The developmentally regulated expression of two linked myosin heavy-chain genes

Heavy and light roles: myosin in the morphogenesis of the heart

Myosin is an essential component of cardiac muscle, from the onset of cardiogenesis through to the adult heart. Although traditionally known for its role in energy transduction and force development, recent studies suggest that both myosin heavy-chain and myosin light-chain proteins are required for a correctly formed heart. Myosins are structural proteins that are not only expressed from early stages of heart development, but when mutated in humans they may give rise to congenital heart defects. This review will discuss the roles of myosin, specifically with regards to the developing heart. The expression of each myosin protein will be described, and the effects that altering expression has on the heart in embryogenesis in different animal models will be discussed. The human molecular genetics of the myosins will also be reviewed.

Knockdown of embryonic myosin heavy chain reveals an essential role in the morphology and function of the developing heart

The expression and function of embryonic myosin heavy chain (eMYH) has not been investigated within the early developing heart. This is despite the knowledge that other structural proteins, such as alpha and beta myosin heavy chains and cardiac alpha actin, play crucial roles in atrial septal development and cardiac function. Most cases of atrial septal defects and cardiomyopathy are not associated with a known causative gene, suggesting that further analysis into candidate genes is required. Expression studies localised eMYH in the developing chick heart. eMYH knockdown was achieved using morpholinos in a temporal manner and functional studies were carried out using electrical and calcium signalling methodologies. Knockdown in the early embryo led to abnormal atrial septal development and heart enlargement. Intriguingly, action potentials of the eMYH knockdown hearts were abnormal in comparison with the alpha and beta myosin heavy chain knockdowns and controls. Although myofibrillogenesis appeared normal, in knockdown hearts the tissue integrity was affected owing to apparent focal points of myocyte loss and an increase in cell death. An expression profile of human skeletal myosin heavy chain genes suggests that human myosin heavy chain 3 is the functional homologue of the chick eMYH gene. These data provide compelling evidence that eMYH plays a crucial role in important processes in the early developing heart and, hence, is a candidate causative gene for atrial septal defects and cardiomyopathy.

Interspecific and intragenic differences in codon usage bias among vertebrate myosin heavy-chain genes

Synonymous codon usage bias is a broadly observed phenomenon in bacteria, plants, and invertebrates and may result from selection. However, the role of selective pressures in shaping codon bias is still controversial in vertebrates, particularly for mammals. The myosin heavy-chain (MyHC) gene family comprises multiple isoforms of the major force-producing contractile protein in cardiac and skeletal muscles. Slow and fast genes are tandemly arrayed on separate chromosomes, and have distinct patterns of functionality and expression in muscle. We analyze both full-length MyHC genes (~5400 bp) and a larger collection of partial sequences at the 3' end (~500 bp). The MyHC isoforms are an interesting system in which to study codon usage bias because of their length, expression, and critical importance to organismal mobility. Codon bias and GC content differs among MyHC genes with regards to functional type, isoform, and position within the gene. Codon bias even varies by isoform within a species. We find evidence in favor of both chromosomal influences on nucleotide composition and selection against nonsense errors (SANE) acting on codon usage in MyHC genes. Intragenic variation in codon bias and elongation rate is significant, with a strong trend for increasing codon bias and elongation rate towards the 3' end of the gene, although the trend is dependent upon the degeneracy class of the codons. Therefore, patterns of codon usage in MyHC genes are consistent with models supporting SANE as a major force shaping codon usage.

Isokinetic work profile of shoulder flexors and extensors in sport climbers and nonclimbers

STUDY DESIGN

Cross-sectional, 2-group comparison, experimental laboratory study.

OBJECTIVES

Examining and comparing the work profiles of the shoulder flexors and extensors between sport climbers and nonclimbers.

BACKGROUND

Sport climbing places high demands on the shoulder, which could lead to unique work profiles of the agonist/antagonist muscle groups.

METHODS AND MEASURES

Isokinetic work output of the dominant shoulder flexors and extensors of 31 sport climbers and 27 nonclimbers were measured from 0 degrees to 180 degrees of flexion at a test speed of 60 degrees /s. Profiles for work data (concentric flexion [conFlex], eccentric flexion [eccFlex], concentric extension [conExt], eccentric extension [eccExt]) normalized to body mass, conventional work ratios (conFlex/conExt and eccFlex/eccExt), and functional work ratios (eccFlex/conExt and eccExt/conFlex) were developed for both climbers and nonclimbers.

RESULTS

All work profiles were different between the 2 groups (P<.001). All normalized work data were higher in climbers than nonclimbers, especially for conExt and eccExt. In the climbers, the conventional ratios were smaller than 1 for conFlex/conExt (0.74) and eccFlex/eccExt (0.74), whereas for the nonclimbers the ratios were 1.13 and 1.05, respectively. For the functional work data, the eccFlex/conExt ratio was 0.9 for the climbers compared to 1.46 for the nonclimbers. Conversely, the eccExt/conFlex ratio was much higher in the climbers (1.73) compared to the nonclimbers (1.28).

CONCLUSION

The differences in work profiles for the shoulder flexors and extensors between the climbers and nonclimbers suggest training-induced adaptations, stronger shoulder flexors, and, especially, stronger extensors, resulting from the sports of climbing.

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

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

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

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

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

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

Mapping of the chicken N-CAM gene and a myosin heavy chain gene: avian microchromosomes are not genetically inert reserves of DNA

Physical mapping of the chicken neural cell adhesion molecule gene and of a chicken myosin fast-white heavy chain gene via in situ hybridization has assigned both loci to different-sized microchromosomes in this species. Two genomic cloned chicken DNA probes of 16 kb (lambda cN6) and 15 kb (DCM 11), respectively, were biotin labeled for fluorescent detection of the genes. The implications derived from these results, together with further data just now beginning to accumulate regarding the genetic content of avian microchromosomes, confirm an unexpectedly active role for these minute chromosomes.

In vitro production of enzymatically active myosin heavy chain

In order to initiate studies on the structural and functional relationships of the myosin heavy chain, we constructed a full-length complementary DNA encoding the isoform that is found in the fast white muscle of the embryonic chicken. The complementary DNA contained 108 basepairs of its 3'-untranslated region and was preceded by a leader sequence derived from the alfalfa mosaic virus. Similarly, a complementary DNA encoding 963 amino acids which encompass the subfragment-1 of myosin and part of the subfragment-2 was also constructed. Each was inserted into the expression vector pMT2 and transiently transfected into COS-1 cells. Both constructs directed the expression of the respective proteins, each of which was immunogenic. The full-length and subfragment-1 proteins interacted with actin and demonstrated high levels of a K(+)-activated, EDTA-resistant ATPase activity, which is characteristic of myosin.

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.

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

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

Gene conversions within the skeletal myosin multigene family

Comparisons of the nucleotide sequences of the light meromyosin (LMM) region of developmentally regulated fast chicken myosin heavy chain (MHC) isoforms indicates that chicken MHC isoforms are more similar to each other than to MHC isoforms in other species. The sequence data provide evidence that gene conversion events have occurred recently among the isoforms. An embryonic (Cemb1) isoform and neonatal isoform have the most extensive regions of sequence identity. Similar gene conversion events are present in the rat alpha- and beta-cardiac MHCs, but were not obvious in the LMM of developmentally regulated fast human MHC isoforms. The data suggest that gene conversion events can play a significant role in the evolution of the MHC multigene families and that concerted evolution of the chicken multigene family occurred after the divergence of mammals and avians.

Structural and phylogenetic analysis of the chicken ventricular myosin heavy chain rod

We have isolated and characterized five overlapping clones that encompass 3.2 kb and encode a part of the short subfragment 2, the hinge, and the light meromyosin regions of the myosin heavy chain rod as well as 143 bp of the 3' untranslated portion of the mRNA. Northern blot analysis showed expression of this mRNA mainly in ventricular muscle of the adult chicken heart, with trace levels detected in the atrium. Transient expression was seen in skeletal muscle during development and in regenerating skeletal muscle following freeze injury. To our knowledge, this is the first report of an avian ventricular myosin heavy chain sequence. Phylogenetic analysis indicated that this isoform is a distant homolog of other ventricular and skeletal muscle myosin heavy chains and represents a distinct member of the multigene family of sarcomeric myosin heavy chains. The ventricular myosin heavy chain of the chicken is either paralogous to its counterpart in other vertebrates or has diverged at a significantly higher rate.

Developmental myosin heavy chain progression in avian type IIB muscle fibres

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

Cardiac myosin heavy chain mRNA expression and myocardial function in the mouse heart

The vertebrate heart contains two myosin heavy chain isoforms, alpha and beta, which are differentially expressed. To establish a murine model for gene-targeting experiments, we defined the precise temporal expression of the myosin isoforms during cardiogenesis and obtained quantitative measurements of cardiac performance. The relative levels of the alpha- and beta-cardiac transcripts were determined by isolating the RNA from the hearts of CD-1 mice during development and hybridizing the preparations to probes that detect specifically the alpha- or beta-cardiac myosin heavy chain mRNAs. The data indicate that, although both isoforms are present from the onset of cardiogenesis, the beta-isoform predominates during embryogenesis and fetal development. This relation is reversed after the first day of life with a significant drop in the absolute transcript levels during the switch; and alpha/beta ratio of 16:1 is maintained in the neonate, and the relatively high levels of the alpha-transcript remain throughout the adult stages. To be able to make functional comparisons between normal and transgenic mice, we obtained indexes of myocardial function in isolated retrogradely perfused and in work-performing heart preparations in normal and hypodynamic mouse hearts. We found that the physiology of the mouse heart is similar to the rat heart in that we observed a positive staircase in the force-frequency relation of the mouse Langendorff preparation. We also saw contractile responses of more than twice control induced by paired stimulation and persistent postextrasystolic potentiation. As is the case for the rat, in the work-performing mouse heart, afterload (Starling resistance, pressure) changes produced a steeper Starling function curve than did changes in preload (volume, venous return).

A multipurpose vector for the study of transcriptional control

To facilitate the insertion of transcriptional control regions next to a reporter gene, plasmid vectors containing multiple cloning sites next to the cat have been constructed. These vectors also contain features which make their use convenient for the construction of deletions in the inserted transcriptional control regions, as well as for the direct sequencing of the deletion series produced. The vectors have been constructed such that the control region cassette (and the deletions produced) may be easily removed with or without the reporter gene for placement into transgenic mice. The system's utility has been demonstrated by deletion analysis of a chicken myosin heavy chain-encoding gene promoter.

Induction and stability of the adult myosin phenotype in striated muscles of dwarf mice after chronic thyroid hormone treatment

It is known that a deficiency in thyroid hormone delays the post-natal maturation of several mammalian tissues. In striated muscle tissue, hypothyroidism delays or inhibits some of the isoform transitions of myosin heavy chains which would occur during normal development. In this paper, using the mouse mutant dwarf, we demonstrate an influence of thyroid hormone on expression of the myosin phenotype in cardiac and skeletal muscle of dwarf mice. Myosin isoforms were identified by gel electrophoresis of native myosin, localised within muscle cells by indirect immunofluorescence and quantified using an ELISA technique. We show that an adult phenotype can be established in both cardiac and skeletal muscle following a treatment involving multiple injections of thyroxine although cardiac muscle responds more rapidly. The skeletal myosin phenotype remains stable until at least five weeks after the last injection. In contrast, the fetal form of cardiac myosin reaccumulates upon cessation of thyroxine treatment. Thus, cardiac and skeletal muscles are not only affected differently by the dwarf mutation but also they respond differently to thyroxine treatment and thyroxine withdrawal.

Temporal resolution and sequential expression of muscle-specific genes revealed by in situ hybridization

The expression of muscle-specific mRNAs was analyzed directly within individual cells by in situ hybridization to chicken skeletal myoblasts undergoing differentiation in vitro. The probes detected mRNAs for sarcomeric myosin heavy chain (MHC) or the skeletal, cardiac, and beta isoforms of actin. Precise information as to the expression of these genes in individual cells was obtained and correlated directly with analyses of cell morphology and interactions, cell cycle stage, and immunofluorescence detection of the corresponding proteins. Results demonstrate that mRNAs for the two major muscle-specific proteins, myosin and actin, are not synchronously activated at the time of cell fusion. The mRNA for alpha-cardiac actin (CAct), known to be the predominant embryonic actin isoform in muscle, is expressed prior to cell fusion and prior to the expression of any isoform of muscle MHC mRNA. MHC mRNA accumulates rapidly immediately after fusion, whereas skeletal actin mRNA is expressed only in larger myofibers. Single cells expressing CAct mRNA have a characteristic short bipolar morphology, are in terminal G1, and do not contain detectable levels of the corresponding protein. In a pattern of expression reciprocal to that of CAct mRNA, beta-actin mRNA diminishes to low or undetectable levels in myofibers and in cells of the morphotype which expresses CAct mRNA. Finally, the intracellular distribution of mRNAs for different actin isoforms was compared using nonisotopic detection of isoform-specific oligonucleotide probes. This work illustrates a generally valuable approach to the analysis of cell differentiation and gene expression which directly integrates molecular, morphological, biochemical, and cell cycle information on individual cells.

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.

Neonatal myosin heavy chains are not expressed in Ni-induced rat rhabdomyosarcoma

Myosin heavy chain (MHC) composition of chemically-induced rhabdomyosarcoma (RMS) was analyzed by gel electrophoresis and Western blotting using a panel of monoclonal antimyosin antibodies specific for embryonic-, neonatal-, slow- and adult fast-type MHC isoforms. Myosin extracted from tumours and electrophoresed on 6%-sodium dodecyl sulfate (SDS)glycerol gels was found to migrate as three distinct MHC components. These polypeptides were present in different relative amounts in the five RMS studied. Western blotting experiments revealed that variable proportions of embryonic-, slow- and adult fast-, but not neonatal-type, MHC isoforms are consistently expressed in RMS. Indirect and double immunofluorescence procedures applied to cryosections of tumoral tissue showed that: (a) RMS cells were unreactive with antineonatal-type-MHC antibody, (b) the majority of neoplastic, desmin-positive, cells contained embryonic- as well as adult fast-type MHCs and (c) a minority of cells were labelled by anti-slow MHC antibody. The results of this study indicate that there is no obligatory sequence of MHC isoform expression in the molecular transition (emb----neo----adult) which occurs during rat skeletal myogenesis.