An electron microscopic study of moderate and virulent virus-cell interactions of the parainfluenza virus SV5

Low-Fidelity Assembly of Influenza A Virus Promotes Escape from Host Cells

Influenza viruses inhabit a wide range of host environments using a limited repertoire of protein components. Unlike viruses with stereotyped shapes, influenza produces virions with significant morphological variability even within clonal populations. Whether this tendency to form pleiomorphic virions is coupled to compositional heterogeneity and whether it affects replicative fitness remains unclear. Here, we address these questions by developing a strain of influenza A virus amenable to rapid compositional characterization through quantitative, site-specific labeling of viral proteins. Using this strain, we find that influenza A produces virions with broad variations in size and composition from even single infected cells. This phenotypic variability contributes to virus survival during environmental challenges, including exposure to antivirals. Complementing genetic adaptations that act over larger populations and longer times, this "low-fidelity" assembly of influenza A virus allows small populations to survive environments that fluctuate over individual replication cycles.

Human Metapneumovirus Induces Formation of Inclusion Bodies for Efficient Genome Replication and Transcription

Human metapneumovirus (HMPV) causes significant upper and lower respiratory disease in all age groups worldwide. The virus possesses a negative-sense single-stranded RNA genome of approximately 13.3 kb encapsidated by multiple copies of the nucleoprotein (N), giving rise to helical nucleocapsids. In addition, copies of the phosphoprotein (P) and the large RNA polymerase (L) decorate the viral nucleocapsids. After viral attachment, endocytosis, and fusion mediated by the viral glycoproteins, HMPV nucleocapsids are released into the cell cytoplasm. To visualize the subsequent steps of genome transcription and replication, a fluorescence in situ hybridization (FISH) protocol was established to detect different viral RNA subpopulations in infected cells. The FISH probes were specific for detection of HMPV positive-sense RNA (+RNA) and viral genomic RNA (vRNA). Time course analysis of human bronchial epithelial BEAS-2B cells infected with HMPV revealed the formation of inclusion bodies (IBs) from early times postinfection. HMPV IBs were shown to be cytoplasmic sites of active transcription and replication, with the translation of viral proteins being closely associated. Inclusion body formation was consistent with an actin-dependent coalescence of multiple early replicative sites. Time course quantitative reverse transcription-PCR analysis suggested that the coalescence of inclusion bodies is a strategy to efficiently replicate and transcribe the viral genome. These results provide a better understanding of the steps following HMPV entry and have important clinical implications.IMPORTANCE Human metapneumovirus (HMPV) is a recently discovered pathogen that affects human populations of all ages worldwide. Reinfections are common throughout life, but no vaccines or antiviral treatments are currently available. In this work, a spatiotemporal analysis of HMPV replication and transcription in bronchial epithelial cell-derived immortal cells was performed. HMPV was shown to induce the formation of large cytoplasmic granules, named inclusion bodies, for genome replication and transcription. Unlike other cytoplasmic structures, such as stress granules and processing bodies, inclusion bodies are exclusively present in infected cells and contain HMPV RNA and proteins to more efficiently transcribe and replicate the viral genome. Though inclusion body formation is nuanced, it corresponds to a more generalized strategy used by different viruses, including filoviruses and rhabdoviruses, for genome transcription and replication. Thus, an understanding of inclusion body formation is crucial for the discovery of innovative therapeutic targets.

Filamentous influenza viruses

Clinical isolates of influenza virus produce pleomorphic virus particles, including extremely long filamentous virions. In contrast, strains of influenza that have adapted to laboratory growth typically produce only spherical virions. As a result, the filamentous phenotype has been overlooked in most influenza virus research. Recent advances in imaging and improved animal models have highlighted the distinct structure and functional relevance of filamentous virions. In this review we summarize what is currently known about these strikingly elongated virus particles and discuss their possible roles in clinical infections.

Paramyxovirus glycoprotein incorporation, assembly and budding: a three way dance for infectious particle production

Paramyxoviruses are a family of negative sense RNA viruses whose members cause serious diseases in humans, such as measles virus, mumps virus and respiratory syncytial virus; and in animals, such as Newcastle disease virus and rinderpest virus. Paramyxovirus particles form by assembly of the viral matrix protein, the ribonucleoprotein complex and the surface glycoproteins at the plasma membrane of infected cells and subsequent viral budding. Two major glycoproteins expressed on the viral envelope, the attachment protein and the fusion protein, promote attachment of the virus to host cells and subsequent virus-cell membrane fusion. Incorporation of the surface glycoproteins into infectious progeny particles requires coordinated interplay between the three viral structural components, driven primarily by the matrix protein. In this review, we discuss recent progress in understanding the contributions of the matrix protein and glycoproteins in driving paramyxovirus assembly and budding while focusing on the viral protein interactions underlying this process and the intracellular trafficking pathways for targeting viral components to assembly sites. Differences in the mechanisms of particle production among the different family members will be highlighted throughout.

An amino acid of human parainfluenza virus type 3 nucleoprotein is critical for template function and cytoplasmic inclusion body formation

The nucleoprotein (N) and phosphoprotein (P) interaction of nonsegmented negative-strand RNA viruses is essential for viral replication; this includes N⁰-P (N⁰, free of RNA) interaction and the interaction of N-RNA with P. The precise site(s) within N that mediates the N-P interaction and the detailed regulating mechanism, however, are less clear. Using a human parainfluenza virus type 3 (HPIV3) minigenome assay, we found that an N mutant (N(L478A) did not support reporter gene expression. Using in vivo and in vitro coimmunoprecipitation, we found that N(L478A) maintains the ability to form N(L478A)⁰-P, to self-assemble, and to form N(L478A)-RNA but that N(L478A)-RNA does not interact with P. Using an immunofluorescence assay, we found that N-P interaction provides the minimal requirement for the formation of cytoplasmic inclusion bodies, which contain viral RNA, N, P, and polymerase in HPIV3-infected cells. N(L478A) was unable to form inclusion bodies when coexpressed with P, but the presence of N rescued the ability of N(L478A) to form inclusion bodies and the transcriptional function of N(L478A), thereby suggesting that hetero-oligomers formed by N and N(L478A) are functional and competent to form inclusion bodies. Furthermore, we found that N(L478A) is also defective in virus growth. To our knowledge, we are the first to use a paramyxovirus to identify a precise amino acid within N that is critical for N-RNA and P interaction but not for N(0)-P interaction for the formation of inclusion bodies, which appear to be bona fide sites of RNA synthesis.

Sendai virus particle production: basic requirements and role of the SYWST motif present in HN cytoplasmic tail

Sendai virus (SeV) HN protein is dispensable for virus particle production. HN incorporation into virions strictly depends on a cytoplasmic domain SYWST motif. HNAFYKD, with SYWST replaced with the analogous sequence of measles virus (MeV) H (AFYKD), is not incorporated in virus particles produced by LLCMK2 cells, although it is normally expressed at the plasma membrane. Unlike HNSYWST, HNAFYKD is not internalized to late endosomes, raising the possibility that HN internalization is required for uptake into virus particles. Various mosaic MeV-H containing increasing amounts of the SeV-HN all failed to be taken up in SeV virions. However, when co-expressed with HNAFYKD these MeV-H chimera induced HNAFYKD uptake into virions showing that internalization is not a prerequisite for HN uptake into particles. We propose that HN incorporation in virus particles requires first neutralization by HN of a putative inhibitor of infectious particle formation.

Refined methods for propagating vesicular stomatitis virus vectors that are defective for G protein expression

Propagation-defective vesicular stomatitis virus (VSV) vectors that encode a truncated G protein (VSV-Gstem) or lack the G gene entirely (VSV-DeltaG) are attractive vaccine vectors because they are immunogenic, cannot replicate and spread after vaccination, and do not express many of the epitopes that elicit neutralizing anti-VSV immunity. To consider advancing non-propagating VSV vectors towards clinical assessment, scalable technology that is compliant with human vaccine manufacturing must be developed to produce clinical trial material. Accordingly, two propagation methods were developed for VSV-Gstem and VSV-DeltaG vectors encoding HIV gag that have the potential to support large-scale production. One method is based on transient expression of G protein after electroporating plasmid DNA into Vero cells and the second is based on a stable Vero cell line that contains a G gene controlled by a heat shock-inducible transcription unit. Both methods reproducibly supported production of 1 x 10(7) to 1 x 10(8) infectious units (I.U.s) of vaccine vector per milliliter. Results from these studies also showed that optimization of the G gene is necessary for abundant G protein expression from electroporated plasmid DNA or from DNA integrated in the genome of a stable cell line, and that the titers of VSV-Gstem vectors generally exceeded VSV-DeltaG.

Parainfluenza virus type 5 (PIV-5) morphology revealed by cryo-electron microscopy

The knowledge of parainfluenza type 5 (PIV-5) virion morphology is essentially based on the observation of negatively stained preparations in conventional transmission electron microscopy (CTEM). In this study, the ultrastructure of frozen-hydrated intact PIV-5 was examined by cryo-electron microscopy (cryo-EM). Cryo-EM revealed a majority of spherical virions (70%), with a lower pleiomorphy than originally observed in CTEM. Phospholipid bilayer thickness, spike length and glycoprotein spikes density were measured. About 2000 glycoprotein spikes were present in an average-sized spherical virion. Altogether, these data depict a more precise view of PIV-5 morphology.

Human metapneumovirus nucleoprotein and phosphoprotein interact and provide the minimal requirements for inclusion body formation

Human metapneumovirus (HMPV) is a recently discovered paramyxovirus of the subfamily Pneumovirinae, which also includes avian pneumovirus and human respiratory syncytial virus (HRSV). HMPV is an important cause of respiratory disease worldwide. To understand early events in HMPV replication, cDNAs encoding the HMPV nucleoprotein (N), phosphoprotein (P), matrix protein (M), M2-1 protein and M2-2 protein were cloned from cells infected with the genotype A1 HMPV wild-type strain TN/96-12. HMPV N and P were shown to interact using a variety of techniques: yeast two-hybrid assays, co-immunoprecipitation and fluorescence resonance energy transfer (FRET). Confocal microscopy studies showed that, when expressed individually, fluorescently tagged HMPV N and P exhibited a diffuse expression pattern in the host-cell cytoplasm of uninfected cells but were recruited to cytoplasmic viral inclusion bodies in HMPV-infected cells. Furthermore, when HMPV N and P were expressed together, they also formed cytoplasmic inclusion-like complexes, even in the absence of viral infection. FRET microscopy revealed that HMPV N and P interacted directly within cytoplasmic inclusion-like complexes. Moreover, it was shown by yeast two-hybrid analysis that the N-terminal 28 aa are required for the recruitment to and formation of cytoplasmic inclusions, but are dispensable for binding to HMPV P. This work showed that HMPV N and P proteins provide the minimal viral requirements for HMPV inclusion body formation, which may be a distinguishing characteristic of members of the subfamily Pneumovirinae.

Membrane Glycoproteins of Enveloped Viruses

This chapter focuses on the recent information of the glycoprotein components of enveloped viruses and points out specific findings on viral envelopes. Although enveloped viruses of different major groups vary in size and shape, as well as in the molecular weight of their structural polypeptides, there are general similarities in the types of polypeptide components present in virions. The types of structural components found in viral membranes are summarized briefly in the chapter. All the enveloped viruses studied to date possess one or more glycoprotein species and lipid as a major structural component. The presence of carbohydrate covalently linked to proteins is demonstrated by the incorporation of a radioactive precursor, such as glucosamine or fucose, into viral polypeptides, which is resolved by sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis. Enveloped viruses share many common features in the organization of their structural components, as indicated by several approaches, including electron microscopy, surface-labeling, and proteolytic digestion experiments, and the isolation of subviral components. The chapter summarizes the detailed structure of the glycoproteins of four virus groups: (1) influenza virus glycoproteins, (2) rhabdovirus G protein, (3) togavirus glycoprotein, and (4) paramyxovirus glycoproteins The information obtained includes the size and shape of viral glycoproteins, the number of polypeptide chains in the complete glycoprotein structure, and compositional data on the polypeptide and oligosaccharide portions of the molecules.

Viral morphogenesis and morphological changes in human neuronal cells following Tioman and Menangle virus infection

Tioman virus (TioPV) and Menangle virus (MenPV) are two antigenically and genetically related paramyxoviruses (genus: Rubulavirus, family: Paramyxoviridae) isolated from Peninsular Malaysia (2001) and Australia (1997), respectively. Both viruses are potential zoonotic agents. In the present study, the infectivity, growth kinetics, morphology and morphogenesis of these two paramyxoviruses in a human neuronal cell (SK-N-SH) line were investigated. Sub-confluent SK-N-SH cells were infected with TioPV and MenPV at similar multiplicity of infection. These cells were examined by conventional and immunoelectron microscopy, and virus titres in the supernatants were assayed. Syncytia were observed for both infections in SK-N-SH cells and were more pronounced during the early stages of TioPV infection. The TioPV titre increased consistently (10(1)) every 12 h after infection. In MenPV-infected cells, cellular material was frequently observed within budding virions, and microfilaments and microtubules were abundant. Viral budding was common, and extracellular MenPVs tended to be more pleomorphic compared to TioPVs, which appeared to be more spherical in appearance. The MenPV cytoplasmic viral inclusion appeared to be comparatively smaller, loose and interspersed with randomly scattered circle-like particles, whereas huge tubule-like cytoplasmic inclusions were observed in TioPV-infected cells. Both viruses also displayed different cellular pathology in the SK-N-SH cells. The intracellular ultrastructural characteristics of these two viruses in infected neuronal cells may allow them to be differentiated by electron microscopy.

Isolation and partial characterization of a novel paramyxovirus from the gills of diseased seawater-reared Atlantic salmon (Salmo salar L)

A formerly undescribed virus has been isolated from the gills of farmed Atlantic salmon post-smolts in Norway suffering from gill disease. Cytopathic effects appeared in RTgill-W1 cells 9 weeks post-inoculation with gill tissue material. Virus production continued for an extended period thereafter. Light and electron microscopic examination revealed inclusions and replication in the cytoplasm. The viral nucleocapsid consisted of approximately 17 nm thick filaments in a herringbone pattern. Certain areas of the plasma membrane were thickened by the alignment of nucleocapsids on the internal surface and projections of 10 nm long viral glycoprotein spikes on the external surface. Virus assembly and release was achieved by budding through the modified plasma membrane. Negatively stained virions were spherical and partly pleomorphic with a diameter of 150-300 nm as seen by electron microscopy. The virus was sensitive to chloroform, heat and low and high pH, and replication was not inhibited by Br-dU or IdU indicating an RNA genome. Both haemagglutination and receptor-destroying enzyme activity were associated with the virions and the formation of syncytia in infected cultures indicated fusion activity. The receptor-destroying enzyme was identified as neuraminidase. The virus contained five major structural polypeptides with estimated molecular masses of 70, 62, 60, 48 and 37 kDa. Its buoyant density was 1.18-1.19 g ml(-1) in CsCl gradients. From the observed properties we conclude that this new virus belongs to the Paramyxoviridae and suggest the name Atlantic salmon paramyxovirus (ASPV). Furthermore, replication occurred at 6-21 degrees C, suggesting a host range confined to cold-blooded animals.

Elucidation of Nipah virus morphogenesis and replication using ultrastructural and molecular approaches

Nipah virus, which was first recognized during an outbreak of encephalitis with high mortality in Peninsular Malaysia during 1998-1999, is most closely related to Hendra virus, another emergent paramyxovirus first recognized in Australia in 1994. We have studied the morphologic features of Nipah virus in infected Vero E6 cells and human brain by using standard and immunogold electron microscopy and ultrastructural in situ hybridization. Nipah virions are enveloped particles composed of a tangle of filamentous nucleocapsids and measured as large as 1900 nm in diameter. The nucleocapsids measured up to 1.67 microm in length and had the herringbone structure characteristic for paramyxoviruses. Cellular infection was associated with multinucleation, intracytoplasmic nucleocapsid inclusions (NCIs), and long cytoplasmic tubules. Previously undescribed for other members of the family Paramyxoviridae, infected cells also contained an inclusion formed of reticular structures. Ultrastructural ISH studies suggest these inclusions play an important role in the transcription process.

"Rule of six": how does the Sendai virus RNA polymerase keep count?

The "rule of six" stipulates that the Paramyxovirus RNA polymerase efficiently replicates only viral genomes counting 6n + 0 nucleotides. Because the nucleocapsid proteins (N) interact with 6 nucleotides, an exact nucleotide-N match at the RNA 3'-OH end (3'-OH congruence) may be required for recognition of an active replication promoter. Alternatively, assuming that the six positions for the interaction of N with the nucleotides are not equivalent, the nucleotide position relative to N may be critical (N phase context). The replication abilities of various minireplicons, designed so that the 3'-OH congruence could be discriminated from the N phase context, were studied. The results strongly suggest that the application of the rule of six depends on the recognition of nucleotides positioned in the proper N phase context.

Filamentous particle formation by human parainfluenza virus type 2

Some paramyxoviruses form long filamentous virus particles: however, the determinants of filament formation and the role of such particles in virus transmission and pathogenicity are not clearly defined. By using conventional immunofluorescence microscopy, we found that human parainfluenza virus type 2 (HPIV2) forms filamentous particles ranging from 5 to 15 microm in length in virus-infected, polarized epithelial cells. The formation of filamentous particles was found to be virus type-specific and was not observed when the same cell types were infected with parainfluenza virus type 3 or Sendai virus, suggesting that different paramyxovirus genera exhibit distinct morphological properties. HPIV2 filamentous particle formation was found to be inhibited by cytochalasin D (CD) or jasplakinolide treatment in a dose-dependent manner. In the presence of 4 microg/ml CD or 1 microM jasplakinolide, the formation of filamentous particles was completely abolished, although similar haemagglutination and p.f.u. titres of virus were found to be released into the culture medium at 24 h post-infection. These observations indicate that host cell components, including the actin microfilament network, are important determinants of the morphology of parainfluenza viruses. The predominance of filamentous particles in polarized epithelial cells may reflect specific pathogenic roles of these particles in infection of human epithelial tissues.

A cell-line-specific defect in the intracellular transport and release of assembled retroviral capsids

Retrovirus assembly involves a complex series of events in which a large number of proteins must be targeted to a point on the plasma membrane where immature viruses bud from the cell. Gag polyproteins of most retroviruses assemble an immature capsid on the cytoplasmic side of the plasma membrane during the budding process (C-type assembly), but a few assemble immature capsids deep in the cytoplasm and are then transported to the plasma membrane (B- or D-type assembly), where they are enveloped. With both assembly phenotypes, Gag polyproteins must be transported to the site of viral budding in either a relatively unassembled form (C type) or a completely assembled form (B and D types). The molecular nature of this transport process and the host cell factors that are involved have remained obscure. During the development of a recombinant baculovirus/insect cell system for the expression of both C-type and D-type Gag polyproteins, we discovered an insect cell line (High Five) with two distinct defects that resulted in the reduced release of virus-like particles. The first of these was a pronounced defect in the transport of D-type but not C-type Gag polyproteins to the plasma membrane. High Five cells expressing wild-type Mason-Pfizer monkey virus (M-PMV) Gag precursors accumulate assembled immature capsids in large cytoplasmic aggregates similar to a transport-defective mutant (MA-A18V). In contrast, a larger fraction of the Gag molecules encoded by the M-PMV C-type morphogenesis mutant (MA-R55W) and those of human immunodeficiency virus were transported to the plasma membrane for assembly and budding of virions. When pulse-labeled Gag precursors from High Five cells were fractionated on velocity gradients, they sedimented more rapidly, indicating that they are sequestered in a higher-molecular-mass complex. Compared to Sf9 insect cells, the High Five cells also demonstrate a defect in the release of C-type virus particles. These findings support the hypothesis that host cell factors are important in the process of Gag transport and in the release of enveloped viral particles.

Rimmed vacuoles with beta-amyloid and ubiquitinated filamentous deposits in the muscles of patients with long-standing denervation (postpoliomyelitis muscular atrophy): similarities with inclusion body myositis

In the chronically denervated muscles of patients with prior paralytic poliomyelitis, there are secondary myopathic features, including endomysial inflammation and rare vacuolated fibers. To assess the frequency and characteristics of the vacuoles and their similarities with those seen in inclusion body myositis (IBM), we examined 58 muscle biopsy specimens from patients with prior paralytic poliomyelitis for (1) the presence of rimmed vacuoles; (2) acid-phosphatase reactivity; (3) Congo-red-positive amyloid deposits; (4) electron microscopy, searching for tubulofilaments; and (5) immunoelectron microscopy, using antibodies against beta-amyloid and ubiquitin. We found vacuolated muscle fibers in 18 of 58 (31%) biopsies, with a mean frequency of 2.06 +/- 0.42 fibers per specimen. The vacuoles contained acid phosphatase-positive material in 6 of the 18 (33.30%) specimens and stained positive for Congo red in five (27.80%). By immunoelectron microscopy, the vacuoles contained 5.17 +/- 0.13 nm fibrils and 14.9 +/- 0.31 nm filaments that immunoreacted with antibodies to beta-amyloid and ubiquitin in a pattern identical to the one seen in IBM. We conclude that vacuolated muscle fibers containing filamentous inclusions positive for amyloid and ubiquitin are not unique to IBM and the other vacuolar myopathies but can also occur in a chronic neurogenic condition, such as postpoliomyelitis. The chronicity of the underlying disease, rather than the cause, may lead to vacuolar formation, amyloid deposition, and accumulation of ubiquitinated filaments.

In vitro cytopathogenicity and in vivo virulence of two strains of canine parainfluenza virus

In vivo and in vitro properties of two strains of canine parainfluenza virus (CPIV) were investigated. One strain, designated CPIV(+), induced syncytial giant cell formation and cytolysis in vitro, whereas the second strain, CPIV(-), caused only a mild strand-forming cytopathic effect with few, small syncytial giant cells. Vero cells infected with CPIV(+) or CPIV(-) were 100% positive for CPIV antigen as determined by immunofluorescent staining; however, 100% of CPIV(+) and less than 10% of CPIV(-) infected cells were hemadsorption positive. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis analysis revealed no differences in electrophoretic mobility of viral polypeptides between both strains; however, in CPIV(-), reduced or absent synthesis of the putative HN and F1 proteins was observed. Isopycnic separation of CPIV(+) progeny virions showed a high proportion of viral particles with a buoyant density of 1.18 g/cm3. In contrast, CPIV(-) progeny virions had a heterogeneous density profile ranging from 1.08 to 1.18 g/cm3. Intracerebral infection of six ferrets with CPIV(+) resulted in moderate lymphocytic and histiocytic choroiditis, meningitis, and ependymitis, whereas CPIV(-) infection caused only mild to moderate inflammation. Immunohistologically, CPIV antigen was prominent in ependymal lining cells of the ventricles in CPIV(+)-infected ferrets and was reduced or lacking in CPIV(-)-infected ferrets (n = 6). Sham-injected ferrets (n = 6) did not have histologic lesions and no viral antigen was identified. The present findings suggest that certain changes in the activities of CPIV glycoproteins may lead to alterations of CPIV virulence in vivo.

Characterization of the cord-like structures emerging from the surface of influenza C virus-infected cells

When HMV-II cells (a human malignant melanoma cell line) infected with a newly isolated influenza C strain (Yamagata/1/88) were examined by simple light microscopy, it was found that a large number of cord-like structures which had lengths up to about 500 microns or greater were emerging from the cell surface. The existence of viral glycoproteins (hemagglutinin-esterase, HE) on the surface of these huge structures was confirmed by hemadsorption experiments with erythrocytes from a variety of species as well as by immunofluorescent staining with anti-HE monoclonal antibody. Furthermore, electron microscopy revealed that numerous filamentous particles in the process of budding, each covered with a layer of surface projections approximately 13 nm in length, aggregated with their long axes parallel to form a cord-like structure visible under a light microscope. An electron-dense layer, which presumably consists of membrane protein (M), was seen in cross-sections of all filamentous virions whereas internal nucleocapsids were rarely seen. SDS-polyacrylamide gel electrophoresis of the purified cords also showed that they contained HE and M polypeptides but not nucleoprotein, confirming that long filamentous particles are mostly devoid of nucleocapsids. The emergence of cords on the cell surface was observed in various cell cultures infected with C/Yamagata/1/88 though their number and length varied markedly depending on cell type. The production of cord-like structures was also evident in HMV-II cells infected with any of several different influenza C strains, which suggests that the cord formation is a common feature of influenza C virus group.

A single amino acid substitution within the matrix protein of a type D retrovirus converts its morphogenesis to that of a type C retrovirus

Two different morphogenic processes of retroviral capsid assembly have been observed: the capsid is either assembled at the plasma membrane during the budding process (type C), or preassembled within the cytoplasm (types B and D). We describe here a gag mutant of Mason-Pfizer monkey virus, a type D retrovirus, in which a tryptophan substituted for an arginine in the matrix protein results in efficient assembly of capsids at the plasma membrane through a morphogenic process similar to that of type C retroviruses. We conclude that a type D retrovirus Gag polyprotein contains an additional, dominant signal that prevents immediate transport of precursors from the site of biosynthesis to the plasma membrane. Instead, they are directed to and retained at a cytoplasmic site where a concentration sufficient for self-assembly into capsids occurs. Thus, capsid assembly processes for different retroviruses appear to differ only in the intracellular site to which capsid precursors are directed.

Myristylation is required for intracellular transport but not for assembly of D-type retrovirus capsids

The role of myristylation, a fatty acid modification of nascent polypeptides, in the assembly and intracellular transport of D-type retroviral capsids was investigated through the use of oligonucleotide-directed mutagenesis. Myristic acid is normally esterified through an amide linkage to a glycine residue at the amino terminus of the Mason-Pfizer monkey virus gag gene products. Mutant pA-1, which has a codon for valine substituted for that of the normally myristylated glycine, is completely noninfectious. While the mutant gag polyprotein precursors are synthesized at normal levels, they are not myristylated and are not cleaved to the mature virion proteins. No extracellular virus particles are released from mutant pA-1-infected cells, but intracytoplasmic A-type particles (capsids) accumulate in the cytoplasm. Since none of the intracellular capsids can be found associated with the plasma membrane, these results strongly suggest that myristylation is a critical signal for intracytoplasmic transport of completed viral capsids to their normal site of budding and release.

Replication of varicella zoster virus in Raji cells

This report provides evidence for the replication of varicella zoster virus (VZV) in Raji cells. Infection was achieved by co-cultivation of Raji cells with VZV-infected human fibroblasts. Replication of VZV, as assessed by immunofluorescence using monoclonal antibodies against VZV-glycoproteins, ranged from 18 to 24% of the cells. Electron microscopy detected complete virions within the membrane-bound cytoplasmic vesicles and free viral particles in the nuclear matrix as late as 12 days post-infection. Western blot analysis of infected Raji cells demonstrated VZV-specific glycoproteins. The availability of a VZV-susceptible cell line growing in suspension culture provides a useful model for future studies.

A rapid screening procedure for the isolation of nonconditional replication mutants of Mason-Pfizer monkey virus: identification of a mutant defective in pol

A rapid, sensitive, and reproducible method for the isolation of human cell clones containing nonconditional, replication-defective (rd) mutants of Mason-Pfizer monkey virus (M-PMV), the prototype of the D-type retroviruses is described. The two mutants, rd1 and rd2, thus far isolated have been analyzed for virus particle production (using radiolabeled precursors and by electron microscopy) and for the status of intracellular viral precursors. Thin sections of rd1 and rd2 infected cells showed typical M-PMV particles when observed under electron microscope. A more direct assay of virus production, by labeling the mutant cell clones with [3H]uridine, also showed a distinct virus peak at an approximate density of 1.16 g/ml when culture fluids from rd1 and rd2 were analyzed. Analyses of these two mutants showed no defect in either gag or env gene products, however, further analysis of rd1 showed that the Pr180gag-pol was altered in its migration on SDS-polyacrylamide gel electrophoresis and no reverse transcriptase activity could be detected in rd1 virions. Mutant rd2, on the other hand, assembles noninfectious virus particles that are otherwise indistinguishable from those produced by wild-type cell clones. The biochemical basis for the defect in this mutant remains to be established.

Analysis and gene assignment of mRNAs of a paramyxovirus, simian virus 5

Polypeptides synthesized by the paramyxovirus SV5 in infected CV-1 cells were readily identified when the host cell was treated with actinomycin D. The unglycosylated forms of HN and Fo synthesized in infected cells in the presence of tunicamycin and HN and Fo synthesized in vitro were identified by immunoprecipitation with specific antibodies. Separation of SV5-specific poly(A)-containing RNAs on methyl-mercury agarose gels and in vitro translation of fractions, indicated that the viral polypeptides were translated from individual mRNAs except P (Mr approximately 44K) and the nonstructural polypeptide V (Mr approximately 24K) for which the mRNAs could not be separated. cDNA copies of SV5-specific mRNAs were synthesized and cloned in plasmid pBR322. Clones to NP, P + V, M, F, and HN were identified by hybrid-arrest and hybrid-selection translation of SV5 mRNAs. Tryptic peptide mapping of polypeptides P and V indicated that the peptides of V were a subset of those of P. Hybridization of cDNA probes to infected cell mRNAs separated on agarose gels permitted identification of the NP, P + V, M, F, and HN mRNAs and presumptive polycistronic mRNAs. The sizes and sequence homologies of these polycistronic mRNAs were used to derive a likely gene order on the SV5 50 S genome RNA.

Experimental Sendai virus-induced labyrinthitis in guinea pigs. An ultrastructural study of cochlear lesions

Guinea pigs were inoculated with a Sendai virus into the scala tympani and subsequent pathological changes of the cochleas were investigated by electron microscopy. Replication of the virus was indicated by buddings at the endolymphatic surface and by the intracytoplasmic occurrence of filamentous substances, ie, nucleocapsids. Budding viruses or free virus particles observed in a series of our experiments were identified as Sendai viruses by means of the immunological labeling with ferritin. Viral lesions, which were defined as the pathological changes of the cells associated with virus multiplication, were found in eight cochleas out of 20. This study revealed that the early lesions of the cochleas infected by Sendai viruses were primarily confined to Reissner's membrane and the stria vascularis, but the sensory cells were not affected.

Studies on the role of glycosylation in the functions and antigenic properties of influenza virus glycoproteins

The biological and antigenic roles of glycosylation were investigated in the influenza hemagglutinin (HA) glycoprotein using the glycosylation inhibitor tunicamycin (TM). Under conditions where only the nonglycosylated form of HA was detected by immunoprecipitation and gel electrophoresis, the migration of glycoproteins to the cell surface was observed by immunofluorescence using either monospecific or monoclonal antibody to the HA polypeptide. Analysis of the surface fluorescence in TM-treated infected cells by a fluorescence-activated cell sorter (FACS) showed that all cells exhibited fluorescence in the complete absence of glycosylation. The relative amount of HA antigen on cell surfaces was found to be reduced by only 30-40% in TM-treated cells, and this reflected a similar reduction in intracellular synthesis. Electron microscopic studies using ferritin labeling also demonstrated that the nonglycosylated HA glycoprotein was present in significant amounts on surfaces of infected cells. Virions with nonglycosylated glycoproteins were purified, and were found to have an approximate 30-fold decrease in both hemagglutinin and neuraminidase specific activities. The possible role of oligosaccharides in antigenic variation among various H1N1 strains was investigated. Immunoprecipitation reactions involving five different monoclonal antibodies and five antigenic variants of A/USSR/90/77 revealed no major antigenic differences between the glycosylated and nonglycosylated forms of HA.

Effect of monensin on Mason-Pfizer monkey virus glycoprotein synthesis

The effect of the monovalent carboxylic ionophore monensin on the biosynthesis, intracellular transport, and surface expression of the glycoproteins of Mason-Pfizer monkey virus was examined. Cells treated with monensin at concentrations of 10(-7) or 10(-6) M continued to synthesize virus particles, which from electron microscopic studies appeared to bud normally from the plasma membrane of the cells. However, the particles released had an altered buoyant density in sucrose gradients and were noninfectious. These noninfectious virions had a normal complement of non-glycosylated polypeptides but showed a significantly reduced amount of glycosylated proteins. The gp70 and gp20 polypeptides appeared to be completely absent, and a heterogeneous, higher-molecular-weight protein was observed on the virions instead. Studies on intracellular protein synthesis indicated that the precursor (Pr86env) to gp70 and gp20 is synthesized normally but is not cleaved to the mature proteins. Immunofluorescence studies showed, however, that the uncleaved molecule is expressed on the cell surface. In this system, therefore, Mason-Pfizer monkey virus glycoprotein migration appears to occur in the presence of monensin, whereas the cleavage and insertion of the glycoproteins into virions are inhibited.

In vitro assembly of the nonglycosylated membrane protein (M) of Sendai virus

The nonglycosylated membrane protein (M) of Sendai virus was purified from virions and conditions were found under which the protein assembled in vitro into three types of ordered structures: narrow tubes, wide tubes, and sheets. These structures were examined by high resolution electron microscopy by using negative staining and metal shadowing techniques. The tubes and sheets are formed from strands 7.2 nm wide that are composed of annular subunits. The wide tubes appear to be formed by the rolling of a sheet into a cylinder in which the 7.2-nm strands are inclined with a pitch of 26-33 degrees and have a left-handed orientation. In addition to the strong reflections corresponding to the 7.2-nm spacings generated by the strands, optical diffraction patterns also showed weak reflections that could be indexed on a lattice corresponding to real-space lattice constants of 7.6 nm and 5.3 nm, with an included angle of 71 degrees. The dimensions and arrangements of these structures formed in vitro are strikingly similar to those of ordered arrays of particles found by others to be associated with the inner surface of the plasma membrane of infected cells. The results support the concept that ordered arrays of M protein, similar to those assembled in vitro, are involved in the assembly of the virus particle by budding from the cell membrane and that they provide specific recognition sites for the viral nucleocapsid at the cytoplasmic surface of the plasma membrane.

In vivo and in vitro models of demyelinating diseases. V. Comparison of the assembly of mouse hepatitis virus, strain JHM, in two murine cell lines

The developmental sequence of a neurotropic strain (JHM) of mouse hepatitis virus was examined by transmission electron microscopy and immunocytology. The nucleoprotein core of this coronavirus, which contains RNA of positive polarity and is helical in configuration, becomes incorporated into enveloped particles in the same manner as the nucleocapsids of the orthomyxo- and paramyxoviruses. However, JHM virus is assembled intracellularly by budding at surfaces of smooth membranous vacuoles. A comparison of JHM virus replication in L2 and 17Cl-1 cell lines revealed that L2 cells undergo more rapid cytopathology and cease virus production much sooner than 17Cl-l cells. In L2 cells the accumulation of core material appears to continue after the abrupt cessation of virus assembly. This is evident by the massive cytoplasmic accumulation of structure resembling nucleocapsids, which react with hybridoma antibody to the nucleocapsid antigen as demonstrated by the immunoperoxidase procedure. The current findings are consistent with our previously published demonstration, using cells of neural and other deviation, of the fundamental role of the host cell type in regulating the replication and expression of coronaviruses.

Tunicamycin resistant glycosylation of coronavirus glycoprotein: demonstration of a novel type of viral glycoprotein

Tunicamycin has different effects on the glycosylation of the two envelope glycoproteins of mouse hepatitis virus (MHV), a coronavirus. Unlike envelope glycoproteins of other viruses, the transmembrane glycoprotein El is glycosylated normally in the presence of tunicamycin. This suggests that glycosylation of El does not involve transfer of core oligosaccharides from dolichol pyrophosphate intermediates to asparagine residues, but may occur by 0-linked glycosylation of serine or threonine residues. Synthesis of the peplomeric glycoprotein E2 is not readily detectable in the presence of tunicamycin. Inhibition of N-linked glycosylation of E2 by tunicamycin either prevents synthesis or facilitates degradation of the protein moiety of E2. Radiolabeling with carbohydrate precursors and borate gel electrophoresis of glycopeptides show that different oligcsaccharide side chains are attached to El and E2. The two coronavirus envelope glycoproteins thus appear to be glycosylated by different mechanisms. In tunicamycin-treated cells, noninfectious virions lacking peplomers are formed at intracytoplasmic membranes and released from the cells. These virions contain normal amounts of nucleocapsid protein and glycosylated El, but lack E2. Thus the transmembrane glycoprotein El is the only viral glycoprotein required for the formation of the viral envelope or for virus maturation and release. The peplomeric glycoprotein E2 appears to be required for attachment to virus receptors on the plasma membrane. The coronavirus envelope envelope glycoprotein E1 appears to be a novel type of viral glycoprotein which is post-translationally glycosylated by a tunicamycin-resistant process that yields oligosaccharide side chains different from those of N-linked glycoproteins. These findings suggest that El may be particularly useful as a model for studying the biosynthesis, glycosylation, and intracellular transport of 0-linked glycoproteins.

Mycoplasma-virus contacts, budding and scars remaining in the membranes of Acholeplasma laidlawii and its virus

Various stages of virus and mycoplasma budding indicated that both virus and, most probably some mycoplasma progeny developed by budding. Besides this alternative, binary fission was the mode of mycoplasma reproduction. Mycoplasma-virus and mycoplasma-mycoplasma connections by stems were observed. Circular scars, 40-80 nm in diameter, often in groups, were left in the membrane of mycoplasmas by the budding bodies. cytoplasmic structures seen in cross-fraction are presented. a relatively small number of globular virus-like bodies, not identical with MV-Lg-L 172, were observed budding from mycoplasma cells in the non-infected host culture.

Effect of tunicamycin on cell fusion induced by Mason-Pfizer monkey virus

Mason-Pfizer monkey virus, a D-type retrovirus, has been shown to induce multinucleate cell (syncytium) formation or cell fusion in several normal primate cells. A series of experiments has been carried out to examine whether a glycosylated "fusion-inducing" product is responsible for this biological property of Mason-Pfizer monkey virus. Treatment of rhesus monkey fetal lung cells with different concentrations of tunicamycin, a potent inhibitor of glycosylation, during infection with Mason-Pfizer monkey virus had no effect on cell fusion even though up to 5 micrograms of the drug per ml was tested. Furthermore, no significant effect on the extent of syncytium formation in rhesus monkey fetal lung cells was observed when the time of addition or duration of treatment with this inhibitor was varied. Nevertheless, tunicamycin was very effective in blocking glycosylation in rhesus cells since virions produced in the presence of this drug completely lacked gp70 and gp20, the two structural glycoproteins of Mason-Pfizer monkey virus. These non-glycosylated virus particles produced in the presence of tunicamycin were noninfectious as determined by a protein A binding assay and were unable to induce syncytium formation when assayed on rhesus cells. These results indicate that glycosylation of the fusion-inducing product is not required for multinucleate cell formation induced by Mason Pfizer monkey virus.

Effect of vanadate on intracellular distribution and function of 10-nm filaments

Earlier reports from this laboratory suggested that 10-nm filaments and microtubules act together in the movement and positioning of nuclei and centrioles. Sodium vanadate has been found to alter the distribution of 10-nm filaments and separate them from microtubules in virus-induced syncytia and uninfected cells. Accompanying this change in cytoskeletal elements in an alteration in the distribution of nuclei, centrioles, and other organelles. Nuclei in vanadate-treated syncytia were found in a circle or horseshoe arrangement, and 10-nm filaments were aggregated within the circle, whereas microtubules, were found in a network throughout the cytoplasm. Vanadate also caused a perinuclear aggregation of 10-nm filaments in single uninfected cells, whereas microtubules were throughout the cytoplasm, as in syncytia. Centrioles, mitochondria, rough endoplasmic reticulum, and lysosomes were scattered in the perinuclear area, with mitochondria and rough endoplasmic reticulum frequently closely associated, whereas the peripheral region of vanadate-treated cells contained ribosomes, microfilament bundles, and microtubules, but not 10-nm filaments. Vanadate limited virus-induced fusion of cells to polykaryocytes with 5--20 nuclei, in contrast to the massive syncytia found in untreated cells. These results indicate that vanadate separates 10-nm filaments and microtubules topologically and functionally, and support previous evidence that 10-nm filaments and microtubules act together in the movement and positioning of nuclei and other organelles.

Polypeptide synthesis in alphavirus-infected Aedes albopictus cells during the establishment of persistent infection

Polypeptide synthesis was examined in mosquito cells during the establishment of a persistent infection with two alphaviruses, Ross River virus (RRV) and Semliki Forest virus (SFV), and in vertebrate cells cytopathically-infected with the same viruses. In Aedes albopictus cell, RRV reached peak titres at 34--48 hours p.i. At 12 hours 85 per cent of cells assayed as infected by infective centre assay; by 48 hours when persistence was established, virus production was reduced and less than 5 per cent of cells assayed as infected. There was no shut-down of host polypeptide synthesis during infection. Viral polypeptide synthesis was maximal between 10 and 24 hours p.i. The major viral polypeptides labelled were nucleocapsid protein and envelope protein(s). The precursor polypeptide p95 which was prominent in infected BHK cells was not detected in mosquito cells. Similar results were obtained on SFV infection. During the establishment of persistence there was a coordinate decline in the synthesis of RRV polypeptides, reaching undetectable levels by 72 hours p.i. Subculturing persitently-infected cells led to a small increase in viral polypeptide synthesis and virus titre. In contrast, during RRV growth in BHK celos host protein synthesis was severly inhibited and by 9--11 hours p.i. virus-specific polypeptide synthesis represented more than 90 per cent of total protein synthetic activity.

Electron microscope study of cultured cells of the chorioallantoic membrane infected with representative paramyxoviruses

Chorioallantoic membrane (CAM) tissue cultures were found to be permissive for representative paramyxoviruses. The CAM cells can be used for plaque assay without the presence of trypsin. Ultrastructures of CAM cells infected with paramyxovirus Yucaipa (PMY), Sendai virus, and NDV were different. Nucleocapsids were readily seen in budding structures of cells infected with PMY, and in Sendai virus infected cells, large clusters of nucleocapsids were clearly evident in the cytoplasm. The site of glycoprotein cleavage does not have any effect on the nature of budding. It appears that a factor or factors in addition to the nature of the plasma membrane influences the morphology of cells infected with paramyxoviruses.

Polarized distribution of viral envelope proteins in the plasma membrane of infected epithelial cells

The surface distribution of the envelope glycoproteins of influenza, Sendai and Vesicular Stomatitis viruses was studied by immunofluorescence and immunoelectromicroscopy in infected epithelial cell monolayers, from which these viruses bud in a polarized fashion. It was found that before the onset of viral budding, the envelope proteins are exclusively localized into the same plasma membrane domains of the epithelial cells from which the virions ultimately bud: the glycoproteins of influenza and Sendai were detected at the apical surface, while the G protein of Vesicular Stomatitis virus was concentrated at the basolateral region. On the other hand, Sendai virus nucleocapsids, which can be easily identified in the cytoplasm before viral assembly, could be observed throughout the cell, not showing any preferential localization near the surface that the virions utilize for budding. These results are consistent with a model in which the asymmetric distribution of viral envelope proteins, rather than a polarized delivery of nucleocapsids, directs the polarity of viral budding. Furthermore, the asymmetric surface localization of viral glycoproteins suggests that these proteins share with intrinsic surface proteins of epithelial cells common biogenetic mechanisms and informational features or "sorting out" signals that determine their compartmentalization in the plasma membrane.

Conformation of the helical nucleocapsids of paramyxoviruses and vesicular stomatitis virus: reversible coiling and uncoiling induced by changes in salt concentration

The conformations of the helical nucleocapsids of the paramyxoviruses Sendai virus and simian virus 5, and of a rhabdovirus, vesicular stomatitis virus, have been found to vary extensively with changes in salt concentration. In 10 mM sodium phosphate buffer at pH 7.2, the nucleocapsids are loosely coiled or almost completely extended; with increasing concentrations of NaCl they become more tightly coiled and less flexible. Under isotonic conditions (150 mM) the Sendai virus nucleocapsid is moderately tightly coiled but still curved and apparently flexible, whereas at 400 mM or higher it is very tightly coiled, with the appearance of a rigid rod. These salt-dependent changes in conformation were also found with nucleocapsids composed of proteolytically cleaved protein subunits. Because of the effect of salt concentration, and the fact that it may change during the preparation of negatively stained samples of electron microscopy, it was necessary to fix that nucleocapsids before negative staining to preserve their original conformation. The striking changes in nucleocapsid conformation in response to the ionic milieu indicate the plasticity of its helical structure and suggest that changes in the microenvironment of the nucleocapsid could influence its conformation during viral RNA transcription and replication or during virus assembly by budding, processes in which changes in the coiling of the nucleocapsid or its flexibility could be important.

Importance of antibodies to the fusion glycoprotein of paramyxoviruses in the prevention of spread of infection

The effects of monospecific antibodies to the viral glycoprotein with hemagglutinating and neuraminidase activity (HN) and the viral glycoprotein with membrane-fusing activity (F) of the paramyxovirus simian virus 5 (SV5) on the spread of infection in two cell types have been investigated. In CV-1 cells, infection can spread by either released progeny virus adsorbing to and infecting other cells, or by fusion of an infected cell with an adjacent cell as a result of the cell-fusing activity of the F glycoprotein. In these cells, antibodies specific for the HN glycoprotein prevented the dissemination of infection by released infectious virus, but spread by cell fusion was not inhibited. Antibodies to the F glycoprotein completely prevented the spread of infection in these cells. In Madin-Darby bovine kidney cells, which are relatively resistant to SV5-induced fusion, antibodies to either the HN or F glycoproteins were capable of preventing the dissemination of infection. These results indicate that effective immunological prevention of the spread of paramyxovirus infection requires the presence of antibodies that inactivate the F glycoprotein. This requirement for anti-F antibodies has obvious implications for the design of effective paramyxovirus vaccines and provides an explanation for previous failures of formalin-inactivated paramyxovirus vaccines as well as additional insight into the possible immunopathological mechanisms involved in the atypical and severe infections that have occurred in individuals who received inactivated paramyxovirus vaccines and were subsequently infected by the virus.

Comparative studies of five strains of mumps virus in vitro and in neonatal hamsters: evaluation of growth, cytopathogenicity, and neurovirulence

The growth and cytopathogenicity of five strains of mumps virus were examined in six types of cell cultures and in neonatal hamsters. These strains included the MJ and RW strains, both recent cerebrospinal fluid isolates: the neuroadapted Kilham strain; the Enders strain adapted to chick embryo; and the Jeryl Lynn vaccine strain adapted to chick cell culture. The MJ, RW, and Kilham strains all produced infectious virus without restriction in vitro, but the RW strain did not cause obvious cytopathic effect; the MJ and Kilham strains were cytopathic. The Enders and Jeryl Lynn strains adapted to chick embryo cells were cytopathic and productive in chick cell culture but were restricted in ability to grow productively in vitro on mammalian cell types, even failing to produce noninfectious particles in some cases. In vitro cytopathogenicity was a host-independent property of a specific virus strain, but the type of cytopathic effect manifest in culture (eg, fusion, cytoplasmic vacuoles) depended on both the strain and the host cell. The ability of a virus strain to invade the brain parenchyma and infect neurons in vivo appeared to correlate with the strain's cytopathogenicity and not with passage history or adaptive status.