Changes in tropomyosin subunit pattern in chronic electrically stimulated rabbit fast muscles

Cultured slow vs. fast skeletal muscle cells differ in physiology and responsiveness to stimulation

In vitro studies have used protein markers to distinguish between myogenic cells isolated from fast and slow skeletal muscles. The protein markers provide some support for the hypothesis that satellite cells from fast and slow muscles are different, but the data are equivocal. To test this hypothesis directly, three-dimensional skeletal muscle constructs were engineered from myogenic cells isolated from fast tibialis anterior (TA) and slow soleus (SOL) muscles of rats and functionality was tested. Time to peak twitch tension (TPT) and half relaxation time (RT(1/2)) were approximately 30% slower in constructs from the SOL. The slower contraction and relaxation times for the SOL constructs resulted in left shift of the force-frequency curve compared with those from the TA. Western blot analysis showed a 60% greater quantity of fast myosin heavy chain in the TA constructs. 14 days of chronic low-frequency electrical stimulation resulted in a 15% slower TPT and a 14% slower RT(1/2), but no change in absolute force production in the TA constructs. In SOL constructs, slow electrical stimulation resulted in an 80% increase in absolute force production with no change in TPT or RT(1/2). The addition of cyclosporine A did not prevent the increase in force in SOL constructs after chronic low-frequency electrical stimulation, suggesting that calcineurin is not responsible for the increase in force. We conclude that myogenic cells associated with a slow muscle are imprinted to produce muscle that contracts and relaxes slowly and that calcineurin activity cannot explain the response to a slow pattern of electrical stimulation.

New dimensions in tissue engineering: possible models for human physiology

Tissue engineering is a discipline of great promise. In some areas, such as the cornea, tissues engineered in the laboratory are already in clinical use. In other areas, where the tissue architecture is more complex, there are a number of obstacles to manoeuvre before clinically relevant tissues can be produced. However, even in areas where clinically relevant tissues are decades away, the tissues being produced at the moment provide powerful new models to aid the understanding of complex physiological processes. This article provides a personal view of the role of tissue engineering in advancing our understanding of physiology, with specific attention being paid to musculoskeletal tissues.

Function, morphology and protein expression of ageing skeletal muscle: a cross-sectional study of elderly men with different training backgrounds

The function and morphology of knee extension/m. vastus lateralis and elbow flexion/m. biceps brachii were studied in young (28 +/- 0.1 years, n = 7) and elderly (68 +/- 0.5 years, n = 8) sedentary subjects and in elderly swimmers (69 +/- 1.9 years, n = 6), runners (70 +/- 0.7 years, n = 5) and strength-trained subjects (68 +/- 0.8 years, n = 7). On average, the training groups had, for the 12-17 years before the measurements were taken, performed their training regimen 3 +/- 0.1 times a week. Compared with the young subjects, the maximal isometric torque of the sedentary elderly subjects was 44% (P less than 0.05) lower in knee extension and 32% (P less than 0.05) lower in elbow flexion, and speed of movement was between 20 and 26% (P less than 0.05) lower in both knee extension and elbow flexion. The cross-sectional area of m. quadriceps femoris and the elbow flexors was also 24% (P less than 0.05) and 20% lower respectively, and the specific tension was 27% (P less than 0.05) lower in m. quadriceps femoris and 14% (P less than 0.05) lower in the elbow flexors. A 27% (P less than 0.05) higher content of myosin heavy chain type I and a 39% (P less than 0.05) higher content of the slow-type myosin light chain--2 was observed in m. vastus lateralis of the sedentary elderly subjects as compared with the young subjects. The same tendency was also seen with m. biceps brachii. Since the histochemical fibre-type distribution was identical and no major co-expression of type I and type II myosin heavy-chain isoforms was observed with immunocytochemistry, the increase in slow myosin isoforms with ageing seems mainly related to a larger relative area of type I fibres, induced by a selective atrophy of type II fibre area. An increased content of the beta-isoform of tropomyosin was also demonstrated with ageing. In contrast to the swimmers and runners, the elderly strength-trained subjects had maximal isometric torques, speed of movements, cross-sectional areas, specific tensions and a content of myosin and tropomyosin isoforms in both muscles studied identical to those of the young controls. These results seem to suggest that strength training can counteract the age-related changes in function and morphology of the ageing human skeletal muscle.

The expression of myosin light chains and tropomyosin in human muscle biopsies with histochemical type 1 and type 2 fiber deficiency

Two-dimensional gel electrophoresis (2DE) was used to compare the protein composition of human muscle biopsies that were shown by histochemical staining to be deficient in either type 1 or type 2 fibers. Distinct quantitative differences were found in the myofibrillar protein composition of muscle from a 43-year-old woman with proximal limb weakness that showed almost total absence of type 1 fibers and muscle from a 2.5-year-old girl with congenital myopathy in which there was severe lack of type 2 fibers. These data indicate that human type 1 and type 2 muscle fibers express distinct isoforms of myosin light chains and alpha-tropomyosin.

Molecular genetics in basic myology: a rapidly evolving perspective

Myology has greatly benefited from the recent unification of concepts in molecular, cellular, and developmental biology. The interplay between intrinsic and extrinsic factors in determining the physiologic characteristics of individual myofibers has emerged as an important theme. Of special note is the manner in which the study of contractile protein gene structure and expression has contributed to our understanding of the development and ultimate plasticity of the contractile apparatus. As mechanistic models of normal myogenesis achieve increasing sophistication, the opportunities for understanding the pathogenesis of progressive muscle disfunction improve. In this article we review recent progress in basic myology which will be of interest to clinicians studying the heritable neuromuscular disorders.

Fiber types in normal and neonatally denervated fast muscles of the rat: immunocytochemical study with an antimyosin monoclonal antibody

The differentiation of fiber types in normal and neonatally denervated gastrocnemius muscles of the rat was compared by myosin ATPase histochemistry and immunocytochemistry using a monoclonal antibody, HM-1.2. The specificity of HM-1.2 for the fast myosin heavy chain was determined by radioimmunoassay, immunoautoradiography, and indirect immunofluorescence techniques. In normal 1-month-old and adult rats, the type IIB (fast glycolytic) fibers of the gastrocnemius could be clearly divided into three subtypes by their graded immunofluorescence staining with the myosin heavy chain-specific monoclonal antibody. In the gastrocnemius muscle of the newborn rat, all fibers were negative with the monoclonal antibody. The transition from negative to three grades of immunoreactivity occurred 1 to 2 weeks postnatally. After neonatal denervation of the gastrocnemius muscle, however, uniformly positive monoclonal antibody immunofluorescence staining for the myosin heavy chain was observed without subtype differentiation. This study, thus, gave clear immunocytochemical evidence that the type IIB muscle fibers are heterogeneous with respect to their myosin isoform and that the expression of this heterogeneity is dependent on the normal developmental influence of motor innervation on the muscle fibers.

Mechanical properties of skinned single fibers of identified types from rat diaphragm

Maximum isometric tension (Po), maximum velocity of shortening (Vmax), and tension-pCa (i.e., -log[Ca2+]) relationships were determined in single skinned fibers from rat diaphragm. Histochemistry (myosin-ATPase) and sodium dodecyl sulfate (SDS) gel electrophoresis were performed on these same fibers to determine fiber type and protein composition. Physiologically fast fibers were found to have larger cross-sectional areas (CSA) and produced more tension per CSA and were less sensitive to [Ca2+] than physiologically slow fibers. Fast fibers were typed histochemically as type II and contained myosin heavy chains (MHC) and light chains (LC) of the fast type, whereas the slow fibers contained slow MHC and LC. There were also corresponding differences in the regulatory protein composition of these two fiber types. The histochemical sections confirmed a significant fiber size difference between the type IIa and IIb fibers. When fiber size was used to separate the fast fibers into two groups, type IIb fibers were found to have significantly greater Vmax and tension per CSA than the type IIa fibers. Although there were no noticeable differences in MHC composition between the type IIa and IIb fibers, there were some differences in the myosin LC and regulatory protein content.

Glyceraldehyde-3-phosphate dehydrogenase mRNA. Activity and amount in dystrophic hamster muscle

The activity and amount of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) in muscle of young dystrophic hamsters was reduced to approximately half the level found in control animals. No changes in brain or liver enzyme activity were found. Several other glycolytic enzyme activities and creatine kinase activity in muscle were unchanged, except for modest decreases in aldolase and pyruvate kinase. To assess the synthesis of glyceraldehyde-3-phosphate dehydrogenase, the poly(A)+ RNA was isolated from muscle polysomes of dystrophic and control animals and its activity was assessed in an mRNA-dependent translation system. The translatability of the mRNA for GAPDH found in the dystrophic muscle preparations also was half of that found in the control muscle preparations. Decreases were also found in the translatability of mRNA for tropomyosin.

Effect of exercise on the motor unit

The motoneuron part of this review deals with the changes in recruitment and firing rates of the motor unit types upon changes from a physically inactive life to endurance or strength training. The muscle fibers react to prolonged exercise by adaptation to a higher level of performance. A matter of discussion is the prerequisites for a transformation between the basic muscle fiber types, slow twitch and fast twitch, during voluntary (transsynaptic) activity, which is demonstrated after artificial nerve stimulation. The review includes current knowledge of muscle fiber transformation as an adaptive response to increased usage either by electrical stimulation or by transsynaptic neuronal activity. The metabolic adaptation related to increased endurance is reviewed with special reference to effects on muscle fibers. The increase in strength as a result of high resistance training is mainly the result of increased muscle cross-section. Whether this is solely the result of an increase in size of individual fibers or an increased fiber number is a controversial matter.

Influence of spinal cord stimulation upon myosin light chain and tropomyosin subunit expression in a fast muscle (posterior latissimus dorsi) of the chick embryo

Latissimus dorsi muscles of the chick consist of a slow (ALD) and a fast (PLD) muscle. The influence of chronic spinal cord stimulation in the chick embryo upon the expression of myosin light chains and tropomyosin subunits was investigated. Early in development the two muscles exhibited the same ratio of alpha- and beta-tropomyosin subunits. Later, in the slow muscle the ratio beta:alpha decreased and in chicken the amounts of the two components were about the same. In the fast muscle, the alpha-subunit increased and reached 66% in young chicken. In the fast muscle, the alpha-subunit increased and reached 66% in young chicken. In the In the early stages of embryonic development, both muscles accumulated slow and fast light chains. However, in ALD the amount of slow light chains was greater than that of fast light chains and the reverse was observed in PLD muscle. Later during development, the slow components decreased in PLD while the fast components increased; the reverse was observed in ALD muscle. The fast myosin LC3f has been detected in 18-day-old embryonic PLD. Chronic spinal cord stimulation at a low rhythm was performed from day 10 of embryonic development to day 15 or 16. In both muscles from spinal cord-stimulated embryos, the beta-tropomyosin subunit was lower than in control embryos. In ALD, the pattern of light chains was unaffected by chronic stimulation while in PLD muscle the slow and fast components were modified. In particular the ratio LCs:LCf was increased in spinal cord-stimulated embryos with regard to controls.

Mechanisms underlying the asynchronous replacement of myosin light chain isoforms during stimulation-induced fibre-type transformation of skeletal muscle

During the fibre-type transformation induced by chronic electrical stimulation of rabbit fast-twitch muscle, replacement of the fast forms of the two classes of myosin light chain by their slow isoforms occurs asynchronously. Studies of total cellular myosin light chains and of the slow-to-fast transition now justify the conclusion that the asynchrony is due to switching between the expression of fast and slow genes for the two light chain classes at sequential stages of the transformation process.

Neural control of phenotypic expression in mammalian muscle fibers

In this review, the present knowledge about the mechanisms involved in the control of the phenotypic expression of mammalian muscle fibers is summarized. There is a discussion as to how the activity imposed on the muscle fibers by the motoneuron finally induces in the muscle cells the expression of those genes that define its particular phenotype. The functional and molecular heterogeneity of skeletal muscle is thus defined by the existence of motor units with varied function, while the homogeneity of muscle fibers belonging to the same motor unit is yet another indication of the importance of activity in the control of gene expression of the mammalian muscle fiber.

Autotransplantation of skeletal muscle into myocardium

In a series of 15 studies in dogs, sternocleidomastoid muscle was used to replace deficits created in left ventricular myocardium and sternohyoid muscle was used to replace portions of right myocardial wall. The five right ventricular autotransplants resulted in a 100% surgical success rate, with animals electively killed between 3 and 55 weeks after surgery. In 10 left ventricular studies excision of areas varying from 12 X 46 mm to 30 X 60 mm and incisions of from 40 mm to 70 mm in length were performed. Left ventricular studies resulted in a 60% surgical success rate, with clinically healthy animals being killed for study between 2 weeks and 50 weeks after surgery. Animals surviving the critical surgical recovery period showed no loss of weight or changes in activity. Gross findings at autopsy confirmed the viability of the skeletal muscle transplants. Borders were well healed and the grafted tissue was pliable. Histologic studies suggest that revascularization of skeletal muscle occurred from the myocardial side, and that there were healthy myocardial and skeletal muscle fibers at the junction, with evidence of regeneration.

Functionality of muscle proteins in gelation mechanisms of structured meat products

Recent advances in muscle biology concerning the discoveries of a large variety of proteins have been described in this review. The existence of polymorphism in several muscle proteins is now well established. Various isoforms of myosin not only account for the difference in physiological functions and biochemical activity of different fiber types or muscles, but also seem to differ in functional properties in food systems. The functionality of various muscle proteins, especially myosin and actin in the gelation process in modal systems which simulate structured meat products, is discussed at length. Besides, the role of different subunits and subfragments of myosin molecule in the gelation mechanism, and the various factors affecting heat-induced gelation of actomyosin in modal systems are also highlighted. Finally, the areas which need further investigation in this discipline have been suggested.

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.

Effect of denervation at birth on the development of skeletal muscle cell types in the rat

In the newborn rat all cells of soleus, extensor digitorum longus (EDL), and tibialis anterior (TA) muscles stained for fast troponin I. A proportion of the cells, that was much higher in the soleus, also stained for slow troponin I. Fast and slow troponin I were segregated in different cell types in all three muscles 10 to 12 days after birth. No subsequent changes in the distribution of the two forms of troponin I occurred with further growth of EDL and TA muscles. The number of type I cells in soleus steadily increased with increasing age to 24 weeks. Three weeks after denervation at birth, almost all cells in soleus muscle stained for fast troponin I but less than 5% stained significantly dark for slow troponin I. All cells stained for myosin ATPase after alkaline preincubation, but very few after acid preincubation. Three weeks after denervation of EDL and to a lesser extent with TA muscle, fast and slow troponin I were still segregated in different cells. After alkaline preincubation all cells stained equally dark for myosin ATPase but only those positive for slow troponin I stained for myosin ATPase after acid preincubation.

The effect of cross-innervation on the tropomyosin composition of rabbit skeletal muscle

Soleus, semitendinosus and crureus muscles of the rabbit were found to contain alpha- and beta-tropomyosin subunits and additional forms that have been provisionally designated gamma and delta. Extensor digitorum longus and psoas muscles contained only alpha and beta subunits, the relative proportions of which varied between single fibres of psoas muscle. On cross-innervation of rabbit soleus and extensor digitorum longus muscles, the fraction of the total tropomyosin present as the beta subunit remained constant. The relative proportions of alpha, gamma and delta subunits changed as would be expected from the change in speed that occurred.

Changes in transcriptional activity of chronically stimulated fast twitch muscle

mRNAs extracted from rabbit soleus, normal and 28-day, indirectly stimulated tibialis anterior muscles were translated in an in vitro system. Analysis for translation products by 2-dimensional electrophoresis showed fast myosin light chains in tibialis anterior, and slow myosin light chains in soleus muscle. The stoichiometry of the in vitro translated light chains varies from that seen in normal fast and slow twitch muscles. The stimulated muscle contained mRNA coding, both for fast and slow myosin light chains, although the pattern of slow myosin light chains appears not to be complete at this point of time of the transformation process.

Polymorphism of myofibrillar proteins of rabbit skeletal-muscle fibres. An electrophoretic study of single fibres

Rabbit predominantly fast-twitch-fibre and predominantly slow-twitch-fibre skeletal muscles of the hind limbs, the psoas, the diaphragm and the masseter muscles were fibre-typed by one-dimensional polyacrylamide-gel electrophoresis of the myofibrillar proteins of chemically skinned single fibres. Investigation of the distribution of fast-twitch-fibre and slow-twitch-fibre isoforms of myosin light chains and the type of myosin heavy chains, based on peptide 'maps' published in Cleveland. Fischer, Kirschner & Laemmli [(1977) J. Biol. Chem. 252, 1102-1106], allowed a classification of muscle fibres into four classes, corresponding to histochemical types I, IIA, IIB and IIC. Type I fibres with a pure slow-twitch-type of myosin were found to be characterized by a unique set of isoforms of troponins I, C and T, in agreement with the immunological data of Dhoot & Perry [(1979) Nature (London) 278, 714-718], by predominance of the beta-tropomyosin subunit and by the presence of a small amount of an additional tropomyosin subunit, apparently dissimilar from fast-twitch-fibre alpha-tropomyosin subunit. The myofibrillar composition of type IIB fast-twitch white fibres was the mirror image of that found for slow-twitch fibres in that the fast-twitch-fibre isoforms only of the troponin subunits were present and the alpha-tropomyosin subunit predominated. Type IIA fast-twitch red fibres showed a troponin subunit composition identical with that of type IIB fast-twitch white fibres. On the other hand, a unique type of myosin heavy chains was found to be associated with type IIA fibres. Furthermore, the myosin light-chain composition of these fibres was invariably characterized by a small amount of LC3F light chain and by a pattern that was either a pure fast-twitch-fibre light-chain pattern or a hybrid LC1F/LC2F/LC3F/LC1Sb light-chain pattern. By these criteria type IIA fibres could be distinguished from type IIC intermediate fibres, which showed coexistence of fast-twitch-fibre and slow-twitch-fibre forms of myosin light chains and of troponin subunits.

A radioimmunoassay method for quantification of alpha-tropomyosin in heart homogenates

A new, extremely sensitive, solid phase radioimmunoassay has been developed to quantify tropomyosin levels in heart tissue homogenates. Specific antibody was coupled to Sepharose 4B and saturation levels of [125I]tropomyosin bound. Release of radiolabel into the supernatant portion occurred when heart homogenates, authentic tropomyosin or tropomyosin in the presence of homogenate were added to these immunobeads. Quantification of the amount of tropomyosin was based on the level of release effected by standard tropomyosin with and without homogenate. The assay was determined to be highly specific and sensitive for tropomyosin. Picomolar quantities of the protein were readily detectable. Linearity extended well over the range of 5-200 ng tropomyosin in the homogenate. The method can be applied to other proteins for quantification during embryonic development.

Dynamic aspects of structural proteins in vertebrate skeletal muscle

In this review, our current knowledge on the structural proteins of vertebrate skeletal muscle is briefly outlined. Structural proteins include the contractile proteins (actin and myosin), the major regulatory proteins (troponin and tropomyosin), the minor regulatory proteins (M-protein, C-protein, F-protein, I-protein, and actinins), and the scaffold proteins (connectin, desmin, and Z-protein). In addition, the relative turnover rates of the muscle proteins (M-protein greater than or equal to troponin greater than soluble protein as a whole greater than tropomyosin not equal to alpha-actinin greater than myosin greater than 10S-actinin greater than actin) are discussed. The changes in the turnover of muscle proteins are compared in denervated and dystrophic muscles. The properties of the various proteases in muscle, including alkaline protease, calcium-activated neutral protease (CANP), and acidic protease (cathepsins), and the structural alterations of myofibrils by these proteases are also described. Finally, the role of proteases and their inhibitors in diseased muscle is summarized, with focus on CANP and its inhibitors, leupeptin and E-64.

The reorganization of subcellular structure in muscle undergoing fast-to-slow type transformation. A stereological study

Transformation of fast-twitch into slow-twitch skeletal muscle was induced in adult rabbits by chronic low-frequency stimulation and studied at the ultrastructural level. With the use of stereological techniques, a time course was established for changes in mitochondrial volume, sarcotubular system, and Z-band thickness for periods of stimulation ranging from 6 h to 24 weeks. T-tubules, terminal cisternae, and sarcoplasmic reticulum decreased at an early stage and reached levels typical of slow muscle after only 2 weeks of stimulation. Transformation of Z-band structure took place between 1 1/2 and 3 weeks after the onset of stimulation. Mitochondrial volume increased several fold over the first 3 weeks of stimulation, and fell rapidly after 7 weeks, although it still remained above the levels typical of slow muscle. Although there was no sign of degradation and regeneration of the muscle fibers themselves, considerable structural reorganization was evident at the subcellular level after 1 week of stimulation. The fibers passed through a less well organized transitional stage in which fibers could not be assigned to a normal ultrastructural category. After 3 weeks all of the stimulated fibers could be assigned to the normal slow-twitch category although some subcellular irregularities persisted even after 24 weeks. The ultrastructural alterations are discussed in relation to functional and biochemical changes in the whole muscle.

The adaptive response of skeletal muscle to increased use

Skeletal muscle undergoes profound changes in morphological, physiological, and biochemical character when subjected to prolonged periods of increased use. Although increased use may be brought about in a variety of ways, the results show consistent features. In particular, endurance exercise and chronic stimulation differ only in degree: the properties which change in response to exercise are also those which change at an early stage of stimulation; the properties which are resistant to change under exercise conditions change only after prolonged stimulation. There is therefore a hierarchy of stability in the properties of skeletal muscle which is revealed in its response to changing functional demands. The adaptive potential of muscle provides a logical framework for understanding neural influences on the emergence of fiber types during muscle development. It is also relevant to the study of pathological conditions which may involve a sustained departure from normal postural and locomotor patterns of activity.