Appearance of acetylcholine receptor in differentiating cultures of embryonic chick breast muscle

BDNF is expressed in skeletal muscle satellite cells and inhibits myogenic differentiation

In skeletal muscle, brain-derived neurotrophic factor (BDNF) has long been thought to serve as a retrograde trophic factor for innervating motor neurons throughout their lifespan. However, its localization in mature muscle fibers has remained elusive. Given the postulated roles of BDNF in skeletal muscle, we performed a series of complementary experiments aimed at defining the localization of BDNF and its transcripts in adult muscle. By reverse transcription-PCR, in situ hybridization, and immunofluorescence, we show that BDNF, along with the receptor p75NTR, is not expressed at significant levels within mature myofibers and that it does not accumulate preferentially within subsynaptic regions of neuromuscular junctions. Interestingly, expression of BDNF correlated with that of Pax3, a marker of muscle progenitor cells, in several different adult skeletal muscles. Additionally, BDNF was expressed in Pax7+ satellite cells where it colocalized with p75NTR. In complementary cell culture experiments, we detected high levels of BDNF and p75NTR in myoblasts. During myogenic differentiation, expression of BDNF became drastically reduced. Using small interfering RNA (siRNA) technology to knock down BDNF expression, we demonstrate enhanced myogenic differentiation of myoblasts. This accelerated rate of myogenic differentiation seen in myoblasts expressing BDNF siRNA was normalized by administration of recombinant BDNF. Collectively, these findings show that BDNF plays an important regulatory function during myogenic differentiation. In addition, the expression of BDNF in satellite cells is coherent with the notion that BDNF serves a key role in maintaining the population of muscle progenitors in adult muscle.

Single K+ currents during differentiation of embryonic muscle cells in vitro

After 3-7 days in culture, chicken myotubes possess five types of K+ channel: two high-conductance channels of 195 and 105 pS which are sensitive to tetraethylammonium (TEA), an ATP-sensitive channel of 64 pS and two low-conductance channels of 40 and 15 pS which are insensitive to TEA and ATP. The same population of channels is to be found in EGTA-treated muscle cells with blocked fusion and, with the exception of the ATP-sensitive channel, also in 1-day-old myoblasts. There are differences between myoblasts and myotubes in the percentage of incidence of individual channel types. High-conductance K+ channels are most frequently to be observed in myotubes, but they are rare in myoblasts and EGTA-treated cells where low-conductance K+ channels predominate.

Muscle-specific gene expression controlled by a regulatory element lacking a MyoD1-binding site

Muscle-specific expression of the gene encoding the delta subunit of the acetylcholine receptor is controlled by a 54-base-pair region that does not contain a binding site for MyoD1, a protein involved in activation of the myogenic program. A MyoD1-binding site is present in the proximal promoter region of the gene encoding the delta-subunit, but is neither sufficient nor necessary for muscle-specific expression in transfected muscle cells.

Opposing effects of calcium entry and phorbol esters on fusion of chick muscle cells

Studies utilizing cultured muscle cells have shown that myoblast fusion requires extracellular Ca2+ and involves transient coordinated changes in cell membrane topography and cytoskeletal organization. However, neither the mechanisms by which Ca2+ influences these changes nor its cellular sites of action are known. We have investigated the effects of Ca2+ channel modulators and phorbol esters on fusion of embryonic chick myoblasts in culture. Myoblast fusion was inhibited by the Ca2+ channel blockers D600 and nitrendipine and stimulated by the Ca2+ channel activator Bay K 8644. We have obtained evidence that the tumor promoting phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) inhibits fusion through activation of protein kinase C. Myoblasts prevented from fusing by Ca2+ channel blockers or TPA display a distinctive elongated morphology that is characteristic of cells prevented from fusion by Ca2+ deprivation. The inhibition of fusion by D600 and TPA is significantly diminished in the presence of the Ca2+ ionophore A23187. TPA arrest of myoblast fusion was found to be accompanied by an increase in phosphorylation of the 20-kDa light chain of cytoplasmic myosin in a dose- and time-dependent manner. The effects of TPA on myoblast fusion and phosphorylation of myosin light chain were mimicked by the cell permeant diacylglycerol sn-1,2-dioctanoylglycerol, a potent activator of protein kinase C. The present results suggest that activators of protein kinase C block fusion by interfering with a Ca2+ signal transduction pathway and that this interference may be associated with a protein kinase C catalyzed inhibitory phosphorylation of myosin light chain.

Purothionin from wheat endosperm reversibly blocks myogenic differentiation of chick embryonic muscle cells in culture

Purothionin from wheat endosperm is a cysteine-rich, basic polypeptide of about 5000 Da, which modifies membrane permeability of cultured mammalian cells. This peptide was found to block fusion of chick embryonic muscle cells in culture but allows proliferation and alignment. A purothionin concentration of 6 micrograms/ml (1.2 microM) was necessary for the complete prevention of myotube formation. Under similar conditions, incorporation of [35S]methionine occurred normally but the synthesis of muscle-specific proteins including creatine kinase and acetylcholine receptor was strongly inhibited. In addition, purothionin blocked the uptake of 86Rb+, immediately after its addition to the cultured myoblasts. No such effects were found with the purothionin chemically modified with acetic or succinic anhydride. Thus, the basic residues in purothionin appear to be associated with the inhibition of myogenic differentiation. These results suggest that purothionin exerts its regulatory effect on the transition from proliferative to differentiative myoblasts by interfering with membrane permeability or intercellular contact and recognition, which are necessary for the initiation of muscle differentiation.

Isolation and characterization of the mouse acetylcholine receptor delta subunit gene: identification of a 148-bp cis-acting region that confers myotube-specific expression

We have isolated the gene encoding the delta subunit of the mouse skeletal muscle acetylcholine receptor (AChR) and have identified a 148-bp cis-acting region that controls cell type-specific and differentiation-dependent gene expression. The 5' flanking region of the delta subunit gene was fused to the protein-coding region of the chloramphenicol acetyltransferase (CAT) gene and gene fusions were transfected into C2 mouse skeletal muscle cells. Both transiently and stably transfected cells were assayed for CAT gene expression. Deletions from the 5' end of the mouse delta gene demonstrate that 148 bp of 5' flanking DNA is sufficient to confer cell type-specific and differentiation-dependent expression: CAT activity is present in transfected myotubes, but not in transfected 3T3 cells or 10T1/2 cells. Moreover, the level of CAT expression in myotubes transfected with constructs containing 148 bp of 5' flanking DNA from the delta subunit gene is identical to that in myotubes transfected with constructs containing 3.2 kb of 5' flanking DNA and similar to expression from the SV-40 early promoter. Increased CAT activity in myotubes is a result of an increased rate of transcription from the delta subunit promoter, since CAT RNA levels are also 35-fold more abundant in myotubes than myoblasts. In contrast, the SV-40 early promoter is similarly active in all cell types. Thus, 148 bp of 5' flanking DNA from the delta subunit gene contains all the information required for cell type-specific and differentiation-dependent expression of the AChR delta subunit.

Lymph heart musculature in birds

Development and innervation of the lymph heart musculature of chicken, emu, rhea, and duck was studied by electron microscopy at post-hatch ages from 3 days to adulthood. Development of innervation was monitored by acetylcholinesterase staining. Horseradish peroxidase was used to determine the extent of the transverse tubule network. Chickens were unusual among these birds in that lymph heart myocytes had already undergone a definitive differentiation and degeneration by 3 days. In ducks and ratite birds, lymph heart myocytes more slowly but progressively differentiate a cytomorphology that does not conform in all characteristics to cardiac or skeletal muscle and even resembles in some aspects, smooth muscle. Myofibrils become the dominant cytoplasmic structure, transverse tubules form "internal couplings" with agranular reticulum cisternae, and "external couplings" are formed between myocytes at myomyal junctions. The myomyal junctions also contain AChE-positive reaction product and some subplasmalemmal vesicles that lack a dense core. The lymph heart myocardium of ducks of 2 weeks demonstrated mitotic figures. In adult ducks the myosatellite cell numbers diminish and a characteristic pattern of myocyte degeneration appears. In juvenile ducks and ratites some myocytes differentiate to conductile cells, much as the conductile myocytes and myofibers of the blood heart. The lymph heart innervation is described, and the role of nerve in differentiation and maintenance of myocyte morphology in the lymph heart is discussed.

Properties of a purified pore-forming protein (perforin 1) isolated from H-2-restricted cytotoxic T cell granules

Histocompatibility-restricted cytotoxic T lymphocytes produce circular lesions on target cell membranes. The pore-forming protein (PFP or perforin 1) that forms these membrane lesions has been purified from lymphocytes. At 37 degrees C, in the presence of Ca2+, this protein polymerizes into a supramolecular tubular complex of Mr greater than 10(6) that partially resists dissociation by SDS and reducing agents. It incorporates spontaneously into planar lipid bilayers during polymerization to form nonselective ion channels, showing heterogeneous size distribution, the smallest conductance per unit being identified as 400 pS in 0.1 M NaCl. PFP/P1 that had been assembled in lipid vesicles before incorporation into planar bilayer show much larger single channel conductance, ranging from 1 to 6 nS in 0.1 M NaCl, suggesting that PFP/P1 may assume multiple functional sizes in proportion to its state of polymerization. The reconstituted channels are relatively voltage-insensitive, with most channels persisting in the open state for seconds to minutes. Nucleated cells are rapidly depolarized by this protein. The purified protein lyses a variety of tumor cells. Polymerization and functional channel activity are absolutely Ca2+-dependent. The activity of this protein may play a direct role in T lymphocyte-mediated cytolysis.

Maintenance of proliferative capacity during avian myogenesis is independent of fusion-permissive calcium ion concentration

Skeletal muscle differentiation is normally accompanied by the permanent withdrawal of myogenic nuclei from the proliferative cycle. However, embryonic Japanese quail (Coturnix coturnix japonica) myoblasts which have been prevented from fusing in vitro by the addition of EGTA to the culture medium retain the capacity to re-enter the cell cycle following accumulation of muscle-specific myosin. We have therefore investigated the roles of Ca2+ and fusion in the withdrawal of myogenic cells from the cell cycle. Using three defined media which differ in Ca2+ and in the ability to promote fusion, we examined the ability of differentiation-competent myoblasts to resume proliferation with increased time in G1. Under these conditions, there is a periodic variation in the ability of the myoblasts to respond to mitogenic stimulation, irrespective of the medium employed. These results indicate that loss of proliferative capacity during myogenesis is independent of Ca2+.

Posttranslational control of membrane-skeleton (ankyrin and alpha beta-spectrin) assembly in early myogenesis

Adult chicken skeletal muscle cells express polypeptides that are antigenically related to alpha-spectrin (Mr 240,000) and beta-spectrin (Mr 220,000-225,000), the major components of the erythrocyte membrane-skeleton, and to ankyrin (Mr 237,000; also termed goblin in chicken erythrocytes), which binds spectrin to the transmembrane anion transporter in erythrocytes. Comparative immunoblotting of SDS-solubilized extracts of presumptive myoblasts and fully differentiated myotubes cultured in vitro demonstrated that there is a dramatic accumulation of ankyrin and alpha- and beta-spectrin during myogenesis and a concomitant switch in the subunit composition of spectrin from alpha gamma to alpha beta. Analysis of early time points in myogenesis (12-96 h) revealed that these changes occur shortly after the main burst of cell fusion. To determine the temporal relationship between cell fusion and the accumulation of ankyrin and alpha- and beta-spectrin, we treated presumptive myoblasts with 2 mM EGTA, which resulted in the complete inhibition of cell fusion. The incorporation of [35S]methionine into total protein and, specifically, into alpha-, gamma-, and beta-spectrin remained the same in EGTA-treated and control cells. Analysis by immunoblotting of the amounts of ankyrin and alpha- and beta-spectrin in fusion-blocked cells revealed that there was no effect on accumulation for the first 19 h. However, there was then a dramatic cessation in their accumulation, and thereafter, the amount of each protein at steady state remained constant. Upon release from the EGTA block, the cells fused rapidly (less than 11 h), and the accumulation of ankyrin and alpha- and beta-spectrin was reinitiated after a lag period of 3-5 h at a rate similar to that in control cells. The inhibition in the accumulation of newly synthesized ankyrin, alpha-spectrin, and beta-spectrin in EGTA-treated myoblasts was not characteristic of all structural proteins, since the accumulation of the muscle-specific intermediate filament protein desmin was the same in control and fusion-blocked cells. These results show that in myogenesis, the synthesis of ankyrin and alpha- and beta-spectrin and their accumulation as a complex, although concurrent, are not coupled events. We hypothesize that the extent of assembly of these components of the membrane-skeleton in muscle cells is determined by a control mechanism(s) operative at the posttranslational level that is triggered near the time of cell fusion and the onset of terminal differentiation.

The all-or-none role of innervation in expression of apamin receptor and of apamin-sensitive Ca2+-activated K+ channel in mammalian skeletal muscle

The long-lasting after-hyperpolarization(s) (AHP) that follows the action potential in rat myotubes differentiated in culture is due to Ca2+-activated K+ channels. These channels have the property to be specifically blocked by the bee venom toxin apamin at low concentrations. Apamin has been used in this work to analyze, by electrophysiological and biochemical techniques, the role of innervation in expression of these important channels. The main results are as follows: (i) Long-lasting AHP that follows the action potential in rat myotubes in culture disappears when myotubes are cocultured with nerve cells from the spinal cord under the conditions of in vitro innervation. (ii) Extensor digitorum longus muscles from adult rats have action potentials that are not followed by AHP but AHP are systematically recorded after muscle denervation and they are blocked by apamin. (iii) Specific 125I-labeled apamin binding is undetectable in innervated muscle fibers but it becomes detectable 2-4 days after muscle denervation to be maximal 10 days after denervation. (iv) Apamin receptors detected with 125I-labeled apamin are present at fetal stages with biochemical characteristics identical to those found in myotubes in culture. The receptor number decreases as maturation proceeds and 125I-labeled apamin receptors completely disappear after the first week of postnatal life, in parallel with the disappearance of multi-innervation. All these results taken together strongly suggest an all-or-none effect of innervation on the expression of apamin-sensitive Ca2+-activated K+ channels.

An hypothesis on the role of cellular colloid osmotic pressure in determining behavior of cells in vitro including anchorage dependency and maintenance of the differentiated state

The osmotic problems involved when cells are isolated from tissues are analyzed. Evidence is considered which indicates that in vivo the Na pump is operating at maximal or near maximal rates and that this depends on low leak rates for salts and water due to various aspects of the tissues structure. Dispersion of the tissue results in breakdown of these barriers on free diffusion and the isolated cell is subjected to an enormous increase in passive influx due to colloid osmotic pressure without being able to increase its pumping rate to the extent needed to maintain volume control. It is proposed that the primary problem the cell faces in vitro is to compensate for the effective increase in its colloid pressure, e.g. the colloid osmotic pressure excess, emerging with the breakdown of the tissue structure. The finding that most normal cells have to adhere to a surface in order to grow or "anchorage dependency" is analyzed in terms of the way adhesion and spreading result in changes in ion and water movements into cells enabling them to achieve fluid balance in the face of the colloid pressure excess. It is also proposed that the differentiated state is more dependent on colloid osmotic balance than proliferation. The failure of conditions used in tissue culture to compensate adequately for the colloid pressure excess results in limiting the amount of protein which can be synthesized, dissipation of cellular energy, and changes in orientation of cellular components which contribute directly to the loss of differentiation which occurs during growth in vitro.

The pericellular boundary layer modulates acetylcholine receptor stability in cultured myotubes

Degradation of acetylcholine receptors in cultured chicken myotubes was measured by release into the medium of radioactivity from 125I-labeled alpha-bungarotoxin. Disturbance of the pericellular boundary layer by stirring of the culture medium shortened the half-life of receptor in the membrane from 24 to 12 h. The effect could not be explained by dissociation of toxin-receptor complexes or by conditioning of the bulk phase of the medium. The rates of synthesis and degradation of total cell protein and the degradation of lactoperoxidase-iodinated surface protein were not affected by medium stirring. The loss of glucosamine-labeled material from the cells was enhanced by stirring, however, and this resulted entirely from the increased shedding of high molecular weight glycosubstances from the cells. Cells in stirred cultures contained lower levels of surface coat material stainable with colloidal thorium. These results indicate that glycosubstances of the pericellular matrix protect ACh receptors from degradation.

Extinction of muscle-specific properties in somatic cell heterokaryons

In studies of gene regulation using somatic cell fusion techniques, the analysis of heterokaryons circumvents several problematic aspects of the more traditional approach utilizing proliferating hybrid cells. We have analyzed the expression of muscle specific properties in heterokaryons between muscle and nonmuscle cells in order to investigate whether differentiating cells contain regulatory factors that repress the expression of alternative developmental pathways. Heterokaryons and cybrids were derived from polyethylene glycol-mediated fusion of differentiated mononucleate chicken myocytes with mouse melanoma cells, mouse melanoma cytoplasts, chicken fibroblasts, or other chicken myocytes. Our results demonstrate that fusion of a myocyte with a nonmyogenic cell generally results in extinction of muscle-specific properties in the immediate fusion product. Myocyte X melanoma heterokaryons ceased to express the skeletal muscle forms of myosin, desmin and creatine kinase, reinitiated DNA synthesis, and showed a loss of spontaneous fusion competence within 96 hr after their formation. Although chicken myocyte X mouse melanoma heterokaryons showed extinction of muscle specific properties, they continued to synthesize protein and to incorporate [3H]hypoxanthine, presumably due to the continued production of constitutive chicken HPRT. That presence of the melanoma nucleus was required for extinction to be observed was demonstrated by the continued expression of muscle proteins in cybrids between chicken myocytes and melanoma cytoplasts. Significantly, heterokaryons between chicken myocytes and chicken fibroblasts also exhibited extinction of muscle proteins, demonstrating for the first time that extinction is not restricted to fusions in which at least one parental cell type was derived from an established cell line. Our results strongly support the notion that extinction reflects cell-type specific gene regulatory mechanisms operative during development.

Alpha-bungarotoxin binding to the myotome and choline acetyltransferase activity in the rabbit embryo

We have previously found incomplete sarcomeres and acetylcholinesterase activity in the myoblasts of the myotome of the rabbit at day 13 of gestation. We now report that an acetylcholine (ACh)-synthesizing enzyme and the nicotinic receptor are present at this stage as well. A study of the myotome using [125I]alpha-bungarotoxin shows that the mononucleated myoblasts have alpha-bungarotoxin binding sites before they migrate away to form multinucleated myotubes. Choline acetylcholinesterase activity and/or a different ACh-synthesizing enzyme are found at early stages of development, even before the spinal nerve has formed. An ACh-synthesizing enzyme is present in the notochord, a neural tube-dorsal root ganglion preparation, as well as in rows of myotomes separated from the latter preparation. Assays of isolated myotomes with very little adherent mesenchyme indicate that the enzyme is located either within the myotome or in its immediate vicinity. Cholinergic components, therefore, are associated with the mononucleated myoblasts of the myotome before they fuse to form myotubes and before they receive their permanent innervation.

Analysis of muscle protein expression in polyethylene glycol-induced chicken: rat myoblast heterokaryons

Heterokaryons derived from polyethylene glycol-mediated fusion of myoblasts at different stages of development were used to investigate the transition of cells in the skeletal muscle lineage from the determined to the differentiated state. Heterokaryons were analyzed by immunofluorescence, using rabbit antibodies against the skeletal muscle isoforms of chicken creatine kinase and myosin, and a mouse monoclonal antibody that cross-reacts with chicken and rat skeletal muscle myosin. When cytochalasin B-treated rat L8(E63) myocytes (Konieczny S.F., J. McKay, and J. R. Coleman, 1982, Dev. Biol., 91:11-26) served as the differentiated parental component and chicken limb myoblasts from stage 23-26 or 10-12-d embryos were used as the determined, undifferentiated parental cell, heterokaryons exhibited a progressive extinction of rat skeletal muscle myosin during a 4-6-d culture period, and no precocious expression of chicken differentiated gene products was detected. In the reciprocal experiment, 85-97% of rat myoblast X chicken myocyte heterokaryons ceased expression of chicken skeletal muscle myosin and the M subunit of chicken creatine kinase within 7 d of culture. Extinction was not observed in heterokaryons produced by fusion of differentiated chicken and differentiated rat myocytes and thus is not due to species incompatibility or to the polyethylene glycol treatment itself. The results suggest that, when confronted in a common cytoplasm, the regulatory factors that maintain myoblasts in a proliferating, undifferentiated state are dominant over those that govern expression of differentiated gene products.

Trifluoperazine, a calmodulin antagonist, inhibits muscle cell fusion

We investigated the effect of trifluoperazine (TFP), a calmodulin antagonist, on the fusion of chick skeletal myoblasts in culture. TFP was found to inhibit myoblast fusion. This effect occurs at concentrations that have been reported to inhibit Ca2+-calmodulin in vitro, and is reversed upon removal of TFP. In addition, other calmodulin antagonists, including chlorpromazine, N-(6-aminohexyl)-5-chloro-1-naphthalene-sulfonamide (W7), and N-(6-aminohexyl)-1-naphthalene-sulfonamide (W5), inhibit fusion at doses that correspond closely to the antagonistic effects of these drugs on calmodulin. The expression of surface acetylcholine receptor, a characteristic aspect of muscle differentiation, is not impaired in TFP-arrested myoblasts. Myoblasts inhibited from fusion by 10 microM TFP display impaired alignment. In the presence of the Ca2+ ionophore A23187, the fusion block by 10 microM TFP is partially reversed and myoblast alignment is restored. The presence and distribution of calmodulin in both prefusional myoblasts and fused muscle cells was established by immunofluorescence. We observed an apparent redistribution of calmodulin staining that is temporally correlated with the onset of myoblast fusion. Our findings suggest a possible role for calmodulin in the regulation of myoblast fusion.

Regulation of membrane transport sites for amino acids in myogenic cells. A differentiation dependent phenomenon

Myoblasts from 12-day chick embryos in cell culture transport the nonmetabolizable amino acid alpha-aminoisobutyric acid (AIB) two to three-fold more rapidly than multinucleated myotubes which form from them. This decrease in transport is due to a relative decrease in the number of transport sites per unit area of cell surface suggesting a compositional change in the plasma membrane during myogenesis. In studies reported here, AIB transport was monitored throughout myogenesis and correlated with other aspects of differentiation. During myogenesis the number of amino acid transport sites remains constant per myotube nucleus. As myogenesis proceeds, there is a marked increase in cellular protein and cell surface without a commensurate increase in amino acid transport sites. The net consequence of the surface area change is fewer amino acid transport sites per unit area of myotube membrane surface. The decrease in membrane transport sites for AIB per unit area of membrane is not a result of length of time in culture per se, medium depletion, or cell density, but is a result of differentiation, since blocking myoblast fusion by deprivation of calcium delays the decrease in AIB transport sites per unit cell surface area while reversal of the calcium deprivation block is accompanied by a rapid decrease in the number of AIB transport sites per unit cell surface area. Thus, the decrease in AIB transport sites is an aspect of differentiation which accompanies the marked elaboration of surface membrane during myogenesis.

Interaction of monoclonal antibodies to Torpedo acetylcholine receptor with the receptor of skeletal muscle

Several monoclonal antibodies (mcAbs) elicited against the nicotinic acetylcholine receptor (AChR) from Torpedo react also with skeletal muscle AChR. Such mcAbs were used to define antigenic determinants on muscle AChR and to elucidate their effect on muscle AChR functions. Primary chick muscle cultures were used as a model for skeletal muscle. Of the four mcAbs studies only mcAb 5.5, which is directed against the cholinergic site in Torpedo AChR, blocks the binding of alpha-bungarotoxin (alpha-Bgt) to AChR in chick muscle cultures and inhibits carbamylcholine-induced sodium transport in these cells. The interaction of mcAb 5.5 with the cholinergic site on muscle AChR demonstrates the conservation of this site. Two mcAbs, 5.5 and 5.34, each of a different antigenic specificity but both directed against conformation-dependent antigenic determinants, accelerate the degradation of AChR in muscle cultures. From the reactivity of the various mcAbs with Triton-solubilized and membranous AChR it appears that there are some antigenic differences between the detergent solubilized and membranous forms of the receptor.

Tunicamycin inhibits the expression of surface Na+ channels in cultured muscle cells

We have investigated the effect of tunicamycin (TM), an inhibitor of protein glycosylation, on surface Na+ channels in cultured chick skeletal muscle cells. The expression of Na+ channels, estimated by the measurement of batrachotoxin (BTX)-activated 22Na+ uptake, was found to be significantly diminished after exposure of muscle cells to TM. This effect is partially reversed by the protease inhibitor leupeptin and is associated with a markedly enhanced rate of disappearance of Na+ channels from the surface of TM-treated cells. Our findings suggest that protein glycosylation contributes to the metabolic stability of voltage-sensitive Na+ channels.

Effect of inhibitors of cholesterol synthesis on muscle differentiation

Aggregation, the first step in the fusion of cultured chick myoblasts to form myotubes, is inhibited by 90% after only 2 h exposure to 25-hydroxycholesterol, an inhibitor of cholesterol synthesis. In this study, the effect of this agent on the concentration of an integral membrane protein in the plasma membrane, the synthesis of two differentiation-controlled proteins and total protein synthesis were examined. While it is possible that any of these mechanisms could be responsible for the inhibition of fusion, none of them appear to account for the attenuation of fusion induced by the inhibition of cholesterol synthesis.

Appearance and distribution "in situ" of nicotinic acetylcholine receptors in cervical myotomes of young chick embryos. Radioautographic studies by light and electron microscopy

Localization of the acetylcholine (nicotinic) receptor sites was investigated in the developing cervical myotomes of the early chick embryo by radioautography at the light and electron microscope level, using 125I-alpha-bungarotoxin. The presence of cholinergic receptor sites was detected in situ as early as 60 hours of incubation (stage 17); their relative density increased in the myotome during the differentiation of the somite. Specific labeling of these receptor sites was detected in the myotomal tissue but not in the notochord, spinal cord or periaxial mesenchyme. The distribution of the receptor sites was uniform in the myotome at 3 days in ovo. An anterior-posterior asymmetry of the density appeared at 4 days in ovo and developed up to the 6th day. The highest density of these toxin-binding receptor sites was observed near the spinal motor nerve bundle as revealed by silver staining. These observations, made in situ, are discussed with respect to the possible neurotrophic or physical effects of the early motor innervation.

Interaction of the cytoskeletal framework with acetylcholine receptor on th surface of embryonic muscle cells in culture

To monitor the interaction of cell surface acetylcholine (AcCho) receptors with the cytoskeleton, cultured muscle cells were labeled with radioactive or fluorescent alpha-bungarotoxin and extracted with Triton X-100, using conditions that preserve internal structure. A significant population of the AcCho receptors is retained on the skeletal framework remaining after detergent extraction. The skeleton organization responsible for restricting AcCho receptors to a patched region may also result in their retention after detergent extraction.

Regulation of muscle differentiation: stimulation of myoblast fusion in vitro by catecholamines

Epinephrine and isoproterenol provoke primary chick myoblasts to initiate precocious cell fusion. Both the rise in intracellular adenosine 3' ,5-monophosphate (cyclic AMP) and cell fusion generated by these effectors are prevented by propranolol, which is a specific blocker of the beta-adrenergic receptor. Propranolol has no effect either on the precocious cell fusion provoked by prostaglandin E or on cell fusion in control cultures. The results support the idea that a rise in cyclic AMP is the critical intracellular change responsible for initiating events that culminate in myoblast differentiation 4 to 5 hours later. They also indicate that the culminate in myoblast differentiation 4 to 5 hours later. They also indicate that the hormone responsible for the positive regulation of myoblast differentiation in vitro is not acting through the beta-adrenergic receptor.

Synthesis of rat myosin light chains in heterokaryons formed between undifferentiated rat myoblasts and chick skeletal myocytes

The control of gene expression during terminal myogenesis was explored in heterokaryons between differentiated and undifferentiated myogenic cells by analyzing the formation of species specific myosin light chains of chick and rat skeletal muscle. Dividing L6 rat myoblasts served as the biochemically undifferentiated parent. The differentiated parental cells were mononucleated muscle cells (myocytes) that were obtained from primary cultures of embryonic chick thigh muscle by blocking myotube formation with EGTA and later incubating the postimitotic cells in cytochalasin B. Heterokaryons were isolated by the selective rescue of fusion products between cells previously treated with lethal doses of different cell poisons. 95-99% pure populations of heterokaryons formed between undifferentiated rat myoblasts and differentiated chick myocytes were obtained. The cells were labeled with [35S]methionine, and whole cell extracts were analyzed on two-dimensional polyacrylamide gels. These heterokaryons synthesize the light chain of chick myosin and both embryonic and adult light chains of rat skeletal myosin. Control homokaryons formed by fusing undifferentiated cells to themselves did not synthesize skeletal myosin light chains. Control heterokaryons formed between undifferentiated rat myoblasts and chick fibroblasts also failed to synthesize myosin light chains. These results indicate that differentiated chick muscle cells provide some factor that induces L6 myoblasts to synthesize rat myosin light chains. This system provides a model for investigating the processes by which differentiated cell functions are induced.

Developmental reorganization of the skeletal framework and its surface lamina in fusing muscle cells

The skeletal framework of cells, composed of internal structural fibers, microtrabeculae, and the surface lamina, is revealed with great clarity after extraction with detergent. When muscle cells fuse to form a multinucleated myotube, their skeletal framework reorganizes extensively. When myoblasts prepare to fuse, the previously continuous surface lamina develops numerous lacunae unique to this stage. The retention of iodinated surface proteins suggests that the lacunae are not formed by the extraction of lamina proteins. The lacunae appear to correspond to extensive patches that do not bind concanavalin A and are probably regions of lipid bilayer devoid of glycoproteins. The lacunae appear to be related to fusion and disappear rapidly after the multinucleated myotube is formed. When muscle cells fuse, their internal structural networks must interconnect to form the framework of the myotube. Transmission electron microscopy of skeletal framework whole mounts shows that proliferating myoblasts have well developed and highly interconnected internal networks. Immediately before fusion, these networks are extensively reorganized and destabilized. After fusion, a stable, extensively cross-linked internal structure is reformed, but with a morphology characteristic of the myotube. Muscle cells therefore undergo extensive reorganization both on the surface and internally at the time of fusion.

The effect of Ca2+ ionophores upon the synthesis of proteins in cultured skeletal muscle

The influence of the Ca2+ ionophores, ionomycin and A23187 upon the incorporation of [35S]methionine into proteins of cultured chicken pectoralis muscle was studied during differentiation of myoblasts into multinucleated myotubes. Fusion was reversibly arrested by growing cells in low-calcium media from the time of plating. Exposure of normal and fusion blocked cultures to 10-6-10-5 M ionomycin or A23187 for 2-6 h on the second to fourth day of growth, resulted in a selective increase in the incorporation of [35S]methionine into two proteins of about 100 000 and 80 000 dalton. When 10-5 M ionomycin or A23187 were added to older cultures, all large myotubes contracted and detached from the plate. Only the adhering myoblasts and small myotubes incorporated [35s[methionine into the muscle proteins and showed increased incorporation of label into 100 000 and 80 000 proteins. After ionophore pulse, the adhering cells retained the ability to differentiate and accumulate myosin. The effect of Ca2+ ionophores upon the rate of protein synthesis is presumably related to increased influx of extracellular Ca2+ with a rise in the Ca2+ concentration of the cytoplasm. We conclude that Ca2+ sensitive mechanisms may regulate the synthesis of a select group of muscle proteins.

Carbohydrate requirement for expression and stability of acetylcholine receptor on the surface of embryonic muscle cells in culture

We have investigated the significance of protein glycosylation for metabolism of acetylcholine receptors (AcChoR) in primary cultures of embryonic chicken muscle cells. Tunicamycin, a specific inhibitor of the glycosylation of asparagine residues on glycoproteins, decreased AcChoR accumulation and accelerated its degradation. In contrast, there was no evidence that tunicamycin treatment affected AcChoR biosynthesis, intracellular transport, or incorporation into surface membranes. Leupeptin, an inhibitor of intracellular proteases, markedly increased accumulation of AcChoR on the external surface of muscle cells treated with tunicamycin. Our findings indicate that impairment of protein glycosylation prevents accumulation of AcChoR by increasing its susceptibility to degradation by cellular proteases.

Synthesis of the calcium transport ATPase of sarcoplasmic reticulum and other muscle proteins during development of muscles cells in vivo and in vitro

The effect of medium Ca2+ concentration upon the concentration and the rate of synthesis of muscle proteins was investigated in chicken pectoralis muscle cultures. There is an easily identifiable class of muscle protein which includes the Ca2+-ATPase of sarcoplasmic reticulum, myosin, troponin C, ATP : creatine phosphotransferase, muscle specific actin, tropomysin 1 and 2, and muscle hemagglutinin, which show a large increase in concentration during normal development. The increased synthesis of these proteins was inhibited, without inhibition of cell proliferation, in culture media of relatively low Ca2+ concentration, 0.05--0.3 mM, where fusion was prevented. Similar medium Ca2+ concentration was required for the expression of all these proteins, suggesting their coordinate regulation. The proteins are denoted as 'calcium-modulated proteins'. The increased Ca2+ transport activity of sarcoplasmic reticulum in cultured chicken pectoralis muscle cells during development at 1.8 mM medium calcium concentration represents de novo synthesis of the Ca2+ transport ATPase, as shown by immunoprecipitation, active site labeling and direct identification of the Ca2+ transport ATPase on two-dimensional gel electropherograms of whole muscle homogenates. The concentration and the turnover rate of the majority of the muscle proteins is not affected significantly by medium Ca2+ concentration between 0.06 and 1.8 mM. It is proposed that increase in cytoplasmic free Ca2+ concentration during fusion plays a central role in the regulation of the synthesis of calcium-modulated proteins.

Cell surface changes during muscle differentiation in vitro: a study with the probe 2,4,6-trinitrobenzene sulphonate

Cell surface changes during muscle differentiation in vitro, were investigated using the non permeant probe 2,4,6-trinitrobenzene sulphonate (TNBS) in order to label the aminogroups of proteins exposed on the outer surface of the plasma membrane. Surface proteins of chick myotubes and 'mature' unfused myoblasts (myoblasts grown for 7 days in a calcium-depleted medium) were found to bind an equal amount of probe, which is twice the amount bound by surface proteins in 'immature' myoblasts (1--2 days of culture) and fibroblasts. This indicates that a 'remodelling' of the plasma membrane outer surface takes place in the course of muscle cell differentiation even in the absence of cell fusion. Moreover, the total amount of TNBS bound to the surface was 4--5 times greater in myotubes than in unfused myoblasts. This appears to result from the surface expansion which occurs in myotubes during the development of the T tubule system.

Appearance of new alpha-bungarotoxin-acetylcholine receptors in cultured sympathetic ganglia of chick embryos

alpha-Bungarotoxin (alpha-BuTX) has been used as a marker for studying the production of alpha-bungarotoxin-acetylcholine receptors (alpha-BuTX-AChRs) in explants of chick embryo sympathetic ganglia cultured in vitro. New alpha-BuTX-AChRs appear rapidly in the explants after blocking of the pre-existent ones with the toxin (40% of the total receptors at 3 h). There is a portion of alpha-BuTX-AChRs in the explants which for a short time is not accessible to the toxin. This portion constitutes the precursor pool of receptors and represents 18% of the total. The precursor pool of receptors supplies the neurons with new receptors for 1-2 h in the absence of protein synthesis. The appearance of new receptors from the precursor pool is an energy-dependent process.

Plasminogen activator in chick embryo muscle cells: induction of enzyme by RSV, PMA and retinoic acid

To explore the generality of the effects of sarcoma viruses, tumor-promoting phorbol esters and retinoic acid, we have studied plasminogen activator production in differentiating chick myogenic cultures. Although slightly higher than in chick fibroblast cultures, the level of spontaneously synthesized enzyme is low; it reaches a peak shortly after maximum cell fusion has been completed and then declines. Rous sarcoma virus (RSV) transformation of differentiating myotubes was accomplished by infecting myoblasts with a temperature-sensitive mutant, maintaining cultures at the nonpermissive temperature until completion of fusion and shifting to permissive temperatures at selected times thereafter. RSV transformation, phorbol myristate acetate (PMA) and retinoic acid all induced high levels of plasminogen activator production by differentiating myotubes in the absence of DNA synthesis. In comparison with fibroblasts, virus-induced enzyme synthesis by myogenic cultures proceeded more slowly but ultimately reached comparably high levels. Whereas cAMP strongly repressed RSV- and PMA-induced plasminogen activator production by chick fibroblasts, it weakly stimulated enzyme synthesis by myotubes. This suggests that enzyme induction by RSV and PMA is not mediated primarily through effects on cAMP metabolism.