Differential distribution of tropomyosin subunits in fast and slow rat muscles and its change in long-term denervated hemidiaphragm

Top-Down Proteomics Reveals Myofilament Proteoform Heterogeneity among Various Rat Skeletal Muscle Tissues

Heterogeneity in skeletal muscle contraction time, peak power output, and resistance to fatigue, among others, is necessary to accommodate the wide range of functional demands imposed on the body. Underlying this functional heterogeneity are a myriad of differences in the myofilament protein isoform expression and post-translational modifications; yet, characterizing this heterogeneity remains challenging. Herein, we have utilized top-down liquid chromatography (LC)-mass spectrometry (MS)-based proteomics to characterize myofilament proteoform heterogeneity in seven rat skeletal muscle tissues including vastus lateralis, vastus medialis, vastus intermedius, rectus femoris, soleus, gastrocnemius, and plantaris. Top-down proteomics revealed that myofilament proteoforms varied greatly across the seven different rat skeletal muscle tissues. Subsequently, we quantified and characterized myofilament proteoforms using online LC-MS. We have comprehensively characterized the fast and slow skeletal troponin I isoforms, which demonstrates the ability of top-down MS to decipher isoforms with high sequence homology. Taken together, we have shown that top-down proteomics can be used as a robust and high-throughput method to characterize the molecular heterogeneity of myofilament proteoforms from various skeletal muscle tissues.

Comprehensive analysis of tropomyosin isoforms in skeletal muscles by top-down proteomics

Mammalian skeletal muscles are heterogeneous in nature and are capable of performing various functions. Tropomyosin (Tpm) is a major component of the thin filament in skeletal muscles and plays an important role in controlling muscle contraction and relaxation. Tpm is known to consist of multiple isoforms resulting from different encoding genes and alternative splicing, along with post-translational modifications. However, a systematic characterization of Tpm isoforms in skeletal muscles is still lacking. Therefore, we employed top-down mass spectrometry (MS) to identify and characterize Tpm isoforms present in different skeletal muscles from multiple species, including swine, rat, and human. Our study revealed that Tpm1.1 and Tpm2.2 are the two major Tpm isoforms in swine and rat skeletal muscles, whereas Tpm1.1, Tpm2.2, and Tpm3.12 are present in human skeletal muscles. Tandem MS was utilized to identify the sequences of the major Tpm isoforms. Furthermore, quantitative analysis revealed muscle-type specific differences in the abundance of un-modified and modified Tpm isoforms in rat and human skeletal muscles. This study represents the first systematic investigation of Tpm isoforms in skeletal muscles, which not only demonstrates the capabilities of top-down MS for the comprehensive characterization of skeletal myofilament proteins but also provides the basis for further studies on these Tpm isoforms in muscle-related diseases.

Mammalian skeletal muscle fiber type transitions

Mammalian skeletal muscle is an extremely heterogeneous tissue, composed of a large variety of fiber types. These fibers, however, are not fixed units but represent highly versatile entities capable of responding to altered functional demands and a variety of signals by changing their phenotypic profiles. This adaptive responsiveness is the basis of fiber type transitions. The fiber population of a given muscle is in a dynamic state, constantly adjusting to the current conditions. The full range of adaptive ability spans fast to slow characteristics. However, it is now clear that fiber type transitions do not proceed in immediate jumps from one extreme to the other, but occur in a graded and orderly sequential manner. At the molecular level, the best examples of these stepwise transitions are myofibrillar protein isoform exchanges. For the myosin heavy chain, this entails a sequence going from the fastest (MHCIIb) to the slowest (MHCI) isoform, and vice-versa. Depending on the basal protein isoform profile and hence the position within the fast-slow spectrum, the adaptive ranges of different fibers vary. A simple transition scheme has emerged from the multitude of data collected on fiber type conversions under a variety of conditions.

Effects of sulphydryl modification on skinned rat skeletal muscle fibres using 5,5'-dithiobis(2-nitrobenzoic acid)

1. The sulphydryl groups of skinned skeletal muscle fibres have been reacted with 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) in order to determine whether the effects of modifications to the contractile proteins are reflected in changes in the physiological properties of the contractile apparatus and Ca(2+)-regulatory system. 2. Results obtained from fast-twitch and slow-twitch rat fibres which were treated with DTNB (10 mM, pH 8.6, 5 degrees C) for various periods of time under relaxing conditions showed that a major effect of the modification was to reduce the level of maximally Ca(2+)-activated force and fibre stiffness. Force and fibre stiffness were found to decline in proportion. Treatment with DTNB under these conditions did not cause a rise in force or fibre stiffness in relaxed fibres of either type. 3. The effects induced by DTNB under relaxing conditions were substantially reversed by exposure to the reducing agent dithiothreitol (DTT) (10 mM, pH 7.1, 23 degrees C). Force abolished by 30-35 s treatment with DTNB recovered after subsequent DTT treatment to 67 +/- 3% (mean +/- S.E.M., n = 4) in fast-twitch fibres and to 91 +/- 2% (n = 7) in slow-twitch fibres. These results were significantly different (t test, P less than 0.001) indicating that the level of force recovery depended upon the fibre type. 4. DTNB was found to affect not only the maximal Ca(2+)-activated force, but also the force-pCa (pCa = -log10[Ca2+]) relationships of the fibres in a complex, fibre-type specific way. DTT treatment partially reversed these DTNB effects. 5. The skinned fibre preparations reacted differently with DTNB under rigor conditions than under relaxing conditions, indicating that rigor modifies the reactivity of the functional sulphydryl groups to the thiol-targeted agents. 6. When superprecipitation assays (an in vitro analogue of fibre contraction) were carried out with recombined myofibrillar proteins which had been previously reacted with DTNB it was found that modification of myosin, but not modification of thin filament proteins, led to changes in the superprecipitation reaction. 7. Both the skinned fibre results and the superprecipitation results indicate that the effects of DTNB upon the fibre characteristics are primarily due to modifications of the sulphydryl groups of myosin. Therefore, these results show that myosin is not only involved in determining the ability of the contractile apparatus to develop force but also in determining the Ca(2+)-regulatory characteristics of the muscle fibre.

Thyroid hormone regulates developmental changes in muscle during flounder metamorphosis

Morphological and biochemical changes in the muscular tissue of metamorphosing flounder were studied in relation to the regulatory role of thyroid hormone. Premetamorphic larvae were reared in seawater alone or seawater containing either thyroxine (T4) or an antithyroid drug (thiourea, TU). Histological changes in the muscle were examined and biochemical changes in the muscle proteins were evaluated by SDS-PAGE and immunoblotting for troponin T (TNT). The muscle tissue of premetamorphic larvae was characterized by abundant vacuoles and basophilic sarcoplasm. In control fish, the larval muscle transformed into the adult type during metamorphic climax; the fibers were filled with abundant myofibrils and the vacuoles disappeared. Analysis by SDS-PAGE showed that the bands at 41.5, 35.5, 34.0, 33.5, 25.5, 23.0, 20.0, and 19.0 kDa clearly increased in density from the climax stage. Premetamorphic larvae possessed two immunoreactive TNT isoforms of 41.5 and 34.0 kDa, the former being predominant. At the climax stage an additional isoform appeared at 33.5 kDa, and the 34.0- and 33.5-kDa TNT became predominant. The administration of T4 precociously induced these histological and biochemical changes in the muscle tissue of flounder larvae. In contrast, TU treatment inhibited these developmental changes in the larval muscle. Our results suggest that the developmental changes in the muscular tissue of metamorphosing flounder are regulated by thyroid hormone.

Structure and complete nucleotide sequence of the gene encoding rat fibroblast tropomyosin 4

We have isolated and determined the complete nucleotide sequence of the gene that encodes the 248 amino acid residue fibroblast tropomyosin, TM-4. The TM-4 sequence is encoded by eight exons, which span approximately 16,000 bases. The position of the intron-exon splice junctions relative to the final transcript are identical to those present in other vertebrate tropomyosin genes and the Drosophila melanogaster TMII gene. We have found no evidence that the rat TM-4 gene is alternatively spliced, unlike all the other tropomyosin genes from multicellular organisms that have been described. Typical vertebrate tropomyosin genes contain some, or all, of alternatively spliced exons 1a and 1b, 2a and 2b, 6a and 6b, and 9a, 9b, 9c and 9d in addition to common exons 3, 4, 5, 7 and 8. The rat fibroblast TM-4 mRNA is encoded by sequences most similar to exons 1b, 3, 4, 5, 6b, 7, 8 and 9d. Two exon-like sequences that are highly similar to alternatively spliced exons 2b and 9a of the rat beta-tropomyosin gene and the human TMnm gene have been located in the appropriate region of the gene encoding rat fibroblast TM-4. However, several mutations in these sequences render them non-functional as tropomyosin coding exons. We have termed these exon-like sequences, vestigial exons. The evolutionary relationship of the rat TM-4 gene relative to other vertebrate tropomyosin genes is discussed.

Caldesmon: anomalous electrophoretic behaviour in polyacrylamide gel

In SDS gels caldesmon (Mr = 140 kDa) and myosin light chain kinase (Mr = 130 kDa) migrate as a closely separated doublet. When glycerol is added to the gel caldesmon is characterized by an anomalous migration. In fact under this latter condition, the distance between caldesmon and myosin light chain kinase is enhanced by two-three times. The nature of putative caldesmon and myosin light chain kinase was confirmed by physicochemical, enzymatic and immunological methods.

Isomyosin changes after functional electrostimulation of denervated sheep muscle

Isomyosin analyses by biochemical, immunochemical, and histochemical investigations have been carried out in five sheep following unilateral recurrent laryngeal nerve paralysis and direct functional electrostimulation of the denervated cricoarytenoid posterior muscle. Myosin light chains were identified by two-dimensional gel electrophoresis. Myosin heavy chains were analyzed by one-dimensional SDS-polyacrylamide gel electrophoresis. Slow myosin heavy chain was identified by orthogonal peptide mapping and immunochemistry. The stimulation effect at cellular level was determined using adenosine triphosphatase (ATPase) histochemistry. A dramatic increase of the type 1 fiber area (slow, fatigue-resistant fibers) could be seen after many weeks of an increasing regime of low-frequency direct electrical stimulation. Biochemically, the amount of slow myosin was always higher than in normal muscles. Some muscles were transformed almost completely to the slow type. At the time they were studied and with the methods employed, the expression of embryonic isomyosin was not observed. In conclusion, after numerous weeks of maintained functional activity, elicited by direct electrostimulation, the denervated muscle regionally showed areas of hypertrophy or at least lack of atrophy of slow myofibers without major signs of muscle damage.

Long-term retention of regenerative capability after denervation of skeletal muscle, and dependency of late differentiation on innervation

The present study examines the influence of denervation on the regenerative ability of skeletal muscle in rats. Muscle denervation was achieved by transecting and ligating the cut ends of the sciatic nerve. Four to 48 weeks after denervation, the extensor digitorum longus (EDL) muscle was autotransplanted to induce muscle regeneration. The transplanted EDL muscles were examined at 1-12 weeks. Normal (i.e., no prior denervation) EDL muscle autotransplants were also examined for comparison. Denervation resulted in progressive atrophy of muscle, marked by a reduction in the size of myofibers and an increase in endomysialperimysial connective tissue. In spite of these alterations, typical events of muscle regeneration were invariably observed after transplantation. Initial myofiber degeneration and subsequent regeneration of myotubes occurred in a manner similar to normal muscle transplants. However, only a partial maturation of myotubes was observed in denervated muscles. These results show that extended denervation does not abolish the capability for muscle regeneration. The precursor myosatellite cells, proposed to be responsible for muscle regeneration, retain their regenerative potential after denervation. It is concluded, however, that the presence of intact innervation is crucial for the terminal differentiation and maturation of regenerating muscle.

Factors determining the subunit composition of tropomyosin in mammalian skeletal muscle

Adult rat fast-twitch skeletal muscle such as extensor digitorum longus contains alpha- and beta-tropomyosin subunits, as is the case in the corresponding muscles of rabbit. Adult rat soleus muscle contains beta-, gamma- and delta-tropomyosins, but no significant amounts of alpha-tropomyosin. Evidence for the presence of phosphorylated forms of at least three of the four tropomyosin subunit isoforms was obtained, particularly in developing muscle. Immediately after birth alpha- and beta-tropomyosins were the major components of skeletal muscle, in both fast-twitch and slow-twitch muscles. Differentiation into slow-twitch skeletal muscles was accompanied by a fall in the amount of alpha-tropomyosin subunit and its replacement with gamma- and delta-subunits. After denervation and during regeneration after injury, the tropomyosin composition of slow-twitch skeletal muscle changed to that associated with fast-twitch muscle. Thyroidectomy slowed down the changes in tropomyosin composition resulting from the denervation of soleus muscle. The results suggest that the 'ground state' of tropomyosin-gene expression in the skeletal muscle gives rise to alpha- and beta-tropomyosin subunits. Innervation by a 'slow-twitch' nerve is essential for the expression of the genes controlling gamma- and delta-subunits. There appears to be reciprocal relationship between expression of the gene controlling the synthesis of alpha-tropomyosin and those controlling the synthesis of gamma- and delta-tropomyosin subunits.

Chronic denervation of rat hemidiaphragm: maintenance of fiber heterogeneity with associated increasing uniformity of myosin isoforms

During several months of denervation, rat mixed muscles lose slow myosin, though with variability among animals. Immunocytochemical studies showed that all the denervated fibers of the hemidiaphragm reacted with anti-fast myosin, while many reacted with anti-slow myosin as well. This has left open the question as to whether multiple forms of myosin co-exist within individual fibers or a unique, possibly embryonic, myosin is present, which shares epitopes with fast and slow myosins. Furthermore, one can ask if the reappearance of embryonic myosin in chronically denervated muscle is related both to its re-expression in the pre-existing fibers and to cell regeneration. To answer these questions we studied the myosin heavy chains from individual fibers of the denervated hemidiaphragm by SDS PAGE and morphologically searched for regenerative events in the long term denervated muscle. 3 mo after denervation the severely atrophic fibers of the hemidiaphragm showed either fast or a mixture of fast and slow myosin heavy chains. Structural analysis of proteins sequentially extracted from muscle cryostat sections showed that slow myosin was still present 16 mo after denervation, in spite of the loss of the selective distribution of fast and slow features. Therefore muscle fibers can express adult fast myosin not only when denervated during their differentiation but also after the slow program has been expressed for a long time. Light and electron microscopy showed that the long-term denervated muscle maintained a steady-state atrophy for the rat's life span. Some of the morphological features indicate that aneural regeneration events continuously occur and significantly contribute to the increasing uniformity of the myosin gene expression in long-term denervated diaphragm.

Loss of fast-twitch isomyosins in skeletal muscles of the diabetic rat

By means of pyrophosphate electrophoresis the myosin isoenzyme pattern of two fast-twitch skeletal muscles (extensor digitorum longus, gastrocnemius) and one slow-twitch muscle (soleus) was investigated in control rats and was compared with that of rats 4 weeks after induction of diabetes mellitus by streptozotocin injection. In the fast-twitch muscles the isomyosin pattern consisting of FM1 (fast isomyosin 1), FM2 and FM3 was strongly affected by diabetes, resulting in an extensive loss of FM1 and a substantial decrease of FM2. These changes were also apparent when the light chains of the fast isomyosins were analysed by two-dimensional electrophoresis: LC3f (myosin light chain 3f) largely disappeared and LC2f was significantly diminished. In contrast, the isomyosin pattern in soleus muscle, consisting of SM1 (slow isomyosin 1) and SM2, was not affected by the diabetic state, and two-dimensional electrophoresis revealed a normal light-chain pattern of LC1sa, LC1sb and LC2s. These results indicate that the isomyosins of slow-twitch oxidative myofibres are more resistant to the hormonal and metabolic disorders during diabetes mellitus than are the isomyosins of fast-twitch fibres.

Improved resolution with one-dimensional polyacrylamide gel electrophoresis: myofibrillar proteins from typed single fibers of human muscle

Standard procedures for one-dimensional discontinuous sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis and silver staining were modified to give more effective separation and an improved resolution of human skeletal muscle proteins. In this system, an electrophoresis buffer composed of 100 mM L-isoleucine, 25 mM Tris base, and 0.1% SDS was used. The separating gel consisted of 16% acrylamide with N,N'-methylenebisacrylamide as a crosslinker (1:23), 0.4% SDS, 1.5 M Tris-HCl, pH 8.8. By the present procedure, the slow and the fast forms of myosin light chains (LCs, LCf) and other contractile proteins from human muscle could be better separated. The silver stain is based on a combination of methods previously described. The modified method requires a small fragment of a single fiber to observe as few as 10 ng of myofibrillar muscle proteins. The described simplifications made it possible to assay and compare up to 40 single fibers in the same electrophoretic run. Improved separation of other proteins migrating at basic pH could be achieved by a similar approach.

Chronic denervation of rat diaphragm: selective maintenance of adult fast myosin heavy chains

After long-term denervation in mixed rat muscles there is a selective loss of slow myosin. The bidimensional electrophoretic pattern of light chains and the results of preliminary studies on heavy chains have left open the question of whether the nature of the residual fast-like myosin is of the immature or adult type. We have further investigated chronically denervated myosin by (1) electrophoresis in nondissociating conditions; (2) acidic electrophoresis of the heavy chains; and (3) proteolytic mapping of the heavy chains. These techniques clearly distinguish adult myosins from those present in immature muscles. Using these criteria, myosin from the chronically denervated diaphragm is of an adult type, even though the presence of trace amounts of embryonic myosin cannot be excluded. The contractile properties also indicate that chronically denervated hemidiaphragm is more similar to an adult fast muscle than to an immature muscle. The selective maintenance after long-term denervation of fast myosin in adult muscle provides good evidence of the independence of the genetic expression of myosin from a direct neural chemotrophic control.