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

Disruption of the actin cytoskeleton of mammalian cells by the capping complex actin-fragmin is inhibited by actin phosphorylation and regulated by Ca2+ ions

Fragmin from Physarum polycephalum is a gelsolin-like actin-binding protein and interferes with the growth of actin filaments in vitro by severing actin filaments and capping their barbed ends through formation of an actin-fragmin dimer in a Ca2+-dependent manner. The actin-fragmin dimer is phosphorylated in vivo and in vitro on the actin subunit by the actin-fragmin kinase. We have studied the properties of these capping proteins and their regulation by actin phosphorylation and Ca2+ ions in living PtK2, CV1 and NIH3T3 cultured cells by microinjection or by expression in conjunction with immunostaining and fluorescence microscopy. Microinjection of the actin-fragmin dimer disintegrated the actin cytoskeleton and altered cell morphology. This in vivo effect could be blocked by phosphorylation of the actin subunit by the actin-fragmin kinase in low Ca2+ conditions, and the capping activity could be recovered by high Ca2+ concentration, probably through activation of the second actin-binding site in fragmin. This suggests that in Physarum microplasmodia, actin polymerization can be controlled in a Ca2+-dependent manner through the phosphorylation of actin. Microinjected or overexpressed recombinant fragmin did not affect the actin-based cytoskeleton or cell morphology of resting cells, unless the cytosolic free Ca2+ concentration was increased by microinjection of a Ca2+-containing buffer. The cells were able to revert to their normal phenotype which indicates that endogenous regulatory mechanisms counteracted fragmin activity, probably by uncapping fragmin from the barbed ends of filaments. Fragmin also antagonized formation of stress fibers induced by lysophosphatidic acid. Our findings demonstrate that the interactions between actin and fragmin are tightly regulated by the cytosolic Ca2+ concentration and this provides a basis for a more general mechanism in higher organisms to regulate microfilament organization.

Effect of DMSO and exchange volume on outflow resistance washout and response to pilocarpine during anterior chamber perfusion in monkeys

PURPOSE

To determine the dependence of outflow resistance washout on anterior chamber (AC) fluid exchange volume (EV); the minimum EV required for complete AC mixing, and the effect of 0.01-5% DMSO in the exchange solution on outflow resistance.

METHODS

Total outflow facility was determined in 63 pentobarbital-anesthetized cynomolgus monkeys, before and after AC exchange, for 10 min, with 1, 2, and 4 ml of Bárány's perfusand, containing 0.01-5% DMSO or containing 1 microgram/ml or 5 micrograms/ml pilocarpine HCL (pilo), each volume.

RESULTS

Post-exchange facility increased by 21% (p < 0.05) vs. baseline for both the 1 and 2 ml Bárány's volumes and by 50% (p < 0.001) for the 4-ml volume. Washout-corrected post-exchange facility was unchanged from baseline, following 1 or 2 ml exchange with 1 microgram/ml pilo; 5 micrograms/ml pilo increased facility by 130 +/- 41% and 174 +/- 40% respectively, relative to baseline. Exchange with 4 ml of 1 microgram/ml pilo increased facility 72% more than the 2 ml (p < 0.029) and 100% more than the 1 ml (p < 0.014) volumes. When corrected for their respective baselines, 2 ml of 5 micrograms/ml pilo increased facility 12.7 +/- 4.4% more than the 1 ml volume (p < 0.05). Post exchange facility was not significantly increased over baseline in eyes exchanged with 2 ml of 0.01-4% DMSO or in eyes exchanged with Bárány's alone. When corrected for their respective baselines, post-exchange facility was significantly lower in eyes exchanged with 5% DMSO vs. Bárány's (p = 0.022).

CONCLUSIONS

One- and 2-ml EVs preserve outflow resistance equally well, and substantially better than 4 ml. Two ml is the minimum volume necessary for adequate mixing of AC drug solutions, to achieve the full facility effect. Concentrations of DMSO up to 4% in a 2-ml EV can solubilize compounds without additionally decreasing outflow resistance.

Microinjection of ADP-ribosylated actin inhibits actin synthesis in hepatocyte-hepatoma hybrid cells

Treatment of hepatocyte-hepatoma hybrid cells with Clostridium botulinum C2 toxin led to a 167% increase in monomeric globular actin (G-actin) and to a 57% decrease in filamentous actin (F-actin) within 2 h. Simultaneously, the level of actin mRNA was specifically decreased to 49% and actin synthesis was significantly diminished. In contrast, treatment of hybrid cells with phalloidin led to a decrease in G-actin to 55% and to a reciprocal increase in actin mRNA to 244% and an increase in actin synthesis. These alterations of actin synthesis depending on the G-actin/F-actin ratio corresponded to the autoregulation of actin synthesis observed in primary cultures of rat hepatocytes. Microinjection of C2 toxin or of phalloidin into hepatocyte-hepatoma hybrid cells had the same effects on actin synthesis as incubation with either toxin in the culture medium. Microinjection of nonpolymerizable ADP-ribosylated G-actin into hepatocyte-hepatoma hybrid cells specifically decreased the incorporation of [35S]methionine into newly synthesized actin within 1 h. This decrease continued for at least 19 h. Microinjection of ADP-ribosylated actin led to rounding of cells and obvious disaggregation of actin filaments, which might be due to capping of actin filaments by the ADP-ribosylated actin. Because stabilization of actin filaments by phalloidin before microinjection of ADP-ribosylated actin also resulted in decreased actin synthesis, the concentration of monomeric G-actin seems to be responsible for the regulation of actin synthesis in hepatocyte-hepatoma hybrid cells, which can be regarded as immortalized hepatocytes.

Microfilament reorganization during apoptosis: the role of Gas2, a possible substrate for ICE-like proteases

Gas2, a component of the microfilament system, belongs to the class of gas genes whose expression is induced at growth arrest. After serum or growth factor addition to quiescent NIH 3T3 cells, Gas2 is hyperphosphorylated and relocalized at the membrane ruffles. By overexpressing gas2wt and a series of deletion mutants of the C-terminal region, we have analysed its role in the organization of the actin cytoskeleton in different cell lines. Overexpression of Gas2 deleted at its C-terminal region (delta 276-314 and delta 236-314), but not its wild-type form, induces dramatic changes in the actin cytoskeleton and cell morphology. These effects are not due to interference of the deleted forms with the endogenous Gas2wt function but could be ascribed to a gain of function. We demonstrate that during apoptosis the C-terminal domain of Gas2 is removed by proteolytic cleavage, resulting in a protein that is similar in size to the described delta 276-314. Moreover, by using in vitro mutagenesis, we also demonstrate that the proteolytic processing of Gas2 during apoptosis is dependent on an aspartic acid residue at position 279. The evidence accumulated here could thus represent a first example of a mechanism linking apoptosis with the co-ordinated microfilament-dependent cell shape changes, as possibly mediated by an interleukin-1 beta-converting enzyme (ICE)-like dependent proteolytic cleavage of the Gas2 protein.

In vivo dynamics of myosin II in Dictyostelium by fluorescent analogue cytochemistry

We used fluorescent analogue cytochemistry to study in vivo dynamics of myosin II in Dictyostelium discoideum. We labeled myosin with biotin or tetramethyl-rhodamine iodoacetamide (IATR). The labeled myosin shows normal activities as reversible filament assembly and Ca2+ and actin-activatable Mg(2+)-ATPase. We used the biotin-myosin as a probe examining the effects of microinjection on the amoebae and the ability to associate with endogenous actin cytoskeleton. The biotin-myosin incorporates into certain actin populations and localizes to the cortex with the highest accumulation in the posterior end of polarized amoebae. The dynamics in live amoebae were probed by TR-myosin. We monitored the dynamics for a long period to determined the dynamic reorganization corresponding specific cellular behaviors. The TR-myosin converges into a discrete actin- and myosin-rich structure located at the posterior end ("myosin-organizing center"). The rod-shaped TR-myosin exhibits linear orderly arrays emanating from the organizing center which extend about two-thirds of the cell length. The myosin arrays show a dynamic reorganization when the amoebae move. To examine if the observed myosin dynamics are related to filamentous (F-) actin, we disrupted the F-actin by cytochalasin D. The ratioed image of TR-myosin (vs. FITC-dextran) demonstrates that myosin in these cells accumulates in the cortex but does not form the organizing center. Overall, the results suggest that the filamentous myosin organizes into orderly arrays in the live cytoplasm and its translocation occurs by means of F-actin cables, converging into the organizing center.

Actin filament barbed-end capping activity in neutrophil lysates: the role of capping protein-beta 2

A barbed-end capping activity was found in high speed supernates of neutrophils lysed in submicromolar calcium. In dilute supernate (> or = 100-fold dilution of cytoplasm), this activity accounted for most of the inhibition of barbed-end elongation of pyrenyl-G-actin from spectrin-F-actin seeds. Pointed-end elongation from gelsolin-capped F-actin seeds was not inhibited at comparable concentrations of supernate, thus excluding actin monomer sequestration as a cause of the observed inhibition. Most of the capping activity was due to capping protein-beta 2 (a homologue of cap Z). Thus, while immunoadsorption of > or = 95% of the gelsolin in the supernate did not decrease capping activity, immunoadsorption of capping protein-beta 2 reduced capping activity proportionally to the amount of capping protein-beta 2 adsorbed. Depletion of > 90% of capping protein-beta 2 from the supernate removed 90% of its capping activity. The functional properties of the capping activity were defined. The dissociation constant for binding to barbed ends (determined by steady state and kinetic analyses) was approximately 1-2 nM; the on-rate of capping was between 7 x 10(5) and 5 x 10(6) M-1 s-1; and the off-rate was approximately 2 x 10(-3) s-1. The concentration of capper free in the intact cell (determined by adsorption of supernate with spectrin-actin seeds) was estimated to be approximately 1-2 microM. Thus, there appeared to be enough high affinity capper to cap all the barbed ends in vivo. Nevertheless, immediately after lysis with detergent, neutrophils contained sites that nucleate barbed-end elongation of pyrenyl-G-actin. These barbed ends subsequently become capped with a time course and concentration dependence similar to that of spectrin-F-actin seeds in high speed supernates. These observations suggest that, despite the excess of high affinity capper, some ends either are not capped in vivo or are transiently uncapped upon lysis and dilution.

The specific NH2-terminal sequence Ac-EEED of alpha-smooth muscle actin plays a role in polymerization in vitro and in vivo

The blocking effect of the NH2-terminal decapeptide of alpha-smooth muscle (SM) actin AcEEED-STALVC on the binding of the specific monoclonal antibody anti-alpha SM-1 (Skalli, O., P. Ropraz, A. Trzeviak, G. Benzonana, D. Gillessen, and G. Gabbiani. 1986. J. Cell Biol. 103:2787-2796) was compared with that of synthetic peptides modified by changing the acetyl group or by substituting an amino acid in positions 1 to 5. Using immunofluorescence and immunoblotting techniques, anti-alpha SM-1 binding was abolished by the native peptide and by peptides with a substitution in position 5, indicating that AcEEED is the epitope for anti-alpha SM-1. Incubation of anti-alpha SM-1 (or of its Fab fragment) with arterial SM actin increased polymerization in physiological salt conditions; the antibody binding did not hinder the incorporation of the actin antibody complex into the filaments. This action was not exerted on skeletal muscle actin. After microinjection of the alpha-SM actin NH2-terminal decapeptide or of the epitopic peptide into cultured aortic smooth muscle cells, double immunofluorescence for alpha-SM actin and total actin showed a selective disappearance of alpha-SM actin staining, detectable at approximately 30 min. When a control peptide (e.g. alpha-skeletal [SK] actin NH2-terminal peptide) was microinjected, this was not seen. This effect is compatible with the possibility that the epitopic peptide traps a protein involved in alpha-SM actin polymerization during the dynamic filament turnover in stress fibers. Whatever the mechanism, this is the first evidence that the NH2 terminus of an actin isoform plays a role in the regulation of polymerization in vitro and in vivo.

Effects of CapG overexpression on agonist-induced motility and second messenger generation

Actin modulating proteins that bind polyphosphoinositides, such as phosphatidylinositol 4, 5-bisphosphate (PIP2), can potentially participate in receptor signaling by restructuring the membrane cytoskeleton and modulating second messenger generation through the phosphoinositide cycle. We examined these possibilities by overexpressing CapG, an actin filament end capping, Ca(2+)- and polyphosphoinositide-binding protein of the gelsolin family. High level transient overexpression decreased actin filament staining in the center of the cells but not in the cell periphery. Moderate overexpression in clonally selected cell lines did not have a detectible effect on actin filament content or organization. Nevertheless, it promoted a dose-dependent increase in rates of wound healing and chemotaxis. The motile phenotype was similar to that observed with gelsolin overexpression, which in addition to capping, also severs and nucleates actin filaments. CapG overexpressing clones are more responsive to platelet-derived growth factor than control-transfected clones. They form more circular dorsal membrane ruffles, have higher phosphoinositide turnover, inositol 1,4,5-trisphosphate generation and Ca2+ signaling. These responses are consistent with enhanced PLC gamma activity. Direct measurements of PIP2 mass showed that the CapG effect on PLC gamma was not due primarily to an increase in the PIP2 substrate concentration. The observed changes in cell motility and membrane signaling are consistent with the hypothesis that PIP(2)-binding actin regulatory proteins modulate phosphoinositide turnover and second messenger generation in vivo. We infer that CapG and related proteins are poised to coordinate membrane signaling with actin filament dynamics following cell stimulation.

Effects of cofilin on actin filamentous structures in cultured muscle cells. Intracellular regulation of cofilin action

The previous investigation (Abe et al. (1989) J. Biochem. 106, 696–702) suggested that cofilin is deeply involved in the regulation of actin assembly in developing skeletal muscle. In this study, to examine further the function of cofilin in living myogenic cells in culture, recombinant cofilin having extra Cys residues at the N terminus was produced in Escherichia coli and was labeled with tetramethylrhodamine-iodoacetamide (IATMR). When the cofilin labeled with IATMR (IATMR-cofilin) was introduced into myogenic cells, actin filaments in the cytoplasm or nascent myofibrils were promptly disrupted, and many cytoplasmic rods which contained both IATMR-cofilin and actin were generated. Sarcomeric myofibrillar structures were not disrupted but tropomyosin was dissociated from the structures by the exogenous cofilin, and the IATMR-cofilin became localized in I-band regions. 24 hours after the injection, however, the actin-cofilin rods disappeared completely and the IATMR-cofilin became diffused in the cytoplasm as endogenous cofilin. Concomitantly, actin filaments were recovered and tropomyosin was re-associated with sarcomeric I-bands. At this point, the IATMR-cofilin in the cells still retained the functional activity to form intranuclear actin-cofilin rods in response to stimulation by DMSO just as endogenous cofilin. FITC-labeled actin introduced into myogenic cells at first failed to assemble into filamentous structures in the presence of the exogenous cofilin, but was gradually incorporated into myofibrils with time. The drastic effects of the exogenous cofilin on actin assembly were suppressed by phosphatidylinositol 4,5-bisphosphate (PIP2). These results indicate that the exogenous cofilin is active and alters actin dynamics remarkably in muscle cells, but its activity in the cytoplasm gradually becomes regulated by the action of some factors including PIP2-binding.

Increasing intracellular concentrations of thymosin beta 4 in PtK2 cells: effects on stress fibers, cytokinesis, and cell spreading

Thymosin beta 4 (T beta 4) binds to G-actin in vitro and inhibits actin polymerization. We studied the effects of increasing T beta 4 concentration within living PtK2 cells, comparing its effects on the disassembly of stress fibers and membrane-associated actin with its ability to inhibit cytokinesis and cell spreading after mitosis. We chose PtK2 cells for the study because these cells have many striking actin bundles in both stress fibers and cleavage furrows. They also have prominent concentrations of membrane-associated actin and remain flattened during mitosis. We have found that PtK2 cells contain an endogenous homologue of T beta 4 at a concentration (approximately 28 microM) sufficient to complex a third or more of the cell's unpolymerized actin. Intracellular T beta 4 concentrations were increased by three different methods: 1) microinjection of an RSV vector containing a cDNA for T beta 4; 2) transfection with the same vector; and 3) microinjection of purified T beta 4 protein. The plasmid coding for T beta 4 was microinjected into PtK2 cells together with fluorescently labeled alpha-actinin as a reporter molecule. Immediately after microinjection fluorescently labeled alpha-actinin was detected in a periodic pattern along the stress fibers just as in control cells injected solely with the reporter. However, after 13 h, cells microinjected with reporter and plasmid showed marked disassembly of the fiber bundles. PtK2 cells transfected with this RSV vector for 2-3 days showed disassembly of stress fibers as detected by rhodamine-phalloidin staining; in these cells the membrane actin was also greatly diminished or absent and the border of the cells was markedly retracted. Microinjection of pure T beta 4 protein into interphase PtK2 cells induced disassembly of the stress fibers within 10 min, while membrane actin appeared only somewhat reduced. If the PtK2 cells were mitotic, similar microinjection of pure thymosin beta 4 protein at times from early prophase to metaphase resulted in an unusual pattern of delayed cytokinesis. Furrowing occurred but at a much slower rate than in controls and the amount of actin in the cleavage furrow was greatly reduced. The cells constricted to apparent completion, but after about 30 min the furrow regressed, forming a binucleate cell, much as after treatment with cytochalasin B or D. Postcytokinesis spreading of these T beta 4-injected cells was often inhibited. These experiments suggest that an insufficient number of actin filaments prolongs the contractile phase of cytokinesis and abolishes the final sealing process.

In vivo analysis of functional domains from villin and gelsolin

Transfected CV1 cells were used to compare the in vivo effects of various domains of villin and gelsolin. These two homologous actin modulating proteins both contain a duplicated severin-like sequence. Villin has in addition a carboxy-terminal domain, the headpiece, which accounts for its bundling activity. The effects of the villin-deleted mutants were compared with those of native villin. Our results show that essential domains of villin required to induce the growth of microvilli and F-actin redistribution are present in the first half of the core and in the headpiece. We also show that the second half of the villin core cannot be exchanged by its homolog in gelsolin. When expressed at high levels of CV1 cells, full length gelsolin completely disrupted stress fibers without change of the cell shape. Addition of the villin headpiece to gelsolin had no effect on the phenotype induced by gelsolin alone. Expression of the first half of gelsolin induced similar modifications as capping proteins and rapid cell mortality; this deleterious effect on the cell structure was also observed when the headpiece was linked to the first half of gelsolin. In cells expressing the second half of gelsolin, a dotted F-actin staining was often seen. Moreover elongated dorsal F-actin structures were observed when the headpiece was linked to the second gelsolin domain. These studies illustrate the patent in vivo severing activity of gelsolin as well as the distinct functional properties of villin core in contrast to gelsolin.

Disruption of the actin cytoskeleton after microinjection of proteolytic fragments of alpha-actinin

Alpha-actinin can be proteolytically cleaved into major fragments of 27 and 53 kD using the enzyme thermolysin. The 27-kD fragment contains an actin-binding site and we have recently shown that the 53-kD fragment binds to the cytoplasmic domain of beta 1 integrin in vitro (Otey, C. A., F. M. Pavalko, and K. Burridge. 1990. J. Cell Biol. 111:721-729). We have explored the behavior of the isolated 27- and 53-kD fragments of alpha-actinin after their microinjection into living cells. Consistent with its containing a binding site for actin, the 27-kD fragment was detected along stress fibers within 10-20 min after injection into rat embryo fibroblasts (REF-52). The 53-kD fragment of alpha-actinin, however, concentrated in focal adhesions of REF-52 cells 10-20 min after injection. The association of this fragment with focal adhesions in vivo is consistent with its interaction in vitro with the cytoplasmic domain of the beta 1 subunit of integrin, which was also localized at these sites. When cells were injected with greater than 5 microM final concentration of either alpha-actinin fragment and cultured for 30-60 min, most stress fibers were disassembled. At this time, however, many of the focal adhesions, particularly those around the cell periphery, remained after most stress fibers had gone. By 2 h after injection only a few small focal adhesions persisted, yet the cells remained spread. Identical results were obtained with other cell types including primary chick fibroblasts, BSC-1, MDCK, and gerbil fibroma cells. Stress fibers and focal adhesions reformed if cells were allowed to recover for 18 h after injection. These data suggest that introduction of the monomeric 27-kD fragment of alpha-actinin into cells may disrupt the actin cytoskeleton by interfering with the function of endogenous, intact alpha-actinin molecules along stress fibers. The 53-kD fragment may interfere with endogenous alpha-actinin function at focal adhesions or by displacing some other component that binds to the rod domain of alpha-actinin and that is needed to maintain stress fiber organization.

A Dictyostelium mutant lacking an F-actin cross-linking protein, the 120-kD gelation factor

Actin-binding proteins are known to regulate in vitro the assembly of actin into supramolecular structures, but evidence for their activities in living nonmuscle cells is scarce. Amebae of Dictyostelium discoideum are nonmuscle cells in which mutants defective in several actin-binding proteins have been described. Here we characterize a mutant deficient in the 120-kD gelation factor, one of the most abundant F-actin cross-linking proteins of D. discoideum cells. No F-actin cross-linking activity attributable to the 120-kD protein was detected in mutant cell extracts, and antibodies recognizing different epitopes on the polypeptide showed the entire protein was lacking. Under the conditions used, elimination of the gelation factor did not substantially alter growth, shape, motility, or chemotactic orientation of the cells towards a cAMP source. Aggregates of the mutant developed into fruiting bodies consisting of normally differentiated spores and stalk cells. In cytoskeleton preparations a dense network of actin filaments as typical of the cell cortex, and bundles as they extend along the axis of filopods, were recognized. A significant alteration found was an enhanced accumulation of actin in cytoskeletons of the mutant when cells were stimulated with cyclic AMP. Our results indicate that control of cell shape and motility does not require the fine-tuned interactions of all proteins that have been identified as actin-binding proteins by in vitro assays.

Disruption of microfilament organization in living nonmuscle cells by microinjection of plasma vitamin D-binding protein or DNase I

Plasma vitamin D-binding protein (DBP), which binds to monomeric actin, causes the breakdown of stress fibers when it is microinjected into nonmuscle cells. Disruption of the stress fiber network is also accompanied by shape changes in the cell that resemble those seen after cytochalasin treatment. When DBP was coinjected with fluorescently labeled alpha-actinin, no fluorescent stress fibers or attachment plaques were visible 30 min after injection. Twelve hours later the cells regained their flattened shape and their stress fibers. Fluorescently labeled DBP causes the same reversible changes in cell shape as the unlabeled protein. Upon injection, the labeled DBP diffuses throughout the cytoplasm, becoming localized by 12 hr in a punctate pattern, presumably due to lysozomal sequestration. Similar injections of DBP into skeletal myotubes and cardiac myocytes did not lead to shape changes or breakdown of nascent and/or fully formed myofibrils, even though DBP has a 2-fold higher binding affinity for muscle actin over that of the nonmuscle isoactins. Similar differential effects in nonmuscle cells were also observed after the microinjection of DNase I, another protein capable of binding monomer actin. The effects of these microinjected monomer actin-binding proteins imply that an accessible pool of monomer actin is needed to maintain stress fiber integrity in nonmuscle cells but not the integrity of the nascent or fully formed myofibrils in muscle cells.

Disruption of the actin cytoskeleton in yeast capping protein mutants

Capping protein controls the addition of actin subunits to the barbed end of actin filaments and nucleates actin polymerization in vitro. Capping protein has been identified in all eukaryotic cells examined so far; it is a heterodimer with subunits of relative molecular masses 32,000-36,000 (alpha-subunit) and 28,000-32,000 (beta-subunit). In skeletal muscle, capping protein (CapZ) probably binds the barbed ends of actin filaments at the Z line. The in vivo role of this protein in non-muscle cells is not known. We report here the characterization of CAP2, the single gene encoding the beta-subunit of capping protein in Saccharomyces cerevisiae. Yeast cells in which the CAP2 gene was disrupted by an insertion or a deletion had an abnormal actin distribution, including the loss of actin cables. The mutant cells were round and large, with a heterogeneous size distribution, and, although viable, grew more slowly than congenic wild-type cells. Chitin, a cell wall component restricted to the mother-bud junction in wild-type budding yeast, was found on the entire mother cell surface in the mutants. The phenotype of CAP2 disruption resembled that of temperature-sensitive mutations in the yeast actin gene ACT1, indicating that capping protein regulates actin-filament distribution in vivo.

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

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

Differential effects of gelsolins on tissue culture cells

Gelsolins, prepared from a number of different sources, showed similar severing activity on F-actin in vitro or on stress fibers of detergent-extracted cells but differed in their effects on actin in stress fibers of microinjected cells. When human gelsolin isolated from plasma was injected into cells in a Ca(++)-containing buffer, stress fibers were degraded, the cellular morphology was changed, and numerous actin patches appeared. These effects were particularly striking when the Ca(++)-insensitive N-terminal proteolytic fragment of this gelsolin was injected. By contrast, Ca(++)-sensitive gelsolins isolated from human platelets, pig stomach smooth muscle and pig plasma showed no comparable activity. Furthermore, the Ca(++)-independent N-terminal proteolytic fragments prepared from these gelsolins also had no effect despite their in vitro actin severing activity. Most striking was the finding that human plasma gelsolin expressed in E. coli did not degrade stress fibers, in contrast to the same protein isolated from plasma; nor was there any stress fiber disruption observed with the N-terminal half of human gelsolin expressed in Escherichia coli. The different behavior of these gelsolins in cells cannot be explained by sequence diversity between plasma and cytoplasmic forms, nor by variability in the Ca++ sensitivity of the preparations. It suggests the presence of factors, as yet unidentified, that may regulate gelsolin activity in the cytoplasm of living cells and discriminate between gelsolins of different origin. Such discrimination could be achieved as a result of post-translational modification of the gelsolin; only in this way can differences between apparently identical proteins isolated from human plasma and expressed in E. coli be reconciled.

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.

Modulation of cellular morphology and locomotory activity by antibodies against myosin

Three monoclonal antibodies directed against chicken brush border myosin were used to study the possible function of myosin in microfilament organization and locomotion of chicken fibroblasts. These antibodies bind to distinct and separate epitopes on the heavy chain of chicken nonmuscle myosin and display differential effects of myosin filament formation and actin-myosin interaction (Citi, S., and J. Kendrick-Jones. 1988. J. Musc. Res. Cell Motil. 9: 306-319). When injected into chicken fibroblasts, all antibodies induced breakdown of stress fibers. Concomitantly, a large proportion of the cells developed extensive lamellae which altered their morphology drastically. These cells showed also increased locomotory activity. All effects were concentration dependent and reversible. The most drastic alterations were observed with cells injected with antibody quantities exceeding the quantity of cellular myosin (molar ratios of antibody to myosin greater than 3:1). The finding that antibodies with different effects on myosin filament formation in vitro all induce similar intracellular processes suggests that it is the antibody-induced decrease in functional myosin that triggers an increase in plasma membrane dynamics and locomotory activity, rather than differences in myosin filament length or conformation.

Synthetic peptide GRGDS induces dissociation of alpha-actinin and vinculin from the sites of focal contacts

The synthetic peptide Gly-Arg-Gly-Asp-Ser (GRGDS) mimics the cellular binding site of many adhesive proteins in the extracellular matrix and causes rounding and detachment of spread cells. We have studied whether its binding affects the associations of two major components, alpha-actinin and vinculin, at the adhesion plaque. Living 3T3 cells were microinjected with fluorescently labeled alpha-actinin and/or vinculin and observed using video microscopy before and after the addition of 50 micrograms/ml GRGDS. As soon as 5 min after treatment, fluorescent alpha-actinin and vinculin became dissociated simultaneously from the sites of many focal contacts. The proteins either moved away as discrete structures or dispersed from adhesion plaques. As a result, the enrichment of alpha-actinin and vinculin at these focal contacts was no longer detected. The focal contacts then faded away slowly without showing detectable movement. These data suggest that the binding state of integrin has a transmembrane effect on the distribution of cytoskeletal components. The dissociation of alpha-actinin and vinculin from adhesion plaques may in turn weaken the contacts and result in rounding and detachment of cells.

Calcium-induced changes in cytoskeleton and motility of cultured human keratinocytes

In normal epidermis keratinocytes migrate upward from the basal layer as they undergo terminal differentiation, yet they also have the capacity for lateral movement during wound healing. The purpose of our experiments was to investigate these two types of movement by manipulating the calcium ion concentration of the medium so that keratinocytes formed monolayers (0.1 mM calcium) or stratified sheets (2.0 mM calcium). Time-lapse video recording indicated that keratinocytes in low-calcium medium were laterally more motile than keratinocytes in normal medium. This was consistent with the ultrastructural appearance of the cells and the lack of desmosomal junctions, determined by scanning and transmission electron microscopy. During calcium-induced stratification keratinocytes moved upward from the basal layer by gliding over their neighbors and forming contacts with other suprabasal cells. Keratinocytes in low-calcium medium migrated into wounds made in the cultures, a process which was inhibited by monensin; however, stratified keratinocytes in normal medium did not enter wounds. Cytochalasin D caused rapid cell rounding and disruption of actin filaments in keratinocytes grown in low-calcium but not in normal medium, indicating more rapid treadmilling of actin and consistent with the greater motility of keratinocytes in low-calcium medium. Our results suggest that desmosome formation may place constraints on the movement of individual keratinocytes and that the actomyosin cytoskeleton is involved in lateral migration.

Microinjection of gelsolin into living cells

Gelsolins are actin-binding proteins that cap, nucleate, and sever actin filaments. Microinjection of cytoplasmic or plasma gelsolin into living fibroblasts and macrophages did not affect the shape, actin distribution, deformability, or ruffling activity of the cells. Gelsolin requires calcium for activity, but the NH2-terminal half is active without calcium. Microinjection of this proteolytic fragment had marked effects: the cells rounded up, stopped ruffling, became soft, and stress fibers disappeared. These changes are similar to those seen with cytochalasin, which also caps barbed ends of actin filaments. Attempts to raise the cytoplasmic calcium concentration and thereby activate the injected gelsolin were unsuccessful, but the increases in calcium concentration were minimal or transient and may not have been sufficient. Our interpretation of these results is that at the low calcium concentrations normally found in cells, gelsolin does not express the activities observed in vitro at higher calcium concentrations. We presume that gelsolin may be active at certain times or places if the calcium concentration is elevated to a sufficient level, but we cannot exclude the existence of another molecule that inhibits gelsolin. Microinjection of a 1:1 gelsolin/actin complex had no effect on the cells. This complex is stable in the absence of calcium and has capping activity but no severing and less nucleation activity as compared with either gelsolin in calcium or the NH2-terminal fragment. The NH2-terminal fragment-actin complex also has capping and nucleating activity but no severing activity. On microinjection it had the same effects as the fragment alone. The basis for the difference between the two complexes is unknown. The native molecular weight of rabbit plasma gelsolin is 82,500, and the extinction coefficient at 280 nm is 1.68 cm2/mg. A new simple procedure for purification of plasma gelsolin is described.

Actin-severing activity copurifies with phosphofructokinase

Microinjection of muscle 6-phosphofructokinase (PFK; EC 2.7.1.11) into tissue culture cells led to a reversible disintegration of microfilament bundles (stress fibers). The mode of disruption as well as of recovery of stress fibers was very similar to that found previously in experiments performed with the actin-severing protein brevin, an extracellular variant of gelsolin. PFK, like brevin, was also capable of disrupting stress fibers in detergent-extracted cells and in ethanol-fixed cells, in a Ca2+-dependent manner. When compared with heart muscle gelsolin, PFK comigrated with the 85- to 90-kDa band. Antibodies against PFK crossreacted with gelsolin from the same species. These results point to a tight association between polypeptides with similar biochemical and immunological parameters present in both preparations. They suggest hitherto unexpected cellular control mechanisms for both microfilament functions and glycolysis.

Transformation-dependent increases in endogenous cytochalasin-like activity in chicken embryo fibroblasts infected by Rous sarcoma virus

Transformation of chicken embryo fibroblasts by infection with Rous sarcoma virus has been shown to cause disruption of actin filament organization as seen with fluorescence staining techniques. This study is an attempt to use quantitative biochemical techniques to compare actin-related parameters in normal and transformed cells. Normal cells and cells infected with a temperature-sensitive mutant virus (NY68) and grown at the restrictive temperature of 41.5 degrees C have normal bundles of actin filaments, or F-actin; these cells also have about the same number of high-affinity cytochalasin binding sites at the ends of F-actin (approximately 5 pmol of sites per mg of cellular protein; Kd, 20 nM). In contrast, infected cells grown at the permissive temperature of 37 degrees C have a more diffuse pattern of actin filaments, and the number of cytochalasin binding sites in these transformed cells was below the level of detection. DNase I inhibition assays showed that the percent of unpolymerized actin, or G-actin, in cell extracts was not significantly different between normal and transformed cells (approximately 50%). In assays of cell extracts for endogenous cytochalasin-like activity on actin filaments (i.e., retardation of filament assembly at the fast-growing end, inhibition of cytochalasin binding to actin "nuclei," and decrease of low-shear viscosity of solutions of actin filaments), infected cells at 37 degrees C showed a higher level of activity per mg of protein than did uninfected cells or infected cells at 41.5 degrees C. These results suggest that the increase in endogenous cytochalasin-like activity in transformed cells may relate to the decrease in measurable cytochalasin binding sites and the abnormal distribution of actin filaments previously seen by fluorescence staining techniques.

Mechanistic aspects on chemical induction of spindle disturbances and abnormal chromosome numbers

Work on the chemical induction of spindle disturbances and abnormal chromosome numbers, and work on the composition and biochemistry of the spindle are reviewed. Some early investigations have shown that there is an unspecific mechanism for chemical induction of spindle disturbances. This mechanism is based on the interaction of compounds with cellular hydrophobic compartments. Some compounds act differently and are more active than predicted from their lipophilic character. Selected compounds of that kind and their possible mechanisms of action are discussed. Changes in sulfhydryl and ATP levels, oxidative damage of membranes and impaired control of cytoplasmic Ca2+ levels are discussed in this context.

Cell motion, contractile networks, and the physics of interpenetrating reactive flow

In this paper we propose a physical model of contractile biological polymer networks based on the notion of reactive interpenetrating flow. We show how our model leads to a mathematical formulation of the dynamical laws governing the behavior of contractile networks. We also develop estimates of the various parameters that appear in our equations, and we discuss some elementary predictions of the model concerning the general scaling principles that pertain to the motions of contractile networks.

Equilibrium constant for binding of an actin filament capping protein to the barbed end of actin filaments

Depolymerization of treadmilling actin filaments by a capping protein isolated from bovine brain was used for determination of the equilibrium constant for binding of the capping protein to the barbed ends of actin filaments. When the capping protein blocks monomer consumption at the lengthening barbed ends, monomers continue to be produced at the shortening pointed ends until a new steady state is reached in which monomer production at the pointed ends is balanced by monomer consumption at the uncapped barbed ends. In this way the ratio of capped to uncapped filaments could be determined as a function of the capping protein concentration. Under the experimental conditions (100 mM KCl and 2 mM MgCl2, pH 7.5, 37 degrees C) the binding constant was found to be about 2 X 10(9) M-1. Capping proteins effect the actin monomer concentration only at capping protein concentrations far above the reciprocal of their binding constant. Half-maximal increase of the monomer concentration requires capping of about 99% of the actin filaments. A low proportion of uncapped filaments has a great weight in determining the monomer concentration because association and dissociation reactions occur at the dynamic barbed ends with higher frequencies than at the pointed ends.

Regulation of actin polymerization by non-polymerizable actin-like proteins

Three functionally distinct actin-capping proteins from the slime mould Physarum are structurally closely related to actin itself. In Physarum, actin polymerization is regulated by a set of non-polymerizable actin-like proteins. It remains to be established whether these proteins and actin are each encoded by separate genes.

Interaction of fluorescently-labeled contractile proteins with the cytoskeleton in cell models

To determine if a living cell is necessary for the incorporation of actin, alpha-actinin, and tropomyosin into the cytoskeleton, we have exposed cell models to fluorescently labeled contractile proteins. In this in vitro system, lissamine rhodamine-labeled actin bound to attachment plaques, ruffles, cleavage furrows and stress fibers, and the binding could not be blocked by prior exposure to unlabeled actin. Fluorescently labeled alpha-actinin also bound to ruffles, attachment plaques, cleavage furrows, and stress fibers. The periodicity of fluorescent alpha-actinin along stress fibers was wider in gerbil fibroma cells than it was in PtK2 cells. The fluorescent alpha-actinin binding in cell models could not be blocked by the prior addition of unlabeled alpha-actinin suggesting that alpha-actinin was binding to itself. While there was only slight binding of fluorescent tropomyosin to the cytoskeleton of interphase cells, there was stronger binding in furrow regions of models of dividing cells. The binding of fluorescently labeled tropomyosin could be blocked by prior exposure of the cell models to unlabeled tropomyosin. If unlabeled actin was permitted to polymerize in the stress fibers in cell models, fluorescently labeled tropomyosin stained the fibers. In contrast to the labeled contractile proteins, fluorescently labeled ovalbumin and BSA did not stain any elements of the cytoskeleton. Our results are discussed in terms of the structure and assembly of stress fibers and cleavage furrows.

'Cap 90', a 90-kDa Ca2+-dependent F-actin-capping protein from vertebrate brain

A Ca2+-dependent actin filament-capping protein of 90 kDa was purified from bovine brain using a new and rapid isolation procedure. This basically includes affinity purification on DNase-I agarose. The protein caps the fast-growing end of actin filaments but has no fragmenting or severing activity. Using Triton X-100-extracted cytoskeletons, capping and severing activities of actin-binding proteins become clearly distinguishable from each other.

Treadmilling of actin

Actin filaments can assemble at the barbed end and disassemble simultaneously at the pointed end. A higher monomer concentration is required to balance the association of actin monomers and the dissociation of filament subunits at the pointed end than at the barbed end. This treadmilling reaction or disparity of the apparent affinity of the two ends for monomers is caused by a continuous hydrolysis of adenosine triphosphate occurring during the association of a monomer with a filament end. In this article, in vitro investigations on treadmilling are reviewed and emerging physiological implications are discussed.

Organization and function of structural elements in focal contacts of tissue culture cells

The role of structural elements in the organization and maintenance of focal contacts was studied by microinjecting into tissue culture cells specific probes which interfere with filamentous actin or with vinculin: actin interaction. Injection of actin capping proteins from Physarum and brain resulted in breakdown of microfilament bundles starting at their distal ends and in loss of focal contacts. This process was fully reversible. Injection of a high affinity antibody against chicken gizzard vinculin led to partial breakdown of microfilament bundles con-concomitant with disruption of focal contacts with vinculin remaining at the plasma membrane. This process was irreversible.