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

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.

Mechanisms of movement in outer hair cells and a possible structural basis

Isolated outer hair cells were found to slowly shorten when subjected to a solution that would induce contraction in a muscle fibre. Two possible mechanisms underlying this behaviour emerge from ultrastructural and immunocytochemical investigations. Antibody labelling at the electron microscopic level demonstrates that actin is present not only in the stereocilia and in the cuticular plate but also along the wall of outer hair cells, between the plasma membrane and the subsurface fenestrated cisternae. The latter are interconnected by regularly spaced pillars, resembling those seen between the T-tubules and sarcoplasmic reticulum in muscle fibres. Contraction also results from the application of positively charged macromolecules to the bathing solution. This implies sensitivity of the membrane-associated complex (the cortex system) to an electrical current. A second contractile system may reside in the cytoplasm, where calmodulin is present in contracted hair cells. This protein is a calcium-binding control protein for contraction-like events in smooth muscle and non-muscle cells. The unique presence of the cortex system in outer hair cells, and its absence in inner hair cells, indicates a functional significance that relates to a motor function of outer hair cells in hearing.

The cytoskeleton, endocytosis and cell polarity in the mouse preimplantation embryo

The process of cell polarization in mouse 8-cell embryos includes the formation of a polar cluster of cytoplasmic endocytotic organelles (endosomes) subjacent to an apical surface pole of microvilli. A similar polar morphology, supplemented by basally localized secondary lysosomes, is evident following division to the 16-cell stage in outside blastomeres, precursors of the trophectodermal lineage. The roles of microfilaments and microtubules in generating and stabilizing endocytotic and surface features of polarity (visualized by horseradish peroxidase incubation and indirect immunofluorescence labeling, respectively) have been evaluated by exposure of 8- and 16-cell embryos and 8-cell couplets to drugs (cytochalasin D, colcemid, nocodazole) that disrupt the cytoskeleton. The generation of endocytotic polarity is dependent upon intact microtubules and microfilaments, but the newly established endocytotic pole in blastomeres from compacted 8-cell embryos appears to be stabilized exclusively by microtubules. Polarized endocytotic organelles at the 16-cell stage are more resistant to drug treatment than at the 8-cell stage (probably due to microfilament interactions) indicating a maturation phase in the polar cell lineage. Microtubules are also responsible for the orientation of endocytotic clusters along the cell's axis of polarity. In contrast, the generation and stability of polarity at the cell surface appears relatively independent of cytoskeletal integrity. The results are discussed in relation to the mechanisms that may control the development and stabilization of polarization during cleavage.

Cytoskeletal organization affects cellular responses to cytochalasins: comparison of a normal line and its transformant

The relationships between cytoskeletal network organization and cellular response to cytochalasin D (CD) in a normal rat fibroblast cell line (Hmf-n) and its spontaneous transformant (tHmf-e), with markedly different cytoskeletal phenotypes, were compared (using immunofluorescence, electron microscopy, and DNAse I assay for actin content). Hmf-n have prominent, polar stress fiber (SF) arrays terminating in vinculin adhesion plaques whereas tHmf-e, which are apolar, epithelioid cells with dense plasma membrane-associated actin networks, lack SF and adhesion plaques. Hmf-n exposed to CD become markedly retracted and dendritic, SF-derived actin aggregates form large endoplasmic masses, and discrete tabular aggregates at the distal ends of retraction processes. Prolonged exposure leads to recession of process, cellular rounding, and development of large cystic vacuoles. tHmf-e cells exposed to similar doses of CD display a diagnostically different response; retraction is less drastic, cells retain broad processes containing scattered actin aggregates in discrete foci often associated with plasma membrane, large tabular aggregates are never found and processes persist throughout long exposure, vacuolation is uncommon. The CD-induced microfilamentous aggregates in Hmf-n are composed of short, kinky filament fragments forming a felt-like skein, often aggregates contain a more ordered array of roughly parallel fragments, while those of tHmf-e are very short, kinky, randomly orientated filaments imparting a distinctly granular nature to the mass. Total actin content and the amount of actin associated with detergent-resistant cytoskeletons increase following CD exposure in both cell types. Throughout exposure to CD, the actin-associated contractile proteins tropomyosin, myosin, and alpha-actinin co-localize within the actin aggregates in both cell types. Fodrin, the protein linking cortical actin to membrane, co-localizes with actin aggregates in tHmf-e cells and most, but not all, such aggregates in Hmf-n cells, consistent with their stress fiber derivation. Vinculin is lost from the tabular aggregates at the distal ends of retraction processes in Hmf-n cells concomitant with the fragmentation and contraction of SF. The aborized processes in both cells types contain strikingly similar axial cores of bundled vimentin filaments associated with passively compressed microtubules. The characteristic CD-induced distribution of actin filament aggregates and redistribution of vimentin in these cell types also occur when cells are allowed to respread from the rounded state in the presence of CD.

Loss of surface fibronectin after infection of cultured cells by HSV-1 and 2

Fibronectin is lost from the surface of HSV infected cells during cell rounding. In order to investigate also the fate of fibronectin during the process of HSV-induced cell-fusion, BHK, Vero as well as primary or secondary rabbit kidney cells were infected with HSV-1 strains producing cell-fusion. By immunofluorescence and immunoelectron microscopy a considerable loss of fibronectin after HSV infection could be demonstrated leaving only irregular clumps of fibronectin containing virus particles on the cell surface. Decrease and disarrangement of fibronectin was similar during cell rounding and cell fusion. Loss of Fibronectin was closely connected with the two types of the cytopathic effect (CPE) and could not be prevented by protease inhibitors. The immediate-early protein 175K is essential for induction of CPE and loss of fibronectin. The damage to the cell membrane during HSV infection shows certain analogous mechanisms with events induced by Cytochalasin B and might be explained by the loss of hypothetical fibronectin receptors.

Rapid polymerization of Entamoeba histolytica actin induced by interaction with target cells

Within 5 s of challenge of Entamoeba histolytica trophozoites with red blood cells (RBC), attachment and deformation of target cells occurred at multiple sites on the amoeba surface. Many trophozoite-target interfaces were outlined with a ring of polymerized amoeba actin, revealed by rhodamine-phalloidin staining of glutaraldehyde-fixed and Triton-X 100-extracted cells. The beginnings of phagocytic pseudopods rimmed many targets. The phagocytic membrane and underlying actin network grew uniformly about a target cell, which became dramatically elongated and constricted, sometimes severed, as it entered the amoeba. Total engulfment of RBC targets occurred within 10 s. By methanol extraction and spectrofluorimetric measurement of bound rhodamine-phalloidin we were able to quantitate polymerized actin in amoebae. Interaction with target cells was accompanied by a net increase of up to twofold in the average polymerized actin content of trophozoites. This reached a maximum during the period of most active phagocytosis (4 min after challenge at 25 degrees C), and declined as phagocytic activity diminished (8-16 min). Challenge with latex beads of similar size and number, which E. histolytica phagocytized more slowly than RBC, induced neither a detectable increase in polymerized actin content nor appearance of polymerized actin at the contact interface. RBC inhibited phagocytosis of latex beads, but the reverse did not occur. The results demonstrate a rapid, recognition-specific stimulation of reorganization of the actin cytoskeleton of E. histolytica induced by binding to target cells. Vigorous phagocytic activity is frequently an immediate consequence of cell-cell contact, which emphasizes the importance of this process in the contact-mediated attack mechanism of this pathogen. The quantitative assay of polymerized actin may be useful in further studies of this mechanism.

Rous sarcoma virus-transformed fibroblasts adhere primarily at discrete protrusions of the ventral membrane called podosomes

Rous sarcoma virus-transformed BHK cells (RSV/B4-BHK) adhere to a fibronectin-coated substratum primarily at specific dot-shaped sites. Such sites contain actin and vinculin and represent close contacts with the substratum as revealed by interference reflection microscopy. Only a few adhesion plaques and actin filament bundles can be detected in these cells as compared to untransformed parental fibroblasts. In thin sections examined with transmission electron microscopy (TEM) these adhesion sites correspond to short protrusions of the ventral cell surface that contact the substratum at their apical portion. These structures, which may represent cellular feet, are therefore called podosomes. By screening a number of different transformed fibroblasts plated on a fibronectin-coated substratum we find that podosomes are common to mammalian and avian cell lines transformed either by Rous sarcoma virus (RSV) or by Fujinami avian sarcoma virus (FSV), whose oncogenes encode specific tyrosine kinases. Using antibodies reacting with phosphotyrosine in immunofluorescence experiments, we show that phosphotyrosine-containing molecules are concentrated in podosomes. Podosomes are not detected in fibroblasts transformed by other retroviruses (Snyder-Theilen sarcoma virus, Abelson leukemia virus and Kirsten sarcoma virus) or by DNA tumor viruses (polyoma, SV40), indicating that podosome-mediated adhesion in transformed fibroblasts is related to the peculiar properties of some oncoproteins and possibly to their tropism for adhesion systems. Podosomes and adhesion plaques, although similar in cytoskeletal protein composition, have different mechanisms and kinetics of formation. Assembly of podosomes, in fact (i) does not require fetal calf serum (FCS) in the adhesion medium, that is necessary for the organization of adhesion plaques; (ii) does not require protein synthesis; and (iii) is insensitive to the ionophore monensin, that prevents adhesion plaque formation. Moreover, during attachment to fibronectin-coated dishes, podosomes appear in the initial phase (60 min) of attachment, while adhesion plaques require a minimum of 180 min. In conclusion podosomes of RSV- and FSV-transformed fibroblasts represent a phenotypic variant of adhesion structures.

An hypothesis regarding the origin of aneuploidy in man: indirect evidence from an experimental model

Recent studies have clearly demonstrated that aneuploidy may be induced in about 10 to 20% of oocytes and recently ovulated eggs when female mice are given an intragastric injection of a dilute solution of ethanol. Similar rates of aneuploidy have also been observed when recently ovulated eggs are briefly exposed in vitro to a dilute solution of ethanol in tissue culture medium. These findings are briefly reviewed, and observations made on the possible underlying mechanism of induction of chromosome malsegregation in the ethanol exposed groups. Attention is drawn to evidence from a wide range of studies on the effect of ethanol, acetaldehyde (its primary metabolite), and anaesthetics on cell division and chromosome segregation in an attempt to substantiate an hypothesis regarding the mode of action of these agents. In the light of this information, it is hypothesised that exposure to ethanol probably interferes with the normal functioning of the cytoskeletal elements of the spindle apparatus, or its precursor elements, during the first or second meiotic divisions. An attempt is also made to account for the very high incidence of aneuploid conceptuses in man, a high proportion of which are spontaneously aborted. It is hypothesised that exposure to ethanol and other spindle active agents during appropriate stages of oogenesis (in particular during the first meiotic division), and possibly also during spermatogenesis, may be important aetiological factors in a proportion of those cases of spontaneous abortion with a numerical chromosome anomaly for which no other obvious cause is recognised. If it is valid to extrapolate from these experimental findings to the clinical situation in man, it is suggested that attention should also be drawn to the potentially greater hazard to the conceptus which could result from maternal alcohol consumption at and shortly before conception.

Organization of cytoskeleton and fibronectin matrix in Rous sarcoma virus (RSV)-transformed fibroblast lines with different metastatic potential

Metastatic clones growing in 0.6% 'hard' agar were selected from the non-metastatic Rous sarcoma virus (RSV)-transformed tumorigenic B77-3T3 mouse fibroblast line. The incidence of spontaneous lung metastases varied among clones around 100%, while it was lower than 5% in the parental tumor line. The organization of microfilaments, microtubules and intermediate filaments as well as the pattern of extracellular fibronectin matrix were analyzed by immunofluorescence in two representative clones (B77-AA6 and B77-AA12) and was compared with the structural features displayed by a highly metastasizing RSV-induced mouse sarcoma line (SR-BALB). In the metastatic clones studied microtubules and intermediate filaments were similarly organized in a pattern not significantly different from that of the non-metastatic parental cell line. The major finding was a marked concentration of actin-containing structures in the periphery of cells and notably at the level of surface protrusions, suggesting a high surface motility. In the same lines the production of fibronectin and its distribution in the cell layer and culture medium were analyzed. Metabolic labelling and immunofluorescence experiments indicated that the nonmetastasizing cells (B77-3T3) retain higher amounts of fibronectin in the cell layer and organize this molecule in extracellular fibers, while the metastatic clones (B77-AA6 and B77-AA12) as well as the metastatic line (SR-BALB) are unable to retain and organize fibronectin at their surface. This paper shows that the progression of tumorigenic cell lines toward a metastatic phenotype involves a redistribution of cytoskeletal actin and a loss of organized fibronectin matrix.

Multiple tropomyosin polypeptides in chicken embryo fibroblasts: differential repression of transcription by Rous sarcoma virus transformation

We have found that cytoskeletal extracts of cultured chicken embryo fibroblasts contain at least seven distinct polypeptides (two major and five minor) which cross-react with antiserum to chicken smooth muscle tropomyosin. These polypeptides range in apparent molecular weight from 31,000 to 47,000, and each is encoded by mRNAs which specifically hybridize to cloned muscle tropomyosin cDNAs. These nonmuscle tropomyosin species and their respective mRNAs are electrophoretically distinct from those of chicken skeletal muscle and appear by genomic DNA blotting to comprise a part of a multigene tropomyosin family. In Rous sarcoma virus-transformed chicken embryo fibroblasts, synthesis of the tropomyosins is differentially repressed such that the synthesis of the major species (cp35 and cp33, cytoskeletal proteins of molecular weight 35,000 and 33,000, respectively) and three minor species is drastically reduced, whereas the synthesis of two of the minor species (cp32 and cp31) remains essentially unchanged. Analysis of cellular mRNA and runoff nuclear transcription experiments indicate that the repression of tropomyosin synthesis by Rous sarcoma virus transformation occurs at the level of transcription. This repression of tropomyosin synthesis is partially mimicked in normal chicken embryo fibroblasts during incubation in high-NaCl medium, a condition in which chicken embryo fibroblasts acquire many characteristics of transformed cells.

Change with age in murine corneal epithelial actin and myosin: immunofluorescent and ELISA analyses

The purpose of this study was to use immunofluorescence and ELISA immunoassay to determine whether the cellular distribution and concentration of corneal epithelial actin and myosin change with chronologic age. Diffuse anti-actin and anti-myosin indirect immunofluorescence was observed within the cytoplasm of the corneal epithelium from mice aged postnatal day (PND) 1-18 months. Additionally, highly fluorescent punctate foci were first observed in cortical cytoplasm consistently for both anti-actin and anti-myosin at PND 14. This fluorescent pattern remained relatively unchanged for the remaining ages examined. An enzyme-linked immunosorbent assay (ELISA) method was used to quantitate the amount of actin and myosin in corneal epithelium from mice aged PND 1 to 24 months. Corneal epithelial sheets were removed from whole eyes and processed for ELISA assay. Actin cellular concentration increased from PND 1-7 and decreased from PND 7-16. These results were statistically significant (p less than .005). No statistically significant difference in actin concentration was found for any of the remaining ages examined (PND 16-24 months). Myosin concentration increased from PND 1-7 and decreased until PND 14. These results also were statistically significant (p less than .005 and p less than .005, respectively). There was no significant change in myosin concentration for any of the remaining ages examined (PND 16-24 months).

The relationship between stress fiber-like structures and nascent myofibrils in cultured cardiac myocytes

The topographical relationship between stress fiber-like structures (SFLS) and nascent myofibrils was examined in cultured chick cardiac myocytes by immunofluorescence microscopy. Antibodies against muscle-specific light meromyosin (anti-LMM) and desmin were used to distinguish cardiac myocytes from fibroblastic cells. By various combinations of staining with rhodamine-labeled phalloidin, anti-LMM, and antibodies against chick brain myosin and smooth muscle alpha-actinin, we observed the following relationships between transitory SFLS and nascent and mature myofibrils: (a) more SFLS were present in immature than mature myocytes; (b) in immature myocytes a single fluorescent fiber would stain as a SFLS distally and as a striated myofibril proximally, towards the center of the cell; (c) in regions of a myocyte not yet penetrated by the elongating myofibrils, SFLS were abundant; and (d) in regions of a myocyte with numerous mature myofibrils, SFLS had totally disappeared. Spontaneously contracting striated myofibrils with definitive Z-band regions were present long before anti-desmin localized in the I-Z-band region and long before morphologically recognizable structures periodically link Z-bands to the sarcolemma. These results suggest a transient one-on-one relationship between individual SFLS and newly emerging individual nascent myofibrils. Based on these and other relevant data, a complex, multistage molecular model is presented for myofibrillar assembly and maturation. Lastly, it is of considerable theoretical interest to note that mature cardiac myocytes, like mature skeletal myotubes, lack readily detectable stress fibers.

Growth of neurites without filopodial or lamellipodial activity in the presence of cytochalasin B

To examine the role in neurite growth of actin-mediated tensions within growth cones, we cultured chick embryo dorsal root ganglion cells on various substrata in the presence of cytochalasin B. Time-lapse video recording was used to monitor behaviors of living cells, and cytoskeletal arrangements in neurites were assessed via immunofluorescence and electron microscopic observations of thin sections and whole, detergent-extracted cells decorated with the S1 fragment of myosin. On highly adhesive substrata, nerve cells were observed to extend numerous (though peculiarly oriented) neurites in the presence of cytochalasin, despite their lack of both filopodia and lamellipodia or the orderly actin networks characteristic of typical growth cones. We concluded that growth cone activity is not necessary for neurite elongation, although actin arrays seem important in mediating characteristics of substratum selectivity and neurite shape.

A tumor promoter induces rapid and coordinated reorganization of actin and vinculin in cultured cells

Treatment of epithelial African green monkey kidney (BSC-1) cells with the potent tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) induces a rapid and reversible redistribution of actin and vinculin that is detectable after only 2 min of treatment. Within 20-40 min, stress fibers disappear, while at the same time large actin-containing ribbons resembling ruffles develop both at the cell periphery and in more central regions. Vinculin is associated with these actin ribbons or bands in a punctate or patchy staining pattern. Adhesion to the substratum is changed from predominantly focal contacts associated with stress fiber ends in untreated cells to broad zones of close contact after TPA treatment. High voltage electron microscopic observations disclose the ribbons to consist of highly cross-linked actin filament networks. Thus, association of vinculin with filament networks, rather than (the ends of) filament bundles, is demonstrated. The integrity of microtubules and vimentin filaments is not affected by TPA treatment, but their distribution is altered to conform with the highly distorted cell shape. The response to TPA is neither prevented nor modified by nocodazole-induced depolymerization or taxol-induced stabilization of microtubules. An intact intermediate filament network seems not required either since colcemid-induced collapse of vimentin filaments towards the nucleus does not affect the cell's response to TPA. Rapid redistribution of actin and vinculin also takes place in enucleated cells and in the presence of cycloheximide, but is prevented by dinitrophenol or oligomycin. TPA-induced cytoskeletal alterations are independent of fibronectin expression and not mimicked, modified, or prevented by calmodulin inhibitors or experimentally elevated levels of calcium and cyclic AMP. Thus the morphological response to TPA involves rapid redistribution of actin and vinculin independent of transcription and translation, fluctuations in the levels of calcium or cyclic AMP, or changes in the organization of microtubules, intermediate filaments, and fibronectin.

Multiple tropomyosin polypeptides in chicken embryo fibroblasts: differential repression of transcription by Rous sarcoma virus transformation

We have found that cytoskeletal extracts of cultured chicken embryo fibroblasts contain at least seven distinct polypeptides (two major and five minor) which cross-react with antiserum to chicken smooth muscle tropomyosin. These polypeptides range in apparent molecular weight from 31,000 to 47,000, and each is encoded by mRNAs which specifically hybridize to cloned muscle tropomyosin cDNAs. These nonmuscle tropomyosin species and their respective mRNAs are electrophoretically distinct from those of chicken skeletal muscle and appear by genomic DNA blotting to comprise a part of a multigene tropomyosin family. In Rous sarcoma virus-transformed chicken embryo fibroblasts, synthesis of the tropomyosins is differentially repressed such that the synthesis of the major species (cp35 and cp33, cytoskeletal proteins of molecular weight 35,000 and 33,000, respectively) and three minor species is drastically reduced, whereas the synthesis of two of the minor species (cp32 and cp31) remains essentially unchanged. Analysis of cellular mRNA and runoff nuclear transcription experiments indicate that the repression of tropomyosin synthesis by Rous sarcoma virus transformation occurs at the level of transcription. This repression of tropomyosin synthesis is partially mimicked in normal chicken embryo fibroblasts during incubation in high-NaCl medium, a condition in which chicken embryo fibroblasts acquire many characteristics of transformed cells.

Induction of chondrogenesis in limb mesenchymal cultures by disruption of the actin cytoskeleton

Cell shape is known to influence the chondrogenic differentiation of cultured limb bud mesenchyme cells (Solursh, M., T. F. Linsenmayer, and K. L. Jensen, 1982, Dev. Biol., 94: 259-264). To test whether specific cytoskeletal components mediate this influence of cell shape, we examined different cytoskeleton disrupting agents for their ability to affect chondrogenesis. Limb bud cells cultured at subconfluent densities on plastic substrata normally become flattened, contain numerous cytoplasmic microtubules and actin bundles, and do not undergo spontaneous chondrogenesis. If such cultures are treated with 2 micrograms/ml cytochalasin D during the initial 3-24 h in culture, the cells round up, lose their actin cables, and undergo chondrogenesis, as indicated by the production of immunologically detectable type II collagen and a pericellular Alcian blue staining matrix. Cytochalasin D also induces cartilage formation by high-density cultures of proximal limb bud cells, which normally become blocked in a protodifferentiated state. In addition, cytochalasin D was found to reverse the normal inhibition by fibronectin of chondrogenesis by proximal limb bud cells cultured in hydrated collagen gels. Agents that disrupt microtubules have no apparent effect on the shape or chondrogenic differentiation of limb bud mesenchymal cells. These results suggest an involvement of the actin cytoskeleton in controlling cell shape and chondrogenic differentiation of limb bud mesenchyme. Interactions of the actin cytoskeleton and extracellular matrix components may provide a regulatory mechanism for mesenchyme cell differentiation into cartilage or fibrous connective tissue in the developing limb.

The effect of cytochalasin D on steroid production and stress fiber organization in cultured bovine adrenocortical cells

The effects of cytochalasin D on basal and adrenocorticotropin (ACTH)-stimulated steroid production as well as stress fiber organization by bovine adrenocortical (BAC) cells grown in monolayer culture have been investigated. Corticosteroid and pregnenolone release was determined by use of radioimmunoassay. The addition of ACTH (1 nM) produced a 30-fold increase in steroid release (principally corticosterone) in a 2 h period. Cytochalasin D (1-10 microM) had no effect on ACTH-stimulated release of corticosteroids. Basal steroid release was elevated by cytochalasin D with lower concentrations being more effective. In addition, hormonal stimulation of pregnenolone formation, the initial step in cholesterol metabolism leading to corticosteroid production, was also unaffected by cytochalasin D (1-50 microM) addition. Observation of stress fibers by fluorescence microscopy using the probe NDB - phalladin revealed that cytochalasin D (10 and 50 microM) caused an aggregation of actin-containing filaments into stellate foci within the cytoplasm. This was confirmed by electron microscopic examination of the cells. ACTH, however, had no observable effect on stress fiber organization or cell morphology. These results differ from the inhibitory effect of cytochalasin on steroid production observed using Y-1 adrenal tumor cells and rat adrenal cells in primary culture. BAC cells in contrast to Y-1 adrenal tumor cells differ in non-esterified cholesterol content and their morphological response to ACTH treatment. We suggest that these differences may influence the lack of inhibition of cytochalasin D on ACTH-stimulated steroid release.

Fc-mediated binding of IgG to vimentin-type intermediate filaments in vascular endothelial cells

Prior studies have shown that vascular endothelial cells bind circulating IgG intracellularly during cell death. We now demonstrate that all endothelial cells have intracellular binding sites for IgG and that these binding sites are exposed to circulating IgG only if the plasma membrane is damaged. The binding sites are located on the cytoskeletal intermediate filaments and can be detected also in other cells containing vimentin-type intermediate filaments. Monoclonal human IgG1 exhibited saturable, high-affinity binding to vimentin-enriched cytoskeletons. Binding was inhibited by Fc fragments but not by Fab, F(ab')2, or pFc' fragments, suggesting that the binding site on IgG is located in the C gamma 2 domain of the Fc fragment. Binding of IgG to intermediate filaments may be important for the destruction and removal of damaged cells.

Cytoskeleton and adhesion patterns of cultured chick embryo chondrocytes during cell spreading and Rous sarcoma virus transformation

The cytoskeleton and the adhesion complex of chick embryo chondrocytes maintained in vitro have been studied by fluorescence and interference reflection microscopy during the process of cell spreading. The pattern of actin-containing microfilaments and the distribution of vinculin speckles on adhesion plaques have been found to change as a function of the culture time. Newly plated chondrocytes adhere to the substratum mostly around a peripheral ring-like region and show a complex tridimensional array of microfilaments. When chondrocytes flatten, they develop stress fibres and show a diffuse system of vinculin-containing adhesion plaques scattered over the entire ventral side of the cells. Upon infection with Rous sarcoma virus (RSV) chondrocytes display one or more actin-containing ruffles located on the dorsal side similar to the 'actin flowers' earlier described in other cell types. These structures have been found to accumulate vinculin too. In chondrocytes infected with two td-ts mutants of RSV, 'actin flowers' have been found to persist at the restrictive temperature. At this temperature, however, in the majority of cells, stress fibres and adhesion plaques reappear.

Effects of bunaftine on morphology, microfilament integrity, and mitotic activity in cultured human fibroblasts and HeLa cells

Human fibroblasts and HeLa cells were treated with bunaftine (N-butyl-N-/2-(diethylamino)ethyl/-1-naphthalenecarboxamide ) in vitro. At concentrations of 0.5-2.0 mM, the drug caused contraction and rounding of the cells with loss of microvilli-like processes. Aggregates of dense, partly granular, partly fibrillar material formed in the cytoplasm and the rough endoplasmic reticulum became vesiculated. Immunofluorescence microscopy with DNase I and anti-DNase I demonstrated that bundles of actin filaments were disrupted, forming rings, coils, and granules. Filaments stained with antibodies to vimentin (fibroblasts) and prekeratin (HeLa cells) showed less characteristic rearrangements, probably related to the rounding up of the cells. 0.4 mM bunaftine increased and 0.8-1.0 mM markedly decreased the percentage of mitotic cells, without accumulation of cells in any particular stage of mitosis. The drug may arrest the cell cycle at some point before mitosis; it may have a critical concentration above which the arrest becomes permanent. These results suggest that bunaftine interferes with the integrity of microfilament bundles in a different manner from that of cytochalasins. It does not cause any depletion of cellular ATP, indicating that its effect is not a result of inhibition of cell metabolism. It is proposed that bunaftine may be used a complement to cytochalasins in studies of the microfilament system of the cell. The possible binding of bunaftine to actin or myosin and further details of its mechanism of action remain to be elucidated.

Antivimentin autoantibodies in angioimmunoblastic lymphadenopathy

We evaluated the prevalence and specificity of smooth-muscle autoantibodies in 20 serum samples obtained from patients with angioimmunoblastic lymphadenopathy. Smooth-muscle antibodies in high titers were detected in 75 per cent of the samples. No such antibodies were found in 30 normal control serum samples or in 10 samples from patients with non-Hodgkin's lymphoma and 1 of 12 from patients with other types of polyclonal hypergammaglobulinemia were positive. The antibodies were polyclonal and belonged to the IgM, IgG, and IgA classes. They reacted with vimentin, the major polypeptide of the intermediate-filament cytoskeleton of mesenchymal cells. The pattern of tissue reactivity and absorption experiments both show that these antibodies recognize special antigenic determinants of the vimentin polypeptide that are shared by vimentin and other classes of intermediate-filament proteins - namely, keratin and desmin. The frequency of this unusual autoantibody activity in angioimmunoblastic lymphadenopathy suggests that, like hypergammaglobulinemia and a positive Coombs' test, it may represent a useful serologic marker for the disease.

Immunohistochemical identification and localization of actin and fimbrin in vestibular hair cells in the normal guinea pig and in a strain of the waltzing guinea pig

Using immunohistochemical methods, actin and fimbrin were identified and localized in vestibular epithelia iin the normal guinea pig and in the waltzing guinea pig. In the normal guinea pig, actin was found in the stereocilia and in the cuticular plate. Fimbrin was detected in the stereocilia but surprisingly not in the cuticular plate. As fimbrin was found in the cuticular plates in cochlear hair cells it is suggested that the hair cells in the two organs have different mechanoreceptor properties which can demand different cuticular plate stability. In the waltzing guinea pig, actin was found in the stereocilia, in the rod and n the cuticular plate. Fimbrin was seen in the stereocilia and in the rod, but could not be detected in the cuticular plate. These results emphasize the resemblance between the rod and the stereocilia. It is suggested that the rod is a genetically induced pathological intracellular type of stereocilium which grows in an uncontrolled manner.

Ultrastructural effects of Clostridium difficile toxin B on smooth muscle cells and fibroblasts

The mechanism by which Clostridium difficile toxin B causes cells in culture to round was investigated. Cultured human lung fibroblasts and rabbit aortic smooth muscle cells were treated with partially purified or purified toxin B and monitored by light and transmission electron microscopy (TEM). Both preparations caused progressive cell rounding which correlated with disorganization of actin-containing myofilament bundles. Thin myofilaments became fragmented and finally disappeared (after 24 h) and dense bodies became more prominent, while all other organelles appeared unaffected.

Distribution patterns of cytoplasmic microtubules in epidermal keratinocytes

The distribution of cytoplasmic microtubules in cultured guinea-pig keratinocytes was investigated using immunofluorescence (IF) microscopy with monospecific anti-tubulin antibodies and electron microscopy (EM). In culture, adherent cells displayed networks of thin fluorescent fibres, while a homogeneous and/or granular cytoplasmic IF was shown in the cells of upper layers as well as in trypsinized cells. By EM many microtubules were shown in adherent cells but there were fewer or none in the upper layers. An increase in calcium ion (Ca2+) concentration and the addition of an ionophore (X537A) to the culture medium caused disassembly of microtubules. This effect was cancelled by a calmodulin inhibitor. Cryostat sections of normal human and guinea-pig epidermis stained with anti-tubulin antibodies showed a homogeneous and/or granular cytoplasmic IF from basal to granular layers but no detectable IF was seen in the horny layer. These results suggest that keratinocytes contain a cellular pool of tubulin in various states of polymerization and that microtubule disassembly may occur during differentiation, probably being regulated by Ca2+-calmodulin complexes.

Disruption of microfilament organization after injection of F-actin capping proteins into living tissue culture cells

Capping proteins are F-actin binding proteins which interfere with the in vitro growth of an actin filament by blocking one of its ends (for recent reviews see refs 1-3). The majority of such proteins described so far "cap' the fast-growing (positive) end of the polar filament, thus reducing the velocity of filament growth while increasing the number of filaments being formed de novo from a monomer pool. We have studied the effects of capping proteins on the organization of actin filaments in living tissue culture cells by microinjection in conjunction with fluorescence, reflection contrast and electron microscopy. Our results, reported here, indicate that capping proteins from different sources disrupt microfilament bundles in a variety of cell types causing their disintegration from the distal end towards the centre of the cell.

Calcium-dependent exocytosis in an in vitro secretory granule plasma membrane preparation from sea urchin eggs and the effects of some inhibitors of cytoskeletal function

Egg cortical granules remain attached to the egg plasma membrane when the egg is ruptured. We present evidence that demonstrates that, when the cytoplasmic face of the egg plasma membrane is exposed to micromolar calcium concentrations, an exocytosis of the cortical granules occurs which corresponds to the cortical granule exocytosis seen when the egg is fertilized. The calcium sensitivity of the preparation is decreased by an increase in magnesium concentration and increased by a decrease in magnesium concentration. Exocytosis is inhibited by trifluoperazine (half inhibition at 6 microM), a drug that inhibits the action of the calcium-dependent regulatory protein calmodulin. Colchicine, vinblastine, nocodazole, cytochalasin B, phalloidin, N-ethylmaleimide-modified myosin subfragment 1, and antibody to actin are without effect on this in vitro exocytosis at concentrations that far exceed those required to disrupt microtubules and microfilaments. Conditions are such that penetration to the exocytotic site is optimal. It is unlikely, therefore, that either actin or tubulin participate intimately in exocytosis. Our data also exclude on quantitative grounds several other mechanisms postulated to account for the fusion of the secretory granule with the plasma membrane.

A processed gene defining a gene family encoding a human non-muscle tropomyosin

We have isolated and characterized a human genomic DNA sequence that defines a family of closely related sequences. At least one member of this family expresses a 2.5 X 10(3) base messenger RNA transcript encoding a 30,000 molecular weight tropomyosin in human fibroblasts. The coding sequence of this mRNA but not the non-coding sequence is also related to that of a 1.1 X 10(3) base mRNA encoding a 36,000 molecular weight non-muscle tropomyosin. This demonstrates the existence of at least two functional genes encoding human non-muscle tropomyosins.

Changes in cell surface antigens during in vitro lizard myogenesis

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

Cytochalasin D induces increased actin synthesis in HEp-2 cells

In HEp-2 cells treated with 0.2 to 2.0 microM cytochalasin D (CD) for 7.5 to 24 h there was a 20 to 50% relative increase in actin content (units of actin per microgram of total cell protein). This augmentation, which was concentration and time dependent, was prevented by treatment with cycloheximide during exposure to CD. A 15 to 20% increase in the relative rate of actin synthesis in CD-treated HEp-2 cells (0.2 to 2.0 microM CD) was detectable after 1 h of treatment and increased to 30 to 50% by 24 h. This increased rate of actin synthesis was apparently responsible for the higher actin content of CD-treated HEp-2 cells. The concentration dependence of these effects of CD on actin metabolism correlated with the pattern seen for CD-triggered changes in cellular morphology and the underlying rearrangements of the actin-containing cytoskeletal structures, suggesting that the effects on metabolism and morphology were interrelated. Since the rapidly occurring cytoskeletal reorganization preceded the effects of CD on actin metabolism, it is proposed that actin synthesis is induced by the cytoskeletal rearrangement resulting from exposure to CD.

Altered cell spreading in cytochalasin B: a possible role for intermediate filaments

Trypsinized chicken embryo dermal fibroblasts plated in the presence of cytochalasin B (CB) quickly attached to the substrate and within 24 h obtained an arborized morphology. This morphology is the result of the pushing out of pseudopodial processes along the substrate from the round central cell body. There were no microfilament bundles in the processes of these cells plated in the presence of CB; however, the processes were packed with highly oriented, parallel-aligned intermediate filaments. Only a few scattered microtubules were seen in these processes. These results demonstrated that in CB, cells are capable of a form of movement, i.e., the extension of pseudopodial processes, without the presence of the microfilament structures usually associated with extensions of the cytoplasm and pseudopodial movements. We also found that arborization did not depend on fibronectin since cells plated in CB did not have fibronectin fibers associated with the processes. Chicken fibroblasts transformed with tsLA24A, a Rous sarcoma virus which is temperature sensitive for pp60src, formed arborized cells with properties similar to those of uninfected fibroblasts when plated in the presence of CB at the nonpermissive temperature (41 degrees C). At the permissive temperature for transformation (36 degrees C), the cells attached to the substrate but remained round. These round cells were not only deficient in microfilament bundles but also lacked the highly organized intermediate filaments found in the processes of the arborized cells at 41 degrees C. Although both microfilament bundles and the fibronectin matrix were decreased after transformation with Rous sarcoma virus, neither was involved in the formation of processes in normal cells plated in CB. Therefore, the inability of the transformed cells to form or maintain processes in CB must be the result of another structural alteration in the transformed cells, such as that of the intermediate filaments.

Novel form of non-muscle tropomyosin in human fibroblasts

The cytoskeletal extracts of cultured human fibroblasts were found to contain at least four distinct polypeptides, each of which demonstrated the resistance to denaturation and the acidic isoelectric point characteristic of tropomyosin. One of these, hscp 36 (heat-stable cytoskeletal protein having an apparent molecular weight of 36,000), cross-reacted efficiently with an antiserum to chicken skeletal muscle tropomyosin. Furthermore, the messenger RNA coding for hscp 36 was selected by a chicken complementary DNA clone containing a tropomyosin sequence. The abundance of mRNA coding for hscp 36 was found to be similar in both normal and simian virus 40 (SV40) transformed human fibroblasts. The apparent molecular weight of hscp 36 is different from non-muscle tropomyosins previously isolated from human sources, which show the apparent molecular weight of 30,000 normally associated with non-muscle tropomyosin. This, together with the complexity of the heat-stable cytoskeletal proteins present in human fibroblasts, suggests the existence of multiple genes coding for human non-muscle tropomyosins.

A primary role for microfilaments, but not microtubules, in hormone-induced cytoplasmic retraction

The distributions of microfilaments and microtubules were studied during transient hormone-induced changes in cell shape (retraction-respreading). Two cell types (fibroblasts and bone cells), differentially responsive to parathyroid hormone (PTH) and prostaglandin E2 (PGE2), were analysed. The cytoplasm of fibroblasts retracted in response to PGE2 but not PTH, whereas bone cells could respond to both PGE2 and PTH. Time-lapse photomicrography indicated that the retraction began within minutes of hormone addition, while respreading occurred over longer times, up to 8 h. Affinity-purified actin and tubulin antibodies were used to follow the appearance of microtubules and microfilaments during both the retraction and the respreading phases. Microtubules appeared not to reorganize noticeably, although they were squeezed closer together in cellular pseudopods; no extensive loss or growth was detectable. Microfilaments did alter drastically their appearance and distributions. Soon after hormone addition when earliest detectable cytoplasmic retraction was evident, microfilament bundles appeared to break down. Remaining microfilament bundles consisted of relatively short, non-aligned fragments or aggregates. During respreading, microfilament bundles regrew and realigned throughout the cytoplasm. These data suggest a primary role for microfilaments, but probably not microtubules, in these cell shape changes.

Altered cell spreading in cytochalasin B: a possible role for intermediate filaments

Trypsinized chicken embryo dermal fibroblasts plated in the presence of cytochalasin B (CB) quickly attached to the substrate and within 24 h obtained an arborized morphology. This morphology is the result of the pushing out of pseudopodial processes along the substrate from the round central cell body. There were no microfilament bundles in the processes of these cells plated in the presence of CB; however, the processes were packed with highly oriented, parallel-aligned intermediate filaments. Only a few scattered microtubules were seen in these processes. These results demonstrated that in CB, cells are capable of a form of movement, i.e., the extension of pseudopodial processes, without the presence of the microfilament structures usually associated with extensions of the cytoplasm and pseudopodial movements. We also found that arborization did not depend on fibronectin since cells plated in CB did not have fibronectin fibers associated with the processes. Chicken fibroblasts transformed with tsLA24A, a Rous sarcoma virus which is temperature sensitive for pp60src, formed arborized cells with properties similar to those of uninfected fibroblasts when plated in the presence of CB at the nonpermissive temperature (41 degrees C). At the permissive temperature for transformation (36 degrees C), the cells attached to the substrate but remained round. These round cells were not only deficient in microfilament bundles but also lacked the highly organized intermediate filaments found in the processes of the arborized cells at 41 degrees C. Although both microfilament bundles and the fibronectin matrix were decreased after transformation with Rous sarcoma virus, neither was involved in the formation of processes in normal cells plated in CB. Therefore, the inability of the transformed cells to form or maintain processes in CB must be the result of another structural alteration in the transformed cells, such as that of the intermediate filaments.

Cytochalasin D induces increased actin synthesis in HEp-2 cells

In HEp-2 cells treated with 0.2 to 2.0 microM cytochalasin D (CD) for 7.5 to 24 h there was a 20 to 50% relative increase in actin content (units of actin per microgram of total cell protein). This augmentation, which was concentration and time dependent, was prevented by treatment with cycloheximide during exposure to CD. A 15 to 20% increase in the relative rate of actin synthesis in CD-treated HEp-2 cells (0.2 to 2.0 microM CD) was detectable after 1 h of treatment and increased to 30 to 50% by 24 h. This increased rate of actin synthesis was apparently responsible for the higher actin content of CD-treated HEp-2 cells. The concentration dependence of these effects of CD on actin metabolism correlated with the pattern seen for CD-triggered changes in cellular morphology and the underlying rearrangements of the actin-containing cytoskeletal structures, suggesting that the effects on metabolism and morphology were interrelated. Since the rapidly occurring cytoskeletal reorganization preceded the effects of CD on actin metabolism, it is proposed that actin synthesis is induced by the cytoskeletal rearrangement resulting from exposure to CD.

Hormonally induced changes in the cytoskeleton of human thyroid cells in culture

Serially cultivated thyroid follicular cells are not active in hormone synthesis but retain a thyrotropin-responsive adenylate cyclase. The exposure of such cells to thyrotropin leads to an increase in the concentration of intracellular cAMP and a drastic change in morphology including a total cytoplasmic arborization. The present communication describes these changes at the cytoskeletal level using a cell line derived from a human functioning thyroid adenoma. Phase contrast microscopy showed that the cytoplasmic arborization was preceded by a total disappearance of stress fibers, visible within 20 min of exposure. Small marginal membrane ruffles could also be seen. These morphological changes could also be induced by the addition of dibutyryl cAMP. The action of both thyrotropin and dibutyryl cAMP was potentiated by theophylline. High voltage electron microscopy of whole mounted cells confirmed the loss of stress fibers (microfilament bundles). In addition, thyrotropin treatment led to an uneven redistribution of the cytoplasmic ground substance and to changes in the organization of the microtrabecular lattice. Stereo images demonstrated numerous minute surface ruffles. The thyrotropin-induced arborization was reversible even in the presence of thyrotropin. After 24 h of treatment, cells had flattened and then contained very straight and condensed microfilament bundles. The results thus demonstrate that thyrotropin induces a disintegration of microfilament bundles in human, partially dedifferentiated, follicular cells and that this effect to all appearances is caused by cAMP, the second messenger in thyrotropin action. The relation of this event in partially dedifferentiated cells to the effect of thyrotropin in the intact thyroid gland is unclear. The fact that several other cultured hormone-responsive cells round up or become arborized in conjunction with an increase in cAMP levels implies that cAMP may be a major factor in the disassembly of microfilament bundles in these cells.

Immunohistochemical localization of several cytoskeletal proteins in inner ear sensory and supporting cells

Several structural and contractile proteins have been searched for with immunohistochemical methods using antibodies directed against these proteins. Three types of preparations from the guinea pig have been used: isolated stereocilia from the utricle, organ of Corti fragments obtained by cellular dissociation and 0.2-1 micrometer sections obtained by cryoultramicrotomy. The main finding is that different sets of proteins compose the cytoskeleton in supporting cells and the mechanoreceptor structures of the sensory cells. Thus, actin was found in association with fimbrin in the mechanoreceptive region of hair cells, whereas supporting cells, although rich in actin, did not reveal fimbrin. Instead tubulin was seen together with actin in supporting cells which also exhibited prekeratin. Fimbrin appears to function as a protein capable of making bundles and networks from actin filaments. Its exclusive presence in the mechanosensitive region of the sensory cells is possibly related to the function of these cells as mechanoreceptors.

Microtubules and microfilaments in fixed and permeabilized cells are selectively decorated by nerve growth factor

A specific antibody against nerve growth factor (NGF) and indirect immunofluorescence microscopy have been used to follow the in vitro binding of NGF to cells made permeable to large molecules. All cells tested, both target (sensory neurons and PCI2 cells) and nontarget (3T3, BKH 2I, C6 glioma cells), revealed a decoration of cytoskeletal structures which on the basis of their form, reactivity with antibodies, and sensitivity to specific drugs may be identified as microtubules (MTs) and microfilaments (MFs). The decoration of either structure depends on the fixation and permeabilization conditions: MFs, in the form of stress fibers, are stained by NGF when the plasma membrane is permeabilized with methanol/acetone; MTs become intensely stained when the plasma membrane is solubilized with a nonionic detergent in the presence of a MT-stabilizing medium. The two procedures do not affect the staining of these structures with specific antibodies. Binding of 125I-labeled NGF to PCI2 cells was not competitively inhibited by a 100-fold excess of several positively charged proteins but it was markedly decreased in the presence of DNase I. 125I-Labeled NGF interacted with MTs and F-actin (fixed with paraformaldehyde) in a range of concentrations similar to that used for their cellular localization with NGF-anti-NGF. Our studies show that the specificity and affinity of NGF binding to MTs and MFs is in the range of that of antibodies against tubulin and actin. The possible relevance of these findings to the mechanism of action of NGF in target cells is discussed.

Correlation between effects of 24 different cytochalasins on cellular structures and cellular events and those on actin in vitro

To compare the effects of cytochalasins on the cellular level with those on the molecular level, 24 cytochalasins, 20 natural compounds and 4 derivatives, were used. The following effects were tested for each of 24 cytochalasins; (a) four high dose (2-20 muM) effects on the cellular level: rounding up of fibroblastic cells, contraction of actin cables, formation of hairy filaments containing actin, and inhibition of lymphocyte capping; (b) a low dose (0.2-2 muM) effect: inhibition of membrane ruffling; and (c) two in vitro effects: an inhibition of actin filament elongation (the high affinity effect [low dose effect] in vitro) and an effect on viscosity of actin filaments(the low affinity effect [high dose effect] in vitro). These results indicated that there are almost the same hierarchic orders of relative effectiveness of different cytochalasins between low and high dose effects and between cellular and molecular effects. From the data obtained with the 24 cytochalasins, we have calculated correlation coefficients of 0.87 and 0.79 between an effect in vivo, inhibition of capping, and an effect in vitro, inhibition of actin filament elongation, as well as between inhibition of capping and another effect in vitro, effect on viscosity of actin filaments, respectively. Furthermore, a correlation coefficient between the high affinity effect and the low affinity effect determined in vitro was calculated to be 0.90 from the data obtained in this study. The strong positive correlation among low and high dose effects in vivo and those in vitro suggests that most of the effects caused by a cytochalasin, irrespective of doses or affected phenomena, might be attributed to the interaction between the drug and the common target protein, actin. In the course of the immunofluorescence microscope study on cytochalasin-treated cells using actin antibody, we have found that aspochalasin D, a 10-isopropylcytochalasin, strongly induced the formation of rodlets containing actin in the cytoplasm of the treated fibroblasts. In contrast, the other cytochalasins, including cytochalasin B, cytochalasin C, cytochalasin D, and cytochalasin H, were found to induce the formation of nuclear rodlets. Both cytoplasmic and nuclear rodlets found in the cytochalasin-treated cells were similar in ultrastructures to those induced by 5 to 10 percent (vol/vol) dimethyl sulfoxide in the same type of cells.

Reactivation of contraction in detergent-lysed teleost retinal cones

Teleost retinal cones contract in the light and elongate in the dark. In the green sunfish, Lepomis cyanellus, the necklike myoid region of the cone contracts from as much as 120 micrometers (midnight dark-adapted) to 6 micrometers in fully light-adapted state. When dark-adapted fish are exposed to light (1.4 lux), cone myoids contract with a linear rate of 1.5 +/- 0.1 micrometers/min. We report here that detergent-lysed motile models of teleost retinal cones exhibit calcium- and ATP-dependent reactivated contraction, with morphology and rate comparable to that observed in vivo. For reactivation studies isolated dark-adapted retinas were lysed with nonionic detergent Brij-58 (0.1-1.0%). In reactivation medium containing 10(-5) M free calcium and 4 mM ATP, the lysed cones contracted with normal morphology at in vivo rates (1.4 +/- 1 micrometer/min). Little contraction was observed if ATP or detergent was deleted from the medium or if free calcium levels were less than 10(-8) M. Ultrastructural examination of cone models lysed with 1% Brij-58 revealed that, in spite of extensive extraction of the cytoplasmic matrix, cytoskeletal components (thin filaments, intermediate filaments, microtubules) were still present. Thus we have produced extensively extracted motile models of teleost retinal cones which undergo calcium- and ATP-dependent reactivated contraction with normal morphology at physiological rate.

Cellular response to rabies virus infection

The effects of rabies virus on host cells were studied and compared to those obtained with another rhabdovirus, vesicular stomatitis virus [J. Virol. 34, 777-781 (1980)]. We show here: (1) that rabies infection has no effect on cell morphology, while infection with vesicular stomatitis virus caused cell retraction. Thus, only vesicular stomatitis virus induced a depolymerization of the microfilaments; and (2) that microtubules and microfilaments do not play a major role in rabies virus production, as it is suggested by results obtained with several effectors (colcemid, colchicine and cytochalasin-B) which directly or indirectly affect cytoskeleton organization. The same properties were observed with directly or indirectly affect cytoskeleton organization. The same properties were observed with vesicular stomatitis virus. Furthermore, the use of cytochalasin-B shows that an inhibition of glycosylation of the virion spike protein occurs only in rabies infected cells. As vesicular stomatitis viral glycosylation is normal in cytochalasin-B treated cells, results obtained indicate that two types of interactions can occur between a virion and the host-cell depending on the rhabdovirus type.

Action of cytochalasin D on cytoskeletal networks

Extraction of SC-1 cells (African green monkey kidney) with the detergent Triton X-100 in combination with stereo high-voltage electron microscopy of whole mount preparations has been used as an approach to determine the mode of action of cytochalasin D on cells. The cytoskeleton of extracted BSC-1 cells consists of substrate-associated filament bundles (stress fibers) and a highly cross-linked network of four major filament types extending throughout the cell body; 10-nm filaments, actin microfilaments, microtubules, and 2- to 3-nm filaments. Actin filaments and 2- to 3-nm filaments form numerous end-to-side contacts with other cytoskeletal filaments. Cytochalasin D treatment severely disrupts network organization, increases the number of actin filament ends, and leads to the formation of filamentous aggregates or foci composed mainly of actin filaments. Metabolic inhibitors prevent filament redistribution, foci formation, and cell arborization, but not disorganization of the three-dimensional filament network. In cells first extracted and then treated with cytochalasin D, network organization is disrupted, and the number of free filament ends is increased. Supernates of preparations treated in this way contain both short actin filaments and network fragments (i.e., actin filaments in end-to-side contact with other actin filaments). It is proposed that the dramatic effects of cytochalasin D on cells result from both a direct interaction of the drug with the actin filament component of cytoskeletal networks and a secondary cellular response. The former leads to an immediate disruption of the ordered cytoskeletal network that appears to involve breaking of actin filaments, rather than inhibition of actin filament-filament interactions (i.e., disruption of end-to-side contacts). The latter engages network fragments in an energy-dependent (contractile) event that leads to the formation of filament foci.

Heterogeneity of intermediate filament expression in vascular smooth muscle: a gradient in desmin positive cells from the rat aortic arch to the level of the arteria iliaca communis

The display of the two distinct intermediate filament proteins, desmin and vimentin, in rat vascular smooth muscle tissue was studied by immunofluorescence microscopy on frozen sections of aorta and other blood vessels. Vascular smooth muscle cells present in these vessels always appeared rich in vimentin. However, staining of sections covering six distinct but contiguous parts of the aorta showed that the number of desmin containing cells was low distal to the truncus brachiocephalicus, but increases until in distal parts of the aorta and in the arteria iliaca communis almost all cells appear positive for desmin. Thus blood vessels show heterogeneity of intermediate filament expression not only in cross-section but can also display heterogeneity along their length. Muscular arteries such as the renal artery femoralis, as well as arterioles and veins including the vena jugularis and the vena cava also contain desmin. Thus it may be that low numbers of desmin-positive cells are typical of elastic arteries, while muscular arteries and other blood vessels are characterized by large numbers of desmin-positive cells. We discuss whether desmin-positive and desmin-negative vascular smooth muscle cells may perform functions and raise the possibility that desmin expression may coincide with the turn on of a specially regulated contractility program.