Inhibition of cellular differentiation by phospholipase C. II. Separation of fusion and recognition among myogenic cells

Cell and molecular biology of myoblast fusion

In organisms from Drosophila to mammals, the musculature is comprised of an elaborate array of distinct fibers that are generated by the fusion of committed myoblasts. These muscle fibers differ from each other in features that include location, pattern of innervation, site of attachment, and size. The sizes of the newly formed muscles of an embryo are controlled in large part by the number of cells that form the syncitial fiber. Over the past few decades, an extensive body of literature has described the process of myoblast fusion in vertebrates, relying primarily on the strengths of tissue culture model systems. More recently, genetic studies in Drosophila embryos have provided new insights into the process. Together, these studies define the steps necessary for myoblast differentiation, the acquisition of fusion competence, the recognition and adhesion between myoblasts, and the fusion of two lipid bilayers into one. In this review, we have attempted to combine insights from both Drosophila and vertebrate studies to trace the processes and molecules involved in myoblast fusion. Implicit in this approach is the assumption that fundamental aspects of myoblast fusion will be similar, independent of the organism in which it is occurring.

Evidence of a non-conventional role for the urokinase tripartite complex (uPAR/uPA/PAI-1) in myogenic cell fusion

Urokinase can form a tripartite complex binding urokinase receptor (uPAR) and plasminogen activator inhibitor type-1 (PAI-1), a component of the extracellular matrix (ECM). The components of the tripartite complex are modulated throughout the in vitro myogenic differentiation process. A series of experiments aimed at elucidating the role of the urokinase tripartite complex in the fusion of human myogenic cells were performed in vitro. Myogenic cell fusion was associated with increased cell-associated urokinase-type plasminogen activator (uPA) activity, cell-associated uPAR, and uPAR occupancy. Incubation of cultures with either uPA anticatalytic antibodies, or the amino-terminal fragment of uPA (ATF), which inhibits competitively uPA binding to its receptor, or anti-PAI-1 antibodies, which inhibit uPA binding to PAI-1, resulted in a 30 to 47% decrease in fusion. Incubation of cultures with the plasmin inhibitor aprotinin did not affect fusion. Decreased fusion rates induced by interfering with uPAR/uPA/PAI-1 interactions were not associated with significant changes in mRNA levels of both the myogenic regulatory factor myogenin and its inhibitor of DNA binding, Id. Incubation of cultures with purified uPA resulted in a decrease in fusion, likely due to a competitive inhibition of PAI-1 binding of endogenous uPA. We conclude that muscle cell fusion largely depends on interactions between the members of the urokinase complex (uPAR/uPA/PAI-1), but does not require proteolytic activation of plasmin. Since the intrinsic muscle cell differentiation program appears poorly affected by the state of integrity of the urokinase complex, and since cell migration is a prerequisite for muscle cell fusion in vitro, it is likely that the urokinase system is instrumental in fusion through its connection with the cell migration process. Our results suggest that the urokinase tripartite complex may be involved in cell migration in a non conventional way, playing the role of an adhesion system bridging cell membrane to ECM.

Is intercellular communication via gap junctions required for myoblast fusion?

Fusion of myoblasts to form syncitial muscle cells results from a complex series of sequential events including cell alignment, cell adhesion and cell communication. The aim of the present investigation was to assess whether intercellular communication through gap junctions would be required for subsequent membrane fusion. The presence of the gap junction protein connexin 43 at areas of contact between prefusing rat L6 myoblasts was established by immunofluorescent staining. These myoblasts were dye-coupled, as demonstrated by the use of the scrape-loading/dye transfer technique. L6 myoblast dye coupling was reversibly blocked by heptanol in short term experiments as well as after chronic treatment. After a single addition of 3.5 mM heptanol, gap junctions remained blocked for up to 8 hours, then this inhibitory effect decreased gradually, likely because the alcohol was evaporated. Changing heptanol solutions every 8 hours during the time course of L6 differentiation resulted in a lasting drastic inhibition of myoblast fusion. We further investigated the effect of heptanol and of other uncoupling agents on the differentiation of primary cultures of embryonic chicken myoblasts. These cells are transiently coupled by gap junctions before myoblast fusion and prolonged application of heptanol, octanol and 18-beta-glycyrrhetinic acid also inhibited their fusion. The effect of heptanol and octanol was neither due to a cytotoxic effect nor to a modification of cell proliferation. Moreover, heptanol treatment did not alter myoblast alignment and adhesion. Taken together these observations suggest that intercellular communication might be a necessary step for myoblast fusion.

N-cadherin and N-CAM in myoblast fusion: compared localisation and effect of blockade by peptides and antibodies

The expression and distribution of two cell adhesion molecules, N-cadherin and N-CAM, at the surface of cultured leg muscle cells from 11-day-old chicken embryos were studied and compared. N-cadherin, which was expressed by fusing myoblasts, was down-regulated on old myotubes while N-CAM was still present. Both molecules, as viewed by confocal microscopy, appeared to have coaccumulated at the areas of contact between fusing myoblasts. However, immunogold electron microscopy did not reveal significant colocalization of N-cadherin and N-CAM, and their segregation after antibody-induced patching suggested the absence of direct interactions between N-cadherin and N-CAM. The role of the Ca2+ dependent cell adhesion molecule N-cadherin in myogenesis was investigated. Myoblast fusion was inhibited (1) with a synthetic peptide containing the H-A-V sequence and (2) with a monoclonal anti-N-cadherin antibody, demonstrating that N-cadherin-mediated cell adhesion is required for myoblast fusion. Under the same conditions no effect of anti-N-CAM antibodies was observed. Taken together these observations suggest that N-cadherin, acting independently from N-CAM, is a major cell adhesion molecule involved in embryonic myoblast fusion in vitro.

Involvement of cyclic GMP in the fusion of chick embryonic myoblasts in culture

We found that a transient rise in cGMP levels, which was closely associated with the Ca2+ influx, occurred concomitant with the onset of myoblast fusion. The Ca2+ channel blocker D600 decreased both the cell fusion and the normal rise in cGMP levels. In contrast, the Ca2+ ionophore A23187 transiently increased cGMP levels and induced precocious fusion. In addition, the cGMP analog 8-Br-cGMP induced precocious fusion as A23187 did. The guanylate cyclase inhibitor, methylene blue delayed the fusion in a dose-dependent manner without significantly affecting cell alignment, proliferation, or muscle-specific protein expression. Furthermore, methylene blue delayed the normal rise in cGMP levels, and the fusion block imposed by methylene blue was significantly recovered by 8-Br-cGMP. On the basis of our present findings, we suggest that a Ca2+ influx-dependent rise in cGMP levels is an important step in myoblast fusion.

Expression of choline acetyltransferase activity in a co-culture of spinal cord and skeletal muscle cells is inhibited by myogenic differentiation inhibitors

The effect of myogenic differentiation on the expression of choline acetyltransferase (ChAT) activity in co-cultured spinal cord neurons was studied. ChAT activity in spinal cord cells dissociated from 14-day mouse embryos was markedly increased when co-cultured with skeletal myotubes from 20-day embryos. This enhancement of ChAT activity was not observed in the presence of concanavalin A (ConA) or N-methyl-1-deoxynojirimycin (MDJN) which inhibits myoblast fusion, creatine phosphokinase and acetylcholinesterase activities in muscle cells. ChAT activity in spinal cord neurons cultured alone was unaffected by these agents. The inhibitory effect of ConA and MDJN was reversible, with an almost full recovery of ChAT activity following removal of the agents. Addition of ConA or MDJN after myotube formation exerted little inhibitory effect on ChAT activity. The effects of ConA and MDJN on ChAT activity in co-cultures were comparable to those on creatine phosphokinase and acetylcholinesterase. These observations indicate that the neurotrophic effects of skeletal muscle cells on spinal cord neurons are dependent on the differentiation state of the muscle cells.

Temporal differences in desmin expression between myoblasts from embryonic and adult chicken skeletal muscle

Desmin expression by myoblasts cultured from embryonic and adult chicken breast muscle was examined employing indirect immunofluorescence. The study was performed in conjunction with [3H]thymidine autoradiography and analysis of skeletal myosin expression in order to determine whether the desmin-expressing cells were terminally differentiated. Following 2 h of labeling with [3H]thymidine, 0.55%, 2.60%, and 15.10% of the cells in mass cultures from 10-day-old embryos, 18-day-old embryos and adults, respectively, incorporated [3H]thymidine and were desmin-positive but did not express skeletal-muscle-specific myosin. Using the same approach we determined that 0.07%, 1.25%, and 7.59% of the mononucleated cells in myogenic clones from 10-day-old embryos, 18-day-old embryos and adults, respectively, were desmin-positive, myosin-negative, [3H]thymidine-positive. We suggest that these desmin-positive, myosin-negative myoblasts are proliferating cells, and we conclude that the progeny of adult myoblasts exhibit more desmin-expressing cells of this type than embryonic myoblasts do.

Role of protein kinase C in chick embryo skeletal myoblast fusion

The involvement of Ca2+ and PGE1 in myoblast fusion has been well documented. Extracellular Ca2+ is essential for myoblast adhesion, alignment, and fusion. There is an obligatory increase in Ca2+ influx immediately preceding fusion and the Ca2+ ionophore A23187 promotes precocious fusion. PGE1 receptors appear just prior to fusion, and an antagonist of PGE1 binding to cell surface receptors blocks fusion when added prior to Ca2+ influx. Finally, exogenous PGE1 induces precocious fusion. The present study was an initial test of the hypothesis that membrane protein phosphorylation by protein kinase C (PKC) links PGE1 receptor occupancy and the increase in Ca2+ influx. Our conclusion that PKC is an essential component in the regulation of myoblast fusion is based in part on the following evidence: (1) an activator of PKC, the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA), at low concentration and for a brief exposure period, induces precocious fusion and stimulates Ca2+ influx; (2) 4 alpha-phorbol-12,13-didecanoate, an inactive analog of TPA, has no discernible effect on fusion or Ca2+ influx; (3) 1-oleoyl-2-acetyl glycerol, an analog of endogenous diacylglycerol (DAG) which activates PKC, promotes precocious fusion, as does the DAG kinase inhibitor R59022 (6-[2-[4-[(4-fluorophenyl)phenylmethylene]-1-piperidinyl]ethyl]-7- methyl-5H-thiazole-[3,2 alpha]-pyrimidin-5-one) which raises the level of endogenous DAG by inhibiting its catabolism; (4) 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7), a highly potent PKC inhibitor, reversibly blocks myogenesis at a point between alignment and fusion; and (5) H-7 also blocks the normal increase in Ca2+ influx preceding fusion.

Effects of cesium on in vitro myoblast differentiation: an electron microscopic study

This paper describes the microscopic evidence supporting a cesium-induced delay in the fusion of chick embryo myoblast membranes during in vitro myogenic differentiation. We have recently demonstrated that the sharp decrease in the conductivity and permittivity of the membranes of these myogenic cells at the time of fusion is delayed 30 h by the addition of cesium to the culture medium (Santini et al., Biochim. Biophys. Acta 945:56-64; 1988). We report here that this delay in fusion is substantiated by direct microscopic observation and that cesium also induces ultrastructural changes in the myoblast cells themselves. Possible mechanisms by which cesium may cause both the delay in fusion as well as the ultrastructural changes observed are discussed.

Correlation between fibronectin and its receptor in chick myoblast differentiation

Alterations in the amount of fibronectin and in the number of its receptors during myoblast differentiation of chicken embryo were investigated. The amount of fibronectin in the cell surface pool as measured by immunoblotting decreased during myogenesis. To identify and characterize the fibronectin receptors on the myoblasts, the interactions of the 28,000 dalton (28 kDa) amino terminal fragment and 85,000 dalton (85 kDa) cell-binding fragment of fibronectin with myoblasts were examined. The binding of the 28 kDa fragment was found to be time-dependent and reached a maximum level within 60 min. The unlabeled 28 kDa fragment inhibited the binding of the radioiodinated 28 kDa fragment, whereas the unlabeled 85 kDa fragment and antibody to integrin did not inhibit it, suggesting that the 28 kDa fragment interacts with the matrix assembly receptors but not with the cell adhesion receptors. There was a single class of 3.4 x 10(5) binding sites per cell with an apparent dissociation constant of 1.4 x 10(-7) M on 30 hr old myoblasts. The specific binding of the radioiodinated 28 kDa fragment to myoblasts decreased as the fusion proceeded. This decrease of binding was consistent with the decrease in the amount of fibronectin. Furthermore, the levels of fibronectin and binding of the radioiodinated 28 kDa fragment in the fusion-blocked myoblasts by EGTA treatment appeared to remain constant. These results suggest that the decrease and/or loss of fibronectin during myoblast fusion is closely correlated with the alteration of fibronectin receptors and with the fusion of myoblasts.

Involvement of cell surface phosphatidylinositol-anchored glycoproteins in cell-cell adhesion of chick embryo myoblasts

During myogenesis myoblasts fuse to form multinucleate cells that express muscle-specific proteins. A specific cell-cell adhesion process precedes lipid bilayer union during myoblast fusion (Knudsen, K. A., and A. F. Horwitz. 1977. Dev. Biol. 58:328-338) and is mediated by cell surface glycoproteins (Knudsen, K. A., 1985. J. Cell Biol. 101:891-897). In this paper we show that myoblast adhesion and myotube formation are inhibited by treating fusion-competent myoblasts with phosphatidylinositol-specific phospholipase C (PI-PLC). The effect of PI-PLC on myoblast adhesion is dose dependent and inhibited by D-myo-inositol 1-monophosphate and the effect on myotube formation is reversible, suggesting a specific, nontoxic effect on myogenesis by the enzyme. A soluble form of adhesion-related glycoproteins is released from fusion-competent myoblasts by treatment with PI-PLC as evidenced by (a) the ability of phospholipase C (PLC)-released material to block the adhesion-perturbing activity of a polyclonal antiserum to intact myoblasts; and (b) the ability of PLC-released glycoprotein to stimulate adhesion-perturbing antisera when injected into mice. PI-PLC treatment of fusion-competent myoblasts releases an isoform of N-CAM into the supernate, suggesting that N-CAM may participate in mediating myoblast interaction during myogenesis.

Expression of major histocompatibility complex class I antigens in rat muscle cultures: the possible developmental role in myogenesis

The expression of major histocompatibility complex class I antigens was demonstrated on aneurally cultured rat muscle cells. Myoblasts showed constitutive expression of class I antigens on their cell surfaces. The presence of the antigens was transitory, disappearing as myoblasts fused and differentiated into multinucleate myotubes. Furthermore, antibody against rat class I antigens showed an inhibitory effect on the generation of myotubes during muscle development. Although mature myotubes did not show any detectable levels of class I antigens on their cell surface, soluble factors from concanavalin A-activated spleen cells or interferon gamma could induce the expression of class I antigens on muscle fibers. These results suggest that the expression of class I antigens on muscle cells is not only immunologically modulated but also developmentally regulated and that the antigens may play a role in cell recognition and interactions during the fusion process of myogenesis.

Isolation and characterization of a variant myoblast cell line that is temperature sensitive for differentiation

A new variant rat myogenic cell line, ts485, was isolated by subcloning the cell line ts3b2 (H. T. Nguyen, R. M. Medford, and B. Nadal-Ginard, Cell 34:281-293, 1983). Unlike the progenitor cell line, ts485 was thermosensitive for differentiation. Experiments with conditioned medium suggested that diffusible extracellular factors were not involved in dictating the differential phenotypes of ts485 cells cultured at the permissive and nonpermissive temperatures. Temperature shift experiments performed on cultures of ts485 cells indicated that the temperature-sensitive lesion was in a factor active during the growth phase and required to trigger a cascade of events leading to terminal differentiation.

Isolation and characterization of a variant myoblast cell line that is temperature sensitive for differentiation

A new variant rat myogenic cell line, ts485, was isolated by subcloning the cell line ts3b2 (H. T. Nguyen, R. M. Medford, and B. Nadal-Ginard, Cell 34:281-293, 1983). Unlike the progenitor cell line, ts485 was thermosensitive for differentiation. Experiments with conditioned medium suggested that diffusible extracellular factors were not involved in dictating the differential phenotypes of ts485 cells cultured at the permissive and nonpermissive temperatures. Temperature shift experiments performed on cultures of ts485 cells indicated that the temperature-sensitive lesion was in a factor active during the growth phase and required to trigger a cascade of events leading to terminal differentiation.

A cell surface phosphoprotein of 48 kDa specific for myoblast fusion

The data we present here permit us to affirm that a 48 kDa phosphoprotein is the target of extracellular Ca2+ during fusion. It is detected only in fusion-competent L6 myoblasts and not in the fusion-defective spontaneous stable variants we isolated. The phosphorylation of this protein species can be totally inhibited by culturing myoblasts in a medium containing low Ca2+ concentrations (0.250 mM). However, under such conditions myoblasts do not fuse, but withdraw from the cell cycle and accumulate the muscle isoform of creatine kinase (M-CK). The results we have obtained support the following conclusions: (1) in fusion-competent cells, overall Ca2+-dependent phosphorylation of cell surface proteins appears to be necessary, but is not sufficient by itself for myoblast fusion; (2) the phosphorylation of a 48 kDa protein species is required for cell fusion; and (3) the phosphorylation of this 48 kDa protein is independent of other main events of cellular differentiation.

Prostaglandin binding does not require direct cell-cell contact during chick myogenesis in vitro

Myogenic differentiation in vitro involves at least three events at the cell surface: binding of prostaglandin to the cells, contact-mediated cell-cell recognition, and fusion of the myoblast membranes into myotubes. While the earlier events are thought to be necessary for subsequent fusion, the sequence of events has not been determined. A major impediment to determining the initial event has been the lack of synchrony of cell differentiation in vitro. To overcome this, we cultured chick embryo myoblasts as a suspension of single cells in gyratory rotation in medium without added Ca2+. Under these conditions, myoblasts exhibited characteristic prostaglandin binding at 34 h. Within 30 min, the cells began to aggregate. Because this occurred without change of medium or conditions of rotation, we termed the process autoaggregation. Within 8-10 h. cells within these autoaggregates began to fuse into syncytia. These results suggest that an early cell surface event in embryonic myogenesis is the characteristic binding of prostaglandin to the myoblasts. The results demonstrate that this binding precedes any direct cell-cell contact and suggest that it causes the subsequent change in myoblast cell-cell adhesion.

Phospholipid biosynthesis during normal and dystrophic avian muscle cell differentiation in culture

The biosynthesis of phospholipids during differentiation of normal and dystrophic muscle cells was studied using [32Pi] added to primary culture media. Labeled phosphatides were separated by two-dimensional thin-layer chromatography, exposing the plates to HCl vapors between solvents to cleave the alk-1-enyl bonds. Phosphatidylethanolamine labeling did not vary during myotube formation. Although lower labeling of phosphatidylcholine was observed in dystrophic myoblasts at the single cell stage, no differences were observed between normal and dystrophic cultures after cell contact and myotube formation. A sharp decrease in phosphoinositide content was observed as myoblasts first achieved cell contact and continued to decline slowly as myotubes developed. During myogenesis, phosphatidylcholine content gradually increased, from 18.4% of the total labeled phospholipid in dividing myoblasts to 48.6% of the labeled phospholipids in fully differentiated myotubes.

Prostaglandin binding activity and myoblast fusion in aggregates of avian myoblasts

Myoblast aggregates provide a system for studying cell interactions which have several advantages over standard, stationary cultures. In gyrotory rotation, aggregate size can be controlled and is independent of cell migration. In muscle aggregates, fibroblasts are excluded, yet myoblast differentiation and fusion occur in a highly synchronous fashion. Specific PG binding occurs in chick or quail myoblast aggregates: in chick the peak of binding is at 35-36 hr. Aggregation is complete 16 hr before PG binding activity appears. This suggests either that gyrotory aggregation is not identical to myoblast recognition, or that PG binding activity occurs subsequent to myoblast recognition. Myoblast aggregates begin to release PG before 18 hr. The amount detected remains constant until binding begins at 34 hr when PG binding to the aggregates begins. Thus, both the release of PG and PG receptor activity are characteristics of the myoblasts and release of prostaglandin precedes appearance of the binding activity. As a first step in identifying the PG receptor and determining its appearance on the myoblast cell surface, we have prepared antisera against myoblast surfaces which blocks receptor-ligand interaction and have absorbed it against both peripheral and intrinsic membrane fractions. The results indicate that the PG receptor is a myoblast peripheral membrane macromolecule.

Temperature-sensitive non-fusing myoblast variant and spontaneous revertant: isolation and characterization

A stable, temperature-sensitive, non-fusing variant of the L6 rat myoblast cell line has been isolated following mild EMS-induced mutagenesis. At the permissive temperature (37 degrees C), the growth characteristics and developmental pattern of the tsA1 variant are essentially identical to those of the parental L6D0 line at either 37 degrees C or 40 degrees C. At the nonpermissive temperature (40 degrees C), the tsA1 variant grows normally but does not align, fuse, or synthesize detectable amounts of beta-tropomyosin or myosin LC2. A peptide corresponding to myosin LClemb is barely detectable. The temperature-sensitive period spans the interval from 4 to 72 h post-plating with a midpoint at approximately 40 h. Under standard culture conditions, commitment to terminal differentiation occurs between days 3 and 4, and alignment and fusion begin on days 4 and 5, respectively. Thus, the temperature-sensitive event occurs very early in the L6 developmental program. A spontaneous revertant of the temperature-sensitive phenotype (tsA1 [R3]) exhibits recovery of the capacities to align, fuse, and synthesize the repertoire of muscle-specific proteins, suggesting that a single pleiotropic mutation in the tsA1 variant may regulate several stages in L6 myogenesis.

Phospholipase activity and plasma membrane homeostasis

Cyclic changes in the physiochemical state of the plasma membrane appear to be necessary for the normal functioning of the cell, especially with respect to division and differentiation. Such changes require a flexible membrane lipid composition to permit the necessary sequence of physicochemical changes to occur during these cellular events. This flexibility can be lost as a result of peroxidation-induced cross linking of membrane constituents, which prevents the normal physico-chemical membrane changes from occurring, resulting in abnormal cellular function. It is proposed that phospholipase A2 and C form a mutually regulatory enzyme system playing an important role with respect to the maintenance of membrane composition and flexibility.

Multiple effects of interferon on myogenesis in chicken myoblast cultures

Effects of chicken interferon on the differentiation of chicken skeletal muscle in vitro were examined. Continuous treatment of chicken myoblast culture with 200 IU/ml of interferon (10 IU/mg protein) resulted in significant inhibition of cell fusion and subsequent myotube formation. However, treatment of myoblast culture with 2 to 200 IU/ml of interferon increased activities of creatine kinase and myokinase in 4- or 6-day cultured muscle cells in a dose-dependent fashion. The effect of interferon on myokinase was less than on creatine kinase. Three-fold increase in creatine kinase activity induced by interferon was not accompanied by the accelerated transition of creatine kinase isozyme from BB- to MM-type. On the other hand, accumulation of acetylcholinesterase in interferon-treated cells at day 6 was suppressed to nearly half the level of control cells. Rates of actin and myosin synthesis in 4-day cultures estimated by pulse-labelling with [35S]methionine were also suppressed to 85% of control cultures. However, a proportion of 35S-labelled actin and myosin in labelled proteins associated with glycerinated cells was not changed by interferon treatment. These results indicate that partially purified interferon has multiple effects on the process of the myogenic differentiation of chicken myoblast in vitro.

The kinetics of myoblast fusion

The kinetics of myoblast fusion were estimated using two complementary assays. Both utilized suspensions of fusion-competent cells, i.e. 48-52-h cultures of chick pectoral muscle grown in a low-calcium medium, thus minimizing contributions arising from cellular migration. One assay, designed to measure the onset of membrane contiguity, relies on the transfer of a lipid dye, diI-C18-[3], from labelled to unlabelled cells. The other assay, designed to estimate the kinetics of appearance of morphologically distinct multinucleate cells in suspension (myoballs), relies on enzymic dissociation of cellular aggregates followed by nuclear staining. The assays demonstrate significant membrane contiguity within 20-30 min after initiating the fusion process; however, the multinucleate myoball morphology does not appear for at least one additional hour.

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.

Changes in cell surface antigens during in vitro lizard myogenesis

Indirect immunofluorescence has been used to examine surface antigens of lizard myogenic cells during in vitro differentiation. At least two developmental stage-specific surface alterations have been identified. One of these is a compositional change and involves the appearance of a cell-surface antigen(s) as the cells differentiate. This antigen(s) (Ag1422) is muscle specific and is characteristic of some rounded-up G0 myosin-positive myocytes, all stretched-back, G0 myosin-positive myocytes, and all identifiable myotubes. The antigen is not found on proliferating myoblasts, extended G1 (myosin-negative) cell-cycle-competent myoblasts or newly differentiated rounded-up, G0 myosin-positive myocytes. Pretreatment of cells with neuraminidase, trypsin, or proteinase K indicates the antigen is not present in "masked" form on normally nonreactive cells. Proteinase K is effective in the removal or destruction of the antigen, indicating it is at least partially protein in nature. The antigen is expressed in a similar developmental stage-specific fashion on early-passage myogenic cells taken from both adult lizard tail regenerates and embryonic muscle. The antibodies identifying Ag1422 can be removed by adsorption with homogenates of mature skeletal muscle. Therefore, Ag1422 is not an artifact due to in vitro conditions or the expression of a transformation antigen unique to the continuous culture line. The second alteration is an apparent restriction in the mobility of surface components (antigens and lectin receptors). Upon treatment with multivalent ligands, undifferentiated myosin-negative myoblasts exhibit rapid patching and capping of cell surface components while well-differentiated myocytes and myotubes do not. This mobility restriction is evident after the appearance of Ag1422. Treatment with cytochalasin B (15 micrograms/ml) and/or colchicine (100 microM) does not alter the restricted mobility of surface components seen on differentiated cells. Therefore, neither microfilaments nor microtubules seem to be involved in the mobility restriction. These observations are discussed in relation to current views of myogenesis.

Manipulation of myogenesis in vitro: reversible inhibition by DMSO

A system has been developed for the detailed analysis of the transition from proliferative myoblast to differentiated muscle cell. Dimethylsulfoxide (DMSO) prevents the terminal differentiation of L8 myoblasts in vitro, and its effect is reversible. DMSO (2%) inhibits the fusion of myoblasts to form multinucleate myotubes, the normal increases in activity of creatine phosphokinase (CPK) and acetylcholinesterase, and the synthesis of alpha-actin and acetylcholine receptor protein. Upon removal of DMSO from the medium, a lag precedes the onset of differentiation. The potential to inhibit muscle differentiation reversibly is not specific to DMSO, but is shared by a number of compounds, including dimethylformamide, hexamethylbisacetamide and butyric acid, all potent inducers of gene expression in Friend erythroleukemia cells. L8 cells routinely cease DNA synthesis and initiate fusion and muscle protein synthesis once they are confluent. In the presence of DMSO, however, nearly all cells continue DNA synthesis, even several days after reaching confluence. Protein synthetic patterns of DMSO-inhibited cells are almost indistinguishable from those of untreated myoblasts and distinct from differentiated myotubes. It appears that cells exposed to DMSO are locked indefinitely in a proliferative myoblast stage of development and are unable to enter the Go phase of the cell cycle necessary for initiation of differentiation. DMSO coordinately inhibits all the differentiative parameters measured. In contrast, cytochalasin B uncouples normally linked differentiative events so that fusion is inhibited while muscle-specific protein synthesis proceeds. DMSO has similar effects on both cytochalasin B-treated and fusing control cultures, suggesting that its primary effect is exerted not at the level of fusion but earlier in the differentiative time-table. Once fusion and the synthesis of muscle-specific proteins are well under way, the addition of DMSO is ineffective and differentiation continues in its presence. The potential to manipulate muscle gene expression in vitro makes this system particularly useful for the detailed analysis of the processes involved in the transition to the differentiated state and for determining the linkage of developmental events.