Distribution of actin and tubulin in cells and in glycerinated cell models after treatment with cytochalasin B (CB)

Cytochalasans and Their Impact on Actin Filament Remodeling

The eukaryotic actin cytoskeleton comprises the protein itself in its monomeric and filamentous forms, G- and F-actin, as well as multiple interaction partners (actin-binding proteins, ABPs). This gives rise to a temporally and spatially controlled, dynamic network, eliciting a plethora of motility-associated processes. To interfere with the complex inter- and intracellular interactions the actin cytoskeleton confers, small molecular inhibitors have been used, foremost of all to study the relevance of actin filaments and their turnover for various cellular processes. The most prominent inhibitors act by, e.g., sequestering monomers or by interfering with the polymerization of new filaments and the elongation of existing filaments. Among these inhibitors used as tool compounds are the cytochalasans, fungal secondary metabolites known for decades and exploited for their F-actin polymerization inhibitory capabilities. In spite of their application as tool compounds for decades, comprehensive data are lacking that explain (i) how the structural deviances of the more than 400 cytochalasans described to date influence their bioactivity mechanistically and (ii) how the intricate network of ABPs reacts (or adapts) to cytochalasan binding. This review thus aims to summarize the information available concerning the structural features of cytochalasans and their influence on the described activities on cell morphology and actin cytoskeleton organization in eukaryotic cells.

The Effect of Cytochalasans on the Actin Cytoskeleton of Eukaryotic Cells and Preliminary Structure⁻Activity Relationships

In our ongoing search for new bioactive fungal metabolites, two new cytochalasans were isolated from stromata of the hypoxylaceous ascomycete Hypoxylon fragiforme. Their structures were elucidated via high-resolution mass spectrometry (HR-MS) and nuclear magnetic resonance (NMR) spectroscopy. Together with 23 additional cytochalasans isolated from ascomata and mycelial cultures of different Ascomycota, they were tested on their ability to disrupt the actin cytoskeleton of mammal cells in a preliminary structure–activity relationship study. Out of all structural features, the presence of hydroxyl group at the C7 and C18 residues, as well as their stereochemistry, were determined as important factors affecting the potential to disrupt the actin cytoskeleton. Moreover, reversibility of the actin disrupting effects was tested, revealing no direct correlations between potency and reversibility in the tested compound group. Since the diverse bioactivity of cytochalasans is interesting for various applications in eukaryotes, the exact effect on eukaryotic cells will need to be determined, e.g., by follow-up studies involving medicinal chemistry and by inclusion of additional natural cytochalasans. The results are also discussed in relation to previous studies in the literature, including a recent report on the anti-Biofilm activities of essentially the same panel of compounds against the pathogenic bacterium, Staphylococcus aureus.

The use of real-time cell analyzer technology in drug discovery: defining optimal cell culture conditions and assay reproducibility with different adherent cellular models

The use of impedance-based label-free technology applied to drug discovery is nowadays receiving more and more attention. Indeed, such a simple and noninvasive assay that interferes minimally with cell morphology and function allows one to perform kinetic measurements and to obtain information on proliferation, migration, cytotoxicity, and receptor-mediated signaling. The objective of the study was to further assess the usefulness of a real-time cell analyzer (RTCA) platform based on impedance in the context of quality control and data reproducibility. The data indicate that this technology is useful to determine the best coating and cellular density conditions for different adherent cellular models including hepatocytes, cardiomyocytes, fibroblasts, and hybrid neuroblastoma/neuronal cells. Based on 31 independent experiments, the reproducibility of cell index data generated from HepG2 cells exposed to DMSO and to Triton X-100 was satisfactory, with a coefficient of variation close to 10%. Cell index data were also well reproduced when cardiomyocytes and fibroblasts were exposed to 21 compounds three times (correlation >0.91, p < 0.0001). The data also show that a cell index decrease is not always associated with cytotoxicity effects and that there are some confounding factors that can affect the analysis. Finally, another drawback is that the correlation analysis between cellular impedance measurements and classical toxicity endpoints has been performed on a limited number of compounds. Overall, despite some limitations, the RTCA technology appears to be a powerful and reliable tool in drug discovery because of the reasonable throughput, rapid and efficient performance, technical optimization, and cell quality control.

The anti-proliferative agent jasplakinolide rearranges the actin cytoskeleton of plant cells

In the present study, we have characterized the action of the natural cyclodepsipeptide jasplakinolide (JAS) on the cytoplasmic architecture, actin-based cytoplasmic motility, and the organization of the actin cytoskeleton in selected examples of green algae (Acetabularia, Pseudobryopsis and Nitella) and higher plant cells (Allium bulb scale cells and Sinapis root hairs). JAS was capable of influencing the actin cytoskeleton and inhibiting cytoplasmic streaming in a differential, cell type-specific manner. With the exception of Nitella, two consecutive responses were observed upon incubation with 2.5 microM JAS: In the first phase cytoplasmic streaming increased transiently alongside with minor modifications of the actin cytoskeleton in the form of adventitious actin spots and spikes appearing throughout the cell cortex in addition to the normal actin bundle system typical for each cell type. In the second phase, cytoplasmic streaming stopped and the actin cytoskeleton became heavily reorganized into shorter, straight, more and more randomly oriented bundle segments. JAS exerted severe long-term effects on the actin cytoskeleton when treatments exceeded 30min at a concentration of 2.5 microM. An in situ competition assay using equimolar concentrations of JAS and FITC-phalloidin suggested that JAS has a phalloidin-like action. Effects of JAS were significantly different from those of cytochalasin D with respect to the resulting degree of perturbance of cytoplasmic organization, the distribution of actin filaments and the speed of reversibility.

Sensitivity of fibroblasts and their cytoskeletons to substratum topographies: topographic guidance and topographic compensation by micromachined grooves of different dimensions

Fibroblasts alter their shape, orientation, and direction of movement to align with the direction of micromachined grooves, exhibiting a phenomenon termed topographic guidance. In this study we examined the ability of the microtubule and actin microfilament bundle systems, either in combination with or independently from each other, to affect alignment of human gingival fibroblasts on sets of micromachined grooves of different dimensions. To assess specifically the role of microtubules and actin microfilament bundles, we examined cell alignment, over time, in the presence or absence of specific inhibitors of microtubules (colcemid) and actin microfilament bundles (cytochalasin B). Using time-lapse videomicroscopy, computer-assisted morphometry and confocal microscopy of the cytoskeleton we found that the dimensions of the grooves influenced the kinetics of cell alignment irrespective of whether cytoskeletons were intact or disturbed. Either an intact microtubule or an intact actin microfilament-bundle system could produce cell alignment with an appropriate substratum. Cells with intact microtubules aligned to smaller topographic features than cells deficient in microtubules. Moreover, cells deficient in microtubules required significantly more time to become aligned. An unexpected finding was that very narrow 0.5-microm-wide and 0.5-microm-deep grooves aligned cells deficient in actin microfilament bundles (cytochalasin B-treated) better than untreated control cells but failed to align cells deficient in microtubules yet containing microfilament bundles (colcemid treated). Thus, the microtubule system appeared to be the principal but not sole cytoskeletal substratum-response mechanism affecting topographic guidance of human gingival fibroblasts. This study also demonstrated that micromachined substrata can be useful in dissecting the role of microtubules and actin microfilament bundles in cell behaviors such as contact guidance and cell migration without the use of drugs such as cytochalasin and colcemid.

The actin network in the ciliary stalk of photoreceptors functions in the generation of new outer segment discs

Cytochalasin D (CD) interferes with the morphogenesis of outer segment disc membrane in photoreceptors. Disruption of either the actin network in the ciliary stalk, where membrane evagination is initiated, or the actin core of the calycal processes, whose position could define the disc perimeter, could be responsible. We have attempted to determine which of these local F-actin populations is involved in membrane morphogenesis and what step in the process is actin-dependent. Biocytin accumulation in nascent discs, detected by fluorescent avidin and laser scanning confocal microscopy (LSCM), provided a means of labeling abnormal discs and a measure of disc membrane addition. F-actin content and distribution were assessed using fluorescent phalloidin and LSCM. First, we examined the effects of a range of CD dosages (0.1, 1.0, or 10.0 microM) on rod photoreceptors in Xenopus laevis eyecup cultures. Ectopic outgrowth of discs, evaluated by LSCM and transmission electron microscopy (TEM), occurred at each concentration. Phalloidin labeling intensified in the ciliary stalk with increasing CD concentration, indicating F-actin aggregation. In contrast, it diminished in the calycal processes, indicating dispersal; TEM showed that calycal process collapse ensued. Disruption was evident at a lower concentration in the ciliary stalk (0.1 microM) than in the calycal processes (1.0 microM). TEM confirmed that the calycal processes remained intact at 0.1 microM. Thus, CD's action on the ciliary stalk network is sufficient to disrupt disc morphogenesis. Second, we examined the effect of CD on temperature-induced acceleration of the rate of disc formation. In the absence of CD, a 10 degrees C temperature shift increased the disc formation rate nearly three-fold. CD (5 microM) caused a 94% inhibition (P < 0.025) of this response; yet, the rate of membrane addition to ectopically growing discs exhibited the expected three-fold increase. Thus, CD's action interferes with the generation of new discs.

Cleavage furrow: timing of emergence of contractile ring actin filaments and establishment of the contractile ring by filament bundling in sea urchin eggs

Cleavage furrow formation at the first cell division of sea urchin and sand dollar eggs was investigated in detail by fluorescence staining of actin filaments with rhodamine-phalloidin of either whole eggs or isolated egg cortices. Cortical actin filaments were clustered at anaphase and then the clusters became fibrillar at the end of anaphase. The timing when the contractile ring actin filaments appear was precisely determined in the course of mitosis: accumulation of the contractile ring actin filaments at the equatorial cell cortex is first noticed at the beginning of telophase (shortly before furrow formation), when the chromosomal vesicles are fusing with each other. The accumulated actin filaments were not well organized at the early stage but were organized into parallel bundles as the furrowing progressed. The bundles were finally fused into a tightly packed filament belt. Wheat germ agglutinin (WGA)-binding sites were distributed on the surface of the egg in a manner similar to the actin filaments after anaphase. The WGA-binding sites became accumulated in the contractile ring together with the contractile ring actin filaments, indicating an intimate relationship between these sites and actin filament-anchoring sites on the plasma membrane. Myosin also appeared in the contractile ring together with the actin filaments. The ‘cleavage stimulus’, a signal hypothesized by Rappaport (reviewed by R. Rappaport (1986) Int. Rev. Cytol. 105, 245–281) was suggested to induce aggregation or bundling of the actin filaments in the cortical layer.

A method for enucleation of Saccharomyces cerevisiae

The first method for enucleation of yeast Saccharomyces cerevisiae is reported. Various strains, including some killer strain and respiratory-deficient mutants of Saccharomyces cerevisiae were enucleated after treatment with cytochalasin B. Removal of nuclei from protruding sphaeroplasts was induced by centrifugation in a Percoll density gradient. The enucleation yield (which averaged about 80%) and the quality of the cytoplasts were best when the yeast culture had been synchronized with nocodazole before the preparation. The presence of 1 mM CaCl2 and ATP (10 microM) in the enucleation medium prevented the formation of fragile products or aggregation. Cytoplasts could be stored for at least 1 day without visible deterioration.

Host cell actin assembly is necessary and likely to provide the propulsive force for intracellular movement of Listeria monocytogenes

Listeria monocytogenes is able to escape from the phagolysosome and grow within the host cell cytoplasm. By 3 h after initiation of infection, actin filaments begin to concentrate at one end of the bacterium. Polarization of F-actin is associated with intracellular bacterial movement, long projections of actin filaments forming directly behind the moving bacteria. New actin monomers are added to the region of the projection in proximity to the bacterium. The rate of new actin filament growth correlates closely with the speed of bacterial migration. This actin structure is anchored within the cytoplasm, serving as a fixed platform for directional expansion of the actin filament network. The actin projection progressively lengthens as the bacterium migrates. Cytochalasin blocks both elongation of the projection and bacterial movement but does not result in complete depolymerization of the bacterially induced actin structure, residual actin and alpha-actinin persisting in proximity to one end of the bacterium. Bacteria initially migrate within the cortical cytoplasm but later move to the peripheral membrane, where they form filopodiumlike structures which pivot and undulate in the extracellular medium. In the filopodia, bacteria are occasionally seen to abruptly change direction, turn 180 degrees, and move back into the medullary region of the host cell. All filopodium movement ceases once the bacterium containing the F-actin projection returns to the cortical cytoplasm. These results indicate that host cell actin polymerization is necessary for intracellular migration of listeriae and suggest that directional actin assembly may in fact generate the propulsive force for bacterial and filopodial movement.

Eicosanoid-mediated contractility of hepatic stellate cells

To approach experimentally the problem of contractility, stellate cells from rats were isolated and grown on a flexible silicone rubber substrate. Increases or decreases in the number of wrinkles of the silicone membrane beneath the cells that were easily observable by microscopy was employed as semi-quantitative measure of stellate cell motility. Contraction of stellate cells accompanied by diminution of cell body size was induced by U46619 (a thromboxane A2 analogue) and prostaglandin (PG) F2 alpha. Wrinkle formation became detectable 1.5 min after addition of 2 microM-U46619 and reached its maximum 10-15 min later. The effect of PGF2 alpha was not so striking, but lasted for a longer period of time. On the other hand, dibutyryl cyclic AMP, Iloprost (a PGI2 analogue) and PGE2 led to the disappearance or decrease in the number of wrinkles, indicating relaxation of contracted stellate cells. For instance, after addition of 2 microM-Iloprost, 47, 75 and 82% of contracted stellate cells had relaxed within 5, 10 and 20 min respectively. Moreover, dibutyryl cyclic AMP induced disappearance of alpha-smooth muscle actin stress fibres. This response became recognizable 10 min after addition of dibutyryl cyclic AMP; 40 min later, 97% of stellate cells were devoid of stress fibres. Thus stellate cells are able to undergo reversible contraction in primary culture, and the contraction of these cells may be mediated by eicosanoids that can be produced within the liver.

Changes in the cytoskeleton of 3T3 fibroblasts induced by the phosphatase inhibitor, calyculin-A

Addition of the protein phosphatase inhibitor, calyculin-A, to 3T3 fibroblasts causes a marked change in cell morphology. Initially the cells become rounded, develop surface blebs and then detach from the substratum. In the detached cells an unusual ball-like structure is observed. This study focuses on the cytoskeleton during these calyculin-A-induced morphological changes. Stress fibres disappear as the cells begin to round and aggregates of actin are formed towards the apical surface of the cell. These aggregates condense, in the detached cells, to form the ball structure of approximately 3 microns diameter. Between the ball and the nucleus are cables of intermediate filaments that appear to be attached to the surface of the ball and to the nuclear lamina. Using a procedure designed for the isolation of nuclei the nucleus-ball complex can be obtained. Analysis of the nucleus-ball preparation by immunofluorescence and electron microscopy demonstrate that the ball contains actin and that intermediate filaments are located between the ball and the nucleus. In this preparation, the intermediate filaments also appear to attach to the surfaces of the ball and the nucleus. Electrophoretic analysis of the nucleus-ball preparation indicates that, in addition to actin, a major component of the ball is myosin. It is suggested that the formation of the ball is caused by an actin-myosin-based contractile process, initiated by the phosphorylation of myosin. The aggregation of the actomyosin draws together the intermediate filaments into the area between the ball and nucleus. This hypothesis requires that vimentin binds both to the nucleus and to some component of the ball.

Cytochalasins induce actin polymerization in human leukocytes

We studied the effect of cytochalasins (B, D, and E) on the F-actin content in human neutrophils and lymphocytes using NBD-phallacidin labeling followed by flow cytometry. All three cytochalasins induced a concentration- and time-dependent increase in the F-actin content in both cell types. The order of potency was cytochalasin D greater than E greater than B. The increase in F-actin content was accompanied by a decrease in the G-actin content as measured by DNase I inhibition assay. These observations suggest that in intact cells cytochalasins may function differently compared to purified and semipurified systems, and their effects may be modified through other actin-binding or sequestering proteins. 2-deoxyglucose (20 mM) caused a decrease in the basal F-actin content and significantly reduced the change induced by the cytochalasins. These results suggest that the state of actin in intact cells is regulated by cytosolic ATP levels, primarily by the integrity of the glycolytic pathway. Based on these observations, we conclude that the mechanism of action of cytochalasins in intact cells is more complex than current models suggest.

Cytochalasin D treatment induces meiotic resumption in follicular sheep oocytes

Ovine cumulus-enclosed oocytes collected from antral follicles (3-5 mm in diameter) were cultured in vitro with 2 x 10(6) granulosa cells/ml in the presence or absence of gonadotropins or in the presence of cytochalasin D (CD). The maturation rate was assessed after 24 h of culture. In the control group, in the presence of gonadotropins (follicle-stimulating hormone-luteinizing hormone (FSH-LH; -10 micrograms/ml) 100% of the oocytes reached metaphase II. Whereas intercellular junctions were no longer present after 6-7 h of culture, germinal vesicle breakdown (GVBD) occurred by the same time. In contrast, in the absence of gonadotropin, the majority of the oocytes (59%) remained blocked in GV stage. The inhibition exerted by the granulosa cells on meiotic resumption was overcome when the cumulus-oocyte complexes (COCs) were incubated in CD (5 micrograms/ml) for 6 h at the beginning of the culture. Under these conditions, 85% of the oocytes matured with extrusion of the first polar body. Cytological analysis by cytofluorescence (NBD phallacidin) and electron microscopy showed that, after 6 h of treatment, CD provoked a redistribution of the microfilaments, mainly in the cumulus cells and to a lesser extent in the oocyte cortex. Intercellular junctions disappeared concomitantly with a significant decrease of the intercellular transport of tritiated uridine. The initiation of GVBD occurred at the same time. These results indicate that the resumption of meiosis was correlated with a loss of both junctional complexes (intermediate and gap junctions) between the cumulus cells and the oocyte.

The characterization of free, cytoskeletal and membrane-bound polysomes in Krebs II ascites and 3T3 cells

Polysomes from Krebs II ascites and 3T3 cells were separated into three populations by using a sequential extraction method. Free polysomes were released by using a combination of low salt (25 mM KCl) and NP-40 detergent in the lysis buffer. The cytoskeletal bound polysomes were subsequently released by raising the salt concentration to 130 mM and finally, polysomes bound to the membranes of the endoplasmic reticulum were extracted by the combined treatment with Triton X-100 and deoxycholate. The results presented here illustrate that the three polysome-containing fractions differ in many parameters such as polysome profiles, cytoskeletal components and phospholipid content. When polyA-containing mRNA was isolated from the three polysome fractions and translated in an in vitro system, some differences were observed in the patterns of proteins being synthesized.

Polarized expression of integrin receptors (alpha 6 beta 4, alpha 2 beta 1, alpha 3 beta 1, and alpha v beta 5) and their relationship with the cytoskeleton and basement membrane matrix in cultured human keratinocytes

In human keratinocytes cultured in conditions which allow differentiation and stratification and are suitable to reconstitute a fully functional epidermis, alpha 6 beta 4 and two members of the beta 1 integrin family (alpha 2 beta 1 and alpha 3 beta 1) were respectively polarized to the basal and lateral domains of the plasmamembrane both in growing colonies and in the reconstituted epidermis. Conversely, the alpha v integrin subunit, presumably in association with beta 5, was expressed at the basal surface in growing and migrating but not in stationary keratinocytes. The integrin alpha 6 beta 4: (a) was organized in typical patches which often showed a "leopard skin" pattern where spots corresponded to microfilament-free areas; (b) was not associated with focal contacts containing vinculin and talin but rather corresponded to relatively removed contact areas of the basal membrane as shown by interference reflection microscopy; and (c) was coherent to patches of laminin secreted and deposited underneath the ventral membrane of individual cells. The two beta 1 integrins (alpha 2 beta 1 and alpha 3 beta 1), both endowed with laminin receptor properties, were not associated with focal adhesions under experimental conditions allowing full epidermal maturation but matched the lateral position of vinculin (but not talin), cingulin, and desmoplakin, all makers of intercellular junctions. Often thin strips of laminin were observed in between the lateral aspects of individual basal keratinocytes. The integrin complex alpha v beta 5 had a topography similar to that of talin- and vinculin-containing focal adhesions mostly in the peripheral cells of expanding keratinocyte colonies and in coincidence with fibronectin strands. The discrete topography of beta 1 and beta 4 integrins has a functional role in the maintenance of the state of aggregation of cultured keratinocytes since lateral aggregation was impaired by antibodies to beta 1 whereas antibodies to beta 4 prevented cell-matrix adhesion (De Luca, M., R. N. Tamura, S. Kajiji, S. Bondanza, P. Rossino, R. Cancedda, P. C. Marchisio, and V. Quaranta. Proc. Natl. Acad. Sci. USA. 87:6888-6892). Moreover, the surface polarization of integrins followed attachment and depended both on the presence of Ca2+ in the medium and on the integrity of the cytoskeleton. We conclude that our in vitro functional tests and structural data suggest a correlation between the pattern of integrin expression on defined plasmamembrane domains and the mechanism of epidermal assembly.

ATP and calmodulin dependent actomyosin aggregates induced by cytochalasin D in goldfish retinal ganglion cell axons in vitro

Growing retinal ganglion cell (RGC) axons of the goldfish have mobile varicosities, which play a role in rapid bulk redistribution of axoplasm (Koenig, Kinsman, Repasky, and Sultz, 1985; Edmonds and Koenig, 1987). Varicosities contain a tubulo-vesicular SER embedded in an actin-containing cytomatrix (Koenig et al., 1985). Cytochalasin D (CD) induces the formation of focal cytoskeletal aggregates throughout preterminal axons and especially in varicosities. The aggregates are visible when labelled with fluoroscein isothiocyanate (FITC)-conjugated phalloidin. Double-labelling experiments show that Texas red-myosin or rhodamine isothiocyanate (RITC)-calmodulin immunofluorescence co-localizes with FITC-phalloidin-labelled aggregates. Formation of aggregates is blocked by calmidazolium, a calmodulin antagonist. Axon models permeabilized with digitonin retain the capacity to form focal aggregates in response to CD, when ATP or adenosine-5'-O(3-thiotriphosphate) (ATP-gamma S) is present in the permeabilization buffer, but not when 5'-adenylylimidodiphosphate (AMP-PNP) is present. The latter result indicates that formation of focal aggregates depends on ATP. The findings suggest that the formation of focal aggregates in immature axons is a manifestation of actomyosin interactions after free actin-filament ends are generated by CD treatment.

Distribution of fodrin in the keratinocyte in vivo and in vitro

Distribution of fodrin in the keratinocyte, both in vivo and in vitro, was examined by immunofluorescence microscopy. In the rat epidermis in vivo, fodrin was localized in the cell periphery of the spinous layer of all the skins studied. In only the basal layer of the thick skin, however, fodrin was seen intensely in the cytoplasm. As in vitro keratinocytes, a mouse cell line (Pam 212) cultured in low (0.06 mM) as well as standard (1.87 mM) Ca2+ was examined. In low Ca2+, fodrin was observed throughout the cytoplasm without marked accumulation irrespective of the cell density. The cytoplasmic labeling in low Ca2+ looked filamentous and became aggregated when cells were treated with cytochalasin B; at least some of the aggregates coexisted with those of F-actin. In contrast, fodrin distribution was not affected with colchicine. On the other hand, in standard Ca2+, the protein became concentrated along the cell periphery and less conspicuous in the cytoplasm as the cells reached confluency. When cells were transferred from low to standard Ca2+, the distribution of fodrin changed accordingly within 180 min. The present results indicate that fodrin in the keratinocyte is likely to be associated with actin filaments and that it takes two different ways of distribution both in vivo and in vitro. The peripheral and the cytoplasmic labeling of in vivo and in vitro cells are likely to correspond. It may be that fodrin changes its localization according to the cell's proliferative activity.

Action of general and alpha-smooth muscle-specific actin antibody microinjection on stress fibers of cultured smooth muscle cells

Arterial smooth muscle cells express alpha- and gamma-smooth muscle, as well as beta- and gamma-cytoplasmic actins. Two actin antibodies, one recognizing smooth muscle and cytoplasmic actin isoforms, the other recognizing specifically alpha-smooth muscle actin, were microinjected into cultured aortic smooth muscle cells. The effect of these antibodies on stress fiber organization was examined by staining with rhodamine-labeled phalloidin and by immunofluorescence with the same antibodies. Microinjection of the general actin antibody abolished most of the stress fiber staining with all reagents, but did not significantly affect the shape of the injected cells. This suggests that stress fiber integrity is not absolutely necessary for the maintenance of cell shape within the time of observation. Microinjection of the specific alpha-smooth muscle antibody abolished to various extents the staining of stress fibers with this antibody, but left practically intact their staining with rhodamine-labeled phalloidin and with the general actin antibody. This suggests that the incorporation of alpha-smooth muscle actin is not absolutely necessary for the maintenance of stress fiber integrity in cultured smooth muscle cells.

Cytochalasin D inhibits basal body migration and ciliary elongation in quail oviduct epithelium

The effects of cytochalasin D (CD) were studied by scanning (SEM) and transmission (TEM) electron-microscopic examination at different stages of ciliary differentiation in epithelial cells of quail oviduct. Immature quails were prestimulated by estradiol benzoate injections to induce ciliogenesis in the undifferentiated oviduct. After 24 h of CD culture, SEM study revealed inhibition of ciliogenesis and dilation of the apex of non-ciliated cells. TEM study showed that 2 h of CD treatment produced dilation of lateral intercellular spaces, after 6 h of treatment, this resulted in intracellular macrovacuolation. Vacuoles were surrounded by aggregates of dense felt-like material. CD also induced the disappearance of microvilli, and rounding of the apical surface of undifferentiated cells and those blocked in ciliogenesis. Centriologenesis was not inhibited by CD; basal bodies assembled in generative complexes in the supranuclear region after 24 h of treatment. However, the migration of mature basal bodies towards the apical surface was impaired. Instead, they anchored onto the membrane of intracellular vacuoles; growth of cilia was induced in the vacuole lumen. Cilium elongation was disturbed, giving abnormally short cilia with a dilated tip; microtubules failed to organize correctly.

Cytoskeletal F-actin patterns in whole-mounted insect Malpighian tubules

The distribution of actin filaments in Malpighian tubules of the fleshfly Sarcophaga bullata (Parker) was investigated before and after metamorphosis by means of the rhodamine phalloidin staining method. The numerous primary cells show a pattern of thick basal actin bundles resembling stress fibres of cultured cells, while the apical microvillar zone shows a bright and homogeneous labelling. The less abundant stellate cells contain no such basal actin bundles and their apical microvillar zone gets only faintly stained. Late larval stages display fingerlike infoldings and an increased actin filament concentration at the apical membrane of the stellate cells. During metamorphosis the Malpighian tubules dedifferentiate and eventually redifferentiate to give rise to adult tubules resembling larval ones. The different types of actin filament organisation in the primary and stellate cells of the Malpighian tubules are discussed.

Cytoskeletal dynamics in rabbit synovial fibroblasts: II. Reformation of stress fibers in cells rounded by treatment with collagenase-inducing agents

Modulation of the synthesis and secretion of extracellular matrix proteins and matrix-degrading metalloproteases by rabbit synovial fibroblasts is an important model system for studying the control of tissue-specific gene expression. Induction of collagenase expression is correlated with changes in cell shape and actin filament distribution, but the role of the cellular cytoskeleton in the sustained synthesis and secretion of metalloproteases has not been closely examined. When cells were allowed to respread after rounding by trypsin or cytochalasin, two known metalloprotease inducers, reformation of stress fibers was observed within 2 h in the presence of serum. In the absence of serum, trypsin-treated cells did not respread substantially, even after 24 h in culture. In contrast, cytochalasin-treated cells recovered almost as rapidly in the absence as in the presence of serum, showing reformation of well-formed microfilament bundles within 30 min of drug removal, especially at the spreading cell edges. High resolution electron-microscopic views of detergent-extracted cytoskeletons confirmed the rapid rebundling of peripheral microfilaments. Acrylamide-treated cells fell between these two extremes, spreading slowly in the absence of serum, but almost as rapidly as cytochalasin-treated cells in its presence. Reestablishment of normal intermediate filament distribution generally lagged slightly behind actin for all treatments, and intermediate filaments always appeared to spread back into the cellular cytoplasm within the confines of the reforming peripheral microfilament bundles. No obvious interaction between these two cytoskeletal elements was observed after any treatment, and no specific role for intermediate filaments in modulating gene expression in these cells is suggested by these results. The serum dependence displayed after trypsin or acrylamide treatment may be due to the disturbances in fibronectin synthesis observed in these cells and is consistent with evidence that both induction and sustained expression of matrix-degrading metalloprotease may involve signals transduced through plasma membrane matrix receptors (integrins).

Cell adhesion to fibronectin and tenascin: quantitative measurements of initial binding and subsequent strengthening response

Cell-substratum adhesion strengths have been quantified using fibroblasts and glioma cells binding to two extracellular matrix proteins, fibronectin and tenascin. A centrifugal force-based adhesion assay was used for the adhesive strength measurements, and the corresponding morphology of the adhesions was visualized by interference reflection microscopy. The initial adhesions as measured at 4 degrees C were on the order of 10(-5)dynes/cell and did not involve the cytoskeleton. Adhesion to fibronectin after 15 min at 37 degrees C were more than an order of magnitude stronger; the strengthening response required cytoskeletal involvement. By contrast to the marked strengthening of adhesion to FN, adhesion to TN was unchanged or weakened after 15 min at 37 degrees C. The absolute strength of adhesion achieved varied according to protein and cell type. When a mixed substratum of fibronectin and tenascin was tested, the presence of tenascin was found to reduce the level of the strengthening of cell adhesion normally observed at 37 degrees C on a substratum of fibronectin alone. Parallel analysis of corresponding interference reflection micrographs showed that differences in the area of cell surface within 10-15 nm of the substratum correlated closely with each of the changes in adhesion observed: after incubation for 15 min on fibronectin at 37 degrees C, glioma cells increased their surface area within close contact to the substrate by integral to 125-fold. Cells on tenascin did not increase their surface area of contact. The increased surface area of contact and the inhibitory activity of cytochalasin b suggest that the adhesive "strengthening" in the 15 min after initial binding brings additional adhesion molecules into the adhesive site and couples the actin cytoskeleton to the adhesion complex.

Membrane-cytoskeleton associations during myogenesis deviate from traditional definitions

Plasma membrane-cytoskeleton associations involving four membrane proteins (A5, H58, H36, and I20) were studied in developing L8E63 rat skeletal muscle cells using immunofluorescence microscopy and photometry on the basis of three criteria: Triton-insolubility, colocalization with cytoskeletal components, and sensitivity to cytoskeleton-directed drugs. The results presented demonstrate that there are developmental stage-specific associations between membrane proteins and the cytoskeleton during skeletal myogenesis. Several inconsistencies were found with traditional expectations of membrane-cytoskeleton associations. For example, although A5 is Triton-insoluble and sensitive to cytochalasin, its distribution generally does not correspond with any known cytoskeletal structure. Furthermore, the topography of A5 is dependent on the integrity of the plasma membrane. H36 and I20 are completely soluble in Triton and therefore by accepted definitions would not be expected to be associated with any cytoskeletal component. Yet H36 and actin codisrupt in the presence of cytochalasin, while I20, whose distribution does not correspond with microtubules, is uniquely sensitive to their disruption. These results demonstrate that (i) neither Triton-solubility nor colocalization alone predicts all membrane-cytoskeleton associations; some associations between the membrane and cytoskeleton are unstable in nonionic detergent; (ii) the native distribution of proteins in the membrane may not reflect their cytoskeletal associations; and (iii) the topography of some membrane proteins with no apparent association with the cytoskeleton may be greatly influenced by the cell cytoskeleton.

Transport across rat trachea in vitro after exposure to cytoskeleton-active drugs in vitro or to ozone in vivo

Full-length tracheas from Sprague-Dawley rats were exposed to cytoskeleton-active drugs in short-term organ culture, and the permeability of the tracheal epithelium was measured by instilling radiotracers into the lumen and assay of the radioactivity appearing in the external bathing medium. In vitro treatment with cytochalasin D (cyto D, 2-10 x 10(-6) M) increased the rate of movement of [14C]mannitol across the epithelium. Exposure to vinblastine (VB, 10(-4) M) alone had no significant effect. However, VB in combination with cyto D increased the permeability in a dose-dependent manner. In vivo exposure to ozone (O3, 0.8 or 2.0 ppm, 2 h) had only a slight effect on the rate of movement of the tracer as measured in vitro immediately after exposure. At 24 h postexposure there was no significant difference in permeability between ozone- and air-exposed tracheas. Prior in vivo O3 exposure sensitized the tracheas to the in vitro effects of cyto D; treatment of O3-exposed tracheas with cyto D immediately after O3 exposure produced a greater than additive effect on permeability measured in vitro. VB at concentrations up to 10(-4) M had no enhancing effect on permeability in O3-exposed tracheas. Sham exposure to clean air did not affect permeability compared to untreated (shelf) controls. Electron microscopic studies demonstrated penetration of horseradish peroxidase into intercellular spaces in the tracheas treated in vitro with cyto D or cyto D plus VB. Cyto D is known to affect intracellular microfilaments that have attachments at or near the cell surface, while VB affects microtubules associated with internal cellular structures. Therefore, the synergistic effect on tracheal permeability observed with O3 and cyto D, but not with O3 and VB, suggests that O3 may change cell surface structures associated with the microfilamentous cytoskeleton.

Latrunculins--novel marine macrolides that disrupt microfilament organization and affect cell growth: I. Comparison with cytochalasin D

The latrunculins are architecturally novel marine compounds isolated from the Red Sea sponge Latrunculia magnifica. In vivo, they alter cell shape, disrupt microfilament organization, and inhibit the microfilament-mediated processes of fertilization and early development. In vitro, latrunculin A was recently found to affect the polymerization of pure actin in a manner consistent with the formation of a 1:1 molar complex with G-actin. These in vitro effects as well as previous indications that the latrunculins are more potent than the cytochalasins suggest differences in the in vivo mode of action of the two classes of drugs. To elucidate these differences we have compared the short- and long-term effects of latrunculins on cell shape and actin organization to those of cytochalasin D. Exposure of hamster fibroblast NIL8 cells for 1-3 hr to latrunculin A, latrunculin B, and cytochalasin D causes concentration-dependent changes in cell shape and actin organization. However, the latrunculin-induced changes were strikingly different from those induced by cytochalasin D. Furthermore, while initial effects were manifest with both latrunculin A and cytochalasin D already at concentrations of about 0.03 microgram/ml, latrunculin A caused complete rounding up of all cells at 0.2 microgram/ml, whereas with cytochalasin D maximum contraction was reached at concentrations 10-20 times higher. The short-term effects of latrunculin B were similar to those of latrunculin A although latrunculin B was slightly less potent. All three drugs inhibited cytokinesis in synchronized cells, but their long-term effects were markedly different. NIL8 cells treated with latrunculin A maintained their altered state for extended periods. In contrast, the effects of cytochalasin D progressed with time in culture, and the latrunculin B-induced changes were transient in the continued presence of the drug. These transient effects were found to be due to a gradual inactivation of latrunculin B by serum and were used to compare recovery patterns of cell shape and actin organization in two different cell lines. This comparison showed that the transient effects of latrunculin B were fully reversible for the NIL8 cells and not for the mouse neuroblastoma N1E-115 cells.

Cytoskeletal involvement in spermiation and sperm transport

The process of spermiation and sperm transport was studied using specific inhibitors of cytoskeletal elements. Within 12-24 hr after the intratesticular injection of taxol, a compound that acts to stabilize microtubules and inhibit microtubule-related processes, an unusually large number of microtubules was seen within the body of the Sertoli cell. At the same time, transport of elements within the seminiferous epithelium was affected. At the end of stage VI of the cycle, step 19 spermatids were maintained in the deep recesses of the Sertoli cell and not transported to the rim of the seminiferous tubule lumen. At stage VIII, residual bodies remained at, or near, the rim of the tubule and were not transported to the base of the tubule. They underwent only partial degradation at this site, indicating that there may have been two phases involved in their dissolution--one autophagic and one phagocytic, but the latter did not occur since the residual bodies were not transported to Sertoli lysosomes at the base of the tubule. The observations suggest that microtubules are involved in transport processes within the seminiferous epithelium. Within 1-12 hr after the intratesticular injection of 500 microM cytochalasin D, a compound which interferes with actin-related processes, normal appearing tubulobulbar complexes were not present. The tubular portion (distal tube) of the complex did not initiate development. It was assumed that filaments (which were identified as such using NBD-phallacidin and the S-1 fragment of myosin) played an important role in the development of this portion of the complex. Cells did not eliminate cytoplasm normally, as evidenced by an enlarged cytoplasmic droplet, further emphasizing the published role for tubulobulbar complexes in cytoplasmic elimination. Although sperm were released normally from stage VIII tubules, many remained within the tubular lumen and did not traverse the duct system. Cytochalasin did not inhibit fluid secretion by the Sertoli cell, as demonstrated by efferent duct ligation, but did alter myoid cell actin cytoskeletal organization, suggesting that myoid cell contractility is primarily responsible for transport of sperm. Overall, the observations suggest that cytoskeletal activity of the Sertoli cell is important for several aspects of the spermiation process as well as sperm transport.

Actions of cytochalasins on the organization of actin filaments and microtubules in a neuronal growth cone

Actions of cytochalasin B (CB) on cytoskeletons and motility of growth cones from cultured Aplysia neurons were studied using a rapid flow perfusion chamber and digital video light microscopy. Living growth cones were observed using differential interference contrast optics and were also fixed at various time points to assay actin filament (F-actin) and microtubule distributions. Treatment with CB reversibly blocked motility and eliminated most of the phalloidin-stainable F-actin from the leading lamella. The loss of F-actin was nearly complete within 2-3 min of CB application and was largely reversed within 5-6 min of CB removal. The loss and recovery of F-actin were found to occur with a very distinctive spatial organization. Within 20-30 s of CB application, F-actin networks receded from the entire peripheral margin of the lamella forming a band devoid of F-actin. This band widened as F-actin receded at rates of 3-6 microns/min. Upon removal of CB, F-actin began to reappear within 20-30 s. The initial reappearance of F-actin took two forms: a coarse isotropic matrix of F-actin bundles throughout the lamella, and a denser matrix along the peripheral margin. The denser peripheral matrix then expanded in width, extending centrally to replace the coarse matrix at rates again between 3-6 microns/min. These results suggest that actin normally polymerizes at the leading edge and then flows rearward at a rate between 3-6 microns/min. CB treatment was also observed to alter the distribution of microtubules, assayed by antitubulin antibody staining. Normally, microtubules are restricted to the neurite shaft and a central growth cone domain. Within approximately 5 min after CB application, however, microtubules began extending into the lamellar region, often reaching the peripheral margin. Upon removal of CB, the microtubules were restored to their former central localization. The timing of these microtubule redistributions is consistent with their being secondary to effects of CB on lamellar F-actin.

Regulation of actin microfilament integrity in living nonmuscle cells by the cAMP-dependent protein kinase and the myosin light chain kinase

Microinjection of the catalytic subunit of cAMP-dependent protein kinase (A-kinase) into living fibroblasts or the treatment of these cells with agents that elevate the intracellular cAMP level caused marked alterations in cell morphology including a rounded phenotype and a complete loss of actin microfilament bundles. These effects were transient and fully reversible. Two-dimensional gel electrophoresis was used to analyze the changes in phosphoproteins from cells injected with A-kinase. These experiments showed that accompanying the disassembly of actin microfilaments, phosphorylation of myosin light chain kinase (MLCK) increased and concomitantly, the phosphorylation of myosin P-light chain decreased. Moreover, inhibiting MLCK activity via microinjection of affinity-purified antibodies specific to native MLCK caused a complete loss of microfilament bundle integrity and a decrease in myosin P-light chain phosphorylation, similar to that seen after injection of A-kinase. These data support the idea that A-kinase may regulate microfilament integrity through the phosphorylation and inhibition of MLCK activity in nonmuscle cells.

Expression of cytoskeleton components in various Epstein-Barr virus infected cell lines

It has been shown that viruses can induce alterations in the content and distribution of cytoskeleton structures, particularly actin microfilaments and microtubules. An immunomorphologic study of the cytoskeleton components of various EBV-infected cell lines with expression of different functions of EBV genome has been performed using antibodies to each of its three major components. Intermediate filaments and microtubules were similarly represented in all examined lymphoblastoid cell lines. The distribution of actin microfilaments, on the contrary, differed significantly from cell line to cell line. It is concluded that the morphologic expression of actin might depend on the expression of EBV genome. Furthermore, some of these cell lines might represent a useful substrate for the identification of anticytoskeleton antibodies, mainly anti-actin antibodies, in human sera.

Acetylcholine receptor clustering and triton solubility: neural effect

Previous studies by Prives et al. (1980, 1982a and b) have shown that acetylcholine receptors (AchRs) are extracted from muscle cells in vitro by Triton X-100 at different rates, and that clustered receptors extract most slowly. The present study was aimed at comparing the relative extractability of receptors in clusters with those in intercluster regions and the role of neural factors in regulating this extractability. Using primary rat muscle cells in vitro we confirmed that receptor extraction with Triton X-100 does not fit a single exponential but has more than one rate, and that in control cells clustered receptors extract more slowly than do receptors in intercluster regions. The major new observation in this study was that neural extract lowered the overall Triton extraction rate of intercluster receptors to that of clustered receptors. Additional new observations include the findings that (1) both clustered and intercluster receptors show multiphasic extraction rates; (2) stabilization of AchRs against Triton extraction increases with time in the surface membrane; (3) the effect of neural extract on Triton extractability of AChR is dependent on factors that control RNA synthesis, cytoskeletal elements, and collagen; (4) fixation and/or buffer washes accelerate receptor extraction only in cells that are treated with Triton, but not in control cells; (5) in control cells (not exposed to neural factors) Triton X-100 causes new clusters to form. From experiments using Con A we suggest that the Triton-induced new clusters may not be formed by a redistribution of receptors but are, most likely, due to the presence of groups of intercluster receptors with extraction rates lower than those of surrounding receptors.

Airway permeability in rats exposed to ozone or treated with cytoskeleton-destabilizing drugs

Ozone (O3) exposure of rats increases airway epithelial permeability. We hypothesized that this increased permeability may be mediated by the epithelial cell cytoskeleton. To test this hypothesis, we studied the effect of cytoskeletal disruption on the transmucosal transport of tracers from airway lumen to blood and compared the results with the effects of O3 exposure. No increase in transport occurred following disruption of microtubules by vinblastine, but disruption of microfilaments with cytochalasin D resulted in increased transport of radiolabeled tracers [99mTc- and 111In-labeled diethylenetriamine-pentacetate (DTPA) and 125I-labeled bovine serum albumin (BSA)]. In control rats, both horseradish peroxidase (HRP) and BSA, localized by cytochemistry and autoradiography, respectively, were detected on the epithelial cell surfaces and in endocytic vesicles. In rats treated with cytochalasin D or exposed to O3, the tracer molecules also penetrated the intercellular spaces, though the apical tight junctions remained devoid of the tracers. Increased numbers of endocytic vesicles containing HRP and aggregation of 125I-labeled BSA autoradiographic grains in the subepithelial region were also seen after either treatment. We conclude that destabilization of cytoskeletal elements following O3 exposure is a possible mechanism of increased transmucosal transport, which may be a combined effect of accelerated transport through both endocytic and paracellular pathways.

Alterations in chondrocyte cytoskeletal architecture during phenotypic modulation by retinoic acid and dihydrocytochalasin B-induced reexpression

The differentiated phenotype of rabbit articular chondrocytes was modulated in primary culture by treatment with 1 microgram/ml retinoic acid (RA) and reexpressed in secondary culture by treatment with the microfilament-disruptive drug dihydrocytochalasin B (DHCB) in the absence of RA. Because the effective dose of DHCB (3 microM) did not elicit detectable cell rounding or retraction, the nature and extent of microfilament modification responsible for induction of reexpression was evaluated. The network of microfilament stress fibers detected with rhodamine-labeled phalloidin in primary control chondrocytes was altered by RA to a "cobblestone" pattern of circularly oriented fibers at the cell periphery. Subsequent treatment with DHCB resulted in rapid changes in this pattern before overt reexpression. Stress fibers decreased in number and were reoriented. Parallel arrays of long fibers that traversed the cell were evident, in addition to fiber fragments and focal condensations of staining. Immunofluorescent staining of intermediate filaments revealed a marked decrease in complexity and intensity during RA treatment but no change during reexpression. An extended microtubular architecture was present throughout the study. These results clearly identify microfilaments as the principal affected cytoskeletal element and demonstrate that their modification, rather than complete disruption, is sufficient for reexpression. The specificity of DHCB and the reorientation of these filaments before reexpression of the differentiated phenotype suggests a causative role in the mechanism of reexpression.

Effects of inhibitors of the cytoplasmic structures and functions on the early phase of infection of cultured cells with simian virus 40

To obtain information about cytoplasmic structures and functions involving the entry of simian virus 40 virions into cells, we examined whether the inhibitors that affect the functions and/or structure of lysosomes, cell membrane, and cytoskeletons inhibit expression of nuclear T antigen in the SV40-inoculated rat 3Y1 and monkey CV-1 cells. Chloroquine, methylamine, and butylamine did not inhibit T-antigen expression, suggesting that lysosomal acidification is not required for establishment of infection. Cytochalasin B had no effect, suggesting that microfilaments are not involved. Monensin, colcemid, and amantadine each inhibited T-antigen expression at doses causing no obvious cytotoxicity. Maximal inhibition was seen when these inhibitors were added to the cultures within 1 hr (monensin), within 4 hr (colcemid), or within 12 hr (amantadine) after virion adsorption to the cell surface. When the inhibitor was present in the virus-inoculated cultures for 24 hr and then removed, nuclear T antigen began to be expressed at 4 hr (monensin), 9 hr (colcemid), or 1 hr (amantadine) after removal of the inhibitors. Results of SDS-PAGE analysis of immunoprecipitated radiolabeled proteins of infected cells revealed that amantadine inhibited synthesis of large and small T antigens as well as general protein synthesis. Inhibition by colcemid may be due to disruption of microtubules, because other microtubule-disrupting agents (colchicine, vinblastine, nocodazole, and podophyllotoxin) also inhibited appearance of nuclear T antigen but lumicolchicine and taxol did not. Electron microscopy revealed that, in the presence of colcemid, although the adsorbed virions were readily internalized to form pinosomes, vectorial movement of the pinosomes to the nucleus appeared to be inhibited. Results of electron microscopy also suggest that inhibition by monensin may occur mainly in internalization of adsorbed virions and that the inhibition is leaky such that the early steps of infection proceed slowly in the presence of monensin. We conclude that monensin, colcemid, and amantadine interfere with mutually different early events of SV40 infection.

Enucleation of normal and transformed cells

A quantitative analysis based on centrifugal force requirements for enucleation was developed to examine the response of a number of untransformed and transformed cell lines to cytochalasin mediated enucleation. Examination of the extent of cell enucleation as a function of centrifugal force resulted in a series of response curves demonstrating that enucleation g force requirements varied between Balb/c 3T3, Swiss 3T3, and Kirsten sarcoma virus transformed Balb/c 3T3 (3T3-K). A four times greater centrifugal force was required to reach 50% enucleation for transformed Balb/c 3T3-K when compared to Swiss 3T3. A qualitative correlation could be observed between ease of enucleation and the existence of a well-formed stress fiber network. A comparison of cytochalasin B and D suggested that cytochalasin D was far more effective in the enucleation of transformed cells. Experiments with 2-deoxyglucose and monensin provided evidence that decreasing cellular ATP levels, either directly or potentially by uncoupling ion transport from ATP generation, can decrease the efficiency of enucleation. It is suggested that the organization of the cytoskeleton is affected by the altered cellular ATP levels which can affect the centrifugal requirements of enucleation.

Cytochalasin releases mRNA from the cytoskeletal framework and inhibits protein synthesis

Cytochalasin D was shown to be a reversible inhibitor of protein synthesis in HeLa cells. The inhibition was detectable at drug levels typically used to perturb cell structure and increased in a dose-dependent manner. The drug also released mRNA from the cytoskeletal framework in direct proportion to the inhibition of protein synthesis. The released mRNA was unaltered in its translatability as measured in vitro but was no longer translated in the cytochalasin-treated HeLa cells. The residual protein synthesis occurred on polyribosomes that were reduced in amount but displayed a normal sedimentation distribution. The results support the hypothesis that mRNA binding to the cytoskeletal framework is necessary although not sufficient for translation. Analysis of the cytoskeletal framework, which binds the polyribosomes, revealed no alterations in composition or amount of protein as a result of treatment with cytochalasin D. Electron microscopy with embedment-free sections shows the framework in great detail. The micrographs revealed the profound reorganization effected by the drug but did not indicate substantial disaggregation of the cytoskeletal elements.

Effects of microfilament disrupters on microfilament distribution and morphology in maize root cells

Maize root tip cells were examined for the distribution of actin microfilaments in various cell types and to determine the effects of microfilament disrupters. Fluorescence microscopy on fixed, stabilized, squashed cells using the F-actin specific probe, rhodamine-labelled phalloidin, allowed for a three-dimensional visualization of actin microfilaments. Microfilaments were observed as long, meandering structures in root cap cells and meristematic cells, while those in immature vascular parenchyma were abundant in the thin band of cytoplasm and were long and less curved. By modifying standard electron microscopic fixation procedures, microfilaments in plant cells could be easily detected in all cell types. Treatment with cytochalasin B, cytochalasin D and lead acetate, compounds that interfere with microfilament related processes, re-organized the microfilaments into abnormal crossed and highly condensed masses. All the treatments affected not only the microfilaments but also the accumulation of secretory vesicles. The vivid demonstration of the effects of all of these microfilament disrupters on the number and size of Golgi vesicles indicates that these vesicles may depend on microfilaments for intracellular movement.

Contraction and relaxation of cultured mesangial cells on a silicone rubber surface

Glomerular mesangial cells (MC) in culture are believed to contract or relax in response to agents such as angiotensin II and cyclic AMP. However, cells grown on glass or plastic surfaces are limited in their response to vasoactive agents because of the rigid surfaces to which they adhere; thus, interpretation of a change in shape as contraction, relaxation, or detachment is difficult. We have grown MC on a flexible silicone rubber (dimethylpolysiloxane) substrate (DMPS), and studied with sequential photographs several models of cell contraction, relaxation, and detachment. When the cells contracted, the DMPS became wrinkled; when the cells relaxed, the DMPS lost wrinkles. In contrast, if the cells detached, the sheet lost wrinkles as the cells became smaller and rounder. Angiotensin II (5 X 10(-7) M), and calcium ionophore A23187 (2 X 10(-6) M) increased wrinkles in more than 30% of cells at 22 degrees C and more than 40% of the cells at 36 degrees C. The earliest effect was visible within five to 10 minutes at 22 degrees C and within one minute at 36 degrees C and increased until 40 minutes; thereafter, the cells relaxed and wrinkles were reduced. 10(-1) M Na azide prevented the increase in wrinkles produced by angiotensin II. Seventy-two percent of the angiotensin II-treated cells whose margins could be seen in their entirety, and 78% of the calcium ionophore-treated cells showed a reduction in surface area at a time when new wrinkles were appearing or wrinkles were increasing in size. Dibutyryl cyclic AMP, a known smooth muscle relaxant, produced a decrease or loss of wrinkles in 90% of the cells, and an accompanying increase in surface area. Untreated control cells, observed in conjunction with the above series, showed little change in wrinkles. Ten percent DMSO, an actin-translocating agent, produced a reversible disappearance of wrinkles. These models of contraction and relaxation could be distinguished from cell detachment; EDTA, for example, in the presence of zero calcium, diminished both cell size and wrinkles, with an accompanying lifting of cells from the surface. Similar results were obtained with cytochalasin B and chlorpromazine. Thus, the silicone rubber system accurately reflects the contraction, relaxation and detachment of cultured mesangial cells in response to a variety of agents.

Cytochalasin D and cationized ferritin as probes for the morphological investigation of blebbing in two human cell lines

The potent fungal metabolite cytochalasin D (CD) and cationized ferritin (CF) are used in combination to test for negative charge distribution on blebs (knobs). Two established human epithelial cell lines, WISH and HeLa, that display blebs in various phases of the cell cycle or under certain culture conditions are investigated. CD alone, applied at a low concentration (1.0 micrograms/ml) and for a short time period (3 min), causes blebs to appear as the prevalent surface feature. These are filled mainly with free ribosomes. Additionally, feltlike mats, presumed to be disorganized, compacted microfilaments, are formed directly beneath the cell membrane. These are especially evident in the cortical cytoplasm below the blebs or bleb clusters. CF (0.345 mg/ml), applied for a 5-min period after CD administration (1.0 microgram/ml) for 3 min, appears along the surface of microvilli, at the base of blebs, and in vesicles beneath the bleb clusters. In some cases, microfilaments (6 nm in diameter) are closely related to the vesicles. CF does not preferentially bind to the apical cell membrane of blebs. Above areas of the subplasmalemmal microfilaments, CF membrane binding is apparent, even under circumstances where the filaments are disorganized by cytochalasin treatment. These results seem to show the following: bleb membranes are different from the remainder of the cell and do exhibit a loss of negative charge and surface charge may be dependent on the presence or structural integrity of membrane-related 6-nm microfilaments.

Characterization of a polypeptide associated with coated vesicles and the cytoskeleton which is recognized by a CREST serum

Serum from an individual with the CREST syndrome (calcinosis, Raynaud's phenomenon, esophageal dismotility, sclerodactyly, telangiectasia) reacts not only with kinetochores, but also with a cytoplasmic, phosphorylatable polypeptide, which is shown by immunofluorescence in whole cells and immunoelectronmicroscopy in sections to be associated with actin stress fibres in cultured mammalian cells. The antigen shows some variation in molecular weight between species, estimated by immunoblotting to range from 68 to 76 kD between mouse, Chinese hamster, sheep and human cells. Much of the polypeptide copurifies with coated vesicles, of which approx. 5% bound antibody from the serum, as detected by immunogold electronmicroscopy.

Involvement of actin-containing microfilaments in HSV-induced cytopathology and the influence of inhibitors of glycosylation

Two and a half hours after infection with a high dose of different strains of HSV-1 which induce rounding of cells, breakdown of actin containing microfilaments can be observed. At the periphery of the cell, actin containing knob-like protuberances were visible. Later on, actin seems to be located exclusively on the surface of cells. Observations were done by immunofluorescence microscopy, scanning electron-microscopy and immunoperoxidase staining of ultrathin sections. The envelope of HSV appears to be stained by anti-actin. Strain IES produces rounding of cells at a high dose of infection before fusion proceeds at 37 degrees C. Similar alterations were not observed with the fusing strains MP and HFEM. Incubation of infected cells at 39 degrees C revealed strain dependent differences of the fusion activity. At 41 degrees C no "fusion from within" of cells but only rounding was detectable. Application of tunicamycin resulted in complete inhibition of fusion by all strains. The fusion activity of some strains of HSV-1 (ANG, HFEM, and MP) was not inhibited by addition of 2-deoxy-D-glucose and 2-fluoro-deoxy-D-glucose. A variant from strain MP could be isolated, which is sensitive to the effects of 2-deoxy-D-glucose. Inhibitors of processing of glycoproteins did not affect fusion of cells.

Unusual morphological changes in cultured oligodendrocytes induced by cytochalasin B

When cultured chick oligodendrocytes were treated with more than 1 microM of cytochalasin B (CB), an unusual effect, namely the extension of curly processes, was observed in a dose-dependent manner within 2 h after CB was added, that is, the processes with long, thin and curled shapes developed. The effect was found to be reversible and to be inhibited by colchicine. It was demonstrated by immunofluorescence that CB induced the depolymerization of actin filaments and the enhanced assembly of microtubules in cellular processes.

Reversible induction of actin rods in mouse C3H-2K cells by incubation in salt buffers and by treatment with non-ionic detergents

Incubating conditions which induced actin paracrystal-like intracellular structures (actin rods) were investigated by using several cell lines. We have found that an incubation of cells of a mouse fibroblastic cell line, C3H-2K, in an isotonic solution of NaCl containing 1 mM MgCl2, 1 mM CaCl2 and 10 mM MES, pH 6.5, induced disintegration of stress fibers and formation of actin rods in the cytoplasm. Actin rods were induced also by incubating in salt buffers in which Na+ of the above solution was substituted by most cations except K+ or Rb+. When the actin rod-forming cells were transferred back to DMEM containing 10% FBS, actin rods disappeared and stress fibers subsequently re-formed within 1 h at 37 degrees C. Although the induction was observed in NaCl buffer at a wide range of pH values (5.5-10), the optimal pH was 6.5. Formation of actin rods is dependent upon cellular metabolism, as it was inhibited at 4 degrees C, or by metabolic inhibitors. Incubation in NaCl buffer induced actin rods in HeLa, L, NRK, BALB/c 3T3 and Swiss 3T3 cells, but not in CEF or MEF cells. A decrease in cell volume was observed parallel with the induction of actin rods, except for CEF and MEF cells. Alterations in intracellular concentrations of Na, K or Ca were not correlated with the induction, however. Actin rods were also induced in C3H-2K cells by a brief treatment with non-ionic detergents. Tween 80 at concentrations as low as 0.003% was effective for the induction, but did not increase the passive membrane transport of p-nitrophenylphosphate. In contrast to the induction by NaCl buffer, treatment with Tween 80 induced numerous tiny actin rods at 4 degrees C, which became larger when further incubated at 37 degrees C. Double immunofluorescence staining with anti-actin antibody and anti-vinculin antibody showed that vinculin plaques remained at least in an early stage of the actin rod formation. We discuss the mechanism for the induction of actin rods based upon the present findings.