Effects of small doses of cytochalasins on fibroblasts: preferential changes of active edges and focal contacts

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.

Actin cytoskeleton in myofibroblast differentiation: ultrastructure defining form and driving function

Myofibroblasts are modified fibroblasts characterized by the presence of a well-developed contractile apparatus and the formation of robust actin stress fibers. These mechanically active cells are thought to orchestrate extracellular matrix remodeling during normal wound healing in response to tissue injury; these cells are found also in aberrant tissue remodeling in fibrosing disorders. This review surveys the understanding of the role of actin stress fibers in myofibroblast biology. Actin stress fibers are discussed as a defining ultrastructural and morphologic feature and well-accepted observations demonstrating its participation in contraction, focal adhesion maturation, and extracellular matrix reorganization are presented. Finally, more recent observations are reviewed, demonstrating its role in transducing mechanical force into biochemical signals, transcriptional control of genes involved in locomotion, contraction, and matrix reorganization, as well as the localized regulation of messenger RNA (mRNA) translation. This breadth of functionality of the actin stress fiber serves to reinforce and amplify its mechanical function, via induced expression of proteins that themselves augment contraction, focal adhesion formation, and matrix remodeling. In composite, the functions of the actin cytoskeleton are most often aligned, allowing for the integration and amplification of signals promoting both myofibroblast differentiation and matrix remodeling during fibrogenesis.

Cytoskeletal disassembly and cell rounding promotes adipogenesis from ES cells

Biomechanical signals such as cell shape and spreading play an important role in controlling stem cell commitment. Cell shape, adhesion and spreading are also affected by calreticulin, a multifunctional calcium-binding protein, which influences several cellular processes, including adipogenesis. Here we show that cytoskeletal disruption in mouse embryonic stem cells using cytochalasin D or nocodazole promotes adipogenesis. While cytochalasin D disrupts stress fibres and inhibits focal adhesion formation, nocodazole depolymerises microtubules and promotes focal adhesion formation. Furthermore, cytochalasin D increases the levels of both total and activated calcium/calmodulin-dependent protein kinase II, whereas nocodazole decreases it. Nevertheless, both treatments significantly increase the adipogenic potential of embryonic stem cells in vitro. Both cytochalasin D and nocodazole exposure caused cell rounding suggesting that it is cell shape that causes the switch towards the adipogenic programme. Calreticulin-containing embryonic stem cells, under baseline conditions, show low adipogenic potential, have low activity of signalling via calcium/calmodulin-dependent protein kinase II and display normal adhesive properties and cellular spreading in comparison to the highly adipogenic but poorly spread calreticulin-deficient ES cells. We conclude that forced cell rounding via cytoskeletal disruption overrides the effects of calreticulin, an ER chaperone, thus negatively regulating adipogenesis via focal adhesion-mediated cell spreading.

The effect of sub-lethal ALA-PDT on the cytoskeleton and adhesion of cultured human cancer cells

5-Aminolevulinic acid (ALA), a precursor of the endogenous photosensitizer protoporphyrin IX, is used in the photodynamic therapy (PDT) of cancer. Sub-lethal ALA-PDT (1-min irradiation with 370-450 nm blue light, 0.6 mW/cm(2) after 2-h incubation with 1 mM ALA) has been earlier shown to change cell morphology and to inhibit both trypsin-induced detachment of cultured cancer cells from the plastic substrata and cell attachment to the bottom of the plastic well plates. In the present study, we found that such treatment of human adenocarcinoma WiDr cells grown in dense colonies stimulated the formation of actin cortex between cells in the colonies and increased the number of actin stress fibres in some, but not in all, cells. However, ALA-PDT did not change the microtubular cytoskeleton in these cells. A similar treatment of glioblastoma D54Mg cells, which grow separately and communicate by protrusions, caused loss of fibrillar actin structures in growth cones, retraction of protrusions, and surface blebbing in some cells. The application of the cytoskeleton inhibitors cytochalasin D, colchicine or taxol showed that the inhibition of trypsin-induced detachment of photosensitized WiDr cells was related to ALA-PDT-induced changes in actin and microtubular cytoskeleton. Some signal transduction processes are suggested to be involved in ALA-PDT-induced changes in cytoskeleton, cell shape, and adhesion.

Cloning and characterization of betaCAP73, a novel regulator of beta-actin assembly

In non-muscle cells, the isoactins are differentially localized, with beta-actin specifically enriched at the cell cortex within motile structures, such as lamellae, while gamma-actin shows no specific localization. To understand the sorting and regulation of beta-actin within moving cells, we previously isolated betaCAP73, a novel beta-actin-specific binding protein (Cell Motil. Cytoskel. 35 (1996) 175). Here, we have cloned and characterized the 4718 nucleotide betaCAP73 cDNA from an endothelial cell library. betaCAP73 cDNA contains six predicted ankyrin-like repeats at the amino terminus and is partially homologous to three previously reported sequences of unknown function. Northern analysis reveals betaCAP73 expression in all tissues tested, with highest levels in skeletal muscle. Consistent with previously demonstrated interactions between native betaCAP73 and beta-actin filament barbed-ends, recombinant betaCAP73 inhibits pyrene-actin assembly in an isoactin-specific manner. Compared to stationary cells betaCAP73 mRNA is down regulated in crawling cells. Similarly, motility-defective cells have increased betaCAP73 protein. Overexpression of full-length betaCAP73 induces the formation of novel membrane protrusions that are enriched in betaCAP73, while overexpression of betaCAP73 domains alters cell morphology. Combined, these results indicate that betaCAP73 modulates isoactin dynamics to regulate the morphological alterations required for cell growth and motility.

Entamoeba invadens: enhancement of excystation and metacystic development by cytochalasin D

Effects of three actin-modifying drugs, cytochalasin D, latrunculin A, and jasplakinolide, on the excystation and metacystic development in vitro of Entamoeba invadens were examined by transfer of the cysts to growth medium with the drugs. Cytochalasin D unexpectedly increased the number of metacystic amoebae of E. invadens strain IP-1 during incubation. Metacystic development, which was determined by the number of nuclei of metacystic amoebae, was faster in the culture with cytochalasin D than in the culture without the drug. These results suggest that cytochalasin D enhances the excystation and metacystic development. In contrast, latrunculin A and jasplakinolide inhibited these process. No excystation occurred in encystation medium even in the presence of cytochalasin D, suggesting that growth medium is essential for excystation. Excystation was further enhanced when the cysts were incubated with cytochalasin D before culture in growth medium with the drug. The enhancing effect of cytochalasin D on the excystation and metacystic development was abrogated by jasplakinolide. Thus, the results indicate that cytochalasin D, unlike latrunculin A and jasplakinolide, caused enhancement of the excystation and metacystic development of this parasite.

Contraction-Dependent Apoptosis of Normal Dermal Fibroblasts

The mechanisms underlying the contraction-dependent apoptosis of primary fibroblasts are of prime importance in understanding anchorage-dependent survival/apoptosis of dermal fibroblasts. As integrins are essential extracellular matrix receptors in fibroblasts, their role in anchorage-dependent apoptosis/survival of fibroblasts was analyzed. Primary human fibroblasts displayed a marked reduction of apoptosis in mechanically relaxed collagen matrices in the presence of adhesion-blocking antibodies against alpha1beta1 or alpha2beta1. Anti-alphavbeta3 antibodies had a considerably weaker effect. In additional experiments RD cells, which lack alpha2 integrin, displayed no apoptosis in mechanically relaxed collagen matrices. Their susceptibility to apoptosis was restored after transfection with functional alpha2 integrin, and it could be blocked again by adhesion-blocking antibodies against alpha2beta1 integrin. Therefore we conclude that apoptosis of human primary fibroblasts in contractile collagen matrices is - at least in part - inhibited by adhesion-blocking anti-integrin antibodies, suggesting that the mode of apoptosis in this case is different from anoikis. Further, apoptosis in a mechanically relaxed collagen matrix could be abrogated by depolymerization of F-actin using cytochalasin D and also by disturbing actin-myosin interaction using 2,3-butanedione monoxime, indicating a possible dependence of apoptosis on mechanical forces and/or cell shape.

A cell-free system to study regulation of focal adhesions and of the connected actin cytoskeleton

Assembly and modulation of focal adhesions during dynamic adhesive processes are poorly understood. We describe here the use of ventral plasma membranes from adherent fibroblasts to explore mechanisms regulating integrin distribution and function in a system that preserves the integration of these receptors into the plasma membrane. We find that partial disruption of the cellular organization responsible for the maintenance of organized adhesive sites allows modulation of integrin distribution by divalent cations. High Ca2+ concentrations induce quasi-reversible diffusion of beta1 integrins out of focal adhesions, whereas low Ca2+ concentrations induce irreversible recruitment of beta1 receptors along extracellular matrix fibrils, as shown by immunofluorescence and electron microscopy. Both effects are independent from the presence of actin stress fibers in this system. Experiments with cells expressing truncated beta1 receptors show that the cytoplasmic portion of beta1 is required for low Ca2+-induced recruitment of the receptors to matrix fibrils. Analysis with function-modulating antibodies indicates that divalent cation-mediated receptor distribution within the membrane correlates with changes in the functional state of the receptors. Moreover, reconstitution experiments show that purified alpha-actinin colocalizes and redistributes with beta1 receptors on ventral plasma membranes depleted of actin, implicating binding of alpha-actinin to the receptors. Finally, we found that recruitment of exogenous actin is specifically restricted to focal adhesions under conditions in which new actin polymerization is inhibited. Our data show that the described system can be exploited to investigate the mechanisms of integrin function in an experimental setup that permits receptor redistribution. The possibility to uncouple, under cell-free conditions, events involved in focal adhesion and actin cytoskeleton assembly should facilitate the comprehension of the underlying molecular mechanisms.

Tyrphostins disrupt stress fibers and cellular attachments in endothelial monolayers

Endothelial permeability, which plays a critical role in many physiologic and pathologic processes, depends on the integrity of intercellular and cell-substrate attachments and the actin cytoskeleton. The proteins located at the cytoplasmic face of adherens and focal contact junctions are rich in sites of tyrosine phosphorylation. To better understand the role of tyrosine phosphorylation in regulating endothelial cell shape, actin stress fibers, and cell junctions, we treated confluent calf pulmonary artery endothelial cells with 14 different tyrphostins, a class of specific tyrosine kinase inhibitors. Using immunofluorescence microscopy to assess cell shape, phosphotyrosine levels, actin stress fibers, and focal contact and junctional proteins, we found that the effects of the tyrphostins could be grouped into three categories. Four tyrphostins had no discernible effect on stress fibers or cell attachments. Seven tyrphostins produced cell retraction with concomitant disruption of both stress fibers and cell-substrate attachments. One member of this group, tyrphostin 25, showed greater specificity for cell-cell junctions than the others, causing cell separation without significantly affecting actin stress fibers or focal contacts. The third group of tyrphostins had the opposite effect, completely disrupting stress fibers and focal contacts without causing cell separation. The ability of specific tyrphostins to disrupt cell-cell or cell-substrate attachments and/or actin stress fibers implies that a certain steady-state level of tyrosine phosphorylation is necessary to maintain these structures and that there may be independent tyrosine kinase signaling pathways controlling them. Comparison of the phosphotyrosinated proteins affected by each group of tyrphostins should provide a useful new approach toward understanding the regulation of endothelial cell-cell and cell-substrate junctions.

Disruption of actin microfilaments by cytochalasin D leads to activation of p53

Activation of p53 plays a central role in the cell's response to various stress signals. We investigated whether p53 is activated upon disruption of actin microfilaments, caused by cytochalasin D (CD). We show that treatment with CD leads to accumulation of p53 in the cells and activation of p53-dependent transcription. Treatment with CD led to arrest of G1-to-S transition in cells retaining wild-type p53, while cells with inactivated p53 showed partial rescue from it. CD also induces apoptosis in p53+/+, but not in p53-/- cells. The obtained data suggest that disruption of the actin microfilaments activates p53-dependent pathways.

Ultrasound inhibits the adhesion and migration of smooth muscle cells in vitro

This study investigated in vitro the effect of therapeutic ultrasound (ULS) on smooth muscle cell (SMC) function as adhesion, migration and proliferation. Experiments were conducted on aortic SMC in culture. The LD50 was established (1.5 W for 15 s at a frequency of 20 kHz) and used as standard dose in all experiments. Control SMC and viable sonicated SMC were compared in each experiment. Migratory capacity decreased 2.4-fold after sonication and stayed reduced for up to 24 h. Adhesion capacity decreased 5.5-fold after ULS. The proliferative capacity was similar to that of nonsonicated SMC. Sonication was accompanied by the disorganization of alpha-SM actin fibers and diminished distribution of vinculin; tyrosinated alpha tubulin and vimentin appeared unaffected. These changes might be responsible for the observed inhibition of SMC adhesion and migration. Sonicated cells exhibited less lamellipodia, membrane collapse and bleb formation. The signal transduction cascade, which involves activation of the phospholipase-C pathway, was unaffected by ULS.

A mechanism for trabecular meshwork cell retraction: ethacrynic acid initiates the dephosphorylation of focal adhesion proteins

Ethacrynic acid (ECA) increases aqueous humor outflow facility in human and animal model systems, and causes cellular retraction in cultured trabecular meshwork (TM) cells. ECA-induced retraction, a possible correlate to the opening of spaces in the outflow pathway in vivo, takes place coincident with disruption of cell-cell attachments and actin stress fibers. Tyrosine phosphorylated proteins are located predominantly where actin filaments terminate at sites of cell-to-cell and cell-to-substrate adhesion, and are understood to regulate cellular adhesions and filamentous (F) actin organization in many cell types. In the present study we investigated whether ECA might affect cell adhesions and F-actin in TM cells by altering levels of phosphotyrosine. We analysed levels of phosphotyrosine in cultured human TM and calf pulmonary artery endothelial cells after exposure to ECA. Using immunoflourescence microscopy and antibodies to phosphotyrosinated proteins we found a rapid decrease in phosphotyrosine levels at the focal contacts of cells treated with ECA. Immunoblots of whole cell extracts showed a decrease in phosphotyrosine predominantly in a band running at about 120 kD, with a more subtle decrease in a band about 65 kD. Reprobing the blot with antibodies to pp120 focal adhesion kinase (FAK) or paxillin indicated that the 120 kD band was FAK and the 65 kD band was likely paxillin. Immunoprecipitation of FAK or paxillin and probing the resulting blot with antibodies to phosphotyrosine confirmed that these proteins were rapidly dephosphorylated after ECA addition. Loss of FAK and paxillin proteins in cells was then confirmed using immunofluorescence microscopy. Dephosphorylation of these proteins was detected before the onset of retraction, stress fiber disruption, or complete disruption of focal adhesions. A pure microtubule inhibitor (colchicine), did not cause stress fiber disruption or decrease focal adhesion phosphorylation. We postulate that dephosphorylation of FAK and paxillin by ECA disrupts signaling pathways that normally maintain the stability of the actin cytoskeleton and cellular adhesions, and that this action leads both to cell shape change in culture, and to facility changes in vivo.

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.

Actin-binding protein, drebrin, accumulates in submembranous regions in parallel with neuronal differentiation

Drebrins are developmentally regulated actin-binding proteins. In this study, we analyzed subcellular distribution of drebrin E in neuroblastoma cells (SH-SY5Y) in culture, especially in terms of its relationship to actin filaments. In undifferentiated cells, drebrin E was scattered as flocculus small dots along the stress fibers and also accumulated at adhesion plaques. In parallel with the neuronal differentiation following retinoic acid treatment, drebrin E was accumulated, accompanying filamentous (F) actin, in the submembranous cortical cytoplasm. Similar submembranous localization of drebrins was observed in primary cultured neurons. In the presence of drebrin E F-actin was more stable against cytochalasin D than F-actin lacking drebrin E. These results suggest that drebrin E plays a role in neuronal morphological differentiation by changing its subcellular localization with stabilized F-actin.

Morphogenetic response of cultured normal and transformed fibroblasts, and epitheliocytes, to a cylindrical substratum surface. Possible role for the actin filament bundle pattern

Morphometric characteristics such as cell area, dispersion, elongation and orientation were studied in normal and transformed fibroblasts, and in epitheliocytes cultured on flat or cylindrical substrata. Cylindrical surfaces with a high degree of curvature (12-13 or 25 microns radii) were shown to affect cell size, shape and alignment. The reaction of the cells to the curvature of cylindrical substrata was different in various cell types studied and depended on the pattern of actin microfilament bundles. The cells containing pronounced straight actin bundles (mouse embryo fibroblasts at the polarization stage of spreading, single spread cells of the ‘normal’ epithelial FBT line or the fully transformed epithelial IAR 6–1 line) were relatively resistant to bending around a cylindrical substratum, and became elongated and oriented along the cylinder. Cells with circular actin bundles as the predominant pattern (mouse embryo fibroblasts at the radial stage of spreading, single spread cells of ‘normal’ epithelial IAR 20 line) and cells with insufficient or no actin bundles (transformed fibroblastic L line) were prone to bending around a cylinder with much less pronounced elongation and orientation along its axis. The data obtained indicate that the reaction of cultured cells to the geometry of the substratum surface and, in particular, to a cylindrical surface is determined not only by the presence or absence of actin microfilament bundles but by their pattern in the cell.

Dissociation of actin microfilament organization from acquisition and maintenance of elongated shape of human dermal fibroblasts in three-dimensional collagen gel

Actin microfilaments of the fibroblasts cultured in a collagen gel were distributed along the inner surface of the entire cell membrane, in either spherical shape at an initial stage of culture or elongated shape at a later stage. The distribution was quite different from that of the fibroblast cultured on a two-dimensional surface, where actin microfilaments were found to be aligned essentially along the inner membrane which is in contact with a flat surface. Timing of morphological change from spherical shape to spread shape or elongated shape was also greatly affected by contact with substrates whether in two-dimension or in three-dimension: distinct morphological change was observed within 6 h on glass or on the collagen gel, and at 30 h or later within the collagen gel. The retardation of cell elongation in the gel was antagonized by a low dose (0.2 microM) of cytochalasin D, although the drug kept the cells in round shape at a concentration of 2 microM. Since a low concentration of cytochalasin was reported to induce actin polymerization in vitro, the organization of actin microfilaments was examined by rhodamine-phalloidin staining. It was found that actin filaments in elongated cells by low cytochalasin D were disrupted. These results suggest that accelerated acquisition of elongated shape by the treatment of a low dose of cytochalasin D might be initiated by destabilization of the actin microfilaments that may scaffold the spherical shape of the cell in the collagen gel. The elongated shape thus formed returned to spherical upon washing of the added free cytochalasin D.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytoskeletal agents inhibit motility and adherence of human tumor cells

Cytoskeletal agents have been demonstrated to inhibit stimulated motility and substrate adherence by the human tumor cell line, A2058. cis-tubulozole, taxol, and cytochalasin D were tested for their effects on chemotaxis in response to a tumor cytokine, autocrine motility factor, and on adherence to several substrata: laminin- and gelatin-coated dishes as well as tissue culture plastic. Cytochalasin D, which inhibits microfilament polymerization, abolished stimulated motility. Taxol, which stabilizes microtubules, decreased stimulated motility to a greater degree than cis-tubulozole, which inhibits microtubular polymerization. In contrast, cis-tubulozole had the greatest inhibitory effect on adherence with a gelatin substratum more affected (100% inhibition) than tissue culture plastic (90%) or laminin substratum (52%). Taxol affected adherence in the same order but less than cis-tubulozole. Cytochalasin D had no significant effect on adherence to laminin with moderate inhibition of adherence to tissue culture plastic or gelatin. These data suggest that, in these tumor cells, microfilaments are more crucial for motility than adherence, but the dynamic polymerization and depolymerization of microtubules are required for both types of cellular activities.

Cellular partitioning of beta-1 integrins and their phosphorylated forms is altered after transformation by Rous sarcoma virus or treatment with cytochalasin D

A sequential extraction procedure of 3-[(3-cholamidopropyl)-dimethylammonio]-1-propane sulfonate (CHAPS) buffer followed by RIPA or Laemmli sample buffer was developed to define two distinct subpopulations of beta-1 integrins in primary chicken embryo fibroblasts. Extraction of cells in culture revealed an association of adhesion plaque-localized integrin with the CHAPS-insoluble fraction. Phosphorylated integrins were found in both fractions, but the specific phosphorylation was 12-fold higher in the CHAPS insoluble fraction. The phosphorylation was evenly distributed between phosphoserine and phosphotyrosine. Transformation by Rous sarcoma virus caused a redistribution of integrin to rosettes and an increase in total integrin phosphorylation. Treatment with cytochalasin D caused a redistribution of the adhesion plaque-associated integrin into lacelike structures and reduced the level of integrin phosphorylation. These treatments also caused an altered distribution of phosphorylated integrin between the CHAPS soluble and insoluble fractions. These results suggest a role for integrin phosphorylation in the assembly and disassembly of cellular adhesion structures.

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.

What structures, besides adhesions, prevent spread cells from rounding up?

The outline of cells in sparse cultures consists predominantly of concave and convex segments; straight segments are rare and ephemeral. The convex segments are areas of active cell expansion. The concave segments are stationary and web-shaped, similar in profile to the cables of a suspension bridge. In 3T3 fibroblasts, we have found a single microfilament bundle following the outline of every webbed edge and have called it the actin edge-bundle (AEB). While the AEB is composed predominantly of actin, alpha-actinin and myosin are also present. In contrast to normal stress fibers, AEBs are more resistant to several treatments that depolymerize F-actin. Once an AEB disassembles, however, the webbed edge collapses and retracts, suggesting that the actin edge-bundle is a specialized cytoskeletal structure that supports the webbed edges of interphase 3T3 fibroblasts. The stability of AEBs is independent of microtubules. We suggest that the microfilament bundles that frequently line the lateral contacts between epithelial cells in vivo may be related to the actin edge-bundle.

Quantitative evaluation of the factors affecting the process of fibroblast-mediated collagen gel contraction by separating the process into three phases

Kinetics of collagen gel contraction by fibroblasts cultured in vitro was examined in detail for quantitative analysis. The process of collagen gel contraction was not expressed by a simple function of time. It appeared to consist of three distinct phases; a lag phase before the initiation of contraction, a rapid contraction phase and a slow contraction phase. Factors affecting the gel contraction can be classified into four groups. The first group includes increase in cell number, in culture temperature or in serum concentration, which strengthened the contraction in all the three phases, suggesting that they affected cellular activity particularly in interacting with collagen. The second group repressed the later two phases of contraction but not the first lag phase, typically increase in collagen concentration and a low dose of nocodazole or colcemid. Increasing population doubling levels of fibroblasts belongs to the third group which caused a reduced lag time but no change in the later two phases. Cytochalasin D at a low dose (0.03-0.1 microgram/ml) is another example of the third group which shortened the lag time. The last group did not change the contraction curves. Donor age of fibroblasts isolated from the skin is an example of this group. The rate of rapid contraction in the second phase was always found to be closely correlated with the degree of contraction at the end of the third phase, in a whole set of the factors above mentioned. The results suggest that the extent of the later two phases might be a reflection of the same cellular activity, particularly cytokinetical one. The lag time is directly related to the time for cells to become elongate in shape as observed by using the video-microscopy, suggesting that the lag phase is also governed by cytokinetical activity. The two cytokinetical activities are closely related, but may be distinct, since the factors affecting the collagen gel contraction can be differentiated into four groups.

Contact formation during fibroblast locomotion: involvement of membrane ruffles and microtubules

We have correlated the motility of the leading edge of fibroblasts, monitored by phase-contrast cinematography, with the relative distributions of several cytoskeletal elements (vinculin, tubulin, and actin) as well as with the contact patterns determined by interference reflection microscopy. This analysis has revealed the involvement of both ruffles and microspikes, as well as microtubules in the initiation of focal contact formation. Nascent vinculin sites within the leading edge or at its base, taken as primordial cell-substrate contacts, were invariably colocalized with sites that showed a history of transient, prolonged, or cyclic ruffling activity. Extended microspike structures, often preceded the formation of ruffles. Immunofluorescent labeling indicated that some of these primordial contacts were in close apposition to the ends of microtubules that penetrated into the leading edge. By fluorescence and electron microscopy short bundles of actin filaments found at the base of the leading edge were identified as presumptive, primordial contacts. It is concluded that ruffles and microspikes, either independently or in combination, initiate and mark the sites for future contact. Plaque proteins then accumulate (within 10-30 s) at the contract site and, beneath ruffles, induce localized bundling of actin filaments. We propose that all primordial contacts support traction for leading edge protrusion but that only some persist long enough to nucleate stress fiber assembly. Microtubules are postulated as the elements that select, stabilize, and potentiate the formation of these latter, long-lived contacts.

Relationship between the organization of actin bundles and vinculin plaques

The temporal pattern of the formation and dissolution of vinculin patches during experimental manipulation of the state of actin within the cell was studied. Cytochalasin D-induced retraction and disappearance of stress fibers is followed, with a brief delay, by the dissolution of vinculin-containing patches and the coordinated redistribution of both actin and vinculin into newly formed amorphous aggregates or foci. Recovery from cytochalasin treatment begins with a transformation of these foci into doughnut-shaped assemblies in which actin and vinculin are precisely co-localized. The emergence and growth of filament bundles is paralleled by the appearance of faint vinculin patches that gradually increase in size in parallel with the stress fibers. If stress fibers are stabilized by microinjected rhodamine-phalloidin against stimuli that normally induce a coordinated redistribution of actin and vinculin, also the vinculin patches persist. These observations indicate that treatments influencing the state of actin in the cell have corresponding effects on the stability of vinculin patches and suggest a strong interdependency of actin and vinculin organization.

Regulation and drug insensitivity of F-actin association with adhesion areas of transformed cells

F-actin aggregates have been found near the substrate attachments in a variety of transformed cells (Carley et al., 1981). Interference reflection microscopy shows that these aggregates are present in central close adhesion areas in Rous sarcoma virus (RSV)-transformed rat kidney cells. If these transformed cells are incubated with N6, O2-dibutyryl 3':5'-cyclic monophosphoric acid (db-cAMP), adenosine 5'-monophosphoric acid (5'-AMP) or adenosine, the F-actin aggregates and their associated close adhesion areas disappear, and the cells flatten out. Treatment of untransformed cells with db-cAMP spreads their focal adhesion plaques and thickens microfilament bundles. Furthermore, F-actin aggregates are substantially more resistant to cytochalasin B and the Ca2+ ionophore A23187 than microfilament bundles in untransformed cells. These differences between F-actin complexes in untransformed and in RSV-transformed cells, with respect to morphology and sensitivities to db-cAMP and cytoskeleton-disrupting drugs, define properties of the change in F-actin regulation and association with the plasma membrane due to transformation.

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.