivalent duplicate enhancer-like elements. This mechanism may be exceptional and restricted to muscle genes, reflecting the specific requirement to mediate rapid muscle responses. However, it may also be a more general mechanism to control the correct levels of gene expression during development in each cell type.

Gene expression of myogenic regulatory factors, nicotinic acetylcholine receptor subunits, and GAP-43 in skeletal muscle following denervation in a rat model

Neuromuscular junction destabilization following nerve injury contributes to irreversible functional impairment. Myogenic Regulatory Factors (MRF's) including myoblast determination factor (MyoD), MRF-4, Myogenin, and myogenic factors-5 (myf-5), and Growth-associated protein 43 KDa (GAP43) regulate gene expression of nicotinic acetylcholine receptor (nAChR) subunits (alpha, beta, delta, gamma, and epsilon). We hypothesized that nerve injury induces altered gene expression of MRF's, nAChRs, and GAP-43 in the skeletal muscle which destabilize neuromuscular junctions. The tibial nerve was transected in 42 juvenile male Sprague-Dawley rats. Denervated and contralateral control gastrocnemius m. mRNA for nAChR subunits, MRF's, and GAP-43 were determined by real time reverse transcription polymerase chain reaction (real time RT-PCR). After transection, muscle mass decreased for 1 year with a nadir of 75% at 3 months. Alpha, gamma, and epsilon subunit genes increased by 3 and peaked at 7 days before returning to control levels (P < 0.05). Beta subunits and GAP-43 tended to increase. Delta subunits peaked at 3 days returning to control levels by 30 days. By one month, most of the nAChR subunits had returned to control levels. Alpha, beta, gamma, and delta subunit expression remained significantly lower than control up to 1 year later (P < 0.05). MRF4, Myogenin, and MyoD expression paralleled that of alpha, gamma, and epsilon nAChR subunits (P < 0.05). Gene expression of nAChR alpha, gamma, delta and epsilon subunits was biphasic in the first month after nerve injury, similar to that of MRF's. nAChR subunits and MRF's may play a critical role in neuromuscular junction stability.

Neuronal control of myogenic regulatory factor accumulation in fetal muscle

The lumbosacral spinal cords of 14.5-day gestation mice (E14.5) were ablated. The number of molecules of each of the four myogenic regulatory factor (MRF) mRNAs per nanogram of total RNA were evaluated in innervated and aneural fetal crural muscles. Accumulation of all four MRF mRNAs was affected in aneural muscle, but was never more than threefold different than in innervated muscles, considerably less than after adult denervation. The effect of the nerve varied with the MRF, the fetal age, and with the muscle (extensor digitorum longus muscle [EDL] vs. soleus muscle), with the nerve having multiple effects including down-regulation of certain MRF genes at specific periods (e.g., myoD and myogenin [E16.5-E18.5] and MRF4 in the EDL only [E18.5-E19.5]); limiting the up-regulation of certain genes, which occurred in the absence of innervation (e.g., myf-5 [E18.5-E19.5] and myogenin [E14.5-E16.5]); and even enhancing the accumulation of MRF4 mRNA (E14.5-E16.5). We hypothesize that factors other than nerve contribute to the down-regulation of myf-5 and myogenin mRNAs to adult levels. Innervation was required for the emergence of the slow, but not the fast, MRF mRNA profile at birth. MyoD, found in both the nuclear and cytoplasmic protein extracts of innervated fetal muscle, increased by approximately 5-fold in the nuclear extracts (approximately 2.5-fold in the cytoplasmic) of E19.5 aneural muscles, significantly less than the 12-fold increase found in the nuclear extract of 4-day denervated adult muscle. This increase in aneural fetal muscle was due primarily to an increased concentration of myoD in muscle lineage nuclei, rather than to the presence of additional myoD(+) muscle lineage nuclei.

Molecular structure and expression patterns of flounder (Paralichthys olivaceus) Myf-5, a myogenic regulatory factor

Myf-5, a member of the myogenic regulatory factors (MRF), has been shown to be expressed in muscle precursors in early stage zebrafish embryos. The MRFs, including MyoD, Myf-5, Myogenin and MRF4, belong to the basic Helix-Loop-Helix transcription factors that contain a conserved basic Helix-Loop-Helix (bHLH) domain. To better understand the role of Myf-5 in the development of fish muscles, we have isolated the Myf-5 genomic sequence and cDNA from Flounder (Paralichthys olivaceus), and analyzed its structures and patterns of expression. Promoter analysis identified several putative transcription factor binding sites such as an E-box, NF-Y sites that might confer muscle-specific expression. Myf-5 transcripts were first detected in the paraxial mesoderm that gives rise to slow muscles. During somitogenesis, Myf-5 expression was found in developing somites. Myf-5 expression decreased gradually in somites in the anterior region, but remained strong in the newly formed somites. In the hatching stage, the expression was also detected in other muscle cells such as head muscle and fin muscle. In the growing fish, RT-PCR results showed that Myf-5 was expressed in the skeletal muscle and intestine.

Overexpression of MyoD-inducible lysosomal sialidase (neu1) inhibits myogenesis in C2C12 cells

Lysosomal sialidase, encoded by neu1, is required for the removal of terminal sialic acid residues from a variety of sialoglycoconjugates. In humans, deficiency of this enzyme results in the inborn error of metabolism sialidosis, characterized by the accumulation of sialoglycoconjugates within the nervous system and in peripheral organs. A subset of sialidosis patients present with symptoms of profound muscle dysfunction, including progressive muscular atrophy. We have previously shown that the 5' regulatory region of murine neu1 is typical of skeletal muscle-specific genes due to the presence of several E-boxes and its responsiveness to stimulation by muscle regulatory factors (MRFs) such as MyoD. Here, we report that sialidase activity is increased 6-fold during the first 24 h of differentiation of C2C12 myoblasts followed by an attenuation to pre-differentiation levels by 48 h. We demonstrate that the lysosomal sialidase promoter is highly upregulated by MyoD through a mechanism that is dependent on the MyoD chromatin remodeling domain. We also show that the sialidase promoter is repressed by activated MEK. Inappropriate overexpression of sialidase 48 h after the onset of differentiation results in downregulation of myogenin as well as myosin heavy chain expression and in a halt of the differentiation cascade. This study indicates that lysosomal sialidase is a potent regulator of the early stages of myogenesis.

Co-operation between enhancers modulates quantitative expression from the Drosophila Paramyosin/miniparamyosin gene in different muscle types

The distinct muscles of an organism accumulate different quantities of structural proteins, but always maintaining their stoichiometry. However, the mechanisms that control the levels of these proteins and that co-ordinate muscle gene expression remain to be defined. The paramyosin/miniparamyosin gene encodes two thick filament proteins transcribed from two different promoters. We have analysed the regulatory regions that control expression of this gene and that are situated in the two promoters, the 5' and the internal promoters, both in vivo and in silico. A distal muscle enhancer containing three conserved MEF2 motifs is essential to drive high levels of paramyosin expression in all the major embryonic, larval and adult muscles. This enhancer shares sequence motifs, as well as its structure and organisation, with at least four co-regulated muscle enhancers that direct similar patterns of expression. However, other elements located downstream of the enhancer are also required for correct gene expression. Other muscle genes with different patterns of expression, such as miniparamyosin, are regulated by other basic mechanisms. The expression of miniparamyosin is controlled by two enhancers, AB and TX, but a BF modulator is required to ensure the correct levels of expression in each particular muscle. We propose a mechanism of transcriptional regulation in which similar enhancers are responsible for the spatio-temporal expression of co-regulated genes. However, it is the interaction between enhancers which ensures that the correct amounts of protein are expressed at any particular time in a cell, adapting these levels to their specific needs. These mechanisms may not be exclusive to neural or muscle tissue and might represent a general mechanism for genes that are spatially and temporally co-regulated.

Tendon injury and tendinopathy: healing and repair

Tendon disorders are frequent and are responsible for substantial morbidity both in sports and in the workplace. Tendinopathy, as opposed to tendinitis or tendinosis, is the best generic descriptive term for the clinical conditions in and around tendons arising from overuse. Tendinopathy is a difficult problem requiring lengthy management, and patients often respond poorly to treatment. Preexisting degeneration has been implicated as a risk factor for acute tendon rupture. Several physical modalities have been developed to treat tendinopathy. There is limited and mixed high-level evidence to support the, albeit common, clinical use of these modalities. Further research and scientific evaluation are required before biological solutions become realistic options.

Cluster characterisation and temporal expression of porcine sarcomeric myosin heavy chain genes

Members of the myosin heavy chain (MyHC) gene family are subjected to temporal regulation of gene switching during development. One strategy to the identification of cis-acting regulatory elements that are involved in temporal or fibre-type specific regulation is to undertake a comparative analysis of the MyHC gene family between the pig, an important target species, and other mammals, like human whose entire genome has been recently sequenced. Towards this end, we report here on the isolation, and characterisation of the porcine cardiac (MyHC slow/beta and alpha) and skeletal MyHC (embryonic, 2a, 2x, 2b and perinatal) gene clusters, and their structural comparisons with mouse and human clusters. The genome organisation of both clusters in the pig, human and mouse is conserved as having the same gene order, similar intergenic distances, and in the same head-to-tail orientation. For a period of pre-natal muscle growth, relative expression of MyHC isoforms, as determined by TaqMan real-time RT-PCR, correlated with the gene order in the skeletal MyHC cluster (embryonic > 2a > 2x > 2b) suggesting the possible presence of DNA elements on the same side as the MyHC embryonic gene that direct temporal regulation.

In vivo characterisation of the human UCP3 gene minimal promoter in mice tibialis anterior muscles

Transcriptional mechanisms controlling human UCP3 gene expression in skeletal muscle remain poorly understood. Experiments based on plasmid electrotransfer into tibialis anterior muscle of C57/BL6 male mice were set up in order to functionally analyze the hUCP3 gene promoter. These transfection experiments showed that a 6300 bp region upstream of the transcription initiation site was sufficient to mediate maximal promoter activity. Further analyses with a series of 5(')-deleted constructs demonstrated that the hUCP3 gene minimal promoter was located between nucleotides -284 and -40. Furthermore, an essential region was identified between nucleotides -284 and -224. The analysis of this region revealed a putative response element for PPAR located between nucleotides -281 and -269. Finally, mutations of potential cis-acting elements within the hUCP3 minimal promoter showed the presence of two TATA boxes (-198/-194 and -45/-41) required for constitutive UCP3 gene expression. To our knowledge, this is the first time that molecular characterization of the UCP3 promoter has been achieved using an in vivo experimental model.

Dual-regulated myoD- and msx1-based interventions in C2C12-derived cells enable precise myogenic/osteogenic/adipogenic lineage control

BACKGROUND

Advanced gene therapy, tissue engineering and biopharmaceutical manufacturing require sophisticated and well-balanced multiregulated multigene interventions to reprogram desired mammalian cell phenotypes.

METHODS

We have combined the streptogramin (PIP)- and tetracycline (TET)-responsive gene regulation systems for independent expression control of the differentiation determinants myoD and msx1 in C2C12-derived cells.

RESULTS

Different dual-regulated expression scenarios which induce either both, only one or none of the lineage control genes triggered differential differentiation and precise control of myogenic, osteogenic or adipogenic cell phenotypes.

CONCLUSIONS

Our findings substantiate the use of multiregulated multigene interventions in reprogramming cellular differentiation pathways in a desired manner.

Aberrant development of motor axons and neuromuscular synapses in MyoD-null mice

Myogenic regulatory factors (MRFs), muscle-specific transcription factors, are implicated in the activity-dependent regulation of nicotinic acetylcholine receptor (AChR) subunit genes. Here we show, with immunohistochemistry, Western blotting, and electron microscopy that MyoD, a member of the MRF family, also plays a role in fetal synapse formation. In the diaphragm of 14.5 d gestation (E14.5) wild-type and MyoD-/- mice, AChR clusters (the formation of which is under a muscle intrinsic program) are confined to a centrally located endplate zone. This distribution persists in wild-type adult muscles. However, beginning at E15.5 and extending to the adult, innervated AChR clusters are distributed all over the diaphragm of MyoD-/- mice, extending as far as the insertion of the diaphragm into the ribs. In wild-type muscle, motor axons terminate on clusters adjacent to the main intramuscular nerve; in MyoD-/- muscle, axonal bundles form extensive secondary branches that terminate on the widely distributed clusters. The number of AChR clusters on adult MyoD-/- and wild-type diaphram muscles is similar. Junctional fold density is reduced at MyoD-/- endplates, and the transition from the fetal (alpha, beta, gamma, delta) to adult-type (alpha, beta, delta, epsilon) AChRs is markedly delayed. However, MyoD-/- mice assemble a complex postsynaptic apparatus that includes muscle-specific kinase (MuSK), rapsyn, erbB, and utrophin.

The mouse Ifi200 gene cluster: genomic sequence, analysis, and comparison with the human HIN-200 gene cluster

The interferon-activatable Ifi200 gene cluster is located on mouse Chromosome 1q21-q23. We report here our analysis of two genomic regions encoding at least 10 closely related 200 family genes (Ifi201, Ifi202a, Ifi202b, Ifi202c, Ifi203a, Ifi203b, Ifi203c, Ifi203', Ifi204, and Ifi204') in 129/SvJ mice. Through a BAC-based sequencing approach, the exact structure and organization of these highly similar Ifi200 genes were obtained. A high degree of conservation (99% identity) was observed between Ifi202a and b and between Ifi203a and b. The presence of an additional transcribed region in intron 4 of Ifi203a and b suggests the possibility of alternative splicing, and a spliced variant of the Ifi204' mRNA exhibits 91% sequence identity with a related but unmapped D3 mRNA. Comparative analysis of the mouse and human clusters indicates an absence of significant sequence conservation in noncoding sequences, suggesting that the 200 family emerged prior to human-mouse speciation and subsequently diverged after gene duplication.

Requirement for down-regulation of the CCAAT-binding activity of the NF-Y transcription factor during skeletal muscle differentiation

NF-Y is composed of three subunits, NF-YA, NF-YB, and NF-YC, all required for DNA binding. All subunits are expressed in proliferating skeletal muscle cells, whereas NF-YA alone is undetectable in terminally differentiated cells in vitro. By immunohistochemistry, we show that the NF-YA protein is not expressed in the nuclei of skeletal and cardiac muscle cells in vivo. By chromatin immunoprecipitation experiments, we demonstrate herein that NF-Y does not bind to the CCAAT boxes of target promoters in differentiated muscle cells. Consistent with this, the activity of these promoters is down-regulated in differentiated muscle cells. Finally, forced expression of the NF-YA protein in cells committed to differentiate leads to an impairment in the down-regulation of cyclin A, cyclin B1, and cdk1 expression and is accompanied by a delay in myogenin expression. Thus, our results indicate that the suppression of NF-Y function is of crucial importance for the inhibition of several cell cycle genes and the induction of the early muscle-specific program in postmitotic muscle cells.

Identification of cultured progenitor cells from human marrow stroma

The marrow stromal cells (MSC) are essential for regulation of bone remodeling and hematopoiesis. It is of prime importance to isolate MSC and to expand the proliferating cells ex vivo. In this study, we analyzed cultured MSC for various cellular parameters, including cell morphology, cell cycle, and expression of cell surface antigens by flow cytometry. MSC were divided based on cell size to small (S-cells) and large (L-cells) and were visualized by light and electron microscope. The S-cells were proliferating cells correlated with G0/G1 phase of cell cycle, and expressed cFOS. The expression of surface markers CD-34, -44, -51, -61, -62E, -62P, -62L was quantified using flow cytometry. CD-44 was ubiquitously expressed by S and L cells, CD-51 and -61 were expressed by 30%-38% of S-cells. CD-34 and -62 expressed 20% positive of the analyzed cells that were of the proliferating progenitors (S-cells). This study enables the identification of subpopulations from MSC with special attention paid to the proliferating cells from ex vivo cultures of marrow stroma.

Retinoid X receptor alpha represses GATA-4-mediated transcription via a retinoid-dependent interaction with the cardiac-enriched repressor FOG-2

Dietary vitamin A and its derivatives, retinoids, regulate cardiac growth and development. To delineate mechanisms involved in retinoid-mediated control of cardiac gene expression, the regulatory effects of the retinoid X receptor alpha (RXR alpha) on atrial naturietic factor (ANF) gene transcription was investigated. The transcriptional activity of an ANF promoter-reporter in rat neonatal ventricular myocytes was repressed by RXR alpha in the presence of 9-cis-RA and by the constitutively active mutant RXR alpha F318A indicating that liganded RXR confers the regulatory effect. The RXR alpha-mediated repression mapped to the proximal 147 bp of the rat ANF promoter, a region lacking a consensus retinoid response element but containing several known cardiogenic cis elements including a well characterized GATA response element. Glutathione S-transferase "pull-down" assays revealed that RXR alpha interacts directly with GATA-4, in a ligand-independent manner, via the DNA binding domain of RXR alpha and the second zinc finger of GATA-4. Liganded RXR alpha repressed the activity of a heterologous promoter-reporter construct containing GATA-response element recognition sites in cardiac myocytes but not in several other cell types, suggesting that additional cardiac-enriched factors participate in the repression complex. Co-transfection of liganded RXR alpha and the known cardiac-enriched GATA-4 repressor, FOG-2, resulted in additive repression of GATA-4 activity in ventricular myocytes. In addition, RXR alpha was found to bind FOG-2, in a 9-cis-RA-dependent manner. These data reveal a novel mechanism by which retinoids regulate cardiogenic gene expression through direct interaction with GATA-4 and its co-repressor, FOG-2.

Origin and differentiation of human and murine stroma

Stromal cells generated in long-term cultures appear to follow a vascular smooth muscle differentiation pathway. Such a pathway, comprising several steps hallmarked by the expression of cytoskeletal and extracellular matrix markers, is found not only for bone marrow stromal cells, but also for stromal cells generated from the different developmental sites of hematopoiesis (yolk sac, aorta-gonad-mesonephros region, fetal liver, and spleen). Factors responsible for this differentiation pathway and its functional significance are discussed. The mesenchymal founder cell might be, at least for bone marrow, a mesenchymal stem cell (MSC), giving rise to stromal cells, endothelial cells, adipocytes, osteoblasts, and chondrocytes. A feature that distinguishes the MSC lineage from that of the hematopoietic stem cell lineage is that differentiation pathways are not strictly delineated, since even apparently fully differentiated cells from a given lineage have the potential to convert into another lineage (phenotype "plasticity") and intermediate cell phenotypes are observed. A stochastic Repression/Induction model that would account for this plasticity is proposed.

The myogenic basic helix-loop-helix family of transcription factors shows similar requirements for SWI/SNF chromatin remodeling enzymes during muscle differentiation in culture

The myogenic basic helix-loop-helix family of transcription factors, MyoD, Myf5, myogenin, and MRF4, can each activate the muscle differentiation program when ectopically expressed in non-muscle cells. SWI/SNF complexes are ATP-dependent chromatin remodeling enzymes. We demonstrated previously that SWI/SNF enzymes promote MyoD-mediated muscle differentiation. To ascertain the requirement for SWI/SNF enzymes in muscle differentiation mediated by different MyoD family members, we examined MyoD, Myf5, MRF4, and myogenin-mediated induction of muscle differentiation in cells expressing dominant negative versions of BRG1 or BRM-based SWI/SNF enzymes. We demonstrated that expression of dominant negative BRG1 or BRM inhibited the induction of muscle-specific gene expression by Myf5 and MRF4; however, myogenin failed to induce measurable quantities of muscle-specific mRNAs, even in cells not expressing dominant negative SWI/SNF. In contrast, all four myogenic regulators induced expression of the cell cycle regulators p21, Rb, and cyclin D3 and promoted cell cycle arrest independently of the SWI/SNF enzymes. We proposed that SWI/SNF enzymes are required for the induction of all muscle-specific gene expression by MyoD, Myf5, and MRF4, whereas induction of the cell cycle regulators, p21, Rb, and cyclin D3 occurred independently of SWI/SNF function.

Peroxisome proliferator-activated receptor gamma ligands differentially modulate muscle cell differentiation and MyoD gene expression via peroxisome proliferator-activated receptor gamma -dependent and -independent pathways

The effects of distinct classes of peroxisome proliferator-activated receptor gamma (PPARgamma) ligands on myogenesis and MyoD gene expression were examined in mouse skeletal muscle C2C12 myoblasts. Treatment of C2C12 cells with the PPARgamma ligand, 15-deoxy-Delta12,14-prostaglandin J2 (15d-PGJ2), repressed morphologically defined myogenesis and reduced endogenous mRNA levels of the myogenic differentiation markers MyoD, myogenin, and alpha-actin. In contrast, two synthetic PPARgamma ligands, L-805645 and ciglitazone, exhibited no effects. In transient transfection assays, 15d-PGJ2 specifically inhibited the expression of a MyoD promoter-luciferase reporter gene (MyoDLuc) in a cell type- and promoter-specific manner, indicating that 15d-PGJ2 functions in part by repressing MyoD gene transcription. The inhibition of MyoD gene expression by 15d-PGJ2 is mediated by the distal region of the MyoD gene promoter. PPARgamma on its own also inhibited MyoDLuc expression and further augmented the 15d-PGJ2 response. In contrast, L-805645 and ciglitazone did not inhibit MyoDLuc expression on their own but did so in the presence of ectopically expressed PPARgamma. Interestingly, a transdominant inhibitor of PPARgamma (hPPARgamma2Delta500) had no effect on the 15d-PGJ2-dependent repression of MyoDLuc expression but overcame L-805645/PPARgamma-dependent repression. Finally, saturating concentrations of L-805645, which did not affect myogenesis, failed to ablate 15d-PGJ2-mediated repression of the myogenic program. Thus, distinct PPARgamma ligands may repress MyoD gene expression through PPARgamma-dependent and -independent pathways, and 15d-PGJ2 can inhibit the myogenic program independent of its cognate receptor, PPARgamma.

Development and composition of skeletal muscle fibres in mouse oesophagus

The development of skeletal muscle in mouse oesophagus was investigated by studying the expression of skeletal muscle type myosin heavy chain (MHC), troponin I (TnI) and tropoinin T (TnT) using immunocytochemical and immunoblotting procedures. Both slow and fast muscle fibres were first detected in outer layer muscularis externa of cranial oesophagus at 14 days gestation. The fast MHC was present in all skeletal muscle fibres of oesophagus while the slow MHC was restricted to only a subset of myotubes during foetal development, indicating that slow and fast fibres emerged during early stages of myogenesis. A small number of cells expressed both slow and fast MHCs in the caudal region of adult mouse oesophagus, suggesting that some muscle fibres did not differentiate fully even in the adult. The conversion of some muscle fibre types, from slow to fast, was apparent during postnatal development. This was indicated by a gradual reduction in the number of slow MHC positive fibres during postnatal growth. The complete suppression of slow MHC was observed in cranial oesophagus by 4 weeks of age. However, the persistence of some slow MHC in the caudal oesophagus was apparent even in the adult. The conversion of muscle fibres from slow to fast type was also evidenced by immunoblotting study of fast and slow TnI. The expression level of slow TnI decreased while that of fast TnI increased during neonatal growth period. Compared with the limb skeletal muscles, the onset of the adult fast TnT isoform expression was delayed in mouse oesophagus and its developmental isoforms were not completely suppressed in the adult, although their expression level was reduced.

Both smooth and skeletal muscle precursors are present in foetal mouse oesophagus and they follow different differentiation pathways

Muscularis externa of mouse oesophagus is composed of two skeletal muscle layers in the adult. Unlike rest of skeletal muscle in the body, the oesophageal skeletal muscle in the mouse has been proposed to be derived from fully differentiated smooth muscle cells by transdifferentiation during later foetal and early postnatal development (Patapoutian et al. [1995] Science 270:1818-1821). Here we characterised the nature of cells in muscularis externa of the mouse oesophagus by ultrastructural and immunoctyochemical analyses. The presence of differentiated skeletal muscle cells identified by positive staining for skeletal muscle specific myosin heavy chain became first apparent in the outer layer of cranial oesophagus at 14 days gestation. The transient expression of smooth muscle type alpha-actin in mouse oesophageal muscle was also apparent during foetal development. This isoform, however, was not smooth muscle specific during early development as it was also detected in foetal skeletal muscles. Compared with oesophagus, the suppression of this smooth muscle type alpha-actin during foetal development was faster in non-oesophageal skeletal muscle cells. The development of skeletal muscle in oesophagus showed a cranial to caudal and an outer layer to inner layer progression. During early foetal development, mouse oesophagus is composed of undifferentiated mesenchymal cells that formed cell clusters. Two types of cells with different staining densities could be distinguished within these cell clusters by electron microscopy. The centrally located pale staining cells gave rise to skeletal muscle cells while the peripherally positioned dense staining cells gave rise to smooth muscle cells, indicating the existence of both skeletal and smooth muscle cell precursors in mouse oesophagus during early foetal development. Further development showed an increase in the proportion of skeletal muscle cells and a decrease in size and number of the smooth muscle type cells. Apart from decrease in cell size, some other morphological features of smooth muscle cell degeneration were also observed during later foetal and early neonatal development. No smooth muscle cells undergoing transdifferentiation were observed. Both immunochemical and ultrastructural observations, thus, demonstrated the presence of skeletal muscle cells in early foetal oesophagus. It is concluded that the transient appearance of smooth muscle cells may provide a scaffold for the laying down of skeletal muscle layers in mouse oesophagus, the final disappearance of which may be triggered by lack of smooth muscle innervation.

Msx1 antagonizes the myogenic activity of Pax3 in migrating limb muscle precursors

The migration of myogenic precursors to the vertebrate limb exemplifies a common problem in development - namely, how migratory cells that are committed to a specific lineage postpone terminal differentiation until they reach their destination. Here we show that in chicken embryos, expression of the Msx1 homeobox gene overlaps with Pax3 in migrating limb muscle precursors, which are committed myoblasts that do not express myogenic differentiation genes such as MyoD. We find that ectopic expression of Msx1 in the forelimb and somites of chicken embryos inhibits MyoD expression as well as muscle differentiation. Conversely, ectopic expression of Pax3 activates MyoD expression, while co-ectopic expression of Msx1 and Pax3 neutralizes their effects on MyoD. Moreover, we find that Msx1 represses and Pax3 activates MyoD regulatory elements in cell culture, while in combination, Msx1 and Pax3 oppose each other's trancriptional actions on MyoD. Finally, we show that the Msx1 protein interacts with Pax3 in vitro, thereby inhibiting DNA binding by Pax3. Thus, we propose that Msx1 antagonizes the myogenic activity of Pax3 in migrating limb muscle precursors via direct protein-protein interaction. Our results implicate functional antagonism through competitive protein-protein interactions as a mechanism for regulating the differentiation state of migrating cells.

Cloning and characterization of the 5' flanking region of the human uncoupling protein 3 (UCP3) gene

Uncoupling protein 3 (UCP3), a member of the UCP family, mainly expressed in skeletal muscle could be responsible for thermogenesis in humans. Since little is known about its regulation, we studied the 5' flanking region of the human UCP3 (hUCP3) gene, which potentially contains the promoter sequences. We report the hUCP3 transcription initiation on a G located 764 nucleotides upstream the A contained in the first translated codon. Therefore, hUCP3 first exon has 669 bases of untranslated sequence. We also report the cloning and sequencing of seven kilobases from the gene 5' end and analyze the features of the potential proximal promoter. The MyoD family binding motif, called E-box, is the most abundant on this region. Other muscle-specific motives present in the potential proximal promoter include a MEF2 site as well as binding sequences for ubiquitous factors such as GC box and two CAAT boxes. Additionally, three putative peroxisome proliferator and one thyroid hormone response elements (PPRE and TRE, respectively) are found, which suggest a potential role for the peroxisome proliferator-activated receptor (PPAR) and thyroid hormone in human UCP3 gene expression. The description of the promoter region of the UCP3 gene will facilitate the elucidation of its transcriptional control.

Novel endothelin B receptor transcripts with the potential of generating a new receptor

Using RT-PCR and rapid amplification of 5' cDNA ends (5' RACE), we have cloned three previously unrecognized endothelin B receptor (EDNRB) transcripts from a human melanoma cell line. Three distinct types of cDNAs (EDNRBDelta1, Delta2 and Delta3) were identified. EDNRBDelta1 starts upstream of the published transcription start site of hEDNRB without splicing, whereas, EDNRBDelta2 and EDNRBDelta3 are alternatively spliced. EDNRBDelta1 and EDNRBDelta2 share the same transcription initiation site and are 560bp upstream of the conventional hEDNRB, whereas that of EDNRBDelta3 is 939bp upstream from that described for the conventional hEDNRB. Interestingly, many transcription factor motifs are detectable in the upstream regions of these transcription initiation sites. The predicted amino acid sequences reveal that EDNRBDelta1 and EDNRBDelta2 produce the same protein as the conventional hEDNRB, but EDNRBDelta3 would give rise to additional in-frame 89- or 83-amino-acid residues at the N-terminus. EDNRBDelta3 generates the same amino acid sequence at the C terminus, but utilizes the polyadenylation signal within the open reading frame, resulting in a shorter 3'UTR. These transcripts are widely expressed in human tissues, but their expression patterns vary between different tissues.

Alteration of myogenic regulatory components in a rat myoblast GLUT 3(-) mutant

Myogenesis is a complex process characterized by both biochemical and morphological differentiation. Recent transfection studies suggested a close relationship between the GLUT 3 transporter and the myogenic ability of rat skeletal L6 myoblast. In this study, the myogenic properties of GLUT 3- mutants were examined. Studies using three different GLUT 3- mutants (D2, D9 and D23) revealed that these mutants were defective not only in the GLUT 3 transporter, but also in the expression of myogenesis-associated genes. The properties of mutant D23 were further characterized. Overexpression of an exogenous functional GLUT 3 transporter was unable to restore the myogenic defects of this mutant. This mutant was subsequently found to be altered in components acting on at least two different sites of the L6 myogenic pathway. Transfection studies revealed that mutant D23 was deficient in component(s) essential for the myogenin promoter activity. The second component was required for the transcription of muscle-specific protein genes, as overexpression of myogenin was unable to rescue the inability of this mutant to express muscle-specific genes and to form myotubes. Mutant D23 was therefore thought to be deficient in a regulatory component which controlled the expression of GLUT 3, myogenin and muscle-specific genes.

Transforming growth factor alpha up-regulates desmin expression during embryonic mouse tongue myogenesis

Myogenesis is determined by a set of myogenic differentiation factors that are, in turn, regulated by a number of peptide growth factors. During embryonic mouse tongue formation, transforming growth factor alpha (TGF alpha), epidermal growth factor (EGF), and their cognate receptor (EGFR) are co-expressed spatially and temporally with desmin, a muscle-specific structural protein. This investigation tested the hypothesis that TGF alpha directly regulates the myogenic program in developing tongue myoblasts. Mandibular processes from the first branchial arch of embryonic day 10.5 (E10.5) mouse embryos were microdissected and explanted into an organ culture system using serumless chemically defined medium. Exogenous TGF alpha at 10 and 20 ng/ml specifically increased the amount of desmin expression and the number of desmin-positive cells without affecting the general growth and development of the mandibles. This inductive response was detected as early as 2 days after treatment and sustained up to 9 days in culture. EGFR antisense oligonucleotides (30 microM) as well as tyrphostin (80 microM) were able to negate TGF alpha-induced up-regulation of desmin expression. These data indicate that autocrine and/or paracrine action of TGF alpha promotes tongue myogenesis, and that this action is mediated through functional kinase activity of the EGFR. We speculate that the myogenic program in the developing mouse tongue is dependent upon growth factor mediated cell-cell communication of mesenchymal cells originating from the occipital somites and ectomesenchymal cells originating from the cranial neural crest.

Disruption of the lama2 gene in embryonic stem cells: laminin alpha 2 is necessary for sustenance of mature muscle cells

Mutations in the gene coding for the alpha 2 chain of laminin-2 and -4 (merosin) cause a severe form of congenital muscular dystrophy in humans and mice. To establish a defined model for in vitro and in vivo studies of the role of laminin alpha 2/merosin in development and cell and tissue function, we generated several lines of mutant embryonic stem (ES) cell with disruption of the laminin alpha 2 chain gene. We find that homozygous mutant ES cells differentiate normally in vitro, giving rise to cardiomyocytes, myotubes, and smooth muscle cells in addition to many other cell types. However, the myotubes that are formed are unstable. They detach, collapse, and degenerate, a process which is initiated at the appearance of the mature, contractile phenotype of the cells. We propose that the detachment and death of contracting myotubes in vitro has its counterpart in vivo and that contraction-induced myofiber damage, along with the lack of survival cues provided by laminin alpha 2/merosin, is a significant contribution to muscle degeneration in merosin-deficient muscular dystrophy.

The structures of the mouse and human urocortin genes (Ucn and UCN)

The mouse and human urocortin genes (Ucn and UCN, respectively) have been isolated, characterized, and found to have very similar structures. Each has two exons, and the entire coding region is located in the second exon, as is the case for the gene of the related peptide, corticotropin-releasing factor. Several putative transcription factor-binding sites were identified in each of the urocortin promoters, including a TATA box, a cyclic AMP response element (CRE), GATA-binding sites, and a C/EBP-binding site as well as a Brn-2-binding site(s). Sequence analyses of the mouse and human genes also revealed the presence of a previously identified gene, Mpv17, in the 5' region upstream of the urocortin gene. Functional studies following transient transfection of urocortin reporter plasmids in PC12 cells revealed that the urocortin promoter is controlled by both positive and negative elements; the CRE is important for basal activity as well as responsiveness to forskolin stimulation.

Identification of regulatory regions which confer muscle-specific gene expression

For many newly sequenced genes, sequence analysis of the putative protein yields no clue on function. It would be beneficial to be able to identify in the genome the regulatory regions that confer temporal and spatial expression patterns for the uncharacterized genes. Additionally, it would be advantageous to identify regulatory regions within genes of known expression pattern without performing the costly and time consuming laboratory studies now required. To achieve these goals, the wealth of case studies performed over the past 15 years will have to be collected into predictive models of expression. Extensive studies of genes expressed in skeletal muscle have identified specific transcription factors which bind to regulatory elements to control gene expression. However, potential binding sites for these factors occur with sufficient frequency that it is rare for a gene to be found without one. Analysis of experimentally determined muscle regulatory sequences indicates that muscle expression requires multiple elements in close proximity. A model is generated with predictive capability for identifying these muscle-specific regulatory modules. Phylogenetic footprinting, the identification of sequences conserved between distantly related species, complements the statistical predictions. Through the use of logistic regression analysis, the model promises to be easily modified to take advantage of the elucidation of additional factors, cooperation rules, and spacing constraints.

In vitro system for differentiating pluripotent neural crest cells into smooth muscle cells

The change in vascular smooth muscle cells (SMC) from a differentiated to a dedifferentiated state is the critical phenotypic response that promotes occlusive arteriosclerotic disease. Despite its importance, research into molecular mechanisms regulating smooth muscle differentiation has been hindered by the lack of an in vitro cell differentiation system. We identified culture conditions that promote efficient differentiation of Monc-1 pluripotent neural crest cells into SMC. Exclusive Monc-1 to SMC differentiation was indicated by cellular morphology and time-dependent induction of the SMC markers smooth muscle alpha-actin, smooth muscle myosin heavy chain, calponin, SM22alpha, and APEG-1. The activity of the SM22alpha promoter was low in Monc-1 cells. Differentiation of these cells into SMC caused a 20-30-fold increase in the activity of the wild-type SM22alpha promoter and that of a hybrid promoter containing three copies of the CArG element. By gel mobility shift analysis, we identified new DNA-protein complexes in nuclear extracts prepared from differentiated Monc-1 cells. One of the new complexes contained serum response factor. This Monc-1 to SMC model should facilitate the identification of nodal regulators of smooth muscle development and differentiation.

A novel site, Mt, in the human desmin enhancer is necessary for maximal expression in skeletal muscle

Previous investigations have shown that expression of the muscle-specific intermediate filament desmin gene in skeletal muscle is controlled in part by a 5' muscle-specific enhancer. This enhancer activity can be divided into myoblast-specific and myotube-specific activation domains. The myotube-specific region contains a MyoD and MEF2 sites, whereas the myoblast-specific region contains Sp1, Krox, and Mb sites. In the present study, we designed mutations in the conserved portion of the myotube-specific region; transfection analysis of these mutations showed that a novel site located between the MyoD and MEF2 sites, named Mt (GGTATTT), is required for full transcriptional activity of the desmin enhancer in skeletal muscle. Although gel mobility shift assays demonstrate that myotube, myoblast, fibroblast, and HeLa nuclear extracts contain a nuclear factor that binds specifically to Mt, four copies of the Mt site function as the native enhancer only in myotubes. Functional synergism among the MyoD, MEF2, and Mt sites in myotubes has been demonstrated. These results show that the novel Mt site cooperates with MyoD and MEF2 to give maximal expression of the desmin gene.

The basic helix-loop-helix transcription factor Mist1 functions as a transcriptional repressor of myoD

A good model system to examine aspects of positive and negative transcriptional regulation is the muscle-specific regulatory factor, MyoD, which is a basic helix-loop-helix (bHLH) transcription factor. Although MyoD has the ability to induce skeletal muscle terminal differentiation in a variety of non-muscle cell types, MyoD activity itself is highly regulated through protein-protein interactions involving several different co-factors. Here we describe the characterization of a novel bHLH protein, Mist1, and how it influences MyoD function. We show that Mist1 accumulates in myogenic stem cells (myoblasts) and then decreases as myoblasts differentiate into myotubes. Mist1 functions as a negative regulator of MyoD activity, preventing muscle differentiation and the concomitant expression of muscle-specific genes. Mist1-induced inhibition occurs through a combination of mechanisms, including the formation of inactive MyoD-Mist1 heterodimers and occupancy of specific E-box target sites by Mist1 homodimers. Mist1 lacks a classic transcription activation domain and instead possesses an N-terminal repressor region capable of inhibiting heterologous activators. Thus, Mist1 may represent a new class of repressor molecules that play a role in controlling the transcriptional activity of MyoD, ensuring that expanding myoblast populations remain undifferentiated during early embryonic muscle formation.

The human pH aldolase A promoter directs widespread but muscle-predominant expression in transgenic mice

In order to identify regulatory elements that direct widespread in vivo expression of a linked gene, we have examined one of the human aldolase A alternative promoters, the ubiquitous pH promoter, which is active in most foetal and adult tissues. We have used the pH promoter region to drive expression of an heterologous CAT reporter gene in transgenic mice. We show that a short 820 bp pH promoter fragment is able to confer a ubiquitous and reproducible activity pattern on the CAT reporter gene in most of the transgenic lines analysed, with a particularly high level of expression in adult skeletal muscle. Activity of this transgene was detected from early embryonic stages. Therefore, this pH promoter region appears to be a powerful tool to direct ubiquitous and early expression of a transgene in vivo. Deletion analysis revealed that: (i) the region between -651 and -369 bp relative to the pH promoter transcription start site includes DNA elements capable of overriding effects of the surrounding chromatin at the integration site, (ii) the region between -285 and -211 bp is involved in pH promoter tissue-specific expression pattern in skeletal muscle and/or nervous tissues, (iii) the region located between -211 and -108 bp is necessary for its ubiquitous and muscle-predominant activity and (iv) the most proximal region downstream from -108 bp is still sufficient to confer an activity in brain and lung.

Increase in p202 expression during skeletal muscle differentiation: inhibition of MyoD protein expression and activity by p202

p202 is a primarily nuclear, interferon-inducible murine protein that is encoded by the Ifi 202 gene. Overexpression of p202 in transfected cells retards cell proliferation. p202 modulates the pattern of gene expression by inhibiting the activity of various transcription factors including NF-kappaB, c-Fos, c-Jun, E2F-1, and p53. Here we report that p202 was constitutively expressed in mouse skeletal muscle and that the levels of 202 RNA and p202 greatly increased during the differentiation of cultured C2C12 myoblasts to myotubes. When overexpressed in transfected myoblasts, p202 inhibited the expression of one muscle protein (MyoD) without affecting the expression of a second one (myogenin). Thus, the decrease in the level of MyoD (but not of myogenin) during muscle differentiation may be the consequence of the increase in p202 level. Overexpressed p202 also inhibited the transcriptional activity of both MyoD and myogenin. This inhibition was correlated with an interaction of p202 with both proteins, as well as the inhibition by p202 of the sequence-specific binding of both proteins to DNA. This inhibition of the expression of MyoD and of the transcriptional activity of MyoD and myogenin may account for the inhibition of the induction of myoblast differentiation by premature overexpression of p202.

An Oct-like binding factor regulates Myf-5 expression in primary avian cells

Myogenic regulatory factors (MRFs) are hierarchical regulators of skeletal myogenesis. Many MRF promoters have been well characterized with respect to flanking sequences that control their expression. Yet the promoter elements that regulate Myf-5, the first MRF expressed during mammalian embryogenesis, are still largely unknown. Comparison of Myf-5 5' flanking regions from bovine, mouse, and chicken genes revealed three evolutionarily conserved elements proximal to the transcription start site: the TATA box, an octamer motif, termed OLS, and a 6-bp C-rich element. Mobility shift assays and DNase I footprinting analysis demonstrated that a nuclear factor(s) present in both bovine and avian muscle and nonmuscle tissues specifically recognized OLS. Furthermore, this binding activity reacted with a polyclonal Oct-1 antibody. In avian primary myoblast and fibroblast cultures, CAT reporter constructs under regulation of the proximal Myf-5 5' flanking sequence were expressed preferentially in myoblasts with CAT levels approximately 12-fold higher than in fibroblasts. The TATA box and octamer motif were important for expression in both myoblasts and fibroblasts: loss of the TATA box abolished activity, and disruption of the OLS resulted in 50-75% loss of promoter activity.

Drosophila myogenesis and insights into the role of nautilus

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

Expression of myogenic regulatory factors (MRFs) in human neuromuscular disorders

Immunohistochemical studies using antibodies to myogenic regulatory factors (MRFs) Myo D, myogenin, myf-5, and myf-6, and transcription factors c-Fos and c-Jun, were performed on muscle biopsies from patients suffering from Duchenne and Becker muscular dystrophies, polymyositis, and denervation atrophy, to investigate whether expression of these factors occurs during degeneration and regeneration of adult muscle fibres. Strong Myo D, myogenin, myf-5 and myf-6 immunoreactivity was observed in the nuclei of small regenerating fibres and satellite cells, as revealed by double-labelling immunohistochemistry with N-CAM antibodies, in Duchenne and Becker muscular dystrophies and in polymyositis. This suggests that the myogenic programme is activated during regeneration of adult human muscle fibres. In addition, strong myf-6 and c-Jun immunoreactivity was found in the cytoplasm of some necrotic muscle fibres in patients with Duchenne and Becker muscular dystrophies and in patients with polymyositis. The latter findings suggest that strong cytoplasmic expression of myf-6 and c-Jun is related to the process of muscle fibre degeneration that occurs in these conditions. Increased Myo D, myogenin, myf-5 and myf-6 immunoreactivity was not observed in the nuclei of denervated muscle fibres, although strong c-Fos and c-Jun immunoreactivity was seen in the nuclei of denervated muscle fibres; this suggests that denervation triggers the expression of these transcription factors. Taken together, these observations demonstrate that MRFs and c-Fos and c-Jun are selectively expressed in different human muscular disorders.

Cardiomyocyte differentiation by GATA-4-deficient embryonic stem cells

In situ hybridization studies, promoter analyses and antisense RNA experiments have implicated transcription factor GATA-4 in the regulation of cardiomyocyte differentiation. In this study, we utilized Gata4−/− embryonic stem (ES) cells to determine whether this transcription factor is essential for cardiomyocyte lineage commitment. First, we assessed the ability of Gata4−/− ES cells form cardiomyocytes during in vitro differentiation of embryoid bodies. Contracting cardiomyocytes were seen in both wild-type and Gata4−/− embryoid bodies, although cardiomyocytes were observed more often in wild type than in mutant embryoid bodies. Electron microscopy of cardiomyocytes in the Gata4−/− embryoid bodies revealed the presence of sarcomeres and junctional complexes, while immunofluorescence confirmed the presence of cardiac myosin. To assess the capacity of Gata4−/− ES cells to differentiate into cardiomyocytes in vivo, we prepared and analyzed chimeric mice. Gata4−/− ES cells were injected into 8-cell-stage embryos derived from ROSA26 mice, a transgenic line that expresses beta-galactosidase in all cell types. Chimeric embryos were stained with X-gal to discriminate ES cell- and host-derived tissue. Gata4−/− ES cells contributed to endocardium, myocardium and epicardium. In situ hybridization showed that myocardium derived from Gata4−/− ES cells expressed several cardiac-specific transcripts, including cardiac alpha-myosin heavy chain, troponin C, myosin light chain-2v, Nkx-2.5/Csx, dHAND, eHAND and GATA-6. Taken together these results indicate that GATA-4 is not essential for terminal differentiation of cardiomyocytes and suggest that additional GATA-binding proteins known to be in cardiac tissue, such as GATA-5 or GATA-6, may compensate for a lack of GATA-4.

The epithelial cell default-phenotype hypothesis and its implications for cancer

The expression of epithelial cell adhesion and cytoskeletal genes is orchestrated by an apparently unique set of rules. No tissue-specific transactivator proteins have been found to drive them; only ubiquitous factors are utilized. In non-epithelial cells, they are actively repressed. Moreover, it was recently found that a single protein (adenovirus E1a) coordinately represses non-epithelial genes while inducing epithelial genes. A simple model is offered to explain how epithelial gene expression is coordinated. Under this model, the epithelial cell gene expression program is a transcriptional 'default'; that is, it occurs in the absence of tissue-specific transactivation. Conversion to this default requires only that mesenchymal transactivators are not expressed, or that central 'integrator' proteins are inactive. In their absence, mesenchymal gene expression cannot occur. Moreover, because the repressors cease to be expressed, the epithelial genes are induced. Oncogenes generally cause the breakdown of the epithelial phenotype--generating carcinomas--so genes such as E1a that cause epithelial conversion may prove useful for both understanding and controlling cancer.

TGF-alpha, EGF, and their cognate EGF receptor are co-expressed with desmin during embryonic, fetal, and neonatal myogenesis in mouse tongue development

The developing mouse tongue provides a model for discrete patterns of morphogenesis during short periods of embryonic development. Occipital somite-derived myogenic cells interact with cranial neural crest-derived ecto-mesenchymal cells to form the musculature of the tongue. The biochemical signals that control close range autocrine and/or paracrine signaling processes required to establish the fast-twitch complex tongue musculature are not known. The present study was designed to test the hypothesis that desmin, epidermal growth factor (EGF), and transforming growth factor-alpha (TGF alpha) and their cognate receptor, epidermal growth factor receptor (EGFr), are co-expressed during tongue myogenesis and define specific developmental stages of tongue muscle cell differentiation. To test this hypothesis, we performed studies to analyze the timing, position, and concentration of desmin, TGF alpha, EGF, and EGFr from embryonic day 9 (E9) through birth in Swiss Webster mouse tongue development. Desmin, TGF alpha, EGF, and EGFr co-localized to cells of myogenic lineage in the four occipital somites and subsequently in myoblasts and myotubes from E9 through E17. By newborn stage, desmin is localized to discrete regions in myofibers corresponding to Z-line delimiting sarcomeres, and A-band within sarcomeres; immunostaining for desmin, TGF alpha, and EGF persisted in differentiated myotubes and striated skeletal muscle. Desmin increased from 0.01% at E11 to 0.51% of the total protein by E17 and at birth. Concomitantly, the patterns and increases in TGF alpha, EGF, and EGFr showed significant increases during the same developmental period. The temporal and positional co-localization of TGF alpha, EGF, and EGFr support the hypothesis that autocrine and paracrine regulation of desmin by actions of growth factor ligand and receptor defines critical stages of tongue myogenesis.

Detection of mouse skeletal muscle-specific product, which includes ZF5 zinc fingers and a VP16 acidic domain, by reverse transcriptase PCR

ZF5, which we have cloned as a repressor on the mouse c-myc promoter, is a zinc finger protein containing Kruppel-type zinc finger and ZiN/POZ domains. In a reverse transcriptase PCR assay using mouse skeletal muscle RNA, we identified a 827 bp PCR product including the zinc finger domain of ZF5 and the acidic domain of VP16. The presence of the VP16 acidic domain induced the reduction of DNA-binding activity of the zinc finger domain. In addition, the inhibitory effect of the VP16 acidic domain was demonstrated on the human immunodeficiency virus (HIV) promoter, but there was no effect on the thymidine kinase (TK) promoter.

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.

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

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