In vitro translation of RNA from embryonic and from adult chicken pectoralis muscle produces different myosin heavy chains

Single molecular assay of individual ATP turnover by a myosin-GFP fusion protein expressed in vitro

Fusion proteins of a truncated mutant of myosin subfragment-1 (S1dC) and green fluorescent protein (GFP) were expressed in vitro by T7 RNA polymerase and rabbit reticulocyte lysate. Single S1dC-GFP fusion proteins were clearly seen and their individual ATP turnovers were directly monitored using low background total internal reflection fluorescence microscopy (LBTIRFM), recently developed by our laboratory. LBTIRFM using GFP as a fluorescent tag allowed us to assay functions of single protein molecules expressed in vitro. Thus, the results suggested that this method may be particularly useful to analyze functions of proteins that cannot be produced in an active form and/or in large quantities in conventional heterologous expression systems.

Expression of Ca2(+)-ATPase isoforms in denervated, regenerating, and dystrophic chicken skeletal muscle

The expression of fast and slow isoforms of the sarcoplasmic reticulum Ca2(+)-ATPase was studied in denervated, regenerating, and dystrophic fast and slow avian skeletal muscles. We found that both fast and slow Ca2(+)-ATPase isoforms were expressed in most myofibers following denervation of adult fast-twitch muscle, but only the slow Ca2(+)-ATPase isoform was found in slow-tonic muscle which had been denervated. Regenerating myotubes in normally innervated and previously denervated adult fast-twitch or slow-tonic muscle expressed both Ca2(+)-ATPase isoforms. Expression of the slow Ca2(+)-ATPase isoform was found to persist in dystrophic fast-twitch muscle, long after it had disappeared from normal fast-twitch muscle. However, the fast Ca2(+)-ATPase isoform disappeared from slow-tonic muscle similarly in normal and dystrophic birds. These results demonstrate that the appearance of myosin heavy chain isoforms characteristic of developing muscle is correlated with similar changes in the expression of sarcoplasmic reticulum Ca2(+)-ATPases.

Determination of myofibrillar and connective tissue protein contents of young and adult avian (Gallus domesticus) skeletal muscles and the N tau-methylhistidine content of avian actins

The myosin, actin, and collagen contents of young and adult avian red (leg) and white (breast) skeletal muscles from White Leghorn chickens have been determined by the use of analytical chromatographic methods developed to quantify the unique amino acids that occur in these proteins. Because one mole of actin purified from the red and white muscles of Leghorn chickens and one mole of myosin contain respectively one and two moles of N tau-methylhistidine, and the molar ratio of myosin and actin in skeletal muscle is known to be 1:6, the myofibrillar myosin and actin contents of avian skeletal muscles can be determined from the amounts of protein-bound N tau-methylhistidine found in acid hydrolysates of this tissue. Actin accounts for an estimated 11.2 to 12.2% of total muscle mass in both muscles or about 21.1% of total myofibrillar protein, whereas myosin ranges from 23.4 to 25.8% of total protein, corresponding to a mean value of 43.8% of total myofibrillar proteins. Total avian collagen ranged from 1.96 to 3.08% in breast and from 5.63 to 6.87% in leg skeletal muscles. With the methods described herein, content of collagen and collagen-like proteins can be calculated from amounts of 5-hydroxylysine present. The content of total connective tissue proteins could also be calculated from amounts of 4-hydroxyproline found. These quantifications are based on total protein content of the selected avian muscles determined by their detailed amino acid composition.

Expression of myosin heavy chain isoforms in regenerating myotubes of innervated and denervated chicken pectoral muscle

Monoclonal antibodies were prepared to stage-specific chicken pectoral muscle myosin heavy chain isoforms. From comparison of serial sections reacted with these antibodies, the myosin heavy chain isoform composition of individual myofibers was determined in denervated pectoral muscle and in regenerating myotubes that developed following cold injury of normal and denervated muscle. It was found that the neonatal myosin heavy chain reappeared in most myofibers following denervation of the pectoral muscle. Regenerating myotubes in both innervated and denervated muscle expressed all of the myosin heavy chain isoforms which have thus far been characterized in developing pectoral muscle. However, the neonatal and adult myosin heavy chains appeared more rapidly in regenerating myotubes compared to myofibers in developing muscle. While the initial expression of these isoforms in the regenerating areas was similar in innervated and denervated muscles, the neonatal myosin heavy chain did not disappear from noninnervated regenerating fibers. These results indicate that innervation is not required for the appearance of fast myosin heavy chain isoforms, but that the nerve plays some role in the repression of the neonatal myosin heavy chain.

Immunochemical analysis of protein isoforms in thick myofilaments of regenerating skeletal muscle

The expression of myosin heavy chain (MHC) and C-protein isoforms has been examined immunocytochemically in regenerating skeletal muscles of adult chickens. Two, five, and eight days after focal freeze injury to the anterior latissimus dorsi (ALD) and posterior latissimus dorsi (PLD) muscles, cryostat sections of injured and control tissues were reacted with a series of monoclonal antibodies previously shown to specifically bind MHC or C-protein isoforms in adult or embryonic muscles. We observed that during the course of regeneration in each of these muscles there was a reproducible sequence of antigenic changes consistent with differential isoform expression for these two proteins. These isoform switches appear to be tissue specific; i.e., the isoforms of MHC and C-protein which are expressed during the regeneration of a "slow" muscle (ALD) differ from those which are synthesized in a regenerating "fast" muscle (PLD). Evidence has been obtained for the transient expression of a "fast-type" MHC and C-protein during ALD regeneration. Furthermore, during early stages of PLD regeneration this muscle contains MHCs which antigenically resemble those found in the pectoralis muscle at embryonic and early posthatch stages of development. Both regenerating muscles express an isoform of C-protein which appears immunochemically identical to that normally expressed in embryonic and adult cardiac muscle. These results support the concept that isoform transitions in regenerating skeletal muscles qualitatively resemble those found in developing muscles but differences may exist in temporal and tissue-specific patterns of gene expression.

Contractile activity is required for the expression of neonatal myosin heavy chain in embryonic chick pectoral muscle cultures

The expression of neonatal myosin heavy chain (MHC) was examined in developing embryonic chicken muscle cultures using a monoclonal antibody (2E9) that has been shown to be specific for that isoform (Bandman, E., 1985, Science (Wash. DC), 227: 780-782). After 1 wk in vitro some myotubes could be stained with the antibody, and the number of cells that reacted with 2E9 increased with time in culture. All myotubes always stained with a second monoclonal antibody that reacted with all MHC isoforms (AG19) or with a third monoclonal antibody that reacted with the embryonic but not the neonatal MHC (EB165). Quantitation by ELISA of an extract from 2-wk cultures demonstrated that the neonatal MHC represented between 10 and 15% of the total myosin. The appearance of the neonatal isoform was inhibited by switching young cultures to medium with a higher [K+] which has been shown to block spontaneous contractions of myotubes in culture. Furthermore, if mature cultures that reacted with the neonatal antibody were placed into high [K+] medium, neonatal MHC disappeared from virtually all myotubes within 3 d. The effect of high [K+] medium was reversible. When cultures maintained in high [K+] medium for 2 wk were placed in standard medium, which permitted the resumption of contractile activity, within 24 h cells began to react with the neonatal specific antibody, and by 72 h many myotubes were strongly positive. Since similar results were also obtained by inhibiting spontaneous contractions with tetrodotoxin, we suggest that the development of contractile activity is not only associated with the maturation of myotubes in culture, but may also be the signal that induces the expression of the neonatal MHC.

Detection of a ventricular-specific myosin heavy chain in adult and developing chicken heart

In the present study, a monoclonal antibody (McAb), ALD19, generated against myosin of slow tonic muscle, was shown to react with the heavy chain of ventricular myosin in the adult chicken heart. With this antibody, it was possible to detect a ventricular-specific myosin during myocardial differentiation and to show that the epitope recognized by ALD19 was present from the earliest stages of ventricular differentiation and maintained throughout development only in the ventricle. A second McAb, specific for atrial myosin heavy chain (MHC) (Gonzalez-Sanchez, A., and D. Bader, 1984, Dev. Biol., 103:151-158), was used as a control to detect an atrial-specific myosin in the caudal portion of the developing heart at Hamburger-Hamilton stage 15. It was found that the appearance of ventricular MHC predated the expression of atrial MHC by approximately 1 d in ovo and that specific MHCs were always differentially distributed. While a common primordial MHC may be present in the early heart, this study showed the tissue-specific expression of a ventricular MHC during the initial stages of heart development and its differential accumulation throughout development.

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.

Characterization of a myosin heavy chain in the conductive system of the adult and developing chicken heart

A monoclonal antibody (anterior latissimus dorsi 58 [ALD58]; antimyosin heavy chain, MHC) directed against myosin from slow tonic muscle was found to react specifically with the striated muscle cells of the conductive system in the adult chicken heart. This monoclonal antibody was used to study the expression of myosin in the conductive system of the adult and developing heart. Using immunofluorescence microscopy with ALD58, muscle cells of the conductive system were demonstrated in both the atria and ventricles of the adult heart as previously shown by Sartore et al. (Sartore, S., S. Pierobon-Bormioli, and S. Schiafinno, 1978, Nature (Lond.), 274: 82-83). Radioactive myosin from adult atria and ventricles was precipitated with ALD58 and subjected to limited proteolysis and subsequent peptide mapping. Peptide maps of ALD58 reactive myosin from atria and ventricles were very similar, if not identical, but differed from peptide maps of ordinary atrial and ventricular myosin. The same antibody was used to study cardiac myogenesis in the chick embryo. When ALD58 was reacted with myosin isolated from atria and ventricles at selected stages of development in radioimmunoassays, reactivity was not observed until the last week of embryonic life (greater than 15 d of egg incubation). Thereafter concomitant and progressively increased reactivity was observed in atrial and ventricular preparations. Also, no ALD58 positive cells were observed in immunofluorescence studies of embryonic hearts until 17 d of egg incubation. Primary cell cultures of embryonic hearts also proved to be negative for this antibody. This study demonstrates that an epitope recognized by ALD58 associated with an antimyosin heavy chain of striated muscle cells of the adult heart conductive system is absent or present in only small amounts in the early embryonic heart.

Myosin components of the latissimus dorsi and the pectoralis major muscles in the dystrophic chicken

The myosin composition of the anterior latissimus dorsi, the posterior latissimus dorsi, and the pectoralis major muscles was examined in the inbred White Leghorn dystrophic chicken and its isogenic normal line at different ages during development and maturation. Using the biochemical methods of native gel electrophoresis, one- and two-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS PAGE), and peptide mapping, it was found that myosin isozyme changes occurred normally in the anterior latissimus dorsi muscle. However, in the posterior latissimus dorsi muscle, slow myosin components which were not present in the adult normal muscle were present in the adult dystrophic muscle. In addition, the pectoralis major muscle of the dystrophic chicken failed to undergo the neonatal to adult fast myosin isozyme transition. Our data also showed that muscle cell cultures derived from the pectoralis major muscle of dystrophic chickens expressed identical myosin components to cultures derived from normal embryos. However, since these cultures only produced embryonic myosins even after 1 month in culture, it implied that cells in tissue culture were phenotypically normal because present cell culture conditions were insufficient to induce the fetal to adult isozyme changes.

Immunochemical analysis of myosin heavy chains in the developing chicken heart

Monoclonal antibodies (McAb) against myosin from the pectoralis muscle of the adult chicken have been generated and shown to react specifically with the myosin heavy chain (MHC). The reactivities of two such McAbs with myosin from adult chicken atrial and ventricular myocardium were further analysed by immunoautoradiography, radioimmunoassay, and immunofluorescence microscopy. Monoclonal antibody MF 20 was found to bind both atrial and ventricular MHC and stain all striated muscle cells of the adult chicken heart. In contrast, McAb B1 bound specifically to atrial myocytes in immunofluorescence studies, while immunoautoradiography and radioimmunoassay demonstrated the specificity of this antibody for the atrial MHC. Upon reacting these McAbs with myosin isolated from embryonic hearts where definitive atria and ventricles were present, the same specificity of antibody binding was observed. Immunofluorescence studies demonstrated that all striated muscle cells of the embryonic heart contained MHCs recognized by MF 20, while only atrial muscle cells were bound by B1. When extracts of presumptive atrial and ventricular tissue were reacted with MF 20 and B1, significant reactivity of MF 20 was first observed at stage 10 in the presumptive ventricle and thereafter this McAb reacted with all regions of the developing myocardium. Binding of B1 was detected approximately 1 day later at stage 15 and was confined to atrial-forming tissues. These data demonstrate antigenic similarity between adult and embryonic MHC isolated from atrial myocardium and suggest the expression of an atrial-specific MHC early in the regional differentiation of the heart.

Immunofluorescence analysis of the primordial myosin detectable in embryonic striated muscle

Immunofluorescence analysis showed that the earliest myosin detectable in both the embryonic chicken heart and somitic myotome, the precursor to skeletal muscle, was strongly reactive with two different monoclonal antibodies specific for the heavy chain of cardiac ventricular myosin, but it showed no reactivity with affinity-purified polyclonal antibodies specific for the heavy chains of either fast-twitch or slow-tonic skeletal myosins. The heart remained reactive exclusively with the antibodies to cardiac myosin throughout development, while late embryonic (day 20) skeletal muscles were strongly reactive only with their homologous skeletal myosin antibodies. Our findings suggest that the primordial myosin heavy chain detectable in both forms of embryonic chicken striated muscle, the myotome and the heart, is immunologically distinct from myosins expressed in later embryonic as well as adult skeletal muscles, but it contains antigenic determinants similar to those present in cardiac ventricular myosin.

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.

Regenerating adult chicken skeletal muscle and satellite cell cultures express embryonic patterns of myosin and tropomyosin isoforms

Regenerating areas of adult chicken fast muscle (pectoralis major) and slow muscle (anterior latissimus dorsi) were examined in order to determine synthesis patterns of myosin light chains, heavy chains and tropomyosin. In addition, these patterns were also examined in muscle cultures derived from satellite cells of adult fast and slow muscle. One week after cold-injury the regenerating fast muscle showed a pattern of synthesis that was predominately embryonic. These muscles synthesized the embryonic myosin heavy chain, beta-tropomyosin and reduced amounts of myosin fast light chain-3 which are characteristic of embryonic fast muscle but synthesized very little myosin slow light chains. The regenerating slow muscle, however, showed a nearly complete array of embryonic peptides including embryonic myosin heavy chain, fast and slow myosin light chains and both alpha-fast and slow tropomyosins. Peptide map analysis of the embryonic myosin heavy chains synthesized by regenerating fast and slow muscles showed them to be identical. Thus, in both muscles there is a return to embryonic patterns during regeneration but this return appears to be incomplete in the pectoralis major. By 4 weeks postinjury both regenerating fast and slow muscles had stopped synthesizing embryonic isoforms of myosin and tropomyosin and had returned to a normal adult pattern of synthesis. Adult fast and slow muscles yielded a satellite cell population that formed muscle fibers in culture. Fibers derived from either population synthesized the embryonic myosin heavy chain in addition to alpha-fast and beta-tropomyosin. Thus, muscle fibers derived in culture from satellite cells of fast and slow muscles synthesized a predominately embryonic pattern of myosin heavy chains and tropomyosin. In addition, however, the satellite cell-derived myotubes from fast muscle synthesized only fast myosin light chains while the myotubes derived from slow muscle satellite cells synthesized both fast and slow myosin light chains. Thus, while both kinds of satellite cells produced embryonic type myotubes in culture the overall patterns were not identical. Satellite cells of fast and slow muscle appear therefore to have diverged from each other in their commitment during maturation in vivo.

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.

Molecular and cell isoforms during development

Development proceeds by way of a discrete yet overlapping series of biosynthetic and restructuring events that result in the continued molding of tissues and organs into highly restricted and specialized states required for adult function. Individual molecules and cells are replaced by molecular and cellular variants, called isoforms; these arise and function during embryonic development or later life. Isoforms, whether molecular or cellular, have been identified by their structural differences, which allow separation and characterization of each variant. These isoforms play a central and controlling role in the continued and dynamic remodeling that takes place during development. Descriptions of the individual phases of the orderly replacement of one isoform for another provides an experimental context in which the process of development can be better understood.

Monoclonal antibodies localize changes on myosin heavy chain isozymes during avian myogenesis

Monoclonal antibodies were used to identify and localize by immunoelectron microscopy epitopes on myosin isozymes. An antibody that reacts with an amino-terminal fragment of the myosin heavy chain maps on the myosin head 140 A distal to the head-rod junction. It identifies an epitope that is shared on adult and embryonic myosin, and detects two transitions in myosin expression during avian pectoralis myogenesis. Another antibody maps to the carboxyl terminus of the myosin rod. It is specific for an adult fast myosin epitope that is not detected in early developing pectoralis muscle. In contrast, an epitope that is present throughout development is identified by an antibody that reacts with a myosin light chain. This light chain epitope is localized at the head-rod junction. These results demonstrate structural changes in widely separated regions of the myosin molecule accompanying the sequential expression of developmental myosin isozymes.

Immunochemical analysis of myosin heavy chain during avian myogenesis in vivo and in vitro

Monoclonal antibodies (McAbs) against the myosin heavy chain (MHC) of adult chicken pectoralis muscle have been tested for reactivity with pectoralis myosin at selected stages of chick development in vivo and in vitro. Three such McAbs, MF 20 and MF 14, which bind to light meromyosin, and MF 30, which binds to myosin subfragment two (S2), were used to assay the appearance and accumulation of specific MHC epitopes with: (a) indirect, solid phase radioimmune assay (RIA), (b) immunoautoradiography, (c) immunofluorescence microscopy. McAb MF 20 bound strongly and equivalently to MHC at all stages of embryonic development in vivo. In contrast, the MF 30 epitope was barely detectable at 12 d of incubation but its concentration rose rapidly just before hatching. No detectable binding of MF 14 to pectoralis myosin could be measured during myogenesis in vivo until 1 wk after hatching. Immunofluorescence studies revealed that all three epitopes accumulate in the same myocytes of the developing pectoralis muscle. Since all three McAbs bound with high activity to native and denatured forms of myosin, it is unlikely that differential antibody reactivity can be explained by conformational changes in myosin during development in vivo. When myogenesis in vitro was monitored using the same McAbs, MF 20 bound to the MHC at all stages tested while reactivity of MF 30 and MF 14 with myosin from cultured muscle was never observed. Thus, this study demonstrates three different immunochemical states of the MHC during development in vivo of chick pectoralis muscle and the absence of later occurring immunochemical transitions in the MHC of cultured embryonic muscle.

Myosin heavy chains from two different adult fast-twitch muscles have different peptide maps but identical mRNAs

Myosin heavy chains prepared from the pectoralis major and from the posterior latissimus dorsi of the same adult chicken exhibit different peptide maps when cleaved with Staphylococcus aureus V8 protease. These differences were observed at five different enzyme concentrations and in chickens of various strains. The cleavage pattern of pectoralis major myosin heavy chain from different adult chickens was always identical, as was that of posterior latissimus dorsi myosin heavy chain, demonstrating the reproducibility of the technique. However, when RNAs extracted from the pectoralis major and from the posterior latissimus dorsi were translated in a cell-free reticulocyte lysate, the myosin heavy chain encoded by pectoralis major RNA and the myosin heavy chain encoded by posterior latissimus dorsi RNA exhibited identical peptide maps. These results suggest that the different peptide maps of myosin heavy chains from the pectoralis major and posterior latissimus dorsi may arise from posttranslational modifications.

Increased K+ inhibits spontaneous contractions reduces myosin accumulation in cultured chick myotubes

Increasing the K+ from 5.4 mM to 12 mM in the culture medium of developing chick myotubes causes an immediate cessation of spontaneous contractions and leads to an inhibition of myosin accumulation. The synthesis of myosin continues at the same rate in 12 mM K+ as in 5.4 mM K+ as measured by [3H]leucine incorporation into myosin corrected for differences in pool specific activity. Total protein synthesis and total protein accumulation are unaffected by growth in 12 mM K+. In addition, growth in 12 mM K+ did not alter the type of myosin heavy-chain isoform expression nor did it alter the pattern of myosin light-chain synthesis. However, the rate of myosin turnover increased threefold in cultures grown in 12 mM K+ compared to cultures grown in 5.4 mM K+, while total protein turnover was only marginally increased. We conclude that suppressed electrical or contractile activity of myotubes leads to an increased rate of myofibrillar protein turnover and that spontaneous mechanical and or electrical activity is required for continued myotube maturation in culture.