SRC mediates a switch from microtubule- to actin-based motility of vaccinia virus

Virus trafficking - learning from single-virus tracking

What could be a better way to study virus trafficking than 'miniaturizing oneself' and 'taking a ride with the virus particle' on its journey into the cell? Single-virus tracking in living cells potentially provides us with the means to visualize the virus journey. This approach allows us to follow the fate of individual virus particles and monitor dynamic interactions between viruses and cellular structures, revealing previously unobservable infection steps. The entry, trafficking and egress mechanisms of various animal viruses have been elucidated using this method. The combination of single-virus trafficking with systems approaches and state-of-the-art imaging technologies should prove exciting in the future.

Slit Diaphragm Junctional Complex and Regulation of the Cytoskeleton

In recent years, identification of proteins found at the slit diaphragm has greatly improved our understanding of the molecular mechanisms responsible for forming and maintaining the glomerular filtration barrier. This review examines the function of proteins identified at the podocyte intercellular junction in coordinating podocyte intercellular junction dynamics or cytoskeletal dynamics.

Porcine reproductive and respiratory syndrome virus (PRRSV) infection spreads by cell-to-cell transfer in cultured MARC-145 cells, is dependent on an intact cytoskeleton, and is suppressed by drug-targeting of cell permissiveness to virus infection

Background

Porcine reproductive and respiratory syndrome virus (PRRSV) is the etiologic agent of PRRS, causing widespread chronic infections which are largely uncontrolled by currently available vaccines or other antiviral measures. Cultured monkey kidney (MARC-145) cells provide an important tool for the study of PRRSV replication. For the present study, flow cytometric and fluorescence antibody (FA) analyses of PRRSV infection of cultured MARC-145 cells were carried out in experiments designed to clarify viral dynamics and the mechanism of viral spread. The roles of viral permissiveness and the cytoskeleton in PRRSV infection and transmission were examined in conjunction with antiviral and cytotoxic drugs.

Results

Flow cytometric and FA analyses of PRRSV antigen expression revealed distinct primary and secondary phases of MARC-145 cell infection. PRRSV antigen was randomly expressed in a few percent of cells during the primary phase of infection (up to about 20–22 h p.i.), but the logarithmic infection phase (days 2–3 p.i.), was characterized by secondary spread to clusters of infected cells. The formation of secondary clusters of PRRSV-infected cells preceded the development of CPE in MARC-145 cells, and both primary and secondary PRRSV infection were inhibited by colchicine and cytochalasin D, demonstrating a critical role of the cytoskeleton in viral permissiveness as well as cell-to-cell transmission from a subpopulation of cells permissive for free virus to secondary targets. Cellular expression of actin also appeared to correlate with PRRSV resistance, suggesting a second role of the actin cytoskeleton as a potential barrier to cell-to-cell transmission. PRRSV infection and cell-to-cell transmission were efficiently suppressed by interferon-γ (IFN-γ), as well as the more-potent experimental antiviral agent AK-2.

Conclusion

The results demonstrate two distinct mechanisms of PRRSV infection: primary infection of a relatively small subpopulation of innately PRRSV-permissive cells, and secondary cell-to-cell transmission to contiguous cells which appear non-permissive to free virus. The results also indicate that an intact cytoskeleton is critical for PRRSV infection, and that viral permissiveness is a highly efficient drug target to control PRRSV infection. The data from this experimental system have important implications for the mechanisms of PRRSV persistence and pathology, as well as for a better understanding of arterivirus regulation.

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.

Kidins220/ARMS is transported by a kinesin-1-based mechanism likely to be involved in neuronal differentiation

Kinase D-interacting substrate of 220 kDa/ankyrin repeat-rich membrane spanning (Kidins220/ARMS) is a conserved membrane protein mainly expressed in brain and neuroendocrine cells, which is a downstream target of the signaling cascades initiated by neurotrophins and ephrins. We identified kinesin light chain 1 (KLC1) as a binding partner for Kidins220/ARMS by a yeast two-hybrid screen. The interaction between Kidins220/ARMS and the kinesin-1 motor complex was confirmed by glutathione S-transferase-pull-down and coimmunoprecipitation experiments. In addition, Kidins220/ARMS and kinesin-1 were shown to colocalize in nerve growth factor (NGF)-differentiated PC12 cells. Using Kidins220/ARMS and KLC1 mutants, we mapped the regions responsible for the binding to a short sequence of Kidins220/ARMS, termed KLC-interacting motif (KIM), which is sufficient for the interaction with KLC1. Optimal binding of KIM requires a region of KLC1 spanning both the tetratricopeptide repeats and the heptad repeats, previously not involved in cargo recognition. Overexpression of KIM in differentiating PC12 cells impairs the formation and transport of EGFP-Kidins220/ARMS carriers to the tips of growing neurites, leaving other kinesin-1 dependent processes unaffected. Furthermore, KIM overexpression interferes with the activation of the mitogen-activated protein kinase signaling and neurite outgrowth in NGF-treated PC12 cells. Our results suggest that Kidins220/ARMS-positive carriers undergo a kinesin-1-dependent transport linked to neurotrophin action.

The spread of Rice dwarf virus among cells of its insect vector exploits virus-induced tubular structures

Various cytopathological structures, known as inclusion bodies, are formed upon infection of cultured leafhopper cells by Rice dwarf virus, a member of the family Reoviridae. These structures include tubules of approximately 85 nm in diameter which are composed of the nonstructural viral protein Pns10 and contain viral particles. Such tubular structures were produced in heterologous non-host insect cells that expressed Pns10 of the virus. These tubules, when associated with actin-based filopodia, were able to protrude from the surface of cells and to penetrate neighboring cells. A binding assay in vitro revealed the specific binding of Pns10 to actin. Infection of clusters of cells was readily apparent 5 days after inoculation at a low multiplicity of infection with the virus, even in the presence of neutralizing antibodies. However, treatment of host cells with drugs that inhibited the elongation of actin filaments abolished the extension of Pns10 tubules from the surface of cells, with a significant simultaneous decrease in the extent of infection of neighboring cells. These results together revealed a previously undescribed aspect of the intercellular spread of Rice dwarf virus, wherein the virus exploits tubules composed of a nonstructural viral protein and actin-based filopodia to move into neighboring cells.

Interaction between vaccinia virus extracellular virus envelope A33 and B5 glycoproteins

The extracellular form of vaccinia virus acquires its outer envelope by wrapping with cytoplasmic membranes that contain at least seven virus-encoded proteins, of which four are glycoproteins. We searched for interactions between the vaccinia virus A33 glycoprotein and proteins A34, A36, B5, F12, and F13. First, when myc epitope-tagged A33 was expressed in combination with other envelope proteins, A33 colocalized with B5 and A36, suggesting that direct A33-B5 and A33-A36 interactions occur in the absence of infection. A recombinant vaccinia virus (vA33Rmyc) was constructed by introduction of the myc-tagged A33 version (A33myc) into A33-deficient vaccinia virus. A33myc partially restored plaque formation and colocalized with enveloped virions in infected cells. Coimmunoprecipitation experiments with extracts of vA33Rmyc-infected cells confirmed the existence of a physical association of A33 with A36 and B5. Of these, the A33-B5 interaction is a novel finding, whereas the interaction between A33 and A36 has been previously characterized. A collection of vaccinia viruses expressing mutated versions of the B5 protein was used to investigate the domain(s) of B5 required for interaction with A33. Both the cytoplasmic domain and most of the extracellular domain, but not the transmembrane domain, of the B5 protein were dispensable for binding to A33. Mutations in the extracellular portions of B5 and A33 that enhance extracellular virus release did not affect the interaction between the two. In contrast, substituting the B5 transmembrane domain with that of the vesicular stomatitis virus G glycoprotein prevented the association with A33. Immunofluorescence experiments on virus mutants indicated that B5 is required for efficient targeting of A33 into enveloped virions. These results point to the transmembrane domain of B5 as the major determinant of the A33-B5 interaction and demonstrate that protein-protein interactions are crucial in determining the composition of the virus envelope.

LC16m8, a highly attenuated vaccinia virus vaccine lacking expression of the membrane protein B5R, protects monkeys from monkeypox

The potential threat of smallpox as a bioweapon has led to the production and stockpiling of smallpox vaccine in some countries. Human monkeypox, a rare but important viral zoonosis endemic to central and western Africa, has recently emerged in the United States. Thus, even though smallpox has been eradicated, a vaccinia virus vaccine that can induce protective immunity against smallpox and monkeypox is still invaluable. The ability of the highly attenuated vaccinia virus vaccine strain LC16m8, with a mutation in the important immunogenic membrane protein B5R, to induce protective immunity against monkeypox in nonhuman primates was evaluated in comparison with the parental Lister strain. Monkeys were immunized with LC16m8 or Lister and then infected intranasally or subcutaneously with monkeypox virus strain Liberia or Zr-599, respectively. Immunized monkeys showed no symptoms of monkeypox in the intranasal-inoculation model, while nonimmunized controls showed typical symptoms. In the subcutaneous-inoculation model, monkeys immunized with LC16m8 showed no symptoms of monkeypox except for a mild ulcer at the site of monkeypox virus inoculation, and those immunized with Lister showed no symptoms of monkeypox, while nonimmunized controls showed lethal and typical symptoms. These results indicate that LC16m8 prevents lethal monkeypox in monkeys, and they suggest that LC16m8 may induce protective immunity against smallpox.

Aggresomes and autophagy generate sites for virus replication

The replication of many viruses is associated with specific intracellular compartments called virus factories or virioplasm. These are thought to provide a physical scaffold to concentrate viral components and thereby increase the efficiency of replication. The formation of virus replication sites often results in rearrangement of cellular membranes and reorganization of the cytoskeleton. Similar rearrangements are seen in cells in response to protein aggregation, where aggresomes and autophagosomes are produced to facilitate protein degradation. Here I review the evidence that some viruses induce aggresomes and autophagosomes to generate sites of replication.

Signaling During Pathogen Infection

Over the millennia, pathogens have coevolved with their hosts and acquired the ability to intercept, disrupt, mimic, and usurp numerous signaling pathways of those hosts. The study of host/pathogen interactions thus not only teaches us about the intricate biology of these parasitic invaders but also provides interesting insights into basic cellular processes both at the level of the individual cell and more globally throughout the organism. Host/pathogen relationships also provide insights into the evolutionary forces that shape biological diversity. Here we review a few recent examples of how viruses, bacteria, and parasites manipulate tyrosine kinase-mediated and Rho guanosine triphosphatase-mediated signaling pathways of their hosts to achieve efficient entry, replication, and exit during their infectious cycles.

Viral interactions with the cytoskeleton: a hitchhiker's guide to the cell

The actin and microtubule cytoskeleton play important roles in the life cycle of every virus. During attachment, internalization, endocytosis, nuclear targeting, transcription, replication, transport of progeny subviral particles, assembly, exocytosis, or cell-to-cell spread, viruses make use of different cellular cues and signals to enlist the cytoskeleton for their mission. Viruses induce rearrangements of cytoskeletal filaments so that they can utilize them as tracks or shove them aside when they represent barriers. Viral particles recruit molecular motors in order to hitchhike rides to different subcellular sites which provide the proper molecular environment for uncoating, replicating and packaging viral genomes. Interactions between subviral components and cytoskeletal tracks also help to orchestrate virus assembly, release and efficient cell-to-cell spread. There is probably not a single virus that does not use cytoskeletal and motor functions in its life cycle. Being well informed intracellular passengers, viruses provide us with unique tools to decipher how a particular cargo recruits one or several motors, how these are activated or tuned down depending on transport needs, and how cargoes switch from actin tracks to microtubules to nuclear pores and back.

Exploiting pathogenic Escherichia coli to model transmembrane receptor signalling

Many microbial pathogens manipulate the actin cytoskeleton of eukaryotic target cells to promote their internalization, intracellular motility and dissemination. Enteropathogenic and enterohaemorrhagic Escherichia coli, which both cause severe diarrhoeal disease, can adhere to mammalian intestinal cells and induce reorganization of the actin cytoskeleton into 'pedestal-like' pseudopods beneath the extracellular bacteria. As pedestal assembly is triggered by E. coli virulence factors that mimic several host cell-signalling components, such as transmembrane receptors, their cognate ligands and cytoplasmic adaptor proteins, it can serve as a powerful model system to study eukaryotic transmembrane signalling. Here, we consider the impact of recent data on our understanding of both E. coli pathogenesis and cell biology, and the rich prospects for exploiting these bacterial factors as versatile tools to probe cellular signalling pathways.

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.

Src-dependent phosphorylation of Scar1 promotes its association with the Arp2/3 complex

The WAVE/Scar proteins regulate actin polymerisation at the leading edge of motile cells via activation of the Arp2/3 complex in response to extracellular cues. Within cells they form part of a pentameric complex that is thought to regulate their ability to interact and activate the Arp2/3 complex. However, the exact mechanism for this is not known. We set out to assess whether phosphorylation of Scar1 by the non-receptor tyrosine kinase Src may influence the function of Scar1 and its ability to regulate Arp2/3-mediated actin polymerisation. We show that Scar1 is phosphorylated by Src in vitro and in vivo and identify tyrosine 125 as the major site in Scar1 to be phosphorylated in cells. Src-dependent phosphorylation of Scar1 on tyrosine 125 enhances its ability to bind to the Arp2/3 complex and regulates its ability to control actin polymerisation in cells. Thus, Src may act as an intermediary to regulate the activity of the Arp2/3 complex in response to external stimuli, via modulation of its interaction with WAVE/Scar proteins.

Vaccinia virus proteome: identification of proteins in vaccinia virus intracellular mature virion particles

Vaccinia virus is a large enveloped poxvirus with more than 200 genes in its genome. Although many poxvirus genomes have been sequenced, knowledge of the host and viral protein components of the virions remains incomplete. In this study, we used gel-free liquid chromatography and tandem mass spectroscopy to identify the viral and host proteins in purified vaccinia intracellular mature virions (IMV). Analysis of the proteins in the IMV showed that it contains 75 viral proteins, including structural proteins, enzymes, transcription factors, and predicted viral proteins not known to be expressed or present in the IMV. We also determined the relative abundances of the individual protein components in the IMV. Finally, 23 IMV-associated host proteins were also identified. This study provides the first comprehensive structural analysis of the infectious vaccinia virus IMV.

Abl collaborates with Src family kinases to stimulate actin-based motility of vaccinia virus

Local activation of Src at the plasma membrane by extracellular vaccinia virus results in a signalling cascade that acts to stimulate actin polymerization beneath the virus to enhance its cell-to-cell spread. Initiation of this signalling cascade involves Src-mediated phosphorylation of tyrosine 112 and 132 of the viral membrane protein A36R. Here we show that recruitment of Src is dependent on its myristoylation and an interaction with A36R upstream of tyrosine 112 and 132. We further show that Src, Fyn and Yes have unique specificities towards these tyrosine residues. Using cell lines deficient in Src, Fyn and Yes, we demonstrate that multiple Src family members can stimulate vaccinia-induced actin polymerization and also uncover a role for Abl family kinases. Additionally, Abl and Arg are able to phosphorylate A36R in vitro and are recruited to vaccinia-induced actin tails. The ability of multiple families of tyrosine kinases to directly phosphorylate A36R ensures robust cell-to-cell spread of vaccinia virus will occur under a variety of cellular conditions.

Molecular motors hijacking by intracellular pathogens

Cargoes are transported intracellularly along cytoskeletal tracks composed of actin or tubulin. Their movement involves the action of molecular motor proteins that generate directed movement along microtubules or actin filaments. The three classes of molecular motors--kinesins, dyneins and myosins--are involved in a multiplicity of biological movements such as mitosis, positioning of organelles, intracellular transports and also vesicular sorting through membrane tubulation and fission and delivery to their target compartment. Intracellular pathogens use this molecular machinery to reach their site of replication, to leave their host or to control the dynamics of membrane exchanges with their replication compartment.

Tyrosine-phosphorylated bacterial effector proteins: the enemies within

The tyrosine phosphorylation of proteins has a central role during signal transduction in eukaryotes. Recent progress shows that tyrosine phosphorylation is also a common feature of several effector proteins translocated by bacterial type III and type IV secretion systems. The involvement of these secretion systems in disease development is exemplified by a variety of pathogenic processes: pedestal formation (Tir of EPEC and Citrobacter), cell scattering (CagA of Helicobacter), invasion (Tarp of Chlamydia) and possibly proinflammatory responses and cell proliferation (BepD-F of Bartonella). The discovery that different bacterial pathogens use this common strategy to subvert host-cell function suggests that more examples will soon emerge.

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.

Viral stop-and-go along microtubules: taking a ride with dynein and kinesins

Incoming viral particles move from the cell surface to sites of viral transcription and replication. By contrast, during assembly and egress, subviral nucleoprotein complexes and virions travel back to the plasma membrane. Because diffusion of large molecules is severely restricted in the cytoplasm, viruses use ATP-hydrolyzing molecular motors of the host for propelling along the microtubules, which are the intracellular highways. Recent studies have revealed that, besides travelling inside endocytic or exocytic vesicles, viral proteins interact directly with dynein or kinesin motors. Understanding the molecular mechanisms of cytoplasmic viral transport will aid in the construction of viral vectors for human gene therapy and the search for new antiviral targets.

The Src family kinase c-Yes is required for maturation of West Nile virus particles

The role of cellular genes in West Nile virus (WNV) replication is not well understood. Examination of cellular transcripts upregulated during WNV infection revealed an increase in the expression of the src family kinase (SFK) c-Yes. WNV-infected cell lines treated with the SFK inhibitor PP2 demonstrated a 2- to 4-log decrease in viral titers, suggesting that SFK activity is required for completion of the viral replication cycle. RNA interference mediated knock-down of c-Yes, but not c-Src, and similarly reduced virus yield, specifically implicating c-Yes in WNV production. Interestingly, PP2 treatment did not reduce intracellular levels of either viral RNA or protein, suggesting that the drug does not act on the early stages of replication. However, endoglycosidase H (endoH) digestion of the viral envelope (E) glycoprotein revealed that the acquisition of endoH-resistant glycans by E, but not endogenous major histocompatibility complex class I, was reduced in PP2-treated cells, demonstrating that E specifically does not traffic beyond the endoplasmic reticulum in the absence of SFK activity. Electron microscopy further revealed that PP2-treated WNV-infected cells accumulated an increased number of virions in the ER compared to untreated cells. Therefore, we conclude that inhibition of SFK activity did not interfere with virus assembly but prevented transit of virions through the secretory pathway. These results identify c-Yes as a cellular protein that is involved in WNV assembly and egress.

Viruses as probes for systems analysis of cellular signalling, cytoskeleton reorganization and endocytosis

It is well known that mammalian viruses hijack the cellular signalling and internalization machineries to enter and to infect their host cells; however, only in the past six years have researchers started to follow individual virus particles and to investigate the events that they induce in living cells. The relative ease of imaging individual virus particles with time-lapse microscopy, despite being limited by light-diffraction, allows for specific and local kinetic analysis of individual events in signalling, cytoskeleton reorganization and endocytosis. Furthermore, virus infection is an easy-to-use endpoint readout, which is ideally suited for functional genomics approaches. The combined information from these studies will be crucial for the development of models that describe the underlying systems of cellular signalling, cytoskeleton reorganization and membrane trafficking during virus entry.

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.

Disabling poxvirus pathogenesis by inhibition of Abl-family tyrosine kinases

The Poxviridae family members vaccinia and variola virus enter mammalian cells, replicate outside the nucleus and produce virions that travel to the cell surface along microtubules, fuse with the plasma membrane and egress from infected cells toward apposing cells on actin-filled membranous protrusions. We show that cell-associated enveloped virions (CEV) use Abl- and Src-family tyrosine kinases for actin motility, and that these kinases act in a redundant fashion, perhaps permitting motility in a greater range of cell types. Additionally, release of CEV from the cell requires Abl- but not Src-family tyrosine kinases, and is blocked by STI-571 (Gleevec), an Abl-family kinase inhibitor used to treat chronic myelogenous leukemia in humans. Finally, we show that STI-571 reduces viral dissemination by five orders of magnitude and promotes survival in infected mice, suggesting possible use for this drug in treating smallpox or complications associated with vaccination. This therapeutic approach may prove generally efficacious in treating microbial infections that rely on host tyrosine kinases, and, because the drug targets host but not viral molecules, this strategy is much less likely to engender resistance compared to conventional antimicrobial therapies.

Epitope-mapping studies define two major neutralization sites on the vaccinia virus extracellular enveloped virus glycoprotein B5R

Vaccinia extracellular enveloped virus (EEV) is critical for cell-to-cell and long-range virus spread both in vitro and in vivo. The B5R gene encodes an EEV-specific type I membrane protein that is essential for efficient EEV formation. The majority of the B5R ectodomain consists of four domains with homology to short consensus repeat domains followed by a stalk. Previous studies have shown that polyclonal antibodies raised against the B5R ectodomain inhibit EEV infection. In this study, our goal was to elucidate the antigenic structure of B5R and relate this to its function. To do this, we produced multimilligram quantities of vaccinia virus B5R as a soluble protein [B5R(275t)] using a baculovirus expression system. We then selected and characterized a panel of 26 monoclonal antibodies (MAbs) that recognize B5R(275t). Five of these MAbs neutralized EEV and inhibited comet formation. Two other MAbs were able only to neutralize EEV, while five others were able only to inhibit comet formation. This suggests that the EEV neutralization and comet inhibition assays measure different viral functions and that at least two different antigenic sites on B5R are important for these activities. We further characterized the MAbs and the antigenic structure of B5R(275t) by peptide mapping and by reciprocal MAb blocking studies using biosensor analysis. The epitopes recognized by neutralizing MAbs were localized to SCR1-SCR2 and/or the stalk of B5R(275t). Furthermore, the peptide and blocking data support the concept that SCR1 and the stalk may be in juxtaposition and may be part of the same functional domain.

Cytoskeletal rearrangements and cell extensions induced by the US3 kinase of an alphaherpesvirus are associated with enhanced spread

The US3 protein is a viral kinase that is conserved among the Alphaherpesvirinae. Here, we show that US3 of the swine alphaherpesvirus pseudorabies virus causes dramatic alterations in the cytoskeleton, resulting in the formation of long actin- and microtubule-containing cell projections in infected and transfected cells. Analysis with a GFP-labeled virus showed that multiple virus particles move inside the projections toward the tip. GFP-labeled virus could also be found in the cytoplasm of neighboring cells that were in contact with the projections. In addition, projection formation could be inhibited by using the actin-stabilizing drug jasplakinolide and could be induced by using the Rho kinase inhibitor Y27632. Analyzing the effect of these drugs on intercellular virus spread indicated that the observed US3-induced alterations in the host cytoskeleton are associated with enhanced intercellular virus spread, thereby suggesting a previously undescribed aspect of alphaherpesvirus spread.

Nef from pathogenic simian immunodeficiency virus is a negative factor for vaccinia virus

The nef gene of human and simian immunodeficiency viruses (HIV and SIV) is important for pathogenicity and maintenance of high virus loads. We previously reported that recombinant vaccinia viruses (rVVs) expressing nef from attenuated SIVmac1A11 (vNef1A11) produced typical plaques on thymidine kinase-deficient 143B cells, whereas rVVs expressing nef derived from the pathogenic SIVmac239 (vNef157) formed plaques with altered morphology. Here, we show that vNef157 is attenuated in normal and nude mice, whereas the pathogenicity of vNef1A11 is similar to that of a control virus. Thus, Nef157 is an attenuating factor in the vaccinia virus (VV) system, contrasting sharply with its function in lentiviruses. We also show that Nef157 inhibits VV cell-to-cell spread, causing formation of atypical plaques regardless of thymidine kinase deficiency, neoplasticity, and species of the infected cell line. We hypothesized that Nef157 interferes with VV spread by association with actin, but no direct colocalization of Nef and the cytoskeletal actin network was detected. Instead, higher levels of Nef157 protein were observed, although mRNAs for both nef genes were produced at comparable levels. Thus, the mechanism behind such Nef157 protein accumulation and Nef157-mediated VV attenuation could be related to the process that causes an opposite effect in its native SIV system, making SIVmac239 more pathogenic than SIVmac1A11.

Actin-based motility of intracellular pathogens

The actin cytoskeleton is harnessed by several pathogenic bacteria that are capable of entering into non-phagocytic cells, the so-called 'invasive bacteria'. Among them, a few also exploit the host actin cytoskeleton to move intra- and inter-cellularly. Our knowledge of the basic mechanisms underlying actin-based motility has dramatically increased and the list of bacteria that are able to move in this way is also increasing including not only Listeria, Shigella and Rickettsia species but also Mycobacterium marinum and Burkholderia pseudomallei. In all cases the central player is the Arp2/3 complex. Vaccinia virus moves intracellularly on microtubules and just after budding, triggers actin polymerization and the formation of protrusions similar to that of adherent enteropathogenic Escherichia coli, that involve the Arp2/3 complex and facilitate its inter-cellular spread.

Tyrosine kinase signaling and type III effectors orchestrating Shigella invasion

Upon epithelial cell contact, Shigella type III effectors activate complex signaling pathways that induce localized membrane ruffling, resulting in Shigella invasion. Bacterial induced membrane ruffles require a timely coordination of cytoskeletal processes, including actin polymerization, filament reorganization and depolymerization, orchestrated by Rho GTPases and tyrosine kinases. An emerging concept is that multiple Shigella effectors act in synergy to promote actin polymerization in membrane extensions at the site of bacterial entry. Recent advances point to the role of Abl/Arg and Src tyrosine kinases as key regulators of bacterial induced cytoskeletal dynamics.

Poxvirus tropism

Despite the success of the WHO-led smallpox eradication programme a quarter of a century ago, there remains considerable fear that variola virus, or other related pathogenic poxviruses such as monkeypox, could re-emerge and spread disease in the human population. Even today, we are still mostly ignorant about why most poxvirus infections of vertebrate hosts show strict species specificity, or how zoonotic poxvirus infections occur when poxviruses occasionally leap into novel host species. Poxvirus tropism at the cellular level seems to be regulated by intracellular events downstream of virus binding and entry, rather than at the level of specific host receptors as is the case for many other viruses. This review summarizes our current understanding of poxvirus tropism and host range, and discusses the prospects of exploiting host-restricted poxvirus vectors for vaccines, gene therapy or tissue-targeted oncolytic viral therapies for the treatment of human cancers.

Poxvirus host range varies markedly ? some viruses, such as variola and molluscum contagiosum virus (both of which are human-specific), exhibit strict species tropism, whereas others such as cowpox virus are able to infect multiple host species.

Members of four of the eight genera of chordopoxviruses can zoonotically infect man. For example, monkeypox virus can cause severe smallpox-like disease in humans that clinically resembles variola virus.

The species tropism that is exhibited by many poxviruses in terms of causing disease is frequently quite different from the range of cultured cells that can be infected by these viruses.

Specific host-cell receptors do not mediate the distinction between cells that are permissive as opposed to non-permissive for poxvirus infection. Rather, restrictive host cells fail to support the full replication cycle of the infecting poxvirus at a point downstream of binding and entry.

A variety of poxviral host-range genes have been identified that contribute to the control of permissive versus non-permissive infection of cultured mammalian cells. The gene products of these host-range genes regulate the ability of the virus to complete its cytoplasmic replication cycle.

The development of host-restricted vaccines, like modified vaccinia Ankara (MVA), that do not replicate in humans but that retain potent immunogenicity, will provide safer platforms for recombinant vaccines.

Another advance has been the development of poxvirus-based oncolytic vectors that replicate preferentially in human tumour cells.

Host-based antipoxvirus therapeutic strategies: turning the tables

The potential threat of the smallpox virus as a bioterror weapon has long been recognized, and the need for developing suitable countermeasures has become especially acute following the events of September 2001. Traditional antiviral agents interfere with viral proteins or functions. In a new study, Yang et al. focus instead on host cellular pathways used by the virus. A drug that interferes with the cellular ErbB-1 signal transduction pathway, activated by smallpox growth factor, sheds new light on how the virus replicates in the cell. Drugs that target the ErbB-signaling pathways represent a promising new class of antiviral agents.

Genetically stable and fully effective smallpox vaccine strain constructed from highly attenuated vaccinia LC16m8

A highly attenuated LC16m8 (m8) smallpox vaccine has been licensed in Japan because of its extremely low neurovirulence profile, which is comparable to that of replication incompetent strains of vaccinia virus. From 1973 to 1975, m8 was administrated to >100,000 infants where it induced levels of immunity similar to that of the originating Lister strain, without any serious side effects. Recently, we observed that m8 reverts spontaneously to large plaque forming clones that possess virulence equivalent to that of LC16mO, a parental virus strain of m8. Here, we report that the B5R gene is responsible for the reversion, and that we could construct a more genetically stable virus by deleting B5R from m8. The protective immunogenicity of the vaccine candidate proved to be equivalent to that of the U.S.-licensed product Dryvax, and much superior to modified vaccinia Ankara in a mouse model. Furthermore, the vaccine strain never elicited any symptoms in severe combined immunodeficiency disease mice, even at a dose 1,000-fold greater than that used in the immune protection experiments, which is in contrast to the lethal pathogenicity induced by Dryvax inoculation of severe combined immunodeficiency disease mice. Our results suggest that this vaccine strain is a good candidate as a suitable smallpox vaccine and a vector virus, and that B5R is not essential for protective immunity against smallpox.

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.