Cell fusion induced by herpes simplex virus glycoproteins gB, gD, and gH-gL requires a gD receptor but not necessarily heparan sulfate

Aescin Inhibits Herpes simplex Virus Type 1 Induced Membrane Fusion

Infections with Herpes simplex virus can cause severe ocular diseases and encephalitis. The present study aimed to investigate potential inhibitors of fusion between HSV-1 and the cellular membrane of the host cell. Fusion and entry of HSV-1 into the host cell is mimicked by a virus-free eukaryotic cell culture system by co-expression of the HSV-1 glycoproteins gD, gH, gL, and gB in presence of a gD receptor, resulting in excessive membrane fusion and polykaryocyte formation. A microscopic read-out was used for the screening of potential inhibitors, whereas luminometric quantification of cell-cell fusion was used in a reporter fusion assay. HSV-1 gB was tagged at its C-terminus with mCherry to express mCherry-gB in both assay systems for the visualization of the polykaryocyte formation. Reporter protein expression of SEAP was regulated by a Tet-On 3 G system. The saponin mixture aescin was identified as the specific inhibitor (IC50 7.4 µM, CC50 24.3 µM, SI 3.3) of membrane fusion. A plaque reduction assay on Vero cells reduced HSV-1 entry into cells and HSV-1 cell-to-cell spread significantly; 15 µM aescin decreased relative plaque counts to 41%, and 10 µM aescin resulted in a residual plaque size of 11% (HSV-1 17 syn+) and 2% (HSV-1 ANG path). Release of the HSV-1 progeny virus was reduced by one log step in the presence of 15 µM aescin. Virus particle integrity was mainly unaffected. Analytical investigation of aescin by UHPLC-MS revealed aescin IA and -IB and isoaescin IA and -IB as the main compounds with different functional activities. Aescin IA had the lowest IC50, the highest CC50, and an SI of > 4.6.

Development of a vesicular stomatitis virus pseudotyped with herpes B virus glycoproteins and its application in a neutralizing antibody detection assay

Herpes B virus (BV) is a zoonotic virus and belongs to the genus Simplexvius, the same genus as human herpes simplex virus (HSV). BV typically establishes asymptomatic infection in its natural hosts, macaque monkeys. However, in humans, BV infection causes serious neurological diseases and death. As such, BV research can only be conducted in a high containment level facility (i.e., biosafety level [BSL] 4), and the mechanisms of BV entry have not been fully elucidated. In this study, we generated a pseudotyped vesicular stomatitis virus (VSV) expressing BV glycoproteins using G-complemented VSV∆G system, which we named VSV/BVpv. We found that four BV glycoproteins (i.e., gB, gD, gH, and gL) were required for the production of a high-titer VSV/BVpv. Moreover, VSV/BVpv cell entry was dependent on the binding of gD to its cellular receptor nectin-1. Pretreatment of Vero cells with endosomal acidification inhibitors did not affect the VSV/BVpv infection. The result indicated that VSV/BVpv entry occurred by direct fusion with the plasma membrane of Vero cells and suggested that the entry pathway was similar to that of native HSV. Furthermore, we developed a VSV/BVpv-based chemiluminescence reduction neutralization test (CRNT), which detected the neutralization antibodies against BV in macaque plasma samples with high sensitivity and specificity. Crucially, the VSV/BVpv generated in this study can be used under BSL-2 condition to study the initial entry process through gD-nectin-1 interaction and the direct fusion of BV with the plasma membrane of Vero cells.IMPORTANCEHerpes B virus (BV) is a highly pathogenic zoonotic virus against humans. BV belongs to the genus Simplexvius, the same genus as human herpes simplex virus (HSV). By contrast to HSV, cell entry mechanisms of BV are not fully understood. The research procedures to manipulate infectious BV should be conducted in biosafety level (BSL)-4 facilities. As pseudotyped viruses provide a safe viral entry model because of their inability to produce infectious progeny virus, we tried to generate a pseudotyped vesicular stomatitis virus bearing BV glycoproteins (VSV/BVpv) by modification of expression constructs of BV glycoproteins, and successfully obtained VSV/BVpv with a high titer. This study has provided novel information for constructing VSV/BVpv and its usefulness to study BV infection.

Models of Herpes Simplex Virus Latency

Our current understanding of HSV latency is based on a variety of clinical observations, and in vivo, ex vivo, and in vitro model systems, each with unique advantages and drawbacks. The criteria for authentically modeling HSV latency include the ability to easily manipulate host genetics and biological pathways, as well as mimicking the immune response and viral pathogenesis in human infections. Although realistically modeling HSV latency is necessary when choosing a model, the cost, time requirement, ethical constraints, and reagent availability are also equally important. Presently, there remains a pressing need for in vivo models that more closely recapitulate human HSV infection. While the current in vivo, ex vivo, and in vitro models used to study HSV latency have limitations, they provide further insights that add to our understanding of latency. In vivo models have shed light on natural infection routes and the interplay between the host immune response and the virus during latency, while in vitro models have been invaluable in elucidating molecular pathways involved in latency. Below, we review the relative advantages and disadvantages of current HSV models and highlight insights gained through each.

Development of a novel, high-efficacy oncolytic herpes simplex virus type 1 platform equipped with two distinct retargeting modalities

To retarget oncolytic herpes simplex virus (oHSV) to cancer-specific antigens, we designed a novel, double-retargeted oHSV platform that uses single-chain antibodies (scFvs) incorporated into both glycoprotein H and a bispecific adapter expressed from the viral genome to mediate infection predominantly via tumor-associated antigens. Successful retargeting was achieved using a nectin-1-detargeted HSV that remains capable of interacting with herpesvirus entry mediator (HVEM), the second canonical HSV entry receptor, and is, therefore, recognized by the adapter consisting of the virus-binding N-terminal 82 residues of HVEM fused to the target-specific scFv. We tested both an epithelial cell adhesion molecule (EpCAM)- and a human epidermal growth factor receptor 2-specific scFv separately and together to target cells expressing one, the other, or both receptors. Our results show not only dose-dependent, target receptor-specific infection in vitro, but also enhanced virus spread compared with single-retargeted virus. In addition, we observed effective infection and spreading of the EpCAM double-retargeted virus in vivo. Remarkably, a single intravenous dose of the EpCAM-specific virus eliminated all detectable tumors in a subcutaneous xenograft model, and the same intravenous dose seemed to be harmless in immunocompetent FVB/N mice. Our findings suggest that our double-retargeted oHSV platform can provide a potent, versatile, and systemically deliverable class of anti-cancer therapeutics that specifically target cancer cells while ensuring safety.

MicroRNA-194-5p/Heparin-binding EGF-like growth factor signaling mediates dexamethasone-induced activation of pseudorabies virus in rat pheochromocytoma cells

Pseudorabies virus (PRV) is a neurotropic virus, which infects a wide range of mammals. The activity of PRV is gradually suppressed in hosts that have tolerated the primary infection. Increased glucocorticoid levels resulting from stressful stimuli overcome repression of PRV activity. However, the host cell mechanism involved in the activation processes under stressful conditions remains unclear. In this study, infection of rat PC-12 pheochromocytoma cells with neuronal properties using PRV at a multiplicity of infection (MOI) = 1 for 24 h made the activity of PRV be the relatively repressed state, and then incubation with 0.5 μM of the corticosteroid dexamethasone (DEX) for 4 h overcomes the relative repression of PRV activity. RNA-seq deep sequencing and bioinformatics analyses revealed different microRNA and mRNA profiles of PC-12 cells with/without PRV and/or DEX treatment. qRT-PCR and western blot analyses confirmed the negative regulatory relationship of miRNA-194-5p and its target heparin-binding EGF-like growth factor (Hbegf); a dual-luciferase reporter assay revealed that Hbegf is directly targeted by miRNA-194-5p. Further, miRNA-194-5p mock transfection contributed to PRV activation, Hbegf was downregulated in DEX-treated PRV infection cells, and Hbegf overexpression contributed to returning activated PRV to the repression state. Moreover, miRNA-194-5p overexpression resulted in reduced levels of HBEGF, c-JUN, and p-EGFR, whereas Hbegf overexpression suppressed the reduction caused by miRNA-194-5p overexpression. Overall, this study is the first to report that changes in the miR-194-5p-HBEGF/EGFR pathway in neurons are involved in DEX-induced activation of PRV, laying a foundation for the clinical prevention of stress-induced PRV activation.

Herpes Simplex Virus 1 Glycoprotein B from a Hyperfusogenic Virus Mediates Enhanced Cell-Cell Fusion

Herpes simplex virus 1 (HSV-1) causes significant morbidity and death in humans worldwide. Herpes simplex virus 1 has a complex fusion mechanism that is incompletely understood. The HSV-1 strain ANG has notable fusion and entry activities that distinguish it from wild type. HSV-1 ANG virions fused with the Vero cell surface at 4 °C and also entered cells more efficiently at 15 °C, relative to wild type HSV-1 strain KOS virions, consistent with a hyperfusogenic phenotype. Understanding the molecular basis for the unique entry and fusion activities of HSV-1 strain ANG will help decipher the HSV fusion reaction and entry process. Sequencing of HSV-1 ANG genes revealed multiple changes in gB, gC, gD, gH, and gL proteins relative to wild type HSV-1 strains. The ANG UL45 gene sequence, which codes for a non-essential envelope protein, was identical to wild type KOS. HSV-1 ANG gB, gD, and gH/gL were necessary and sufficient to mediate cell-cell fusion in a virus-free reporter assay. ANG gB, when expressed with wild type KOS gD and gH/gL, increased membrane fusion, suggesting that ANG gB has hyperfusogenic cell-cell fusion activity. Replacing the KOS gD, gH, or gL with the corresponding ANG alleles did not enhance cell-cell fusion. The novel mutations in the ANG fusion and entry glycoproteins provide a platform for dissecting the cascade of interactions that culminate in HSV fusion and entry.

Membrane fusion activity of herpes simplex virus 1 glycoproteins from a hyperfusogenic virus

Herpes simplex virus 1 (HSV-1) causes significant morbidity and death in humans worldwide. Herpes simplex virus 1 has a complex fusion mechanism that is incompletely understood. The HSV-1 strain ANG has notable fusion and entry activities that distinguish it from wild type. HSV-1 ANG virions fused with the Vero cell surface at 4°C and also entered cells more efficiently at 15°C relative to wild type virions, consistent with a hyperfusogenic phenotype. Understanding the molecular basis for the unique entry and fusion activities of HSV-1 strain ANG will help decipher the HSV fusion reaction and entry process. Sequencing of HSV-1 ANG genes revealed multiple changes in gB, gC, gD, gH, and gL proteins relative to wild type HSV-1 strains. The ANG UL45 gene sequence, which codes for a non-essential envelope protein, was identical to wild type. HSV-1 ANG gB, gD, and gH/gL were necessary and sufficient to mediate cell-cell fusion in a virus-free reporter assay. ANG gB, when expressed with wild type gD and gH/gL, increased membrane fusion, suggesting that ANG gB has hyperfusogenic cell-cell fusion activity. Replacing the wild type gD, gH, or gL with the corresponding ANG alleles did not enhance cell-cell fusion. Wild type gC is proposed to facilitate fusion and entry into epithelial cells by optimizing conformational changes in the fusion protein gB. ANG gC substitution or addition also had no effect on cell-cell fusion. The novel mutations in the ANG fusion and entry glycoproteins provide a platform for dissecting the cascade of interactions that culminate in HSV fusion and entry.

Impact of actin polymerization and filopodia formation on herpes simplex virus entry in epithelial, neuronal, and T lymphocyte cells

Herpes simplex virus type 1 (HSV-1) has been known as a common viral pathogen that can infect several parts of the body, leading to various clinical manifestations. According to this diverse manifestation, HSV-1 infection in many cell types was demonstrated. Besides the HSV-1 cell tropism, e.g., fibroblast, epithelial, mucosal cells, and neurons, HSV-1 infections can occur in human T lymphocyte cells, especially in activated T cells. In addition, several studies found that actin polymerization and filopodia formation support HSV-1 infection in diverse cell types. Hence, the goal of this review is to explore the mechanism of HSV-1 infection in various types of cells involving filopodia formation and highlight potential future directions for HSV-1 entry-related research. Moreover, this review covers several strategies for possible anti-HSV drugs focused on the entry step, offering insights into potential therapeutic interventions.

Multiple Sites on Glycoprotein H (gH) Functionally Interact with the gB Fusion Protein to Promote Fusion during Herpes Simplex Virus (HSV) Entry

Enveloped virus entry requires fusion of the viral envelope with a host cell membrane. Herpes simplex virus 1 (HSV-1) entry is mediated by a set of glycoproteins that interact to trigger the viral fusion protein glycoprotein B (gB). In the current model, receptor-binding by gD signals a gH/gL heterodimer to trigger a refolding event in gB that fuses the membranes. To explore functional interactions between gB and gH/gL, we used a bacterial artificial chromosome (BAC) to generate two HSV-1 mutants that show a small plaque phenotype due to changes in gB. We passaged the viruses to select for restoration of plaque size and analyzed second-site mutations that arose in gH. HSV-1 gB was replaced either by gB from saimiriine herpesvirus 1 (SaHV-1) or by a mutant form of HSV-1 gB with three alanine substitutions in domain V (gB3A). To shift the selective pressure away from gB, the gB3A virus was passaged in cells expressing gB3A. Sequencing of passaged viruses identified two interesting mutations in gH, including gH-H789Y in domain IV and gH-S830N in the cytoplasmic tail (CT). Characterization of these gH mutations indicated they are responsible for the enhanced plaque size. Rather than being globally hyperfusogenic, both gH mutations partially rescued function of the specific gB version present during their selection. These sites may represent functional interaction sites on gH/gL for gB. gH-H789 may alter the positioning of a membrane-proximal flap in the gH ectodomain, whereas gH-S830 may contribute to an interaction between the gB and gH CTs. IMPORTANCE Enveloped viruses enter cells by fusing their envelope with the host cell membrane. Herpes simplex virus 1 (HSV-1) entry requires the coordinated interaction of several viral glycoproteins, including gH/gL and gB. gH/gL and gB are essential for virus replication and both proteins are targets of neutralizing antibodies. gB fuses the membranes after being activated by gH/gL, but the details of how gH/gL activates gB are not known. This study examined the gH/gL-gB interaction using HSV-1 mutants that displayed reduced virus entry due to changes in gB. The mutant viruses were grown over time to select for additional mutations that could partially restore entry. Two mutations in gH (H789Y and S830N) were identified. The positions of the mutations in gH/gL may represent sites that contribute to gB activation during virus entry.

Herpes Simplex Virus 1 Entry Glycoproteins Form Complexes before and during Membrane Fusion

Herpesviruses—ubiquitous pathogens that cause persistent infections—have some of the most complex cell entry mechanisms. Entry of the prototypical herpes simplex virus 1 (HSV-1) requires coordinated efforts of 4 glycoproteins, gB, gD, gH, and gL. The current model posits that the glycoproteins do not interact before receptor engagement and that binding of gD to its receptor causes a “cascade” of sequential pairwise interactions, first activating the gH/gL complex and subsequently activating gB, the viral fusogen. But how these glycoproteins interact remains unresolved. Here, using a quantitative split-luciferase approach, we show that pairwise HSV-1 glycoprotein complexes form before fusion, interact at a steady level throughout fusion, and do not depend on the presence of the cellular receptor. Based on our findings, we propose a revised “conformational cascade” model of HSV-1 entry. We hypothesize that all 4 glycoproteins assemble into a complex before fusion, with gH/gL positioned between gD and gB. Once gD binds to a cognate receptor, the proximity of the glycoproteins within this complex allows for efficient transmission of the activating signal from the receptor-activated gD to gH/gL to gB through sequential conformational changes, ultimately triggering the fusogenic refolding of gB. Our results also highlight previously unappreciated contributions of the transmembrane and cytoplasmic domains to glycoprotein interactions and fusion. Similar principles could be at play in other multicomponent viral entry systems, and the split-luciferase approach used here is a powerful tool for investigating protein-protein interactions in these and a variety of other systems.

Broadly Applicable, Virus-Free Dual Reporter Assay to Identify Compounds Interfering with Membrane Fusion: Performance for HSV-1 and SARS-CoV-2

Membrane fusion constitutes an essential step in the replication cycle of numerous viral pathogens, hence it represents an important druggable target. In the present study, we established a virus-free, stable reporter fusion inhibition assay (SRFIA) specifically designed to identify compounds interfering with virus-induced membrane fusion. The dual reporter assay is based on two stable Vero cell lines harboring the third-generation tetracycline (Tet3G) transactivator and a bicistronic reporter gene cassette under the control of the tetracycline responsive element (TRE3G), respectively. Cell–cell fusion by the transient transfection of viral fusogens in the presence of doxycycline results in the expression of the reporter enzyme secreted alkaline phosphatase (SEAP) and the fluorescent nuclear localization marker EYFPNuc. A constitutively expressed, secreted form of nanoluciferase (secNLuc) functioned as the internal control. The performance of the SRFIA was tested for the quantification of SARS-CoV-2- and HSV-1-induced cell–cell fusion, respectively, showing high sensitivity and specificity, as well as the reliable identification of known fusion inhibitors. Parallel quantification of secNLuc enabled the detection of cytotoxic compounds or insufficient transfection efficacy. In conclusion, the SRFIA reported here is well suited for high-throughput screening for new antiviral agents and essentially will be applicable to all viral fusogens causing cell–cell fusion in Vero cells.

A surface pocket in the cytoplasmic domain of the herpes simplex virus fusogen gB controls membrane fusion

Membrane fusion during the entry of herpesviruses is carried out by the viral fusogen gB that is activated by its partner protein gH in some manner. The fusogenic activity of gB is controlled by its cytoplasmic (or intraviral) domain (gBCTD) and, according to the current model, the gBCTD is a trimeric, inhibitory clamp that restrains gB in the prefusion conformation. But how the gBCTD clamp is released by gH is unclear. Here, we identified two new regulatory elements within gB and gH from the prototypical herpes simplex virus 1: a surface pocket within the gBCTD and residue V831 within the gH cytoplasmic tail. Mutagenesis and structural modeling suggest that gH V831 interacts with the gB pocket. The gB pocket is located above the interface between adjacent protomers, and we hypothesize that insertion of the gH V831 wedge into the pocket serves to push the protomers apart, which releases the inhibitory clamp. In this manner, gH activates the fusogenic activity of gB. Both gB and gH are conserved across all herpesviruses, and this activation mechanism could be used by other gB homologs. Our proposed mechanism emphasizes a central role for the cytoplasmic regions in regulating the activity of a viral fusogen.

Glycosylation of viral proteins: Implication in virus-host interaction and virulence

Glycans are among the most important cell molecular components. However, given their structural diversity, their functions have not been fully explored. Glycosylation is a vital post-translational modification for various proteins. Many bacteria and viruses rely on N-linked and O-linked glycosylation to perform critical biological functions. The diverse functions of glycosylation on viral proteins during viral infections, including Dengue, Zika, influenza, and human immunodeficiency viruses as well as coronaviruses have been reported. N-linked glycosylation is the most common form of protein modification, and it modulates folding, transportation and receptor binding. Compared to N-linked glycosylation, the functions of O-linked viral protein glycosylation have not been comprehensively evaluated. In this review, we summarize findings on viral protein glycosylation, with particular attention to studies on N-linked glycosylation in viral life cycles. This review informs the development of virus-specific vaccines or inhibitors.

Opportunities, Technology, and the Joy of Discovery

My grandparents were immigrants. My paternal grandfather was illiterate. Yet my parents were able to complete college and to become teachers. I had a conventional upbringing in a small town in Florida, graduating from high school in 1960. I was fortunate enough to graduate cum laude from Florida State University and to earn other credentials leading to faculty positions at outstanding institutions of higher education: the University of Chicago and Northwestern University. At a time when women were rarely the leaders of research groups, I was able to establish a well-funded research program and to make contributions to our understanding of viral entry into cells. My best research was done after I became confident enough to seek productive interactions with collaborators. I am grateful for the collaborators and collaborations that moved our field forward and for my trainees who have gone on to successes in many different careers. Expected final online publication date for the Annual Review of Virology, Volume 9 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Luciferase-Based Biosensors in the Era of the COVID-19 Pandemic

Luciferase-based biosensors have a wide range of applications and assay formats, including their relatively recent use in the study of viruses. Split luciferase, bioluminescence resonance energy transfer, circularly permuted luciferase, cyclic luciferase, and dual luciferase systems have all been used to interrogate the structure and function of prominent viruses infecting humans, animals, and plants. The utility of these assays is demonstrated by numerous studies which have not only successfully characterized interactions between viral and host cell proteins but that have also used these systems to identify viral inhibitors. In the present COVID-19 pandemic, luciferase-based biosensors are already playing a critical role in the study of the culprit virus SARS-CoV-2 as well as in the development of serological assays and drug development via high-throughput screening. In this review paper, we provide a summary of existing luciferase-based biosensors and their applications in virology.

Herpes Simplex Virus Glycoprotein B Mutations Define Structural Sites in Domain I, the Membrane Proximal Region, and the Cytodomain That Regulate Entry

The viral fusion protein glycoprotein B (gB) is conserved in all herpesviruses and is essential for virus entry. During entry, gB fuses viral and host cell membranes by refolding from a prefusion to a postfusion form. We previously introduced three structure-based mutations (gB-I671A/H681A/F683A) into the domain V arm of the gB ectodomain that resulted in reduced cell-cell fusion. A virus carrying these three mutations (called gB3A) displayed a small-plaque phenotype and remarkably delayed entry into cells. To identify mutations that could counteract this phenotype, we serially passaged the gB3A virus and selected for revertant viruses with increased plaque sizes. Genomic sequencing revealed that the revertant viruses had second-site mutations in gB, including E187A, M742T, and S383F/G645R/V705I/V880G. Using expression constructs encoding these mutations, only gB-V880G was shown to enhance cell-cell fusion. In contrast, all of the revertant viruses showed enhanced entry kinetics, underscoring the fact that cell-cell fusion and virus-cell fusion are different. The results indicate that mutations in three different regions of gB (domain I, the membrane proximal region, and the cytoplasmic tail domain) can counteract the slow-entry phenotype of gB3A virus. Mapping these compensatory mutations to prefusion and postfusion structural models suggests sites of intramolecular functional interactions with the gB domain V arm that may contribute to the gB fusion function. IMPORTANCE The nine human herpesviruses are ubiquitous and cause a range of diseases in humans. Glycoprotein B (gB) is an essential viral fusion protein that is conserved in all herpesviruses. During host cell entry, gB mediates virus-cell membrane fusion by undergoing a conformational change. Structural models for the prefusion and postfusion forms of gB exist, but the details of how the protein converts from one to the other are unclear. We previously introduced structure-based mutations into gB that inhibited virus entry and fusion. By passaging this entry-deficient virus over time, we selected second-site mutations that partially restore virus entry. The locations of these mutations suggest regulatory sites that contribute to fusion and gB refolding during entry. gB is a target of neutralizing antibodies, and defining how gB refolds during entry could provide a basis for the development of fusion inhibitors for future research or clinical use.

Harringtonine Inhibits Herpes Simplex Virus Type 1 Infection by Reducing Herpes Virus Entry Mediator Expression

Herpes simplex virus type 1 (HSV-1) infection induces various clinical disorders, such as herpes simplex encephalitis (HSE), herpes simplex keratitis (HSK), and genital herpes. In clinical intervention, acyclovir (ACV) is the major therapeutic drug used to suppress HSV-1; however, ACV-resistant strains have gradually increased. In the present study, harringtonine (HT) significantly inhibited infection of HSV-1 as well as two ACV-resistant strains, including HSV-1 blue and HSV-1 153. Time-of-drug addition assay further revealed that HT mainly reduced the early stage of HSV-1 infection. We also demonstrated that HT mainly affected herpes virus entry mediator (HVEM) expression as shown by qPCR, Western Blot, and Immunofluorescence. Collectively, HT showed antiviral activity against HSV-1 and ACV-resistant strains by targeting HVEM and could be a promising therapeutic candidate for mitigating HSV-1-induced-pathogenesis.

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.

Protease, Growth Factor, and Heparanase-Mediated Syndecan-1 Shedding Leads to Enhanced HSV-1 Egress

Heparan sulfate (HS) and heparan sulfate proteoglycans (HSPGs) are considered important for the entry of many different viruses. Previously, we demonstrated that heparanase (HPSE), the host enzyme responsible for cleaving HS chains, is upregulated by herpes simplex virus-1 (HSV-1) infection. Higher levels of HPSE accelerate HS removal from the cell surface, facilitating viral release from infected cells. Here, we study the effects of overexpressing HPSE on viral entry, cell-to-cell fusion, plaque formation, and viral egress. We provide new information that higher levels of HPSE reduce syncytial plaque formation while promoting egress and extracellular release of the virions. We also found that transiently enhanced expression of HPSE did not affect HSV-1 entry into host cells or HSV-1-induced cell-to-cell fusion, suggesting that HPSE activation is tightly regulated and facilitates extracellular release of the maturing virions. We demonstrate that an HSPG-shedding agonist, PMA; a protease, thrombin; and a growth factor, EGF as well as bacterially produced recombinant heparinases resulted in enhanced HSV-1 release from HeLa and human corneal epithelial (HCE) cells. Our findings here underscore the significance of syndecan-1 functions in the HSV-1 lifecycle, provide evidence that the shedding of syndecan-1 ectodomain is another way HPSE works to facilitate HSV-1 release, and add new evidence on the significance of various HSPG shedding agonists in HSV-1 release from infected cells.

Varicella-zoster virus: molecular controls of cell fusion-dependent pathogenesis

Varicella-zoster virus (VZV) is the causative agent of chicken pox (varicella) and shingles (zoster). Although considered benign diseases, both varicella and zoster can cause complications. Zoster is painful and can lead to post herpetic neuralgia. VZV has also been linked to stroke, related to giant cell arteritis in some cases. Vaccines are available but the attenuated vaccine is not recommended in immunocompromised individuals and the efficacy of the glycoprotein E (gE) based subunit vaccine has not been evaluated for the prevention of varicella. A hallmark of VZV pathology is the formation of multinucleated cells termed polykaryocytes in skin lesions. This cell-cell fusion (abbreviated as cell fusion) is mediated by the VZV glycoproteins gB, gH and gL, which constitute the fusion complex of VZV, also needed for virion entry. Expression of gB, gH and gL during VZV infection and trafficking to the cell surface enables cell fusion. Recent evidence supports the concept that cellular processes are required for regulating cell fusion induced by gB/gH-gL. Mutations within the carboxyl domains of either gB or gH have profound effects on fusion regulation and dramatically restrict the ability of VZV to replicate in human skin. This loss of regulation modifies the transcriptome of VZV infected cells. Furthermore, cellular proteins have significant effects on the regulation of gB/gH-gL-mediated cell fusion and the replication of VZV, exemplified by the cellular phosphatase, calcineurin. This review provides the current state-of-the-art knowledge about the molecular controls of cell fusion-dependent pathogenesis caused by VZV.

Formation of Large Intracellular Actin Networks Following Plasmonic Cell Fusion

Following fusion between two or more individual cells, the resulting cellular entity must undergo extensive restructuring of its plasma membrane and cytoskeleton in order to maintain its mechanical and physiological function. In artificial cell fusion that is executed by external triggering, such restructuring could be problematic due to the absence of preconditioning biological signals. In this work we study the reorganization of the actin filaments in adenocarcinoma cells that were fused using plasmonic triggering, i.e. the irradiation by resonant femtosecond laser pulses of cells specifically targeted by gold nanoparticles. Time-lapse confocal microscopy of the fusing cells has revealed the formation of large-scale actin networks that preserve the local orientations of the original actin cytoskeletons. The results confirm the local nature of the plasmonic interactions that were confined to the cells' plasma membranes and would help studying the development and dynamics of actin networks by offering a relatively stable, living cellular environment that supports large-scale actin growth.

Antibody Screening System Using a Herpes Simplex Virus (HSV)-Based Probe To Identify a Novel Target for Receptor-Retargeted Oncolytic HSVs

Herpes simplex virus (HSV) is a promising tool for developing oncolytic virotherapy. We recently reported a platform for receptor-retargeted oncolytic HSVs that incorporates single-chain antibodies (scFvs) into envelope glycoprotein D (gD) to mediate virus entry via tumor-associated antigens. Therefore, it would be useful to develop an efficient system that can screen antibodies that might mediate HSV entry when they are incorporated as scFvs into gD. We created an HSV-based screening probe by the genetic fusion of a gD mutant with ablated binding capability to the authentic HSV entry receptors and the antibody-binding C domain of streptococcal protein G. This engineered virus failed to enter cells through authentic receptors. In contrast, when this virus was conjugated with an antibody specific to an antigen on the cell membrane, it specifically entered cells expressing the cognate antigen. This virus was used as a probe to identify antibodies that mediate virus entry via recognition of certain molecules on the cell membrane other than authentic receptors. Using this method, we identified an antibody specific to epiregulin (EREG), which has been investigated mainly as a secreted growth factor and not necessarily for its precursor that is expressed in a transmembrane form. We constructed an scFv from the anti-EREG antibody for insertion into the retargeted HSV platform and found that the recombinant virus entered cells specifically through EREG expressed by the cells. This novel antibody-screening system may contribute to the discovery of unique and unexpected molecules that might be used for the entry of receptor-retargeted oncolytic HSVs.IMPORTANCE The tropism of the cellular entry of HSV is dependent on the binding of the envelope gD to one of its authentic receptors. This can be fully retargeted to other receptors by inserting scFvs into gD with appropriate modifications. In theory, upon binding to the engineered gD, receptors other than authentic receptors should induce a conformational change in the gD, which activates downstream mechanisms required for viral entry. However, prerequisite factors for receptors to be used as targets of a retargeted virus remain poorly understood, and it is difficult to predict which molecules might be suitable for our retargeted HSV construct. Our HSV-based probe will allow unbiased screening of antibody-antigen pairs that mediate virus entry and might be a useful tool to identify suitable pairs for our construct and to enhance our understanding of virus-cell interactions during infection by HSV and possibly other viruses.

Localization of the Interaction Site of Herpes Simplex Virus Glycoprotein D (gD) on the Membrane Fusion Regulator, gH/gL

A cascade of protein-protein interactions between four herpes simplex virus (HSV) glycoproteins (gD, gH/gL, and gB) drive fusion between the HSV envelope and host membrane, thereby allowing for virus entry and infection. Specifically, binding of gD to one of its receptors induces a conformational change that allows gD to bind to the regulatory complex gH/gL, which then activates the fusogen gB, resulting in membrane fusion. Using surface plasmon resonance and a panel of anti-gD monoclonal antibodies (MAbs) that sterically blocked the interaction, we previously showed that gH/gL binds directly to gD at sites distinct from the gD receptor binding site. Here, using an analogous strategy, we first evaluated the ability of a panel of uncharacterized anti-gH/gL MAbs to block binding to gD and/or inhibit fusion. We found that the epitopes of four gD-gH/gL-blocking MAbs were located within flexible regions of the gH N terminus and the gL C terminus, while the fifth was placed around gL residue 77. Taken together, our data localized the gD binding region on gH/gL to a group of gH and gL residues at the membrane distal region of the heterodimer. Surprisingly, a second set of MAbs did not block gD-gH/gL binding but instead stabilized the complex by altering the kinetic binding. However, despite this prolonged gD-gH/gL interaction, "stabilizing" MAbs also inhibited cell-cell fusion, suggesting a unique mechanism by which the fusion process is halted. Our findings support targeting the gD-gH/gL interaction to prevent fusion in both therapeutic and vaccine strategies against HSV.IMPORTANCE Key to developing a human HSV vaccine is an understanding of the virion glycoproteins involved in entry. HSV employs multiple glycoproteins for attachment, receptor interaction, and membrane fusion. Determining how these proteins function was resolved, in part, by structural biology coupled with immunological and biologic evidence. After binding, virion gD interacts with a receptor to activate the regulator gH/gL complex, triggering gB to drive fusion. Multiple questions remain, one being the physical location of each glycoprotein interaction site. Using protective antibodies with known epitopes, we documented the long-sought interaction between gD and gH/gL, detailing the region on gD important to create the gD-gH/gL triplex. Now, we have identified the corresponding gD contact sites on gH/gL. Concurrently we discovered a novel mechanism whereby gH/gL antibodies stabilize the complex and inhibit fusion progression. Our model for the gD-gH/gL triplex provides a new framework for studying fusion, which identifies targets for vaccine development.

Anti-HSV-1 activity of Aspergillipeptide D, a cyclic pentapepetide isolated from fungus Aspergillus sp. SCSIO 41501

Background

Herpes simplex virus 1, an enveloped DNA virus belonging to the Herpesviridae family, spreads to neurons and causes pathological changes in the central nervous system. The purpose of this study was to investigate the potency and mechanism of antiviral activity of Aspergillipeptide D, a cyclic pentapeptide isolated from a culture broth of marine gorgonian-derived fungus Aspergillus sp. SCSIO 41501, At present, there are many studies on the anti-tumor, anti-clotting, anti-oxidant and immunoinflammatory effects of Aspergillipeptide D, but little research has been done on the anti-HSV-1 activity of Aspergillipeptide D.

Methods

The anti-HSV-1 activity of Aspergillipeptide D was evaluated by plaque reduction assay. The mechanism of action against HSV-1 was determined from the effective stage. Then we assayed the viral DNA replication, viral RNA synthesis and protein expression, respectively. We also identified the proteins that interact with gB by mass spectrometry, and assayed the effect of Aspergillipeptide D on the interaction between the virus gB protein and cell proteins.

Results

Plaque reduction experiments showed that Aspergillipeptide D did not affect HSV-1 early infection events, including viral inactivation, attachment and penetration. Interestingly, Aspergillipeptide D dramatically reduced both the gene and protein levels of viral late protein gB, and suppressed its location in the endoplasmic reticulum and Golgi apparatus. In contrast, overexpression of gB restored viral production. Finally, proteomic analysis revealed that the numbers of cellular proteins that interacted with gB protein was largely decreased by Aspergillipeptide D. These results suggested that Aspergillipeptide D inhibited gB function to affect HSV-1 intercellular spread.

Conclusions

Our results indicated that Aspergillipeptide D might be a potential candidate for HSV-1 therapy, especially for ACV-resistant strains.

Herpes Simplex Virus Glycoprotein C Regulates Low-pH Entry

Herpes simplex viruses (HSVs) cause significant morbidity and mortality in humans worldwide. Herpesviruses mediate entry by a multicomponent virus-encoded machinery. Herpesviruses enter cells by endosomal low-pH and pH-neutral mechanisms in a cell-specific manner. HSV mediates cell entry via the envelope glycoproteins gB and gD and the heterodimer gH/gL regardless of pH or endocytosis requirements. Specifics concerning HSV envelope proteins that function selectively in a given entry pathway have been elusive. Here, we demonstrate that gC regulates cell entry and infection by a low-pH pathway. Conformational changes in the core herpesviral fusogen gB are critical for membrane fusion. The presence of gC conferred a higher pH threshold for acid-induced antigenic changes in gB. Thus, gC may selectively facilitate low-pH entry by regulating conformational changes in the fusion protein gB. We propose that gC modulates the HSV fusion machinery during entry into pathophysiologically relevant cells, such as human epidermal keratinocytes.IMPORTANCE Herpesviruses are ubiquitous pathogens that cause lifelong latent infections and that are characterized by multiple entry pathways. We propose that herpes simplex virus (HSV) gC plays a selective role in modulating HSV entry, such as entry into epithelial cells, by a low-pH pathway. gC facilitates a conformational change of the main fusogen gB, a class III fusion protein. We propose a model whereby gC functions with gB, gD, and gH/gL to allow low-pH entry. In the absence of gC, HSV entry occurs at a lower pH, coincident with trafficking to a lower pH compartment where gB changes occur at more acidic pHs. This report identifies a new function for gC and provides novel insight into the complex mechanism of HSV entry and fusion.

Point Mutations in Retargeted gD Eliminate the Sensitivity of EGFR/EGFRvIII-Targeted HSV to Key Neutralizing Antibodies

Effective oncolytic virotherapy may require systemic delivery, tumor targeting, and resistance to virus-neutralizing (VN) antibodies. Since herpes simplex virus (HSV) glycoprotein D (gD) is the viral attachment/entry protein and predominant VN target, we examined the impact of gD retargeting alone and in combination with alterations in dominant VN epitopes on virus susceptibility to VN antibodies. We compared the binding of a panel of anti-gD monoclonal antibodies (mAbs) that mimic antibody specificities in human HSV-immune sera to the purified ectodomains of wild-type and retargeted gD, revealing the retention of two prominent epitopes. Substitution of a key residue in each epitope, separately and together, revealed that both substitutions (1) blocked retargeted gD recognition by mAbs to the respective epitopes, and, in combination, caused a global reduction in mAb binding; (2) protected against fusion inhibition by VN mAbs reactive with each epitope in virus-free cell-cell fusion assays; and (3) increased the resistance of retargeted HSV-1 to these VN mAbs. Although the combined modifications of retargeted gD allowed bona fide retargeting, incorporation into virions was partially compromised. Our results indicate that stacking of epitope mutations can additively block retargeted gD recognition by VN antibodies but also that improvements in gD incorporation into virus particles may be required.

Common characteristics and unique features: A comparison of the fusion machinery of the alphaherpesviruses Pseudorabies virus and Herpes simplex virus

Membrane fusion is a fundamental biological process that allows different cellular compartments delimited by a lipid membrane to release or exchange their respective contents. Similarly, enveloped viruses such as alphaherpesviruses exploit membrane fusion to enter and infect their host cells. For infectious entry the prototypic human Herpes simplex viruses 1 and 2 (HSV-1 and -2, collectively termed HSVs) and the porcine Pseudorabies virus (PrV) utilize four different essential envelope glycoproteins (g): the bona fide fusion protein gB and the regulatory heterodimeric gH/gL complex that constitute the "core fusion machinery" conserved in all members of the Herpesviridae; and the subfamily specific receptor binding protein gD. These four components mediate attachment and fusion of the virion envelope with the host cell plasma membrane through a tightly regulated sequential activation process. Although PrV and the HSVs are closely related and employ the same set of glycoproteins for entry, they show remarkable differences in the requirements for fusion. Whereas the HSVs strictly require all four components for membrane fusion, PrV can mediate cell-cell fusion without gD. Moreover, in contrast to the HSVs, PrV provides a unique opportunity for reversion analyses of gL-negative mutants by serial cell culture passaging, due to a limited cell-cell spread capacity of gL-negative PrV not observed in the HSVs. This allows a more direct analysis of the function of gH/gL during membrane fusion. Unraveling the molecular mechanism of herpesvirus fusion has been a goal of fundamental research for years, and yet important mechanistic details remain to be uncovered. Nevertheless, the elucidation of the crystal structures of all key players involved in PrV and HSV membrane fusion, coupled with a wealth of functional data, has shed some light on this complex puzzle. In this review, we summarize and discuss the contemporary knowledge on the molecular mechanism of entry and membrane fusion utilized by the alphaherpesvirus PrV, and highlight similarities but also remarkable differences in the requirements for fusion between PrV and the HSVs.

Using Antibodies and Mutants To Localize the Presumptive gH/gL Binding Site on Herpes Simplex Virus gD

HSV virus-cell and cell-cell fusion requires multiple interactions between four essential virion envelope glycoproteins, gD, gB, gH, and gL, and between gD and a cellular receptor, nectin-1 or herpesvirus entry mediator (HVEM). Current models suggest that binding of gD to receptors induces a conformational change that leads to activation of gH/gL and consequent triggering of the prefusion form of gB to promote membrane fusion. Since protein-protein interactions guide each step of fusion, identifying the sites of interaction may lead to the identification of potential therapeutic targets that block this process. We have previously identified two "faces" on gD: one for receptor binding and the other for its presumed interaction with gH/gL. We previously separated the gD monoclonal antibodies (MAbs) into five competition communities. MAbs from two communities (MC2 and MC5) neutralize virus infection and block cell-cell fusion but do not block receptor binding, suggesting that they block binding of gD to gH/gL. Using a combination of classical epitope mapping of gD mutants with fusion and entry assays, we identified two residues (R67 and P54) on the presumed gH/gL interaction face of gD that allowed for fusion and viral entry but were no longer sensitive to inhibition by MC2 or MC5, yet both were blocked by other MAbs. As neutralizing antibodies interfere with essential steps in the fusion pathway, our studies strongly suggest that these key residues block the interaction of gD with gH/gL.IMPORTANCE Virus entry and cell-cell fusion mediated by HSV require gD, gH/gL, gB, and a gD receptor. Neutralizing antibodies directed against any of these proteins bind to residues within key functional sites and interfere with an essential step in the fusion pathway. Thus, the epitopes of these MAbs identify critical, functional sites on their target proteins. Unlike many anti-gD MAbs, which block binding of gD to a cellular receptor, two, MC2 and MC5, block a separate, downstream step in the fusion pathway which is presumed to be the activation of the modulator of fusion, gH/gL. By combining epitope mapping of a panel of gD mutants with fusion and virus entry assays, we have identified residues that are critical in the binding and function of these two MAbs. This new information helps to define the site of the presumptive interaction of gD with gH/gL, of which we have limited knowledge.

Natural Selection of Glycoprotein B Mutations That Rescue the Small-Plaque Phenotype of a Fusion-Impaired Herpes Simplex Virus Mutant

Glycoprotein B (gB) is a conserved viral fusion protein that is required for herpesvirus entry. To mediate fusion with the cellular membrane, gB refolds from a prefusion to a postfusion conformation. We hypothesize that an interaction between the C-terminal arm and the central coiled coil of the herpes simplex virus 1 (HSV-1) gB ectodomain is critical for fusion. We previously reported that three mutations in the C-terminal arm (I671A/H681A/F683A, called gB3A) greatly reduced cell-cell fusion and that virus carrying these mutations had a small-plaque phenotype and delayed entry into cells. By serially passaging gB3A virus, we selected three revertant viruses with larger plaques. These revertant viruses acquired mutations in gB that restore the fusion function of gB3A, including gB-A683V, gB-S383F/G645R/V705I/A855V, and gB-T509M/N709H. V705I and N709H are novel mutations that map to the portion of domain V that enters domain I in the postfusion structure. S383F, G645R, and T509M are novel mutations that map to an intersection of three domains in a prefusion model of gB. We introduced these second-site mutations individually and in combination into wild-type gB and gB3A to examine the impact of the mutations on fusion and expression. V705I and A855V (a known hyperfusogenic mutation) restored the fusion function of gB3A, whereas S383F and G645R dampened fusion and T509M and N709H worked in concert to restore gB3A fusion. The results identify two regions in the gB ectodomain that modulate the fusion activity of gB, potentially by impacting intramolecular interactions and stability of the prefusion and/or postfusion gB trimer.IMPORTANCE Glycoprotein B (gB) is an essential viral protein that is conserved in all herpesviruses and is required for virus entry. gB is thought to undergo a conformational change that provides the energy to fuse the viral and cellular membranes; however, the details of this conformational change and the structure of the prefusion and intermediate conformations of gB are not known. Previously, we demonstrated that mutations in the gB "arm" region inhibit fusion and impart a small-plaque phenotype. Using serial passage of a virus carrying these mutations, we identified revertants with restored plaque size. The revertant viruses acquired novel mutations in gB that restored fusion function and mapped to two sites in the gB ectodomain. This work supports our hypothesis that an interaction between the gB arm and the core of gB is critical for gB refolding and provides details about the function of gB in herpesvirus-mediated fusion and subsequent virus entry.

Acidic pH Mediates Changes in Antigenic and Oligomeric Conformation of Herpes Simplex Virus gB and Is a Determinant of Cell-Specific Entry

Herpes simplex virus (HSV) is an important human pathogen with a high worldwide seroprevalence. HSV enters epithelial cells, the primary site of infection, by a low-pH pathway. HSV glycoprotein B (gB) undergoes low pH-induced conformational changes, which are thought to drive membrane fusion. When neutralized back to physiological pH, these changes become reversible. Here, HSV-infected cells were subjected to short pulses of radiolabeling, followed by immunoprecipitation with a panel of gB monoclonal antibodies (MAbs), demonstrating that gB folds and oligomerizes rapidly and cotranslationally in the endoplasmic reticulum. Full-length gB from transfected cells underwent low-pH-triggered changes in oligomeric conformation in the absence of other viral proteins. MAbs to gB neutralized HSV entry into cells regardless of the pH dependence of the entry pathway, suggesting a conservation of gB function in distinct fusion mechanisms. The combination of heat and acidic pH triggered irreversible changes in the antigenic conformation of the gB fusion domain, while changes in the gB oligomer remained reversible. An elevated temperature alone was not sufficient to induce gB conformational change. Together, these results shed light on the conformation and function of the HSV-1 gB oligomer, which serves as part of the core fusion machinery during viral entry.IMPORTANCE Herpes simplex virus (HSV) causes infection of the mouth, skin, eyes, and genitals and establishes lifelong latency in humans. gB is conserved among all herpesviruses. HSV gB undergoes reversible conformational changes following exposure to acidic pH which are thought to mediate fusion and entry into epithelial cells. Here, we identified cotranslational folding and oligomerization of newly synthesized gB. A panel of antibodies to gB blocked both low-pH and pH-neutral entry of HSV, suggesting conserved conformational changes in gB regardless of cell entry route. Changes in HSV gB conformation were not triggered by increased temperature alone, in contrast to results with EBV gB. Acid pH-induced changes in the oligomeric conformation of gB are related but distinct from pH-triggered changes in gB antigenic conformation. These results highlight critical aspects of the class III fusion protein, gB, and inform strategies to block HSV infection at the level of fusion and entry.

Functional Relevance of the Transmembrane Domain and Cytoplasmic Tail of the Pseudorabies Virus Glycoprotein H for Membrane Fusion

Herpesvirus membrane fusion depends on the core fusion machinery, comprised of glycoproteins B (gB) and gH/gL. Although gB structurally resembles autonomous class III fusion proteins, it strictly depends on gH/gL to drive membrane fusion. Whether the gH/gL complex needs to be membrane anchored to fulfill its function and which role the gH cytoplasmic (CD) and transmembrane domains (TMD) play in fusion is unclear. While the gH CD and TMD play an important role during infection, soluble gH/gL of herpes simplex virus 1 (HSV-1) seems to be sufficient to mediate cell-cell fusion in transient assays, arguing against an essential contribution of the CD and TMD. To shed more light on this apparent discrepancy, we investigated the role of the CD and TMD of the related alphaherpesvirus pseudorabies virus (PrV) gH. For this purpose, we expressed C-terminally truncated and soluble gH and replaced the TMD with a glycosylphosphatidylinositol (gpi) anchor. We also generated chimeras containing the TMD and/or CD of PrV gD or HSV-1 gH. Proteins were characterized in cell-based fusion assays and during virus infection. Although truncation of the CD resulted in decreased membrane fusion activity, the mutant proteins still supported replication of gH-negative PrV, indicating that the PrV gH CD is dispensable for viral replication. In contrast, PrV gH lacking the TMD, membrane-anchored via a lipid linker, or comprising the PrV gD TMD were nonfunctional, highlighting the essential role of the gH TMD for function. Interestingly, despite low sequence identity, the HSV-1 gH TMD could substitute for the PrV gH TMD, pointing to functional conservation.IMPORTANCE Enveloped viruses depend on membrane fusion for virus entry. While this process can be mediated by only one or two proteins, herpesviruses depend on the concerted action of at least three different glycoproteins. Although gB has features of bona fide fusion proteins, it depends on gH and its complex partner, gL, for fusion. Whether gH/gL prevents premature fusion or actively triggers gB-mediated fusion is unclear, and there are contradictory results on whether gH/gL function requires stable membrane anchorage or whether the ectodomains alone are sufficient. Our results show that in pseudorabies virus gH, the transmembrane anchor plays an essential role for gB-mediated fusion while the cytoplasmic tail is not strictly required.

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 Neutralizing Linear Epitope of Human Herpesvirus 6A Glycoprotein B Does Not Affect Virus Infectivity

Human herpesvirus 6A (HHV-6A) glycoprotein B (gB) is a glycoprotein consisting of 830 amino acids and is essential for the growth of the virus. Previously, we reported that a neutralizing monoclonal antibody (MAb) called 87-y-13 specifically reacts with HHV-6A gB, and we identified its epitope residue at asparagine (Asn) 347 on gB. In this study, we examined whether the epitope recognized by the neutralizing MAb is essential for HHV-6A infection. We constructed HHV-6A bacterial artificial chromosome (BAC) genomes harboring substitutions at Asn347, namely, HHV-6A BACgB(N347K) and HHV-6A BACgB(N347A). These mutant viruses could be reconstituted and propagated in the same manner as the wild type and their revertants, and MAb 87-y-13 could not inhibit infection by either mutant. In a cell-cell fusion assay, Asn at position 347 on gB was found to be nonessential for cell-cell fusion. In addition, in building an HHV-6A gB homology model, we found that the epitope of the neutralizing MAb is located on domain II of gB and is accessible to solvents. These results indicate that Asn at position 347, the linear epitope of the neutralizing MAb, does not affect HHV-6A infectivity.IMPORTANCE Glycoprotein B (gB) is one of the most conserved glycoproteins among all herpesviruses and is a key factor for virus entry. Therefore, antibodies targeted to gB may neutralize virus entry. Human herpesvirus 6A (HHV-6A) encodes gB, which is translated to a protein of about 830 amino acids (aa). Using a monoclonal antibody (MAb) for HHV-6A gB, which has a neutralizing linear epitope, we analyzed the role of its epitope residue, N347, in HHV-6A infectivity. Interestingly, this gB linear epitope residue, N347, was not essential for HHV-6A growth. By constructing a homology model of HHV-6A gB, we found that N347 was located in the region corresponding to domain II. Therefore, with regard to its neutralizing activity against HHV-6A infection, the epitope on gB might be exposed to solvents, suggesting that it might be a target of the immune system.

Prediction of conserved sites and domains in glycoproteins B, C and D of herpes viruses

Glycoprotein B (gB), C (gC) and D (gD) of herpes simplex virus are implicated in virus adsorption and penetration. The gB, gC and gD are glycoproteins for different processes of virus binding and attachment to the host cells. Moreover, their expression is necessary and sufficient to induce cell fusion in the absence of other glycoproteins. Egress of herpes simplex virus (HSV) and other herpes viruses from cells involves extensive modification of cellular membranes and sequential envelopment, de-envelopment and re-envelopment steps. Viral glycoproteins are important in these processes, and frequently two or more glycoproteins can largely suffice in any step. Hence, we target the 3 important glycoproteins (B, C and D) of eight different herpes viruses of different species. These species include human (HSV1 and 2), bovine (BHV1), equine (EHV1 and 4), chicken (ILT1 and MDV2) and pig (PRV1). By applying different bioinformatics tools, we highlighted the conserved sites in these glycoproteins which might be most significant regarding attachment and infection of the viruses. Moreover the conserved domains in these glycoproteins are also highlighted. From this study, we will able to analyze the role of different viral glycoproteins of different species during herpes virus adsorption and penetration. Moreover, this study will help to construct the antivirals that target the glycoproteins of different herpes viruses.

Targeting Herpes Simplex Virus-1 gD by a DNA Aptamer Can Be an Effective New Strategy to Curb Viral Infection

Herpes simplex virus type 1 (HSV-1) is an important factor for vision loss in developed countries. A challenging aspect of the ocular infection by HSV-1 is that common treatments, such as acyclovir, fail to provide effective topical remedies. Furthermore, it is not very clear whether the viral glycoproteins, required for HSV-1 entry into the host, can be targeted for an effective therapy against ocular herpes in vivo. Here, we demonstrate that HSV-1 envelope glycoprotein gD, which is essential for viral entry and spread, can be specifically targeted by topical applications of a small DNA aptamer to effectively control ocular infection by the virus. Our 45-nt-long DNA aptamer showed high affinity for HSV-1 gD (binding affinity constant [K_d] = 50 nM), which is strong enough to disrupt the binding of gD to its cognate host receptors. Our studies showed significant restriction of viral entry and replication in both in vitro and ex vivo studies. In vivo experiments in mice also resulted in loss of ocular infection under prophylactic treatment and statistically significant lower infection under therapeutic modality compared to random DNA controls. Thus, our studies validate the possibility that targeting HSV-1 entry glycoproteins, such as gD, can locally reduce the spread of infection and define a novel DNA aptamer-based approach to control HSV-1 infection of the eye.

The Fusion Loops of the Initial Prefusion Conformation of Herpes Simplex Virus 1 Fusion Protein Point Toward the Membrane

All enveloped viruses, including herpesviruses, must fuse their envelope with the host membrane to deliver their genomes into target cells, making this essential step subject to interference by antibodies and drugs. Viral fusion is mediated by a viral surface protein that transits from an initial prefusion conformation to a final postfusion conformation. Strikingly, the prefusion conformation of the herpesvirus fusion protein, gB, is poorly understood. Herpes simplex virus (HSV), a model system for herpesviruses, causes diseases ranging from mild skin lesions to serious encephalitis and neonatal infections. Using cryo-electron tomography and subtomogram averaging, we have characterized the structure of the prefusion conformation and fusion intermediates of HSV-1 gB. To this end, we have set up a system that generates microvesicles displaying full-length gB on their envelope. We confirmed proper folding of gB by nondenaturing electrophoresis-Western blotting with a panel of monoclonal antibodies (MAbs) covering all gB domains. To elucidate the arrangement of gB domains, we labeled them by using (i) mutagenesis to insert fluorescent proteins at specific positions, (ii) coexpression of gB with Fabs for a neutralizing MAb with known binding sites, and (iii) incubation of gB with an antibody directed against the fusion loops. Our results show that gB starts in a compact prefusion conformation with the fusion loops pointing toward the viral membrane and suggest, for the first time, a model for gB’s conformational rearrangements during fusion. These experiments further illustrate how neutralizing antibodies can interfere with the essential gB structural transitions that mediate viral entry and therefore infectivity.

The herpesvirus family includes herpes simplex virus (HSV) and other human viruses that cause lifelong infections and a variety of diseases, like skin lesions, encephalitis, and cancers. As enveloped viruses, herpesviruses must fuse their envelope with the host membrane to start an infection. This process is mediated by a viral surface protein that transitions from an initial conformation (prefusion) to a final, more stable, conformation (postfusion). However, the prefusion conformation of the herpesvirus fusion protein (gB) is poorly understood. To elucidate the structure of the prefusion conformation of HSV type 1 gB, we have employed cryo-electron microscopy to study gB molecules expressed on the surface of vesicles. Using different approaches to label gB’s domains allowed us to model the structures of the prefusion and intermediate conformations of gB. Overall, our findings enhance our understanding of HSV fusion and lay the groundwork for the development of new ways to prevent and block HSV infection.

Anti-herpes simplex virus type 1 activity of Houttuynoid A, a flavonoid from Houttuynia cordata Thunb

Early events in herpes simplex virus type 1 (HSV-1) infection reactivate latent human immunodeficiency virus, Epstein-Barr virus, and human papillomavirus in the presence of acyclovir (ACV). The common use of nucleoside analog medications, such as ACV and pencyclovir, has resulted in the emergence of drug-resistant HSV-1 strains in clinical therapy. Therefore, new antiherpetics that can inhibit early events in HSV-1 infection should be developed. An example of this treatment is Houttuynia cordata Thunb. water extract, which can inhibit HSV-1 infection through multiple mechanisms. In this study, the anti-HSV-1 activity of Houttuynoid A, a new type of flavonoid isolated from H. cordata, was investigated. Three different assays confirmed that this compound could exhibit strong in vitro anti-HSV-1 activity. One assay verified that this compound could inhibit HSV-1 multiplication and prevent lesion formation in a HSV-1 infection mouse model. Mechanism analysis revealed that this compound could inactivate HSV-1 infectivity by blocking viral membrane fusion. Moreover, Houttuynoid A exhibited antiviral activities against other alpha herpes viruses, such as HSV-2 and varicella zoster virus (VZV). In conclusion, Houttuynoid A may be a useful antiviral agent for HSV-1.

Structural basis of nectin-1 recognition by pseudorabies virus glycoprotein D

An early and yet indispensable step in the alphaherpesvirus infection is the engagement of host receptors by the viral envelope glycoprotein D (gD). Of the thus-far identified gD receptors, nectin-1 is likely the most effective in terms of its wide usage by multiple alphaherpesviruses for cell entry. The molecular basis of nectin-1 recognition by the gD protein is therefore an interesting scientific question in the alphaherpesvirus field. Previous studies focused on the herpes simplex virus (HSV) of the Simplexvirus genus, for which both the free gD structure and the gD/nectin-1 complex structure were reported at high resolutions. The structural and functional features of other alphaherpesviral gDs, however, remain poorly characterized. In the current study, we systematically studied the characteristics of nectin-1 binding by the gD of a Varicellovirus genus member, the pseudorabies virus (PRV). We first showed that PRV infects host cells via both human and swine nectin-1, and that its gD exhibits similar binding affinities for nectin-1 of the two species. Furthermore, we demonstrated that removal of the PRV gD membrane-proximal residues could significantly increase its affinity for the receptor binding. The structures of PRV gD in the free and the nectin-1-bound states were then solved, revealing a similar overall 3D fold as well as a homologous nectin-1 binding mode to its HSV counterpart. However, several unique features were observed at the binding interface of PRV gD, enabling the viral ligand to utilize different gD residues (from those of HSV) for nectin-1 engagement. These observed binding characteristics were further verified by the mutagenesis study using the key-residue mutants of nectin-1. The structural and functional data obtained in this study, therefore, provide the basis of receptor recognition by PRV gD.

Structure-Based Mutations in the Herpes Simplex Virus 1 Glycoprotein B Ectodomain Arm Impart a Slow-Entry Phenotype

Glycoprotein B (gB) is the conserved herpesvirus fusion protein, and it is required for the entry of herpesviruses. The structure of the postfusion conformation of gB has been solved for several herpesviruses; however, the gB prefusion crystal structure and the details of how the protein refolds from a prefusion to a postfusion form to mediate fusion have not been determined. Using structure-based mutagenesis, we previously reported that three mutations (I671A, H681A, and F683A) in the C-terminal arm of the gB ectodomain greatly reduced cell-cell fusion. This fusion deficit could be rescued by the addition of a hyperfusogenic mutation, suggesting that the gB triple mutant was not misfolded. Using a bacterial artificial chromosome (BAC), we constructed two independent herpes simplex virus 1 mutant strains (gB 3A) carrying the three arm mutations. The gB 3A viruses have 200-fold smaller plaques than the wild-type virus and demonstrate remarkably delayed entry into cells. Single-step and multistep growth curves show that gB 3A viruses have delayed replication kinetics. Interestingly, incubation at 40°C promoted the entry of the gB 3A viruses. We propose that the gB 3A viruses' entry deficit is due to a loss of interactions between residues in the gB C-terminal arm and the coiled-coil core of gB. The results suggest that the triple alanine mutation may destabilize the postfusion gB conformation and/or stabilize the prefusion gB conformation and that exposure to elevated temperatures can overcome the defect in gB 3A viruses.IMPORTANCE Because of its complexity, the mechanism of herpesvirus entry into cells is not well understood. Our study investigated one of the most important unanswered questions about herpesvirus entry; i.e., how does the herpesvirus fusion protein gB mediate membrane fusion? gB is an essential protein that is conserved in all herpesviruses and is thought to undergo a conformational change to provide the energy to fuse the viral and cellular membranes. Using our understanding of the structure of gB, we designed mutations in the gB "arm" region that we predicted would impede gB function. We introduced these mutations into herpes simplex virus 1 by using a bacterial artificial chromosome, and the mutant virus exhibited a drastically delayed rate of entry. This entry defect was rescued by incubation at elevated temperatures, supporting a hypothesis that the engineered mutations altered the energetics of gB refolding. This study supports our hypothesis that an interaction between the gB arm and the core of gB is critical for gB refolding and the execution of membrane fusion, thus providing key details about the function of gB in herpesvirus-mediated fusion and subsequent virus entry.

Insertion of a ligand to HER2 in gB retargets HSV tropism and obviates the need for activation of the other entry glycoproteins

Herpes simplex virus (HSV) entry into the cells requires glycoproteins gD, gH/gL and gB, activated in a cascade fashion by conformational modifications induced by cognate receptors and intermolecular signaling. The receptors are nectin1 and HVEM (Herpes virus entry mediator) for gD, and αvβ6 or αvβ8 integrin for gH. In earlier work, insertion of a single chain antibody (scFv) to the cancer receptor HER2 (human epidermal growth factor receptor 2) in gD, or in gH, resulted in HSVs specifically retargeted to the HER2-positive cancer cells, hence in highly specific non-attenuated oncolytic agents. Here, the scFv to HER2 was inserted in gB (gBHER2). The insertion re-targeted the virus tropism to the HER2-positive cancer cells. This was unexpected since gB is known to be a fusogenic glycoprotein, not a tropism determinant. The gB-retargeted recombinant offered the possibility to investigate how HER2 mediated entry. In contrast to wt-gB, the activation of the chimeric gBHER2 did not require the activation of the gD and of gH/gL by their respective receptors. Furthermore, a soluble form of HER2 could replace the membrane-bound HER2 in mediating virus entry, hinting that HER2 acted by inducing conformational changes to the chimeric gB. This study shows that (i) gB can be modified and become the major determinant of HSV tropism; (ii) the chimeric gBHER2 bypasses the requirement for receptor-mediated activation of other essential entry glycoproteins.

Mapping sites of herpes simplex virus type 1 glycoprotein D that permit insertions and impact gD and gB receptors usage

Glycoprotein D (gD) of herpes simplex virus type 1 (HSV-1) is one of four glycoproteins essential for HSV entry and cell fusion. The purpose of this study was to determine the plasticity of gD to tolerate insertion or deletion mutations and to construct an oncolytic HSV-1 that utilizes the disialoganglioside GD2 as a HSV-1 entry receptor. We found that the N-terminus of gD tolerates long insertions, whereas residues adjacent to the gD Ig-like V-type core tolerated shorter insertions (up to 15 amino acids), but not greater than 60 amino acids. Recombinant HSV-1 containing the ch14.18 single chain variable fragment (scFv) at the N-terminus of gD failed to mediate entry, even though the ch14.18 scFv-gD chimera Fc bound to neuroblastoma cells expressing GD2. Finally, we found that hyperfusogenic gB mutants enhanced fusion to a greater degree with the gB receptor the paired immunoglobulin-like type 2 receptor alpha (PILRα) than with gD receptors HVEM and nectin-1. Hyperfusogenic gB could restore the fusion function with PILRα when a gD constructed contained only the “profusion domain” (PFD), suggesting the hyperfusogenic form of gB may regulate fusion of PILRα via a novel mechanism through gH/gL and the gD PFD.

Ovine Herpesvirus 2 Glycoproteins B, H, and L Are Sufficient for, and Viral Glycoprotein Ov8 Can Enhance, Cell-Cell Membrane Fusion

Ovine herpesvirus 2 (OvHV-2) is a gammaherpesvirus in the genus Macavirus that is carried asymptomatically by sheep. Infection of poorly adapted animals with OvHV-2 results in sheep-associated malignant catarrhal fever, a fatal disease characterized by lymphoproliferation and vasculitis. There is no treatment or vaccine for the disease and no cell culture system to propagate the virus. The lack of cell culture has hindered studies of OvHV-2 biology, including its entry mechanism. As an alternative method to study OvHV-2 glycoproteins responsible for membrane fusion as a part of the entry mechanism, we developed a virus-free cell-to-cell membrane fusion assay to identify the minimum required OvHV-2 glycoproteins to induce membrane fusion. OvHV-2 glycoproteins B, H, and L (gB, gH, and gL) were able to induce membrane fusion together but not when expressed individually. Additionally, open reading frame Ov8, unique to OvHV-2, was found to encode a transmembrane glycoprotein that can significantly enhance membrane fusion. Thus, OvHV-2 gB, gH, and gL are sufficient to induce membrane fusion, while glycoprotein Ov8 plays an enhancing role by an unknown mechanism.IMPORTANCE Herpesviruses enter cells via attachment of the virion to the cellular surface and fusion of the viral envelope with cellular membranes. Virus-cell membrane fusion is an important step for a successful viral infection. Elucidating the roles of viral glycoproteins responsible for membrane fusion is critical toward understanding viral entry. Entry of ovine herpesvirus 2 (OvHV-2), the causative agent of sheep associated-malignant catarrhal fever, which is one of the leading causes of death in bison and other ungulates, has not been well studied due to the lack of a cell culture system to propagate the virus. The identification of OvHV-2 glycoproteins that mediate membrane fusion may help identify viral and/or cellular factors involved in OvHV-2 cell tropism and will advance investigation of cellular factors necessary for virus-cell membrane fusion. We found that OvHV-2 glycoproteins B, H, and L are sufficient for, and viral glycoprotein Ov8 can significantly enhance, cell-cell membrane fusion.

Infectivity inhibition by overlapping synthetic peptides derived from the gH/gL heterodimer of herpes simplex virus type 1

Herpes simplex virus (HSV) is a human pathogen that infects epithelial cells. The cutaneous lesions, caused by the virus, spread to the nervous system creating several complications. Fusion of host membranes with the viral envelope is mandatory and mediated by a group of glycoproteins conserved in all Herpesviridae subfamilies, such as the glycoproteins B (gB), H (gH), L (gL) and D (gD). We investigated the inhibitory activity mediated by synthetic overlapping peptides spanning the entire ectodomains of gH and gL glycoproteins. We have performed a brute analysis of the complete gH/gL heterodimer in order to explore the inhibitory activity of peptides modelled on these glycoproteins against HSV‐1 infection. Twenty‐four of the gH peptides at a concentration of 150 μM reached the 50% of inhibition cut‐off. Interestingly, they are mainly located in the gH carboxy‐terminal domain. None of the gL peptides had a clear inhibiting effect. No peptide toxicity was observed by lactate dehydrogenase assay at the concentrations used in our experimental conditions. HSV‐1 therapy is based on acyclovir treatment, but some resistant strains are emerging. In this scenario, innovative approaches for HSV‐1 treatment are necessary. Our data support the direct involvement of the described domains in the process of virus penetration; therefore, these results are of relevance to the potential development of novel therapeutic compounds to prevent HSV‐1 infections. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.

Rapid Screening by Cell-Based Fusion Assay for Identifying Novel Antivirals of Glycoprotein B-Mediated Herpes Simplex Virus Type 1 Infection

Herpes simplex virus type 1 (HSV-1) is a causative agent for a variety of diseases. Although antiherpetic drugs such as acyclovir have been developed to inhibit virus replication through interaction with DNA kinases, their continuous administration leads to an increase in the frequency of drug-resistant HSV-1, which is an important clinical issue that requires urgent solution. Recently, we reported that the sialylated O-linked sugar T antigen (sTn) and its attached peptide region (O-glycosylated sTn peptide) derived from the HSV-1 glycoprotein B (gB) protein inhibited HSV-1 infection by specifically targeting paired immunoglobulin-like type 2 receptor alpha (PILRα) in vitro. In this study, to further identify novel inhibitors of gB-mediated HSV-1 infection in vitro, we established a cell-based fusion assay for rapid drug screening. Chinese hamster ovary (CHO) cells were transfected with expression plasmids for HSV-1 gB, gD, gH, and gL, and T7 RNA polymerase, and were designated as the effector cells. The CHO-K1 cells stably expressing PILRα were transfected with the expression plasmid for firefly luciferase under the T7 promoter, and were designated as the target cells. The effector and target cells were co-cultured, and luminescence was measured when both cells were successfully fused. Importantly, we found that cell-to-cell fusion was specifically inhibited by O-glycosylated sTn peptide in a dose dependent manner. Our results suggested that this virus-free cell-based fusion assay system could be a useful and promising approach to identify novel inhibitors of gB-mediated HSV-1 infection, and will aid in the development of antiviral therapeutic strategies for HSV-1-associated diseases.

Herpes simplex virus Membrane Fusion

Herpes simplex virus mediates multiple distinct fusion events during infection. HSV entry is initiated by fusion of the viral envelope with either the limiting membrane of a host cell endocytic compartment or the plasma membrane. In the infected cell during viral assembly, immature, enveloped HSV particles in the perinuclear space fuse with the outer nuclear membrane in a process termed de-envelopment. A cell infected with some strains of HSV with defined mutations spread to neighboring cells by a fusion event called syncytium formation. Two experimental methods, the transient cell-cell fusion approach and fusion from without, are useful surrogate assays of HSV fusion. These five fusion processes are considered in terms of their requirements, mechanism, and regulation. The execution and modulation of these events require distinct yet often overlapping sets of viral proteins and host cell factors. The core machinery of HSV gB, gD, and the heterodimer gH/gL is required for most if not all of the HSV fusion mechanisms.

Regulation of Herpes Simplex Virus Glycoprotein-Induced Cascade of Events Governing Cell-Cell Fusion

Receptor-dependent herpes simplex virus (HSV)-induced cell-cell fusion requires glycoproteins gD, gH/gL, and gB. Our current model posits that during fusion, receptor-activated conformational changes in gD activate gH/gL, which subsequently triggers the transformation of the prefusion form of gB into a fusogenic state. To examine the role of each glycoprotein in receptor-dependent cell-cell fusion, we took advantage of our discovery that fusion by wild-type herpes simplex virus 2 (HSV-2) glycoproteins occurs twice as fast as that achieved by HSV-1 glycoproteins. By sequentially swapping each glycoprotein between the two serotypes, we established that fusion speed was governed by gH/gL, with gH being the main contributor. While the mutant forms of gB fuse at distinct rates that are dictated by their molecular structure, these restrictions can be overcome by gH/gL of HSV-2 (gH2/gL2), thereby enhancing their activity. We also found that deregulated forms of gD of HSV-1 (gD1) and gH2/gL2 can alter the fusogenic potential of gB, promoting cell fusion in the absence of a cellular receptor, and that deregulated forms of gB can drive the fusion machinery to even higher levels. Low pH enhanced fusion by affecting the structure of both gB and gH/gL mutants. Together, our data highlight the complexity of the fusion machinery, the impact of the activation state of each glycoprotein on the fusion process, and the critical role of gH/gL in regulating HSV-induced fusion. IMPORTANCE Cell-cell fusion mediated by HSV glycoproteins requires gD, gH/gL, gB, and a gD receptor. Here, we show that fusion by wild-type HSV-2 glycoproteins occurs twice as fast as that achieved by HSV-1 glycoproteins. By sequentially swapping each glycoprotein between the two serotypes, we found that the fusion process was controlled by gH/gL. Restrictions imposed on the gB structure by mutations could be overcome by gH2/gL2, enhancing the activity of the mutants. Under low-pH conditions or when using deregulated forms of gD1 and gH2/gL2, the fusogenic potential of gB could only be increased in the absence of receptor, underlining the exquisite regulation that occurs in the presence of receptor. Our data highlight the complexity of the fusion machinery, the impact of the activation state of each glycoprotein on the fusion process, and the critical role of gH/gL in regulating HSV-induced fusion.

Multiple Roles of the Cytoplasmic Domain of Herpes Simplex Virus 1 Envelope Glycoprotein D in Infected Cells

Herpes simplex virus 1 (HSV-1) envelope glycoprotein D (gD) plays an essential role in viral entry. The functional regions of gD responsible for viral entry have been mapped to its extracellular domain, whereas the gD cytoplasmic domain plays no obvious role in viral entry. Thus far, the role(s) of the gD cytoplasmic domain in HSV-1 replication has remained to be elucidated. In this study, we show that ectopic expression of gD induces microvillus-like tubular structures at the plasma membrane which resemble the reported projection structures of the plasma membrane induced in HSV-1-infected cells. Mutations in the arginine cluster (residues 365 to 367) in the gD cytoplasmic domain greatly reduced gD-induced plasma membrane remodeling. In agreement with this, the mutations in the arginine cluster in the gD cytoplasmic domain reduced the number of microvillus-like tubular structures at the plasma membrane in HSV-1-infected cells. In addition, the mutations produced an accumulation of unenveloped nucleocapsids in the cytoplasm and reduced viral replication and cell-cell spread. These results suggest that the arginine cluster in the gD cytoplasmic domain is required for the efficient induction of plasma membrane projections and viral final envelopment, and these functions of the gD domain may lead to efficient viral replication and cell-cell spread.

IMPORTANCE

The cytoplasmic domain of HSV-1 gD, an envelope glycoprotein essential for viral entry, was reported to promote viral replication and cell-cell spread, but the role(s) of the domain during HSV-1 infection has remained unknown. In this study, we clarify two functions of the arginine cluster in the HSV-1 gD cytoplasmic domain, both of which require host cell membrane remodeling, i.e., the formation of microvillus-like projections at the plasma membrane and viral final envelopment in HSV-1-infected cells. We also show that the gD arginine cluster is required for efficient HSV-1 replication and cell-cell spread. This is the first report clarifying not only the functions of the gD cytoplasmic domain but also identifying the gD arginine cluster to be the HSV-1 factor responsible for the induction of plasma membrane projections in HSV-1-infected cells. Our results elucidate some of the functions of this multifunctional envelope glycoprotein during HSV-1 infection.

Characterization of Vesicular Stomatitis Virus Pseudotypes Bearing Essential Entry Glycoproteins gB, gD, gH, and gL of Herpes Simplex Virus 1

Herpes simplex viruses (HSVs) are unusual in that unlike most enveloped viruses, they require at least four entry glycoproteins, gB, gD, gH, and gL, for entry into target cells in addition to a cellular receptor for gD. The dissection of the herpes simplex virus 1 (HSV-1) entry mechanism is complicated by the presence of more than a dozen proteins on the viral envelope. To investigate HSV-1 entry requirements in a simplified system, we generated vesicular stomatitis virus (VSV) virions pseudotyped with HSV-1 essential entry glycoproteins gB, gD, gH, and gL but lacking the native VSV fusogen G. These virions, referred to here as VSVΔG-BHLD virions, infected a cell line expressing a gD receptor, demonstrating for the first time that the four essential entry glycoproteins of HSV-1 are not only required but also sufficient for cell entry. To our knowledge, this is the first time the VSV pseudotyping system has been successfully extended beyond two proteins. Entry of pseudotyped virions required a gD receptor and was inhibited by HSV-1 specific anti-gB or anti-gH/gL neutralizing antibodies, which suggests that membrane fusion during the entry of the pseudotyped virions shares common requirements with the membrane fusion involved in HSV-1 entry and HSV-1-mediated syncytium formation. The HSV pseudotyping system established in this study presents a novel tool for systematic exploration of the HSV entry and membrane fusion mechanisms.

IMPORTANCE

Herpes simplex viruses (HSVs) are human pathogens that can cause cold sores, genital herpes, and blindness. No vaccines or preventatives are available. HSV entry into cells-a prerequisite for a successful infection-is a complex process that involves multiple viral and host proteins and occurs by different routes. Detailed mechanistic knowledge of the HSV entry is important for understanding its pathogenesis and would benefit antiviral and vaccine development, yet the presence of more than a dozen proteins on the viral envelope complicates the dissection of the HSV entry mechanisms. In this study, we generated heterologous virions displaying the four essential entry proteins of HSV-1 and showed that they are capable of cell entry and, like HSV-1, require all four entry glycoproteins along with a gD receptor. This HSV pseudotyping system pioneered in this work opens doors for future systematic exploration of the herpesvirus entry mechanisms.

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

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

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

Constitutive Expression of GATA4 Dramatically Increases the Cardiogenic Potential of D3 Mouse Embryonic Stem Cells

The transcription factor GATA binding protein 4 (GATA4) is a vital regulator of cardiac programming that acts by inducing the expression of many different genes involved in cardiomyogenesis. Here we generated a D3 mouse embryonic stem cell line that constitutively expresses high levels of GATA4 and show that these cells have dramatically increased cardiogenic potential compared to an eGFP-expressing control cell line. Embryoid bodies (EB) derived from the D3-GATA4 line displayed increased levels of cardiac gene expression and showed more abundant cardiomyocyte differentiation than control eGFP EB. These cells and two additional lines expressing lower levels of GATA4 provide a platform to screen previously untested cardiac genes and gene combinations for their ability to further increase the efficiency of cardiomyocyte differentiation beyond that achieved by transgenic GATA4 alone. Non-integrative delivery of identified gene combinations will aid in the production of differentiated cells for the treatment of ischemic cardiomyopathy.