Developmental appearance of myosin heavy and light chain isoforms in vivo and in vitro in chicken skeletal muscle

Histopathologic and Myogenic Gene Expression Changes Associated with Wooden Breast in Broiler Breast Muscles

The wooden breast condition is a myopathy affecting the pectoralis major (p. major) muscle in fast-growing commercial broiler lines. Currently, wooden breast-affected birds are phenotypically detected by palpation of the breast area, with affected birds having a very hard p. major muscle that is of lower value. The objective of this study was to compare the wooden breast myopathy in two fast-growing broiler lines (Lines A and B) with incidence of wooden breast to a slower growing broiler Line C with no phenotypically observable wooden breast. One of the characteristics of the wooden breast condition is fibrosis of the p. major muscle. Morphologic assessment of Lines A and B showed significant fibrosis in both lines, but the collagen distribution and arrangement of the collagen fibrils was different. In Line A, the collagen fibrils were tightly packed, whereas in Line B the collagen fibrils were diffuse. This difference in collagen organization may be due to the expression of the extracellular matrix proteoglycan decorin. Decorin is a regulator of collagen crosslinking and is expressed at significantly higher levels in Line A wooden breast-affected p. major muscle, which would lead to tightly packed collagen fibers due to high levels of collagen crosslinking. Furthermore, expression of the muscle-specific transcriptional regulatory factors for proliferation and differentiation of muscle cells leading to the regeneration of muscle in response to muscle damage was significantly elevated in Line A, and only the factor for differentiation, myogenin, was increased in Line B. The results from this study provide initial evidence that the etiology of the wooden breast myopathy may vary between fast-growing commercial broiler lines.

Genome-wide expression analysis and EMX2 gene expression in embryonic myoblasts committed to diverse skeletal muscle fiber type fates

BACKGROUND

Primary skeletal muscle fibers form during embryonic development and are characterized as fast or slow fibers based on contractile protein gene expression. Different avian primary muscle fiber types arise from myoblast lineages committed to formation of diverse fiber types. To understand the basis of embryonic muscle fiber type diversity and the distinct myoblast lineages that generate this diversity, gene expression analyses were conducted on differentiated muscle fiber types and their respective myoblast precursor lineages.

RESULTS

Embryonic fast muscle fibers preferentially expressed 718 genes, and embryonic fast/slow muscle fibers differentially expressed 799 genes. Fast and fast/slow myoblast lineages displayed appreciable diversity in their gene expression profiles, indicating diversity of precursor myoblasts. Several genes, including the transcriptional regulator EMX2, were differentially expressed in both fast/slow myoblasts and muscle fibers vs. fast myoblasts and muscle fibers. EMX2 was localized to nuclei of fast/slow myoblasts and muscle fibers and was not detected in fast lineage cells. Furthermore, EMX2 overexpression and knockdown studies indicated that EMX2 is a positive transcriptional regulator of the slow myosin heavy chain 2 (MyHC2) gene promoter activity in fast/slow muscle fibers.

CONCLUSIONS

These results indicate the presence of distinct molecular signatures that characterize diverse embryonic myoblast lineages before differentiation.

Lineage-based primary muscle fiber type diversification independent of MEF2 and NFAT in chick embryos

Differences in primary avian skeletal muscle fiber types are based on myoblast cell lineages and independent of innervation. To understand the basis for this mode of myogenesis, embryonic myoblasts specifically committed to the formation of either fast or fast/slow muscle fiber types were isolated, characterized, and examined for their capacities to transcriptionally regulate the slow myosin heavy chain 2 (MyHC2) gene. Myogenic basic helix-loop-helix protein binding sites within the slow MyHC2 promoter were mutated and did not direct fast versus fast/slow muscle fiber type development. Using promoter analyses coupled with overexpression studies and transcriptional sensors, the roles of Nuclear Factor of Activated T cells (NFATc1), and MEF2A in regulation of the slow MyHC2 gene were determined. MEF2A activated the slow MyHC2 promoter in both fast and fast/slow primary muscle fibers. In contrast, NFATc1 repressed promoter activity. These results do not support the roles of MEF2 and NFAT as direct regulators of primary muscle fiber type differences. Rather, the results reflect intrinsic differences in the modes of regulation of the slow MyHC2 gene in primary muscle fiber types.

Analysis of myosin isoform transitions during growth and development in diverse chicken genotypes

The temporal expression of chicken skeletal fast myosin heavy chain (MyHC) isoforms in pectoralis major muscle was characterized in 3 commercial broiler lines at embryonic d 19 and at 7, 14, and 21 d posthatch. Lines A and B have been selected for breast yield, and line C is a fast' growing commercial line with limited selection for carcass traits. The isoform transitions in breast muscle samples were compared with samples from Single Comb White Leghorns (line D) using a semiquantitative immunoassay. The hypothesis was that selection for growth and carcass development in broilers would be accompanied by changes in the temporal expression of one or more of the chicken fast MyHC isoforms. Embryos from all lines were sampled at 19 d of incubation, and chicks were randomly sampled at 7, 14, and 21 d post-hatch. Myosin was extracted from pectoralis major muscle and assayed for purity and total protein concentration by SDS-PAGE and bincinchoninic acid protein analyses, respectively. The relative concentration of MyHC isoforms was evaluated by semiquantitative ELISA with 3 monoclonal antibodies specific for chicken skeletal fast embryonic and adult (eMyHC, aMyHC; EB165), neonatal (nMyHC; 2E9), and adult (aMyHC; AB8) myosin, respectively. The overall temporal expression of the myosin isoforms, eMyHC, nMyHC, and aMyHC, was similar in all lines. With eMyHC, at 19 d of incubation, line B had lower expression than lines A, C, and D. Expression of nMyHC, in lines C and D was similar with expression being highest at 7 d and lower at 14 d and 21 d. In lines A and B, however, nMyHC expression was higher at hatch than lines C and D. In line D, aMyHC was expressed at 14 d and increased through 21 d, whereas in lines A, B, and C, aMyHC isoform was expressed and was higher at 7 d and increased through 21 d. The results of this experiment support our hypothesis that commercial broilers have different temporal expression patterns of the developmental chicken fast MyHC isoforms.

Anatomy and histochemistry of spread-wing posture in birds. 3. Immunohistochemistry of flight muscles and the ?shoulder lock? in albatrosses

As a postural behavior, gliding and soaring flight in birds requires less energy than flapping flight. Slow tonic and slow twitch muscle fibers are specialized for sustained contraction with high fatigue resistance and are typically found in muscles associated with posture. Albatrosses are the elite of avian gliders; as such, we wanted to learn how their musculoskeletal system enables them to maintain spread-wing posture for prolonged gliding bouts. We used dissection and immunohistochemistry to evaluate muscle function for gliding flight in Laysan and Black-footed albatrosses. Albatrosses possess a locking mechanism at the shoulder composed of a tendinous sheet that extends from origin to insertion throughout the length of the deep layer of the pectoralis muscle. This fascial "strut" passively maintains horizontal wing orientation during gliding and soaring flight. A number of muscles, which likely facilitate gliding posture, are composed exclusively of slow fibers. These include Mm. coracobrachialis cranialis, extensor metacarpi radialis dorsalis, and deep pectoralis. In addition, a number of other muscles, including triceps scapularis, triceps humeralis, supracoracoideus, and extensor metacarpi radialis ventralis, were found to have populations of slow fibers. We believe that this extensive suite of uniformly slow muscles is associated with sustained gliding and is unique to birds that glide and soar for extended periods. These findings suggest that albatrosses utilize a combination of slow muscle fibers and a rigid limiting tendon for maintaining a prolonged, gliding posture.

Ventricular myosin heavy chain isoform expression is altered in vitro in low score normal chickens

The low score normal (LSN) chicken exhibits a genetic muscle weakness and altered in vitro myogenesis compared to the normal White Leghorn chicken. The ventricular myosin heavy chain isoform has been reported to be the initial muscle-specific contractile protein expressed during myogenesis. The goals of this study were to determine whether altered myogenesis of the LSN satellite cells in culture was accompanied by delayed ventricular myosin heavy chain expression and to further characterize the altered myogenic events exhibited by the LSN chicken. Immunocytochemical and ELISA analyses were employed to document the temporal expression of the ventricular myosin heavy chain during LSN chicken myogenesis. Satellite cells derived from the LSN chicken pectoralis major exhibited lower (P </= 0.05) expression of ventricular myosin heavy chain during proliferation and differentiation in culture than did satellite cells derived from White Leghorn chickens. Low score normal cells failing to express the ventricular myosin heavy chain generally remained mononucleated and unfused, whereas cells that were multinucleated appeared to express ventricular myosin heavy chain regardless of the avian source. These results suggest that the altered myogenesis observed in LSN chickens is associated with delayed ventricular myosin heavy chain expression.

Effect of selection for growth rate on myosin heavy chain temporal and spatial localization during turkey breast muscle development

The temporal and spatial localization of the heavy chain fast form of myosin was studied during turkey pectoralis major muscle development in a randombred control line (RBC2), a subline (F) of RBC2 selected only for increased 16-wk BW, a commercial sire line (B), and reciprocal crosses of the F and B lines. Pectoralis major muscle samples were obtained from three females and three males from each group in a manner to avoid contraction. After fixing and sectioning, the muscle samples were stained with a monoclonal antibody to determine the temporal and spatial localization of the heavy chain fast form of myosin. The percentage of muscle fibers at 25 d of incubation and 1 wk posthatch expressing the fast form of myosin heavy chain was calculated. The average percentage of muscle fibers expressing the fast form of myosin heavy chain for all genetic lines combined at embryonic d 25 for males was 76.1 and for females 66.6, whereas at 1 wk posthatch the average percentage for males was 24.2 and 36.1 for females. No interaction of sex and genetic group was noted at either age. At 25 d of embryonic development and at 1 wk of age, additive and nonadditive genetic effects were important in the inheritance of the fast form of myosin heavy chain. Heterosis was negative at both ages but significant at 1 wk of age. By 4 wk posthatch, all the muscle fibers in each genetic group were expressing the myosin heavy chain fast form, and no sex differences were observed. At 16 wk posthatch muscle fiber fragmentation was noted in the samples having reduced endomysial spacing. In the fragmenting muscle fiber areas, expression of the heavy chain fast form of myosin was observed. These muscle fiber changes were predominant in the growth selected F-line suggesting that growth selection may be associated with muscle damage.

Evolutionary implications of three novel members of the human sarcomeric myosin heavy chain gene family

Sarcomeric myosin heavy chain (MyHC) is the major contractile protein of striated muscle. Six tandemly linked skeletal MyHC genes on chromosome 17 and two cardiac MyHC genes on chromosome 14 have been previously described in the human genome. We report the identification of three novel human sarcomeric MyHC genes on chromosomes 3, 7, and 20, which are notable for their atypical size and intron-exon structure. Two of the encoded proteins are structurally most like the slow-beta MyHC, whereas the third one is closest to the adult fast IIb isoform. Data from pairwise comparisons of aligned coding sequences imply the existence of ancestral genomes with four sarcomeric genes before the emergence of a dedicated smooth muscle MyHC gene. To further address the evolutionary relationships of the distinct sarcomeric and nonsarcomeric rod sequences, we have identified and further annotated human genomic DNA sequences corresponding to 14 class-II MyHCs. An extensive analysis provides a timeline for intron gain and loss, gene contraction and expansion, and gene conversion among genes encoding class-II myosins. One of the novel human genes is found to have introns at positions shared only with the molluscan catchin/MyHC gene, providing evidence for the structure of a pre-Cambrian ancestral gene.

Evolutionary significance of myosin heavy chain heterogeneity in birds

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

Evidence for the expression of neonatal skeletal myosin heavy chain in primary myocardium and cardiac conduction tissue in the developing chick heart

We isolated a neonatal skeletal myosin heavy chain (MHC) cDNA clone, CV11E1, from a cDNA library of embryonic chick ventricle. At early cardiogenesis, diffuse expression of neonatal skeletal MHC mRNA was first detected in the heart tube at stage 10. During subsequent embryonic stages, the expression of the mRNA in the atrium was upregulated until shortly after birth. It then diminished, dramatically, and disappeared in the adult. On the other hand, in the ventricle, only a trace of the expression was detected throughout embryonic life and in the adult. However, transient expression of mRNA in the ventricle was observed, post-hatching. At the protein level, during the embryonic stage, the atrial myocardium was stained diffusely with monoclonal antibody 2E9, specific for chick neonatal skeletal MHC, whereas the ventricles showed weak reactivity with 2E9. At the late embryonic and newly hatched stages, 2E9-positive cells were located clearly in the subendocardial layer, and around the blood vessels of the atrial and ventricular myocardium. These results provide the first evidence that the neonatal skeletal MHC gene is expressed in developing chick hearts. This MHC appears during early cardiogenesis and is then localized in cardiac conduction cells. Dev Dyn 2000;217:37-49.

Regulation of alpha-helical coiled-coil dimerization in chicken skeletal muscle light meromyosin

The dimerization specificity of the light meromyosin (LMM) domain of chicken neonatal and adult myosin isoforms was analyzed by metal chelation chromatography. Our results show that neonatal and adult LMMs associate preferentially, although not exclusively, as homodimeric coiled-coils. Using chimeric LMM constructs combining neonatal and adult sequences, we observed that a stretch of 183 amino acids of sequence identity at the N terminus of the LMM was sufficient to allow the adult LMM to dimerize in a non-selective manner. In contrast, sequence identity in the remaining C-terminal 465 amino acids had only a modest effect on the dimerization selectivity of the adult isoform. Sequence identity at the N terminus also promoted dimerization of the neonatal LMM to a greater degree than sequence identity at the C terminus. However, the N terminus had only a partial effect on the dimerization specificity of the neonatal sequence, and residues distributed throughout the LMM were capable of affecting dimerization selectivity of this isoform. These results indicated that dimerization preference of the neonatal and adult isoforms was affected to a different extent by sequence identity at a given region of the LMM.

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.

The generation and interpretation of positional information within the vertebrate myotome

How somitic cells become restricted to the muscle fate has been investigated on a number of levels. Classical embryological manipulations have attempted to define the source of inductive signals that control the formation of the myotome. Recently, these studies have converged with others dissecting the role of secreted proteins in embryonic patterning to demonstrate a role for specific peptides in inducing individual cell types of the myotome. Collectively, these investigations have implicated the products of the Wnt, Hedgehog (Hh) and Bone morphogenetic protein (Bmp) gene families as key myogenic regulators; simultaneously controlling both the initiation of myogenesis and the fate of individual myoblasts.

Protein and mRNA analysis of myosin heavy chains in the developing avian pectoralis major muscle

While the existence of post-hatch and adult myosin heavy chain isoforms in the large, avian type IIB pectoralis major muscle has been clearly established, the number and nature of fast myosin heavy chains during in ovo development and the perihatch period have not been resolved. In the present study, developmental fast heavy chain proteins purified by high resolution anion-exchange have been characterized by sequence analysis of a unique CNBr peptide and by complementary mRNA analysis. The four proteins present at 15/16 days in ovo are shown to differ uniquely in primary structure. They correlate with heavy chains II, IV, VI and VII, characterized recently as major or minor species in adult fast muscles using similar methods. These four heavy chains are expressed in a time-dependent fashion from 8 to 16 days in ovo. At the mRNA level, heavy chain VI predominates until 12 days in ovo. Heavy chain IV mRNA is upregulated dramatically at 16 days in ovo preparatory to its protein's predominance in the peri-hatch period. Heavy chains II, IV and V (the post-hatch isoform which replaces heavy chain IV) have major roles in adult fast muscles.

Characterization of the myosin heavy chains of avian adult fast muscles at the protein and mRNA levels

High resolution anion-exchange chromatography of myosin subfragment-1 in avian fast muscles revealed five fast heavy chains (I-V) expressed in muscle-specific patterns. Sequence analysis of a unique peptide established that the proteins differed in primary structure and suggested correlation with heavy chain genes identified independently by Robbins and coworkers. The identities of the isoforms and their expression patterns were confirmed at the mRNA level by a reverse-transcription, 5'-anchored PCR procedure. The fast white pectoralis major muscle possessed heavy chain I, the posterior latissimus dorsi muscle, of similar fibre type, expressed heavy chains I, III and IV. The fast red adductor superficialis muscle expressed either, or both, of heavy chains II and IV. The lateral gastocnemius muscle, of mixed fibre type, expressed heavy chains II-V. In general, heavy chains I, III and V appeared to be favoured in fast white fibres, while heavy chains II and IV were characteristic of fast red fibres. These results imply a greater subtlety of fast muscle function than has previously been appreciated.

Skeletal muscle satellite cells cultured in simulated microgravity

Satellite cells are postnatal myoblasts responsible for providing additional nuclei to growing or regenerating muscle cells. Satellite cells retain the capacity to proliferate and differentiate in vitro and, therefore, provide a useful model to study postnatal muscle development. Most culture systems used to study postnatal muscle development are limited by the two-dimensional (2-D) confines of the culture dish. Limiting proliferation and differentiation of satellite cells in 2-D could potentially limit cell-cell contacts important for developing the level of organization in skeletal muscle obtained in vivo. Culturing satellite cells on microcarrier beads suspended in the High-Aspect-Ratio-Vessel (HARV) designed by NASA provides a low shear, three-dimensional (3-D) environment to study muscle development. Primary cultures established from anterior tibialis muscles of growing rats (approximately 200 gm) were used for all studies and were composed of greater than 75% satellite cells. Different inoculation densities did not affect the proliferative potential of satellite cells in the HARV. Plating efficiency, proliferation, and glucose utilization were compared between 2-D culture and 3-D HARV culture. Plating efficiency (cells attached divided by cells plated x 100) was similar between the two culture systems. Proliferation was reduced in HARV cultures and this reduction was apparent for both satellite cells and nonsatellite cells. Furthermore, reduction in proliferation within the HARV could not be attributed to reduced substrate availability because glucose levels in medium from HARV and 2-D cell culture were similar. Morphologically, microcarrier beads within the HARV were joined together by cells into 3-D aggregates composed of greater than 10 beads/aggregate. Aggregation of beads did not occur in the absence of cells. Myotubes were often seen on individual beads or spanning the surface of two beads. In summary, proliferation and differentiation of satellite cells on microcarrier beads within the HARV bioreactor results in a 3-D level of organization that could provide a more suitable model to study postnatal muscle development than is currently available with standard culture methods.

Expression of fast myosin heavy chain transcripts in developing and dystrophic chicken skeletal muscle

The expression of fast myosin heavy chain (MyHC) genes was examined in vivo during fast skeletal muscle development in the inbred White Leghorn chicken (line 03) and in adult muscles from the genetically related dystrophic White Leghorn chicken (line 433). RNA dotblot and northern hybridization was employed to monitor MyHC transcript levels utilizing specific oligonucleotide probes. The developmental pattern of MyHC gene expression in the pectoralis major (PM) and the gastrocnemius muscles was similar during embryonic development with three embryonic MyHC isoform genes, Cemb1, Cemb2, and Cemb3, sequentially expressed. Following hatching, MyHC expression patterns in each muscle differed. The expression of MyHC genes was also studied in muscle cell cultures derived from 12-day embryonic pectoralis muscles. In vitro, Cvent, Cemb1, and Cemb2 MyHC genes were expressed; however, little if any Cemb3 MyHC gene expression could be detected, even though Cemb3 was the predominant MyHC gene expressed during late embryonic development in vivo. In most adult muscles other than the PM and anterior latissimus dorsi (ALD), the Cemb3 MyHC gene was the major adult MyHC isoform. In addition, two general patterns of expression were identified in fast muscle. The fast muscles of the leg expressed neonatal (Cneo) and Cemb3 MyHC genes, while other fast muscles expressed adult (Cadult) and Cemb3 MyHC genes. MyHC gene expression in adult dystrophic muscles was found to reflect the expression patterns found in corresponding normal muscles during the neonatal or early post-hatch developmental period, providing additional evidence that avian muscular dystrophy inhibits muscle maturation.

Tubulin tyrosine ligase: protein and mRNA expression in developing rat skeletal muscle

Alpha tubulin can be post-translationally tyrosinated at the carboxy-terminus by a specific enzyme: tubulin tyrosine ligase. The expression of tubulin tyrosine ligase mRNA and protein during the development of rat skeletal muscle was examined in the present study. A portion of the coding region of the rat ligase cDNA was isolated and sequenced. The nucleotide and amino acid sequences showed about 90% homology with previously reported porcine and bovine ligase sequences. In newborn rats, ligase mRNA and protein were highly expressed in skeletal muscle. During early postnatal development, however, both ligase mRNA and protein dropped down dramatically. Quantitative measurements revealed that ligase protein at postnatal day 20 represented only 10% or less of the level at postnatal day 1. Ligase mRNA expression was also examined during the myogenesis in vitro. A strong ligase mRNA signal was detected in both undifferentiated myoblasts and cross-striated, contractile myotubes. The present results suggest that, during muscle differentiation, ligase function may be regulated by the amount of available mRNA. The discrepancy in the ligase expression between the in vivo and in vitro myogenesis suggests that factors controlling the levels of mRNA in vivo are lost in vitro.

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.

Heterogeneity of myosin heavy-chain expression in fast-twitch fiber types of mature avian pectoralis muscle

The aims of this study are to investigate the diversity of myosin heavy-chain (MyHC) expression among avian fast-twitch fibers, and to test the hypothesis that dissimilar MyHC isoforms are found in each of the principal avian fast-twitch fiber types. MyHCs within the muscle fibers of the pectoralis of 31 species of bird are characterized using immunocytochemical methods. A library of 11 monoclonal antibodies previously produced against chicken MyHCs is used. The specificity of these antibodies for MyHCs in each of the muscles studied is confirmed by Western blots. The results show that avian fast-twitch glycolytic fibers and fast-twitch oxidative-glycolytic fibers can contain different MyHCs. Among the species studied, there is also a conspicuous variety of MyHC isoforms expressed. In addition, the results suggest that two epitopes are restricted to chickens and closely allied gallinaceous birds. There are no apparent correlations between between MyHC epitope and presupposed contractile properties. However, the presence of different isoforms in different fast-twitch fiber types suggests a correlation between isoform and contractile function.

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.

Neural tube can induce fast myosin heavy chain isoform expression during embryonic development

We investigated the role of the neural tube in muscle cell differentiation in developing somitic myotome of chick embryo, particularly through fast myosin heavy chain (MHC) isoform expression. An embryonic fast MHC labeled with EB165 mAb was expressed in somitic cells from stage 15 of Hamburger and Hamilton (H.H.) (24 somites). Moreover, a distinct early embryonic fast MHC was expressed only from stage 15 of H.H. to stage 36 (E10). Like neonatal MHC, this isoform was labeled with 2E9 mAb but differed in its immunopeptide mapping. Expression of EB165-labeled embryonic fast MHC occurred in somitic myotomes deprived of neural tube influence by in ovo ablation as well as in somite explants cultured alone in vitro. Conversely, ablation of the neural tube prevented somitic expression of MHC labeled with 2E9 mAb. The neural tube induced in vitro expression of this MHC in explants of somites which failed to express it when cultured alone. These results indicate that signals emanating from the neural tube are required for the expression of early embryonic fast MHC isoform in developing somitic myotome.

Development of the excitation-contraction coupling apparatus in skeletal muscle: peripheral and internal calcium release units are formed sequentially

The development of calcium release units and of transverse tubules has been studied in skeletal muscle fibres from embryonal and newborn chicken. Three constituents of calcium release units: the tetrads, the feet and an internal protein directly associated with junctional surface of the sarcoplasmic reticulum are visualized by various electron microscope techniques. Evidence in the literature indicates that the three components correspond to the voltage sensors, the sarcoplasmic reticulum calcium release channels and the calcium binding protein calsequestrin respectively. We recognize two stages at which important events in membrane morphogenesis occur. The first stage coincides with early myofibrillogenesis (starting at approximately embryonal day E5.5), and it involves the assembly of calcium release units at the periphery of the muscle fibre in which feet and the internal protein are identified. Groups of tetrads also are present at very early stages and their disposition indicates a relation to the feet of peripheral couplings. Thus three major components of the excitation-contraction coupling pathway are in place as soon as myofibrils develop. The density of groups of tetrads in the surface membrane of primary and secondary fibres is similar, despite differences in developmental stages. The second stage involves the formation of a complex transverse tubule network and of internal sarcoplasmic reticulum-transverse tubule junctions, while peripheral couplings disappear. This stage starts abruptly (between E15 and E16) and simultaneously in primary and secondary fibres. It coincides with the myotube-to-myofibre transition. The two stages are separated by a relatively long intervening period (between E9 and E16). During the latter part of this period some primitive transverse tubules appear, and form junctions with the sarcoplasmic reticulum, but they remain strictly located at the periphery of the fibre and are not numerous. Finally, after the second stage there is a prolonged (up to 4 weeks) period of maturation, during which density of free sarcoplasmic reticulum increases, triads acquire a location at the A-I junction and fibre type differences appear. We conclude that a system for calcium uptake, storage and release exists at the periphery of the myotube during early myogenesis. The complexity of the system and its ability to deliver calcium through the entire fibre develop in parallel to the formation of myofibrils.

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.

Denervated chicken breast muscle displays discoordinate regulation and differential patterns of expression of alpha f and beta tropomyosin genes

The expression of the alpha fast (alpha f) and beta tropomyosin (TM) genes has been analysed with muscle-specific and common cDNA probes after unilateral nerve section of the pectoralis major muscle (PM) in 4-week-old chickens. The following were observed in denervated muscles. (1) The beta TM mRNA, which was repressed during development, reaccumulates in a biphasic curve with the increase in the beta TM protein lagging behind the changes in its mRNA. Accordingly, no beta TM is seen in products translated in vitro from total and polyA+ RNA obtained 1 week after denervation. No such translation block is seen with RNA obtained from control or muscles denervated for 6 weeks. (2) No changes in the alpha fTM mRNA and corresponding protein are observed. (3) RNA processing of the two genes is not changed. (4) In the contralateral muscles, transitory increases in alpha f and beta TM mRNAs are observed while the corresponding proteins remain unchanged. Our data suggest that muscle fibres display early and long-term responses to the loss of neural input which might result from a combination of changes produced by regenerative processes and reprogramming of existing fibres. Moreover, in contrast to normal development, no reciprocal changes of alpha f and beta TM expression are seen in denervated muscles.

Satellite cells from slow rat muscle express slow myosin under appropriate culture conditions

Satellite cells were isolated at high yields from slow-twitch soleus and fast-twitch tibialis anterior (TA) muscles of adult male Wistar rats. The number of satellite cells isolated from soleus muscle exceeded that from TA muscles by a factor of three. A comparison of satellite cells grown on gelatin- or Matrigel-coated dishes revealed that Matrigel greatly enhances the maturation of the satellite-cell-derived myotubes. As judged from immunohistochemistry, myosin heavy chain electrophoresis and immunoblot analyses, only cells grown on Matrigel, but not on gelatin, expressed adult myosin isoforms. Slow myosin expression was only detected in Matrigel cultures. Soleus cultures contained, in addition to the majority of myotubes expressing fast myosin, a small fraction (maximally 10%) of myotubes coexpressing fast and slow myosins. The number of fast/slow myosin-containing myotubes was negligible in TA cultures. The expression of slow myosin increased with age. Slow myosin was nonuniformly distributed along the length of specific myotubes and accumulated around some myonuclei. These results point to the existence of myotubes with a heterogeneous population of myonuclei, probably resulting from fusion of differently preprogrammed satellite cells. We suggest that the patch-like expression of slow myosin results from local accumulation of myonuclei of slow-type satellite cells.

Developmental transitions in the myosin patterns of two fast muscles

Transitions in myosin patterns were examined in situ by immunofluorescence in two fast muscles of the developing chicken, the pectoralis and the posterior latissimus dorsi. Myosin isoforms were localized using stage-specific monoclonal antibodies against the heavy chain of pectoralis myosin. Two antibodies (12C5 and 10H10) recognize adult and late embryonic myosin. They reacted weakly with both the pectoralis and posterior latissimus dorsi at 10 days in ovo, but intensely at 18 days in ovo. Both muscles were completely unreactive with an adult-specific antibody (5C3), indicating that the staining with 12C5 and 10H10 at 18 days in ovo reflects embryonic myosin. Thus two different embryonic isoforms are expressed sequentially in each muscle. Both 12C5 and 10H10 reacted weakly again with these muscles after hatching. The reappearance of a strong positive response to both antibodies, at 28 days in the pectoralis and after 60 days in the posterior latissimus dorsi, correlated well with the first appearance of a response to the adult-specific antibody, 5C3, signalling the beginning of the adult pattern. Both muscles reacted strongly with an antibody (5B4) specific for 'neonatal' myosin between 18 days in ovo and 60 days after hatching. In the pectoralis, embryonic was replaced by neonatal myosin in most fibres by 14 days after hatching; by 28 days, both adult and neonatal myosin were expressed in most fibres; and in the adult, neonatal myosin was replaced entirely by the adult isoform. In contrast, many fibres in the posterior latissimus dorsi still expressed both embryonic and neonatal myosins up to at least 60 days post-hatch, and the remaining fibres expressed the neonatal isoform; the neonatal isoform was present in some fibres even in the adult posterior latissimus dorsi. We have therefore demonstrated in situ four different heavy chain isoforms in two different fast muscles. 'Early embryonic', 'late embryonic', 'neonatal' and eventually 'adult' isoforms are expressed in each muscle and more than one isoform often coexists in the same fibre.

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.

Contractile protein isoforms in muscle development

The contractile proteins of skeletal muscle are often represented by families of very similar isoforms. Protein isoforms can result from the differential expression of multigene families or from multiple transcripts from a single gene via alternative splicing. In many cases the regulatory mechanisms that determine the accumulation of specific isoforms via alternative splicing or differential gene expression are being unraveled. However, the functional significance of expressing different proteins during muscle development remains a key issue that has not been resolved. It is widely believed that distinct isoforms within a family are uniquely adapted to muscles with different physiological properties, since separate isoform families are often coordinately regulated within functionally distinct muscle fiber types. It is also possible that different isoforms are functionally indistinguishable and represent an inherent genetic redundancy among critically important muscle proteins. The goal of this review is to assess the evidence that muscle proteins which exist as different isoforms in developing and mature skeletal and cardiac muscles are functionally unique. Since regulation of both transcription and alternative splicing within multigene families may also be an important factor determining the accumulation of specific protein isoforms, evidence that genetic regulation rather than protein coding information provides the functional basis of isoform diversity is also examined.

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.

Skeletal muscle satellite cells appear during late chicken embryogenesis

The emergence of avian satellite cells during development has been studied using markers that distinguish adult from fetal cells. Previous studies by us have shown that myogenic cultures from fetal (Embryonic Day 10) and adult 12-16 weeks) chicken pectoralis muscle (PM) each regulate expression of the embryonic isoform of fast myosin heavy chain (MHC) differently. In fetal cultures, embryonic MHC is coexpressed with a ventricular MHC in both myocytes (differentiated myoblasts) and myotubes. In contrast, myocytes and newly formed myotubes in adult cultures express ventricular but not embryonic MHC. In the current study, the appearance of myocytes and myotubes which express ventricular but not embryonic MHC was used to determine when adult myoblasts first emerge during avian development. By examining patterns of MHC expression in mass and clonal cultures prepared from embryonic and posthatch chicken skeletal muscle using double-label immunofluorescence with isoform-specific monoclonal antibodies, we show that a significant number of myocytes and myotubes which stain for ventricular but not embryonic MHC are first seen in cultures derived from PM during fetal development (Embryonic Day 18) and comprise the majority, if not all, of the myoblasts present at hatching and beyond. These results suggest that adult type myoblasts become dominant in late embryogenesis. We also show that satellite cell cultures derived from adult slow muscle give results similar to those of cultures derived from adult fast muscle. Cultures derived from Embryonic Day 10 hindlimb form myocytes and myotubes that coexpress ventricular and embryonic MHCs in a manner similar to cells of the Embryonic Day 10 PM. Thus, adult and fetal expression patterns of ventricular and embryonic MHCs are correlated with developmental age but not muscle fiber type.

Developmental modulation of myosin expression by thyroid hormone in avian skeletal muscle

It is well established that a rise in circulating thyroid hormone during the second half of chick embryo development significantly influences muscle weight gain and bone growth. We studied thyroid influence on differentiation in slow anterior latissimus dorsi (ALD) and fast posterior latissimus dorsi (PLD) muscles of embryos rendered hypothyroid by hypophysectomy or administration of an anti-thyroid drug. The expression of native myosins and myosin light chains (MLCs) was studied by electrophoretic analysis, and the myosin heavy chain (MHC) was characterized by immunohistochemistry. The first effects of hypothyroid status were observed at day 21 of embryonic development (stage 46 according to Hamburger and Hamilton). Analysis of myosin isoform expression in PLD muscles of hypothyroid embryos showed persistence of slow migrating native myosins and slow MLCs as well as inhibition of neonatal fast MHC expression, indicating retarded differentiation of this muscle. In ALD muscle, hypothyroidism maintained fast embryonic MHC and induced noticeable amounts of fast MLCs, thus delaying slow muscle differentiation. Our results suggest that thyroid hormones play a role in modulating the appearance of neonatal fast MHC and the disappearance of isomyosins transiently present during embryogenesis. However, T3 supplemental treatment would seem to compensate in part for the effects of hypothyroidism induced by hypophysectomy, suggesting that thyroid hormone might interfere with other factors also accounting for the observed effects.

Analysis of the chicken fast myosin heavy chain family. Localization of isoform-specific antibody epitopes and regions of divergence

cDNAs encoding the rod region of four different fast myosin heavy chains (MYCHs) in the chicken were identified, using anti-MYCH monoclonal antibodies, in two expression libraries prepared from 19-day embryonic and adult chicken muscle. These clones were used to determine the amino acid sequences that encompass the epitopes of five anti-MYHC monoclonal antibodies. Additionally, the amino acid sequences were compared to each other and to a full length embryonic MYHC. Although there is extensive homology in the chicken fast myosin rods, sequences within the hinge, within the central portion of the light meromyosin fragment, and at the carboxy terminus exhibit the largest number of amino acid substitutions. We propose that divergence within these subdomains may contribute to isoform-specific properties associated with skeletal myosin rods.

Repression of the embryonic myosin heavy chain phenotype in regenerating chicken slow muscle is dependent on innervation

Ventricular-like and fast myosin heavy chains (VL-MHC and FMHC) are transiently expressed during slow skeletal muscle development. The influence of innervation on repression of these MHC isoforms is investigated over an 84-day time course in: (1) normal anterior latissimus dorsi (N-ALD) muscles, (2) regenerating ALD (R-ALD) muscles, (3) denervated ALD (D-ALD) muscles, and (4) regenerating and denervated ALD (RD-ALD) muscles. Western blotting demonstrates that the VL-MHC is expressed in R-, D-, and RD-ALD muscles, but not in N-ALD muscles. Expression of the VL-MHC is transient in R-ALD muscles. In contrast, VL-MHC expression persists in RD-ALD muscles, and appears with time in D-ALD muscles. FMHC was not detected in N-ALD muscles by Western blotting. Two FMHCs are seen in R-ALD and RD-ALD muscles, and in 13-day embryonic ALD muscles. The slower migrating FMHC (FMHCA) comigrates with developmentally regulated FMHCs in fast pectoralis muscle, while the faster migrating FMHC (FMHCB) comigrates with the faster migrating FMHC in embryonic ALD muscle (13 days in ovo). FMHCB decreases in amount over the time course in R-ALD muscles, while FMHCA persists. In contrast, substantial levels of both FMHCs persist in RD-ALD muscles, and appear with time in D-ALD muscles. The cellular distribution of MHCs is followed by immunocytochemistry. Regenerating cells expressing VL-MHC and FMHC are replaced by a mature population in R-ALD muscles. Some of the mature myofibers in R-ALD muscles express FMHC, but not VL-MHC. In RD-ALD and D-ALD muscles, both regenerating and mature muscle cells are seen which express VL-MHC and FMHC. Our results indicate that innervation is required for the repression of VL-MHC and FMHCB during regeneration of slow muscle.

Myoblasts from fetal and adult skeletal muscle regulate myosin expression differently

We compared the expression of myosin heavy chains in myogenic cultures prepared from fetal (embryonic Day 10) and adult (12-16 weeks) chicken pectoralis muscle using immunofluorescence with isoform-specific monoclonal antibodies. We found that the majority of fetal myocytes (differentiated myoblasts) and myotubes coexpressed ventricular and embryonic myosin heavy chains in culture. Also, when fetal cells were plated at a clonal density most clones coexpressed both ventricular and embryonic isoforms. In contrast, all adult myocytes and newly formed adult myotubes expressed just ventricular myosin, whether plated at mass or clonal densities. Within 12-24 hr of the onset of fusion, adult myotubes began to express embryonic myosin as well. Eventually, the majority of adult myotubes coexpressed both ventricular and embryonic myosin. The delay of embryonic myosin expression until after fusion was also seen in passaged adult myoblasts and in myoblasts isolated from regenerating adult muscle. The expression of embryonic myosin can be abolished by inhibiting fusion with EGTA in adult but not in fetal cultures. We conclude that both fetal and adult myotubes express ventricular and embryonic myosins but only fetal myocytes express the embryonic isoform prior to fusion. This difference in the regulation of embryonic myosin expression between fetal and adult myoblasts supports the hypothesis that these cells may represent two distinct populations of myogenic precursors.

Myosin isoform expression in skeletal muscles of turkeys at various ages

The appearance of myosin isoforms in skeletal muscles of turkey embryos, poults, and toms was studied, using monoclonal antibodies raised against myosin isoforms in chicken fast-twitch muscle (Pectoralis). The myosin extract was prepared by repeated salt extraction-precipitation. The reactivity of monoclonal antibodies with turkey myosin isoforms was tested by an enzyme linked immunosorbent assay using alkaline phosphatase-conjugated antibody and detection by color development with p-nitrophenyl phosphate. Detection was also effected by protein slot blotting using peroxidase-conjugated antibody and color development with 3,3'-diaminobenzidine tetrahydrochloride. The monoclonal antibody AB8 was found to be specific for the adult myosin isoform, present in Pectoralis muscle of 14-day-old and adult turkeys and adult chickens. Subsequent peptide mapping also indicated that the adult myosin isoform of turkey Pectoralis muscle was nearly identical to the adult isoform from chickens. The monoclonal antibody 2E9 reacted with the myosin extract only from poults at ages of 7 days and 14 days posthatch, indicating that 2E9 is specific for the neonatal myosin isoform. The reactivity of 2E9 was noted with the muscle of the mixed fiber type (the thigh muscle group) as well as with the fast-twitch muscle (Pectoralis). Monoclonal antibodies EB 165 and AG6 were found to react with the myosin extract from all ages tested. Based on the reactivity with monoclonal antibodies, it was concluded that myosin in turkey muscles existed as at least three discrete isoforms that were expressed sequentially in the course of muscle development.

Expression of embryonic myosin heavy chain mRNA in stretched adult chicken skeletal muscle

Chronic stretch of the chicken fast-twitch patagialis muscle increases the rate of growth and percentage of fast-twitch oxidative fibers. We have analyzed the effects of stretch on the expression of two previously identified "embryonic" myosin heavy chain (MHC) mRNAs (p251 and p110). Both MHC mRNAs were expressed in the patagialis at their highest levels in the embryo and 1 wk after hatching. During posthatch development (7-52 wk), the p110 mRNA was expressed in only trace quantities while the p251 mRNA was not detectable. After 2 wk of stretch of the patagialis in 7- or 38-wk-old birds, the p110 mRNA was increased to levels similar to that found in patagialis of newly hatched chicks, whereas expression of the p251 transcript was not affected. The existence of two other MHC mRNAs homologous to the p110 mRNA was suggested by the S1 mapping analysis, one of which was expressed at dramatically reduced levels in the stretched patagialis. It is concluded that stretch can cause selective alterations in the expression of developmentally regulated MHC isoforms in chicken fast-twitch muscle.

Developmental myosin heavy chain progression in avian type IIB muscle fibres

Myosin heavy chain species were investigated during development in avian pectoralis major muscles (type IIB fibres) by high resolution anion-exchange chromatography of the myosin head region, subfragment-1. At 15 days in ovo four distinct fast-type heavy chain species, I, II, III and IV, in order of elution, were identified. By 19 days in ovo, form IV had become predominant and remained the major species through 3-days post-hatch. This form has been named the peri-hatch form. Between 3 and 5 days post-hatch, a second massive change occurred such that by 5 days post-hatch a new species, V, apparent at 19 days in ovo in small amounts, dominated and at 8 days post-hatch was the only heavy chain species present. Form V, which corresponds to that previously identified as the post-hatch form, continued as the major species through 20 days post-hatch and was replaced slowly by the adult form. N-terminal sequencing of CNBr peptides from three subfragment-1 heavy chain species, the peri-hatch (form IV), the post-hatch (form V) and adult, revealed differences in amino acid sequence consistent with the three being products of different genes. These results confirm and extend recent reports of complexity in fast heavy chain expression prior to hatching in the chicken (Hofmann et al., 1988; Van Horn & Crow, 1989).

Neonatal and adult myosin heavy chains form homodimers during avian skeletal muscle development

Myosin isoforms contribute to the heterogeneity and adaptability of skeletal muscle fibers. Besides the well-characterized slow and fast muscle myosins, there are those isoforms that appear transiently during the course of muscle development. At a stage of development when two different myosins are coexpressed, the possibility arises for the existence of heterodimers, molecules containing two different heavy chains, or homodimers, molecules with two identical heavy chains. The question of whether neonatal and adult myosin isoforms can associate to form a stable heterodimer was addressed by using stage-specific monoclonal antibodies in conjunction with immunological and electron microscopic techniques. We find that independent of the ratio of adult to neonatal myosin, depending on the age of the animal, the myosin heavy chains form predominantly homodimeric molecules. The small amount of hybrid species present suggests that either the rod portion of the two heavy chain isoforms differs too much in sequence to form a stable alpha-helical coiled coil, or that the biosynthesis of the heavy chains precludes the formation of heterodimeric molecules.

Assembly of avian skeletal muscle myosins: evidence that homodimers of the heavy chain subunit are the thermodynamically stable form

Using a double antibody sandwich ELISA we examined the heavy chain isoform composition of myosin molecules isolated from chicken pectoralis major muscle during different stages of development. At 2- and 40-d posthatch, when multiple myosin heavy chain isoforms are being synthesized, we detected no heterodimeric myosins, suggesting that myosins are homodimers of the heavy chain subunit. Chymotryptic rod fragments of embryonic, neonatal, and adult myosins were prepared and equimolar mixtures of embryonic and neonatal rods and neonatal and adult rods were denatured in 8 M guanidine. The guanidine denatured myosin heavy chain fragments were either dialyzed or diluted into renaturation buffer and reformed dimers which were electrophoretically indistinguishable from native rods. Analysis of these renatured rods using double antibody sandwich ELISA showed them to be predominantly homodimers of each of the isoforms. Although hybrids between the different heavy chain fragments were not detected, exchange was possible under these conditions since mixture of biotinylated neonatal rods and fluoresceinated neonatal rods formed a heterodimeric biotinylated-fluoresceinated species upon renaturation. Therefore, we propose that homodimers are the thermodynamically stable form of skeletal muscle myosin isoforms and that there is no need to invoke compartmentalization or other cellular regulatory processes to explain the lack of heavy chain heterodimers in vivo.

Myosin expression in hypertrophied fast twitch and slow tonic muscles of normal and dystrophic chickens

Disruption of the development program of myosin gene expression has been reported in chicken muscular dystrophy. In the present report, the relationship between muscular dystrophy and the ability of muscle to respond to an increased work load with a transition in the myosin phenotype has been investigated. Hypertrophy of slow tonic anterior latissimus dorsi (ALD) and fast twitch patagialis (PAT) muscles was induced by overloading for 35 days and myosin expression was analyzed by electrophoresis and immunocytochemistry. Normal and dystrophic chicken ALD muscles have nearly identical proportions of SM-1 and SM-2 isomyosins and both exhibit an age-related repression of the SM-1 isomyosin which is enhanced and accelerated by overloading. Immunocytochemistry with anti-myosin heavy chain (MHC) antibodies demonstrates the appearance of nascent myofibers in overloaded ALD muscles from both normal and dystrophic chickens. A minor fast twitch fiber population is also identified which doubles in number with overloading in normal ALD muscles. There are only half as many fast twitch fibers in control dystrophic ALD muscles and this number does not increase with overloading. In contrast to ALD muscles, the isomyosin profile of normal and dystrophic PAT muscles is quite different. There is significantly more FM-3 and significantly less FM-1 isomyosin in the dystrophic PAT muscle. However, both normal and dystrophic PAT muscles exhibit an overload-induced accumulation of the FM-3 isomyosin. Immunocytochemistry reveals that, unlike the normal PAT muscle, the dystrophic PAT muscle contains a population of myofibers which express slow MHCs. As in the ALD muscle, overload-induced hypertrophy is associated with a repression of the SM-1 MHC in these fibers. Nascent myofiber formation does not occur in either normal or dystrophic overloaded PAT muscles.

Alterations in the expression of muscle-specific genes mediated by troponin C antisense oligodeoxynucleotide

The effectiveness of an antisense oligodeoxynucleotide to troponin C (TnC) mRNA in blocking expression of TnC in differentiated chicken myotubes was examined. An 18-nucleotide-long sequence common to both fast and slow isoforms of TnC mRNAs was chosen as the target sequence. The oligomer was found to be efficiently taken up by myotubes. However, the intracellular half-life of the oligomer was found to be only 3 h. Results of studies using different concentrations of oligomer for 3 h in the culture medium showed that compared to the untreated control culture, myotubes incubated with 20 microns antisense oligomer showed a 30% reduction in the steady-state level of TnC mRNAs. An increase of incubation period to 12 h with additions of fresh culture medium containing 20 microns antisense oligomer every 3 h failed to produce any further reduction of TnC mRNA level. Concomitant to the decrease of TnC mRNAs in antisense oligomer-treated cells, the steady-state levels of alpha-actin and alpha-tropomyosin mRNAs were also reduced by approximately 20 to 40%. Analysis of the homology of the sense sequence of this oligomer with that of alpha-actin and alpha-tropomyosin mRNAs suggested that reduction in the level of alpha-actin and alpha-tropomyosin mRNAs was not due to direct hybridization of the antisense oligomer to these mRNAs. Comparison of TnC polypeptide synthesis in untreated and oligomer-treated myotubes showed approximately 70% reduction of fast TnC polypeptide synthesis in antisense oligomer-treated cells. In contrast, slow TnC polypeptide synthesis was not significantly reduced in treated cells. Similarly, alpha-actin and alpha-tropomyosin polypeptide synthesis remained close to the level of untreated cells. Furthermore, analysis of transcription of various muscle-specific mRNAs showed increased synthesis of both TnC and alpha-tropomyosin mRNAs in antisense oligomer-treated myotubes. On the other hand, synthesis of actin mRNAs was not altered by this treatment. These results showed that antisense oligomer was effective in significantly reducing TnC polypeptide synthesis in chicken myotubes. Furthermore, these results suggest that treatment of myotubes with antisense oligomer to TnC mRNA may have triggered a complex array of compensatory processes.

Relationship of primary and secondary myogenesis to fiber type development in embryonic chick muscle

The formation of fast and slow myotubes was investigated in embryonic chick muscle during primary and secondary myogenesis by immunocytochemistry for myosin heavy chain and Ca2(+)-ATPase. When antibodies to fast or slow isoforms of these two molecules were used to visualize myotubes in the posterior iliotibialis and iliofibularis muscles, one of the isoforms was observed in all primary and secondary myotubes until very late in development. In the case of myosin, the fast antibody stained virtually all myotubes until after stage 40, when fast myosin expression was lost in the slow myotubes of the iliofibularis. In the case of Ca2(+)-ATPase, the slow antibody also stained all myotubes until after stage 40, when staining was lost in secondary myotubes and in the fast primary myotubes of the posterior iliotibialis and the fast region of the iliofibularis. In contrast, the antibodies against slow muscle myosin heavy chain and fast muscle Ca2(+)-ATPase stained mutually exclusive populations of myotubes at all developmental stages investigated. During primary myogenesis, fast Ca2(+)-ATPase staining was restricted to the primary myotubes of the posterior iliotibialis and the fast region of the iliofibularis, whereas slow myosin heavy chain staining was confined to all of the primary myotubes of the slow region of the iliofibularis. During secondary myogenesis, the fast Ca2(+)-ATPase antibody stained nearly all secondary myotubes, while primaries in the slow region of the iliofibularis remained negative. Thus, in the slow region of the iliofibularis muscle, these two antibodies could be used in combination to distinguish primary and secondary myotubes. EM analysis of staining with the fast Ca2(+)-ATPase antibody confirmed that it recognizes only secondary myotubes in this region. This study establishes that antibodies to slow myosin heavy chain and fast Ca2(+)-ATPase are suitable markers for selective labeling of primary and secondary myotubes in the iliofibularis; these markers are used in the following article to describe and quantify the effects that chronic blockade of neuromuscular activity or denervation has on these populations of myotubes.

Long-term maintenance of primary myogenic cultures on a reconstituted basement membrane

We describe a simple technique for maintaining highly contractile long-term chicken myogenic cultures on Matrigel, a gel composed of basement membrane components extracted from the Engelbreth-Holm-Swarm mouse tumor. Cultures grown on Matrigel consist of three-dimensional multilayers of cylindrical, contracting myotubes which endure for at least 60 d without myotube detachment. A Matrigel substrate increases the initial plating efficiency but does not effect cell proliferation. Large-scale differentiation in cultures maintained on Matrigel is delayed by 1 to 2 d, compared to cultures grown on gelatin-coated dishes. Long-term maintenance on Matrigel also results in increased expression of the neonatal and adult fast myosin heavy chain isoforms. Culturing of cells on a Matrigel substrate could thus facilitate the study of later events of in vitro myogenesis.