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

Viral cell-to-cell spread: Conventional and non-conventional ways

A critical step in the life cycle of a virus is spread to a new target cell, which generally involves the release of new viral particles from the infected cell which can then initiate infection in the next target cell. While cell-free viral particles released into the extracellular environment are necessary for long distance spread, there are disadvantages to this mechanism. These include the presence of immune system components, the low success rate of infection by single particles, and the relative fragility of viral particles in the environment. Several mechanisms of direct cell-to-cell spread have been reported for animal viruses which would avoid the issues associated with cell-free particles. A number of viruses can utilize several different mechanisms of direct cell-to-cell spread, but our understanding of the differential usage by these pathogens is modest. Although the mechanisms of cell-to-cell spread differ among viruses, there is a common exploitation of key pathways and components of the cellular cytoskeleton. Remarkably, some of the viral mechanisms of cell-to-cell spread are surprisingly similar to those used by bacteria. Here we summarize the current knowledge of the conventional and non-conventional mechanisms of viral spread, the common methods used to detect viral spread, and the impact that these mechanisms can have on viral pathogenesis.

Lamellipodin tunes cell migration by stabilizing protrusions and promoting adhesion formation

Efficient migration on adhesive surfaces involves the protrusion of lamellipodial actin networks and their subsequent stabilization by nascent adhesions. The actin-binding protein lamellipodin (Lpd) is thought to play a critical role in lamellipodium protrusion, by delivering Ena/VASP proteins onto the growing plus ends of actin filaments and by interacting with the WAVE regulatory complex, an activator of the Arp2/3 complex, at the leading edge. Using B16-F1 melanoma cell lines, we demonstrate that genetic ablation of Lpd compromises protrusion efficiency and coincident cell migration without altering essential parameters of lamellipodia, including their maximal rate of forward advancement and actin polymerization. We also confirmed lamellipodia and migration phenotypes with CRISPR/Cas9-mediated Lpd knockout Rat2 fibroblasts, excluding cell type-specific effects. Moreover, computer-aided analysis of cell-edge morphodynamics on B16-F1 cell lamellipodia revealed that loss of Lpd correlates with reduced temporal protrusion maintenance as a prerequisite of nascent adhesion formation. We conclude that Lpd optimizes protrusion and nascent adhesion formation by counteracting frequent, chaotic retraction and membrane ruffling.This article has an associated First Person interview with the first author of the paper.

A fruitful tree: developing the dendritic nucleation model of actin-based cell motility

A fundamental question in cell biology concerns how cells move, and this has been the subject of intense research for decades. In the 1990s, a major leap forward was made in our understanding of cell motility, with the proposal of the dendritic nucleation model. This essay describes the events leading to the development of the model, including findings from many laboratories and scientific disciplines. The story is an excellent example of the scientific process in action, with the combination of multiple perspectives leading to robust conclusions.

Phototracking Vaccinia Virus Transport Reveals Dynamics of Cytoplasmic Dispersal and a Requirement for A36R and F12L for Exit from the Site of Wrapping

The microtubule cytoskeleton is a primary organizer of viral infections for delivering virus particles to their sites of replication, establishing and maintaining subcellular compartments where distinct steps of viral morphogenesis take place, and ultimately dispersing viral progeny. One of the best characterized examples of virus motility is the anterograde transport of the wrapped virus form of vaccinia virus (VACV) from the trans-Golgi network (TGN) to the cell periphery by kinesin-1. Yet many aspects of this transport event are elusive due to the speed of motility and the challenges of imaging this stage at high resolution over extended time periods. We have established a novel imaging technology to track virus transport that uses photoconvertible fluorescent recombinant viruses to track subsets of virus particles from their site of origin and determine their destination. Here we image virus exit from the TGN and their rate of egress to the cell periphery. We demonstrate a role for kinesin-1 engagement in regulating virus exit from the TGN by removing A36 and F12 function, critical viral mediators of kinesin-1 recruitment to virus particles. Phototracking viral particles and components during infection is a powerful new imaging approach to elucidate mechanisms of virus replication.

Cytoskeletons in the Closet-Subversion in Alphaherpesvirus Infections

Actin filaments, microtubules and intermediate filaments form the cytoskeleton of vertebrate cells. Involved in maintaining cell integrity and structure, facilitating cargo and vesicle transport, remodelling surface structures and motility, the cytoskeleton is necessary for the successful life of a cell. Because of the broad range of functions these filaments are involved in, they are common targets for viral pathogens, including the alphaherpesviruses. Human-tropic alphaherpesviruses are prevalent pathogens carried by more than half of the world’s population; comprising herpes simplex virus (types 1 and 2) and varicella-zoster virus, these viruses are characterised by their ability to establish latency in sensory neurons. This review will discuss the known mechanisms involved in subversion of and transport via the cytoskeleton during alphaherpesvirus infections, focusing on protein-protein interactions and pathways that have recently been identified. Studies on related alphaherpesviruses whose primary host is not human, along with comparisons to more distantly related beta and gammaherpesviruses, are also presented in this review. The need to decipher as-yet-unknown mechanisms exploited by viruses to hijack cytoskeletal components—to reveal the hidden cytoskeletons in the closet—will also be addressed.

An Integrative Approach to Virus-Host Protein-Protein Interactions

Since cell regulation and protein expression can be dramatically altered upon infection by viruses, studying the mechanisms by which viruses infect cells and the regulatory networks they disrupt is essential to understanding viral pathogenicity. This line of study can also lead to discoveries about the workings of host cells themselves. Computational methods are rapidly being developed to investigate viral-host interactions, and here we highlight recent methods and the insights that they have revealed so far, with a particular focus on methods that integrate different types of data. We also review the challenges of working with viruses compared with traditional cellular biology, and the limitations of current experimental and informatics methods.

Divergent roles of β- and γ-actin isoforms during spread of vaccinia virus

Actin is a major component of the cytoskeleton and is present as two isoforms in non‐muscle cells: β‐ and γ‐cytoplasmic actin. These isoforms are strikingly conserved, differing by only four N‐terminal amino acids. During spread from infected cells, vaccinia virus (VACV) particles induce localized actin nucleation that propel virus to surrounding cells and facilitate cell‐to‐cell spread of infection. Here we show that virus‐tipped actin comets are composed of β‐ and γ‐actin. We employed isoform‐specific siRNA knockdown to examine the role of the two isoforms in VACV‐induced actin comets. Despite the high level of similarity between the actin isoforms, and their colocalization, VACV‐induced actin nucleation was dependent exclusively on β‐actin. Knockdown of β‐actin led to a reduction in the release of virus from infected cells, a phenotype dependent on virus‐induced Arp2/3 complex activity. We suggest that local concentrations of actin isoforms may regulate the activity of cellular actin nucleator complexes.

Viruses That Exploit Actin-Based Motility for Their Replication and Spread

The actin cytoskeleton is a crucial part of the eukaryotic cell. Viruses depend on host cells for their replication, and, as a result, many have developed ways of manipulating the actin network to promote their spread. This chapter reviews the various ways in which viruses utilize the actin cytoskeleton at discrete steps in their life cycle, from entry into the host cell, replication, and assembly of new progeny to virus release. Various actin inhibitors that function in different ways to affect proper actin dynamics can be used to parse the role of actin at these steps.

Viruses that ride on the coat-tails of actin nucleation

Actin nucleation drives a diversity of critical cellular processes and the motility of a select group of viral pathogens. Vaccinia virus and baculovirus, Autographa californica multiple nucleopolyhedrovirus, recruit and activate the cellular actin nucleator, the Arp2/3 complex, at the surface of virus particles thereby instigating highly localized actin nucleation. The extension of these filaments provides a mechanical force that bestows the ability to navigate the intracellular environment and promote their infectious cycles. This review outlines the viral and cellular proteins that initiate and regulate the signalling networks leading to viral modification of the actin cytoskeleton and summarizes recent insights into the role of actin-based virus transport.

Pathogenic microbes manipulate cofilin activity to subvert actin cytoskeleton

Actin-depolymerizing factor (ADF)/cofilin proteins are key players in controlling the temporal and spatial extent of actin dynamics, which is crucial for mediating host-pathogen interactions. Pathogenic microbes have evolved molecular mechanisms to manipulate cofilin activity to subvert the actin cytoskeletal system in host cells, promoting their internalization into the target cells, modifying the replication niche and facilitating their intracellular and intercellular dissemination. The study of how these pathogens exploit cofilin pathways is crucial for understanding infectious disease and providing potential targets for drug therapies.

The serine/threonine kinase STK11 promotes Shigella flexneri dissemination through establishment of cell-cell contacts competent for tyrosine kinase signaling

Shigella flexneri is an intracellular pathogen that disseminates in the intestinal epithelium by displaying actin-based motility. We found that although S. flexneri displayed comparable actin-based motilities in the cytosols of HeLa229 and HT-29 epithelial cell lines, the overall dissemination process was much more efficient in HT-29 cells. Time-lapse microscopy demonstrated that as motile bacteria reached the cell cortex in HT-29 cells, they formed membrane protrusions that resolved into vacuoles, from which the bacteria escaped and gained access to the cytosol of adjacent cells. In HeLa229 cells, S. flexneri also formed membrane protrusions that extended into adjacent cells, but the protrusions rarely resolved into vacuoles. Instead, the formed protrusions collapsed and retracted, bringing the bacteria back to the cytosol of the primary infected cells. Silencing the serine/threonine kinase STK11 (also known as LKB1) in HT-29 cells decreased the efficiency of protrusion resolution into vacuoles. Conversely, expressing STK11 in HeLa229 cells, which lack the STK11 locus, dramatically increased the efficiency of protrusion resolution into vacuoles. S. flexneri dissemination in HT-29 cells led to the local phosphorylation of tyrosine residues in protrusions, a signaling event that was not observed in HeLa229 cells but was restored in STK11-expressing HeLa229 cells. Treatment of HT-29 cells with the tyrosine kinase inhibitor imatinib abrogated tyrosine kinase signaling in protrusions, which correlated with a severe decrease in the efficiency of protrusion resolution into vacuoles. We suggest that the formation of STK11-dependent lateral cell-cell contacts competent for tyrosine kinase signaling promotes S. flexneri dissemination in epithelial cells.

Viral exploitation of actin: force-generation and scaffolding functions in viral infection

As a fundamental component of the host cellular cytoskeleton, actin is routinely engaged by infecting viruses. Furthermore, viruses from diverse groups, and infecting diverse hosts, have convergently evolved an array of mechanisms for manipulating the actin cytoskeleton for efficacious infection. An ongoing chorus of research now indicates that the actin cytoskeleton is critical for viral replication at many stages of the viral life cycle, including binding, entry, nuclear localization, genomic transcription and reverse transcription, assembly, and egress/dissemination. Specifically, viruses subvert the force-generating and macromolecular scaffolding properties of the actin cytoskeleton to propel viral surfing, internalization, and migration within the cell. Additionally, viruses utilize the actin cytoskeleton to support and organize assembly sites, and eject budding virions for cell-to-cell transmission. It is the purpose of this review to provide an overview of current research, focusing on the various mechanisms and themes of virus-mediated actin modulation described therein.

Novel anti-HIV therapeutics targeting chemokine receptors and actin regulatory pathways

The human immunodeficiency virus-1 (HIV-1) infects helper CD4(+) T cells, and causes CD4(+) T-cell depletion and immunodeficiency. In the past 30 years, significant progress has been made in antiretroviral therapy, and the disease has become manageable. Nevertheless, an effective vaccine is still nowhere in sight, and a cure or a functional cure awaits discovery. Among possible curative therapies, traditional antiretroviral therapy, mostly targeting viral proteins, has been proven ineffective. It is possible that targeting HIV-dependent host cofactors may offer alternatives, both for preventing HIV transmission and for forestalling disease progression. Recently, the actin cytoskeleton and its regulators in blood CD4(+) T cells have emerged as major host cofactors that could be targeted. The novel concept that the cortical actin is a barrier to viral entry and early post-entry migration has led to the nascent model of virus-host interaction at the cortical actin layer. Deciphering the cellular regulatory pathways has manifested exciting prospects for future therapeutics. In this review, we describe the study of HIV interactions with actin cytoskeleton. We also examine potential pharmacological targets that emerge from this interaction. In addition, we briefly discuss several actin pathway-based anti-HIV drugs that are currently in development or testing.

Arp2/3-mediated actin-based motility: a tail of pathogen abuse

Intracellular pathogens have developed elaborate mechanisms to exploit the different cellular systems of their unwilling hosts to facilitate their entry, replication, and survival. In particular, a diverse range of bacteria and viruses have evolved unique strategies to harness the power of Arp2/3-mediated actin polymerization to enhance their cell-to-cell spread. In this review, we discuss how studying these pathogens has revolutionized our molecular understanding of Arp2/3-dependent actin assembly and revealed key signaling pathways regulating actin assembly in cells. Future analyses of microbe-host interactions are likely to continue uncovering new mechanisms regulating actin assembly and dynamics, as well as unexpected cellular functions for actin. Further, studies with known and newly emerging pathogens will also undoubtedly continue to enhance our understanding of the role of the actin cytoskeleton during pathogenesis and potentially highlight future therapeutic approaches.

Phosphatidic acid regulation of PIPKI is critical for actin cytoskeletal reorganization

Type I phosphatidylinositol-4-phosphate 5-kinase (PIPKI) is the main enzyme generating the lipid second messenger phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2], which has critical functions in many cellular processes, such as cytoskeletal reorganization, membrane trafficking, and signal transduction. All three members of the PIPKI family are activated by phosphatidic acid (PA). However, how PA regulates the activity and functions of PIPKI have not been fully elucidated. In this study, we identify a PA-binding site on PIPKIγ. Mutation of this site inhibited the PA-stimulated activity and membrane localization of PIPKIγ as well as the formation of actin comets and foci induced by PIPKIγ. We also demonstrate that phospholipase D (PLD) generates a pool of PA involved in PIPKIγ regulation by showing that PLD inhibitors blocked the membrane localization of PIPKIγ and its ability to induce actin cytoskeletal reorganization. Targeting the PIPKIγ PA-binding-deficient mutant to membranes by a membrane localization sequence failed to restore the actin reorganization activity of PIPKIγ, suggesting that PA binding is not only involved in recruiting PIPKIγ to membranes but also may induce a conformational change. Taken together, these results reveal a new molecular mechanism through which PA regulates PIPKI and provides direct evidence that PA is important for the localization and functions of PIPKI in intact cells.

Loss of cytoskeletal transport during egress critically attenuates ectromelia virus infection in vivo

Egress of wrapped virus (WV) to the cell periphery following vaccinia virus (VACV) replication is dependent on interactions with the microtubule motor complex kinesin-1 and is mediated by the viral envelope protein A36. Here we report that ectromelia virus (ECTV), a related orthopoxvirus and the causative agent of mousepox, encodes an A36 homologue (ECTV-Mos-142) that is highly conserved despite a large truncation at the C terminus. Deleting the ECTV A36R gene leads to a reduction in the number of extracellular viruses formed and to a reduced plaque size, consistent with a role in microtubule transport. We also observed a complete loss of virus-associated actin comets, another phenotype dependent on A36 expression during VACV infection. ECTV ΔA36R was severely attenuated when used to infect the normally susceptible BALB/c mouse strain. ECTV ΔA36R replication and spread from the draining lymph nodes to the liver and spleen were significantly reduced in BALB/c mice and in Rag-1-deficient mice, which lack T and B lymphocytes. The dramatic reduction in ECTV ΔA36R titers early during the course of infection was not associated with an augmented immune response. Taken together, these findings demonstrate the critical role that subcellular transport pathways play not only in orthopoxvirus infection in an in vitro context but also during orthopoxvirus pathogenesis in a natural host. Furthermore, despite the attenuation of the mutant virus, we found that infection nonetheless induced protective immunity in mice, suggesting that orthopoxvirus vectors with A36 deletions may be considered another safe vaccine alternative.

Mobilization of HIV spread by diaphanous 2 dependent filopodia in infected dendritic cells

Paramount to the success of persistent viral infection is the ability of viruses to navigate hostile environments en route to future targets. In response to such obstacles, many viruses have developed the ability of establishing actin rich-membrane bridges to aid in future infections. Herein through dynamic imaging of HIV infected dendritic cells, we have observed how viral high-jacking of the actin/membrane network facilitates one of the most efficient forms of HIV spread. Within infected DC, viral egress is coupled to viral filopodia formation, with more than 90% of filopodia bearing immature HIV on their tips at extensions of 10 to 20 µm. Live imaging showed HIV filopodia routinely pivoting at their base, and projecting HIV virions at µm.sec⁻¹ along repetitive arc trajectories. HIV filopodial dynamics lead to up to 800 DC to CD4 T cell contacts per hour, with selection of T cells culminating in multiple filopodia tethering and converging to envelope the CD4 T-cell membrane with budding HIV particles. Long viral filopodial formation was dependent on the formin diaphanous 2 (Diaph2), and not a dominant Arp2/3 filopodial pathway often associated with pathogenic actin polymerization. Manipulation of HIV Nef reduced HIV transfer 25-fold by reducing viral filopodia frequency, supporting the potency of DC HIV transfer was dependent on viral filopodia abundance. Thus our observations show HIV corrupts DC to CD4 T cell interactions by physically embedding at the leading edge contacts of long DC filopodial networks.

Dendritic cells represent a unique cell type with respect to HIV, as they are the first point of contact for the virus in the genital mucosa and have the ability to spread virus efficiently in very low numbers to the primary HIV target, CD4 T cells. During the primary immune response, dendritic cells work in small numbers to make numerous and repetitive contacts, in order to filter and communicate with appropriate CD4 T cells. Thus HIV is hypothesized to be hijacking the same DC-CD4 T cell communication. Attempts to observe how HIV would achieve this have largely been limited, as introduction of imaging markers in the virus has often led to significant viral attenuation. Herein by using novel HIV constructs that permit imaging of HIV in infected dendritic cells, we observed newly forming HIV virions on the tips of long finger-like projections known as filopodia. In real-time imaging filopodia pivoted at their base and moved virions along trajectories that led to numerous CD4 T cell contacts. By manipulating filopodial formation we conclude the location of the virus on long filopodial tips allows the virus to corrupt the promiscuous dendritic cell to CD4 T cell contacts for efficient viral spread.

SP600125 inhibits Orthopoxviruses replication in a JNK1/2 -independent manner: Implication as a potential antipoxviral

The pharmacological inhibitor SP600125 [anthra(1,9-cd)pyrazol-6(2H)-one 1,9-pyrazoloanthrone] has been largely employed as a c-JUN N-terminal kinase (JNK1/2) inhibitor. In this study, we evaluated whether pretreatment with SP600125 was able to prevent Orthopoxviruses Vaccinia virus (VACV), Cowpox virus (CPXV) and modified Vaccinia virus Ankara (MVA) replication. We found that incubation with SP600125 not only blocked virus-stimulated JNK phosphorylation, but also, significantly reduced virus production. We observed 1-3 log decline in viral yield depending on the cell line infected (A31, BSC-40 or BHK-21). The reduction in viral yield correlated with a dramatic impact on virus morphogenesis progress, intracellular mature viruses (IMV) were barely detected. Despite the fact that SP600125 can act as an efficient anti-orthopoxviral compound, we also provide evidence that this antiviral effect is not specifically exerted through JNK1/2 inhibition. This conclusion is supported by the fact that viral titers measured after infections of JNK1/2 knockout cells were not altered as compared to those of wild-type cells. In contrast, a decline in viral titers was verified when the infection of KO cells was carried out in the presence of the pharmacological inhibitor. SP600125 has been the focus of recent studies that have evaluated its action on diverse viral infections including DNA viruses. Our data support the notion that SP600125 can be regarded as a potential antipoxviral compound.

The host phosphoinositide 5-phosphatase SHIP2 regulates dissemination of vaccinia virus

After fusing with the plasma membrane, enveloped poxvirus virions form actin-filled membranous protrusions, called tails, beneath themselves and move toward adjacent uninfected cells. While much is known about the host and viral proteins that mediate formation of actin tails, much less is known about the factors controlling release. We found that the phosphoinositide 5-phosphatase SHIP2 localizes to actin tails. Localization requires phosphotyrosine, Abl and Src family tyrosine kinases, and neural Wiskott-Aldrich syndrome protein (N-WASP) but not the Arp2/Arp3 complex or actin. Cells lacking SHIP2 have normal actin tails but release more virus. Moreover, cells infected with viral strains with mutations in the release inhibitor A34 release more virus but recruit less SHIP2 to tails. Thus, the inhibitory effects of A34 on virus release are mediated by SHIP2. Together, these data suggest that SHIP2 and A34 may act as gatekeepers to regulate dissemination of poxviruses when environmental conditions are conducive.

Anaplasma phagocytophilum induces actin phosphorylation to selectively regulate gene transcription in Ixodes scapularis ticks

Anaplasma phagocytophilum, the agent of human anaplasmosis, persists in ticks and mammals. We show that A. phagocytophilum induces the phosphorylation of actin in an Ixodes ricinus tick cell line and Ixodes scapularis ticks, to alter the ratio of monomeric/filamentous (G/F) actin. A. phagocytophilum–induced actin phosphorylation was dependent on Ixodes p21-activated kinase (IPAK1)–mediated signaling. A. phagocytophilum stimulated IPAK1 activity via the G protein–coupled receptor Gβγ subunits, which mediated phosphoinositide 3-kinase (PI3K) activation. Disruption of Ixodes gβγ, pi3k, and pak1 reduced actin phosphorylation and bacterial acquisition by ticks. A. phagocytophilum–induced actin phosphorylation resulted in increased nuclear G actin and phosphorylated actin. The latter, in association with RNA polymerase II (RNAPII), enhanced binding of TATA box–binding protein to RNAPII and selectively promoted expression of salp16, a gene crucial for A. phagocytophilum survival. These data define a mechanism that A. phagocytophilum uses to selectively alter arthropod gene expression for its benefit and suggest new strategies to interfere with the life cycle of this intracellular pathogen, and perhaps other Rickettsia-related microbes of medical importance.

Vaccinia protein F12 has structural similarity to kinesin light chain and contains a motor binding motif required for virion export

Vaccinia virus (VACV) uses microtubules for export of virions to the cell surface and this process requires the viral protein F12. Here we show that F12 has structural similarity to kinesin light chain (KLC), a subunit of the kinesin-1 motor that binds cargo. F12 and KLC share similar size, pI, hydropathy and cargo-binding tetratricopeptide repeats (TPRs). Moreover, molecular modeling of F12 TPRs upon the crystal structure of KLC2 TPRs showed a striking conservation of structure. We also identified multiple TPRs in VACV proteins E2 and A36. Data presented demonstrate that F12 is critical for recruitment of kinesin-1 to virions and that a conserved tryptophan and aspartic acid (WD) motif, which is conserved in the kinesin-1-binding sequence (KBS) of the neuronal protein calsyntenin/alcadein and several other cellular kinesin-1 binding proteins, is essential for kinesin-1 recruitment and virion transport. In contrast, mutation of WD motifs in protein A36 revealed they were not required for kinesin-1 recruitment or IEV transport. This report of a viral KLC-like protein containing a KBS that is conserved in several cellular proteins advances our understanding of how VACV recruits the kinesin motor to virions, and exemplifies how viruses use molecular mimicry of cellular components to their advantage.

A novel pseudopodial component of the dendritic cell anti-fungal response: the fungipod

Fungal pathologies are seen in immunocompromised and healthy humans. C-type lectins expressed on immature dendritic cells (DC) recognize fungi. We report a novel dorsal pseudopodial protrusion, the “fungipod”, formed by DC after contact with yeast cell walls. These structures have a convoluted cell-proximal end and a smooth distal end. They persist for hours, exhibit noticeable growth and total 13.7±5.6 µm long and 1.8±0.67 µm wide at the contact. Fungipods contain clathrin and an actin core surrounded by a sheath of cortactin. The actin cytoskeleton, but not microtubules, is required for fungipod integrity and growth. An apparent rearward flow (225±55 nm/second) exists from the zymosan contact site into the distal fungipod. The phagocytic receptor Dectin-1 is not required for fungipod formation, but CD206 (Mannose Receptor) is the generative receptor for these protrusions. The human pathogen Candida parapsilosis induces DC fungipod formation strongly, but the response is species specific since the related fungal pathogens Candida tropicalis and Candida albicans induce very few and no fungipods, respectively. Our findings show that fungipods are dynamic actin-driven cellular structures involved in fungal recognition by DC. They may promote yeast particle phagocytosis by DC and are a specific response to large (i.e., 5 µm) particulate ligands. Our work also highlights the importance of this novel protrusive structure to innate immune recognition of medically significant Candida yeasts in a species specific fashion.

Yeasts are normal microbial commensals of humans and a significant source of opportunistic infections, especially in immunocompromised individuals. We report a novel cellular protrusive structure, the fungipod, which participates in the host-microbe interaction between human immature dendritic cells (DC) and yeasts. The fungipod's structure is based on and propelled by a robust process of local actin cytoskeleton growth at the DC-yeast contact site, and this cytoskeletal remodeling results in a durable tubular structure over 10 µm long connecting the dorsal DC membrane and yeast. The fungal cell wall polysaccharides mannan and chitin trigger fungipod formation by stimulating the carbohydrate pattern recognition receptor CD206. Fungipods are part of a specific response to large particulate objects (i.e., yeast), and they may promote the human immature DC's relatively poor phagocytosis of yeast. The human fungal pathogen, Candida parapsilosis, induces a strong fungipod response from DC, and this response is highly species specific since the related pathogens Candida albicans and Candida tropicalis induce fungipods rarely. Our work highlights a novel cell biological element of fungal recognition by the innate immune system.

A reciprocal interdependence between Nck and PI(4,5)P(2) promotes localized N-WASp-mediated actin polymerization in living cells

Modulation of actin dynamics through the N-WASp/Arp2/3 pathway is important in cell locomotion, membrane trafficking, and pathogen infection. Here, we demonstrate that Nck is essential for actin remodeling stimulated by phosphatidylinositol 4,5 bisphosphate (PI(4,5)P(2)) and, conversely, that PI(4,5)P(2) is necessary for localized actin polymerization induced by Nck in vivo. Nck knockdown or knockout suppressed actin comets induced by phosphatidylinositol 5-kinase (PIP5K), and PIP5K stimulated tyrosine phosphorylation of an Nck SH2 domain binding partner, suggesting that Nck couples phosphotyrosine- and phosphoinositide-dependent signals. We show that PI(4,5)P(2) and PIP5K are both enriched at actin comets induced by Nck aggregates and that formation of actin comets was strongly inhibited by coclustering with an inositol 5-phosphatase domain to decrease local PI(4,5)P(2) levels. The extent of Nck-induced actin polymerization was also modulated by PI(4,5)P(2)-sensitive N-WASp mutants. This study uncovers a strong reciprocal interdependence between Nck and PI(4,5)P(2) in promoting localized N-WASp-mediated actin polymerization in cells.

Using fluorescent proteins to study poxvirus morphogenesis

Fluorescent protein (FP) fusions not only allow for the convenient visualization of a protein of -interest's subcellular localization but also permit the real-time monitoring of their subcellular trafficking. The subcellular fluorescent pattern of FP-fusions can also serve as a visual marker for various subcellular processes using either live or static microscopy. We have employed FP-fusions for the study of poxvirus morphogenesis. Fusion of FP with either a viral core protein or an extracellular virion-specific protein can serve as a visual read-out for normal poxvirus morphogenesis at the subcellular level. Recombinant viruses expressing a FP-fusion, in conjunction with the deletion of a gene involved in either morphogenesis or egress, usually display an aberrant FP pattern. Functional domains in the missing protein are then mapped by complementation in-trans followed by fluorescent microscopy for analysis of the FP pattern. The methods presented here describe how to infect and transfect cells for trans-complementation for the purpose of functional domain mapping. The imaging and analysis of these cells is described.

Endosomal trafficking of Src tyrosine kinase

Endosomal trafficking is an essential cellular process involved in the transport of proteins such as integrins, hormone receptors, growth factor receptors, receptor tyrosine kinases, and lipids (e.g. sphingomyelin). Regulation of this process is highly complex and involves Arf GAPs, SNAREs, Rab proteins, Rho GTPases and the actin cytoskeleton. In this article, we focus on the intracellular targeting of the Src family of non-receptor tyrosine kinases (nRTKs), and the role of endosomes in the delivery of nRTKs to the plasma membrane. Furthermore, we discuss the role of the actin cytoskeleton in this process and consider how endosome-regulated intracellular trafficking affects cell signalling.

Mutagenesis of the Shigella flexneri autotransporter IcsA reveals novel functional regions involved in IcsA biogenesis and recruitment of host neural Wiscott-Aldrich syndrome protein

The IcsA (VirG) protein of Shigella flexneri is a polarly localized, outer membrane protein that is essential for virulence. Within host cells, IcsA activates the host actin regulatory protein, neural Wiskott-Aldrich syndrome protein (N-WASP), which in turn recruits the Arp2/3 complex, which nucleates host actin to form F-actin comet tails and initiate bacterial motility. Linker insertion mutagenesis was undertaken to randomly introduce 5-amino-acid in-frame insertions within IcsA. Forty-seven linker insertion mutants were isolated and expressed in S. flexneri Delta icsA strains. Mutants were characterized for IcsA protein production, cell surface expression and localization, intercellular spreading, F-actin comet tail formation, and N-WASP recruitment. Using this approach, we have identified a putative autochaperone region required for IcsA biogenesis, and our data suggest an additional region, not previously identified, is required for N-WASP recruitment.

Adapter protein SH2-Bbeta stimulates actin-based motility of Listeria monocytogenes in a vasodilator-stimulated phosphoprotein (VASP)-dependent fashion

SH2-Bbeta (Src homology 2 Bbeta) is an adapter protein that is required for maximal growth hormone-dependent actin reorganization in membrane ruffling and cell motility. Here we show that SH2-Bbeta is also required for maximal actin-based motility of Listeria monocytogenes. SH2-Bbeta localizes to Listeria-induced actin tails and increases the rate of bacterial propulsion in infected cells and in cell extracts. Furthermore, Listeria motility is decreased in mouse embryo fibroblasts from SH2-B(-/-) mice. Both recruitment of SH2-Bbeta to Listeria and SH2-Bbeta stimulation of actin-based propulsion require the vasodilator-stimulated phosphoprotein (VASP), which binds ActA at the surfaces of Listeria cells and enhances bacterial actin-based motility. SH2-Bbeta enhances actin-based movement of ActA-coated beads in a biomimetic actin-based motility assay, provided that VASP is present. In vitro binding assays show that SH2-Bbeta binds ActA but not VASP; however, binding to ActA is greater in the presence of VASP. Because VASP also plays an essential regulatory role in actin-based processes in eukaryotic cells, the present results provide mechanistic insight into the functions of both SH2-Bbeta and VASP in motility and also increase our understanding of the fundamental mechanism by which Listeria spreads.

Cytoskeletal protein transformation in HIV-1-infected macrophage giant cells

The mechanisms linking HIV-1 replication, macrophage biology, and multinucleated giant cell formation are incompletely understood. With the advent of functional proteomics, the characterization, regulation, and transformation of HIV-1-infected macrophage-secreted proteins can be ascertained. To these ends, we performed proteomic analyses of culture fluids derived from HIV-1 infected monocyte-derived macrophages. Robust reorganization, phosphorylation, and exosomal secretion of the cytoskeletal proteins profilin 1 and actin were observed in conjunction with productive viral replication and giant cell formation. Actin and profilin 1 recruitment to the macrophage plasma membrane paralleled virus-induced cytopathicity, podosome formation, and cellular fusion. Poly-l-proline, an inhibitor of profilin 1-mediated actin polymerization, inhibited cytoskeletal transformations and suppressed, in part, progeny virion production. These data support the idea that actin and profilin 1 rearrangement along with exosomal secretion affect viral replication and cytopathicity. Such events favor the virus over the host cell and provide insights into macrophage defense mechanisms used to contain viral growth and how they may be affected during progressive HIV-1 infection.

Dynamin is required for F-actin assembly and pedestal formation by enteropathogenic Escherichia coli (EPEC)

After attaching to human intestinal epithelial cells, enteropathogenic Escherichia coli (EPEC) induces the formation of an actin-rich pedestal-like structure. The signalling pathway leading to pedestal formation is initiated by the bacterial protein Tir, which is inserted into the host cell plasma membrane. The domain exposed on the cell surface binds to another bacterial protein, intimin, while one of the cytoplasmic domains binds the adaptor protein Nck. This leads to recruitment of other cytoskeletal proteins including neural Wiskott-Aldrich syndrome protein (N-WASP) and Arp2/3, resulting in focused actin polymerization at the site of bacterial attachment. In this study we investigated the role of the large GTPase dynamin 2 (Dyn2) in pedestal formation. We found that in HeLa cells, both endogenous and overexpressed Dyn2 were recruited to sites of EPEC attachment. Recruitment of endogenous Dyn2 required the presence of Tir, Nck and N-WASP but was independent of cortactin and Arp2/3. Knock-down of Dyn2 expression by RNA interference reduced actin polymerization and pedestal formation. Overexpression of dominant-negative mutants of Dyn2 also reduced pedestal formation and prevented recruitment of N-WASP, Arp3 and cortactin, but not Nck. Together, our results indicate that Dyn2 is an integral component of the signalling cascade leading to actin polymerization in EPEC pedestals.

Actin dynamics at sites of extracellular matrix degradation

The degradation of extracellular matrix (ECM) by proteases is crucial in physiological and pathological cell invasion alike. In vitro, degradation occurs at specific sites where invasive cells make contact with the ECM via specialized plasma membrane protrusions termed invadopodia. Here we present an extensive morpho-functional analysis of invadopodia actively engaged in ECM degradation and show that they are actin comet-based structures, not unlike the well-known bacteria-propelling actin tails. The relative mapping of the basic molecular components of invadopodia to actin tails is also provided. Finally, a live-imaging analysis of invadopodia highlights the intrinsic long-term stability of the structures coupled to a highly dynamic actin turnover. The results offer new insight into the tight coordination between signalling, actin remodelling and trafficking activities occurring at sites of focalized ECM degradation by invadopodia. In conclusion, invadopodia-associated actin comets are a striking example of consistently arising, spontaneous expression of actin-driven propulsion events that also represent a valuable experimental paradigm.

African swine fever virus induces filopodia-like projections at the plasma membrane

When exiting the cell vaccinia virus induces actin polymerization and formation of a characteristic actin tail on the cytosolic face of the plasma membrane, directly beneath the extracellular particle. The actin tail acts to propel the virus away from the cell surface to enhance its cell-to-cell spread. We now demonstrate that African swine fever virus (ASFV), a member of the Asfarviridae family, also stimulates the polymerization of actin at the cell surface. Intracellular ASFV particles project out at the tip of long filopodia-like protrusions, at an average rate of 1.8 microm min(-1). Actin was arranged in long unbranched parallel arrays inside these virus-tipped projections. In contrast to vaccinia, this outward movement did not involve recruitment of Grb2, Nck1 or N-WASP. Actin polymerization was not nucleated by virus particles in transit to the cell periphery, and projections were not produced when the secretory pathway was disrupted by brefeldin A treatment. Our results show that when ASFV particles reach the plasma membrane they induce a localized nucleation of actin, and that this process requires interaction with virus-encoded and/or host proteins at the plasma membrane. We suggest that ASFV represents a valuable new model for studying pathways that regulate the formation of filopodia.

A superhighway to virus infection

Microtubule-mediated transport of macromolecules and organelles (also known as "cargo") is essential for cells to function. Deficiencies in cytoplasmic transport are frequently associated with severe diseases and syndromes. Cytoplasmic transport also provides viruses with the means to reach their site of replication and is the route for newly assembled progeny to leave the infected cell. This parasitic relationship of viruses with the host cytoskeleton provides an excellent basis for cell biologists to unlock the secrets of cytoplasmic transport and unravel mechanisms of disease. Recent advances in live cell imaging and computational tracking of fluorescently labeled viruses are now revealing how complex the movements of single viruses are in infected cells. This review focuses on microtubule-based motility of viruses and highlights the mechanisms regulating cytoplasmic transport.

The longest micron; transporting poxviruses out of the cell

The large size of poxvirus virions (approximately 250-300 microm) makes them dependent on active transport for intracellular movement during infection. Several recent papers have reported the utilization of the microtubule network by poxviruses during viral egress and their use of conventional kinesin for intracellular transport. This review looks at recent reports of poxvirus intracellular transport for virion egress and their interaction with the microtubule network.

Abl kinases regulate actin comet tail elongation via an N-WASP-dependent pathway

Microbial pathogens have evolved diverse strategies to modulate the host cell cytoskeleton to achieve a productive infection and have proven instrumental for unraveling the molecular machinery that regulates actin polymerization. Here we uncover a mechanism for Shigella flexneri-induced actin comet tail elongation that links Abl family kinases to N-WASP-dependent actin polymerization. We show that the Abl kinases are required for Shigella actin comet tail formation, maximal intracellular motility, and cell-to-cell spread. Abl phosphorylates N-WASP, a host cell protein required for actin comet tail formation, and mutation of the Abl phosphorylation sites on N-WASP impairs comet tail elongation. Furthermore, we show that defective comet tail formation in cells lacking Abl kinases is rescued by activated forms of N-WASP. These data demonstrate for the first time that the Abl kinases play a role in the intracellular motility and intercellular dissemination of Shigella and uncover a new role for Abl kinases in the regulation of pathogen motility.

Wiskott-Aldrich syndrome protein is needed for vaccinia virus pathogenesis

Smallpox, caused by variola virus, was a devastating disease in humans, but how the virus evolved a strategy to spread to tissue remains unknown. Through the use of microarrays, we identified the gene encoding the Wiskott-Aldrich syndrome protein (WASP), one of the five known WASP family members, which has been induced in the course of infection of human cells with different strains of vaccinia virus (VV) (S. Guerra, L. A. Lopez-Fernandez, A. Pascual-Montano, M. Munoz, K. Harshman, and M. Esteban, J. Virol. 77:6493-6506, 2003; S. Guerra, L. A. Lopez-Fernandez, R. Conde, A. Pascual-Montano, K. Harshman, and M. Esteban, J. Virol. 78:5820-5834, 2004). In a mouse model, we evaluated the role of WASP in infection with VV, a close relative of variola virus. WASP(-/-) (KO) mice infected intranasally and intraperitoneally with VV showed reduced weight loss and mortality compared to wild-type (WT) mice. WASP expression correlated with VV replication in the ovaries but not in the liver or spleen. WT mouse macrophages express WASP but not N-WASP; after VV infection, WASP levels increase threefold. KO macrophages lack N-WASP expression and, when VV infected, are incapable of inducing actin tails and producing extracellular virus. These functions were rescued in KO macrophages after ectopic WASP expression. Overall, our findings demonstrate that WASP has a role in orthopoxvirus infections. Use of WASP proteins for virus spread via the actin tail provides a selective advantage for VV, and probably variola virus, dissemination to distant tissues.

Cortactin: an Achilles' heel of the actin cytoskeleton targeted by pathogens

Cortactin is an actin-binding protein and a central regulator of the actin cytoskeleton. Importantly, cortactin is also a common target exploited by microbes during infection. Its involvement in disease development is exemplified by a variety of pathogenic processes, such as pedestal formation [enteropathogenic and enterohaemorrhagic Escherichia coli (EPEC and EHEC)], invasion (Shigella, Neisseria, Rickettsia, Chlamydia, Staphylococcus and Cryptosporidium), actin-based motility (Listeria, Shigella and vaccinia virus) and cell scattering (Helicobacter). Recent progress turns our attention to how cortactin function can be regulated by serine and tyrosine phosphorylation. This has an important impact on how pathogens abuse cortactin to modulate the architecture of the host actin cytoskeleton.

Cortactin phosphorylation as a switch for actin cytoskeletal network and cell dynamics control

Cortactin is an important molecular scaffold for actin assembly and organization. Novel mechanistic functions of cortactin have emerged with more interacting partners identified, revealing its multifaceted roles in regulating actin cytoskeletal networks that are necessary for endocytosis, cell migration and invasion, adhesion, synaptic organization and cell morphogenesis. These processes are mediated by its multi-domains binding to F-actin and Arp2/3 complex and various SH3 targets. Furthermore, its role in actin remodeling is subjected to regulation by tyrosine and serine/threonine kinases. Elucidating the mechanisms underlying cortactin phosphorylation and its functional consequences would provide new insights to various aspects of cell dynamics control.

Cortactin signalling and dynamic actin networks

Cortactin was first identified over a decade ago, and its initial characterization as both an F-actin binding protein and v-Src substrate suggested that it was likely to be a key regulator of actin rearrangements in response to tyrosine kinase signalling. The recent discovery that cortactin binds and activates the actin related protein (Arp)2/3 complex, and thus regulates the formation of branched actin networks, together with the identification of multiple protein targets of the cortactin SH3 domain, have revealed diverse cellular roles for this protein. This article reviews current knowledge regarding the role of cortactin in signalling to the actin cytoskeleton in the context of these developments.

Identification of a bacterial factor required for actin-based motility of Burkholderia pseudomallei

Burkholderia pseudomallei is a Gram-negative facultative intracellular pathogen that enters and escapes from eukaryotic cells using the power of actin polymerization. We have identified a bacterial protein (BimA) that is required for the ability of B. pseudomallei to induce the formation of actin tails. BimA contains proline-rich motifs and WH2-like domains and shares limited homology at the C-terminus with the Yersinia autosecreted adhesin YadA. BimA is located at the pole of the bacterial cell at which actin polymerization occurs and mutation of bimA abolished actin-based motility of the pathogen in J774.2 cells. Transient expression of BimA in HeLa cells resulted in F-actin clustering reminiscent of that seen on WASP overexpression. Antibody-mediated clustering of a CD32 chimera in which the cytoplasmic domain was replaced with BimA resulted in localization of the chimera to the tips of F-actin enriched membrane protrusions. We report that purified truncated BimA protein binds monomeric actin in a concentration-dependent manner in cosedimentation assays and that BimA stimulates actin polymerization in vitro in a manner independent of the cellular Arp2/3 complex.

RhoB and actin polymerization coordinate Src activation with endosome-mediated delivery to the membrane

We have used a c-Src-GFP fusion protein to address the spatial control of Src activation and the nature of Src-associated intracellular structures during stimulus-induced transit to the membrane. Src is activated during transit, particularly in RhoB-containing cytoplasmic endosomes associated with the perinuclear recycling compartment. Knocking out RhoB or expressing a dominant-interfering Rab11 mutant suppresses both catalytic activation of Src and translocation of active kinase to peripheral membrane structures. In addition, the Src- and RhoB-containing endosomes harbor proteins involved in actin polymerization and filament assembly, for example Scar1, and newly polymerized actin can associate with these endosomes in a Src-dependent manner. This implies that Src may regulate an endosome-associated actin nucleation activity. In keeping with this, Src controls the actin dependence of RhoB endosome movement toward the plasma membrane. This work identifies RhoB as a component of "outside-in" signaling pathways that coordinate Src activation with translocation to transmembrane receptors.

Actin dynamics coupled to clathrin-coated vesicle formation at the trans-Golgi network

In diverse species, actin assembly facilitates clathrin-coated vesicle (CCV) formation during endocytosis. This role might be an adaptation specific to the unique environment at the cell cortex, or it might be fundamental, facilitating CCV formation on different membranes. Proteins of the Sla2p/Hip1R family bind to actin and clathrin at endocytic sites in yeast and mammals. We hypothesized that Hip1R might also coordinate actin assembly with clathrin budding at the trans-Golgi network (TGN). Using deconvolution and time-lapse microscopy, we showed that Hip1R is present on CCVs emerging from the TGN. These vesicles contain the mannose 6-phosphate receptor involved in targeting proteins to the lysosome, and the actin nucleating Arp2/3 complex. Silencing of Hip1R expression by RNAi resulted in disruption of Golgi organization and accumulation of F-actin structures associated with CCVs on the TGN. Hip1R silencing and actin poisons slowed cathepsin D exit from the TGN. These studies establish roles for Hip1R and actin in CCV budding from the TGN for lysosome biogenesis.

Erk/Src phosphorylation of cortactin acts as a switch on-switch off mechanism that controls its ability to activate N-WASP

The Arp2/3 complex can be independently activated to initiate actin polymerization by the VCA domain of WASP family members and by the acidic N-terminal and F-actin-binding repeat region of cortactin, which possesses a C-terminal SH3 domain. Cortactin is a target for phosphorylation by Src tyrosine kinases and by serine/threonine kinases that include Erk. Here we demonstrate that cortactin binds N-WASP and WASP via its SH3 domain, induces in vitro N-WASP-mediated actin polymerization, and colocalizes with N-WASP and WASP at sites of active actin polymerization. Erk phosphorylation and a mimicking S405,418D double mutation enhanced cortactin binding and activation of N-WASP. In contrast, Src phosphorylation inhibited the ability of cortactin previously phosphorylated by Erk, and that of S405,418D double mutant cortactin, to bind and activate N-WASP. Furthermore, Y-->D mutation of three tyrosine residues targeted by Src (Y421, Y466, and Y482) inhibited the ability of S405,418D cortactin to activate N-WASP. We propose that Erk phosphorylation liberates the SH3 domain of cortactin from intramolecular interactions with proline-rich regions, causing it to synergize with WASP and N-WASP in activating the Arp2/3 complex, and that Src phosphorylation terminates cortactin activation of N-WASP and WASP.

A dynamic podosome-like structure of epithelial cells

Focal contacts and hemidesmosomes are cell-matrix adhesion structures of cultured epithelial cells. While focal contacts link the extracellular matrix to microfilaments, hemidesmosomes make connections with intermediate filaments. We have analyzed hemidesmosome assembly in 804G carcinoma cells. Our data show that hemidesmosomes are organized around a core of actin filaments that appears early during cell adhesion. These actin structures look similar to podosomes described in cells of mesenchymal origin. These podosome-like structures are distinct from focal contacts and specifically contain Arp3 (Arp2/3 complex), cortactin, dynamin, gelsolin, N-WASP, VASP, Grb2 and src-like kinase(s). The integrin alpha3beta1 is localized circularly around F-actin cores and co-distributes with paxillin, vinculin, and zyxin. We also show that the maintenance of the actin core and hemidesmosomes is dependent on actin polymerization, src-family kinases, and Grb2, but not on microtubules. Video microscopy analysis reveals that assembly of hemidesmosomes is preceded by recruitment of beta4 integrin subunit to the actin core before its positioning at hemidesmosomes. When 804G cells are induced to migrate, actin cores as well as hemidesmosomes disappear and beta4 integrin subunit becomes co-localized with dynamic actin at leading edges. We show that podosome-like structures are not unique to cells of mesenchymal origin, but also appear in epithelial cells, where they seem to be related to basement membrane adhesion.

Inducible clustering of membrane-targeted SH3 domains of the adaptor protein Nck triggers localized actin polymerization

BACKGROUND

SH2/SH3 adaptor proteins play a critical role in tyrosine kinase signaling pathways, regulating essential cell functions by increasing the local concentration or altering the subcellular localization of downstream effectors. The SH2 domain of the Nck adaptor can bind tyrosine-phosphorylated proteins, while its SH3 domains can modulate actin polymerization by interacting with effectors such as WASp/Scar family proteins. Although several studies have implicated Nck in regulating actin polymerization, its role in living cells is not well understood.

RESULTS

We used an antibody-based system to experimentally modulate the local concentration of Nck SH3 domains on the plasma membrane of living cells. Clustering of fusion proteins containing all three Nck SH3 domains induced localized polymerization of actin, including the formation of actin tails and spots, accompanied by general cytoskeletal rearrangements. All three Nck SH3 domains were required, as clustering of individual SH3 domains or a combination of the two N-terminal Nck SH3 domains failed to promote significant local polymerization of actin in vivo. Changes in actin dynamics induced by Nck SH3 domain clustering required the recruitment of N-WASp, but not WAVE1, and were unaffected by downregulation of Cdc42.

CONCLUSIONS

We show that high local concentrations of Nck SH3 domains are sufficient to stimulate localized, Cdc42-independent actin polymerization in living cells. This study provides strong evidence of a pivotal role for Nck in directly coupling ligand-induced tyrosine phosphorylation at the plasma membrane to localized changes in organization of the actin cytoskeleton through a signaling pathway that requires N-WASp.

Vaccinia virus motility

Vaccinia virus (VV), the virus smallpox vaccine, replicates in the cytoplasm of infected cells. The intracellular movement of this large virus would be inefficient without specific transport mechanisms; therefore, VV uses microtubules for movement during both entry and egress. In addition, the dissemination of virus from infected cells to adjacent cells is promoted by the polymerization of actin beneath cell surface virions to drive virus particles away from the cell. Last, the roles of different VV particles in virus movement within and between hosts are discussed.

Annexin 2 binding to phosphatidylinositol 4,5-bisphosphate on endocytic vesicles is regulated by the stress response pathway

Annexin 2 is a Ca(2+)-binding protein that has an essential role in actin-dependent macropinosome motility. We show here that macropinosome rocketing can be induced by hyperosmotic shock, either alone or synergistically when combined with phorbol ester or pervanadate. Rocketing was blocked by inhibitors of phosphatidylinositol-3-kinase(s), p38 mitogen-activated protein (MAP) kinase, and calcium, suggesting the involvement of phosphoinositide signaling. Since various phosphoinositides are enriched on inwardly mobile vesicles, we examined whether or not annexin 2 binds to any of this class of phospholipid. In liposome sedimentation assays, we show that recombinant annexin 2 binds to phosphatidylinositol 4,5-bisphosphate (PtdIns-4,5P(2)) but not to other poly- and mono-phosphoinositides. The affinity of annexin 2 for PtdIns-4,5P(2) (K(D) approximately 5 microm) is comparable with those reported for a variety of PtdIns-4,5P(2)-binding proteins and is enhanced in the presence of Ca(2+). Although annexin 1 also bound to PtdIns-4,5P(2), annexin 5 did not, indicating that this is not a generic annexin property. To test whether annexin 2 binds to PtdIns-4,5P(2) in vivo, we microinjected rat basophilic leukemia cells stably expressing annexin 2-green fluorescent protein (GFP) with fluorescently tagged antibodies to PtdIns-4,5P(2). Annexin 2-GFP and anti-PtdIns-4,5P(2) IgG co-localize at sites of pinosome formation, and annexin 2-GFP relocalizes to intracellular membranes in Ptk cells microinjected with Arf6Q67L, which has been shown to stimulate PtdIns-4,5P(2) synthesis on pinosomes through activation of phosphatidylinositol 5 kinase. These results establish a novel phospholipid-binding specificity for annexin 2 consistent with a role in mediating the interaction between the macropinosome surface and the polymerized actin tail.

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.

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.

Cellular gene expression survey of vaccinia virus infection of human HeLa cells

Vaccinia virus (VV) is a cytocidal virus that causes major changes in host cell machinery shortly after infecting cells. To define the consequences of virus infection on host gene expression, we used microarrays of approximately 15,000 human cDNAs to examine expression levels of mRNAs isolated at 2, 6, and 16 h postinfection from cultures of infected HeLa cells. The majority of profiling changes during VV infection corresponded to downregulation of genes at 16 h postinfection. Differentially expressed genes were clustered into seven groups to identify common regulatory pathways, with most of them (90%) belonging to clusters 6 and 7, which represent genes whose expression was repressed after infection. Cluster 1, however, contained 37 transcripts (2.81%) showing a robust pattern of induction that was maintained during the course of infection. Genes in cluster 1 included those for Wiskott-Aldrich syndrome protein (WASP) family member WASF1, thymosine, adenosine A2a receptor, glutamate decarboxylase 2, CD-80 antigen, KIAA0888 protein, selenophosphate synthetase, pericentrin, and attractin as well as several expressed sequence tags. We analyzed in more detail the fate of WASP protein in VV-infected cells, because a related family member, N-WASP, is involved in viral motility. WASP protein accumulated in the course of infection; its increase required viral DNA replication and de novo protein synthesis, and it localized in cytoplasmic structures distinct from uninfected cells. This study is the first quantitative analysis of host gene expression following VV infection of cultured human cells, demonstrating global changes in the expression profile, and identifies upregulated genes with potential roles in the virus replication cycle.

Actin-based motility of Burkholderia pseudomallei involves the Arp 2/3 complex, but not N-WASP and Ena/VASP proteins

The facultative intracellular bacterium Burkholderia pseudomallei induces actin rearrangement within infected host cells leading to formation of actin tails and membrane protrusions. To investigate the underlying mechanism we analysed the contribution of cytoskeletal proteins to B. pseudomallei-induced actin tail assembly. By using green fluorescent protein (GFP)-fusion constructs, the recruitment of the Arp2/3 complex, vasodilator-stimulated phosphoprotein (VASP), Neural Wiskott-Aldrich syndrome protein (N-WASP), zyxin, vinculin, paxillin and alpha-actinin to the surface of B. pseudomallei and into corresponding actin tails was studied. In addition, antibodies against the same panel of proteins were used for immunolocalization. Whereas the Arp2/3 complex and alpha-actinin were incorporated into B. pseudomallei-induced actin tails, none of the other proteins were detected in these structures. The overexpression of an Arp2/3 binding fragment of the Scar1 protein, shown previously to block actin-based motility of Listeria, had no effect on B. pseudomallei tail formation. Infections of either N-WASP- or Ena/VASP-defective cells showed that these proteins are not essential for B. pseudomallei-induced actin polymerization. In conclusion, our results suggest that B. pseudomallei induces actin polymerization through a mechanism that differs from those evolved by Listeria, Shigella, Rickettsia or vaccinia virus.