Myosin isoenzyme transitions in muscle development, maturation, and disease

Effects of perinatal undernutrition on the circumvallate papilla of developing Wistar rats

During the gestation and the lactating periods the gustatory papillae contain taste buds that respond to different flavors and aversive stimuli. The current study analyzed the effects of pre-and neonatal undernutrition on the circumvallate papillae of rats at 12, 20, and 30days of age. Early undernourishment occurred from gestational days G6 to G19 when dams received low percentages of food followed by a balanced diet from G20-21. After birth pups were underfed by rotating two lactating dams every 12h; in one of them, the nipples were tied. The pups were weaned at 25days of age, and then given an ad libitum diet. Under anesthesia the tongues were removed and stained with the hematoxylin-eosin (H-E) procedure. The results indicated that young underfed rats had significantly body weight reductions. The tongue measurements in underfed rats showed reduced total area and length of the anterior portion, but there were negligible effects on the posterior portion. The circumvallate papillae in underfed rats was significantly reduced in major length, major diameter, and total and upper areas, but unaffected in the lateral wall trench region. The taste bud areas and minor diameter were unaffected by undernutrition, but there were significant reductions in the total number of visible taste buds and the major diameter, delayed opening of taste bud pores, and an increased number of closed pores were also observed. These alterations by undernutrition reflect the vulnerability of structures in the gustatory oral cavity and suggest a possible interference with the receptorś activation, and transduction and perhaps with the taste encoding of signals to generate the gustatory sensory and hedonic responses.

Temporal embryonic transcription of chicken fast skeletal myosin heavy chain isoforms in the single comb white leghorn

There are numerous factors that can significantly influence embryonic development in poultry and thus make simple days of incubation (chronological age) a less than perfect metric for studying embryonic physiology. The developmental fast skeletal muscle myosin (MyHC), the predominant protein in the Pectoralis major (PM), is temporally expressed as a cadre of highly specific developmental isoforms. In the study described herein, a novel molecular technology (NanoString) was used to characterize the myosin isoform transcriptional patterns in the PM of Single Comb White Leghorn (SCWL) embryos. NanoString technology is based on quantitative analysis of the transcriptome through digital detection and quantification of target mRNA transcripts. Total RNA was isolated and gene transcription quantified using NanoString in embryonic muscle samples collected daily from 6 through 19 days of incubation. Data were analyzed using the LOESS smoothing function at a 95% confidence level. The temporal transcription of MyHC isoforms obtained in this study was consistent with the literature at higher specificity and resolution, thus validating NanoString for use in gene transcription analyses. The results support a hypothesis that the transcription patterns of the embryonic MyHC isoforms may be used as molecular clocks to further investigate the developmental relationships underlying embryonic fast skeletal muscle growth and development.

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.

Feed restriction delays developmental fast skeletal muscle myosin heavy chain isoforms in turkey poults selected for differential growth

Genetic selection has been very successful at significantly increasing BW and breast muscle proportion in commercial broiler and turkey strains. The mechanisms of breast muscle growth in poultry and the interactive effects of nutritional status and selection are not fully understood. The hypothesis underlying the current study is that feed restriction, simply as a vehicle for controlling early growth, would delay the temporal expression pattern of neonatal (nMyHC) and adult (aMyHC) fast skeletal muscle myosin heavy chain (MyHC) isoforms in the pectoralis major muscle of turkey poults. The poultry growth model used to evaluate this hypothesis consisted of a randombred control turkey line (RBC2) that represents commercial turkeys of the 1960s and a line developed from the RBC2 by selection for BW at 16 wk of age (F line). The F line has significantly heavier breast muscles than the RBC2 concomitant with increased BW, but the proportion of breast muscle relative to BW is similar. A quantitative indirect ELISA using fast skeletal MyHC isoform specific monoclonal antibodies revealed no significant line differences in the temporal expression of posthatch fast skeletal muscle MyHC in ad libitum fed poults. Feed restriction, however, altered the temporal expression patterns of nMyHC and aMyHC in both F line and RBC2 poults compared with the poults fed ad libitum.

The ontogeny of muscle structure and locomotory function in the long-finned squid Doryteuthis pealeii

Understanding the extent to which changes in muscle form and function underlie ontogenetic changes in locomotory behaviors and performance is important in understanding the evolution of musculoskeletal systems and also the ecology of different life stages. We explored ontogenetic changes in the structure, myosin heavy chain (MHC) expression and contractile properties of the circular muscles that provide power for jet locomotion in the long-finned squid Doryteuthis pealeii. The circular muscle fibers of newly hatched paralarvae had different sizes, shapes, thick filament lengths, thin:thick filament ratio, myofilament organization and sarcoplasmic reticulum (SR) distribution than those of adults. Viewed in cross section, most circular muscle cells were roughly triangular or ovoid in shape with a core of mitochondria; however, numerous muscle cells with crescent or other unusual cross-sectional shapes and muscle cells with unequal distributions of mitochondria were present in the paralarvae. The frequency of these muscle cells relative to ‘normal’ circular muscle cells ranged from 1:6 to 1:10 among the 19 paralarvae we surveyed. The thick filaments of the two types of circular fibers, superficial mitochondria-rich (SMR) and central mitochondria-poor (CMP), differed slightly in length among paralarvae with thick filament lengths of 0.83±0.15 μm and 0.71±0.1 μm for the SMR and CMP fibers, respectively (P 0.05; ANOVA). During ontogeny the thick filament lengths of both the CMP and SMR fibers increased significantly to 1.78±0.27 μm and 3.12±0.56 μm, respectively, in adults (P<0.0001 for both comparisons; ANOVA with Tukey's highly significant difference post hoc tests). When sectioned parallel to their long axes, the SMR and CMP fibers of both paralarvae and adults exhibited the myofilament arrangements typical of obliquely striated muscle cells but the angle of obliquity of the dense bodies was 22.8±2.4 deg. and 4.6±0.87 deg. for paralarvae and adults, respectively. There were also differences in the distribution of the anastomosing network of SR. In paralarvae, the outer and central zones of SR were well developed but the intramyoplasmic zone was greatly reduced in some cells or was scattered non-uniformly across the myoplasm. Whereas in adults the intramyoplasmic SR region was composed primarily of flattened tubules, it was composed primarily of rounded vesicles or tubules when present in the paralarvae. The ontogenetic differences in circular muscle structure were correlated with significant differences in their contractile properties. In brief tetanus at 20°C, the mean unloaded shortening velocity of the paralarval circular muscle preparations was 9.1 L0 s−1 (where L0 was the preparation length that generated the peak isometric stress), nearly twice that measured in other studies for the CMP fibers of adults. The mean peak isometric stress was 119±15 mN mm−2 physiological cross section, nearly half that measured for the CMP fibers of adults. Reverse transcriptase-polymerase chain reaction analysis of paralarval and adult mantle samples revealed very similar expression patterns of the two known isoforms of squid MHC. The ontogenetic differences in the structure and physiology of the circular muscles may result in more rapid mantle movements during locomotion. This prediction is consistent with jet pulse durations observed in other studies, with shorter jet pulses providing hydrodynamic advantages for paralarvae.

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.

Dynamics of Z-band based proteins in developing skeletal muscle cells

During myofibril formation, Z-bodies, small complexes of alpha-actinin and associated proteins, grow in size, fuse and align to produce Z-bands. To determine if there were changes in protein dynamics during the assembly process, Fluorescence Recovery after Photobleaching was used to measure the exchange of Z-body and Z-band proteins with cytoplasmic pools in cultures of quail myotubes. Myotubes were transfected with plasmids encoding Yellow, Green, or Cyan Fluorescent Protein linked to the Z-band proteins: actin, alpha-actinin, cypher, FATZ, myotilin, and telethonin. Each Z-band protein showed a characteristic recovery rate and mobility. All except telethonin were localized in both Z-bodies and Z-bands. Proteins that were present both early in development in Z-bodies and later in Z-bands had faster exchange rates in Z-bodies. These results suggest that during myofibrillogenesis, molecular interactions develop between the Z-band proteins that decrease their mobility and increase the stability of the Z-bands. A truncated construct of alpha-actinin, which localized in Z-bands in myotubes and exhibited a very low rate of exchange, led to disruption of myofibrils, suggesting the importance of dynamic, intact alpha-actinin molecules for the formation and maintenance of Z-bands. Our experiments reveal the Z-band to be a much more dynamic structure than its appearance in electron micrographs of cross-striated muscle cells might suggest.

Invertebrate Muscles: Muscle Specific Genes and Proteins

This is the first of a projected series of canonic reviews covering all invertebrate muscle literature prior to 2005 and covers muscle genes and proteins except those involved in excitation-contraction coupling (e.g., the ryanodine receptor) and those forming ligand- and voltage-dependent channels. Two themes are of primary importance. The first is the evolutionary antiquity of muscle proteins. Actin, myosin, and tropomyosin (at least, the presence of other muscle proteins in these organisms has not been examined) exist in muscle-like cells in Radiata, and almost all muscle proteins are present across Bilateria, implying that the first Bilaterian had a complete, or near-complete, complement of present-day muscle proteins. The second is the extraordinary diversity of protein isoforms and genetic mechanisms for producing them. This rich diversity suggests that studying invertebrate muscle proteins and genes can be usefully applied to resolve phylogenetic relationships and to understand protein assembly coevolution. Fully achieving these goals, however, will require examination of a much broader range of species than has been heretofore performed.

Masticatory function and properties of masseter muscle fibers in microphthalmic (mi/mi) mice during postnatal development

It is well known that teeth do not erupt in microphthalmic (mi/mi) mice, a type of osteopetrotic mice, due to bone resorption failure. Therefore, we surmise that the masticatory function of these mice is different from that of normal mice. In this study, the differences to the properties of masseter muscle fibers were clarified morphologically and immunohistochemically in the mi/mi and normal mice. Morphological observations revealed that the muscle fibers in the mi/mi mice were smaller than those in normal mice at 9 weeks of age. However, no marked differences between mi/mi and normal mice at 2 and 4 weeks of age. Immunohistochemical observations showed myosin heavy chain (MHC) slow type fibers, which were usually seen at only early stages of development, in 4-week old mi/mi mice. There were also differences in isoform compositions between the mi/mi and normal mice. These results suggest that differences in masticatory function affect the properties of its muscle fibers.

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.

RNA-protein interactions of the 3' untranslated regions of myosin heavy chain transcripts

The RNA-protein interactions of the myosin heavy chain (MyHC) 3' untranslated regions (3'UTRs) were investigated using gel mobility shift assays. Marine skeletal myosin heavy chain mRNAs were amplified using reverse transcription coupled with the polymerase chain reaction (RT-PCR). Four cloned MyHC sequences were identified as slow type 1, fast 2a, fast 2b and fast 2x. The 3'UTRs of the four MyHC mRNAs were shown to interact with muscle protein in a tissue-specific manner as illustrated by gel retardation assays with protein extracts from various tissues. Competition assays indicate that this interaction is specific to the MyHC 3'UTR sequence. UV cross-linking suggests that several small proteins bind to the 3'UTR's. Peptide sequencing identified aldolase A and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) as MyHC 3'UTR RNA-binding proteins. The implications of these interactions and post-transcriptional regulation of the MyHC genes are discussed.

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.

A discrepancy resolved: human satellite cells are not preprogrammed to fast and slow lineages

Satellite cells from chicken and mouse muscle when differentiated in vitro have been shown to display a myosin heavy chain phenotype that corresponds to the fibre from which they originated. Indirect evidence has suggested that this might not be the case for human satellite cells. In the present study we have compared the myosin heavy chain (MHC) profile expressed by differentiated cultures of satellite cells isolated from single fast or slow muscle fibres. The MHC composition of the isolated fibres was determined by sodium dodecyl sulfate glycerol gel electrophoresis and Western blotting. The MHC profile expressed by the differentiated myotubes was identified by immunostaining using specific antibodies. Our results show that all human satellite cells isolated from either fast or slow fibres form myotubes in vitro which co-express both fast and slow MHCs independently of the fibre type from which they originated. These results confirm that human satellite cells, in contrast to those of birds and rodents, are not confined to distinct fast and slow lineages.

Repression of myosin isoforms in developing and denervated skeletal muscle fibers originates near motor endplates

During development of chicken pectoralis muscle, a neonatal myosin heavy-chain isoform is supplanted progressively by an adult isoform. This expression is under neuronal control. In this study we test the hypothesis that developmental myosin transformations are initiated near the motor endplate of each muscle fiber, thereafter progressing toward the fiber ends. By using immunocytochemical methods, pectoralis muscle from chickens aged 1-115 days after hatching were labeled by antibody against neonatal isoform. Ellipse minor axis and mean optical density of labeled and/or unlabeled fiber profiles from each bird were measured by computer image analysis. Acetylcholinesterase (AChE) activity was demonstrated histochemically. Using serial cross sections, we show that smaller fiber profiles are the tapered ends of larger fiber profiles. The largest fiber profiles (central regions of the fibers) were the first to lose their neonatal myosin during development. Motor endplates were localized by AChE activity to the central regions of the fibers. The pectoralis of mature chickens was denervated for 3, 7, 15, or 21 days. After 2 weeks' denervation, neonatal myosin is first reexpressed in the fiber ends. Dev Dyn 2000;217:50-61.

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.

Differential expression of C-protein isoforms in developing and degenerating mouse striated muscles

With the aim of clarifying the roles of C-protein isoforms in developing mammalian skeletal muscle, we cloned the complementary DNA (cDNAs) encoding mouse fast (F) and slow (S) skeletal muscle C-proteins and determined their entire sequences. Northern blotting with these cDNAs together with mouse cardiac (C) C-protein cDNA was performed. It revealed that in adult mice, C, F, and S isoforms are expressed in a tissue-specific fashion, although the messages for both F and S isoforms are transcribed in extensor digitorum longus muscle, which has been categorized as a fast muscle. In addition, although C isoform is expressed first and transiently during development of chicken skeletal muscles, C isoform is not expressed in mouse skeletal muscles at all through the developmental stages; S isoform is first expressed, followed by the appearance of F isoform. Finally, in dystrophic mouse skeletal muscles, the expression of S isoform is increased as it is in dystrophic chicken muscle. These observations suggest that mutations in C isoform (MyBP-C) do not lead to any disturbance in skeletal muscle, although they may lead to familial hypertrophic cardiomyopathy. We also suggest that the expression of S isoform may be stimulated in degenerating human dystrophic muscles.

The role of interhelical ionic interactions in myosin rod assembly

Interhelical electrostatic interactions at specific heptad positions can regulate dimerization specificity of alpha-helical coiled-coils. We have analyzed 20 vertebrate myosin sequences from a variety of organisms and tissues in order to determine if interhelical ionic interactions correlate with the observed myosin dimerization specificity. We find that the sites for potential interhelical ion pairing are identical in virtually all sarcomeric myosins whether they form homo- or heterodimers. We also show that smooth muscle and non-muscle myosin rod sequences exhibit a different conserved pattern of potential interhelical ion pairing. These observations suggest that myosin rod residues involved in interhelical electrostatic interactions do not regulate dimerization specificity, but may contribute to the specific arrangements of myosin molecules that determine differences in the filament morphology of sarcomeric and non-sarcomeric muscles.

Mammalian skeletal muscle fiber type transitions

Mammalian skeletal muscle is an extremely heterogeneous tissue, composed of a large variety of fiber types. These fibers, however, are not fixed units but represent highly versatile entities capable of responding to altered functional demands and a variety of signals by changing their phenotypic profiles. This adaptive responsiveness is the basis of fiber type transitions. The fiber population of a given muscle is in a dynamic state, constantly adjusting to the current conditions. The full range of adaptive ability spans fast to slow characteristics. However, it is now clear that fiber type transitions do not proceed in immediate jumps from one extreme to the other, but occur in a graded and orderly sequential manner. At the molecular level, the best examples of these stepwise transitions are myofibrillar protein isoform exchanges. For the myosin heavy chain, this entails a sequence going from the fastest (MHCIIb) to the slowest (MHCI) isoform, and vice-versa. Depending on the basal protein isoform profile and hence the position within the fast-slow spectrum, the adaptive ranges of different fibers vary. A simple transition scheme has emerged from the multitude of data collected on fiber type conversions under a variety of conditions.

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.

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.

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.

Characterization of a human perinatal myosin heavy-chain transcript

Using a monoclonal antibody specific to the neonatal myosin heavy chain, we have cloned the full-length heavy chain cDNA from an 18-week human fetal cDNA library. Ribonuclease protection assays were used to survey a human muscle collection ranging from 11 weeks gestation to 16 years. Expression of the RNA encoded by this cDNA was observed at 20 and 21 weeks gestation and at 2 days after birth. No expression was observed at 13.5 weeks, before 2 years, at 2 years, or after 2 years gestation. Due to the timing of its expression, this cDNA appears to represent of the human fetal myosin heavy chain. Sequencing of the entire 6010 bases showed high similarity to the rat perinatal myosin heavy chain [Periasamy, M., Wieczorek, D. F. & Nadal-Ginard, B. (1984) J. Biol. Chem. 21, 13,573-13,578]. However, moderate divergence was observed when compared to a previously described human perinatal myosin heavy chain [Karsch-Mizrachi, I., Feghali, R., Shows, T. B. & Leinwand, L. A. (1990) Gene 89, 289-294; Feghali, R. & Leinwand, L. A. (1989) J. Cell Biol. 108, 1791-1797]. Restriction fragment-length polymorphism analyses of sites in both the S1 and rod domains showed the presence of this fetal myosin heavy chain sequence in all 27 genomic samples examined. Restriction fragment-length polymorphism analysis failed to find the previously described perinatal isoform in any sample.

Sarcoplasmic reticulum function in newborn ferret cremaster skeletal muscles

A study of the properties of the sarcoplasmic reticulum was performed with newborn ferret cremaster muscles at two different development stages: at 8 and 21 days. The effects of extracellular Ca2+, caffeine and cyclopiazonic acid, a specific sarcoplasmic reticulum Ca(2+)-ATPase inhibitor, were examined on intact cremaster skeletal muscles. The uptake and release of Ca2+ were explored on saponin-skinned fibres with or without cyclopiazonic acid and some results obtained were compared with those obtained with adult cremaster muscle. The results have shown that skeletal muscle sarcoplasmic reticulum of newborn animals possesses the ability to accumulate and release Ca2+. Furthermore, application of cyclopiazonic acid modified the twitch, the caffeine responses and decreased the amount of Ca2+ taken up by the sarcoplasmic reticulum in saponin-skinned fibres. In contrast to adult skeletal muscle, in newborn cremaster muscles, the Ca2+ dependence of the twitch suggests that the Ca2+ influx at the sarcolemma level was mainly involved in the activation of the contraction. Furthermore, the results obtained in the presence of cyclopiazonic acid were in favour, as in adult muscle, of a participation of the sarcoplasmic reticulum in the relaxation process.

The effect of denervation on myosin isoform synthesis in rabbit slow-type and fast-type muscles during terminal differentiation. Denervation induces differentiation into slow-type muscles

The soleus and gastrocnemius medialis of eight-day-old rabbits were denervated and the effects were examined after fifty-two days by biochemical, cytochemical and mechanical methods. The contralateral soleus exhibited the properties of slow-type muscle, namely a predominance of slow-type myosin isoforms and slow-type oxidative fibers, slow twitch and low maximal velocity for shortening. The contralateral gastrocnemius exhibited the properties of fast-type muscle, namely a predominance of fast-type myosin isoforms and fast-type non-oxidative fibers, fast twitch and high maximal velocity of shortening. Denervation of muscles caused the differentiation of the two muscles towards slow-type muscles. Both denervated soleus and gastrocnemius muscles exhibited a predominance of slow-type myosins (either the normal type, made up of slow heavy and light chains, or the hybrid type, made up of slow heavy and regulatory light chains and fast essential light chains), a predominance of slow-type fibers, and slow mechanical properties. Thus, innervation in rabbit appears to be a determining factor for differentiation into fast-type muscle, but it is not necessary for differentiation into slow-type muscle. This conclusion contradicts the findings of previous studies in rat and thus raises new questions concerning the role of nerves in controlling the expression of myosin isoforms and the differentiation of muscle fibers.

Ontogeny of the pectoralis muscle in the little brown bat, Myotis lucifugus

The ontogeny of a primary flight muscle, the pectoralis, in the little brown bat (Myotis lucifugus: Vespertilionidae) was studied using histochemical, immunocytochemical, and electrophoretic techniques. In fetal and early neonatal (postnatal age 1-6 days) Myotis, histochemical techniques for myofibrillar ATPase (mATPase) and antibodies for slow and fast myosins demonstrated the presence of two fiber types, here called types I and IIa. These data correlated with multiple transitional myosin heavy chain isoforms and native myosin isoforms demonstrated with SDS-PAGE and 4% pyrophosphate PAGE. There was a decrease in the distribution and number of type I fibers with increasing postnatal age. At postnatal age 8-9 days, the adult phenotype was observed with regard to muscle fiber type (100% type IIa fibers) and myosin isoform profile (single adult MHC and native myosin isoforms). This "adult" fiber type profile and myosin isoform composition preceded adult function by about 2 weeks. For example, little brown bats were incapable of sustained flight until approximately postnatal day 24, and myofiber size did not achieve adult size until approximately postnatal day 25. Although Myotis pectoralis is unique in being composed of 100% type IIa fibers, transitional fiber types and isoforms were present. These transitional forms had been observed previously in other mammals bearing mixed adult muscle fibers and which undergo transitional stages in muscle ontogeny. However, in Myotis pectoralis, this transition transpires relatively early in development.

Different temporal patterns of expression result in the same type, amount, and distribution of filamin (ABP) in cardiac and skeletal myofibrils

The morphogenesis of functional myofibrils in chick skeletal and cardiac muscle occurs in greatly different time spans, in about 7 and 2 days, respectively. In chick skeletal myogenic cells, one isoform of the 250 kD actin-binding protein (ABP) filamin is associated with stress fiber-like structures of myoblasts and early myotubes, then disappears for approximately 4 days, whereupon a second filamin isoform reappears at the Z-disc periphery. We sought to determine if cardiac myogenesis involves this sequence of appearance, disappearance, and reappearance of a new filamin isoform in a compressed time scale. It was known that in mature heart, filamin is localized at the Z-disc periphery as in mature (fast) skeletal muscle, and is also associated with intercalated discs. We find that myocardial filamin has an apparent molecular weight similar to that of adult skeletal muscle filamin and lower than that of smooth muscle filamin, and that both skeletal and cardiac muscle contain roughly 200 filamin monomers per sarcomere. Two-dimensional peptide mapping shows that myocardial filamin is very similar to skeletal muscle filamin. Myocardial, slow skeletal, and fast skeletal muscle filamins are all phosphorylated, as previously shown for filamin of non-striated muscle. Using immunofluorescence, we found that filamin could not be detected in the developing heart until the 14-somite stage, when functional myofibrils exist and the heart has been beating for 3 to 4 hours. We conclude that in cardiac and skeletal myogenesis, different sequences of filamin gene expression result in myofibrils with similar filamin distributions and isoforms.

Role of extracellular matrix proteins in heart function

The cardiac interstitium is populated by nonmyocyte cell types including transcriptionally active cardiac fibroblasts and endothelial cells. Since these cells are the source of many components of the cardiac extracellular matrix, and because changes in cardiac extracellular matrix are suspected of contributing to the genesis of cardiovascular complications in disease states such as diabetes, hypertension, cardiac hypertrophy and congestive heart failure, interest in the mechanisms of activation of fibroblasts and endothelial cells has led to progress in understanding these processes. Recent work provides evidence for the role of the renin-angiotensin-aldosterone system in the pathogenesis of abnormal deposition of extracellular matrix in the cardiac interstitium during the development of inappropriate cardiac hypertrophy and failure. The cardiac extracellular matrix is also known to change in response to altered cardiac performance associated with post-natal aging, and in response to environmental stimuli including intermittent hypoxia and abnormal nutrition. It is becoming clear that the extracellular matrix mainly consists of molecules of collagen types I and III; they form fibrils and provide most of the connective material for typing together myocytes and other structures in the myocardium and thus is involved in the transmission of developed mechanical force. The data available in the literature support the view that the extracellular matrix is a dynamic entity and alterations in this structure result in the development of heart dysfunction.

Myosin polymorphism and differential expression in adult human skeletal muscle

1. Myosin heavy chain (HC) and light chain (LC) isoforms are expressed in a tissue-specific and developmentally-regulated manner in human skeletal muscle. 2. At least seven myosin HC isoforms are expressed in skeletal muscle of the adult. 3. Histochemically-delineated fibre types (based on the stability of myofibrillar actomyosin adenosine triphosphatase activity) in limb muscles correlate with the myosin HC content. 4. Alterations in the phenotypic expression of myosin provides a mechanism of adaptation to stresses placed upon the muscle (e.g. increased and decreased usage).

Plasticity of human satellite cells

Satellite cells were isolated from human quadriceps and masseter muscles and the phenotype of these cells examined in vitro. The expression of the different isoforms of the myosin heavy chains (embryonic, fetal, fast and slow) and light chain isoforms was used to assay myotube diversification. We found that fused cultures of human satellite cells express adult fast and slow MHCs in addition to the embryonic and fetal isoforms. Only the four fast light chains (MLC1emb, MLC1F, MLC2F and MLC3F) were synthesized. No slow MLCs were ever detected in these cultures. In order to determine if the human satellite cells were committed to distinct fast and slow myogenic lineages, a clonal analysis was carried out on both cell populations. All myogenic clones expressed fast and slow MHCs, suggesting that there is no evidence for different fast and slow satellite cell lineages in human skeletal muscle.

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

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

Stretch receptors and body wall muscle in leeches

1. A new kind of muscle receptor has been described in the medicinal leech, the first stretch receptor in a soft-bodied invertebrate to be identified and characterized. 2. The sensory responses to stretch in these receptors are different from those of other more well known muscle receptors such as vertebrate spindles or crustacean stretch receptors. 3. The leech receptors provide the sensory innervation for the tubular muscle layers of the body wall, and they respond to stretch of the muscle with hyperpolarizing potentials that are conducted passively to the CNS. 4. This review summarizes their unusual morphology, membrane properties and sensory responses to stretch; their association with particular muscle fibre types in leech body wall; their synaptic connections; and their role in a well characterized rhythmic behaviour in the leech-swimming.

Expression of myogenic factors in denervated chicken breast muscle: isolation of the chicken Myf5 gene

In this study, we have isolated and characterized the chicken Myf5 gene, and cDNA clones encoding chicken MyoD1 and myogenin. The chicken Myf5 and MRF4 genes are tandemly located on a single genomic DNA fragment, and the chicken Myf5 gene is organized into at least three exons. Using genomic and cDNA probes, we further analyzed the mRNA levels of four myogenic factors during chicken breast muscle development. This analysis revealed that myogenin expression is restricted to in ovo stages in breast muscle, and is not detectable in neonatal and adult stages. On the other hand, Myf5 expression is detectable until day 7 post-hatching, and is not found in adult muscle, whereas high levels of MyoD1 and MRF4 are detectable at all stages. To further understand the roles of innervation on muscle maturation, we analyzed the expression of the four myogenic factors in denervated adult breast muscle. We found that MyoD1, myogenin, and MRF4 are induced at high levels in denervated muscle, whereas no change occurs in the level of Myf5. These studies suggest that innervation controls the relative abundance and type of myogenic factors that are expressed in adult muscle, and that when nerve control is removed, the muscle reverts to a neonatal phenotype, with the enhanced expression of three myogenic factors (MyoD1, myogenin, and MRF4).

The evolutionary relationship of avian and mammalian myosin heavy-chain genes

Sequence comparisons of avian and mammalian skeletal and cardiac myosin heavy-chain isoforms are used to examine the evolutionary relationships of sarcomeric myosin multigene families. Mammalian fast-myosin heavy-chain isoforms from different species, with comparable developmental expression, are more similar to each other than they are to other fast isoforms within the same genome. In contrast, the developmentally regulated chicken fast isoforms are more similar to each other than they are to myosin heavy-chain isoforms in other species. Extensive regions of nucleotide identity among the chicken fast myosin heavy chains and in the mouse and rat alpha- and beta-cardiac myosin heavy-chain sequences suggest that gene-conversion-like mechanisms have played a major role in the concerted evolution of these gene families. We also conclude that the chicken fast myosin heavy-chain multigene family has undergone recent expansion subsequent to the divergence of birds and mammals and that both the developmental regulation and the specialization of myosin isoforms have likely developed independently in birds and mammals.

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.

The proximal promoter of the aldolase A gene remains active during myogenesis in vitro and muscle development in vivo

The gene for aldolase A in mouse has been shown to be regulated by alternative promoters with attendant alternative first exons. The distal promoter/exon M functions only in muscle while the proximal promoter/exon H is active in early muscle development and in most other tissues. We have analyzed the developmental expression of M and H promoters in mouse throughout myogenesis both in vitro and in vivo. In C2C12 cells RNase protection assays revealed the M promoter is induced within 24 hr of the onset of myogenic differentiation, and both M- and H-specific mRNAs accumulate over 5 days in culture. Nuclear run-on transcription and in situ hybridization with an exon-specific probe demonstrate that the H promoter remains transcriptionally active even in differentiated myotubes. The in vitro results were then compared to similar RNase protection studies of M and H expression during muscle development in vivo. These data show a marked similarity between promoter activation and steady-state transcript accumulation in vivo and in vitro, but within a limited developmental time frame (E15 to 1 week postnatal). In situ hybridizations suggest that simultaneous transcription from both promoters may also occur early in muscle development. Furthermore, the M promoter shows no fiber-type restriction until 1 to 3 weeks postnatally, coincident with muscle maturation, while the H promoter remains transcriptionally active at all stages of development and in all fiber types.

Control of myosin heavy chain expression: interaction of hypothyroidism and hindlimb suspension

The aim of this study was to contrast competing influences, hypothyroidism and hindlimb suspension, on myosin heavy chain (MHC) expression studied at the protein level and mRNA level. Female Sprague-Dawley rats were assigned to either normal control (NC), normal suspended (NS), or hypothyroid (thyroidectomized) control (TC) and suspended (TS) groups. NS and TS animals were suspended for 14 days following which myofibrils and total RNA were purified from the hindlimb muscles. In the soleus and vastus intermedius (VI), there was an increase in type I MHC and a decrease in type IIa MHC in both the TC and TS groups and a decrease in type I and increase in type IIa MHC in the NS group. At the mRNA level, similar shifts were observed with the exception that 1) the increased type IIa MHC seen in the soleus and VI of the NS animals was not accompanied by an increase in IIa mRNA and 2) type IIb mRNA was increased in the NS soleus without concomitant changes in IIb protein levels. These data suggest the following: 1) a hypothyroid state predominates over mechanical unweighting factors in the control of MHC distribution in slow muscles; and 2) translational or posttranslational factors may be important in the regulation of type IIa and IIb MHC expression during hindlimb suspension.

Identification of sequences necessary for the association of cardiac myosin subunits

To begin to understand the nature of myosin subunit assembly, we determined the region of a vertebrate sarcomeric myosin heavy chain required for binding of light chain 1. We coexpressed in Escherichia coli segments of the rat alpha cardiac myosin heavy chain which spanned the carboxyl terminus of subfragment 1 and the amino terminus of subfragment 2 with a full-length rat cardiac myosin light chain 1. A 16 amino acid region of the myosin heavy chain (residues 792-808) was shown to be required for myosin light chain 1 binding in an immunoprecipitation assay.