Vinculin proteolysis unmasks an ActA homolog for actin-based Shigella motility

Immunopeptidomics-based design of mRNA vaccine formulations against Listeria monocytogenes

Listeria monocytogenes is a foodborne intracellular bacterial pathogen leading to human listeriosis. Despite a high mortality rate and increasing antibiotic resistance no clinically approved vaccine against Listeria is available. Attenuated Listeria strains offer protection and are tested as antitumor vaccine vectors, but would benefit from a better knowledge on immunodominant vector antigens. To identify novel antigens, we screen for Listeria peptides presented on the surface of infected human cell lines by mass spectrometry-based immunopeptidomics. In between more than 15,000 human self-peptides, we detect 68 Listeria immunopeptides from 42 different bacterial proteins, including several known antigens. Peptides presented on different cell lines are often derived from the same bacterial surface proteins, classifying these antigens as potential vaccine candidates. Encoding these highly presented antigens in lipid nanoparticle mRNA vaccine formulations results in specific CD8+ T-cell responses and induces protection in vaccination challenge experiments in mice. Our results can serve as a starting point for the development of a clinical mRNA vaccine against Listeria and aid to improve attenuated Listeria vaccines and vectors, demonstrating the power of immunopeptidomics for next-generation bacterial vaccine development.

Transfer of HTLV-1 p8 and Gag to target T-cells depends on VASP, a novel interaction partner of p8

The Human T-cell leukemia virus type 1 (HTLV-1) orf I-encoded accessory protein p8 is cleaved from its precursor p12, and both proteins contribute to viral persistence. p8 induces cellular protrusions, which are thought to facilitate transfer of p8 to target cells and virus transmission. Host factors interacting with p8 and mediating p8 transfer are unknown. Here, we report that vasodilator-stimulated phosphoprotein (VASP), which promotes actin filament elongation, is a novel interaction partner of p8 and important for p8 and HTLV-1 Gag cell-to-cell transfer. VASP contains an Ena/VASP homology 1 (EVH1) domain that targets the protein to focal adhesions. Bioinformatics identified a short stretch in p8 (amino acids (aa) 24–45) which may mediate interactions with the EVH1 domain of VASP. Co-immunoprecipitations confirmed interactions of VASP:p8 in 293T, Jurkat and HTLV-1-infected MT-2 cells. Co-precipitation of VASP:p8 could be significantly blocked by peptides mimicking aa 26–37 of p8. Mutational studies revealed that the EVH1-domain of VASP is necessary, but not sufficient for the interaction with p8. Further, deletion of the VASP G- and F-actin binding domains significantly diminished co-precipitation of p8. Imaging identified areas of partial co-localization of VASP with p8 at the plasma membrane and in protrusive structures, which was confirmed by proximity ligation assays. Co-culture experiments revealed that p8 is transferred between Jurkat T-cells via VASP-containing conduits. Imaging and flow cytometry revealed that repression of both endogenous and overexpressed VASP by RNA interference or by CRISPR/Cas9 reduced p8 transfer to the cell surface and to target Jurkat T-cells. Stable repression of VASP by RNA interference in chronically infected MT-2 cells impaired both p8 and HTLV-1 Gag transfer to target Jurkat T-cells, while virus release was unaffected. Thus, we identified VASP as a novel interaction partner of p8, which is important for transfer of HTLV-1 p8 and Gag to target T-cells.

The delta-retrovirus Human T-cell leukemia virus type 1 encodes the accessory protein p8, which is generated by proteolytic cleavage from p12. Earlier work has shown that p8 enhances the formation of cellular conduits between T-cells, is transferred through these conduits to target T-cells and increases HTLV-1 transmission. It was suggested that p8 dampens T-cell responses in target T-cells, thus facilitating HTLV-1 infection. Our work sheds light on the mechanism of p8 transfer to target T-cells. We show that vasodilator-stimulated phosphoprotein (VASP), a novel interaction partner of p8, contributes to transfer of p8 to target T-cells. Mechanistically, VASP is crucial for recruitment of p8 to the cell surface. Since VASP is known to promote elongation of actin filaments by preventing them from capping, interactions of p8 with VASP are an elegant strategy to exploit the host cell machinery for being transported to the cell surface, and as a consequence, to other cells. Given that VASP is also important for cell-to-cell transfer of the HTLV-1 Gag protein, our work proposes that VASP is a new cellular target to counteract HTLV-1 cell-to-cell transmission.

Mechanisms and Functions of Vinculin Interactions with Phospholipids at Cell Adhesion Sites

The cytoskeletal protein vinculin is a major regulator of cell adhesion and attaches to the cell surface by binding to specific phospholipids. Structural, biochemical, and biological studies provided much insight into how vinculin binds to membranes, what components it recognizes, and how lipid binding is regulated. Here we discuss the roles and mechanisms of phospholipids in regulating the structure and function of vinculin and of its muscle-specific metavinculin splice variant. A full appreciation of these processes is necessary for understanding how vinculin regulates cell motility, migration, and wound healing, and for understanding of its role in cancer and cardiovascular diseases.

Actoclampin (+)-end-tracking motors: How the pursuit of profilin's role(s) in actin-based motility twice led to the discovery of how cells crawl

The path to the discovery of the actoclampins began with efforts to define profilin's role in actin-based pathogen and endosome rocketing. That research identified a set of FPPPP-containing cargo proteins and FPPPP-binding proteins that are consistently stationed within the polymerization zone during episodes of active motility. The very same biophysical clues that forced us to abandon Brownian Ratchet models guided us to the Actoclampin Hypothesis, which asserts that every propulsive filament possesses a (+)-end-tracking motor that generates the forces cells need to crawl. Each actoclampin motor is a multi-arm oligomeric complex, employing one arm to recruit/deliver Profilin•Actin•ATP to a growth-site located at the (+)-end of the lagging subfilament, while a second arm maintains an affinity-modulated binding interaction with the extreme (+)-end of the other subfilament. The alternating actions of these arms define a true molecular motor, the processivity of which explains why propelling filaments maintain full possession of their cargo. The Actoclampin Hypothesis also suggests how the energetics of tracker interactions with the (+)-end determines whether a given actoclampin is a passive (low force-producing) or active (high force-producing) motor, the latter requiring the Gibbs free energy of ATP hydrolysis. Another aim of this review is to acknowledge an earlier notional model that emerged from efforts to comprehend profilin's pivotal role(s) in actin-based cell motility.

Brain abscess following rituximab infusion in a patient with pemphigus vulgaris

Patient: Female, 52

Final Diagnosis: Brain abscess

Symptoms: Fever • headache • weakness, left sided

Medication: Prednisolone • Azathioprine • Rituximab

Clinical Procedure: Stereotactic brain biopsy and LP

Specialty: Neurology

Co-evolution and exploitation of host cell signaling pathways by bacterial pathogens

Bacterial pathogens have evolved by combinations of gene acquisition, deletion, and modification, which increases their fitness. Additionally, bacteria are able to evolve in "quantum leaps" via the ability to promiscuously acquire new genes. Many bacterial pathogens - especially Gram-negative enteric pathogens - have evolved mechanisms by which to subvert signal transduction pathways of eukaryotic cells by expressing genes that mimic or regulate host protein factors involved in a variety of signaling cascades. This results in the ability to cause diseases ranging from tumor formation in plants to gastroenteritis and bubonic plague. Here, we present recent advances on mechanisms of bacterial pathogen evolution, including specific signaling cascades targeted by their virulence genes with an emphasis on the ubiquitin modification system, Rho GTPase regulators, cytoskeletal modulators, and host innate immunity. We also comment briefly on evolution of host defense mechanisms in place that limit disease caused by bacterial pathogens.

Anthrax lethal toxin paralyzes actin-based motility by blocking Hsp27 phosphorylation

Inhalation of anthrax causes fatal bacteremia, indicating a meager host immune response. We previously showed that anthrax lethal toxin (LT) paralyzes neutrophils, a major component of innate immunity. Here, we have found that LT also inhibits actin-based motility of the intracellular pathogen Listeria monocytogenes. LT inhibition of actin assembly is mediated by blockade of Hsp27 phosphorylation, and can be reproduced by treating cells with the p38 mitogen-activated protein (MAP) kinase inhibitor SB203580. Nonphosphorylated Hsp27 inhibits Listeria actin-based motility in cell extracts, and binds to and sequesters purified actin monomers. Phosphorylation of Hsp27 reverses these effects. RNA interference knockdown of Hsp27 blocks LT inhibition of Listeria actin-based motility. Rescue with wild-type Hsp27 accelerates Listeria speed in knockdown cells, whereas introduction of Hsp27 mutants incapable of phosphorylation or dephosphorylation causes slowing down. We propose that Hsp27 facilitates actin-based motility through a phosphorylation cycle that shuttles actin monomers to regions of new actin filament assembly. Our findings provide a previously unappreciated mechanism for LT virulence, and emphasize a central role for p38 MAP kinase-mediated phosphorylation of Hsp27 in actin-based motility and innate immunity.

IpaA targets beta1 integrins and rho to promote actin cytoskeleton rearrangements necessary for Shigella entry

Shigella invasion into the colonic epithelium involves many steps including the formation of large membrane protrusions by the epithelial cells that facilitate bacterial engulfment. IpaA, a Shigella protein secreted into target cells upon cell contact induces a loss of actin stress fibers in cells and promotes the reorganization of actin at the site of entry. The mechanism for this is not known but is thought to involve recruitment of the focal adhesion protein vinculin to IpaA. Here we have examined the mechanism for the effects of IpaA on the actin cytoskeleton. We show that IpaA-induced loss of actin stress fibers and cell rounding do not require vinculin expression or an intact vinculin binding site on IpaA. Rather, we find that cells expressing IpaA exhibited elevated Rho activity and increased myosin light chain phosphorylation. In addition, IpaA decreases integrin affinity for extracellular matrix ligands by interfering with talin recruitment to the integrin cytoplasmic tail. The combination of these two effects, namely weakened adhesion and increased contractility, account for the loss of actin stress fibers and cell rounding observed in cells exposed to IpaA.

Trypanosoma cruzi infection disrupts vinculin costameres in cardiomyocytes

Chagas' disease cardiomyopathy is an important manifestation of Trypanosoma cruzi infection, leading to cardiac dysfunction and serious arrhythmias. We have here investigated by indirect immunofluorescence assay the distribution of vinculin, a focal adhesion protein with a major role in the transmission of contraction force, during the T. cruzi-cardiomyocyte infection in vitro and in vivo. No change in vinculin distribution was observed after 24 h of infection, where control and T. cruzi-infected cardiomyocytes displayed vinculin localized at costameres and intercalated discs. On the other hand, a clear disruption of vinculin costameric distribution was noted after 72 h of infection. A significant reduction in the levels of vinculin expression was observed at all times of infection. In murine experimental Chagas' disease, alteration in the vinculin distribution was also detected in the infected myocardium, with no costameric staining in infected myocytes and irregular alignment of intercalated discs in cardiac fibers. These data suggest that the disruption of costameric vinculin distribution and the enlargement of interstitial space due to inflammatory infiltration may contribute to the reduction of transmission of cardiac contraction force, leading to alterations in the heart function in Chagas' disease.

Gelsolin mediates calcium-dependent disassembly of Listeria actin tails

The role of intracellular Ca2+ in the regulation of actin filament assembly and disassembly has not been clearly defined. We show that reduction of intracellular free Ca2+ concentration ([Ca2+]i) to <40 nM in Listeria monocytogenes-infected, EGFP-actin-transfected Madin-Darby canine kidney cells results in a 3-fold lengthening of actin filament tails. This increase in tail length is the consequence of marked slowing of the actin filament disassembly rate, without a significant change in assembly rate. The Ca2+-sensitive actin-severing protein gelsolin concentrates in the Listeria rocket tails at normal resting [Ca2+]i and disassociates from the tails when [Ca2+]i is lowered. Reduction in [Ca2+]i also blocks the severing activity of gelsolin, but not actin-depolymerizing factor (ADF)/cofilin microinjected into Listeria-infected cells. In Xenopus extracts, Listeria tail lengths are also calcium-sensitive, markedly shortening on addition of calcium. Immunodepletion of gelsolin, but not Xenopus ADF/cofilin, eliminates calcium-sensitive actin-filament shortening. Listeria tail length is also calcium-insensitive in gelsolin-null mouse embryo fibroblasts. We conclude that gelsolin is the primary Ca2+-sensitive actin filament recycling protein in the cell and is capable of enhancing Listeria actin tail disassembly at normal resting [Ca2+]i (145 nM). These experiments illustrate the unique and complementary functions of gelsolin and ADF/cofilin in the recycling of actin filaments.

Listeria's right-handed helical rocket-tail trajectories: Mechanistic implications for force generation in actin-based motility

Listeria monocytogenes forms right-handed helical rocket tail trajectories during actin-based motility in cell-free extracts, and this stereochemical feature is consistent with actoclampin's affinity-modulated, clamped-filament elongation model [Dickinson and Purich, 2002: Biophys J 82:605-617]. In that mechanism, right-handed torque is generated by an end-tracking molecular motor, each comprised of a filament barbed end and clamping protein that processively traces the right-handed helix of its filament partner. By contrast, torque is not a predicted property of those models (e.g., elastic propulsion, elastic Brownian ratchet, tethered ratchet, and insertional polymerization models) requiring filament barbed ends to depart/detach from the motile object's surface during/after each monomer-addition step. Helical trajectories also explain why Listeria undergoes longitudinal-axis rotation on a length-scale matching the helical periodicity of Listeria's rocket tails.

Comparative biochemical analysis suggests that vinculin and metavinculin cooperate in muscular adhesion sites

Metavinculin, the muscle-specific splice variant of the cell adhesion protein vinculin, is characterized by a 68-amino acid insert within the C-terminal tail domain. The findings that mutations within this region correlate with hereditary idiopathic dilated cardiomyopathy in man suggest a specific contribution of metavinculin to the molecular architecture of muscular actin-membrane attachment sites, the nature of which, however, is still unknown. In mice, metavinculin is expressed in smooth and skeletal muscle, where it co-localizes with vinculin in dense plaques and costameres, respectively, but is of conspicuously low abundance in the heart. Immunoprecipitates suggest that both isoforms are present in the same complex. On the molecular level, both vinculin isoforms are regulated via an intramolecular head-tail interaction, with the metavinculin tail domain having a lower affinity for the head as compared with the vinculin tail. In addition, metavinculin displays impaired binding to acidic phospholipids and reduced homodimerization. Only in the presence of phospholipid-activated vinculin tail, the metavinculin tail domain is readily incorporated into heterodimers. Mutational analysis revealed that the metavinculin insert significantly alters binding of the C-terminal hairpin loop to acidic phospholipids. In summary, our data lead to a model in which unfurling of the metavinculin tail domain is impaired by the negative charges of the 68-amino acid insert, thus requiring vinculin to fully activate the metavinculin molecule. As a consequence, microfilament anchorage may be modulated at muscular adhesion sites through heterodimer formation.

Scavenger receptor-A mediates gp96/GRP94 and calreticulin internalization by antigen-presenting cells

gp96 (GRP94) elicits antigen-presenting cell (APC) activation and can direct peptides into the cross- presentation pathways of APC. These responses arise through interactions of gp96 with Toll-like (APC activation) and endocytic (cross-presentation) receptors of APC. Previously, CD91, the alpha2-macroglobulin receptor, was identified as the heat shock/chaperone protein receptor of APC. Recent data indicates, however, that inhibition of CD91 ligand binding does not alter gp96 recognition and uptake. Furthermore, CD91 expression is not itself sufficient for gp96 binding and internalization. We now report that scavenger receptor class-A (SR-A), a prominent scavenger receptor of macrophages and dendritic cells, serves a primary role in gp96 and calreticulin recognition and internalization. gp96 internalization and peptide re-presentation are inhibited by the SR-A inhibitory ligand fucoidin, although fucoidin was without effect on alpha2-macroglobulin binding or uptake. Ectopic expression of SR-A in HEK 293 cells yielded gp96 recognition and uptake activity. In addition, macrophages derived from SR-A-/- mice were substantially impaired in gp96 binding and uptake. These data identify new roles for SR-A in the regulation of cellular responses to heat shock proteins.

Shigella interactions with the actin cytoskeleton in the absence of Ena/VASP family proteins

Shigella move through the cytosol of infected cells by assembly of a propulsive actin tail at one end of the bacterium. Vasodilator-stimulated phosphoprotein (VASP), a member of the Ena/VASP family of proteins, is important in cellular actin dynamics and is present on intracellular Shigella. VASP binds both profilin, an actin monomer-binding protein, and vinculin, a component of intercellular contacts that also binds the Shigella actin assembly protein IcsA. It has been postulated that VASP might serve as a linker between vinculin and profilin on intracellular Shigella, thereby delivering profilin to the Shigella actin assembly machinery. We show that Shigella actin-based motility is unaltered in cells that are deficient for the Ena/VASP family of proteins. In these cells, Shigella form normal-appearing actin tails and move at rates that are comparable to the rates of bacterial movement in Ena/VASP-deficient cells complemented with the Ena/VASP family member Mena. Finally, whereas vinculin can bind the Arp2/3 complex, we show that Arp2/3 recruitment to Shigella is not correlated with vinculin recruitment, indicating that the role of vinculin in Shigella motility is not recruitment of Arp2/3. Thus, although VASP is recruited to the surface of intracellular Shigella, it is not essential for Shigella actin-based motility.

Vinculin is proteolyzed by calpain during platelet aggregation: 95 kDa cleavage fragment associates with the platelet cytoskeleton

The focal adhesion protein vinculin contributes to cell attachment and spreading through strengthening of mechanical interactions between cell cytoskeletal proteins and surface membrane glycoproteins. To investigate whether vinculin proteolysis plays a role in the influence vinculin exerts on the cytoskeleton, we studied the fate of vinculin in activated and aggregating platelets by Western blot analysis of the platelet lysate and the cytoskeletal fractions of differentially activated platelets. Vinculin was proteolyzed into at least three fragments (the major one being approximately 95 kDa) within 5 min of platelet activation with thrombin or calcium ionophore. The 95 kDa vinculin fragment shifted cellular compartments from the membrane skeletal fraction to the cortical cytoskeletal fraction of lysed platelets in a platelet aggregation-dependent manner. Vinculin cleavage was inhibited by calpeptin and E64d, indicating that the enzyme responsible for vinculin proteolysis is calpain. These calpain inhibitors also inhibited the translocation of full-length vinculin to the cytoskeleton. We conclude that cleavage of vinculin and association of vinculin cleavage fragment(s) with the platelet cytoskeleton is an activation response that may be important in the cytoskeletal remodeling of aggregating platelets.

Disruption of cardiac Ena-VASP protein localization in intercalated disks causes dilated cardiomyopathy

Vasodilator-stimulated phosphoprotein (VASP) and mammalian enabled (Mena) are actin cytoskeleton and signaling modulators. Ena-VASP proteins share an identical domain organization with an NH2-terminal Ena VASP homology (EVH1) domain, which mediates the binding of these proteins to FPPPP-motif containing partners such as zyxin and vinculin. VASP and Mena are abundantly expressed in the heart. However, previous studies showed that disruption by gene targeting of VASP or Mena genes in mice did not reveal any cardiac phenotype, whereas mice lacking both VASP and Mena died during embryonic development. To determine the in vivo function of Ena-VASP proteins in the heart, we used a dominant negative strategy with cardiac-specific expression of the VASP-EVH1 domain. Transgenic mice with cardiac myocyte-restricted, alpha-myosin heavy chain promoter-directed expression of the VASP-EVH1 domain were generated. Overexpression of the EVH1 domain resulted in specific displacement of both VASP and Mena from cardiac intercalated disks. VASP-EVH1 transgenic mice developed dilated cardiomyopathy with myocyte hypertrophy and bradycardia, which resulted in early postnatal lethality in mice with high levels of transgene expression. The results demonstrate that Ena-VASP proteins may play an important role in intercalated disk function at the interface between cardiac myocytes.

A role for VASP in RhoA-Diaphanous signalling to actin dynamics and SRF activity

Vasodilator-stimulated phosphoprotein (VASP) is involved in multiple actin-mediated processes, including regulation of serum response factor (SRF) activity. We used the SRF transcriptional assay to define functional domains in VASP and to show that they coincide with those required for F-actin accumulation, as determined by a quantitative FACS assay. We identified inactive VASP mutants that can interfere both with F-actin assembly and with SRF activation by wild-type VASP. These VASP mutants also inhibit actin-based motility of Vaccinia virus and Shigella flexneri. VASP-induced F-actin accumulation and SRF activation require both functional Rho and its effector mDia, and conversely, mDia-mediated SRF activation is critically dependent on functional VASP. VASP and mDia also associate physically in vivo. These findings show that VASP and mDia function cooperatively downstream of Rho to control F-actin assembly and SRF activity.

Actin comet tails, endosomes and endosymbionts

The Arp2/3 complex consists of seven highly conserved and tightly associated subunits, two of which are the actin-related proteins Arp2 and Arp3. One of the best-studied functions of the Arp2/3 complex is to stimulate actin nucleation and force production at the leading edge of motile cells. What is now clear is that Arp2/3-complex-mediated force production drives many intracellular movements, including movement of bacterial pathogens in infected host cells, internalization of extracellular materials via phagocytosis and endocytosis, and movement of mitochondria during cell division in budding yeast. Here, we describe recent advances in the mechanisms underlying Arp2/3 complex-driven intracellular movement.

Shigella interaction with intestinal epithelial cells determines the innate immune response in shigellosis

Shigellae are Gram-negative bacilli that cause bacillary dysentery in humans. This review summarizes current knowledge of Shigella pathogenesis and pathogenicity factors, invasion of epithelial cells, intracellular motility and cell-to-cell spreading, as well as components of the host cell involved in innate immune responses.

Actin-based endosome and phagosome rocketing in macrophages: activation by the secretagogue antagonists lanthanum and zinc

Although motile endocytic vesicles form actin-rich rocket tails [Merrifield et al., 1999: Nature Cell Biol 1:72-74], the mechanism of intracellular organelle locomotion remains poorly understood. We now demonstrate that bone marrow macrophages treated with lanthanum and zinc ions, well-known secretagogue antagonists, reliably exhibit vesicle motility. This treatment results in accentuated membrane ruffling and the formation of phagosomes and early endosomes that move rapidly through the cytoplasm by assembling actin filament rocket tails. Protein-specific immunolocalization demonstrated the presence of Arp2/3 complex in the polymerization zone and throughout the actin-rich tail, whereas N-WASP was most abundant in the polymerization zone. Although Arp2/3 and N-WASP play essential roles in nucleating filament assembly, other processes (i.e., elongation and filament cross-linking) are required to produce forces needed for motility. Efficient elongation was found to require zyxin, VASP, and profilin, proteins that interact by means of their ABM-1 and ABM-2 proline-rich motifs. The functional significance of these motifs was demonstrated by inhibition of vesicle motility by the motif-specific ABM-1 and ABM-2 analogues. Furthermore, lanthanum/zinc treatment also facilitated the early onset of actin-based vaccinia motility, a process that also utilizes Arp2/3 and N-WASP for nucleation and the zyxin-VASP-profilin complex for efficient elongation. Although earlier studies using cell extracts clouded the role of oligoproline sequences in activating the polymerization zone, our studies emphasize the importance of evaluating motility in living cells.

Actin-based phagosome motility

Despite abundant evidence of actin's involvement at the particle internalization stage of phagocytosis, little is known about whether phagosomes undergo the same type of actin-based motility as observed with endocytic vesicles or such intracellular pathogens as Listeria and Shigella. By employing video microscopy to follow the fate of latex bead-containing phagosomes within the cytoplasm of bone marrow macrophages, we have made the novel observation of actin-based phagosome motility. Immunofluorescence microscopy confirmed that phagosomes containing IgG-opsonized, bovine serum albumin (or BSA) -coated or uncoated latex beads all formed actin-rich rocket tails that persisted only during a brief, 1-2 min period of actin-based motility. Average speeds of actin-based phagosome motility were 0.13 +/- 0.06 microm/s for IgG-coated beads, 0.14 +/- 0.04 microm/s for BSA-coated beads, and 0.11+/- 0.03 microm/s for uncoated beads. Moreover, the speeds and motile-phase duration of each type of phagosome were comparable to the behavior of pinosomes [Merrifield et al., 1999: Nat. Cell Biol. 1:72-74.]. Determination of optimal conditions for observing and analyzing actin-based phagosome motility should facilitate future investigations of phagocytosis and phagosome maturation.

Shigella apyrase--a novel variant of bacterial acid phosphatases?

A virulence-associated ATP diphosphohydrolase activity in the periplasm of Shigella, identified as apyrase, was found to be markedly similar to bacterial non-specific acid phosphatases in primary structure. When the Shigella apyrase sequence was threaded in to the recently published 3D structure of the highly similar (73%) Escherichia blattae acid phosphatase it was found to have a highly overlapping 3D structure. Our analysis, which included assays for phosphatase, haloperoxidase and catalase activities, led us to hypothesize that Shigella apyrase might belong to a new class of pyrophosphatase originating as one more variant in the family of bacterial non-specific acid phosphatases. It revealed interesting structure-function relationships and probable roles relevant to pathogenesis.

Clamped-filament elongation model for actin-based motors

Although actin-based motility drives cell crawling and intracellular locomotion of organelles and certain pathogens, the underlying mechanism of force generation remains a mystery. Recent experiments demonstrated that Listeria exhibit episodes of 5.4-nm stepwise motion corresponding to the periodicity of the actin filament subunits, and extremely small positional fluctuations during the intermittent pauses [S. C. Kuo and J. L. McGrath. 2000. Nature. 407:1026-1029]. These findings suggest that motile bacteria remain firmly bound to actin filament ends as they elongate, a behavior that appears to rule out previous models for actin-based motility. We propose and analyze a new mechanochemical model (called the "Lock, Load & Fire" mechanism) for force generation by means of affinity-modulated, clamped-filament elongation. During the locking step, the filament's terminal ATP-containing subunit binds tightly to a clamp situated on the surface of a motile object; in the loading step, actin.ATP monomer(s) bind to the filament end, an event that triggers the firing step, wherein ATP hydrolysis on the clamped subunit attenuates the filament's affinity for the clamp. This last step initiates translocation of the new ATP-containing terminus to the clamp, whereupon another cycle begins anew. This model explains how surface-tethered filaments can grow while exerting flexural or tensile force on the motile surface. Moreover, stochastic simulations of the model reproduce the signature motions of Listeria. This elongation motor, which we term actoclampin, exploits actin's intrinsic ATPase activity to provide a simple, high-fidelity enzymatic reaction cycle for force production that does not require elongating filaments to dissociate from the motile surface. This mechanism may operate whenever actin polymerization is called upon to generate the forces that drive cell crawling or intracellular organelle motility.

Actin-based motility of intracellular microbial pathogens

A diverse group of intracellular microorganisms, including Listeria monocytogenes, Shigella spp., Rickettsia spp., and vaccinia virus, utilize actin-based motility to move within and spread between mammalian host cells. These organisms have in common a pathogenic life cycle that involves a stage within the cytoplasm of mammalian host cells. Within the cytoplasm of host cells, these organisms activate components of the cellular actin assembly machinery to induce the formation of actin tails on the microbial surface. The assembly of these actin tails provides force that propels the organisms through the cell cytoplasm to the cell periphery or into adjacent cells. Each of these organisms utilizes preexisting mammalian pathways of actin rearrangement to induce its own actin-based motility. Particularly remarkable is that while all of these microbes use the same or overlapping pathways, each intercepts the pathway at a different step. In addition, the microbial molecules involved are each distinctly different from the others. Taken together, these observations suggest that each of these microbes separately and convergently evolved a mechanism to utilize the cellular actin assembly machinery. The current understanding of the molecular mechanisms of microbial actin-based motility is the subject of this review.

Exploitation of host factors for efficient infection by Shigella

Shigellosis is a worldwide endemic ulcerating disease of the large intestine caused by enteroinvasive bacteria. Shigella takes the route via M-cells and macrophages to access the basolateral pole of enterocytes. After invasion of and cell-to-cell spread within the epithelial cell layer, the bacterium multiplies within the cytoplasm of enterocytes. Induced by a limited number of bacterial effector proteins, Shigella makes use of established signaling pathways of the host cell to achieve internalization, transcytosis, apoptosis or cell-to-cell spread. This review addresses the host factors required for efficient infection focusing on Shigella-induced cytoskeletal rearrangements and associated signaling.

Regulating cellular actin assembly

Cellular actin assembly is tightly regulated. The study of pathogen motility has led to the identification of several cellular factors that are critical for controlling this process. Pathogens such as Listeria require Ena/VASP and Arp2/3 proteins to translate actin polymerization into movement. Recent work has extended these observations and uncovered some similarities and surprising differences in the way cells and pathogens utilize the actin cytoskeleton.

Enhanced xenobiotic transporter expression in normal teleost hepatocytes: response to environmental and chemotherapeutic toxins

Many aquatic organisms are resistant to environmental pollutants, probably because their inherent multi-drug-resistant protein extrusion pump (pgp) can be co-opted to handle man-made pollutants. This mechanism of multixenobiotic resistance is similar to the mechanism of multidrug resistance exhibited in chemotherapy-resistant human tumor cells. In the present study, a variety of techniques were used to characterize this toxin defense system in killifish (Fundulus heteroclitus) hepatocytes. The cellular localization and activity of the putative drug efflux system were evaluated. In addition, in vitro and in vivo studies were used to examine the range of expression of this putative drug transporter in the presence of environmental and chemotherapeutic toxins. The broad range of pgp expression generally observed in transformed mammalian cells was found in normal cells of our teleost model. Our findings suggest that the expression of the pgp gene in the killifish could be an excellent indicator of toxin levels or stressors in the environment.

Rho family GTPase Cdc42 is essential for the actin-based motility of Shigella in mammalian cells

Shigella, the causative agent of bacillary dysentery, is capable of directing its movement within host cells by exploiting actin dynamics. The VirG protein expressed at one pole of the bacterium can recruit neural Wiskott-Aldrich syndrome protein (N-WASP), a downstream effector of Cdc42. Here, we show that Cdc42 is required for the actin-based motility of Shigella. Microinjection of a dominant active mutant Cdc42, but not Rac1 or RhoA, into Swiss 3T3 cells accelerated Shigella motility. In add-back experiments in Xenopus egg extracts, addition of a guanine nucleotide dissociation inhibitor for the Rho family, RhoGDI, greatly diminished the bacterial motility or actin assembly, which was restored by adding activated Cdc42. In N-WASP-depleted extracts, the bacterial movement almost arrested was restored by adding exogenous N-WASP but not H208D, an N-WASP mutant defective in binding to Cdc42. In pyrene actin assay, Cdc42 enhanced VirG-stimulating actin polymerization by N-WASP-actin-related protein (Arp)2/3 complex. Actually, Cdc42 stimulated actin cloud formation on the surface of bacteria expressing VirG in a solution containing N-WASP, Arp2/3 complex, and G-actin. Immunohistological study of Shigella-infected cells expressing green fluorescent protein-tagged Cdc42 revealed that Cdc42 accumulated by being colocalized with actin cloud at one pole of intracellular bacterium. Furthermore, overexpression of H208D mutant in cells interfered with the actin assembly of infected Shigella and diminished the intra- and intercellular spreading. These results suggest that Cdc42 activity is involved in initiating actin nucleation mediated by VirG-N-WASP-Arp2/3 complex formed on intracellular Shigella.

Actin-based motility of pathogens: the Arp2/3 complex is a central player

Bacterial actin-based motility has provided cell biologists with tools that led to the recent discovery that, in many forms of actin-based motilities, a key player is a protein complex named the Arp2/3 complex. The Arp2/3 complex is evolutionally conserved and made up of seven polypeptides involved in both actin filament nucleation and organization. Interestingly, this complex is inactive by itself and recent work has highlighted the fact that its activation is achieved differently in the different types of actin-based motilities, including the well-known examples of Listeria and Shigella motilities. Proteins of the WASP family and small G-proteins are involved in most cases. It is interesting that bacteria bypass or mimic some of the events occurring in eukaryotic systems. The Shigella protein IcsA recruits N-WASP and activates it in a Cdc42-like fashion. This activation leads to Arp2/3 complex recruitment, activation of the complex and ultimately actin polymerization and movement. The Listeria ActA protein activates Arp2/3 directly and, thus, seems to mimic proteins of the WASP family. A breakthrough in the field is the recent reconstitution of the actin-based motilities of Listeria and N-WASP-coated E. coli (IcsA) using a restricted number of purified cellular proteins including F-actin, the Arp2/3 complex, actin depolymerizing factor (ADF or cofilin) and capping protein. The movement was more effective upon addition of profilin, alpha-actinin and VASP (for Listeria). Bacterial actin-based motility is now one of the best-documented examples of the exploitation of mammalian cell machineries by bacterial pathogens.

Recruitment of the Arp2/3 complex and mena for the stimulation of actin polymerization in growth cones by nerve growth factor

The growth of axons and dendrites during development and regeneration is regulated by cues in the environment. Many of these cues regulate the actin cytoskeleton of the protrusive structures (like filopodia) of the growth cone that are essential for detecting and responding to cues. Nerve growth factor, which promotes the formation of protrusive structures, stimulated actin polymerization in rat sympathetic growth cones, resulting within 1-2 min in accumulations of F-actin at the distal edge and in splotches of F-actin farther back. We examined the potential involvement of a protein machinery important in at least certain types of actin polymerization in non-neuronal cells. Members of the Arp2/3 complex, p34-Arc and p21-Arc, heavily concentrated in the early accumulations of F-actin, as did one member of the Ena/VASP family (Mena) but not another (VASP). Retention of Arc proteins at preferred sites of actin polymerization did not require polymerization itself. Growth cones of differentiated PC12 cells were similar to sympathetic growth cones in their response to NGF. Introduction into these cells of a peptide that should block the binding of Ena/VASP family proteins to the protein complex at sites of actin polymerization reduced the formation of splotches and filopodia in response to NGF. These results point to the early involvement of the Arp2/3 complex and the Ena/VASP family in growth factor-stimulated actin polymerization that gives rise to protrusive structures at the growth cone.

Shigella actin-based motility in the presence of truncated vinculin

Mounting evidence supports the role of truncated vinculin in the intracellular actin-based motility of Shigella flexneri. Vinculin's role was recently questioned by Goldberg [1997: Cell Motil Cytoskeleton 37:44-53] who observed Shigella motility in mouse embryonal carcinoma 5.51 cells, a genetically modified cell line that reputedly lacked vinculin. That challenge implicitly relied on the assumption that 5.51 cells had no detectable vinculin polypeptide and lacked full-length vinculin mRNA. Despite the appearance of being an unambiguous test of vinculin's role in Shigella motility, 5.51 cells were shown to contain adequate amounts of truncated vinculin (as well as the corresponding mRNA transcript) to support bacterial locomotion. We also examined Shigella locomotion in gamma229 cells, a related embryonal carcinoma cell line containing approximately one-half the vinculin content found in 5.51 cells. We observed that there was a commensurate twofold decrease in the Shigella motility rate, as compared to 5.51 cells; this finding raises the possibility that vinculin can become a rate-limiting factor under some circumstances. Immunofluorescence microscopy using vin 11-5 monoclonal antibody directed against the vinculin head domain showed intense staining of Shigella rocket tails in both gamma229 and 5.51 cells. Our findings clearly demonstrate that motility in 5.51 cells cannot be regarded as a valid criterion for evaluating the role of truncated vinculin in Shigella motility.

Directed actin polymerization is the driving force for epithelial cell-cell adhesion

We have found that epithelial cells engage in a process of cadherin-mediated intercellular adhesion that utilizes calcium and actin polymerization in unexpected ways. Calcium stimulates filopodia, which penetrate and embed into neighboring cells. E-cadherin complexes cluster at filopodia tips, generating a two-rowed zipper of embedded puncta. Opposing cell surfaces are clamped by desmosomes, while vinculin, zyxin, VASP, and Mena are recruited to adhesion zippers by a mechanism that requires alpha-catenin. Actin reorganizes and polymerizes to merge puncta into a single row and seal cell borders. In keratinocytes either null for alpha-catenin or blocked in VASP/Mena function, filopodia embed, but actin reorganization/polymerization is prevented, and membranes cannot seal. Taken together, a dynamic mechanism for intercellular adhesion is unveiled involving calcium-activated filopodia penetration and VASP/Mena-dependent actin reorganization/polymerization.

Profilin promotes barbed-end actin filament assembly without lowering the critical concentration

The mechanism of profilin-promoted actin polymerization has been systematically reinvestigated. Rates of barbed-end elongation onto Spectrin.4.1. Actin seeds were measured by right angle light scattering to avoid confounding effects of pyrenyl-actin, and KINSIM was used to analyze elongation progress curves. Without thymosin-beta4, both actin and Profilin. Actin (P.A) are competent in barbed-end polymerization, and kinetic simulations yielded the same bimolecular rate constant ( approximately 10 x 10(6) M(-1) s(-1)) for actin monomer or Profilin. Actin. When measured in the absence of profilin, actin assembly curves over a 0.7-4 microM thymosin-beta4 concentration range fit a simple monomer sequestering model (1 microM K(D) for Thymosin-beta4. Actin). The corresponding constant for thymosin-beta4.pyrenyl-Actin, however, was significantly higher ( approximately 9-10 microM), suggesting that the fluorophore markedly weakens binding to thymosin-beta4. With solutions of actin (2 microM) and thymosin-beta4 (2 or 4 microM), the barbed-end assembly rate rose with increasing profilin concentration (0.7-2 microM). Actin assembly in presence of thymosin-beta4 and profilin fit a simple thermodynamic energy cycle, thereby disproving an earlier claim (D. Pantaloni and M.-F. Carlier (1993) Cell 75, 1007-1014) that profilin promotes nonequilibrium filament assembly by accelerating hydrolysis of filament-bound ATP. Our findings indicate that profilin serves as a polymerization catalyst that captures actin monomers from Thymosin-beta4. Actin and ushers actin as a Profilin. Actin complex onto growing barbed filament ends.

Activation of the CDC42 effector N-WASP by the Shigella flexneri IcsA protein promotes actin nucleation by Arp2/3 complex and bacterial actin-based motility

To propel itself in infected cells, the pathogen Shigella flexneri subverts the Cdc42-controlled machinery responsible for actin assembly during filopodia formation. Using a combination of bacterial motility assays in platelet extracts with Escherichia coli expressing the Shigella IcsA protein and in vitro analysis of reconstituted systems from purified proteins, we show here that the bacterial protein IcsA binds N-WASP and activates it in a Cdc42-like fashion. Dramatic stimulation of actin assembly is linked to the formation of a ternary IcsA-N-WASP-Arp2/3 complex, which nucleates actin polymerization. The Arp2/3 complex is essential in initiation of actin assembly and Shigella movement, as previously observed for Listeria monocytogenes. Activation of N-WASP by IcsA unmasks two domains acting together in insertional actin polymerization. The isolated COOH-terminal domain of N-WASP containing a verprolin-homology region, a cofilin-homology sequence, and an acidic terminal segment (VCA) interacts with G-actin in a unique profilin-like functional fashion. Hence, when N-WASP is activated, its COOH-terminal domain feeds barbed end growth of filaments and lowers the critical concentration at the bacterial surface. On the other hand, the NH(2)-terminal domain of N-WASP interacts with F-actin, mediating the attachment of the actin tail to the bacterium surface. VASP is not involved in Shigella movement, and the function of profilin does not require its binding to proline-rich regions.

Dynamics of actin-based movement by Rickettsia rickettsii in vero cells

Actin-based motility (ABM) is a virulence mechanism exploited by invasive bacterial pathogens in the genera Listeria, Shigella, and Rickettsia. Due to experimental constraints imposed by the lack of genetic tools and their obligate intracellular nature, little is known about rickettsial ABM relative to Listeria and Shigella ABM systems. In this study, we directly compared the dynamics and behavior of ABM of Rickettsia rickettsii and Listeria monocytogenes. A time-lapse video of moving intracellular bacteria was obtained by laser-scanning confocal microscopy of infected Vero cells synthesizing beta-actin coupled to green fluorescent protein (GFP). Analysis of time-lapse images demonstrated that R. rickettsii organisms move through the cell cytoplasm at an average rate of 4.8 +/- 0.6 micrometer/min (mean +/- standard deviation). This speed was 2.5 times slower than that of L. monocytogenes, which moved at an average rate of 12.0 +/- 3.1 micrometers/min. Although rickettsiae moved more slowly, the actin filaments comprising the actin comet tail were significantly more stable, with an average half-life approximately three times that of L. monocytogenes (100.6 +/- 19.2 s versus 33.0 +/- 7.6 s, respectively). The actin tail associated with intracytoplasmic rickettsiae remained stationary in the cytoplasm as the organism moved forward. In contrast, actin tails of rickettsiae trapped within the nucleus displayed dramatic movements. The observed phenotypic differences between the ABM of Listeria and Rickettsia may indicate fundamental differences in the mechanisms of actin recruitment and polymerization.

Actin-based motility of the intracellular pathogen Listeria monocytogenes: assessing the inhibitory specificity of ABM-1 peptide analogues

Actin-Based Motility motifs [ABM-1 sequence = (D/E)FPPPPX(D/E), where X = P or T, and ABM-2 sequence = XPPPPP, where X denotes G, A, L, P, and S] facilitate assembly of an activated motility complex. Potent inhibition of intracellular motility of pathogens by ABM-1 and ABM-2 peptide analogues has served as a criterion for investigating actin-based motility. To assess the specificity of ABM-1 peptide inhibitors, we microinjected proline-rich peptides into Listeria-infected PtK2 host cells. Use of a combinatorial ABM-1 peptide library (empirical formula = D1E2F2P4T1) demonstrated that high-potency inhibition requires a precise sequence, and not merely a particular amino acid composition. Calculated concentrations of specific sequences in this library indicate that the entire (D/E)FPPPPX(D/E) motif is needed to achieve high-affinity inhibition in living cells. The failure of the well known proline-rich SH3 binding antagonists VSL-12 or APP-12 to inhibit Listeria motility also indicates that SH3 interactions are unlikely to control actin-based motility directly.

A comparative study of the actin-based motilities of the pathogenic bacteria Listeria monocytogenes, Shigella flexneri and Rickettsia conorii

Listeria monocytogenes, Shigella flexneri, and Rickettsia conorii are three bacterial pathogens that are able to polymerize actin into ‘comet tail’ structures and move within the cytosol of infected cells. The actin-based motilities of L. monocytogenes and S. flexneri are known to require the bacterial proteins ActA and IcsA, respectively, and several mammalian cytoskeleton proteins including the Arp2/3 complex and VASP (vasodilator-stimulated phosphoprotein) for L. monocytogenes and vinculin and N-WASP (the neural Wiskott-Aldrich syndrome protein) for S. flexneri. In contrast, little is known about the motility of R. conorii. In the present study, we have analysed the actin-based motility of this bacterium in comparison to that of L. monocytogenes and S. flexneri. Rickettsia moved at least three times more slowly than Listeria and Shigella in both infected cells and Xenopus laevis egg extracts. Decoration of actin with the S1 subfragment of myosin in infected cells showed that the comet tails of Rickettsia have a structure strikingly different from those of L. monocytogenes or S. flexneri. In Listeria and Shigella tails, actin filaments form a branching network while Rickettsia tails display longer and not cross-linked actin filaments. Immunofluorescence studies revealed that the two host proteins, VASP and (α)-actinin colocalized with actin in the tails of Rickettsia but neither the Arp2/3 complex which we detected in the Shigella actin tails, nor N-WASP, were detected in Rickettsia actin tails. Taken together, these results suggest that R. conorii may use a different mechanism of actin polymerization.

Tyrosine phosphorylation is required for actin-based motility of vaccinia but not Listeria or Shigella

Studies of the actin-based motility of pathogens have provided important insights into the events occurring at the leading edge of motile cells [1] [2] [3]. To date, several actin-cytoskeleton-associated proteins have been implicated in the motility of Listeria or Shigella: vasodilator-stimulated phosphoprotein (VASP), vinculin and the actin-related protein complex of Arp2 and Arp3 [4] [5] [6] [7]. To further investigate the underlying mechanism of actin-tail assembly, we examined the localization of components of the actin cytoskeleton including Arp3, VASP, vinculin and zyxin during vaccinia, Listeria and Shigella infections. The most striking difference between the systems was that a phosphotyrosine signal was observed only at the site of vaccinia actin-tail assembly. Micro-injection experiments demonstrated that a phosphotyrosine protein plays an important role in vaccinia actin-tail formation. In addition, we observed a phosphotyrosine signal on clathrin-coated vesicles that have associated actin-tail-like structures and on endogenous vesicles in Xenopus egg extracts which are able to nucleate actin tails [8] [9]. Our observations indicate that a host phosphotyrosine protein is required for the nucleation of actin filaments by vaccinia and suggest that this phosphoprotein might be associated with cellular membranes that can nucleate actin.

Intracellular pathogens and the actin cytoskeleton

Many pathogens actively exploit the actin cytoskeleton during infection. This exploitation may take place during entry into mammalian cells after engagement of a receptor and/or as series of signaling events culminating in the engulfment of the microorganism. Although actin rearrangements are a common feature of most internalization events (e.g. entry of Listeria, Salmonella, Shigella, Yersinia, Neisseria, and Bartonella), bacterial and other cellular factors involved in entry are specific to each bacterium. Another step during which pathogens harness the actin cytoskeleton takes place in the cytosol, within which some bacteria (Listeria, Shigella, Rickettsia) or viruses (vaccinia virus) are able to move. Movement is coupled to a polarized actin polymerization process, with the formation of characteristic actin tails. Increasing attention has focused on this phenomenon due to its striking similarity to cellular events occurring at the leading edge of locomoting cells. Thus pathogens are convenient systems in which to study actin cytoskeleton rearrangements in response to stimuli at the plasma membrane or inside cells.

Enteropathogenic E. coli, Salmonella, and Shigella: masters of host cell cytoskeletal exploitation

Bacterial pathogens have evolved numerous strategies to exploit their host's cellular processes so that they can survive and persist. Often, a bacterium must adhere very tightly to the cells and mediate its effects extracellularly, or it must find a way to invade the host's cells and survive intracellularly. In either case, the pathogen hijacks the host's cytoskeleton. The cytoskeleton provides a flexible framework for the cell and is involved in mediating numerous cellular functions, from cell shape and structure to programmed cell death. Altering the host cytoskeleton is crucial for mediating pathogen adherence, invasion, and intracellular locomotion. We highlight recent advances in the pathogenesis of enteropathogenic Escherichia coli, Salmonella Typhimurium, and Shigella flexneri. Each illustrates how bacterial pathogens can exert dramatic effects on the host cytoskeleton.

Bacterial infections of the small intestine and colon

Enterotoxigenic Escherichia coli, Vibrio cholerae O1, Campylobacter jejuni, Salmonella species, and Shigella species are major causes of morbidity and death in diarrheal disease. More recently recognized pathogens are V. cholerae O139 and enterohemorrhagic E. coli. In addition to this, several presumptive virulence factors have been identified in diarrheagenic E. coli and in other species. To confirm these as virulence factors we need good diagnostic tools and good epidemiological studies. These are of vital importance to create vaccines for diarrheal diseases.

Vaccinia locomotion in host cells: evidence for the universal involvement of actin-based motility sequences ABM-1 and ABM-2

Vaccinia uses actin-based motility for virion movement in host cells, but the specific protein components have yet to be defined. A cardinal feature of Listeria and Shigella actin-based motility is the involvement of vasodilator-stimulated phosphoprotein (VASP). This essential adapter recognizes and binds to actin-based motility 1 (ABM-1) consensus sequences [(D/E)FPPPPX(D/E), X = P or T] contained in Listeria ActA and in the p90 host-cell vinculin fragment generated by Shigella infection. VASP, in turn, provides the ABM-2 sequences [XPPPPP, X = G, P, L, S, A] for binding profilin, an actin-regulatory protein that stimulates actin filament assembly. Immunolocalization using rabbit anti-VASP antibody revealed that VASP concentrates behind motile virions in HeLa cells. Profilin was also present in these actin-rich rocket tails, and microinjection of 10 microM (intracellular) ABM-2 peptide (GPPPPP)3 blocked vaccinia actin-based motility. Vinculin did not colocalize with VASP on motile virions and remained in focal adhesion contacts; however, another ABM-1-containing host protein, zyxin, was concentrated at the rear of motile virions. We also examined time-dependent changes in the location of these cytoskeletal proteins during vaccinia infection. VASP and zyxin were redistributed dramatically several hours before the formation of actin rocket tails, concentrating in the viral factories of the perinuclear cytoplasm. Our findings underscore the universal involvement of ABM-1 and ABM-2 docking sites in actin-based motility of Listeria, Shigella, and now vaccinia.