Molecular cloning of two fast myosin heavy chain cDNAs from chicken embryo skeletal muscle

Regulation of myosin expression during myotome formation

The first skeletal muscle fibers to form in vertebrate embryos appear in the somitic myotome. PCR analysis and in situ hybridization with isoform-specific probes reveal differences in the temporal appearance and spatial distribution of fast and slow myosin heavy chain mRNA transcripts within myotomal fibers. Embryonic fast myosin heavy chain was the first isoform expressed, followed rapidly by slow myosin heavy chains 1 and 3, with slow myosin heavy chain 2 appearing several hours later. Neonatal fast myosin heavy chain is not expressed in myotomal fibers. Although transcripts of embryonic fast myosin heavy chain were always distributed throughout the length of myotomal fibers, the mRNA for each slow myosin heavy chain isoform was initially restricted to the centrally located myotomal fiber nuclei. As development proceeded, slow myosin heavy chain transcripts spread throughout the length of myotomal fibers in order of their appearance. Explants of segments from embryos containing neural tube, notochord and somites 7-10, when incubated overnight, become innervated by motor neurons from the neural tube and express all four myosin heavy chain genes. Removal of the neural tube and/or notochord from explants prior to incubation or addition of d-tubocurare to intact explants prevented expression of slow myosin chain 2 but expression of genes encoding the other myosin heavy chain isoforms was unaffected. Thus, expression of slow myosin heavy chain 2 is dependent on functional innervation, whereas expression of embryonic fast and slow myosin heavy chain 1 and 3 are innervation independent. Implantation of sonic-hedgehog-soaked beads in vivo increased the accumulation of both fast and slow myosin heavy chain transcripts, as well as overall myotome size and individual fiber size, but had no effect on myotomal fiber phenotype. Transcripts encoding embryonic fast myosin heavy chain first appear ventrolaterally in the myotome, whereas slow myosin heavy chain transcripts first appear in fibers positioned midway between the ventrolateral and dorsomedial lips of the myotome. Therefore, models of epaxial myotome formation must account for the positioning of the oldest fibers in the more ventral-lateral region of the myotome and the youngest fibers in the dorsomedial region.

Evolutionary significance of myosin heavy chain heterogeneity in birds

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

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

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

Tissue specific distribution of Drosophila sarcomeric myosin heavy-chain protein isoforms

The sarcomeric myosin heavy-chain (sMHC) gene of Drosophila is single-copy and RNA transcription from this gene is developmentally regulated. Numerous sMHC mRNAs that differ in exon composition can be formed by alternate RNA processing. These transcriptional events result in the presence of multiple sMHC isoforms in the developing organism. We have developed and characterized two antibodies which are specific for different types of sarcomeric myosin heavy-chain protein isoforms in Drosophila and have begun to examine the tissue distribution and function of these various protein isoforms. One of the antibodies (anti-A) is capable of distinguishing between two classes of sMHC protein isoforms which differ in their carboxy terminal amino acid sequences. The second antibody (anti-MHC) recognizes a separate and distinct domain in sMHC protein isoforms. We demonstrate the specificity and the utility of these antibodies in examining the developmental and tissue-specific expression of sMHC protein isoforms in the developing fly.

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

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

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

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

Mitogen stimulation affects contractile protein mRNA abundance and translation in embryonic quail myocytes

In cultures of differentiated, fusion-blocked muscle cells obtained from embryonic Japanese quail (Coturnix coturnix japonica), mitogen stimulation leads to an immediate reduction in the rates of synthesis of skeletal muscle myosin heavy chain (MHC) and alpha-actin. The molecular mechanisms responsible for this downregulation were examined. The cellular abundances of the alpha-actin and MHC mRNAs were affected differently by mitogen stimulation; alpha-actin mRNA abundance declined by an amount which quantitatively accounted for the observed decrease in alpha-actin synthesis, whereas MHC mRNA abundance remained virtually unchanged during the first 6 h following mitogen stimulation, a period during which MHC synthesis declined by more than 70%. MHC mRNA abundance did decline between 6 and 12 h after mitogen stimulation. Downregulation of MHC synthesis therefore involves an initial block in mRNA translation combined with a later loss of MHC mRNA from the cytoplasma, while alpha-actin synthesis is regulated at the level of mRNA abundance. These observations are consistent with the hypothesis that, in addition to transcriptional activation of muscle-specific genes, skeletal muscle differentiation normally involves cell cycle-dependent modulations in cellular factors which control message stability and message translation.

Mitogen stimulation affects contractile protein mRNA abundance and translation in embryonic quail myocytes

In cultures of differentiated, fusion-blocked muscle cells obtained from embryonic Japanese quail (Coturnix coturnix japonica), mitogen stimulation leads to an immediate reduction in the rates of synthesis of skeletal muscle myosin heavy chain (MHC) and alpha-actin. The molecular mechanisms responsible for this downregulation were examined. The cellular abundances of the alpha-actin and MHC mRNAs were affected differently by mitogen stimulation; alpha-actin mRNA abundance declined by an amount which quantitatively accounted for the observed decrease in alpha-actin synthesis, whereas MHC mRNA abundance remained virtually unchanged during the first 6 h following mitogen stimulation, a period during which MHC synthesis declined by more than 70%. MHC mRNA abundance did decline between 6 and 12 h after mitogen stimulation. Downregulation of MHC synthesis therefore involves an initial block in mRNA translation combined with a later loss of MHC mRNA from the cytoplasma, while alpha-actin synthesis is regulated at the level of mRNA abundance. These observations are consistent with the hypothesis that, in addition to transcriptional activation of muscle-specific genes, skeletal muscle differentiation normally involves cell cycle-dependent modulations in cellular factors which control message stability and message translation.

Regulation of myosin heavy-chain gene expression during skeletal-muscle hypertrophy

Changes in the myosin phenotype of differentiated muscle are a prominent feature of the adaptation of the tissue to a variety of physiological stimuli. In the present study the molecular basis of changes in the proportion of myosin isoenzymes in rat skeletal muscle which occur during compensatory hypertrophy caused by the combined removal of synergist muscles and spontaneous running exercise was investigated. The relative amounts of sarcomeric myosin heavy (MHC)- and light (MLC)-chain mRNAs in the plantaris (fast) and soleus (slow) muscles from rats was assessed with cDNA probes specific for different MHC and MLC genes. Changes in the proportion of specific MHC mRNA levels were in the same direction as, and of similar magnitude to, changes in the proportion of myosin isoenzymes encoded for by the mRNAs. No significant changes in the proportion of MLC proteins or mRNA were detected. However, high levels of MLC3 mRNA were measured in both normal and hypertrophied soleus muscles which contained only trace amounts of MLC3 protein. Small amounts of embryonic and neonatal MHC mRNAs were induced in both muscles during hypertrophy. We conclude that the change in the pattern of myosin isoenzymes during skeletal-muscle adaptation to work overload is a consequence of changes in specific MHC mRNA levels.

Temporal and tissue-specific expression of myosin heavy chain isoforms in developing and adult avian muscle

We have raised monoclonal antibodies (Mabs) to myosin heavy chain isoforms (MHCs) that have specific patterns of temporal expression during the development of quail pectoral muscle and that are expressed in very restricted, tissue-specific patterns in adult birds. We find that an early embryonic, a perinatal, and an adult-specific, fast myosin heavy chain are co-expressed at different levels in the pectoral muscle of 8-12 day quail embryos. The early embryonic MHC disappears from the pectoral muscle at approximately 14 days in ovo, whereas the perinatal MHC persists until 26 days post-hatching. The adult-specific MHC accumulates preferentially and eventually completely replaces the other isoforms. These Mabs cross-react with the homologous isoforms of the chick and detect a similar pattern of MHC expression in the pectoral muscle of developing chicks. Although the early embryonic and perinatal MHC isoforms recognized by our Mabs are expressed in the pectoral muscle only during distinct developmental stages, our Mabs also recognize MHC isoforms present in the heart and extraocular muscle of adult quail. Immunofingerprinting using Staphylococcus aureus protease V8 suggests that the early embryonic and perinatal MHC isoforms that we see are strongly homologous with the adult ventricular and extraocular muscle isoforms, respectively. These observations suggest that at least three distinct MHC isoforms, which are normally expressed in adult muscles, are co-expressed during the early development of the pectoral muscle in birds. In this respect, the pattern of expression of the MHCs recognized by our Mabs in developing, fast muscle is very similar to the patterns described for other muscle contractile proteins.

Characterization of cDNA coding for the complete light meromyosin portion of a rabbit fast skeletal muscle myosin heavy chain

Myosin-heavy-chain-specific cDNA clones have been isolated from a cDNA library prepared from hind leg muscle of a 14-day-old rabbit. According to restriction enzyme analysis these can be grouped into at least two, probably three different classes. RNA dot-blot hybridization shows that all of these clones correspond to mRNAs expressed in fast skeletal muscle. The clones of the most abundant form, class I, can be aligned to cover the complete light meromyosin portion of myosin heavy chain. The sequence of the coding and the 3'-untranslated region, together comprising 2143 base pairs, has been determined. The class I clone detects a multigene family of 8-12 members on a Southern blot of rabbit genomic DNA.

Developmental regulation of the multiple myogenic cell lineages of the avian embryo

The developmental regulation of myoblasts committed to fast, mixed fast/slow, and slow myogenic cell lineages was determined by analyzing myotube formation in high density and clonal cultures of myoblasts isolated from chicken and quail embryos of different ages. To identify cells of different myogenic lineages, myotubes were analyzed for content of fast and slow classes of myosin heavy chain (MHC) isoforms by immunocytochemistry and immunoblotting using specific monoclonal antibodies. Myoblasts from the hindlimb bud, forelimb bud, trunk, and pectoral regions of the early chicken embryo and hindlimb bud of the early quail embryo (days 3-6 in ovo) were committed to three distinct lineages with 60-90% of the myoblasts in the fast lineage, 10-40% in the mixed fast/slow lineage, and 0-3% in the slow lineage depending on the age and species of the myoblast donor. In contrast, 99-100% of the myoblasts in the later embryos (days 9-12 in ovo) were in the fast lineage. Serial subculturing from a single myoblast demonstrated that commitment to a particular lineage was stably inherited for over 30 cell doublings. When myoblasts from embryos of the same age were cultured, the percentage of muscle colonies of the fast, fast/slow, and slow types that formed in clonal cultures was the same as the percentage of myotubes of each of these types that formed in high density cultures, indicating that intercellular contact between myoblasts of different lineages did not affect the type of myotube formed. An analysis in vivo showed that three types of primary myotubes--fast, fast/slow, and slow--were also found in the chicken thigh at day 7 in ovo and that synthesis of both the fast and slow classes of MHC isoforms was concomitant with the formation of primary myotubes. On the basis of these results, we propose that in the avian embryo, there is an early phase of muscle fiber formation in which primary myotubes with differing MHC contents are formed from myoblasts committed to three intrinsically different primary myogenic lineages independent of innervation and a later phase in which secondary myotubes are formed from myoblasts in a single, secondary myogenic lineage with maturation and maintenance of fiber diversity dependent on innervation.

Developmental origins of skeletal muscle fibers: clonal analysis of myogenic cell lineages based on expression of fast and slow myosin heavy chains

A clonal analysis was used to show that skeletal muscle myoblasts are committed to distinct cell lineages during development. Myoblasts taken from embryonic chicken hindlimb muscles of different ages were cultured at clonal density. The content of fast and slow classes of the myosin heavy chain isoforms in the myotubes of the resulting muscle colonies was determined immunocytochemically with specific monoclonal antibodies that served as markers for the different fiber types. The muscle colonies formed by cloning myoblasts from early hindlimbs (days 4-6 in ovo) were of three types: the most numerous type, in which all myotubes in a colony contained only the fast class of myosin heavy chain; a less numerous type, in which all myotubes in a colony contained both the fast and slow classes of myosin heavy chain isoforms; and a rare type, in which all myotubes in a colony contained only the slow class of myosin heavy chain. The muscle colonies formed by cloning myoblasts from later hindlimbs (days 10-12 in ovo) were, however, all of one type, in which every myotube in a colony contained only fast myosin heavy chain. Thus, myoblasts in the early embryo (days 4-6 in ovo) were a heterogeneous population committed to three myogenic lineages: fast, mixed fast/slow, and slow, whereas myoblasts from the later embryo (days 10-12 in ovo) were only in the fast myogenic lineage. These results suggest that muscle fiber formation is rooted in two developmental phases--an early phase in which diverse fiber types are formed from intrinsically diverse populations of myoblasts and a later phase in which fibers are formed from a single population of myoblasts.

Slow and fast myosin heavy chain content defines three types of myotubes in early muscle cell cultures

We prepared monoclonal antibodies specific for fast or slow classes of myosin heavy chain isoforms in the chicken and used them to probe myosin expression in cultures of myotubes derived from embryonic chicken myoblasts. Myosin heavy chain expression was assayed by gel electrophoresis and immunoblotting of extracted myosin and by immunostaining of cultures of myotubes. Myotubes that formed from embryonic day 5-6 pectoral myoblasts synthesized both a fast and a slow class of myosin heavy chain, which were electrophoretically and immunologically distinct, but only the fast class of myosin heavy chain was synthesized by myotubes that formed in cultures of embryonic day 8 or older myoblasts. Furthermore, three types of myotubes formed in cultures of embryonic day 5-6 myoblasts: one that contained only a fast myosin heavy chain, a second that contained only a slow myosin heavy chain, and a third that contained both a fast and a slow heavy chain. Myotubes that formed in cultures of embryonic day 8 or older myoblasts, however, were of a single type that synthesized only a fast class of myosin heavy chain. Regardless of whether myoblasts from embryonic day 6 pectoral muscle were cultured alone or mixed with an equal number of myoblasts from embryonic day 12 muscle, the number of myotubes that formed and contained a slow class of myosin was the same. These results demonstrate that the slow class of myosin heavy chain can be synthesized by myotubes formed in cell culture, and that three types of myotubes form in culture from pectoral muscle myoblasts that are isolated early in development, but only one type of myotube forms from older myoblasts; and they suggest that muscle fiber formation probably depends upon different populations of myoblasts that co-exist and remain distinct during myogenesis.

Unusual fast myosin isozyme pattern in the lateral gastrocnemius of the chicken

The myosin isozyme composition of the lateral gastrocnemius muscle of the chicken leg was investigated during various stages of development utilizing non-denaturing pyrophosphate gel electrophoresis, two-dimensional gel electrophoresis and peptide mapping techniques. An unusual isoform pattern for fast myosin in the lateral gastrocnemius muscle of the adult chicken leg was demonstrated which consisted of a predominance of myosin homodimers and lesser amounts of myosin heterodimer. In addition, a different myosin heavy chain isoform was present in the adult chicken lateral gastrocnemius muscle when compared to other adult fast-twitch muscles. While the adult lateral gastrocnemius muscle contained a different myosin heavy chain isoform from other adult fast-twitch muscles, the embryonic lateral gastrocnemius muscle contained a myosin heavy chain identical to that of the embryonic pectoralis major.

Co-expression of multiple myosin heavy chain genes, in addition to a tissue-specific one, in extraocular musculature

We have investigated the developmental transitions of myosin heavy chain (MHC) gene expression in the rat extraocular musculature (EOM) at the mRNA level using S1-nuclease mapping techniques and at the protein level by polypeptide mapping and immunochemistry. We have isolated a genomic clone, designated lambda 10B3, corresponding to an MHC gene which is expressed in the EOM fibers (recti and oblique muscles) of the adult rat but not in hind limb muscles. Using cDNA and genomic probes for MHC genes expressed in skeletal (embryonic, neonatal, fast oxidative, fast glycolytic, and slow/cardiac beta-MHC), cardiac (alpha-MHC), and EOM (lambda 10B3) muscles, we demonstrate the concomitant expression at the mRNA level of at least six different MHC genes in adult EOM. Protein and immunochemical analyses confirm the presence of at least four different MHC types in EOM. Immunocytochemistry demonstrates that different myosin isozymes tend to segregate into individual myofibers, although some fibers seem to contain more than one MHC type. The results also show that the EOM fibers exhibit multiple patterns of MHC gene regulation. One set of fibers undergoes a sequence of isoform transitions similar to the one described for limb skeletal muscles, whereas other EOM myofiber populations arrest the MHC transition at the embryonic, neonatal/adult, or adult EOM-specific stage. Thus, the MHC gene family is not under the control of a strict developmental clock, but the individual genes can modify their expression by tissue-specific and/or environmental factors.

Evidence for transcriptional regulation of the myosin heavy chain gene during myogenesis

One of the changes accompanying skeletal muscle cell (myoblast) fusion is a dramatic increase in synthesis of muscle specific proteins, one of which is myosin. The underlying mechanism for this burst in synthesis is not yet understood but may occur by two mechanisms: (a) gradual storage of mRNA and translational control as found by others or (b) gene activation and rapid synthesis of mRNA for immediate translation. In this paper we show that the myosin gene changes its organization such that postfusion skeletal muscle cells show an increased susceptibility to DNase I, a recognized probe for gene activation. We also show that this change accompanies an increase in rate of transcription and an increased cell content of myosin heavy chain mRNA. This work shows that transcriptional control is an important mechanism during muscle cell development in addition to the translational control shown by other workers.

Denervated skeletal muscle displays discoordinate regulation for the synthesis of several myofibrillar proteins

Synthesis patterns of myosin heavy- and light-chain isoforms, tropomyosin and troponin, have been studied in chicken fast muscle denervated at both neonatal and adult stages. Denervated neonatal muscle does not synthesize the adult myosin heavy-chain isoform at the time of denervation, but it does synthesize the adult isoform several months after denervation. Thus, innervation does not appear to be necessary for the normal sequential replacement of embryonic and neonatal myosin heavy chain by the adult variant. Nerve is required, however, for the regulation of tropomyosin and troponin expression. Normally the pectoralis major muscle represses synthesis of both beta-tropomyosin and leg-type troponin T during late embryonic development. After denervation, however, the muscle relaxes its ongoing repression of these proteins and significant amounts of both beta-tropomyosin and leg-type troponin T are synthesized by the muscle. Denervation also results in an altered pattern of myosin light-chain synthesis so that the ratio of fast light-chain 3/fast light-chain 1 decreases. Similar results are found in muscle denervated at the adult stage. In denervated muscle, therefore, synthesis of these myofibrillar proteins is not coordinated: ongoing isoform shifts proceed to express an adult pattern of myosin heavy chain while tropomyosin, troponin, and myosin light-chain patterns appear to revert to embryonic configurations.

Myosin isozymes in avian skeletal muscles. I. Sequential expression of myosin isozymes in developing chicken pectoralis muscles

Myosin has been purified from chicken pectoralis muscle at various stages of development, from 10 days' incubation to approximately 10 months after hatching. Embryonic myosin from the earliest stage showed a high level of ATPase activity, similar to that obtained for adult pectoralis myosin. Two-dimensional peptide mapping of partial chymotryptic digests showed, however, that is heavy chain is quite different from that of adult fast myosin. The immunological crossreactivity observed between embryonic myosin and adult fast (pectoralis) myosin is therefore due to shared antigenic determinants rather than the presence of any adult isoforms. In an accompanying paper we will show that embryonic myosin at 10 days' incubation is not a single species, but consists of at least two heavy chain isozymes. The minor fraction binds slow light chains preferentially, and appears to be largely responsible for the observed crossreactivity with slow (ALD) myosin. None of the embryonic myosins is equivalent to the adult forms. Prior to hatching, LC3f is present only in very small amounts (less than 5%), and the adult light chain pattern, containing LC1f and LC3f in equimolar amounts, is not generated until after one week post-hatching. At about that time a new heavy chain population is detected, different from either the embryonic heavy chain or the adult heavy chain. The adult heavy chain peptide pattern appears from about three weeks' post-hatching, but a map indistinguishable from that of adult myosin is not observed until about 26 weeks. None of the observed differences in peptide maps can be related to different strains of chicken; pectoralis myosin from adult White Rock gave an identical map to that from White Leghorn. Unexpectedly, posterior latissimus dorsi (PLD) myosin from White Leghorn appears to be different from pectoralis myosin from the same strain, despite the histochemical and immunocytochemical similarity of the two muscles. We conclude that myosin polymorphism is widespread in muscle tissue, and that the expression of myosin isozymes and their subunits is under developmental regulation.

Characterization of a human genomic DNA fragment coding for a myosin heavy chain

A DNA segment from the human genome with information for myosin heavy chain (MHC) was isolated from a human genomic DNA library cloned in lambda Charon 4A phages. The isolation was accomplished by a myosin cDNA probe obtained from rabbit heart muscle mRNA (Sinha et al. 1982). The selected human DNA clone, designated lambda gMHC1, contains a genomic DNA fragment of about 14 kilobase pairs. The transcriptional polarity of this DNA was determined. The 5'-end of the gene is missing from the cloned fragment. This human gene exhibits sequence homology to MHC DNA of rabbit and chicken, but not to an MHC sequence of nematode. The isolated gene fragment is a member of the human MHC multi-gene family, which is presumed to consist of probably more than ten separate sarcomeric MHC genes per haploid genome.

Chromosomal localization of a human myosin heavy-chain gene by in situ hybridization

A cloned rabbit heart muscle myosin heavy-chain cDNA was hybridized in situ with human metaphase chromosomes. The probe was known to have sequence homology with human genomic heavy-chain DNA. Only one site in the human haploid karyotype was labeled with the cDNA, and this site was found on the short arm of chromosome 17. The localization of autoradiographic grains suggests a subregional assignment of the myosin heavy-chain locus to 17p 1,2-pter.

Drosophila muscle myosin heavy chain encoded by a single gene in a cluster of muscle mutations

Drosophila muscle myosin heavy chain is encoded by a single-copy gene which is transcribed during both larval and adult development. This myosin gene maps to a chromosomal locus distant from any of the actin genes, but is within a cluster of flight muscle mutations.

The two myosin isoenzymes of chicken anterior latissimus dorsi muscle contain different myosin heavy chains encoded by separate mRNAs

The two myosin isozymes (SM1 and SM2) of the anterior latissimus dorsi muscle of the chicken change in relative concentration during development. As SM1 decreases from 13 days of embryonic growth through 1 year of adult maturation, SM2 increases. In the adult muscle SM2 accounts for over 95% of the total myosin. The myosin heavy chains of the two isozymes are distinctly different and may be separated from each other by 5% SDS polyacrylamide gel electrophoresis. The faster migrating myosin heavy chain is identified as originating from SM1 and the slower migrating myosin heavy chain from SM2 myosin isozymes. The myosin heavy chains change in relative concentration during development exactly parallel with changes in SM1 and SM2 isozyme levels. Peptide map analysis also reveals that SM1 myosin heavy chains and SM2 myosin heavy chains are distinctly different. When RNA from the ALD muscle is added to reticulocyte lysate protein synthesizing systems the translation products are shown to include both SM1 and SM2 myosin heavy chains. These comigrate exactly on 5% SDS polyacrylamide gels with authentic counterparts from ALD muscle. Finally, when peptide maps of SM1 and SM2 myosin heavy chains synthesized in the reticulocyte lysate are compared they are again found to be distinctly different and each is identical to a peptide map of respective authentic SM1 and SM2 myosin heavy chains. It is concluded that the myosin heavy chains of SM1 and SM2 myosin isozymes of ALD muscle have different primary structures and that they are encoded by two distinctly different mRNAs.

Molecular cloning of mRNA sequences for cardiac alpha- and beta-form myosin heavy chains: expression in ventricles of normal, hypothyroid, and thyrotoxic rabbits

We have isolated cDNA clones from thyrotoxic (pMHC alpha) and normal (pMHC beta) adult rabbit hearts. Restriction map analysis and DNA sequence analyses show that, although there is strong homology between overlapping regions of the two clones, they are distinctly different. The two clones exhibited 78-83% homology between the derived amino acid sequences and those determined by direct amino acid sequence analysis of rabbit fast skeletal muscle myosin heavy chains. The clones specify a segment of the myosin heavy chain corresponding to subfragment 2 and the COOH-terminal portions of subfragment 1. Nuclease S1 mapping was used to compare transcription of the two clones with expression of the alpha and beta forms of myosin heavy chains in the ventricles of thyrotoxic, hypothyroid (propylthiouracil-treated), and normal rabbits. Thyrotoxic ventricles contained only pMHC alpha transcripts whereas hypothyroid ventricles contained exclusively pMHC beta transcripts. These data correlate well with the presence of alpha- and beta-form myosin heavy chains. In the normal young adult rabbit, pMHC beta transcripts predominate, agreeing with the known beta form/alpha form ratio of 4:1. We therefore conclude that pMHC alpha and pMHC beta contain sequences of the alpha- and beta-form myosin heavy chain genes, respectively.

Sarcomeric myosin heavy chain is coded by a highly conserved multigene family

pMHC25, a recombinant plasmid containing myosin heavy chain (MHC) cDNA sequences from differentiated myotubes of the L6E9 rat cell line, has been shown to hybridize to all sarcomeric MHC mRNAs so far tested but not to nonsarcomeric MHC mRNAs. In addition, pMHC25 hybridizes to multiple restriction endonuclease-digested fragments of rat genomic DNA corresponding to different MHC genomic sequences. Thus, the MHC gene represented by pMHC25 is a member of a sarcomeric MHC multigene family that has regions of sequence homology shared among its members. This sarcomeric MHC multigene family has been estimated to be composed of a minimum of seven genes, some of which are polymorphic in the rat. We have also determined that pMHC25 hybridizes to MHC gene sequences in genomic DNA of all species that have striated muscle, ranging from nematodes to man. Sarcomeric MHC genes, therefore, have been horizontally and vertically conserved in evolution. Additionally, we have used the pMHC25 plasmid to demonstrate that MHC genes do not undergo rearrangement or amplification during muscle cell differentiation.