Vaccinia virus-induced microtubule-dependent cellular rearrangements

TRIM Proteins and Antiviral Microtubule Reorganization: A Novel Component in Innate Immune Responses?

TRIM proteins are a family of innate immune factors that play diverse roles in innate immunity and protect the cell against viral and bacterial aggression. As part of this special issue on TRIM proteins, we will take advantage of our findings on TRIM69, which acts by reorganizing the microtubules (MTs) in a manner that is fundamentally antiviral, to more generally discuss how host-pathogen interactions that take place for the control of the MT network represent a crucial facet of the struggle that opposes viruses to their cell environment. In this context, we will present several other TRIM proteins that are known to interact with microtubules in situations other than viral infection, and we will discuss evidence that may suggest a possible contribution to viral control. Overall, the present review will highlight the importance that the control of the microtubule network bears in host-pathogen interactions.

Poxvirus A51R Proteins Negatively Regulate Microtubule-Dependent Transport by Kinesin-1

Microtubule (MT)-dependent transport is a critical means of intracellular movement of cellular cargo by kinesin and dynein motors. MT-dependent transport is tightly regulated by cellular MT-associated proteins (MAPs) that directly bind to MTs and either promote or impede motor protein function. Viruses have been widely shown to usurp MT-dependent transport to facilitate their virion movement to sites of replication and/or for exit from the cell. However, it is unclear if viruses also negatively regulate MT-dependent transport. Using single-molecule motility and cellular transport assays, we show that the vaccinia virus (VV)-encoded MAP, A51R, inhibits kinesin-1-dependent transport along MTs in vitro and in cells. This inhibition is selective as the function of kinesin-3 is largely unaffected by VV A51R. Interestingly, we show that A51R promotes the perinuclear accumulation of cellular cargo transported by kinesin-1 such as lysosomes and mitochondria during infection. Moreover, A51R also regulates the release of specialized VV virions that exit the cell using kinesin-1-dependent movement. Using a fluorescently tagged rigor mutant of kinesin-1, we show that these motors accumulate on A51R-stabilized MTs, suggesting these stabilized MTs may form a "kinesin-1 sink" to regulate MT-dependent transport in the cell. Collectively, our findings uncover a new mechanism by which viruses regulate host cytoskeletal processes.

Poxvirus A51R proteins regulate microtubule stability and antagonize a cell-intrinsic antiviral response

Numerous viruses alter host microtubule (MT) networks during infection, but how and why they induce these changes is unclear in many cases. We show that the vaccinia virus (VV)-encoded A51R protein is a MT-associated protein (MAP) that directly binds MTs and stabilizes them by both promoting their growth and preventing their depolymerization. Furthermore, we demonstrate that A51R-MT interactions are conserved across A51R proteins from multiple poxvirus genera, and highly conserved, positively charged residues in A51R proteins mediate these interactions. Strikingly, we find that viruses encoding MT interaction-deficient A51R proteins fail to suppress a reactive oxygen species (ROS)-dependent antiviral response in macrophages that leads to a block in virion morphogenesis. Moreover, A51R-MT interactions are required for VV virulence in mice. Collectively, our data show that poxviral MAP-MT interactions overcome a cell-intrinsic antiviral ROS response in macrophages that would otherwise block virus morphogenesis and replication in animals.

The poxvirus F17 protein counteracts mitochondrially orchestrated antiviral responses

Poxviruses are unusual DNA viruses that replicate in the cytoplasm. To do so, they encode approximately 100 immunomodulatory proteins that counteract cytosolic nucleic acid sensors such as cGAMP synthase (cGAS) along with several other antiviral response pathways. Yet most of these immunomodulators are expressed very early in infection while many are variable host range determinants, and significant gaps remain in our understanding of poxvirus sensing and evasion strategies. Here, we show that after infection is established, subsequent progression of the viral lifecycle is sensed through specific changes to mitochondria that coordinate distinct aspects of the antiviral response. Unlike other viruses that cause extensive mitochondrial damage, poxviruses sustain key mitochondrial functions including membrane potential and respiration while reducing reactive oxygen species that drive inflammation. However, poxvirus replication induces mitochondrial hyperfusion that independently controls the release of mitochondrial DNA (mtDNA) to prime nucleic acid sensors and enables an increase in glycolysis that is necessary to support interferon stimulated gene (ISG) production. To counter this, the poxvirus F17 protein localizes to mitochondria and dysregulates mTOR to simultaneously destabilize cGAS and block increases in glycolysis. Our findings reveal how the poxvirus F17 protein disarms specific mitochondrially orchestrated responses to later stages of poxvirus replication.

The Host Cytoskeleton Functions as a Pleiotropic Scaffold: Orchestrating Regulation of the Viral Life Cycle and Mediating Host Antiviral Innate Immune Responses

Viruses are obligate intracellular parasites that critically depend on their hosts to initiate infection, complete replication cycles, and generate new progeny virions. To achieve these goals, viruses have evolved numerous elegant strategies to subvert and utilize different cellular machinery. The cytoskeleton is often one of the first components to be hijacked as it provides a convenient transport system for viruses to enter the cell and reach the site of replication. The cytoskeleton is an intricate network involved in controlling the cell shape, cargo transport, signal transduction, and cell division. The host cytoskeleton has complex interactions with viruses during the viral life cycle, as well as cell-to-cell transmission once the life cycle is completed. Additionally, the host also develops unique, cytoskeleton-mediated antiviral innate immune responses. These processes are also involved in pathological damages, although the comprehensive mechanisms remain elusive. In this review, we briefly summarize the functions of some prominent viruses in inducing or hijacking cytoskeletal structures and the related antiviral responses in order to provide new insights into the crosstalk between the cytoskeleton and viruses, which may contribute to the design of novel antivirals targeting the cytoskeleton.

Microtubules and viral infection

Microtubules (MTs) form rapidly adaptable, complex intracellular networks of filaments that not only provide structural support, but also form the tracks along which motors traffic macromolecular cargos to specific sub-cellular sites. These dynamic arrays play a central role in regulating various cellular processes including cell shape and motility as well as cell division and polarization. Given their complex organization and functional importance, MT arrays are carefully controlled by many highly specialized proteins that regulate the nucleation of MT filaments at distinct sites, their dynamic growth and stability, and their engagement with other subcellular structures and cargoes destined for transport. This review focuses on recent advances in our understanding of how MTs and their regulatory proteins function, including their active targeting and exploitation, during infection by viruses that utilize a wide variety of replication strategies that occur within different cellular sub-compartments or regions of the cell.

Ultrastructural analysis of monkeypox virus replication in Vero cells

In early May 2022, the first worldwide monkeypox virus (MPXV) outbreak was reported, with different clinical aspects from previously studied human monkeypox infections. Despite monkeypox medical importance, much of its biological aspects remain to be further investigated. In the present work, we evaluated ultrastructural aspects of MPXV asynchronous infections in Vero cells by transmission electron microscopy (TEM). The viral strain was isolated from a male patient infected during the 2022 outbreak. TEM analysis showed: (i) adhered intracellular mature virus particles before entry of the host cell; (ii) a reorganization of the rough endoplasmic reticulum cisternae into the so-called "mini-nuclei" structure associated with genome replication; and (iii) noticeably different sites within the viral factory presenting granular or fibrillar aspects. We also observed viral crescents, different MPXV particle morphotypes, and cellular alterations induced by infection, such as changes in the cytoskeleton structure and multimembrane vesicles abundance. Taken together, to the best of our knowledge, these results revealed for the first-time ultrastructural aspects of different steps of the MPXV cycle.

Amoebal Tubulin Cleavage Late during Infection Is a Characteristic Feature of Mimivirus but Not of Marseillevirus

Mimivirus and Marseillevirus infections of Acanthamoeba castellanii, like most other viral infections, induce cytopathic effects (CPE). The details of how they bring about CPE and to what extent and how they modify the host cytoskeletal network are unclear. In this study, we compared the rearrangement of the host cytoskeletal network induced by Mimivirus and Marseillevirus upon infection. We show that while both Mimivirus and Marseillevirus infections of A. castellanii cells cause retraction of acanthopodia and depolymerization of the host actin filament network, the Mimivirus infection also results in characteristic cleavage of the host tubulin, a phenomenon not previously reported with any intracellular pathogens. Furthermore, we show that the amoebal tubulin cleavage during Mimivirus infection is a post-replicative event. Because time-lapse microscopy showed that Mimivirus infection leads to the bursting of cells, releasing the virus, we hypothesize that tubulin cleavage together with actin depolymerization during the later stages of Mimivirus assembly is essential for cell lysis due to apoptotic/necrotic cell death. We also characterize the Mimivirus-encoded gp560, a Zn metalloprotease, however, the purified gp560 protein was unable to cleave the commercially available porcine brain tubulin. While protein synthesis is essential for causing the morphological changes in the case of Mimivirus, the proteins which are packaged in the viral capsid along with the genome are sufficient to induce CPE in the case of Marseillevirus. IMPORTANCE In general, intracellular pathogens target the cytoskeletal network to enable their life cycle inside the host. Pathogen-induced changes in the host cell morphology usually accompany global changes in the cytoskeleton resulting in cytopathic effects. While viruses have been shown to use the host actin cytoskeleton for entry and transport during early infection, the role of microtubules in the viral life cycle is only beginning to emerge. Here, we show that the giant viruses Mimivirus and Marseillevirus both induce depolymerization of the actin filament, Mimivirus also causes a characteristic cleavage of tubulin not previously reported for any intracellular pathogen. Because tubulin cleavage occurs late during infection, we hypothesize that tubulin cleavage aids in cell death and lysis rather than establishing infection. The different strategies used by viruses with similar host niches may help them survive in competition.

Manipulation of Host Microtubule Networks by Viral Microtubule-Associated Proteins

Diverse DNA and RNA viruses utilize cytoskeletal networks to efficiently enter, replicate, and exit the host cell, while evading host immune responses. It is well established that the microtubule (MT) network is commonly hijacked by viruses to traffic to sites of replication after entry and to promote egress from the cell. However, mounting evidence suggests that the MT network is also a key regulator of host immune responses to infection. At the same time, viruses have acquired mechanisms to manipulate and/or usurp MT networks to evade these immune responses. Central to most interactions of viruses with the MT network are virally encoded microtubule-associated proteins (MAPs) that bind to MTs directly or indirectly. These MAPs associate with MTs and other viral or cellular MAPs to regulate various aspects of the MT network, including MT dynamics, MT-dependent transport via motor proteins such as kinesins and dyneins, and MT-dependent regulation of innate immune responses. In this review, we examine how viral MAP interactions with the MT network facilitate viral replication and immune evasion.

A new microtubule-stabilizing agent shows potent antiviral effects against African swine fever virus with no cytotoxicity

African swine fever virus (ASFV) is the causal agent of a fatal disease of domestic swine for which no effective antiviral drugs are available. Recently, it has been shown that microtubule-targeting agents hamper the infection cycle of different viruses. In this study, we conducted in silico screening against the colchicine binding site (CBS) of tubulin and found three new compounds with anti-ASFV activity. The most promising antiviral compound (6b) reduced ASFV replication in a dose-dependent manner (IC50 = 19.5 μM) with no cellular (CC50 > 500 μM) and animal toxicity (up to 100 mg/kg). Results also revealed that compound 6b interfered with ASFV attachment, internalization and egress, with time-of-addition assays, showing that compound 6b has higher antiviral effects when added within 2-8 h post-infection. This compound significantly inhibited viral DNA replication and disrupted viral protein synthesis. Experiments with ASFV-infected porcine macrophages disclosed that antiviral effects of the compound 6b were similar to its effects in Vero cells. Tubulin polymerization assay and confocal microscopy demonstrated that compound 6b promoted tubulin polymerization, acting as a microtubule-stabilizing, rather than a destabilizing agent in cells. In conclusion, this work emphasizes the idea that microtubules can be targets for drug development against ASFV.

Cytoplasmic factories, virus assembly, and DNA replication kinetics collectively constrain the formation of poxvirus recombinants

Poxviruses replicate in cytoplasmic structures called factories and each factory begins as a single infecting particle. Sixty-years ago Cairns predicted that this might have effects on vaccinia virus (VACV) recombination because the factories would have to collide and mix their contents to permit recombination. We've since shown that factories collide irregularly and that even then the viroplasm mixes poorly. We’ve also observed that while intragenic recombination occurs frequently early in infection, intergenic recombination is less efficient and happens late in infection. Something inhibits factory fusion and viroplasm mixing but what is unclear. To study this, we’ve used optical and electron microscopy to track factory movement in co-infected cells and correlate these observations with virus development and recombinant formation. While the technical complexity of the experiments limited the number of cells that are amenable to extensive statistical analysis, these studies do show that intergenic recombination coincides with virion assembly and when VACV replication has declined to ≤10% of earlier levels. Along the boundaries between colliding factories, one sees ER membrane remnants and other cell constituents like mitochondria. These collisions don't always cause factory fusion, but when factories do fuse, they still entrain cell constituents like mitochondria and ER-wrapped microtubules. However, these materials wouldn’t seem to pose much of a further barrier to DNA mixing and so it’s likely that the viroplasm also presents an omnipresent impediment to DNA mixing. Late packaging reactions might help to disrupt the viroplasm, but packaging would sequester the DNA just as the replication and recombination machinery goes into decline and further reduce recombinant yields. Many factors thus appear to conspire to limit recombination between co-infecting poxviruses.

Kinetics of Mimivirus Infection Stages Quantified Using Image Flow Cytometry

Due to the heterogeneity of viruses and their hosts, a comprehensive view of viral infection is best achieved by analyzing large populations of infected cells. However, information regarding variation in infected cell populations is lost in bulk measurements. Motivated by an interest in the temporal progression of events in virally infected cells, we used image flow cytometry (IFC) to monitor changes in Acanthamoeba polyphaga cells infected with Mimivirus. This first use of IFC to study viral infection required the development of methods to preserve morphological features of adherent amoeba cells prior to detachment and analysis in suspension. It also required the identification of IFC parameters that best report on key events in the Mimivirus infection cycle. The optimized IFC protocol enabled the simultaneous monitoring of diverse processes including generation of viral factories, transport, and fusion of replication centers within the cell, accumulation of viral progeny, and changes in cell morphology for tens of thousands of cells. After obtaining the time windows for these processes, we used IFC to evaluate the effects of perturbations such as oxidative stress and cytoskeletal disruptors on viral infection. Accurate dose‐response curves could be generated, and we found that mild oxidative stress delayed multiple stages of virus production, but eventually infection processes occurred with approximately the same amplitudes. We also found that functional actin cytoskeleton is required for fusion of viral replication centers and later for the production of viral progeny. Through this report, we demonstrate that IFC offers a quantitative, high‐throughput, and highly robust approach to study viral infection cycles and virus–host interactions. © The Authors. Cytometry Part A published by Wiley Periodicals, Inc. on behalf of International Society for Advancement of Cytometry.

The Virology of Taterapox Virus In Vitro

Taterapox virus (TATV) is phylogenetically the closest related virus to variola—the etiological agent of smallpox. Despite the similarity, few studies have evaluated the virus. In vivo, TATV can infect several animals but produces an inapparent infection in wild-type mice; however, TATV does cause morbidity and mortality in some immunocompromised strains. We employed in vitro techniques to compare TATV to ectromelia (ECTV) and vaccinia (VACV) viruses. Both ECTV and TATV replicate efficiently in primate cell lines but TATV replicates poorly in murine cells lines. Furthermore, TATV induces cytopathic effects, but to a lesser extent than ECTV, and changes cytoskeletal networks differently than both ECTV and VACV. Bioinformatic studies revealed differences in several immunomodulator open reading frames that could contribute to the reduced virulence of TATV, which were supported by in vitro cytokine assays.

RhoD Inhibits RhoC-ROCK-Dependent Cell Contraction via PAK6

RhoA-mediated regulation of myosin-II activity in the actin cortex controls the ability of cells to contract and bleb during a variety of cellular processes, including cell migration and division. Cell contraction and blebbing also frequently occur as part of the cytopathic effect seen during many different viral infections. We now demonstrate that the vaccinia virus protein F11, which localizes to the plasma membrane, is required for ROCK-mediated cell contraction from 2 hr post infection. Curiously, F11-induced cell contraction is dependent on RhoC and not RhoA signaling to ROCK. Moreover, RhoC-driven cell contraction depends on the upstream inhibition of RhoD signaling by F11. This inhibition prevents RhoD from regulating its downstream effector Pak6, alleviating the suppression of RhoC by the kinase. Our observations with vaccinia have now demonstrated that RhoD recruits Pak6 to the plasma membrane to antagonize RhoC signaling during cell contraction and blebbing.

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Vaccinia F11 protein is required for virus-induced cell contraction and blebbing

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F11-induced cell contraction depends on RhoC, but not RhoA, signaling to ROCK

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RhoD recruits Pak6 to the plasma membrane to antagonize RhoC signaling

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F11 inhibits RhoD signaling to its downstream effector Pak6

Microtubule Regulation and Function during Virus Infection

Microtubules (MTs) form a rapidly adaptable network of filaments that radiate throughout the cell. These dynamic arrays facilitate a wide range of cellular processes, including the capture, transport, and spatial organization of cargos and organelles, as well as changes in cell shape, polarity, and motility. Nucleating from MT-organizing centers, including but by no means limited to the centrosome, MTs undergo rapid transitions through phases of growth, pause, and catastrophe, continuously exploring and adapting to the intracellular environment. Subsets of MTs can become stabilized in response to environmental cues, acquiring distinguishing posttranslational modifications and performing discrete functions as specialized tracks for cargo trafficking. The dynamic behavior and organization of the MT array is regulated by MT-associated proteins (MAPs), which include a subset of highly specialized plus-end-tracking proteins (+TIPs) that respond to signaling cues to alter MT behavior. As pathogenic cargos, viruses require MTs to transport to and from their intracellular sites of replication. While interactions with and functions for MT motor proteins are well characterized and extensively reviewed for many viruses, this review focuses on MT filaments themselves. Changes in the spatial organization and dynamics of the MT array, mediated by virus- or host-induced changes to MT regulatory proteins, not only play a central role in the intracellular transport of virus particles but also regulate a wider range of processes critical to the outcome of infection.

Proteomic Screen for Cellular Targets of the Vaccinia Virus F10 Protein Kinase Reveals that Phosphorylation of mDia Regulates Stress Fiber Formation

Vaccinia virus, a complex dsDNA virus, is unusual in replicating exclusively within the cytoplasm of infected cells. Although this prototypic poxvirus encodes >200 proteins utilized during infection, a significant role for host proteins and cellular architecture is increasingly evident. The viral B1 kinase and H1 phosphatase are known to target cellular proteins as well as viral substrates, but little is known about the cellular substrates of the F10 kinase. F10 is essential for virion morphogenesis, beginning with the poorly understood process of diversion of membranes from the ER for the purpose of virion membrane biogenesis. To better understand the function of F10, we generated a cell line that carries a single, inducible F10 transgene. Using uninduced and induced cells, we performed stable isotope labeling of amino acids in cell culture (SILAC) coupled with phosphopeptide analysis to identify cellular targets of F10-mediated phosphorylation. We identified 27 proteins that showed statistically significant changes in phosphorylation upon the expression of the F10 kinase: 18 proteins showed an increase in phosphorylation whereas 9 proteins showed a decrease in phosphorylation. These proteins participate in several distinct cellular processes including cytoskeleton dynamics, membrane trafficking and cellular metabolism. One of the proteins with the greatest change in phosphorylation was mDia, a member of the formin family of cytoskeleton regulators; F10 induction led to increased phosphorylation on Ser22 Induction of F10 induced a statistically significant decrease in the percentage of cells with actin stress fibers; however, this change was abrogated when an mDia Ser22Ala variant was expressed. Moreover, expression of a Ser22Asp variant leads to a reduction of stress fibers even in cells not expressing F10. In sum, we present the first unbiased screen for cellular targets of F10-mediated phosphorylation, and in so doing describe a heretofore unknown mechanism for regulating stress fiber formation through phosphorylation of mDia. Data are available via ProteomeXchange with identifier PXD005246.

Dynamic monitoring of membrane nanotubes formation induced by vaccinia virus on a high throughput microfluidic chip

Membrane nanotubes (MNTs) are physical connections for intercellular communication and induced by various viruses. However, the formation of vaccinia virus (VACV)-induced MNTs has never been studied. In this report, VACV-induced MNTs formation process was monitored on a microfluidic chip equipped with a series of side chambers, which protected MNTs from fluidic shear stress. MNTs were formed between susceptible cells and be facilitated by VACV infection through three patterns. The formed MNTs varied with cell migration and virus concentration. The length of MNTs was positively correlated with the distance of cell migration. With increasing virus titer, the peak value of the ratio of MNT-carried cell appeared earlier. The immunofluorescence assay indicated that the rearrangement of actin fibers induced by VACV infection may lead to the formation of MNTs. This study presents evidence for the formation of MNTs induced by virus and helps us to understand the relationship between pathogens and MNTs.

Disruption of Microtubules Post-Virus Entry Enhances Adeno-Associated Virus Vector Transduction

Perinuclear retention of viral particles is a poorly understood phenomenon observed during many virus infections. In this study, we investigated whether perinuclear accumulation acts as a barrier to limit recombinant adeno-associated virus (rAAV) transduction. After nocodazole treatment to disrupt microtubules at microtubule-organization center (MT-MTOC) after virus entry, we observed higher rAAV transduction. To elucidate the role of MT-MTOC in rAAV infection and study its underlying mechanisms, we demonstrated that rAAV's perinuclear localization was retained by MT-MTOC with fluorescent analysis, and enhanced rAAV transduction from MT-MTOC disruption was dependent on the rAAV capsid's nuclear import signals. Interestingly, after knocking down RhoA or inhibiting its downstream effectors (ROCK and Actin), MT-MTOC disruption failed to increase rAAV transduction or nuclear entry. These data suggest that enhancement of rAAV transduction is the result of increased trafficking to the nucleus via the RhoA-ROCK-Actin pathway. Ten-fold higher rAAV transduction was also observed by disrupting MT-MTOC in brain, liver, and tumor in vivo. In summary, this study indicates that virus perinuclear accumulation at MT-MTOC is a barrier-limiting parameter for effective rAAV transduction and defines a novel defense mechanism by which host cells restrain viral invasion.

Vaccinia virus dissemination requires p21-activated kinase 1

The orthopoxvirus vaccinia virus (VACV) interacts with both actin and microtubule cytoskeletons in order to generate and spread progeny virions. Here, we present evidence demonstrating the involvement of PAK1 (p21-activated kinase 1) in the dissemination of VACV. Although PAK1 activation has previously been associated with optimal VACV entry via macropinocytosis, its absence does not affect the production of intracellular mature virions (IMVs) and extracellular enveloped virions (EEVs). Our data demonstrate that low-multiplicity infection of PAK1(-/-) MEFs leads to a reduction in plaque size followed by decreased production of both IMVs and EEVs, strongly suggesting that virus spread was impaired in the absence of PAK1. Confocal and scanning electron microscopy showed a substantial reduction in the amount of VACV-induced actin tails in PAK1(-/-) MEFs, but no significant alteration in the total amount of cell-associated enveloped virions (CEVs). Furthermore, the decreased VACV dissemination in PAK1(-/-) cells was correlated with the absence of phosphorylated ARPC1 (Thr21), a downstream target of PAK1 and a key regulatory subunit of the ARP2/3 complex, which is necessary for the formation of actin tails and viral spread. We conclude that PAK1, besides its role in virus entry, also plays a relevant role in VACV dissemination.

A single vertebrate DNA virus protein disarms invertebrate immunity to RNA virus infection

Virus-host interactions drive a remarkable diversity of immune responses and countermeasures. We found that two RNA viruses with broad host ranges, vesicular stomatitis virus (VSV) and Sindbis virus (SINV), are completely restricted in their replication after entry into Lepidopteran cells. This restriction is overcome when cells are co-infected with vaccinia virus (VACV), a vertebrate DNA virus. Using RNAi screening, we show that Lepidopteran RNAi, Nuclear Factor-κB, and ubiquitin-proteasome pathways restrict RNA virus infection. Surprisingly, a highly conserved, uncharacterized VACV protein, A51R, can partially overcome this virus restriction. We show that A51R is also critical for VACV replication in vertebrate cells and for pathogenesis in mice. Interestingly, A51R colocalizes with, and stabilizes, host microtubules and also associates with ubiquitin. We show that A51R promotes viral protein stability, possibly by preventing ubiquitin-dependent targeting of viral proteins for destruction. Importantly, our studies reveal exciting new opportunities to study virus-host interactions in experimentally-tractable Lepidopteran systems.

A novel mode of poxvirus superinfection exclusion that prevents fusion of the lipid bilayers of viral and cellular membranes

Superinfection exclusion is a widespread phenomenon that prevents secondary infections by closely related viruses. The vaccinia virus A56 and K2 proteins in the cell membrane can prevent superinfection by interacting with the entry-fusion complex of subsequent viruses. Here, we described another form of exclusion that is established earlier in infection and does not require the A56 or K2 protein. Cells infected with one or more infectious virions excluded hundreds of superinfecting vaccinia virus particles. A related orthopoxvirus, but neither a flavivirus nor a rhabdovirus, was also excluded, indicating selectivity. Although superinfecting vaccinia virus bound to cells, infection was inhibited at the membrane fusion step, thereby preventing core entry into the cytoplasm and early gene expression. In contrast, A56/K2 protein-mediated exclusion occurred subsequent to membrane fusion. Induction of resistance to superinfection depended on viral RNA and protein synthesis by the primary virus but did not require DNA replication. Although superinfection resistance correlated with virus-induced changes in the cytoskeleton, studies with mutant vaccinia viruses indicated that the cytoskeletal changes were not necessary for resistance to superinfection. Interferon-inducible transmembrane proteins, which can inhibit membrane fusion in other viral systems, did not prevent vaccinia virus membrane fusion, suggesting that these interferon-inducible proteins are not involved in superinfection exclusion. While the mechanism remains to be determined, the early establishment of superinfection exclusion may provide a "winner-take-all" reward to the first poxvirus particles that successfully initiate infection and prevent the entry and genome reproduction of defective or less fit particles.

IMPORTANCE

The replication of a virus usually follows a defined sequence of events: attachment, entry into the cytoplasm or nucleus, gene expression, genome replication, assembly of infectious particles, and spread to other cells. Although multiple virus particles may enter a cell at the same time, mechanisms exist to prevent infection by subsequent viruses. The latter phenomenon, known as superinfection exclusion, can occur by a variety of mechanisms that are not well understood. We showed that superinfection by vaccinia virus was prevented at the membrane fusion step, which closely followed virion attachment. Thus, neither gene expression nor genome replication of the superinfecting virus occurred. Expression of early proteins by the primary virus was necessary and sufficient to induce the superinfection-resistant state. Superinfection exclusion may be beneficial to vaccinia virus by selecting particles that can infect cells rapidly, excluding defective particles and synchronizing the replication cycle.

Orf virus (ORFV) ANK-1 protein mitochondrial localization is mediated by ankyrin repeat motifs

Orf virus (ORFV) strain D1701-V, a Parapoxvirus belonging to the family Poxviridae, became attractive as a novel virus vector system that we successfully used for the generation of recombinant vaccines. Therefore, the identification of viral genes involved in host tropisms or immune modulation is of great interest, as for instance the ORFV-encoded ankyrin-repeat (AR) containing proteins. The present study shows for the first time that the ANK-1 designated gene product of ORFV126 is targeted to mitochondria of ORFV-infected and in ANK-1 transiently expressing cells. Taking advantage of ANK-1 EGFP fusion proteins and confocal fluorescence microscopy mutational and deletion analyses indicated the importance of AR8 and AR9, which may contain a novel class of mitochondria-targeting sequence (MTS) in the central to C-terminal part of this AR-containing protein. The fluorescent findings were corroborated by cell fractionation and Western blotting experiments. The presented results open the avenue for more detailed investigations on cellular binding partners and the function of ANK-1 in viral replication or virulence.

TRAF2 facilitates vaccinia virus replication by promoting rapid virus entry

Tumor necrosis factor receptor (TNFR)-associated factor 2 (TRAF2) is a pivotal intracellular mediator of signaling pathways downstream of TNFR1 and -2 with known pro- and antiviral effects. We investigated its role in the replication of the prototype poxvirus vaccinia virus (VACV). Loss of TRAF2 expression, either through small interfering RNA treatment of HeLa cells or through genetic knockout in murine embryonic fibroblasts (MEFs), led to significant reductions in VACV growth following low-multiplicity infection. In single-cycle infections, there was delayed production of both early and late VACV proteins as well as accelerated virus-induced alterations to cell morphology, indicating that TRAF2 influences early stages of virus replication. Consistent with an early role, uncoating assays showed normal virus attachment but delayed virus entry in the absence of TRAF2. Although alterations to c-Jun N-terminal kinase (JNK) signaling were apparent in VACV-infected TRAF2^−/− MEFs, treatment of wild-type cells with a JNK inhibitor did not affect virus entry. Instead, treatment with an inhibitor of endosomal acidification greatly reduced virus entry into TRAF2^−/− MEFs, suggesting that VACV is reliant on the endosomal route of entry in the absence of TRAF2. Thus, TRAF2 is a proviral factor for VACV that plays a role in promoting efficient viral entry, most likely via the plasma membrane.

IMPORTANCE Tumor necrosis factor receptor-associated factors (TRAFs) are key facilitators of intracellular signaling with roles in innate and adaptive immunity and stress responses. We have discovered that TRAF2 is a proviral factor in vaccinia virus replication in both HeLa cells and mouse embryonic fibroblasts and that its influence is exercised through promotion of efficient virus entry.

Repair of base damage and genome maintenance in the nucleo-cytoplasmic large DNA viruses

Among the DNA viruses, the so-called nucleo-cytoplasmic large DNA viruses (NCLDV) constitute a monophyletic group that currently consists of seven families of viruses infecting a very broad variety of eukaryotes, from unicellular marine protists to humans. Many recent papers have analyzed the sequence and structure of NCLDV genomes and their phylogeny, providing detailed analysis about their genomic structure and evolutionary history and proposing their inclusion in a new viral order named Megavirales that, according to some authors, should be considered as a fourth domain of life, aside from Bacteria, Archaea and Eukarya. The maintenance of genetic information protected from environmental attacks and mutations is essential not only for the survival of cellular organisms but also viruses. In cellular organisms, damaged DNA bases are removed in two major repair pathways: base excision repair (BER) and nucleotide incision repair (NIR) that constitute the major pathways responsible for repairing most endogenous base lesions and abnormal bases in the genome by precise repair procedures. Like cells, many NCLDV encode proteins that might constitute viral DNA repair pathways that would remove damages through BER/NIR pathways. However, the molecular mechanisms and, specially, the biological roles of those viral repair pathways have not been deeply addressed in the literature so far. In this paper, we review viral-encoded BER proteins and the genetic and biochemical data available about them. We propose and discuss probable viral-encoded DNA repair mechanisms and pathways, as compared with the functional and molecular features of known homologs proteins.

Viral interactions with microtubules: orchestrators of host cell biology?

Viral interaction with the microtubule (MT) cytoskeleton is critical to infection by many viruses. Most data regarding virus–MT interaction indicate key roles in the subcellular transport of virions/viral genomic material to sites of replication, assembly and egress. However, the MT cytoskeleton orchestrates diverse processes in addition to subcellular cargo transport, including regulation of signaling pathways, cell survival and mitosis, suggesting that viruses, expert manipulators of the host cell, may use the virus–MT interface to control multiple aspects of cell biology. Several lines of evidence support this idea, indicating that specific viral proteins can modify MT dynamics and/or structure and regulate processes such as apoptosis and innate immune signaling through MT-dependent mechanisms. Here, the authors review general aspects of virus–MT interactions, with emphasis on viral mechanisms that modify MT dynamics and functions to affect processes beyond virion transport. The emerging importance of discrete viral protein–MT interactions in pathogenic processes indicates that these interfaces may represent new targets for future therapeutics and vaccine development.

Orthopoxvirus genes that mediate disease virulence and host tropism

In the course of evolution, viruses have developed various molecular mechanisms to evade the defense reactions of the host organism. When understanding the mechanisms used by viruses to overcome manifold defense systems of the animal organism, represented by molecular factors and cells of the immune system, we would not only comprehend better but also discover new patterns of organization and function of these most important reactions directed against infectious agents. Here, study of the orthopoxviruses pathogenic for humans, such as variola (smallpox), monkeypox, cowpox, and vaccinia viruses, may be most important. Analysis of the experimental data, presented in this paper, allows to infer that variola virus and other orthopoxviruses possess an unexampled set of genes whose protein products efficiently modulate the manifold defense mechanisms of the host organisms compared with the viruses from other families.

A vaccinia virus-driven interplay between the MKK4/7-JNK1/2 pathway and cytoskeleton reorganization

Viral manipulation of transduction pathways associated with key cellular functions such as survival, response to microbial infection, and cytoskeleton reorganization can provide the supportive milieu for a productive infection. Here, we demonstrate that vaccinia virus (VACV) infection leads to activation of the stress-activated protein kinase (SAPK)/extracellular signal-regulated kinase (ERK) 4/7 (MKK4/7)-c-Jun N-terminal protein kinase 1/2 (JNK1/2) pathway; further, the stimulation of this pathway requires postpenetration, prereplicative events in the viral replication cycle. Although the formation of intracellular mature virus (IMV) was not affected in MKK4/7- or JNK1/2-knockout (KO) cells, we did note an accentuated deregulation of microtubule and actin network organization in infected JNK1/2-KO cells. This was followed by deregulated viral trafficking to the periphery and enhanced enveloped particle release. Furthermore, VACV infection induced alterations in the cell contractility and morphology, and cell migration was reduced in the JNK-KO cells. In addition, phosphorylation of proteins implicated with early cell contractility and cell migration, such as microtubule-associated protein 1B and paxillin, respectively, was not detected in the VACV-infected KO cells. In sum, our findings uncover a regulatory role played by the MKK4/7-JNK1/2 pathway in cytoskeleton reorganization during VACV infection.

Premature chromosome condensation induced by caffeine, 2-aminopurine, staurosporine and sodium metavanadate in S-phase arrested HeLa cells is associated with a decrease in Chk1 phosphorylation, formation of phospho-H2AX and minor cytoskeletal rearrangements

Here, we demonstrate that in HeLa cells, Ser317 of Chk1 undergoes phosphorylation in response to replication stress induced by hydroxyurea. We also demonstrate the existence of constitutive (interphase and mitotic) Chk1 kinase phosphorylation, the translocation of its phosphorylated form from the nucleus to cytoplasm in prometaphase as well as strong labeling of apoptotic nuclei with α-Chk1(S317) antibodies. Additionally, we show that caffeine, 2-aminopurine, staurosporine and sodium metavanadate can induce premature chromosome condensation (PCC) by the abrogation of the S-M checkpoint. Staurosporine appeared to be the most effective PCC inductor, and as in the case of the remaining inductors, the addition of hydroxyurea each time brought about an increase in the number of cells showing PCC symptoms (synergic effect). The forced premature mitosis was accompanied by an increasing index of double-strand breaks marked by the phosphorylation of histone H2AX on Ser139. Moreover, we found that the chemicals used brought about minor actin and tubulin network rearrangements that occurred following either replication stress or drug-induced cell cycle delay. At the same time, it was found that the extent of the cytoskeleton rearrangement did not hinder PCC in all its subperiods, i.e., from PCC-type prophase to PCC-type telophase.

Uncoupling uncoating of herpes simplex virus genomes from their nuclear import and gene expression

Incoming capsids of herpes simplex virus type 1 (HSV-1) enter the cytosol by fusion of the viral envelopes with host cell membranes and use microtubules and microtubule motors for transport to the nucleus. Upon docking to the nuclear pores, capsids release their genomes into the nucleoplasm. Progeny genomes are replicated in the nucleoplasm and subsequently packaged into newly assembled capsids. The minor capsid protein pUL25 of alphaherpesviruses is required for capsid stabilization after genome packaging and for nuclear targeting of incoming genomes. Here, we show that HSV-1 pUL25 bound to mature capsids within the nucleus and remained capsid associated during assembly and nuclear targeting. Furthermore, we tested potential interactions between parental pUL25 bound to incoming HSV-1 capsids and host factors by competing for such interactions with an experimental excess of cytosolic pUL25. Overexpression of pUL25, GFPUL25, or UL25GFP prior to infection reduced gene expression of HSV-1. Electron microscopy and in situ hybridization studies revealed that an excess of GFPUL25 or UL25GFP prevented efficient nuclear import and/or transcription of parental HSV-1 genomes, but not nuclear targeting of capsids or the uncoating of the incoming genomes at the nuclear pore. Thus, the uncoating of HSV-1 genomes could be uncoupled from their nuclear import and gene expression. Most likely, surplus pUL25 competed with important interactions between the parental capsids, and possibly between authentic capsid-associated pUL25, and cytosolic or nuclear host factors required for functional interaction of the incoming genomes with the nuclear machinery.

Interaction of orthopoxviruses with the cellular ubiquitin-ligase system

Protein modification by ubiquitin or ubiquitin-like polypeptides is important for the fate and functions of the majority of proteins in the eukaryotic cell and can be involved in regulation of various biological processes, including protein metabolism (degradation), protein transport to several cellular compartments, rearrangement of cytoskeleton, and transcription of cytoprotective genes. The accumulated experimental data suggest that the ankyrin-F-box-like and BTB-kelch-like proteins of orthopoxviruses, represented by the largest viral multigene families, interact with the cellular Cullin-1- and Cullin-3-containing ubiquitin-protein ligases, respectively. In addition, orthopoxviruses code for their own RING-domain-containing ubiquitin ligase. In this review, this author discusses the differences between variola (smallpox), monkeypox, cowpox, vaccinia, and ectromelia (mousepox) viruses in the organization of ankyrin-F-box and BTB-kelch protein families and their likely functions.

F11-mediated inhibition of RhoA signalling enhances the spread of vaccinia virus in vitro and in vivo in an intranasal mouse model of infection

The cortical actin cytoskeleton beneath the plasma membrane represents a physical barrier that vaccinia virus has to overcome during its exit from an infected cell. Previous observations using overexpression and pharmacological approaches suggest that vaccinia enhances its release by modulating the cortical actin cytoskeleton by inhibiting RhoA signalling using the viral protein F11. We have now examined the role of F11 and its ability to interact with RhoA to inhibit its downstream signalling in the spread of vaccinia infection both in vitro and in vivo. Live cell imaging over 48 hours reveals that loss of F11 or its ability to bind RhoA dramatically reduces the rate of cell-to-cell spread of the virus in a cell monolayer. Cells infected with the DeltaF11L virus also maintained their cell-to-cell contacts, and did not undergo virus-induced motility as observed during wild-type infections. The DeltaF11L virus is also attenuated in intranasal mouse models of infection, as it is impaired in its ability to spread from the initial sites of infection to the lungs and spleen. Loss of the ability of F11 to bind RhoA also reduces viral spread in vivo. Our results clearly establish that viral-mediated inhibition of RhoA signalling can enhance the spread of infection not only in cell monolayers, but also in vivo.

Dynamics of coronavirus replication-transcription complexes

Coronaviruses induce in infected cells the formation of double-membrane vesicles (DMVs) in which the replication-transcription complexes (RTCs) are anchored. To study the dynamics of these coronavirus replicative structures, we generated recombinant murine hepatitis coronaviruses that express tagged versions of the nonstructural protein nsp2. We demonstrated by using immunofluorescence assays and electron microscopy that this protein is recruited to the DMV-anchored RTCs, for which its C terminus is essential. Live-cell imaging of infected cells demonstrated that small nsp2-positive structures move through the cytoplasm in a microtubule-dependent manner. In contrast, large fluorescent structures are rather immobile. Microtubule-mediated transport of DMVs, however, is not required for efficient replication. Biochemical analyses indicated that the nsp2 protein is associated with the cytoplasmic side of the DMVs. Yet, no recovery of fluorescence was observed when (part of) the nsp2-positive foci were bleached. This result was confirmed by the observation that preexisting RTCs did not exchange fluorescence after fusion of cells expressing either a green or a red fluorescent nsp2. Apparently, nsp2, once recruited to the RTCs, is not exchanged with nsp2 present in the cytoplasm or at other DMVs. Our data show a remarkable resemblance to results obtained recently by others with hepatitis C virus. The observations point to intriguing and as yet unrecognized similarities between the RTC dynamics of different plus-strand RNA viruses.

The purified and recombinant Legionella pneumophila chaperonin alters mitochondrial trafficking and microfilament organization

A portion of the total cellular pool of the Legionella pneumophila chaperonin, HtpB, is found on the bacterial cell surface, where it can mediate invasion of nonphagocytic cells. HtpB continues to be abundantly produced and released by internalized L. pneumophila and may thus have postinvasion functions. We used here two functional models (protein-coated beads and expression of recombinant proteins in CHO cells) to investigate the competence of HtpB in mimicking early intracellular trafficking events of L. pneumophila, including the recruitment of mitochondria, cytoskeletal alterations, the inhibition of phagosome-lysosome fusion, and association with the endoplasmic reticulum. Microscopy and flow cytometry studies indicated that HtpB-coated beads recruited mitochondria in CHO cells and U937-derived macrophages and induced transient changes in the organization of actin microfilaments in CHO cells. Ectopic expression of HtpB in the cytoplasm of transfected CHO cells also led to modifications in actin microfilaments similar to those produced by HtpB-coated beads but did not change the distribution of mitochondria. Association of phagosomes containing HtpB-coated beads with the endoplasmic reticulum was not consistently detected by either fluorescence or electron microscopy studies, and only a modest delay in the fusion of TrOv-labeled lysosomes with phagosomes containing HtpB-coated beads was observed. HtpB is the first Legionella protein and the first chaperonin shown to, by means of our functional models, induce mitochondrial recruitment and microfilament rearrangements, two postinternalization events that typify the early trafficking of virulent L. pneumophila.

Death by committee: organellar trafficking and communication in apoptosis

Apoptosis proceeds through a set of evolutionarily conserved processes that co-ordinate the elimination of damaged or unneeded cells. This program of cell death is carried out by organelle-directed regulators, including the Bcl-2 proteins, and ultimately executed by proteases of the caspase family. Although the biochemical mechanisms of apoptosis are increasingly understood, the underlying cell biology orchestrating programmed cell death remains enigmatic. In this review, we summarize the current understanding of Bcl-2 protein regulation and caspase activation while examining cell biological mechanisms and consequences of apoptotic induction. Organellar contributions to apoptotic induction include death receptor endocytosis, mitochondrial and lysosomal permeabilization, endoplasmic reticulum calcium release and fragmentation of the Golgi apparatus. These early apoptotic events are accompanied by stabilization of the microtubule cytoskeleton and translocation of organelles to the microtubule organizing center. Together, these phenomena establish a model of apoptotic induction whereby a cytoskeletal-dependent coalescence and 'scrambling' of organelles in the paranuclear region co-ordinates apoptotic communication, caspase activation and cell death.

Tiger frog virus can infect zebrafish cells for studying up- or down-regulated genes by proteomics approach

Tiger frog virus (TFV), a member of the iridovirus family, causes high mortality of cultured tiger frog tadpoles in southern China. To better understand TFV infection and its interaction with host cells, zebrafish embryonic fibroblast (ZF4) cells, a stable polyploid cell line with most clear genetic map, was used for our present study. Our results showed that TFV caused typical lytic plaque forming cytopathic effect (CPE) and that various stages of viral proliferation were observed using electron microscopy and indirect immunofluorescence assay. Two-dimensional electrophoresis also showed that some cellular proteins were differentially expressed in the ZF4 cells infected with TFV. A total of 10 proteins were identified using the matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) technique, including 7 that were up-regulated and 3 that were down-regulated after infection. Among the 10 identified proteins, alterations in Hsp90 and alpha-tubulin expression were further confirmed by Western blot analysis. Furthermore, reorganization of microtubules was also observed in TFV-infected cells and can probably be attributed to the overexpression of translationally controlled tumor protein. The present study is the first attempt to reveal cellular responses to TFV infection by proteomics. The results suggest that the ZF4 cell line could be used as a model to study TFV infection.

Activation of the PI3K/Akt pathway early during vaccinia and cowpox virus infections is required for both host survival and viral replication

Viral manipulation of the transduction pathways associated with key cellular functions such as actin remodeling, microtubule stabilization, and survival may favor a productive viral infection. Here we show that consistent with the vaccinia virus (VACV) and cowpox virus (CPXV) requirement for cytoskeleton alterations early during the infection cycle, PBK/Akt was phosphorylated at S473 [Akt(S473-P)], a modification associated with the mammalian target of rapamycin complex 2 (mTORC2), which was paralleled by phosphorylation at T308 [Akt(T308-P)] by PI3K/PDK1, which is required for host survival. Notably, while VACV stimulated Akt(S473-P/T308-P) at early (1 h postinfection [p.i.]) and late (24 h p.i.) times during the infective cycle, CPXV stimulated Akt at early times only. Pharmacological and genetic inhibition of PI3K (LY294002) or Akt (Akt-X and a dominant-negative form of Akt-K179M) resulted in a significant decline in virus yield (from 80% to >/=90%). This decline was secondary to the inhibition of late viral gene expression, which in turn led to an arrest of virion morphogenesis at the immature-virion stage of the viral growth cycle. Furthermore, the cleavage of both caspase-3 and poly(ADP-ribose) polymerase and terminal deoxynucleotidyl transferase-mediated deoxyuridine nick end labeling assays confirmed that permissive, spontaneously immortalized cells such as A31 cells and mouse embryonic fibroblasts (MEFs) underwent apoptosis upon orthopoxvirus infection plus LY294002 treatment. Thus, in A31 cells and MEFs, early viral receptor-mediated signals transmitted via the PI3K/Akt pathway are required and precede the expression of viral antiapoptotic genes. Additionally, the inhibition of these signals resulted in the apoptosis of the infected cells and a significant decline in viral titers.

Foot-and-mouth disease virus, but not bovine enterovirus, targets the host cell cytoskeleton via the nonstructural protein 3Cpro

Foot-and-mouth disease virus (FMDV), a member of the Picornaviridae, is a pathogen of cloven-hoofed animals and causes a disease of major economic importance. Picornavirus-infected cells show changes in cell morphology and rearrangement of cytoplasmic membranes, which are a consequence of virus replication. We show here, by confocal immunofluorescence and electron microscopy, that the changes in morphology of FMDV-infected cells involve changes in the distribution of microtubule and intermediate filament components during infection. Despite the continued presence of centrosomes in infected cells, there is a loss of tethering of microtubules to the microtubule organizing center (MTOC) region. Loss of labeling for gamma-tubulin, but not pericentrin, from the MTOC suggests a targeting of gamma-tubulin (or associated proteins) rather than a total breakdown in MTOC structure. The identity of the FMDV protein(s) responsible was determined by the expression of individual viral nonstructural proteins and their precursors in uninfected cells. We report that the only viral nonstructural protein able to reproduce the loss of gamma-tubulin from the MTOC and the loss of integrity of the microtubule system is FMDV 3C(pro). In contrast, infection of cells with another picornavirus, bovine enterovirus, did not affect gamma-tubulin distribution, and the microtubule network remained relatively unaffected.

The vaccinia virus F11L gene product facilitates cell detachment and promotes migration

We previously showed that infection with vaccinia virus (VV) induces cell motility, characterized by contractility and directed migration. Motility is temporally regulated because cells are motile immediately after infection, whereas late in infection motility ceases and cells resettle. Motility and its cessation are accompanied by temporal rearrangements of both the microtubule and the actin networks. Because the F11L gene has previously been implicated in VV-induced migration, we now explore the role of F11L in contractility, migration, the cessation of motility and the cytoskeletal rearrangements. By live cell imaging using a VV that lacks an intact F11L gene, we show that F11L facilitates cell detachment and is required for migration but not for contractility. By light microscopy, F11L expression induces a remodeling of the actin, but not the microtubule, network. The lack of migration correlates with smaller plaques, indicating that this process facilitates cell-to-cell spreading of VV. Late in infection, when motility ceases, cells re-establish cell-to-cell contacts in an F11L-independent manner. We finally show that VV-induced motility and its cessation correlate with a temporal regulation of the guanosine triphosphatase RhoA as well as the expression levels of F11L during the infectious cycle.

Host-cell-dependent role of actin cytoskeleton during the replication of a human strain of influenza A virus

This study was aimed at investigating the possible involvement of the actin cytoskeleton in the modulation of host permissiveness to A/NWS/33 human influenza virus infection in two mammalian (MDCK and LLC-MK2) cell lines in vitro. During the early stages of infection, no appreciable association between incoming NWS/33 virions and cortical actin was detectable in the permissive MDCK model by confocal microscopy, while extensive colocalization and a slower infection progression were observed in LLC-MK2 cells. In the latter model, we also demonstrated the inability of the virus to carry out multiple replication cycles, irrespective of the presence of cleaved HA subunits in the released virions. Treatment with the actin-depolymerizing agent cytochalasin D significantly increased the infection efficiency in LLC-MK2 cells, while a detrimental effect was observed in the MDCK cell line. Our data suggest a selective role of the actin network in inducing a restriction to influenza virus replication, mostly depending on its molecular organization, the host cell type and virus replication phase.

Aggresomes and pericentriolar sites of virus assembly: cellular defense or viral design?

Virus replication and virus assembly often occur in virus inclusions or virus factories that form at pericentriolar sites close to the microtubule organizing center or in specialized nuclear domains called ND10/PML bodies. Similar inclusions called aggresomes form in response to protein aggregation. Protein aggregates are toxic to cells and are transported along microtubules to aggresomes for immobilization and subsequent degradation by proteasomes and/or autophagy. The similarity between aggresomes and virus inclusions raises the possibility that viruses use aggresome pathways to concentrate cellular and viral proteins to facilitate replication and assembly. Alternatively, aggresomes may be part of an innate cellular response that recognizes virus components as foreign or misfolded and targets them for storage and degradation. Insights into the possible roles played by aggresomes during virus assembly are emerging from an understanding of how virus inclusions form and how viral proteins are targeted to them.

A guide to viral inclusions, membrane rearrangements, factories, and viroplasm produced during virus replication

Virus replication can cause extensive rearrangement of host cell cytoskeletal and membrane compartments leading to the “cytopathic effect” that has been the hallmark of virus infection in tissue culture for many years. Recent studies are beginning to redefine these signs of viral infection in terms of specific effects of viruses on cellular processes. In this chapter, these concepts have been illustrated by describing the replication sites produced by many different viruses. In many cases, the cellular rearrangements caused during virus infection lead to the construction of sophisticated platforms in the cell that concentrate replicase proteins, virus genomes, and host proteins required for replication, and thereby increase the efficiency of replication. Interestingly, these same structures, called virus factories, virus inclusions, or virosomes, can recruit host components that are associated with cellular defences against infection and cell stress. It is possible that cellular defence pathways can be subverted by viruses to generate sites of replication. The recruitment of cellular membranes and cytoskeleton to generate virus replication sites can also benefit viruses in other ways. Disruption of cellular membranes can, for example, slow the transport of immunomodulatory proteins to the surface of infected cells and protect against innate and acquired immune responses, and rearrangements to cytoskeleton can facilitate virus release.

Cross-priming of cytotoxic T cells dictates antigen requisites for modified vaccinia virus Ankara vector vaccines

Recombinant vaccines based on modified vaccinia virus Ankara (MVA) have an excellent record concerning safety and immunogenicity and are currently being evaluated in numerous clinical studies for immunotherapy of infectious diseases and cancer. However, knowledge about the biological properties of target antigens to efficiently induce MVA vaccine-mediated immunity in vivo is sparse. Here, we examined distinct antigen presentation pathways and different antigen formulations contained in MVA vaccines for their capability to induce cytotoxic CD8(+) T-cell (CTL) responses. Strikingly, we found that CTL responses against MVA-produced antigens were dominated by cross-priming in vivo, despite the ability of the virus to efficiently infect professional antigen-presenting cells such as dendritic cells. Moreover, stable mature protein was preferred to preprocessed antigen as the substrate for cross-priming. Our data are essential for improved MVA vaccine design, as they demonstrate the need for optimal adjustment of the target antigen properties to the intrinsic requirements of the delivering vector system.

Kinesin-1 plays multiple roles during the vaccinia virus life cycle

The cytoplasmic distribution of cellular structures is known to depend on the balance between plus- and minus-end-directed motor complexes. Among the plus-end-directed kinesins, kinesin-1 and -2 have been implicated in the outward movement of many organelles. To test for a role of kinesin-1 previous studies mostly relied on the overexpression of dominant-negative kinesin-1 constructs. The latter are often cytotoxic, modify the microtubule network and indirect effects related to altered microtubule dynamics should be excluded. In the present study we present a novel kinesin-1 construct, encompassing the first 330 amino acids of kinesin heavy chain fused to GFP (kin330-GFP) that does not alter microtubules upon its overexpression. Kin330-GFP functionally inhibits kinesin-1 because it induces the peri-nuclear accumulation of mitochondria and intermediate filaments. Using this construct and previously established siRNA-mediated knock-down of kinesin-2 function, we assess the role of both motors in the subcellular distribution of distinct steps of the vaccinia virus (VV) life cycle. We show that kinesin-1, but not kinesin-2, contributes to the specific cytoplasmic distribution of three of the four steps of VV morphogenesis tested. These results are discussed with respect to the possible regulation of kinesin-1 during VV infection.

Vaccinia-virus-induced cellular contractility facilitates the subcellular localization of the viral replication sites

Poxviruses, such as vaccinia virus (VV), replicate their DNA in endoplasmic-reticulum-enclosed cytoplasmic sites. Here, we compare the dynamics of the VV replication sites with those of the attenuated strain, modified VV Ankara (MVA). By live-cell imaging, small, early replication sites of both viruses undergo motility typical of microtubule (MT)-motor-mediated movement. Over time, growing replication sites of VV collect around the nucleus in a MT-dependent fashion, whereas those of MVA remain mostly scattered in the cytoplasm. Surprisingly, blocking the dynein function does not impair the perinuclear accumulation of large VV replication sites. Live-cell imaging demonstrates that in contrast to small replication sites, large sites do not display MT-motor-mediated motility. Instead, VV infection induces cellular contractility that facilitates the collection of growing replication sites around the nucleus. In a subset of cells (30-40%), this VV-induced contractility is alternated by phases of directed cell migration, suggesting that the two processes may be linked. The MVA-infected cells do not display contractility or cell migration, supporting the idea that these cellular activities facilitate the efficient accumulation of the VV replication sites around the nucleus. We propose that the recently described cytoskeletal rearrangements induced by VV are a prerequisite for the observed cell contractility and migration activities that apparently contribute to the organization of the complex cytoplasmic life cycle of VV.

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.