Role of plasma membrane lipid microdomains in respiratory syncytial virus filament formation

The fusion protein (F) of respiratory syncytial virus (RSV) is the envelope glycoprotein responsible for the characteristic cytopathology of syncytium formation. RSV has been shown to bud from selective areas of the plasma membrane as pleomorphic virions, including both filamentous and round particles. With immunofluorescent microscopy, we demonstrated evidence of RSV filaments incorporating the fusion protein F and colocalizing with a lipid microdomain-specific fluorescent dye, 1,1-dihexadecyl-3,3,3,3-tetramethylindocarbocyanine perchlorate. Western blot analysis of Triton X-100 cold-extracted membrane fractions confirmed the presence of RSV proteins within the lipid microdomains. RSV proteins also colocalized with cellular proteins associated with lipid microdomains, caveolin-1, and CD44, as well as with RhoA, a small GTPase. ADP-ribosylation of RhoA by Clostridium botulinum exotoxin inactivated RhoA signaling and resulted in the absence of RSV-induced syncytia despite no significant change in viral titer. We demonstrated an overall decrease in both the number and length of the viral filaments and a shift in the localization of F to nonlipid microdomain regions of the membrane in the presence of C3 toxin. This suggests that the selective incorporation of RSV proteins into lipid microdomains during virus assembly may lead to critical interactions of F with cellular proteins, resulting in microvillus projections necessary for the formation of filamentous virus particles and syncytium formation. Thus, manipulation of membrane lipid microdomains may lead to alterations in the production of viral filaments and RSV pathogenesis and provide a new pharmacologic target for RSV therapy.     

GP96 Interacts with HHV-6 during Viral Entry and Directs It for Cellular Degradation

CD46 and CD134 mediate attachment of Human Herpesvirus 6A (HHV-6A) and HHV-6B to host cell, respectively. But many cell types interfere with viral infection through rapid degradation of viral DNA. Hence, not all cells expressing these receptors are permissive to HHV-6 DNA replication and production of infective virions suggesting the involvement of additional factors that influence HHV-6 propagation. Here, we used a proteomics approach to identify other host cell proteins necessary for HHV-6 binding and entry. We found host cell chaperone protein GP96 to interact with HHV-6A and HHV-6B and to interfere with virus propagation within the host cell. In human peripheral blood mononuclear cells (PBMCs), GP96 is transported to the cell surface upon infection with HHV-6 and interacts with HHV-6A and -6B through its C-terminal end. Suppression of GP96 expression decreased initial viral binding but increased viral DNA replication. Transient expression of human GP96 allowed HHV-6 entry into CHO-K1 cells even in the absence of CD46. Thus, our results suggest an important role for GP96 during HHV-6 infection, which possibly supports the cellular degradation of the virus.     

Heat shock protein 70 is necessary for Rice stripe virus infection in plants

Heat shock proteins 70 (HSP70s) are a highly conserved family of genes in eukaryotes, and are involved in a remarkable variety of cellular processes. In many plant positive‐stranded RNA viruses, HSP70 participates in the construction of a viral replication complex and plays various roles during viral infection. Here, we found increased expression of HSP70 following infection by Rice stripe virus (RSV), a negative‐stranded RNA virus, in both rice (the natural host) and Nicotiana benthamiana (an experimental host). Heat treatment of N. benthamiana (Nb) plants enhanced viral infection, whereas RSV infection was retarded and viral RNAs accumulated at a low level when HSP70 was silenced. In both bimolecular fluorescence complement and in vitro pull‐down assays, the N‐terminus of RSV RNA‐dependent RNA polymerase (RdRp) interacted and co‐localized with the HSP70s of both plants (OsHSP70 and NbHSP70). The localization of the N‐terminus of RdRp when expressed alone was not obviously different from when it was co‐expressed with OsHSP or NbHSP, and vice versa. RSV infection also had no effect on the localization of host HSP70. These results demonstrate that host HSP70 is necessary for RSV infection and probably plays a role in viral replication by interacting with viral RdRp, which provides the first evidence of an interacting host protein related to RSV replication, which has been little studied to date.     

Late activation of the Raf/MEK/ERK pathway is required for translocation of the respiratory syncytial virus F protein to the plasma membrane and efficient viral replication

Activation of the Raf/MEK/ERK cascade is required for efficient propagation of several RNA and DNA viruses, including human respiratory syncytial virus (RSV). In RSV infection, activation of the Raf/MEK/ERK cascade is biphasic. An early induction within minutes after infection is associated with viral attachment. Subsequently, a second activation occurs with, so far, unknown function in the viral life cycle. In this study, we aimed to characterise the role of Raf/MEK/ERK‐mediated signalling during ongoing RSV infection. Our data show that inhibition of the kinase MEK after the virus has been internalised results in a reduction of viral titers. Further functional investigations revealed that the late‐stage activation of ERK is required for a specific step in RSV replication, namely, the secretory transport of the RSV fusion protein F. Thus, MEK inhibition resulted in impaired surface accumulation of the F protein. F protein surface expression is essential for efficient replication as it is involved in viral filament formation, cell fusion, and viral transmission. In summary, we provide detailed insights of how host cell signalling interferes with RSV replication and identified the Raf/MEK/ERK kinase cascade as potential target for novel anti‐RSV strategies.     

Foot-and-Mouth Disease Virus Modulates Cellular Vimentin for Virus Survival

Foot-and-mouth disease virus (FMDV), the causative agent of foot-and-mouth disease, is an Aphthovirus within the Picornaviridae family. During infection with FMDV, several host cell membrane rearrangements occur to form sites of viral replication. FMDV protein 2C is part of the replication complex and thought to have multiple roles during virus replication. To better understand the role of 2C in the process of virus replication, we have been using a yeast two-hybrid approach to identify host proteins that interact with 2C. We recently reported that cellular Beclin1 is a natural ligand of 2C and that it is involved in the autophagy pathway, which was shown to be important for FMDV replication. Here, we report that cellular vimentin is also a specific host binding partner for 2C. The 2C-vimentin interaction was further confirmed by coimmunoprecipitation and immunofluorescence staining to occur in FMDV-infected cells. It was shown that upon infection a vimentin structure forms around 2C and that this structure is later resolved or disappears. Interestingly, overexpression of vimentin had no effect on virus replication; however, overexpression of a truncated dominant-negative form of vimentin resulted in a significant decrease in viral yield. Acrylamide, which causes disruption of vimentin filaments, also inhibited viral yield. Alanine scanning mutagenesis was used to map the specific amino acid residues in 2C critical for vimentin binding. Using reverse genetics, we identified 2C residues that are necessary for virus growth, suggesting that the interaction between FMDV 2C and cellular vimentin is essential for virus replication.     

Interactions between respiratory syncytial virus and the host cell: opportunities for antivirus strategies?

At the start of the 21st century, respiratory syncytial virus (RSV) remains a serious global health concern. Although RSV has traditionally been acknowledged as a leading cause of morbidity and mortality in the paediatric population, the elderly and people with suppressed immune systems are now also recognised as being at risk from serious RSV infection. This problem is currently exacerbated by the lack of an effective vaccine to prevent RSV infection. Although the virus proteins play a variety of roles during the virus replication cycle, in many cases these tasks are performed via specific interactions with host-cell factors, including proteins, carbohydrates and lipids. The way in which RSV interacts with the host cell is currently being examined using a battery of different techniques, which encompass several scientific disciplines. This is providing new and interesting insights into how RSV interacts with the host cell at the molecular level, which in turn is offering the hope of new strategies to prevent RSV infection.     

Interaction of frog virus-3 with the cytoskeleton. I. Altered organization of microtubules, intermediate filaments, and microfilaments

The progressive cytoskeletal alterations of frog virus 3-infected baby hamster kidney (BHK) and fathead minnow (FHM) cells were studied by immunofluorescence and electron microscopy. The virus assembly sites, which contain viral genomes and viral proteins, were detected in the cytoplasm at 4 h (FHM) or 6 h (BHK) and mature virions appeared 2 h later. When infected cells were treated with Triton X-100, the assembly sites were found in association with the cytoskeleton. In infected cells, the number of microtubules progressively decreased but a few microtubules traversing in the vicinity of the assembly sites remained intact. Early in infection, the intermediate filaments retracted from the cell periphery, delimited the forming assembly sites, and remained there throughout infection. We suggest that intermediate filaments are involved in the formation of assembly sites. In addition, the filaments either by themselves or in conjunction with microtubules may anchor the assembly sites near the nucleus. The microfilament bundles (stress fibers) disappeared with the formation of assembly sites, and late in infection many projections containing microfilaments and virus particles appeared at the cell surface. The observation suggests a role for microfilaments in virus release. Taken together, these results provide the first example of a virus-infected cell in which all three cytoskeletal filaments show profound organizational changes and suggest an active participation of the host cytoskeleton in viral functions.     

A novel viral strategy for host factor recruitment: The co-opted proteasomal Rpn11 protein interaction hub in cooperation with subverted actin filaments are targeted to deliver cytosolic host factors for viral replication

Positive-strand (+)RNA viruses take advantage of the host cells by subverting a long list of host protein factors and transport vesicles and cellular organelles to build membranous viral replication organelles (VROs) that support robust RNA replication. How RNA viruses accomplish major recruitment tasks of a large number of cellular proteins are intensively studied. In case of tomato bushy stunt virus (TBSV), a single viral replication protein, named p33, carries out most of the recruitment duties. Yet, it is currently unknown how the viral p33 replication protein, which is membrane associated, is capable of the rapid and efficient recruitment of numerous cytosolic host proteins to facilitate the formation of large VROs. In this paper, we show that, TBSV p33 molecules do not recruit each cytosolic host factor one-by-one into VROs, but p33 targets a cytosolic protein interaction hub, namely Rpn11, which interacts with numerous other cytosolic proteins. The highly conserved Rpn11, called POH1 in humans, is the metalloprotease subunit of the proteasome, which couples deubiquitination and degradation of proteasome substrates. However, TBSV takes advantage of a noncanonical function of Rpn11 by exploiting Rpn11’s interaction with highly abundant cytosolic proteins and the actin network. We provide supporting evidence that the co-opted Rpn11 in coordination with the subverted actin network is used for delivering cytosolic proteins, such as glycolytic and fermentation enzymes, which are readily subverted into VROs to produce ATP locally in support of VRO formation, viral replicase complex assembly and viral RNA replication. Using several approaches, including knockdown of Rpn11 level, sequestering Rpn11 from the cytosol into the nucleus in plants or temperature-sensitive mutation in Rpn11 in yeast, we show the inhibition of recruitment of glycolytic and fermentation enzymes into VROs. The Rpn11-assisted recruitment of the cytosolic enzymes by p33, however, also requires the combined and coordinated role of the subverted actin network. Accordingly, stabilization of the actin filaments by expression of the Legionella VipA effector in yeast and plant, or via a mutation of ACT1 in yeast resulted in more efficient and rapid recruitment of Rpn11 and the selected glycolytic and fermentation enzymes into VROs. On the contrary, destruction of the actin filaments via expression of the Legionella RavK effector led to poor recruitment of Rpn11 and glycolytic and fermentation enzymes. Finally, we confirmed the key roles of Rpn11 and the actin filaments in situ ATP production within TBSV VROs via using a FRET-based ATP-biosensor. The novel emerging theme is that TBSV targets Rpn11 cytosolic protein interaction hub driven by the p33 replication protein and aided by the subverted actin filaments to deliver several co-opted cytosolic pro-viral factors for robust replication within VROs.     

Genetic and molecular in vivo analysis of herpes simplex virus assembly in murine visual system neurons

Herpes simplex virus (HSV) infects both epithelial cells and neuronal cells of the human host. Although HSV assembly has been studied extensively for cultured epithelial and neuronal cells, cultured neurons are biochemically, physiologically, and anatomically significantly different than mature neurons in vivo. Therefore, it is imperative that viral maturation and assembly be studied in vivo. To study viral assembly in vivo, we inoculated wild-type and replication-defective viruses into the posterior chamber of mouse eyes and followed infection in retinal ganglion cell bodies and axons. We used PCR techniques to detect viral DNA and RNA and electron microscopy immunohistochemistry and Western blotting to detect viral proteins in specific portions of the optic tract. This approach has shown that viral DNA replication is necessary for viral DNA movement into axons. Movement of viral DNA along ganglion cell axons occurs within capsid-like structures at the speed of fast axonal transport. These studies show that the combined use of intravitreal injections of replication-defective viruses and molecular probes allows the genetic analysis of essential viral replication and maturation processes in neurons in vivo. The studies also provide novel direct evidence for the axonal transport of viral DNA and support for the subassembly hypothesis of viral maturation in situ.     

RhoA signaling is required for respiratory syncytial virus-induced syncytium formation and filamentous virion morphology

Respiratory syncytial virus (RSV) is an important human pathogen that can cause severe and life-threatening respiratory infections in infants, the elderly, and immunocompromised adults. RSV infection of HEp-2 cells induces the activation of RhoA, a small GTPase. We therefore asked whether RhoA signaling is important for RSV replication or syncytium formation. The treatment of HEp-2 cells with Clostridium botulinum C3, an enzyme that ADP-ribosylates and specifically inactivates RhoA, inhibited RSV-induced syncytium formation and cell-to-cell fusion, although similar levels of PFU were released into the medium and viral protein expression levels were equivalent. Treatment with another inhibitor of RhoA signaling, the Rho kinase inhibitor Y-27632, yielded similar results. Scanning electron microscopy of C3-treated infected cells showed reduced numbers of single blunted filaments, in contrast to the large clumps of long filaments in untreated infected cells. These data suggest that RhoA signaling is associated with filamentous virus morphology, cell-to-cell fusion, and syncytium formation but is dispensable for the efficient infection and production of infectious virus in vitro. Next, we developed a semiquantitative method to measure spherical and filamentous virus particles by using sucrose gradient velocity sedimentation. Fluorescence and transmission electron microscopy confirmed the separation of spherical and filamentous forms of infectious virus into two identifiable peaks. The C3 treatment of RSV-infected cells resulted in a shift to relatively more spherical virions than those from untreated cells. These data suggest that viral filamentous protuberances characteristic of RSV infection are associated with RhoA signaling, are important for filamentous virion morphology, and may play a role in initiating cell-to-cell fusion.     

Role of an internal and two 3'-terminal RNA elements in assembly of tombusvirus replicase

Plus-strand RNA virus replication requires the assembly of the viral replicase complexes on intracellular membranes in the host cells. The replicase of Cucumber necrosis virus (CNV), a tombusvirus, contains the viral p33 and p92 replication proteins and possible host factors. In addition, the assembly of CNV replicase is stimulated in the presence of plus-stranded viral RNA (Z. Panaviene et al., J. Virol. 78:8254-8263, 2004). To define cis-acting viral RNA sequences that stimulate replicase assembly, we performed a systematic deletion approach with a model tombusvirus replicon RNA in Saccharomyces cerevisiae, which also coexpressed p33 and p92 replication proteins. In vitro replicase assays performed with purified CNV replicase preparations from yeast revealed critical roles for three RNA elements in CNV replicase assembly: the internal p33 recognition element (p33RE), the replication silencer element (RSE), and the 3'-terminal minus-strand initiation promoter (gPR). Deletion or mutagenesis of these elements reduced the activity of the CNV replicase to a minimal level. In addition to the primary sequences of gPR, RSE, and p33RE, formation of two alternative structures among these elements may also play a role in replicase assembly. Altogether, the role of multiple RNA elements in tombusvirus replicase assembly could be an important factor to ensure fidelity of template selection during replication.     

Proteomics analysis of the tombusvirus replicase: Hsp70 molecular chaperone is associated with the replicase and enhances viral RNA replication

Plus-strand RNA virus replication occurs via the assembly of viral replicase complexes involving multiple viral and host proteins. To identify host proteins present in the cucumber necrosis tombusvirus (CNV) replicase, we affinity purified functional viral replicase complexes from yeast. Mass spectrometry analysis of proteins resolved by two-dimensional gel electrophoresis revealed the presence of CNV p33 and p92 replicase proteins as well as four major host proteins in the CNV replicase. The host proteins included the Ssa1/2p molecular chaperones (yeast homologues of Hsp70 proteins), Tdh2/3p (glyceraldehyde-3-phosphate dehydrogenase, an RNA-binding protein), Pdc1p (pyruvate decarboxylase), and an unknown approximately 35-kDa acidic protein. Copurification experiments demonstrated that Ssa1p bound to p33 replication protein in vivo, and surface plasmon resonance measurements with purified recombinant proteins confirmed this interaction in vitro. The double mutant strain (ssa1 ssa2) showed 75% reduction in viral RNA accumulation, whereas overexpression of either Ssa1p or Ssa2p stimulated viral RNA replication by approximately threefold. The activity of the purified CNV replicase correlated with viral RNA replication in the above-mentioned ssa1 ssa2 mutant and in the Ssa overexpression strains, suggesting that Ssa1/2p likely plays an important role in the assembly of the CNV replicase.     

Cellular VPS4 is required for efficient entry and egress of budded virions of Autographa californica multiple nucleopolyhedrovirus

Membrane budding is essential for the egress of many enveloped viruses, and this process shares similarities with the biogenesis of multivesicular bodies (MVBs). In eukaryotic cells, the budding of intraluminal vesicles (IVLs) is mediated by the endosomal sorting complex required for transport (ESCRT) machinery and some viruses require ESCRT machinery components or functions to bud from host cells. Baculoviruses, such as Autographa californica multiple nucleopolyhedrovirus (AcMNPV), enter host cells by clathrin-mediated endocytosis. Viral DNA replication and nucleocapsid assembly occur within the nucleus. Some progeny nucleocapsids are subsequently trafficked to, and bud from, the plasma membrane, forming budded virions (BV). To determine whether the host ESCRT machinery is important or necessary for AcMNPV replication, we cloned a cDNA of Spodoptera frugiperda VPS4, a key regulator for disassembly and recycling of ESCRT III. We then examined viral infection and budding in the presence of wild-type (WT) or dominant negative (DN) forms of VPS4. First, we used a viral complementation system, in combination with fluorescent tags, to examine the effects of transiently expressed WT or DN VPS4 on viral entry. We found that dominant negative VPS4 substantially inhibited virus entry. Entering virus was observed within aberrant compartments containing the DN VPS4 protein. We next used recombinant bacmids expressing WT or DN VPS4 proteins to examine virus egress. We found that production of infectious AcMNPV BV was substantially reduced by expression of DN VPS4 but not by WT VPS4. Together, these results indicate that a functional VPS4 is necessary for efficient AcMNPV BV entry into, and egress from, insect cells.     

甲型流感病毒蛋白和遗传物质在宿主细胞质内顺向转运过程及其机制

流感病毒的蛋白和基因组在宿主细胞内能否正确地转运到相关部位,直接影响到病毒颗粒的形态发生。流感病毒跨膜蛋白(HA、NA和M2)主要通过宿主细胞的运输膜泡实现转运,而宿主细胞的蛋白转运机器参与了这一过程。新合成的流感病毒核糖核蛋白复合物(vRNPs)出核后,通过与活化的Rab11相结合,聚集于邻近微管组织中心(MTOC)的胞内体。然后以运输小膜泡的形式,沿着MTOC的微管网络向细胞膜方向转运。跨膜蛋白和基因组在细胞质内的转运受一些宿主因子的调控,如ARHGAP21和小G蛋白Cdc42能够调节NA蛋白向细胞膜转运,Rab11协助vRNPs从MTOC向细胞膜转运。文中主要讨论新合成的流感病毒跨膜蛋白和遗传物质在宿主细胞质内的顺向转运(Anterograde transport)过程与调控。     

The Hepatitis C Virus-Induced Membranous Web and Associated Nuclear Transport Machinery Limit Access of Pattern Recognition Receptors to Viral Replication Sites

Hepatitis C virus (HCV) is a positive-strand RNA virus of the Flaviviridae family and a major cause of liver disease worldwide. HCV replicates in the cytoplasm, and the synthesis of viral proteins induces extensive rearrangements of host cell membranes producing structures, collectively termed the membranous web (MW). The MW contains the sites of viral replication and assembly, and we have identified distinct membrane fractions derived from HCV-infected cells that contain replication and assembly complexes enriched for viral RNA and infectious virus, respectively. The complex membrane structure of the MW is thought to protect the viral genome limiting its interactions with cytoplasmic pattern recognition receptors (PRRs) and thereby preventing activation of cellular innate immune responses. Here we show that PRRs, including RIG-I and MDA5, and ribosomes are excluded from viral replication and assembly centers within the MW. Furthermore, we present evidence that components of the nuclear transport machinery regulate access of proteins to MW compartments. We show that the restricted assess of RIG-I to the MW can be overcome by the addition of a nuclear localization signal sequence, and that expression of a NLS-RIG-I construct leads to increased immune activation and the inhibition of viral replication.