Varicella-zoster virus glycoprotein oligosaccharides are phosphorylated during posttranslational maturation

The Biology of Varicella-Zoster Virus Replication in the Skin

The replication of varicella-zoster virus (VZV) in skin is critical to its pathogenesis and spread. Primary infection causes chickenpox, which is characterised by centrally distributed skin blistering lesions that are rich in infectious virus. Cell-free virus in the cutaneous blistering lesions not only spreads to cause further cases, but infects sensory nerve endings, leading to the establishment of lifelong latency in sensory and autonomic ganglia. The reactivation of virus to cause herpes zoster is again characterised by localised painful skin blistering rash containing infectious virus. The development of in vitro and in vivo models of VZV skin replication has revealed aspects of VZV replication and pathogenesis in this important target organ and improved our understanding of the vaccine strain vOKa attenuation. In this review, we outline the current knowledge on VZV interaction with host signalling pathways, the viral association with proteins associated with epidermal terminal differentiation, and how these interconnect with the VZV life cycle to facilitate viral replication and shedding.

Microfluidic electrical cell lysis for high-throughput and continuous production of cell-free varicella-zoster virus

Varicella-zoster virus (VZV), the causative agent of varicella and herpes zoster, is highly cell-associated and spreads via cell-to-cell contact in tissue culture. The lack of cell-free VZV hampers studies on VZV biology as well as antiviral and vaccine development. In the present study, a poly(methylmethacrylate) microfluidic device integrated with arrays of microelectrode was fabricated to continuously electrolyse VZV-infected cells to produce cell-free viruses. By designing multiple constrictions and microelectrode arrays, a high electric field is focused on the constricted region of the microchannel to disrupt large numbers of virus-infected cells with high-throughput on a microfluidic platform. Plaque assay and scanning electron microscopy were conducted to quantify and characterize cell-free VZV produced using the microfluidic continuous-flow electrical cell lysis device. The process of microfluidic electrical cell lysis followed by subsequent filtration and virus concentration process yielded a 1.4-2.1 × 10⁴ plaque-forming units (PFUs) per mL of cell-free VZV from 7.0 × 10⁶ VZV-infected human foreskin fibroblasts (HFF) cells. The high electric field formed inside a microfluidic channel combined with the continuous-flow of virus-infected cells within the microchannel enabled the rapid and efficient production of high-titer cell-free virus in large quantities with relatively low input of the voltage.

Tissue plasminogen activator is a ligand of cation-independent mannose 6-phosphate receptor and consists of glycoforms that contain mannose 6-phosphate

Plasmin is the key enzyme in fibrinolysis. Upon interaction with plasminogen activators, the zymogen plasminogen is converted to active plasmin. Some studies indicate plasminogen activation is regulated by cation-independent mannose 6-phosphate receptor (CI-MPR), a protein that facilitates lysosomal enzyme trafficking and insulin-like growth factor 2 downregulation. Plasminogen regulation may be accomplished by CI-MPR binding to plasminogen or urokinase plasminogen activator receptor. We asked whether other members of the plasminogen activation system, such as tissue plasminogen activator (tPA), also interact with CI-MPR. Because tPA is a glycoprotein with three N-linked glycosylation sites, we hypothesized that tPA contains mannose 6-phosphate (M6P) and binds CI-MPR in a M6P-dependent manner. Using surface plasmon resonance, we found that two sources of tPA bound the extracellular region of human and bovine CI-MPR with low-mid nanomolar affinities. Binding was partially inhibited with phosphatase treatment or M6P. Subsequent studies revealed that the five N-terminal domains of CI-MPR were sufficient for tPA binding, and this interaction was also partially mediated by M6P. The three glycosylation sites of tPA were analyzed by mass spectrometry, and glycoforms containing M6P and M6P-N-acetylglucosamine were identified at position N448 of tPA. In summary, we found that tPA contains M6P and is a CI-MPR ligand.

Exocytosis of Progeny Infectious Varicella-Zoster Virus Particles via a Mannose-6-Phosphate Receptor Pathway without Xenophagy following Secondary Envelopment

The literature on the egress of different herpesviruses after secondary envelopment is contradictory. In this report, we investigated varicella-zoster virus (VZV) egress in a cell line from a child with Pompe disease, a glycogen storage disease caused by a defect in the enzyme required for glycogen digestion. In Pompe cells, both the late autophagy pathway and the mannose-6-phosphate receptor (M6PR) pathway are interrupted. We have postulated that intact autophagic flux is required for higher recoveries of VZV infectivity. To test that hypothesis, we infected Pompe cells and then assessed the VZV infectious cycle. We discovered that the infectious cycle in Pompe cells was remarkably different from that of either fibroblasts or melanoma cells. No large late endosomes filled with VZV particles were observed in Pompe cells; only individual viral particles in small vacuoles were seen. The distribution of the M6PR pathway (trans-Golgi network to late endosomes) was constrained in infected Pompe cells. When cells were analyzed with two different anti-M6PR antibodies, extensive colocalization of the major VZV glycoprotein gE (known to contain M6P residues) and the M6P receptor (M6PR) was documented in the viral highways at the surfaces of non-Pompe cells after maximum-intensity projection of confocal z-stacks, but neither gE nor the M6PR was seen in abundance at the surfaces of infected Pompe cells. Taken together, our results suggested that (i) Pompe cells lack a VZV trafficking pathway within M6PR-positive large endosomes and (ii) most infectious VZV particles in conventional cell substrates are transported via large M6PR-positive vacuoles without degradative xenophagy to the plasma membrane.IMPORTANCE The long-term goal of this research has been to determine why VZV, when grown in cultured cells, invariably is more cell associated and has a lower titer than other alphaherpesviruses, such as herpes simplex virus 1 (HSV1) or pseudorabies virus (PRV). Data from both HSV1 and PRV laboratories have identified a Rab6 secretory pathway for the transport of single enveloped viral particles from the trans-Golgi network within small vacuoles to the plasma membrane. In contrast, after secondary envelopment in fibroblasts or melanoma cells, multiple infectious VZV particles accumulated within large M6PR-positive late endosomes that were not degraded en route to the plasma membrane. We propose that this M6PR pathway is most utilized in VZV infection and least utilized in HSV1 infection, with PRV's usage being closer to HSV1's usage. Supportive data from other VZV, PRV, and HSV1 laboratories about evidence for two egress pathways are included.

Most rotavirus strains require the cation-independent mannose-6-phosphate receptor, sortilin-1, and cathepsins to enter cells

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Rotaviruses require the TGN to LE transporter CI-M6PR for cell entry.

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Sortilin-1 was identified as a cell factor involved in rotavirus replication.

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Rotaviruses require cathepsins also to enter Caco-2 cells.

Cathepsins, endosomal acid proteases, are transported from the trans-Golgi network to late endosomes by the mannose-6-phosphate receptor (M6PR). We have previously demonstrated that some rotavirus strains, like UK, Wa, WI61, DS-1, and YM, require the cation-dependent (CD-) M6PR and cathepsins to enter from late endosomes to the cytoplasm in MA104 cells, while other strains, like the simian strain RRV, which enter cells from maturing endosomes, do not. However, the role of other trans-Golgi network-late endosome transporters, such as the cation-independent (CI-) M6PR and sortillin-1, has not been evaluated. In this work, we found that several rotavirus strains that require the CD-M6PR for cell entry are also dependent on CI-M6PR and sortilin-1. Furthermore, we showed that the infectivity of all these rotavirus strains also requires cathepsins to enter not only MA104 cells, but also human intestinal Caco-2 cells. This study identifies sortilin-1 as a novel cell factor necessary for the infectivity of a virus; in addition, our results strongly suggest that cathepsins could be common cell factors needed for the infectivity of most rotavirus strains.

Human Embryonic Stem Cell-Derived Neurons Are Highly Permissive for Varicella-Zoster Virus Lytic Infection

Varicella-zoster virus (VZV) is highly cell associated when grown in culture and has a much higher (4,000- to 20,000-fold increased) particle-to-PFU ratio in vitro than herpes simplex virus (HSV). In contrast, VZV is highly infectious in vivo by airborne transmission. Neurons are major targets for VZV in vivo; in neurons, the virus can establish latency and reactivate to produce infectious virus. Using neurons derived from human embryonic stem cells (hESC) and cell-free wild-type (WT) VZV, we demonstrated that neurons are nearly 100 times more permissive for WT VZV infection than very-early-passage human embryonic lung cells or MRC-5 diploid human fibroblasts, the cells used for vaccine production or virus isolation. The peak titers achieved after infection were ∼10-fold higher in human neurons than in MRC-5 cells, and the viral genome copy number-to-PFU ratio for VZV in human neurons was 500, compared with 50,000 for MRC-5 cells. Thus, VZV may not necessarily have a higher particle-to-PFU ratio than other herpesviruses; instead, the cells previously used to propagate virus in vitro may have been suboptimal. Furthermore, based on electron microscopy, neurons infected with VZV produced fewer defective or incomplete viral particles than MRC-5 cells. Our data suggest that neurons derived from hESC may have advantages compared to other cells for studies of VZV pathogenesis, for obtaining stocks of virus with high titers, and for isolating VZV from clinical specimens.IMPORTANCE Varicella-zoster virus (VZV) causes chickenpox and shingles. Cell-free VZV has been difficult to obtain, both for in vitro studies and for vaccine production. While numerous cells lines have been tested for their ability to produce high titers of VZV, the number of total virus particles relative to the number of viral particles that can form plaques in culture has been reported to be extremely high relative to that in other viruses. We show that VZV grows to much higher titers in human neurons than in other cell types in vitro and that the number of total virus genomes relative to the number of viral particles that can form plaques in culture is much lower in human neurons than other cultured cells. These findings indicate that human neurons may be useful for studying VZV in vitro, for growing preparations of virus with high titers, and for isolating the virus from human samples.

Varicella-Zoster Virus Glycoproteins: Entry, Replication, and Pathogenesis

Varicella-zoster virus (VZV), an alphaherpesvirus that causes chicken pox (varicella) and shingles (herpes zoster), is a medically important pathogen that causes considerable morbidity and, on occasion, mortality in immunocompromised patients. Herpes zoster can afflict the elderly with a debilitating condition, postherpetic neuralgia, triggering severe, untreatable pain for months or years. The lipid envelope of VZV, similar to all herpesviruses, contains numerous glycoproteins required for replication and pathogenesis.

PURPOSE OF REVIEW

To summarize the current knowledge about VZV glycoproteins and their roles in cell entry, replication and pathogenesis.

RECENT FINDINGS

The functions for some VZV glycoproteins are known, such as gB, gH and gL in membrane fusion, cell-cell fusion regulation, and receptor binding properties. However, the molecular mechanisms that trigger or mediate VZV glycoproteins remains poorly understood.

SUMMARY

VZV glycoproteins are central to successful replication but their modus operandi during replication and pathogenesis remain elusive requiring further mechanistic based studies.

Role for the αV Integrin Subunit in Varicella-Zoster Virus-Mediated Fusion and Infection

Varicella-zoster virus (VZV) is an alphaherpesvirus that causes varicella and herpes zoster. Membrane fusion is essential for VZV entry and the distinctive syncytium formation in VZV-infected skin and neuronal tissue. Herpesvirus fusion is mediated by a complex of glycoproteins gB and gH-gL, which are necessary and sufficient for VZV to induce membrane fusion. However, the cellular requirements of fusion are poorly understood. Integrins have been implicated to facilitate entry of several human herpesviruses, but their role in VZV entry has not yet been explored. To determine the involvement of integrins in VZV fusion, a quantitative cell-cell fusion assay was developed using a VZV-permissive melanoma cell line. The cells constitutively expressed a reporter protein and short hairpin RNAs (shRNAs) to knock down the expression of integrin subunits shown to be expressed in these cells by RNA sequencing. The αV integrin subunit was identified as mediating VZV gB/gH-gL fusion, as its knockdown by shRNAs reduced fusion levels to 60% of that of control cells. A comparable reduction in fusion levels was observed when an anti-αV antibody specific to its extracellular domain was tested in the fusion assay, confirming that the domain was important for VZV fusion. In addition, reduced spread was observed in αV knockdown cells infected with the VZV pOka strain relative to that of the control cells. This was demonstrated by reductions in plaque size, replication kinetics, and virion entry in the αV subunit knockdown cells. Thus, the αV integrin subunit is important for VZV gB/gH-gL fusion and infection.

IMPORTANCE

Varicella-zoster virus (VZV) is a highly infectious pathogen that causes chickenpox and shingles. A common complication of shingles is the excruciating condition called postherpetic neuralgia, which has proven difficult to treat. While a vaccine is now available, it is not recommended for immunocompromised individuals and its efficacy decreases with the recipient's age. These limitations highlight the need for new therapies. This study examines the role of integrins in membrane fusion mediated by VZV glycoproteins gB and gH-gL, a required process for VZV infection. This knowledge will further the understanding of VZV entry and provide insight into the development of better therapies.

Fasciola hepatica Surface Coat Glycoproteins Contain Mannosylated and Phosphorylated N-glycans and Exhibit Immune Modulatory Properties Independent of the Mannose Receptor

Fascioliasis, caused by the liver fluke Fasciola hepatica, is a neglected tropical disease infecting over 1 million individuals annually with 17 million people at risk of infection. Like other helminths, F. hepatica employs mechanisms of immune suppression in order to evade its host immune system. In this study the N-glycosylation of F. hepatica’s tegumental coat (FhTeg) and its carbohydrate-dependent interactions with bone marrow derived dendritic cells (BMDCs) were investigated. Mass spectrometric analysis demonstrated that FhTeg N-glycans comprised mainly of oligomannose and to a lesser extent truncated and complex type glycans, including a phosphorylated subset. The interaction of FhTeg with the mannose receptor (MR) was investigated. Binding of FhTeg to MR-transfected CHO cells and BMDCs was blocked when pre-incubated with mannan. We further elucidated the role played by MR in the immunomodulatory mechanism of FhTeg and demonstrated that while FhTeg’s binding was significantly reduced in BMDCs generated from MR knockout mice, the absence of MR did not alter FhTeg’s ability to induce SOCS3 or suppress cytokine secretion from LPS activated BMDCs. A panel of negatively charged monosaccharides (i.e. GlcNAc-4P, Man-6P and GalNAc-4S) were used in an attempt to inhibit the immunoregulatory properties of phosphorylated oligosaccharides. Notably, GalNAc-4S, a known inhibitor of the Cys-domain of MR, efficiently suppressed FhTeg binding to BMDCs and inhibited the expression of suppressor of cytokine signalling (SOCS) 3, a negative regulator the TLR and STAT3 pathway. We conclude that F. hepatica contains high levels of mannose residues and phosphorylated glycoproteins that are crucial in modulating its host’s immune system, however the role played by MR appears to be limited to the initial binding event suggesting that other C-type lectin receptors are involved in the immunomodulatory mechanism of FhTeg.

Fascioliasis, caused by the liver fluke Fasciola hepatica, is a neglected tropical disease infecting over 1 million individuals annually with 17 million people at risk of infection. These worms infect the liver and can survive for many years in its animal or human host because they supress the host’s immune system that is important in clearing worm infection. Worms are similar to humans in that they are made of proteins, fats and sugars, and while there are many studies on worm proteins, few studies have examined the sugars. We are interested in the sugars because we believe that they help the parasite survive for many years within its host. To examine this, we have used a technique called mass spectrometric analysis to characterise the sugars present in F. hepatica. We also have developed systems in the laboratory to test if these sugars can suppress the host’s immune system. We conclude that F. hepatica sugars are crucial in suppressing its host’s immune system; however, the exact way the sugars can do this requires further studies. These studies are important for the development of worm vaccines or therapies.

Infected peripheral blood mononuclear cells transmit latent varicella zoster virus infection to the guinea pig enteric nervous system

Latent wild-type (WT) and vaccine (vOka) varicella zoster virus (VZV) are found in the human enteric nervous system (ENS). VZV also infects guinea pig enteric neurons in vitro, establishes latency and can be reactivated. We therefore determined whether lymphocytes infected in vitro with VZV secrete infectious virions and can transfer infection in vivo to the ENS of recipient guinea pigs. T lymphocytes (CD3-immunoreactive) were preferentially infected following co-culture of guinea pig or human peripheral blood mononuclear cells with VZV-infected HELF. VZV proliferated in the infected T cells and expressed immediate early and late VZV genes. Electron microscopy confirmed that VZV-infected T cells produced encapsulated virions. Extracellular virus, however, was pleomorphic, suggesting degradation occurred prior to release, which was confirmed by the failure of VZV-infected T cells to secrete infectious virions. Intravenous injection of WT- or vOka-infected PBMCs, nevertheless, transmitted VZV to recipient animals (guinea pig > human lymphocytes). Two days post-inoculation, lung and liver, but not gut, contained DNA and transcripts encoding ORFs 4, 40, 66 and 67. Twenty-eight days after infection, gut contained DNA and transcripts encoding ORFs 4 and 66 but neither DNA nor transcripts could any longer be found in lung or liver. In situ hybridization revealed VZV DNA in enteric neurons, which also expressed ORF63p (but not ORF68p) immunoreactivity. Observations suggest that VZV infects T cells, which can transfer VZV to and establish latency in enteric neurons in vivo. Guinea pigs may be useful for studies of VZV pathogenesis in the ENS.

Varicella-zoster virus ORF49 functions in the efficient production of progeny virus through its interaction with essential tegument protein ORF44

The ORF49 tegument protein of varicella-zoster virus (VZV) is one of the core gene products that is conserved among herpesvirus family members. Although ORF49 is known to be a cell-tropic factor, its detailed functions remain elusive. ORF44 is another core gene product reported to be essential, although its characterization and detailed functional analysis have not been reported. These two core gene products form a complex in other herpesviruses beyond the host species and herpesvirus subfamilies. Here, we show that complex formation between ORF44 and ORF49 is conserved in VZV. We serendipitously found that binding is eliminated by an amino acid substitution at position 129 (phenylalanine 129), and four amino acids in the carboxyl-terminal half of the acidic cluster in ORF49 (i.e., aspartate-phenylalanine-aspartate-glutamate from positions 41 to 44 [41DFDE44]) were identified as its binding motif. Alanine substitutions in each domain rendered the ORF44F129A mutation lethal for VZV, similar to deletion of the entire ORF44. The phenotype of the ORF49-41AAAA44 mutation was comparable to that of the ORF49-defective virus, including small-plaque formation, impaired growth, and low infectious virus production. These results suggest that the interaction between ORF44 and ORF49 is essential for their role in VZV infection and that ORF49 is required for the efficient production of infectious progeny virus mediated by the conserved interaction between the two proteins.

Pathogenesis and current approaches to control of varicella-zoster virus infections

Varicella-zoster virus (VZV) was once thought to be a fairly innocuous pathogen. That view is no longer tenable. The morbidity and mortality due to the primary and secondary diseases that VZV causes, varicella and herpes zoster (HZ), are significant. Fortunately, modern advances, including an available vaccine to prevent varicella, a therapeutic vaccine to diminish the incidence and ameliorate sequelae of HZ, effective antiviral drugs, a better understanding of VZV pathogenesis, and advances in diagnostic virology have made it possible to control VZV in the United States. Occult forms of VZV-induced disease have been recognized, including zoster sine herpete and enteric zoster, which have expanded the field. Future progress should include development of more effective vaccines to prevent HZ and a more complete understanding of the consequences of VZV latency in the enteric nervous system.

Varicella zoster vaccines and their implications for development of HSV vaccines

Live attenuated vaccines to prevent varicella and zoster have been available in the US for the past 17 years, with a resultant dramatic decrease in varicella incidence and a predicted future decrease in the incidence of zoster. The pathogenesis and immune responses to varicella zoster virus (VZV) as well as the safety and effectiveness of VZV vaccines are reviewed. The lack of sterilizing immunity provided by VZV vaccines has not prevented them from being safe and effective. Virological and pathological information concerning parallels and differences between VZV and herpes simplex virus (HSV) are highlighted. Although VZV and HSV are distinct pathogens, they appear to have similarities in target organs and immunity that provide an expectation of a high likelihood for the success of vaccination against HSV, and predicted to be similar to that of VZV.

Varicella zoster virus (VZV) infects and establishes latency in enteric neurons

Case reports have linked varicella-zoster virus (VZV) to gastrointestinal disorders, including severe abdominal pain preceding fatal varicella and acute colonic pseudoobstruction (Ogilvie's syndrome). Because we had previously detected DNA and transcripts encoding latency-associated VZV gene products in the human gut, we sought to determine whether latent VZV is present in the human enteric nervous system (ENS) and, if so, to identify the cells in which it is located and its route to the bowel. Neither DNA, nor transcripts encoding VZV gene products, could be detected in resected gut from any of seven control children (<1 year old) who had not received the varicella vaccine or experienced varicella; however, VZV DNA and transcripts were each found to be present in resected bowel from 6/6 of children with a past history of varicella and in that of 6/7 of children who received the varicella vaccine. Both wild-type (WT) and vaccine-type (vOka) VZV thus establish latent infection in human gut. To determine routes by which VZV might gain access to the bowel, we injected guinea pigs with human or guinea pig lymphocytes expressing green fluorescent protein (GFP) under the control of the VZV ORF66 gene (VZV(OKA66.GFP)). GFP-expressing enteric neurons were found throughout the bowel within 2 days and continued to be present for greater than 6 weeks. DNA encoding VZV gene products also appeared in enteric and dorsal root ganglion (DRG) neurons following intradermal administration of WT-VZV and in enteric neurons after intradermal injection of VZV(OKA66.GFP); moreover, a small number of guinea pig DRG neurons were found to project both to the skin and the intraperitoneal viscera. Viremia, in which lymphocytes carry VZV, or axonal transport from DRG neurons infected through their epidermal projections are thus each potential routes that enable VZV to gain access to the ENS.

VZV infection of keratinocytes: production of cell-free infectious virions in vivo

Varicella-zoster virus (VZV) is the cause of varicella (chickenpox) and zoster (shingles). Varicella is a primary infection that spreads rapidly in epidemics while zoster is a secondary infection that occurs sporadically as a result of the reactivation of previously acquired VZV. Reactivation is made possible by the establishment of latency during the initial episode of varicella. The signature lesions of both varicella and zoster are cutaneous vesicles, which are filled with a clear fluid that is rich in infectious viral particles. It has been postulated that the skin is the critical organ in which both host-to-host transmission of VZV and the infection of neurons to establish latency occur. This hypothesis is built on evidence that the large cation-independent mannose 6-phosphate receptor (MPR(ci)) interacts with VZV in virtually all infected cells, except those of the suprabasal epidermis, in a way that prevents the release of infectious viral particles. Specifically, the virus is diverted in an MPR(ci)-dependent manner from the secretory pathway to late endosomes where VZV is degraded. Because nonepidermal cells are thus prevented from releasing infectious VZV, a slow process, possibly involving fusion of infected cells with their neighbors, becomes the means by which VZV is disseminated. In the epidermis, however, the maturation of keratinocytes to give rise to corneocytes in the suprabasal epidermis is associated uniquely with a downregulation of the MPR(ci). As a result, the diversion of VZV to late endosomes does not occur in the suprabasal epidermis where vesicular lesions occur. The formation of the waterproof, chemically resistant barrier of the epidermis, however, requires that constitutive secretion outlast the downregulation of the endosomal pathway. Infectious VZV is therefore secreted by default, accounting for the presence of infectious virions in vesicular fluid. Sloughing of corneocytes, aided by scratching, then aerosolizes the virus, which can float with dust to be inhaled by susceptible hosts. Infectious virions also bathe the terminals of those sensory neurons that innervate the epidermis. These terminals become infected with VZV and provide a route, retrograde transport, which can conduct VZV to cranial nerve (CNG), dorsal root ganglia (DRG), and enteric ganglia (EG) to establish latency. Reactivation returns VZV to the skin, now via anterograde transport in axons, to cause the lesions of zoster. Evidence in support of these hypotheses includes observations of the VZV-infected human epidermis and studies of guinea pig neurons in an in vitro model system.

Myelin-associated glycoprotein mediates membrane fusion and entry of neurotropic herpesviruses

Varicella-zoster virus (VZV) and herpes simplex virus (HSV) are prevalent neurotropic herpesviruses that cause various nervous system diseases. Similar to other enveloped viruses, membrane fusion is an essential process for viral entry. Therefore, identification of host molecules that mediate membrane fusion is important to understand the mechanism of viral infection. Here, we demonstrate that myelin-associated glycoprotein (MAG), mainly distributed in neural tissues, associates with VZV glycoprotein B (gB) and promotes cell-cell fusion when coexpressed with VZV gB and gH/gL. VZV preferentially infected MAG-transfected oligodendroglial cells. MAG also associated with HSV-1 gB and enhanced HSV-1 infection of promyelocytes. These findings suggested that MAG is involved in VZV and HSV infection of neural tissues.

Human cytomegalovirus final envelopment on membranes containing both trans-Golgi network and endosomal markers

The human cytomegalovirus (HCMV) has been shown to complete its final envelopment on cytoplasmic membranes prior to its secretion to the extracellular medium. However, the nature of these membranes has not been characterized. It is thought that HCMV acquires its final envelope from the trans-Golgi network (TGN), though we and others have previously reported a role for endocytic membranes. Here we studied the localization of cellular markers in HCMV-infected cells and in isolated viruses. Immunofluorescence staining indicated that HCMV induces the recruitment of TGN and endosomal markers to the virus factory. Immuno-gold labelling of isolated viral particles and electron microscopy demonstrated the incorporation of TGN46, endosomal markers early endosomal antigen 1, annexin I, transferrin receptor and CD63, and the cation-independent mannose 6-phosphate receptor, which traffics between the TGN and endosomes into the viral envelope. Virus immunoprecipitation assays demonstrated that virions containing TGN46 and CD63 were infectious. This study reconciles the apparent controversy regarding the nature of the HCMV assembly site and suggests that HCMV has the ability to generate a novel membrane compartment containing markers for both TGN and endosomes, or that the membranes that HCMV uses for its envelope may be vesicles in transit between the TGN and endosomes.

Strategies for carbohydrate recognition by the mannose 6-phosphate receptors

The two members of the P-type lectin family, the 46 kDa cation-dependent mannose 6-phosphate receptor (CD-MPR) and the 300 kDa cation-independent mannose 6-phosphate receptor (CI-MPR), are ubiquitously expressed throughout the animal kingdom and are distinguished from all other lectins by their ability to recognize phosphorylated mannose residues. The best-characterized function of the MPRs is their ability to direct the delivery of approximately 60 different newly synthesized soluble lysosomal enzymes bearing mannose 6-phosphate (Man-6-P) on their N-linked oligosaccharides to the lysosome. In addition to its intracellular role in lysosome biogenesis, the CI-MPR, but not the CD-MPR, participates in a number of other biological processes by interacting with various molecules at the cell surface. The list of extracellular ligands recognized by this multifunctional receptor has grown to include a diverse spectrum of Man-6-P-containing proteins as well as several non-Man-6-P-containing ligands. Recent structural studies have given us a clearer view of how these two receptors use related, but yet distinct, approaches in the recognition of phosphomannosyl residues.

In vitro replication of varicella-zoster virus in human retinal pigment epithelial cells

Here we describe for the first time the productive in vitro infection of human retinal pigment epithelial cells by varicella-zoster virus (VZV), resulting in a typical cytopathic effect (CPE) that is characterized by enlarged cells with increased granularity. Depending on the CPE dissemination, high titers of up to 1.6 x 10(6) PFU of cell-free and cryostable VZV/ml can be recovered.

Deletion in open reading frame 49 of varicella-zoster virus reduces virus growth in human malignant melanoma cells but not in human embryonic fibroblasts

The ORF49 gene product (ORF49p) of the varicella-zoster virus (VZV) is likely a myristylated tegument protein, and its homologs are conserved across the herpesvirus subfamilies. The UL11 gene of herpes simplex virus type 1 and of pseudorabies virus and the UL99 gene of human cytomegalovirus are the homologs of ORF49 and have been well characterized by using mutant viruses; however, little research on the VZV ORF49 gene has been reported. Here we report on VZV ORF49p expression, subcellular localization, and effect on viral spread in vitro. ORF49p was expressed during the late phase of infection and located in the juxtanuclear region of the cytoplasm, where it colocalized mainly with the trans-Golgi network-associated protein. ORF49p was incorporated into virions and showed a molecular mass of 13 kDa in VZV-infected cells and virions. To elucidate the role of the ORF49 gene, we constructed a mutant virus that lacked a functional ORF49. No differences in plaque size or cell-cell spread were observed in human embryonic fibroblast cells, MRC-5 cells, infected with the wild-type or the mutant virus. However, the mutant virus showed diminished cell-cell infection in a human malignant melanoma cell line, MeWo cells. Therefore, VZV ORF49p is important for virus growth in MeWo cells, but not in MRC-5 cells. VZV may use different mechanisms for virus growth in MeWo and MRC-5 cells. If so, understanding the role of ORF49p should help elucidate how VZV accomplishes cell-cell infections in different cell types.

The amino terminus of varicella-zoster virus (VZV) glycoprotein E is required for binding to insulin-degrading enzyme, a VZV receptor

Varicella-zoster virus (VZV) glycoprotein E (gE) is required for VZV infection. Although gE is well conserved among alphaherpesviruses, the amino terminus of VZV gE is unique. Previously, we showed that gE interacts with insulin-degrading enzyme (IDE) and facilitates VZV infection and cell-to-cell spread of the virus. Here we define the region of VZV gE required to bind IDE. Deletion of amino acids 32 to 71 of gE, located immediately after the predicted signal peptide, resulted in loss of the ability of gE to bind IDE. A synthetic peptide corresponding to amino acids 24 to 50 of gE blocked its interaction with IDE in a concentration-dependent manner. However, a chimeric gE in which amino acids 1 to 71 of VZV gE were fused to amino acids 30 to 545 of herpes simplex virus type 2 gE did not show an increased level of binding to IDE compared with that of full-length HSV gE. Thus, amino acids 24 to 71 of gE are required for IDE binding, and the secondary structure of gE is critical for the interaction. VZV gE also forms a heterodimer with glycoprotein gI. Deletion of amino acids 163 to 208 of gE severely reduced its ability to form a complex with gI. The amino portion of IDE, as well an IDE mutant in the catalytic domain of the protein, bound to gE. Therefore, distinct motifs of VZV gE are important for binding to IDE or to gI.

Cholesterol dependence of varicella-zoster virion entry into target cells

The entry of inhaled virions into airway cells is presumably the initiating step of varicella-zoster infection. In order to characterize viral entry, we studied the relative roles played by lipid rafts and clathrin-mediated transport. Virus and target cells were pretreated with agents designed to perturb selected aspects of endocytosis and membrane composition, and the effects of these perturbations on infectious focus formation were monitored. Infectivity was exquisitely sensitive to methyl-beta-cyclodextrin (M beta CD) and nystatin, which disrupt lipid rafts by removing cholesterol. These agents inhibited infection by enveloped, but not cell-associated, varicella-zoster virus (VZV) in a dose-dependent manner and exerted these effects on both target cell and viral membranes. Inhibition by M beta CD, which could be reversed by cholesterol replenishment, rapidly declined as a function of time after exposure of target cells to VZV, suggesting that an early step in viral infection requires cholesterol. No effect of cholesterol depletion, however, was seen on viral binding; moreover, there was no reduction in the surface expression or internalization of mannose 6-phosphate receptors, which are required for VZV entry. Viral entry was energy dependent and showed concentration-dependent inhibition by chlorpromazine, which, among other actions, blocks clathrin-mediated endocytosis. These data suggest that both membrane lipid composition and clathrin-mediated transport are critical for VZV entry. Lipid rafts are likely to contribute directly to viral envelope integrity and, in the host membrane, may influence endocytosis, evoke downstream signaling, and/or facilitate membrane fusion.

Antiviral activity obtained from aqueous extracts of the Chilean soapbark tree (Quillaja saponaria Molina)

Natural, aqueous extracts of Quillaja saponaria, the Chilean soapbark tree, contain several physiologically active triterpenoid saponins that display strong adjuvant activity when used in either human or animal vaccines. In this paper, we describe studies that demonstrate a novel antiviral activity of Quillaja extracts against six viruses: vaccinia virus, herpes simplex virus type 1, varicella zoster virus, human immunodeficiency viruses 1 and 2 (HIV-1, HIV-2) and reovirus. We demonstrate that microgram amounts of extract, while exhibiting no cell cytotoxicity or direct virucidal activity, prevent each of the six viruses tested from infecting their host cells. In addition, the presence of residual amounts of extract continue to block virus infection and render cells resistant to infection for at least 16 h after the removal of the extract from the cell culture medium. We demonstrate that a Quillaja extract possesses strong antiviral activity at concentrations more than 100-fold lower than concentrations that exhibit cell cytotoxicity. Extract concentrations as high as 100 microg ml(-1) are not cytotoxic, but concentrations as low as 0.1 microg ml(-1) are able to block HIV-1 and HIV-2 virus attachment and infection.

Insulin degrading enzyme is a cellular receptor mediating varicella-zoster virus infection and cell-to-cell spread

Varicella-zoster virus (VZV) causes chickenpox and shingles. While varicella is likely spread as cell-free virus to susceptible hosts, the virus is transmitted by cell-to-cell spread in the body and in vitro. Since VZV glycoprotein E (gE) is essential for virus infection, we postulated that gE binds to a cellular receptor. We found that insulin-degrading enzyme (IDE) interacts with gE through its extracellular domain. Downregulation of IDE by siRNA, or blocking of IDE with antibody, with soluble IDE protein extracted from liver, or with bacitracin inhibited VZV infection. Cell-to-cell spread of virus was also impaired by blocking IDE. Transfection of cell lines impaired for VZV infection with a plasmid expressing human IDE resulted in increased entry and enhanced infection with cell-free and cell-associated virus. These studies indicate that IDE is a cellular receptor for both cell-free and cell-associated VZV.

Postentry events are responsible for restriction of productive varicella-zoster virus infection in Chinese hamster ovary cells

Productive infection of varicella-zoster virus (VZV) in vitro is restricted almost exclusively to cells derived from humans and other primates. We demonstrate that the restriction of productive VZV infection in CHO-K1 cells occurs downstream of virus entry. Entry of VZV into CHO-K1 cells was characterized by utilizing an ICP4/beta-galactosidase reporter gene that has been used previously to study herpes simplex virus type 1 entry. Entry of VZV into CHO-K1 cells involved cell surface interactions with heparan sulfate glycosaminoglycans and a cation-independent mannose-6-phosphate receptor. Lysosomotropic agents inhibited the entry of VZV into CHO-K1 cells, consistent with a low-pH-dependent endocytic mechanism of entry. Infection of CHO-K1 cells by VZV resulted in the production of both immediate early and late gene products, indicating that a block to progeny virus production occurs after the initiation of virus gene expression.

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.

Chlamydia pneumoniae uses the mannose 6-phosphate/insulin-like growth factor 2 receptor for infection of endothelial cells

Several mechanisms for attachment and entry of Chlamydia have been proposed. We previously determined that the major outer membrane protein of Chlamydia trachomatis is glycosylated with a high-mannose oligosaccharide, and a similar structure inhibited the attachment and infectivity of C. trachomatis in epithelial cells. Because insulin-like growth factor 2 (IGF2) was shown to enhance the infectivity of Chlamydia pneumoniae but not C. trachomatis in endothelial cells, a hapten inhibition assay was used to analyze whether the mannose 6-phosphate (M6P)/IGF2 receptor that also binds M6P could be involved in infection of endothelial cells (HMEC-1) by Chlamydia. M6P and mannose 6-phosphate-poly[N-(2-hydroxyethyl)-acrylamide] (M6P-PAA) inhibited the infectivity of C. pneumoniae AR-39, but not C. trachomatis serovar UW5 or L2, while mannan inhibited the growth of C. trachomatis, but not C. pneumoniae. Using metabolically labeled organisms incubated with cells at 4 degrees C (organisms attach but do not enter) or at 37 degrees C (organisms attach and are internalized), M6P-PAA was shown to inhibit attachment and internalization of C. pneumoniae in endothelial cells but did not inhibit attachment or internalization of C. trachomatis serovar E or L2. These findings indicate that C. pneumoniae can utilize the M6P/IGF2 receptor and that the use of this receptor for attachment and entry differs between C. pneumoniae and C. trachomatis.

Chickenpox

PURPOSE OF REVIEW

Varicella-zoster virus (VZV) remains a public health issue around the globe despite the availability of a live attenuated vaccine and several highly active antiviral agents. A program of universal infant vaccination against varicella was introduced in the US almost 10 years ago. Epidemiological data continue to accumulate that will inform decision-making on vaccine use elsewhere. These findings, together with relevant advances in VZV virology, form the substance of this review.

RECENT FINDINGS

Understanding of the pathogenesis of varicella has significantly advanced with the demonstration that the cation-independent mannose 6-phosphate receptor is critical to both entry and egress of enveloped VZV. While our knowledge of intervening events remains sketchy, the future study of VZV will be facilitated by the recent successful cloning of the VZV genome into a bacterial artificial chromosome. Models of latency and reactivation are also being developed, which may help us to understand the epidemiology of herpes zoster in vaccinated populations. Continued evidence of decline in the incidence of varicella, associated hospitalizations and deaths suggests that the vaccine as used in the US is highly effective. However, rates of breakthrough disease are significant and sufficient to sustain outbreaks, even among highly vaccinated populations. This is so despite the generally reduced infectiousness of varicella occurring in vaccinated individuals. There is some evidence of attrition of the immune response over time following immunization in a small proportion of vaccinees.

SUMMARY

Our ability to prevent and treat varicella still outstrips our knowledge of pathogenetic and immune mechanisms. Further clinical advances are likely to arise from growing understanding of VZV biology.

Mannose 6-phosphate receptor dependence of varicella zoster virus infection in vitro and in the epidermis during varicella and zoster

Varicella zoster virus (VZV) is a highly infectious human pathogen; nevertheless, infectious virions are not released in vitro where infection is cell associated. Four VZV envelope glycoproteins contain mannose 6-phosphate (Man 6-P), and Man 6-P blocks infection of cells by cell-free VZV. Expression of antisense cDNA or siRNA-like transcripts were used to generate five stable human cell lines deficient in cation-independent mannose 6-phosphate receptors (MPRci). All 5 MPRci-deficient lines resisted infection by cell-free, but not cell-associated, VZV, secreted lysosomal enzymes, and released infectious virions when infected by cell-associated VZV. Intracellular MPRci thus appear to divert newly enveloped VZV to late endosomes, and plasmalemmal MPRci are necessary for entry by cell-free VZV. Biopsies from VZV-infected human skin supported the idea that because MPRci expression is naturally lost in maturing superficial epidermal cells, these cells do not divert VZV to endosomes and constitutively secrete infectious VZV.

Localization of the carbohydrate recognition sites of the insulin-like growth factor II/mannose 6-phosphate receptor to domains 3 and 9 of the extracytoplasmic region

The insulin-like growth factor II/mannose 6-phosphate receptor is a multifunctional receptor that binds to a diverse array of mannose 6-phosphate (Man-6-P) modified proteins as well as nonglycosylated ligands. Previous studies have mapped its two Man-6-P binding sites to a minimum of three domains, 1-3 and 7-9, within its 15-domain extracytoplasmic region. Since the primary amino acid determinants of carbohydrate recognition by the insulin-like growth factor II/mannose 6-phosphate receptor are predicted by sequence alignment to the cation-dependent mannose 6-phosphate receptor to reside within domains 3 and 9, constructs encoding either domain 3 alone or domain 9 alone were expressed in a Pichia pastoris expression system and tested for their ability to bind several carbohydrate ligands, including Man-6-P, pentamannosyl phosphate, the lysosomal enzyme, beta-glucuronidase, and the carbohydrate modifications (mannose 6-sulfate and Man-6-P methyl ester) found on Dictyostelium discoideum lysosomal enzymes. Although both constructs were functional in ligand binding and dissociation, these studies demonstrate the ability of domain 9 alone to fold into a high affinity (K(d) = 0.3 +/- 0.1 nm) carbohydrate-recognition domain whereas the domain 3 alone construct is capable of only low affinity binding (K(d) approximately 500 nm) toward beta-glucuronidase, suggesting that residues in adjacent domains (domains 1 and/or 2) are important, either directly or indirectly, for optimal binding by domain 3.

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.

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.

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.

Putative dengue virus receptors from mosquito cells

Dengue viruses are arthropod-borne, single-stranded RNA viruses. Aëdes aegypti and Aëdes albopictus are the principal vectors. In order to understand the molecular basis of dengue virus infections we explored the biochemical identity of dengue-2 (DEN-2) virus receptors in the Aëdes albopictus-derived cell line C6/36. We show here that DEN-2 interacts with two major polypeptides of 80 and 67 kDa. Polyclonal anti-C6/36 membrane antibodies block DEN-2 binding to intact C6/36 monolayers as well as to membrane extracts. Our results strongly suggest that the identified polypeptides are part of the DEN-2 receptors.

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

The varicella-zoster virus (VZV) is the etiological agent of two different human pathologies, chickenpox (varicella) and shingles (zoster). This alphaherpesvirus is believed to acquire its lipidic envelope in the trans-Golgi network (TGN). This is consistent with previous data showing that the most abundant VZV envelope glycoprotein gE accumulates at steady-state in this organelle when expressed from cloned cDNA. In the present study, we have investigated the intracellular trafficking of gI, another VZV envelope glycoprotein. In transfected cells, this protein shows a very slow biosynthetic transport to the cell surface where it accumulates. However, upon co-expression of gE, gI experiences a dramatic increase in its exit rate from the endoplasmic reticulum, it accumulates in a sialyltransferase-positive compartment, presumably the TGN, and cycles between this compartment and the cell surface. This differential behavior results from the ability of gE and gI to form a complex in the early stages of the biosynthetic pathway whose intracellular traffic is exclusively determined by the sorting information in the tail of gE. Thus, gI provides the first example of a molecule localized to the TGN by means of its association with another TGN protein. We also show that, during the early stages of VZV infection, both proteins are also found in the TGN of the host cell. This suggests the existence of an intermediate stage during VZV biogenesis in which the envelope glycoproteins, transiently arrested in the TGN, could promote the envelopment of newly synthesized nucleocapsids into this compartment and, therefore, the assembly of infective viruses.

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.

Varicella-zoster virus Fc receptor gE glycoprotein: serine/threonine and tyrosine phosphorylation of monomeric and dimeric forms

Varicella-zoster virus (VZV) glycoprotein gE is the predominant viral cell surface molecule; it behaves as an Fc receptor for immunoglobulin G, but its central function may be more closely related to viral egress and cell-to-cell spread. To further analyze the receptor properties of VZV gE, the gE gene (also called open reading frame 68) was expressed by a baculovirus vector in insect cells. The recombinant baculovirus gE product had a molecular mass of 64 kDa, smaller than the previously documented 98 kDa of mature gE expressed in mammalian cells. The major reason for the lowered molecular mass was diminished glycosylation. In addition to the 64-kDa form, a larger (130-kDa) form was observed in insect cells and represented dimerized 64-kDa molecules. Both the monomeric and dimeric gE forms were highly phosphorylated in insect cells. Protein kinase assays conducted in vitro with [gamma-32P]ATP and [gamma-32P]GTP indicated that endogenous casein kinase II was phosphorylating monomeric gE, while the dimeric gE form was phosphorylated by another kinase which did not utilize [gamma-32P]GTP. When immobilized recombinant gE molecules were probed with a monoclonal antibody which specifically recognizes a phosphotyrosine linkage, the gE dimer was found to be tyrosine phosphorylated whereas the monomer was not similarly modified. When recombinant gE produced in HeLa cells was probed with the same antiphosphotyrosine antibody, a dimeric gE form at 130 kDa was detected on the cell surface. These results suggested that VZV gE closely resembled other cell surface receptors, being modified on its various forms by both serine/threonine and tyrosine protein kinases. In this case, tyrosine phosphorylation occurred on a previously unrecognized and underglycosylated VZV gE dimeric product.

A tyrosine-based motif and a casein kinase II phosphorylation site regulate the intracellular trafficking of the varicella-zoster virus glycoprotein I, a protein localized in the trans-Golgi network

We have studied the intracellular trafficking of the envelope glycoprotein I (gpI) of the varicella-zoster virus, a human herpes virus whose assembly is believed to occur in the trans-Golgi network (TGN) and/or in endocytic compartments. When expressed in HeLa cells in the absence of additional virally encoded factors, this type-I membrane protein localizes to the TGN and cycles between this compartment and the cell surface. The expression of gpI promotes the recruitment of the AP-1 Golgi-specific assembly proteins onto TGN membranes, strongly suggesting that gpI, like the mannose 6-phosphate receptors, can leave the TGN in clathrin-coated vesicles for subsequent transport to endosomes. Its return from the cell surface to the TGN also occurs through endosomes. The transfer of the gpI cytoplasmic domain onto a reporter molecule shows that this domain is sufficient to confer TGN localization. Mutational analysis of this domain indicates that proper subcellular localization and cycling of gpI depend on two different determinants, a tyrosine-containing tetrapeptide related to endocytosis sorting signals and a cluster of acidic amino acids containing casein kinase II phosphorylatable residues. Thus, the VZV gpI and the mannose 6-phosphate receptors, albeit localized in different intracellular compartments at steady-state, follow similar trafficking pathways and share similar sorting mechanisms.

Live attenuated varicella vaccine

Varicella-zoster virus (VZV) is a ubiquitous human pathogen that causes varicella, commonly called chicken pox; establishes latency; and reactivates as herpes zoster, referred to as shingles. A live attenuated varicella vaccine, derived from the Oka strain of VZV has clinical efficacy for the prevention of varicella. The vaccine induces persistent immunity to VZV in healthy children and adults. Immunization against VZV also has the potential to lower the risk of reactivation of latent virus. The varicella vaccine may eventually reduce or eliminate herpes zoster, which is a serious problem for elderly and immunocompromised individuals.

Retrieval of human cytomegalovirus glycoprotein B from the infected cell surface for virus envelopment

Surface biotinylation of human cytomegalovirus (HCMV)-infected fibroblasts under pulse-chase conditions was used to define the cellular route of the dominant viral envelope glycoprotein gB into the cytoplasmic compartment of viral maturational envelopment. The results showed that a major fraction of gB was re-internalized from the infected cell surface prior to incorporation into the viral envelope. Viral particles carrying biotinylated gB were subsequently released into the culture medium. Viral release appeared to be inhibited in the presence of gB-specific antibody or when infected cultures were incubated at room temperature, but was not reduced by inhibitors of cellular glycoprotein transport. To our knowledge this is the first report describing that HCMV gB is retrieved from the infected cell surface prior to viral envelopment.

Identification of bovine viral diarrhea virus receptor in different cell types

Anti-idiotypic antibodies (anti-ids) have been used successfully in studies on bovine viral diarrhea virus (BVDV) receptor(s) in our laboratory. The anti-ids specifically bound to cultured cells and identified a 50 kDa cellular membrane protein, which is thought to be a specific receptor for BVDV. In this study, flow cytometric analyses demonstrated that the anti-ids also specifically bound to different cell types, namely MDBK, EBK, BT, PK15, MA1O4, and Vero. Experiments on virus attachment and replication showed that BVDV adsorbed to all cells and replicated in them except monkey kidney cells MA 104 and Vero (non-permissive). Results from plaque reduction assays indicated that cellular membrane proteins from all cell lines competitively inhibited BVDV attachment to cultured MDBK cells, suggesting the presence of BVDV receptor on all cells. Immunoblotting of cell membrane proteins with the anti-ids revealed a 50 kDa protein in both permissive and nonpermissive cells. Subcloned or synchronized MDBK cells demonstrated no significant difference of binding with anti-ids as compared to normal cultured cells.

Varicella-zoster virus: aspects of pathogenesis and host response to natural infection and varicella vaccine

Events in the pathogenesis of infection and the host response to VZV are very closely linked. Our experiments demonstrate that CD4- and CD8+ T-lymphocyte populations that are targets of cell-associated VZV viremia also mediate protection against severe infection. Diminished cell-mediated immunity predisposes the host to progressive primary or recurrent VZV disease because infected lymphocytes persist in the circulation and carry the virus to major organs, causing pneumonitis, hepatitis, or other life-threatening complications. The live attenuated varicella vaccine induces cell-mediated immunity and protects against or significantly reduces the morbidity associated with primary VZV infections. The universal administration of varicella vaccine is likely to generate new insights about host-virus interactions, particularly in relation to how VZV immunity is maintained, that will be relevant to the design of vaccines for other human herpesviruses.

Envelopment of varicella-zoster virus: targeting of viral glycoproteins to the trans-Golgi network

Previous studies suggested that varicella-zoster virus derives its final envelope from the trans-Golgi network (TGN) and that envelope glycoproteins (gps) are transported to the TGN independently of nucleocapsids. We tested the hypothesis that gpI is targeted to the TGN as a result of a signal sequence or patch encoded in its cytosolic domain. cDNAs encoding gpI wild type (wt) and a truncated mutant gpI(trc) lacking transmembrane and cytosolic domains were cloned by using the PCR. Cells transfected with cDNA encoding gpI(wt) or gpI(trc) synthesized and N glycosylated the proteins. gpI(wt) accumulated in the TGN, some reached the plasmalemma, but none was secreted. In contrast, gpI(trc) was retained and probably degraded in the endoplasmic reticulum; none was found on cell surfaces, but some was secreted. The distribution of gpI(trc) was not affected by deletion of potential glycosylation sites. To locate a potential gpI-targeting sequence, cells were transfected with cDNA encoding chimeric proteins in which the ectodomain of a plasmalemmal marker, the interleukin-2 receptor (tac), was fused to different domains of gpI. A chimeric protein in which tac was fused with the transmembrane and cytoplasmic domains of gpI was targeted to the TGN. In contrast, a chimeric protein in which tac was fused only with the gpI transmembrane domain passed through the TGN and concentrated in endosomes. We conclude that gpI is targeted to the TGN as a result of a targeting sequence or patch in its cytosolic domain.

Role of mannose-6-phosphate receptors in herpes simplex virus entry into cells and cell-to-cell transmission

Herpes simplex virus (HSV) glycoprotein D (gD) is essential for virus entry into cells, is modified with mannose-6-phosphate (M-6-P), and binds to both the 275-kDa M-6-P receptor (MPR) and the 46-kDa MPR (C. R. Brunetti, R. L. Burke, S. Kornfeld, W. Gregory, K. S. Dingwell, F. Masiarz, and D. C. Johnson, J. Biol. Chem. 269:17067-17074, 1994). Since MPRs are found on the surfaces of mammalian cells, we tested the hypothesis that MPRs could serve as receptors for HSV during virus entry into cells. A soluble form of the 275-kDa MPR, derived from fetal bovine serum, inhibited HSV plaques on monkey Vero cells, as did polyclonal rabbit anti-MPR antibodies. In addition, the number and size of HSV plaques were reduced when cells were treated with bovine serum albumin conjugated with pentamannose-phosphate (PM-PO4-BSA), a bulky ligand which can serve as a high-affinity ligand for MPRs. These data imply that HSV can use MPRs to enter cells; however, other molecules must also serve as receptors for HSV because a reasonable fraction of virus could enter cells treated with even the highest concentrations of these inhibitors. Consistent with the possibility that there are other receptors, HSV produced the same number of plaques on MPR-deficient mouse fibroblasts as were produced on normal mouse fibroblasts, but there was no inhibition with PM-PO4-BSA with either of these embryonic mouse cells. Together, these results demonstrate that HSV does not rely solely on MPRs to enter cells, although MPRs apparently play some role in virus entry into some cell types and, perhaps, act as one of a number of cell surface molecules that can facilitate entry. We also found that HSV produced small plaques on human fibroblasts derived from patients with pseudo-Hurler's polydystrophy, cells in which glycoproteins are not modified with M-6-P residues and yet production of infectious HSV particles was not altered in the pseudo-Hurler cells. In addition, HSV plaque size was reduced by PM-PO4-BSA; therefore, it appears that M-6-P residues and MPRs are required for efficient transmission of HSV between cells, a process which differs in some respects from entry of exogenous virus particles.

Infection of cells by varicella zoster virus: inhibition of viral entry by mannose 6-phosphate and heparin

Envelope glycoproteins of varicella zoster virus (VZV) contain mannose 6-phosphate (Man6P) residues. We now report that Man6P competitively and selectively inhibits infection of cells in vitro by cell-free VZV; furthermore, dephosphorylation of VZV by exposure to alkaline phosphatase rapidly destroys infectivity. Cells are also protected from VZV in a concentration-dependent manner by heparin (ED50 = 0.23 micrograms/ml; 95% confidence limits = 0.16-0.26 microgram/ml) but not by chondroitin sulfate. Both heparin and Man6P are protective only when present about the time of inoculation. Heparin but not Man6P interferes with the attachment of VZV to cell surfaces; moreover, VZV binds to heparin-affinity columns. These data are compatible with a working hypothesis, whereby VZV attaches to cell surfaces by binding to a heparin sulfate proteoglycan. This binding stabilizes VZV, making possible a low-affinity interaction with another Man6P-dependent receptor, which is necessary for viral entry.

Intracellular transport of newly synthesized varicella-zoster virus: final envelopment in the trans-Golgi network

The maturation and envelopment of varicella-zoster virus (VZV) was studied in infected human embryonic lung fibroblasts. Transmission electron microscopy confirmed that nucleocapsids acquire an envelope from the inner nuclear membrane as they enter the perinuclear-cisterna-rough endoplasmic reticulum (RER). Tegument is not detectable in these virions; moreover, in contrast to the mature VZV envelope, the envelope of VZV in the RER is not radioautographically labeled in pulse-chase experiments with [3H]mannose, and it lacks gpI immunoreactivity and complex oligosaccharides. This primary envelope fuses with the RER membrane (detected in cells incubated at 20 degrees C), thereby releasing nucleocapsids to the cytosol. Viral glycoproteins, traced by transmission electron microscopy radioautography in pulse-chase experiments with [3H]mannose, are transported to the trans-Golgi network (TGN) by a pathway that runs from the RER through an intermediate compartment and the Golgi stack. At later chase intervals, [3H]mannose labeling becomes associated with enveloped virions in post-Golgi locations (prelysosomes and plasma membrane). Nucleocapsids appear to be enveloped by wrapping in specialized cisternae, identified as the TGN with specific markers. Tegument-like material adheres to the cytosolic face of the concave surface of TGN sacs; nucleocapsids adhere to this protein, which is thus trapped between the nucleocapsid and the TGN-derived membrane that wraps around it. Experiments with brefeldin A suggest that tegument may bind to the cytosolic tails of viral glycoproteins. Fusion and fission convert the TGN-derived wrapping sacs into an inner enveloped virion and an outer transport vesicle that carries newly enveloped virions to cytoplasmic vacuoles. These vacuoles are acidic and were identified as prelysosomes. It is postulated that secreted virions are partially degraded by their exposure to the prelysosomal internal milieu and rendered noninfectious. This process explains the cell-associated nature of VZV in vitro; however, the mechanism by which the virus escapes diversion from the secretory pathway to the lysosomal pathway in vivo remains to be determined.