Association of bluetongue virus with the cytoskeleton

Virological applications of the grid-cell-culture technique

Whole mounts of intact virus-infected cells have been used for several decades to examine virus-cell relationships and virus structure. The general concept of studying virus structure in association with the host cell has recently been expanded to reveal interactions between viruses and the cytoskeleton. The procedure permits utilization of immuno-gold protocols using both the transmission and scanning electron microscopes. The grid-cell-culture technique is reviewed to explain how it can be exploited to provide valuable information about virus structure and replication in both diagnostic and research laboratories. The use of the technique at the research level is discussed using bluetongue virus as a model. The procedure can provide basic structural information about intact virions and additional data on the intracellular location of viruses and virus-specific structures and about the mode of virus release from infected cells. Application of immunoelectron microscopy reveals information on the protein composition of not only released virus particles but also cell surface and cytoskeletal-associated viruses and virus-specific structures. Collectively, this simple and physically gentle technique has provided information which would otherwise be difficult to obtain.

Identification of bluetongue virus VP6 protein as a nucleic acid-binding protein and the localization of VP6 in virus-infected vertebrate cells

Recently the insect baculovirus Autographa californica nuclear polyhedrosis virus (AcNPV) has been effectively adapted as a highly efficient vector in insect cells for the expression of various genes. A cDNA sequence of RNA segment 9 of bluetongue virus serotype 10 (BTV-10, an orbivirus member of the Reoviridae family) encoding a minor core protein (VP6) has been inserted into the BamHI site of the pAcYM1 transfer vector derived from AcNPV. Spodoptera frugiperda cells were cotransfected with the derived vector in the presence of authentic AcNPV DNA to produce recombinant viruses. These synthesized significant amounts of a protein (representing ca. 50% of the stained cellular protein) similar in size and antigenicity to the authentic BTV VP6. The expressed protein was identified as a nucleic acid-binding protein by using an RNA overlay-protein blot assay. A polyclonal anti-VP6 serum prepared by using the expressed VP6 protein has been used in an immunogold procedure to locate VP6 in BTV-infected mammalian cells. Gold was found to be associated with the matrix of virus inclusion bodies (VIB), with viruslike particles in the VIB, as well as with mature virion particles that were in close proximity to the VIB or were released from cells and adsorbed to cell surfaces. The recombinant virus antigen has also been used to identify antibodies to different BTV serotypes in infected sheep sera, indicating the potential of the expressed protein as a group-reactive antigen for the diagnosis of BTV infections.

Receptor activity of rotavirus nonstructural glycoprotein NS28

Rotavirus morphogenesis involves the budding of subviral particles through the rough endoplasmic reticulum (RER) membrane of infected cells. During this process, particles acquire the outer capsid proteins and a transient envelope. Previous immunocytochemical and biochemical studies have suggested that a rotavirus nonstructural glycoprotein, NS28, encoded by genome segment 10, is a transmembrane RER protein and that about 10,000 Mr of its carboxy terminus is exposed on the cytoplasmic side of the RER. We have used in vitro binding experiments to examine whether NS28 serves as a receptor that binds subviral particles and mediates the budding process. Specific binding was observed between purified simian rotavirus SA11 single-shelled particles and RER membranes from SA11-infected monkey kidney cells and from SA11 gene 10 baculovirus recombinant-infected insect cells. Membranes from insect cells synthesizing VP1, VP4, NS53, VP6, VP7, or NS26 did not possess binding activity. Comparison of the binding of single-shelled particles to microsomes from infected monkey kidney cells and from insect cells indicated that a membrane-associated component(s) from SA11-infected monkey kidney cells interfered with binding. Direct evidence showing the interaction of NS28 and its nonglycosylated 20,000-Mr precursor expressed in rabbit reticulocyte lysates and single-shelled particles was obtained by cosedimentation of preformed receptor-ligand complexes through sucrose gradients. The domain on NS28 responsible for binding also was characterized. Reduced binding of single-shelled particles to membranes was seen with membranes treated with (i) a monoclonal antibody previously shown to interact with the C terminus of NS28, (ii) proteases known to cleave the C terminus of NS28, and (iii) the Enzymobead reagent. VP6 on single-shelled particles was suggested to interact with NS28 because (i) a monoclonal antibody to the subgroup I epitope on VP6 reduced particle binding, (ii) a purified polyclonal antiserum raised against recombinant baculovirus-produced VP6 reduced ligand binding, and (iii) a monoclonal antibody to a conserved epitope on VP6 augmented ligand binding. These experimental data provide support for the hypothesized receptor role of NS28 before the budding stage of rotavirus morphogenesis.

The release of bluetongue virus from infected cells and their superinfection by progeny virus

Immunoelectron microscopy using an anti-VP2 monoclonal antibody complexed to colloidal gold has been used to study the mechanism of bluetongue virus (BTV) release from infected cells. Examination of the BTV-infected cell surface revealed that viruses are released both as enveloped particles, by budding through the plasma membrane, and as nonenveloped particles by "extrusion" through the membrane. Particles being released and those remaining on the cell surface retain an association with the cortical layer of the cytoskeleton. Analyses of virus particles released from infected cells and the intracellular viruses in the cytosol and attached to the cytoskeleton indicate that although the three populations have similar particle to infectivity ratios they differ in their ability to bind gold-labeled anti-VP2 antibody. The fact that released viruses bind less antibody than intracellular viruses suggests that virus release from infected cells may be associated with either a loss of VP2 or a rearrangement of the virus outer coat which obscures a proportion of the reactive epitopes on the virus surface. Electron microscopic observations also indicated that, in addition to virus release, events at the plasma membrane resulted in the uptake of progeny virus by endocytosis. Elevation of intraendosomal/lysosomal pH by lysomotropic bases and an acidic ionophore inhibited BTV replication when added to cells concurrently with the virus. Addition of such agents to infected cells at 4 hr p.i. decreased both the maximum titer of released virus and the rate at which virus antigen was synthesized in infected cells. Addition of anti-BTV antiserum 4 hr p.i. also resulted in a decreased rate of intracellular virus antigen accumulation. These results suggest that superinfection of BTV-infected cells by progeny virions effectively increases the multiplicity of infection and enhances the kinetics of BTV replication.

Association of African swine fever virus with the cytoskeleton

The association of African swine fever virus (ASFV) with the cytoskeleton was investigated. Immunofluorescent studies of ASFV infected cells with anti-ASFV serum showed a temporal and spatial development of viral inclusions which moved from a peripheral to a perinuclear location and fused to give a single large perinuclear factory. The migration and fusion of viral inclusions was inhibited by colchicine suggesting a function for microtubules in assembly site organization not previously described. Accumulation of virions outside the inclusions and inhibition of viral release was also observed in colchicine treated cells. Viral antigens and structural elements were retained on the cytoskeleton fraction of Triton X-100 extracted cells. Reorganization of cytoskeletal elements around the assembly sites was demonstrated by transmission electronmicroscopy and by immunofluorescent studies using monoclonal antibodies against actin, tubulin and vimentin. Intermediate filaments accumulated around the viral factories, microtubules were greatly decreased in number and microfilaments were reorganized in association with the plasma membrane. Bundles of 15 nm tubules of unknown origin were also observed around the assembly sites. The distribution of viral proteins in soluble, cytoskeleton and detergent insoluble nuclear fractions was studied by pulse-chase experiments with [35S]methionine. SDS-PAGE analysis showed the presence in the cytoskeletal and nuclear fractions of 150, 72, 38, 28, 19 and 15 kDa virus structural proteins which increased after a 5 h chase. Our results indicate a close association of ASFV replication with the cytoskeleton similar to events described during FV3 replication but which differ from those occurring in poxvirus-infected cells.

Morphogenesis of a bluetongue virus variant with an amino acid alteration at a neutralization site in the outer coat protein, VP2

Neutralization-resistant variants of bluetongue virus, selected with a monoclonal antibody to the outer coat protein VP2, have been used to delineate a neutralization epitope on the VP2 protein. Comparison of the RNA 2 sequence of four variants with that of the wild-type virus indicated that each variant contained a single nucleotide substitution which in turn resulted in a single amino acid alteration in VP2. The changes were clustered within a span of eight amino acids at positions 328 to 335 in the VP2 protein. In addition, analyses of cells infected with wild-type and a variant virus V35B2 have provided information on the site of VP2 addition to virus particles during morphogenesis. Electron microscopic examination revealed few virus-like particles around virus inclusion bodies (VIB) in wild-type virus-infected cells and cytoskeletons. In contrast, VIB in cells infected with the neutralization-resistant variant V35B2 were surrounded by particles identified as virus cores on the basis of their size and morphology. Probing of cytoskeletons with gold-labeled anti-VP2 monoclonal antibody revealed that in wild-type virus-infected cells the antibodies reacted weakly with VIB and only at locations where virus particles appeared to be leaving. The core-like particles surrounding VIB in V35B2-infected cells labeled very weakly with the anti-VP2 antibody. In contrast, wild-type and V35B2 virus particles which bound to the cytoskeleton at locations distal to VIB and those outside the infected cell bound significant amounts of antibody. These results suggest that although some VP2 may be added to developing virus particles at the periphery of VIB, the remainder of the VP2 protein is added outside the VIB either in the cytosol or following attachment of the particles to the cytoskeleton.

Localization of the nonstructural protein NS1 in bluetongue virus-infected cells and its presence in virus particles

Seven monoclonal antibodies to the nonstructural protein NS1 of an Australian isolate of bluetongue virus (BTV) have been used in immunofluoresence and immunogold procedures to locate NS1 in virus-infected cells and cytoskeletons. The antibodies fall into three groups indicating that NS1 contains at least three antigenic sites. One group consists of four antibodies which react solely with cytoskeleton-associated virus-specific tubules. A second group contains one antibody which reacts with cytoskeleton-associated virus particles, released viruses, and purified virus and core particles. Two antibodies constituting a third group react with both tubules and cytoskeleton-associated and released virus particles. NS1 was found in [35S]methionine-labeled, purified virus and core particles. Immunofluorescence tests reveal that those antibodies which react with virus particles also bind to cytoskeleton-associated virus inclusion bodies (VIB). The nature of this association was examined by probing cytoskeletons of BTV-infected cells with antibodies to NS1 and protein A-gold. VIB observed in thin sections were not uniformly labeled. Gold was associated with fibrillar arrays found around virus particles either leaving or in close proximity to the VIB. Fibrillar material was not found in association with all virus particles elsewhere in the cell and this suggests that fibril-virus complexes may be intermediate in virus morphogenesis.

A functional role for intermediate filaments in the formation of frog virus 3 assembly sites

During the course of frog virus 3 (FV3) infection in baby hamster kidney 21 (BHK) cells, vimentin-type intermediate filaments reorganize to surround the virus's cytoplasmic assembly sites. To determine whether the association between vimentin filaments and viral assembly sites has a functional role in the virus life-cycle, we treated cells with the antimicrotubule drugs taxol or colchicine, or injected them with monoclonal antivimentin antibodies prior to FV3 infection. Each of these reagents caused the collapse of the normally extended BHK intermediate filament system. In the case of taxol-treated or antivimentin-injected cells, the collapsed vimentin filaments were unable to reorganize around the newly forming viral assembly sites. The viral assembly sites that did form were aberrant and there was a significant reduction in the number of mature virions present. Colchicine, which also caused the collapse of vimentin filament organization, did not block the reorganization of vimentin filaments in response to viral infection and viral assembly sites appeared normal. These results suggest that intermediate filaments play an important role in maintaining the structural and functional integrity of FV3 assembly sites.

The grid-cell-culture technique: the direct examination of virus-infected cells and progeny viruses

We describe a method for the structural analysis and identification of viruses, without purification or concentration steps which could alter virus morphology. Virus-infected cells grown on carbon-Parlodion-coated electron microscope grids release large numbers of progeny viruses which adsorb to the surface of the grid and are revealed by negative staining. The technique is rapid, sensitive and can be used at three levels. Negative staining of whole cell preparations revealed both extracellular and intracellular viruses or nucleocapsids beneath the plasma membrane; non-ionic detergent extraction of cells infected with certain viruses reveals cytoskeleton-associated, virus-specific structures normally only observed after thin sectioning; cultures prepared by either procedure are suitable for colloidal gold immunological studies. Extracellular and cytoskeletal-associated viruses were heavily and specifically labelled with gold. The results indicate that the technique may be used to rapidly identify unknown viruses on the basis of size, topography, morphology and mode of maturation from the infected cell, as well as the presence of characteristic intracellular cytoskeletal-associated structures. The technique also has potential use in the sero-grouping and sero-typing of viruses with specific monoclonal antibodies.