The Ras-related protein Cdc42Hs and bradykinin promote formation of peripheral actin microspikes and filopodia in Swiss 3T3 fibroblasts

The Ras-related protein Cdc42 plays a role in yeast cell budding and polarity. Two related proteins, Rac1 and RhoA, promote formation in mammalian cells of membrane ruffles and stress fibers, respectively, which contain actin microfilaments. We now show that microinjection of the related human Cdc42Hs into Swiss 3T3 fibroblasts induced the formation of peripheral actin microspikes, determined by staining with phalloidin. A proportion of these microspikes was found to be components of filopodia, as analyzed by time-lapse phase-contrast microscopy. The formation of filopodia was also found to be promoted by Cdc42Hs microinjection. This was followed by activation of Rac1-mediated membrane ruffling. Treatment with bradykinin also promoted formation of microspikes and filopodia as well as subsequent effects similar to that seen upon Cdc42Hs microinjection. These effects of bradykinin were specifically inhibited by prior microinjection of dominant negative Cdc42HsT17N, suggesting that bradykinin acts by activating cellular Cdc42Hs. Since filopodia have been ascribed an important sensory function in fibroblasts and are required for guidance of neuronal growth cones, these results indicate that Cdc42Hs plays an important role in determining mammalian cell morphology.

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

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

Two distinct actin networks drive the protrusion of migrating cells

Cell migration initiates by extension of the actin cytoskeleton at the leading edge. Computational analysis of fluorescent speckle microscopy movies of migrating epithelial cells revealed this process is mediated by two spatially colocalized but kinematically, kinetically, molecularly, and functionally distinct actin networks. A lamellipodium network assembled at the leading edge but completely disassembled within 1 to 3 micrometers. It was weakly coupled to the rest of the cytoskeleton and promoted the random protrusion and retraction of the leading edge. Productive cell advance was a function of the second colocalized network, the lamella, where actomyosin contraction was integrated with substrate adhesion.

Microtubule-mediated transport of incoming herpes simplex virus 1 capsids to the nucleus

Herpes simplex virus 1 fuses with the plasma membrane of a host cell, and the incoming capsids are efficiently and rapidly transported across the cytosol to the nuclear pore complexes, where the viral DNA genomes are released into the nucleoplasm. Using biochemical assays, immunofluorescence, and immunoelectron microscopy in the presence and absence of microtubule depolymerizing agents, it was shown that the cytosolic capsid transport in Vero cells was mediated by microtubules. Antibody labeling revealed the attachment of dynein, a minus end-directed, microtubule-dependent motor, to the viral capsids. We propose that the incoming capsids bind to microtubules and use dynein to propel them from the cell periphery to the nucleus.

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.

Microtubules and actin filaments: dynamic targets for cancer chemotherapy

Microtubules and actin filaments play important roles in mitosis, cell signaling, and motility. Thus these cytoskeletal filaments are the targets of a growing number of anti-cancer drugs. In this review we summarize the current understanding of the mechanisms of these drugs in relation to microtubule and actin filament polymerization and dynamics. In addition, we outline how, by targeting microtubules, drugs inhibit cell proliferation by blocking mitosis at the mitotic checkpoint and inducing apoptosis. The beta-tubulin isotype specificities of new anticancer drugs and the antitumor potential of agents that act on the actin cytoskeleton are also discussed.

Mechanics of stimulated neutrophils: cell stiffening induces retention in capillaries

The effect of peptide chemoattractants on neutrophil mechanical properties was studied to test the hypothesis that stimulated neutrophils (diameter, 8 micrometers) are retained in pulmonary capillaries (5.5 micrometers) as a result of a decreased ability of the cell to deform within the capillary in response to the hydrodynamic forces of the bloodstream. Increased neutrophil stiffness, actin assembly, and retention in both 5-micrometer pores and the pulmonary vasculature were seen in response to N-formyl-methionyl-leucyl-phenylalanine. These changes were abolished in cells that had been incubated with 2 micromolar cytochalasin D, an agent that disrupts cellular actin organization. A monoclonal antibody directed at the CD11-CD18 adhesive glycoprotein complex did not inhibit the increase in stiffness or retention in pores. These data suggest that neutrophil stiffening may be both necessary and sufficient for the retention that is observed. Hence, neutrophil sequestration in lung and other capillaries in the acute inflammatory process may be the result of increased stiffness stimulated by chemoattractants.

Characterization of two classes of small molecule inhibitors of Arp2/3 complex

Polymerization of actin filaments directed by the actin-related protein (Arp)2/3 complex supports many types of cellular movements. However, questions remain regarding the relative contributions of Arp2/3 complex versus other mechanisms of actin filament nucleation to processes such as path finding by neuronal growth cones; this is because of the lack of simple methods to inhibit Arp2/3 complex reversibly in living cells. Here we describe two classes of small molecules that bind to different sites on the Arp2/3 complex and inhibit its ability to nucleate actin filaments. CK-0944636 binds between Arp2 and Arp3, where it appears to block movement of Arp2 and Arp3 into their active conformation. CK-0993548 inserts into the hydrophobic core of Arp3 and alters its conformation. Both classes of compounds inhibit formation of actin filament comet tails by Listeria and podosomes by monocytes. Two inhibitors with different mechanisms of action provide a powerful approach for studying the Arp2/3 complex in living cells.

Single-molecule speckle analysis of actin filament turnover in lamellipodia

Lamellipodia are thin, veil-like extensions at the edge of cells that contain a dynamic array of actin filaments. We describe an approach for analyzing spatial regulation of actin polymerization and depolymerization in vivo in which we tracked single molecules of actin fused to the green fluorescent protein. Polymerization and the lifetime of actin filaments in lamellipodia were measured with high spatial precision. Basal polymerization and depolymerization occurred throughout lamellipodia with largely constant kinetics, and polymerization was promoted within one micron of the lamellipodium tip. Most of the actin filaments in the lamellipodium were generated by polymerization away from the tip.

The mammalian G protein rhoC is ADP-ribosylated by Clostridium botulinum exoenzyme C3 and affects actin microfilaments in Vero cells

Clostridium botulinum C3 is a recently discovered exoenzyme that ADP-ribosylates a eukaryotic GTP-binding protein of the ras superfamily. We show now that the bacterially-expressed product of the human rhoC gene is ADP-ribosylated by C3 and corresponds in size, charge and behavior to the dominant C3 substrate of eukaryotic cells. C3 treatment of Vero cells results in the disappearance of microfilaments and in actinomorphic shape changes without any apparent direct effect upon actin. Thus the ADP-ribosylation of a rho protein seems to be responsible for microfilament disassembly and we infer that the unmodified form of a rho protein may be involved in cytoskeletal control.

Clathrin- and caveolin-1-independent endocytosis: entry of simian virus 40 into cells devoid of caveolae

Simian Virus 40 (SV40) has been shown to enter host cells by caveolar endocytosis followed by transport via caveosomes to the endoplasmic reticulum (ER). Using a caveolin-1 (cav-1)-deficient cell line (human hepatoma 7) and embryonic fibroblasts from a cav-1 knockout mouse, we found that in the absence of caveolae, but also in wild-type embryonic fibroblasts, the virus exploits an alternative, cav-1-independent pathway. Internalization was rapid (t1/2 = 20 min) and cholesterol and tyrosine kinase dependent but independent of clathrin, dynamin II, and ARF6. The viruses were internalized in small, tight-fitting vesicles and transported to membrane-bounded, pH-neutral organelles similar to caveosomes but devoid of cav-1 and -2. The viruses were next transferred by microtubule-dependent vesicular transport to the ER, a step that was required for infectivity. Our results revealed the existence of a virus-activated endocytic pathway from the plasma membrane to the ER that involves neither clathrin nor caveolae and that can be activated also in the presence of cav-1.

A 25-kD inhibitor of actin polymerization is a low molecular mass heat shock protein

The 25-kD inhibitor of actin polymerization (25-kD IAP), isolated from turkey smooth muscle (Miron, T., M. Wilchek, and B. Geiger, 1988. Eur. J. Biochem. 178:543-553), is shown here to be a low molecular mass heat shock protein (HSP). Direct sequence analysis of the purified protein, as well as cloning and sequencing of the respective cDNA, disclosed a high degree of homology (67% identity, 80% similarity) to the human 27-kD HSP. Southern blot of chicken genomic DNA disclosed one band, suggesting the presence of a single gene, and Northern blot analysis revealed abundant transcript of approximately 1 kb in gizzard and heart tissues and lower amounts in total 18-d chick embryo RNA and in cultured fibroblasts. Exposure of the latter cells to 45 degrees C resulted in over 15-fold increase in the apparent level of the 25-kD IAP protein, confirming that its expression is regulated by heat shock. Immunofluorescent microscopic localization indicated that after heat treatment, the levels of the 25-kD IAP were markedly increased and the protein was apparently associated with cytoplasmic granules. Heat shock also had a transient, yet prominent, effect on the microfilament system in cultured fibroblasts: stress fibers disintegrated within 10-15 min after incubation at 45 degrees C, yet upon further incubation at the elevated temperature, conspicuous actin bundles were apparently reformed.

p38 mitogen-activated protein kinase is required for TGFβ-mediated fibroblastic transdifferentiation and cell migration

Transforming growth factor β (TGFβ) contributes to tumor progression by inducing an epithelial to mesenchymal transdifferentiation(EMT) and cell migration. We found that TGFβ-induced EMT was blocked by inhibiting activation of p38 mitogen-activated protein kinase (MAPK) with H-7,a protein kinase C inhibitor, and with SB202190, a direct inhibitor of p38MAPK. Inhibition of the p38MAPK pathway affected TGFβ-mediated phosphorylation of ATF2, but did not inhibit phosphorylation of Smad2. SB202190 impaired TGFβ-mediated changes in cell shape and reorganization of the actin cytoskeleton. Forced expression of dominant-negative (DN) MAPK kinase 3 (MKK3) inhibited TGFβ-mediated activation of p38MAPK and EMT. Expression of DN-p38α impaired TGFβ-induced EMT. Inhibition of p38MAPK blocked TGFβ-induced migration of non-tumor and tumor mammary epithelial cells. TGFβ induced activation of the p38MAPK pathway within 15 minutes. Expression of TGFβ type II (TβRII) and type I(TβRI/Alk5) kinase-inactive receptors blocked EMT and activation of p38MAPK, whereas expression of constitutively active Alk5-T204D resulted in EMT and phosphorylation of MKK3/6 and p38MAPK. Finally, dominant-negative Rac1N17 blocked TGFβ-induced activation of the p38MAPK pathway and EMT,suggesting that Rac1 mediates activation of the p38MAPK pathway. These studies suggest that the p38MAPK pathway is required for TGFβ-mediated EMT and cell migration.

Mechanism of endothelial cell shape change and cytoskeletal remodeling in response to fluid shear stress

Endothelium exposed to fluid shear stress (FSS) undergoes cell shape change, alignment and microfilament network remodeling in the direction of flow by an unknown mechanism. In this study we explore the role of tyrosine kinase (TK) activity, intracellular calcium ([Ca2+]i), mechanosensitive channels and cytoskeleton in the mechanism of cell shape change and actin stress fiber induction in bovine aortic endothelium (BAE). We report that FSS induces beta-actin mRNA in a time- and magnitude-dependent fashion. Treatment with quin2-AM to chelate intracellular calcium release and herbimycin A to inhibit TK activity abolished BAE shape change and actin stress fiber induction by FSS, while inhibition of protein kinase C with chelerythrine had no effect. Altering intermediate filament structure with acrylamide did not affect alignment or F-actin induction by FSS. Examining the role of the BAE cytoskeleton revealed a critical role for microtubules (MT). MT disruption with nocodazole blocked both FSS-induced morphological change and actin stress fiber induction. In contrast, MT hyperpolymerization with taxol attenuated the cell shape change but did not prevent actin stress fiber induction under flow. Mechanosensitive channels were found not to be involved in the FSS-induced shape change. Blocking the shear-activated current (IK.S) with barium and the stretch-activated cation channels (ISA) with gadolinium had no effect on the shear-induced changes in morphology and cytoskeleton. In summary, FSS has a profound effect on endothelial shape and F-actin network by a mechanism which depends on TK activity, intracellular calcium, and an intact microtubule network, but is independent of protein kinase C, intermediate filaments and shear- and stretch-activated mechanosensitive channels.

Hypertrophic cardiomyopathy in cardiac myosin binding protein-C knockout mice

Familial hypertrophic cardiomyopathy (FHC) is an inherited autosomal dominant disease caused by mutations in sarcomeric proteins. Among these, mutations that affect myosin binding protein-C (MyBP-C), an abundant component of the thick filaments, account for 20% to 30% of all mutations linked to FHC. However, the mechanisms by which MyBP-C mutations cause disease and the function of MyBP-C are not well understood. Therefore, to assess deficits due to elimination of MyBP-C, we used gene targeting to produce a knockout mouse that lacks MyBP-C in the heart. Knockout mice were produced by deletion of exons 3 to 10 from the endogenous cardiac (c) MyBP-C gene in murine embryonic stem (ES) cells and subsequent breeding of chimeric founder mice to obtain mice heterozygous (+/-) and homozygous (-/-) for the knockout allele. Wild-type (+/+), cMyBP-C(+/-), and cMyBP-C(-/-) mice were born in accordance with Mendelian inheritance ratios, survived into adulthood, and were fertile. Western blot analyses confirmed that cMyBP-C was absent in hearts of homozygous knockout mice. Whereas cMyBP-C(+/-) mice were indistinguishable from wild-type littermates, cMyBP-C(-/-) mice exhibited significant cardiac hypertrophy. Cardiac function, assessed using 2-dimensionally guided M-mode echocardiography, showed significantly depressed indices of diastolic and systolic function only in cMyBP-C(-/-) mice. Ca2+ sensitivity of tension, measured in single skinned myocytes, was reduced in cMyBP-C(-/-) but not cMyBP-C(+/-) mice. These results establish that cMyBP-C is not essential for cardiac development but that the absence of cMyBP-C results in profound cardiac hypertrophy and impaired contractile function.

Involvement of microtubules in the control of adhesion-dependent signal transduction

BACKGROUND

The adhesion of cells to the extracellular matrix (ECM) generates transmembrane signals that affect cell proliferation, differentiation and survival. These signals are triggered by interactions between integrin and the ECM and involve tyrosine phosphorylation of specific proteins, including focal adhesion kinase (FAK) and paxillin, and the assembly of focal adhesions and actin bundles. In matrix-adherent, serum-starved Swiss 3T3 cells, the system of focal adhesions and actin bundles is poorly developed, and the level of tyrosine phosphorylation of FAK and paxillin is low. A number of growth factors rapidly stimulate tyrosine phosphorylation of these proteins and the assembly of focal adhesions and actin bundles. Growth factors and adhesion to the ECM are both necessary for the subsequent transition of cells to the S-phase of the cell cycle.

RESULTS

In serum-starved Swiss 3T3 cells, the disruption of microtubules by nocodazole or vinblastine, without the addition of external growth factors, induces the rapid assembly of focal adhesions and microfilament bundles, tyrosine phosphorylation of FAK and paxillin, and subsequent enhancement of DNA synthesis. All these effects require cell adhesion to the ECM and do not occur when cells are plated on substrates coated with poly-L-lysine or concanavalin A. Inhibitors of tyrosine phosphorylation and cell contractility also eliminate the effects of microtubule disruption on adhesion-dependent signal transduction.

CONCLUSIONS

In ECM-attached cells, microtubule disruption activates the integrin-dependent signaling cascade, which leads to the assembly of matrix adhesions and the induction of DNA synthesis. The increase in cell contractility is an indispensable intermediate step in this signaling process.

Intracellular targeting of the Yersinia YopE cytotoxin in mammalian cells induces actin microfilament disruption

Pathogenic Yersinia spp., including the etiological agent of plague, Y. pestis, all carry a common plasmid that encodes a number of essential virulence determinants, the Yop proteins. One of these, YopE, has been shown to be involved in the obstruction of the primary host defense by a molecular mechanism leading to inhibition of phagocytosis (R. Rosqvist, A. Forsberg, M. Rimpiläinen, T. Bergman, and H. Wolf-Watz, Mol. Microbiol. 4:657-667, 1990). Although the Yop proteins are secreted into the culture supernatant in vast amounts, in vitro studies of the function of the Yop proteins have so far been unsuccessful. We show that isolated Yop proteins indeed can cause cytotoxic effects in vitro if the proteins are introduced intracellularly into the eukaryotic cell. Isolated Yop proteins of Yersinia pseudotuberculosis were found to disrupt the microfilament structure when microinjected intracellularly into the host cell. In particular, YopE was demonstrated to be directly involved in the cytotoxic action, whereas YopD seems to have a critical role in translocating the YopE protein through the host cell membrane. These results elucidate the requirement for at least some of the Yop proteins to leave the pathogen during infection.

Polymerization of tau into filaments in the presence of heparin: the minimal sequence required for tau-tau interaction

Paired helical filaments isolated from the brains of patients with Alzheimer's disease are composed of a major protein component, the microtubule-associated protein termed tau, together with other nonprotein components, including heparan, a glycosaminoglycan, the more extensively sulfated form of which is heparin. As some of these nonprotein components may modulate the assembly of tau into filamentous structures, we have analyzed the ability of the whole tau protein or some of its fragments to self-assemble in the presence of heparin. Different tau fragments, all of them containing some sequences of the tubulin-binding motif, can assemble in vitro into filaments. We have also found formation of polymers with the 18-residue-long peptide corresponding to the third tubulin-binding motif of tau. This suggests that the ability of tau for self-assembly could be localized in a short sequence of amino acids present in the tubulin-binding repeats of the tau molecule.

Actin-dependent organelle movement in squid axoplasm

Studies of organelle movement in axoplasm extruded from the squid giant axon have led to the basic discoveries of microtubule-dependent organelle motility and the characterization of the microtubule-based motor proteins kinesin and cytoplasmic dynein. Rapid organelle movement in higher animal cells, especially in neurons, is considered to be microtubule-based. The role of actin filaments, which are also abundant in axonal cytoplasm, has remained unclear. The inhibition of organelle movement in axoplasm by actin-binding proteins such as DNase I, gelsolin and synapsin I has been attributed to their ability to disorganize the microtubule domains where most of the actin-filaments are located. Here we provide evidence of a new type of organelle movement in squid axoplasm which is independent of both microtubules and microtubule-based motors. This movement is ATP-dependent, unidirectional, actin-dependent, and probably generated by a myosin-like motor. These results demonstrate that an actomyosin-like mechanism can be directly involved in the generation of rapid organelle transport in nerve cells.

Reevaluation of in vitro differentiation protocols for bone marrow stromal cells: disruption of actin cytoskeleton induces rapid morphological changes and mimics neuronal phenotype

Bone marrow stromal cells (MSC), which represent a population of multipotential mesenchymal stem cells, have been reported to undergo rapid and robust transformation into neuron-like phenotypes in vitro following treatment with chemical induction medium including dimethyl sulfoxide (DMSO; Woodbury et al. [2002] J. Neurosci. Res. 96:908). In this study, we confirmed the ability of cultured rat MSC to undergo in vitro osteogenesis, chondrogenesis, and adipogenesis, demonstrating differentiation of these cells to three mesenchymal cell fates. We then evaluated the potential for in vitro neuronal differentiation of these MSC, finding that changes in morphology upon addition of the chemical induction medium were caused by rapid disruption of the actin cytoskeleton. Retraction of the cytoplasm left behind long processes, which, although strikingly resembling neurites, showed essentially no motility and no further elaboration during time-lapse studies. Similar neurite-like processes were induced by treating MSC with DMSO only or with actin filament-depolymerizing agents. Although process formation was accompanied by rapid expression of some neuronal and glial markers, the absence of other essential neuronal proteins pointed toward aberrantly induced gene expression rather than toward a sequence of gene expression as is required for neurogenesis. Moreover, rat dermal fibroblasts responded to neuronal induction by forming similar processes and expressing similar markers. These studies do not rule out the possibility that MSC can differentiate into neurons; however, we do want to caution that in vitro differentiation protocols may have unexpected, misleading effects. A dissection of molecular signaling and commitment events may be necessary to verify the ability of MSC transdifferentiation to neuronal lineages.

An actin network is present in the cytoplasm throughout the cell cycle of carrot cells and associates with the dividing nucleus

We have studied the F-actin network in cycling suspension culture cells of carrot (Daucus carota L.) using rhodaminyl lysine phallotoxin (RLP). In addition to conventional fixation with formaldehyde, we have used two different nonfixation methods before adding RLP: extracting cells in a stabilizing buffer; inducing transient pores in the plasma membrane with pulses of direct current (electroporation). These alternative methods for introducing RLP revealed additional features of the actin network not seen in aldehyde-fixed cells. The three-dimensional organization of this network in nonflattened cells was demonstrated by projecting stereopairs derived from through-focal series of computer-enhanced images. F-actin is present in interphase cells in four interconnected configurations: a meshwork surrounding the nucleus; thick cables in transvacuolar strands and deep in the cytoplasm; a finer network of bundles within the cortical cytoplasm; even finer filaments that run in ordered transverse array around the cell periphery. The actin network is organized differently during division but it does not disappear as do the cortical microtubules. RLP stains a central filamentous cortical band as the chromatin begins to condense (preprophase); it stains the mitotic spindle (as recently shown by Seagull et al. [Seagull, R. W., M. Falconer, and C. A. Weerdenburg, 1987, J. Cell Biol., 104:995-1004] for aldehyde fixed suspension cells) and the cytokinetic apparatus (as shown by Clayton, L., and C. W. Lloyd, 1985, Exp. Cell Res., 156:231-238). However, it is now shown that an additional network of F-actin persists in the cytoplasm throughout division associating in turn with the preprophase band, the mitotic spindle, and the cytokinetic phragmoplast.

Actin polymerization-driven molecular movement of mDia1 in living cells

mDia1, a Rho effector, belongs to the Formin family of proteins, which shares the conserved tandem FH1-FH2 unit structure. Formins including mDia1 accelerate actin nucleation while interacting with actin filament fast-growing ends. Here our single-molecule imaging revealed fast directional movement of mDia1 FH1-FH2 for tens of microns in living cells. The movement of mDia1 FH1-FH2 was blocked by actin-perturbing drugs, and the speed of mDia1 FH1-FH2 movement appeared to correlate with actin elongation rates. In vitro, mDia1 FH1-FH2 associated persistently with the growing actin barbed end. mDia1 probably moves processively along the growing end of actin filaments in cells, and Formins may be a molecular motility machinery that is independent from motor proteins.

Clostridium difficile toxin CDT induces formation of microtubule-based protrusions and increases adherence of bacteria

Clostridium difficile causes antibiotic-associated diarrhea and pseudomembranous colitis by production of the Rho GTPase-glucosylating toxins A and B. Recently emerging hypervirulent Clostridium difficile strains additionally produce the binary ADP-ribosyltransferase toxin CDT (Clostridium difficile transferase), which ADP-ribosylates actin and inhibits actin polymerization. Thus far, the role of CDT as a virulence factor is not understood. Here we report by using time-lapse- and immunofluorescence microscopy that CDT and other binary actin-ADP-ribosylating toxins, including Clostridium botulinum C2 toxin and Clostridium perfringens iota toxin, induce redistribution of microtubules and formation of long (up to >150 µm) microtubule-based protrusions at the surface of intestinal epithelial cells. The toxins increase the length of decoration of microtubule plus-ends by EB1/3, CLIP-170 and CLIP-115 proteins and cause redistribution of the capture proteins CLASP2 and ACF7 from microtubules at the cell cortex into the cell interior. The CDT-induced microtubule protrusions form a dense meshwork at the cell surface, which wrap and embed bacterial cells, thereby largely increasing the adherence of Clostridia. The study describes a novel type of microtubule structure caused by less efficient microtubule capture and offers a new perspective for the pathogenetic role of CDT and other binary actin-ADP-ribosylating toxins in host–pathogen interactions.

Clostridium difficile is responsible for ∼20 percent of antibiotic-related cases of diarrhea and nearly all cases of pseudomembranous colitis. The pathogens produce two protein toxins (toxins A and B), which inactivate Rho-GTPases of host cells by glucosylation. Recently emerging hypervirulent strains of C. difficile release higher amounts of toxins A and B, are resistant towards fluoroquinolones and produce an additional protein toxin called C. difficile transferase (CDT). CDT is a binary toxin, which modifies G-actin by ADP-ribosylation, thereby inhibiting actin polymerization. So far the pathogenetic role of CDT is not clear. Here we studied the effects of CDT on human colon carcinoma cells and show that the toxin causes rearrangement of microtubules and formation of long cellular protrusions. The microtubule-based protrusions form a dense meshwork at the cell surface, which wrap and embed Clostridia, thereby increasing adherence of the pathogens. We observed similar effects with other members of the family of binary actin-ADP-ribosylating toxins like C. botulinum C2 toxin and C. perfringens iota toxin. Our findings show a novel type of microtubule structures induced by actin-ADP-ribosylating toxins and propose an important role of these toxins in host–pathogen interactions by their effects on adherence and colonization of Clostridia.

Fc-receptor-mediated phagocytosis is regulated by mechanical properties of the target

Phagocytosis is an actin-based process used by macrophages to clear particles greater than 0.5 μm in diameter. In addition to its role in immunological responses, phagocytosis is also necessary for tissue remodeling and repair. To prevent catastrophic autoimmune reactions, phagocytosis must be tightly regulated. It is commonly assumed that the recognition/selection of phagocytic targets is based solely upon receptor-ligand binding. Here we report an important new criterion, that mechanical parameters of the target can dramatically affect the efficiency of phagocytosis. When presented with particles of identical chemical properties but different rigidity, macrophages showed a strong preference to engulf rigid objects. Furthermore, phagocytosis of soft particles can be stimulated with the microinjection of constitutively active Rac1 but not RhoA, and with lysophosphatidic acid, an agent known to activate the small GTP-binding proteins of the Rho family. These data suggest a Rac1-dependent mechanosensory mechanism for phagocytosis, which probably plays an important role in a number of physiological and pathological processes from embryonic development to autoimmune diseases.