Function of dynein and dynactin in herpes simplex virus capsid transport

Tissue-specific and transcription factor-mediated nuclear entry of DNA

Low levels of gene transfer and a lack of tissue-specific targeting of vectors have limited the therapeutic potential of non-viral gene therapy. This is due to the numerous cellular barriers that hinder nuclear delivery of vectors and the paucity of methods that restrict expression to specific cells types. In non-dividing cells, the nuclear envelope is an especially problematic hurdle to gene transfer. Given that the majority of target tissues are non-dividing in vivo, the nuclear membrane is a major obstacle to therapeutic gene transfer. In this review, the various barriers to gene transfer are discussed. In particular, the role of the nuclear pore complex (NPC) in regulating passage of plasmid vectors during interphase is reviewed. Several methods of modifying plasmid (pDNA) vectors to enhance nuclear import through the NPC are also discussed, including the use of tissue-specific transcription factors to mediate nuclear entry of pDNA in a cell-specific manner.

Functional characterization of Negri bodies (NBs) in rabies virus-infected cells: Evidence that NBs are sites of viral transcription and replication

Rabies virus infection induces the formation of cytoplasmic inclusion bodies that resemble Negri bodies found in the cytoplasm of some infected nerve cells. We have studied the morphogenesis and the role of these Negri body-like structures (NBLs) during viral infection. The results indicate that these spherical structures (one or two per cell in the initial stage of infection), composed of the viral N and P proteins, grow during the virus cycle before appearing as smaller structures at late stages of infection. We have shown that the microtubule network is not necessary for the formation of these inclusion bodies but is involved in their dynamics. In contrast, the actin network does not play any detectable role in these processes. These inclusion bodies contain Hsp70 and ubiquitinylated proteins, but they are not misfolded protein aggregates. NBLs, in fact, appear to be functional structures involved in the viral life cycle. Specifically, using in situ fluorescent hybridization techniques, we show that all viral RNAs (genome, antigenome, and every mRNA) are located inside the inclusion bodies. Significantly, short-term RNA labeling in the presence of BrUTP strongly suggests that the NBLs are the sites where viral transcription and replication take place.

Herpes simplex virus type 1 glycoprotein E mediates retrograde spread from epithelial cells to neurites

In animal models of infection, glycoprotein E (gE) is required for efficient herpes simplex virus type 1 (HSV-1) spread from the inoculation site to the cell bodies of innervating neurons (retrograde direction). Retrograde spread in vivo is a multistep process, in that HSV-1 first spreads between epithelial cells at the inoculation site, then infects neurites, and finally travels by retrograde axonal transport to the neuron cell body. To better understand the role of gE in retrograde spread, we used a compartmentalized neuron culture system, in which neurons were infected in the presence or absence of epithelial cells. We found that gE-deleted HSV-1 (NS-gEnull) retained retrograde axonal transport activity when added directly to neurites, in contrast to the retrograde spread defect of this virus in animals. To better mimic the in vivo milieu, we overlaid neurites with epithelial cells prior to infection. In this modified system, virus infects epithelial cells and then spreads to neurites, revealing a 100-fold retrograde spread defect for NS-gEnull. We measured the retrograde spread defect of NS-gEnull from a variety of epithelial cell lines and found that the magnitude of the spread defect from epithelial cells to neurons correlated with epithelial cell plaque size defect, indicating that gE plays a similar role in both types of spread. Therefore, gE-mediated spread between epithelial cells and neurites likely explains the retrograde spread defect of gE-deleted HSV-1 in vivo.

Contribution of direct and cross-presentation to CTL immunity against herpes simplex virus 1

Dendritic cells (DC), which can be subdivided into different phenotypic and functional subsets, play a pivotal role in the generation of cytotoxic T cell immunity against viral infections. Understanding the modes of Ag acquisition, processing and presentation by DC is essential for the design of effective antiviral vaccines. We aimed to assess the contribution of direct vs cross-presentation for the induction of HSV1-specific CD8(+) T lymphocyte responses in mice. Using HSV1 strains expressing fluorescence proteins, we provide evidence for the ability of HSV1 to induce viral transcription. Using HSV1-wild-type as well as gB- or gH-deficient mutants to either directly inoculate DC or to infect target cells, which then were given to DC, we show that DC acquired viral Ag via phagocytosis of target cells and via direct inoculation of virus being released from target cells. Our study corroborates the function of the CD8(+) DC specialized in Ag cross-presentation and confirms this specific feature for Ags that these DC acquire directly from HSV1. However, although infection of cross-presenting DC amplified T cell responses, it was not a requirement for presentation of viral Ags, both in vitro and in vivo. Finally, we provide evidence that direct presentation did not contribute to the Ag presentation capacity of CD8(+) DC after phagocytosis of infected target cells. We conclude that cross-presentation is of major importance for the induction of CTL immunity in mice.

Microtubule-dependent and microtubule-independent steps in Crimean-Congo hemorrhagic fever virus replication cycle

Following binding and entry many viruses exploit the host cell cytoskeleton to ensure intracellular transport, assembly or egress. For Crimean-Congo hemorrhagic fever virus (CCHFV), the causative agent of a severe hemorrhagic disease, virus-host interactions are poorly investigated. In this study we demonstrated that drug-induced suppression of microtubule dynamics and especially microtubule disassembly, impaired CCHFV biogenesis. Our results showed that intact microtubules were required early during virus internalization, and late, during virus assembly and egress. Furthermore, disruption of microtubules resulted in reduced levels of viral RNA while preservation of microtubule dynamics was most important during viral egress. Finally, although CCHFV proteins were redistributed in drug-treated cells, the glycoprotein remained associated with the Golgi apparatus, the organelle of virus budding. Taken together, our results suggest that manipulation of microtubules affects CCHFV entry, replication, assembly and egress.

Translocation of incoming pseudorabies virus capsids to the cell nucleus is delayed in the absence of tegument protein pUL37

After fusion of the envelope of herpesvirus particles with the host cell plasma membrane, incoming nucleocapsids are transported to nuclear pores. Inner tegument proteins pUL36, pUL37, and pUS3 remain attached to the nucleocapsid after entry and therefore might mediate interactions between the nucleocapsid and cellular microtubule-associated motor proteins during transport. To assay for the role of pUL37 in this process, we constructed a pUL37-deleted pseudorabies virus mutant, PrV-DeltaUL37/UL35GFP, which expresses a fusion protein of green fluorescent protein (GFP) and the nonessential small capsid protein pUL35, resulting in the formation of fluorescently labeled capsids. Confocal laser-scanning microscopy of rabbit kidney cells infected with PrV-DeltaUL37/UL35GFP revealed that, whereas penetration was not affected in the absence of pUL37, nuclear translocation of incoming particles was delayed by approximately 1 h compared to PrV-UL35GFP, but not abolished. In contrast, phenotypically complemented pUL37-containing virions of PrV-DeltaUL37/UL35GFP exhibited wild type-like entry kinetics. Thus, the presence of pUL37 is required for rapid nuclear translocation of incoming nucleocapsids.

Making it to the synapse: measles virus spread in and among neurons

Measles virus (MV) is one of the most transmissible microorganisms known, continuing to result in extensive morbidity and mortality worldwide. While rare, MV can infect the human central nervous system, triggering fatal CNS diseases weeks to years after exposure. The advent of crucial laboratory tools to dissect MV neuropathogenesis, including permissive transgenic mouse models, the capacity to manipulate the viral genome using reverse genetics, and cell biology advances in understanding the processes that govern intracellular trafficking of viral components, have substantially clarified how MV infects, spreads, and persists in this unique cell population. This review highlights some of these technical advances, followed by a discussion of our present understanding of MV neuronal infection and transport. Because some of these processes may be shared among diverse viruses, comparisons are made to parallel studies with other neurotropic viruses. While a crystallized view of how the unique environment of the neuron affects MV replication, spread, and, ultimately, neuropathogenesis is not fully realized, the tools and ideas are in place for exciting advances in the coming years.

Peptide-modified vectors for nucleic acid delivery to neurons

Neuron-targeted nucleic acid delivery systems are important technologies for realizing the potential of gene therapy for nervous system disorders. However, neurons are difficult cells to transfect using non-viral vectors due in part to the specific and unique delivery challenges present in these cells. We have investigated several bioactive peptides for their ability to assist in overcoming delivery barriers in mammalian cells. We summarize here our recent progress in developing and applying peptide-modified polycations for nucleic acid delivery. In addition, we present data demonstrating the potential of using multicomponent, peptide-modified polycations for nucleic acid delivery to neurons.

Herpes simplex virus replication: roles of viral proteins and nucleoporins in capsid-nucleus attachment

Replication of herpes simplex virus type 1 (HSV-1) involves a step in which a parental capsid docks onto a host nuclear pore complex (NPC). The viral genome then translocates through the nuclear pore into the nucleoplasm, where it is transcribed and replicated to propagate infection. We investigated the roles of viral and cellular proteins in the process of capsid-nucleus attachment. Vero cells were preloaded with antibodies specific for proteins of interest and infected with HSV-1 containing a green fluorescent protein-labeled capsid, and capsids bound to the nuclear surface were quantified by fluorescence microscopy. Results showed that nuclear capsid attachment was attenuated by antibodies specific for the viral tegument protein VP1/2 (UL36 gene) but not by similar antibodies specific for UL37 (a tegument protein), the major capsid protein (VP5), or VP23 (a minor capsid protein). Similar studies with antibodies specific for nucleoporins demonstrated attenuation by antibodies specific for Nup358 but not Nup214. The role of nucleoporins was further investigated with the use of small interfering RNA (siRNA). Capsid attachment to the nucleus was attenuated in cells treated with siRNA specific for either Nup214 or Nup358 but not TPR. The results are interpreted to suggest that VP1/2 is involved in specific attachment to the NPC and/or in migration of capsids to the nuclear surface. Capsids are suggested to attach to the NPC by way of the complex of Nup358 and Nup214, with high-resolution immunofluorescence studies favoring binding to Nup358.

Two viral kinases are required for sustained long distance axon transport of a neuroinvasive herpesvirus

Axonal transport is essential for the successful establishment of neuroinvasive herpesvirus infections in peripheral ganglia (retrograde transport) and the subsequent spread to exposed body surfaces following reactivation from latency (anterograde transport). We examined two components of pseudorabies virus (US3 and UL13), both of which are protein kinases, as potential regulators of axon transport. Following replication of mutant viruses lacking kinase activity, newly assembled capsids displayed an increase in retrograde motion that prevented efficient delivery of capsids to the distal axon. The aberrant increase in retrograde motion was accompanied by loss of a viral membrane marker from the transported capsids, indicating that the viral kinases allow for efficient anterograde transport by stabilizing membrane-capsid interactions during the long transit from the neuron cell body to the distal axon.

Native 3D intermediates of membrane fusion in herpes simplex virus 1 entry

The concerted action of four viral glycoproteins and at least one cellular receptor is required to catalyze herpes simplex virus 1 entry into host cells either by fusion at the plasma membrane or intracellularly after internalization by endocytosis. Here, we applied cryo electron tomography to capture 3D intermediates from Herpes simplex virus 1 fusion at the plasma membrane in their native environment by using two model systems: adherent cells and synaptosomes. The fusion process was delineated as a series of structurally different steps. The incoming capsid separated from the tegument and was closely surrounded by the cortical cytoskeleton. After entry, the viral membrane curvature changed concomitantly with a reorganization of the envelope glycoprotein spikes. Individual glycoprotein complexes in transitional conformations during pore formation and dilation revealed the complex viral fusion mechanism in action. Snapshots of the fusion intermediates provide unprecedented details concerning the overall structural changes occurring during herpesvirus entry. Moreover, our data suggest that there are two functional "poles" of the asymmetric herpesvirion: one related to cell entry, and the other formed during virus assembly.

Comprehensive characterization of extracellular herpes simplex virus type 1 virions

The herpes simplex virus type 1 (HSV-1) genome is contained in a capsid wrapped by a complex tegument layer and an external envelope. The poorly defined tegument plays a critical role throughout the viral life cycle, including delivery of capsids to the nucleus, viral gene expression, capsid egress, and acquisition of the viral envelope. Current data suggest tegumentation is a dynamic and sequential process that starts in the nucleus and continues in the cytoplasm. Over two dozen proteins are assumed to be or are known to ultimately be added to virions as tegument, but its precise composition is currently unknown. Moreover, a comprehensive analysis of all proteins found in HSV-1 virions is still lacking. To better understand the implication of the tegument and host proteins incorporated into the virions, highly purified mature extracellular viruses were analyzed by mass spectrometry. The method proved accurate (95%) and sensitive and hinted at 8 different viral capsid proteins, 13 viral glycoproteins, and 23 potential viral teguments. Interestingly, four novel virion components were identified (U(L)7, U(L)23, U(L)50, and U(L)55), and two teguments were confirmed (ICP0 and ICP4). In contrast, U(L)4, U(L)24, the U(L)31/U(L)34 complex, and the viral U(L)15/U(L)28/U(L)33 terminase were undetected, as was most of the viral replication machinery, with the notable exception of U(L)23. Surprisingly, the viral glycoproteins gJ, gK, gN, and U(L)43 were absent. Analyses of virions produced by two unrelated cell lines suggest their protein compositions are largely cell type independent. Finally, but not least, up to 49 distinct host proteins were identified in the virions.

Does chronic stress lead to increased rates of recurrences of genital herpes - a review of the psychoneuroimmunological evidence?

Summary:

It is a commonly expressed expert view that stress is associated with frequent recurrences of genital herpes (GH) but the evidence for this is poor and it is often asserted that stress is the result of GH rather than any other cause. We have reviewed the recent literature on this topic, restricting evidence to only prospective studies. We have further combined and integrated this evidence with both human and animal work in the psychoneuroimmunological field to come up with suggestive evidence that recurrence rates of GH are indeed associated with antecedent chronic stress/depression, whatever might be the cause. We further discuss the psychoneuroimmunological underpinning of the systemic features of the recurrent GH prodrome. Recommendations about holistic management of recurrent GH that include both pharmacological and psychological therapies are given.

Transport and egress of herpes simplex virus in neurons

The mechanisms of axonal transport of the alphaherpesviruses, HSV and pseudorabies virus (PrV), in neuronal axons are of fundamental interest, particularly in comparison with other viruses, and offer potential sites for antiviral intervention or development of gene therapy vectors. These herpesviruses are transported rapidly along microtubules (MTs) in the retrograde direction from the axon terminus to the dorsal root ganglion and then anterogradely in the opposite direction. Retrograde transport follows fusion and deenvelopment of the viral capsid at the axonal membrane followed by loss of most of the tegument proteins and then binding of the capsid via one or more viral proteins (VPs) to the retrograde molecular motor dynein. The HSV capsid protein pUL35 has been shown to bind to the dynein light chain Tctex1 but is likely to be accompanied by additional dynein binding of an inner tegument protein. The mechanism of anterograde transport is much more controversial with different processes being claimed for PrV and HSV: separate transport of HSV capsid/tegument and glycoproteins versus PrV transport as an enveloped virion. The controversy has not been resolved despite application, in several laboratories, of confocal microscopy (CFM), real-time fluorescence with viruses dual labelled on capsid and glycoprotein, electron microscopy in situ and immuno-electron microscopy. Different processes for each virus seem counterintuitive although they are the most divergent in the alphaherpesvirus subfamily. Current hypotheses suggest that unenveloped HSV capsids complete assembly in the axonal growth cones and varicosities, whereas with PrV unenveloped capsids are only found travelling in a retrograde direction.

Cellular proteasome activity facilitates herpes simplex virus entry at a postpenetration step

Herpes simplex virus (HSV) entry into cells is a multistep process that engages the host cell machinery. The proteasome is a large, ATP-dependent, multisubunit protease that plays a critical role in the maintenance of cell homeostasis. A battery of assays were used to demonstrate that proteasome inhibitors blocked an early step in HSV entry that occurred after capsid penetration into the cytosol but prior to capsid arrival at the nuclear periphery. Proteasome-dependent viral entry was not reliant on host or viral protein synthesis. MG132, a peptide aldehyde that competitively inhibits the degradative activity of the proteasome, had a reversible inhibitory effect on HSV entry. HSV can use endocytic or nonendocytic pathways to enter cells. These distinct entry routes were both dependent on proteasome-mediated proteolysis. In addition, HSV successfully entered cells in the absence of a functional host ubiquitin-activating enzyme, suggesting that viral entry is ubiquitin independent. We propose that proteasomal degradation of virion and/or host proteins is required for efficient delivery of incoming HSV capsids to the nucleus.

Lissencephaly 1 linking to multiple diseases: mental retardation, neurodegeneration, schizophrenia, male sterility, and more

Lissencephaly 1 (LIS1) was the first gene implicated in the pathogenesis of type-1 lissencephaly. More than a decade of research by multiple laboratories has revealed that LIS1 is a key node protein, which participates in several pathways, including association with the molecular motor cytoplasmic dynein, the reelin signaling pathway, and the platelet-activating factor pathway. Mutations in LIS1-interacting proteins, either in human, or in mouse models has suggested that LIS1 might play a role in the pathogenesis of numerous diseases such as male sterility, schizophrenia, neuronal degeneration, and viral infections.

Functions of human cytomegalovirus tegument proteins prior to immediate early gene expression

Proteins within the tegument layer of herpesviruses such as human cytomegalovirus (HCMV) are released into the cell upon entry when the viral envelope fuses with the cell membrane. These proteins are fully formed and active, and they mediate key events at the very start of the lytic infectious cycle, including the delivery of the viral genome to the nucleus and the initiation of viral gene expression. This review examines what is known about tegument protein function prior to the immediate early (IE) phase of the viral lytic replication cycle and identifies key questions that need to be answered to better understand how these proteins promote HCMV infection so that antiviral treatments that target these important viral regulators can be developed.

Nuclear egress and envelopment of herpes simplex virus capsids analyzed with dual-color fluorescence HSV1(17+)

To analyze the assembly of herpes simplex virus type 1 (HSV1) by triple-label fluorescence microscopy, we generated a bacterial artificial chromosome (BAC) and inserted eukaryotic Cre recombinase, as well as beta-galactosidase expression cassettes. When the BAC pHSV1(17(+))blueLox was transfected back into eukaryotic cells, the Cre recombinase excised the BAC sequences, which had been flanked with loxP sites, from the viral genome, leading to HSV1(17(+))blueLox. We then tagged the capsid protein VP26 and the envelope protein glycoprotein D (gD) with fluorescent protein domains to obtain HSV1(17(+))blueLox-GFPVP26-gDRFP and -RFPVP26-gDGFP. All HSV1 BACs had variations in the a-sequences and lost the oriL but were fully infectious. The tagged proteins behaved as their corresponding wild type, and were incorporated into virions. Fluorescent gD first accumulated in cytoplasmic membranes but was later also detected in the endoplasmic reticulum and the plasma membrane. Initially, cytoplasmic capsids did not colocalize with viral glycoproteins, indicating that they were naked, cytosolic capsids. As the infection progressed, they were enveloped and colocalized with the viral membrane proteins. We then analyzed the subcellular distribution of capsids, envelope proteins, and nuclear pores during a synchronous infection. Although the nuclear pore network had changed in ca. 20% of the cells, an HSV1-induced reorganization of the nuclear pore architecture was not required for efficient nuclear egress of capsids. Our data are consistent with an HSV1 assembly model involving primary envelopment of nuclear capsids at the inner nuclear membrane and primary fusion to transfer capsids into the cytosol, followed by their secondary envelopment on cytoplasmic membranes.

HIV-1 Nef employs two distinct mechanisms to modulate Lck subcellular localization and TCR induced actin remodeling

The Nef protein acts as critical factor during HIV pathogenesis by increasing HIV replication in vivo via the modulation of host cell vesicle transport and signal transduction processes. Recent studies suggested that Nef alters formation and function of immunological synapses (IS), thereby modulating exogenous T-cell receptor (TCR) stimulation to balance between partial T cell activation required for HIV-1 spread and prevention of activation induced cell death. Alterations of IS function by Nef include interference with cell spreading and actin polymerization upon TCR engagement, a pronounced intracellular accumulation of the Src kinase Lck and its reduced IS recruitment. Here we use a combination of Nef mutagenesis and pharmacological inhibition to analyze the relative contribution of these effects to Nef mediated alterations of IS organization and function on TCR stimulatory surfaces. Inhibition of actin polymerization and IS recruitment of Lck were governed by identical Nef determinants and correlated well with Nef's association with Pak2 kinase activity. In contrast, Nef mediated Lck endosomal accumulation was separable from these effects, occurred independently of Pak2, required integrity of the microtubule rather than the actin filament system and thus represents a distinct Nef activity. Finally, reduction of TCR signal transmission by Nef was linked to altered actin remodeling and Lck IS recruitment but did not require endosomal Lck rerouting. Thus, Nef affects IS function via multiple independent mechanisms to optimize virus replication in the infected host.

Nonviral approaches for neuronal delivery of nucleic acids

The delivery of therapeutic nucleic acids to neurons has the potential to treat neurological disease and spinal cord injury. While select viral vectors have shown promise as gene carriers to neurons, their potential as therapeutic agents is limited by their toxicity and immunogenicity, their broad tropism, and the cost of large-scale formulation. Nonviral vectors are an attractive alternative in that they offer improved safety profiles compared to viruses, are less expensive to produce, and can be targeted to specific neuronal subpopulations. However, most nonviral vectors suffer from significantly lower transfection efficiencies than neurotropic viruses, severely limiting their utility in neuron-targeted delivery applications. To realize the potential of nonviral delivery technology in neurons, vectors must be designed to overcome a series of extra- and intracellular barriers. In this article, we describe the challenges preventing successful nonviral delivery of nucleic acids to neurons and review strategies aimed at overcoming these challenges.

Identification of functional domains within the essential large tegument protein pUL36 of pseudorabies virus

Proteins of the capsid proximal tegument are involved in the transport of incoming capsids to the nucleus and secondary envelopment after nuclear egress. Homologs of the essential large capsid proximal tegument protein pUL36 are conserved within the Herpesviridae. They interact with another tegument component, pUL37, and contain a deubiquitinating activity in their N termini which, however, is not essential for virus replication. Whereas an internal deletion of 709 amino acids (aa) within the C-terminal half of the alphaherpesvirus pseudorabies virus (PrV) pUL36 does not impair its function (S. Böttcher, B. G. Klupp, H. Granzow, W. Fuchs, K. Michael, and T. C. Mettenleiter, J. Virol. 80:9910-9915, 2006), deletion of the very C terminus does (J. Lee, G. Luxton, and G. A. Smith, J. Virol. 80:12086-12094, 2006). For further characterization we deleted several predicted functional and structural motifs within PrV pUL36 and analyzed the resulting phenotypes in cell culture and a mouse infection model. Extension of the internal deletion to encompass aa 2087 to 2981 exerted only minor effects on virus replication but resulted in prolonged mean survival times of infected mice. Any additional extension did not yield viable virus. Deletion of an N-terminal region containing the deubiquitinating activity (aa 22 to 248) only slightly impaired viral replication in cell culture but slowed neuroinvasion in our mouse model, whereas a strong impairment of viral replication was observed after simultaneous removal of both nonessential domains. Absence of a region containing two predicted leucine zipper motifs (aa 748 to 991) also strongly impaired virus replication and spread. Thus, we identify several domains within the PrV UL36 protein, which, though not essential, are nevertheless important for virus replication.

Three-dimensional structure of the human cytomegalovirus cytoplasmic virion assembly complex includes a reoriented secretory apparatus

Human cytomegalovirus (HCMV) induces profound changes in infected cell morphology, including a large cytoplasmic inclusion that corresponds to the virion assembly complex (AC). In electron micrographs, the AC is a highly vacuolated part of the cytoplasm. Markers of cellular secretory organelles have been visualized at the outer edge of the AC, and we recently showed that a marker for early endosomes (i.e., early endosome antigen 1) localizes to the center of the AC. Here, we examined the relationship between the AC and components of the secretory apparatus, studied temporal aspects of the dramatic infection-induced cytoplasmic remodeling, examined the three-dimensional structure of the AC, and considered the implications of our observations for models of HCMV virion maturation and egress. We made three major observations. First, in addition to being relocated, the expression levels of some organelle markers change markedly during the period while the AC is developing. Second, based on three-dimensional reconstructions from z-series confocal microscopic images, the observed concentric rings of vesicles derived from the several compartments (Golgi bodies, the trans-Golgi network [TGN], and early endosomes) are arranged as nested cylinders of organelle-specific vesicles. Third, the membrane protein biosynthetic and exocytic pathways from the endoplasmic reticulum to the Golgi bodies, TGN, and early endosomes are in an unusual arrangement that nonetheless allows for a conventional order of biosynthesis and transport. Our model of AC structure suggests a mechanism by which the virus can regulate the order of tegument assembly.

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.

The Herpesvirus capsid surface protein, VP26, and the majority of the tegument proteins are dispensable for capsid transport toward the nucleus

Upon entering a cell, alphaherpesvirus capsids are transported toward the minus ends of microtubules and ultimately deposit virus DNA within the host nucleus. The virus proteins that mediate this centripetal transport are unknown but are expected to be either viral tegument proteins, which are a group of capsid-associated proteins, or a surface component of the capsid itself. Starting with derivatives of pseudorabies virus that encode a fluorescent protein fused to a structural component of the virus, we have made a collection of 12 mutant viruses that lack either the VP26 capsid protein or an individual tegument protein. Using live-cell fluorescence microscopy, we tracked individual virus particles in axons following infection of primary sensory neurons. Quantitative analysis of the VP26-null virus indicates that this protein plays no observable role in capsid transport. Furthermore, viruses lacking tegument proteins that are nonessential for virus propagation in cell culture were also competent for axonal transport. These results indicate that a protein essential for viral propagation mediates transport of the capsid to the nucleus.

Dynein-dependent transport of the hantaan virus nucleocapsid protein to the endoplasmic reticulum-Golgi intermediate compartment

In contrast to most negative-stranded RNA viruses, hantaviruses and other viruses in the family Bunyaviridae mature intracellularly, deriving the virion envelope from the endoplasmic reticulum (ER) or Golgi compartment. While it is generally accepted that Old World hantaviruses assemble and bud into the Golgi compartment, some studies with New World hantaviruses have raised the possibility of maturation at the plasma membrane as well. Overall, the steps leading to virion assembly remain largely undetermined for hantaviruses. Because hantaviruses do not have matrix proteins, the nucleocapsid protein (N) has been proposed to play a key role in assembly. Herein, we examine the intracellular trafficking and morphogenesis of the prototype Old World hantavirus, Hantaan virus (HTNV). Using confocal microscopy, we show that N colocalized with the ER-Golgi intermediate compartment (ERGIC) in HTNV-infected Vero E6 cells, not with the ER, Golgi compartment, or early endosomes. Brefeldin A, which effectively disperses the ER, the ERGIC, and Golgi membranes, redistributed N with the ERGIC, implicating membrane association; however, subcellular fractionation experiments showed the majority of N in particulate fractions. Confocal microscopy revealed that N was juxtaposed to and distributed along microtubules and, over time, became surrounded by vimentin cages. To probe cytoskeletal association further, we probed trafficking of N in cells treated with nocodazole and cytochalasin D, which depolymerize microtubules and actin, respectively. We show that nocodazole, but not cytochalasin D, affected the distribution of N and reduced levels of intracellular viral RNA. These results suggested the involvement of microtubules in trafficking of N, whose movement could occur via molecular motors such as dynein. Overexpression of dynamitin, which is associated with dynein-mediated transport, creates a dominant-negative phenotype blocking transport on microtubules. Overexpression of dynamitin reduced N accumulation in the perinuclear region, which further supports microtubule components in N trafficking. The combined results of these experiments support targeting of N to the ERGIC prior to its movement to the Golgi compartment and the requirement of an intact ERGIC for viral replication and, thus, the possibility of virus factories in this region.

Requirement for the dynein light chain km23-1 in a Smad2-dependent transforming growth factor-beta signaling pathway

We have identified km23-1 as a novel transforming growth factor-beta (TGFbeta) receptor (TbetaR)-interacting protein that is also a light chain of the motor protein dynein (dynein light chain). Herein, we demonstrate by sucrose gradient analyses that, in the presence of TGFbeta but not in the absence, km23-1 was present in early endosomes with the TbetaRs. Further, confocal microscopy studies indicate that endogenous km23-1 was co-localized with endogenous Smad2 at early times after TGFbeta treatment, prior to Smad2 translocation to the nucleus. In addition, immunoprecipitation/blot analyses showed that TGFbeta regulated the interaction between endogenous km23-1 and endogenous Smad2 in vivo. Blockade of km23-1 using a small interfering RNA approach resulted in a reduction in both total intracellular Smad2 levels and in nuclear levels of phosphorylated Smad2 after TGFbeta treatment. This decrease was reversed by lactacystin, a specific inhibitor of the 26 S proteasome, suggesting that knockdown of km23-1 causes proteasomal degradation of phosphorylated (i.e. activated) Smad2. Blockade of km23-1 also resulted in a reduction in TGFbeta/Smad2-dependent ARE-Lux transcriptional activity, which was rescued by a km23-1 small interfering RNA-resistant construct. In contrast, a reduction in TGFbeta/Smad3-dependent SBE2-Luc transcriptional activity did not occur under similar conditions. Furthermore, overexpression of the dynactin subunit dynamitin, which is known to disrupt dynein-mediated intracellular transport, blocked TGFbeta-stimulated nuclear translocation of Smad2. Collectively, our findings indicate for the first time that a dynein light chain is required for a Smad2-dependent TGFbeta signaling pathway.

Immune synapses formed with measles virus-infected dendritic cells are unstable and fail to sustain T cell activation

Interaction with dendritic cells (DCs) is considered as central to immunosuppression induced by viruses, including measles virus (MV). Commonly, viral infection of DCs abrogates their ability to promote T cell expansion, yet underlying mechanisms at a cellular level are undefined. We found that MV-infected DCs only subtly differed from LPS-matured with regard to integrin activation, acquisition of a migratory phenotype and motility. Similarly, the organization of MV-DC/T cell interfaces was consistent with that of functional immune synapses with regard to CD3 clustering and MHC class II surface recruitment. The majority of MV-DC/T cell conjugates was, however, unstable and only promoted abortive T cell activation. Thus, MV-infected DCs retain activities required for initiating, but not sustaining T cell conjugation and activation. This is partially rescued if surface expression of the MV glycoproteins on DCs is abolished by infection with a recombinant MV encoding VSV G protein instead, indicating that these contribute directly to synapse destabilization and thereby act as effectors of T cell inhibition.

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.

In vitro interaction of poliovirus with cytoplasmic dynein

OBJECTIVE

Poliovirus (PV) enters the host by the oral route and can infect the central nervous system (CNS) by two mechanisms: crossing the blood-brain barrier and traveling along the nerves from the muscle to the spinal cord. In the latter mechanism, the PV receptor, CD155, and the motor protein, dynein, have been implicated in the transport of PV to the CNS. In this work we analyzed the possible interaction of PV with dynein.

METHODS

PV was bound to a Sepharose 4B beads and they were used to analyze the interaction of PV with cytoplasmic proteins from neuroblastoma cells by affinity chromatography and Western blot.

RESULTS

The interaction with cytoplasmic dynein was observed only when the Sepharose beads bound to PV were used and not in the control ones, where proteins from uninfected cells were coupled.

CONCLUSION

These preliminary results open the possibility that PV uses the dynein directly in its retrograde axonal transport.

The pseudorabies virus VP1/2 tegument protein is required for intracellular capsid transport

Transport of capsids in cells is critical to alphaherpesvirus infection and pathogenesis; however, viral factors required for transport have yet to be identified. Here we provide a detailed examination of capsid dynamics during the egress phase of infection in Vero cells infected with pseudorabies virus. We demonstrate that the VP1/2 tegument protein is required for processive microtubule-based transport of capsids in the cytoplasm. A second tegument protein that binds to VP1/2, UL37, was necessary for wild-type transport but was not essential for this process. Both proteins were also required for efficient nuclear egress of capsids to the cytoplasm.

Bovine herpesvirus 1 infection and infectious bovine rhinotracheitis

Bovine herpesvirus 1 (BoHV-1), classified as an alphaherpesvirus, is a major pathogen of cattle. Primary infection is accompanied by various clinical manifestations such as infectious bovine rhinotracheitis, abortion, infectious pustular vulvovaginitis, and systemic infection in neonates. When animals survive, a life-long latent infection is established in nervous sensory ganglia. Several reactivation stimuli can lead to viral re-excretion, which is responsible for the maintenance of BoHV-1 within a cattle herd. This paper focuses on an updated pathogenesis based on a molecular characterization of BoHV-1 and the description of the virus cycle. Special emphasis is accorded to the impact of the latency and reactivation cycle on the epidemiology and the control of BoHV-1. Several European countries have initiated BoHV-1 eradication schemes because of the significant losses incurred by disease and trading restrictions. The vaccines used against BoHV-1 are described in this context where the differentiation of infected from vaccinated animals is of critical importance to achieve BoHV-1 eradication.

Intracellular transport of hepatitis B virus

For genome multiplication hepadnaviruses use the transcriptional machinery of the cell that is found within the nucleus. Thus the viral genome has to be transported through the cytoplasm and nuclear pore. The intracytosolic translocation is facilitated by the viral capsid that surrounds the genome and that interacts with cellular microtubules. The subsequent passage through the nuclear pore complexes (NPC) is mediated by the nuclear transport receptors importin alpha and beta. Importin alpha binds to the C-terminus of the capsid protein that comprises a nuclear localization signal (NLS). The exposure of the NLS is regulated and depends upon genome maturation and/or phosphorylation of the capsid protein. As for other karyophilic cargos using this pathway importin alpha interacts with importin beta that facilitates docking of the import complex to the NPC and the passage through the pore. Being a unique strategy, the import of the viral capsid is incomplete in that it becomes arrested inside the nuclear basket, which is a cage-like structure on the karyoplasmic face of the NPC. Presumably only this compartment provides the factors that are required for capsid disassembly and genome release that is restricted to those capsids comprising a mature viral DNA genome.

Evaluation of an LC8-binding peptide for the attachment of artificial cargo to dynein

The limited cytoplasmic mobility of nonviral gene carriers is likely to contribute to their low transfection efficiency. This limitation could be overcome by mimicking the viral strategy of recruiting the dynein motor complex for efficient transport toward the host cell nucleus. A promising approach for attaching artificial cargo to dynein is through an adaptor peptide that binds the 8 kDa light chain (LC8) found in the cargo-binding region of the dynein complex. Several viral proteins that bind LC8 have in common an LC8-binding motif defined by (K/R)XTQT. Short peptides containing this motif have also been shown to bind recombinant LC8 in vitro. However, since the majority of intracellular LC8 exists outside of the dynein complex, it remains unclear whether peptides displaying this LC8-binding motif can access and bind to dynein-associated LC8. In this study, we employed biochemical analysis to investigate the feasibility of attaching artificial cargo to the dynein motor complex using a peptide displaying the well-characterized LC8-binding motif. We report that free intracellular LC8 bound specifically to an LC8-binding (TQT) peptide and not to a control peptide with a mutated LC8-binding motif. However, a similar binding interaction between the TQT peptide and intracellular dynein was not detected. To determine whether dynein binding of the TQT peptide was prevented by competition with free intracellular LC8 or due to the inability of the peptide to access its LC8 binding site in the dynein complex, the TQT peptide was evaluated for its ability to bind either purified LC8 or purified dynein. Our results demonstrate that, while the TQT peptide readily binds free LC8, it cannot bind to dynein-associated LC8. The results emphasize the need to identify functional dynein-binding peptides and highlight the importance of designing peptides that bind to the intact dynein motor complex.

Intracellular trafficking of plasmids for gene therapy: mechanisms of cytoplasmic movement and nuclear import

Under physiologically relevant conditions, the levels of non-viral gene transfer are low at best. The reason for this is that many barriers exist for the efficient transfer of genes to cells, even before any gene expression can occur. While many transfection strategies focus on DNA condensation and overcoming the plasma membrane, events associated with the intracellular trafficking of the DNA complexes have not been as extensively studied. Once internalized, plasmids must travel potentially long distances through the cytoplasm to reach their next barrier, the nuclear envelope. This review summarizes the current progress on the cytoplasmic trafficking and nuclear transport of plasmids used for gene therapy applications. Both of these processes utilize specific and defined mechanisms to facilitate movement of DNA complexes through the cell. The continued elucidation and exploitation of these mechanisms will lead to improved strategies for transfection and successful gene therapy.

Two modes of herpesvirus trafficking in neurons: membrane acquisition directs motion

Alphaherpesvirus infection of the mammalian nervous system is dependent upon the long-distance intracellular transport of viral particles in axons. How viral particles are effectively trafficked in axons to either sensory ganglia following initial infection or back out to peripheral sites of innervation following reactivation remains unknown. The mechanism of axonal transport has, in part, been obscured by contradictory findings regarding whether capsids are transported in axons in the absence of membrane components or as enveloped virions. By imaging actively translocated viral structural components in living peripheral neurons, we demonstrate that herpesviruses use two distinct pathways to move in axons. Following entry into cells, exposure of the capsid to the cytosol resulted in efficient retrograde transport to the neuronal cell body. In contrast, progeny virus particles moved in the anterograde direction following acquisition of virion envelope proteins and membrane lipids. Retrograde transport was effectively shut down in this membrane-bound state, allowing for efficient delivery of progeny viral particles to the distal axon. Notably, progeny viral particles that lacked a membrane were misdirected back to the cell body. These findings show that cytosolic capsids are trafficked to the neuronal cell body and that viral egress in axons occurs after capsids are enshrouded in a membrane envelope.

The cycle of human herpes simplex virus infection: virus transport and immune control

After infection of skin or mucosa, herpes simplex virus enters the sensory nerve endings and is conveyed by retrograde axonal transport to the dorsal root ganglion, where the virus develops lifelong latency. Intermittent reactivation, which is spontaneous in humans, leads to anterograde transport of virus particles and proteins to the skin or mucosa, where the virus is shed and/or causes disease. Immune control of viral infection and replication occurs at the level of skin or mucosa during initial or recurrent infection and also within the dorsal root ganglion, where immune mechanisms control latency and reactivation. This article examines current views on the mechanisms of retrograde and anterograde transport of the virus in axons and the mechanisms of innate and adaptive immunity that control infection in the skin or mucosa and in the dorsal root ganglion--in particular, the role of interferons, myeloid and plasmacytoid dendritic cells, CD4(+) and CD8(+) T cells, and interferon- gamma and other cytokines, including their significance in the development of vaccines for genital herpes.

Betaherpesvirus-conserved cytomegalovirus tegument protein ppUL32 (pp150) controls cytoplasmic events during virion maturation

The UL32 gene of human cytomegalovirus (CMV) encodes a prominent betaherpesvirus-conserved virion tegument protein, called pp150 (basic phosphoprotein/ppUL32), that accumulates within a cytoplasmic inclusion adjacent to the nucleus at late times during infection. Using a UL32 deletion mutant (DeltaUL32-BAC) (where BAC is bacterial artificial chromosome), we demonstrate that pp150 is critical for virion maturation in the cytoplasmic compartment. Cotransfection of a pp150 expression plasmid with DeltaUL32-BAC DNA led to complementation of the replication defect with focus formation due to secondary spread. Deletion of the amino terminus of pp150 or disruption of the betaherpesvirus conserved regions, CR1 and CR2, revealed these regions to be critical for replication. In contrast, deletion of the carboxyl terminus only partially compromised maturation while disruption of glycosylation sites had no effect. An African green monkey CMV UL32 homolog complemented DeltaUL32-BAC replication but murine CMV M32 failed to complement, consistent with evolutionary divergence of rodent and primate cytomegaloviruses. Infection with DeltaUL32-BAC showed normal expression of all kinetic classes of viral genes and replication of viral DNA, with accumulation of viral DNA-containing particles in the cytoplasm; however, mutant virus did not spread to adjacent cells. In contrast to this block in virion infectivity, cell-to-cell transfer of pp65-containing particles was observed, suggesting that release of dense bodies continued in the absence of pp150. These observations demonstrate that pp150 is critical for virion egress, possibly at the stage of final envelopment.

Measles virus contact with T cells impedes cytoskeletal remodeling associated with spreading, polarization, and CD3 clustering

CD3/CD28-induced activation of the PI3/Akt kinase pathway and proliferation is impaired in T cells after contact with the measles virus (MV) glycoprotein (gp) complex. We now show that this signal also impairs actin cytoskeletal remodeling in T cells, which loose their ability to adhere and to promote microvilli formation. MV exposure results in an almost complete collapse of membrane protrusions associated with reduced phosphorylation levels of cofilin and ezrin/radixin/moesin (ERM) proteins. Consistent with their inability to activate Cdc42 and Rac1 in response to the ligation of CD3/CD28, T cells exposed to MV fail to acquire a morphology consistent with spreading and lamellopodia formation. In spite of these impairments of cytoskeleton-driven morphological alterations, these cells are recruited into conjugates with dendritic cells as efficiently as control T cells. The signal elicited by MV, however, prevents T cells to polarize as documented by a failure to redistribute the microtubule organizing center toward the synapse. Moreover, CD3 cannot be efficiently clustered and redistributed to the central region of the immunological synapse. Thus, by inducing microvillar collapse and interfering with cytoskeletal remodeling, MV signaling disturbs the ability of T cells to adhere, spread, and cluster receptors essential for sustained T-cell activation.

A common mechanism for cytoplasmic dynein-dependent microtubule binding shared among adeno-associated virus and adenovirus serotypes

During infection, adenovirus-associated virus (AAV) undergoes microtubule-dependent retrograde transport as part of a program of vectorial transport of viral genome to the nucleus. A microtubule binding assay was used to evaluate the hypothesis that cytoplasmic dynein mediates AAV interaction with microtubules. Binding of AAV serotype 2 (AAV2) was enhanced in a nucleotide-dependent manner by the presence of total cellular microtubule-associated proteins (MAPs) but not cytoplasmic dynein-depleted MAPs. Excess AAV2 capsid protein prevented microtubule binding by AAV serotypes 2, 5, and rh.10, as well as adenovirus serotype 5, indicating that similar binding sites are used by these viruses.

Role of pseudorabies virus Us3 protein kinase during neuronal infection

The pseudorabies virus (PRV) Us3 gene is conserved among the alphaherpesviruses and encodes a serine/threonine protein kinase that is not required for growth in standard cell lines. In this report, we used a compartmented culture system to investigate the role of PRV Us3 in viral replication in neurons, in spread from neurons to PK15 cells, and in axon-mediated spread of infection. We also examined the role of Us3 in neuroinvasion and virulence in rodents. Us3 null mutants produce about 10-fold less infectious virus from neurons than wild-type virus and have no discernible phenotypes for axonal targeting of viral components in cultured peripheral nervous system neurons. After eye infection in rodents, Us3 null mutants were slightly attenuated for virulence, with a delayed onset of symptoms compared to the wild type or a Us3 null revertant. While initially delayed, the symptoms increased in severity until they approximated those of the wild-type virus. Us3 null mutants were neuroinvasive, spreading in both efferent and afferent circuits innervating eye tissues.

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.

Eclipse phase of herpes simplex virus type 1 infection: Efficient dynein-mediated capsid transport without the small capsid protein VP26

Cytoplasmic dynein,together with its cofactor dynactin, transports incoming herpes simplex virus type 1 (HSV-1) capsids along microtubules (MT) to the MT-organizing center (MTOC). From the MTOC, capsids move further to the nuclear pore, where the viral genome is released into the nucleoplasm. The small capsid protein VP26 can interact with the dynein light chains Tctex1 (DYNLT1) and rp3 (DYNLT3) and may recruit dynein to the capsid. Therefore, we analyzed nuclear targeting of incoming HSV1-DeltaVP26 capsids devoid of VP26 and of HSV1-GFPVP26 capsids expressing a GFPVP26 fusion instead of VP26. To compare the cell entry of different strains, we characterized the inocula with respect to infectivity, viral genome content, protein composition, and particle composition. Preparations with a low particle-to-PFU ratio showed efficient nuclear targeting and were considered to be of higher quality than those containing many defective particles, which were unable to induce plaque formation. When cells were infected with HSV-1 wild type, HSV1-DeltaVP26, or HSV1-GFPVP26, viral capsids were transported along MT to the nucleus. Moreover, when dynein function was inhibited by overexpression of the dynactin subunit dynamitin, fewer capsids of HSV-1 wild type, HSV1-DeltaVP26, and HSV1-GFPVP26 arrived at the nucleus. Thus, even in the absence of the potential viral dynein receptor VP26, HSV-1 used MT and dynein for efficient nuclear targeting. These data suggest that besides VP26, HSV-1 encodes other receptors for dynein or dynactin.

Reconstitution of herpes simplex virus microtubule-dependent trafficking in vitro

Microtubule-mediated anterograde transport of herpes simplex virus (HSV) from the neuronal cell body to the axon terminal is crucial for the spread and transmission of the virus. It is therefore of central importance to identify the cellular and viral factors responsible for this trafficking event. In previous studies, we isolated HSV-containing cytoplasmic organelles from infected cells and showed that they represent the first and only destination for HSV capsids after they emerge from the nucleus. In the present study, we tested whether these cytoplasmic compartments were capable of microtubule-dependent traffic. Organelles containing green fluorescent protein-labeled HSV capsids were isolated and found to be able to bind rhodamine-labeled microtubules polymerized in vitro. Following the addition of ATP, the HSV-associated organelles trafficked along the microtubules, as visualized by time lapse microscopy in an imaging microchamber. The velocity and processivity of trafficking resembled those seen for neurotropic herpesvirus traffic in living axons. The use of motor-specific inhibitors indicated that traffic was predominantly kinesin mediated, consistent with the reconstitution of anterograde traffic. Immunocytochemical studies revealed that the majority of HSV-containing organelles attached to the microtubules contained the trans-Golgi network marker TGN46. This simple, minimal reconstitution of microtubule-mediated anterograde traffic should facilitate and complement molecular analysis of HSV egress in vivo.

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.

A motor neuron disease-associated mutation in p150Glued perturbs dynactin function and induces protein aggregation

The microtubule motor cytoplasmic dynein and its activator dynactin drive vesicular transport and mitotic spindle organization. Dynactin is ubiquitously expressed in eukaryotes, but a G59S mutation in the p150^Glued subunit of dynactin results in the specific degeneration of motor neurons. This mutation in the conserved cytoskeleton-associated protein, glycine-rich (CAP-Gly) domain lowers the affinity of p150^Glued for microtubules and EB1. Cell lines from patients are morphologically normal but show delayed recovery after nocodazole treatment, consistent with a subtle disruption of dynein/dynactin function. The G59S mutation disrupts the folding of the CAP-Gly domain, resulting in aggregation of the p150^Glued protein both in vitro and in vivo, which is accompanied by an increase in cell death in a motor neuron cell line. Overexpression of the chaperone Hsp70 inhibits aggregate formation and prevents cell death. These data support a model in which a point mutation in p150^Glued causes both loss of dynein/dynactin function and gain of toxic function, which together lead to motor neuron cell death.

The inner tegument promotes herpes simplex virus capsid motility along microtubules in vitro

After viral fusion, capsids of the neurotropic herpes simplex virus are transported along microtubules (MT) to the nuclear pores for viral genome uncoating, nuclear transcription and replication. After assembly and egress from the nucleus, cytosolic capsids are transported to host membranes for secondary envelopment or to the axon terminal for further viral spread. Using GFP-tagged capsids, Cy3-labelled MT and cytosol, we have reconstituted viral capsid transport in vitro. In the presence of ATP, capsids moved along MT up to 30 microm. Blocking the function of dynactin, a cofactor of dynein and kinesin-2, inhibited the transport. Removing outer tegument proteins from the capsids increased in vitro motility. In contrast, capsids isolated from infected nuclei that were devoid of inner as well as outer tegument proteins showed little interaction with dynein and its cofactor dynactin. Our data suggest that the inner tegument of alphaherpesviruses contains viral receptors for MT motors.

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.