Herpes simplex virus-1 utilizes the host actin cytoskeleton for its release from axonal growth cones

Herpes simplex virus type 1 (HSV-1) has evolved mechanisms to exploit the host cytoskeleton during entry, replication and exit from cells. In this study, we determined the role of actin and the molecular motor proteins, myosin II and myosin V, in the transport and release of HSV-1 from axon termini, or growth cones. Using compartmentalized neuronal devices, we showed that inhibition of actin polymerization, but not actin branching, significantly reduced the release of HSV-1 from axons. Furthermore, we showed that inhibition of myosin V, but not myosin II, also significantly reduced the release of HSV-1 from axons. Using confocal and electron microscopy, we determined that viral components are transported along axons to growth cones, despite actin or myosin inhibition. Overall, our study supports the role of actin in virus release from axonal growth cones and suggests myosin V as a likely candidate involved in this process.

Herpes simplex virus type 1 (HSV-1) is a ubiquitous human pathogen causing cold sores and genital herpes. HSV-1 infects sensory neurons of the peripheral nervous system where it establishes a lifelong infection and cannot be cured. Reactivation is common, with the virus transported back along sensory nerves, forming new lesions, or is shed asymptomatically. Antiviral resistance is emerging to current antivirals that target viral replication, indicating the need to identify new targets for future treatment. The host cell cytoskeleton plays an important role during transport of the virus. HSV-1 is transported along axons via microtubules; however, how the virus is released from axon termini, where actin predominates, is unknown. Here we show that an intact actin cytoskeleton is required for efficient virus release from axon termini. Furthermore, we show that myosin V, an actin based molecular motor that drives transport, is essential in virus release from axon termini. Together, this study defines the mechanisms behind HSV-1 release from axon termini which will guide future directions in identifying possible therapeutic targets for HSV-1.

A "Driver Switchover" Mechanism of Influenza Virus Transport from Microfilaments to Microtubules

When infecting host cells, influenza virus must move on microfilaments (MFs) at the cell periphery and then move along microtubules (MTs) through the cytosol to reach the perinuclear region for genome release. But how viruses switch from the actin roadway to the microtubule highway remains obscure. To settle this issue, we systematically dissected the role of related motor proteins in the transport of influenza virus between cytoskeletal filaments in situ and in real-time using quantum dot (QD)-based single-virus tracking (SVT) and multicolor imaging. We found that the switch between MF- and MT-based retrograde motor proteins, myosin VI (myoVI) and dynein, was responsible for the seamless transport of viruses from MFs to MTs during their infection. After virus entry by endocytosis, both the two types of motor proteins are attached to virus-carrying vesicles. MyoVI drives the viruses on MFs with dynein on the virus-carrying vesicle hitchhiking. After role exchanges at actin-microtubule intersections, dynein drives the virus along MTs toward the perinuclear region with myoVI remaining on the vesicle moving together. Such a "driver switchover" mechanism has answered the long-pending question of how viruses switch from MFs to MTs for their infection. It will also facilitate in-depth understanding of endocytosis.

Tick-borne encephalitis virus infects rat astrocytes but does not affect their viability

Tick-borne encephalitis virus (TBEV) causes one of the most dangerous human neuroinfections in Europe and Asia. To infect neurons it must cross the blood-brain-barrier (BBB), and presumably also cells adjacent to the BBB, such as astrocytes, the most abundant glial cell type. However, the knowledge about the viral infection of glial cells is fragmental. Here we studied whether TBEV infects rat astrocytes. Rats belong to an animal group serving as a TBEV amplifying host. We employed high resolution quantitative fluorescence microscopy to investigate cell entry and cytoplasmic mobility of TBEV particles along with the effect on the cell cytoskeleton and cell survival. We report that infection of astrocytes with TBEV increases with time of exposure to TBEV and that with post-infection time TBEV particles gained higher mobility. After several days of infection actin cytoskeleton was affected, but cell survival was unchanged, indicating that rat astrocytes resist TBEV-mediated cell death, as reported for other mammalian cells. Therefore, astrocytes may present an important pool of dormant TBEV infections and a new target for therapeutic intervention.

The non-canonical roles of clathrin and actin in pathogen internalization, egress and spread

The role of clathrin in pathogen entry has received much attention and has highlighted the adaptability of clathrin during internalization. Recent studies have now uncovered additional roles for clathrin and have put the spotlight on its role in pathogen spread. Here, we discuss the manipulation of clathrin by pathogens, with specific attention to the processes that occur at the plasma membrane. In the majority of cases, both clathrin and the actin cytoskeleton are hijacked, so we also examine the interplay between these two systems and their role during pathogen internalization, egress and spread.

The tobamovirus Turnip Vein Clearing Virus 30-kilodalton movement protein localizes to novel nuclear filaments to enhance virus infection

Plant viruses overcome the barrier of the plant cell wall by encoding cell-to-cell movement proteins (MPs), which direct newly replicated viral genomes to, and across, the wall. The paradigm for how a single MP regulates and coordinates these activities is the Tobacco mosaic virus (TMV) 30-kDa protein (MP(TMV)). Detailed studies demonstrate that TMV multiplies exclusively in the cytoplasm and have documented associations of MP(TMV) with endoplasmic reticulum (ER) membrane, microtubules, and plasmodesmata throughout the course of infection. As TMV poorly infects Arabidopsis thaliana, Turnip vein clearing virus (TVCV) is the tobamovirus of choice for studies in this model plant. A key problem, which has contributed to confusion in the field, is the unproven assumption that the TVCV and TMV life cycles are identical. We engineered an infectious TVCV replicon that expressed a functional fluorescence-tagged MP(TVCV) and report here the unexpected discovery that MP(TVCV), beyond localizing to ER membrane and plasmodesmata, targeted to the nucleus in a nuclear localization signal (NLS)-dependent manner, where it localized to novel F-actin-containing filaments that associated with chromatin. The MP(TVCV) NLS appeared to be conserved in the subgroup 3 tobamoviruses, and our mutational analyses showed that nuclear localization of MP(TVCV) was necessary for efficient TVCV cell-to-cell movement and systemic infection in Nicotiana benthamiana and Arabidopsis thaliana. Our studies identify a novel nuclear stage in TVCV infection and suggest that nuclear MP encoded by TVCV and other subgroup 3 tobamoviruses interacts with F-actin and chromatin to modulate host defenses or cellular physiology to favor virus movement and infection.

Human immunodeficiency virus (HIV)-1 proteins and cytoskeleton: partners in viral life and host cell death

Cytoskeletal components play a major role in the human immunodeficiency virus-1 (HIV-1) infection. A wide variety of molecules belonging to the microfilament system, including actin filaments and actin binding proteins, as well as microtubules have a key role in regulating both cell life and death. Cell shape maintenance, cell polarity and cell movements as well as cytoplasmic trafficking of molecules determining cell fate, including apoptosis, are in fact instructed by the cytoskeleton components. HIV infection and viral particle production seem to be controlled by cytoskeleton as well. Furthermore, HIV-associated apoptosis failure can also be regulated by the actin network function. In fact, HIV protein gp120 is able to induce cytoskeleton-driven polarization, thus sensitizing T cells to CD95/Fas-mediated apoptosis. The microfilament system seems thus to be a sort of cytoplasmic supervisor of the viral particle, the host cell and the bystander cell's very fate.

Infectious entry of West Nile virus occurs through a clathrin-mediated endocytic pathway

The pathway of West Nile flavivirus early internalization events was mapped in detail in this study. Overexpression of dominant-negative mutants of Eps15 strongly inhibits West Nile virus (WNV) internalization, and pharmacological drugs that blocks clathrin also caused a marked reduction in virus entry but not caveola-dependent endocytosis inhibitory agent, filipin. Using immunocryoelectron microscopy, WNV particles were seen within clathrin-coated pits after 2 min postinfection. Double-labeling immunofluorescence assays and immunoelectron microscopy performed with anti-WNV envelope or capsid proteins and cellular markers (EEA1 and LAMP1) revealed the trafficking pathway of internalized virus particles from early endosomes to lysosomes and finally the uncoating of the virus particles. Disruption of host cell cytoskeleton (actin filaments and microtubules) with cytochalasin D and nocodazole showed significant reduction in virus infectivity. Actin filaments are shown to be essential during the initial penetration of the virus across the plasma membrane, whereas microtubules are involved in the trafficking of internalized virus from early endosomes to lysosomes for uncoating. Cells treated with lysosomotropic agents were largely resistant to infection, indicating that a low-pH-dependent step is required for WNV infection. In situ hybridization of DNA probes specific for viral RNA demonstrated the trafficking of uncoated viral RNA genomes to the endoplasmic reticulum.

Role of cytoskeleton components in measles virus replication

Cellular factors have been indicated to be essential for the replication of Measles virus (MV), but the exact roles of these components are, however, not understood in detail. In this study, we investigated the role of actin and tubulin in productive MV infection by inducing disassembly of the microfilaments and microtubules. Vero cells were treated with latrunculin-A, which sequesters actin monomers, or nocodazole, which breaks the microtubules. The disruption of either of the structures efficiently inhibited the maturation of new infectious virus. Interestingly, virus spreading to neighboring cells still occurred by fusion and large syncytia typical for MV infection appeared. We also investigated a possible role for proteins of the ERM-family. Our results support an important role for actin filaments and microtubules for efficient MV replication.

Anchorage of HIV on permissive cells leads to coaggregation of viral particles with surface nucleolin at membrane raft microdomains

The cross-linking of HIV on permissive cells results aggregation of HIV particles with surface nucleolin, CD4, and CXCR4, but without affecting the organization of CD45. In addition, HIV particles and nucleolin coaggregate with glycolipid-enriched membrane microdomains (GEMs) containing ganglioside, and glycosylphosphatidylinositol-linked proteins CD90 and CD59, pointing out that HIV anchorage induces lateral assemblies of specific membrane components into lipid rafts in which surface nucleolin is also incorporated. Consequently, equilibrium density fractionation of extracts from infected cells revealed that HIV proteins and nucleolin copurify with Triton X-100-resistant GEM-associated proteins. After HIV entry, nucleolin is recovered also in fractions containing HIV DNA, viral matrix, and reverse transcriptase, thus suggesting that it could accompany viral entry. We show that surface nucleolin is markedly down-regulated a few hours following HIV entry into permissive cells; an effect that appears to be the consequence of its translocation into the cytoplasm. Our findings demonstrate that anchorage of HIV particles on permissive cells induces aggegation of surface nucleolin and its association with detergent-insoluble lipid raft components. Moreover, they support the suggestion that surface nucleolin and lipid rafts are implicated in early events in the HIV entry process.

Temporary disturbance of actin stress fibers in swine kidney cells during pseudorabies virus infection

Rounding and loosening of cells is a consequence of infection with pseudorabies virus (PrV), both in vitro and in vivo. These changes in the normal structure of the cell may be the result of cytoskeletal changes. Immunofluorescence staining of actin filaments and microtubule bundles was performed to examine whether PrV induces a reorganization of these cytoskeletal components in infected swine kidney (SK) cells. Every 2h until 12h post-inoculation (p.i.), cells were washed in cytoskeleton stabilizing buffer (CSB), fixed with paraformaldehyde and washed again with CSB. Cells were permeabilized with a 1/1000 dilution of Triton X-100 and actin filaments were stained by incubating cells with phalloidin-Texas Red. Staining of microtubules was done by incubating the cells subsequently with mouse monoclonal anti-alpha-tubulin and goat anti-mouse IgG-FITC. During the course of infection, actin fibers of SK cells were rearranged in the following sequence: (1) disappearance of thick actin stress fibers between 4 and 6h p.i., (2) complete loss of stress fibers between 6 and 8h p.i., and (3) reappearance of thin stress fibers starting from 10h p.i. In contrast to herpes simplex virus 1 (HSV1) or equine herpesvirus 1 (EHV1), PrV infection did not induce changes in the cellular microtubule network. PrV infection induces a temporary disassembly of actin stress fibers.

A multi-mutant herpes simplex virus vector has minimal cytotoxic effects on the distribution of filamentous actin, alpha-actinin 2 and a glutamate receptor in differentiated PC12 cells

To develop effective gene therapy techniques that target populations of neurons in the spinal cord, suitable vectors must be developed that will undergo efficient, retrograde transport from an appropriate peripheral site and will not be cytotoxic. Our previous work (LeVatte et al, 1998a) has demonstrated that a replication defective herpes simplex virus vector 14Hdelta3vhsZ, that has been substantially detoxified, is retrogradely transported from peripheral sites and can infect large numbers of the targeted spinal neurons. We plan to develop targeted gene therapy approaches designed to modulate the excitatory glutamatergic methyl-D-aspartate (NMDA) receptor in spinal cord neurons as a means of ameliorating a form of episodic high blood pressure that occurs after spinal cord injury. In this report, we demonstrate that, in differentiated PC12 cells, a neuronal-like cell line, the virus vector does not appear to alter aspects of the cytoskeletal architecture important to the proper distribution of the NMDA receptor. In turn, the distribution of endogenous NMDA receptor 1 subunit protein (NMDAR1) or a transfected NMDAR1-green fluorescent fusion protein was also found to be unaltered after vector infection. However, whereas endogenous NMDAR1 distribution was maintained, vector infection did tend to reduce the level of its expression. This drop in endogenous NMDAR1 expression coincided with the expression of the HSV immediate early genes ICP0 and ICP27 over the first 24-48 h. These results indicate that the 14Hdelta3vhsZ herpes simplex virus vector is suitable to use in future strategies to alter the level of gene expression in targeted populations of spinal cord neurons.

Ultrastructural analysis of hepatitis B virus in HepG2-transfected cells with special emphasis on subviral filament morphogenesis

The intracellular accumulation of empty hepatitis B virus (HBV) particles of filamentous shape leads to a direct cytopathic effect in so-called ground-glass hepatocytes. The aim of this study was to investigate how these filaments can be structurally formed at the cellular level. By electron microscopy, we reexamined the HBV-producer HepG2T-14 cells, which have been described as producing a substantial amount of empty HBV filaments compared with the other forms of HBV particles. Examination of ultrathin sections of HepG2T14 cells revealed the presence of HBV virions and filaments at the periphery of extremely large intracellular cisternae, probably related to a pre-Golgi compartment. Very long filaments appeared to be formed by a tubular budding of a long portion of the cisterna membrane. This phenomenon may be identical to that observed in the hepatocytes of HBV chronic carriers, in which the inability of the infected cell to export long HBV filamentous particles through the cellular secretion pathway seems to be at the origin of a direct cytopathic effect.

Modification of cytoskeleton and prosome networks in relation to protein synthesis in influenza A virus-infected LLC-MK2 cells

Modifications of the cytoskeleton and protein synthesis were investigated in LLC-MK2 cells during infection by FPV/Ulster 73, an avian strain of influenza A virus. During infection, the cytoskeleton and the prosome networks undergo a dramatic reorganization, which seems to be at least temporally differentiated for each cytoskeletal system, i.e. microfilaments (MFs), microtubules (MTs), intermediate filaments (IFs). In order to evaluate the role of the three different cytoskeletal networks during FPV/Ulster infection, studies were carried out on cellular and virus-specific protein synthesis and viral production, using drugs which selectively affect individual cytoskeletal systems. Our data show that the perturbation of the IF system, but not that of the MFs or MTs, seems to have a strong inhibitory effect on virus production and cellular and viral protein synthesis. Furthermore, the dynamics of IFs and prosomes were investigated during viral infection and, at no time, dissociation of the prosome and IF networks was observed. Taken together, these results strongly support the idea that the interactions between the protein synthesis machinery, the cytoskeleton, and the prosomes are all affected by viral infection in a partially coordinated manner.