Cellular and molecular diversity in skeletal muscle development: news from in vitro and in vivo

miR-100-5p Regulates Skeletal Muscle Myogenesis through the Trib2/mTOR/S6K Signaling Pathway

MicroRNAs (miRNAs) are endogenous small non-coding RNAs that play crucial regulatory roles in many biological processes, including the growth and development of skeletal muscle. miRNA-100-5p is often associated with tumor cell proliferation and migration. This study aimed to uncover the regulatory mechanism of miRNA-100-5p in myogenesis. In our study, we found that the miRNA-100-5p expression level was significantly higher in muscle tissue than in other tissues in pigs. Functionally, this study shows that miR-100-5p overexpression significantly promotes the proliferation and inhibits the differentiation of C2C12 myoblasts, whereas miR-100-5p inhibition results in the opposite effects. Bioinformatic analysis predicted that Trib2 has potential binding sites for miR-100-5p at the 3'UTR region. A dual-luciferase assay, qRT-qPCR, and Western blot confirmed that Trib2 is a target gene of miR-100-5p. We further explored the function of Trib2 in myogenesis and found that Trib2 knockdown markedly facilitated proliferation but suppressed the differentiation of C2C12 myoblasts, which is contrary to the effects of miR-100-5p. In addition, co-transfection experiments demonstrated that Trib2 knockdown could attenuate the effects of miR-100-5p inhibition on C2C12 myoblasts differentiation. In terms of the molecular mechanism, miR-100-5p suppressed C2C12 myoblasts differentiation by inactivating the mTOR/S6K signaling pathway. Taken together, our study results indicate that miR-100-5p regulates skeletal muscle myogenesis through the Trib2/mTOR/S6K signaling pathway.

Generation of human muscle fibers and satellite-like cells from human pluripotent stem cells in vitro

Progress toward finding a cure for muscle diseases has been slow because of the absence of relevant cellular models and the lack of a reliable source of muscle progenitors for biomedical investigation. Here we report an optimized serum-free differentiation protocol to efficiently produce striated, millimeter-long muscle fibers together with satellite-like cells from human pluripotent stem cells (hPSCs) in vitro. By mimicking key signaling events leading to muscle formation in the embryo, in particular the dual modulation of Wnt and bone morphogenetic protein (BMP) pathway signaling, this directed differentiation protocol avoids the requirement for genetic modifications or cell sorting. Robust myogenesis can be achieved in vitro within 1 month by personnel experienced in hPSC culture. The differentiating culture can be subcultured to produce large amounts of myogenic progenitors amenable to numerous downstream applications. Beyond the study of myogenesis, this differentiation method offers an attractive platform for the development of relevant in vitro models of muscle dystrophies and drug screening strategies, as well as providing a source of cells for tissue engineering and cell therapy approaches.

Rapid and simple method for in vivo ex utero development of mouse embryo explants

The in utero development of mammals drastically reduces the accessibility of the mammalian embryo and therefore limits the range of experimental manipulation that can be done to study functions of genes or signaling pathways during embryo development. Over the past decades, tissue and organ-like culture methods have been developed with the intention of reproducing in vivo situations. Developing accessible and simple techniques to study and manipulate embryos is an everlasting challenge. Herein, we describe a reliable and quick technique to culture mid-gestation explanted mouse embryos on top of a floating membrane filter in a defined medium. Viability of the cultured tissues was assessed by apoptosis and proliferation analysis showing that cell proliferation is normal and there is only a slight increase in apoptosis after 12h of culture compared to embryos developing in utero. Moreover, differentiation and morphogenesis proceed normally as assessed by 3D imaging of the transformation of the myotome into deep back muscles. Not only does muscle cell differentiation occur as expected, but so do extracellular matrix organization and the characteristic splitting of the myotome into the three epaxial muscle groups. Our culture method allows for the culture and manipulation of mammalian embryo explants in a very efficient way, and it permits the manipulation of in vivo developmental events in a controlled environment. Explants grown under these ex utero conditions simulate real developmental events that occur in utero.

Decorin-induced proliferation of avian myoblasts involves the myostatin/Smad signaling pathway

Decorin, a small leucine-rich proteoglycan as a component of the extracellular matrix, plays an important role in the skeletal muscle development. It has been reported that decorin promoted proliferation and differentiation of muscle cells by restraining myostatin activity in rodents. However, the effects and mechanisms of decorin on avian myoblast proliferation are not understood clearly. Thus, in our research, decorin overexpressing and knocking-down quail myoblast-7 (QM7) myoblasts were established to explore the effects of decorin on avian myoblast proliferation by flow cytometry. The results showed that overexpression of decorin enhanced the proliferation of QM7 myoblasts, which was accompanied by the upregulation of follistatin and primary muscle regulatory factors (i.e., myogenic factor 5, myogenic factor 1, myogenin), and downregulation of myostatin expression, as well as the decreased phosphorylation level of SMAD family member 3 (Smad3). In line with expectations, decorin RNAi displayed an opposite effect on the proliferation and gene expression pattern of QM7 cells. In conclusion, our in vitro studies suggested the decorin-mediated myostatin/Smad signaling pathway might be involved in the regulation of avian myoblast proliferation.

Biofunctional hydrogels for skeletal muscle constructs

Hydrogel scaffolds encapsulating C2C12 mouse skeletal muscle cells have been engineered as in vitro constructs towards regenerative medicine therapies for the enhancement and inducement of functional skeletal muscle formation. Previous work has largely involved two-dimensional (2D) muscle strips, naturally occurring hydrogels and incomplete examination of the effects of the scaffold and/or biological functionalization on myogenic differentiation in a controllable manner. The goal of this study was to identify key properties in functionalized poly(ethylene glycol) (PEG)-maleimide (MAL) synthetic hydrogels that promote cell attachment, proliferation and differentiation for the formation of multinucleated myotubes and functional skeletal muscle tissue constructs. Significant differences in myoblast viability were observed as a function of cell seeding density, polymer weight percentage and bioadhesive ligands. The identified optimized conditions for cell survival, required for myotube development, were carried over for differentiation assays. PEG hydrogels (5% weight/volume), functionalized with 2.0 mm RGD adhesive peptide and crosslinked with protease-cleavable peptides, incubated for 3 days before supplementation with 2% horse serum, significantly increased expression of differentiated skeletal muscle markers by 50%; 17% more multinucleated cells and a 40% increase in the number of nuclei/differentiated cell compared to other conditions. Functionality of cell-laden hydrogels was demonstrated by a 20% decrease in the extruded length of the hydrogel when stimulated with a contractile agent, compared to 7% for a saline control. This study provided strategies to engineer a three-dimensional (3D) microenvironment, using synthetic hydrogels to promote the development of differentiated muscle tissue from skeletal muscle progenitor cells to form contractile units. Copyright © 2014 John Wiley & Sons, Ltd.

TGF-β1 enhances contractility in engineered skeletal muscle

Scaffoldless engineered 3D skeletal muscle tissue created from satellite cells offers the potential to replace muscle tissue that is lost due to severe trauma or disease. Transforming growth factor-beta 1 (TGF-β1) plays a vital role in mediating migration and differentiation of satellite cells during the early stages of muscle development. Additionally, TGF-β1 promotes collagen type I synthesis in the extracellular matrix (ECM) of skeletal muscle, which provides a passive elastic substrate to support myofibres and facilitate the transmission of force. To determine the role of TGF-β1 in skeletal muscle construct formation and contractile function in vitro, we created tissue-engineered 3D skeletal muscle constructs with varying levels of recombinant TGF-β1 added to the cell culture medium. Prior to the addition of TGF-β1, the primary cell population was composed of 75% Pax7-positive cells. The peak force for twitch, tetanus and spontaneous force were significantly increased in the presence of 2.0 ng/ml TGF-β1 when compared to 0, 0.5 and 1.0 ng/ml TGF-β1. Visualization of the cellular structure with H&E and with immunofluorescence staining for sarcomeric myosin heavy chains and collagen type I showed denser regions of better organized myofibres in the presence of 2.0 ng/ml TGF-β1 versus 0, 0.5 and 1.0 ng/ml. The addition of 2.0 ng/ml TGF-β1 to the culture medium of engineered 3D skeletal muscle constructs enhanced contractility and extracellular matrix organization.

Fiber phenotype and coenzyme Q₁₀ content in Turkey skeletal muscles

Phenotypical differences between muscle fibers are associated with a source of cellular energy. Coenzyme Q(10) (CoQ(10)) is a major component of the mitochondrial oxidative phosphorylation process, and it significantly contributes to the production of cellular energy in the form of ATP. The objective of this study was to determine the relationship between whole-tissue CoQ(10) content, mitochondrial CoQ(10) content, mitochondrial protein, and muscle phenotype in turkeys. Four specialized muscles (anterior latissimus dorsi, ALD; posterior latissimus dorsi, PLD; pectoralis major, PM, and biceps femoris, BF) were evaluated in 9- and 20-week-old turkey toms. The amount of muscle mitochondrial protein was determined using the Bradford assay and CoQ(10) content was measured using HPLC-UV. The amount of mitochondrial protein relative to total protein was significantly lower (p < 0.05) at 9 compared to 20 weeks of age. All ALD fibers stained positive for anti-slow (S35) MyHC antibody. The PLD and PM muscle fibers revealed no staining for slow myosin heavy chain (S35 MyHC), whereas half of BF muscle fibers exhibited staining for S35 MyHC at 9 weeks and 70% at 20 weeks of age. The succinate dehydrogenase (SDH) staining data revealed that SDH significantly increases (p < 0.05) in ALD and BF muscles and significantly decreases (p < 0.05) in PLD and PM muscles with age. The study reveals age-related decreases in mitochondrial CoQ(10) content in muscles with fast/glycolytic profile, and demonstrates that muscles with a slow/oxidative phenotypic profile contain a higher proportion of CoQ(10) than muscles with a fast/glycolytic phenotypic profile.

Consequences of birth weight for postnatal growth performance and carcass quality in pigs as related to myogenesis

In polytocous species such as the pig there is intralitter variation in birth weight and skeletal muscle fiber number. It is commonly recognized that low birth weight in piglets correlates with decreased survival and lower postnatal growth rates. In the majority of low birth weight piglets low numbers of muscle fibers differentiate during prenatal myogenesis, for genetic or maternal reasons, and those low birth weight piglets with reduced fiber numbers are unable to exhibit postnatal catch-up growth. Pigs of low birth weight show the lowest growth performance and the lowest lean percentage at slaughter. In addition, they tend to develop extremely large muscle fibers (giant fibers) and poor meat quality, which results in part from the inverse correlation between fiber number and fiber size. Prenatal growth and myogenesis are under the control of various genetic and environmental factors, which can be targeted for growth manipulation. Genetic selection is considered a suitable tool to improve fetal growth and myogenesis. Prenatal development is mainly dependent on a close interrelation between nutritional supply/use and regulation by hormones and growth factors. In particular, the maternal somatotropic axis plays a significant role in the control of myogenesis. Thus, treatment of sows with GH until mid-gestation was able to increase birth weight and the number of muscle fibers in the small littermates of the progeny that are disadvantaged by insufficient nutrient supply. Growth hormone treatment was associated with increased nutrient availability to the embryos and changes in regulatory proteins of the GH-IGF axis. Interactions between maternal nutrition and the somatotropic axis in determining prenatal growth and myogenesis are worthy of further investigation.

A combinatorial role for NFAT5 in both myoblast migration and differentiation during skeletal muscle myogenesis

Skeletal muscle regeneration depends on myoblast migration, differentiation and myofiber formation. Isoforms of the nuclear factor of activated T cells (NFAT) family of transcription factors display nonredundant roles in skeletal muscle. NFAT5, a new isoform of NFAT, displays many differences from NFATc1-c4. Here, we examine the role of NFAT5 in myogenesis. NFAT5+/- mice displayed a defect in muscle regeneration with fewer myofibers formed at early times after injury. NFAT5 has a muscle-intrinsic function because inhibition of NFAT5 transcriptional activity caused both a migratory and differentiation defect in cultured myoblasts. We identified Cyr61 as a target of NFAT5 signaling in skeletal muscle cells. Addition of Cyr61 to cells expressing inhibitory forms of NFAT5 rescued the migratory phenotype. These results demonstrate a role for NFAT5 in skeletal muscle cell migration and differentiation. Furthermore, as cell-cell interactions are crucial for myoblast differentiation, these data suggest that myoblast migration and differentiation are coupled and that NFAT5 is a key regulator.

Altered primary myogenesis in NFATC3(-/-) mice leads to decreased muscle size in the adult

Signal transduction pathways involving calcineurin and its downstream effector NFAT have been implicated in regulating myogenesis. Several isoforms of NFAT exist that may differentially contribute to regulating skeletal muscle physiology. The purpose of this study was to determine the role of the NFATC3 isoform in skeletal muscle development. Adult mice lacking NFATC3 have reduced muscle mass compared to control mice. The smaller size of the muscles is not due to atrophy or blunted myofiber growth, but rather to a reduced number of myofibers. This reduction in myofiber number is not limited to a specific fiber type nor are the proportions of fiber types altered. The lower fiber number found in the adult NFATC3(-/-) mice is a consequence of impaired muscle development during embryogenesis. Immunohistochemical studies of E15 EDL muscles indicate that the total number of primary myofibers is decreased in NFATC3(-/-) embryos. At E17.5 no further decrease in primary myofiber number occurs; the size and organization of the myofibers are unaltered, and secondary myogenesis proceeds normally, suggesting a role for NFATC3 during early events in primary myogenesis. These results suggest a heretofore unknown role for the transcription factor NFAT in early skeletal muscle development.

Evolutionary significance of myosin heavy chain heterogeneity in birds

This article reviews the complexity, expression, genetics, regulation, function, and evolution of the avian myosin heavy chain (MyHC). The majority of pertinent studies thus far published have focussed on domestic chicken and, to a much lesser extent, Japanese quail. Where possible, information available about wild species has also been incorporated into this review. While studies of additional species might modify current interpretations, existing data suggest that some fundamental properties of myosin proteins and genes in birds are unique among higher vertebrates. We compare the characteristics of myosins in birds to those of mammals, and discuss potential molecular mechanisms and evolutionary forces that may explain how avian MyHCs acquired these properties.

Myocyte enhancer factor 2C and myogenin up-regulate each other's expression and induce the development of skeletal muscle in P19 cells

Two families of transcription factors, myogenic regulatory factors (MRFs) and myocyte enhancer factor 2 (MEF2), function synergistically to regulate myogenesis. In addition to activating structural muscle-specific genes, MRFs and MEF2 activate each other's expression. The MRF, myogenin, can activate MEF2 DNA binding activity when transfected into fibroblasts and, in turn, the myogenin promoter contains essential MEF2 DNA binding elements. To determine which MEF2 is involved in this regulation, P19 cells stably expressing MyoD and myogenin were compared for their ability to activate the expression of MEF2 family members. There was very little cross-activation of MyoD expression by myogenin and vice versa. Myogenin expression, and not MyoD, was found to up-regulate MEF2C expression. MEF2A, -B, and -D expression levels were not up-regulated by overexpression of either MyoD or myogenin. To examine whether MEF2C can differentially regulate MyoD or myogenin expression, P19 cell lines overexpressing MEF2C were analyzed. MEF2C induced myogenesis in P19 cells and up-regulated the expression of myogenin with 25-fold greater efficiency than that of MyoD. Therefore, myogenin and MEF2C participate in a regulatory loop in differentiating stem cells. This positive regulation does not extend to MyoD or the other MEF2 family members. Consequently, MEF2C appears to play a specific role in early events of myogenesis.

Myotube heterogeneity in developing chick craniofacial skeletal muscles

Avian skeletal muscles consist of myotubes that can be categorized according to contraction and fatigue properties, which are based largely on the types of myosins and metabolic enzymes present in the cells. Most mature muscles in the head are mixed, but they display a variety of ratios and distributions of fast and slow muscle cells. We examine the development of all head muscles in chick and quail embryos, using immunohistochemical assays that distinguish between fast and slow myosin heavy chain (MyHC) isoforms. Some muscles exhibit the mature spatial organization from the onset of primary myotube differentiation (e.g., jaw adductor complex). Many other muscles undergo substantial transformation during the transition from primary to secondary myogenesis, becoming mixed after having started as exclusively slow (e.g., oculorotatory, neck muscles) or fast (e.g., mandibular depressor) myotube populations. A few muscles are comprised exclusively of fast myotubes throughout their development and in the adult (e.g., the quail quadratus and pyramidalis muscles, chick stylohyoideus muscles). Most developing quail and chick head muscles exhibit identical fiber type composition; exceptions include the genioglossal (chick: initially slow, quail: mixed), quadratus and pyramidalis (chick: mixed, quail: fast), and stylohyoid (chick: fast, quail: mixed). The great diversity of spatial and temporal scenarios during myogenesis of head muscles exceeds that observed in the limbs and trunk, and these observations, coupled with the results of precursor mapping studies, make it unlikely that a lineage based model, in which individual myoblasts are restricted to fast or slow fates, is in operation. More likely, spatiotemporal patterning of muscle fiber types is coupled with the interactions that direct the movements of muscle precursors and subsequent segregation of individual muscles from common myogenic condensations. In the head, most of these events are facilitated by connective tissue precursors derived from the neural crest. Whether these influences act upon uncommitted, or biased but not restricted, myogenic mesenchymal cells remains to be tested.

Germ-line regulation of the Caenorhabditis elegans sex-determining gene tra-2

The Caenorhabditis elegans sex-determining gene tra-2 promotes female development of the XX hermaphrodite soma and germ line. We previously showed that a 4.7-kb tra-2 mRNA, which encodes the membrane protein TRA-2A, provides the primary feminizing activity of the tra-2 locus. This paper focuses on the germ-line activity and regulation of tra-2. First, we characterize a 1.8-kb tra-2 mRNA, which is hermaphrodite-specific and germ-line-dependent. This mRNA encodes TRA-2B, a protein identical to a predicted intracellular domain of TRA-2A. We show that the 1.8-kb mRNA is oocyte-specific, suggesting that it is involved in germ-line or embryonic sex determination. Second, we identify a tra-2 maternal effect on brood size that may be associated with the 1.8-kb mRNA. Third, we investigate seven dominant tra-2(mx) (for mixed character) mutations that sexually transform hermaphrodites to females by eliminating hermaphrodite spermatogenesis. Each of the tra-2(mx) mutants possesses a nonconserved missense change in a 22-amino-acid region common to both TRA-2A and TRA-2B, called the MX region. We propose that the MX region mediates a posttranslational regulation of tra-2 essential for the onset of hermaphrodite spermatogenesis. Finally, we discuss aspects of tra-2 function and regulation that are specific to the unusual control of cell fate in the hermaphrodite germ line.

A variety of cytokines and immunologically relevant surface molecules are expressed by normal human skeletal muscle cells under proinflammatory stimuli

Muscle is an attractive target for gene therapy and for immunization with DNA vaccines and is also the target of immunological injury in myositis. It is important therefore to understand the immunologic capabilities of muscle cells themselves. In this study, we show that proinflammatory stimuli induce the expression of other cytokines such as IL-6, transforming growth factor-beta (TGF-beta), and granulocyte-macrophage colony-stimulating factor (GM-CSF) by muscle cells themselves, as well as the up-regulation of human leucocyte antigen (HLA) class I, class II and intercellular adhesion molecule-1 (ICAM-1). Thus, muscle cells have an inherent ability to express and respond to a variety of cytokines and chemokines. The levels of HLA class I, class II and ICAM-1 in inflamed muscle may be affected by the secreted products of the stimulation.

Proximal sequences of the aldolase A fast muscle-specific promoter direct nerve- and activity-dependent expression in transgenic mice

Muscle activity is known to modulate the muscle fiber phenotype. Changes in muscle activity (normal or experimentally induced) lead to modifications of the expression status of several muscle-specific genes. However, the transcription regulatory elements involved in the adaptative response are mainly unknown. The aldolase A muscle-specific promoter, pM, is expressed in adult fast twitch muscle with a preferential expression in fast glycolytic-2B fibers. Its activity is induced during postnatal muscle maturation, suggesting a role of nerve and/or muscle activity. Indeed, denervation of gastrocnemius in newborn mice prevented the activation of the promoter in this muscle, despite the nerve-independent formation of 2B fibers. Although the nerve was necessary for pM onset during development, denervating the gastrocnemius in adults had only mild effects on pM activity. By contrast, a transgene including the pM proximal regulatory sequences that are sufficient to reproduce the 2B fiber-specific expression of the endogenous promoter was shown to be highly sensitive to both neonatal and adult denervation. Transgenes containing muscle-specific pM proximal promoter elements were used to delineate the regulatory elements involved in this response to innervation and changes in the contractile activity pattern. Nerve- and activity-dependent elements could be localized in the 130-base pair-long proximal promoter region of the human aldolase A gene.

Expression of lactate dehydrogenase, myosin heavy chain and myogenic regulatory factor genes in rabbit embryonic muscle cell cultures

The expression of myogenic regulatory factors (MRFs), lactate dehydrogenase (LDH) and myosin heavy chains (MyHC), as markers of myogenesis, metabolism and contractility respectively, were investigated during differentiation of rabbit embryonic muscle cells in primary culture. Myf5, MyoD and myogenin mRNAs were abundantly expressed at day 1 of culture. The expression of Myf5 and MyoD mRNA transcripts decreased sharply as myoblasts fused and differentiated into myotubes, whilst myogenin mRNA was maintained throughout the duration of the culture. In contrast, MRF4 mRNA was weakly expressed on day 1 of culture, its expression increased slightly as myoblasts fused and reached a maximum level in 7-day-old cultures containing striated myofibres. The specific activity of LDH increased linearly during myoblast proliferation and fusion. In 7-day-old cultures, LDH-M mRNA (dominant in glycolytic muscles) and LDH-H mRNA (predominant in perinatal and oxidative muscles) represented 38% and 62% of total LDH mRNA respectively. At this stage, immunocytochemical staining with perinatal and adult-type MyHC antibodies showed that embryonic and perinatal MyHC isoforms were expressed in all myotubes, while few of them were stained by type I MyHC antibody. However, none of them expressed adult type II MyHC. The latter results were further supported by RT-PCR analysis of adult-type MyHC mRNA which showed that only the type I MyHC mRNA transcript was expressed. These data were in agreement with those reported in vivo on perinatal rabbit muscles. They differed from those obtained on cultured satellite cells isolated from adult rabbit fast-twitch or slow-twitch muscles which did not express embryonic MyHC, and instead expressed fast- or slow-type MyHC according to their muscle origin. Taken together, these results further suggest that myogenic mononucleated cells express different properties in vitro according to their developmental origin as well as properties related to those of the muscles from which they were isolated.

Studies of the dynamics of skeletal muscle regeneration: the mouse came back!

Regeneration of skeletal muscle tissue includes sequential processes of muscle cell proliferation and commitment, cell fusion, muscle fiber differentiation, and communication between cells of various tissues of origin. Central to the process is the myosatellite cell, a quiescent precursor cell located between the mature muscle fiber and its sheath of external lamina. To form new fibers in a muscle damaged by disease or direct injury, satellite cells must be activated, proliferate, and subsequently fuse into an elongated multinucleated cell. Current investigations in the field concern modulation of the effectiveness of skeletal muscle regeneration, the regeneration-specific role of myogenic regulatory gene expression distinct from expression during development, the impact of growth and scatter factors and their respective receptors in amplifying precursor numbers, and promoting fusion and maturation of new fibers and the ultimate clinical therapeutic applications of such information to alleviate disease. One approach to muscle regeneration integrates observations of muscle gene expression, proliferation, myoblast fusion, and fiber growth in vivo with parallel studies of cell cycling behaviour, endocrine perturbation, and potential biochemical markers of steps in the disease-repair process detected by magnetic resonance spectroscopy techniques. Experiments on muscles from limb, diaphragm, and heart of the mdx dystrophic mouse, made to parallel clinical trials on human Duchenne muscular dystrophy, help to elucidate mechanisms underlying the positive treatment effects of the glucocorticoid drug deflazacort. This review illustrates an effective combination of in vivo and in vitro experiments to integrate the distinctive complexities of post-natal myogenesis in regeneration of skeletal muscle tissue.Key words: satellite cell, cell cycling, HGF/SF, c-met receptor, MyoD, myogenin, magnetic resonance spectroscopy, mdx dystrophic mouse, deflazacort.

Changes in mRNA levels of the sarcoplasmic/endoplasmic-reticulum Ca(2+)-ATPase isoforms in the rat soleus muscle regenerating from notexin-induced necrosis

The relative mRNA levels corresponding to the different sarcoplasmic/endoplasmic-reticulum Ca(2+)-ATPase isoforms (SERCA1a, SERCA1b, SERCA2a, SERCA2b and SERCA3) were measured by reverse transcriptase-PCR in rat soleus muscles regenerating after notexin-induced necrosis. The succession of appearance of the different types of SERCA mRNA species in regenerating muscle largely recapitulates those observed during normal ontogenesis. The mRNA levels of the muscle-specific isoforms SERCA1a and SERCA2a became very low on the first and third days after injection of the snake venom. It was only on the fifth day of regeneration that the mRNA of the neonatal variant of the fast-twitch skeletal SERCA1b isoform began to rise, well before the other SERCA transcripts. At 7 and 10 days, i.e. at a time when the new myofibres normally become reinnervated, the mRNA level of SERCA1a and SERCA2a increased markedly, but the fast-twitch skeletal SERCA1a isoform was still the most prominent. On day 21, in the advanced stage of regeneration, a switch in the relative expression levels of SERCA1a and SERCA2a mRNA was observed and the ratio of both isoforms became similar to that found in the normal soleus muscles. This was followed by a decline in the level of all SERCA mRNA species, so that on day 28 the levels of the sarcoplasmic/endoplasmatic-reticulum Ca(2+)-pump RNAs was again lower but their ratio remained similar to that of the untreated control soleus.

E-box sites and a proximal regulatory region of the muscle creatine kinase gene differentially regulate expression in diverse skeletal muscles and cardiac muscle of transgenic mice

Previous analysis of the muscle creatine kinase (MCK) gene indicated that control elements required for transcription in adult mouse muscle differed from those required in cell culture, suggesting that distinct modes of muscle gene regulation occur in vivo. To examine this further, we measured the activity of MCK transgenes containing E-box and promoter deletions in a variety of striated muscles. Simultaneous mutation of three E boxes in the 1,256-bp MCK 5' region, which abolished transcription in muscle cultures, had strikingly different effects in mice. The mutations abolished transgene expression in cardiac and tongue muscle and caused a reduction in expression in the soleus muscle (a muscle with many slow fibers) but did not affect expression in predominantly fast muscles: quadriceps, abdominals, and extensor digitorum longus. Other regulatory sequences with muscle-type-specific activities were found within the 358-bp 5'-flanking region. This proximal region conferred relatively strong expression in limb and abdominal skeletal muscles but was inactive in cardiac and tongue muscles. However, when the 206-bp 5' enhancer was ligated to the 358-bp region, high levels of tissue-specific expression were restored in all muscle types. These results indicate that E boxes and a proximal regulatory region are differentially required for maximal MCK transgene expression in different striated muscles. The overall results also imply that within skeletal muscles, the steady-state expression of the MCK gene and possibly other muscle genes depends on transcriptional mechanisms that differ between fast and slow fibers as well as between the anatomical and physiological attributes of each specific muscle.

Extrinsic regulation of domestic animal-derived satellite cells

Satellite cells are the postnatal myogenic cells, as they provide myonuclei to support skeletal muscle hypertrophy and are principal cells responsible for myofiber repair and regeneration. Even though research with satellite cells from meat animals is new, considerable data exist to suggest that these cells are regulated through both intrinsic and extrinsic mechanisms. This review covers the present status of the extrinsic factors known or postulated to modulate meat animal satellite cell growth and development.

Fast-muscle-specific DNA-protein interactions occurring in vivo at the human aldolase A M promoter are necessary for correct promoter activity in transgenic mice

The human aldolase A tissue-specific M promoter (pM) has served as a model system for identifying pathways that lead to fast-muscle-specialized expression. The current study has delimited the sequences necessary and sufficient for fast-muscle-specific expression in transgenic mice to a short 209-bp fragment extending from bp -164 to +45 relative to the pM transcription start site. Genomic footprinting methods showed that in this proximal region, the same elements that bind muscle nuclear proteins in vitro are involved in DNA-protein interactions in intact muscle nuclei of transgenic mice. Furthermore, these experiments provided the first evidence that different DNA-binding activities exist between slow and fast muscles in vivo. Fast-muscle-specific interactions occur at an element named M1 and at a muscle-specific DNase I-hypersensitive site that was previously detected by in vitro methods. The formation of the muscle-specific DNase I-hypersensitive site reflects binding of proteins to a close element, named M2, which contains a binding site for nuclear factors of the NF1 family. Mutational analysis performed with transgenic mice confirmed the importance of the M1 element for high-level fast-muscle-specific pM activity and suggested that the M2/NF1 element is differently required for correct pM expression in distinct fast muscles. In addition, two other protein binding sites, the MEF3 motif and the USF site, seem to act as stage-specific activators and/or as participants in the establishment of an active chromatin configuration at pM.

Differentiation and growth of muscle in the fish Sparus aurata (L): I. Myosin expression and organization of fibre types in lateral muscle from hatching to adult

Post-hatching development of lateral muscle in a teleost fish, Sparus aurata (L) was examined. At hatching only two fibre types were present, several layers of mitochondria-poor, myofibril-rich deep muscle fibres surrounded the notochord and were covered by a superficial monolayer of mitochondria-rich, myofibril-poor A third ultrastructurally distinct fibre type first appeared as one or two fibres located just under the lateral line at 6 days post-hatching. This type, which gradually increased in number during larval life, contained a slow isoform of myosin, identified by mATPase staining and immunostaining with myosin isoform-specific antibodies. Deep muscle fibres--the presumptive fast-white type--contained a fast myosin, and superficial monolayer fibres an isoform similar but not identical to that in adult pink muscle fibres. The only fibres present during larval life which showed a clear change in myosin expression were the superficial monolayer fibres, which gradually transformed into the slow type post-larvally. Pink muscle fibres first appeared near the end of larval life. Both slow and pink muscle fibres remained concentrated around the horizontal septum under the lateral line during larval life, expanding outwards towards the apices of the myotomes only after metamorphosis. Between 60 and 90 days very small diameter fibres with a distinct mATPase profile appeared scattered throughout the deep, fast-white muscle layer, giving it a 'mosaic' appearance, which persisted into adult life. A marked expansion in the slow muscle layer began at the same time, partly by transformation of superficial monolayer fibres, but mainly by addition of new fibres both on the deep surface of the superficial monolayer and close to the lateral line. The order of appearance of these fibre types, their myosin composition, and the significance of the superficial monolayer layer are discussed and compared to muscle fibre type development in higher vertebrates.

Angiotensin and bradykinin metabolism by peptidases identified in cultured human skeletal muscle myocytes and fibroblasts

Angiotensin (ANG) and kinin metabolizing enzymes, angiotensin-converting enzyme (ACE; EC 3.4.15.1), neutral endopeptidase-24.11 (NEP-24.11; EC 3.4.24.11), and aminopeptidase M (AmM; EC 3.4.11.2), have recently been identified in a purified skeletal muscle glycoprotein fraction. We have analyzed the cellular localization of these enzymes. In cultured human skeletal muscle adult myoblasts, myotubes, and fibroblasts, kinins and angiotensins were metabolized by NEP-24.11 and AmM but not by ACE. NEP-24.11 degraded ANG II, ANG III. and bradykinin (BK) and converted ANG I to the active metabolite ANG(1-7). ANG III was converted to the novel ANG IV metabolite [des-Arg1]ANG III by AmM. These data suggest that, due to their abundance in the body, skeletal muscle myocytes and fibroblasts may play a major role in modulation of the systemic and local effects of angiotensins and kinins. This role could be particularly important in individuals receiving treatment with ACE inhibitors.

Fast-muscle-specific expression of human aldolase A transgenes

The expression of the human aldolase A gene is controlled by three alternative promoters. In transgenic mice, pN and pH are active in all tissues whereas pM is activated specifically in adult muscles composed mainly of fast, glycolytic fibers. To detect potential regulatory regions involved in the fast-muscle-specific activation of pM, we analyzed DNase I hypersensitivity in a 4.3-kbp fragment from the 5' end of the human aldolase A gene. Five hypersensitive sites were located near the transcription initiation site of each promoter in those transgenic-mouse tissues in which the corresponding promoter was active. Only one muscle-specific hypersensitive site was detected, mapping near pM. To functionally delimit the elements required for muscle-specific activity of pM, we performed a deletion analysis of the aldolase A 5' region in transgenic mice. Our results show that a 280-bp fragment containing 235 bp of pM proximal upstream sequences together with the noncoding M exon is sufficient for tissue-specific expression of pM. When a putative MEF-2-binding site residing in this proximal pM region is mutated, pM is still active and no change in its tissue specificity is detected. Furthermore, we observed a modulation of pM activity by elements lying further upstream and downstream from pM. Interestingly, pM was expressed in a tissue-specific way in all transgenic mice in which the 280-bp region was present (32 lines and six founder animals). This observation led us to suggest that the proximal pM region contains elements that are able to override to some extent the effects of the surrounding chromatin.

Fast-muscle-specific expression of human aldolase A transgenes

The expression of the human aldolase A gene is controlled by three alternative promoters. In transgenic mice, pN and pH are active in all tissues whereas pM is activated specifically in adult muscles composed mainly of fast, glycolytic fibers. To detect potential regulatory regions involved in the fast-muscle-specific activation of pM, we analyzed DNase I hypersensitivity in a 4.3-kbp fragment from the 5' end of the human aldolase A gene. Five hypersensitive sites were located near the transcription initiation site of each promoter in those transgenic-mouse tissues in which the corresponding promoter was active. Only one muscle-specific hypersensitive site was detected, mapping near pM. To functionally delimit the elements required for muscle-specific activity of pM, we performed a deletion analysis of the aldolase A 5' region in transgenic mice. Our results show that a 280-bp fragment containing 235 bp of pM proximal upstream sequences together with the noncoding M exon is sufficient for tissue-specific expression of pM. When a putative MEF-2-binding site residing in this proximal pM region is mutated, pM is still active and no change in its tissue specificity is detected. Furthermore, we observed a modulation of pM activity by elements lying further upstream and downstream from pM. Interestingly, pM was expressed in a tissue-specific way in all transgenic mice in which the 280-bp region was present (32 lines and six founder animals). This observation led us to suggest that the proximal pM region contains elements that are able to override to some extent the effects of the surrounding chromatin.

Somite subdomains, muscle cell origins, and the four muscle regulatory factor proteins

We show by immunohistology that distinct expression patterns of the four muscle regulatory factor (MRF) proteins identify subdomains of mouse somites. Myf-5 and MyoD are, at specific stages, each expressed in both myotome and dermatome cells. Myf-5 expression is initially restricted to dorsal cells in all somites, as is MyoD expression in neck somites. In trunk somites, however, MyoD is initially expressed in ventral cells. Myogenin and MRF4 are restricted to myotome cells, though the MRF4-expressing cells are initially less widely distributed than the myogenin-expressing cells, which are at all stages found throughout the myotome. All somitic myocytes express one or more MRFs. The transiently distinct expression patterns of the four MRF proteins identify dorsal and ventral subdomains of somites, and suggest that skeletal muscle cells in somites originate at multiple sites and via multiple molecular pathways.

Cardiovascular development. Prospects for a genetic approach

Genetics is a powerful tool, especially when used in combination with embryology, in the seeking of genes necessary for assembly of the cardiovasculature. The first questions must address the types of cellular decisions that are made during development. As for simpler systems in C elegans and D melanogaster, the lineage and cell-fate decisions of the cardiovascular progenitors need to be assessed. In addition it is likely that new paradigms will emerge for multicellular assembly. The study of cardiovascular mutations will define individual genetic steps that define organotypic decisions. A genetic approach is a natural extension of embryology, physiology, and anatomy, fields of great sophistication with regard to the cardiovasculature, because, like them, it focuses on integrative biology and on the intact organism. The zebrafish is particularly well suited to a combination genetic-embryologic study of the fashioning of the cardiovasculature.