Intracellular transport of the glycoproteins gE and gI of the varicella-zoster virus. gE accelerates the maturation of gI and determines its accumulation in the trans-Golgi network

Comparative Antibody Responses to the Live-Attenuated and Recombinant Herpes Zoster Vaccines

Two herpes zoster (HZ) vaccines licensed in the United States are recommended by the Advisory Committee on Immunization Practices (ACIP): (i) live-attenuated vaccine (ZVL) using vOka strain varicella-zoster virus (VZV) and (ii) recombinant adjuvanted vaccine (RZV) containing recombinant varicella-zoster virus (VZV) glycoprotein E (gE). Two phase 3 clinical trials of RZV led the Advisory Committee on Immunization Practices (ACIP) to recommend it with preferred status. VZV T cell-mediated immunity (CMI), but not humoral immunity, is considered essential for protection against HZ. Published studies of humoral immunity focused on VZV-specific IgG concentration. To complement reports comparing the CMI responses to these vaccines, we compared humoral responses in ZVL and RZV recipients, emphasizing functional qualities (avidity and neutralization). Baseline avidities to a VZV glycoprotein mixture (gp) were near the upper limit of detection, but avidity to gE was much lower. Small increases in gp avidity were observed for both RZV and ZVL vaccination (19 and 12 avidity index units [AIU], respectively). RZV boosted both gE avidity and VZV neutralizing antibody significantly more than ZVL (mean gE avidity boost, 47 AIU versus 22 AIU; mean neutralizing antibody boost, 22-fold versus 8-fold). Increases in neutralizing antibodies strongly correlated with gE avidity increases (r = 0.5) and moderately with gp avidity increases (r = 0.23). After 1 year, 81% of RZV recipients and only 18% of ZVL recipients retained >50% of their peak avidity boosts. These results are consistent with the CMI responses to these vaccines: RZV responses are skewed to long-term memory, whereas ZVL preferentially induces transient effector responses.IMPORTANCE These observations further distinguish the immunogenicity and duration of the immune response of the two vaccines. In addition, measurements of functional humoral immunity (IgG avidity and neutralizing antibody) in response to zoster immunization, alone or combined with other immune markers, might contribute to practical in vitro correlates of protection. Combined with previous observations of the cell-mediated response to these vaccines, this study suggests that vaccine development will benefit from more expansive and granular assessments of acquired immunity during early phase 1 immunogenicity trials.

The herpes virus Fc receptor gE-gI mediates antibody bipolar bridging to clear viral antigens from the cell surface

The Herpes Simplex Virus 1 (HSV-1) glycoprotein gE-gI is a transmembrane Fc receptor found on the surface of infected cells and virions that binds human immunoglobulin G (hIgG). gE-gI can also participate in antibody bipolar bridging (ABB), a process by which the antigen-binding fragments (Fabs) of the IgG bind a viral antigen while the Fc binds to gE-gI. IgG Fc binds gE-gI at basic, but not acidic, pH, suggesting that IgG bound at extracellular pH by cell surface gE-gI would dissociate and be degraded in acidic endosomes/lysosomes if endocytosed. The fate of viral antigens associated with gE-gI-bound IgG had been unknown: they could remain at the cell surface or be endocytosed with IgG. Here, we developed an in vitro model system for ABB and investigated the trafficking of ABB complexes using 4-D confocal fluorescence imaging of ABB complexes with transferrin or epidermal growth factor, well-characterized intracellular trafficking markers. Our data showed that cells expressing gE-gI and the viral antigen HSV-1 gD endocytosed anti-gD IgG and gD in a gE-gI-dependent process, resulting in lysosomal localization. These results suggest that gE-gI can mediate clearance of infected cell surfaces of anti-viral host IgG and viral antigens to evade IgG-mediated responses, representing a general mechanism for viral Fc receptors in immune evasion and viral pathogenesis.

Virion incorporation of the herpes simplex virus type 1 tegument protein VP22 is facilitated by trans-Golgi network localization and is independent of interaction with glycoprotein E

HSV-1 virions contain a proteinaceous layer termed the tegument that lies between the nucleocapsid and viral envelope. The molecular mechanisms that facilitate incorporation of tegument proteins are poorly characterized. The tegument protein VP22 interacts with VP16 and the cytoplasmic tail of glycoprotein E (gE). Virion incorporation of VP22 occurs independently of interaction with VP16; however, the contribution of gE binding remains undefined. Site-directed mutagenesis was used to identify VP22 mutants which abrogate interaction with gE but retain VP16 binding. Virion incorporation assays demonstrated that failure to bind gE did not abrogate VP22 packaging. A region of VP22 which binds to both VP16 and gE failed to be packaged efficiently, with wild-type levels of incorporation only attained when residues 43-86 of VP22 were present. Mutational analysis of an acidic cluster of amino acids within this region indicates that this motif facilitates trans-Golgi network (TGN) localization and optimal virion incorporation of VP22.

Optimal replication of human cytomegalovirus correlates with endocytosis of glycoprotein gpUL132

Envelopment of a herpesvirus particle is a complex process of which much is still to be learned. We previously identified the glycoprotein gpUL132 of human cytomegalovirus (HCMV) as an envelope component of the virion. In its carboxy-terminal portion, gpUL132 contains at least four motifs for sorting of transmembrane proteins to endosomes; among them are one dileucine-based signal and three tyrosine-based signals of the YXXØ and NPXY (where X stands for any amino acid, and Ø stands for any bulky hydrophobic amino acid) types. To investigate the role of each of these trafficking signals in intracellular localization and viral replication, we constructed a panel of expression plasmids and recombinant viruses in which the signals were rendered nonfunctional by mutagenesis. In transfected cells wild-type gpUL132 was mainly associated with the trans-Golgi network. Consecutive mutation of the trafficking signals resulted in increasing fractions of the protein localized at the cell surface, with gpUL132 mutated in all four trafficking motifs predominantly associated with the plasma membrane. Concomitant with increased surface expression, endocytosis of mutant gpUL132 was reduced, with a gpUL132 expressing all four motifs in mutated form being almost completely impaired in endocytosis. The replication of recombinant viruses harboring mutations in single trafficking motifs was comparable to replication of wild-type virus. In contrast, viruses containing mutations in three or four of the trafficking signals showed pronounced deficits in replication with a reduction of approximately 100-fold. Moreover, recombinant viruses expressing gpUL132 with three or four trafficking motifs mutated failed to incorporate the mutant protein into the virus particle. These results demonstrate a role of endocytosis of an HCMV envelope glycoprotein for incorporation into the virion and optimal virus replication.

EBV-positive human sera contain antibodies against the EBV BMRF-2 protein

We previously showed that the EBV glycoprotein BMRF-2 contains a functional integrin-binding Arg-Gly-Asp (RGD) domain that plays an important role in viral infection and cell-to-cell spread of progeny virions in oral epithelial cells. In this study, we found that EBV-seropositive human sera contain antibodies against BMRF-2. The inhibitory effect of EBV-positive sera on EBV infection of oral epithelial cells was substantially reduced by pre-incubation of serum samples with the BMRF-2 RGD peptide, suggesting that anti-BMRF-2 human antibodies possess neutralizing activity. EBV-specific sera reacted strongly with the BMRF-2 extracellular domain (170-213 aa) containing the RGD motif, whereas they reacted only weakly or not at all with a mutated form of the BMRF-2 extracellular domain containing AAA instead of RGD. These data indicate that RGD motif of BMRF-2 is part of an immunodominant antigenic determinant within the extracellular domain of BMRF-2 that may contribute to EBV neutralization during EBV reactivation.

EBV BMRF-2 facilitates cell-to-cell spread of virus within polarized oral epithelial cells

We previously reported that the Epstein-Barr virus (EBV) BMRF-2 protein plays an important role in EBV infection of polarized oral epithelial cells by interacting with beta1 and alphav family integrins. Here we show that infection of polarized oral epithelial cells with B27-BMRF-2(low) recombinant virus, expressing a low level of BMRF-2, resulted in significantly smaller plaques compared with infection by parental B95-8 virus. BMRF-2 localized in the trans-Golgi network (TGN) and basolateral sorting vesicles and was transported to the basolateral membranes of polarized epithelial cells. Mutation of the tyrosine- and dileucine-containing basolateral sorting signal, YLLV, in the cytoplasmic domain of BMRF-2 led to the failure of its accumulation in the TGN and its basolateral transport. These data show that BMRF-2 may play an important role in promoting the spread of EBV progeny virions through lateral membranes of oral epithelial cells.

Contribution of endocytic motifs in the cytoplasmic tail of herpes simplex virus type 1 glycoprotein B to virus replication and cell-cell fusion

The use of endocytic pathways by viral glycoproteins is thought to play various functions during viral infection. We previously showed in transfection assays that herpes simplex virus type 1 (HSV-1) glycoprotein B (gB) is transported from the cell surface back to the trans-Golgi network (TGN) and that two motifs of gB cytoplasmic tail, YTQV and LL, function distinctly in this process. To investigate the role of each of these gB trafficking signals in HSV-1 infection, we constructed recombinant viruses in which each motif was rendered nonfunctional by alanine mutagenesis. In infected cells, wild-type gB was internalized from the cell surface and concentrated in the TGN. Disruption of YTQV abolished internalization of gB during infection, whereas disruption of LL induced accumulation of internalized gB in early recycling endosomes and impaired its return to the TGN. The growth of both recombinants was moderately diminished. Moreover, the fusion phenotype of cells infected with the gB recombinants differed from that of cells infected with the wild-type virus. Cells infected with the YTQV-mutated virus displayed reduced cell-cell fusion, whereas giant syncytia were observed in cells infected with the LL-mutated virus. Furthermore, blocking gB internalization or impairing gB recycling to the cell surface, using drugs or a transdominant negative form of Rab11, significantly reduced cell-cell fusion. These results favor a role for endocytosis in virus replication and suggest that gB intracellular trafficking is involved in the regulation of cell-cell fusion.

Intracellular localization of varicella-zoster virus ORF39 protein and its functional relationship to glycoprotein K

Varicella-zoster virus (VZV) encodes two multiply inserted membrane proteins, open reading frame (ORF) 39 protein (ORF39p) and glycoprotein K (gK). The HSV-1 homologs of these proteins are believed to act in conjunction with each other during viral egress and cell-cell fusion, and they directly influence each other's intracellular trafficking. However, ORF39p and VZV gK have received very limited study largely due to difficulties in producing antibodies to these highly hydrophobic proteins. To overcome this obstacle, we introduced epitope tags into both ORF39p and gK and examined their intracellular distributions in transfected and infected cells. Our data demonstrate that both ORF39p and gK accumulate predominately in the ER of cultured cells when expressed in the absence of other VZV proteins or when coexpressed in isolation from other VZV proteins. Therefore, the transport of VZV ORF39p and gK does not exhibit the functional interdependence seen in their HSV-1 homologs. However, during infection, the primary distributions of ORF39p and gK shift from the ER to the Golgi, and they are also found in the plasma membrane indicating that their intracellular trafficking during infection depends on other VZV-encoded proteins. During infection, ORF39p and gK tightly colocalize with VZV envelope glycoproteins B, E and H; however, the coexpression of ORF39p or gK with other individual viral glycoproteins is insufficient to alter the transport of either ORF39p or gK.

Molecular studies of Varicella zoster virus

VZV is a highly cell-associated member of the Herpesviridae family and one of the eight herpesviruses to infect humans. The virus is ubiquitous in most populations worldwide, primary infection with which causes varicella, more commonly known as chickenpox. Characteristic of members of the alphaherpesvirus sub-family, VZV is neurotropic and establishes latency in sensory neurones. Reactivation from latency, usually during periods of impaired cellular immunity, causes herpes zoster (shingles). Despite being one of the most genetically stable human herpesviruses, nucleotide alterations in the virus genome have been used to classify VZV strains from different geographical regions into distinct clades. Such studies have also provided evidence that, despite pre-existing immunity to VZV, subclinical reinfection and reactivation of reinfecting strains to cause zoster is also occurring. During both primary infection and reactivation, VZV infects several PBMC and skin cell lineages. Difficulties in studying the pathogenesis of VZV because of its high cell association and narrow host range have been overcome through the development of the VZV severe combined immunodeficient mouse model carrying human tissue implants. This model has provided a valuable tool for studying the importance of individual viral proteins during both the complex intracellular replication and assembly of new virions and for understanding the underlying mechanism of attenuation of the live varicella vaccine. In addition, a rat model has been developed and successfully used to uncover which viral proteins are important for both the establishment and maintenance of latent VZV infection.

Reconstitution of herpes simplex virus microtubule-dependent trafficking in vitro

Microtubule-mediated anterograde transport of herpes simplex virus (HSV) from the neuronal cell body to the axon terminal is crucial for the spread and transmission of the virus. It is therefore of central importance to identify the cellular and viral factors responsible for this trafficking event. In previous studies, we isolated HSV-containing cytoplasmic organelles from infected cells and showed that they represent the first and only destination for HSV capsids after they emerge from the nucleus. In the present study, we tested whether these cytoplasmic compartments were capable of microtubule-dependent traffic. Organelles containing green fluorescent protein-labeled HSV capsids were isolated and found to be able to bind rhodamine-labeled microtubules polymerized in vitro. Following the addition of ATP, the HSV-associated organelles trafficked along the microtubules, as visualized by time lapse microscopy in an imaging microchamber. The velocity and processivity of trafficking resembled those seen for neurotropic herpesvirus traffic in living axons. The use of motor-specific inhibitors indicated that traffic was predominantly kinesin mediated, consistent with the reconstitution of anterograde traffic. Immunocytochemical studies revealed that the majority of HSV-containing organelles attached to the microtubules contained the trans-Golgi network marker TGN46. This simple, minimal reconstitution of microtubule-mediated anterograde traffic should facilitate and complement molecular analysis of HSV egress in vivo.

Comparison of virus transcription during lytic infection of the Oka parental and vaccine strains of Varicella-Zoster virus

The attenuated Oka vaccine (V-Oka) strain of varicella-zoster virus (VZV) effectively reduces disease produced by primary infection and virus reactivation. V-Oka was developed by propagation of the Oka parental (P-Oka) strain of VZV in guinea pig and human embryo fibroblasts. Complete DNA sequencing of both viruses has revealed 63 sites that differ between P-Oka and V-Oka, 37 of which are located within 21 unique open reading frames (ORFs). Of the ORFs that differ, ORF 62 contains the greatest number (10) of mutated sites. ORF 62 encodes IE 62, the major immediate-early transactivator of virus genes, and is essential for lytic virus replication. To determine whether a disproportionate number of mutations in ORF 62 might account for virus attenuation, we compared the global pattern of V-Oka gene expression to that of P-Oka. Transcription of ORFs 62, 65, 66, and 67 was suppressed, whereas ORF 41 was elevated in V-Oka-infected cells compared to P-Oka-infected cells (P < 0.01; z test). Suppression of ORF 62, 65, and 66 transcription was confirmed by quantitative dot blot and Western blot analyses. Transient-transfection assays to determine whether mutations within V-Oka-derived IE 62 affected its ability to transactivate VZV gene promoters revealed similar IE 62 transactivation of VZV gene 20, 21, 28, 29, 65, and 66 promoters in both P-Oka and V-Oka. Together, our results indicate that mutations in V-Oka IE 62 alone are unlikely to account for vaccine virus attenuation.

The why's of Y-based motifs in alphaherpesvirus envelope proteins

The Alphaherpesvirinae are large DNA viruses and represent the largest subfamily of the Herpesviridae with closely related members of man and animal, including herpes simplex virus, varicella-zoster virus, pseudorabies virus, bovine herpesvirus 1, and many others. The viral envelope proteins of alphaherpesviruses are remarkably diverse and are incorporated in the ER, Golgi, and plasma membrane of infected cells. The cytoplasmic domain of many of these envelope proteins contain specific tyrosine-based amino acids. During recent years, accumulating evidence indicates that these tyrosine-based motifs serve different important functions during the virus life cycle, and are implicated in endocytosis processes, intracellular trafficking, basolateral and axonal sorting, and signal transduction events. The current minireview will discuss the functions associated with these tyrosine-based motifs in alphaherpesvirus envelope proteins.

Trafficking of viral membrane proteins

Many viruses express membrane proteins. For enveloped viruses in particular, membrane proteins are frequently structural components of the virus that mediate the essential tasks of receptor recognition and membrane fusion. The functional activities of these proteins require that they are sorted correctly in infected cells. These sorting events often depend on the ability of the virus to mimic cellular protein trafficking signals and to interact with the cellular trafficking machinery. Importantly, loss or modification of these signals can influence virus infectivity and pathogenesis.

Viral and cellular gene transcription in fibroblasts infected with small plaque mutants of varicella-zoster virus

Varicella-zoster virus (VZV) is an alphaherpesvirus that causes varicella and herpes zoster. In these experiments, cDNA corresponding to 69 VZV open reading frames was added to 42K human cDNA microarrays and used to examine viral as well as cellular gene transcription concurrently in fibroblasts infected with two genetically distinct small plaque VZV mutants, rOka/ORF63rev[T171] and rOkaDeltagI. rOka/ORF63rev[T171] has a point mutation in ORF63, which encodes the immediate early regulatory protein, IE63, and rOkaDeltagI has a deletion of ORF67, encoding glycoprotein I (gI). rOka/ORF63rev[T171] was deficient in the transcription of several viral genes compared to the recombinant rOka control virus. Deletion of ORF67 had minimal effects on viral gene transcription. Effects of rOka/ORF63rev[T171] and rOkaDeltagI on host cell gene transcription were similar to the rOka control, but a few host cell genes were regulated differently in rOkaDeltagI-infected cells. Infection of fibroblasts with intact or small plaque VZV mutants was associated with down-regulation of NF-kappaB and interferon responsive genes, down-regulation of TGF-beta responsive genes accompanied by reduced amounts of fibrotic/wound healing response genes (e.g. collagens, follistatin) and activation of cellular proliferation genes, and alteration of neuronal growth markers, as well as cellular genes encoding proteins important in protein and vesicle trafficking. These observations suggest that replication of VZV small plaque mutant viruses and intact VZV have similar consequences for host cell gene transcription in infected cells, and that the small plaque phenotype in these mutants reflects deficiencies in viral gene expression.

Herpes simplex virus type 1 capsids transit by the trans-Golgi network, where viral glycoproteins accumulate independently of capsid egress

Egress of herpes capsids from the nucleus to the plasma membrane is a complex multistep transport event that is poorly understood. The current model proposes an initial envelopment at the inner nuclear membrane of capsids newly assembled in the nucleus. The capsids are then released in cytosol by fusion with the outer nuclear membrane. They are finally reenveloped at a downstream organelle before traveling to the plasma membrane for their extracellular release. Although the trans-Golgi network (TGN) is often cited as a potential site of reenvelopment, other organelles have also been proposed, including the Golgi, endoplasmic reticulum-Golgi intermediate compartment, aggresomes, tegusomes, and early or late endosomes. To clarify this important issue, we followed herpes simplex virus type 1 egress by immunofluorescence under conditions that slowed intracellular transport and promoted the accumulation of the otherwise transient reenvelopment intermediate. The data show that the capsids transit by the TGN and point to this compartment as the main reenvelopment site, although a contribution by endosomes cannot formally be excluded. Given that viral glycoproteins are expected to accumulate where capsids acquire their envelope, we examined this prediction and found that all tested could indeed be detected at the TGN. Moreover, this accumulation occurred independently of capsid egress. Surprisingly, capsids were often found immediately adjacent to the viral glycoproteins at the TGN.

Incorporation of three endocytosed varicella-zoster virus glycoproteins, gE, gH, and gB, into the virion envelope

The cytoplasmic tails of all three major varicella-zoster virus (VZV) glycoproteins, gE, gH, and gB, harbor functional tyrosine-based endocytosis motifs that mediate internalization. The aim of the present study was to examine whether endocytosis from the plasma membrane is a cellular route by which VZV glycoproteins are delivered to the final envelopment compartment. In this study, we demonstrated that internalization of the glycoproteins occurred in the first 24 h postinfection but was reduced later in infection. Using surface biotinylation of VZV-infected cells followed by a glutathione cleavage assay, we showed that endocytosis was independent of antibody binding to gE, gH, and gB. Subsequently, with this assay, we demonstrated that biotinylated gE, gH, and gB retrieved from the cell surface were incorporated into nascent virus particles isolated after density gradient sedimentation. To confirm and extend this finding, we repeated the above sedimentation step and specifically detected envelopes decorated with Streptavidin-conjugated gold beads on a majority of complete virions through examination by transmission electron microscopy. In addition, a gE-gI complex and a gE-gH complex were found on the virions. Therefore, the above studies established that VZV subsumed a postendocytosis trafficking pathway as one mechanism by which to deliver viral glycoproteins to the site of virion assembly in the cytoplasm. Furthermore, since a recombinant VZV genome lacking only endocytosis-competent gE cannot replicate, these results supported the conclusion that the endocytosis-envelopment pathway is an essential component of the VZV life cycle.

Functions of the C-terminal domain of varicella-zoster virus glycoprotein E in viral replication in vitro and skin and T-cell tropism in vivo

Varicella-zoster virus (VZV) glycoprotein E (gE) is essential for VZV replication. To further analyze the functions of gE in VZV replication, a full deletion and point mutations were made in the 62-amino-acid (aa) C-terminal domain. Targeted mutations were introduced in YAGL (aa 582 to 585), which mediates gE endocytosis, AYRV (aa 568 to 571), which targets gE to the trans-Golgi network (TGN), and SSTT, an "acid cluster" comprising a phosphorylation motif (aa 588 to 601). Substitutions Y582G in YAGL, Y569A in AYRV, and S593A, S595A, T596A, and T598A in SSTT were introduced into the viral genome by using VZV cosmids. These experiments demonstrated a hierarchy in the contributions of these C-terminal motifs to VZV replication and virulence. Deletion of the gE C terminus and mutation of YAGL were lethal for VZV replication in vitro. Mutations of AYRV and SSTT were compatible with recovery of VZV, but the AYRV mutation resulted in rapid virus spread in vitro and the SSTT mutation resulted in higher virus titers than were observed for the parental rOka strain. When the rOka-gE-AYRV and rOka-gE-SSTT mutants were evaluated in skin and T-cell xenografts in SCIDhu mice, interference with TGN targeting was associated with substantial attenuation, especially in skin, whereas the SSTT mutation did not alter VZV infectivity in vivo. These results provide the first information about how targeted mutations of this essential VZV glycoprotein affect viral replication in vitro and VZV virulence in dermal and epidermal cells and T cells within intact tissue microenvironments in vivo.

Viral and cellular kinases are potential antiviral targets and have a central role in varicella zoster virus pathogenesis

Herpesviruses utilize viral and cellular kinases for replication, and these mediate essential functions that are important for viral pathogenesis. Elucidating the roles of kinases in herpesvirus infections may highlight virus-host interactions that are possible targets for kinase inhibitors with antiviral activity. Varicella zoster virus (VZV) encodes two kinases that phosphorylate viral proteins involved in regulation, assembly, and virulence. VZV infection also induces the activity of host cell cyclin-dependent kinases (cdk4 and cdk2) in nondividing cells, causing a disregulation of the cell cycle. Roscovitine and Purvalanol, kinase inhibitors that target cdks, prevent VZV replication at concentrations with few cytotoxic effects. Cdk inhibitors therefore have potential as antivirals that may extend to a broad range of viruses and have the added advantage that resistance does not arise easily.

Mutational analysis of open reading frames 62 and 71, encoding the varicella-zoster virus immediate-early transactivating protein, IE62, and effects on replication in vitro and in skin xenografts in the SCID-hu mouse in vivo

The varicella-zoster virus (VZV) genome has unique long (U(L)) and unique short (U(S)) segments which are flanked by internal repeat (IR) and terminal repeat (TR) sequences. The immediate-early 62 (IE62) protein, encoded by open reading frame 62 (ORF62) and ORF71 in these repeats, is the major VZV transactivating protein. Mutational analyses were done with VZV cosmids generated from parent Oka (pOka), a low-passage clinical isolate, and repair experiments were done with ORF62 from pOka and vaccine Oka (vOka), which is derived from pOka. Transfections using VZV cosmids from which ORF62, ORF71, or the ORF62/71 gene pair was deleted showed that VZV replication required at least one copy of ORF62. The insertion of ORF62 from pOka or vOka into a nonnative site in U(S) allowed VZV replication in cell culture in vitro, although the plaque size and yields of infectious virus were decreased. Targeted mutations in binding sites reported to affect interaction with IE4 protein and a putative ORF9 protein binding site were not lethal. Single deletions of ORF62 or ORF71 from cosmids permitted recovery of infectious virus, but recombination events repaired the defective repeat region in some progeny viruses, as verified by PCR and Southern hybridization. VZV infectivity in skin xenografts in the SCID-hu model required ORF62 expression; mixtures of single-copy recombinant Oka Delta 62 (rOka Delta 62) or rOka Delta 71 and repaired rOka generated by recombination of the single-copy deletion mutants were detected in some skin implants. Although insertion of ORF62 into the nonnative site permitted replication in cell culture, ORF62 expression from its native site was necessary for cell-cell spread in differentiated human skin tissues in vivo.

Membrane association of VP22, a herpes simplex virus type 1 tegument protein

Tegument proteins of herpes simplex virus type 1 (HSV-1) are hypothesized to contain the functional information required for the budding or envelopment process proposed to occur at cytoplasmic compartments of the host cell. One of the most abundant tegument proteins of HSV-1 is the U(L)49 gene product, VP22, a 38-kDa protein of unknown function. To study its subcellular localization, a VP22-green fluorescent protein chimera was expressed in transfected human melanoma (A7) cells. In the absence of other HSV-1 proteins, VP22 localizes to acidic compartments of the cell that may include the trans-Golgi network (TGN), suggesting that this protein is membrane associated. Membrane pelleting and membrane flotation assays confirmed that VP22 partitions with the cellular membrane fraction. Through truncation mutagenesis, we determined that the membrane association of VP22 is a property attributed to amino acids 120 to 225 of this 301-amino-acid protein. The above results demonstrate that VP22 contains specific information required for targeting to membranes of acidic compartments of the cell which may be derived from the TGN, suggesting a potential role for VP22 during tegumentation and/or final envelopment.

A functional YNKI motif in the short cytoplasmic tail of varicella-zoster virus glycoprotein gH mediates clathrin-dependent and antibody-independent endocytosis

The trafficking of varicella-zoster virus (VZV) gH was investigated under both infection and transfection conditions. In initial endocytosis assays performed in infected cells, the three glycoproteins gE, gI, and gB served as positive controls for internalization from the plasma membrane. Subsequently, we discovered that gH in VZV-infected cells was also internalized and followed a similar trafficking pattern. This observation was unexpected because all herpesvirus gH homologues have short endodomains not known to contain trafficking motifs. Further investigation demonstrated that VZV gH, when expressed alone with its chaperone gL, was capable of endocytosis in a clathrin-dependent manner, independent of gE, gI, or gB. Upon inspection of the short gH cytoplasmic tail, we discovered a putative tyrosine-based endocytosis motif (YNKI). When the tyrosine was replaced with an alanine, endocytosis of gH was blocked. Utilizing an endocytosis assay dependent on biotin labeling, we further documented that endocytosis of VZV gH was antibody independent. In control experiments, we showed that gE, gI, and gB also internalized in an antibody-independent manner. Alignment analysis of the VZV gH cytoplasmic tail to other herpesvirus gH homologues revealed two important findings: (i) herpes simplex virus type 1 and 2 homologues lacked an endocytosis motif, while all other alphaherpesvirus gH homologues contained a potential motif, and (ii) the VZV gH and simian varicella virus gH cytoplasmic tails were likely longer in length (18 amino acids) than predicted in the original sequence analyses (12 and 16 amino acids, respectively). The longer tails provided the proper context for a functional endocytosis motif.

Plasma membrane topology of syncytial domains of herpes simplex virus type 1 glycoprotein K (gK): the UL20 protein enables cell surface localization of gK but not gK-mediated cell-to-cell fusion

Most spontaneously occurring mutations that cause extensive herpes simplex virus type 1 (HSV-1)-induced cell fusion are single amino acid changes within glycoprotein K (gK). Despite the strong genetic association of gK with virus-induced cell fusion, its direct involvement in cellular membrane fusion has been controversial, largely due to previously unsuccessful efforts to detect gK expression on virion and cellular surfaces. Recently, we showed that gK is expressed on HSV-1 virions and functioned in virus entry (T. P. Foster, G. V. Rybachuk, and K. G. Kousoulas, J. Virol. 75:12431-12438, 2001). To determine whether gK is expressed on cellular surfaces, as well as its membrane topology, we generated the recombinant viruses gKV5DI, gKV5DII, gKV5DIII, and gKV5DIVcontaining insertions of the V5 antigenic epitope within each of four domains of gK predicted to localize either in the cytoplasmic side or in the extracytoplasmic side of cellular membranes. Immunohistochemical and confocal microscopy analyses of infected cells showed that both wild-type and syncytial forms of gK were expressed on cell surfaces. Analysis of the topology of the V5-tagged gK revealed that gK domains I and IV were located extracellularly, whereas domains II and III were localized intracellularly. Transiently expressed gK failed to localize in cellular plasma membranes. In contrast, infection of gK-transfected cells with the gK-null virus DeltagK enabled expression of gK on cell surfaces, as well as gK-mediated membrane fusion. Transient-coexpression experiments revealed that the UL20 protein enabled cell surface expression of gK, but not gK-mediated cell-to-cell fusion, indicating that additional viral proteins are required for expression of the gK syncytial phenotype.

Promoter sequences of varicella-zoster virus glycoprotein I targeted by cellular transactivating factors Sp1 and USF determine virulence in skin and T cells in SCIDhu mice in vivo

Varicella-zoster virus (VZV) glycoprotein I is dispensable in cell culture but necessary for infection of human skin and T cells in SCIDhu mice in vivo. The gI promoter contains an activating upstream sequence that binds the cellular transactivators specificity factor 1 (Sp1) and upstream stimulatory factor (USF) and an open reading frame 29 (ORF29)-responsive element (29RE), which mediates enhancement by ORF29 DNA binding protein of immediate-early 62 (IE62)-induced transcription. Recombinants, rOKAgI-Sp1 and rOKAgI-USF, with two base pair substitutions in Sp1 or USF sites, replicated like rOKA in vitro, but infectivity of rOKAgI-Sp1 was significantly impaired in skin and T cells in vivo. A double mutant, rOKAgI-Sp1/USF, did not replicate in skin but yielded low titers of infectious virus in T cells. The repaired protein, rOKAgI:rep-Sp1/USF, was as infectious as rOKA. Thus, disrupting gI promoter sites for cellular transactivators altered VZV virulence in vivo, with variable consequences related to the cellular factor and the host cell type. Mutations in the 29RE of the gI promoter were made by substituting each of four 10-bp blocks in this region with a 10-bp sequence, GATAACTACA, that was predicted to interfere with enhancer effects of the ORF29 protein. One of these mutants, which was designated rOKAgI-29RE-3, had diminished replication in skin and T cells, indicating that ORF29 protein-mediated enhancement of gI expression contributes to VZV virulence. Mutations within promoters of viral genes that are nonessential in vitro should allow construction of recombinant herpesviruses that have altered virulence in specific host cells in vivo and may be useful for designing herpesviral gene therapy vectors and attenuated viral vaccines.

The requirement of varicella zoster virus glycoprotein E (gE) for viral replication and effects of glycoprotein I on gE in melanoma cells

The glycoprotein E (gE) of varicella zoster virus (VZV), encoded by ORF68, is the most abundant viral glycoprotein. In the current experiments, we demonstrated that ORF68 deletion was incompatible with recovery of infectious virus from VZV cosmids. Replacing ORF68 at a nonnative AvrII site in the genome restored infectivity. Further, we found that VZV gE could be expressed under the control of the Tet-On promoter in stably transfected melanoma cell lines (Met-gE cells) without evidence of toxicity. In these Met-gE cells, gE colocalized with gamma-adaptin, a trans Golgi network marker, in perinuclear sites, but did not reach plasma membranes. In order to investigate how infection altered gE localization, we made a recombinant virus, vOka-MSPgE, with ORF68 from the VZV MSP strain. VZV MSP encodes a mutant gE protein (D150N) that lacks the mAb epitope, 3B3 (Santos et al., Virology 275, 306-317, 2000), whereas Met-gE protein binds mAb 3B3. Within 48 h after Met-gE cells were infected with vOka-MSPgE, the steady-state distribution of Met-gE protein extended beyond the perinuclear areas to other cytoplasmic sites and to plasma membranes. A second recombinant, vOka-MSPgE without gI (vOka-MSPgEdeltagI), was constructed to investigate Met-gE protein distribution in the absence of gI. The redistribution of Met-gE protein which was observed by 48 h after vOka-MSPgE infection did not occur until 5 days (140 h) within vOka-MSPgEdeltagI infected cells. After vOka-MSPgE infection of Met-gE cells, most Met-gE protein was in the final 94K mature form by 72 h. However, progression to predominance of mature gE was delayed in Met-gE cells infected with vOka-MSPgEdeltagI. These observations confirm our hypothesis that VZV gE is essential, based upon the demonstration of restored infectivity after replacing ORF68 in a nonnative site in the genome, and provide further evidence of the role of gI in facilitating the maturation and intracellular distribution of this critical VZV glycoprotein.

Glycoprotein I of varicella-zoster virus is required for viral replication in skin and T cells

Varicella-zoster virus (VZV) glycoprotein I (gI) is dispensable in cell culture; the SCIDhu model of VZV pathogenesis was used to determine whether gI is necessary in vivo. The parental and repaired viruses grew in human skin and thymus/liver implants, but the gI deletion mutant was not infectious. Thus, gI is essential for VZV infectivity in skin and T cells.

Glycoproteins E and I of Marek's disease virus serotype 1 are essential for virus growth in cultured cells

The role of glycoprotein E (gE) and gI of Marek's disease virus serotype 1 (MDV-1) for growth in cultured cells was investigated. MDV-1 mutants lacking either gE (20DeltagE), gI (20DeltagI), or both gE and gI (20DeltagEI) were constructed by recE/T-mediated mutagenesis of a recently established infectious bacterial artificial chromosome (BAC) clone of MDV-1 (D. Schumacher, B. K. Tischer, W. Fuchs, and N. Osterrieder, J. Virol. 74:11088-11098, 2000). Deletion of either gE or gI, which form a complex in MDV-1-infected cells, resulted in the production of virus progeny that were unable to spread from cell to cell in either chicken embryo fibroblasts or quail muscle cells. This was reflected by the absence of virus plaques and the detection of only single infected cells after transfection, even after coseeding of transfected cells with uninfected cells. In contrast, growth of rescuant viruses, in which the deleted glycoprotein genes were reinserted by homologous recombination, was indistinguishable from that of parental BAC20 virus. In addition, the 20DeltagE mutant virus was able to spread from cell to cell when cotransfected into chicken embryo fibroblasts with an expression plasmid encoding MDV-1 gE, and the 20DeltagI mutant virus exhibited cell-to-cell spread capability after cotransfection with a gI expression plasmid. The 20DeltagEI mutant virus, however, was not able to spread in the presence of either a gE or gI expression plasmid, and only single infected cells were detected by indirect immunofluorescence. The results reported here demonstrate for the first time that both gE and gI are absolutely essential for cell-to-cell spread of a member of the Alphaherpesvirinae.

Essential role played by the C-terminal domain of glycoprotein I in envelopment of varicella-zoster virus in the trans-Golgi network: interactions of glycoproteins with tegument

Varicella-zoster virus (VZV) is enveloped in the trans-Golgi network (TGN). Here we report that glycoprotein I (gI) is required within the TGN for VZV envelopment. Enveloping membranous TGN cisternae were microscopically identified in cells infected with intact VZV. These sacs curved around, and ultimately enclosed, nucleocapsids. Tegument coated the concave face of these sacs, which formed the viral envelope, but the convex surface was tegument-free. TGN cisternae of cells infected with VZV mutants lacking gI (gI(Delta)) or its C (gI(DeltaC))- or N-terminal (gI(DeltaN))-terminal domains were uniformly tegument coated and adhered to one another, forming bizarre membranous stacks. Viral envelopment was compromised, and no virions were delivered to post-Golgi structures. The TGN was not gI-immunoreactive in cells infected with the gI(Delta) or gI(DeltaN) mutants, but it was in cells infected with gI(DeltaC) (because the ectodomains of gI and gE interact). The presence in the TGN of gI lacking a C-terminal domain, therefore, was not sufficient to maintain enveloping cisternae. In cells infected with intact VZV or with gI(Delta), gI(DeltaN), or gI(DeltaC) mutants, ORF10p immunoreactivity was concentrated on the cytosolic face of TGN membranes, suggesting that it interacts with the cytosolic domains of glycoproteins. Because of the gE-gI interaction, cotransfected cells that expressed gE or gI were able to target truncated forms of the other to the TGN. Our data suggest that the C-terminal domain of gI is required to segregate viral and cellular proteins in enveloping TGN cisternae.

Glycoprotein E of varicella-zoster virus enhances cell-cell contact in polarized epithelial cells

Varicella-zoster virus (VZV) infection involves the cell-cell spread of virions, but how viral proteins interact with the host cell membranes that comprise intercellular junctions is not known. Madin-Darby canine kidney (MDCK) cells were constructed to express the glycoproteins gE, gI, or gE/gI constitutively and were used to examine the effects of these VZV glycoproteins in polarized epithelial cells. At low cell density, VZV gE induced partial tight junction (TJ) formation under low-calcium conditions, whether expressed alone or with gI. Although most VZV gE was intracellular, gE was also shown to colocalize with the TJ protein ZO-1 with or without concomitant expression of gI. Freeze fracture electron microscopy revealed normal TJ strand morphology in gE-expressing MDCK cells. Functionally, the expression of gE was associated with a marked acceleration in the establishment of maximum transepithelial electrical resistance (TER) in MDCK-gE cells; MDCK-gI and MDCK-gE/gI cells exhibited a similar pattern of early TER compared to MDCK cells, although peak resistances were lower than those of gE alone. VZV gE expression altered F-actin organization and lipid distribution, but coexpression of gI modulated these effects. Two regions of the gE ectodomain, amino acids (aa) 278 to 355 and aa 467 to 498, although lacking Ca(2+) binding motifs, exhibit similarities with corresponding regions of the cell adhesion molecules, E-cadherin and desmocollin. These observations suggest that VZV gE and gE/gI may contribute to viral pathogenesis by facilitating epithelial cell-cell contacts.

The role of virion membrane protein endocytosis in the herpesvirus life cycle

Endocytosis of cellular surface membrane proteins is a well-characterized, common occurrence. Internalization of cell surface receptors, often with bound ligands, aid in global events, such as cellular metabolism, as well as in specific, directed functions, such as the induction of signal transduction cascades or immune function. Some, but not all, herpesvirus membrane proteins are internalized from the plasma membrane by a process similar to receptor-mediated endocytosis. No known functions, however, have been ascribed to endocytosis of these proteins. In this review, we consider the function of herpesvirus membrane protein endocytosis. We compare and contrast the endocytosis and intracellular trafficking of two pseudorabies virus membrane proteins, the type I glycoprotein, gE, and the type II, tail-anchored membrane protein, Us9, and discuss the possible function of their internalization during the virus life cycle.

Trafficking of varicella-zoster virus glycoprotein gI: T(338)-dependent retention in the trans-Golgi network, secretion, and mannose 6-phosphate-inhibitable uptake of the ectodomain

The trans-Golgi network (TGN) is putatively the site where varicella-zoster virus is enveloped. gE is targeted to the TGN by selective retrieval from the plasmalemma in response to signaling sequences in its endodomain. gI lacks these sequences but forms a complex with gE. We now find that gI is targeted to the TGN and plasma membrane when expressed in Cos-7 cells; nevertheless, surface labeling revealed that gI is not retrieved from the plasma membrane. TGN targeting of gI depended on the T(338) of its endodomain and was lost when T(338) was deleted or mutated to A, S, or D. The endodomain of gI was sufficient, if it contained T(338), to target a fusion protein containing the ectodomain of the human interleukin-2 receptor to the TGN. A truncated protein consisting only of the gI ectodomain was secreted and taken up by nontransfected cells. This uptake of the secreted gI ectodomain was blocked by mannose 6-phosphate. Following cotransfection, both gI and gE were retrieved to the TGN from the plasma membrane in 26.7% of cells, neither gI nor gE was internalized in 18.3%, and gE was retrieved to the TGN while gI remained at the plasma membrane in 55%. We suggest that the T(338) of its endodomain is necessary to retain gI in the TGN; moreover, because gI and gE interact, the signaling sequences of each glycoprotein reinforce one another in ensuring that both glycoproteins are concentrated in the TGN yet remain on the cell surface.

Directional transneuronal infection by pseudorabies virus is dependent on an acidic internalization motif in the Us9 cytoplasmic tail

The Us9 gene is conserved among most alphaherpesviruses. In pseudorabies virus (PRV), the Us9 protein is a 98-amino-acid, type II membrane protein found in the virion envelope. It localizes to the trans-Golgi network (TGN) region in infected and transfected cells and is maintained in this compartment by endocytosis from the plasma membrane. Viruses with Us9 deleted have no observable defects in tissue culture yet have reduced virulence and restricted spread to retinorecipient neurons in the rodent brain. In this report, we demonstrate that Us9-promoted transneuronal spread in vivo is dependent on a conserved acidic motif previously shown to be essential for the maintenance of Us9 in the TGN region and recycling from the plasma membrane. Mutant viruses with the acidic motif deleted have an anterograde spread defect indistinguishable from that of Us9 null viruses. Transneuronal spread, however, is not dependent on a dileucine endocytosis motif in the Us9 cytoplasmic tail. Through alanine scanning mutagenesis of the acidic motif, we have identified two conserved tyrosine residues that are essential for Us9-mediated spread as well as two serine residues, comprising putative consensus casein kinase II sites, that modulate the rate of PRV transneuronal spread in vivo.

Intracellular trafficking and localization of the pseudorabies virus Us9 type II envelope protein to host and viral membranes

The Us9 protein is a phosphorylated membrane protein present in the lipid envelope of pseudorabies virus (PRV) particles in a unique tail-anchored type II membrane topology. In this report, we demonstrate that the steady-state residence of the Us9 protein is in a cellular compartment in or near the trans-Golgi network (TGN). Through internalization assays with an enhanced green fluorescent protein epitope-tagged Us9 protein, we demonstrate that the maintenance of Us9 to the TGN region is a dynamic process involving retrieval of molecules from the cell surface. Deletion analysis of the cytoplasmic tail reveals that an acidic cluster containing putative phosphorylation sites is necessary for the recycling of Us9 from the plasma membrane. The absence of this cluster results in the relocalization of Us9 to the plasma membrane due to a defect in endocytosis. The acidic motif, however, does not contain signals needed to direct the incorporation of Us9 into viral envelopes. In this study, we also investigate the role of a dileucine endocytosis signal in the Us9 cytoplasmic tail in the recycling and retention of Us9 to the TGN region. Site-directed mutagenesis of the dileucine motif results in an increase in Us9 plasma membrane staining and a partial internalization defect.

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.

Mutation of the YXXL endocytosis motif in the cytoplasmic tail of pseudorabies virus gE

The role of alphaherpesvirus membrane protein internalization during the course of viral infection remains a matter of speculation. To determine the role of internalization of the pseudorabies virus (PRV) gE and gI proteins, we constructed viral mutants encoding specific mutations in the cytoplasmic tail of the gE gene that inhibited internalization of the gE-gI complex. We used these mutants to assess the role of gE-gI endocytosis in incorporation of the proteins into the viral envelope and in gE-mediated spread or gE-promoted virulence. In addition, we report that another viral mutant, PRV 25, which encodes a gE protein defective in endocytosis, contains an additional, previously uncharacterized mutation in the gE gene. We compared PRV 25 to another viral mutant, PRV 107, that does not express the cytoplasmic tail of the gE protein. The gE protein encoded by PRV 107 is also defective in endocytosis. We conclude that efficient endocytosis of gE is not required for gE incorporation into virions, gE-mediated virulence, or spread of virus in the rat central nervous system. However, we do correlate the defect in endocytosis to a small-plaque phenotype in cultured cells.

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.