Characterization of the tropomyosin present in various chick embryo muscle types and in muscle cells differentiated in vitro

Separation of muscle proteins

This review covers various methods used in the separation and isolation of individual muscle contractile proteins. It is shown which methods have been most useful for the separation of contractile proteins and their fragments and in extending our knowledge of muscle biochemistry and physiology.

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.

Potential phasic and tonic muscles express a common set of fast and slow myosin light chains and fast tropomyosin during early development of chick embryo

We investigated the expression of myosin light chains and tropomyosin subunits during chick embryonic development of the anterior (ALD) and posterior (PLD) parts of the latissimus dorsi muscles. As early as day 8 in ovo, both muscles accumulate a common set of myosin light chains (LC) in similar ratios (LC1F: 55 per cent; LC2S: 25 per cent; LC2F: 12 per cent; LC1S: 8 per cent) and a common set of tropomyosin (TM) subunits (beta 2, beta 1, alpha 2F). Later during development, the slow components of the LC regularly disappear in the PLD and the fast components of the LC and the alpha 2FTM disappear in the ALD, so that the adult pattern is almost established at the time of hatching. Thus, early in development, the two muscles accumulate a common set of fast and slow myosin light chains and fast tropomyosin and some isoforms are repressed at a later stage during development. These data might suggest that during development, the regulatory mechanisms of muscle specific isoform expression differ from one contractile protein to another.

Heterogeneity of tropomyosin and actin in normal and diseased muscle

Actin and tropomyosin in muscle samples from normal humans, from human fetuses between 12 and 17 gestational weeks, and from patients with a variety of neuromuscular disorders were studied with two-dimensional electrophoresis using isoelectric focusing with either a broad pH range (8.6-4.5) or a narrow pH range (5.9-3.8) for the first dimension and either SDS or SDS-urea for the second dimension. With the broad pH range, two brothers with Duchenne muscular dystrophy were noted to have a less acidic variant of alpha-tropomyosin in biceps muscle which was not found in biceps muscle from other patients or controls. Studies of 8 additional biopsy specimens from patients with Duchenne muscular dystrophy and comparison with both fetal and normal human muscle using the narrow pH range revealed multiple forms of actin and tropomyosin which varied from individual to individual. This heterogeneity appeared to be unrelated to the dystrophic state but also obscured the ability to detect a change in actin or tropomyosin which could be related to dystrophy.

Glycosylation as a criterion for defining subpopulations of fast-transported proteins

The role carbohydrate residues may play in the sorting of newly synthesized fast-transported proteins during the initiation of fast axonal transport has been examined by identifying individual fast-transported glycoproteins that contain either or both fucose and galactose. [3H]Fucose or [3H]galactose was incorporated together with [35S]methionine in vitro in bullfrog dorsal root ganglia. Fast-transported proteins that accumulated proximal to a ligature on the spinal nerve were separated via two-dimensional gel electrophoresis, and 92 gel spots were analyzed quantitatively for the presence of 35S and 3H. Of these spots, 56 (61%) contained either or both fucose and galactose. Glycomoieties were generally associated with families of charged spots whose isoelectric points could be altered with neuraminidase treatment. Single spots tended to be unglycosylated and were unaffected by neuraminidase. The prevalence of glycoproteins was considerably greater in the higher-molecular weight range. Of the 55 spots analyzed with molecular weight greater than approximately 35,000 daltons, 89% were glycosylated, whereas only 19% of the 37 spots with lower molecular weight contained sugar moieties. When considered in light of previous studies in which similar subpopulations have been described, the current findings suggest that the presence or absence of glycomoieties may represent another criterion by which proteins are sorted during the initiation of fast axonal transport.

Dimerization of the polypeptide chains of skeletal muscle tropomyosin

The composition of alpha and beta chains in tropomyosin dimers present in fetal and adult skeletal muscle of cow has been analysed by SDS-polyacrylamide gel electrophoresis after cross-linking of the chains by disulphide bridges. The results indicate that in vivo alpha beta heterodimers of tropomyosin are assembled preferentially and only the excess of particular chains forms homodimers, i.e., alpha alpha dimers in adult and beta beta ones in fetal muscle. The original dimers of tropomyosin were dissociated with urea in the presence of dithiothreitol. Subsequent reassembly of the tropomyosin dimers from the mixture of alpha and beta chains approaches the random model.

The comparative effects of tumor-related agents on developmental marker expression in presumptive myoblasts and myotubes

The effect of a tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA), on differentiation marker expression in skeletal myotubes and their immediate progenitors, presumptive myoblasts (PMbs), was investigated. The markers employed included the K-isozyme of pyruvate kinase (PK-K) and a newly characterized phosphoprotein (pp(65;4.5)), which are both normally concentrated in PMbs, and the M-isozyme of pyruvate kinase (PK-M) and phosphorylated derivatives of alpha- and beta-tropomyosin (pT), which are normally concentrated in myotubes. PMbs treated with TPA for 3, 6 or 10 days continued to express PK-K and pp(65;4.5), and did not express PK-M or pT at detectable levels. Myotubes which were treated for 3 or 6 days were inhibited in their expression of PK-M and pT, but were not stimulated in their expression of PK-K or pp(65;4.5). The effects of TPA were reversible upon discontinuation of treatment. In parallel experiments, where pp(65;4.5) and pT were monitored, infection with Rous sarcoma virus (RSV) had similar effects. The data suggest that TPA treatment and RSV transformation do not alter the state of differentiation of target cells per se but, in the case of myotubes, do suppress the expression of that state.

Regional differences in the expression of myosin light chains and tropomyosin subunits during development of chicken breast muscle

Types of myosin light chains and tropomyosins present in various regions and at different developmental stages of embryonic and posthatched chicken breast muscle (pectoralis major) have been characterized by two-dimensional gel electrophoresis. In the embryonic muscle all areas appear to accumulate both slow and fast forms of myosin light chains in addition to alpha and beta forms of tropomyosin. During development regional differences in myosin and tropomyosin expression become apparent. Slow myosin subunits become gradually restricted to areas of the anterior region of the muscle and finally become localized to a small red strip found on its anterior deep surface. This red region is characterized by the presence of slow and fast myosin light chains, alpha-fast, alpha-slow, and beta-tropomyosin. In all other areas of the muscle examined only fast myosin light chains, beta-tropomyosin and the alpha-fast form of tropomyosin, are found. In addition, beta-tropomyosin also gradually becomes lost in the posterior regions of the developing breast muscle. In the adult, the red strip area represents less than 1% of the total pectoralis major mass and of the myosin extracted from this area approximately 15% was present as an isozyme that comigrated on nondenaturing gels with myosin from a slow muscle (anterior latissimus dorsi). The red region accumulates therefore fast as well as slow muscle myosin. Thus while the adult chicken pectoralis major is over 99% fast white muscle, the embryonic muscle displays a significant and changing capacity to accumulate both fast and slow muscle peptides.

Fast and slow chicken skeletal muscles contain different alpha and beta tropomyosins

Avian tropomyosin has been purified from fast skeletal muscles (breast muscle and posterior latissimus dorsi : PLD) and from a slow skeletal muscle (anterior latissimus dorsi : ALD) and the alpha and beta subunits have been further separated using preparative gel electrophoresis. These subunits have been subjected to partial proteolysis using different proteolytic enzymes. In this communication we show that this procedure allows to distinguish not only between fast and slow alpha tropomyosin but also between fast and slow beta tropomyosin. Furthermore we have raised an antiserum against the fast alpha tropomyosin and we present evidence to show that this antiserum does not cross-react with the slow alpha tropomyosin. These results are taken to indicate that all these tropomyosin subunits represent different gene products.

Tropomyosin from adult human skeletal muscle is partially phosphorylated

Alpha and beta tropomyosins were isolated from postmortem adult human psoas and pectoralis major muscles. 31P nuclear magnetic resonance and amino acid analysis were used to show that 10% of the major alpha tropomyosin component was phosphorylated. The 31P NMR spectra also suggested that human beta tropomyosin was phosphorylated, but to a lesser extent.

Distinct alpha-tropomyosin mRNA sequences in chicken skeletal muscle

Recombinant plasmids have been isolated which contain sequences complementary to two distinct alpha-tropomyosin mRNA species present in chicken leg muscle. The proteins coded for by these different mRNAs could be distinguished by their electrophoretic behaviour in the presence of 3.5 M urea. The properties of the minor alpha-tropomyosin of chicken leg muscle were similar to those reported for the alpha-tropomyosin of slow twitch chicken skeletal muscle. Sequence analysis of available plasmids showed that the deduced protein sequences of both types of alpha-tropomyosin were very similar and closely related to the known protein sequence of rabbit alpha-tropomyosin. However considerable variation in nucleotide coding sequence of the two alpha-tropomyosin mRNAs was found.

Cytoskeleton organization in differentiating mouse teratocarcinoma cells

In addition to the well-known cytoskeleton actin and tubulin, new intermediate-sized filaments have been isolated that ensure cellular connections. Their proteins, referred to here as IFP (intermediate filament proteins), comprise keratin desmin, vimentin, glialin and neurofilin. We review some of these new findings, with emphasis on the IFP expressions in the differentiation of embryonic cells and the deviations encountered in tumor cells. The first measurements on embryonic and differentiated mouse teratoma carcinoma cells established that all cell lines contain cytoplasmic microtubules. Although actin is also present in all cells, its organization differs markedly in differentiated derivatives. Since then, extensive studies have confirmed these findings and refined our understanding of the adaptation of cytoskeletons during differentiation to fulfill thier function. Fibrillar IFP structures have diameters ranging from 80 to 150 A, by electromicroscopic measurements. The specificity established in various structural functions have emerged from protein assays by two-dimensional electrophoresis and by immunofluorescence. With mouse teratoma cells, the incipient formation of the three structural components is followed by applying antibodies of high specificity for keratin, desmin and vimentin. During in vitro differentiation of teratoma carcinoma cells, as in animal tissue, keratin is expressed always in the endodermal cells and desmin in the muscle cells. Vimentin, which is known to be restricted to mesenchymal tissue in animals, occurs in all cells which have acquired the potential for unlimited growth in culture. In embryonic developments, all cells of mouse blastocyst express microtubules which is consistent with the function attributed to tubulin. The structure of actin, by contrast, changes during the first morphological step of differentiation. The outer trophectodermal cells contain actin cables, whereas the inner cell mass contains actin in a diffuse state. It has been established that keratin fibers appear in trophoblastic cells, i.e., in the earliest embryonic differentiation of epithelial character. Research on the development of IFP is rapidly progressing. Recent results of several groups are discussed.