Characterization of cDNA and genomic sequences corresponding to an embryonic myosin heavy chain

Striated myosin heavy chain gene is a crucial regulator of larval myogenesis in the pacific oyster Crassostrea gigas

Pacific oyster (Crassostrea gigas), the most productive economical bivalve mollusc, is identified as an attractive model for developmental studies due to its classical mosaic developmental pattern. Myosin heavy chain is a structural and functional component of myosin, the key muscle protein of thick filament. Here, full length cDNA of striated myosin heavy chains in C. gigas (CgSmhc) was obtained, and the expression profiles were examined in different development stage. CgSmhc had a high expression level in trochophore and D-shaped stage during embryo-larval stage. In adult, CgSmhc was a muscle-specific gene and primarily expressed in muscle tissues. Then, activity of 5' flanking region of CgSmhc were examined through an reconstructed EGFP vector. The results indicated that 3098 bp 5'-flanking region of CgSmhc owned various conserved binding sites of myogenesis-related regulatory elements, and the 2000 bp 5'-flanking sequence was sufficient to induce the CgSmhc expression. Subsequently, the CRISPR/Cas9-mediated target disruption of CgSmhc was generated by co-injection of Cas9mRNA and CgSmhc-sgRNAs into one-cell stage embryos of C. gigas. Loss of CgSmhc had a visible effect on the sarcomeric organization of thin filaments in larval musculature, indicating that CgSmhc was required during larval myogenesis to regulate the correct assembly of sarcomere.

Myosin heavy chain-embryonic regulates skeletal muscle differentiation during mammalian development

Myosin heavy chain-embryonic (MyHC-emb) is a skeletal muscle-specific contractile protein expressed during muscle development. Mutations in MYH3, the gene encoding MyHC-emb, lead to Freeman-Sheldon and Sheldon-Hall congenital contracture syndromes. Here, we characterize the role of MyHC-emb during mammalian development using targeted mouse alleles. Germline loss of MyHC-emb leads to neonatal and postnatal alterations in muscle fiber size, fiber number, fiber type and misregulation of genes involved in muscle differentiation. Deletion of Myh3 during embryonic myogenesis leads to the depletion of the myogenic progenitor cell pool and an increase in the myoblast pool, whereas fetal myogenesis-specific deletion of Myh3 causes the depletion of both myogenic progenitor and myoblast pools. We reveal that the non-cell-autonomous effect of MyHC-emb on myogenic progenitors and myoblasts is mediated by the fibroblast growth factor (FGF) signaling pathway, and exogenous FGF rescues the myogenic differentiation defects upon loss of MyHC-emb function in vitro Adult Myh3 null mice exhibit scoliosis, a characteristic phenotype exhibited by individuals with Freeman-Sheldon and Sheldon-Hall congenital contracture syndrome. Thus, we have identified MyHC-emb as a crucial myogenic regulator during development, performing dual cell-autonomous and non-cell-autonomous functions.This article has an associated 'The people behind the papers' interview.

Exogenous skeletal muscle satellite cells promote the repair of levator palpebrae superioris mechanical damage in rat

AIM OF THE STUDY

Blepharoptosis is a drooping of the upper eyelid, usually due to dysfunction of the levator palpebrae superioris (LPS). Recently, skeletal muscle satellite cells (SSCs) have been reported to promote the repair of damaged skeletal muscle. This study aims to investigate the potential contribution of exogenous SSCs to the regeneration of mechanically damaged LPS.

MATERIALS AND METHODS

Thirty-two rats were randomly divided into four groups, including control group, SSCs-treated group, SSCs-treated injury group and non-treated injury group. After rats in injury groups were artificially lacerated on both the left and right LPS, HBBS (Hank's Balanced Salt Solution) containing SSCs was injected into upper eyelid tissue. After 7 days, the LPS muscle tissues were excised. In addition, skeletal muscle cells (SMCs) and SSCs were cocultured for use as an in vitro model, and the protective effects of SSCs on cultured SMCs were also investigated.

RESULTS

Histological staining revealed that exogenous SSCs repaired the damaged muscle fibers and attenuated the fibrosis of LPS, possibly due to the increased level of IGF-1. In contrast, the level of IL-1β, IL-6, TGF-β1 and Smad2/3 (phospho-T8) were significantly reduced in the SSCs-treated group. The in vitro model using coculture of skeletal muscle cells (SMCs) and SSCs also revealed an increased level of IGF-1 and reduced level of inflammatory factors, resulting in a better cell survival rate.

CONCLUSIONS

This study found that exogenous SSCs can promote the repair of LPS mechanical damage and provides new insight into the development of novel therapeutic approaches for blepharoptosis.

Fast skeletal muscle regulatory light chain is required for fast and slow skeletal muscle development

In skeletal muscle, the myosin molecule contains two sets of noncovalently attached low molecular weight proteins, the regulatory (RLC) and essential (ELC) light chains. To assess the functional and developmental significance of the fast skeletal isoform of the RLC (RLC-f), the murine fast skeletal RLC gene (Mylpf) was disrupted by homologous recombination. Heterozygotes containing an intronic neo cassette (RLC-/+) had approximately one-half of the amount of the RLC-f mRNA compared to wild-type (WT) mice but their muscles were histologically normal in both adults and neonates. In contrast, homozygous mice (RLC-/-) had no RLC-f mRNA or protein and completely lacked both fast and slow skeletal muscle. This was likely due to interference with mRNA processing in the presence of the neo cassette. These RLC-f null mice died immediately after birth, presumably due to respiratory failure since their diaphragms lacked skeletal muscle. The body weight of newborn RLC-f null mice was decreased 30% compared to heterozygous or WT newborn mice. The lack of skeletal muscle formation in the null mice did not affect the development of other organs including the heart. In addition, we found that WT mice did not express the ventricular/slow skeletal RLC isoform (RLC-v/s) until after birth, while it was expressed normally in the embryonic heart. The lack of skeletal muscle formation observed in RLC-f null mice indicates the total dependence of skeletal muscle development on the presence of RLC-f during embryogenesis. This observation, along with the normal function of the RLC-v/s in the heart, implicates a coupled, diverse pathway for RLC-v/s and RLC-f during embryogenesis, where RLC-v/s is responsible for heart development and RLC-f is necessary for skeletal muscle formation. In conclusion, in this study we demonstrate that the Mylpf gene is critically important for fast and slow skeletal muscle development.

Insulin-like growth factor-I downregulates embryonic myosin heavy chain (eMyHC) in myoblast nuclei

The obscure ability of the insulin-like growth factors (IGF-I & -II) to stimulate both myoblast proliferation and differentiation suggests that the latter effect may be mediated locally, possibly by IGF binding proteins (IGFBPs). In some cells, the growth inhibitory actions of IGFBP-5 require plasma membrane translocation and nuclear localization. Immunoreactivity of presumably endogenous IGFBP-5 was identified within proliferating rat L6 myoblast nuclei using fluorescent and confocal microscopy in separate experiments and was reduced by 100 ng/ml IGF-I in a time-dependent manner. Western blotting of nuclear and cytosolic protein identified a single anti-IGFBP-5 immunoreactive protein of approximately 200 kDa, primarily in nuclear fractions, that was downregulated in cells treated with IGF-I for 12 h. The unknown protein was immunopurified from nuclear fractions and identified as the rat homologue for embryonic myosin heavy chain (eMyHC) using matrix-associated laser desorption ionization time of flight (MALDI-TOF) mass spectroscopy. Cross-reactivity of the IGFBP-5 antiserum with eMyHC was confirmed by blotting anti-IGFBP-5 nuclear immunoprecipitates with eMyHC monoclonal antibodies (F1.652). These data indicate that eMyHC is located predominantly within the nuclei of proliferating L6 myoblasts and suggest that IGF-stimulated differentiation is associated with the rapid downregulation of nuclear eMyHC as these cells stop expressing this myosin II isoform as they differentiate. Myosin Ibeta has been identified within the nuclei of non-muscle cells where it helps to regulate gene transcription. Thus, eMyHC may serve a similar role in myoblasts that is specific only to the undifferentiated state.

Stress-dependent and -independent expression of the myogenic regulatory factors and the MARP genes after eccentric contractions in rats

The relationship between muscle mechanical conditions and gene expression was investigated by varying both stress and contraction mode imposed upon rat dorsiflexors (n= 25), activating them at high or low frequencies (150 Hz or 40 Hz) either eccentrically or isometrically. Muscle physiological, immunohistochemical and gene expression changes were then measured 24 h after the exercise bout. Peak stress was the best predictor of muscle injury, independent of contraction mode (i.e. eccentric or isometric). When peak stresses were matched, no physiological or immunohistochemical differences were detected between isometric and eccentric contractions. The expression of certain myogenic regulatory and muscle ankyrin repeat protein (MARP) genes (myoD, myogenin, MLP and CARP) depended both on peak muscle stress achieved during contraction and contraction mode. In contrast, Arpp/Ankrd2 was dramatically upregulated only by eccentric contractions, but not by isometric contractions, even though the stress level of the eccentric contractions varied over a three-fold range and overlapped with that of the isometric group. The role that Arpp/Ankrd2 upregulation plays in the biological response to eccentric contraction remains to be determined, as does the control mechanism whereby the expression of certain genes (such as myoD, myogenin, MLP and CARP) is sensitive to muscle stress while another (Arpp/Ankrd2) is sensitive only to contraction mode.

Asynchronous functional, cellular and transcriptional changes after a bout of eccentric exercise in the rat

Thirty eccentric contractions (ECs) were imposed upon rat dorsiflexors (n = 46) by activating the peroneal nerve and plantarflexing the foot ~40 deg, corresponding to a sarcomere length change over the range 2.27-2.39 microm for the tibialis anterior and 2.52-2.66 microm for the extensor digitorum longus. Animals were allowed to recover for one of 10 time periods ranging from 0.5 to 240 h, at which time muscle contractile properties, immunohistochemical labelling and gene expression were measured. Peak isometric torque dropped significantly by ~40 % from an initial level of 0.0530 +/- 0.0009 Nm to 0.0298 +/- 0.0008 Nm (P < 0.0001) immediately after EC, and then recovered in a linear fashion to control levels 168 h later. Immunohistochemical labelling of cellular proteins revealed a generally asynchronous sequence of events at the cellular level, with the earliest event measured being loss of immunostaining for the intermediate filament protein, desmin. Soon after the first signs of desmin loss, infiltration of inflammatory cells occurred, followed by a transient increase in membrane permeability, manifested as inclusion of plasma fibronectin. The quantitative polymerase chain reaction (QPCR) was used to measure transcript levels of desmin, vimentin, embryonic myosin heavy chain (MHC), myostatin, myoD and myogenin. Compared to control levels, myostatin transcripts were significantly elevated after only 0.5 h, myogenic regulatory factors significantly elevated after 3 h and desmin transcripts were significantly increased 12 h after EC. None of the measured parameters provide a mechanistic explanation for muscle force loss after EC. Future studies are required to investigate whether there is a causal relationship among desmin loss, increased cellular permeability, upregulation of the myoD and desmin genes, and, ultimately, an increase in the desmin content per sarcomere of the muscle.

Expression profiling reveals metabolic and structural components of extraocular muscles

The extraocular muscles (EOM) are anatomically and physiologically distinct from other skeletal muscles. EOM are preferentially affected in mitochondrial myopathies, but spared in Duchenne's muscular dystrophy. The anatomical and pathophysiological properties of EOM have been attributed to their unique molecular makeup: an allotype. We used expression profiling to define molecular features of the EOM allotype. We found 346 differentially expressed genes in rat EOM compared with tibialis anterior, based on a twofold difference cutoff. Genes required for efficient, fatigue-resistant, oxidative metabolism were increased in EOM, whereas genes for glycogen metabolism were decreased. EOM also showed increased expression of genes related to structural components of EOM such as vessels, nerves, mitochondria, and neuromuscular junctions. Additionally, genes related to specialized functional roles of EOM such as the embryonic and EOM-specific myosin heavy chains and genes for muscle growth, development, and/or regeneration were increased. The EOM expression profile was validated using biochemical, structural, and molecular methods. Characterization of the EOM expression profile begins to define gene transcription patterns associated with the unique anatomical, metabolic, and pathophysiological properties of EOM.

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

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

Identification of genes that are differentially expressed in extraocular and limb muscle

The extraocular muscles (EOM) are anatomically and physiologically distinct from other striated muscles in mammals. Among other differences, they can be driven to generate individual twitch contractions at an extremely high frequency and are resistant to [Ca(2+)]-induced myonecrosis. While EOM are preferentially targeted in some neuromuscular diseases such as myasthenia gravis and congenital fibrosis of the extraocular muscles, they are enigmatically spared in Duchenne's muscular dystrophy, despite the widespread damage seen in all other skeletal muscle groups during the course of this disease. To address the molecular mechanisms that specify the EOM-phenotype, we characterized the transcriptional profile of genes expressed in rat EOM versus limb muscle using a differential display strategy. Ninety-five putative differentially expressed cDNA tags were cloned, from which fourteen were confirmed as being differentially expressed by RNA slot blot and Northern blot analysis. Ten of these cDNAs were homologous to known human or murine genes and ESTs, while four genes that were upregulated in EOM were novel, and have been named expressed in ocular muscle (eom) 1-4. The identification of these differentially expressed genes may provide mechanistic clues toward understanding the unique patho-physiological phenotype of EOM.

Quantification of myosin heavy chain mRNA in somatic and branchial arch muscles using competitive PCR

The purpose of this study was to quantify the type and amount of myosin heavy chain (MHC) mRNA within muscles of different developmental origins to determine whether the regulation of gene expression is comparable. Seven MHC isoforms were analyzed in rat adult limb (extensor digitorum longus, tibialis anterior, and soleus) and nonlimb (extraocular, thyroarytenoid, diaphragm, and masseter) muscles using a competitive PCR assay. An exogenous template that included oligonucleotide sequences specific for seven rat sarcomeric MHC isoforms (beta-cardiac, 2A, 2X, 2B, extraocular, embryonic, and neonatal) as well as beta-actin was constructed and used as the competitor. Only the extraocular muscle contained all seven isoforms. All seven muscles contained type 2A and type 2X MHC transcripts in varying percentages. As expected, the soleus muscle contained primarily beta-cardiac MHC (87.8 +/- 2.6%). Extraocular MHC was found only in the extraocular and thyroarytenoid muscles and in relatively small proportions (7.4 +/- 1.5% and 4.0 +/- 0.7%, respectively). Neonatal MHC was identified in extraocular (7.9 +/- 0. 3%), thyroarytenoid (4.4 +/- 0.4%), and masseter (1.0 +/- 0.2%) muscles, and embryonic MHC was identified both in extraocular (1.2 +/- 0.5%) and, unexpectedly, in soleus (0.6 +/- 0.1%) muscles. Absolute MHC mRNA mass was greatest in the masseter (106 pg/0.5 microg RNA) and least for the tibialis anterior (64 pg/0.5 microg RNA). These values suggest that MHC mRNA represents from 4 to 17% of the total mRNA pool in various skeletal muscles. Differences in MHC profile between somatic and branchial arch muscles suggest that the developmental origin of a muscle may, at least in part, be responsible for the MHC expression program that is implemented in the adult. An inverse relationship between the expression of beta-cardiac and type 2B MHC transcripts across muscles was noted, suggesting that the expression of these two isoforms may be reciprocally regulated.

Myosin heavy chain isoforms in smooth muscle

In recent years, significant progress has been made toward understanding smooth muscle myosin heavy chain (SMHC) structure. Molecular cloning analysis has identified four different MHC isoforms. They are products of a single gene and result from alternative mRNA splicing. In addition, two non-muscle MHC isoforms are also expressed in smooth muscle cells. Studies show that SMHC expression is highly tissue specific and does not appear in cardiac or skeletal muscle cells. Each smooth muscle tissue is characterized by a specific pattern of MHC isoform expression that changes during development and disease. This review essentially focuses on SMHC isoforms and their expression in mammals.

Cardiomyopathy in transgenic myf5 mice

To explore the compatibility of skeletal and cardiac programs of gene expression, transgenic mice that express a skeletal muscle myogenic regulator, bmyf5, in the heart were analyzed. These mice develop a severe cardiomyopathy and exhibit a significantly shorter life span than do their nontransgenic littermates. The transgene was expressed from day 7.5 post coitum forward, resulting in activation of skeletal muscle genes not normally seen in the myocardium. Cardiac pathology was not apparent at midgestation but was evident by day 2 of postnatal life, and by 42 days, hearts exhibited multifocal interstitial inflammation, fibrosis, cellular hypertrophy, and occasional myocyte degeneration. All four chambers of the heart were enlarged to varying degrees, with the atria demonstrating the most significant hypertrophy (>100% in 42-day-old mice). The transgene and several skeletal muscle-specific genes were expressed only in patchy areas of the heart in heterozygous mice. However, molecular markers of hypertrophy (such as alpha-skeletal actin and atrial myosin light chain- 1) were expressed with a wider distribution, suggesting that their induction was secondary to the expression of the transgene, In older (28-week-old) mice, lung weights were also significantly increased, consistent with congestive heart failure. The life span of bmyf5 mice was significantly shortened, with an average life span of 109 days, compared with at least a twofold longer life expectancy for nontransgenic littermates. Expression of the transgene was associated with an increase in Ca2+-stimulated myofibrillar ATPase in myofibrils obtained from the left ventricles of 42-day-old bmyf5 mice. Myocardial bmyf5 expression therefore induces a program of skeletal muscle gene expression that results in progressive cardiomyopathy that may be due to incompatibility of heart and skeletal muscle structural proteins.

Neural regulation of differentiation of rat skeletal muscle cell types

Three monoclonal antibodies (LM5, F2 and F39) to the fast class of myosin heavy chain (MHC) were used to study the effect of denervation on the differentiation of muscle cell types in some rat skeletal muscles. Antibody LM5 in immunocytochemical investigations did not stain any myotubes during early fetal development but presumptive fast muscle cells started to stain during later fetal development. Unlike antibody LM5, antibodies F2 and F39 stained all myotubes during fetal development. The suppression of fast myosin heavy chains recognised in presumptive slow muscle cells was observed within 1-2 days after birth with antibody F39 but not until 10-14 days after birth with antibody F2. The emergence of subsets of fast muscle fibre types in rat extensor digitorum longus (EDL) and tibialis anteri (TA) detectable by F39 and F2 antibodies was not observed until 2-3 weeks after birth. Denervation of developing muscles led to marked changes in the expression of myosins identified by these antibodies.

Transfection of chicken skeletal muscle alpha-actinin cDNA into nonmuscle and myogenic cells: dimerization is not essential for alpha-actinin to bind to microfilaments

alpha-Actinins from striated muscle, smooth muscle, and nonmuscle cells are distinctive in their primary structure and Ca2+ sensitivity for the binding to F-actin. We isolated alpha-actinin cDNA clones from a cDNA library constructed from poly(A)+ RNA of embryonic chicken skeletal muscle. The amino acid sequence deduced from the nucleotide sequence of these cDNAs was identical to that of adult chicken skeletal muscle alpha-actinin. To examine whether the differences in the structure and Ca2+ sensitivity of alpha-actinin molecules from various tissues are responsible for their tissue-specific localization, the cDNA cloned into a mammarian expression vector was transfected into cell lines of mouse fibroblasts and skeletal muscle myoblasts. Immunofluorescence microscopy located the exogenous alpha-actinin by use of an antibody specific for skeletal muscle alpha-actinin. When the protein was expressed at moderate levels, it coexisted with endogenous alpha-actinin in microfilament bundles in the fibroblasts or myoblasts and in Z-bands of sarcomeres in the myotubes. These results indicate that Ca2+ sensitivity or insensitivity of the molecules does not determine the tissue-specific localization. In the cells expressing high levels of the exogenous protein, however, the protein was diffusely present and few microfilament bundles were found. Transfection with cDNAs deleted in their 3' portions showed that the expressed truncated proteins, which contained the actin-binding domain but lacked the domain responsible for dimerization, were able to localize, though less efficiently in microfilament bundles. Thus, dimer formation is not essential for alpha-actinin molecules to bind to microfilaments.

Isolation and characterization of a previously unrecognized myosin heavy chain gene present in the Syrian hamster

A full length (25,000 base-pair) myosin heavy chain gene completely contained within a single cosmid clone was isolated from a Syrian hamster cosmid genomic library. Sequence comparison of the 3' untranslated region indicated the presence of a 75% homology with the rat embryonic myosin heavy chain gene. Extensive 5' flanking region regulatory element conservation was also found when the sequence was compared to the rat myosin heavy chain gene. S1 nuclease digestion analysis, however, indicated that the Syrian hamster myosin heavy chain gene exhibited expression in adult Syrian hamster ventricular tissue, as well as the adult vastus medialis, a fast twitch skeletal muscle. Expression also appears to be enhanced in myopathic relative to control hearts. This myosin heavy chain gene is neither the alpha nor beta cardiac myosin heavy chain gene, but is a unique, previously unrecognized, myosin heavy chain gene present in both myocardial and skeletal muscle tissues.

Development of muscle fiber types in the prenatal rat hindlimb

Immunohistochemistry was used to examine the expression of embryonic, slow, and neonatal isoforms of myosin heavy chain in muscle fibers of the embryonic rat hindlimb. While the embryonic isoform is present in every fiber throughout prenatal development, by the time of birth the expression of the slow and neonatal isoforms occurs, for the most part, in separate, complementary populations of fibers. The pattern of slow and neonatal expression is highly stereotyped in individual muscles and mirrors the distribution of slow and fast fibers found in the adult. This pattern is not present at the early stages of myogenesis but unfolds gradually as different generations of fibers are added. As has been noted by previous investigators (e.g., Narusawa et al., 1987, J. Cell Biol. 104, 447-459), all of the earliest generation (primary) muscle fibers initially express the slow isoform but some of these primary fibers later lose this expression. In this study we show that loss of slow myosin in these fibers is accompanied by the expression of neonatal myosin. This switch in isoform expression occurs in all primary fibers located in specific regions of particular muscles. However, in other muscles primary fibers which retain their slow expression are extensively intermixed with those that switch to neonatal expression. Later generated (secondary) muscle fibers, which are interspersed among the primary fibers, express neonatal myosin, although a few of them in stereotyped locations later switch from neonatal to slow myosin expression. Many of the observed changes in myosin expression occur coincidentally with the arrival of axons in the limb or the invasion of axons into individual muscles. Thus, although both fiber birth date and intramuscular position are grossly predictive of fiber fate, neither factor is sufficient to account for the final pattern of fiber types seen in the rat hindlimb. The possibility that fiber diversification is dependent upon innervation is tested in the accompanying paper (K. Condon, L. Silberstein, H.M. Blau, and W.J. Thompson, 1990, Dev. Biol. 138, 275-295).

Myogenin, a factor regulating myogenesis, has a domain homologous to MyoD

In this report, we describe the isolation, sequence, and initial characterization of the cDNA for the muscle-specific regulatory factor skeletal myogenin. Transfection of myogenin into the mesenchymal cell line C3H10T1/2 produces cells expressing muscle-specific markers. Myogenin is absent in undifferentiated cells, peaks, and then declines following a stimulus to differentiate, and is overexpressed in myoblasts selected with 5-bromodeoxyuridine for the overproduction of factors that regulate the decision to differentiate. High levels of myogenin transcripts are present in the myotomal region of somites at 8.5 days of gestation in the mouse. Although myogenin and MyoD are different genes, they share the myc homology domain. Myogenin and MyoD thus form part of a gene family regulating myogenesis, and together with myd may constitute a set of factors that interact to regulate the determination and differentiation of muscle cells.

Structure and expression of a newt cardio-skeletal myosin gene. Implications for the C value paradox

As part of our studies on the fate of the muscle lineage during amphibian limb regeneration, we have isolated genomic and cDNA sequences from a myosin heavy chain in the newt (Notophthalmus viridescens). Notwithstanding the technical problems inherent in analysing the large newt genome, genomic and cDNA sequences have been isolated and subjected to analysis by restriction mapping. Northern hybridization, Southern hybridization and DNA sequencing. We believe these to be the first single copy newt gene sequences to have been subjected to this type of analysis. The newt gene sequences showed a striking difference from mammalian myosins in both the estimated sizes of the gene and its intervening sequences; these being much larger than in the mammalian models, it is speculated that this could contribute to the exceptional size of the newt genome. By contrast, the coding sequences displayed very high levels of sequence homology to mammalian myosins. In particular, the amino acid sequence of the newt myosin was found to have greatest homology with rat and human myosin isotypes having a similar cardio-skeletal muscle expression pattern. Despite a long evolutionary separation, newt and mammalian cardio-skeletal myosins have remained more similar to each other than have the human or rat cardiac forms to skeletal myosins within their own respective species.

Embryonic and neonatal myosin heavy chain in denervated and paralyzed rat skeletal muscle

Using immunofluorescence procedures with specific polyclonal and monoclonal antimyosin antibodies we have found that embryonic and neonatal myosin heavy chains (MHCs), which in rat skeletal muscle disappear during the first weeks after birth, are reexpressed in adult muscle after denervation. Reactivity for embryonic and neonatal MHCs was detected in some fibers as early as 3 days after denervation, became more evident by 7 days, and occurred exclusively in the type 2A fiber population. Paralysis of innervated muscles by tetrodotoxin block of the sciatic nerve also resulted in the reappearance of embryonic and neonatal MHCs in type 2A fibers. Significant variation in the degree of immunoreactivity was observed in different segments of the same muscle fiber, suggesting that coordination of muscle fiber nuclei in the control of myosin heavy chain gene expression is partially lost following denervation.

Immunocytochemical and electrophoretic analyses of changes in myosin gene expression in cat limb fast and slow muscles during postnatal development

Changes in myosin synthesis during the postnatal development of the fast extensor digitorum longus (EDL) and the slow soleus muscles of the kitten were examined using immunocytochemical techniques supplemented by pyrophosphate gel electrophoresis and gel electrophoresis-derived enzyme linked immunosorbent assay (GEDELISA) of myosin isoforms. The antibodies used were monoclonals against heavy chains of slow and fast myosins and a polyclonal against foetal/embryonic myosin. In both muscles in the newborn kitten, there was a population of more mature fibres which stained strongly for slow but weakly for foetal/embryonic myosin. These fibres were considered to be primary fibres. They formed 4.8% of EDL fibres and 26% of soleus fibres at birth, and continued to express slow myosin in adult muscles. The less mature secondary fibres stained strongly for foetal/embryonic myosin, and these could be divided into two subpopulations; fast secondaries in which foetal/embryonic myosin was replaced by fast myosin, and slow secondaries in which the myosin was replaced by slow myosin. At 50 days the EDL had a large population of fast secondaries (83% of total fibres) and a small population of slow secondaries which gradually transformed into fast fibres with maturity. The vast majority of secondary fibres in the soleus were slow secondaries, in which slow myosin synthesis persisted in adult life. There was a restricted zone of fast secondaries in the soleus, and these gradually transformed into slow fibres in adult life. It is proposed that the emergence of primary fibres and the two populations of secondary fibres is myogenically determined.

Molecular cloning of Schistosoma mansoni myosin

A cDNA library representative of adult Schistosoma mansoni mRNA populations was screened with serum from infected rats (refractory hosts), positive plaques being rescreened with serum from infected mice and humans. Based on general reactivity, one clone was selected for further study. As judged by immunofluorescence data, size of corresponding mRNA, and nucleotide sequence analysis, the recombinant expresses approximately 625 amino acids of a schistosome muscle myosin rod. Antibodies evoked by the protein do not cross-react with human cardiac or skeletal muscle, are not invariably stimulated in naturally infected human beings, and rise in titer after chemotherapeutic cure, findings which suggest that the antigen is not a causative agent of Katayama fever, and is probably presented by degenerating worms. The schistosome sarcomeric myosin gene, the most primitive examined to date, appears to be unique inasmuch as it may not be a member of a multigene family and encodes a single mRNA transcript; nonetheless, predicted higher order structure of its translation product is consistent with expected function.

Developmental and hormonal regulation of sarcomeric myosin heavy chain gene family

Sarcomeric myosin heavy chain (MHC), the main component of the sarcomere, contains the ATPase activity that generates the contractile force of cardiac and skeletal muscles. The different MHC isoforms are encoded by a closely related multigene family. Most members (seven) of this gene family have been isolated and characterized in the rat, including the alpha- and beta-cardiac, skeletal embryonic, neonatal, fast IIA, fast IIB, and extraocular specific MHC. The slow type I skeletal MHC is encoded by the same gene that codes for the cardiac beta-MHC. Each MHC gene studied displays a pattern of expression that is tissue and developmental stage specific, both in cardiac and skeletal muscles. Furthermore, more than one MHC gene is expressed in each muscle while each gene is expressed in more than one tissue. The expression of each MHC gene in cardiac and skeletal muscles is modulated by thyroid hormone. Surprisingly, however, the same MHC gene can be regulated by the hormone in a significantly different manner, even in opposite directions, depending on the muscle in which it is expressed. Moreover, the skeletal embryonic and neonatal MHC genes, so far considered specific to these 2 developmental stages, are normally expressed in certain adult muscles and can be reinduced by hypothyroidism in specific muscles. This complex pattern of expression and regulation of the MHC gene family in cardiac and skeletal muscle sheds new light on the mechanisms involved in determining the biochemical basis of the contractile state. It also indicates that the cardiac contractile system needs to be examined in a broader context, including skeletal muscles, in order to understand fully its developmental and physiologic regulation.

Developmental pattern of mouse skeletal myosin heavy chain gene transcripts in vivo and in vitro

We have studied the transcripts of the embryonic, perinatal, and adult fast myosin heavy chain (MHC) genes in mouse skeletal muscle in vivo before and after birth, and in vitro in myogenic cell lines. In vivo, in 15-day fetal muscle, embryonic and perinatal MHC mRNAs are both present, and the former is the major transcript. By 18 days the perinatal is predominant and the adult MHC mRNA appears. In beta-bungarotoxin-treated fetuses, a similar developmental pattern is detected, suggesting that it is nerve-independent and that primary myotubes alone undergo the same developmental changes. In vitro, in the absence of the nerve, embryonic, perinatal, and adult IIB MHC mRNAs accumulate. The level of the latter two isomRNAs is influenced by culture conditions.

Intron-dependent evolution: preferred types of exons and introns

Exon insertions and exon duplications, two major mechanisms of exon shuffling, are shown to involve modules that have introns of the same phase class at both their 5'- and 3'-ends. At the sites of intronic recombinations exon insertions and duplications create new introns which belong to the same phase class as the recipient introns. As a consequence of repeated exon insertions and exon duplications introns of a single phase class predominate in the resulting genes, i.e. gene assembly by exon shuffling is reflected both by this nonrandom intron phase usage and by the correlation between the domain organization of the proteins and exon-intron organization of their genes. Genes that appeared before the eukaryote-prokaryote split do not show these diagnostic signs of exon shuffling. Since ancestral introns (e.g. self-splicing introns) did not favour intronic recombination, exon shuffling may not have been significant in the early part of protein evolution.

Complete nucleotide and encoded amino acid sequence of a mammalian myosin heavy chain gene. Evidence against intron-dependent evolution of the rod

The complete nucleotide sequence and exon/intron structure of the rat embryonic skeletal muscle myosin heavy chain (MHC) gene has been determined. This gene comprises 24 X 10(3) bases of DNA and is split into 41 exons. The exons encode a 6035 nucleotide (nt) long mRNA consisting of 90 nt of 5' untranslated, 5820 nt of protein coding and 125 nt of 3' untranslated sequence. The rat embryonic MHC polypeptide is encoded by exons 3 to 41 and contains 1939 amino acid residues with a calculated Mr of 223,900. Its amino acid sequence displays the structural features typical for all sarcomeric MHCs, i.e. an amino-terminal "globular" head region and a carboxy-terminal alpha-helical rod portion that shows the characteristics of a coiled coil with a superimposed 28-residue repeat pattern interrupted at only four positions by "skip" residues. The complex structure of the rat embryonic MHC gene and the conservation of intron locations in this and other MHC genes are indicative of a highly split ancestral sarcomeric MHC gene. Introns in the rat embryonic gene interrupt the coding sequence at the boundaries separating the proteolytic subfragments of the head, but not at the head/rod junction or between the 28-residue repeats present within the rod. Therefore, there is little evidence for exon shuffling and intron-dependent evolution by gene duplication as a mechanism for the generation of the ancestral MHC gene. Rather, intron insertion into a previously non-split ancestral MHC rod gene consisting of multiple tandemly arranged 28-residue-encoding repeats, or convergent evolution of an originally non-repetitive ancestral MHC rod gene must account for the observed structure of the rod-encoding portion of present-day MHC genes.

All members of the MHC multigene family respond to thyroid hormone in a highly tissue-specific manner

In mammals different isoforms of myosin heavy chain are encoded by the members of a multigene family. The expression of each gene of this family is regulated in a tissue- and developmental stage-specific manner as well as by hormonal and various pathological stimuli. In this study the molecular basis of isoform switches induced in myosin heavy chain by thyroid hormone was investigated. The expression of the myosin heavy chain gene family was analyzed in seven different muscles of adult rats subjected to hypo- or hyperthyroidism with complementary DNA probes specific for six different myosin heavy chain genes. The results demonstrate that all six genes are responsive to thyroid hormone. More interestingly, the same myosin heavy chain gene can be regulated by thyroid hormone in highly different modes, even in opposite directions, depending on the tissue in which it is expressed. Furthermore, the skeletal embryonic and neonatal myosin heavy chain genes, so far considered specific to these two developmental stages, can be reinduced by hypothyroidism in specific adult muscles.