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

Myofibril assembly and the roles of the ubiquitin proteasome system

De novo assembly of myofibrils in vertebrate cross-striated muscles progresses in three distinct steps, first from a minisarcomeric alignment of several nonmuscle and muscle proteins in premyofibrils, followed by insertions of additional proteins and increased organization in nascent myofibrils, ending with mature contractile myofibrils. In a search for controls of the process of myofibril assembly, we discovered that the transition from nascent to mature myofibrils could be halted by inhibitors of three distinct functions of the ubiquitin proteasome system (UPS). First, inhibition of pathway to E3 Cullin ligases that ubiquitinate proteins led to an arrest of myofibrillogenesis at the nascent myofibril stage. Second, inhibition of p97 protein extractions of ubiquitinated proteins led to a similar arrest of myofibrillogenesis at the nascent myofibril stage. Third, inhibitors of proteolytic action by proteasomes also blocked nascent myofibrils from transitioning to mature myofibrils. In contrast, inhibitors of autophagy or lysosomes did not affect myofibrillogenesis. To probe for differences in the effects of UPS inhibitors during myofibrillogenesis, we analyzed by fluorescence recovery after photobleaching the exchange rates of two selected sarcomeric proteins (muscle myosin II heavy chains and light chains). In the presence of p97 and proteasomal inhibitors, the dynamics of each of these two myosin proteins decreased in the nascent myofibril stage, but were unaffected in the mature myofibril stage. The increased stability of myofibrils occurring in the transition from nascent to mature myofibril assembly indicates the importance of dynamics and selective destruction in the muscle myosin II proteins for the remodeling of nascent to mature myofibrils.

In vitro Differentiation of Functional Human Skeletal Myotubes in a Defined System

In vitro human skeletal muscle systems are valuable tools for the study of human muscular development, disease and treatment. However, published in vitro human muscle systems have so far only demonstrated limited differentiation capacities. Advanced differentiation features such as cross-striations and contractility have only been observed in co-cultures with motoneurons. Furthermore, it is commonly regarded that cultured human myotubes do not spontaneously contract, and any contraction has been considered to originate from innervation. This study developed a serum-free culture system in which human skeletal myotubes demonstrated advanced differentiation. Characterization by immunocytochemistry, electrophysiology and analysis of contractile function revealed these major features: A) well defined sarcomeric development, as demonstrated by the presence of cross-striations. B) finely developed excitation-contraction coupling apparatus characterized by the close apposition of dihydropyridine receptors on T-tubules and Ryanodine receptors on sarcoplasmic reticulum membranes. C) spontaneous and electrically controlled contractility. This report not only demonstrates an improved level of differentiation of cultured human skeletal myotubes, but also provides the first published evidence that such myotubes are capable of spontaneous contraction. Use of this functional in vitro human skeletal muscle system would advance studies concerning human skeletal muscle development and physiology, as well as muscle-related disease and therapy.

Focal adhesion kinase is essential for costamerogenesis in cultured skeletal muscle cells

A central question in muscle biology is how costameres are formed and become aligned with underlying myofibrils in mature tissues. Costameres are composed of focal adhesion proteins, including vinculin and paxillin, and anchor myofibril Z-bands to the sarcolemma. In the present study, we investigated the process of costamere formation ("costamerogenesis") in differentiating primary mouse myoblasts. Using vinculin and paxillin as costameric markers, we found that two additional focal adhesion components, alpha5beta1 integrin and focal adhesion kinase (FAK), are associated with costameres. We have characterized costamerogenesis as occurring in three distinct stages based on the organizational pattern of these costameric proteins. We show that both costamerogenesis and myofibrillogenesis are initiated at sites of membrane contacts with the extracellular matrix and that their maturation is tightly coupled. To test the importance of FAK signaling in these processes, we analyzed cells expressing a dominant negative form of FAK (dnFAK). When cells expressing dnFAK were induced to differentiate, both costamerogenesis and myofibrillogenesis were disrupted although the expression of constituent proteins was not inhibited. Likewise, inhibiting FAK activity by reducing FAK levels using an siRNA approach also resulted in an inhibition of costamerogenesis and myofibrillogenesis. The relationship between costamere and myofibril formation was tested further by treating myotube cultures with potassium or tetrodotoxin to block contraction and disrupt myofibril organization. This also resulted in inhibition of costamere maturation. We present a model of costamerogenesis whereby signaling through FAK is essential for both normal costamerogenesis and normal myofibrillogenesis which are tightly coupled during skeletal myogenesis.

Myocyte differentiation generates nuclear invaginations traversed by myofibrils associating with sarcomeric protein mRNAs

Certain types of cell both in vivo and in vitro contain invaginated or convoluted nuclei. However, the mechanisms and functional significance of the deformation of the nuclear shape remain enigmatic. Recent studies have suggested that three types of cytoskeleton, microfilaments, microtubules and intermediate filaments, are involved in the formation of nuclear invaginations, depending upon cell type or conditions. Here, we show that undifferentiated mouse C2C12 skeletal muscle myoblasts had smoothsurfaced spherical or ellipsoidal nuclei, whereas prominent nuclear grooves and invaginations were formed in multinucleated myotubes during terminal differentiation. Conversion of mouse fibroblasts to myocytes by the transfection of MyoD also resulted in the formation of nuclear invaginations after differentiation. C2C12 cells prevented from differentiation did not have nuclear invaginations, but biochemically differentiated cells without cell fusion exhibited nuclear invaginations. Thus, biochemical differentiation is sufficient for the nuclear deformation. Although vimentin markedly decreased both in the biochemically and in the terminally differentiated cells, exogenous expression of vimentin in myotubes did not rescue nuclei from the deformation. On the other hand, non-striated premyofibrils consisting of sarcomeric actinmyosin filament bundles and cross-striated myofibrils traversed the grooves and invaginations. Time-lapse microscopy showed that the preformed myofibrillar structures cut horizontally into the nuclei. Prevention of myofibril formation retarded the generation of nuclear invaginations. These results indicate that the myofibrillar structures are, at least in part, responsible for the formation of nuclear grooves and invaginations in these myocytes. mRNA of sarcomeric proteins including myosin heavy chain and alpha-actin were frequently associated with the myofibrillar structures running along the nuclear grooves and invaginations. Consequently, the grooves and invaginations might function in efficient sarcomeric protein mRNA transport from the nucleus along the traversing myofibrillar structures for active myofibril formation.

Lengthening of muscle during distraction osteogenesis

Chronic lengthening of immobilized, neurally intact muscle leads to the addition of sarcomeres in series. Confirmation of a similar adaptation during distraction osteogenesis is crucial for providing a rationale for a successful outcome of the intervention. When distraction osteogenesis (at < or = 1.4 mm/day) is done in skeletally immature animals, muscle adapts by creating a longer and functionally intact muscle. This is achieved through muscle growth, the proliferation of myogenic cells ultimately leading to serial addition of sarcomeres. When distraction osteogenesis is done in skeletally mature animals, however, the same distraction regimen leads to a lengthened muscle that has significant fibrosis and weakness, the latter possibly a result of partial denervation. Despite a modest but significant elevation of local insulinlike growth factor-1 in the lengthened muscles from adult animals, muscle growth is not adequate and leads to a loss of function. In adult animals, the distraction osteogenesis-induced increase in insulinlike growth factor-1 is insufficient to facilitate muscle growth during lengthening. Muscle can be targeted for future therapeutic use of insulinlike growth factor-1; however, such a therapy also may lead to increased fibrosis.

Formation of sarcomeres in developing myotubes: role of mechanical stretch and contractile activation

In a series of experiments, cultured myotubes were exposed to passive stretch or pharmacological agents that block contractile activation. Under these experimental conditions, the formation of Z lines and A bands (morphological structures, resulting from the specific structural alignment of sarcomeric proteins, necessary for contraction) was assessed by immunofluorescence. The addition of an antagonist of the voltage-gated Na(+) channels [tetrodotoxin (TTX)] for 2 days in developing rat myotube cultures led to a nearly total absence of Z lines and A bands. When contractile activation was allowed to resume for 2 days, the Z lines and A bands reappeared in a significant way. The appearance of Z lines or A bands could not be inhibited nor facilitated by the application of a uniaxial passive stretch. Electrical stimulation of the cultures increased sarcomere assembly significantly. Antagonists of L-type Ca(2+) channels (verapamil, nifedipine) combined with electrical stimulation led to the absence of Z lines and A bands to the same degree as the TTX treatment. Western blot analysis did not show a major change in the amount of sarcomeric alpha-actinin nor a shift in myosin heavy chain phenotype as a result of a 2-day passive stretch or TTX treatment. Results of experiments suggest that temporal Ca(2+) transients play an important factor in the assembly and maintenance of sarcomeric structures during muscle fiber development.

Regulation of cardiac myocyte protein turnover and myofibrillar structure in vitro by specific directions of stretch

We have examined how different degrees (0.5%, 1.0%, 2.5%, 5.0%, and 10.0%) and directions of stretch regulate the turnover and accumulation of contractile proteins in cultured neonatal cardiac myocytes (NCMs). In pulse-chase experiments, stellate-shaped NCMs with random arrays of myofibrils (MFs) exhibited a threshold response to stretch. With respect to unstretched controls, the turnover of the contractile protein pool was suppressed 50% to 100% in stellate NCMs stretched 1.0% to 5.0% and was unaltered in stellate NCMs stretched 0.5% or 10.0%. The posttranslational metabolism of myosin heavy chain (MHC) and actin was regulated in parallel with the total contractile protein pool. The turnover of the cytoplasmic protein pool remained unchanged in response to stretch. NCMs plated onto an aligned matrix of type I collagen expressed an elongated, rod-like cell shape. The MFs of these cells were distributed in parallel with one another along a single unique axis. The tissue-like pattern of organization of these cultures made it possible to assay how specific directions of stretch affected cardiac protein turnover and MF organization. In pulse-chase experiments, stretch in parallel with the MFs did not alter the turnover of the total contractile protein pool, the cytoplasmic protein pool, MHC, or actin. The total cellular concentration of MHC and actin remained constant, and MF alignment was not overtly affected. In contrast, even modest degrees of stretch across the short axis of the MFs suppressed total contractile protein turnover, the turnover of MHC and actin, and promoted the accumulation of these MF subunits. The parallel alignment of MFs deteriorated in myocytes stretched greater than 5%. The characteristic response of aligned myocytes to stretch was not affected by the contractile state of the cells. Isoproterenol (ISO) treatment in concert with stretch in parallel with the MFs modestly accelerated contractile protein turnover. Conversely, contractile protein turnover was suppressed in cells treated with ISO and stretched across the short axis of the MFs. Contractile arrest with nifedipine (NIFED) accelerated total myofibrillar protein turnover. Stretch across the short axis, but not in parallel with the MFs, suppressed protein turnover in cells treated with NIFED. The turnover of the cytosolic proteins remained constant under all conditions assayed. These data suggest that specific directions of stretch may play a crucial role in regulating MF organization and the metabolism of contractile proteins in the cardiac myocyte.

Effect of atrophy and contractions on myogenin mRNA concentration in chick and rat myoblast omega muscle cells

The skeletal rat myoblast omega (RMo) cell line forms myotubes that exhibit spontaneous contractions under appropriate conditions in culture. We examined if the RMo cells would provide a model for studying atrophy and muscle contraction. To better understand how to obtain contractile cultures, we examined levels of contraction under different growing conditions. The proliferation medium and density of plating affected the subsequent proportion of spontaneously contracting myotubes. Using a ribonuclease protection assay, we found that exponentially growing RMo myoblasts contained no detectable myogenin or herculin mRNA, while differentiating myoblasts contained high levels of myogenin mRNA but no herculin mRNA. There was no increase in myogenin mRNA concentration in either primary chick or RMo myotubes whose contractions were inhibited by depolarizing concentrations of potassium (K+). Thus, altered myogenin mRNA concentrations are not involved in atrophy of chick myotubes. Depolarizing concentrations of potassium inhibited spontaneous contractions in both RMo cultures and primary chick myotube cultures. However, we found that the myosin concentration of 6-d-old contracting RMo cells fed medium plus AraC was 11 +/- 3 micrograms myosin/microgram DNA, not significantly different from 12 +/- 4 micrograms myosin/microgram DNA (n = 3), the myosin concentration of noncontracting RMo cells (treated with 12 mM K+ for 6 d). Resolving how RMo cells maintained their myosin content when contraction is inhibited may be important for understanding atrophy.

Embryonic chicken skeletal muscle cells fail to develop normal excitation-contraction coupling in the absence of the alpha ryanodine receptor. Implications for a two-ryanodine receptor system

Two ryanodine receptor (RyR), sarcoplasmic reticulum Ca2+ release channels, alpha and beta, co-exist in chicken skeletal muscles. To investigate a two-RyR Ca2+ release system, we compared electrically evoked Ca2+ transients in Crooked Neck Dwarf (cn/cn) cultured muscle cells, which do not make alpha RyR, and normal (+/?) cells. At day 3 in culture, Ca2+ release in +/? cells required extracellular Ca2+ (Ca2+o), and Ca2+ transients had slow kinetics. At day 5, Ca2+ release was Ca2+o-independent in 40% of the cells, and transients were more rapid. By day 7, all +/? cells had Ca2+o-independent Ca2+ release. Contractions were observed in +/? cells on all days. Ca2+ transients were observed in cn/cn cells on days 3, 5, and 7, but in each case they were Ca2+o-dependent and exhibited slow kinetics. Localized vesiculations, not contractions, occurred in cn/cn cells. By day 10, Ca2+ transients were no longer observed in cn/cn cells even in Ca2+o. Sarcoplasmic reticulum Ca2+ was not depleted, as caffeine induced Ca2+ transients. Thus, in the absence of alpha RyR there is a failure to develop Ca2+o-independent Ca2+ release and contractions and to sustain Ca2+o-dependent release. Moreover, contributions by the alpha RyR cannot be duplicated by the beta RyR alone.

Effects of chronic stimulation with different impulse patterns on the expression of myosin isoforms in rat myotube cultures

In order to study maturation and differentiation of aneural myotubes in vitro, long-term myotube cultures were established from hindlimb musculature of newborn rats. The developmental state of the myotubes was judged by their myosin heavy chain (HC) patterns. Newly formed myotubes only expressed the embryonic isoform, HCemb, older myotubes expressed the neonatal isoform HCneo, as well as the fast adult isoforms HCIIb and HCIId. HCIId increased continuously, reaching a relative concentration of 47% in 37-day-old cultures. The third fast isoform, HCIIa, was not detected and also the slow isoform HCI was absent. Effects of chronic (20 days) electrostimulation were studied by exposing the cultures to various stimulus patterns. Bursts of 250 ms duration at various pulse frequencies were applied at low and high burst frequencies. Although HCemb remained the predominant isoform under all conditions, different stimulus patterns induced specific changes in the patterns of fast and slow HC isoforms. Bursts of 250 ms duration at 15 Hz, 40 Hz, or 100 Hz, repeated every second or every 4 s, induced the expression of slow myosin, i.e., HCl. Bursts of 250 ms duration at 100 Hz, repeated every 100 s, enhanced the expression of HCIId, but not of HCI. Because slow myosin was induced at high burst frequency with low and high pulse rates, we suggest that burst frequency rather than pulse frequency has a specifying effect on myosin expression. Our results show that the basal program of myosin expression during myogenesis in vitro can be modulated by electrostimulation, suggesting a possible influence of neuromuscular activity on the development of adult fiber types.

Failure to make normal alpha ryanodine receptor is an early event associated with the crooked neck dwarf (cn) mutation in chicken

We have investigated the molecular basis of the Crooked Neck Dwarf (cn) mutation in embryonic chickens. Using biochemical and pharmacological techniques we are unable to detect normal alpha ryanodine receptor (RyR) protein in intact cn/cn skeletal muscle. Extremely low levels of alpha RyR immunoreactivity can be observed in mutant muscles, but the distribution of this staining differs from that in normal muscle and colocalizes with the rough endoplasmic reticulum immunoglobulin binding protein, BiP. This suggests the existence of an abnormal alpha RyR protein in mutant muscle. In day E12 cn/cn muscle the levels of RyR mRNA are reduced by approximately 80%, while the levels of other muscle proteins, including the alpha 1 subunit of the dihydropyridine receptor, the SR Ca(2+)-ATPase, calsequestrin, and glyceraldehyde-3-phosphate dehydrogenase, and their associated mRNAs are essentially normal in cn/cn muscle. There is also a failure to express alpha RyR in cn/cn cerebellar Purkinje neurons. Expression of the beta RyR, a second RyR isoform, is not initiated in normal skeletal muscle until day E18. In cn/cn skeletal muscle significant muscle degeneration has occurred by this time and the beta RyR is found at low levels in only a subset of fibers suggesting the reduced levels of this isoform are a secondary consequence of the mutation. The cardiac RyR isoform is found in cn/cn cardiac muscle, which contracts in a vigorous manner. In summary, a failure to make normal alpha RyR receptor appears to be an event closely associated with the cn mutation and one which may be largely responsible for development of the cn/cn phenotype in embryonic skeletal muscle.

Crooked neck dwarf (cn) mutant chicken skeletal muscle cells in low density primary cultures fail to express normal alpha ryanodine receptor and exhibit a partial mutant phenotype

The Crooked Neck Dwarf (cn) mutation in chickens causes marked changes in intact embryonic skeletal muscle. We have investigated whether the cn/cn phenotype develops in vitro, and if cultured muscle cells are suitable for studies of this mutation. The properties of cn/cn muscle cells maintained in low density primary cultures (6.25 x 10(3) cells/cm2) are described in this report. In normal muscle cells, the alpha ryanodine receptor (RyR) isoform appears prior to, and at greater levels than, the beta RyR, and is detected in mononucleated myocytes. The beta RyR isoform appears within 24 hr after the initiation of myotube formation, which is earlier than anticipated from studies with intact embryonic muscle. Normal alpha RyR protein is not detected in cultured cn/cn muscle cells, whereas the beta RyR, the alpha 1-subunit of the dihydropyridine receptor, the sarcoplasmic reticulum Ca(2+)-ATPase, and calsequestrin are expressed at comparable levels in normal and mutant muscle cells. Calcium transients elicited by electrical stimulation, acetylcholine, and caffeine are similar in normal and cn/cn cultured myotubes and are blocked by ryanodine in both cell types. In addition, comparable L- and T-type calcium currents are observed in normal and mutant muscle cells, suggesting that both the alpha 1-subunit of the dihydropyridine receptor and the beta RyR in mutant muscle cells are functional. Normal and cn/cn muscle cells proliferate and form myotubes in a similar manner. These latter events do not appear to depend on sarcoplasmic reticulum calcium release, as they also occur in normal muscle cells in which calcium release is prevented by chronic treatment with 100 microM ryanodine. Both cn/cn and ryanodine-treated normal muscle cells exhibit morphological changes similar to those observed in intact cn/cn skeletal muscle. Thus, the mutant phenotype observed in ovo is partially expressed under low density culture conditions, and neither beta RyR protein nor its function appear to be capable of preventing the associated changes.

Contractile arrest accelerates myosin heavy chain degradation in neonatal rat heart cells

Mechanical forces influence the growth and metabolism of a variety of cells, including cultured neonatal rat ventricular myocytes. To determine whether mechanical activity affected the synthesis and turnover of myosin heavy chain (MHC) in these striated muscle cells, MHC fractional degradative rates were measured in spontaneously beating cells and in arrested myocytes in which contractile activity was prevented by L-channel blockade (with verapamil, nifedipine, nisoldipine, and diltiazem) or K+ depolarization. MHC degradative rates were measured as the difference between rates of MHC synthesis and accumulation and in pulse-chase biosynthetic labeling experiments. Both methods indicated that contractile arrest markedly increased MHC degradation. Contractile arrest produced by L-channel blockade accelerated MHC degradation to a greater extent than K+ depolarization. The signal transduction pathway linking contractile activity to alterations in MHC degradation did not involve protein kinase C (PKC), because MHC degradation was unaffected by activating PKC in arrested cells or inhibiting PKC in spontaneously beating cells. Chloroquine and E-64 did not suppress the accelerated MHC degradation, suggesting that the rate-limiting step in MHC turnover occurred before degradative processing by cellular proteinases. Using a computer simulation, we hypothesize that the rate-limiting step in MHC turnover preceded (or was coincident with) MHC release from thick filaments. Thus mechanical forces may influence MHC half-life by regulating the rate of myosin disassembly.

The influence of muscle contractile activity versus neural factors on morphologic properties of innervated cultured human muscle

In contrast to aneurally cultured human muscle, which is immature in regard to its morphologic phenotype and only rarely and weakly contracts spontaneously, innervated cultured human muscle fibres have: (1) nearly continuous, d-tubocurarine-inhibitable contractions; (2) well-developed cross-striations, basal lamina, t-tubules, and postsynaptic folds of the neuromuscular junctions; (3) the majority of their nuclei peripheralized; and (4) acetylcholinesterase-positive sites present only at the neuromuscular junctions. To see whether the expression of the muscle morphologic phenotype is induced only by neural factors generated from the spinal cord explants or also by their frequent contractile activity, we paralyzed innervated cultured human muscle fibres with 2 microM tetrodotoxin for four weeks, either from the first day of muscle contractions or following four weeks of muscle contractions. In both experimental designs, by light microscopy tetrodotoxin paralysis abolished cross-striations and caused prominent internalization of muscle nuclei; however, it did not influence the intensity of acetylcholinesterase staining at the neuromuscular junctions. By electron microscopy, there was no difference between paralyzed and contracting muscle fibres in development of t-tubules, basal lamina and postsynaptic folds. Our study demonstrates that in human muscle contractile activity: (1) regulates peripheral migration of nuclei and development of cross-striations; and (2) does not influence development of the neuromuscular junction, basal lamina, and t-tubules, which are mainly regulated by neural influences. This culture model may be useful for studying detailed mechanisms of human muscle fibre development and structural abnormalities in human neuromuscular diseases.

Sarcoplasmic-reticulum biogenesis in contraction-inhibited skeletal-muscle cultures

We have previously shown that inhibition of the spontaneous contractile activity of cultured embryonic-chick skeletal-muscle fibres with tetrodotoxin (TTX) leads to decreased sarcoplasmic-reticulum Ca(2+)-transport rates and steady-state concentrations of the high-energy Ca(2+)-ATPase phosphoenzyme intermediate [Charuk & Holland (1983) Exp. Cell Res. 144, 143-157]. In the present study we used a monoclonal antibody to the Ca(2+)-ATPase to show that there is a decreased amount of enzyme accumulated by contraction-inhibited myotubes. Indirect immunofluorescence microscopy using the monoclonal antibody to the Ca(2+)-ATPase also revealed a disordered subcellular organization of the sarcotubular system in contraction-inhibited myotubes. The biogenesis of sarcoplasmic-reticulum proteins in TTX-paralysed myofibres was studied by labelling cells with [35S]methionine before isolation of the active Ca(2+)-pump membrane fraction. Protein turnover was selectively increased in that fraction from TTX-treated muscle cultures. Electrophoretic analysis and quantitative fluorography confirmed that decreased accumulation of the Ca(2+)-ATPase enzyme in contraction-inhibited myotubes was associated with increased turnover of this protein. The present results demonstrate that biogenesis of the sarcoplasmic-reticulum Ca(2+)-ATPase is regulated by the contractile activity of skeletal-muscle fibres.

Developmental study of the expression of dystrophin in cultured human muscle aneurally and innervated with fetal rat spinal cord

So far there have been no developmental studies including the influences of innervation and contractile activity on the expression of dystrophin in cultured human muscle. We performed immunocytochemical studies of the localization of dystrophin on aneurally cultured non-contracting (AMs) and innervated continuously contracting cross-striated human muscle fibers (ICMs) with fetal rat spinal cord from normal and Duchenne muscular dystrophy (DMD) biopsied muscles. In normal AMs, myoblasts and some immature AMs showed negative staining of dystrophin, but many AMs had a patchy (discontinuous) distribution of dystrophin in the subplasmalemmal region and with some granularity near the sarcolemma and in the deeper cytoplasm. In normal ICMs, dystrophin was localized continuously at the inner aspect of the sarcolemmal membrane and some periodic dense patterns were detected in some areas. Both AMs and ICMs from DMD had negative staining of dystrophin. To investigate the muscle contractile activity on the distribution of dystrophin, we paralyzed ICMs with tetrodotoxin (TTX) for two weeks from the first appearance of muscle contractions. In paralyzed innervated muscles (PIMs), dystrophin remained in a patchy (discontinuous) pattern at the inner aspect of the plasmalemma similar to that in AMs. It is strongly suggested that muscle contractile activity plays an important role in the continuous and even distribution of dystrophin at the sarcolemma during development.

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.

Skeletal muscle growth is stimulated by intermittent stretch-relaxation in tissue culture

Avian pectoralis muscle cells differentiated in vitro are mechanically stimulated by repetitive stretch-relaxation of the cell's substratum using a computerized mechanical cell stimulator device. Initiation of mechanical stimulation increases the efflux of creatine kinase from the cells during the first 8-10 h of activity, but the efflux rate returns to control levels after this time period. Decreased total cell protein content accompanies the temporary elevation of creatine kinase efflux. With continued mechanical stimulation for 48-72 h, total cell protein loss recovers and significantly increases in medium supplemented with serum and embryo extract. Myotube diameters increase and cell hyperplasia occurs in the stimulated cultures. In basal medium without supplements, mechanical activity prevents myotube atrophy but does not lead to cell growth. Mechanically induced growth is accompanied by significant increases in protein synthesis rates. The increases in protein synthesis and accumulation induced by mechanical stimulation are not inhibited by tetrodotoxin but are significantly reduced in basal medium without supplements. Mechanically stimulated cell growth is thus dependent on medium growth factors but independent of electrical activity.

Contraction modulates the capacity for protein synthesis during growth of neonatal heart cells in culture

Neonatal ventricular myocytes that were incubated in a well-defined serum-free medium containing 50 mM KCl did not contract and maintained stable cell size, as assessed by the protein/DNA ratio. The present study utilized KCl-arrested cells to examine the effect of constant rates of synchronous contraction in normal [K+]o (4 mM) as a physiological stimulus for myocyte growth. Cell growth increased following the onset of contraction when measured over 3 days. The rate of protein synthesis was accelerated in parallel by contraction, but the rate of protein degradation remained similar to rates in noncontracting cells. The capacity for protein synthesis was estimated by total RNA content and was increased in contracting as compared with KCl-arrested cells. This increase was accompanied by faster rates of RNA synthesis as determined from the incorporation of [3H]uridine into RNA and the specific activity of the cellular UTP pool. The rate of RNA degradation was accelerated during contraction but the difference between the rates of RNA synthesis and degradation resulted in net RNA accumulation of 49% after 3 days. These data demonstrated that 1) contractile activity stimulated myocyte growth through an increased capacity for protein synthesis and 2) the increased capacity for protein synthesis involved acceleration of the rate of RNA synthesis. Since enhancement of protein synthetic capacity is a common feature of myocyte hypertrophy in vivo and in vitro, this model can be used to examine the regulation of ribosome synthesis during hypertrophic growth.

Early cross-striation formation in twitching Xenopus myocytes in culture

Spontaneous release of neurotransmitter has been demonstrated in various types of synapses. Its physiological significance, however, is still unknown. In nerve-muscle cultures of embryonic Xenopus laevis, we observed that acetylcholine, which is released spontaneously at the synaptic terminal, caused frequent twitches of muscle cells. These muscle cells developed cross-striations earlier than neighboring non-twitching cells. This effect of innervation was unaffected by tetrodotoxin but was blocked by alpha-bungarotoxin. Repeated iontophoretic application of acetylcholine or KCl to muscle cells caused twitches and also accelerated the formation of cross-striations. Thus twitching apparently promotes lateral alignment of myofibrils. It is also known that myosin synthesis is higher in twitching muscle cells. Therefore, successfully innervated twitching muscle cells may have an advantage for faster differentiation over neighboring non-twitching muscle cells. We suggest that spontaneously released transmitter may serve as a mediator for trophic interaction at forming synapses.

Heparin inhibits skeletal muscle growth in vitro

Heparin or heparan sulfate proteoglycan (HeSPG), but not chondroitin sulfate or hyaluronic acid, exerts a pronounced inhibitory effect on muscle growth in vitro, as determined by total protein, myosin accumulation or synthesis, and [3H]thymidine incorporation studies. Primary muscle fibroblast culture growth is also inhibited by heparin but to a substantially lesser degree compared to muscle (30% and over 90% inhibition of growth, respectively). Heparin-induced inhibition of skeletal muscle growth is a consequence of its interaction with a growth factor(s) present in the media used to support myogenesis; heparin-Sepharose column absorbed horse serum can support muscle growth only in the presence of added heparin-binding growth factors like fibroblast growth factor (FGF) or chicken muscle growth factor (CMGF). Furthermore, heparin prevents the binding of iodinated FGF to the myoblast surface. We also show that the extent of muscle growth is a function of the relative amounts of heparin and FGF in culture. Finally, we provide evidence indicating that FGF can combine with endogenously occurring heparin-like components: immobilized FGF binds sodium-[35S]sulfate labeled components secreted in muscle culture conditioned medium, an interaction inhibited by anti-HeSPG antibodies or heparin, but not by other sulfated glycosaminoglycans. Since heparin binding growth factors not only stimulate myoblast proliferation but also actively inhibit the onset of muscle differentiation (G. Spitzz, D. Roman, and A. Strauss (1986). J. Biol. Chem. 261, 9483-9488), their interaction with naturally occurring heparin-like components may be an important physiological mechanism for modulating muscle growth and differentiation in development and regeneration.

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.

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.

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.

Role of stress fiber-like structures in assembling nascent myofibrils in myosheets recovering from exposure to ethyl methanesulfonate

When day 1 cultures of chick myogenic cells were exposed to the mutagenic alkylating agent ethyl methanesulfonate (EMS) for 3 d, 80% of the replicating cells were killed, but postmitotic myoblasts survived. The myoblasts fused to form unusual multinucleated "myosheets": extraordinarily wide, flattened structures that were devoid of myofibrils but displayed extensive, submembranous stress fiber-like structures (SFLS). Immunoblots of the myosheets indicated that the carcinogen blocked the synthesis and accumulation of the myofibrillar myosin isoforms but not that of the cytoplasmic myosin isoform. When removed from EMS, widely spaced nascent myofibrils gradually emerged in the myosheets after 3 d. Striking co-localization of fluorescent reagents that stained SFLS and those that specifically stained myofibrils was observed for the next 2 d. By both immunofluorescence and electron microscopy, individual nascent myofibrils appeared to be part of, or juxtaposed to, preexisting individual SFLS. By day 6, all SFLS had disappeared, and the definitive myofibrils were displaced from their submembranous site into the interior of the myosheet. Immunoblots from recovering myosheets demonstrated a temporal correlation between the appearance of the myofibrillar myosin isoforms and the assembly of thick filaments. The assembly of definitive myofibrils did not appear to involve desmin intermediate filaments, but a striking aggregation of sarcoplasmic reticulum elements was seen at the level of each I-Z-band. Our findings suggest that SFLS in the EMS myosheets function as early, transitory assembly sites for nascent myofibrils.

Selected muscle and nerve extracts contain an activity which stimulates myoblast proliferation and which is distinct from transferrin

Extracts from normal chicken anterior latissimus dorsi and dystrophic pectoralis major muscles and from normal chicken sciatic nerves induce a growth stimulation in chicken and rat myogenic cell cultures. Transferrin is only partially responsible for the observed stimulation since the addition of the extracts to transferrin-saturated cultures induces a further growth response and extracts from which transferrin has been removed by immunoabsorption still retain a substantial portion of their stimulation activity. The active fractions of muscle and nerve extracts display heat, acid, and organic solvent inactivation. Gel filtration of ammonium sulfate fractionated activity from the anterior latissimus dorsi muscle suggests the presence of a growth factor in the molecular weight range of 10,000 to 30,000.

Regulation of myofibrillar accumulation in chick muscle cultures: evidence for the involvement of calcium and lysosomes in non-uniform turnover of contractile proteins

The effect of calcium on myofibrillar turnover in primary chick leg skeletal muscle cultures was examined. Addition of the calcium ionophore A23187 at subcontraction threshold levels (0.38 microM) increased significantly rates of efflux of preloaded 45Ca+2 but had no effect on total protein accumulation. However, A23187 as well as ionomycin caused decreased accumulation of the myofibrillar proteins, myosin heavy chain (MHC), myosin light chain 1f (LC1f), 2f (LC2f), alpha-actin (Ac), and tropomyosin (TM). A23187 increased the degradation rate of LC1f, LC2f, and TM after 24 h. In contrast, the calcium ionophore caused decreased degradation of Ac and troponin-C and had no effect on the degradation of MHC, troponin-T, troponin-I, or alpha, beta-desmin (Dm). In addition, A23187 did not alter degradation of total myotube protein. The ionophore had little or no effect on the synthesis of total myotube proteins, but caused a marked decrease in the synthesis of MHC, LC1f, LC2f, Ac, TM, and Dm after 48 h. The mechanisms involved in calcium-stimulated degradation of the myofibrillar proteins were also investigated. Increased proteolysis appeared to involve a lysosomal pathway, since the effect of the Ca++ ionophore could be blocked by the protease inhibitor leupeptin and the lysosomotropic agents methylamine and chloroquine. The effects of A23187 occur in the presence of serum, a condition in which no lysosomal component of overall protein degradation is detected. The differential effect of A23187 on the degradative rates of the myofibrillar proteins suggests a dynamic structure for the contractile apparatus.

Myogenic growth factor present in skeletal muscle is purified by heparin-affinity chromatography

A myogenic growth factor has been purified from a skeletal muscle, the anterior latissimus dorsi, of adult chickens. In the range of 1-10 ng, this factor stimulates DNA synthesis as well as protein and muscle-specific myosin accumulation in myogenic cell cultures. Purification is achieved through binding of the factor to heparin. The factor is distinct from transferrin and works synergistically with transferrin in stimulating myogenesis in vitro.

Simultaneous response of myocardial contractility and a major proteolytic process to beta-adrenergic-receptor occupancy in the Langendorff isolated perfused rat heart

The Langendorff isolated rat heart was adapted to the study of minute-to-minute percentage changes in bulk protein degradation by using non-recirculating perfusion. Hearts were perfused at 8 ml/min at 35 degrees C with Krebs-Henseleit buffer containing 11 mM-glucose, and only hearts with regular ventricular rhythm were employed. Proteins were labelled by infusion of [3H]leucine for 0.5 h in vitro. A complete amino acid mixture was then added at 3 times normal rat extracellular concentrations. After labelling, the re-incorporation of [3H]leucine was competitively inhibited by addition of either 4 mM-leucine or 20 microM-cycloheximide. The residual unincorporated radioactivity and the preferentially labelled rapid-turnover proteins were eliminated during a 3 h preliminary perfusion period. The basal rate of release of [3H]leucine and percentage changes were then determined at 1 min intervals, by using each heart as its own control. Leucine metabolism was inconsequential to results. Exchange of intracellular leucine pools with extracellular leucine and subsequent release in effluent perfusate was 95% complete within approx. 2 min. The basal rate of protein degradation was unchanged by electrical stimulation of the heart rate to 360 beats/min or cessation of contractile activity by membrane depolarization under 25 mM-KCl. Infusion of the beta-agonist isoprenaline at 5-500 nM caused a graded inhibition of myocardial protein degradation within 5-6 min, with a maximum inhibition of 30%. This inhibition was sustained for at least 1 h of drug administration and was reversed within 4-6 min of cessation of isoprenaline or simultaneous infusion of 1 microM of the beta-receptor antagonist propranolol. Minute-to-minute adrenergic proteolytic control was a simultaneous co-variable with beta-receptor-mediated inotropic changes in right-intraventricular systolic pressure. Stoppage of the heart in asystole by the Ca2+-channel blocker nifedipine (0.7 microM) delayed the onset, but did not cause sustained reversal, of adrenergic-inhibited degradation, indicating the absence of a direct obligatory mechanistic linkage between the events of the contraction-relaxation cycle and protein degradation in this preparation.

Development of heart cells in culture: studies using an affinity purified antibody to a myosin light chain

Cultured neonatal rat heart cells can be used to study the factors that regulate cardiac contractility and myocyte development in vitro. An antibody to the 26,000 dalton light chain of myosin (MLC1), has been produced and purified on a Sepharose 4B affinity column prepared with rat heart myosin. When primary cultures of myocytes are studied by indirect immunofluorescence using this antibody a predictable pattern of myofibrillar structure is observed to develop over 72 h. This myosin cytoskeleton is highly organized and the myosin fibrils exhibit cross striations. The antibody does not stain non-muscle heart cells and there is no evidence for myocyte division in culture. The qualitative immunofluorescent pattern of myosin organization is the same in both spontaneously beating and in non-contracting cells.

Relationship of muscle growth in vitro to sodium pump activity and transmembrane potential

Serum stimulates embryonic avian skeletal muscle growth in vitro and the growth-related processes of amino acid transport and protein synthesis. Serum also stimulates myotube Na pump activity (measured as ouabain-sensitive rubidium-86 uptake) for at least 2 h after serum addition. Serum-stimulated growth depends on this Na pump activity since ouabain added at the same time as serum totally inhibits the growth responses. The relationship of myotube growth, Na pump activity, and transmembrane potential was studied to determine whether serum-stimulated Na pump activation and growth are coupled by long-term membrane hyperpolarization. When myotube amino acid transport and protein synthesis are prestimulated by serum, ouabain was found to have little inhibitory effect, indicating that the already stimulated growth-related processes are not tightly coupled to continued Na pump activity. Serum-stimulated protein synthesis is tightly coupled to Na pump activity, but only during the first 5-10 min after serum addition. When myotube transmembrane potentials were measured using the lipophilic cation tetraphenylphosphonium, serum at concentrations that stimulate myotube growth and Na pump activity was found to have little effect on the cell's transmembrane potential. Furthermore, partial depolarization of the myotubes with 12- to 55-mM extracellular potassium does not prevent serum stimulation of myotube growth. Monensin was found to hyperpolarize the myotubes, but causes myotube atrophy. These results indicate that although Na pump activity is associated with initiation of serum-stimulated myotube growth, continued Na pump activity is not essential, and there is little relationship between myotube growth and the myotube's transmembrane potential.

There is selective accumulation of a growth factor in chicken skeletal muscle. I. Transferrin accumulation in adult anterior latissimus dorsi

Chick embryo myoblasts in culture will respond to extracts of adult anterior latissimus dorsi muscle with an increase in cell number and an increase in total protein and in myosin heavy chain in fused myotubes. Extracts of adult pectoralis major and of posterior latissimus muscles are only marginally active. The active adult muscle extracts are fractionated by DEAE-cellulose column chromatography and transferrin is identified as the active component based on the following findings: (1) the active fractions are shown to contain an 80K protein that comigrates with chicken transferrin on SDS-PAGE, (2) the active extract from the anterior latissimus dorsi completely replaced embryo extract in the culture medium and supported normal myogenesis, (3) the active extract requires iron for its ability to support myogenesis, (4) the peptide map of the 80K protein is identical to a peptide map of transferrin. Under conditions where the 80K protein is detected in adult anterior latissimus dorsi muscles it is shown that the protein is nevertheless not synthesized in the muscle. These results support the idea that tissues of selective muscles in the adult chicken accumulate transferrin. An accompanying paper shows that transferrin also accumulates in early developmental stages of fast muscle tissue but that accumulation ceases after hatching in these muscles in normal chickens but not in animals of congenic strains with inherited muscular dystrophy.

Switching of subunit composition of muscle spectrin during myogenesis in vitro

Spectrin comprises a family of polypeptides thought to be involved in mediating linkage of actin filaments to the plasma membrane in a wide variety of cell types (for reviews see refs 1-3). Spectrin is present as a tetramer composed of two non-identical subunits. Most cells express a common subunit with a molecular weight (Mr) of 240,000 (240 K; termed alpha-spectrin) in association with a polymorphic cell type-specific subunit: Mr 260 K in the intestinal terminal web (termed TW260), Mr 235 K in nervous tissue., liver, lymphocytes and fibroblasts (termed gamma-spectrin; also referred to as fodrin), and Mr 225/220 K in erythrocytes and adult cardiac and skeletal muscle (termed beta'- and beta-spectrin, respectively). We show here that primary chicken myoblasts express predominantly alpha gamma-spectrin, but on terminal differentiation in vitro the cells gradually switch to alpha beta-spectrin as a result of the onset of beta- and beta'-spectrin synthesis and by the subsequent differential stabilization of beta- and gamma-spectrin. This switching correlates with known changes in the biophysical properties and function of the developing muscle sarcolemma and cytoskeleton.

Extracellular potassium and the regulation of acetylcholine receptor synthesis in embryonic chick muscle cells

The effect of elevated extracellular potassium on acetylcholine receptor synthesis was studied in chick embryonic muscle cultures. At physiological ionic strength, potassium chloride, in the 3.3 to 50 mM range, gave rise to a complex dose-response curve whose prominent features are a considerable reduction of receptor appearance rate at 20 mM and a more than 2-fold increase at higher concentrations. The effect of potassium chloride on receptor synthesis appears to be fairly specific: neither was there a duplication of its effect by other electrolytes or solutes, nor did it alter total protein synthesis or receptor stability by more than 30% at any concentration tested; cellular acetylcholinesterase levels actually declined with increasing KCl concentrations. In order to explore the mechanism of the potassium effect, tetrodotoxin (10(-6) M), veratridine (3 X 10(-6) M), D-600 (1.6 X 10(-5) M), and ryanodine (3 X 10(-7) M) were tested in the presence of various concentrations of potassium. Sodium channel toxins as well as calcium effectors modified the potassium response. Based on these findings we propose that the effects of potassium are due to: (a) cessation of spontaneous muscle activity upon raising KCl from 3 to 10 mM; (b) depolarization of the muscle membrane and persistent activation of a calcium channel as concentration is raised from 10 to 20 mM; (c) finally, inactivation or desensitization of the calcium channel, or some other signaling element proximal to the sarcoplasmic reticulum, upon further depolarization.