Slow myosin in developing rat skeletal muscle

NADH-O2 oxidoreductase activity and mRNA expression of complex I (51 kDa, ND1) in postnatal intrinsic muscle of rat tongue

Complex I is one of the respiratory chain enzymes related to NADH dehydrogenase and is an encoded gene product derived from both nuclear and mitochondrial genomes. Transcription levels of ND1 (mitochondrial) and 51 kDa (nuclear) subunits of complex I in the postnatal development of the intrinsic muscle in rat tongues were determined by Northern blot analysis. Enzyme activity levels were determined by NADH staining with tetrazolum salt, and oxygen consumption of NADH-O2 oxidoreductase activity using a Clark-type electrode. The detailed structure of the mitochondria was observed using electron microscopy. The cross-sectional area of the mitochondria gradually increased during postnatal development, and the cristae also became complex, despite the length of mitochondria in muscle fibre being constant. The mitochondria density increased from birth to 15 days of age, and declined slightly afterwards. This pattern of density resembled that of NADH-O2 oxidoreductase activity. The level of mRNA for ND1 through Northern blot analysis gradually increased from birth to 15 days of age and was highest at 21 days. For 51 kDa, the level was highest at 0 days and fell thereafter to a constant low. This suggests that the production of NADH dehydrogenase is limited by 51 kDa of Complex I derived from nuclear genomes rather than by the increase in mitochondria and composition of muscle fibre types due to changes in feeding behaviour.

Slow myosins in muscle development

Myogenesis has been a system central to investigations on mechanisms of diversification within groups of differentiating cells. Diversity among cell types has been well described in striated muscle tissue at the protein and enzymatic-function levels for decades, but it is only in recent years that some understanding of the molecular mechanisms responsible for this diversity has begun to emerge. Study of the expression of the slow isoforms of the myosin heavy chain has contributed to our understanding of how cell diversity arises within skeletal and cardiac muscle. Slow MyHc isoforms are developmentally responsive to a number of cues provided by the nervous systems, the endocrine system and, later in development, to functional demands on these developing tissues. Perhaps most informative have been studies on the mechanism for regulation of slow MyHc expression in mammals and birds where studies on the calcineurin-NF-AT pathways and nuclear hormone action have been shown to control MyHC gene expression in skeletal muscle and in the developing heart. The mechanisms involved in cell diversification in myogenesis are undoubtedly more varied and complex than those currently offered to explain cell diversification, but these recent studies have broadened our understanding of the interplay between the nervous system, the endocrine system and cell lineages in controlling cell diversification. Greater focus on the first fibers and cardiomyocytes to form in the embryo are likely to bring additional insights into the mechanism crucial for establishing the patterns of diversity required for successful formation of embryonic tissues.

Myosin heavy chain and physiological adaptation of the rat diaphragm in elastase-induced emphysema

BACKGROUND

Several physiological adaptations occur in the respiratory muscles in rodent models of elastase-induced emphysema. Although the contractile properties of the diaphragm are altered in a way that suggests expression of slower isoforms of myosin heavy chain (MHC), it has been difficult to demonstrate a shift in MHCs in an animal model that corresponds to the shift toward slower MHCs seen in human emphysema.

METHODS

We sought to identify MHC and corresponding physiological changes in the diaphragms of rats with elastase-induced emphysema. Nine rats with emphysema and 11 control rats were studied 10 months after instillation with elastase. MHC isoform composition was determined by both reverse transcriptase polymerase chain reaction (RT-PCR) and immunocytochemistry by using specific probes able to identify all known adult isoforms. Physiological adaptation was studied on diaphragm strips stimulated in vitro.

RESULTS

In addition to confirming that emphysematous diaphragm has a decreased fatigability, we identified a significantly longer time-to-peak-tension (63.9 +/- 2.7 ms versus 53.9 +/- 2.4 ms). At both the RNA (RT-PCR) and protein (immunocytochemistry) levels, we found a significant decrease in the fastest, MHC isoform (IIb) in emphysema.

CONCLUSION

This is the first demonstration of MHC shifts and corresponding physiological changes in the diaphragm in an animal model of emphysema. It is established that rodent emphysema, like human emphysema, does result in a physiologically significant shift toward slower diaphragmatic MHC isoforms. In the rat, this occurs at the faster end of the MHC spectrum than in humans.

Molecular and cellular mechanisms involved in the generation of fiber diversity during myogenesis

Skeletal muscles have a characteristic proportion and distribution of fiber types, a pattern which is set up early in development. It is becoming clear that different mechanisms produce this pattern during early and late stages of myogenesis. In addition, there are significant differences between the formation of muscles in head and those found in rest of the body. Early fiber type differentiation is dependent upon an interplay between patterning systems which include the Wnt and Hox gene families and different myoblast populations. During later stages, innervation, hormones, and functional demand increasingly act to determine fiber type, but individual muscles still retain an intrinsic commitment to form particular fiber types. Head muscle is the only muscle not derived from the somites and follows a different development pathway which leads to the formation of particular fiber types not found elsewhere. This review discusses the formation of fiber types in both head and other muscles using results from both chick and mammalian systems.

Neurotrophic factors enhance the survival of muscle fibers in EDL, but not SOL, after neonatal nerve injury

Neonatal sciatic nerve crush results in a sustained reduction of the mass of both extensor digitorum longus (EDL) and soleus (SOL) muscles in the rat. Type IIB fibers are selectively lost from EDL. We have investigated the effects of ciliary neurotrophic factor (CNTF) combined with neurotrophin (NT)-3 or NT-4 on muscle mass, as well as the number, cross-sectional area, and distribution of muscle fiber types and the number of motor neurons innervating EDL and SOL 3 mo after transient axotomy 5 days after birth. Both NT treatments prevented the axotomy-induced loss of muscle mass in both EDL and SOL and of total number of muscle fibers in EDL but not in SOL. Although IIB fiber loss was not prevented, both NT treatments resulted in altered fiber type distribution. Both NT combinations also reduced the loss of EDL motor neurons. These data suggest that a differential distribution of NT receptors on either motor neurons or muscle fibers may lead to different levels of susceptibility to neonatal axotomy.

Fibre type regionalisation in lower hindlimb muscles of rabbit, rat and mouse: a comparative study

The topographical distribution of different fibre types in muscles of the lower hindlimb in rabbits and mice was quantitatively determined. The results were compared to those previously obtained, using the same new quantification methods, in homologous muscles of the rat. Type I fibres ('slow') were identified using myofibrillar ATPase histochemistry and mapped out at the mid proximo-distal level for 11 'fast' muscles in the rabbit and 7 'fast' muscles in the mouse. For the slow soleus muscle the procedure was undertaken for the type II fibres. Furthermore, for 5 of the 'fast' muscles from each animal species (extensor digitorum longus; flexor digitorum and hallucis longus; gastrocnemius medialis; peroneus longus; tibialis anterior), several more proximal and distal cross-sectional levels were also analysed. All the investigated 'fast' muscles showed a significant degree of topographical eccentricity in the midlevel distribution of type I fibres. For most muscles, the direction of this 'vector regionalisation' of type I fibres was similar between the three animal species. For homologous muscles, the degree of vector regionalisation was significantly different: mouse > rat > rabbit. The relative area of the region containing the type I fibres, inversely related to the degree of 'area regionalisation', was also significantly different: mouse < rat < rabbit. Also within each animal species, muscles with a marked degree of vector regionalisation tended to show a marked area regionalisation. Proximo-distal differences in type I fibre density were observed in all the three species of animals; also these patterns showed marked inter-species differences. The findings demonstrate the general occurrence of, and systematic relationships between, different aspects of type I fibre regionalisation. The observed interspecies differences suggest that the expression of this phenomenon is adapted to differing functional needs.

Expression of the thrombin receptor (PAR-1) during rat skeletal muscle differentiation

The serine protease thrombin has been proposed to be involved in neuromuscular plasticity. Its specific receptor "protease activated receptor-1" (PAR-1), a G protein-coupled receptor, has been shown to be expressed in myoblasts but not after fusion (Suidan et al., 1996 J Biol Chem 271:29162-29169). In the present work we have investigated the expression of PAR-1 during rat skeletal muscle differentiation both in vitro and in vivo. Primary cultures of rat foetal skeletal muscle, characterized by their spontaneous contractile activity, were used for exploration of PAR-1 by RT-PCR, immunocytochemistry and Western blotting. Our results show that PAR-1 mRNA and protein are both present in myoblasts and myotubes. Incubation of myotubes loaded with fluo-3-AM in presence of thrombin (200 nM) or PAR-1 agonist peptide (SFLLRN, 500 microM), induced the intracellular release of calcium indicating the activation of PAR-1. Blockade of contractile activity by tetrodotoxin (TTX, 6 nM) did not modify either PAR-1 synthesis or its cellular localization. Investigation of PAR-1 on rat muscle cryostat sections at Day 18 of embryogenesis and postnatal Days 1, 5, and 10 indicated that this protein is first expressed in the cytoplasm and that it later localizes to the membrane. Moreover, its expression correlates with myosin heavy chain transitions occurring during post-natal period and is restricted to primary fibers. Taken together, these results suggest that PAR-1 expression is not related to contractile activity but to myogenic differentiation.

Interrelations of myogenic response, progressive atrophy of muscle fibers, and cell death in denervated skeletal muscle

Little is known concerning the time-course and structural dynamics of reactivation of compensatory myogenesis in denervated muscle, its initiating cellular mechanisms, and the relationship between this process and the progression of postdenervation atrophy. The purpose of this study was to investigate the interrelations between temporal and spatial patterns of the myogenic response in denervated muscle and progressive atrophy of muscle fibers. Another objective was to study whether reactivation of myogenesis correlates with destabilization of the differentiated state and death of denervated muscle cells. It has remained unclear whether muscle fiber atrophy was the primary factor activating the myogenic response, what levels of cellular atrophy were associated with its activation, and whether the initiation and intensity of myogenesis depended on the local and individual heterogeneity of atrophic changes among fibers. For this reason, our objective was also to identify the levels of atrophic and degenerative changes in denervated muscle fibers that are correlated with activation of the myogenic response. We found that the reactivation of myogenesis in the tibialis anterior and extensor digitorum longus muscles of the rat starts between days 10-21 following nerve transection, before atrophy has attained advanced level, long before dead cells are found in the tissue. Formation of new muscle fibers reaches its maximum between 2 and 4 months following denervation and gradually decreases with progressive postdenervation atrophy. The myogenic response is biphasic and includes two distinct processes. The first process resembles the formation of secondary and tertiary generations of myotubes during normal muscle development and dominates during the first 2 months of denervation. During this period, activated satellite cells form new myotubes on live differentiated muscle fibers. Most of the daughter myotubes in 1- and 2-month denervated muscle develop on the surface of fast type parent muscle fibers, and some of the newly formed muscle fibers express slow myosin. Some fast type parent fibers are weakly or, more rarely, moderately immunopositive for embryonic isomyosin. This indicates that reactivation of myogenesis may also depend on the fiber type. The level of atrophy, destabilization of the differentiated myofiber phenotype, and degenerative changes of individual fibers in denervated muscle are very heterogeneous. The myogenic response of the first type is associated predominantly with fibers of average and higher than average levels of atrophy. Muscle cells that undergo a lesser degree of atrophy also form daughter fibers, although with a lower incidence. We did not find any correlation between the size of newly formed fibers and the level of atrophy of parent fibers. The topographical distribution of new myotubes both in the peripheral and central areas of the mid-belly equatorial sections at the early stages following nerve transection indicates that myogenesis of the first type represents a systemic reaction of muscle to the loss of neural control. These data indicate that activation of the myogenic response does not depend on cell death and degenerative processes per se. The second type of myogenesis is a typical regenerative reaction that occurs mainly within the spaces surrounded by the basal laminae of dead muscle fibers. Myocytes of different sizes are susceptible to degeneration and death, which indicates that cell death in denervated muscle does not correlate with levels of muscle cell atrophy. The regenerative process frequently results in development of abnormal muscle cells that branch or form small clusters. Replacement of lost fibers becomes activated between 2 and 4 months following nerve transection, i.e., mainly at advanced stages of postdenervation atrophy, when cell death becomes a contributing factor of the atrophic process. In long-term denervated muscle, the first and second types of myogenesisoccur concurrently, and the topographical distribution of the myogenic response becomes more heterogeneous than during the first weeks following denervation. Thus, our data demonstrate differential temporal and spatial expression of two patterns of myogenesis in denervated muscle that appear to be controlled by different regulatory mechanisms during the postdenervation period. (c) 2001 Wiley-Liss, Inc.

Myosin heavy chain expression and plasticity: role of myoblast diversity

Muscle fibers exhibit a limited capacity to alter their myosin heavy chain expression in response to various stimuli. Recent results pertinent to this observation are discussed, and a hypothetical scheme is presented whereby the contribution of myonuclei from distinct populations of myogenic precursors may account for this limited adaptive range.

Regulation of myosin heavy chain expression during rat skeletal muscle development in vitro

Signals that determine fast- and slow-twitch phenotypes of skeletal muscle fibers are thought to stem from depolarization, with concomitant contraction and activation of calcium-dependent pathways. We examined the roles of contraction and activation of calcineurin (CN) in regulation of slow and fast myosin heavy chain (MHC) protein expression during muscle fiber formation in vitro. Myotubes formed from embryonic day 21 rat myoblasts contracted spontaneously, and approximately 10% expressed slow MHC after 12 d in culture, as seen by immunofluorescent staining. Transfection with a constitutively active form of calcineurin (CN*) increased slow MHC by 2.5-fold as determined by Western blot. This effect was attenuated 35% by treatment with tetrodotoxin and 90% by administration of the selective inhibitor of CN, cyclosporin A. Conversely, cyclosporin A alone increased fast MHC by twofold. Cotransfection with VIVIT, a peptide that selectively inhibits calcineurin-induced activation of the nuclear factor of activated T-cells, blocked the effect of CN* on slow MHC by 70% but had no effect on fast MHC. The results suggest that contractile activity-dependent expression of slow MHC is mediated largely through the CN-nuclear factor of activated T-cells pathway, whereas suppression of fast MHC expression may be independent of nuclear factor of activated T-cells.

Proximo-distal organization and fibre type regionalization in rat hindlimb muscles

Five muscles of the rat's lower hindlimb were compared with regard to their histochemical fibre type distribution at seven different proximo-distal levels. The muscles were: extensor digitorum longus (ED), flexor digitorum and hallucis longus (FD), gastrocnemius medialis (GM), peroneus longus (PE) and tibialis anterior (TA). In all the five muscles, the relative density of the 'slow' type I fibres showed a striking and similar decrease from proximal toward more distal levels. In addition, the type I fibres were concentrated within smaller and more eccentrically placed regions at distal than at more proximal levels. As a background for the further analysis of these lengthwise aspects of type I fibre regionalization, architectural features of the muscles were determined. Pinnation angles and the position of major tendons and tendon sheets were assessed in fresh specimens. Muscle fibre lengths were measured for single fibres dissected from different regions of macerated muscles. In all cases, fibre length was much shorter than muscle length (mean fraction ranging from 21 to 55%), implying that the proximo-distal changes in histochemical fibre properties were indeed explainable as being due to gradual lengthwise changes in fibre type populations. The similarity of these lengthwise changes across the muscles was in contrast to their differences in other aspects of functional organization, such as the average density of type I fibres and architectural features determining their relative capacities for shortening and force generation. The possible functional role of the proximal accumulation of type I fibres was discussed in relation to circulation and thermal balance; besides, the proximo-distal regionalization of type I fibres might (partly) reflect processes that had been associated with early stages of muscle differentiation. Furthermore, the results underline that, when determining the fibre type composition of rat hindlimb muscles, identifying the proximo-distal level of sampling is a matter of great importance.

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.

Invited Review: plasticity and energetic demands of contraction in skeletal and cardiac muscle

Numerous studies have explored the energetic properties of skeletal and cardiac muscle fibers. In this mini-review, we specifically explore the interactions between actin and myosin during cross-bridge cycling and provide a conceptual framework for the chemomechanical transduction that drives muscle fiber energetic demands. Because the myosin heavy chain (MHC) is the site of ATP hydrolysis and actin binding, we focus on the mechanical and energetic properties of different MHC isoforms. Based on the conceptual framework that is provided, we discuss possible sites where muscle remodeling may impact the energetic demands of contraction in skeletal and cardiac muscle.

Quantification of fibre type regionalisation: an analysis of lower hindlimb muscles in the rat

Newly developed concepts and methods for the quantification of fibre type regionalisation were used for comparison between all muscles traversing the ankle of the rat lower hindlimb (n = 12). For each muscle, cross-sections from the proximodistal midlevel were stained for myofibrillar ATPase and classified as type I ('slow') or II ('fast'). For the 11 'fast' muscles (i.e. all except soleus), the muscle outline and the position of each type I fibre were digitised for further computer processing. Two potentially independent aspects of type I fibre regionalisation were evaluated quantitatively: (1) the degree to which type I fibres were restricted to a limited portion of the total cross-sectional area ('area-regionalisation'): (2) the extent and direction of the difference (if any) between the centre of the muscle cross-section and the calculated centre for the type I fibre cluster ('vector regionalisation'). Statistical analysis showed that type I fibres were vector regionalised in practically all investigated muscles and area regionalised within most of them, the only consistent exceptions being peroneus brevis and peroneus digitorum 4, 5. In muscles with a high degree of area regionalisation the population of type I fibres also had a markedly eccentric intramuscular position (i.e. high vector regionalisation). A significant relationship was observed between the relative position of a muscle within the hindlimb (transverse plane) and the direction and degree of its type I fibre eccentricity. On average, the degree of type I fibre eccentricity was greater for muscles remote from the limb centre than for those situated more centrally. In addition, the intramuscular concentration of type I fibres was typically greatest towards the centre of the limb, the most striking exception being tibialis posterior. For the slow soleus muscle, which is centrally placed within the limb, our analysis concerned the type II fibres, which were found to be weakly vector regionalised but not significantly area regionalised. It is concluded that, within muscles of the rat's lower hindlimb, fibre type regionalisation is a general and graded phenomenon which may reflect differentiating (embryological?) mechanisms of a transmuscular significance. Furthermore, the analysis demonstrated the usefulness of our new methods and concepts for the quantification of fibre type regionalisation.

Monospecific antibodies against the three mammalian fast limb myosin heavy chains

Skeletal muscle fibres in mammalian limb muscles are of four types: slow, 2A, 2X, and 2B, each characterized by a distinct myosin heavy chain (MyHC) isoform. Existing monoclonal antibodies (mabs) against fast MyHCs lack fibre-type specificity across species and could not positively identify 2X fibres. In this work, mabs were raised against each of the fast MyHCs. These mabs were shown to be monospecific by Western blots and immunohistochemistry in the rat. The advantages of using these mabs for identifying the three fast fibre types and hybrid fibres expressing multiple isoforms were illustrated using rat tibialis anterior muscle. Immunohistochemical analyses confirmed the monospecificity of these mabs in the following additional species: mouse, guinea pig, rabbit, cat, and baboon. 2B fibres were absent in limb muscles of the cat and baboon. These mabs constitute a set of powerful tools for studying muscle fibre types and their transformations.

MRF-4 exhibits fiber type- and muscle-specific pattern of expression in postnatal rat muscle

The crucial role played by the myogenic regulatory factors (MRFs) in the development of skeletal muscle has been well characterized. The continued expression of these factors in skeletal muscle of the postnatal animal has led to the suggestion that they may play a role in the regulation of muscle fiber phenotype. The few studies that have examined the expression of MRF-4 in postnatal muscle have been carried out at the whole muscle level. These studies demonstrated that this factor is expressed at a higher level than any other MRF but suggested that this was not affected by muscle phenotype. In this study, the expression of the MRF-4 transcript has been examined at the cellular level by in situ hybridization. It was observed that in the mixed fiber type muscle the gastrocnemius, MRF-4 was preferentially expressed in slow muscle fibers, but in the slow postural soleus, no fiber type specificity was observed. These observations suggest that MRF-4 may play a role in the regulation of muscle fiber phenotype in the postnatal animal.

Single-fiber myosin heavy chain polymorphism during postnatal development: modulation by hypothyroidism

The primary objective of this study was to follow the developmental time course of myosin heavy chain (MHC) isoform transitions in single fibers of the rodent plantaris muscle. Hypothyroidism was used in conjunction with single-fiber analyses to better describe a possible linkage between the neonatal and fast type IIB MHC isoforms during development. In contrast to the general concept that developmental MHC isoform transitions give rise to muscle fibers that express only a single MHC isoform, the single-fiber analyses revealed a very high degree of MHC polymorphism throughout postnatal development. In the adult state, MHC polymorphism was so pervasive that the rodent plantaris muscles contained approximately 12-15 different pools of fibers (i.e., fiber types). The degree of polymorphism observed at the single-fiber level made it difficult to determine specific developmental schemes analogous to those observed previously for the rodent soleus muscle. However, hypothyroidism was useful in that it confirmed a possible link between the developmental regulation of the neonatal and fast type IIB MHC isoforms.

Developmental myosin heavy chains in the adult human diaphragm: coexpression patterns and effect of COPD

In preliminary experiments we noted developmental (i.e., embryonic and neonatal) myosin heavy chains (MHCs) in the diaphragms of patients with severe chronic obstructive pulmonary disease (COPD). We hypothesized that this finding represented new fiber formation secondary to injury associated with the mechanical stress of COPD or previously undescribed MHCs in the human diaphragm. To distinguish between these possibilities, we analyzed diaphragmatic biopsies obtained from 9 patients with severe COPD (forced expiratory volume in 1 s = 21 +/- 2% predicted, residual volume = 283 +/- 22% predicted) and 10 age-matched controls. First, using immunocytochemistry with specific monoclonal antibodies, we noted that control diaphragms had greater proportions of fibers expressing embryonic (50 +/- 2 vs. 28 +/- 3%, P < 0.0001) and neonatal (52 +/- 2 vs. 32 +/- 3%, P < 0.001) MHCs than COPD diaphragms. Second, SDS-PAGE demonstrated that these developmental MHCs represented only a very small fraction of the diaphragmatic MHC content. Third, the RT-PCR demonstrated mRNA coding for embryonic and neonatal MHCs in COPD and control diaphragms. Last, COPD and control diaphragms exhibited normal histology on light microscopy. We conclude that the presence of developmental MHC isoforms does not indicate new fiber formation in diaphragms of patients with severe COPD. Although these results represent the first systematic description of embryonic and neonatal MHCs in normal adult human diaphragms, their function remains to be elucidated.

Development of skeletal muscles in transforming growth factor-beta 1 (TGF-beta1) null-mutant mice

Fetal transforming growth factor-beta 1 (TGF-beta1) has been postulated to regulate the onset of myotube formation and/or pattern formation in developing skeletal muscles. In apparent contradiction of these hypotheses, the development of the extensor digitorum longus and soleus in TGF-beta1 null-mutant muscle was normal. The onset of secondary myotube formation, the numbers of myotubes formed, the proportion of fast and slow fibers, and the patterns of fiber types and connective tissues were essentially identical in TGF-beta1(+/+) and TGF-beta1(-/-) mice. A portion of the TGFbeta1 in skeletal muscles is derived from the mother, via the placenta. This maternal-derived TGF-beta1 was also not essential for the development of skeletal muscles, as the characteristics of pups born to a TGF-beta1(-/-) mother were normal TGF-beta1(-/-) mice die at weaning due to a generalized autoimmune attack. This postnatal death was circumvented by breeding the TGF-beta1 null mutation into nude mice (Whn(-/-)). Like many other strains of TGF-beta1(-/-) mice, extensive loss of Whn(-/-), TGF-beta1(-/-) embryos occurred in utero. However, a portion of the Whn(-/-), TGF-beta1(-/-) mice survived past weaning, remained healthy, and were fertile. The TGF-beta1(-/-) x Whn(-/-) mouse thus represents a valuable tool for the study of the function of TGF-beta1 in the adult, including its putative role as a pregnancy-related hormone.

Evidence for differential post-translational modifications of slow myosin heavy chain during murine skeletal muscle development

The contractile properties of muscle fibres are, in part, determined by the myosin heavy chain (MyHC) isoforms they express. Using monoclonal antibodies, we show that at least three forms of slow twitch MyHC accumulate sequentially during mouse fetal development and that slow MyHC maturation in slow fibres occurs before expression of the adult fast MyHCs in fast fibres. Expression of deletion derivatives of beta-cardiac MyHC cDNA shows that the slow MyHC epitopes that are detected in adult but not in young animals are located near the N-terminus. The same N-terminal region of various fast MyHC molecules contains a conserved epitope that can, on occasions, be observed when slow MyHC cDNA is expressed in non-muscle cells. The results raise the possibility that the N-terminal epitopes result from post-translational modification of the MyHC and that a sequence of slow and fast MyHC isoform post-translational modifications plays a significant role during development of murine muscle fibres.

The induction of skeletal muscle hypertrophy by a ski transgene is promoter-dependent

The chicken c-ski gene expresses at least three alternatively spliced messages. Transgenic mice expressing proteins from cDNA corresponding to two of these messages (FB27 and FB29) under the control of a murine sarcoma virus (MSV) long terminal repeat (LTR) express the transgene in skeletal muscle and develop a muscular phenotype. Both a biologically active form of c-ski and the MSV LTR are required for the development of the muscular phenotype. The normal c-ski gene linked to two other tissue-specific promoters failed to induce muscle growth in transgenic mice, as did an inactive mutant of c-ski expressed under the control of the MSV LTR.

Myosin heavy chain profiles in regenerated fast and slow muscles innervated by the same motor nerve become nearly identical

Plasticity of mature muscles exposed to different activation patterns is limited, probably due to restricted adaptive range of their muscle fibres. In this study, we tested whether satellite cells derived from slow muscles can give rise to a normal fast muscle, if transplanted to the fast muscle bed. Marcaine-treated rat soleus and extensor digitorum longus (EDL) muscles were transplanted to the EDL muscle bed and innervated by the 'EDL' nerve. Six months later expression of myosin heavy chain isoforms was analysed by areal densities of fibres, binding specific monoclonal antibodies, and by SDS gel electrophoresis. Both regenerated muscles closely resembled each other. Their myosin heavy chain profiles were similar to those in fast muscles although they were not identical to that in the control EDL muscle. Since not even regenerated EDL was able to reach the myosin heavy chain isoform profile of mature EDL muscle, our experimental model did not permit studying the adaptive capacity of satellite cells in different muscles in its whole extent. However, the results favour the multipotential myoblast stem cell population in rat muscles and underline the importance of the extrinsic regulation of muscle phenotype.

Identification and expression analysis of two developmentally regulated myosin heavy chain gene transcripts in carp (Cyprinus carpio)

Whilst developmentally regulated genes for the myosin heavy chain (MyoHC) have been characterised in mammalian, avian and amphibian species, no developmental MyoHC gene has previously been characterised in a species of fish. In this study, we identify two developmentally regulated MyoHC gene transcripts (named Eggs22 and Eggs24) in carp (Cyprinus carpio) and characterise their expression patterns during embryonic and larval development. The transcripts showed an identical temporal pattern of expression commencing 22 h post-fertilisation (18 degrees C incubation temperature), coincident with the switch from exclusive expression of genes for beta-actin to expression of genes for both beta- and alpha-actin, and continuing for 2 weeks post-hatching. No expression of these myosin transcripts was detected in juvenile or adult carp. Wholemount in situ hybridisation showed that both transcripts are expressed initially in the rostral region of the developing trunk and progress caudally. Both are expressed in the developing pectoral fin and protractor hyoideus muscles. However, the muscles of the lower jaw express only the Eggs22 transcript. No expression of either transcript was detected in cardiac or smooth muscle. A distinct chevron pattern of expression was observed in the myotomal muscle. This was shown to be caused by localisation of the mRNAs to the myoseptal regions of the fibres, the sites of new sarcomere addition during muscle growth, suggesting transport of MyoHC mRNA transcripts. The 3′ untranslated region of the Eggs24 transcript contains a 10 base pair motif (AAAATGTGAA) which is shown to be also present in the 3′ untranslated regions of MyoHC genes from a wide range of species. Possible reasons for the need for developmental isoforms of myosin heavy chain isoforms are discussed.

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.

Primary myotubes preferentially mature into either the fastest or slowest muscle fibers

Myoblasts and myotubes are heterogeneous, but what is the significance of this heterogeneity? Is it a vital component of the mechanism by which a muscle develops or is it part of the process that generates mature fibers with diverse sizes, speeds of contracture, and metabolisms? We have begun to explore these questions by using BrdU to selectively label rat primary myotubes, thus enabling their mature characteristics to be defined for the first time. In the soleus, the type I fibers of primary myotube origin were 21% larger than those of secondary myotube origin, indicating that the origin of a fiber can affect its mature force production. In the extensor digitorum longus (EDL), the primary myotubes differentiated into all known fibers types, but with marked variation in frequency. In the superficial portion of the EDL, 97% of primary myotubes became IIB fibers, even though approximately 41% of the fibers in this region are IIA or IIX. In the deep portion, primary myotubes preferentially developed into type I fibers. Thus, primary myotubes in the EDL predominantly differentiate into the two most dissimilar fiber types: the slowest, smallest, most oxidative, type I fibers and the largest, fastest, most glycolytic, type IIB fibers. Each of the subtypes of primary myotubes had a different fate. In the EDL, the slow and fast primary myotubes appeared to differentiate into type I and IIB fibers, respectively. This implies that spatial and temporal signals operating in the limb are major determinants of the mature pattern of fiber types and that innervation of a muscle involves a selective matching between the various types of motoneurons and muscle fibers.

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.

Alteration in myosatellite cell commitment with muscle maturation

Myosatellite cells are myoblasts found between the basal lamina and sarcolemma of myofibers of postnatal mice. The extent to which these cells are programmed, upon differentiation, to express isoforms of contractile protein genes specific to the type of fiber with which they are associated has been evaluated in vitro using myosatellite cells derived from the soleus and the extensor digitorum longus muscles (EDL) of 4-day-old and adult transgenic mice, which express nuclear localizing beta-galactosidase (nlsbeta-gal) under the control of the promoter and 3' enhancer of the gene encoding fast myosin light chain 3F (MLC3F) (Kelly et al. [1995] J. Cell Biol. 129:383-396). Cultures were allowed to differentiate either as myocytes (mononucleated cells), to prevent possible modification of the myosatellite phenotype by other myonuclei in mosaic myotubes, or as myotubes. Transgene expression was age related, with 90% and 70% of the myocytes derived from the neonatal EDL and soleus muscles (muscles that had not yet achieved their mature phenotype), respectively, having nuclei encoding beta-gal; 61% and 32% of the myocyte nuclei derived from myosatellite cells of the adult EDL (a fast muscle) and the adult soleus muscle (a mixed muscle containing many slow myofibers), respectively, expressed this transgene. Because myosatellite cells found in adult muscles are the progeny of those found in the neonate, an alteration of myosatellite cell commitment to express this transgene occurs with muscle maturation. Because expression of the transgene in neonatal and adult muscle in vivo reflects the expression of the endogenous MLC3F gene (Kelly et al. [1995] J. Cell Biol. 129:383-396), it is likely that expression of the transgene by differentiated myosatellite cells reflects the extent of commitment of these cells to produce MLC3F. A hypothesis is presented that MLC3F is widely expressed in developing muscles but eliminated in myofibers that undergo maturation toward a slower phenotype.

Role of the nerve in determining fetal skeletal muscle phenotype

To determine the role of the nerve on the establishment of myofiber diversity in skeletal muscles, the lumbosacral spinal cord of 14-day gestation mice (E14) was laser ablated, and the accumulation of the myosin alkali light chains (MLC) mRNAs in crural (hindleg) muscles was evaluated just prior to birth with in situ hybridization. Numbers of molecules of each alkali MLC/ng total RNA in the extensor digitorum longus (EDL) and soleus muscles were determined with competitive polymerase chain reaction. Transcripts for all four alkali MLCs accumulate in aneural muscles. Data suggest that: (1) the absence of the nerve to either future fast or slow muscles results in less accumulation of MLC1V transcript. Moreover, the presence of the nerve is required for the enhanced accumulation of this transcript in future slow muscles; (2) the absence of innervation of future slow, but not fast, muscles decreases the accumulation of MLC1A transcript. Since increased accumulation of MLC1A and MLC1V transcripts are found in future slow muscles at birth, the nerve is necessary for the development of the slow phenotype during myogenesis; (3) MLC1F and MLC3F transcripts do not display any preferential accumulation in future fast muscles during the fetal period. Therefore, the establishment of the differential distribution of these mRNAs, based on fiber type, is a postnatal phenomenon. The nerve is required during the fetal period to allow accumulation of MLC3F messages above a basal level in future fast as well as slow muscles; whereas, the absence of the innervation to future fast, but not slow, muscles reduces the accumulation of MLC1F. Thus, the accumulation of the various alkali MLC mRNAs shows a differential, rather than coordinate, response to the absence of the nerve, and this response may vary depending on the future fiber type of the muscles.

Cellular adaptations in the diaphragm in chronic obstructive pulmonary disease

BACKGROUND

In patients with severe chronic obstructive pulmonary disease, the diaphragm undergoes physiologic adaptations characterized by an increase in energy expenditure and relative resistance to fatigue. We hypothesized that these physiologic characteristics would be associated with structural adaptations consisting of an increased proportion of less-fatigable slow-twitch muscle fibers and slow isoforms of myofibrillar proteins.

METHODS

We obtained biopsy specimens of the diaphragm from 6 patients with severe chronic obstructive pulmonary disease (mean [+/-SE] forced expiratory volume in one second, 33+/-4 percent of the predicted value; residual volume, 259+/-25 percent of the predicted value) and 10 control subjects. The proportions of the various isoforms of myosin heavy chains, myosin light chains, troponin, and tropomyosin were determined by sodium dodecyl sulfate-polyacrylamide-gel electrophoresis. We also used immunocytochemical techniques to determine the proportions of the various types of muscle fibers.

RESULTS

The diaphragm-biopsy specimens from the patients had higher percentages of slow myosin heavy chain I (64+/-3 vs. 45+/-2 percent, P<0.001), and lower percentages of fast myosin heavy chains IIa (29+/-3 vs. 39+/-2 percent, P=0.01) and IIb (8+/-1 vs. 17+/-1 percent, P<0.001) than the diaphragms of the controls. Similar differences were noted when immunohistochemical techniques were used to compare the percentages of these fiber types in the two groups. In addition, the patients had higher percentages of the slow isoforms of myosin light chains, troponins, and tropomyosin, whereas the controls had higher percentages of the fast isoforms of these proteins.

CONCLUSIONS

Severe chronic obstructive pulmonary disease increases the slow-twitch characteristics of the muscle fibers in the diaphragm, an adaptation that increases resistance to fatigue.

Transitory expression of alpha cardiac myosin heavy chain in a subpopulation of secondary generation muscle fibers in the pig

Unlike the random distribution of fiber types seen in skeletal muscles of most mammals, pig muscle exhibits a rosette pattern consisting of islets of slow fibers surrounded by concentric circles of type IIA and IIB fibers. Within each islet of slow fibers, one of the central fibers is a primary myofiber, whereas all others are secondary fibers. The present study demonstrates that a subpopulation of the slow secondary fibers transiently expresses alpha-myosin heavy chain (MHC). Two cDNA libraries were made from longissimus dorsi skeletal muscle of 14-day-old piglet and adult pig atrium; the latter muscle is mainly composed of alpha-MHC. Screening of the libraries with a human anti-alpha-MHC mAb (F8812F8) demonstrated the presence of positive MHC clones in both libraries; the nucleotide sequence of the 3'-untranslated region (3'-UTR) was identical in both libraries. As this MHC 3'-UTR had 75% homology with the human alpha-MHC, it was identified as pig alpha-MHC. Using specific cRNA probes and mAbs against pig alpha-cardiac and beta/slow/type I MHC, we studied the expression of these MHCs in developing pig semitendinosus muscle by combining in situ hybridization and immunocytochemistry on serial sections at 90 days of gestation, and at 1, 6, 35 days and 6 months of age. The results showed that a subpopulation of secondary fibers that directly abut primary fibers, transiently produced alpha-MHC, both at the levels of the protein and its transcript. Subsequently, these fibres expressed beta-MHC. At 1 day, immunocytochemistry showed that 16% of the secondary fibers expressed alpha-MHC, among which 20% did not yet express beta-MHC. At 6 days, alpha- and beta-MHCs were mostly present in the same fibers, i.e., 23% of the secondary fibers. Thereafter, the proportion of secondary fibers reacting with alpha-MHC mAb decreased to 10% at 5 weeks and 0% at 6 months, whereas beta-MHC was still accumulating in about 38% of the secondary fibers. During the period studied, the distribution of alpha- and beta-MHC transcripts closely matched that of the corresponding proteins. Expression of alpha-MHC was not detected in primary type I muscle fibers and slow type I secondary fibers at the periphery of the rosettes of slow fibers. This study is the first unequivocal demonstration of a transitory expression of alpha-MHC in a subpopulation of secondary fibers in a limb skeletal muscle during mammalian 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.

Contractile properties and myosin heavy chain composition of newborn rat soleus muscles at different stages of postnatal development

This study was undertaken to correlate some of the functional characteristics with the myofibrillar composition in myosin heavy chain isoforms on newborn and adult rat soleus muscles. The following postnatal ages were chosen in order to determine the role of innervation in the establishment of the mature muscle phenotype: before (postnatal day 6), when (postnatal day 12), and after (days 17 and 23) the monosynaptic innervation appeared. The steady state of definitive innervation was controlled on adult muscles (i.e. approximately 13 weeks). Muscle maturation was followed by ATPase staining and fibre diversity was observed at postnatal day 12. The functional properties of skinned bundles isolated from newborn rats were determined by Calcium/Strontium activation characteristics (Tension/pCa and pSr relationships). From postnatal days 6 to 17, the Soleus bundles exhibited Calcium/Strontium activation characteristics intermediate between slow and fast fibre populations previously described in muscles. At day 23, the Calcium/Strontium activation characteristics of the soleus were closer to those of a slow type. Moreover, we observed a decrease in Ca affinity concomitant with the installation of the monosynaptic innervation, and an increase of the slow type I during postnatal development. Finally, this work reported a greater correlation between the Calcium/Strontium activation parameters and the myosin heavy chain isoform composition at the postnatal days when the mature monosynaptic innervation pattern occurred.

Regenerated rat fast muscle transplanted to the slow muscle bed and innervated by the slow nerve, exhibits an identical myosin heavy chain repertoire to that of the slow muscle

The hypothesis that the limited adaptive range observed in fast rat muscles in regard to expression of the slow myosin is due to intrinsic properties of their myogenic stem cells was tested by examining myosin heavy chain (MHC) expression in regenerated rat extensor digitorum longus (EDL) and soleus (SOL) muscles. The muscles were injured by bupivacaine, transplanted to the SOL muscle bed and innervated by the SOL nerve. Three months later, muscle fibre types were determined. MHC expression in muscle fibres was demonstrated immunohistochemically and analysed by SDS-glycerol gel electrophoresis. Regenerated EDL transplants became very similar to the control SOL muscles and indistinguishable from the SOL transplants. Slow type 1 fibres predominated and the slow MHC-1 isoform was present in more than 90% of all muscle fibres. It contributed more than 80% of total MHC content in the EDL transplants. About 7% of fibres exhibited MHC-2a and about 7% of fibres coexpressed MHC-1 and MHC-2a. MHC-2x/d contributed about 5-10% of the whole MHCs in regenerated EDL and SOL transplants. The restricted adaptive range of adult rat EDL muscle in regard to the synthesis of MHC-1 is not rooted in muscle progenitor cells; it is probably due to an irreversible maturation-related change switching off the gene for the slow MHC isoform.

Patterns of abnormal protein expression in target formations and unstructured cores

Streaming of Z-disks and focal myofibrillar degeneration occur in target formations (TF) and unstructured cores (UC). Similar myofibrillar alterations are also part of the spectrum of ultrastructural reactions that can occur in the myopathies associated with myofibrillar degeneration and abnormal foci of desmin positivity. In the latter disorders, there is ectopic overexpression of dystrophin, neural cell adhesion molecule (NCAM), gelsolin, beta-amyloid precursor protein (beta APP) epitopes, alpha 1-antichymotrypsin (alpha 1-ACT), and many abnormal fiber regions are also strongly congophilic. Therefore, we searched for similar abnormalities in TF and UC. The UC and the center of TF show increased immunoreactivity for actin, alpha-actinin, gelsolin, dystrophin, beta APP epitopes, alpha 1-ACT, beta 2-microglobulin, desmin, and NCAM, but minimal or no congophilia. The periphery of the TF reacts strongly for nebulin but not for actin. The observed immunocytochemical alterations in TF and UC may represent a stereotyped cellular response associated with myofibrillar degeneration due to any cause. However, the three-dimensional profile of the TF and UC as well as their fiber-type specificity distinguish them from lesions that have similar immunocytochemical profiles in other myopathies.

Adaptive range of myosin heavy chain expression in regenerating soleus is broader than in mature muscle

In adult rat muscles experimentally exposed to various patterns of activation, expression of myosin heavy chain isoforms changes, but only within a certain adaptive range. It is characteristic and different in fast or slow muscles. This may be due either to different intrinsic properties of the myogenic cells of the two types of muscles or to extrinsic factors. To test these assumptions, either rat soleus or extensor digitorum longus muscles were injured and transplanted to the bed of the extensor digitorum longus muscle. They regenerated and were reinnervated by the extensor digitorum longus nerve. Expression of myosin heavy chain isoforms was demonstrated immunohistochemically and by in situ hybridization, and analysed by SDS-gel electrophoresis. Three months after cross-transplantation, regenerated soleus expressed all adult myosin heavy chain isoforms, including the myosin heavy chain-2B. The latter was detected in about 50% of muscle fibres and contributed about 10-20% of all myosin heavy chains. The same percentage of myosin heavy chain-2B was found in regenerated extensor digitorum longus. In this regard therefore, the adaptive range of the regenerated soleus muscle was not significantly different from that of the extensor digitorum longus regenerating under the same conditions. This indicates that restriction of the adaptive range in a mature soleus muscle is not due to intrinsic properties of its myogenic cells. It is probably imposed by an extrinsic factor leading to irreversible shut-down of individual myosin heavy chain genes. On the other hand, myosin heavy chain-1 expression was significantly greater in the regenerated soleus than in the extensor digitorum longus innervated by the same nerve. Myosin heavy chain-1 and myosin heavy chain-2B were co-expressed in some regenerated soleus muscle fibres.

Isolation and characterization of an avian slow myosin heavy chain gene expressed during embryonic skeletal muscle fiber formation

We have isolated and begun characterization of the quail slow myosin heavy chain (MyHC) 3 gene, the first reported avian slow MyHC gene. Expression of slow MyHC 3 in skeletal muscle is restricted to the embryonic period of development, when the fiber pattern of future fast and slow muscle is established. In embryonic hindlimb development, slow MyHC 3 gene expression coincides with slow muscle fiber formation as distinguished by slow MyHC-specific antibody staining. In addition to expression in embryonic appendicular muscle, slow MyHC 3 is expressed continuously in the atria. Transfection of slow MyHC 3 promoter-reporter constructs into embryonic myoblasts that form slow MyHC-expressing fibers identified two regions regulating expression of this gene in skeletal muscle. The proximal promoter, containing potential muscle-specific regulatory motifs, permits expression of a reporter gene in embryonic slow muscle fibers, while a distal element, located greater than 2600 base pairs upstream, further enhances expression 3-fold. The slow muscle fiber-restricted expression of slow MyHC 3 during embryonic development, and expression of slow MyHC 3 promoter-reporter constructs in embryonic muscle fibers in vitro, makes this gene a useful marker to study the mechanism establishing the slow fiber lineage in the embryo.

Developmental Modulation of a beta myosin heavy chain promoter-driven transgene

The molecular mechanisms underlying heart and skeletal muscle-specific gene expression during development and in response to physioloic stimuli are largely unknown. Using a novel immunohistochemical procedure to detect chloramphenicol acetyltransferase (CAT), we have investigated, in vivo at high resolution, the ability of cis-acting DNA sequences within the 5' flanking region of the mouse beta myosin heavy chain (MyHC) gene (beta-MyHC) to direct appropriate gene expression throughout development. A 5.6-kb fragment 5' to the beta-MyHC's transcriptional start site was linked to the reporter gene encoding CAT (cat) and used to generate transgenic mice. The anti-CAT in situ assay described in this report allowed us to define the ability of the promoter fragment to direct appropriate temporal, tissue- and muscle fiber type-specific gene expression throughout early development. In skeletal muscles, the transgene expression profile mimics the endogenous beta-myHC's at all developmental stages and is appropriately restricted to slow (type I) skeletal fibers in the adult. Surprisingly, transgene expression was detected in both the atria and ventricles during embryonic and fetal development, indicating that ventricular specification involves elements outside the 5.6-kb fragment. In contrast, in the adult, hypothyroid conditions led to transgene induction specifically in the ventricles, suggesting that distinct regulatory mechanisms control fetal versus adult beta-MyHC expression in the cardiac compartment.

Spatial and temporal patterns of myosin heavy chain expression in developing rat extraocular muscle

The present study describes transitions in myosin heavy chain expression in the extraocular muscles of rats between the ages of E17 and adult. The unique phenotype of the extraocular muscle is reflected in its fibre type composition, which is comprised by six distinct profiles, each defined by location (orbital versus global layer) and innervation pattern (single versus multiple terminals). During extraocular muscle myogenesis, developmental myosin heavy chains were expressed in both primary and secondary fibres from embryonic day E17 through the first postnatal week. At this time, the downregulation of developmental myosin heavy chain isoforms began in the global layer in a fibre type-specific manner, reaching completion only after the first postnatal month. By contrast, developmental isoforms were retained in the overwhelming majority of orbital layer fibres into adulthood and expressed differentially along the length of these fibres. Fast myosin heavy chain was detected pre- and postnatally in developing secondary fibres and in all of the singly innervated fibre types and one of the multiply innervated fibre types in the adult. As many as four fast isoforms were detected in maturing extraocular muscle, including the extraocular muscle-specific myosin heavy chain. Slow myosin heavy chain was expressed in primary fibres throughout development and in one of the multiply innervated fibre types in the adult. In contrast, the pure fast-twitch retractor bulbi initially expressed slow myosin heavy chain in fibres destined to switch to the fast myosin heavy chain developmental programme. Based upon spatial and temporal patterns of myosin heavy chain isoform transitions, we suggest that epigenetic influences, rather than purely myogenic stage-specific factors, are critical in determining the unique extraocular muscle phenotype.

Muscle fiber type correlates with innervation topography in the rat serratus anterior muscle

Previous studies have reported that motoneurons from the sixth spinal nerve (C6) innervate the majority of muscle fibers in the rat serratus anterior (SA) muscle. The seventh spinal nerve (C7) innervates a limited number of SA fibers, increasing caudally. This topographic map is partially reestablished following denervation. In the present study, muscle fibers of the SA were stained with monoclonal antibodies for the muscle-specific fast myosin heavy chain (F-MHC) and slow myosin heavy chain (S-MHC) proteins. We found that the majority of fibers in the SA muscle stained for F-MHC antibody, and the percentage of muscle fibers staining for S-MHC antibody increased caudally. When newborn SA muscles were denervated and then reinnervated by the entire long thoracic (LT) nerve or only the C6 branch to the LT nerve, the reinnervated muscle had the normal proportion of muscle fibers expressing S-MHC protein. However, if the LT nerve was crushed and only C7 motoneurons allowed to reinnervate the SA muscle, a greater percentage of muscle fibers stained for S-MHC antibody than normal. We conclude that there is a correlation between muscle fiber type and innervation topography in the SA muscle of the rat.

Phenotype of adult mouse muscle myoblasts reflects their fiber type of origin

Phenotypic diversity among mature skeletal muscle fibers originates from muscle progenitor cells, primary and secondary myoblasts, each of which is intrinsically committed to express a characteristic complement of developmentally regulated myosin heavy chain genes when differentiated. Similarly, postnatal muscle myoblasts, the satellite cells nestling beneath basement membranes of mature skeletal muscle fibers, have been shown to exhibit diversity, related to whether the muscle in which they reside is of a slow, fast or superfast type. Here we analyzed this association in more detail, evaluating the myosin heavy chain gene expression in immature muscle fibers (myotubes) formed in vitro from satellite cells extracted from isolated, living, single muscle-fibers of mature murine muscle. We identified a population of satellite cells that form myotubes expressing type I (slow) myosin heavy chain and found this population to be preferentially associated with individual slow muscle-fibers. These results not only confirm diversity among mammalian satellite cells, but also demonstrate that the phenotype of satellite cells is indicative of the type of fiber from which they derive.

Selective expression of a ski transgene affects IIb fast muscles and skeletal structure

The expression of a ski transgene in the bind leg muscles of mice follows a spatial and temporal pattern reminiscent of the pattern of myogenic development. Anterior muscles, which are formed earliest during development, are also the first muscles to express ski mRNA. Muscles derived from the posterior muscle group, formed later during development, exhibit delayed expression of ski mRNA. In addition, there is regional variation in ski mRNA levels within a particular muscle. Superficial regions of fast muscles, which contain a large percentage of type IIb fibers and have a high ATPase activity, express a higher level of ski mRNA than the deep portions of the same muscles. The deep regions contain a lower percentage of type IIb fibers and lower ATPase activity. The soleus, a slow muscle composed predominantly of type I fibers, expresses low ATPase activity and contains much lower levels of ski mRNA. mRNA from the ski transgene is also expressed at high levels in the osteocytes of the leg bones of 15-day and older transgenic mice. High levels of Ski protein is present in the osteocytes of the leg bones. ski expression appears to cause remodeling of the tibia and fibula. The cross-sectional area of the tibia and fibula of ski transgenic mice is significantly decreased compared to controls. X-rays of the skeletons of ski transgenic mice suggest that the bones of the entire skeleton are thinner than the bones in normal mice. Pathological stress fractures were found in several bones in the ski transgenic mice.

Postnatal development and plasticity of specialized muscle fiber characteristics in the hindlimb

Recent progress in defining molecular components of pathways controlling early stages of myogenesis has been substantial, but regulatory factors that govern the striking functional specialization of adult skeletal muscle fibers in vertebrate organisms have not yet been identified. A more detailed understanding of the temporal and spatial patterns by which specialized fiber characteristics arise may provide clues to the identity of the relevant regulatory factors. In this study, we used immunohistochemical, in situ hybridization, and Northern blot analyses to examine the time course and spatial characteristics of expression of myoglobin protein and mRNA during development of the distal hindlimb in the mouse. In adult animals, myoglobin is expressed selectively in oxidative, mitochondria-rich, fatigue-resistant myofibers, and it provides a convenient marker for this particular subset of specialized fibers. We observed only minimal expression of myoglobin in the hindlimb prior to the second day after birth, but a rapid and large (50-fold) induction of this gene in the ensuing neonatal period. Myoglobin expression was limited, however, to fibers located centrally within the limb which coexpress myosin isoforms characteristic of type I, IIA, and IIX fibers. This induction of myoglobin expression within the early postnatal period was accompanied by increased expression of nuclear genes encoding mitochondrial proteins, and exhibited a time course similar to the upregulation of myoglobin and mitochondrial proteins, and exhibited a time course similar to the upregulation of myoglobin and mitochondrial protein expression that can be induced in adult muscle fibers by continuous motor nerve stimulation. This comparison suggests that progressive locomotor activity of neonatal animals may provide signals which trigger the development of the specialized features of oxidative, fatigue-resistant skeletal muscle fibers.

Myosin heavy chain expression within the tapered ends of skeletal muscle fibers

BACKGROUND

The pectoralis muscle of the chicken contains fast-twitch glycolytic fibers, which during development undergo a transformation in their myosin heavy chain (MyHC) content from embryonic to a neonatal to an adult isoform (Bandman et al., 1990). Little, however, is known of MyHC expression within the ends of these or other muscle fibers. Here we test the hypothesis that the tapered ends of mature skeletal muscle fibers contain a less mature MyHC isoform than that typically found throughout their lengths.

METHODS

We apply an ammoniacal silver histological stain for endomysium and monoclonal antibodies against neonatal and adult MyHCs of chicken pectoralis to transverse serial sections of pectoralis from five mature chickens. The "lesser fiber diameters" of populations of fibers from each bird are also measured.

RESULTS

Most (approximately 81.8%) of the small (< 12 microns) and none of the larger (> 20 microns) diameter fibers contain the neonatal MyHC. Following these smaller fibers through serial sections, we show that they are the tapered ends of the larger fibers. Whereas neonatal MyHC is restricted to the tapered fiber ends, adult MyHC is present throughout the entire lengths of all fibers. We also demonstrate acetylcholinesterase (AChE) activity at some of these fiber ends.

CONCLUSIONS

We postulate that longitudinal growth of myofibrils in adult muscle is characterized by the sequential expression of MyHC isoforms similar to that observed in rapidly growing muscle and that the presence of the neurotransmitter hydrolase AChE at the tapered fiber ends may be related to the retention of neonatal MyHC.

An immunocytochemical marker for early type I muscle fibers in the developing rat hindlimb

Muscle fibers develop sequentially from several generations of myotubes that express specific isoforms of myosin heavy chain (MHC). We observed that the chicken-derived monoclonal antibody (mAb) S46 binds to myotubes of the fetal rat hindlimb in a specific temporal and spatial pattern. To determine the type and fate of the S46-reactive myotubes, we immunoreacted sections of fetal, neonatal and postnatal hindlimb muscles to this antibody. The mAb S46 bound to a subpopulation of primary myotubes in the tibialis anterior, and to all primary and slow/fast secondary myotubes in the soleus muscle. The S46-reactive primary myotubes represented the oldest set of myotubes in the muscles. Reactivity to S46 was present from the earliest stages of muscle development, peaked in the late fetal period, and dissipated in the first postnatal week, suggesting that mAb S46 binds to a developmental form of slow myosin. The regional distribution of myotubes that bound S46 in fetal muscles was identical to the distribution of type I (slow-twitch) fibers in the adult, indicating that S46-reactive myotubes ultimately develop into type I extrafusal fibers. Thus, mAb S46 can be used as a marker for prospective type I extrafusal fibers in the rat hindlimb.

Expression of extraocular myosin heavy chain in rabbit laryngeal muscle

The intrinsic laryngeal muscles of mammals are functionally heterogeneous, some of these muscles (e.g. the thyroarytenoid) contract extremely rapidly, like extraocular muscle, whilst others (e.g. the cricothyroid) contract as fast as limb fast muscle. The extraordinarily rapid contraction speed of extraocular muscles is associated with a fast myosin not found in limb muscles. In this work we explored the possibility that the thyroarytenoid muscle may also express this extraocular-specific fast myosin by raising a monoclonal antibody (mab 4A6) against its heavy chain. Electrophoretic separation of native isomyosins revealed that both the extraocular and the thyroarytenoid have two similar bands migrating ahead of bands found in limb fast or cricothyroid myosins. These two bands bound mab 4A6. The thyroarytenoid muscle can be divided into two divisions, a vocalis division which is important in phonation and an external division which functions in closing the glottis. Fibres in the vocalis are heterogeneous, some stain with mab 4A6, whilst others stain with mabs against limb myosin heavy chains. Fibres in the external division stain almost homogeneous with mab 4A6. The immunohistochemical staining pattern in the cricothyroid muscle resembled that of fast limb muscle: no fibres stained with mab 4A6. Thus, the high speed of contraction of the thyroarytenoid is associated with the same myosin heavy chain found in extraocular muscles, this characteristic is presumably an evolutionary adaptation for rapid closure of the glottis to enhance airway defense mechanisms.

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.