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

Knockout of signal peptide peptidase in the eye reduces HSV-1 replication and eye disease in ocularly infected mice

We previously reported that knocking out signal peptide peptidase (SPP), a glycoprotein K (gK) binding partner, in mouse peripheral sensory neurons reduced latency-reactivation in infected mice without affecting primary virus replication or eye disease. Since virus replication in the eye plays an essential role in eye disease, we generated a conditional knockout mouse lacking SPP expression in the eye by crossing Pax6 (paired box 6)-Cre mice that have intact Pax6 expression with SPPflox/flox mice. Significantly less SPP protein expression was detected in the eyes of Pax6-SPP-/- mice than in WT control mice. HSV-1 replication in the eyes of Pax6-SPP-/- mice was significantly lower than in WT control mice. Levels of gB, gK, and ICP0 transcripts in corneas, but not trigeminal ganglia (TG), of Pax6-SPP-/- infected mice were also significantly lower than in WT mice. Corneal scarring and angiogenesis were significantly lower in Pax6-SPP-/- mice than in WT control mice, while corneal sensitivity was significantly higher in Pax6-SPP-/- mice compared with WT control mice. During acute viral infection, absence of SPP in the eye did not affect CD4 expression but did affect CD8α and IFNγ expression in the eye. However, in the absence of SPP, latency-reactivation was similar in Pax6-SPP-/- and WT control groups. Overall, our results showed that deleting SPP expression in the eyes reduced primary virus replication in the eyes, reduced CD8α and IFNγ mRNA expression, reduced eye disease and reduced angiogenesis but did not alter corneal sensitivity or latency reactivation to HSV-1 infection. Thus, blocking gK binding to SPP in the eye may have therapeutic potential by reducing both virus replication in the eye and eye disease associated with virus replication.

A novel H129-based anterograde monosynaptic tracer exhibits features of strong labeling intensity, high tracing efficiency, and reduced retrograde labeling

Background

Viral tracers are important tools for mapping brain connectomes. The feature of predominant anterograde transneuronal transmission offers herpes simplex virus-1 (HSV-1) strain H129 (HSV1-H129) as a promising candidate to be developed as anterograde viral tracers. In our earlier studies, we developed H129-derived anterograde polysynaptic tracers and TK deficient (H129-dTK) monosynaptic tracers. However, their broad application is limited by some intrinsic drawbacks of the H129-dTK tracers, such as low labeling intensity due to TK deficiency and potential retrograde labeling caused by axon terminal invasion. The glycoprotein K (gK) of HSV-1 plays important roles in virus entry, egress, and virus-induced cell fusion. Its deficiency severely disables virus egress and spread, while only slightly limits viral genome replication and expression of viral proteins. Therefore, we created a novel H129-derived anterograde monosynaptic tracer (H129-dgK) by targeting gK, which overcomes the limitations of H129-dTK.

Methods

Using our established platform and pipeline for developing viral tracers, we generated a novel tracer by deleting the gK gene from the H129-G4. The gK-deleted virus (H129-dgK-G4) was reconstituted and propagated in the Vero cell expressing wildtype H129 gK (gK_wt) or the mutant gK (gK_mut, A40V, C82S, M223I, L224V, V309M), respectively. Then the obtained viral tracers of gK_mut pseudotyped and gK_wt coated H129-dgK-G4 were tested in vitro and in vivo to characterize their tracing properties.

Results

H129-dgK-G4 expresses high levels of fluorescent proteins, eliminating the requirement of immunostaining for imaging detection. Compared to the TK deficient monosynaptic tracer H129-dTK-G4, H129-dgK-G4 labeled neurons with 1.76-fold stronger fluorescence intensity, and visualized 2.00-fold more postsynaptic neurons in the downstream brain regions. gK_mut pseudotyping leads to a 77% decrease in retrograde labeling by reducing axon terminal invasion, and thus dramatically improves the anterograde-specific tracing of H129-dgK-G4. In addition, assisted by the AAV helper trans-complementarily expressing gK_wt, H129-dgK-G4 allows for mapping monosynaptic connections and quantifying the circuit connectivity difference in the Alzheimer’s disease and control mouse brains.

Conclusions

gK_mut pseudotyped H129-dgK-G4, a novel anterograde monosynaptic tracer, overcomes the limitations of H129-dTK tracers, and demonstrates desirable features of strong labeling intensity, high tracing efficiency, and improved anterograde specificity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13024-021-00508-6.

The Structures and Functions of VZV Glycoproteins

The virions of all enveloped viruses, including those of the Herpesviridae, must bind to the cell surface then undergo a process of membrane fusion between the cell plasma membrane and the virus particle envelope. As for all herpesviruses, glycoproteins in the virion envelope are the modus operandi of these events.

Two Sides to Every Story: Herpes Simplex Type-1 Viral Glycoproteins gB, gD, gH/gL, gK, and Cellular Receptors Function as Key Players in Membrane Fusion

Herpes simplex virus type-1 (HSV-1) and type-2 (HSV-2) are prototypical alphaherpesviruses that are characterized by their unique properties to infect trigeminal and dorsal root ganglionic neurons, respectively, and establish life-long latent infections. These viruses initially infect mucosal epithelial tissues and subsequently spread to neurons. They are associated with a significant disease spectrum, including orofacial and ocular infections for HSV-1 and genital and neonatal infections for HSV-2. Viral glycoproteins within the virion envelope bind to specific cellular receptors to mediate virus entry into cells. This is achieved by the fusion of the viral envelope with the plasma membrane. Similarly, viral glycoproteins expressed on cell surfaces mediate cell-to-cell fusion and facilitate virus spread. An interactive complex of viral glycoproteins gB, gD/gH/gL, and gK and other proteins mediate these membrane fusion phenomena with glycoprotein B (gB), the principal membrane fusogen. The requirement for the virion to enter neuronal axons suggests that the heterodimeric protein complex of gK and membrane protein UL20, found only in alphaherpesviruses, constitute a critical determinant for neuronal entry. This hypothesis was substantiated by the observation that a small deletion in the amino terminus of gK prevents entry into neuronal axons while allowing entry into other cells via endocytosis. Cellular receptors and receptor-mediated signaling synergize with the viral membrane fusion machinery to facilitate virus entry and intercellular spread. Unraveling the underlying interactions among viral glycoproteins, envelope proteins, and cellular receptors will provide new innovative approaches for antiviral therapy against herpesviruses and other neurotropic viruses.

Prohibitin-1 Contributes to Cell-to-Cell Transmission of Herpes Simplex Virus 1 via the MAPK/ERK Signaling Pathway

Viral cell-to-cell spread, a method employed by several viral families for entrance via cell junctions, is highly relevant to the pathogenesis of various viral infections. Cell-to-cell spread of herpes simplex virus 1 (HSV-1) is known to depend greatly on envelope glycoprotein E (gE). However, the molecular mechanism by which gE acts in HSV-1 cell-to-cell spread and the mechanisms of cell-to-cell spread by other herpesviruses remain poorly understood. Here, we describe our identification of prohibitin-1 as a novel gE-interacting host cell protein. Ectopic expression of prohibitin-1 increased gE-dependent HSV-1 cell-to-cell spread. As observed with the gE-null mutation, decreased expression or pharmacological inhibition of prohibitin-1 reduced HSV-1 cell-to-cell spread without affecting the yield of virus progeny. Similar effects were produced by pharmacological inhibition of the mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) pathway, wherein prohibitin-1 acts as a protein scaffold and is required for induction of this pathway. Furthermore, artificial activation of the MAPK/ERK pathway restored HSV-1 cell-to-cell spread impaired by the gE-null mutation. Notably, pharmacological inhibition of prohibitins or the MAPK/ERK pathway reduced viral cell-to-cell spread of representative members in all herpesvirus subfamilies. Our results suggest that prohibitin-1 contributes to gE-dependent HSV-1 cell-to-cell spread via the MAPK/ERK pathway and that this mechanism is conserved throughout the Herpesviridae, whereas gE is conserved only in the Alphaherpesvirinae subfamily.IMPORTANCE Herpesviruses are ubiquitous pathogens of various animals, including humans. These viruses primarily pass through cell junctions to spread to uninfected cells. This method of cell-to-cell spread is an important pathogenic characteristic of these viruses. Here, we show that the host cell protein prohibitin-1 contributes to HSV-1 cell-to-cell spread via a downstream intracellular signaling cascade, the MAPK/ERK pathway. We also demonstrate that the role of the prohibitin-1-mediated MAPK/ERK pathway in viral cell-to-cell spread is conserved in representative members of every herpesvirus subfamily. This study has revealed a common molecular mechanism of the cell-to-cell spread of herpesviruses.

Viral Vectors for Neural Circuit Mapping and Recent Advances in Trans-synaptic Anterograde Tracers

Viral tracers are important tools for neuroanatomical mapping and genetic payload delivery. Genetically modified viruses allow for cell-type-specific targeting and overcome many limitations of non-viral tracers. Here, we summarize the viruses that have been developed for neural circuit mapping, and we provide a primer on currently applied anterograde and retrograde viral tracers with practical guidance on experimental uses. We also discuss and highlight key technical and conceptual considerations for developing new safer and more effective anterograde trans-synaptic viral vectors for neural circuit analysis in multiple species.

Identification and Visualization of Functionally Important Domains and Residues in Herpes Simplex Virus Glycoprotein K(gK) Using a Combination of Phylogenetics and Protein Modeling

Alphaherpesviruses are a subfamily of herpesviruses that include the significant human pathogens herpes simplex viruses (HSV) and varicella zoster virus (VZV). Glycoprotein K (gK), conserved in all alphaherpesviruses, is a multi-membrane spanning virion glycoprotein essential for virus entry into neuronal axons, virion assembly, and pathogenesis. Despite these critical functions, little is known about which gK domains and residues are most important for maintaining these functions across all alphaherpesviruses. Herein, we employed phylogenetic and structural analyses including the use of a novel model for evolutionary rate variation across residues to predict conserved gK functional domains. We found marked heterogeneity in the evolutionary rate at the level of both individual residues and domains, presumably as a result of varying selective constraints. To clarify the potential role of conserved sequence features, we predicted the structures of several gK orthologs. Congruent with our phylogenetic analysis, slowly evolving residues were identified at potentially structurally significant positions across domains. We found that using a quantitative measure of amino acid rate variation combined with molecular modeling we were able to identify amino acids predicted to be critical for gK protein structure/function. This analysis yields targets for the design of anti-herpesvirus therapeutic strategies across all alphaherpesvirus species that would be absent from more traditional analyses of conservation.

Role of Herpes Simplex Virus Type 1 (HSV-1) Glycoprotein K (gK) Pathogenic CD8+ T Cells in Exacerbation of Eye Disease

HSV-1-induced corneal scarring (CS), also broadly referred to as Herpes Stromal Keratitis (HSK), is the leading cause of infectious blindness in developed countries. It is well-established that HSK is in fact an immunopathological disease. The contribution of the potentially harmful T cell effectors that lead to CS remains an area of intense study. Although the HSV-1 gene(s) involved in eye disease is not yet known, we have demonstrated that gK, which is one of the 12 known HSV-1 glycoproteins, has a crucial role in CS. Immunization of HSV-1 infected mice with gK, but not with any other known HSV-1 glycoprotein, significantly exacerbates CS, and dermatitis. The gK-induced eye disease occurs independently of the strain of the virus or mouse. HSV-1 mutants that lack gK are unable to efficiently infect and establish latency in neurons. HSV-1 recombinant viruses expressing two additional copies of the gK (total of three gK genes) exacerbated CS as compared with wild type HSV-1 strain McKrae that contains one copy of gK. Furthermore, we have shown that an 8mer (ITAYGLVL) within the signal sequence of gK enhanced CS in ocularly infected BALB/c mice, C57BL/6 mice, and NZW rabbits. In HSV-infected “humanized” HLA-A^*0201 transgenic mice, this gK 8mer induced strong IFN-γ-producing cytotoxic CD8⁺ T cell responses. gK induced CS is dependent on gK binding to signal peptide peptidase (SPP). gK also binds to HSV-1 UL20, while UL20 binds GODZ (DHHC3) and these quadruple interactions are required for gK induced pathology. Thus, potential therapies might include blocking of gK-SPP, gK-UL20, UL20-GODZ interactions, or a combination of these strategies.

The Amino Terminus of Herpes Simplex Virus 1 Glycoprotein K (gK) Is Required for gB Binding to Akt, Release of Intracellular Calcium, and Fusion of the Viral Envelope with Plasma Membranes

Previously, we have shown that the amino terminus of glycoprotein K (gK) binds to the amino terminus of gB and that deletion of the amino-terminal 38 amino acids of gK prevents herpes simplex virus 1 (HSV-1) infection of mouse trigeminal ganglia after ocular infection and virus entry into neuronal axons. Recently, it has been shown that gB binds to Akt during virus entry and induces Akt phosphorylation and intracellular calcium release. Proximity ligation and two-way immunoprecipitation assays using monoclonal antibodies against gB and Akt-1 phosphorylated at S473 [Akt-1(S473)] confirmed that HSV-1(McKrae) gB interacted with Akt-1(S473) during virus entry into human neuroblastoma (SK-N-SH) cells and induced the release of intracellular calcium. In contrast, the gB specified by HSV-1(McKrae) gKΔ31-68, lacking the amino-terminal 38 amino acids of gK, failed to interact with Akt-1(S473) and induce intracellular calcium release. The Akt inhibitor miltefosine inhibited the entry of McKrae but not the gKΔ31-68 mutant into SK-N-SH cells. Importantly, the entry of the gKΔ31-68 mutant but not McKrae into SK-N-SH cells treated with the endocytosis inhibitors pitstop-2 and dynasore hydrate was significantly inhibited, indicating that McKrae gKΔ31-68 entered via endocytosis. These results suggest that the amino terminus of gK functions to regulate the fusion of the viral envelope with cellular plasma membranes.IMPORTANCE HSV-1 glycoprotein B (gB) functions in the fusion of the viral envelope with cellular membranes during virus entry. Herein, we show that a deletion in the amino terminus of glycoprotein K (gK) inhibits gB binding to Akt-1(S473), the release of intracellular calcium, and virus entry via fusion of the viral envelope with cellular plasma membranes.

The Absence of DHHC3 Affects Primary and Latent Herpes Simplex Virus 1 Infection

UL20, an essential herpes simplex virus 1 (HSV-1) protein, is involved in cytoplasmic envelopment of virions and virus egress. We reported recently that UL20 can bind to a host protein encoded by the zinc finger DHHC-type containing 3 (ZDHHC3) gene (also known as Golgi-specific DHHC zinc finger protein [GODZ]). Here, we show for the first time that HSV-1 replication is compromised in murine embryonic fibroblasts (MEFs) isolated from GODZ^-/- mice. The absence of GODZ resulted in blocking palmitoylation of UL20 and altered localization and expression of UL20 and glycoprotein K (gK); the expression of gB and gC; and the localization and expression of tegument and capsid proteins within HSV-1-infected MEFs. Electron microscopy revealed that the absence of GODZ limited the maturation of virions at multiple steps and affected the localization of virus and endoplasmic reticulum morphology. Virus replication in the eyes of ocularly HSV-1-infected GODZ^-/- mice was significantly lower than in HSV-1-infected wild-type (WT) mice. The levels of UL20, gK, and gB transcripts in the corneas of HSV-1-infected GODZ^-/- mice on day 5 postinfection were markedly lower than in WT mice, whereas only UL20 transcripts were reduced in trigeminal ganglia (TG). In addition, HSV-1-infected GODZ^-/- mice showed notably lower levels of corneal scarring, and HSV-1 latency reactivation was also reduced. Thus, normal HSV-1 infectivity and viral pathogenesis are critically dependent on GODZ-mediated palmitoylation of viral UL20.IMPORTANCE HSV-1 infection is widespread. Ocular infection can cause corneal blindness; however, approximately 70 to 90% of American adults exposed to the virus show no clinical symptoms. In this study, we show for the first time that the absence of a zinc finger protein called GODZ affects primary and latent infection, as well as reactivation, in ocularly infected mice. The reduced virus infectivity is due to the absence of the GODZ interaction with HSV-1 UL20. These results strongly suggest that binding of UL20 to GODZ promotes virus infectivity in vitro and viral pathogenesis in vivo.

Role of L-Particles during Herpes Simplex Virus Infection

Infection of eukaryotic cells with α-herpesviruses results in the formation and secretion of infectious heavy particles (virions; H-particles) and non-infectious light particles (L-particles). Herpes simplex virus type 1 (HSV-1) H-particles consist of a genome-containing capsid surrounded by tegument proteins and a glycoprotein-rich lipid bilayer. Non-infectious L-particles are composed mainly of envelope and tegument proteins and are devoid of capsids and viral DNA. L-particles were first described in the early nineties and from then on investigated for their formation and role during virus infection. The development and secretion of L-particles occur simultaneously to the assembly of complete viral particles. HSV-1 L-particles are assembled by budding of condensed tegument into Golgi-delivered vesicles and are capable of delivering their functional content to non-infected cells. Thereby, HSV-1 L-particles contribute to viral pathogenesis within the infected host by enhancing virion infectivity and providing immune evasion functions. In this review we discuss the emergence of HSV-1 L-particles during virus replication and their biological functions described thus far.

Binding of Herpes Simplex Virus 1 UL20 to GODZ (DHHC3) Affects Its Palmitoylation and Is Essential for Infectivity and Proper Targeting and Localization of UL20 and Glycoprotein K

Herpes simplex virus 1 (HSV-1) UL20 plays a crucial role in the envelopment of the cytoplasmic virion and its egress. It is a nonglycosylated envelope protein that is regulated as a γ1 gene. Two-hybrid and pulldown assays demonstrated that UL20, but no other HSV-1 gene-encoded proteins, binds specifically to GODZ (also known as DHHC3), a cellular Golgi apparatus-specific Asp-His-His-Cys (DHHC) zinc finger protein. A catalytically inactive dominant-negative GODZ construct significantly reduced HSV-1 replication in vitro and affected the localization of UL20 and glycoprotein K (gK) and their interactions but not glycoprotein C (gC). GODZ is involved in palmitoylation, and we found that UL20 is palmitoylated by GODZ using a GODZ dominant-negative plasmid. Blocking of palmitoylation using 2-bromopalmitate (2-BP) affected the virus titer and the interaction of UL20 and gK but did not affect the levels of these proteins. In conclusion, we have shown that binding of UL20 to GODZ in the Golgi apparatus regulates trafficking of UL20 and its subsequent effects on gK localization and virus replication. We also have demonstrated that GODZ-mediated UL20 palmitoylation is critical for UL20 membrane targeting and thus gK cell surface expression, providing new mechanistic insights into how UL20 palmitoylation regulates HSV-1 infectivity.IMPORTANCE HSV-1 UL20 is a nonglycosylated essential envelope protein that is highly conserved among herpesviruses. In this study, we show that (i) HSV-1 UL20 binds to GODZ (also known as DHHC3), a Golgi apparatus-specific Asp-His-His-Cys (DHHC) zinc finger protein; (ii) a GODZ dominant-negative mutant and an inhibitor of palmitoylation reduced HSV-1 titers and altered the localization of UL20 and glycoprotein K; and (iii) UL20 is palmitoylated by GODZ, and this UL20 palmitoylation is required for HSV-1 infectivity. Thus, blocking of the interaction of UL20 with GODZ, using a GODZ dominant-negative mutant or possibly GODZ shRNA, should be considered a potential alternative therapy in not only HSV-1 but also other conditions in which GODZ processing is an integral component of pathogenesis.

Bovine herpesvirus type-1 glycoprotein K (gK) interacts with UL20 and is required for infectious virus production

We have previously shown that the HSV-1 gK and UL20 proteins interact and function in virion envelopment, membrane fusion, and neuronal entry. Alignment of the predicted secondary structures of gKs encoded by BoHV-1, HSV-1, HSV-2, EHV-1 and VZV indicated a high degree of domain conservation. Two BoHV-1 gK-null mutant viruses were created by either gK gene deletion or stop codon insertion. In addition, a V5 epitope-tag was inserted at the carboxyl terminus of gK gene to detect gK. The engineered gK-null mutant viruses failed to replicate and produce viral plaques. Co-immunoprecipitation of gK and UL20 expressed via different methods revealed that gK and UL20 physically interacted in the presence or absence of other viral proteins. Confocal microscopy showed that gK and UL20 colocalized in infected cells. These results indicate that BoHV-1 gK and UL20 may function in a similar manner to other alphaherpesvirus orthologues specified by HSV-1, PRV and EHV-1.

Deletion of a Predicted β-Sheet Domain within the Amino Terminus of Herpes Simplex Virus Glycoprotein K Conserved among Alphaherpesviruses Prevents Virus Entry into Neuronal Axons

We have shown previously that herpes simplex virus 1 (HSV-1) lacking expression of the entire glycoprotein K (gK) or expressing gK with a 38-amino-acid deletion (gKΔ31-68 mutation) failed to infect ganglionic neurons after ocular infection of mice. We constructed a new model for the predicted three-dimensional structure of gK, revealing that the gKΔ31-68 mutation spans a well-defined β-sheet structure within the amino terminus of gK, which is conserved among alphaherpesviruses. The HSV-1(McKrae) gKΔ31-68 virus was tested for the ability to enter into ganglionic neuronal axons in cell culture of explanted rat ganglia using a novel virus entry proximity ligation assay (VEPLA). In this assay, cell surface-bound virions were detected by the colocalization of gD and its cognate receptor nectin-1 on infected neuronal surfaces. Capsids that have entered into the cytoplasm were detected by the colocalization of the virion tegument protein UL37, with dynein required for loading of virion capsids onto microtubules for retrograde transport to the nucleus. HSV-1(McKrae) gKΔ31-68 attached to cell surfaces of Vero cells and ganglionic axons in cell culture as efficiently as wild-type HSV-1(McKrae). However, unlike the wild-type virus, the mutant virus failed to enter into the axoplasm of ganglionic neurons. This work suggests that the amino terminus of gK is a critical determinant for entry into neuronal axons and may serve similar conserved functions for other alphaherpesviruses.

IMPORTANCE

Alphaherpesviruses, unlike beta- and gammaherpesviruses, have the unique ability to infect and establish latency in neurons. Glycoprotein K (gK) and the membrane protein UL20 are conserved among all alphaherpesviruses. We show here that a predicted β-sheet domain, which is conserved among alphaherpesviruses, functions in HSV-1 entry into neuronal axons, suggesting that it may serve similar functions for other herpesviruses. These results are in agreement with our previous observations that deletion of this gK domain prevents the virus from successfully infecting ganglionic neurons after ocular infection of mice.

The immunomodulator, ammonium trichloro[1,2-ethanediolato-O,O']-tellurate, suppresses the propagation of herpes simplex virus 2 by reducing the infectivity of the virus progeny

Persistent investigations for the identification of novel anti-herpetic drugs are being conducted worldwide, as current treatment options are sometimes insufficient. The immunomodulator, ammonium trichloro[1,2‑ethanediolato‑O,O']‑tellurate (AS101), a non‑toxic tellurium (Ⅳ) compound, has been shown to exhibit anti‑viral activity against a variety of viruses in cell cultures and in animal models. In the present study, the anti‑viral activity of AS101 against herpes simplex virus (HSV)‑1 and 2 was investigated in vitro. The results demonstrated that AS101 significantly restricted HSV‑2-induced plaque formation and reduced the infectivity of the HSV‑2 yield, while HSV‑1 was affected to a lesser extent. The incubation of mature HSV‑1 and HSV‑2 viruses with AS101 had no effect on viral infectivity, indicating that the compound interrupts de novo viral synthesis. The addition of AS101 at up to 9 h post‑infection had almost the same effect as did the addition of the drug together with the virus (it maintained 80% of its total anti‑viral capacity). Quantitative PCR and immunofluoresence staining of viral structural proteins revealed that the viral DNA and protein synthesis stages were not interrupted by the administration of AS101. By contrast, in the presence of the compound, significantly fewer viable viruses (≥2 log reduction) were recovered from the AS10‑treated cell cultures. Of note, when we determined the viability of the intracellular virus, formed in the presence of the compound, a less severe (≤1 log) effect was observed. Taken together, these data strongly suggest that AS101 primarily interferes with late stages of viral replication, such as viral particle envelopment or egress, leading to the production of a defective virus progeny.

HSV-1 gM and the gK/pUL20 complex are important for the localization of gD and gH/L to viral assembly sites

Herpes simplex virus-1 (HSV-1), like all herpesviruses, is a large complex DNA virus containing up to 16 different viral membrane proteins in its envelope. The assembly of HSV-1 particles occurs by budding/wrapping at intracellular membranes producing infectious virions contained within the lumen of cytoplasmic membrane-bound compartments that are then released by secretion. To ensure incorporation of all viral membrane proteins into the envelope, they need to be localized to the appropriate intracellular membranes either via the endocytic pathway or by direct targeting to assembly sites from the biosynthetic secretory pathway. Many HSV-1 envelope proteins encode targeting motifs that direct their endocytosis and targeting, while others do not, including the essential entry proteins gD and the gH/gL complex, and so it has been unclear how these envelope proteins reach the appropriate assembly compartments. We now show that efficient endocytosis of gD and gH/gL and their incorporation into mature virions relies upon the presence of the HSV-1 envelope proteins gM and the gK/pUL20 complex. Our data demonstrate both redundant and synergistic roles for gM and gK/pUL20 in controlling the targeting of gD and gH/L to the appropriate intracellular virus assembly compartments.

Mutations within the pathogenic region of herpes simplex virus 1 gK signal sequences alter cell surface expression and neurovirulence

To investigate the role of the signal sequences of herpes simplex virus 1 (HSV-1) gK on virus replication and viral pathogenesis, we constructed recombinant viruses with or without mutations within the signal sequences of gK. These recombinant viruses expressed two additional copies of the mutated (MgK) or native (NgK) form of the gK gene in place of the latency-associated transcript with a myc epitope tag to facilitate detection at their 3' ends. The replication of MgK virus was similar to that of NgK both in vitro and in vivo, as well as in the trigeminal ganglia (TG) of latently infected mice. The levels of gB and gK transcripts in the corneas, TG, and brains of infected mice on days 3 and 5 postinfection were markedly virus and time dependent, as well as tissue specific. Mutation in the signal sequence of gK in MgK virus blocked cell surface expression of gK-myc in rabbit skin cells, increased 50% lethal dose, and decreased corneal scarring in ocularly infected mice compared to the NgK or revertant (RgK) virus. MgK and NgK viruses, and not the RgK virus, showed a reduced extent of explant reactivation at the lower dose of ocular infection but not at the higher dose. However, the time of reactivation was not affected by overexpression of the different forms of gK. Taken together, these results strongly suggest that the 8mer peptide (ITAYGLVL) within the signal sequence of gK promotes cell surface expression of gK in infected cells and ocular pathogenesis in infected mice.

IMPORTANCE

In this study, we show for the first time that mutations within the signal sequence of gK blocked cell surface expression of inserted recombinant gK in vitro. Furthermore, this blockage in cell surface expression was correlated with higher 50% lethal dose and less corneal scarring in vivo. Thus, these studies point to a key role for the 8mer within the signal sequence of gK in HSV-1-induced pathogenicity.

Visualization of mouse neuronal ganglia infected by Herpes Simplex Virus 1 (HSV-1) using multimodal non-linear optical microscopy

Herpes simplex virus 1 (HSV-1) is a neurotropic virus that causes skin lesions and goes on to enter a latent state in neurons of the trigeminal ganglia. Following stress, the virus may reactivate from latency leading to recurrent lesions. The in situ study of neuronal infections by HSV-1 is critical to understanding the mechanisms involved in the biology of this virus and how it causes disease; however, this normally requires fixation and sectioning of the target tissues followed by treatment with contrast agents to visualize key structures, which can lead to artifacts. To further our ability to study HSV-1 neuropathogenesis, we have generated a recombinant virus expressing a second generation red fluorescent protein (mCherry), which behaves like the parental virus in vivo. By optimizing the application of a multimodal non-linear optical microscopy platform, we have successfully visualized in unsectioned trigeminal ganglia of mice both infected cells by two-photon fluorescence microscopy, and myelinated axons of uninfected surrounding cells by coherent anti-Stokes Raman scattering (CARS) microscopy. These results represent the first report of CARS microscopy being combined with 2-photon fluorescence microscopy to visualize virus-infected cells deep within unsectioned explanted tissue, and demonstrate the application of multimodal non-linear optical microscopy for high spatial resolution biological imaging of tissues without the use of stains or fixatives.

Inhibitors of signal peptide peptidase (SPP) affect HSV-1 infectivity in vitro and in vivo

Recently we have shown that the highly conserved herpes simplex virus glycoprotein K (gK) binds to signal peptide peptidase (SPP), also known as minor histocompatibility antigen H13. In this study we have demonstrated for the first time that inhibitors of SPP, such as L685,458, (Z-LL)2 ketone, aspirin, ibuprofen and DAPT, significantly reduced HSV-1 replication in tissue culture. Inhibition of SPP activity via (Z-LL)2 ketone significantly reduced viral transcripts in the nucleus of infected cells. Finally, when administered during primary infection, (Z-LL)2 ketone inhibitor reduced HSV-1 replication in the eyes of ocularly infected mice. Thus, blocking SPP activity may represent a clinically effective and expedient approach to the reduction of viral replication and the resulting pathology.

Phenylalanine residues at the carboxyl terminus of the herpes simplex virus 1 UL20 membrane protein regulate cytoplasmic virion envelopment and infectious virus production

The herpes simplex virus type 1 (HSV-1) UL20 gene encodes a 222-amino-acid nonglycosylated envelope protein which forms a complex with viral glycoprotein K (gK) that functions in virion envelopment, egress, and virus-induced cell fusion. To investigate the role of the carboxyl terminus of the UL20 protein (UL20p) in cytoplasmic virion envelopment, a cadre of mutant viruses was constructed and characterized. The deletion of six amino acids from the carboxyl terminus of UL20p caused an approximately 1-log reduction in infectious virus production compared to that of the wild-type virus. Surprisingly, a phenylalanine-to-alanine replacement at amino acid position 210 caused a gain-of-function phenotype, increasing infectious virus production up to 1 log more than in the wild-type virus. In contrast, the replacement of two membrane-proximal phenylalanines with alanines caused drastic inhibition of infectious virion production and cytoplasmic virion envelopment. Prediction of the membrane topology of UL20p revealed that these two amino acid changes cause retraction of the carboxyl terminus of UL20p from the intracellular space. Confocal microscopy revealed that none of the engineered UL20 mutations affected intracellular transport of UL20p to trans-Golgi network membranes. In addition, a proximity ligation assay showed that none of the UL20 mutations affected UL20p colocalization and potential interactions with the UL37 protein recently found to interact with the gK/UL20 protein complex. Collectively, these studies show that phenylalanine residues within the carboxyl terminus of UL20p are involved in the regulation of cytoplasmic virion envelopment and infectious virus production.

IMPORTANCE

We have shown previously that the UL20/gK protein complex serves crucial roles in cytoplasmic virion envelopment and that it interacts with the UL37 tegument protein to facilitate cytoplasmic virion envelopment. In this study, we investigated the role of phenylalanine residues within the carboxyl terminus of UL20p, since aromatic and hydrophobic amino acids are known to be involved in protein-protein interactions through stacking of their aromatic structures. Characterization of mutant viruses carrying phenylalanine (Phe)-to-alanine (Ala) mutations revealed that the two membrane-proximal Phe residues were critical for the proper UL20p membrane topology and efficient virion envelopment and infectious virus production. Surprisingly, a Phe-to-Ala change located approximately in the middle of the UL20p carboxyl terminus substantially enhanced cytoplasmic envelopment and overall production of infectious virions. This work revealed that Phe residues within the UL20p carboxyl terminus are involved in the regulation of cytoplasmic virion envelopment and infectious virus production.

Overexpression of herpes simplex virus glycoprotein K (gK) alters expression of HSV receptors in ocularly-infected mice

PURPOSE

We have shown previously that HSV-1 glycoprotein K (gK) exacerbates corneal scarring (CS) in mice and rabbits. Here, we investigated the relative impact of gK overexpression on host responses during primary corneal infection and latency in trigeminal ganglia (TG) of infected mice.

METHODS

Mice were infected ocularly with HSV-gK(3) (expressing two extra copies of gK replacing latency associated transcript [LAT]), HSV-gK(3) revertant (HSV-gK(3)R), or wild-type HSV-1 strain McKrae. Individual corneas on day 5 post infection (PI) and TG on day 28 PI were isolated and used for detection of gB DNA in the TG, HSV-1 receptors in the cornea and TG, and inflammatory infiltrates in TG.

RESULTS

During primary HSV-1 infection, gK overexpression resulted in altered expression of herpesvirus entry mediator (HVEM), 3-O-sulfated heparin sulfate (3-OS-HS), paired immunoglobulin-like type 2 receptor-α (PILR-α), nectin-1, and nectin-2 in cornea of BALB/c, but not C57BL/6 mice. However, gK overexpression did have an effect on 3-OS-HS, PILR-α, nectin-1, and nectin-2 expression (but not HVEM expression) in TG of C57BL/6 mice during latency. These differences did not affect the level of latency, but instead were correlated with the presence of CS. The presence of LAT increased HVEM expression and this effect was enhanced further by the presence of CS in latently-infected mice. Finally, the presence of LAT, but not overexpression of gK, affected CD4, CD8, TNF-α, Tim-3, PD-1, IL-21, IL-2, and IFN-γ expression in TG.

CONCLUSIONS

We demonstrate a novel link between gK exacerbation of CS and HSV-1 receptors, suggesting a gK-induced molecular route for the pathogenesis as well as selective advantage of these entry routes for the pathogen during latency-reactivation cycle.

Herpes simplex virus 1 protein UL37 interacts with viral glycoprotein gK and membrane protein UL20 and functions in cytoplasmic virion envelopment

We have shown that glycoprotein K (gK) and its interacting partner, the UL20 protein, play crucial roles in virion envelopment. Specifically, virions lacking either gK or UL20 fail to acquire an envelope, thus causing accumulation of capsids in the cytoplasm of infected cells. The herpes simplex virus 1 (HSV-1) UL37 protein has also been implicated in cytoplasmic virion envelopment. To further investigate the role of UL37 in virion envelopment, the recombinant virus DC480 was constructed by insertion of a 12-amino-acid protein C (protC) epitope tag within the UL37 amino acid sequence immediately after amino acid 480. The DC480 mutant virus expressed full-size UL37 as detected by the anti-protC antibody in Western immunoblots, accumulated unenveloped capsids in the cytoplasm of infected cells, and produced very small plaques on African green monkey kidney (Vero) cells that were similar in size to those produced by the UL20-null and UL37-null viruses. The DC480 virus replicated nearly 4 log less efficiently than the parental wild-type virus when grown on Vero cells. However, DC480 mutant virus titers increased nearly 20-fold when the virus was grown on FRT cells engineered to express the UL20 gene in comparison to the titers on Vero cells, while the UL37-null virus replicated approximately 20-fold less efficiently than the DC480 virus on FRT cells. Coimmunoprecipitation experiments and proximity ligation assays showed that gK and UL20 interact with the UL37 protein in infected cells. Collectively, these results indicate that UL37 interacts with the gK-UL20 protein complex to facilitate cytoplasmic virion envelopment.

IMPORTANCE

Herpes simplex viruses acquire their final envelopes by budding into cytoplasmic membranes derived from the trans-Golgi network (TGN). The tegument proteins UL36 and UL37 are known to be transported to the TGN sites of virus envelopment and to function in virion envelopment, since mutants lacking UL37 accumulate capsids in the cytoplasm that are unable to bud into TGN membranes. Viral glycoprotein K (gK) also functions in cytoplasmic envelopment, in a protein complex with the membrane-associated protein UL20 (UL20mp). This work shows for the first time that the UL37 protein functionally interacts with gK and UL20 to facilitate cytoplasmic virion envelopment. This work may lead to the design of specific drugs that can interrupt UL37 interactions with the gK-UL20 protein complex, providing new ways to combat herpesviral infections.

Binding of HSV-1 glycoprotein K (gK) to signal peptide peptidase (SPP) is required for virus infectivity

Glycoprotein K (gK) is a virion envelope protein of herpes simplex virus types 1 (HSV-1) and 2 (HSV-2), which plays important roles in virion entry, morphogenesis and egress. Two-hybrid and pull-down assays were utilized to demonstrate that gK and no other HSV-1 genes specifically binds to signal peptide peptidase (SPP), also known as minor histocompatibility antigen H13. SPP dominant negative mutants, shRNA against SPP significantly reduced HSV-1 replication in vitro. SPP also affected lysosomes and ER responses to HSV-1 infection. Thus, in this study we have shown for the first time that gK, despite its role in fusion and egress, is also involved in binding the cytoplasmic protein SPP. These results also suggest that SPP plays an important role in viral replication and possibly virus pathogenesis. This makes SPP unique in that its function appears to be required by the virus as no other protein can compensate its loss in terms of viral replication.

A replication competent HSV-1(McKrae) with a mutation in the amino-terminus of glycoprotein K (gK) is unable to infect mouse trigeminal ganglia after cornea infection

PURPOSE

To determine the role of the amino terminus of herpes simplex virus-1 (HSV-1) glycoprotein K (gK) in corneal infection, neuroinvasion, and establishment of virus latency in trigeminal ganglia of mice.

METHODS

The recombinant virus HSV-1 (McKΔgK31-68) was constructed by engineering gK genes encoding gK lacking 38 amino acids immediately after the gK signal sequence. A rescued virus was also produced. Mouse eyes were scarified and infected with 10(5) plaque forming units (PFU) in each eye. Clinical signs of ocular disease were monitored daily. Thirty days postinfection trigeminal ganglia were collected and processed for quantitative PCR (qPCR) analysis of viral DNA and recovery of infectious virions by cell culture of ganglionic tissues.

RESULTS

Deletion of the amino terminus of gK encoded by the McKΔgK31-68 mutant virus did not substantially affect its replication kinetics on African green monkey kidney cells (Vero), while it reduced cell-to-cell spread. McK viral infection of scarified mouse corneas with 10(5) PFU produced severe ocular disease. In contrast, McKΔgK31-68 viral infection with 10(5) PFU produced no significant ocular disease symptoms. All ganglia from mice infected with the McK virus produced high numbers of infectious virions upon explant culture in Vero cells, in agreement with qPCR results detecting high number of HSV-1 viral DNA in ganglionic tissues. In contrast, qPCR failed to detect any viral genomes in McKΔgK31-68 ganglia, while two of the ten ganglia revealed the presence of low numbers of infectious virions upon explant culture in Vero cells.

CONCLUSIONS

The results show that the amino terminus of gK is essential for neuroinvasiveness and acute herpes keratitis in the mouse eye model. It is likely that gK is involved in efficient infection of axonal termini, since mouse eye scarification provided a direct access to the high density of neuronal axons innervating mouse corneas.

Contributions of herpes simplex virus 1 envelope proteins to entry by endocytosis

Herpes simplex virus (HSV) proteins specifically required for endocytic entry but not direct penetration have not been identified. HSVs deleted of gE, gG, gI, gJ, gM, UL45, or Us9 entered cells via either pH-dependent or pH-independent endocytosis and were inactivated by mildly acidic pH. Thus, the required HSV glycoproteins, gB, gD, and gH-gL, may be sufficient for entry regardless of entry route taken. This may be distinct from entry mechanisms employed by other human herpesviruses.

The amino terminus of herpes simplex virus 1 glycoprotein K is required for virion entry via the paired immunoglobulin-like type-2 receptor alpha

The herpes simplex virus 1 (HSV-1) glycoprotein K (gK)/UL20 protein complex is incorporated into virion envelopes and cellular membranes and functions during virus entry and cell-to-cell spread. To investigate the role of gK/UL20 in the context of a highly neurovirulent virus strain, the HSV-1(McKrae) genome was cloned into a bacterial artificial chromosome plasmid (McKbac) and utilized to construct the mutant virus McK(gKΔ31-68), carrying a 37-amino-acid deletion within the gK amino terminus. The McKbac virus entered efficiently into Chinese hamster ovary (CHO) cells constitutively expressing HSV-1 human receptors, nectin-1, herpesvirus entry mediator (HVEM), or paired immunoglobulin-like type-2 receptor alpha (PILRα). In contrast, the McK(gKΔ31-68) virus failed to enter into CHO-PILRα cells, while it entered CHO cells expressing HVEM and nectin-1 more efficiently than the McKbac virus. Both McKbac and McK(gKΔ31-68) viruses entered all CHO cells expressing HSV-1 receptors via a pH-independent pathway. The HSV-1(F) gBΔ28syn mutant virus, encoding a carboxyl-terminal truncated gB, causes extensive cell fusion. Previously, we showed that the gKΔ31-68 amino acid deletion abrogated gBΔ28syn virus-induced cell fusion, indicating that the amino terminus of gK is required for gB-mediated virus-induced cell fusion (V. N. Chouljenko, A. V. Iyer, S. Chowdhury, D. V. Chouljenko, and K. G. J. Kousoulas, Virology 83:12301-12313, 2009). Surprisingly, the gKΔ31-68/gBΔ28syn virus caused extensive fusion of CHO-nectin-1 cells but limited cell fusion of CHO-PILRα cells. Coimmunoprecipitation experiments revealed that both gK and PILRα bound gB in infected cells. Collectively, these results indicate that the amino terminus of gK is functionally and physically associated with the gB-PILRα protein complex and regulates membrane fusion of the viral envelope with cellular membranes during virus entry as well as virus-induced cell-to-cell fusion.

The role of equine herpesvirus type 4 glycoprotein k in virus replication

Equine herpesvirus 4 (EHV-4) is an important equine pathogen that causes respiratory tract disease among horses worldwide. Glycoprotein K (gK) homologues have been identified in several alphaherpesviruses as a major player in virus entry, replication, and spread. In the present study, EHV-4 gK-deletion mutant has been generated by using bacterial artificial chromosome technology and Red mutagenesis to investigate the role of gK in EHV-4 replication. Our findings reported here show that gK is essential for virus replication in vitro and that the gK-negative strain was not able to be reconstituted in equine cells. It is noteworthy that these findings agree with the previously published study describing gK deletion in other alphaherpesviruses.

A herpes simplex virus 1 (McKrae) mutant lacking the glycoprotein K gene is unable to infect via neuronal axons and egress from neuronal cell bodies

We have shown that the herpes simplex virus 1 (HSV-1) gK gene is essential for efficient replication and spread in the corneal epithelium and trigeminal ganglion neuroinvasion in mice (A. T. David, A. Baghian, T. P. Foster, V. N. Chouljenko, and K. G. Kousoulas, Curr. Eye Res. 33:455-467, 2008). To further investigate the role of gK in neuronal infection, we utilized a microfluidic chamber system separating neuronal cell bodies and axonal termini. HSV-1 (McKrae) engineered virus constitutively expressing enhanced green fluorescence protein (GFP) was efficiently transmitted in both a retrograde and an anterograde manner. These results were corroborated by expression of virion structural proteins in either chamber, as well as detection of viral genomes and infectious viruses. In contrast, efficient infection of either chamber with a gK-null virus did not result in infection of the apposed chamber. These results show that gK is an important determinant in virion axonal infection. Moreover, the inability of the gK-null virus to be transmitted in an anterograde manner suggests that virions acquire cytoplasmic envelopes prior to entering axons.

IMPORTANCE

Herpes simplex virus 1 (HSV-1) enters mucosal epithelial cells and neurons via fusion of the viral envelope with cellular membranes, mediated by viral glycoprotein B (gB) in cooperation with other viral glycoproteins. Retrograde transport of virions to neuronal cell bodies (somata) establishes lifelong latent infection in ganglionic neurons. We have previously reported that gK binds gB and is required for gB-mediated membrane fusion (Jambunatathan et al., J. Virol. 85:12910-12918, 2011; V. N. Chouljenko, A. V. Iyer, S. Chowdhury, J. Kim, and K. G. Kousoulas, J. Virol. 84:8596-8606, 2010). In the current study, we constructed a recombinant virus with the gK gene deleted in the highly virulent ocular HSV-1 strain McKrae. This recombinant virus failed to infect rat ganglionic neuronal axons alone or cocultured with Vero cells in microfluidic chambers. In addition, lack of gK expression prevented anterograde transmission of virions. These results suggest that gK is a critical determinant for neuronal infection and transmission.

Amino acid differences in glycoproteins B (gB), C (gC), H (gH) and L (gL) are associated with enhanced herpes simplex virus type-1 (McKrae) entry via the paired immunoglobulin-like type-2 receptor α

BACKGROUND

Herpes simplex virus type-1 (HSV-1) enters into cells via membrane fusion of the viral envelope with plasma or endosomal membranes mediated by viral glycoproteins. HSV-1 virions attach to cell surfaces by binding of viral glycoproteins gC, gD and gB to specific cellular receptors. Here we show that the human ocular and highly neurovirulent HSV-1 strain McKrae enters substantially more efficiently into cells via the gB-specific human paired immunoglobulin-like type-2 receptor-α (hPILR-α). Comparison of the predicted amino acid sequences between HSV-1(F) and McKrae strains indicates that amino acid changes within gB, gC, gH and gL may cause increased entry via the hPILR- α receptor.

RESULTS

HSV-1 (McKrae) entered substantially more efficiently than viral strain F in Chinese hamster ovary (CHO) cells expressing hPIRL-α but not within CHO-human nectin-1, -(CHO-hNectin-1), CHO-human HVEM (CHO-hHVEM) or Vero cells. The McKrae genes encoding viral glycoproteins gB, gC, gD, gH, gL, gK and the membrane protein UL20 were sequenced and their predicted amino acid (aa) sequences were compared with virulent strains F, H129, and the attenuated laboratory strain KOS. Most aa differences between McKrae and F were located at their gB amino termini known to bind with the PILRα receptor. These aa changes included a C10R change, also seen in the neurovirulent strain ANG, as well as redistribution and increase of proline residues. Comparison of gC aa sequences revealed multiple aa changes including an L132P change within the 129-247 aa region known to bind to heparan sulfate (HS) receptors. Two aa changes were located within the H1 domain of gH that binds gL. Multiple aa changes were located within the McKrae gL sequence, which were preserved in the H129 isolate, but differed for the F strain. Viral glycoproteins gD and gK and the membrane protein UL20 were conserved between McKrae and F strains.

CONCLUSIONS

The results indicate that the observed entry phenotype of the McKrae strain is most likely due to a combination of increased binding to heparan sulfate receptors and enhanced virus entry via gB-mediated fusion of the viral envelope with plasma membranes.

Functional hierarchy of herpes simplex virus 1 viral glycoproteins in cytoplasmic virion envelopment and egress

Herpes simplex virus 1 (HSV-1) viral glycoproteins gD (carboxyl terminus), gE, gK, and gM, the membrane protein UL20, and membrane-associated protein UL11 play important roles in cytoplasmic virion envelopment and egress from infected cells. We showed previously that a recombinant virus carrying a deletion of the carboxyl-terminal 29 amino acids of gD (gDΔct) and the entire gE gene (ΔgE) did not exhibit substantial defects in cytoplasmic virion envelopment and egress (H. C. Lee et al., J. Virol. 83:6115-6124, 2009). The recombinant virus ΔgM2, engineered not to express gM, produced a 3- to 4-fold decrease in viral titers and a 50% reduction in average plaque sizes in comparison to the HSV-1(F) parental virus. The recombinant virus containing all three mutations, gDΔct-ΔgM2-ΔgE, replicated approximately 1 log unit less efficiently than the HSV-1(F) parental virus and produced viral plaques which were on average one-third the size of those of HSV-1(F). The recombinant virus ΔUL11-ΔgM2, engineered not to express either UL11 or gM, replicated more than 1 log unit less efficiently and produced significantly smaller plaques than UL11-null or gM-null viruses alone, in agreement with the results of Leege et al. (T. Leege et al., J. Virol. 83:896-907, 2009). Analyses of particle-to-PFU ratios, relative plaque size, and kinetics of virus growth and ultrastructural visualization of glycoprotein-deficient mutant and wild-type virions indicate that gDΔct, gE, and gM function in a cooperative but not redundant manner in infectious virion morphogenesis. Overall, comparisons of single, double, and triple mutant viruses generated in the same HSV-1(F) genetic background indicated that lack of either UL20 or gK expression caused the most severe defects in cytoplasmic envelopment, egress, and infectious virus production, followed by the double deletion of UL11 and gM.

Glycoproteins D of equine herpesvirus type 1 (EHV-1) and EHV-4 determine cellular tropism independently of integrins

Equine herpesvirus type 1 (EHV-1) and EHV-4 are genetically and antigenically very similar, but their pathogenic potentials are strikingly different. The differences in pathogenicity between both viruses seem to be reflected in cellular host range: EHV-1 can readily be propagated in many cell types of multiple species, while EHV-4 entry and replication appear to be restricted mainly to equine cells. The clear difference in cellular tropism may well be associated with differences in the gene products involved in virus entry and/or spread from cell to cell. Here we show that (i) most of the EHV-1 permissive cell lines became resistant to EHV-1 expressing EHV-4 glycoprotein D (gD4) and the opposite was observed for EHV-4 harboring EHV-1 gD (gD1). (ii) The absence of integrins did not inhibit entry into and replication of EHV-1 in CHO-K1 or peripheral blood mononuclear cells (PBMC). Furthermore, integrin-negative K562 cells did not acquire the ability to bind to gD1 when αVβ3 integrin was overexpressed. (iii) PBMC could be infected with similar efficiencies by both EHV-1 and EHV-4 in vitro. (iv) In contrast to results for equine fibroblasts and cells of endothelial or epithelial origin, we were unable to block entry of EHV-1 or EHV-4 into PBMC with antibodies directed against major histocompatibility complex class I (MHC-I), a result that indicates that these viruses utilize a different receptor(s) to infect PBMC. Cumulatively, we provide evidence that efficient EHV-1 and EHV-4 entry is dependent mainly on gD, which can bind to multiple cell surface receptors, and that gD has a defining role with respect to cellular host range of EHV-1 and EHV-4.

Efficient generation and rapid isolation via stoplight recombination of Herpes simplex viruses expressing model antigenic and immunological epitopes

Generation and isolation of recombinant herpesviruses by traditional homologous recombination methods can be a tedious, time-consuming process. Therefore, a novel stoplight recombination selection method was developed that facilitated rapid identification and purification of recombinant viruses expressing fusions of immunological epitopes with EGFP. This "traffic-light" approach provided a visual indication of the presence and purity of recombinant HSV-1 isolates by producing three identifying signals: (1) red fluorescence indicates non-recombinant viruses that should be avoided; (2) yellow fluorescence indicates cells co-infected with non-recombinant and recombinant viruses that are chosen with caution; (3) green fluorescence indicates pure recombinant isolates and to proceed with preparation of viral stocks. Adaptability of this system was demonstrated by creating three recombinant viruses that expressed model immunological epitopes. Diagnostic PCR established that the fluorescent stoplight indicators were effective at differentiating between the presence of background virus contamination and pure recombinant viruses specifying immunological epitopes. This enabled isolation of pure recombinant viral stocks that exhibited wildtype-like viral replication and cell-to-cell spread following three rounds of plaque purification. Expression of specific immunological epitopes was confirmed by western analysis, and the utility of these viruses for examining host immune responses to HSV-1 was determined by a functional T cell assay.

Site-specific proteolytic cleavage of the amino terminus of herpes simplex virus glycoprotein K on virion particles inhibits virus entry

Herpes simplex virus 1 (HSV-1) glycoprotein K (gK) is expressed on virions and functions in entry, inasmuch as HSV-1(KOS) virions devoid of gK enter cells substantially slower than is the case for the parental KOS virus (T. P. Foster, G. V. Rybachuk, and K. G. Kousoulas, J. Virol. 75:12431-12438, 2001). Deletion of the amino-terminal 68-amino-acid (aa) portion of gK caused a reduction in efficiency and kinetics of virus entry similar to that of the gK-null virus in comparison to the HSV-1(F) parental virus. The UL20 membrane protein and gK were readily detected on double-gradient-purified virion preparations. Immuno-electron microscopy confirmed the presence of gK and UL20 on purified virions. Coimmunoprecipitation experiments using purified virions revealed that gK interacted with UL20, as has been shown in virus-infected cells (T. P. Foster, V. N. Chouljenko, and K. G. Kousoulas, J. Virol. 82:6310-6323, 2008). Scanning of the HSV-1(F) viral genome revealed the presence of a single putative tobacco etch virus (TEV) protease site within gD, while additional TEV predicted sites were found within the UL5 (helicase-primase helicase subunit), UL23 (thymidine kinase), UL25 (DNA packaging tegument protein), and UL52 (helicase-primase primase subunit) proteins. The recombinant virus gDΔTEV was engineered to eliminate the single predicted gD TEV protease site without appreciably affecting its replication characteristics. The mutant virus gK-V5-TEV was subsequently constructed by insertion of a gene sequence encoding a V5 epitope tag in frame with the TEV protease site immediately after gK amino acid 68. The gK-V5-TEV, R-gK-V5-TEV (revertant virus), and gDΔTEV viruses exhibited similar plaque morphologies and replication characteristics. Treatment of the gK-V5-TEV virions with TEV protease caused approximately 32 to 34% reduction of virus entry, while treatment of gDΔTEV virions caused slightly increased virus entry. These results provide direct evidence that the gK and UL20 proteins, which are genetically and functionally linked to gB-mediated virus-induced cell fusion, are structural components of virions and function in virus entry. Site-specific cleavage of viral glycoproteins on mature and fully infectious virions utilizing unique protease sites may serve as a generalizable method of uncoupling the roles of viral glycoproteins in virus entry and virion assembly.

Escape of herpesviruses from the nucleus

The nuclear envelope of eukaryotic cells is composed of double lipid-bilayer membranes, the membrane-connected nuclear pore complexes and an underlying nuclear lamina network. The nuclear pore complexes serve as gates for regulating the transport of macromolecules between cytoplasm and nucleus. The nuclear lamina not only provides an intact meshwork for maintaining the nuclear stiffness but also presents a natural barrier against most DNA viruses. Herpesviruses are large DNA viruses associated with multiple human and animal diseases. The complex herpesviral virion contains more than 30 viral proteins. After viral DNA replication, the newly synthesised genome is packaged into the pre-assembled intranuclear capsid. The nucleocapsid must then transverse through the nuclear envelope to the cytoplasm for the subsequent maturation process. Information regarding how nucleocapsid breaches the rigid nuclear lamina barrier and accesses the inner nuclear membrane for primary envelopment has emerged recently. From the point of view of both viral components and nuclear structure, this review summarises recent advances in the complicated protein-protein interactions and the phosphorylation regulations involved in the nuclear egress of herpesviral nucleocapsids.

Exacerbation of corneal scarring in HSV-1 gK-immunized mice correlates with elevation of CD8+CD25+ T cells in corneas of ocularly infected mice

We have shown previously that exacerbation of corneal scarring (CS) in HSV-1 glycoprotein K (gK) immunized mice was associated with CD8+ T cells. In this study, we investigated the type and the nature of the immune responses that are involved in the exacerbation of CS in gK-immunized animals. BALB/c mice were vaccinated with baculovirus expressed gK, gD, or mock-immunized. Twenty-one days after the third immunization, mice were ocularly infected with 2 x 10(5) PFU/eye of virulent HSV-1 strain McKrae. Infiltration of the cornea by CD4+, CD8+, CD25+, CD4+CD25+, CD8+CD25+, CD19+, CD40+, CD40L+, CD62L+, CD95+, B7-1+, B7-2+, MHC-I+, and MHC-II+ cells was monitored by immunohistochemistry, qRT-PCR and FACS at various times post-infection (PI). This study demonstrated for the first time that the presence of CD8+CD25+ T cells in the cornea is correlated with exacerbation of CS in the gK-immunized group.

Differential importance of highly conserved residues in UL24 for herpes simplex virus 1 replication in vivo and reactivation

The UL24 gene of herpes simplex virus 1 (HSV-1) is widely conserved among all subfamilies of the Herpesviridae. It is one of only four HSV-1 genes for which mutations have been mapped that confer a syncytial plaque phenotype. In a mouse model of infection, UL24-deficient viruses exhibit reduced titres, particularly in neurons, and an apparent defect in reactivation from latency. There are several highly conserved residues in UL24; however, their importance in the role of UL24 in vivo is unknown. In this study, we compared virus strains with substitution mutations corresponding to the PD-(D/E)XK endonuclease motif of UL24 (vUL24-E99A/K101A) or a substitution of another highly conserved residue (vUL24-G121A). Both mutant viruses cause the formation of syncytial plaques at 39 degrees C; however, we found that the viruses differed dramatically when tested in a mouse model of infection. vUL24-E99A/K101A exhibited titres in the eye that were 10-fold lower than those of the wild-type virus KOS, and titres in trigeminal ganglia (TG) that were more than 2 log10 lower. Clinical signs were barely detectable with vUL24-E99A/K101A. Furthermore, the percentage of TG from which virus reactivated was also significantly lower for this mutant than for KOS. In contrast, vUL24-G121A behaved similarly to the wild-type virus in mice. These results are consistent with the endonuclease motif being important for the role of UL24 in vivo and also imply that the UL24 temperature-dependent syncytial plaque phenotype can be separated genetically from several in vivo phenotypes.

The amino terminus of herpes simplex virus type 1 glycoprotein K (gK) modulates gB-mediated virus-induced cell fusion and virion egress

Herpes simplex virus type 1 (HSV-1)-induced cell fusion is mediated by viral glycoproteins and other membrane proteins expressed on infected cell surfaces. Certain mutations in the carboxyl terminus of HSV-1 glycoprotein B (gB) and in the amino terminus of gK cause extensive virus-induced cell fusion. Although gB is known to be a fusogenic glycoprotein, the mechanism by which gK is involved in virus-induced cell fusion remains elusive. To delineate the amino-terminal domains of gK involved in virus-induced cell fusion, the recombinant viruses gKDelta31-47, gKDelta31-68, and gKDelta31-117, expressing gK carrying in-frame deletions spanning the amino terminus of gK immediately after the gK signal sequence (amino acids [aa] 1 to 30), were constructed. Mutant viruses gKDelta31-47 and gKDelta31-117 exhibited a gK-null (DeltagK) phenotype characterized by the formation of very small viral plaques and up to a 2-log reduction in the production of infectious virus in comparison to that for the parental HSV-1(F) wild-type virus. The gKDelta31-68 mutant virus formed substantially larger plaques and produced 1-log-higher titers than the gKDelta31-47 and gKDelta31-117 mutant virions at low multiplicities of infection. Deletion of 28 aa from the carboxyl terminus of gB (gBDelta28syn) caused extensive virus-induced cell fusion. However, the gBDelta28syn mutation was unable to cause virus-induced cell fusion in the presence of the gKDelta31-68 mutation. Transient expression of a peptide composed of the amino-terminal 82 aa of gK (gKa) produced a glycosylated peptide that was efficiently expressed on cell surfaces only after infection with the HSV-1(F), gKDelta31-68, DeltagK, or UL20-null virus. The gKa peptide complemented the gKDelta31-47 and gKDelta31-68 mutant viruses for infectious-virus production and for gKDelta31-68/gBDelta28syn-mediated cell fusion. These data show that the amino terminus of gK modulates gB-mediated virus-induced cell fusion and virion egress.

The herpes simplex virus type 1 glycoprotein D (gD) cytoplasmic terminus and full-length gE are not essential and do not function in a redundant manner for cytoplasmic virion envelopment and egress

Herpes simplex virus type 1 (HSV-1) acquires its final envelope by budding into cytoplasmic vesicles thought to be derived from trans-Golgi network membranes. This process is facilitated by interactions among the carboxyl termini of viral glycoproteins and tegument proteins. To directly investigate the relative importance of the carboxyl terminus of glycoprotein D (gD) in the presence or absence of gE, a recombinant virus (gDDeltact) was constructed to specify a truncated gD lacking the carboxy-terminal 29 amino acids. Furthermore, two additional recombinant viruses were constructed by mutating from ATG to CTG the initiation codons of gE (gEctg) or both gE and gM (gEctg+gMctg), causing lack of expression of gE or both gE and gM, respectively. A fourth mutant virus was constructed to specify the gEctg+gDDeltact mutations. The replication properties of these viruses were compared to those of a newly constructed recombinant virus unable to express UL20 due to alteration of the two initiation codons of UL20 (UL20ctgctg). All recombinant viruses were constructed by using the double-Red, site-directed mutagenesis system implemented on the HSV-1(F) genome cloned into a bacterial artificial chromosome. The gEctg, gEctg+gMctg, gDDeltact, and gEctg+gDDeltact viruses produced viral plaques on African monkey kidney cells (Vero), as well as other cells, that were on average approximately 30 to 50% smaller than those produced by the wild-type virus HSV-1(F). In contrast, the UL20ctgctg virus produced very small plaques containing three to five cells, as reported previously for the DeltaUL20 virus lacking the entire UL20 gene. Viral replication kinetics of intracellular and extracellular viruses revealed that all recombinant viruses produced viral titers similar to those produced by the wild-type HSV-1(F) virus intracellularly and extracellularly at late times postinfection, with the exception of the UL20ctgctg and DeltaUL20 viruses, which replicated more than two-and-a-half logs less efficiently than HSV-1(F). Electron microscopy confirmed that all viruses, regardless of their different gene mutations, efficiently produced enveloped virions within infected cells, with the exception of the UL20ctgctg and DeltaUL20 viruses, which accumulated high levels of unenveloped virions in the cytoplasm. These results show that the carboxyl terminus of gD and the full-length gE, either alone or in a redundant manner, are not essential in cytoplasmic virion envelopment and egress from infected cells. Similarly, gM and gE do not function alone or in a redundant manner in cytoplasmic envelopment and virion egress, confirming previous findings.

The herpes simplex virus type 1 (HSV-1) glycoprotein K(gK) is essential for viral corneal spread and neuroinvasiveness

PURPOSE

To determine the role of herpes simplex virus-1 (HSV-1) glycoprotein K(gK) in corneal infection, neuroinvasion, and virus latency in trigeminal ganglia of mice.

METHODS

The recombinant virus HSV-1 (McKrae) Delta gK (MKDelta gK) carrying a deletion of the gK gene was constructed by insertional/deletion mutagenesis and replaced by a gene cassette constitutively expressing the enhanced green fluorescence protein. The gK deletion of the MKDelta gK virus was rescued to produce the wild-type-like virus MKgK. Balb/c mice were infected ocularly with either virus, and the infection pattern in the eye, clinical disease progression, and establishment of viral latency was monitored.

RESULTS

Mice infected with the MKDelta gK strain produced in a gK complementing cell line did not exhibit clinical signs when compared with mice infected with the MKgK virus. Direct visualization of infected eyes revealed that the MKDelta gK virus was unable to spread in mouse corneas, while the MKgK rescued virus spread efficiently. Nineteen of 20 scarified and 5/12 unscarified mice infected with the MKgK virus produced infectious virus after coculture with permissive cells, while 0/20 scarified and 0/12 unscarified mice infected with the MKDelta gK virus produced infectious virus. HSV DNA was detected in trigeminal ganglia by PCR in 19/20 scarified and 9/12 unscarified mice inoculated with MKgK, while HSV DNA was detected in the trigeminal ganglia of 3/20 scarified and 0/12 unscarified mice inoculated with MKDelta gK.

CONCLUSIONS

The results show that HSV-1 gK is essential for efficient replication and spread in the corneal epithelium and trigeminal ganglia neuroinvasion in MKDelta gK inoculated mice.

Functional and physical interactions of the herpes simplex virus type 1 UL20 membrane protein with glycoprotein K

Herpes simplex virus type 1 glycoprotein K (gK) and the UL20 protein (UL20p) are coordinately transported to the trans-Golgi network (TGN) and cell surfaces and are required for cytoplasmic virion envelopment at the TGN. In addition, cell surface expression of gK and UL20p is required for virus-induced cell fusion. Previously, confocal microscopy colocalization and intracellular transport experiments strongly suggested direct protein-protein interactions between gK and UL20p. Direct protein-protein interactions between gK and UL20p were demonstrated through reciprocal coimmunoprecipitation experiments, as well as with glutathione S-transferase (GST) pull-down experiments. A fusion protein consisting of the amino-terminal 66 amino acids of UL20p fused in-frame with GST was expressed in Escherichia coli and purified via glutathione column chromatography. Precipitation of GST-UL20p from mixtures of GST-UL20p fusion protein with cellular extracts containing gK specifically coprecipitated gK but not other viral glycoproteins. The purified UL20p-GST fusion protein reacted with all gK-associated protein species. It was concluded that the amino terminus of UL20p, most likely, interacted with gK domain III, which is predicted to lie intracellularly. UL20p-gK domain-specific interactions must serve important functions in the coordinate transport of UL20p and gK to the TGN, because retention of UL20p in the endoplasmic reticulum (ER) via the addition of an ER retention signal at the carboxyl terminus of UL20p forced the ER retention of gK and drastically inhibited intracellular virion envelopment and virus-induced cell fusion.

Glycoproteins required for entry are not necessary for egress of pseudorabies virus

In the current perception of the herpesvirus replication cycle, two fusion processes are thought to occur during entry and nuclear egress. For penetration, glycoproteins gB and gH/gL have been shown to be essential, whereas a possible role of these glycoproteins in nuclear egress remains unclear. Viral envelope glycoproteins have been detected by immunolabeling in the nuclear membrane as well as in primary enveloped particles in several herpesviruses, indicating that they might be involved in the fusion process. Moreover, a herpes simplex virus type 1 mutant simultaneously lacking gB and gH was described to be deficient in nuclear egress (A. Farnsworth, T. W. Wisner, M. Webb, R. Roller, G. Cohen, R. Eisenberg, and D. C. Johnson, Proc. Natl. Acad. Sci. USA 104:10187-10192, 2007). To analyze the situation in the related alphaherpesvirus pseudorabies virus (PrV), mutants carrying single and double deletions of glycoproteins gB, gD, gH, and gL were constructed and characterized. We show here that the simultaneous deletion of gB and gD, gB and gH, gD and gH, or gH and gL has no detectable effect on PrV egress, implying that none of these glycoproteins either singly or in the tested combinations is required for nuclear egress. In addition, immunolabeling studies using different mono- or polyclonal sera raised against various PrV glycoproteins did not reveal the presence of viral glycoproteins in the inner nuclear membrane or in primary virions. Thus, our data strongly suggest that different fusion mechanisms are active during virus entry and egress.

The herpes simplex virus UL20 protein functions in glycoprotein K (gK) intracellular transport and virus-induced cell fusion are independent of UL20 functions in cytoplasmic virion envelopment

The HSV-1 UL20 protein (UL20p) and glycoprotein K (gK) are both important determinants of cytoplasmic virion morphogenesis and virus-induced cell fusion. In this manuscript, we examined the effect of UL20 mutations on the coordinate transport and Trans Golgi Network (TGN) localization of UL20p and gK, virus-induced cell fusion and infectious virus production. Deletion of 18 amino acids from the UL20p carboxyl terminus (UL20 mutant 204t) inhibited intracellular transport and cell-surface expression of both gK and UL20, resulting in accumulation of UL20p and gK in the endoplasmic reticulum (ER) in agreement with the inability of 204t to complement UL20-null virus replication and virus-induced cell fusion. In contrast, less severe carboxyl terminal deletions of either 11 or six amino acids (UL20 mutants 211t and 216t, respectively) allowed efficient UL20p and gK intracellular transport, cell-surface expression and TGN colocalization. However, while both 211t and 216t failed to complement for infectious virus production, 216t complemented for virus-induced cell fusion, but 211t did not. These results indicated that the carboxyl terminal six amino acids of UL20p were crucial for infectious virus production, but not involved in intracellular localization of UL20p/gK and concomitant virus-induced cell fusion. In the amino terminus of UL20, UL20p mutants were produced changing one or both of the Y38 and Y49 residues found within putative phosphorylation sites. UL20p tyrosine-modified mutants with both tyrosine residues changed enabled efficient intracellular transport and TGN localization of UL20p and gK, but failed to complement for either infectious virus production, or virus-induced cell fusion. These results show that UL20p functions in cytoplasmic envelopment are separable from UL20 functions in UL20p intracellular transport, cell surface expression and virus-induced cell fusion.

A recombinant herpes simplex virus type 1 expressing two additional copies of gK is more pathogenic than wild-type virus in two different strains of mice

The effect of glycoprotein K (gK) overexpression on herpes simplex virus type 1 (HSV-1) infection in two different strains of mice was evaluated using a recombinant HSV-1 virus that expresses two additional copies of the gK gene in place of the latency-associated transcript (LAT). This mutant virus (HSV-gK3) expressed higher levels of gK than either the wild-type McKrae virus or the parental dLAT2903 virus both in vitro (in cultured cells) and in vivo (in infected mouse corneas and trigeminal ganglia [TG] of BALB/c and C57BL/6 mice). gK transcripts were detected in the TG of both HSV-gK3-infected mouse strains on day 30 postinfection (p.i.), while gB transcripts were detected only in the TG of the HSV-gK3-infected C57BL/6 mice, a finding that suggests that increased gK levels promote chronic infection. C57BL/6 mice infected with HSV-gK3 also contained free virus in their TG on day 30 p.i. Both HSV-gK3-infected mouse strains had significantly higher corneal scarring (CS) than did McKrae-infected mice. T-cell depletion studies in C57BL/6 mice suggested that this CS enhancement in the HSV-gK3-infected mice was mediated by a CD8+ T-cell response. Taken together, these results strongly suggest that increased gK levels promote eye disease and chronic infection in infected mice.

Effective treatment of human breast tumor in a mouse xenograft model with herpes simplex virus type 1 specifying the NV1020 genomic deletion and the gBsyn3 syncytial mutation enabling high viral replication and spread in breast cancer cells

A new oncolytic and fusogenic herpes simplex virus type 1 (HSV-1) was constructed on the basis of the wildtype HSV-1(F) strain. To provide for safety and tumor selectivity, the virus carried a large deletion including one of the two alpha4, gamma(1)34.5, alpha0 genes and the latency-associated transcript region. The gamma(1)34.5 gene, a major neurovirulence factor, was replaced by a gene cassette constitutively expressing the red fluorescent protein gene. Homologous recombination was used to transfer the fusogenic gBsyn3 mutation to the viral genome to produce the OncSyn virus. OncSyn causes extensive virus-induced cell fusion (syncytia) and replicates to higher titers than the parental Onc and HSV-1(F) strains in breast cancer cells. Biochemical analysis revealed that the OncSyn virus retains a stable genome and expresses all major viral glycoproteins. A xenograft mouse model system using MDA-MB-435S-luc (MM4L) human breast cancer cells constitutively expressing the luciferase gene implanted within the interscapular region of animals was used to test the ability of the virus to inactivate breast tumor cells in vivo. Seventy-two mice bearing MM4L breast cancer xenografts were randomly divided into three groups and given two rounds of three consecutive intratumoral injections of OncSyn, inactivated OncSyn, or phosphate-buffered saline 3 days apart. A single round of virus injections resulted in a drastic reduction of tumor sizes (p <or= 0.0001) and diminution of chemiluminescence emitted by the cancer cells (p <or= 0.0002). This effect was enhanced by a second round of virus injections into the tumors 3 days after the first round (p <or= 0.0001). Systematic necropsy and pathological evaluation of the primary tumors revealed that the single round of injections resulted in extensive necrosis of tumor cells (p <or= 0.0001), which was enhanced by the second round of injections (p <or= 0.0002). Internal organs were not affected by virus inoculation. Mouse weights were not significantly impacted by any treatment during the course of the entire study (p = 0.46). These results show that the attenuated, fusogenic, and oncolytic HSV-1(F) virus strain OncSyn may effectively treat human breast tumors in vivo.

Epitope mapping of HSV-1 glycoprotein K (gK) reveals a T cell epitope located within the signal domain of gK

Glycoprotein K (gK) is a virion envelope component of herpes simplex virus types 1 (HSV-1) and 2 (HSV-2), which plays an important role in virion morphogenesis and egress. We previously demonstrated that immunization of mice with gK, but not with any of the 10 other HSV-1 glycoproteins, resulted in exacerbation of corneal scarring and herpetic dermatitis following ocular HSV-1 infection. However, little is known about the gK epitope(s) that is (are) involved in T cell activities in vitro or in vivo. Thus, epitope mapping of gK was performed using a panel of 15-mer peptides with five-amino acid overlaps spanning the full-length gK, and four expressed gK recombinant proteins representing different regions of gK. Epitope mapping within the gK polypeptide defined the amino acid sequence STVVLITAYGLVLVW as the predominant CD4(+) and CD8(+) T cell stimulatory region both in vitro and in vivo. IFN-gamma expression by CD4(+) T cells was CD8(+) T cells-dependent. This immunodominant epitope is located within the signal sequence of the gK polypeptide and is highly conserved in HSV-1 and HSV-2 strains. Using prediction algorithms, the peptide is predicted to bind to numerous MHC class I and class II molecules.

Involvement of UL24 in herpes-simplex-virus-1-induced dispersal of nucleolin

UL24 of herpes simplex virus 1 is important for efficient viral replication, but its function is unknown. We generated a recombinant virus, vHA-UL24, encoding UL24 with an N-terminal hemagglutinin tag. By indirect immunofluorescence at 9 h post-infection (hpi), we detected HA-UL24 in nuclear foci and in cytoplasmic speckles. HA-UL24 partially co-localized with nucleolin, but not with ICP8 or coilin, markers for nucleoli, viral replication compartments, and Cajal bodies respectively. HA-UL24 staining was often juxtaposed to that of another nucleolar protein, fibrillarin. Analysis of HSV-1-induced nucleolar modifications revealed that by 18 hpi, nucleolin staining had dispersed, and fibrillarin staining went from clusters of small spots to a few separate but prominent spots. Fibrillarin redistribution appeared to be independent of UL24. In contrast, cells infected with a UL24-deficient virus retained foci of nucleolin staining. Our results demonstrate involvement of UL24 in dispersal of nucleolin during infection.

UL20 protein functions precede and are required for the UL11 functions of herpes simplex virus type 1 cytoplasmic virion envelopment

Egress of herpes simplex virus type 1 (HSV-1) from the nucleus of the infected cell to extracellular spaces involves a number of distinct steps, including primary envelopment by budding into the perinuclear space, de-envelopment into the cytoplasm, cytoplasmic reenvelopment, and translocation of enveloped virions to extracellular spaces. UL20/gK-null viruses are blocked in cytoplasmic virion envelopment and egress, as indicated by an accumulation of unenveloped or partially enveloped capsids in the cytoplasm. Similarly, UL11-null mutants accumulate unenveloped capsids in the cytoplasm. To assess whether UL11 and UL20/gK function independently or synergistically in cytoplasmic envelopment, recombinant viruses having either the UL20 or UL11 gene deleted were generated. In addition, a recombinant virus containing a deletion of both UL20 and UL11 genes was constructed using the HSV-1(F) genome cloned into a bacterial artificial chromosome. Ultrastructural examination of virus-infected cells showed that both UL20- and UL11-null viruses accumulated unenveloped capsids in the cytoplasm. However, the morphology and distribution of the accumulated capsids appeared to be distinct, with the UL11-null virions forming aggregates of capsids having diffuse tegument-derived material and the UL20-null virus producing individual capsids in close juxtaposition to cytoplasmic membranes. The UL20/UL11 double-null virions appeared morphologically similar to the UL20-null viruses. Experiments on the kinetics of viral replication revealed that the UL20/UL11 double-null virus replicated in a manner similar to the UL20-null virus. Additional experiments revealed that transiently expressed UL11 localized to the trans-Golgi network (TGN) independently of either gK or UL20. Furthermore, virus infection with the UL11/UL20 double-null virus did not alter the TGN localization of transiently expressed UL11 or UL20 proteins, indicating that these proteins did not interact. Taken together, these results show that the intracellular transport and TGN localization of UL11 is independent of UL20/gK functions, and that UL20/gK are required and function prior to UL11 protein in virion cytoplasmic envelopment.

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.

Intracellular transport and stability of varicella-zoster virus glycoprotein K

VZV gK, an essential glycoprotein that is conserved among the alphaherpesviruses, is believed to participate in membrane fusion and cytoplasmic virion morphogenesis based on analogy to its HSV-1 homolog. However, the production of VZV gK-specific antibodies has proven difficult presumably due to its highly hydrophobic nature and, therefore, VZV gK has received limited study. To overcome this obstacle, we inserted a FLAG epitope into gK near its amino terminus and produced VZV recombinants expressing epitope-tagged gK (VZV gK-F). These recombinants grew indistinguishably from native VZV, and FLAG-tagged gK could be readily detected in VZV gK-F-infected cells. FACS analysis established that gK is transported to the plasma membrane of infected cells, while indirect immunofluorescence demonstrated that gK accumulates predominately in the Golgi. Using VZV gK-F-infected cells we demonstrated that VZV gK, like several other herpesvirus glycoproteins, is efficiently endocytosed from the plasma membrane. However, pulse-labeling experiments revealed that the half-life of gK is considerably shorter than that of other VZV glycoproteins including gB, gE and gH. This finding suggests that gK may be required in lower abundance than other viral glycoproteins during virion morphogenesis or viral entry.