Cross talk among the glycoproteins involved in herpes simplex virus entry and fusion: the interaction between gB and gH/gL does not necessarily require gD

Herpes Simplex Virus 1 Envelope Glycoprotein C Shields Glycoprotein D to Protect Virions from Entry-Blocking Antibodies

Herpes simplex virus 1 (HSV-1) gD interaction with the host cell receptor nectin-1 triggers the membrane fusion cascade during viral entry. Potent neutralizing antibodies to gD prevent receptor-binding or prevent gD interaction with gH/gL critical for fusion. HSV has many strategies to evade host immune responses. We investigated the ability of virion envelope gC to protect envelope gD from antibody neutralization. HSV-1 lacking gC was more sensitive to neutralization by anti-gD monoclonal antibodies than a wild type rescuant virus. gD in the HSV-1 gC-null viral envelope had enhanced reactivity to anti-gD antibodies compared to wild type. HSV-1 ΔgC binding to the nectin-1 receptor was more readily inhibited by a neutralizing anti-gD monoclonal antibody. HSV-1 ΔgC was also more sensitive to inhibition by soluble nectin-1 receptor. The viral membrane protein composition of HSV-1 ΔgC was equivalent to that of wild type, suggesting that the lack of gC is responsible for the increased reactivity of gD-specific antibodies and the consequent increased susceptibility to neutralization by those antibodies. Together, the results suggest that gC in the HSV-1 envelope shields both receptor-binding domains and gH/gL-interacting domains of gD from neutralizing antibodies, facilitating HSV cell entry.

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.

A Single Amino Acid Substitution in the Transmembrane Domain of Glycoprotein H Functionally Compensates for the Absence of gL in Pseudorabies Virus

Herpesvirus entry requires the coordinated action of at least four viral glycoproteins. Virus-specific binding to a cellular receptor triggers a membrane fusion cascade involving the conserved gH/gL complex and gB. Although gB is the genuine herpesvirus fusogen, it requires gH/gL for fusion, but how activation occurs is still unclear. To study the underlying mechanism, we used a gL-deleted pseudorabies virus (PrV) mutant characterized by its limited capability to directly infect neighboring cells that was exploited for several independent serial passages in cell culture. Unlike previous revertants that acquired mutations in the gL-binding N-terminus of gH, we obtained a variant, PrV-ΔgLPassV99, that unexpectedly contained two amino acid substitutions in the gH transmembrane domain (TMD). One of these mutations, I662S, was sufficient to compensate for gL function in virus entry and in in vitro cell-cell fusion assays in presence of wild type gB, but barely for cell-to-cell spread. Additional expression of receptor-binding PrV gD, which is dispensable for cell-cell fusion mediated by native gB, gH and gL, resulted in hyperfusion in combination with gH V99. Overall, our results uncover a yet-underestimated role of the gH TMD in fusion regulation, further shedding light on the complexity of herpesvirus fusion involving all structural domains of the conserved entry glycoproteins.

A critical review on antiviral peptides derived from viral glycoproteins and host receptors to decoy herpes simplex virus

The health of the human population has been continuously challenged by viral infections. Herpes simplex virus (HSV) is one of the common causes of illness and can lead to death in immunocompromised patients. Existing anti-HSV therapies are not completely successful in eliminating the infection due to anti-viral drug resistance, ineffectiveness against the latent virus and high toxicity over prolonged use. There is a need to update our knowledge of the current challenges faced in anti-HSV therapeutics and realize the necessity of developing alternative treatment approaches. Protein therapeutics are now being explored as a novel approach due to their high specificity and low toxicity. This review highlights the significance of HSV viral glycoproteins and host receptors in the pathogenesis of HSV infection. Proteins or peptides derived from HSV glycoproteins gC, gB, gD, gH and host cell receptors (HSPG, nectin and HVEM) that act as decoys to inhibit HSV attachment, entry, or fusion have been discussed. Few researchers have tried to improve the efficacy and stability of the identified peptides by modifying them using a peptidomimetic approach. With these efforts, we think developing an alternative treatment option for immunocompromised patients and drug-resistant organisms is not far off.

Comparative full genome sequence analysis of wild-type and chicken embryo origin vaccine-like infectious laryngotracheitis virus field isolates from Canada

Infectious laryngotracheitis (ILT), caused by infectious laryngotracheitis virus (ILTV), occurs sporadically in poultry flocks in Canada. Live attenuated chicken embryo origin (CEO) vaccines are being used routinely to prevent and control ILTV infections. However, ILT outbreaks still occur since vaccine strains could revert to virulence in the field. In this study, 7 Canadian ILTV isolates linked to ILT outbreaks across different time in Eastern Canada (Ontario; ON and Quebec; QC) were whole genome sequenced. Phylogenetic analysis confirmed the close relationship between the ON isolates and the CEO vaccines, whereas the QC isolates clustered with strains previously known as CEO revertant and wild-type ILTVs. Recombination network analysis of ILTV sequences revealed clear evidence of historical recombination between ILTV strains circulating in Canada and other geographical regions. The comparison of ON CEO clustered and QC CEO revertant clustered isolates with the LT Blen® CEO vaccine reference sequence showed amino acid differences in 5 and 12 open reading frames (ORFs), respectively. Similar analysis revealed amino acid differences in 32 ORFs in QC wild-type isolates. Compared to all CEO vaccine strains in the public domain, the QC wild-type isolates showed 15 unique mutational sites leading to amino acid changes in 13 ORFs. Our outcomes add to the knowledge of the molecular mechanisms behind ILTV genetic variance and provide genetic markers between wild-type and vaccine strains.

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.

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.

Targeting Host Cellular Factors as a Strategy of Therapeutic Intervention for Herpesvirus Infections

Herpesviruses utilize various host factors to establish latent infection, survival, and spread disease in the host. These factors include host cellular machinery, host proteins, gene expression, multiple transcription factors, cellular signal pathways, immune cell activation, transcription factors, cytokines, angiogenesis, invasion, and factors promoting metastasis. The knowledge and understanding of host genes, protein products, and biochemical pathways lead to discovering safe and effective antivirals to prevent viral reactivation and spread infection. Here, we focus on the contribution of pro-inflammatory, anti-inflammatory, and resolution lipid metabolites of the arachidonic acid (AA) pathway in the lifecycle of herpesvirus infections. We discuss how various herpesviruses utilize these lipid pathways to their advantage and how we target them to combat herpesvirus infection. We also summarize recent development in anti-herpesvirus therapeutics and new strategies proposed or under clinical trials. These anti-herpesvirus therapeutics include inhibitors blocking viral life cycle events, engineered anticancer agents, epigenome influencing factors, immunomodulators, and therapeutic compounds from natural extracts.

In vitro assessment of dual (antiviral and antitumor) activity of a novel lectin produced by the newly cyanobacterium isolate, Oscillatoria acuminate MHM-632 MK014210.1

A novel lectin was purified from newly cyanobacterium isolate, Oscillatoria acuminate MHM-632 MK014210.1 using affinity chromatography with a molecular weight of 120 kDa under native-PAGE and 30 kDa on reducing-PAGE, represented tetramer nature of this lectin. Oscillatorial lectin showed stability at 60 °C for 30 min, pH-dependent, with the highest activities over the pH range of 6-8, and required zinc ions to express its full activity. Oscillatorial lectin is a glycan-binding protein with a neutral carbohydrate content of 7.0% as evaluated by the phenol-sulfuric acid method. Polyols and α- glycosides polymer of mannose sugar or sugars alcohol were completely inhibited oscillatorial lectin with MIC of 0.195 mM, while β-glycosides sugars did not show any inhibition effect. The oscillatorial lectin has anti-proliferative activity against Huh-7 and MCF-7 cancer cells and inhibited their proliferation with EC₅₀ values of 106.75 µg/ml and 254.14 µg/ml, respectively. Besides the anticancer effect, oscillatorial lectin also has potent antiviral activity against HSV-1 in a dose-dependent manner via virions neutralization and inhibition of viral replication with IC₅₀ values of 90.95 ng/ml and 131.3 ng/ml, respectively. The unique carbohydrate affinity of oscillatorial lectin provides insight into its use as a promising candidate in many biotechnological applications, like fighting viral infection and combating cancer disease.Communicated by Ramaswamy H. Sarma.

A glycoprotein B-neutralizing antibody structure at 2.8 Å uncovers a critical domain for herpesvirus fusion initiation

Members of the Herpesviridae, including the medically important alphaherpesvirus varicella-zoster virus (VZV), induce fusion of the virion envelope with cell membranes during entry, and between cells to form polykaryocytes in infected tissues. The conserved glycoproteins, gB, gH and gL, are the core functional proteins of the herpesvirus fusion complex. gB serves as the primary fusogen via its fusion loops, but functions for the remaining gB domains remain unexplained. As a pathway for biological discovery of domain function, our approach used structure-based analysis of the viral fusogen together with a neutralizing antibody. We report here a 2.8 Å cryogenic-electron microscopy structure of native gB recovered from VZV-infected cells, in complex with a human monoclonal antibody, 93k. This high-resolution structure guided targeted mutagenesis at the gB-93k interface, providing compelling evidence that a domain spatially distant from the gB fusion loops is critical for herpesvirus fusion, revealing a potential new target for antiviral therapies.

Herpesvirus virions have an outer lipid membrane dotted with glycoproteins that enable fusion with cell membranes to initiate entry and establish infection. Here the authors elucidate the structural mechanism of a neutralizing antibody derived from a patient infected by the herpesvirus varicella-zoster virus and targeted to its fusogen, glycoprotein-B.

High Preventive Effect of G2-S16 Anionic Carbosilane Dendrimer against Sexually Transmitted HSV-2 Infection

Anionic carbosilane dendrimers such as G2-S16 are very effective in preventing HSV-2 infection both in vitro and in vivo. We present the main achievements obtained for the G2-S16 dendrimer in vivo, especially related to its efficacy against HSV-2 infection. Moreover, we discuss the mechanisms by which the G2-S16 dendrimer applied vaginally as a topical microbicide has been demonstrated to be safe and harmless for the vaginal microbiome balance, as both conditions present an essential step that has to be overcome during microbicide development. This review points to the marked protective effect of the G2-S16 dendrimer against sexually transmitted HSV-2 infection, supporting its role as a possible microbicide against HSV-2 infection.

Herpesvirus membrane fusion - a team effort

One of the essential steps in every viral 'life' cycle is entry into the host cell. Membrane-enveloped viruses carry dedicated proteins to catalyse the fusion of the viral and cellular membrane. Herpesviruses feature a set of essential, structurally diverse glycoproteins on the viral surface that form a multicomponent fusion machinery, necessary for the entry mechanism. For Herpes simplex virus 1, these essential glycoproteins are gD, gH, gL and gB. In this review we describe the functions of the individual components, the potential interactions between them as well as the influence of post-translational modifications on the fusion mechanism.

Viral, bacterial, and fungal infections of the oral mucosa: Types, incidence, predisposing factors, diagnostic algorithms, and management

For millions of years, microbiota residing within us, including those in the oral cavity, coexisted in a harmonious symbiotic fashion that provided a quintessential foundation for human health. It is now clear that disruption of such a healthy relationship leading to microbial dysbiosis causes a wide array of infections, ranging from localized, mild, superficial infections to deep, disseminated life-threatening diseases. With recent advances in research, diagnostics, and improved surveillance we are witnessing an array of emerging and re-emerging oral infections and orofacial manifestations of systemic infections. Orofacial infections may cause significant discomfort to the patients and unnecessary economic burden. Thus, the early recognition of such infections is paramount for holistic patient management, and oral clinicians have a critical role in recognizing, diagnosing, managing, and preventing either new or old orofacial infections. This paper aims to provide an update on current understanding of well-established and emerging viral, bacterial, and fungal infections manifesting in the human oral cavity.

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.

Towards Better Understanding of KSHV Life Cycle: from Transcription and Posttranscriptional Regulations to Pathogenesis

Kaposi’s sarcoma-associated herpesvirus (KSHV), also known as human herpesvirus-8 (HHV-8), is etiologically linked to the development of Kaposi’s sarcoma, primary effusion lymphoma, and multicentric Castleman’s disease. These malignancies often occur in immunosuppressed individuals, making KSHV infection-associated diseases an increasing global health concern with persistence of the AIDS epidemic. KSHV exhibits biphasic life cycles between latent and lytic infection and extensive transcriptional and posttranscriptional regulation of gene expression. As a member of the herpesvirus family, KSHV has evolved many strategies to evade the host immune response, which help the virus establish a successful lifelong infection. In this review, we summarize the current research status on the biology of latent and lytic viral infection, the regulation of viral life cycles and the related pathogenesis.

Dual Ligand Insertion in gB and gD of Oncolytic Herpes Simplex Viruses for Retargeting to a Producer Vero Cell Line and to Cancer Cells

Oncolytic viruses gain cancer specificity in several ways. Like the majority of viruses, they grow better in cancer cells that are defective in mounting the host response to viruses. Often, they are attenuated by deletion or mutation of virulence genes that counteract the host response or are naturally occurring oncolytic mutants. In contrast, retargeted viruses are not attenuated or deleted; their cancer specificity rests on a modified, specific tropism for cancer receptors. For herpes simplex virus (HSV)-based oncolytics, the detargeting-retargeting strategies employed so far were based on genetic modifications of gD. Recently, we showed that even gH or gB can serve as retargeting tools. To enable the growth of retargeted HSVs in cells that can be used for clinical-grade virus production, a double-retargeting strategy has been developed. Here we show that several sites in the N terminus of gB are suitable to harbor the 20-amino-acid (aa)-long GCN4 peptide, which readdresses HSV tropism to Vero cells expressing the artificial GCN4 receptor and thus enables virus cultivation in the producer noncancer Vero-GCN4R cell line. The gB modifications can be combined with a minimal detargeting modification in gD, consisting in the deletion of two residues, aa 30 and 38, and replacement of aa 38 with the scFv to human epidermal growth factor receptor 2 (HER2), for retargeting to the cancer receptor. The panel of recombinants was analyzed comparatively in terms of virus growth, cell-to-cell spread, cytotoxicity, and in vivo antitumor efficacy to define the best double-retargeting strategy.

IMPORTANCE There is increasing interest in oncolytic viruses, following FDA and the European Medicines Agency (EMA) approval of HSV Oncovex^GM-CSF, and, mainly, because they greatly boost the immune response to the tumor and can be combined with immunotherapeutic agents, particularly checkpoint inhibitors. A strategy to gain cancer specificity and avoid virus attenuation is to retarget the virus tropism to cancer-specific receptors of choice. Cultivation of fully retargeted viruses is challenging, since they require cells that express the cancer receptor. We devised a strategy for their cultivation in producer noncancer Vero cell derivatives. Here, we developed a double-retargeting strategy, based on insertion of one ligand in gB for retargeting to a Vero cell derivative and of anti-HER2 ligand in gD for cancer retargeting. These modifications were combined with a minimally destructive detargeting strategy. This study and its companion paper explain the clinical-grade cultivation of retargeted oncolytic HSVs and promote their translation to the clinic.

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.

An Interleukin 12 Adjuvanted Herpes Simplex Virus 2 DNA Vaccine Is More Protective Than a Glycoprotein D Subunit Vaccine in a High-Dose Murine Challenge Model

Vaccination is a proven intervention against human viral diseases; however, success against Herpes Simplex Virus 2 (HSV-2) remains elusive. Most HSV-2 vaccines tested in humans to date contained just one or two immunogens, such as the virion attachment receptor glycoprotein D (gD) and/or the envelope fusion protein, glycoprotein B (gB). At least three factors may have contributed to the failures of subunit-based HSV-2 vaccines. First, immune responses directed against one or two viral antigens may lack sufficient antigenic breadth for efficacy. Second, the antibody responses elicited by these vaccines may have lacked necessary Fc-mediated effector functions. Third, these subunit vaccines may not have generated necessary protective cellular immune responses. We hypothesized that a polyvalent combination of HSV-2 antigens expressed from a DNA vaccine with an adjuvant that polarizes immune responses toward a T helper 1 (Th1) phenotype would compose a more effective vaccine. We demonstrate that delivery of DNA expressing full-length HSV-2 glycoprotein immunogens by electroporation with the adjuvant interleukin 12 (IL-12) generates substantially greater protection against a high-dose HSV-2 vaginal challenge than a recombinant gD subunit vaccine adjuvanted with alum and monophosphoryl lipid A (MPL). Our results further show that DNA vaccines targeting optimal combinations of surface glycoproteins provide better protection than gD alone and provide similar survival benefits and disease symptom reductions compared with a potent live attenuated HSV-2 0ΔNLS vaccine, but that mice vaccinated with HSV-2 0ΔNLS clear the virus much faster. Together, our data indicate that adjuvanted multivalent DNA vaccines hold promise for an effective HSV-2 vaccine, but that further improvements may be required.

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.

Functional Characterization of Glycoprotein H Chimeras Composed of Conserved Domains of the Pseudorabies Virus and Herpes Simplex Virus 1 Homologs

Membrane fusion is indispensable for entry of enveloped viruses into host cells. The conserved core fusion machinery of the Herpesviridae consists of glycoprotein B (gB) and the gH/gL complex. Recently, crystal structures of gH/gL of herpes simplex virus 2 (HSV-2) and Epstein-Barr virus and of a core fragment of pseudorabies virus (PrV) gH identified four structurally conserved gH domains. To investigate functional conservation, chimeric genes encoding combinations of individual domains of PrV and herpes simplex virus 1 (HSV-1) gH were expressed in rabbit kidney cells, and their processing and transport to the cell surface, as well as activity in fusion assays including gB, gD, and gL of PrV or HSV-1, were analyzed. Chimeric gH containing domain I of HSV-1 and domains II to IV of PrV exhibited limited fusion activity in the presence of PrV gB and gD and HSV-1 gL, but not of PrV gL. More strikingly, chimeric gH consisting of PrV domains I to III and HSV-1 domain IV exhibited considerable fusion activity together with PrV gB, gD, and gL. Replacing PrV gB with the HSV-1 protein significantly enhanced this activity. A cell line stably expressing this chimeric gH supported replication of gH-deleted PrV. Our results confirm the specificity of domain I for gL binding, demonstrate functional conservation of domain IV in two alphaherpesviruses from different genera, and indicate species-specific interactions of this domain with gB. They also suggest that gH domains II and III might form a structural and functional unit which does not tolerate major substitutions.

IMPORTANCE

Envelope glycoprotein H (gH) is essential for herpesvirus-induced membrane fusion, which is required for host cell entry and viral spread. Although gH is structurally conserved within the Herpesviridae, its precise role and its interactions with other components of the viral fusion machinery are not fully understood. Chimeric proteins containing domains of gH proteins from different herpesviruses can serve as tools to elucidate the molecular basis of gH function. The present study shows that the C-terminal part of human herpesvirus 1 (herpes simplex virus 1) gH can functionally substitute for the corresponding part of suid herpesvirus 1 (pseudorabies virus) gH, whereas other tested combinations proved to be nonfunctional. Interestingly, the exchangeable fragment included the membrane-proximal end of the gH ectodomain (domain IV), which is most conserved in sequence and structure and might be capable of transient membrane interaction during fusion.

Structure of HCMV glycoprotein B in the postfusion conformation bound to a neutralizing human antibody

Human cytomegalovirus (HCMV) poses a significant threat to immunocompromised individuals and neonates infected in utero. Glycoprotein B (gB), the herpesvirus fusion protein, is a target for neutralizing antibodies and a vaccine candidate due to its indispensable role in infection. Here we show the crystal structure of the HCMV gB ectodomain bound to the Fab fragment of 1G2, a neutralizing human monoclonal antibody isolated from a seropositive subject. The gB/1G2 interaction is dominated by aromatic residues in the 1G2 heavy chain CDR3 protruding into a hydrophobic cleft in the gB antigenic domain 5 (AD-5). Structural analysis and comparison with HSV gB suggest the location of additional neutralizing antibody binding sites on HCMV gB. Finally, immunoprecipitation experiments reveal that 1G2 can bind to HCMV virion gB suggesting that its epitope is exposed and accessible on the virus surface. Our data will support the development of vaccines and therapeutic antibodies against HCMV infection.

Cytomegalovirus is a danger to individuals with compromised immune systems and neonates infected in utero. Here the authors show the structure of a neutralizing antibody-bound viral fusion protein glycoprotein B, supporting the development of therapeutic antibodies and vaccines.

Using a split luciferase assay (SLA) to measure the kinetics of cell-cell fusion mediated by herpes simplex virus glycoproteins

Herpes simplex virus (HSV) entry and cell-cell fusion require the envelope proteins gD, gH/gL and gB. We propose that receptor-activated conformational changes to gD activate gH/gL, which then triggers gB (the fusogen) into an active form. To study this dynamic process, we have adapted a dual split protein assay originally developed to study the kinetics of human immunodeficiency virus (HIV) mediated fusion. This assay uses a chimera of split forms of renilla luciferase (RL) and green fluorescent protein (GFP). Effector cells are co-transfected with the glycoproteins and one of the split reporters. Receptor-bearing target cells are transfected with the second reporter. Co-culture results in fusion and restoration of RL, which can convert a membrane permeable substrate into a luminescent product, thereby enabling one to monitor initiation and extent of fusion in live cells in real time. Restoration of GFP can also be studied by fluorescence microscopy. Two sets of split reporters have been developed: the original one allows one to measure fusion kinetics over hours whereas the more recent version was designed to enhance the sensitivity of RL activity allowing one to monitor both initiation and rates of fusion in minutes. Here, we provide a detailed, step-by-step protocol for the optimization of the assay (which we call the SLA for split luciferase assay) using the HSV system. We also show several examples of the power of this assay to examine both the initiation and kinetics of cell-cell fusion by wild type forms of gD, gB, gH/gL of both serotypes of HSV as well as the effect of mutations and antibodies that alter the kinetics of fusion. The SLA can be applied to other viral systems that carry out membrane fusion.

The Engineering of a Novel Ligand in gH Confers to HSV an Expanded Tropism Independent of gD Activation by Its Receptors

Herpes simplex virus (HSV) enters cells by means of four essential glycoproteins - gD, gH/gL, gB, activated in a cascade fashion by gD binding to one of its receptors, nectin1 and HVEM. We report that the engineering in gH of a heterologous ligand – a single-chain antibody (scFv) to the cancer-specific HER2 receptor – expands the HSV tropism to cells which express HER2 as the sole receptor. The significance of this finding is twofold. It impacts on our understanding of HSV entry mechanism and the design of retargeted oncolytic-HSVs. Specifically, entry of the recombinant viruses carrying the scFv-HER2–gH chimera into HER2⁺ cells occurred in the absence of gD receptors, or upon deletion of key residues in gD that constitute the nectin1/HVEM binding sites. In essence, the scFv in gH substituted for gD-mediated activation and rendered a functional gD non-essential for entry via HER2. The activation of the gH moiety in the chimera was carried out by the scFv in cis, not in trans as it occurs with wt-gD. With respect to the design of oncolytic-HSVs, previous retargeting strategies were based exclusively on insertion in gD of ligands to cancer-specific receptors. The current findings show that (i) gH accepts a heterologous ligand. The viruses retargeted via gH (ii) do not require the gD-dependent activation, and (iii) replicate and kill cells at high efficiency. Thus, gH represents an additional tool for the design of fully-virulent oncolytic-HSVs retargeted to cancer receptors and detargeted from gD receptors.

To enter cells, all herpesviruses use the core fusion glycoproteins gH/gL and gB, in addition to species-specific glycoproteins responsible for specific tropism, etc. In HSV, the additional glycoprotein is the essential gD. We engineered in gH a heterologous ligand to the HER2 cancer receptor. The recombinant viruses entered cells through HER2, independently of gD activation by its receptors, or despite deletion of key residues that are part of the receptors’ binding sites in gD. The ligand activated gH in cis. Cumulatively, the receptor-binding and activating functions of gD were no longer essential and were replaced by the heterologous ligand in gH. Relevance to translational medicine rests in the fact that gH can serve as a tool to retarget HSV tropism to cancer-specific receptors. This expands the toolkit for the design of fully-virulent oncolytic-HSVs.

Using proximity biotinylation to detect herpesvirus entry glycoprotein interactions: Limitations for integral membrane glycoproteins

Herpesvirus entry into cells requires coordinated interactions among several viral transmembrane glycoproteins. Viral glycoproteins bind to receptors and interact with other glycoproteins to trigger virus-cell membrane fusion. Details of these glycoprotein interactions are not well understood because they are likely transient and/or low affinity. Proximity biotinylation is a promising protein-protein interaction assay that can capture transient interactions in live cells. One protein is linked to a biotin ligase and a second protein is linked to a short specific acceptor peptide (AP). If the two proteins interact, the ligase will biotinylate the AP, without requiring a sustained interaction. To examine herpesvirus glycoprotein interactions, the ligase and AP were linked to herpes simplex virus 1 (HSV1) gD and Epstein Barr virus (EBV) gB. Interactions between monomers of these oligomeric proteins (homotypic interactions) served as positive controls to demonstrate assay sensitivity. Heterotypic combinations served as negative controls to determine assay specificity, since HSV1 gD and EBV gB do not interact functionally. Positive controls showed strong biotinylation, indicating that viral glycoprotein proximity can be detected. Unexpectedly, the negative controls also showed biotinylation. These results demonstrate the special circumstances that must be considered when examining interactions among glycosylated proteins that are constrained within a membrane.

A Functional Interaction between Herpes Simplex Virus 1 Glycoprotein gH/gL Domains I and II and gD Is Defined by Using Alphaherpesvirus gH and gL Chimeras

Whereas most viruses require only a single protein to bind to and fuse with cells, herpesviruses use multiple glycoproteins to mediate virus entry, and thus communication among these proteins is required. For most alphaherpesviruses, the minimal set of viral proteins required for fusion with the host cell includes glycoproteins gD, gB, and a gH/gL heterodimer. In the current model of entry, gD binds to a cellular receptor and transmits a signal to gH/gL. This signal then triggers gB, the conserved fusion protein, to insert into the target membrane and refold to merge the viral and cellular membranes. We previously demonstrated that gB homologs from two alphaherpesviruses, herpes simplex virus 1 (HSV-1) and saimiriine herpesvirus 1 (SaHV-1), were interchangeable. In contrast, neither gD nor gH/gL functioned with heterotypic entry glycoproteins, indicating that gD and gH/gL exhibit an essential type-specific functional interaction. To map this homotypic interaction site on gH/gL, we generated HSV-1/SaHV-1 gH and gL chimeras. The functional interaction with HSV-1 gD mapped to the N-terminal domains I and II of the HSV-1 gH ectodomain. The core of HSV-1 gL that interacts with gH also was required for functional homotypic interaction. The N-terminal gH/gL domains I and II are the least conserved and may have evolved to support species-specific glycoprotein interactions.

IMPORTANCE

The first step of the herpesvirus life cycle is entry into a host cell. A coordinated interaction among multiple viral glycoproteins is required to mediate fusion of the viral envelope with the cell membrane. The details of how these glycoproteins interact to trigger fusion are unclear. By swapping the entry glycoproteins of two alphaherpesviruses (HSV-1 and SaHV-1), we previously demonstrated a functional homotypic interaction between gD and gH/gL. To define the gH and gL requirements for homotypic interaction, we evaluated the function of a panel of HSV-1/SaHV-1 gH and gL chimeras. We demonstrate that domains I and II of HSV-1 gH are sufficient to promote a functional, albeit reduced, interaction with HSV-1 gD. These findings contribute to our model of how the entry glycoproteins cooperate to mediate herpesvirus entry into the cell.

A site of varicella-zoster virus vulnerability identified by structural studies of neutralizing antibodies bound to the glycoprotein complex gHgL

Mapping neutralizing epitopes on viral entry glycoproteins allows the identification of potentially important functional regions. The structure of varicella-zoster virus (VZV) gHgL bound to two antibodies isolated from immune donors reveals a common binding site. Functional experiments demonstrate that the two antibodies neutralize VZV infection and inhibit glycoprotein gB/glycoprotein complex gHgL-mediated membrane fusion. Immunization experiments in mice demonstrate that VZV gHgL elicits potently neutralizing antibodies and confirm the key role of this antigenic site in antibody-mediated virus neutralization. This manuscript sheds light on the molecular mechanism of herpesvirus cell entry and will guide the design of subunit-based vaccines against VZV.

Varicella-zoster virus (VZV), of the family Alphaherpesvirinae, causes varicella in children and young adults, potentially leading to herpes zoster later in life on reactivation from latency. The conserved herpesvirus glycoprotein gB and the heterodimer gHgL mediate virion envelope fusion with cell membranes during virus entry. Naturally occurring neutralizing antibodies against herpesviruses target these entry proteins. To determine the molecular basis for VZV neutralization, crystal structures of gHgL were determined in complex with fragments of antigen binding (Fabs) from two human monoclonal antibodies, IgG-94 and IgG-RC, isolated from seropositive subjects. These structures reveal that the antibodies target the same site, composed of residues from both gH and gL, distinct from two other neutralizing epitopes identified by negative-stain electron microscopy and mutational analysis. Inhibition of gB/gHgL-mediated membrane fusion and structural comparisons with herpesvirus homologs suggest that the IgG-RC/94 epitope is in proximity to the site on VZV gHgL that activates gB. Immunization studies proved that the anti-gHgL IgG-RC/94 epitope is a critical target for antibodies that neutralize VZV. Thus, the gHgL/Fab structures delineate a site of herpesvirus vulnerability targeted by natural immunity.

Evasion of early antiviral responses by herpes simplex viruses

Besides overcoming physical constraints, such as extreme temperatures, reduced humidity, elevated pressure, and natural predators, human pathogens further need to overcome an arsenal of antimicrobial components evolved by the host to limit infection, replication and optimally, reinfection. Herpes simplex virus-1 (HSV-1) and herpes simplex virus-2 (HSV-2) infect humans at a high frequency and persist within the host for life by establishing latency in neurons. To gain access to these cells, herpes simplex viruses (HSVs) must replicate and block immediate host antiviral responses elicited by epithelial cells and innate immune components early after infection. During these processes, infected and noninfected neighboring cells, as well as tissue-resident and patrolling immune cells, will sense viral components and cell-associated danger signals and secrete soluble mediators. While type-I interferons aim at limiting virus spread, cytokines and chemokines will modulate resident and incoming immune cells. In this paper, we discuss recent findings relative to the early steps taking place during HSV infection and replication. Further, we discuss how HSVs evade detection by host cells and the molecular mechanisms evolved by these viruses to circumvent early antiviral mechanisms, ultimately leading to neuron infection and the establishment of latency.

Animal models of herpes simplex virus immunity and pathogenesis

Herpes simplex viruses are ubiquitous human pathogens represented by two distinct serotypes: herpes simplex virus (HSV) type 1 (HSV-1); and HSV type 2 (HSV-2). In the general population, adult seropositivity rates approach 90% for HSV-1 and 20-25% for HSV-2. These viruses cause significant morbidity, primarily as mucosal membrane lesions in the form of facial cold sores and genital ulcers, with much less common but more severe manifestations causing death from encephalitis. HSV infections in humans are difficult to study in many cases because many primary infections are asymptomatic. Moreover, the neurotropic properties of HSV make it much more difficult to study the immune mechanisms controlling reactivation of latent infection within the corresponding sensory ganglia and crossover into the central nervous system of infected humans. This is because samples from the nervous system can only be routinely obtained at the time of autopsy. Thus, animal models have been developed whose use has led to a better understanding of multiple aspects of HSV biology, molecular biology, pathogenesis, disease, and immunity. The course of HSV infection in a spectrum of animal models depends on important experimental parameters including animal species, age, and genotype; route of infection; and viral serotype, strain, and dose. This review summarizes the animal models most commonly used to study HSV pathogenesis and its establishment, maintenance, and reactivation from latency. It focuses particularly on the immune response to HSV during acute primary infection and the initial invasion of the ganglion with comparisons to the events governing maintenance of viral latency.

The cytoplasmic domain of varicella-zoster virus glycoprotein H regulates syncytia formation and skin pathogenesis

The conserved herpesvirus fusion complex consists of glycoproteins gB, gH, and gL which is critical for virion envelope fusion with the cell membrane during entry. For Varicella Zoster Virus (VZV), the complex is necessary for cell-cell fusion and presumed to mediate entry. VZV causes syncytia formation via cell-cell fusion in skin and in sensory ganglia during VZV reactivation, leading to neuronal damage, a potential contributory factor for the debilitating condition of postherpetic neuralgia. The gH cytoplasmic domain (gHcyt) is linked to the regulation of gB/gH-gL-mediated cell fusion as demonstrated by increased cell fusion in vitro by an eight amino acid (aa834-841) truncation of the gHcyt. The gHcyt regulation was identified to be dependent on the physical presence of the domain, and not of specific motifs or biochemical properties as substitution of aa834-841 with V5, cMyc, and hydrophobic or hydrophilic sequences did not affect fusion. The importance of the gHcyt length was corroborated by stepwise deletions of aa834-841 causing incremental increases in cell fusion, independent of gH surface expression and endocytosis. Consistent with the fusion assay, truncating the gHcyt in the viral genome caused exaggerated syncytia formation and significant reduction in viral titers. Importantly, infection of human skin xenografts in SCID mice was severely impaired by the truncation while maintaining the gHcyt length with the V5 substitution preserved typical replication in vitro and in skin. A role for the gHcyt in modulating the functions of the gB cytoplasmic domain (gBcyt) is proposed as the gHcyt truncation substantially enhanced cell fusion in the presence of the gB[Y881F] mutation. The significant reduction in skin infection caused by hyperfusogenic mutations in either the gHcyt or gBcyt demonstrates that both domains are critical for regulating syncytia formation and failure to control cell fusion, rather than enhancing viral spread, is severely detrimental to VZV pathogenesis.

Varicella zoster virus (VZV) infects the human population globally, causing chickenpox in children and shingles in adults. While those afflicted with shingles experience severe pain that might last from weeks to months, the cause is not known. Biopsies of VZV infected skin and specimens of nerve ganglia collected at autopsy from patients with shingles at the time of death contain multi-nucleated cells, indicating that the virus is able to cause fusion between infected cells. Since the destruction of nerve cells that results from this process is likely to contribute to the pain associated with shingles, it is important to understand how the virus causes infected cells to fuse. We find that VZV cell-cell fusion is regulated by the intracellular facing domain of glycoprotein H (gH), a viral protein present on the surface of infected cells. This regulation was dependent upon the physical length of the domain, not a specific sequence. Loss of this regulation increased cell-cell fusion causing the formation of larger multi-nucleated cells that limited the ability of the virus to effectively spread in human skin. Our study provides new insight into how VZV manipulates host cells during infection and controls the spread of the virus in tissues.

Mechanism for neutralizing activity by the anti-CMV gH/gL monoclonal antibody MSL-109

Cytomegalovirus (CMV) is a widespread opportunistic pathogen that causes birth defects when transmitted transplacentally and severe systemic illness in immunocompromised individuals. MSL-109, a human monoclonal IgG isolated from a CMV seropositive individual, binds to the essential CMV entry glycoprotein H (gH) and prevents infection of cells. Here, we suggest a mechanism for neutralization activity by MSL-109. We define a genetic basis for resistance to MSL-109 and have generated a structural model of gH that reveals the epitope of this neutralizing antibody. Using surface-based, time-resolved FRET, we demonstrate that gH/gL interacts with glycoprotein B (gB). Additionally, we detect homodimers of soluble gH/gL heterodimers and confirm this novel oligomeric assembly on full-length gH/gL expressed on the cell surface. We show that MSL-109 perturbs the dimerization of gH/gL:gH/gL, suggesting that dimerization of gH/gL may be required for infectivity. gH/gL homodimerization may be conserved between alpha- and betaherpesviruses, because both CMV and HSV gH/gL demonstrate self-association in the FRET system. This study provides evidence for a novel mechanism of action for MSL-109 and reveals a previously undescribed aspect of viral entry that may be susceptible to therapeutic intervention.

αvβ6- and αvβ8-integrins serve as interchangeable receptors for HSV gH/gL to promote endocytosis and activation of membrane fusion

Herpes simplex virus (HSV) - and herpesviruses in general - encode for a multipartite entry/fusion apparatus. In HSV it consists of the HSV-specific glycoprotein D (gD), and three additional glycoproteins, gH/gL and gB, conserved across the Herpesviridae family and responsible for the execution of fusion. According to the current model, upon receptor binding, gD propagates the activation to gH/gL and to gB in a cascade fashion. Questions remain about how the cascade of activation is controlled and how it is synchronized with virion endocytosis, to avoid premature activation and exhaustion of the glycoproteins. We considered the possibility that such control might be carried out by as yet unknown receptors. Indeed, receptors for HSV gB, but not for gH/gL, have been described. In other members of the Herpesviridae family, such as Epstein-Barr virus, integrin receptors bind gH/gL and trigger conformational changes in the glycoproteins. We report that αvβ6- and αvβ8-integrins serve as receptors for HSV entry into experimental models of keratinocytes and other epithelial and neuronal cells. Evidence rests on loss of function experiments, in which integrins were blocked by antibodies or silenced, and gain of function experiments in which αvβ6-integrin was expressed in integrin-negative cells. αvβ6- and αvβ8-integrins acted independently and are thus interchangeable. Both bind gH/gL with high affinity. The interaction profoundly affects the route of HSV entry and directs the virus to acidic endosomes. In the case of αvβ8, but not αvβ6-integrin, the portal of entry is located at lipid microdomains and requires dynamin 2. Thus, a major role of αvβ6- or αvβ8-integrin in HSV infection appears to be to function as gH/gL receptors and to promote virus endocytosis. We propose that placing the gH/gL activation under the integrin trigger point enables HSV to synchronize virion endocytosis with the cascade of glycoprotein activation that culminates in execution of fusion.

In order to infect their hosts and cause disease, viruses must enter their host cells. The human pathogen herpes simplex virus (HSV) - and herpesviruses in general - are equipped with a complex, multipartite entry apparatus, made of four glycoproteins – gD, gH/gL, gB. These glycoproteins must be activated in a timely, coordinated manner. According to the current model, the flux of activation goes from receptor-bound gD, to gH/gL and gB. The premature activation, and hence exhaustion of the glycoproteins must also be prevented. We report on a checkpoint at the gH/gL level. Specifically, αvβ6- and αvβ8-integrins serve as receptors for HSV entry into keratinocytes and other epithelial and neuronal cells. Both bind gH/gL with high affinity. The interaction profoundly affects the pathway of HSV entry, promoting HSV endocytosis into acidic endosomes. For αvβ8-integrin, the portal of entry is at lipid microdomains and requires dynamin 2. We propose that, by placing the activation of gH/gL under control of an integrin trigger point, HSV can synchronize virion endocytosis with the cascade of activation that culminates in the execution of fusion between the virion envelope and cellular membranes.

Dual split protein-based fusion assay reveals that mutations to herpes simplex virus (HSV) glycoprotein gB alter the kinetics of cell-cell fusion induced by HSV entry glycoproteins

Herpes simplex virus (HSV) entry and cell-cell fusion require glycoproteins gD, gH/gL, and gB. We propose that receptor-activated changes to gD cause it to activate gH/gL, which then triggers gB into an active form. We employed a dual split-protein (DSP) assay to monitor the kinetics of HSV glycoprotein-induced cell-cell fusion. This assay measures content mixing between two cells, i.e., fusion, within the same cell population in real time (minutes to hours). Titration experiments suggest that both gD and gH/gL act in a catalytic fashion to trigger gB. In fact, fusion rates are governed by the amount of gB on the cell surface. We then used the DSP assay to focus on mutants in two functional regions (FRs) of gB, FR1 and FR3. FR1 contains the fusion loops (FL1 and FL2), and FR3 encompasses the crown at the trimer top. All FL mutants initiated fusion very slowly, if at all. However, the fusion rates caused by some FL2 mutants increased over time, so that total fusion by 8 h looked much like that of the WT. Two distinct kinetic patterns, "slow and fast," emerged for mutants in the crown of gB (FR3), again showing differences in initiation and ongoing fusion. Of note are the fusion kinetics of the gB syn mutant (LL871/872AA). Although this mutant was originally included as an ongoing high-rate-of-fusion control, its initiation of fusion is so rapid that it appears to be on a "hair trigger." Thus, the DSP assay affords a unique way to examine the dynamics of HSV glycoprotein-induced cell fusion.

Exploiting herpes simplex virus entry for novel therapeutics

Herpes Simplex virus (HSV) is associated with a variety of diseases such as genital herpes and numerous ocular diseases. At the global level, high prevalence of individuals who are seropositive for HSV, combined with its inconspicuous infection, remains a cause for major concern. At the molecular level, HSV entry into a host cell involves multiple steps, primarily the interaction of viral glycoproteins with various cell surface receptors, many of which have alternate substitutes. The molecular complexity of the virus to enter a cell is also enhanced by the existence of different modes of viral entry. The availability of many entry receptors, along with a variety of entry mechanisms, has resulted in a virus that is capable of infecting virtually all cell types. While HSV uses a wide repertoire of viral and host factors in establishing infection, current therapeutics aimed against the virus are not as diversified. In this particular review, we will focus on the initial entry of the virus into the cell, while highlighting potential novel therapeutics that can control this process. Virus entry is a decisive step and effective therapeutics can translate to less virus replication, reduced cell death, and detrimental symptoms.

Herpes virus fusion and entry: a story with many characters

Herpesviridae comprise a large family of enveloped DNA viruses all of whom employ orthologs of the same three glycoproteins, gB, gH and gL. Additionally, herpesviruses often employ accessory proteins to bind receptors and/or bind the heterodimer gH/gL or even to determine cell tropism. Sorting out how these proteins function has been resolved to a large extent by structural biology coupled with supporting biochemical and biologic evidence. Together with the G protein of vesicular stomatitis virus, gB is a charter member of the Class III fusion proteins. Unlike VSV G, gB only functions when partnered with gH/gL. However, gH/gL does not resemble any known viral fusion protein and there is evidence that its function is to upregulate the fusogenic activity of gB. In the case of herpes simplex virus, gH/gL itself is upregulated into an active state by the conformational change that occurs when gD, the receptor binding protein, binds one of its receptors. In this review we focus primarily on prototypes of the three subfamilies of herpesviruses. We will present our model for how herpes simplex virus (HSV) regulates fusion in series of highly regulated steps. Our model highlights what is known and also provides a framework to address mechanistic questions about fusion by HSV and herpesviruses in general.

Glycoprotein B of herpes simplex virus 2 has more than one intracellular conformation and is altered by low pH

The crystal structure of herpes simplex virus (HSV) gB identifies it as a class III fusion protein, and comparison with other such proteins suggests this is the postfusion rather than prefusion conformation, although this is not proven. Other class III proteins undergo a pH-dependent switch between pre- and postfusion conformations, and a low pH requirement for HSV entry into some cell types suggests that this may also be true for gB. Both gB and gH undergo structural changes at low pH, but there is debate about the extent and significance of the changes in gB, possibly due to the use of different soluble forms of the protein and different assays for antigenic changes. In this study, a complementary approach was taken, examining the conformations of full-length intracellular gB by quantitative confocal microscopy with a panel of 26 antibodies. Three conformations were distinguished, and low pH was found to be a major influence. Comparison with previous studies indicates that the intracellular conformation in low-pH environments may be the same as that of the soluble form known as s-gB at low pH. Interestingly, the antibodies whose binding was most affected by low pH both have neutralizing activity and consequently must block either the function of a neutral pH conformation or its switch from an inactive form to an activated form. If one of the intracellular conformations is the fusion-active form, another factor required for fusion is presumably absent from wherever that conformation is present in infected cells so that inappropriate fusion is avoided.

Herpes simplex virus glycoproteins gH/gL and gB bind Toll-like receptor 2, and soluble gH/gL is sufficient to activate NF-κB

A number of sentinels sense incoming herpes simplex virus (HSV) virions and initiate an immediate innate response. The first line of defense at the cell surface is TLR2 (Toll-like receptor 2), whose signature signaling activity leads to activation of the key transcription factor NF-κB. We report that the HSV pathogen-associated molecular patterns for TLR2 are the virion glycoproteins gH/gL and gB, which constitute the conserved fusion core apparatus across the members of the Herpesviridae family. Specifically, virions devoid singly of one of essential fusion glycoproteins (gD, gB, or gH null), able to attach to cells but defective in fusion/entry, were sufficient to elicit the first wave of NF-κB response to HSV. The most effective were the gD-null virions, positive for gH/gL and gB. A soluble form of gB, truncated upstream of the transmembrane sequence (gB(730t-st)), was produced in human cells and purified by means of a Strep tag. gH/gL and gB were each able to physically interact with TLR2 in coimmunoprecipitation assays, one independently of the other, yet gH(t-st)/gL, but not gB(730t-st), elicited an NF-κB response. Thus, whereas both HSV gH/gL and gB are ligands to TLR2, only gH/gL is sufficient to initiate a signaling cascade which leads to NF-κB activation.

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

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

αVβ3-integrin relocalizes nectin1 and routes herpes simplex virus to lipid rafts

Herpes simplex virus (HSV) enters cells by fusion at plasma membranes or endosomes. Cellular factors route the virus to different pathways. αVβ3-integrin directs HSV to a lipid raft and acidic endosome pathway. We report that infection mediated by nectin1 plus αVβ3-integrin exhibits the same characteristics as entry mediated by raft-located forms of nectin. αVβ3-integrin relocalizes nectin1 to lipid rafts, independently of virus. Thus, HSV routing to the lipid raft-dependent pathway is consequent to the integrin-induced relocalization of nectin1. Inhibition by the Na+/H+ exchanger 5-(N-ethyl-N-isopropyl)amirolide suggests that αVβ3-integrin overexpression favors HSV macropinocytic uptake in some cells but not in others.

Molecular network pathways and functional analysis of tumor signatures associated with development of resistance to viral gene therapy

Replication-competent attenuated herpes simplex viruses have proven effective in killing many cancer cell lines. However, determinants of resistance to oncolytic therapy are mostly unknown. We developed viral therapy-resistant cells and examined changes in gene-expression pattern compared with therapy-sensitive parental cells. Colon cancer cell line HT29 and hepatoma cell line PLC5 were exposed to increasing concentrations of virus G207. Therapy-resistant cells were isolated and grown in vitro. Tumorigenicity was confirmed by ability of cell lines to form tumors in mice. Human Genome U133A complementary DNA microarray chips were used to determine gene-expression patterns, which were analyzed in the context of molecular network interactions, pathways and gene ontology. In parental cell lines, 90-100% of cells were killed by day 7 at 1.0 multiplicity of infection. In resistant cell lines, cytotoxicity assay confirmed 200- to 400-fold resistance. Microarray analysis confirmed changes in gene expressions associated with resistance: cell surface proteins affecting viral attachment and entry, cellular proteins affecting nucleotide pools and proteins altering apoptotic pathways. These changes would decrease viral infection and replication. Our study identifies gene-expression signatures associated with resistance to oncolytic viral therapy. These data provide potential targets to overcome resistance, and suggest that molecular assays may be useful in selecting patients for trial with this novel treatment.

Human herpesvirus 6 glycoprotein complex formation is required for folding and trafficking of the gH/gL/gQ1/gQ2 complex and its cellular receptor binding

Human herpesvirus 6 (HHV-6) is a T-cell-tropic betaherpesvirus. A glycoprotein (g) complex that is unique to HHV-6, gH/gL/gQ1/gQ2, is a viral ligand for its cellular receptor, human CD46. However, whether complex formation or one component of the complex is required for CD46 binding and how the complex is transported in cells are open questions. Furthermore, in HHV-6-infected cells the gQ1 protein modified with N-linked glycans is expressed in two forms with different molecular masses: an 80-kDa form (gQ1-80K) and a 74-kDa form (gQ1-74K). Only gQ1-80K, but not gQ1-74K, forms the complex with gQ2, gH, and gL, and this four-component complex is incorporated into mature virions. Here, we characterized the molecular context leading to the maturation of gQ1 by expressing combinations of the individual gH/gL/gQ1/gQ2 components in 293T cells. Surprisingly, only when all four molecules were expressed was a substantial amount of gQ1-80K detected, indicating that all three of the other molecules (gQ2, gH, and gL) were necessary and sufficient for gQ1 maturation. We also found that only the tetrameric complex, and not its subsets, binds to CD46. Finally, a gQ2-null virus constructed in the BAC (bacterial artificial chromosome) system could not be reconstituted, indicating that gQ2 is essential for virus growth. These results show that gH, gL, gQ1, and gQ2 are all essential for the trafficking and proper folding of the gH/gL/gQ1/gQ2 complex and, thus, for HHV-6 infection.

Low-pH-dependent changes in the conformation and oligomeric state of the prefusion form of herpes simplex virus glycoprotein B are separable from fusion activity

The cellular requirements for activation of herpesvirus fusion and entry remain poorly understood. Low pH triggers change in the antigenic reactivity of the prefusion form of the herpes simplex virus (HSV) fusion protein gB in virions, both in vitro and during viral entry via endocytosis (S. Dollery et al., J. Virol. 84:3759-3766, 2010). However, the mechanism and magnitude of gB conformational change are not clear. Here we show that the conformation and oligomeric state of gB with mutations in the bipartite fusion loops were similarly altered despite the fusion-inactivating mutations. Together with previous studies, this suggests that fusion loop mutants undergo conformational changes but are defective for fusion because they fail to make productive contact with the outer leaflet of the host target membrane. A direct, reversible effect of low pH on the structure of gB was detected by fluorescence spectroscopy. A soluble form of gB containing cytoplasmic tail sequences (s-gB) was triggered by mildly acidic pH to undergo changes in tryptophan fluorescence emission, hydrophobicity, antigenic conformation, and oligomeric structure and thus resembled the prefusion form of gB in the virion. In contrast, soluble gB730, for which the postfusion crystal structure is known, was only marginally affected by pH using these measures. The results underscore the importance of using a prefusion form of gB to assess the activation and extent of conformation change. Further, acidic pH had little to no effect on the conformation or hydrophobicity of gD or on gD's ability to bind nectin-1 or HVEM receptors. Our results support a model in which endosomal low pH serves as a cellular trigger of fusion by activating conformational changes in the fusion protein gB.

Capturing the herpes simplex virus core fusion complex (gB-gH/gL) in an acidic environment

Herpes simplex virus (HSV) entry requires the core fusion machinery of gH/gL and gB as well as gD and a gD receptor. When gD binds receptor, it undergoes conformational changes that presumably activate gH/gL, which then activates gB to carry out fusion. gB is a class III viral fusion protein, while gH/gL does not resemble any known viral fusion protein. One hallmark of fusion proteins is their ability to bind lipid membranes. We previously used a liposome coflotation assay to show that truncated soluble gB, but not gH/gL or gD, can associate with liposomes at neutral pH. Here, we show that gH/gL cofloats with liposomes but only when it is incubated with gB at pH 5. When gB mutants with single amino acid changes in the fusion loops (known to inhibit the binding of soluble gB to liposomes) were mixed with gH/gL and liposomes at pH 5, gH/gL failed to cofloat with liposomes. These data suggest that gH/gL does not directly associate with liposomes but instead binds to gB, which then binds to liposomes via its fusion loops. Using monoclonal antibodies, we found that many gH and gL epitopes were altered by low pH, whereas the effect on gB epitopes was more limited. Our liposome data support the concept that low pH triggers conformational changes to both proteins that allow gH/gL to physically interact with gB.

Fusing structure and function: a structural view of the herpesvirus entry machinery

Herpesviruses are double-stranded DNA, enveloped viruses that infect host cells through fusion with either the host cell plasma membrane or endocytic vesicle membranes. Efficient infection of host cells by herpesviruses is remarkably more complex than infection by other viruses, as it requires the concerted effort of multiple glycoproteins and involves multiple host receptors. The structures of the major viral glycoproteins and a number of host receptors involved in the entry of the prototypical herpesviruses, the herpes simplex viruses (HSVs) and Epstein-Barr virus (EBV), are now known. These structural studies have accelerated our understanding of HSV and EBV binding and fusion by revealing the conformational changes that occur on virus-receptor binding, depicting potential sites of functional protein and lipid interactions, and identifying the probable viral fusogen.

Mapping regions of Epstein-Barr virus (EBV) glycoprotein B (gB) important for fusion function with gH/gL

Glycoproteins gB and gH/gL are required for entry of Epstein-Barr virus (EBV) into cells, but the role of each glycoprotein and how they function together to mediate fusion is unclear. Analysis of the functional homology of gB from the closely related primate gammaherpesvirus, rhesus lymphocryptovirus (Rh-LCV), showed that EBV gB could not complement Rh gB due to a species-specific dependence between gB and gL. To map domains of gB required for this interaction, we constructed a panel of EBV/Rh gB chimeric proteins. Analysis showed that insertion of Rh gB from residues 456 to 807 restored fusion function of EBV gB with Rh gH/gL, suggesting this region of gB is important for interaction with gH/gL. Split YFP bimolecular complementation (BiFC) provided evidence of an interaction between EBV gB and gH/gL. Together, our results suggest the importance of a gB-gH/gL interaction in EBV-mediated fusion with B cells requiring the region of EBV gB from 456 to 807.

Cascade of events governing cell-cell fusion induced by herpes simplex virus glycoproteins gD, gH/gL, and gB

Herpesviruses minimally require the envelope proteins gB and gH/gL for virus entry and cell-cell fusion; herpes simplex virus (HSV) additionally requires the receptor-binding protein gD. Although gB is a class III fusion protein, gH/gL does not resemble any documented viral fusion protein at a structural level. Based on those data, we proposed that gH/gL does not function as a cofusogen with gB but instead regulates the fusogenic activity of gB. Here, we present data to support that hypothesis. First, receptor-positive B78H1-C10 cells expressing gH/gL fused with receptor-negative B78H1 cells expressing gB and gD (fusion in trans). Second, fusion occurred when gH/gL-expressing C10 cells preexposed to soluble gD were subsequently cocultured with gB-expressing B78 cells. In contrast, prior exposure of gB-expressing C10 cells to soluble gD did not promote subsequent fusion with gH/gL-expressing B78 cells. These data suggest that fusion involves activation of gH/gL by receptor-bound gD. Most importantly, soluble gH/gL triggered a low level of fusion of C10 cells expressing gD and gB; a much higher level was achieved when gB-expressing C10 cells were exposed to a combination of soluble gH/gL and gD. These data clearly show that gB acts as the HSV fusogen following activation by gD and gH/gL. We suggest the following steps leading to fusion: (i) conformational changes to gD upon receptor binding, (ii) alteration of gH/gL by receptor-activated gD, and (iii) upregulation of the fusogenic potential of gB following its interaction with activated gH/gL. The third step may be common to other herpesviruses.

A double mutation in glycoprotein gB compensates for ineffective gD-dependent initiation of herpes simplex virus type 1 infection

Herpes simplex virus (HSV) entry into cells is triggered by the binding of envelope glycoprotein D (gD) to a specific receptor, such as nectin-1 or herpesvirus entry mediator (HVEM), resulting in activation of the fusion effectors gB and gH and virus penetration. Here we report the identification of a hyperactive gB allele, D285N/A549T, selected by repeat passage of a gD mutant virus defective for nectin-1 binding through cells that express a gD-binding-impaired mutant nectin-1. The gB allele in a wild-type virus background enabled the use of other nectins as virus entry receptors. In addition, combination of the mutant allele with an epidermal growth factor receptor (EGFR)-retargeted gD gene yielded dramatically increased EGFR-specific virus entry compared to retargeted virus carrying wild-type gB. Entry of the gB mutant virus into nectin-1-bearing cells was markedly accelerated compared to that of wild-type virus, suggesting that the gB mutations affect a rate-limiting step in entry. Our observations indicate that ineffective gD activation can be complemented by hypersensitization of a downstream component of the entry cascade to gD signaling.

Reevaluating herpes simplex virus hemifusion

Membrane fusion induced by enveloped viruses proceeds through the actions of viral fusion proteins. Once activated, viral fusion proteins undergo large protein conformational changes to execute membrane fusion. Fusion is thought to proceed through a "hemifusion" intermediate in which the outer membrane leaflets of target and viral membranes mix (lipid mixing) prior to fusion pore formation, enlargement, and completion of fusion. Herpes simplex virus type 1 (HSV-1) requires four glycoproteins-glycoprotein D (gD), glycoprotein B (gB), and a heterodimer of glycoprotein H and L (gH/gL)-to accomplish fusion. gD is primarily thought of as a receptor-binding protein and gB as a fusion protein. The role of gH/gL in fusion has remained enigmatic. Despite experimental evidence that gH/gL may be a fusion protein capable of inducing hemifusion in the absence of gB, the recently solved crystal structure of HSV-2 gH/gL has no structural homology to any known viral fusion protein. We found that in our hands, all HSV entry proteins-gD, gB, and gH/gL-were required to observe lipid mixing in both cell-cell- and virus-cell-based hemifusion assays. To verify that our hemifusion assay was capable of detecting hemifusion, we used glycosylphosphatidylinositol (GPI)-linked hemagglutinin (HA), a variant of the influenza virus fusion protein, HA, known to stall the fusion process before productive fusion pores are formed. Additionally, we found that a mutant carrying an insertion within the short gH cytoplasmic tail, 824L gH, is incapable of executing hemifusion despite normal cell surface expression. Collectively, our findings suggest that HSV gH/gL may not function as a fusion protein and that all HSV entry glycoproteins are required for both hemifusion and fusion. The previously described gH 824L mutation blocks gH/gL function prior to HSV-induced lipid mixing.