Glycoproteins E and I facilitate neuron-to-neuron spread of herpes simplex virus

Varicella-zoster virus glycoproteins E and I expressed in insect cells form a heterodimer that requires the N-terminal domain of glycoprotein I

Varicella-zoster virus (VZV) glycoproteins E and I (gE and gI), which are major components of the virion envelope, form a noncovalently linked complex. To understand their properties and functions, we expressed and purified soluble forms of gE and gI in the baculovirus system. Extracellular domains of gE and gI were cloned into baculoviruses, using either native or insect-derived signal peptides. Each recombinant virus yielded soluble protein in culture medium although a higher level of secretion was achieved with insect-derived signal peptides in recombinant gE baculoviruses. A soluble gE-gI complex was formed by co-infecting insect cells with recombinant gE and gI baculoviruses and detected by immunoprecipitation followed by Western blotting analyses. By gel filtration and cross-linking studies, we showed that the VZV gE-gI complex expressed in insect cells is a heterodimer. Interestingly, two recombinant gI proteins in which signal peptides were replaced with insect-derived signal peptides did not associate with gE. Amino-terminal sequencing and site-specific mutational studies showed that the replacement of only the signal peptides did not prevent complex formation but alterations in the processed amino-terminus of gI abrogated its ability to complex with gE. These findings indicate that the mature amino-terminus of gI is required for gE-gI complex formation by the external domains of VZV gE and gI.

The herpes simplex virus-1 glycoprotein E (gE) mediates IgG binding and cell-to-cell spread through distinct gE domains

Herpes simplex virus-1 (HSV-1) glycoprotein E (gE) is a multifunctional protein capable of both binding the Fc portion of IgG and mediating cell-to-cell spread of HSV-1. Here we report that the domain on gE involved in IgG binding is distinct from the domain involved in mediating cell-to-cell spread. To do this we have used five mutants of the HSV-1 strain NS: NS-gE(null), a gE deletion virus; rNS-gE(null), a gE rescued virus; NS-gE339, a gE mutant virus with a four amino acid insert at position 339; rNS-gE339, a gE rescue of NS-gE339; and NS-gE406, a gE mutant virus with the same four amino acids inserted at position 406. Using IgG coated sheep red blood cells in rosetting assays, we show that the NS-gE339 does not bind IgG, yet retains the ability to mediate normal cell-to-cell spread. These results demonstrate that the gE domain involved in IgG binding differs from the domain involved in cell-to-cell spread.

Endocytosis and recycling of varicella-zoster virus Fc receptor glycoprotein gE: internalization mediated by a YXXL motif in the cytoplasmic tail

Varicella-zoster virus (VZV) encodes a cell surface Fc receptor, glycoprotein gE. VZV gE has previously been shown to display several features common to nonviral cell surface receptors. Most recently, VZV gE was reported to be tyrosine phosphorylated on a dimeric form (J. K. Olson, G. A. Bishop, and C. Grose, J. Virol. 71:110-119, 1997). Thereafter, attention focused on the ability of VZV gE to undergo receptor-mediated endocytosis. The current transient transfection studies demonstrated by confocal microscopy and internalization assays that VZV gE was endocytosed when expressed in HeLa cells. Endocytosis of gE was shown to be dependent on clathrin-coated vesicle formation within the cells. Subsequent colocalization studies showed that endocytosis of VZV gE closely mimicked endocytosis of the transferrin receptor. The gE cytoplasmic tail and more specifically tyrosine residue 582 were determined by mutagenesis studies to be important for efficient internalization of the protein; this tyrosine residue is part of a conserved YXXL motif. The amount of gE internalized at any given time reached a steady state of 32%. In addition, like the transferrin receptor, internalized gE recycled to the cell surface. The finding of gE endocytosis provided insight into earlier documentation of gE serine/threonine and tyrosine phosphorylation, since these phosphorylation events may serve as sorting signals for internalized receptors. Taken together with the previous discovery that both human and simian immunodeficiency virus envelope proteins can undergo endocytosis, the gE findings suggest that endocytosis of envelope components may be a posttranslational regulatory mechanism among divergent families of enveloped viruses.

An equine herpesvirus-1 gene 71 deletant is attenuated and elicits a protective immune response in mice

The pathogenesis of pulmonary infection and the immune response following intranasal inoculation of mice with two equine herpesvirus type 1 (EHV-1) deletion mutants have been assessed. The mutants, ED71 and ED75, have deletions in genes 71 (EUS4) and 75 (10K), respectively. Deletions were replaced by the Escherichia coli lacZ gene driven by the simian virus 40 (SV40) early promoter. It has previously been shown that the protein products of genes 71 and 75 are dispensable in vitro but that removal of gene 71 results in a defect in virus maturation and capsid envelopment which impairs the ability of mutant virus to spread via release and readsorption. This study demonstrated that the 192-kDa gene 71 product is required for full expression of virulence in mice, whereas the putative 10-kDa product of gene 75 has minimal effect. Both mutants exhibited the same tissue and cytotropism as wild-type EHV-1 and induced both humoral and cell-mediated immune responses indistinguishable from those induced by the parental strain. Irrespective of the reduced pathogenicity of the gene 71 mutant, infected mice were protected against a challenge with wild-type EHV-1. These findings highlight the potential of ED71 as a vaccine candidate.

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

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

Laminin reduces HSV-1 spread from cell to cell in human keratinocyte cultures

Herpes simplex virus-1 (HSV-1) infects epidermal cells where it replicates and spreads from cell to cell. While some of the viral factors responsible for cell-to-cell spread are known, the host cell molecules and structures which are utilized by HSV-1 during spread are not well studied. Here we report that a laminin substrate reduced the ability of HSV-1 to spread from cell to cell in cultures of a human keratinocyte cell line (HaCaT). Laminin did not reduce spread of the virus by decreasing the viral replication rate. However, laminin did stimulate the formation of tight junctions between HaCaT cells, suggesting that tight junctions can affect cell-to-cell spread of HSV-1. Since laminin is an abundant component of the basement membrane in vivo, culturing cells on laminin may provide an assay which more accurately reflects the rate and mechanism of HSV-1 cell-to-cell spread in vivo.

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.

Accessory human cytomegalovirus glycoprotein US9 in the unique short component of the viral genome promotes cell-to-cell transmission of virus in polarized epithelial cells

Human cytomegalovirus (CMV) encodes accessory glycoproteins that are dispensable for virus growth in nonpolarized cells in culture. We report that CMV deletion mutants lacking the gene for accessory glycoprotein US9 in the unique short component of the viral genome are impaired in plaque formation in polarized human retinal pigment epithelial (ARPE-19) cells. Comparison of CMV deletion mutants in US9 with herpes simplex virus type 1 deletion mutants lacking glycoproteins gE and gI showed that both of these mutants are impaired in altering junctional complexes and increasing paracellular permeability in polarized ARPE-19 cells cultured on permeable filter supports. Results of functional studies indicate that CMV US9 and homologs of gE have analogous roles in promoting virus spread across lateral membranes of polarized epithelial cells.

Biosynthesis of glycoproteins E and I of feline herpesvirus: gE-gI interaction is required for intracellular transport

The biosynthesis of glycoproteins E and I of feline herpesvirus was studied by using the vaccinia virus vTF7-3 expression system. gE and gI were synthesized as N-glycosylated, endoglycosidase H (EndoH)-sensitive precursors with Mrs of 83,000 and 67,000, respectively. When coexpressed, gE and gI formed sodium dodecyl sulfate-sensitive hetero-oligomeric complexes that were readily transported from the endoplasmic reticulum (ER). Concomitantly, the glycoproteins acquired extensive posttranslational modifications, including O glycosylation, leading to an increase in their apparent molecular weights to 95,000 and 80,000 to 100,000 for gE and gI, respectively. In the absence of gE, most gI remained EndoH sensitive. Only a minor population became EndoH resistant, but these molecules were processed aberrantly as indicated by their Mrs (100,000 to 120,000). By immunofluorescence microscopy, gI was detected primarily in the ER but also at the plasma membrane. gE, when expressed by itself, remained EndoH sensitive and was found only in the ER and the nuclear envelope. These results were corroborated by studying the biosynthesis of gE in feline herpesvirus (FHV)-infected cells. In cells infected with wild-type FHV, gE acquired the same co- and posttranslational modifications as during vTF7-3-driven expression. However, an FHV mutant lacking gI failed to produce mature gE. We conclude that gE is retained in the ER, presumably by associating with molecular chaperones, and becomes transport competent only when in a complex with gI.

Anterograde, transneuronal transport of herpes simplex virus type 1 strain H129 in the murine visual system

Herpes simplex virus (HSV) undergoes retrograde and anterograde axonal transport as it establishes latency and later intermittently reactivates. Most strains of HSV show preferential retrograde transport within the central nervous system (CNS), however. Previous experiments suggest that an exception to this is HSV type 1 (HSV-1) strain H129, since this virus appears to spread primarily in the CNS via anterograde, transneuronal movement. The objective of the present study was to test how specifically this virus spreads in the visual system, a system with well-described neuronal connections. In the present study, the pattern of viral spread was examined following inoculation into the murine vitreous body. Virus was initially detected in the retina and optic tract. Virus then appeared in all known primary targets of the retina, including those in the thalamus (e.g., lateral geniculate complex), hypothalamus (suprachiasmatic nucleus), and superior colliculus (superficial layers). In previous studies, many strains of HSV were shown to infect these structures, even though they spread predominantly in a retrograde direction. However, the H129 strain was unique in then spreading, via anterograde transport, to the primary visual cortex (layer 4 of area 17) via thalamocortical connections. At later times after infection, specific labeling was also detected in other cortical and subcortical areas known to receive projections from the visual cortex. No labeling was ever detected in the contralateral retina, which is consistent with a lack of retrograde spread of HSV-1 strain H129. These results demonstrate the specific anterograde movement of this virus from the retina to subcortical and cortical regions, with no clear evidence for retrograde spread. HSV-1 strain H129 should be generally useful for tracing sensory pathways and may provide the basis for designing a virus vector capable of delivering genetic material via anterograde pathways within the CNS.

Disulfide bond structure determination and biochemical analysis of glycoprotein C from herpes simplex virus

A biochemical analysis of glycoprotein C (gC of herpes simplex virus was undertaken to further characterize the structure of the glycoprotein and to determine its disulfide bond arrangement. We used three recombinant forms of gC, gC1(457t), gC1(delta33-123t), and gC2(426t), each truncated prior to the transmembrane region. The proteins were expressed and secreted by using a baculovirus expression system and have been shown to bind to monoclonal antibodies which recognize discontinuous epitopes and to complement component C3b in a dose-dependent manner. We confirmed the N-terminal residues of each mature protein by Edman degradation and confirmed the internal deletion in gC1(delta33-123t). The molecular weight and extent of glycosylation of gC1 (457t), gC1(delta33-123t), and gC2(426t) were determined by treating each protein with endoglycosidases and then subjecting it to sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and mass spectrometric analysis. The data indicate that eight to nine of the predicted N-linked oligosaccharide sites on gC1(457t) are occupied by glycans of approximately 1,000 Da. In addition, O-linked oligosaccharides are present on gC1(457t), primarily localized to the N-terminal region (amino acids [aa] 33 to 123) of the protein. gC2(426t) contains N-linked oligosaccharides, but no O-linked oligosaccharides were detected. To determine the disulfide bond arrangement of the eight cysteines of gC1(457t),the protein was cleaved with cyanogen bromide. SDS-PAGE analysis followed by Edman degradation identified three cysteine-containing fragments which are not connected by disulfide linkages. Chemical modification of cysteines combined with matrix-assisted laser desorption ionization mass spectrometry identified disulfide bonds between cysteine 1 (aa 127) and cysteine 2 (aa 144) and between cysteine 3 (aa 286) and cysteine 4 (aa 347). Further proteolysis of the cyanogen bromide-generated fragment containing cysteine 5 through cysteine 8, combined with mass spectrometry and Edman degradation, showed that disulfide bonds link cysteine 5 (aa 386) to cysteine 8 (aa 442) and cysteine 6 (aa 390) to cysteine 7 (aa 419). A similar disulfide bond arrangement is postulated to exist in gC homologs from other herpesviruses.