Myosin light chains in normal and electrostimulated cultures of embryonic chicken breast muscle

Effect of electrical stimulation on beta-adrenergic receptor population and cyclic amp production in chicken and rat skeletal muscle cell cultures

Expression of the beta-adrenergic receptor (betaAR) and its coupling to cyclic AMP (cAMP) synthesis are important components of the signaling system that controls muscle atrophy and hypertrophy, and the goal of this study was to determine if electrical stimulation in a pattern simulating slow muscle contraction would alter the betaAR response in primary cultures of avian and mammalian skeletal muscle cells. Specifically, chicken skeletal muscle cells and rat skeletal muscle cells that had been grown for 7 d in culture were subjected to electrical stimulation for an additional 2 d at a pulse frequency of 0.5 pulses/sec and a pulse duration of 200 msec. In chicken skeletal muscle cells, the betaAR population was not significantly affected by electrical stimulation; however, the ability of these cells to synthesize cyclic AMP was reduced by approximately one-half. In contrast, the betaAR population in rat muscle cells was increased slightly but not significantly by electrical stimulation, and the ability of these cells to synthesize cyclic AMP was increased by almost twofold. The basal levels of intracellular cyclic AMP in neither rat muscle cells nor chicken muscle cells were affected by electrical stimulation.

Adult fast myosin pattern and Ca2+-induced slow myosin pattern in primary skeletal muscle culture

A primary muscle cell culture derived from newborn rabbit muscle and growing on microcarriers in suspension was established. When cultured for several weeks, the myotubes in this model develop the completely adult pattern of fast myosin light and heavy chains. When Ca2+ ionophore is added to the culture medium on day 11, raising intracellular [Ca2+] about 10-fold, the myotubes develop to exhibit properties of an adult slow muscle by day 30, expressing slow myosin light as well as heavy chains, elevated citrate synthase, and reduced lactate dehydrogenase. The remarkable plasticity of these myotubes becomes apparent, when 8 days after withdrawal of the ionophore a marked slow-to-fast transition, as judged from the expression of isomyosins and metabolic enzymes, occurs.

Electrical stimulation of contractile activity accelerates growth of cultured neonatal cardiocytes

An electrical stimulation system was designed to regulate synchronized contractile activity of neonatal rat cardiocytes and to examine the effects of mechanical contraction on cardiocyte growth. Continuous electrical stimulation at a pulse duration of 5 milliseconds and frequency of 3 Hz resulted in a time-dependent accumulation of cell protein that reached 34% above initial values, as measured by the protein-to-DNA ratio. The growth response did not occur using voltage amplitudes that were subthreshold for contraction and was independent of contraction frequencies set at > or = 0.5 Hz. The RNA-to-DNA ratio increased in parallel to cell protein, indicating that the capacity for protein synthesis was enhanced by contraction. Rates of 28S rRNA synthesis were accelerated twofold in contracting cardiocytes. By comparison, protein and RNA accumulation did not occur in electrically stimulated cardiocytes in which contraction was blocked by either 10 mumol/L verapamil or by 5 mmol/L 2,3-butanedione monoxime, an inhibitor of actomyosin crossbridge cycling. Electrical stimulation of cardiocyte contraction did not enhance alpha-cardiac actin or myosin heavy chain (alpha+beta) mRNA transcript levels relative to 28S rRNA during the period of rapid growth that occurred over the first 48 hours. It is concluded that (1) electrical stimulation of contraction accelerates cardiocyte growth and RNA accumulation, (2) mechanical contraction is involved in regulating the growth of electrically stimulated cardiocytes, and (3) the levels of alpha-actin and myosin heavy chain mRNA increase in proportion to rRNA during the growth of contracting cardiocytes.

Normal expression of myosin fast alkali light chain 3 in the hindlimb muscle of chick embryos paralyzed with curare

We have used a monoclonal antibody (Mab) raised against the fast alkali light chains of quail pectoral muscle myosin to study the expression of MLC1f and MLC3f in the hindlimb muscle of a staged series of control chick embryos and 16-day embryos that had been paralyzed with curare. The Mab (QBM-2) is highly specific for the fast myosin alkali light chains of chick, hamster, and human muscle myosin. On Western blots, MLC1f is detected in hindlimb actomyosin at all of the stages examined, whereas MLC3f cannot be detected until Embryonic Day 14 (E14). Most of the E16 embryos that had been paralyzed with curare since E4 express detectable levels of both MLC1f and MLC3f in their hindlimb muscles, even though embryos incubated under these conditions do not exhibit spontaneous limb movements. Contrary to other reports, our results demonstrate that muscle contraction is not required for the accumulation of MLC3f. In light of our previous finding that innervation is essential for MLC3f accumulation in limb buds grafted onto the chorioallantoic membrane of chick hosts, these results suggest that some neural influence other than contraction, possibly a trophic factor, may play a role in the developmentally regulated expression of MLC3f in avian limb muscle.

Effects of electrically induced contractile activity on cultured embryonic chick breast muscle cells

Development of chicken breast muscle is characterized by the sequential appearance of six electrophoretically distinct myosin heavy chain (HC) isoforms. Cultured secondary myotubes, derived from 12-day embryonic chick breast muscle, mainly express the early embryonic HC isoform HCemb/e, normally present in 8-day embryonic breast muscle, and the two fast light chain isoforms LC1f and LC2f. Direct low-frequency (2.5 Hz) stimulation of these myotubes via platinum electrodes leads to a shift in myosin HC expression with increases in the late embryonic HC isoform HCemb/l amounting to 35% of total HC in 19-day-stimulated cultures. Measurements of 35S-methionine incorporation and immunohistochemical analyses demonstrate increases in LC3f. This increase is also seen at the mRNA level. These results indicate that induced contractile activity promotes myotube maturation in vitro. The observation that chronic stimulation enhances the expression of the slow isoform LC2s at the RNA, as well as the protein level, suggests an additional effect consisting of a fast-to-slow change in phenotype expression. In view of the fact that muscle maturation and phenotype expression is under neural control during development in vivo, our results on directly stimulated, aneural myotubes indicate that neurally transmitted contractile activity may be an important factor in modulating phenotype expression of secondary myotubes.

Paralysis of innervated cultured human muscle fibers affects enzymes differentially

Increased accumulation of muscle-specific isozyme (MSI) of creatine kinase (CK), lactate dehydrogenase (LDH), glycogen phosphorylase (GP), and phosphoglycerate mutase (PGAM) occurs with development and indicates muscle fiber maturation. The expression of MSIs of those four enzymes is greatly enhanced in innervated-contracting as compared to noninnervated and noncontracting cultured human muscle fibers. We have now studied the effect of contractile activity on developmental accumulation of MSIs in innervated-contracting, innervated-paralyzed (2 microM tetrodotoxin for 30 days), and noninnervated-noncontracting cultured human muscle fibers. Muscle acetylcholinesterase (AChE) and total enzyme activities were also studied under the same conditions. We observed a different dependency on contractile activity between total enzymatic activities of CK, LDH, and AChE, which were substantially reduced after paralysis, and GP and PGAM, which were unchanged. The expression of MSIs of CK, GP, PGAM, and LDH was always significantly increased in innervated as compared to noninnervated fibers. While the expression of MSIs of GP and PGAM was the same in contracting-innervated and paralyzed-innervated muscle fibers, the expression of MSIs of CK and LDH in paralyzed-innervated muscle fibers was very slightly decreased as compared to their contracting-innervated controls. Our studies demonstrate that in human muscle: (1) total enzymatic activities and the expression of MSIs of GP and PGAM are regulated by neuronal effect(s); (2) total enzymatic activities of CK, LDH, and AChE depend mainly on muscle contractile activity; and (3) MSIs of CK and LDH are regulated predominantly by neuronal factors and to a much lesser degree by muscle contractile activity.

Differential expression of creatine kinase and phosphoglycerate mutase isozymes during development in aneural and innervated human muscle culture

Several enzymes that occur in multimolecular forms undergo transitions during myogenesis. Studies of such developmentally regulated isozymes (e.g. creatine kinase) indicate that muscle cells, cultured in the absence of neural tissue never develop fully mature isozyme patterns, but continue to express large amounts of 'housekeeping' isozymes that are characteristically present in fetal muscle. We studied two developmentally controlled isozymes, creatine kinase (CK) and phosphoglycerate mutase (PGAM) in normal human muscle, both aneurally cultured and co-cultured with fetal mouse spinal cord complex. Innervated cultures attain a greater degree of maturity than non-innervated cultures, as revealed by light and electron microscopy, showing well-developed sarcomeres and motor endplates after several weeks in vitro. During early stages of muscle regeneration in co-culture, characteristic fetal isozyme patterns of CK-BB and PGAM-BB activity predominate, as in aneural cultures. The muscle-specific isozymes (CK-MM; PGAM-MM) begin to appear as the muscle differentiates, and after 2-3 months in co-culture only, virtually all enzyme activity is due to the muscle-specific forms of CK and PGAM, as is normally observed in mature skeletal muscle in vivo.

Effects of electrical stimulation and tetrodotoxin paralysis on expression of muscle-specific isozymes of four enzymes in aneurally cultured embryonic rat muscle

We studied the effect of electrical stimulation and a sodium channel blocker (tetrodotoxin) on the expression of muscle-specific isozymes of creatine kinase, glycogen phosphorylase, phosphoglycerate mutase, and lactate dehydrogenase in aneurally cultured embryonic rat muscle. Muscle contractile activity slightly accelerated the accumulation of muscle-specific isozyme of creatine kinase in early cultures (4 days of experiment), but no increase in the expression of muscle-specific isozymes of any enzyme was present in older cultures (11 days of experiment). We conclude that muscle contractile activity is not a main regulator of isozyme maturation in this system.

Altered synthesis of myosin light chains is associated with contractility in cultures of differentiating chick embryo breast muscle

Cultured chick embryo skeletal muscle cells normally synthesize only the embryonic isoform of mysoin. We have found that aneural muscle cultures that become or are provoked into an extremely contractile state will begin to synthesize a pattern of myosin light chains typical of maturing muscle. Immunoblots with neonatal and adult specific monoclonal antibodies did not reveal a corresponding isozyme transition in myosin heavy chain. These results demonstrate a correlation between contractility and the regulation of myosin light chain maturation, and also suggest that the transitions of heavy and light chain synthesis during development do not appear to be under close coordinate regulation.

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.

PC12 cells stimulate slow-myosin light chain 2 synthesis in chick breast muscle culture

Skeletal muscle fibers developing in vitro synthesize predominantly fast-myosin light chains, with a small contribution (less than 10%) from slow-myosin light chain 2. Muscle fibers can be cocultured with a rat adrenal pheochromocytoma-derived nerve cell line (PC12) known to display properties similar to sympathetic neurons. PC12 cells cultured alone synthesize catecholamines and respond to nerve growth factor by synthesizing acetylcholine and extending neurite structures. They also synthesize significant amounts of acetylcholine in the presence of nonneuronal cell types, including muscle. When cocultures of skeletal muscle fibers and PC12 cells are established, the muscle cells respond with an increased level of slow light chain 2 synthesis. Myosin light chains were identified by two-dimensional gel electrophoresis and immunoblotting with an antiserum specific to slow light chain 2.

Axonal transport blockade and denervation have qualitatively different effects upon skeletal muscle metabolism

The activity and isoenzyme pattern of muscle lactic dehydrogenase (LDH) was measured at different times after axonal transport blockade by colchicine or after denervation. After denervation, total LDH activity decreased and the isoenzyme pattern was altered, LDH-1 being the most affected form. In contrast, after axonal transport blockade there was a decrease in LDH activity but the isoenzyme pattern was not modified. Denervation abolishes both nerve-evoked muscle activity and the release of neuro trophic substances from the nerve whereas colchicine blocks axonal transport without affecting the nerve capacity to conduct action potentials or neuromuscular transmission. It is then concluded that nerve-evoked muscle activity is the most important factor in the regulation of muscle LDH isoenzyme distribution. On the other hand, muscle metabolism can also be regulated by axonally transported molecules. The results presented here show that there is a qualitative difference between the effects of denervation and those of axonal transport blockade upon the muscle, since only denervation altered the isoenzyme pattern of muscle LDH.

Differentiation of muscle fiber types in aneurogenic brachial muscles of the chick embryo

Cross-reinnervation studies performed ex ovo with newly hatched chicks demonstrate that peripheral motor neurons control the phenotypic characteristics of avian muscles. The present experiments were designed to determine whether or not nerves play a similar role during the initial expression of muscle fiber types. Previous experiments indicated that differentiation of specific fiber types occurs during the first week of embryogenesis, temporally coincident with the penetration of nerves within muscle masses. These observations suggested that peripheral nerves may be associated with the initial differentiation of fiber types. To test this hypothesis directly, anterior limb buds of the chick embryo were rendered aneurogenic by deletion of the brachial segment of the neural tube. To ensure a completely aneurogenic environment for developing brachial muscles, surgery was performed at day 2 in ovo before the exit of ventral root fibers. Experimental and control embryos from Stage (St) 25 (4.5 d) through St 45 (19d) were analyzed histochemically by a silver-cholinesterase reaction to detect nerves and by the myosin ATPase reaction, following alkali and acid preincubation, to determine the fiber type composition of the muscles. In addition, the total volume of aneurogenic and control muscles was compared. Results demonstrate that the characteristic myosin ATPase profiles of individual aneurogenic and innervated (control) muscles were identical throughout the entire period analyzed. Therefore, we conclude that these enzymic profiles are endogenously expressed and are not under neuronal control during early embryogenesis. Furthermore, the entire sequence of events from the migration of myogenic cells to the anterior limb bud through the division of the primary muscle masses to form individual brachial muscles proceeded on schedule in the absence of nerves. Since the growth of aneurogenic muscles was impaired, we conclude that during embryogenesis peripheral motor nerves are necessary initially for the proper growth of muscles and ultimately, for their survival. They are not involved, however, with either the initial formation or initial differentiation of individual brachial muscles.

Isoforms of heavy and light chains of cardiac myosins from rat and rabbit

The light chains of myosin from atrial and ventricular tissues from rat and rabbit were examined by one- and two-dimensional polyacrylamide gel electrophoresis. The myosin heavy chains were electrophoretically isolated, digested after denaturation in sodium dodecyl sulfate with papain and proteinase from S. aureus V8, and the resulting peptides resolved in one-dimensional gel electrophoresis. The peptide patterns of myosin heavy chains from atrial and ventricular tissues of adult rabbits were different, indicating differences in their primary structures. No such differences could be detected in a total of around 180 peptides produced by the two proteinases from the myosin heavy chains of adult rat atrial and ventricular tissues. With regard to light chains, the same migration pattern was observed for atrial and ventricular tissues from both rat and rabbit. The atrial light chains ALC1 and ALC2 migrated with molecular weights lying between those of the ventricular light chains VLC1 and VLC2. In two-dimensional electrophoresis, the corresponding light chains from rat and rabbit co-migrated. An additional light chain was observed in foetal ventricles, which exhibited identical electrophoretic properties to ALC1 from adult atrial tissues. In rat myofibrillar preparations from atrium and ventricle, an unidentified protein (x) occurred in the region of light chain-1 but with a more acidic isoelectric point, which seems to be related to the developmental stage of these tissues and which could not be detected in rabbit heart tissues or in any skeletal muscles.

Two major allelic forms of myosin light chain-1 in strains of normal and dystrophic chickens

Evidence is presented from electrophoresis and peptide-mapping for the existence of two major allelic forms of myosin light chain-1 in the fast white muscle fibers of domestic chickens. One form predominates in birds of White Leghorn stock, the other in birds of New Hampshire Red stock. The two light chain-1 forms were invariant during development. Variability was not detected in light chains-2 or -3. The distribution of the two forms in two strains homozygous for the am gene for muscular dystrophy--Connecticut dystrophic and line 413--and their controls, White Leghorn and line 412, respectively, while clearly unrelated to avian dystrophy, emphasizes the heterogeneity in background genes of these non-inbred lines and indicates caution in their use in studies of avian dystrophy.

Distribution and properties of myosin isozymes in developing avian and mammalian skeletal muscle fibers

Isozymes of myosin have been localized with respect to individual fibers in differentiating skeletal muscles of the rat and chicken using immunocytochemistry. The myosin light chain pattern has been analyzed in the same muscles by two-dimensional PAGE. In the muscles of both species, the response to antibodies against fast and slow adult myosin is consistent with the speed of contraction of the muscle. During early development, when speed of contraction is slow in future fast and slow muscles, all the fibers react strongly with anti-slow as well as with anti-fast myosin. As adult contractile properties are acquired, the fibers react with antibodies specific for either fast or slow myosin, but few fibers react with both antibodies. The myosin light chain pattern slow shows a change with development: the initial light chains (LC) are principally of the fast type, LC1(f), and LC2(f), independent of whether the embryonic muscle is destined to become a fast or a slow muscle in the adult. The LC3(f), light chain does not appear in significant amounts until after birth, in agreement with earlier reports. The predominance of fast light chains during early stages of development is especially evident in the rat soleus and chicken ALD, both slow muscles, in which LC1(f), is gradually replaced by the slow light chain, LC1(s), as development proceeds. Other features of the light chain pattern include an "embryonic" light chain in fetal and neonatal muscles of the rat, as originally demonstrated by R.G. Whalen, G.S. Butler- Browne, and F. Gros. (1978. J. Mol. Biol. 126:415-431.); and the presence of approximately 10 percent slow light chains in embryonic pectoralis, a fast white muscle in the adult chicken. The response of differentiating muscle fibers to anti-slow myosin antibody cannot, however, be ascribed solely to the presence of slow light chains, since antibody specific for the slow heavy chain continues to react with all the fibers. We conclude that during early development, the myosin consists of a population of molecules in which the heavy chain can be associated with a fast, slow, or embryonic light chain. Biochemical analysis has shown that this embryonic heavy chain (or chains) is distinct from adult fast or slow myosin (R.G. Whalen, K. Schwartz, P. Bouveret, S.M. Sell, and F. Gros. 1979. Proc. Natl. Acad. Sci. U.S.A. 76:5197-5201. J.I. Rushbrook, and A. Stracher. 1979. Proc Natl. Acad. Sci. U.S.A. 76:4331-4334. P.A. Benfield, S. Lowey, and D.D. LeBlanc. 1981. Biophys. J. 33(2, Pt. 2):243a[Abstr.]). Embryonic myosin, therefore, constitutes a unique class of molecules, whose synthesis ceases before the muscle differentiates into an adult pattern of fiber types.

Electrophoretic analyses of atrial and ventricular cardiac myosins from foetal and adult rabbits

1. Rabbit cardiac myosins from atrium and ventricle were found to differ in their native form in pyrophosphate gel electrophoresis as well as in their light chain pattern in one- and two-dimensional electrophoretic systems. 2. The myosin light chain pattern in sodium dodecylsulfate gel electrophoresis differs between atrial and ventricular tissues in the mammalians (rabbit, dog, human, pig, sheep and rat) but not in the chicken. 3. In foetal rabbit ventricle an additional light chain is observed which, in two-dimensional electrophoresis with adult cardiac light chains, was found to be the same as atrial light chain-1.