Monensin inhibits the processing of herpes simplex virus glycoproteins, their transport to the cell surface, and the egress of virions from infected cells

The significance of the Golgi complex in envelopment of bovine herpesvirus 1 (BHV-1) as revealed by cryobased electron microscopy

Nucleocapsids of herpesviruses originate in the nucleus of host cells and bud through the inner nuclear membrane acquiring tegument and envelope. The release of the enveloped virus particle from the perinuclear space is unknown. Cryobased electron microscopic imaging revealed enveloped virus particles within cisterns associated with the perinuclear space, a pre-Golgi compartment connecting Golgi cisterns to the perinuclear space, and enveloped virus particles in Golgi cisterns where they are packaged into transport vacuoles by membrane fission. To our knowledge, our images show for the first time the connectivity from the perinuclear space to Golgi cisterns. The data strongly indicate an intracisternal transport of enveloped virus particles from the budding site to the packaging site. Budding starts by condensation at the inner membrane. Condensation involving the viral envelope and peripheral tegument was persistent in virus particles within perinuclear space and associated cisterns. Virus particles within Golgi cisterns and transport vacuoles originating by Golgi membrane fission, however, lacked condensation. Instead, spikes were clearly evident. The phenomenon of condensation is considered likely to be responsible for preventing fusion of the viral envelope with cisternal membranes and/or for driving virions from the perinuclear space to Golgi cisterns. Glycoprotein K is discussed to likely play a role in the intracisternal transportation of virions. In addition to the pathway including intracisternal transport and packaging, there were clear indications for the well-known pathway involving wrapping of cytoplasmic nucleocapsids by Golgi membranes. The origin of the cytoplasmic nucleocapsids, however, remains obscure. Lack of evidence for release of nucleocapsids at the outer nuclear membrane suggests that the process is very rapid, or that nucleocapsids pass the nucleocytoplasmic barrier via an alternative route.

Human herpesvirus 8 glycoprotein B (gB), gH, and gL can mediate cell fusion

Herpesvirus entry into cells and herpesvirus-induced cell fusion are related processes in that virus penetration proceeds by fusion of the viral envelope and cell membrane. To characterize the human herpesvirus 8 (HHV-8) glycoproteins that can mediate cell fusion, a luciferase reporter gene activation assay was used. Chinese hamster ovary (CHO) cells expressing the HHV-8 glycoproteins of interest along with a luciferase reporter gene under the control of the T7 promoter were cocultivated with human cells transfected with T7 RNA polymerase. Because HHV-8 glycoprotein B (gB) expressed in CHO cells localizes to the perinuclear region, a truncated form of gB (designated gB(MUT)) that lacks putative endocytosis signals was constructed by deletion of the distal 58 amino acids of the cytoplasmic tail. HHV-8 gB(MUT) was expressed efficiently on the surface of CHO cells. HHV-8 gB, gH, and gL could mediate the fusion of CHO cells with two different human cell types, embryonic kidney cells and B lymphocytes. Substituting gB(MUT) for gB significantly enhanced the fusion of CHO cells with human embryonic kidney cells but not B lymphocytes. Thus, two human cell types known to be susceptible to HHV-8 entry were also suitable targets for cell fusion induced by HHV-8 gB, gH, and gL. For human embryonic kidney cells and B cells at least, optimal fusion was noted with the expression of all three HHV-8 glycoproteins.

Morphologic, immunohistochemical, immunologic, ultrastructural, and time-related study of herpes simplex virus type 1-infected cultured human fibroblasts

Membrane glycoproteins of enveloped animal viruses are synthesized, processed, and transported inside infected cells. Expression of viral glycoproteins on the surface of viral particles and host cells are essential for many biologic functions. In the case of herpes simplex virus, the glycoprotein molecules may act as nucleation points for virus assembly and budding at the nuclear membrane. The temporal distribution of herpes simplex virus type 1 particles and glycoproteins in cultured human fibroblasts was studied by titration plaque assay, immunoblots, immunofluorescence light microscopy, and immunogold cryosection electron microscopy to describe the virus-cell interactions. These concordant analyses revealed significant release of infectious viral particles to the medium at 6 hours postinfection, that the capacity of the host cells to make infectious viral particles was complete at 18 hours postinfection, and that the infection brought time-related modifications of tubulin, cell morphology, and viral glycoproteins. The data presented is in accord with the theory of envelopment at the nuclear membranes containing immature glycoproteins followed by multiple deenvelopments and reenvelopments of the virus particles during the transport and maturation in the endoplasmic reticulum and the Golgi complex.

The interacting UL31 and UL34 gene products of pseudorabies virus are involved in egress from the host-cell nucleus and represent components of primary enveloped but not mature virions

A 2.6-kbp fragment of the pseudorabies virus (PrV) genome was sequenced and shown to contain the homologues of the highly conserved herpesvirus genes UL31 and UL32. By use of a monospecific antiserum, the UL31 gene product was identified as a nuclear protein with an apparent molecular mass of 29 kDa. For functional analysis, UL31 was deleted by mutagenesis in Escherichia coli of an infectious full-length clone of the PrV genome. The resulting virus mutants were deficient in plaque formation, and titers were reduced more than 100-fold from those of wild-type PrV. Ultrastructural analyses demonstrated that capsid maturation and DNA packaging were not affected. However, neither budding at the inner nuclear membrane nor cytoplasmic or extracellular virus particles were observed. These replication defects were similar to those of a UL34 deletion mutant (B. G. Klupp, H. Granzow, and T. C. Mettenleiter, J. Virol. 74:10063-10073, 2000) and could be completely repaired in a cell line which constitutively expresses the UL31 protein. Yeast two-hybrid studies revealed that a UL31 fusion protein specifically interacts with plasmids of a PrV genome library expressing the N-terminal part of UL34. Vice versa, UL34 selected UL31-encoding plasmids from the library. Immunofluorescence studies and immune electron microscopy demonstrated that in cells infected with wild-type PrV, both proteins accumulate at the nuclear membrane, whereas in the absence of UL34 the UL31 protein is dispersed throughout the nucleus. Like the UL34 protein, the UL31 gene product is a component of enveloped virus particles within the perinuclear space and absent from mature virions. Our findings suggest that physical interaction between these two virus proteins might be a prerequisite for primary envelopment of PrV at the inner nuclear membrane and that this envelope is removed by fusion with the outer nuclear membrane.

Herpes simplex virus nucleocapsids mature to progeny virions by an envelopment --> deenvelopment --> reenvelopment pathway

Herpes simplex virus (HSV) nucleocapsids acquire an envelope by budding through the inner nuclear membrane, but it is uncertain whether this envelope is retained during virus maturation and egress or whether mature progeny virions are derived by deenvelopment at the outer nuclear membrane followed by reenvelopment in a cytoplasmic compartment. To resolve this issue, we used immunogold electron microscopy to examine the distribution of glycoprotein D (gD) in cells infected with HSV-1 encoding a wild-type gD or a gD which is retrieved to the endoplasmic reticulum (ER). In cells infected with wild-type HSV-1, extracellular virions and virions in the perinuclear space bound approximately equal amounts of gD antibody. In cells infected with HSV-1 encoding an ER-retrieved gD, the inner and outer nuclear membranes were heavily gold labeled, as were perinuclear enveloped virions. Extracellular virions exhibited very little gold decoration (10- to 30-fold less than perinuclear virions). We conclude that the envelope of perinuclear virions must be lost during maturation and egress and that mature progeny virions must acquire an envelope from a post-ER cytoplasmic compartment. We noted also that gD appears to be excluded from the plasma membrane in cells infected with wild-type virus.

Egress of alphaherpesviruses: comparative ultrastructural study

Egress of four important alphaherpesviruses, equine herpesvirus 1 (EHV-1), herpes simplex virus type 1 (HSV-1), infectious laryngotracheitis virus (ILTV), and pseudorabies virus (PrV), was investigated by electron microscopy of infected cell lines of different origins. In all virus-cell systems analyzed, similar observations were made concerning the different stages of virion morphogenesis. After intranuclear assembly, nucleocapsids bud at the inner leaflet of the nuclear membrane, resulting in enveloped particles in the perinuclear space that contain a sharply bordered rim of tegument and a smooth envelope surface. Egress from the perinuclear cisterna primarily occurs by fusion of the primary envelope with the outer leaflet of the nuclear membrane, which has been visualized for HSV-1 and EHV-1 for the first time. The resulting intracytoplasmic naked nucleocapsids are enveloped at membranes of the trans-Golgi network (TGN), as shown by immunogold labeling with a TGN-specific antiserum. Virions containing their final envelope differ in morphology from particles within the perinuclear cisterna by visible surface projections and a diffuse tegument. Particularly striking was the addition of a large amount of tegument material to ILTV capsids in the cytoplasm. Extracellular virions were morphologically identical to virions within Golgi-derived vesicles, but distinct from virions in the perinuclear space. Studies with gB- and gH-deleted PrV mutants indicated that these two glycoproteins, which are essential for virus entry and direct cell-to-cell spread, are dispensable for egress. Taken together, our studies indicate that the deenvelopment-reenvelopment process of herpesvirus maturation also occurs in EHV-1, HSV-1, and ILTV and that membrane fusion processes occurring during egress are substantially different from those during entry and direct viral cell-to-cell spread.

Characterization of herpes simplex virus-containing organelles by subcellular fractionation: role for organelle acidification in assembly of infectious particles

The cytoplasmic compartments occupied by exocytosing herpes simplex virus (HSV) are poorly defined. It is unclear which organelles contain the majority of trafficking virions and which are occupied by virions on a productive rather than defective assembly pathway. These problems are compounded by the fact that HSV-infected cells produce virus continuously over many hours. All stages in viral assembly and export therefore coexist, making it impossible to determine the sequence of events and their kinetics. To address these problems, we have established assays to monitor the presence of capsids and enveloped virions in cell extracts and prepared HSV-containing organelles from normally infected cells and from cells undergoing a single synchronized wave of viral egress. We find that, in both cases, HSV particles exit the nucleus and accumulate in organelles which cofractionate with the trans-Golgi network (TGN) and endosomes. In addition to carrying enveloped infectious virions in their lumen, HSV-bearing organelles also displayed nonenveloped capsids attached to their cytoplasmic surface. Neutralization of organellar pH by chloroquine or bafilomycin A resulted in the accumulation of noninfectious enveloped particles. We conclude that the organelles of the TGN/endocytic network play a key role in the assembly and trafficking of infectious HSV.

Group IIA and group V secretory phospholipase A(2): quantitative analysis of expression and secretion and determination of the localization and routing in rat mesangial cells

Mesangial cells can be induced to express group IIA and group V secretory phospholipase A(2) (sPLA(2)) at the mRNA level and at the protein level. In this report we quantitatively analyze the expression of both proteins in stimulated cells by Western blot techniques. We found that 75-80% of the total amount of synthesized group IIA sPLA(2) was secreted. The synthesized group V sPLA(2), however, was present almost exclusively intracellularly. The amount of group V present in the cell was comparable to the intracellular amount of group IIA sPLA(2). We furthermore studied the localization and routing of both proteins. Using fusion proteins of the group IIA or group V pre-sPLA(2) with green fluorescent protein it was established that both presequences are able to direct the proteins to the Golgi system. In immunofluorescence studies group V sPLA(2) expressed by rat mesangial cells was located in a punctate pattern in the cytosol with an enrichment near the nucleus. Immunofluorescent confocal laser scanning microscopy revealed that the group V and IIA sPLA(2) show partial colocalization in a Golgi-like structure in the inner part in the cell, but no colocalization was seen in the vesicles in the cytoplasm. The images also showed that group IIA sPLA(2) was located throughout the cell while group V was mainly present in the inner part of the cell. After treatment of the cells with brefeldin A or monensin the group IIA enzyme could no longer be detected, while group V sPLA(2) was still present although its localization was somewhat dependent on the treatment. Collectively, these results indicate that the two enzymes differ in both localization and routing in the cell, which underscores the hypothesis that the enzymes might have different functions.

Characterization of inhibitory action of concanamycins against herpes simplex virus

Concanamycins A (Conmy A) and B (Conmy B), well-known inhibitors of the vacuolar proton-ATPase, were isolated from the culture broth of Streptomyces sp. strain FK51 as antiherpetic agents. These compounds showed potent inhibition of herpes simplex virus type 1 (HSV-1) replication in an in vitro assay system, having antiviral activities with 50% inhibitory concentrations of 0.072 and 0.51 ng/ml for Conmy A and Conmy B, respectively. While the attachment of HSV-1 to Vero cells was not inhibited, both of the compounds blocked the penetration of virus into host cells. When added to the late stages of virus replication, the concanamycins also exerted marked inhibitory effects on the production of viruses. Release of progeny viruses was found to be suppressed by the agents. SDS-PAGE analysis of isotope-labelled HSV-specific proteins revealed that the synthesis of beta proteins was moderately inhibited and some of the glycoproteins were synthesized with reduced molecular weights. Western blot analysis using antibodies against two HSV-specific glycoproteins (gC and gD) showed differences in their syntheses between untreated and Conmy A-treated cells. Syncytium formation by HSV-1 strain HF was inhibited, and small plaques with rounded cells were formed in Conmy A-treated cell cultures. When wild-type HSV-1 was serially propagated under the selective pressure of Conmy A, and the resulting progeny viruses were grown in drug-free medium, their plaque morphology of syncytium and sensitivity to Conmy A were the same as those of parent virus. From these findings, antiherpetic activities of Conmy A and B might be mainly dependent on their activities as vacuolar proton-ATPase inhibitors with intracellular translocation of glycoproteins and the inhibition of the maturation of virus glycoproteins.

Herpes simplex virus gE/gI sorts nascent virions to epithelial cell junctions, promoting virus spread

Alphaherpesviruses spread rapidly through dermal tissues and within synaptically connected neuronal circuitry. Spread of virus particles in epithelial tissues involves movement across cell junctions. Herpes simplex virus (HSV), varicella-zoster virus (VZV), and pseudorabies virus (PRV) all utilize a complex of two glycoproteins, gE and gI, to move from cell to cell. HSV gE/gI appears to function primarily, if not exclusively, in polarized cells such as epithelial cells and neurons and not in nonpolarized cells or cells that form less extensive cell junctions. Here, we show that HSV particles are specifically sorted to cell junctions and few virions reach the apical surfaces of polarized epithelial cells. gE/gI participates in this sorting. Mutant HSV virions lacking gE or just the cytoplasmic domain of gE were rarely found at cell junctions; instead, they were found on apical surfaces and in cell culture fluids and accumulated in the cytoplasm. A component of the AP-1 clathrin adapter complexes, mu1B, that is involved in sorting of proteins to basolateral surfaces was involved in targeting of PRV particles to lateral surfaces. These results are related to recent observations that (i) HSV gE/gI localizes specifically to the trans-Golgi network (TGN) during early phases of infection but moves out to cell junctions at intermediate to late times (T. McMillan and D. C. Johnson, J. Virol., in press) and (ii) PRV gE/gI participates in envelopment of nucleocapsids into cytoplasmic membrane vesicles (A. R. Brack, B. G. Klupp, H. Granzow, R. Tirabassi, L. W. Enquist, and T. C. Mettenleiter, J. Virol. 74:4004-4016, 2000). Therefore, interactions between the cytoplasmic domains of gE/gI and the AP-1 cellular sorting machinery cause glycoprotein accumulation and envelopment into specific TGN compartments that are sorted to lateral cell surfaces. Delivery of virus particles to cell junctions would be expected to enhance virus spread and enable viruses to avoid host immune defenses.

A null mutation in the UL36 gene of herpes simplex virus type 1 results in accumulation of unenveloped DNA-filled capsids in the cytoplasm of infected cells

The UL36 open reading frame (ORF) encodes the largest herpes simplex virus type 1 (HSV-1) protein, a 270-kDa polypeptide designated VP1/2, which is also a component of the virion tegument. A null mutation was generated in the UL36 gene to elucidate its role in the virus life cycle. Since the UL36 gene specifies an essential function, complementing cell lines transformed for sequences encoding the UL36 ORF were made. A mutant virus, designated KDeltaUL36, that encodes a null mutation in the UL36 gene was isolated and propagated in these cell lines. When noncomplementing cells infected with KDeltaUL36 were analyzed, both terminal genomic DNA fragments and DNA-containing capsids (C capsids) were detected; therefore, UL36 is not required for cleavage or packaging of DNA. Sedimentation analysis of lysates from mutant-infected cells revealed the presence of particles that have the physical characteristics of C capsids. In agreement with this, polypeptide profiles of the mutant particles revealed an absence of the major envelope and tegument components. Ultrastructural analysis revealed the presence of numerous unenveloped DNA containing capsids in the cytoplasm of KDeltaUL36-infected cells. The UL36 mutant particles were tagged with the VP26-green fluorescent protein marker, and their movement was monitored in living cells. In KDeltaUL36-infected cells, extensive particulate fluorescence corresponding to the capsid particles was observed throughout the cytosol. Accumulation of fluorescence at the plasma membrane which indicated maturation and egress of virions was observed in wild-type-infected cells but was absent in KDeltaUL36-infected cells. In the absence of UL36 function, DNA-filled capsids are produced; these capsids enter the cytosol after traversing the nuclear envelope and do not mature into enveloped virus. The maturation and egress of the UL36 mutant particles are abrogated, possibly due to a late function of this complex polypeptide, i.e., to target capsids to the correct maturation pathway.

Primary envelopment of pseudorabies virus at the nuclear membrane requires the UL34 gene product

Primary envelopment of several herpesviruses has been shown to occur by budding of intranuclear capsids through the inner nuclear membrane. By subsequent fusion of the primary envelope with the outer nuclear membrane, capsids are released into the cytoplasm and gain their final envelope by budding into vesicles in the trans-Golgi area. We show here that the product of the UL34 gene of pseudorabies virus, an alphaherpesvirus of swine, is localized in transfected and infected cells in the nuclear membrane. It is also detected in the envelope of virions in the perinuclear space but is undetectable in intracytoplasmic and extracellular enveloped virus particles. Conversely, the tegument protein UL49 is present in mature virus particles and absent from perinuclear virions. In the absence of the UL34 protein, acquisition of the primary envelope is blocked and neither virus particles in the perinuclear space nor intracytoplasmic capsids or virions are observed. However, light particles which label with the anti-UL49 serum are formed in the cytoplasm. We conclude that the UL34 protein is required for primary envelopment, that the primary envelope is biochemically different from the final envelope in that it contains the UL34 protein, and that perinuclear virions lack the tegument protein UL49, which is present in mature virions. Thus, we provide additional evidence for a two-step envelopment process in herpesviruses.

Human cytomegalovirus virions differentially incorporate viral and host cell RNA during the assembly process

While analyzing human cytomegalovirus (HCMV) gene expression in infected cells by RNA-specific nucleic acid sequence-based amplification (NASBA), positive results were observed for HCMV RNA encoded by several viral genes immediately after the addition of the virus. UV-inactivated virus also gave a positive NASBA result without establishing active infection, suggesting that RNA was associated with the inoculum. Highly purified virions devoid of cellular contamination proved to be positive for viral RNA encoding both immediate-early (UL123) and late (UL65) gene products. Virion-associated RNA might be incorporated specifically or without selection during the virion assembly. In the latter case, cellular RNA would also be present in the virion. A high-abundant cellular RNA encoded by GAPDH and even U1A RNA, which is expressed at low levels, were detected in the virion fraction, whereas cellular DNA was absent. Virion fractionation revealed that cellular RNA was absent in purified de-enveloped capsids. In conclusion, cellular and viral RNA was present between the capsid and envelope of the virion, whereas in the capsid only viral RNA could be detected. The results suggest that virion-associated viral and cellular RNA is incorporated nonspecifically during virion assembly.

Effects of truncation of the carboxy terminus of pseudorabies virus glycoprotein B on infectivity

Glycoproteins homologous to the type I membrane glycoprotein B (gB) of herpes simplex virus 1 (HSV-1) are the most highly conserved glycoproteins within the family Herpesviridae and are present in members of each herpesvirus subfamily. In the alphaherpesvirus pseudorabies virus (PrV), gB is required for entry into target cells and for direct viral cell-to-cell spread. These processes, though related, appear to be distinct, and thus it was interesting to analyze whether they require different functions of gB. To this end, we established cell lines stably expressing different carboxy-terminally truncated versions of PrV gB by deleting either (i) one predicted intracytoplasmic alpha-helical domain encompassing putative YQRL and dileucine internalization signals, (ii) two predicted intracytoplasmic alpha-helical domains, (iii) the complete intracytoplasmic domain, or (iv) the intracytoplasmic domain and the transmembrane anchor region. Confocal laser scanning microscopy showed that gB derivatives lacking at least the last 29 amino acids (aa) localize close to the plasma membrane, while the full-length protein accumulates in intracellular aggregations. Trans-complementation studies with a gB-deleted PrV (PrV-gB(-)) demonstrated that the 29-aa truncated form lacking the putative internalization signals and the C-terminal alpha-helical domain (gB-008) was efficiently incorporated into PrV-gB(-) virions and efficiently complemented infectivity and cell-to-cell spread. Moreover, gB-008 exhibited an enhanced fusogenic activity. In contrast, gB proteins lacking both alpha-helical domains (gB-007), the complete intracytoplasmic domain, or the intracytoplasmic domain and transmembrane anchor were only inefficiently or not at all incorporated into PrV-gB(-) virions and did not complement infectivity. However, gB-007 was able to mediate cell-to-cell spread of PrV-gB(-). Similar phenotypes were observed when virus recombinants expressing gB-008 or gB-007, respectively, instead of wild-type gB were isolated and analyzed. Thus, our data show that internalization of gB is not required for gB incorporation into virions nor for its function in either entry or cell-to-cell spread. Moreover, they indicate different requirements for gB in these membrane fusion processes.

Evidence that herpes simplex virus VP16 is required for viral egress downstream of the initial envelopment event

During infection with herpes simplex virus type 1 (HSV-1), VP16 serves multiple functions, including transcriptional activation of viral immediate early genes and downregulation of the virion host shutoff protein vhs. Furthermore, VP16 has been shown to be involved in some aspect of virus assembly and/or maturation. Experiments with a VP16 null virus, 8MA, suggested that VP16 plays a direct role in virion assembly, since removal of VP16 from the HSV-1 genome results in reduced levels of encapsidated DNA and a failure to produce extracellular enveloped particles. However, VP16 null mutants display a severe translational arrest due to unrestrained vhs activity, thus complicating interpretation of these data. We examine here the role of VP16 in virion assembly and egress in the context of a vhs null background, using the virus 8MA/DeltaSma (VP16(-) vhs(-)). Comparison of 8MA and 8MA/DeltaSma with respect to viral DNA accumulation and encapsidation and accumulation of the major capsid protein, VP5, revealed that the 8MA lethal phenotype is only partially due to uncontrolled vhs activity, indicating that VP16 is required in HSV-1 virion formation. Electron microscopy confirmed these results and further showed that VP16 is required for HSV-1 egress beyond the perinuclear space. In addition, we describe the isolation and characterization of an 8MA derivative capable of propagation on Vero cells, due to second site mutations in the vhs and UL53 (gK) genes. Taken together, these results show that VP16 is required for viral egress downstream of the initial envelopment step and further underscore the importance of VP16 in controlling vhs activity within an infected cell.

Role of the cytoplasmic tail of pseudorabies virus glycoprotein E in virion formation

Glycoproteins M (gM), E (gE), and I (gI) of pseudorabies virus (PrV) are required for efficient formation of mature virions. The simultaneous absence of gM and the gE/gI complex results in severe deficiencies in virion morphogenesis and cell-to-cell spread, leading to drastically decreased virus titers and a small-plaque phenotype (A. Brack, J. Dijkstra, H. Granzow, B. G. Klupp, and T. C. Mettenleiter, J. Virol. 73:5364-5372, 1999). Serial passaging in noncomplementing cells of a virus mutant unable to express gM, gE, and gI resulted in a reversion of the small-plaque phenotype and restoration of infectious virus formation to the level of a gM(-) mutant. Genetic analyses showed that reversion of the phenotype was accompanied by a genomic rearrangement which led to the fusion of a portion of the gE gene encoding the cytoplasmic domain to the 3' end of the glycoprotein D gene, resulting in expression of a chimeric gD-gE protein. Since this indicated that the intracytoplasmic domain of gE was responsible for the observed phenotypic alterations, the UL10 (gM) gene was deleted in a PrV mutant, PrV-107, which specifically lacked the cytoplasmic tail of gE. Regarding one-step growth, plaque size, and virion formation as observed under the electron microscope, the mutant lacking gM and the gE cytoplasmic tail proved to be very similar to the gE/I/M triple mutant. Thus, our data indicate that it is the cytoplasmic tail of gE which is responsible for the observed phenotypic effects in conjunction with deletion of gM. We hypothesize that the cytoplasmic domain of gE specifically interacts with components of the capsid and/or tegument, leading to efficient secondary envelopment of intracytoplasmic capsids.

Human cytomegalovirus pp28 (UL99) localizes to a cytoplasmic compartment which overlaps the endoplasmic reticulum-golgi-intermediate compartment

Although the assembly of herpesviruses has remained an active area of investigation, considerable controversy continues to surround the cellular location of tegument and envelope acquisition. This controversy is particularly evident when the proposed pathways for alpha- and beta-herpesvirus assembly are compared. We have approached this aspect of human cytomegalovirus (HCMV) assembly, specifically, envelopment, by investigating the intracellular trafficking of viral tegument proteins which localize in the cytoplasms of infected cells. In this study we have demonstrated that the virion tegument protein pp28 (UL99), a true late protein, was membrane associated as a result of myristoylation. A mutation in this protein which prevented incorporation of [(3)H]myristic acid also altered the detergent solubility and intracellular distribution of the protein when it was expressed in transfected cells. Using a panel of markers for intracellular compartments, we could localize the expression of wild-type pp28 to an intracellular compartment which colocalized with the endoplasmic reticulum-Golgi-intermediate compartment (ERGIC), a dynamic compartment of the secretory pathway which interfaces with both the ER and Golgi apparatus. The localization of this viral tegument protein within an early secretory compartment of the cell provided further evidence that the assembly of the HCMV tegument likely includes a cytoplasmic phase. Because pp28 has been shown to be localized to a cytoplasmic assembly compartment in HCMV-infected cells, our findings also suggested that viral tegument protein interactions within the secretory pathway may have an important role in the assembly of the virion.

Pseudotyping of glycoprotein D-deficient herpes simplex virus type 1 with vesicular stomatitis virus glycoprotein G enables mutant virus attachment and entry

The use of herpes simplex virus (HSV) vectors for in vivo gene therapy will require the targeting of vector infection to specific cell types in certain in vivo applications. Because HSV glycoprotein D (gD) imparts a broad host range for viral infection through recognition of ubiquitous host cell receptors, vector targeting will require the manipulation of gD to provide new cell recognition specificities in a manner designed to preserve gD's essential role in virus entry. In this study, we have determined whether an entry-incompetent HSV mutant with deletions of all Us glycoproteins, including gD, can be complemented by a foreign attachment/entry protein with a different receptor-binding specificity, the vesicular stomatitis virus glycoprotein G (VSV-G). The results showed that transiently expressed VSV-G was incorporated into gD-deficient HSV envelopes and that the resulting pseudotyped virus formed plaques on gD-expressing VD60 cells, albeit at a 50-fold-reduced level compared to that of wild-type gD. This reduction may be related to differences in the entry pathways used by VSV and HSV or to the observed lower rate of incorporation of VSV-G into virus envelopes than that of gD. The rate of VSV-G incorporation was greatly improved by using recombinant molecules in which the transmembrane domain of HSV glycoprotein B or D was substituted for that of VSV-G, but these recombinant molecules failed to promote virus entry. These results show that foreign glycoproteins can be incorporated into the HSV envelope during replication and that gD can be dispensed with on the condition that a suitable attachment/entry function is provided.

Effects of targeting herpes simplex virus type 1 gD to the endoplasmic reticulum and trans-Golgi network

Glycoprotein D (gD) of herpes simplex virus type 1 (HSV-1) was modified to encode targeting signals known to localize proteins to either the endoplasmic reticulum (ER) or the trans-Golgi network. These motifs conferred the predicted targeting properties on gD in transfected cells as judged by immunofluorescence staining, and the exclusion of targeted gD from the cell surface was confirmed by the fact that these molecules exhibited substantially reduced activity in cell-cell fusion assays. Recombinant viruses expressing Golgi-targeted forms of gD grew to wild-type levels in noncomplementing cells, exhibited unaltered particle/infectivity ratios, and were found to contain wild-type levels of gD, whereas a recombinant expressing ER-retained gD was helper cell dependent and, when grown on noncomplementing cells, produced virions of low specific infectivity with greatly reduced levels of gD. These data imply that HSV-1 acquires its final membrane from a post-ER compartment and lend support to the view that the virus undergoes de-envelopment and reenvelopment steps during virus egress.

Structural maturation of the transmissible gastroenteritis coronavirus

During the life cycle of the transmissible gastroenteritis coronavirus (TGEV), two types of virus-related particles are detected in infected swine testis cells: large annular viruses and small dense viruses. We have studied the relationships between these two types of particles. Immunoelectron microscopy showed that they are closely related, since both large and small particles reacted equally with polyclonal and monoclonal antibodies specific for TGEV proteins. Monensin, a drug that selectively affects the Golgi complex, caused an accumulation of large annular viral particles in perinuclear elements of the endoplasmic reticulum-Golgi intermediate compartment. A partial reversion of the monensin blockade was obtained in both the absence and presence of cycloheximide, a drug that prevented the formation of new viral particles. After removal of monensin, the Golgi complex recovered its perinuclear location, and a decrease in the number of perinuclear large viral particles was observed. The release of small dense viral particles into secretory vesicles and the extracellular medium was also observed, as was a partial recovery of infectivity in culture supernatants. Small viral particles started to be seen between the third and the fourth Golgi cisternae of normally infected cells. All of these data strongly indicate that the large annular particles are the immature precursors of the small dense viruses, which are the infectious TGEV virions. The immature viral particles need to reach a particular location at the trans side of the Golgi stack to complete their morphological maturation.

Genetic analysis of the role of herpes simplex virus type 1 glycoprotein K in infectious virus production and egress

Herpes simplex virus type 1 (KOS)DeltagK is a mutant virus which lacks glycoprotein K (gK) and exhibits defects in virion egress (S. Jayachandra, A. Baghian, and K. G. Kousoulas, J. Virol. 69:5401-5413, 1997). To further understand the role of gK in virus egress, we constructed recombinant viruses, DeltagKhpd-1, -2, -3, and -4, that specified gK amino-terminal portions of 139, 239, 268, and 326 amino acids, respectively, corresponding to truncations immediately after each of the four putative membrane-spanning domains of gK. DeltagKhpd-1 and DeltagKhpd-2 viruses produced lower yields and smaller plaques than DeltagK. Numerous DeltagKhpd-1 capsids accumulated predominately within large double-membrane vesicles of which the inner membrane appeared to be derived from viral envelopes while the outer membrane appeared to originate from the outer nuclear membrane. The mutant virus DeltagKhpd-3 produced higher yields and larger plaques than the DeltagK virus. The mutant virus DeltagKhpd-4 produced yields and plaques similar to those of the wild-type virus strain KOS, indicating that deletion of the carboxy-terminal 12 amino acids did not adversely affect virus replication and egress. Comparisons of the gK primary sequences specified by alphaherpesviruses revealed the presence of a cysteine-rich motif (CXXCC), located within domain III in the lumen side of gK, and a tyrosine-based motif, YTKPhi (where Phi is any bulky hydrophobic amino acid), located between the second and third hydrophobic domains (domain II) in the cytoplasmic side of gK. The mutant virus gK/Y183S, which was constructed to specify gK with a single-amino-acid change (Y to S) within the YTKPhi motif, replicated less efficiently than the DeltagK virus. The mutant virus gK/C304S-C307S, which was constructed to specify two serine instead of cysteine residues within the cysteine-rich motif (CXXCC changed to SXXSC) of gK domain III, replicated more efficiently than the DeltagK virus. Our data suggests that gK contains domains in its amino-terminal portion that promote aberrant nucleocapsid envelopment and/or membrane fusion between different virion envelopes and contains domains within its domains II and III that function in virus replication and egress.

The genome of cowpox virus contains a gene related to those encoding the epidermal growth factor, transforming growth factor alpha and vaccinia growth factor

Cowpox virus (CPV) is a member of the Orthopoxvirus genus and has the genetic capacity to encode a multitude of genes that interfere with the host inflammatory and immune response or modulate the physiological state of infected and non-infected cells. Among these CPV factors are receptors homologous to interferon and tumor necrosis factor receptors and also a viral cellular serine-proteinase analog. Here we describe the detection of a CPV gene that encodes a protein homologous to epidermal growth factor, transforming growth factor alpha and poxvirus growth factors, such as the vaccinia growth factor (VGF). The VGF and other poxvirus growth factors are produced early in the infection and are secreted into the medium where they bind to the EGF receptors, generating mytotic responses. The cowpox growth factor (CGF) gene was detected in three copies on the virus genome by PCR, and by northern and southern blot hybridization using VGF nucleotide sequences as primers and probes. The CPV gene has a strong nucleotide and predicted amino acid similarity with VGF, and is also produced early in the infection.

Glycoprotein K of herpes simplex virus: a transmembrane protein encoded by the UL53 gene which regulates membrane fusion

Glycoprotein K (gK) encoded by the UL53 gene is the ninth out of eleven HSV glycoproteins (gps). The precursor gK (pgK) is a transmembrane protein with four hydrophobic domains, which consists of 338 amino acids. The UL53 gene has two initiation codons: the upper overlaps with the UL52 ORF, while the lower is located 55 codons downstream and specifies a truncated precursor of the gK polypeptide. The UL53 gene and the upstream located UL52 gene have a common polyadenylation signal downstream from the UL53 stop codon so that the UL53 mRNA is completely nested within the UL52 transcript. The syn1 mutations in several KOSsyn mutants and in the MPsyn virus, which had been fine mapped to DNA coordinates 0.735-0.740, were later on located to the UL53 gene, especially to its portion which specifies the first 120 amino acids (aa) from the N-terminus (most frequently residue 40) and to a less precisely defined locus between aa 301-310 (close to the C-terminus). Point mutations in the N-terminal ectodomain of gK, which are related to syn formation, impair the putative ability of this region to down-regulate membrane fusion. The two N-glycosylated mannose core oligosaccharides are attached to the Asn residues of the gK polypeptide at positions 48 and 58, respectively. In infected cells, gK is localized mainly in the nuclear and endoplasmic reticulum (ER) membranes. It is not clear, whether gK becomes incorporated into the envelope of mature HSV particles. Studies with the insertion/deletion gK mutants showed the importance of gK for capsid envelopment, for the transportation and egress or virions from infected cells. It seems that gK has an essential role in virion egress, even though this glycoprotein acts in accord with gH and with another membrane protein encoded by the UL20 gene.

Intracellular transport and maturation pathway of human herpesvirus 6

A peculiar characteristic of cells infected with human herpesvirus 6 (HHV6) is the absence of viral glycoproteins on the plasma membrane, which may reflect an atypical intracellular transport of the virions and/or the viral glycoproteins, different from that of the other members of the herpesvirus family. To investigate the maturation pathway of HHV-6 in the human T lymphoid cell line HSB-2, we used lectin cytochemistry and immunogold labeling combined with several electron microscopical techniques, such as ultrathin frozen sections, postembedding, and fracture-label. Immunolabeling with anti-gp116 and anti-gp82-gp105 monoclonal antibodies revealed that the viral glycoproteins are undetectable on nuclear membranes and that at the inner nuclear membrane nucleocapsids acquire a primary envelope lacking viral glycoproteins. After de-envelopment, cytoplasmic nucleocapsids acquire a thick tegument and a secondary envelope with viral glycoproteins at the level of neo-formed annulate lamellae or at the cis-side of the Golgi complex. Cytochemical labeling using helix pomatia lectin revealed that the newly acquired secondary viral envelopes contain intermediate forms of glycocomponents, suggesting a sequential glycosylation of the virions during their transit through the Golgi area before their final release into the extracellular space. Immunogold labeling also showed that the viral glycoproteins, which are not involved in the budding process, reach and accumulate in the endosomal/lysosomal compartment. Pulse-chase analysis indicated degradation of the gp116, consistent with its endosomal localization and with the absence of viral glycoproteins on the cell surface of the infected cells.

Characterization of Varicella-Zoster virus glycoprotein K (open reading frame 5) and its role in virus growth

Varicella-zoster virus (VZV) is an alphaherpesvirus that is the causative agent of chickenpox and herpes zoster. VZV open reading frame 5 (ORF5) encodes glycoprotein K (gK), which is conserved among alphaherpesviruses. While VZV gK has not been characterized, and its role in viral replication is unknown, homologs of VZV gK in herpes simplex virus type 1 (HSV-1) and pseudorabies virus (PRV) have been well studied. To identify the VZV ORF5 gene product, we raised a polyclonal antibody against a fusion protein of ORF5 codons 25 to 122 with glutathione S-transferase and used it to study the protein in infected cells. A 40,000-molecular-weight protein was detected in cell-free virus by Western blotting. In immunogold electron microscopic studies, VZV gK was in enveloped virions and was evenly distributed in the cytoplasm in infected cells. To determine the function of VZV gK in virus growth, a series of gK deletion mutants were constructed with VZV cosmid DNA derived from the Oka strain. Full and partial deletions in gK prevented viral replication when the gK mutant cosmids were transfected into melanoma cells. Insertion of the HSV-1 (KOS) gK gene into the endogenous VZV gK site did not compensate for the deletion of VZV gK. The replacement of VZV gK at a nonnative AvrII site in the VZV genome restored the phenotypic characteristics of intact recombinant Oka (rOka) virus. Moreover, gK complementing cells transfected with a full gK deletion mutant exhibited viral plaques indistinguishable from those of rOka. Our results are consistent with the studies of gK proteins of HSV-1 and PRV showing that gK is indispensable for viral replication.

Intrastrain variants of herpes simplex virus type 1 isolated from a neonate with fatal disseminated infection differ in the ICP34.5 gene, glycoprotein processing, and neuroinvasiveness

Two intrastrain variants of herpes simplex virus type 1 (HSV-1) were isolated from a newborn with fatal disseminated infection. A small-plaque-producing variant (SP7) was the predominant virus (>99%) in the brain, and a large-plaque-producing variant (LP5) was the predominant virus (>99%) in the lung and gastrointestinal tract. EcoRI and BamHI restriction fragment patterns indicated that SP7 and LP5 are related strains. The large-plaque variants produced plaques similar in size to those produced by HSV-1 KOS. Unlike LP5 or KOS, SP7 was highly cell associated and processing of glycoprotein C and glycoprotein D was limited to precursor forms in infected Vero cells. The large-plaque phenotype from KOS could be transferred into SP7 by cotransfection of plasmids containing the EK or JK EcoRI fragment or a 3-kb plasmid with the UL34.5 gene of HSV-1 KOS together with SP7 DNA. PCR analysis using primers from within the ICP34.5 gene indicated differences for SP7, LP5, and KOS. Sequencing data indicated two sets of deletions in the UL34.5 gene that distinguish SP7 from LP5. Both SP7 and LP5 variants were neurovirulent (lethal following intracranial inoculation of young BALB/c mice); however, the LP5 variant was much less able to cause lethal neuroinvasive disease (footpad inoculation) whereas KOS caused no disease. Passage of SP7 selected for viruses (SLP-5 and SLP-10) which were attenuated for lethal neuroinvasive disease, were not cell-associated, and differed in the UL34.5 gene. UL34.5 from SLP-5 or SLP-10 resembled that of KOS. These findings support a role for UL34.5 in promoting virus egress and for neuroinvasive disease.

Electrorotation studies of baby hamster kidney fibroblasts infected with herpes simplex virus type 1

The dielectric properties of baby hamster kidney fibroblast (BHK(C-13)) cells have been measured using electrorotation before and after infection with herpes simplex virus type 1 (HSV-1). The dielectric properties and morphology of the cells were investigated as a function of time after infection. The mean specific capacitance of the uninfected cells was 2.0 microF/cm2, reducing to a value of 1. 5 microF/cm2 at 12 h after infection. This change was interpreted as arising from changes in the cell membrane morphology coupled with alterations in the composition of the cell membrane as infection progressed. The measured changes in the cell capacitance were correlated with alterations in cellular morphology determined from scanning electron microscope (SEM) images. Between 9 and 12 h after infection the internal permittivity of the cell exhibited a rapid change, reducing in value from 75epsilono to 58epsilono, which can be correlated with the generation of large numbers of Golgi-derived membrane vesicles and enveloped viral capsids. The data are discussed in relation to the known life cycle of HSV-1 and indicate that electrorotation can be used to observe dynamic changes in both the dielectric and morphological properties of virus-infected cells. Calculations of the dielectrophoretic spectrum of uninfected and infected cells have been performed, and the results show that cells in the two states could be separated using appropriate frequencies and electrode arrays.

Immunogenicity of herpes simplex virus type 1 mutants containing deletions in one or more alpha-genes: ICP4, ICP27, ICP22, and ICP0

Replication defective mutants of HSV have been proposed both as vaccine candidates and as vehicles for gene therapy because of their inability to produce infectious progeny. The immunogenicity of these HSV replication mutants, at both qualitative and quantitative levels, will directly determine their effectiveness for either of these applications. We have previously reported (Brehm et al., J. Virol., 71, 3534, 1997) that a replication defective mutant of HSV-1, which expresses a substantial level of viral genes without producing virus particles, is as efficient as wild-type HSV-1 in eliciting an HSV-specific cytotoxic T-lymphocyte (CTL) response. In this report, we have further evaluated the immunogenic potential of HSV-1-derived replication defective mutants by examining the generation of HSV-specific CTL following immunization with viruses that are severely restricted in viral gene expression due to mutations in one or more HSV alpha genes (ICP4, ICP27, ICP22, and ICP0). To measure the CTL responses induced by the HSV alpha-mutants, we have targeted two H-2Kb-restricted CTL epitopes: an epitope in a virion protein, gB (498-505), and an epitope in a nonvirion protein, ribonucleotide reductase (RR1 822-829). The HSV mutants used in this study are impaired in their ability to express gB while a majority of them still express RR1. Our findings demonstrate that a single immunization with these mutants is able to generate a strong CTL response not only to RR1 822-829, but also to gB498-505 despite their inability to express wild-type levels of gB. Furthermore, a single immunization with any individual mutant can also provide immune protection against HSV challenge. These results suggest that mutants which are restricted in gene expression may be used as effective immunogens in vivo.

Intracellular traffic of herpes simplex virus glycoprotein gE: characterization of the sorting signals required for its trans-Golgi network localization

Herpes simplex virus (HSV) and varicella-zoster virus (VZV) are two pathogenic human alphaherpesviruses whose intracellular assembly is thought to follow different pathways. VZV presumably acquires its envelope in the trans-Golgi network (TGN), and it has recently been shown that its major envelope glycoprotein, VZV-gE, accumulates in this compartment when expressed alone. In contrast, the envelopment of HSV has been proposed to occur at the inner nuclear membrane, although to which compartment the gE homolog (HSV-gE) is transported is unknown. For this reason, we have studied the intracellular traffic of HSV-gE and have found that this glycoprotein accumulates at steady state in the TGN, both when expressed from cloned cDNA and in HSV-infected cells. In addition, HSV-gE cycles between the TGN and the cell surface and requires a conserved tyrosine-containing motif within its cytoplasmic tail for proper trafficking. These results show that VZV-gE and HSV-gE have similar intracellular trafficking pathways, probably reflecting the presence of similar sorting signals in the cytoplasmic domains of both molecules, and suggest that the respective viruses, VZV and HSV, could use the same subcellular organelle, the TGN, for their envelopment.

Human herpesvirus-8 glycoprotein B interacts with Epstein-Barr virus (EBV) glycoprotein 110 but fails to complement the infectivity of EBV mutants

To characterize human herpesvirus 8 (HHV-8) gB, the open reading frame was PCR amplified from the HHV-8-infected cell line BCBL-1 and cloned into an expression vector. To facilitate detection of expressed HHV-8 gB, the cytoplasmic tail of the glycoprotein was tagged with the influenza hemagglutinin (HA) epitope. Expression of tagged HHV-8 gB (gB-HA), as well as the untagged form, was readily detected in CHO-K1 cells and several lymphoblastoid cell lines (LCLs). HHV-8 gB-HA was sensitive to endoglycosidase H treatment, and immunofluorescence revealed that HHV-8 gB-HA was detectable in the perinuclear region of CHO-K1 cells. These observations suggest that HHV-8 gB is not processed in the Golgi and localizes to the endoplasmic reticulum or nuclear membrane. Because both HHV-8 and EBV are gamma-herpesviruses, the ability of HHV-8 gB to interact with and functionally complement EBV gp110 was examined. HHV-8 gB-HA and EBV gp110 co-immunoprecipitated, indicating formation of hetero-oligomers. However, HHV-8 gB-HA and HHV-8 gB failed to restore the infectivity of gp110-negative EBV mutants. These findings indicate that although HHV-8 gB and EBV gp110 have similar patterns of intracellular localization and can interact, there is not sufficient functional homology to allow efficient complementation.

Herpes simplex virus as a transneuronal tracer

Determining the connections of neural systems is critical for determining how they function. In this review, we focus on the use of HSV-1 and HSV-2 as transneuronal tracers. Using HSV to examine neural circuits is technically simple. HSV is injected into the area of interest, and after several days, the animals are perfused and processed for immunohistochemistry with antibodies to HSV proteins. Variables which influence HSV infection include species of host, age of host, titre of virus, strain of virus and phenotype of infected cell. The choice of strain of HSV is critically important. Several strains of HSV-1 and HSV-2 have been utilized for purposes of transneuronal tract-tracing. HSV has been used successfully to study neuronal circuitry in a variety of different neuroanatomical systems including the somatosensory, olfactory, visual, motor, autonomic and limbic systems.

Nuclear import and export of viruses and virus genomes

Many viruses replicate in the nucleus of their animal and plant host cells. Nuclear import, export, and nucleo-cytoplasmic shuttling play a central role in their replication cycle. Although the trafficking of individual virus proteins into and out of the nucleus has been well studied for some virus systems, the nuclear transport of larger entities such as viral genomes and capsids has only recently become a subject of molecular analysis. In this review, the general concepts emerging are discussed and a survey is provided of current information on both plant and animal viruses. Summarizing the main findings in this emerging field, it is evident that most viruses that enter or exit the nucleus take advantage of the cell's nuclear import and export machinery. With a few exceptions, viruses seem to cross the nuclear envelope through the nuclear pore complexes, making use of cellular nuclear import and export signals, receptors, and transport factors. In many cases, they capitalize on subtle control systems such as phosphorylation that regulate traffic of cellular components into and out of the nucleus. The large size of viral capsids and their composition (they contain large RNA and DNA molecules for which there are few precedents in normal nuclear transport) make the processes unique and complicated. Prior capsid disassembly (or deformation) is required before entry of viral genomes and accessory proteins can occur through nuclear pores. Capsids of different virus families display diverse uncoating programs which culminate in genome transfer through the nuclear pores.

The Us9 gene product of pseudorabies virus, an alphaherpesvirus, is a phosphorylated, tail-anchored type II membrane protein

The Us9 gene is highly conserved among the alphaherpesviruses sequenced to date, yet its function remains unknown. In this report, we demonstrate that the pseudorabies virus (PRV) Us9 protein is present in infected cell lysates as several phosphorylated polypeptides ranging from 17 to 20 kDa. Synthesis is first detected at 6 h postinfection and is sensitive to the DNA synthesis inhibitor phosphonoacetic acid. Unlike the herpes simplex virus type 1 Us9 homolog, which was reported to be associated with nucleocapsids in the nuclei of infected cells (M. C. Frame, D. J. McGeoch, F. J. Rixon, A. C. Orr, and H. S. Marsden, Virology 150:321-332, 1986), PRV Us9 localizes to the secretory pathway (predominately to the Golgi apparatus) and not to the nucleus. By fusing the enhanced green fluorescent protein (EGFP) reporter molecule to the carboxy terminus of Us9, we demonstrated that Us9 not only is capable of targeting a Us9-EGFP fusion protein to the Golgi compartment but also is able to direct efficient incorporation of such chimeric molecules into infectious viral particles. Moreover, through protease digestion experiments with Us9-EGFP-containing viral particles, we demonstrated that the Us9 protein is inserted into the viral envelope as a type II, tail-anchored membrane protein.

Herpes simplex virus type 1 glycoprotein B requires a cysteine residue at position 633 for folding, processing, and incorporation into mature infectious virus particles

Herpes simplex virus type 1 (HSV-1) glycoprotein B (gB) resides in the virus envelope in an oligomeric form and plays an essential role in virus entry into susceptible host cells. The oligomerizing domain is a movable element consisting of amino acids 626 to 653 in the gB external domain. This domain contains a single cysteine residue at position 633 (Cys-633) that is predicted to form an intramolecular disulfide bridge with Cys-596. In this study, we examined gB oligomerization, processing, and incorporation into mature virus during infection by two mutant viruses in which either the gB Cys-633 [KgB(C633S)] or both Cys-633 and Cys-596 [KgB(C596S/C633S)] residues were mutated to serine. The result of immunofluorescence studies and analyses of released virus particles showed that the mutant gB molecules were not transported to the cell surface or incorporated into mature virus envelopes and thus infectious virus was not produced. Immunoprecipitation studies revealed that the mutant gB molecules were in an oligomeric configuration and that these mutants produced hetero-oligomers with a truncated form of gB consisting of residues 1 to 43 and 595 to 904, the latter containing the oligomerization domain. Pulse-chase experiments in combination with endoglycosidase H treatment determined that the mutant molecules were improperly processed, having been retained in the endoplasmic reticulum (ER). Coimmunoprecipitation experiments revealed that the cysteine mutations resulted in gB misfolding and retention by the molecular chaperones calnexin, calreticulin, and Grp78 in the ER. The altered conformation of the gB mutant glycoproteins was directly detected by a reduction in monoclonal antibody recognition of two previously defined distinct antigenic sites located within residues 381 to 441 and 595 to 737. The misfolded molecules were not transported to the cell surface as hetero-oligomers with wild-type gB, suggesting that the conformational change could not be corrected by intermolecular interactions with the wild-type molecule. Together, these experiments confirmed that a disulfide bridge involving Cys-633 and Cys-596 is not essential for oligomerization but rather is required for proper folding and maintenance of a gB domain essential to complete posttranslational modification, transport, and incorporation into mature virus particles.

Membranes of herpes simplex virus type-1-infected human corneal epithelial cells are not permeabilized to macromolecules and therefore do not release IL-1alpha

Nanogram amounts of the proinflammatory cytokine interleukin-1alpha (IL-1alpha) were detected in uninfected cultures of human corneal epithelial cells (HCEC). Although HSV-1 replicated >10(4)-fold in these cells and caused extensive cytopathic effects, virus infection was not accompanied by significant extracellular release of IL-1alpha. Additional studies showed that release of radiolabeled cytosolic proteins from virus-infected HCEC was no greater than that released by mock-infected cells. These findings indicate that HSV-1 infection of HCEC does not result in IL-1alpha release because newly formed virus progeny can escape infected cells without disrupting cell membranes.

Herpes simplex virus gD and virions accumulate in endosomes by mannose 6-phosphate-dependent and -independent mechanisms

Herpes simplex virus (HSV) glycoprotein D (gD) is modified with mannose 6-phosphate (M6P) and binds to M6P receptors (MPRs). MPRs are involved in the well-characterized pathway by which lysosomal enzymes are directed to lysosomes via a network of endosomal membranes. Based on the impaired ability of HSV to form plaques under conditions in which glycoproteins could not interact with MPRs, we proposed that MPRs may function during HSV egress or cell-to-cell spread (C. R. Brunetti, R. L. Burke, B. Hoflack, T. Ludwig, K. S. Dingwell, and D. C. Johnson, J. Virol. 69:3517-3528, 1995). To further analyze M6P modification and intracellular trafficking of gD in the absence of other HSV proteins, adenovirus (Ad) vectors were used to express soluble and membrane-anchored forms of gD. Both membrane-bound and soluble gD were modified with M6P residues and were localized to endosomes that contained the 275-kDa MPR or the transferrin receptor. Similar results were observed in HSV-infected cells. Cell fractionation experiments showed that gD was not present in lysosomes. However, a mutant form of gD and another HSV glycoprotein, gI, that were not modified with M6P were also found in endosomes in HSV-infected cells. Moreover, a substantial fraction of the HSV nucleocapsid protein VP6 was found in endosomes, consistent with accumulation of virions in an endosomal compartment. Therefore, it appears that HSV glycoproteins and virions are directed to endosomes, by M6P-dependent as well as by M6P-independent mechanisms, either as part of the virus egress pathway or by endocytosis from the cell surface.

Pseudorabies virus glycoprotein gK is a virion structural component involved in virus release but is not required for entry

The pseudorabies virus (PrV) gene homologous to herpes simplex virus type 1 (HSV-1) UL53, which encodes HSV-1 glycoprotein K (gK), has recently been sequenced (J. Baumeister, B. G. Klupp, and T. C. Mettenleiter, J. Virol. 69:5560-5567, 1995). To identify the corresponding protein, a rabbit antiserum was raised against a 40-kDa glutathione S-transferase-gK fusion protein expressed in Escherichia coli. In Western blot analysis, this serum detected a 32-kDa polypeptide in PrV-infected cell lysates as well as a 36-kDa protein in purified virion preparations, demonstrating that PrV gK is a structural component of virions. After treatment of purified virions with endoglycosidase H, a 34-kDa protein was detected, while after incubation with N-glycosidase F, a 32-kDa protein was specifically recognized. This finding indicates that virion gK is modified by N-linked glycans of complex as well as high-mannose type. For functional analysis, the UL53 open reading frame was interrupted after codon 164 by insertion of a gG-lacZ expression cassette into the wild-type PrV genome (PrV-gKbeta) or by insertion of the bovine herpesvirus 1 gB gene into a PrV gB- genome (PrV-gK(gB)). Infectious mutant virus progeny was obtained only on complementing gK-expressing cells, suggesting that gK has an important function in the replication cycle. After infection of Vero cells with either gK mutant, only single infected cells or small foci of infected cells were visible. In addition, virus yield was reduced approximately 30-fold, and penetration kinetics showed a delay in entry which could be compensated for by phenotypic gK complementation. Interestingly, the plating efficiency of PrV-gKbeta was similar to that of wild-type PrV on complementing and noncomplementing cells, pointing to an essential function of gK in virus egress but not entry. Ultrastructurally, virus assembly and morphogenesis of PrV gK mutants in noncomplementing cells were similar to wild-type virus. However, late in infection, numerous nucleocapsids were found directly underneath the plasma membrane in stages typical for the entry process, a phenomenon not observed after wild-type virus infection and also not visible after infection of gK-complementing cells. Thus, we postulate that presence of gK is important to inhibit immediate reinfection.

A simian virus 40 large T-antigen segment containing amino acids 1 to 127 and expressed under the control of the rat elastase-1 promoter produces pancreatic acinar carcinomas in transgenic mice

The simian virus 40 large T antigen induces tumors in a wide variety of tissues in transgenic mice, the precise tissues depending on the tissue specificity of the upstream region controlling T-antigen expression. Expression of mutant T antigens that contain a subset of the protein's activities restricts the spectrum of tumors induced. Others showed previously that expression of a mutant large T antigen containing the N-terminal 121 amino acids (T1-121) under control of the lymphotropic papovavirus promoter resulted in slow-growing choroid plexus tumors, whereas full-length T antigen under the same promoter induced rapidly growing CPR tumors, T-cell lymphomas, and B-cell lymphomas. In those instances, the alteration in tumor induction or progression correlated with inability of the mutant large T antigen to bind the tumor suppressor p53. In the study reported here, we investigated the capacity of an N-terminal T antigen segment (T1-127) expressed in conjunction with small t antigen under control of the rat elastase-1 (E1) promoter to induce pancreatic tumors. The results show that pancreases of transgenic mice expressing T1-127 and small t antigen display acinar cell dysplasia at birth that progresses to neoplasia. The average age to death in these mice is within the range reported for transgenic mice expressing full-length T antigen under control of the E1 promoter. These results indicate that sequestering p53 by binding is not required for the development of rapidly growing acinar cell carcinomas. In addition, we provide evidence that small t antigen is unlikely to be required. Finally, we show that the p53 protein in acinar cell carcinomas is wild type in conformation.

Antibody-induced and cytoskeleton-mediated redistribution and shedding of viral glycoproteins, expressed on pseudorabies virus-infected cells

Fluorescein isothiocyanate-labeled porcine pseudorabies virus (PrV) polyclonal antibodies were added to PrV-infected swine kidney cells in vitro at 37 degrees C. In approximately 47% of the infected cells, the addition induced passive patching and subsequent energy- and microtubule-dependent capping of all viral envelope glycoproteins, expressed on the plasma membranes of the infected cells. Further contraction and extrusion of the capped viral glycoproteins occurred in approximately 30% of the capped cells 2 h after the addition of antibodies and was accompanied by a concentration of F-actin beneath the caps. At that time, about 18% of the extruded caps were shed spontaneously into the surrounding medium. Mechanical force released 85% of the extruded caps, leaving viable cells with no microscopically detectable levels of viral glycoproteins on their plasma membranes. Experiments with PrV deletion mutants showed that viral glycoproteins gE and gI are important in triggering viral glycoprotein redistribution. Since the PrV gE-gI complex exhibits Fc receptor activity which facilitates capping, the importance of gE and gI may be partially explained by antibody bipolar bridging.

Intracellular processing of pseudorabies virus glycoprotein M (gM): gM of strain Bartha lacks N-glycosylation

Genes encoding homologs of the herpes simplex virus type 1 UL10 product, glycoprotein M, are conserved in all herpesviruses investigated so far. Recently, we identified pseudorabies virus (PrV) gM as a 45-kDa structural component of purified virions. A gM-PrV mutant could be propagated in cell culture, albeit at lower titers and with delayed penetration kinetics. Thus, gM has a nonessential but modulatory function in PrV infection. PrV gM is modified by addition of an N-linked glycan at a consensus sequence located between the predicted first and second hydrophobic region of the protein. This N-glycosylation site is conserved in all gM homologs sequenced so far, indicating an important functional role. To analyze intracellular processing of PrV gM, Western blot analyses were performed. In PrV-infected cells, mature 45-kDa gM as well as 33- and 35-kDa precursor forms were detectable. Presumably dimeric 90- and 70-kDa proteins were also observed. The 33- and 35-kDa proteins represent nonglycosylated and glycosylated precursors as shown by endoglycosidase digestions. Investigation of several PrV strains revealed that the UL10 product of PrV strain Bartha, an attenuated virus used as vaccine, was not modified by N-glycosylation. Sequence analysis showed that the N-glycosylation consensus sequence was altered from NDT to NDA, which resulted in loss of the N-glycosylation signal. To our knowledge, this is the only gM homolog identified so far which is not N-glycosylated. To investigate whether this form of the protein is functionally competent, the UL10 gene of strain Bartha was inserted into PrV strain Kaplan by substitution of the wild-type UL10 gene. The resulting recombinant expressed a UL10 protein lacking N-glycans. In vitro replication analyses did not reveal any difference in virus production, but plaque size and penetration kinetics were slightly reduced. In summary, we show that wild-type gM is modified by N-glycosylation at one conserved site. However, although this site is highly conserved throughout the herpesviruses, loss of N-glycans due to mutation of the consensus sequence had only a minor effect on propagation of PrV in cell culture.

Progress in development of herpes simplex virus gene vectors for treatment of rheumatoid arthritis

Arthritis is presently incurable and poorly treatable, but there are good grounds for expecting gene therapy to improve matters considerably. Although local ex vivo delivery of anti-arthritic genes to the synovial lining of joints has shown considerable promise, intraarticular gene delivery may be desirable. Herpes simplex virus (HSV) may be a viable vector for in vivo transfer of anti-arthritic genes to joints. HSV has the advantages of high infectivity, large carrying capacity and high titer. The large packaging capacity would permit the inclusion of multiple anti-arthritic genes and necessary regulatory elements. Recombinant vectors produced by this laboratory infect synovial cells efficiently, permitting prolonged expression of transgenes in vitro and in vivo without evidence of cytotoxicity. Further improvements to this vector system include taking advantage of an endogenous HSV 'stealthing' gene, ICP47, which interferes with formation of antigen-class I complexes. Inclusion of inducible promoters to appropriately regulate expression of anti-arthritic genes should further improve this system.

Herpes simplex virus type 1 glycoprotein K is not essential for infectious virus production in actively replicating cells but is required for efficient envelopment and translocation of infectious virions from the cytoplasm to the extracellular space

We characterized the glycoprotein K (gK)-null herpes simplex virus type 1 [HSV-1] (KOS) delta gK and compared it to the gK-null virus HSV-1 F-gKbeta (L. Hutchinson et al., J. Virol. 69:5401-5413, 1995). delta gK and F-gKbeta mutant viruses produced small plaques on Vero cell monolayers at 48 h postinfection. F-gKbeta caused extensive fusion of 143TK cells that was sensitive to melittin, a specific inhibitor of gK-induced cell fusion, while delta gK virus did not fuse 143TK cells. A recombinant plasmid containing the truncated gK gene specified by F-gKbeta failed to rescue the ICP27-null virus KOS (d27-1), while a plasmid with the delta gK deletion rescued the d27-1 virus efficiently. delta gK virus yield was approximately 100,000-fold lower in stationary cells than in actively replicating Vero cells. The plaquing efficiencies of delta gK and F-gKbeta virus stocks on VK302 cells were similar, while the plaquing efficiency of F-gKbeta virus stocks on Vero cells was reduced nearly 10,000-fold in comparison to that of delta gK virus. Mutant delta gK and F-gKbeta infectious virions accumulated within Vero and HEp-2 cells but failed to translocate to extracellular spaces. delta gK capsids accumulated in the nuclei of Vero but not HEp-2 cells. Enveloped delta gK virions were visualized in the cytoplasms of both Vero and HEp-2 cells, and viral capsids were found in the cytoplasm of HEp-2 cells within vesicles. Glycoproteins B, C, D, and H were expressed on the surface of delta gK-infected Vero cells in amounts similar to those for KOS-infected Vero cells. These results indicate that gK is involved in nucleocapsid envelopment, and more importantly in the translocation of infectious virions from the cytoplasm to the extracellular spaces, and that actively replicating cells can partially compensate for the envelopment but not for the cellular egress deficiency of the delta gK virus. Comparison of delta gK and F-gKbeta viruses suggests that the inefficient viral replication and plaquing efficiency of F-gKbeta virus in Vero cells and its syncytial phenotype in 143TK- cells are most likely due to expression of a truncated gK.

Immunization with a replication-deficient mutant of herpes simplex virus type 1 (HSV-1) induces a CD8+ cytotoxic T-lymphocyte response and confers a level of protection comparable to that of wild-type HSV-1

Replication-deficient viruses provide an attractive alternative to conventional approaches used in the induction of antiviral immunity. We have quantitatively evaluated both the primary and memory cytotoxic T-lymphocyte (CTL) responses elicited by immunization with a replication-deficient mutant of herpes simplex virus type 1 (HSV-1). In addition, we have examined the potential role of these CTL in protection against HSV infection. Using bulk culture analysis and limiting-dilution analysis, we have shown that a replication-deficient virus, d301, generates a strong primary CTL response that is comparable to the response induced by the wild type-strain, KOS1.1. Furthermore, the CTL induced by d301 immunization recognized the immunodominant, H-2Kb-restricted, CTL recognition epitope gB498-505 to a level similar to that for CTL from KOS1.1-immunized mice. The memory CTL response evoked by d301 was strong and persistent, even though the frequencies of CTL were slightly lower than the frequencies of CTL induced by KOS1.1. Adoptive transfer studies indicated that both the CD8+ and the CD4+ T-cell responses generated by immunization with d301 and KOS1.1 were able to limit the extent of a cutaneous HSV infection to comparable levels. Overall, these results indicate that viral replication is not necessary to elicit a potent and durable HSV-specific immune response and suggest that replication-deficient viruses may be effective in eliciting protection against viral pathogens.

Adding an Rb-binding site to an N-terminally truncated simian virus 40 T antigen restores growth to high cell density, and the T common region in trans provides anchorage-independent growth and rapid growth in low serum concentrations

The simian virus 40 large T antigen is sufficient to confer on cells multiple transformed cell growth characteristics, including growth to a high cell density, rapid growth in medium containing low serum concentrations, and anchorage-independent growth. We showed previously that distinct regions of the protein were involved in conferring these properties and that removal of the first 127 amino acids of T antigen abrogated all three activities. At least three large-T-antigen transformation-related activities have been localized to that region: binding of the tumor suppressor gene product Rb and two independent activities contained within the common region shared by large T and small t antigens. The experiments described here were directed toward determining whether these were the only activities from the N terminus that were needed. To do so we reintroduced an Rb-binding region into the N-terminally truncated T antigen (T128-708) and examined the growth properties of cells immortalized by it in the presence and absence of small t antigen, which can provide the T-common-region transformation-related activities in trans. We show that an Rb-binding region consisting of amino acids 101 to 118, when introduced into a heterologous site in T128-708, is capable of physically binding Rb and that binding is sufficient for cells expressing the protein to acquire the ability to grow to a high saturation density. However, in low-serum medium, the growth rate of the cells and maximal cell density are reduced relative to those of wild-type-T-antigen-expressing cells, and the cells cannot divide without anchorage. This result suggests that although Rb binding is sufficient in the context of T128-708 to confer growth to a high density, one or more other N-terminally located T-antigen activities are needed for cells to acquire the additional growth properties. Small t antigen in trans supplied those activities. These results indicate that the T-common-region activities and Rb binding are the only activities from the T-antigen N terminus needed to restore full transforming activity to the N-terminally truncated T antigen.

Ultrastructural analysis of the replication cycle of pseudorabies virus in cell culture: a reassessment

We reinvestigated major steps in the replicative cycle of pseudorabies virus (PrV) by electron microscopy of infected cultured cells. Virions attached to the cell surface were found in two distinct stages, with a distance of 12 to 14 nm or 6 to 8 nm between virion envelope and cell surface, respectively. After fusion of virion envelope and cell membrane, immunogold labeling using a monoclonal antibody against the envelope glycoprotein gE demonstrated a rapid drift of gE from the fusion site, indicating significant lateral movement of viral glycoproteins during or immediately after the fusion event. Naked nucleocapsids in the cytoplasm frequently appeared close to microtubules prior to transport to nuclear pores. At the nuclear pore, nucleocapsids invariably were oriented with one vertex pointing to the central granulum at a distance of about 40 nm and viral DNA appeared to be released via the vertex region into the nucleoplasm. Intranuclear maturation followed the typical herpesvirus nucleocapsid morphogenesis pathway. Regarding egress, our observations indicate that primary envelopment of nucleocapsids occurred at the inner leaflet of the nuclear membrane by budding into the perinuclear cisterna. This nuclear membrane-derived envelope exhibited a smooth surface which contrasts the envelope obtained by putative reenvelopment at tubular vesicles in the Golgi area which is characterized by distinct surface projections. Loss of the primary envelope and release of the nucleocapsid into the cytoplasm appeared to occur by fusion of envelope and outer leaflet of the nuclear membrane. Nucleocapsids were also found engulfed by both lamella of the nuclear membrane. This vesiculation process released nucleocapsids surrounded by two membranes into the cytoplasm. Our data also indicate that fusion between the two membranes then leads to release of naked nucleocapsids in the Golgi area. Egress of virions appeared to occur via transport vesicles containing one or more virus particles by fusion of vesicle and cell membrane. Our data thus support biochemical data and mutant virus studies of (i) two steps of attachment, (ii) the involvement of microtubules in the transport of nucleocapsids to the nuclear pore, and (iii) secondary envelopment in the trans-Golgi area in PrV infection.

Antiviral effects of 6-diazo-5-oxo-L-norleucin on replication of herpes simplex virus type 1

An L-glutamine antagonist, 6-diazo-5-oxo-L-norleucin (L-DON), inhibits replication of vesicular stomatitis virus, poliovirus and paramyxoviruses in cultured cells. We tested the antiviral activity of L-DON against different strains of herpes simplex virus type 1 (HSV-1) in Vero cells. In the presence of a physiological plasma concentration of L-glutamine (0.5mM) L-Don inhibited 50% production of virus plaques at concentrations ranging from 7.9 to 16 microM. At concentrations of 40 microM L-Don inhibited infectious virus yield by 99%. The antiviral activity of L-DON decreased with increasing L-glutamine concentrations. A concentration of 5000 microM of L-Don had no significant effects on the viability of Vero cells. Transmission electron microscopical investigations showed that L-DON prevented mainly envelopment of viral nucleocapsids in the cytoplasm. The immunoprecipitation experiments demonstrated selective inhibition of synthesis of HSV-1 glycoproteins in L-DON treated cells. The results showed that L-DON inhibits HSV-1 replication at a late stage in the virus replication cycle, probably the cytoplasmic maturation of virions and subsequent virion egress from the cells.

Involvement of the endoplasmic reticulum in the assembly and envelopment of African swine fever virus

African swine fever (ASF) virus is a large enveloped DNA virus assembled in the cytoplasm of cells. In this study, the membrane compartments involved in the envelopment of ASF virus were investigated. A monoclonal antibody recognizing p73, the major structural protein of ASF virus, was generated to analyze the binding of p73 to membranes during the assembly of the virus. Approximately 50% of the intracellular pool of p73 associated with membranes as a peripheral membrane protein. Binding was rapid and complete within 15 min of synthesis. Subcellular membrane fractionation showed that newly synthesized p73 molecules cosedimented with endoplasmic reticulum (ER) membranes and remained associated with the ER during a 2-h chase. A similar distribution on gradients was recorded for p17, a structural membrane protein of ASF virus. The results suggested that the ER was involved in the assembly of ASF virus. A protease protection assay demonstrated a time-dependent envelopment of the membrane bound, but not cytosolic, pool of p73. Envelopment of p73 took place 1 h after binding to membranes and was completed 1 h before the first detection of p73 in virions secreted from cells. Envelopment was unaffected by brefeldin A and monensin, drugs that block membrane transport between the ER and Golgi. Taken together the results provide evidence for the binding of ASF virus structural proteins to a specific membrane compartment and implicate a role for the ER in the assembly and envelopment of ASF virus.

Human interferon reduces surface expression but not total production of herpes simplex virus type 1 glycoproteins gC and gE in heterologous hamster cells

The effect of interferon (IFN) on herpes simplex virus type 1 (HSV-1)-induced glycoproteins gC and gE was investigated in a heterologous IFN/cell model. In this model, the effect on surface expression of the glycoproteins could be studied separately from the effect on virus multiplication. Pretreatment of baby hamster kidney cells (BHK) with heterologous human leukocyte IFN suppressed surface expression of HSV-1-encoded gC and gE but had no influence on total production of the glycoproteins. This was in contrast to the effect on human embryonic fibroblast cells (HE) (homologous IFN and cells), where surface expression as well as total production of glycoproteins were reduced. The surface expression was demonstrated by antibody-sensitized monodisperse polystyrene beads, and immunoblotting and two-dimensional electrophoretic analysis of radioisotope-labeled proteins were used to study the total production.