The myoD gene family: nodal point during specification of the muscle cell lineage

The aryl hydrocarbon receptor complex and the control of gene expression

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that controls the expression of a diverse set of genes. The toxicity of the potent AhR ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin is almost exclusively mediated through this receptor. However, the key alterations in gene expression that mediate toxicity are poorly understood. It has been established through characterization of AhR-null mice that the AhR has a required physiological function, yet how endogenous mediators regulate this orphan receptor remains to be established. A picture as to how the AhR/ARNT heterodimer actually mediates gene transcription is starting to emerge. The AhR/ARNT complex can alter transcription both by binding to its cognate response element and through tethering to other transcription factors. In addition, many of the coregulatory proteins necessary for AhR-mediated transcription have been identified. Cross talk between the estrogen receptor and the AhR at the promoter of target genes appears to be an important mode of regulation. Inflammatory signaling pathways and the AhR also appear to be another important site of cross talk at the level of transcription. A major focus of this review is to highlight experimental efforts to characterize nonclassical mechanisms of AhR-mediated modulation of gene transcription.

The juxtaposition of a promoter with a locus control region transcriptional domain activates gene expression

Nonlinear chromatin configurations can juxtapose widely separated elements within a genomic locus; however, it remains unclear how these structures are established and contribute to transcriptional control. A 5'-remote locus control region (LCR) regulates the human growth hormone (hGH-N) gene. HSI, a pituitary-specific component of the hGH LCR, establishes a domain of polymerase II (PolII) transcription 5' to hGH-N. Repression of this transcriptional domain by HSI deletion or PolII blockade decreases hGH-N expression. Here, we show that hGH-N activation is accompanied by positioning of the hGH-N promoter to this LCR transcriptional domain. Selectively blocking LCR transcription inhibits the formation of this active 'looped' conformation. Thus, HSI is crucial for establishing a domain of noncoding PolII transcription, and this domain is intimately linked with chromatin organization of the active hGH-N locus. This integration of LCR transcription with chromatin reconfiguration constitutes a robust pathway for long-range gene activation.

SCREAM/ICE1 and SCREAM2 specify three cell-state transitional steps leading to arabidopsis stomatal differentiation

Differentiation of specialized cell types in multicellular organisms requires orchestrated actions of cell fate determinants. Stomata, valves on the plant epidermis, are formed through a series of differentiation events mediated by three closely related basic-helix-loop-helix proteins: SPEECHLESS (SPCH), MUTE, and FAMA. However, it is not known what mechanism coordinates their actions. Here, we identify two paralogous proteins, SCREAM (SCRM) and SCRM2, which directly interact with and specify the sequential actions of SPCH, MUTE, and FAMA. The gain-of-function mutation in SCRM exhibited constitutive stomatal differentiation in the epidermis. Conversely, successive loss of SCRM and SCRM2 recapitulated the phenotypes of fama, mute, and spch, indicating that SCRM and SCRM2 together determined successive initiation, proliferation, and terminal differentiation of stomatal cell lineages. Our findings identify the core regulatory units of stomatal differentiation and suggest a model strikingly similar to cell-type differentiation in animals. Surprisingly, map-based cloning revealed that SCRM is INDUCER OF CBF EXPRESSION1, a master regulator of freezing tolerance, thus implicating a potential link between the transcriptional regulation of environmental adaptation and development in plants.

Protein interactions: analysis using allele libraries

Interaction defective alleles (IDAs) are alleles that contain mutations affecting their ability to interact with their wild type binding partners. The locations of the mutations may lead to the identification of protein interaction domains and interaction interfaces. IDAs may also distinguish different binding interfaces of multidomain proteins that are part of large complexes, thus shedding light on large protein structures that have yet to be determined. IDAs may also be used in conjunction with RNAi to dissect protein interaction networks. Here, the wild type allele is knocked down and replaced with an IDA that has lost the ability to interact with a specific binding partner. As a result, interactions are disrupted rather than knocking out the entire gene. Thus, IDAs have the potential to be extremely valuable tools in protein interaction network analysis. IDAs can be isolated by reverse two-hybrid analysis, which was demonstrated over a decade ago, but high background levels caused by truncated IDAs have prevented its widespread adoption. We recently described a novel method for full-length allele library generation that eliminates this background and increases the efficiency of the reverse two-hybrid protocol (and IDA isolation) significantly. Here we discuss our strategy for allele library generation, the potential uses of IDAs as outlined above, and additional applications of allele libraries.

Cell-lineage regulated myogenesis for dystrophin replacement: a novel therapeutic approach for treatment of muscular dystrophy

Duchenne muscular dystrophy (DMD) is characterized in skeletal muscle by cycles of myofiber necrosis and regeneration leading to loss of muscle fibers and replacement with fibrotic connective and adipose tissue. The ongoing activation and recruitment of muscle satellite cells for myofiber regeneration results in loss of regenerative capacity in part due to proliferative senescence. We explored a method whereby new myoblasts could be generated in dystrophic muscles by transplantation of primary fibroblasts engineered to express a micro-dystrophin/enhanced green fluorescent protein (muDys/eGFP) fusion gene together with a tamoxifen-inducible form of the myogenic regulator MyoD [MyoD-ER(T)]. Fibroblasts isolated from mdx(4cv) mice, a mouse model for DMD, were efficiently transduced with lentiviral vectors expressing muDys/eGFP and MyoD-ER(T) and underwent myogenic conversion when exposed to tamoxifen. These cells could also be induced to differentiate into muDys/eGFP-expressing myocytes and myotubes. Transplantation of transduced mdx(4cv) fibroblasts into mdx(4cv) muscles enabled tamoxifen-dependent regeneration of myofibers that express muDys. This lineage control method therefore allows replenishment of myogenic stem cells using autologous fibroblasts carrying an exogenous dystrophin gene. This strategy carries several potential advantages over conventional myoblast transplantation methods including: (i) the relative simplicity of culturing fibroblasts compared with myoblasts, (ii) a readily available cell source and ease of expansion and (iii) the ability to induce MyoD gene expression in vivo via administration of a medication. Our study provides a proof of concept for a novel gene/stem cell therapy technique and opens another potential therapeutic approach for degenerative muscle disorders.

Key aspects of phrenic motoneuron and diaphragm muscle development during the perinatal period

At the time of birth, respiratory muscles must be activated to sustain ventilation. The perinatal development of respiratory motor units (comprising an individual motoneuron and the muscle fibers it innervates) shows remarkable features that enable mammals to transition from in utero conditions to the air environment in which the remainder of their life will occur. In addition, significant postnatal maturation is necessary to provide for the range of motor behaviors necessary during breathing, swallowing, and speech. As the main inspiratory muscle, the diaphragm muscle (and the phrenic motoneurons that innervate it) plays a key role in accomplishing these behaviors. Considerable diversity exists across diaphragm motor units, but the determinant factors for this diversity are unknown. In recent years, the mechanisms underlying the development of respiratory motor units have received great attention, and this knowledge may provide the opportunity to design appropriate interventions for the treatment of respiratory disease not only in the perinatal period but likely also in the adult.

Multilineage potential of STRO-1+ rat dental pulp cells in vitro

The aim of the current study was to determine whether STRO-1 selection is an effective approach for purifying rat dental pulp stem cells, and especially whether such selection is beneficial on the multilineage differentiation capacity, i.e. whether selection will account for a higher rate of differentiation or lesser variability. In this study, two cell populations (STRO-1(+) and non-sorted cells) were cultured under conditions promoting neurogenic, adipogenic, myogenic and chondrogenic differentiation. Results of light microscopy, histochemistry, and immunohistochemistry showed that STRO-1(+) cells were capable of advancing into all four differentiation pathways under the influence of inductive media. Quantitative PCR and statistical analysis on specific differentiation markers confirmed that there were significant upregulations in STRO-1(+) cells compared to the other populations, during induction culture. On the basis of our results, we concluded that: (a) rat STRO-1(+) dental pulp stem cells are capable of differentiating towards multilineage cell types, including neural cells, adipocytes, myocytes and chondrocytes; (b) the STRO-1(+) population has a more defined multilineage potential compared to non-sorted cells, probably because of its more homogeneous nature. .

MesP1 drives vertebrate cardiovascular differentiation through Dkk-1-mediated blockade of Wnt-signalling

ES-cell-based cardiovascular repair requires an in-depth understanding of the molecular mechanisms underlying the differentiation of cardiovascular ES cells. A candidate cardiovascular-fate inducer is the bHLH transcription factor MesP1. As one of the earliest markers, it is expressed specifically in almost all cardiovascular precursors and is required for cardiac morphogenesis. Here we show that MesP1 is a key factor sufficient to induce the formation of ectopic heart tissue in vertebrates and increase cardiovasculogenesis by ES cells. Electrophysiological analysis showed all subtypes of cardiac ES-cell differentiation. MesP1 overexpression and knockdown experiments revealed a prominent function of MesP1 in a gene regulatory cascade, causing Dkk-1-mediated blockade of canonical Wnt-signalling. Independent evidence from ChIP and in vitro DNA-binding studies, expression analysis in wild-type and MesP knockout mice, and reporter assays confirm that Dkk-1 is a direct target of MesP1. Further analysis of the regulatory networks involving MesP1 will be required to preprogramme ES cells towards a cardiovascular fate for cell therapy and cardiovascular tissue engineering. This may also provide a tool to elicit cardiac transdifferentiation in native human adult stem cells.

Three-dimensional epidermis-like growth of human mesenchymal stem cells on dermal equivalents: contribution to tissue organization by adaptation of myofibroblastic phenotype and function

Human mesenchymal stem cells (hMSC) are able to differentiate into mature cells of various mesenchymal tissues. Recent studies have reported that hMSC may even give rise to cells of ectodermal origin. This indication of plasticity makes hMSC a promising donor source for cell-based therapies. This study explores the differentiation potential of hMSC in a tissue-specific microenvironment simulated in vitro. HMSC were cultured air-exposed on dermal equivalents (DEs) consisting of collagen types I and III with dermal fibroblasts and subjected to conditions similar to those used for tissue engineering of skin with keratinocytes. Culture conditions were additionally modified by pre-treating the cells with 5-azacytidine or supplementing the medium with all trans retinoic acid (RA). HMSC were capable of adaptation to epidermis-specific conditions without losing their mesenchymal multipotency. However, despite the viability and evident three-dimensional epidermis-like growth pattern, hMSC showed a persistent expression of mesenchymal but not of epithelial markers, thus indicating a lack of epidermal (trans) differentiation. Further, electron microscopy and immunohistochemical analyses demonstrated that hMSC cultured under epidermis-specific conditions adopted a myofibroblastic phenotype and function, promoted in particular by air exposure. In conclusion, multipotent hMSC failed to differentiate into E-cadherin- or cytokeratin-expressing cells under optimized organotypic culture conditions for keratinocytes but differentiated into myofibroblast-like cells contracting the extracellular matrix, a phenomenon that was enhanced by RA and 5-azacytidine. These results indicate that hMSC might contribute to wound-healing processes by extracellular matrix reorganization and wound contraction but not by differentiation into keratinocytes.

Sox6 is required for normal fiber type differentiation of fetal skeletal muscle in mice

Sox6, a member of the Sox family of transcription factors, is highly expressed in skeletal muscle. Despite its abundant expression, the role of Sox6 in muscle development is not well understood. We hypothesize that, in fetal muscle, Sox6 functions as a repressor of slow fiber type-specific genes. In the wild-type mouse, differentiation of fast and slow fibers becomes apparent during late fetal stages (after approximately embryonic day 16). However, in the Sox6 null-p(100H) mutant mouse, all fetal muscle fibers maintain slow fiber characteristics, as evidenced by expression of the slow myosin heavy chain MyHC-beta. Knockdown of Sox6 expression in wild-type myotubes results in a significant increase in MyHC-beta expression, supporting our hypothesis. Analysis of the MyHC-beta promoter revealed a Sox consensus sequence that likely functions as a negative cis-regulatory element. Together, our results suggest that Sox6 plays a critical role in the fiber type differentiation of fetal skeletal muscle.

Myogenesis in the mouse

The first striated muscle to form during mouse embryogenesis is the heart followed by skeletal muscle which is derived from the somites. The expression of genes encoding muscle structural proteins and myogenic regulatory sequences of the MyoD1 family has been examined using 35S-labelled riboprobes. In the cardiac tube, actin and myosin genes are expressed together from an early stage, whereas in the myotome, the earliest skeletal muscle, they are activated asynchronously over days. They are not expressed in the somite prior to myotome formation. One potential muscle marker, carbonic anhydrase III, is expressed in early mesoderm and subsequently in the notochord, similarly to the Brachyury gene. The myogenic sequences are not detectable in the heart. In the myotome they show distinct patterns of expression; this is discussed in the context of their role as muscle transcription factors. myf-5 is the only myogenic factor sequence present in the somite prior to muscle formation and thus is potentially involved in an earlier step of muscle determination. It is also present in the early limb bud, but the status of myogenic precursor cells in the limb in this context is less clear.

A role for the myogenic determination gene Myf5 in adult regenerative myogenesis

The myogenic determination genes Myf5, Myod and Mrf4 direct skeletal muscle cell fate prenatally. In adult myogenesis, Myod has been shown to regulate myoblast differentiation, however, our understanding of satellite cell regulation is incomplete since the roles of Myf5 and Mrf4 had not been clearly defined. Here we examine the function of Myf5 and Mrf4 in the adult using recently generated alleles. Mrf4 is not expressed in normal or Myf5 null satellite cells and myoblasts, therefore excluding a role for this determination gene in adult muscle progenitors. Skeletal muscles of adult Myf5 null mice exhibit a subtle progressive myopathy. Crucially, adult Myf5 null mice exhibit perturbed muscle regeneration with a significant increase in muscle fibre hypertrophy, delayed differentiation, adipocyte accumulation, and fibrosis after freeze-injury. Satellite cell numbers are not significantly altered in Myf5 null animals and they show a modest impaired proliferation under some conditions in vitro. Mice double mutant for Myf5 and Dystrophin were more severely affected than single mutants, with enhanced necrosis and regeneration. Therefore, we show that Myf5 is a regulator of regenerative myogenesis and homeostasis, with functions distinct from those of Myod and Mrf4.

Temperature influences the coordinated expression of myogenic regulatory factors during embryonic myogenesis in Atlantic salmon (Salmo salarL.)

Potential molecular mechanisms regulating developmental plasticity to temperature were investigated in Atlantic salmon embryos (Salmo salarL.). Six orthologues of the four myogenic regulatory factors (MRFs:individually: smyf5, smyoD1a/1b/1c, smyoG and sMRF4), the master transcription factors regulating vertebrate myogenesis, were characterised at the mRNA/genomic level. In situ hybridisation was performed with specific cRNA probes to determine the expression patterns of each gene during embryonic myogenesis. To place the MRF data in the context of known muscle fibre differentiation events, the expression of slow myosin light chain-1 and Pax7 were also investigated. Adaxial myoblasts expressed smyoD1a prior to and during somitogenesis followed by smyoD1c (20-somite stage, ss),and sMRF4 (25–30 ss), before spreading laterally across the myotome, followed closely by the adaxial cells. Smyf5 was detected prior to somitogenesis, but not in the adaxial cells in contrast to other teleosts studied. The expression domains of smyf5, smyoD1band smyoG were not confined to the s-smlc1 expression field,indicating a role in fast muscle myogenesis. From the end of segmentation,each MRF was expressed to a greater or lesser extent in zones of new muscle fibre production, the precursor cells for which probably originated from the Pax7 expressing cell layer external to the single layer of s-smlc1+ fibres. SmyoD1a and smyoGshowed similar expression patterns with respect to somite stage at three different temperatures investigated (2°C, 5°C and 8°C) in spite of different rates of somite formation (one somite added each 5 h, 8 h and 15 h at 8°C, 5°C and 2°C, respectively). In contrast, the expression of smyf5, sMRF4 and s-smlc1 was retarded with respect to somite stage at 2°C compared to 8°C, potentially resulting in heterochronies in downstream pathways influencing later muscle phenotype.

TGF-beta mediated Msx2 expression controls occipital somites-derived caudal region of skull development

Craniofacial development involves cranial neural crest (CNC) and mesoderm-derived cells. TGF-beta signaling plays a critical role in instructing CNC cells to form the craniofacial skeleton. However, it is not known how TGF-beta signaling regulates the fate of mesoderm-derived cells during craniofacial development. In this study, we show that occipital somites contribute to the caudal region of mammalian skull development. Conditional inactivation of Tgfbr2 in mesoderm-derived cells results in defects of the supraoccipital bone with meningoencephalocele and discontinuity of the neural arch of the C1 vertebra. At the cellular level, loss of TGF-beta signaling causes decreased chondrocyte proliferation and premature differentiation of cartilage to bone. Expression of Msx2, a critical factor in the formation of the dorsoventral axis, is diminished in the Tgfbr2 mutant. Significantly, overexpression of Msx2 in Myf5-Cre;Tgfbr2flox/flox mice partially rescues supraoccipital bone development. These results suggest that the TGF-beta/Msx2 signaling cascade is critical for development of the caudal region of the skull.

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

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

Stem Cell Therapy: A Hope for Dying Hearts

The restoration of functional myocardium following heart failure still remains a formidable challenge among researchers. Irreversible damage caused by myocardial infarction is followed by left ventricular remodeling. The current pharmacologic and interventional strategies fail to regenerate dead myocardium and are usually insufficient to meet the challenge caused by necrotic cardiac myocytes. There is growing evidence, suggesting that the heart has the ability to regenerate through the activation of resident cardiac stem cells or through the recruitment of a stem cell population from other tissues such as bone marrow. These new findings belie the earlier conception about the poor regenerating ability of myocardial tissue. Stem cell therapy is a promising new approach for myocardial repair. However, it has been limited by the paucity of cell sources for functional human cardiomyocytes. Moreover, cells isolated from different sources exhibit idiosyncratic characteristics including modes of isolation, ease of expansion in culture, proliferative ability, characteristic markers, etc., which are the basis for several technical manipulations to achieve successful engraftment. Clinical trials show some evidence for the successful integration of stem cells of extracardiac origin in adult human heart with an improved functional outcome. This may be attributed to the discrepancies in the methods of detection, study subject selection (early or late post transplantation), presence of inflammation, and false identification of infiltrating leukocytes. This review discusses these issues in a comprehensive manner so that their physiological significance in animal as well as in human studies can be better understood.

Embryonic stem cells as a source of models for drug discovery

Embryonic stem cells (ESCs) will become a source of models for a wide range of adult differentiated cells, providing that reliable protocols for directed differentiation can be established. Stem-cell technology has the potential to revolutionize drug discovery, making models available for primary screens, secondary pharmacology, safety pharmacology, metabolic profiling and toxicity evaluation. Models of differentiated cells that are derived from mouse ESCs are already in use in drug discovery, and are beginning to find uses in high-throughput screens. Before analogous human models can be obtained in adequate numbers, reliable methods for the expansion of human ESC cultures will be needed. For applications in drug discovery, involving either species, protocols for directed differentiation will need to be robust and affordable. Here, we explore current challenges and future opportunities in relation to the use of stem-cell technology in drug discovery, and address the use of both mouse and human models.

Cell lineage regulators in B and T cell development

This special issue highlights a pivotal set of regulatory molecules that have emerged as central controllers of cell-type identity in the immune system. Each in its own way has been considered as a kind of 'master' regulator of a particular cell fate choice, but the actual modes of action of these factors vary widely. The comparison among them sheds light on the different ways that an essential regulatory input can affect cellular identity.

Effects of compressive force on the differentiation of pluripotent mesenchymal cells

The purpose of this study was to determine the effect of mechanical stress on the differentiation of the pluripotent mesenchymal cell line C2C12. C2C12 cells were cultured continuously under compressive force (0.25-2.0 g/cm(2)). After mechanical stress loading, the levels of expression of mRNAs and proteins for phenotype-specific markers of osteoblasts (Runx2, Msx2, Dlx5, Osterix, AJ18), chondroblasts (Sox5, Sox9), myoblasts (MyoD), and adipocytes (PPAR gamma) were measured by real-time polymerase chain reaction analysis and Western blot analysis, respectively. The expression of activated p38 mitogen-activated protein kinase (p38 MAPK) was measured by Western blotting and/or ELISA. Loading 0.5 g/cm(2) of compressive force significantly increased the expression levels of Runx2, Msx2, Dlx5, Osterix, Sox5, and Sox9. In contrast, the expression levels of AJ18, MyoD, and PPAR gamma were decreased by exposure to 0.5 g/cm(2) of compressive force. Loading 0.5 g/cm(2) of compressive force also induced the phosphorylation of p38 MAPK. SB203580, which is a specific inhibitor of p38 MAPK, inhibited the compressive force-induced phosphorylation of p38 MAPK and partially blocked compressive force-induced Runx2 mRNA expression. These results demonstrate that compressive force stimulation directs the differentiation pathway of C2C12 cells into the osteoblast and chondroblast lineage via activated phosphorylation of p38 MAPK.

Multilineage differentiation potential of stem cells derived from human dental pulp after cryopreservation

The current study aimed to prove that human dental pulp stem cells (hDPSCs) isolated from the pulp of third molars can show multilineage differentiation after cryopreservation. First, hDPSC were isolated via enzymatic procedures, and frozen in liquid nitrogen until use. After defrosting, cells were analyzed for proliferative potential and the expression of the stem cell marker STRO-1. Subsequently, cells were cultured in neurogenic, osteogenic/odontogenic, adipogenic, myogenic, and chondrogenic inductive media, and analyzed on basis of morphology, immunohistochemistry, and reverse transcriptase-polymerase chain reaction (RT-PCR) for specific marker genes. All data were replicated, and the results of the primary cells were compared to similar tests with an additional primary dental pulp stem cell strain, obtained from the National Institutes of Health (NIH). Results showed that our cell population could be maintained for at least 25 passages. The existence of stem/ progenitor cells in both cell strains was proven by the STRO-1 staining. Under the influence of the 5 different media, both cell strains were capable to advance into all 5 differentiation pathways. Still differences between both strains were found. In general, our primary culture performed better in myogenic differentiation, while the externally obtained cells were superior in the odontogenic/osteogenic and chondrogenic differentiation pathways. In conclusion, the pulp tissue of the third molar may serve as a suitable source of multipotent stem cells for future tissue engineering strategies and cell-based therapies, even after cryopreservation.

Biochemical pathways analysis of microarray results: regulation of myogenesis in pigs

BACKGROUND

Combining microarray results and biological pathway information will add insight into biological processes. Pathway information is widely available in databases through the internet. Mammalian muscle formation has been previously studied using microarray technology in pigs because these animals are an interesting animal model for muscle formation due to selection for increased muscle mass. Results indicated regulation of the expression of genes involved in proliferation and differentiation of myoblasts, and energy metabolism. The aim of the present study was to analyse microarrays studying myogenesis in pigs. It was necessary to develop methods to search biochemical pathways databases.

RESULTS

PERL scripts were developed that used the names of the genes on the microarray to search databases. Synonyms of gene names were added to the list by searching the Gene Ontology database. The KEGG database was searched for pathway information using this updated gene list. The KEGG database returned 88 pathways. Most genes were found in a single pathway, but others were found in up to seven pathways. Combining the pathways and the microarray information 21 pathways showed sufficient information content for further analysis. These pathways were related to regulation of several steps in myogenesis and energy metabolism. Pathways regulating myoblast proliferation and muscle fibre formation were described. Furthermore, two networks of pathways describing the formation of the myoblast cytoskeleton and regulation of the energy metabolism during myogenesis were presented.

CONCLUSION

Combining microarray results and pathways information available through the internet provide biological insight in how the process of porcine myogenesis is regulated.

BM stem cell transplantation rescues pathophysiologic features of aged dystrophic mdx muscle

BACKGROUND

The value of transplantation of BM stem cells in aged (12-month-old) mdx was evaluated because it is thought to be a more ideal model for studying the praxiology of Duchenne muscular dystrophy (DMD). The possible mechanisms of stem cell differentiation were then discussed.

METHODS

BM was isolated from 8-10-week-old male C57 BL/10 mice. After injecting BM cells into 12-month-old female mdx mice through the tail vein, the expression of dystrophin and MyoD was detected at different time points by immunofluorescence staining, RT-PCR and Western blot.

RESULTS

The C57 male mice donor-specific and Y-chromosome-specific sequence could be detected in all female aged mdx mice, implying the success of the transplantation. Expression of dystrophin and MyoD was detected and increased over time.

DISCUSSION

BM cells were recruited to the muscle and partially restored specific pathophysiologic features of the dystrophic muscle in aged mdx mice. Muscle differentiation of BM cells recapitulated embryonic myogenesis.

Regulation of skeletal muscle differentiation in fibroblasts by exogenous MyoD gene in vitro and in vivo

MyoD of the myogenic regulatory factors (MRFs) family regulates the skeletal muscle differentiation program. In this study, stably transfected NIH3T3-derived cell lines were established, in which exogenous MyoD was expressed at high levels. Transcriptional activation of endogenous muscle regulatory gene and induction towards the skeletal muscle lineages were observed with phase-contrast microscopy when continuously cultured in vitro. Moreover, to determine their ability of myogenic formation in vivo, the transfected cells were implanted in nude mice subcutaneously for up to 10 weeks. The morphological characterization of inductive cells was observed using transmission electron microscope and histological staining. Myogenesis of fibroblasts incubated in the medium was activated by overexpression of MyoD, and the cells were accumulated and fused into multinucleated myotubes. Correlatively, RT-PCR and immunohistochemistry confirmed the increased expression of characteristic downstream molecule myogenin and mysion heavy chains during myogenic differentiation. Ecoptic myogenesis was found and remained stable phenotype when the transfected cells were seeded in vivo. Our results suggest that MyoD can be considered to be a determining factor of myogenic lineages, and it may play an important role in the cell therapy and cell-mediated gene therapy of the skeletal muscle.

Differential regulation of multiple alternatively spliced transcripts of MyoD

Splice variants of the basic helix-loop-helix myoblast determination factor (myoD) have not been previously found in vertebrates. Here we report the identification and characterization of three alternative transcripts of a myoD paralogue from the tiger pufferfish (Takifugu rubripes). The T. rubripes myoD1 gene (TmyoD1) has 3 exons and 2 introns and it is present on scaffold 104, in a region of conserved synteny with zebrafish. The isoform TMyoD1-alpha is a putative protein of 281 residues that contains the basic, helix-loop-helix and helix III domains and shares 61%, 56%, 51%, 49% and 56% overall identity with zebrafish, Xenopus, mouse, human and chicken MyoD1, respectively. TMyoD1-beta arises from an alternative 3' splice site and differs from TMyoD1-alpha by a 26-residue insertion adjacent to helix III, which is one of the functional domains required for chromatin remodelling. The third alternative transcript, TmyoD1-gamma, retains intron I and has two premature termination codons far from the 3'-most exon-exon junction. TmyoD1-gamma is therefore likely to be degraded by nonsense-mediated decay, an important widespread post-transcriptional mechanism that regulates transcript levels. Analysis of gene expression by qPCR revealed that TmyoD1-alpha was the most abundant transcript in fast and slow myotomal muscle. TmyoD1-alpha expression was 2-fold higher in fast muscle of juvenile fish that were actively producing new myotubes compared to adult stages that had stopped recruiting fast muscle fibres. A similar expression pattern was observed for TmyoD1-alpha in slow muscle but the differences were not significant. Transcript levels of TmyoD1-gamma only varied significantly in fast muscle and were 5-fold higher in adult compared to juvenile stages. Significant differences in expression of TmyoD1 splice variants were also observed during embryonic development. The differential expression of three alternative transcripts of myoD1 in developing and adult myotomal muscle of T. rubripes supports the hypothesis that diversity generated by alternative splicing may be of functional significance in muscle development in this species.

A Novel Approach for Generating Full-length, High Coverage Allele Libraries for the Analysis of Protein Interactions

The yeast reverse two-hybrid method was developed to identify mutations disrupting protein-protein interactions. Adoption of the method has been slow, in large part, due to the high frequency of truncation and frameshift mutants typically observed with current protocols. We have developed a new strategy, based on in vitro recombinational cloning and full-length selection in Escherichia coli, to eliminate this background and dramatically increase the efficiency of the reverse two-hybrid protocol. The method was tested by generating an allele library of MyoD1 and selecting for alleles with defective interaction with Id1. Our results confirm that most of the interaction-defective alleles contain a single point mutation in the known interaction domain, the basic helix-loop-helix region. Moreover analysis of the crystal structure of MyoD reveals that the majority of these mutations occurred at the interaction interface. The results obtained using this novel approach for allele library generation demonstrate a significant advancement in the application of yeast reverse two-hybrid screens. Furthermore this method is applicable to any loss-of-function mutant screen where truncated proteins are a source of high background.

Breaking the silence: three bHLH proteins direct cell-fate decisions during stomatal development

Stomata are microscopic pores on the surface of land plants used for gas and water vapor exchange. A pair of highly specialized guard cells surround the pore and adjust pore size. Studies in Arabidopsis have revealed that cell-cell communication is essential to coordinate the asymmetric cell divisions required for proper stomatal patterning. Initial research in this area identified signaling molecules that negatively regulate stomatal differentiation. However, genes promoting cell-fate transition leading to mature guard cells remained elusive. Now, three closely related basic helix-loop-helix (bHLH) proteins, SPEECHLESS, MUTE and FAMA have been identified as positive regulators that direct three consecutive cell-fate decisions during stomatal development. The identification of these genes opens a new direction to investigate the evolution of stomatal development and the conserved functions of bHLH proteins in cell type differentiation adopted by plants and animals.

Glucocorticoids induce the differentiation of a mesenchymal progenitor cell line, ROB-C26 into adipocytes and osteoblasts, but fail to induce terminal osteoblast differentiation

To clarify the effects of glucocorticoids (GCs) on osteoblast and adipocyte differentiation, we investigated the effects of dexamethasone (Dex), a GC analogue on transcription factors for osteoblasts (Runx2, Dlx5 and Osterix) and adipocytes (C/EBPs such as C/EBPalpha, C/EBPbeta and C/EBPdelta, and PPARgamma2), late osteoblastic markers, bone sialoprotein (BSP) and osteocalcin (OC), and adipocyte differentiation-dependent protein, aP2 in a clonal mesenchymal progenitor cell line, ROB-C26 (C26). C26 cells were dose- and time-dependently responsive to Dex in terms of an increase in not only mRNA and protein expressions of the C/EBPs, PPARgamma2 and aP2, but also Runx2, Dlx5, BSP and OC with no induction of Osterix, which is considered to act mainly on terminal osteoblast differentiation. Cycloheximide pretreatment indicated that Dex signaling immediately increases expressions of the C/EBPs and Dlx5, while expressions of the rest of the genes require de novo protein synthesis. Continuous Dex treatment stimulated adipocyte formation, but failed to induce Osterix expression and mineralized matrix formation. However, BMP-2 treatment of Dex-treated cells induced Osterix expression and subsequent mineralized matrix formation. These results indicate that Dex up-regulates the C/EBPs followed by increasing PPARgamma2 and aP2 expressions in C26 cells to induce adipocyte differentiation, while Dex enhances Dlx5 followed by increasing Runx2, BSP and OC expressions at gene and protein levels, but cannot induce Osterix expression, suggesting that Dex does not promote their terminal osteoblast differentiation.

Osteosarcoma and osteoblastic differentiation: a new perspective on oncogenesis

In addition to changes in cellular pathways, loss of differentiation is a notable feature of osteosarcoma. We hypothesized that blocks to normal differentiation may be a common feature of osteosarcoma, and may be one of many critical events that occur during oncogenesis in osteosarcoma. Furthermore, therapies that restore normal programs of differentiation may be attractive new treatment strategies for chemo-therapy and/or chemoprevention. We exposed an osteosarcoma cell line to two highly osteogenic bone morphogenetic proteins and noted increased tumor volume and no evidence of osteoinduction in vivo. We then used expression profile analysis to identify downstream targets of the osteogenic bone morphogenetic proteins, revealing up-regulation of the inhibitor of differentiation genes 1, 2, and 3, and the nuclear receptor, peroxisome proliferator activated receptor gamma. We then evaluated the use of nuclear receptor agonists, including peroxisome proliferator activated receptor gamma, to circumvent the apparent block to bone morphogenetic protein-induced differentiation in osteosarcoma cell lines. The peroxisome proliferator activated receptor gamma/retinoid X receptor agonists induced terminal differentiation in all four osteosarcoma cell lines and were synergistic when combined. In osteosarcoma cells, there are inherent blocks to normal bone morphogenetic protein-induced differentiation; however, they do not prevent nuclear receptor agonists from inducing terminal differentiation.

Association analyses of polymorphisms in porcine MYF5 and MYOD1 genes with carcass traits

The objective of this study was to assess the effect of polymorphisms of myogenic factor 5 (MYF5) and myogenic differentiation 1 (MYOD1) genes on carcass traits in pigs. PCR-RFLP was used to identify three and one SNP(s) from the MYF5 and the MYOD1 gene, respectively. Association analysis performed on the four polymorphisms in a series of three Large White × Meishan F2 populations totalling near 400 pigs showed: (1) an MYF5 exon 1 Hsp92II polymorphism causing a Met→Leu substitution was significantly associated with fat meat percentage, shoulder fat thickness, thorax-waist fat thickness, average backfat thickness and carcass length to 1st rib (P < 0.05); (2) an MYF5 exon 2 MspI polymorphism and an intron 1 HaeIII polymorphism, which were completely linked, were significantly associated with thorax-waist fat thickness, 6–7th rib fat thickness and carcass length to 1st rib (P < 0.05); (3) an MYOD1 intron 1 DdeI polymorphism was significantly associated with carcass length to 1st rib.

Premature myogenic differentiation and depletion of progenitor cells cause severe muscle hypotrophy in Delta1 mutants

In vertebrates, skeletal myogenesis is initiated by the generation of myoblasts followed by their differentiation to myocytes and the formation of myofibers. The determination of myoblasts and their differentiation are controlled by muscle regulatory factors that are activated at specific stages during myogenesis. During late embryonic and fetal stages a distinct population of resident proliferating progenitor cells is the major source of myogenic cells. How the differentiation of myoblasts and progenitor cells is regulated is not clear. We show that in mouse embryos the Notch ligand Delta1 (Dll1) controls both differentiation of early myoblasts and maintenance of myogenic progenitor cells. Early dermomyotome-derived myoblasts are determined normally in Dll1 mutant embryos, but their differentiation is accelerated, leading to a transient excess of myotomal muscle fibers. Similarly, migratory hypaxial myogenic cells colonize the limb buds and activate muscle regulatory factor expression normally, but muscle differentiation progresses more rapidly. Resident progenitor cells defined by Pax3/Pax7 expression are formed initially, but they are progressively lost and virtually absent at embryonic day 14.5. Muscle growth declines beginning around embryonic day 12, leading to subsequent severe muscle hypotrophy in hypomorphic Dll1 fetuses. We suggest that premature and excessive differentiation leads to depletion of progenitor cells and cessation of muscle growth, and we conclude that Dll1 provides essential signals that are required to prevent uncontrolled differentiation early and ensure sustained muscle differentiation during development.

Transcription factor control of asymmetric cell divisions that establish the stomatal lineage

The establishment of new cell lineages during development often requires a symmetry-breaking event. An asymmetric division in the epidermis of plants initiates a lineage that ultimately produces stomatal guard cells. Stomata are pores in the epidermis that serve as the main conduits for gas exchange between plants and the atmosphere; they are critical for photosynthesis and exert a major influence on global carbon and water cycles. Recent studies implicated intercellular signalling in preventing the inappropriate production of stomatal complexes. Genes required to make stomata, however, remained elusive. Here we report the identification of a gene, SPEECHLESS (SPCH), encoding a basic helix-loop-helix (bHLH) transcription factor that is necessary and sufficient for the asymmetric divisions that establish the stomatal lineage in Arabidopsis thaliana. We demonstrate that SPCH and two paralogues are successively required for the initiation, proliferation and terminal differentiation of cells in the stomatal lineage. The stomatal bHLHs define a molecular pathway sufficient to create one of the key cell types in plants. Similar molecules and regulatory mechanisms are used during muscle and neural development, highlighting a conserved use of closely related bHLHs for cell fate specification and differentiation.

Down-regulation of Sox10 with specific small interfering RNA promotes transdifferentiation of Schwannoma cells into myofibroblasts

Neural crest-derived cells are noted for the long lasting plasticity during lineage commitment process and the potential for transdifferentiation into other neural crest derivatives. Schwann cells in particular have been reported to transdifferentiate into melanocytes and myofibroblasts. Detailed studies of transdifferentiation at the molecular level have been hampered by difficulty in isolating sufficient quantity of primary cells or cellular materials. Here, we describe a robust in vitro system in which Schwannoma cells undergo an apparent transdifferentiation into myofibroblasts. Importantly, we induce the transdifferentiation by down-regulating a single transcription factor, Sox10, thereby identifying a key molecular event in this process. Myofibroblasts thus generated showed carbachol-stimulated contraction and calcium transients and express several established myofibroblast-specific genes. These results suggest that generating desired cell types based on "knock-down" of critical genes may be a viable strategy.

β-catenin relieves I-mfa-mediated suppression of LEF-1 in mammalian cells

We have previously shown that β-catenin interacts with a transcription suppressor I-mfa and, through this interaction, canonical Wnt signaling could relieve I-mfa-mediated suppression of myogenic regulatory factors (MRFs). In this study, we found that, based on this interaction, I-mfa-mediated suppression of the Wnt transcription factor T-cell factor/lymphoid enhancing factor-1 (TCF/LEF-1) can also be relieved. Our work showed that knocking down endogenous I-mfa expression mimics canonical Wnt treatment by inducing myogenesis and increasing Wnt reporter gene activity, endogenous Wnt target gene expression and expression of MRFs in P19 cells. More importantly, these I-mfa small interfering RNA (siRNA)-induced effects could be blocked by a dominant-negative mutant of LEF-1, confirming the involvement of the TCF/LEF-1 pathway. In addition, we found that β-catenin could compete with I-mfa for binding to LEF-1 and relieve the inhibitory effects of I-mfa in overexpression systems. Furthermore, canonical Wnt was able to reduce the levels of endogenous I-mfa associated with LEF-1, while increasing that of I-mfa associated with β-catenin. All of the evidence supports a conclusion that I-mfa can suppress myogenesis by inhibiting TCF/LEF-1 and that canonical Wnt signaling may relieve the suppression through elevating β-catenin levels, which in turn relieve I-mfa-mediated suppression.

Effects of the MyoG gene on the partial growth traits in pigs

Single-nucleotide polymorphisms of the MyoG gene were tested using PCR-SSCP in different pig breeds including Landrace, Large White, Duroc, Shanxi Black, and Mashen pigs, and the effects of the MyoG gene on the birth weight, the weaning weight, the 6-month body weight, and the backfat thickness were also analyzed. On the basis of the published sequence of the porcine MyoG gene, ten pairs of primers were designed, and one polymorphism was found in the PCR product amplified with In2-3 primers. The results showed that: (1) the Landrace, the Large White, and the Duroc breeds differ significantly (P < 0.05) in genotype distribution from the Shanxi Black and the Mashen breeds; (2) On the basis of the fixed effect model, significant differences were found in the birth weight and the backfat thickness among the different MyoG genotypes, whereas no significant differences existed in the weaning weight and the 6-month body weight; (3) Using least square analysis, it was seen that individuals of the BB genotype had significantly less (P < 0.01) birth weight than those of the AA and AB genotypes, with the order being AA>AB>BB; the pigs of the AA genotype had significantly lower (P < 0.01) backfat thickness than those of the AB and BB genotypes, with the order being AA<AB<BB. These results suggest that the genotype has significant effects on the individual birth weight and the backfat thickness, and that the selection of the A allele is favored with regard to the birth weight and the backfat thickness.

Effects of low-intensity pulsed ultrasound on the differentiation of C2C12 cells

Low-intensity pulsed ultrasound (LIPUS) is known to accelerate bone regeneration, but the precise cellular mechanism is still unclear. The purpose of this study was to determine the effect of LIPUS on the differentiation of pluripotent mesenchymal cell line C2C12. The cells were cultured in differentiation medium with or without the addition of LIPUS stimulation. The ultrasound signal consisted of 1.5 MHz at an intensity of 70 mW/cm2 for 20 min for all cultures. To verify the cell lineage after LIPUS stimulation, mRNA expression of cellular phenotype-specific markers characterizing osteoblasts (Runx2, Msx2, Dlx5, AJ18), chondroblasts (Sox9), myoblasts (MyoD), and adipocytes (C/EBP, PPARgamma) was studied using real-time polymerase chain reaction analysis. The protein expression of Runx2 and activated phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) and p38 mitogen-activated protein kinase (p38 MAPK) were performed using Western blotting. The mRNA expression of Runx2, Msx2, Dlx5, AJ18, and Sox9 was increased markedly by the LIPUS stimulation, whereas the expression of MyoD, C/EBP, and PPARgamma was drastically decreased. In the Western blot analysis, LIPUS stimulation increased Runx2 protein expression and phosphorylation of ERK1/2 and p38 MAPK. Our study demonstrated that LIPUS stimulation converts the differentiation pathway of C2C12 cells into the osteoblast and/or chondroblast lineage via activated phosphorylation of ERK1/2 and p38 MAPK.

Arabidopsis FAMA controls the final proliferation/differentiation switch during stomatal development

Coordination between cell proliferation and differentiation is essential to create organized and functional tissues. Arabidopsis thaliana stomata are created through a stereotyped series of symmetric and asymmetric cell divisions whose frequency and orientation are informed by cell-cell interactions. Receptor-like proteins and a mitogen-activated protein kinase kinase kinase were previously identified as negative regulators of stomatal development; here, we present the characterization of a bona fide positive regulator. FAMA is a putative basic helix-loop-helix transcription factor whose activity is required to promote differentiation of stomatal guard cells and to halt proliferative divisions in their immediate precursors. Ectopic FAMA expression is also sufficient to confer stomatal character. Physical and genetic interaction studies combined with functional characterization of FAMA domains suggest that stomatal development relies on regulatory complexes distinct from those used to specify other plant epidermal cells. FAMA behavior provides insights into the control of differentiation in cells produced through the activity of self-renewing populations.

Myogenin (Myf4) upregulation in trans-differentiating fibroblasts from a congenital myopathy with arrest of myogenesis and defects of myotube formation

Congenital myopathies often have an unclear aetiology. Here, we studied a novel case of a severe congenital myopathy with a failure of myotube formation. Polymerase chain reaction-based analysis was performed to characterize the expression patterns of the Desmin, p21, p57, and muscle regulatory factors (MRFs) MyoD, Myf4, Myf5 and Myf6 in differentiating skeletal muscle cells (SkMCs), normal human fibroblasts and patient-derived fibroblasts during trans-differentiation. The temporal and spatial pattern of MRFs was further characterized by immunocyto- and immunohistochemical stainings. In differentiating SkMCs, each MRF showed a characteristic expression pattern. Normal trans-differentiating fibroblasts formed myotubes and expressed all of the MRFs, which were detected. Interestingly, the patient's fibroblasts also showed some fusion events during trans-differentiation with a comparable expression profile for the MRFs, particularly, with increased expression of Myf4 and p21. Immunohistochemical analysis of normal and patient-derived skeletal musculature revealed that Myf4, which is downregulated during normal fetal development, was still present in patient-derived skeletal head muscle, which was also positive for Desmin and sarcomeric actin. The abnormal upregulation of Myf4 and p21 in the patient who suffered from a severe congenital myopathy suggests that the regulation of Myf4 and p21 gene expression during myogenesis might be of interest for further studies.

Effect of MYOG genotypes on growth rate and production traits in Hungarian large white pigs

The effect of the porcine myogenin (Myog) 3' polymorphism on birth weight, growth rate, carcass weight, lean weight, lean meat percentage and back-fat thickness has been investigated in Hungarian Large White pigs. MYOG genotypes were determined by PCR-RFLP assay. The obtained MYOGA frequency value was 0.6275. Due to the small number of BB piglets the effect of the MYOG genotypes on birth weight was not significant; however, an increasing tendency was observed from genotype AA to BB. The growth rate difference between MYOG genotypes was significant: BB animals showed the highest growth rate values during the fattening period. Since few results are available on the possible use of MYOG gene polymorphism in selection to improve carcass and growth traits, by this study the authors hope to provide additional data on this particular subject.

Adenoviral mediated MyoD gene transfer into fibroblasts: myogenic disease diagnosis

MyoD, a master regulatory gene for myogenesis, converts mesoderm derived cells to the skeletal muscle phenotype MyoD gene transfer into skin fibroblasts has been attempted in an effort to diagnose genetic muscle diseases. Although the gene transduction efficiency of adenoviral gene delivery systems is higher than that of various other systems, the rate of myo-conversion is insignificant. Since high adenovirus doses are cytotoxic and exogenous MyoD expression is insufficient for skin fibroblasts to re-differentiate into muscle cells, we constructed the novel adeno-MyoD vector, Ad.CAGMyoD using the recombinant CAG promoter. Even at a lower multiplicities of infection most skin fibroblasts infected with Ad.CAGMyoD could convert into myotubes without vector-induced cytotoxicity. The converted cells expressed muscle-specific desmin and full-length dystrophin, both of which were detected by Western blotting. Genetic and immunohistochemical analyses using skin fibroblasts and our vector system are reliable and useful for the clinical diagnosis of genetic muscle diseases.

Somite formation and expression ofMyoD,myogeninandmyosinin Atlantic halibut (Hippoglossus hippoglossusL.)embryos incubated at different temperatures: transient asymmetric expression ofMyoD

Genes encoding the myogenic regulating factors MyoD and myogenin and the structural muscle proteins myosin light chain 2 (MyLC2) and myosin heavy chain(MyHC) were isolated from juvenile Atlantic halibut (Hippoglossus hippoglossus L.). The impact of temperature on their temporal and spatial expression during somitogenesis were examined by incubating halibut embryos at 4, 6 and 8°C, and regularly sampling for whole-mount in situhybridisation and reverse transcription (RT)–PCR.

There were no significant effects of temperature on the onset of somitogenesis or number of somites at hatching. The rate of somite formation increased with increasing temperature, and the expression of MyoD, myogenin and MyHC followed the cranial-to-caudal somite formation. Hence, no significant effect of temperature on the spatial and temporal expression of the genes studied was found in relation to somite stage. MyoD, which has subsequently been shown to encode the MyoD2 isoform, displayed a novel bilaterally asymmetric expression pattern only in white muscle precursor cells during early halibut somitogenesis. The expression of myogenin resembled that previously described for other fish species, and preceded the MyHC expression by approximately five somites. Two MyLC2 cDNA sequences were for the first time described for a flatfish, probably representing embryonic (MyLC2a) and larval/juvenile(MyLC2b) isoforms.

Factors regulating muscle determination, differentiation and development have so far mostly been studied in vertebrates with external bilateral symmetry. The findings of the present study suggest that more such investigations of flatfish species could provide valuable information on how muscle-regulating mechanisms work in species with different anatomical,physiological and ecological traits.

How muscles recover from paresis and atrophy after intramuscular injection of botulinum toxin A: Study in juvenile rats

Botulinum toxin A (BoNT-A) is a potent biological toxin widely used for the management of skeletal muscle spasticity or dynamic joint contracture. Intramuscular injection of BoNT-A causes muscle denervation, paresis, and atrophy. This clinical effect of botulinum toxin A lasts 3 to 6 months, and injected muscle eventually regains muscle mass and recovers muscle function. The goal of the present study was to characterize the molecular and cellular mechanisms leading to neuromuscular junction (NMJ) regeneration and skeletal muscle functional recovery after BoNT-A injection. Fifty-six 1-month-old Sprague-Dawley rats were used. Botulinum toxin A was injected into the left gastrocnemius muscle at a dosage of 6 units/kg body weight. An equivalent volume of saline was injected into the right gastrocnemius muscle to serve as control. The gastrocnemius muscle samples were harvested from both hind limbs at 3 days, 7 days, 15 days, 30 days, 60 days, 90 days, 180 days, and 360 days after administration of toxin. In addition, the gastrocnemius muscles from 1-month-old rats with no injections were harvested to serve as uninjected control group. Muscle samples were processed and mRNA was extracted. Real-time polymerase chain reaction (PCR) and gene microarray technology were used to identify key molecules involved in NMJ stabilization and muscle functional recovery. More than 28,000 rat genes were analyzed and approximately 9000 genes are expressed in the rat gastrocnemius muscle. Seven days following BoNT-A injection, 105 genes were upregulated and 59 genes were downregulated. Key molecules involved in neuromuscular junction (NMJ) stabilization and muscle functional recovery were identified and their time course of gene expression following BoNT-A injection were characterized. This animal study demonstrates that following intramuscular injection of BoNT-A, there is a sequence of cellular events that eventually leads to NMJ stabilization, remodeling, and myogenesis and muscle functional recovery. This recovery process is divided into two stages (aneural and neural) and that the IGF-1 signaling pathway play a central role in the process.

Immunohistochemical study of cytoskeletal and extracellular matrix components in the notochord and notochordal sheath of amphioxus

A major cytoskeletal and extracellular matrix proteins of the amphioxus notochordal cells and sheath were detected by immunohistochemical techniques. The three-layered amphioxus notochordal sheath strongly expressed fish collagen type I in its outer and middle layers, while in the innermost layer expression did not occur. The amphioxus notochordal sheath was reactive to applied anti-human antibodies for intermediate filament proteins such as cytokeratins, desmin and vimentin, as well as to microtubule components (beta-tubulin), particularly in the area close to the epipharyngeal groove. Alpha-smooth muscle actin was expressed in some notochordal cells and in the area of the notochordal attachment to the sheath. Thus muscular nature of notochordal cells was shown by immunohistochemistry in tissue section. Our results confirm that genes encoding intermediate filament proteins, microtubules and microfilaments are highly conserved during evolution. Collagen type I was proven to be the key extracellular matrix protein that forms the amphioxus notochordal sheath.

NOV/CCN3 impairs muscle cell commitment and differentiation

NOV (nephroblastoma overexpressed) is a member of a family of proteins which encodes secreted matrix-associated proteins. NOV is expressed during development in dermomyotome and limb buds, but its functions are still poorly defined. In order to understand the role of NOV in myogenic differentiation, C2C12 cells overexpressing NOV (C2-NOV) were generated. These cells failed to engage into myogenic differentiation, whereas they retained the ability to differentiate into osteoblasts. In differentiating conditions, C2-NOV cells remained proliferative, failed to express differentiation markers and lost their ability to form myotubes. Inhibition of differentiation by NOV was also observed with human primary muscle cells. Further examination of C2-NOV cells revealed a strong downregulation of the myogenic determination genes MyoD and Myf5 and of IGF-II expression. MyoD forced expression in C2-NOV was sufficient to restore differentiation and IGF-II induction whereas 10(-6) M insulin treatment had no effects. NOV therefore acts upstream of MyoD and does not affect IGF-II induction and signaling. HES1, a target of Notch, previously proposed to mediate NOV action, was not implicated in the inhibition of differentiation. We propose that NOV is a specific cell fate regulator in the myogenic lineage, acting negatively on key myogenic genes thus controlling the transition from progenitor cells to myoblasts.

Extracellular signal-regulated kinase 1/2 mitogen-activated protein kinase pathway is involved in myostatin-regulated differentiation repression

The cytokines of transforming growth factor beta (TGF-beta) and its superfamily members are potent regulators of tumorigenesis and multiple cellular events. Myostatin is a member of TGF-beta superfamily and plays a negative role in the control of cell proliferation and differentiation. We now show that myostatin rapidly activated the extracellular signal-regulated kinase 1/2 (Erk1/2) cascade in C2C12 myoblasts. A more remarkable Erk1/2 activation stimulated by myostatin was observed in differentiating cells than proliferating cells. The results also showed that Ras was the upstream regulator and participated in myostatin-induced Erk1/2 activation because the expression of a dominant-negative Ras prevented myostatin-mediated inhibition of Erk1/2 activation and proliferation. Importantly, the myostatin-suppressed myotube fusion and differentiation marker gene expression were attenuated by blockade of Erk1/2 mitogen-activated protein kinase (MAPK) pathway through pretreatment with MAPK/Erk kinase 1 (MEK1) inhibitor PD98059, indicating that myostatin-stimulated activation of Erk1/2 negatively regulates myogenic differentiation. Activin receptor type IIb (ActRIIb) was previously suggested as the only type II membrane receptor triggering myostatin signaling. In this study, by using synthesized small interfering RNAs and dominant-negative ActRIIb, we show that myostatin failed to stimulate Erk1/2 phosphorylation and could not inhibit myoblast differentiation in ActRIIb-knockdown C2C12 cells, indicating that ActRIIb was required for myostatin-stimulated differentiation suppression. Altogether, our findings in this report provide the first evidence to reveal functional role of the Erk1/2 MAPK pathway in myostatin action as a negative regulator of muscle cell growth.

TWIST is expressed in human gliomas and promotes invasion

TWIST, a basic helix-loop-helix (bHLH) transcription factor that regulates mesodermal development, has been shown to promote tumor cell metastasis and to enhance survival in response to cytotoxic stress. Our analysis of rat C6 glioma cell-derived cDNA revealed TWIST expression, suggesting that the gene may play a role in the genesis and physiology of primary brain tumors. To further delineate a possible oncogenic role for TWIST in the central nervous system (CNS), we analyzed TWIST expression in human gliomas and normal brain by using reverse transcription polymerase chain reaction, Northern blot analysis, in situ hybridization, and immunohistochemistry. TWIST expression was detected in the large majority of human glioma-derived cell lines and human gliomas examined. Levels of TWIST mRNA were associated with the highest grade gliomas, and increased TWIST expression accompanied transition from low grade to high grade in vivo, suggesting a role for TWIST in promoting malignant progression. In accord, elevated TWIST mRNA abundance preceded the spontaneous malignant transformation of cultured mouse astrocytes hemizygous for p53. Overexpression of TWIST protein in a human glioma cell line significantly enhanced tumor cell invasion, a hallmark of high-grade gliomas. These findings support roles for TWIST both in early glial tumorigenesis and subsequent malignant progression. TWIST was also expressed in embryonic and fetal human brain, and in neurons, but not glia, of mature brain, indicating that, in gliomas, TWIST may promote the functions also critical for CNS development or normal neuronal physiology.

MyoD induces myogenic differentiation through cooperation of its NH2- and COOH-terminal regions

MyoD and Myf5 are basic helix-loop-helix transcription factors that play key but redundant roles in specifying myogenic progenitors during embryogenesis. However, there are functional differences between the two transcription factors that impact myoblast proliferation and differentiation. Target gene activation could be one such difference. We have used microarray and polymerase chain reaction approaches to measure the induction of muscle gene expression by MyoD and Myf5 in an in vitro model. In proliferating cells, MyoD and Myf5 function very similarly to activate the expression of likely growth phase target genes such as L-myc, m-cadherin, Mcpt8, Runx1, Spp1, Six1, IGFBP5, and Chrnbeta1. MyoD, however, is strikingly more effective than Myf5 at inducing differentiation-phase target genes. This distinction between MyoD and Myf5 results from a novel and unanticipated cooperation between the MyoD NH2- and COOH-terminal regions. Together, these results support the notion that Myf5 functions toward myoblast proliferation, whereas MyoD prepares myoblasts for efficient differentiation.

Comparison of prenatal muscle tissue expression profiles of two pig breeds differing in muscle characteristics1

The objective of this study was to compare purebred Duroc and Pietrain prenatal muscle tissue transcriptome expression levels at different stages of prenatal development to gain insight into the differences in muscle tissue development in these pig breeds. Commercial western pig breeds have been selected for muscle growth for the past 2 decades. Pig breeds differ for their muscle phenotypes (i.e., myofiber numbers and myofiber types). Duroc and Pietrain pig breeds are extremes; Duroc pigs have redder muscle fiber types with more intramuscular fat, and Pietrain pigs have faster-growing and whiter muscle fiber types. Pietrain pigs are more muscular than Duroc pigs, whereas Duroc pigs are fatter than Pietrain pigs. The genomic background underlying these breed-specific differences is poorly known. Myogenesis is a complex exclusive prenatal process involving proliferation and differentiation (i.e., fusion) of precursor cells called myoblasts. We investigated the difference in the prenatal muscle-specific transcriptome profiles of Duroc and Pietrain pigs using microarray technology. The microarray contained more than 500 genes affecting myogenesis, energy metabolism, muscle structural genes, and other genes from a porcine muscle cDNA library. The results indicated that the expression of the myogenesis-related genes was greater in early Duroc embryos than in early Pietrain embryos (14 to 49 d of gestation), whereas the opposite was found in late embryos (63 to 91 d of gestation). These findings suggest that the myogenesis process is more intense in early Duroc embryos than in Pietrain embryos but that myogenesis is more intense in late Pietrain fetuses than in Duroc fetuses. Transcriptomes of muscle structural genes followed that pattern. The energy metabolism genes were expressed at a higher level in prenatal Pietrain pigs than in prenatal Duroc pigs, except for d 35, when the opposite situation was found. Fatty acid metabolism genes were expressed at a higher level in early (14 to 49 d of gestation) Duroc embryos than in Pietrain embryos. Better understanding of the genomic regulation of tissue formation leads to improved knowledge of the genome under selection and may lead to directed breed-specific changes in the future.

Rhabdomyosarcoma: molecular diagnostics of patients classified by morphology and immunohistochemistry with emphasis on bone marrow and purged peripheral blood progenitor cells involvement

Two histologically distinct subtypes of rhabdomyosarcomas (RMS), embryonal and alveolar, are different in many aspects, such as age distribution, primary site, and clinical outcome. We analyzed a group of 30 patients with RMS. The aim was to broaden the spectrum of diagnostic tools in evaluating the primary tumors, their recurrences and/or metastases, and to extend the diagnostic boundary to bone marrow and purged peripheral progenitor blood cell samples. We have performed the RT-PCR assay to analyze RMS for the presence of expression of MyoD1 gene and for the presence of chimeric transcripts PAX3/FKHR or PAX7/FKHR. MyoD1 gene expression was found in all 30 patients in samples from primary tumors. The chimeric transcripts PAX/FKHR were identified in 13 of 15 patients with alveolar RMS. Furthermore, the fusion transcript PAX7/FKHR was identified in 2 of 15 patients with RMS classified as embryonal by histology. Bone marrow samples (12) and peripheral blood progenitor cell specimens (13) in ten patients were examined by RT-PCR. We were able to identify 7 patients with bone marrow involvement and/or with contamination of peripheral blood progenitor cells by the tumor cells. We demonstrate that employing molecular diagnostics has an impact on staging, therapy monitoring and recognition of malignant cells at the tumor resection margins.

A novel role for lbx1 in Xenopus hypaxial myogenesis

We have examined lbx1 expression in early X. laevis tadpoles. In contrast to amniotes, lbx1 is expressed in all of the myoblasts that contribute to the body wall musculature, as well as in a group of cells that migrate into the head. Despite this different expression, the function of lbx1 appears to be conserved. Morpholino (MO) knockdown of lbx1 causes a specific reduction of body wall muscles and hypoglossal muscles originating from the somites. Although myoblast migratory defects are observed in antisense MO injected tadpoles targeting lbx1, this results at least in part from a lack of myoblast proliferation in the hypaxial muscle domain. Conversely, overexpression of lbx1 mRNA results in enlarged somites, an increase in cell proliferation, but a lack of differentiated muscle. The control of cell proliferation is linked to a strong downregulation of myoD expression in gain-of-function experiments. Co-injection of myoD mRNA with lbx1 mRNA eliminates the overproliferation phenotype observed when lbx1 is injected alone. The results indicate that a primary function of lbx1 in hypaxial muscle development is to repress myoD, allowing myoblasts to proliferate before the eventual onset of terminal differentiation.

Fusion and differentiation of murine C2C12 skeletal muscle cells that express Trichinella spiralis p43 protein

The ability of a 43 kDa stichocyte protein from Trichinella spiralis (Tsp43) to interfere with mammalian skeletal muscle gene expression was investigated. A MYC-tagged Tsp43 construct was expressed as a recombinant protein in C2C12 myoblasts. Transfection with low amounts of expression plasmid was required for successful expression of the protein. This construct had apparent toxic effects on transfected myoblasts and ectopic green fluorescent protein expression was suppressed in myoblasts co-transfected with the Tsp43 construct. These effects may result from similarities of Tsp43 to DNase II. Use of the general DNase inhibitor aurintricarboxylic acid (ATA) enhanced expression of MYC-Tsp43 in transfected muscle cells. Myoblasts transfected with Tsp43 did not fuse well when cultured under differentiation conditions without ATA. In contrast, transfected myoblasts transiently cultured with ATA underwent fusion and differentiation. Under short-term differentiation conditions without ATA, unfused myoblasts nevertheless expressed both MYC-Tsp43 and myosin heavy chain. Collectively, the results support that Tsp43 has a role in the T. spiralis life cycle that is distinct from repressing muscle gene expression during the muscle phase of infection. While the function of Tsp43 as a DNase is under debate, the effects of ATA on transfected muscle cells were consistent with this possibility.