Effects of cytochalasin B on muscle cells in tissue culture

Distinctive Effects of Cytochalasin B in Chick Primary Myoblasts and Fibroblasts

Actin-based structures play fundamental roles in cellular functions. However it remains controversial how cells cope with the absence of F-actin structures. This report focuses on short- and long-term effects of cytochalasin B (CB) on actin-complexes in fibroblasts and myoblasts. Thirty min of CB treatment dispersed subplasma actin cortices, lamellipodia, ruffled membranes, stress fibers and adhesion plaques into actin patches in fibroblasts and muscle cells. In contrast, 72 hrs CB treatment showed distinct morphological effects. Fibroblasts became giant multinucleated-finger shaped with 5 to 10 protrusions, 3-8 μm in width, and >200 μm in length. They lacked cortical actin, stress fibers, adhesion plaques and ruffled membranes but contained immense lamelliopodia with abnormal adhesion plaque protein complexes. Muscle cells transformed into multinucleated globular-shaped but contained normal I-Z-I and A-bands, indicating that CB did not interfere with the assembly of myofibrils. Within 30 min after CB removal, finger-shaped fibroblasts returned to their original shape and actin-containing structures rapidly reappeared, whereas muscle cells respond slowly to form elongated myotubes following CB washout. The capacity to grow, complete several nuclear cycles, assemble intermediate filaments and microtubules without a morphologically recognizable actin cytoskeleton raises interesting issues related to the role of the actin compartments in eukaryotic cells.

Localization of sarcomeric proteins during myofibril assembly in cultured mouse primary skeletal myotubes

It is important to understand how muscle forms normally in order to understand muscle diseases that result in abnormal muscle formation. Although the structure of myofibrils is well understood, the process through which the myofibril components form organized contractile units is not clear. Based on the staining of muscle proteins in avian embryonic cardiomyocytes, we previously proposed that myofibrils formation occurred in steps that began with premyofibrils followed by nascent myofibrils and ending with mature myofibrils. The purpose of this study was to determine whether the premyofibril model of myofibrillogenesis developed from studies developed from studies in avian cardiomyocytes was supported by our current studies of myofibril assembly in mouse skeletal muscle. Emphasis was on establishing how the key sarcomeric proteins, F-actin, nonmuscle myosin II, muscle myosin II, and α-actinin were organized in the three stages of myofibril assembly. The results also test previous reports that nonmuscle myosins II A and B are components of the Z-bands of mature myofibrils, data that are inconsistent with the premyofibril model. We have also determined that in mouse muscle cells, telethonin is a late assembling protein that is present only in the Z-bands of mature myofibrils. This result of using specific telethonin antibodies supports the approach of using YFP-tagged proteins to determine where and when these YFP-sarcomeric fusion proteins are localized. The data presented in this study on cultures of primary mouse skeletal myocytes are consistent with the premyofibril model of myofibrillogenesis previously proposed for both avian cardiac and skeletal muscle cells.

Morphological changes and spatial regulation of diacylglycerol kinase-zeta, syntrophins, and Rac1 during myoblast fusion

The fusion of mononuclear myoblasts into multinucleated myofibers is essential for the formation and growth of skeletal muscle. Myoblast fusion follows a well-defined sequence of cellular events, from initial recognition and adhesion, to alignment, and finally plasma membrane fusion. These processes depend upon coordinated remodeling of the actin cytoskeleton. Our recent studies suggest diacylglycerol kinase-zeta (DGK-zeta), an enzyme that metabolizes diacylglycerol to yield phosphatidic acid, plays an important role in actin reorganization. Here, we investigated whether DGK-zeta has a role in the fusion of cultured C2C12 myoblasts. We show that DGK-zeta and syntrophins, scaffold proteins of the dystrophin glycoprotein complex that bind directly to DGK-zeta, are spatially regulated during fusion. Both proteins accumulated with the GTPase Rac1 at sites where fine filopodia mediate the initial contact between myoblasts. In addition, DGK-zeta codistributed with the Ca(2+)-dependent cell adhesion molecule N-cadherin at nascent, but not previously established cell contacts. We provide evidence that C2 cells are pulled together at cell-cell junctions by N-cadherin-containing filopodia reminiscent of epithelial adhesion zippers, which guide the advance of lamellipodia from apposing cells. At later times, vesicles with properties of macropinosomes formed close to cell-cell junctions. Reconstruction of confocal optical sections showed these form dome-like protrusions from the dorsal surface of contacting cells. Collectively, these results suggest DGK-zeta and syntrophins play a role at multiple stages of the fusion process. Moreover, our findings provide a potential link between changes in the lipid content of the membrane bilayer and reorganization of the actin cytoskeleton during myoblast fusion.

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.

Myofibrillogenesis in skeletal muscle cells

How are myofibrils assembled in skeletal muscles? The current authors present evidence that myofibrils assemble through a three-step model: premyofibrils to nascent myofibrils to mature myofibrils. This three-step sequence was based initially on studies of living and fixed cultured cells from cardiac muscle. Data from avian primary muscle cells and from a transgenic skeletal mouse cell line indicate that a premyofibril model for myofibrillogenesis also holds for skeletal muscle cells. Premyofibrils are characterized by minisarcomeres bounded by Z-bodies composed of the muscle isoform of alpha-actinin. Actin filaments are connected to these Z-bodies and to the mini-A-bands composed of nonmuscle myosin II filaments. Nascent myofibrils are formed when premyofibrils align and are modified by the addition of titin and muscle myosin II filaments. Mature myofibrils result when nonmuscle myosin II is eliminated from the myofibrils and the alpha-actinin rich Z-bodies fuse as the distance between them increases from 0.5 microm in premyofibrils to 2 to 2.5 microm in the mature myofibrils.

Goniodomin A, an antifungal polyether macrolide, exhibits antiangiogenic activities via inhibition of actin reorganization in endothelial cells

Goniodomin A (GDA) is an antifungal polyether macrolide isolated from the dinoflagellate Goniodoma pseudogoniaulax. Previous studies revealed that GDA profoundly affected cytoskeletal reorganization. We examined the effect of GDA on the angiogenic properties of vascular endothelial cells. GDA itself did not affect proliferation of, migration of, and tube formation in type I collagen gels by, bovine aortic endothelial cells (BAECs). Proliferation of BAECs stimulated by bFGF was not affected by GDA at concentrations of up to 10 nM. However, at similar concentrations, GDA significantly inhibited bFGF-induced migration and tube formation in type I collagen gels by BAECs. Actin reorganization is required for cell migration. GDA caused the perinuclear aggregation of filamentous actin and inhibited stress fiber formation in bFGF- or VEGF-stimulated BAECs and lysophosphatidic acid-stimulated HeLa cells. However, GDA did not affect stress fiber structures already formed through Gbetagamma expression or in constitutively active RhoA mutant HeLa cells. Finally, GDA inhibited forming of vasucular system in a chorioallantoic membrane. Our results indicated that GDA suppressed angiogenic properties of ECs at least in part through the inhibition of actin reorganization and inhibited angiogenesis in vivo.

Myoblast city, the Drosophila homolog of DOCK180/CED-5, is required in a Rac signaling pathway utilized for multiple developmental processes

The Rac and Cdc42 GTPases share several regulators and effectors, yet perform distinct biological functions. The factors determining such specificity in vivo have not been identified. In a mutational screen in Drosophila to identify Rac-specific signaling components, we isolated 11 alleles of myoblast city (mbc). mbc mutant embryos exhibit defects in dorsal closure, myogenesis, and neural development. DOCK180, the mammalian homolog of Mbc, associates with Rac, but not Cdc42, in a nucleotide-independent manner. These results suggest that Mbc is a specific upstream regulator of Rac activity that mediates several morphogenetic processes in Drosophila embryogenesis.

Identification of Kel1p, a kelch domain-containing protein involved in cell fusion and morphology in Saccharomyces cerevisiae

We showed previously that protein kinase C, which is required to maintain cell integrity, negatively regulates cell fusion (Philips, J., and I. Herskowitz. 1997. J. Cell Biol. 138:961-974). To identify additional genes involved in cell fusion, we looked for genes whose overexpression relieved the defect caused by activated alleles of Pkc1p. This strategy led to the identification of a novel gene, KEL1, which encodes a protein composed of two domains, one containing six kelch repeats, a motif initially described in the Drosophila protein Kelch (Xue, F., and L. Cooley. 1993. Cell. 72:681- 693), and another domain predicted to form coiled coils. Overexpression of KEL1 also suppressed the defect in cell fusion of spa2Delta and fps1Delta mutants. KEL2, which corresponds to ORF YGR238c, encodes a protein highly similar to Kel1p. Its overexpression also suppressed the mating defect associated with activated Pkc1p. Mutants lacking KEL1 exhibited a moderate defect in cell fusion that was exacerbated by activated alleles of Pkc1p or loss of FUS1, FUS2, or FPS1, but not by loss of SPA2. kel1Delta mutants form cells that are elongated and heterogeneous in shape, indicating that Kel1p is also required for proper morphology during vegetative growth. In contrast, kel2Delta mutants were not impaired in cell fusion or morphology. Both Kel1p and Kel2p localized to the site where cell fusion occurs during mating and to regions of polarized growth during vegetative growth. Coimmunoprecipitation and two-hybrid analyses indicated that Kel1p and Kel2p physically interact. We conclude that Kel1p has a role in cell morphogenesis and cell fusion and may antagonize the Pkc1p pathway.

Drosophila myoblast city encodes a conserved protein that is essential for myoblast fusion, dorsal closure, and cytoskeletal organization

The Drosophila myoblast city (mbc) locus was previously identified on the basis of a defect in myoblast fusion (Rushton et al., 1995. Development [Camb.]. 121:1979-1988). We describe herein the isolation and characterization of the mbc gene. The mbc transcript and its encoded protein are expressed in a broad range of tissues, including somatic myoblasts, cardial cells, and visceral mesoderm. It is also expressed in the pole cells and in ectodermally derived tissues, including the epidermis. Consistent with this latter expression, mbc mutant embryos exhibit defects in dorsal closure and cytoskeletal organization in the migrating epidermis. Both the mesodermal and ectodermal defects are reminiscent of those induced by altered forms of Drac1 and suggest that mbc may function in the same pathway. MBC bears striking homology to human DOCK180, which interacts with the SH2-SH3 adapter protein Crk and may play a role in signal transduction from focal adhesions. Taken together, these results suggest the possibility that MBC is an intermediate in a signal transduction pathway from the rho/rac family of GTPases to events in the cytoskeleton and that this pathway may be used during myoblast fusion and dorsal closure.

Effect of concanavalin A and vegetalizing factor on the outer and inner ectoderm layers of early gastrulae of Xenopus laevis after treatment with cytochalasin B

Neural (archencephalic) structures have been evoked in the competent ectoderm (consisting of both ectodermal layers) of Xenopus laevis by treatment with Concanavalin A (Con A), which probably acts on the plasma membrane. The size of the neural structures is increased when the ectoderm is incubated in Cytochalasin B prior to the Con A treatment. The results indicate that Cytochalasin B could have an influence on the binding of Con A to receptors on the plasma membrane. On the other hand, Cytochalasin B seems to have an inhibitory effect on the action of the vegetalizing factor, which could be correlated with the decline of endocytotic processes and internalization. In further series, it could be shown that the isolated superficial ectoderm, in contrast to the inner ectoderm layer, does not react to Con A treatment with the differentiation of neural structures. Studies with FITC-Con A indicate that the marker binds less to the outer ectoderm than to the inner ectoderm layer. However, by xenoplastic combinations of the outer ectoderm layer of X. laevis as reacting tissue and chordamesoderm of Triturus vulgaris as inducer, it could be demonstrated that the superficial layer, which is normogenesis does not come into contact with the inducing chordamesoderm but forms the ependymal part of the brain only, is also able to form archencephalic brain structures under in vitro conditions.

Localization of cytoplasmic and skeletal myosins in developing muscle cells by double-label immunofluorescence

Antibodies to a cytoplasmic myosin, rat lymphoma myosin, and to rat skeletal myosin were prepared in rabbits and shown to be specific for their corresponding antigens. The two antibodies did not cross-react. The skeletal myosin antibody was directly labeled with rhodamine, and the cytoplasmic myosin antibody was detected by indirect immunofluorescence with fluorescein-labeled goat anti-rabbit antibody. The two antibodies were used to examine developing rat muscle cultures for the presence and location of the antigens. The antibody to cytoplasmic myosin reacted with multinucleated myotubes and with all the mononucleated cells in the culture. The antibody to skeletal myosin reacted with myotubes and with a small fraction of the mononucleated cells. In the myotubes, the cytoplasmic myosin appeared to be localized primarily in two structures: fine stress fibers, often visible also by phase microscopy and present predominantly in the ends of the cells, and in a submembranous rim all along the cell's border. In addition, a diffuse fluorescence within the cells was observed. The skeletal myosin was localized in the central part of the myotubes in sarcomeres or in fibers without periodicities and was excluded from the ends of the myotubes. When the same cells were doubly stained with the two antibodies, the complementary distribution of the two isozymes was very clear. There was also a narrow region of overlap of staining, with cytoplasmic myosin present in some stress fibers that appeared to be continuous with fibrous elements containing skeletal myosin. Myotubes that rounded up with cytochalasin B or with trypsin displayed a diffuse distribution of both isozymes. When these cells were allowed to respread into extended configurations, the location of the two myosins were essentially the same as in untreated cells. The ability of myotubes to adhere to the surface and to move in culture may be related to the presence of cytoplasmic myosin. Our results show that in myotubes and myoblasts the two isozymes differ sufficiently to be localized in distinct regions of the cell and to be sorted out into different structures, even after the cytoplasmic contents have been reshuffled. The cell can, by some unknown mechanism, distinguish the two myosins.

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.

Response of myogenic and fibrogenic cells to cytochalasin B and to colcemid. I. Light microscope observations

Cytochalasin B (CB) induces a biphasic retraction is some cell types. The rapid response that peaks in 30 min leads to the "dendritic" condition. Replicating myogenic and fibrogenic cells, as well as postmitotic myoblasts and myotubes, participate in this reaction. This is followed by a slower phase that requires 40 h for stabilization and leads to the fully "absorized" state. Only replicating myogenic and fibrogenic cells participate in this reaction. Postmitotic myoblasts and myotubes do not arborize but round up and float off into the medium. Pretreatment with Colcemid does not block the rapid response to CB, but does block arborization. CB-arborized cells exposed to Colcemid while in the presence of CB develop sufficient tension to pull themselves apart. If CB depolymerizes actin-like filaments, and if such filaments constitute the only contractile system in the cell, then it is difficult to visualize how cells in CB develop such tension. Colcemid induces twisting, birefringent bands in interphase- and metaphase-arrested myogenic and fibrogenic cells, and in postmitotic myotubes. Such bands are more evident when CB-arborized cells are removed from CB and allowed to relax in Colcemid. These birefringent bands assemble in the prescence of cycloheximide, and may constitute 20% of the volume of the cell.

In vitro ovulation of trout oocytes: effect of prostaglandins on smooth muscle-like cells of the theca

Ovulation (active expulsion of oocyte from the mature follicle) of trout follicles matured in vivo can be induced in vitro by adding PGF2alpha at doses of 1 and 5 mug/ml. PGE2 is ineffective. The in vitro induction of ovulation by PGF2alpha is inhibited in a calcium free medium or by inhibitors of calcium influx, particularly by Mn++ and La+++, suggesting the ovulation process implies active contraction of the smooth muscle cells of the theca. A significant but partial inhibition is also observed with cytochalasin B (1 and 5 mug/ml) demonstrating that contraction of other cell types than muscle, containing actin-like filaments, may also participate in the process.

Effects of cytochaslasin B and colcemide on myogenic cultures

Muscle cultures treated with cytochalasin B yield mono- and oligonucleated cells of two kinds: (i) arborized, replicating precursor myogenic cells and fibroblasts; and (ii) round, post-mitotic, terminally differentiating myoblasts and myotubes. The arborized cells do not bind fluorescein-labeled antibody against myosin, do not contract rhythmically, and do not display hexagonally stacked thick and thin filaments. The round, mono-nucleated myoblasts and round, oligonucleated myotubes bind the fluorescein-labeled antibody against myosin, contract rhythmically, and display clusters of hexagonally-stacked thick and thin filaments. When cytochalasin B is removed and replaced by colcemide, the arborized cells, but not the post-mitotic muscle cells, acquire a radial symmetry and are induced to assemble massive, meandering cables that may occupy over 25% of the cell volume. These tortuous calbes are positively birefringent and consist exclusively of enormous numbers of 100-A, intermediate-sized filaments.

The use of cytochalasin B to distinguish myoblasts from fibroblasts in cultures of developing chick striated muscle

Cytochalasin B (5 mug/ml) elicits a differential effect on myoblasts and fibroblasts in culture. After 1 day in culture in the presence of the drug, two types of cells were observed, round cells and cells with elongated arms, designated "arborized" cells. Both cell types were examined in the electron microscope. The round cells contained aggregates of thin and thick filaments as well as a few intact sarcomeres. Within the arms of the arborized cells were bundles of intermediate sized filaments (100 A in diameter). The round cells could be shaken off the culture dish, washed free of cytochalasin B, and recultured to form myotubes. The remaining arborized cells lost their stellate shape when the drug was removed. The progeny of these cells gave rise to normal fibroblasts. Cytochalasin B, thus could be used to identify and isolate myoblasts prior to their fusion into developing muscle. It is suggested that this differential effect of the drug can be used to prepare pure cultures of fusible muscle cells uncontaminated by fibroblasts.

Action of cytochalasin D on cells of established lines. I. Early events

HeLa, Vero, L, HEp2, and MDBK cells respond immediately to 0.2-0.5 microg/ml cytochalasin D (CD) with sustained contraction (contracture), loss of microvilli, expression of endoplasmic contents (zeiosis), nuclear protrusion, and extension of cytoplasmic processes. The development of these changes is depicted, and the dose-response patterns in these cell lines are described. MDBK is generally most resistant and HeLa most sensitive to these effects of CD. Cells in G(1) are most sensitive to CD; responsiveness decreases progressively during early S and is least in mid S through G(2). CD inhibits transport of [(14)C]deoxyglucose in HeLa by about 45% but has no significant effect on hexose uptake in Vero and MDBK; sugar transport is thus apparently unrelated to any morphologic effect of CD. Although spreading and attachment are impeded, CD does not decrease and may even enhance the adhesiveness of established monolayers. Contraction appears to be a primary early effect of CD, upon which other visible changes follow. It is prevented by some inhibitors of energy metabolism (deoxyglucose and dinitrophenol) and does not occur in glycerinated models without ATP. The possible bases of the contractile response to CD are discussed. Although direct or indirect action of CD on some microfilaments may occur, a generalized structural disruption of contractile filaments by CD is considered unlikely.

Selective release of lysosomal hydrolases from phagocytic cells by cytochalasin B

1. Cytochalasin B (10mug/ml) enhances the release of rabbit polymorphonuclear leucocyte lysosomal acid hydrolases induced by retinol (vitamin A alcohol). 2. This effect is seen at doses of the vitamin that cause selective release of acid hydrolases and those causing more general enzyme release indicated by the loss of lactate dehydrogenase. 3. Cytochalasin B (2-50mug/ml) has no effect on the release of sedimentable acid hydrolases of intact granules obtained from disrupted polymorphonuclear leucocytes. 4. Cytochalasin B (2-10mug/ml) causes a time- and dose-dependent release of mouse peritoneal macrophage acid hydrolases. 5. This effect is selective at all doses of cytochalasin B used, since no release of lactate dehydrogenase, malate dehydrogenase and leucine 2-naphthylamidase was detected. 6. Treatment with cytochalasin B at doses of up to 10mug/ml for as long as 72h did not significantly change the total activities of any of the enzymes measured. 7. The lack of toxicity of cytochalasin B was shown by dye-exclusion tests and its failure to release radioactive colloidal gold stored in secondary lysosomes.

The effects of cytochalasin B on the microfilaments of baby hamster kidney (BHK-21) cells

Attempts were made to test the motile functions of bundles of microfilaments found in baby hamster kidney (BHK-21) cells, by using cytochalasin B (CB). It was found that individual cells respond differently to the drug. These differential effects are quite obvious in both light and electron microscope preparations. Some cells contain normal bundles of microfilaments even after 24 hr in CB, and other cells form muscle-like configurations which also contain arrays of microfilaments. These varied effects suggest the existence of several types of microfilaments in BHK-21 cells, and make the interpretation of the motile role of microfilaments difficult to evaluate at the present time.

The sensitivity of developing cardiac myofibrils to cytochalasin-B (electron microscopy-polarized light-Z-bands-heartbeat)

Developing cardiac muscle cells of 11- to 13-somite chick embryos are sensitive to cytochalasin-B. In cultured chick embryos, ranging in development from 11 to 13 somites, hearts stop beating in the presence of this agent. Both polarized light and electron microscopic examination show that cytochalasin-B disrupts existing myofibrils and inhibits the formation of new ones. Discrete Z-bands are not present in treated heart cells and thick, presumably myosin, filaments are found in disarray. These effects are reversible; after cytochalasin-B is removed from the medium, heartbeat recovers and myofibrils with discrete Z-bands reappear. Fibrillar sensitivity appears to be a function of age since fibrils in hearts of embryos having from 22 to 28 pairs of somites are more resistant.

Cytochalasin B: effects on cell morphology, cell adhesion, and mucopolysaccharide synthesis (cultured cells-contractile microfilaments-glycoproteins-embryonic cells-sorting-out)

Cytochalasin B reversibly causes extensive branching of myoblasts, fibroblasts, and nonencapsulated chondroblasts; it does not induce the formation of similar processes in myotubes, erythrocytes, amnion cells, encapsulated chondroblasts, or HeLa cells. The drug has no effect on the spontaneous contractions of isolated skeletal, cardiac, or smooth-muscle cells. Within 60 min, it depresses the incorporation of [(14)C]glucosamine into total mucopolysaccharide and glycoproteins by over 50%. The drug interferes with adhesion and sorting-out of dissociated embryonic cells. Cytochalasin B is likely to produce changes in components of the cell surface whose function is not readily or solely related to a system of "primitive contractile microfilaments."