The cellular basis of myosin heavy chain isoform expression during development of avian skeletal muscles

Thyroid development in relation to the development of endothermy in the red-winged blackbird (Agelaius phoeniceus)

We investigated the development of thyroid function during the transition to endothermy in red-winged blackbirds (Agelaius phoeniceus). Thermoregulatory capabilities of blackbirds improve markedly over their relatively short nestling period (10-12 days), with the most striking improvements occurring between days 6 and 8. We hypothesized that the development of endothermy in these birds is dependent in part on the development of thyroid function. We assessed thyroid development by measuring changes in thyroid gland histology and plasma concentrations of thyroxine (T4) and triiodothyronine (T3) during the nestling period. To gain insight into the role of thyroid maturation in the context of thermoregulation, we compared plasma thyroid hormone profiles in nestlings exposed to cold temperatures to those maintained at thermoneutral temperatures. The overall size of the thyroid (as cross-sectional area) increased during nestling development, with the fastest growth occurring just before the development of endothermy. By day 8, it reached the size typical of that in adults. Follicular cell height of the thyroid glands increased in nestlings up to day 6 and then decreased for the rest of the nestling period. The mean area of individual follicles increased up to day 8 of nestling life and then decreased. Individual nestlings were capable of strong endothermic responses at 7 to 8 days of age and had significantly decreased plasma T4 concentrations following cold exposure, suggesting increased T4 to T3 deiodination to maintain the plasma concentrations of the more metabolically active T3. The patterns of plasma T4 and T3 during nestling development were consistent with those of nestlings of other altricial species of birds that have been studied. Overall, the patterns of thyroid development observed were consistent with the hypothesis that the functional development of the thyroid is critical to the development of endothermic capabilities and that thyroid hormones play a role in endothermic responses to cold temperatures.

Functional properties of myosin isoforms in avian muscle

Sarcomeric myosin is the major skeletal muscle protein and is encoded by a large and complex multigene family whose members are differentially expressed in developing and adult muscle cells. The structure and function of sarcomeric myosins have been extensively analyzed and many myosin genes have now been cloned and sequenced. This manuscript reviews the broad spectrum of myosin research with emphasis on studies in avian systems and discusses how advances in myosin isoform analysis have contributed to muscle and meat science.

Maturation of the myogenic program is induced by postmitotic expression of insulin-like growth factor I

The molecular mechanisms underlying myogenic induction by insulin-like growth factor I (IGF-I) are distinct from its proliferative effects on myoblasts. To determine the postmitotic role of IGF-I on muscle cell differentiation, we derived L6E9 muscle cell lines carrying a stably transfected rat IGF-I gene under the control of a myosin light chain (MLC) promoter-enhancer cassette. Expression of MLC-IGF-I exclusively in differentiated L6E9 myotubes, which express the embryonic form of myosin heavy chain (MyHC) and no endogenous IGF-I, resulted in pronounced myotube hypertrophy, accompanied by activation of the neonatal MyHC isoform. The hypertrophic myotubes dramatically increased expression of myogenin, muscle creatine kinase, beta-enolase, and IGF binding protein 5 and activated the myocyte enhancer factor 2C gene which is normally silent in this cell line. MLC-IGF-I induction in differentiated L6E9 cells also increased the expression of a transiently transfected LacZ reporter driven by the myogenin promoter, demonstrating activation of the differentiation program at the transcriptional level. Nuclear reorganization, accumulation of skeletal actin protein, and an increased expression of beta1D integrin were also observed. Inhibition of the phosphatidyl inositol (PI) 3-kinase intermediate in IGF-I-mediated signal transduction confirmed that the PI 3-kinase pathway is required only at early stages for IGF-I-mediated hypertrophy and neonatal MyHC induction in these cells. Expression of IGF-I in postmitotic muscle may therefore play an important role in the maturation of the myogenic program.

The four populations of myoblasts involved in human limb muscle formation are present from the onset of primary myotube formation

To understand how and when myogenic precursor cells become committed to their particular developmental programs, we have analysed the different populations of myoblasts which grow out from explants of muscle tissue isolated from human limb buds from the beginning of primary fibre formation throughout subsequent development and post-natal growth. Four phenotypically distinct types of myoblasts were identified on the basis of their expression of desmin, myogenin and myosin heavy chain isoforms (MyHC), and after 5 and 20 divisions, cells were cloned. All four types of myoblasts were present at the beginning of primary myogenesis. Each respective phenotype was stably heritable through cloning and subsequent proliferation. The type 1 clones correspond to a novel class of myoblasts never described during human development, that biochemically differentiates, but does not fuse. Type 2 clones are composed of small myotubes expressing only embryonic MyHC. Type 3 clones are composed of thin and long myotubes expressing both embryonic and fetal MyHCs. The type 4 clones are composed of myotubes that have a phenotype very similar to human satellite cells. Contrasting with others species, no other population of myoblasts appear during fetal development and only the relative number of these four types changes.

Both avian and mammalian embryonic myoblasts are intrinsically heterogeneous

Adult skeletal muscles are composed of different fibre types. What initiates the distinctive muscle fibre type-specific specialization in a developing embryo is still controversial. In vitro studies of avian muscles have shown the expression of one of the slow myosin heavy chains, SM2, in only some myotubes. In this report we demonstrate the expression of another slow myosin heavy chain, SM1, restricted to only some chicken myotubes (presumptive slow) in vitro. We also demonstrate that as is the case for avian species, distinct fast and slow myogenic cells are detectable in mammalian species, human and rat, during in vitro development in the absence of innervation. While antibodies to fast myosin heavy chains stained all myotubes dark in these muscle cell cultures, antibodies to slow myosin heavy chains stained only a proportion of the myotubes (presumptive slow). The other myotubes were either unstained or only weakly stained with slow myosin heavy chain antibodies. The muscle cell cultures prepared from different developmental stages of rat skeletal muscles showed a reduction in the number of slow myosin heavy chain-positive myotubes with advancing foetal growth. It is concluded that embryonic myogenic cells that are likely to form distinct fast or slow muscle fibre types are intrinsically heterogeneous, not only in avian but also in mammalian species, although extrinsic factors reinforce and modify such commitment throughout subsequent development.

Cytochrome c promoter activity in soleus and white vastus lateralis muscles in rats

Cytochrome c protein and mRNA are 300 and 100% higher, respectively, in the soleus muscle (predominantly slow-twitch oxidative) than the white vastus lateralis (predominately fast-twitch glycolytic) muscle (W. W. Winder, K. M. Baldwin, and J. O. Holloszy. Eur. J. Biochem. 47: 461-467, 1974; M. M. Lai and F. W. Booth. J. Appl. Physiol. 69: 843-848, 1990). However, the mechanisms controlling these differences in cytochrome c mRNA are largely unknown. The present study employed direct plasmid injection techniques to determine whether the proximal promoter (-726 to +610) of the rat somatic cytochrome c gene was more active in the soleus than in white vastus lateralis muscles in rats. No difference between the soleus and white vastus lateralis muscles for the activities of the -726, -631, -489, -326, -215, -159 and -149 cytochrome c promoters was noted. The results of this study suggest that additional elements (outside of -726 to +610) in the cytochrome c gene may be required, or posttranscriptional regulation may account, for the higher cytochrome c mRNA in the slow-twitch oxidative muscle.

Evidence for distinct fast and slow myogenic cell lineages in human foetal skeletal muscle

To analyse the myogenic cell lineages in human foetal skeletal muscle, muscle cell cultures were prepared from different foetal stages of development. The in vitro muscle cell phenotype was defined by staining the myotubes with antibodies to fast and slow skeletal muscle type myosin heavy chains using immunoperoxidase or double immunofluorescence procedures. The antibodies to fast skeletal muscle myosin heavy chains stained nearly all myotubes dark in cell cultures prepared from quadriceps muscles at 10-18 weeks of gestation. The antibodies to slow skeletal muscle myosin heavy chains, in contrast, stained only 10-40% of the myotubes very dark. The remaining myotubes were further subdivided into two populations, one of which was unstained while the other stained with variable intensity for slow myosin heavy chain. The slow myosin heavy chain staining was not influenced by the nature of the substratum used to culture these cells, although the growth of muscle cell cultures was greatly improved on matrigel-coated dishes. The presence of both slow and fast myosin heavy chains was detected even when myotubes were grown on uncoated petri dishes. The myotube diversity was further investigated by analysing the clonal populations of human foetal skeletal muscle cells in vitro. When cultured at clonal densities, two types of myogenic clones were identified by their differential staining with antibodies to slow myosin heavy chain. As was the case with the high density muscle cell cultures, virtually all myotubes in both groups of clones stained with antibodies to fast myosin heavy chains. Antibodies to slow myosin heavy chains stained nearly all myotubes dark in one group of myogenic clones, but only a subset of the myotubes stained dark for slow myosin heavy chain in the second group of clones. The proportion of slow myosin heavy chain positive myotubes in this group varied in different clones. The myogenic diversity was thus apparent in both high density and clonal human muscle cell cultures, and myogenic cells retained their ability to modify their muscle cell phenotype.

Localized and limited changes in the expression of myosin heavy chains in injured skeletal muscle fibers being repaired

The process of skeletal muscle repair was investigated by immunocytochemical evaluation of chicken leg muscles injured by a localized crush or superficial cut. Only the damaged parts of the muscle fibers, approximately 400-500 microm across, along the longitudinal axis, expressed ventricular myosin heavy chain. The level of this myosin heavy chain along the fiber length further decreased with time. Unlike the newly generated independent regenerating myotubes, even the injured parts of original mature muscle fibers positive for ventricular myosin heavy chain in the immediate vicinity of injury did not show changes in the expression of slow or fast myosin heavy chains in these regions. It is concluded that muscle fibers injured by superficial cut or crush methods used in this study despite being multinucleated were rapidly repaired by localized changes without affecting the major gene expression in the uninjured parts of the fibers.

Interferon regulatory factor-2 is a transcriptional activator in muscle where It regulates expression of vascular cell adhesion molecule-1

Previously, we have suggested that vascular cell adhesion molecule-1 (VCAM-1) and its integrin receptor alpha4beta1 mediate cell-cell interactions important for skeletal myogenesis. Expression of the receptors subsequently subsides in muscle after birth. Here, we examine the mechanism regulating VCAM-1 gene expression in muscle. An enhancer located between the TATA box and the transcriptional start site is responsible for VCAM-1 gene expression in muscle-this element is inactive in endothelial cells where VCAM-1 expression is dependent on nuclear factor kappaB sites and inflammatory cytokines. We identify interferon regulatory factor-2 (IRF-2), a member of the interferon regulatory factor family, as the enhancer-binding transcription factor and show that expression of IRF-2 parallels that of VCAM-1 during mouse skeletal myogenesis. IRF-2 is not dependent upon cytokines for expression or activity, and it has been shown to act as a repressor in other nonmuscle cell types. We show that the basic repressor motif located near the COOH-terminal of IRF-2 is not active in muscle cells, but instead an acidic region in the center of the molecule functions as a transactivating domain. Although IRF-2 and VCAM-1 expression diminishes on adult muscle fiber, they are retained on myogenic stem cells (satellite cells). These satellite cells proliferate and fuse to regenerate muscle fiber after injury or disease. We present evidence that VCAM-1 on satellite cells mediates their interaction with alpha4beta1(+) leukocytes that invade the muscle after injury or disease. We propose that VCAM-1 on endothelium generally recruits leukocytes to muscle after injury, whereas subsequent interaction with VCAM-1 on regenerating muscle cells focuses the invading leukocytes specifically to the sites of regeneration.

Myosin heavy chain expression in skeletal muscle autografts under neural or aneural conditions

BACKGROUND

Our purpose was to investigate (1) the heterogeneity of satellite cells derived from adult fast-twitch and slow-twitch skeletal muscles, (2) the influence of innervation on muscle regeneration, and (3) the differences between developmental myoblasts and satellite cells with regard to myosin heavy chain (MHC) expression.

MATERIALS AND METHODS

Autografts under neural (nerve-intact graft; brief denervation interval) or aneural (aneural graft; prolonged denervation interval) conditions of the fast-twitch extensor digitorum longus (EDL) muscle or the slow-twitch soleus muscle were performed in adult rat hindlimbs. MHC expression during skeletal muscle regeneration was determined sequentially using immunocytochemistry.

RESULTS

After grafting, most muscle fibers in the EDL and soleus underwent ischemic degeneration and regeneration; at the periphery of each muscle, a few adult fibers survived. All regenerating fibers initially expressed embryonic/fetal (developmental) MHC alone, and subsequently both developmental and fast MHC. During the first week, no expression of slow MHC was observed in regenerating fibers in either the EDL or the soleus. In nerve-intact grafts, regenerating fibers expressed slow MHC as early as the second week; under aneural conditions, no regenerating fibers expressed slow MHC even 4 weeks after grafting. On the other hand, some persisting fibers in aneural grafts could maintain expression of slow MHC 4 weeks after grafting; other fibers underwent MHC transformation induced by denervation. No significant difference in MHC expression during regeneration was observed for slow compared with fast muscles, under either neural or aneural condition.

CONCLUSIONS

These data suggest that regenerating adult skeletal muscle fibers, derived only from satellite cells, cannot express slow MHC without motor innervation, and that persisting muscle fibers, derived from both myoblasts in fetal development and satellite cells, may be intrinsically distinct from regenerating fibers. Satellite cells derived from slow and from fast muscles may be a single, homogenous population and may be the same population as fetal (secondary) myoblasts with regard to MHC expression.

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

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

Natural occurrence of fast- and fast/slow-muscle chimeric fibers in the expression of troponin T isoforms

Rhomboideus, one of the back muscle tissues, and its single fibers were studied in chickens by immunostaining with antisera against fast- and slow-muscle-type troponin T isoforms. Nonuniform distribution of slow-muscle-type isoforms was for the first time detected in single fibers isolated from the muscle, although fast-muscle-type troponin T isoforms were distributed over the whole length of the fiber. Based on these observations, we conclude that fast- and fast/slow-muscle chimeric fibers exist in normal skeletal muscle tissue and that the existence of chimeric fibers is direct evidence showing that myonuclei subjected to different determination in troponin T isoform expression can together form a single muscle fiber.

Tail regression, apoptosis and thyroid hormone regulation of myosin heavy chain isoforms in Xenopus tadpoles

T3 effects on myosin heavy chain gene expression were analysed in muscles undergoing different fates during metamorphosis. Muscle fate was followed by somatic gene transfer of a constitutively expressed luciferase vector. Persistent expression was found in dorsal muscle which is remodelled during metamorphosis whilst the signal disappeared in apoptosing caudal muscle. RNAse protection assay was used to follow production of myosin heavy chain isoforms: two isoforms identified as embryonic (E3 and E19) and one adult form (A7). The effects of T3 treatment were followed over 120 h. During this time frame E3 and A7 expression patterns were found to be similar in both caudal and dorsal muscles. Most notably, at 48 h E3 expression was significantly down-regulated and production of A7 significantly upregulated in both caudal and dorsal muscle. Thus T3-induced transitions in muscle gene expression are independent of muscle fate during amphibian metamorphosis.

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

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

Regionalized expression of myosin isoforms in heterotypic myotubes formed from embryonic and fetal rat myoblasts in vitro

The development of mammalian limb muscles involves the appearance and fusion of at least two separate populations of muscle precursor cells. These two populations, termed embryonic and fetal myoblasts, are first detected within the limb bud at different stages of development. We have previously demonstrated that, in the rat, each myoblast population expresses a unique pattern of myosin heavy chains (MyHCs) during differentiation in vitro (Pin and Merrifield [1993] Dev. Genet. 14:356-368). Embryonic myoblasts accumulate embryonic and slow MyHCs, whereas fetal myoblasts accumulate embryonic, neonatal, and adult fast MyHCs but not slow MyHC. To determine if the two populations can fuse with each other and whether the pattern of MyHC expression is altered in the resulting heterokaryons, embryonic and fetal myoblasts were labelled with the lipophilic dye PKH26, [3H]-thymidine, or 5-bromodeoxyuridine (BRDU) and cocultured for 24-48 hr. Our results demonstrate that fusion occurs between embryonic and fetal myoblasts in vitro. Moreover, analysis of the resulting heterokaryons revealed regionalized accumulations of MyHC around individual nuclei. Interestingly, these accumulations were typical of the default pattern of expression that individual nuclei would have normally expressed in single culture. Nuclei contributed by embryonic myoblasts were surrounded by localized accumulations of slow MyHC, whereas nuclei from fetal myoblasts were surrounded by neonatal/fast MyHC. The occurrence of such nuclear domains indicates that the myoblast-specific expression of MyHC isoforms is dictated by cis-acting factors established prior to fusion.

Inhibition of troponin C production without affecting other muscle protein synthesis by the antisense oligodeoxynucleotide

The effect of blocking expression of a specific gene with antisense phosphodiester oligodeoxynucleotides on the coordinate regulation of myogenesis was studied. Different regions of both fast and slow troponin C (TnC) mRNAs were targeted for binding of the antisense oligomer. The 5'-cap region of both mRNAs was found to be the most effective target for inhibiting the expression of these genes. Approximately 40%-60% inhibition of expression of a specific isoform of TnC was achieved. However, inhibition of the TnC expression did not appreciably alter the pattern of myogenesis of mouse C2C12 cells. The differentiated murine muscle cells were able to cope with this reduced level of the target gene expression by antisense phosphodiester oligomers. We have also used a phosphorothioate oligomer targeted against a common sequence within the coding region of both fast and slow TnC mRNAs. This oligomer was found to be ineffective in blocking TnC gene expression.

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

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

Myosin isoform transitions in four rabbit muscles during postnatal growth

Four rabbit muscles (i.e. semimembranosus proprius, psoas major, biceps femoris and longissimus lumborum), differing in their fibre type composition in the adult, were investigated during postnatal development. Muscle samples were taken at 1, 7, 14, 21, 28, 35, 49 and 77 days of age. Complementary techniques were used to characterize myosin heavy chain (MHC) isoform transitions, i.e. SDS-PAGE, immunocytochemistry and conventional histochemistry. Good accordance was found between electrophoretic and immunocytochemical techniques. Our results show that rabbit muscles were phenotypically immature at birth. At 1 day of age, perinatal isoform represented 70-90% of the total isoform content of the muscles. Two generations of myofibres could be observed on the basis of their morphology and reaction to specific antibodies. In all muscles, primary fibres expressed slow MHC. In contrast, secondary generation of fibres never expressed slow MHC in future fast muscles, while half of them expressed slow MHC in the future slow-twitch muscle, the semimembranosus proprius. During the postnatal period, all muscles displayed a transition from embryonic to perinatal MHC isoforms, followed by a transition from perinatal to adult MHC isoforms. These transitions occured mainly during the first postnatal month. The embryonic isoform was no longer expressed after 14 days, except in longissimus where it disappeared after 28 days. On the contrary, large differences were found in the timing of disappearance of the perinatal isoform between the four muscles. The perinatal isoform disappeared between 28 and 35 days in semimembranosus proprius and 35 and 49 days in psoas and biceps femoris. Interestingly, the perinatal isoform was still present in 6% of the fibres in longissimus at 77 days, the commercial slaughter age, denoting a great delay in the maturation. Fate of each generation of fibres differed between muscles.

Changes in some troponin and insulin-like growth factor messenger ribonucleic acids in regenerating and denervated skeletal muscles

To investigate the role of innervation and to determine if the process of muscle differentiation is preprogrammed, the expression of insulin-like growth factors (IGF-I and IGF-II), troponin I and troponin T mRNAs was studied in regenerating transplants of rat Extensor digitorum longus muscle in the presence and absence of nerve. The role of innervation was further investigated by denervating some adult fast (Gastrocnemius and Plantaris) and slow (Soleus) skeletal muscles. In normal adult skeletal muscles, IGF-I, IGF-II and developmental fast troponin T mRNA containing exon y, are undetectable or present at very low levels. Induction of all these mRNAs was observed in regenerating muscles in both the presence and absence of nerve as well as following denervation of adult fast and slow skeletal muscles. Their low level expression was maintained in adult denervated skeletal muscles but gradually suppressed in both innervated and noninnervated regenerating extensor digitorum longus muscle transplants after 2 months. Fast troponin T mRNA was synthesized in both innervated and noninnervated EDL transplants although the level of this transcript changed markedly in response to denervation of both adult fast and slow skeletal muscles. The fast troponin T mRNA containing exon 17 was also initially expressed in both regenerating muscles but its level was reduced with time in both transplants and in all adult denervated skeletal muscles. Fast and slow troponin I mRNAs were synthesised during EDL muscle regeneration in both the presence and absence of nerve but the slow troponin I expression was not maintained in noninnervated transplants. The level of fast troponin I mRNA decreased in denervated fast skeletal muscles but markedly increased in denervated Soleus. The level of slow troponin I mRNA was slightly increased in denervated fast skeletal muscles but considerably reduced in denervated Soleus.

Expression of chicken troponin T isoforms in cultured muscle cells

Cells prepared from chicken skeletal muscles of different developmental stages were cultured to study their troponin T isoform expression, using antisera specific to the fast- and slow-muscle-type isoforms. We found that the cultured myogenic cells from chickens and chick embryos were classified into two types, fast type and fast/slow type in which fast- and slow-muscle-type isoforms were coexpressed. Cells expressing only slow-muscle-type troponin T isoforms could not be found. Most cells prepared from pectoralis major (fast muscle) and gastrocnemius (mixed muscle) of 11-day old embryos belonged to the latter, with only a small fraction belonging to the former. The percentage of fast type cells in those cells prepared from pectoralis major increased along development to over 90% by the 17th day of incubation, while, in the cells prepared from gastrocnemius, it reached a plateau of 30-40% by the 13th day of incubation. All the cells from anterior latissimus dorsi (slow muscle) belonged to the fast/slow type. Ratios of these two types of muscle cells varied depending on their origins and stages. The in vitro expression of troponin T isoforms was different from the in vivo expression, and each muscle seems to be determined differently in the composition of cell types during the developmental course.

Common core sequences are found in skeletal muscle slow- and fast-fiber-type-specific regulatory elements

The molecular mechanisms generating muscle diversity during development are unknown. The phenotypic properties of slow- and fast-twitch myofibers are determined by the selective transcription of genes coding for contractile proteins and metabolic enzymes in these muscles, properties that fail to develop in cultured muscle. Using transgenic mice, we have identified regulatory elements in the evolutionarily related troponin slow (TnIs) and fast (TnIf) genes that confer specific transcription in either slow or fast muscles. Analysis of serial deletions of the rat TnIs upstream region revealed that sequences between kb -0.95 and -0.5 are necessary to confer slow-fiber-specific transcription; the -0.5-kb fragment containing the basal promoter was inactive in five transgenic mouse lines tested. We identified a 128-bp regulatory element residing at kb -0.8 that, when linked to the -0.5-kb TnIs promoter, specifically confers transcription to slow-twitch muscles. To identify sequences directing fast-fiber-specific transcription, we generated transgenic mice harboring a construct containing the TnIs kb -0.5 promoter fused to a 144-bp enhancer derived from the quail TnIf gene. Mice harboring the TnIf/TnIs chimera construct expressed the transgene in fast but not in slow muscles, indicating that these regulatory elements are sufficient to confer fiber-type-specific transcription. Alignment of rat TnIs and quail TnIf regulatory sequences indicates that there is a conserved spatial organization of core elements, namely, an E box, a CCAC box, a MEF-2-like sequence, and a previously uncharacterized motif. The core elements were shown to bind their cognate factors by electrophoretic mobility shift assays, and their mutation demonstrated that the TnIs CCAC and E boxes are necessary for transgene expression. Our results suggest that the interaction of closely related transcriptional protein-DNA complexes is utilized to specify fiber type diversity.

Effects of growth hormone injection to embryos on growth and myosin heavy chain isoforms in growing turkeys (Meleagris gallopavo)

Effects of embryonic imprinting with growth hormone (GH) on growth and myosin heavy chain (MyHC) isoforms in pectoralis muscle were determined by injecting turkey embryos with ovine growth hormone (oGH) at a dose of 10 micrograms three times a day. Injections were made on days 20 and 26 (Treatment 1), days 14 and 20 (Treatment 2) or days 14 and 26 (Treatment 3) of incubation. In Treatment 1 poults, plasma GH concentrations were elevated at 3 days posthatch and in Treatment 3 poults, plasma GH concentrations were elevated at 15 days posthatch, as compared to control poults. At 4 weeks of age, in males, body weights, shank length and weights of pectoralis, gastrocnemius and sartorius muscles were increased in Treatment 3, and in females, body weights, shank length and weights of gastrocnemius muscle of female turkeys were increased in Treatment 1. The growth rate of female turkeys from 4 weeks through 16 weeks was increased by Treatment 1. Treatment 1 resulted in a delay in the transition from the embryonic MyHC isoform to the neonatal MyHC isoform and to the adult MyHC isoform. Treatment 3 induced an earlier appearance of the adult MyHC isoform. No effects on body and muscle growth and MyHC isoforms were observed by Treatment 2.

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.

Expression of myosin heavy chain isoforms and myogenesis of intrafusal fibres in rat muscle spindles

This review concerns the pattern of expression and regulation of myosin heavy chain (MHC) isoforms in intrafusal fibres of rat muscle spindles detected by immunocytochemistry. The three types of intrafusal fibres--nuclear bag1, nuclear bag2, and nuclear chain fibres--are unique in co-expressing several MHCs including special isoforms such as slow tonic and alpha cardiac-like MHC and isoforms typical of muscle development, such as embryonic and neonatal MHC. The distinct intrafusal fibre types appear sequentially during rat hind limb development, the nuclear bag2 precursors being first identifiable at 17-18 days in utero as the only primary myotubes expressing slow tonic MHC. Sensory innervation is required for the expression of "spindle-specific" MHC isoforms. Motor innervation contributes to the diversity in distribution of the different MHCs along the length of the nuclear bag fibres. It is suggested that unique populations of myoblasts are destined to become intrafusal fibres during development in the rat hind limb muscles and that the regional heterogeneity in MHC expression is related both to sensory and motor innervation and to the properties of the myoblast lineages. These distinct features make intrafusal fibres an attractive in situ model for investigating myogenesis, myofibrillogenesis, and the mechanisms regulating MHC expression.

Development and postnatal regulation of adult myoblasts

The myogenic precursor cells of postnatal and adult skeletal muscle are situated underneath the basement membrane of the myofibers. It is because of their unique positions that these precursor cells are often referred to as satellite cells. Such defined satellite cells can first be detected following the formation of a distinct basement membrane around the fiber, which takes place in late stages of embryogenesis. Like myoblasts found during development, satellite cells can proliferate, differentiate, and fuse into myofibers. However, in the normal, uninjured adult muscle, satellite cells are mitotically quiescent. In recent years several important questions concerning the biology of satellite cells have been asked. One aspect has been the relationship between satellite cells and myoblasts found in the developing muscle: are these myogenic populations identical or different? Another aspect has been the physiological cues that control the quiescent, proliferative, and differentiative states of these myogenic precursors: what are the growth regulators and how do they function? These issues are discussed, referring to previous work by others and further emphasizing our own studies on avian and rodent satellite cells. Collectively, the studies presented indicate that satellite cells represent a distinct myogenic population that becomes dominant in late stages of embryogenesis. Moreover, although satellite cells are already destined to be myogenic precursors, they do not express any of the four known myogenic regulatory genes unless their activation is induced in the animal or in culture. Furthermore, multiple growth factors are important regulators of satellite cell proliferation and differentiation. Our work on the role of one of these growth factors [platelet-derived growth factor (PDGF)] during proliferation of adult myoblasts is further discussed with greater detail and the possibility that PDGF is involved in the transition from fetal to adult myoblasts in late embryogenesis is brought forward.

Mammalian myoblasts become fast or slow myocytes within the somitic myotome

A monoclonal antibody has been prepared to slow skeletal muscle type myosin heavy chain which in the rat distinguishes fast and slow myocytes within the somitic myotome at 11.5 days in utero. The distribution of slow myotubes identified by this antibody in developing limb buds is also restricted to presumptive slow muscle cell type regions only. No slow myoblasts in hindlimb buds, however, were detected at 14.5 day in utero when a small number of fast muscle cells were already present. The presence of slow muscle cell population detected by this specific antibody became apparent a day later. This study thus demonstrated the diversification into different muscle cell types during both early embryonic and foetal development.

All muscles are not created equal

The muscles of an individual vary in their morphological and contractile properties and in their patterns of innervation. At one time, it was thought that a fairly homogeneous population of myoblasts gave rise to myotubes that were subsequently instructed to diversify by various extramuscular factors. There is now, however, considerable evidence that myoblasts are heterogeneous, and that heritable differences among them interact with environmental influences to give each muscle its distinct characteristics.

Identification of alpha-cardiac myosin heavy chain mRNA and protein in extraocular muscle of the adult rabbit

Extraocular muscles contain both fast-twitch and multiply-innervated, tonic-contracting fibres. In rat, these fibres collectively express numerous myosin heavy chain isoforms including fast-type embryonic and neonatal, adult slow twitch type I and fast twitch type II, and a fast isoform unique to extraocular muscle. Immunocytochemical and Western blotting results are presented which suggest that, in rabbit, an additional species, the alpha-cardiac myosin heavy chain, is present. The immunoreactive species is found in all rabbit extraocular muscles and in the extraocular muscles is expressed in almost all fibres which do not contain a fast myosin heavy chain. Positive identification of this isoform as the alpha-cardiac myosin heavy chain was obtained by sequencing a cloned PCR product derived from extraocular muscle mRNA unique to the 3'-end of rabbit alpha-cardiac myosin heavy chain mRNA. This is the first unequivocal demonstration of alpha-cardiac myosin heavy chain expression in extraocular muscle.

Isolation and structure of the rat gene encoding troponin I(slow)

Troponin I is a myofibrillar protein involved in the Ca(2+)-mediated regulation of actomyosin ATPase activity. We report here the isolation and characterization of the gene coding for the slow-muscle-specific isoform of the rat troponin I polypeptide (TpnI). Using restriction mapping, PCR mapping and partial DNA sequencing, we have determined the exon/intron arrangement of this gene. The transcription unit is 10.5-kb long and contains nine exons ranging in size from 4 bp to 330 bp. The rat TpnI(slow) gene is interrupted by large intervening sequences; a 3.3-kb intron separates the 5' untranslated exons from the protein-coding exons. Comparison of the structure of rat TpnI(slow) with that of quail TpnI(fast) reveals that they have a similar intron/exon organization. The 5' untranslated region of the rat gene contains an additional exon, otherwise, the positions of introns and coding exons map to essentially identical regions in both genes.

Myogenic specification in somites: induction by axial structures

Specification of the myogenic phenotype in somites was examined in the early chick embryo using organotypic explant cultures stained with monoclonal antibodies to myosin heavy chain. It was found that myogenic specification (formation of muscle fibers in explants of somites or segmental plates cultured alone) does not occur until Hamburger and Hamilton stage 11 (12-14 somites). At this stage, only the somites in the rostral half of the embryo are myogenically specified. By Hamburger and Hamilton stage 12 (15-17 somites), the three most caudal somites were not specified to be myogenic while most or all of the more rostral somites are specified to myogenesis. Somites from older embryos (stage 13–15, 18–26 somites) showed the same pattern of myogenic specification--all but the three most caudal somites were specified. We investigated the effects of the axial structures, the notochord and neural tube, on myogenic specification. Both the notochord and neural tube were able to induce myogenesis in unspecified somites. In contrast, the neural tube, but not the notochord, was able to induce myogenesis in explants of segmental plate, a structure which is not myogenic when cultured alone. When explants of specified somites were stained with antibodies to slow or fast MyHC, it was found that myofiber diversity (fast and fast slow fibers) was established very early in development (as early as Hamburger and Hamilton stage 11). We also found fiber diversity in explants of unspecified somites (the three most caudal somites from stage 11 to 15) when they were recombined with notochord or neural tube. We conclude that myogenic specification in the embryo results in diverse fiber types and is an inductive process which is mediated by factors produced by the neural tube and notochord.

Differential response of embryonic and fetal myoblasts to TGF beta: a possible regulatory mechanism of skeletal muscle histogenesis

Embryonic and fetal skeletal myoblasts were grown in culture in the presence of TGF beta. Under the conditions employed, TGF beta inhibited differentiation of fetal but not of embryonic myoblasts. To investigate the possible relevance of these data to skeletal muscle histogenesis in vivo, we studied the proliferation/differentiation state of mesodermal cells in the proximal region of the limb bud at the time of primary fiber formation. BrdU labeling and immunostaining for myosin heavy chains revealed that very few mesodermal cells enter the S phase of the cycle when differentiated primary fibers first appear. However, a few hours later, many cells in S phase surround newly formed muscle fibers, suggesting that the latter may be a source of mitogens for undifferentiated myoblasts. Co-culture experiments supported this hypothesis, showing that medium conditioned by fiber-containing explants can stimulate myoblast proliferation. Taken together these data suggested a possible mechanism for the regulation of muscle fiber formation. The model assumes that fibers form in the proximal region of the limb bud, where TGF beta is known to be present, and BrdU labeling experiments did not reveal cells in S phase. It is conceivable that non-dividing embryonic myoblasts (which do not respond to TGF beta) can undergo differentiation, while fetal myoblasts are inhibited by TGF beta. Once formed, primary fibers may stimulate a new wave of proliferation in fetal myoblasts, in order to expand the pool of cells needed to form secondary fibers.(ABSTRACT TRUNCATED AT 250 WORDS)

Persistent expression of tissue-specific troponin T isoforms in transplanted chicken skeletal muscle

This study attempted to investigate the expression of skeletal muscle troponin T isoforms in chicken reared for six months after muscle transplantations of breast muscle into leg muscle, leg muscle into breast muscle, and slow muscle into breast and leg muscles of the same animal. The regenerated muscle after transplantation was studied by histological observation, two-dimensional SDS-polyacrylamide gel electrophoresis, and immunoblotting with anti-troponin T antibodies. Persistent expression of troponin T isoforms specific to donor tissue was observed in the regenerated muscle, and compared with their expression in the normal developing muscles. During the regeneration, the cells grew up and expressed troponin T isoforms in a manner similar to that in normal developing muscles, and on around the 178th day after the transplantation, the regenerated muscle expressed the adult type troponin T isoforms. Based on the troponin T isoforms expressed in the transplants, we consider that one type of skeletal muscle has some inherent potential to grow in and coexist with other types for a long term.

Effects of chronic stimulation with different impulse patterns on the expression of myosin isoforms in rat myotube cultures

In order to study maturation and differentiation of aneural myotubes in vitro, long-term myotube cultures were established from hindlimb musculature of newborn rats. The developmental state of the myotubes was judged by their myosin heavy chain (HC) patterns. Newly formed myotubes only expressed the embryonic isoform, HCemb, older myotubes expressed the neonatal isoform HCneo, as well as the fast adult isoforms HCIIb and HCIId. HCIId increased continuously, reaching a relative concentration of 47% in 37-day-old cultures. The third fast isoform, HCIIa, was not detected and also the slow isoform HCI was absent. Effects of chronic (20 days) electrostimulation were studied by exposing the cultures to various stimulus patterns. Bursts of 250 ms duration at various pulse frequencies were applied at low and high burst frequencies. Although HCemb remained the predominant isoform under all conditions, different stimulus patterns induced specific changes in the patterns of fast and slow HC isoforms. Bursts of 250 ms duration at 15 Hz, 40 Hz, or 100 Hz, repeated every second or every 4 s, induced the expression of slow myosin, i.e., HCl. Bursts of 250 ms duration at 100 Hz, repeated every 100 s, enhanced the expression of HCIId, but not of HCI. Because slow myosin was induced at high burst frequency with low and high pulse rates, we suggest that burst frequency rather than pulse frequency has a specifying effect on myosin expression. Our results show that the basal program of myosin expression during myogenesis in vitro can be modulated by electrostimulation, suggesting a possible influence of neuromuscular activity on the development of adult fiber types.

Skeletal muscle satellite cells

Evidence now suggests that satellite cells constitute a class of myogenic cells that differ distinctly from other embryonic myoblasts. Satellite cells arise from somites and first appear as a distinct myoblast type well before birth. Satellite cells from different muscles cannot be functionally distinguished from one another and are able to provide nuclei to all fibers without regard to phenotype. Thus, it is difficult to ascribe any significant function to establishing or stabilizing fiber type, even during regeneration. Within a muscle, satellite cells exhibit marked heterogeneity with respect to their proliferative behavior. The satellite cell population on a fiber can be partitioned into those that function as stem cells and those which are readily available for fusion. Recent studies have shown that the cells are not simply spindle shaped, but are very diverse in their morphology and have multiple branches emanating from the poles of the cells. This finding is consistent with other studies indicating that the cells have the capacity for extensive migration within, and perhaps between, muscles. Complexity of cell shape usually reflects increased cytoplasmic volume and organelles including a well developed Golgi, and is usually associated with growing postnatal muscle or muscles undergoing some form of induced adaptive change or repair. The appearance of activated satellite cells suggests some function of the cells in the adaptive process through elaboration and secretion of a product. Significant advances have been made in determining the potential secretion products that satellite cells make. The manner in which satellite cell proliferative and fusion behavior is controlled has also been studied. There seems to be little doubt that cellcell coupling is not how satellite cells and myofibers communicate. Rather satellite cell regulation is through a number of potential growth factors that arise from a number of sources. Critical to the understanding of this form of control is to determine which of the many growth factors that can alter satellite cell behavior in vitro are at work in vivo. Little work has been done to determine what controls are at work after a regeneration response has been initiated. It seems likely that, after injury, growth factors are liberated through proteolytic activity and initiate an activation process whereby cells enter into a proliferative phase. After myofibers are formed, it also seems likely that satellite cell behavior is regulated through diffusible factors arising from the fibers rather than continuous control by circulating factors.(ABSTRACT TRUNCATED AT 400 WORDS)

The integrin alpha 4 beta 1 and its counter receptor VCAM-1 in development and immune function

The integrin alpha 4 beta 1 and its counter receptor vascular cell adhesion molecule-1 (VCAM-1) mediate well-described cell-cell interactions that are critical for immune function. However, these receptors also mediate cell-cell interactions that are important for skeletal muscle differentiation. We have found that contrasting transcriptional mechanisms control their patterns of expression in the immune system and in muscle. Recent studies indicate that alpha 4 beta 1 and VCAM-1 are also expressed in a number of developing tissues, implying that these receptors have a general role in facilitating cell-cell interactions during development.

Differentiation of membrane systems during development of slow and fast skeletal muscle fibres in chicken

The disposition of transverse (T) tubules, sarcoplasmic reticulum (SR) and T-SR junctions (triads) and the width of Z lines are matched to contractile properties in adult muscle fibres. We have studied the development of the membrane systems in the slow anterior (ALD) and the fast posterior (PLD) latissimus dorsi of the chicken in ovo (E14-E21) and after hatching (D1-D30). T tubules, SR, triads and Z lines were visualized using DiIC16[3] labelling for confocal microscopy and either Ca-osmium-ferrocyanide or standard procedures for electron microscopy. Anterior latissimus dorsi and PLD have similar, slow twitches in early development (E14-E16), but PLD suddenly becomes faster starting at E17-E18. We find that in coincidence with the differentiation of faster contraction properties (starting at E18-E19) density of triads is significantly higher and width of Z lines is narrower in PLD. The SR also begins to acquire fibre-type specific characteristics at this time. Early development of T tubules, on the other hand, is quite similar in the two muscles. Peripherally-located, longitudinally-oriented T tubules, and the first T networks crossing the fibre center appear earlier in ALD (E14-E15 and E16) than in PLD (E14-E16 and E17), but have similar dispositions. The final fibre-type specific distribution of T tubules is achieved after hatching. Some T tubules-rich fibres in the ALD, presumably future fast fibres, develop extensive T tubules networks at early stages. Location of triads at the Z line in pectoralis occurs in three steps: an initial location of longitudinally oriented triads at the A-I junction; a subsequent move to the Z lines and finally a rotation to a transverse orientation.

cis-acting sequences of the rat troponin I slow gene confer tissue- and development-specific transcription in cultured muscle cells as well as fiber type specificity in transgenic mice

Transcription of the genes coding for troponin I slow (TnIslow) and other contractile proteins is activated during skeletal muscle differentiation, and their expression is later restricted to specific fiber types during maturation. We have isolated and characterized the rat TnIslow gene in order to begin elucidating its regulation during myogenesis. Transcriptional regulatory regions were delineated by using constructs, containing TnIslow gene sequences driving the expression of the chloramphenicol acetyltransferase (CAT) reporter gene, that were transiently transfected into undifferentiated and differentiated C2C12 cells. TnIslow 5'-flanking sequences directed transcription specifically in differentiated cells. However, transcription rates were approximately 10-fold higher in myotubes transfected with constructs containing the 5'-flanking sequences plus the intragenic region residing upstream of the translation initiation site (introns 1 and 2), indicative of interactions between elements residing upstream and in the introns of the gene. Deletion analysis of the 5' region of the TnIslow gene showed that the 200 bp upstream of the transcription initiation site is sufficient to confer differentiation-specific transcription in C2C12 myocytes. MyoD consensus binding sites were found both in the upstream 200-bp region and in a region residing in the second intron that is highly homologous to the quail TnIfast enhancer. Transactivation experiments using transfected NIH 3T3 fibroblasts with TnI-CAT constructs containing intragenic and/or upstream sequences and with the myogenic factors MyoD, myogenin, and MRF4 showed different potentials of these factors to induce transcription. Transgenic mice harboring the rat TnI-CAT fusion gene expressed the reporter specifically in the skeletal muscle. Furthermore, CAT levels were approximately 50-fold higher in the soleus than in the extensor digitorum longus, gastrocnemius, or tibialis muscle, indicating that the regulatory elements that restrict TnI transcription to slow-twitch myofibers reside in the sequences we have analyzed.

cis-acting sequences of the rat troponin I slow gene confer tissue- and development-specific transcription in cultured muscle cells as well as fiber type specificity in transgenic mice

Transcription of the genes coding for troponin I slow (TnIslow) and other contractile proteins is activated during skeletal muscle differentiation, and their expression is later restricted to specific fiber types during maturation. We have isolated and characterized the rat TnIslow gene in order to begin elucidating its regulation during myogenesis. Transcriptional regulatory regions were delineated by using constructs, containing TnIslow gene sequences driving the expression of the chloramphenicol acetyltransferase (CAT) reporter gene, that were transiently transfected into undifferentiated and differentiated C2C12 cells. TnIslow 5'-flanking sequences directed transcription specifically in differentiated cells. However, transcription rates were approximately 10-fold higher in myotubes transfected with constructs containing the 5'-flanking sequences plus the intragenic region residing upstream of the translation initiation site (introns 1 and 2), indicative of interactions between elements residing upstream and in the introns of the gene. Deletion analysis of the 5' region of the TnIslow gene showed that the 200 bp upstream of the transcription initiation site is sufficient to confer differentiation-specific transcription in C2C12 myocytes. MyoD consensus binding sites were found both in the upstream 200-bp region and in a region residing in the second intron that is highly homologous to the quail TnIfast enhancer. Transactivation experiments using transfected NIH 3T3 fibroblasts with TnI-CAT constructs containing intragenic and/or upstream sequences and with the myogenic factors MyoD, myogenin, and MRF4 showed different potentials of these factors to induce transcription. Transgenic mice harboring the rat TnI-CAT fusion gene expressed the reporter specifically in the skeletal muscle. Furthermore, CAT levels were approximately 50-fold higher in the soleus than in the extensor digitorum longus, gastrocnemius, or tibialis muscle, indicating that the regulatory elements that restrict TnI transcription to slow-twitch myofibers reside in the sequences we have analyzed.

Modulation of contractile protein gene expression in fetal murine crural muscles: emergence of muscle diversity

The modulation of contractile protein gene expression in mouse crural muscles (i.e., muscles located in the region between the knee and ankle) during the fetal period (defined as 15 days gestation to birth), resulting in diversity among and within these muscles, has been evaluated with in situ hybridization and correlated with morphogenetic events in the extensor digitorum longus and soleus muscles. During the fetal period extensive secondary myotube formation occurs in the crural muscles, and the myotubes become innervated (Ontell and Kozeka [1984a,b] Am. J. Anat. 171:133-148, 149-161; Ontell et al. [1988a,b] Am. J. Anat. 181:267-278, 181:278-288). At 15 days gestation, hybridization with 35S-labeled antisense cRNA probes demonstrates the accumulation of transcripts for alpha-cardiac and alpha-skeletal actin; MLC 1A, MLC 1F, and MLC 3F; and MHC emb, MHC pn, and MHC beta/slow. At 16 days gestation, accumulation of MHC emb transcripts is reduced (as compared with earlier developmental stages); intensity of signal following hybridization with the probe for alpha-skeletal actin is, for the first time, equal to that for the cardiac isoform; and MLC 1V mRNA accumulation is discernible. At this stage, variation in transcript accumulation for some mRNAs among and within crural muscles becomes evident. Two factors may play a role in the selective distribution of these transcripts: 1) the stage of muscle maturation; and 2) the future myofiber type. At 16 days gestation anterior crural muscles (which mature approximately 2 days before posterior crural muscles; Ontell and Kozeka [1984a,b], ibid., Ontell et al. [1988a,b], ibid.) exhibit a greater accumulation of transcripts for alpha-skeletal actin and for MLC 3F than is found in posterior crural muscles. In muscles that in the neonate are composed, in large part, of slow myofibers, MHC beta/slow and MLC 1V mRNAs accumulate in greater amounts, whereas MHC pn transcripts are less abundant in the soleus muscle than in other crural muscles. By 19 days gestation regionalization of transcript accumulation is more pronounced. The soleus muscle, a predominantly slow twitch muscle in the newborn mouse (Wirtz et al. [1983] J. Anat. 137:109-126) exhibits strong signal after hybridization with probes specific for MHC beta/slow and MLC 1V. While the level of transcript accumulation for the development isoforms, MHC emb, MLC 1A, and alpha-cardiac actin, is greatly reduced in most crural muscles at 19 days gestation, these transcripts persist in the soleus muscle at levels equal ot or exceeding their amount in limb muscles of 13 day gestation mouse embryos.(ABSTRACT TRUNCATED AT 400 WORDS)

A new approach of urodele amphibian limb regeneration: study of myosin isoforms and their control by thyroid hormone

In P. waltlii, an urodele amphibian species which undergoes spontaneous metamorphosis, study of native myosin in pyrophosphate gels at various stages of normal development demonstrates a complete larval to fast myosin isoforms transition, which occurs more precociously in forelimb muscles than in the dorsal and ventral muscles. In the neotenic species A. mexicanum, forelimb muscles development also presents a complete myosin isoforms transition which is in contrast with the partial myosin isoforms transition observed in the dorsal muscle. In metamorphosed or neotenic animals of both species aged 1 year, forelimb regeneration is characterized by a complete transition from larval to fast myosin isoforms, that occurs earlier and more rapidly than in normal forelimb development. When forelimb regeneration is studied in P. waltlii aged 4 years, the adult fast and slow isomyosins are expressed very early in the regeneration process. In experimental hypothyroidian P. waltlii, the larval to fast isoforms transition in regenerating forelimb muscles is slightly delayed. Experimental hyperthyroidism accelerates the disappearance of larval isomyosins in regenerating forelimb muscles, both in P. waltlii and A. mexicanum aged 1 year. This work demonstrates that changes in myosin isoform pattern during forelimb regeneration in adult urodele amphibians are different from changes occurring in the normal forelimb development. They take place without any thyroid hormone influence, as opposed to normal development, and appear to be age-dependent.

Myoblasts transferred to the limbs of embryos are committed to specific fibre fates

In the limb bud of the 5-day-old avian embryo, when primary muscle fibre formation is beginning and before specific muscles appear, differences in the expression of fast and slow myosin heavy chain genes can be detected among primary fibres of the premuscle masses. Myoblasts that form colonies of fibres of specific types can be isolated from these limb buds. To assess the role of myoblast commitment in specifying fibre types during embryonic development, we cloned myoblasts of specific types from embryonic and adult muscles, transfected them with a reporter gene, and transferred them into developing limb buds. After transfer, cloned myoblasts formed fibres in the limb with the same patterns of myosin heavy chain gene expression as the fibres they formed in cell culture. These results demonstrate that initial skeletal muscle fibre type diversity during avian limb development can originate, in part, from the commitment of distinct myoblast types to the formation of specific fibre types.

Identification of a program of contractile protein gene expression initiated upon skeletal muscle differentiation

The functional diversity of skeletal muscle is largely determined by the combinations of contractile protein isoforms that are expressed in different fibers. Just how the developmental expression of this large array of genes is regulated to give functional phenotypes is thus of great interest. In the present study, we performed a comprehensive analysis of contractile protein isoform mRNA profiles in skeletal muscle systems representing each generation of fiber formed: primary, secondary, and regenerating fibers. We find that in each system examined there is a common pattern of isoform gene expression during early differentiation for 5 of the 6 gene families we have investigated: myosin light chain (MLC)1, MLC2, tropomyosin, troponin (Tn)C, and TnI. We suggest that the common isoform patterns observed together represent a genetic program of skeletal muscle differentiation that is independent of the mature fiber phenotype and is found in all newly formed myotubes. Within each of these contractile protein gene families the program is independent of the isoforms of myosin heavy chain (MHC) expressed. The maintenance of such a program may reflect a specific requirement of the initial differentiation process.

Embryonic and fetal rat myoblasts express different phenotypes following differentiation in vitro

Myosin heavy chain (MHC) is encoded by a multigene family containing members which are expressed in developmental and fiber type-specific patterns. In developing rats, primary (1 degree) and secondary (2 degrees) myotubes can be distinguished by difference in MHC expression: 1 degree myotubes coexpress embryonic and slow MHC, while 2 degrees myotubes initially express only embryonic MHC. We have used monoclonal antibodies which recognize the embryonic, slow, neonatal, and adult fast IIB/IIX MHCs to examine MHC accumulation in myoblasts obtained from hindlimbs of embryonic day (ED) 14 and ED 20 Sprague-Dawley rats during differentiation in vitro. Embryonic myoblasts (ED 14), which develop into 1 degree myotubes in vivo, differentiate as myocytes or small myotubes (i.e., 1-4 nuclei) which express both embryonic and slow MHC. They do not accumulate detectable levels of neonatal or adult fast IIB/IIX MHC. Fetal myoblasts, which develop into secondary myotubes in vivo, fuse to form large myotubes (i.e., 10-50 nuclei) and express predominantly embryonic MHC at 3 days in culture. These myotubes accumulate neonatal and adult fast IIB/IIX isoforms of MHC and eventually contract spontaneously. In contrast to embryonic myotubes, they do not accumulate slow MHC. Our results demonstrate that embryonic and fetal rat myoblasts express different phenotypes in vitro and suggest that they represent distinct myoblast lineages similar to those previously described in chickens and mice. These two lineages may be responsible for the generation of distinct populations of 1 degree and 2 degrees myotubes in vivo.

Myogenic cell lineages

For many years the mechanisms by which skeletal muscles in higher vertebrates come to be composed of diverse fiber types distributed in distinctive patterns has interested cell and developmental biologists. The fiber composition of skeletal muscles varies from class to class and from muscle to muscle within the vertebrates. The developmental basis for these events is the subject of this review. Because an individual multinucleate vertebrate skeletal muscle fiber is formed by the fusion of many individual myoblasts, more attention, in recent times, has been directed toward the origins and differences among myoblasts, and more emphasis has been placed on the lineal relationship of myoblasts to fibers. This is a review of studies related to the concepts of myogenic cell lineage in higher vertebrate development with emphases on some of the most challenging problems of myogenesis including the embryonic origins of myogenic precursor cells, the mechanisms of fiber type diversity and patterning, the distinctions among myoblasts during myogenesis, and the current hypotheses of how a variety of factors, intrinsic and extrinsic to the myoblast, determine the definitive phenotype of a muscle fiber.

Developmentally regulated expression of three slow isoforms of myosin heavy chain: diversity among the first fibers to form in avian muscle

At least three slow myosin heavy chain (MHC) isoforms were expressed in skeletal muscles of the developing chicken hindlimb, and differential expression of these slow MHC isoforms produced distinct fiber types from the outset of skeletal muscle myogenesis. Immunohistochemistry with isoform-specific monoclonal antibodies demonstrated differences in MHC content among the fibers of the dorsal and ventral premuscle masses and distinctions among fibers before splitting of the premuscle masses into individual muscles (Hamburger and Hamilton Stage 25). Immunoblot analyses by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of myosin extracted from the hindlimb demonstrated the presence throughout development of different mobility classes of MHCs with epitopes associated with slow MHC isoforms. Immunopeptide mapping showed that one of the MHCs expressed in the embryonic limb was the same slow MHC isoform, slow MHC1 (SMHC1), that is expressed in adult slow muscles. SMHC1 was expressed in the dorsal and ventral premuscle masses, embryonic, fetal, and some neonatal and adult hindlimb muscles. In the embryo and fetus SMHC1 was expressed in future fast, as well as future slow muscles, whereas in the adult only the slow muscles retained expression of SMHC1. Those embryonic muscles destined in the adult to contain slow fibers or mixed fast/slow fibers not only expressed SMHC1, but also an additional slow MHC not previously described, designated as slow MHC3 (SMHC3). Slow MHC3 was shown by immunopeptide mapping to contain a slow MHC epitope (reactive with mAb S58) and to be structurally similar to a MHC expressed in the atria of the adult chicken heart. SMHC3 was designated as a slow MHC isoform because (i) it was expressed only in those muscles destined to be of the slow type in the adult, (ii) it was expressed only in primary fibers of muscles that subsequently are of the slow type, and (iii) it had an epitope demonstrated to be present on other slow, but not fast, isoforms of avian MHC. This study demonstrates that a difference in phenotype between fibers is established very early in the chicken embryo and is based on the fiber type-specific expression of three slow MHC isoforms.