Low pH-induced conformational change in herpes simplex virus glycoprotein B

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.

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.

The possibility of spreading herpes simplex virus type 1 via food handling and sharing

AIMS

Herpes simplex virus type 1 (HSV-1) is an enveloped virus that causes recurrent and incurable diseases in 67% of the world population. Although it is not listed as a foodborne virus, some studies have shown that it can be recovered from surfaces as well as food.

METHODS AND RESULTS

We investigated its persistence at -20°C, 4°C, 20°C, or 37°C for up to 7 days on stainless steel, aluminum, glass, polypropylene, cheddar cheese, sliced almond, and apple skin and in cola soft drink, orange juice, coffee, and milk, as well as its transferability from stainless steel to dry or moistened nitrile or latex gloves over time at typical ambient temperatures. Based on the plaque assay on Vero cells, HSV-1 persisted at least 24 h on all surfaces and at least 1 h on food matrices but was inactivated quickly in cola soft drink. Temperature and pH affected HSV-1 infectivity. Transfer of HSV-1 at a contact pressure of 1 kg cm2-1 for 10 s occurred only on latex, especially moistened.

CONCLUSIONS

Our data on the persistence of HSV-1 on food-related surfaces suggest that some risk may be associated with sharing foods with infected carriers.

Herpesvirus-Mimicking DNAzyme-Loaded Nanoparticles as a Mitochondrial DNA Stress Inducer to Activate Innate Immunity for Tumor Therapy

Virus-based immunotherapy is a promising approach to treat tumor. Closely mimicking the structure and sequential infection processes of natural viruses is highly desirable for effective tumor immunotherapy but remains challenging. Here, inspired by the robust innate immunity induced by herpesvirus, a herpesvirus-mimicking nanoparticle (named Vir-ZM@TD) is engineered for tumor therapy by mimicking the structure and infection processes of herpesvirus. In this biomimetic system, DNAzyme-loaded manganese-doped zeolitic imidazolate framework-90 (ZIF-90) nanoparticles (ZM@TD) mimic the virus nucleocapsid containing the genome; the erythrocyte membrane mimics the viral envelope; and two functional peptides, RGD and HA2 peptides, resemble the surface glycoprotein spikes of herpesvirus. Vir-ZM@TD can both effectively evade rapid clearance in the blood circulation and closely mimic the serial infection processes of herpesvirus, including specific tumor targeting, membrane fusion-mediated endosomal escape, and TFAM (transcription factor A, mitochondrial) deficiency-triggered mitochondrial DNA stress, as well as the release of manganese ions (Mn²⁺ ) from organelles into the cytosol, ultimately effectively priming cGAS-STING pathway-mediated innate immunity with 68% complete regression of primary tumors and extending by 32 days the median survival time of 4T1-tumor-bearing mice.

Conformational Changes in Herpes Simplex Virus Glycoprotein C

Low endosomal pH facilitates herpesvirus entry in a cell-specific manner. Herpes simplex virus 1 (HSV-1) causes significant morbidity and death in humans worldwide. HSV-1 enters cells by low-pH and neutral-pH pathways. Low-pH-induced conformational changes in the HSV envelope glycoprotein B (gB) may mediate membrane fusion during viral entry. HSV-1 gC, a 511-amino acid, type I integral membrane glycoprotein, mediates HSV-1 attachment to host cell surface glycosaminoglycans, but this interaction is not essential for viral entry. We previously demonstrated that gC regulates low-pH viral entry independent of its known role in cell attachment. Low-pH-triggered conformational changes in gB occur at a lower pH when gC is absent, suggesting that gC positively regulates gB conformational changes. Here, we demonstrate that mildly acidic pH triggers conformational changes in gC itself. Low-pH treatment of virions induced antigenic changes in distinct gC epitopes, and those changes were reversible. One of these gC epitopes is recognized by a monoclonal antibody that binds to a linear sequence that includes residues within gC amino acids 33 to 123. This antibody inhibited low-pH entry of HSV, suggesting that its gC N-terminal epitope is particularly important. We propose that gC plays a critical role in HSV entry through a low-pH endocytosis pathway, which is a major entry route in human epithelial cells. IMPORTANCE Herpesviruses are ubiquitous pathogens that cause lifelong latent infections and are characterized by multiple entry pathways. The HSV envelope gC regulates HSV entry by a low-pH entry route. The fusion protein gB undergoes pH-triggered conformational changes that are facilitated by gC. Here, we report that gC itself undergoes a conformational change at low pH. A monoclonal antibody to gC that binds to a region that undergoes pH-induced changes also selectively inhibits HSV low-pH entry, corroborating the importance of gC in the low-pH entry pathway. This study illustrates the complex role of endosomal pH during HSV entry and provides novel insights into the functions of gC.

The ins and outs of virus trafficking through acidic Ca2+ stores

Many viruses exploit host-cell Ca2+ signaling processes throughout their life cycle. This is especially relevant for viruses that translocate through the endolysosomal system, where cellular infection is keyed to the microenvironment of these acidic Ca2+ stores and Ca2+-dependent trafficking pathways. As regulators of the endolysosomal ionic milieu and trafficking dynamics, two families of endolysosomal Ca2+-permeable cation channels - two pore channels (TPCs) and transient receptor potential mucolipins (TRPMLs) - have emerged as important host-cell factors in viral entry. Here, we review: (i) current evidence implicating Ca2+ signaling in viral translocation through the endolysosomal system, (ii) the roles of these ion channels in supporting cellular infection by different viruses, and (iii) areas for future research that will help define the potential of TPC and TRPML ligands as progressible antiviral agents.

Extracellular pH, osmolarity, temperature and humidity could discourage SARS-CoV-2 cell docking and propagation via intercellular signaling pathways

The COVID-19 pandemic and its virus variants continue to pose a serious and long-lasting threat worldwide. To combat the pandemic, the world's largest COVID-19 vaccination campaign is currently ongoing. As of July 19th 2021, 26.2% of the world population has received at least one dose of a COVID-19 vaccine (1.04 billion), and one billion has been fully vaccinated, with very high vaccination rates in countries like Israel, Malta, and the UEA. Conversely, only 1% of people in low-income countries have received at least one dose with examples of vaccination frequency as low as 0.07% in the Democratic Republic of Congo. It is thus of paramount importance that more research on alternate methods to counter cell infection and propagation is undertaken that could be implemented in low-income countries. Moreover, an adjunctive therapeutic intervention would help to avoid disease exacerbation in high-rate vaccinated countries too. Based on experimental biochemical evidence on viral cell fusion and propagation, herein we identify (i) extracellular pH (epH), (ii) temperature, and (iii) humidity and osmolarity as critical factors. These factors are here in discussed along with their implications on mucus thick layer, proteases, abundance of sialic acid, vascular permeability and exudate/edema. Heated, humidified air containing sodium bicarbonate has long been used in the treatment of certain diseases, and here we argue that warm inhalation of sodium bicarbonate might successfully target these endpoints. Although we highlight the molecular/cellular basis and the signalling pathways to support this intervention, we underscore the need for clinical investigations to encourage further research and clinical trials. In addition, we think that such an approach is also important in light of the high mutation rate of this virus originating from a rapid increase.

Herpes Simplex Virus Cell Entry Mechanisms: An Update

Herpes simplex virus (HSV) can infect a broad host range and cause mild to life threating infections in humans. The surface glycoproteins of HSV are evolutionarily conserved and show an extraordinary ability to bind more than one receptor on the host cell surface. Following attachment, the virus fuses its lipid envelope with the host cell membrane and releases its nucleocapsid along with tegument proteins into the cytosol. With the help of tegument proteins and host cell factors, the nucleocapsid is then docked into the nuclear pore. The viral double stranded DNA is then released into the host cell’s nucleus. Released viral DNA either replicates rapidly (more commonly in non-neuronal cells) or stays latent inside the nucleus (in sensory neurons). The fusion of the viral envelope with host cell membrane is a key step. Blocking this step can prevent entry of HSV into the host cell and the subsequent interactions that ultimately lead to production of viral progeny and cell death or latency. In this review, we have discussed viral entry mechanisms including the pH-independent as well as pH-dependent endocytic entry, cell to cell spread of HSV and use of viral glycoproteins as an antiviral target.

Herpes Simplex Virus Entry by a Nonconventional Endocytic Pathway

Herpes simplex virus 1 (HSV-1) causes significant morbidity and mortality in humans worldwide. HSV-1 enters epithelial cells via an endocytosis mechanism that is low-pH dependent. However, the precise intracellular pathway has not been identified, including the compartment where fusion occurs. In this study, we utilized a combination of molecular and pharmacological approaches to better characterize HSV entry by endocytosis. HSV-1 entry was unaltered in both cells treated with small interfering RNA (siRNA) to Rab5 or Rab7 and cells expressing dominant negative forms of these GTPases, suggesting entry is independent of the conventional endo-lysosomal network. The fungal metabolite brefeldin A (BFA) and the quinoline compound Golgicide A (GCA) inhibited HSV-1 entry via beta-galactosidase reporter assay and impaired incoming virus transport to the nuclear periphery, suggesting a role for trans-Golgi network (TGN) functions and retrograde transport in HSV entry. Silencing of Rab9 or Rab11 GTPases, which are involved in the retrograde transport pathway, resulted in only a slight reduction in HSV infection. Together, these results suggest that HSV enters host cells by an intracellular route independent of the lysosome-terminal endocytic pathway.IMPORTANCE Herpes simplex virus 1 (HSV-1), the prototype alphaherpesvirus, is ubiquitous in the human population and causes lifelong infection that can be fatal in neonatal and immunocompromised individuals. HSV enters many cell types by endocytosis, including epithelial cells, the site of primary infection in the host. The intracellular itinerary for HSV entry remains unclear. We probed the potential involvement of several Rab GTPases in HSV-1 entry and suggest that endocytic entry of HSV-1 is independent of the canonical lysosome-terminal pathway. A nontraditional endocytic route may be employed, such as one that intersects with the trans-Golgi network (TGN). These results may lead to novel targets for intervention.

Glycoprotein C of Herpes Simplex Virus 1 Shields Glycoprotein B from Antibody Neutralization

Viruses have evolved strategies to avoid neutralization by the host antibody response. Herpes simplex virus (HSV) glycoprotein C (gC) functions in viral entry and binds to complement component C3b, inhibiting complement-mediated immunity. We investigated whether gC protects HSV from antibody neutralization. HSV-1 that lacks gC was more sensitive to complement-independent neutralization by a panel of gB monoclonal antibodies than a wild-type gC rescuant virus. The presence of gC decreased neutralization by 2- to 16-fold. The gB in the native envelope of HSV-1 had reduced reactivity with antibodies in comparison to gB from the gC-null virus, suggesting that gC hampers the recognition of gB epitopes in the viral particle. The protein composition of the gC-null virus, including the surface glycoproteins essential for entry, was equivalent to that of the wild type, suggesting that gC is directly responsible for the reduced antibody recognition and neutralization. The neutralizing activity of antibodies to gD and gH antibodies was also increased in HSV lacking gC. Together, the data suggest that HSV-1 gC protects the viral envelope glycoproteins essential for entry, including gB, by shielding them from neutralization as a potential mechanism of immune evasion.IMPORTANCE HSV-1 causes lifelong infection in the human population and can be fatal in neonatal and immunocompromised individuals. There is no vaccine or cure, in part due to the ability of HSV to escape the host immune response by various mechanisms. The HSV particle contains at least 15 envelope proteins, four of which are required for entry and replication. This work suggests a novel role for gC in shielding the HSV entry glycoproteins. gC may function to help HSV escape neutralization by antibodies.

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.

Conformational Change in Herpes Simplex Virus Entry Glycoproteins Detected by Dot Blot

Conformational changes in viral membrane proteins drive membrane fusion, a critical step in virus entry and infection. Here we describe a simple and rapid virus blotting immunoassay to define conformational changes with a panel of monoclonal antibodies to distinct sites across a viral glycoprotein. This dot blot technique has been utilized to define low pH-triggered changes in the prefusion form of the herpesviral fusogen gB. At pH of <6.2 there are specific changes in herpes simplex virus 1 gB domains I and V. This corresponds broadly to host cell endosomal pH. Many of the identified changes are at least partially reversible. This method can be adapted to document changes in viral proteins that are not fusion proteins, including those induced by alternate triggers such as receptor-binding or protease cleavage.

Towards Understanding KSHV Fusion and Entry

How viruses enter cells is of critical importance to pathogenesis in the host and for treatment strategies. Over the last several years, the herpesvirus field has made numerous and thoroughly fascinating discoveries about the entry of alpha-, beta-, and gamma-herpesviruses, giving rise to knowledge of entry at the amino acid level and the realization that, in some cases, researchers had overlooked whole sets of molecules essential for entry into critical cell types. Herpesviruses come equipped with multiple envelope glycoproteins which have several roles in many aspects of infection. For herpesvirus entry, it is usual that a collective of glycoproteins is involved in attachment to the cell surface, specific interactions then take place between viral glycoproteins and host cell receptors, and then molecular interactions and triggers occur, ultimately leading to viral envelope fusion with the host cell membrane. The fact that there are multiple cell and virus molecules involved with the build-up to fusion enhances the diversity and specificity of target cell types, the cellular entry pathways the virus commandeers, and the final triggers of fusion. This review will examine discoveries relating to how Kaposi's sarcoma-associated herpesvirus (KSHV) encounters and binds to critical cell types, how cells internalize the virus, and how the fusion may occur between the viral membrane and the host cell membrane. Particular focus is given to viral glycoproteins and what is known about their mechanisms of action.

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.

Low-pH Endocytic Entry of the Porcine Alphaherpesvirus Pseudorabies Virus

The alphaherpesvirus pseudorabies virus (PRV) is the causative agent of pseudorabies, a disease of great economic and welfare importance in swine. Other alphaherpesviruses, including herpes simplex virus (HSV), utilize low-pH-mediated endocytosis to enter a subset of cell types. We investigated whether PRV used this entry pathway in multiple laboratory model cell lines. Inhibition of receptor-mediated endocytosis by treatment with hypertonic medium prevented PRV entry. PRV entry into several cell lines, including porcine kidney (PK15) cells and African green monkey kidney (Vero) cells, was inhibited by noncytotoxic concentrations of the lysosomotropic agents ammonium chloride and monensin, which block the acidification of endosomes. Inactivation of virions by acid pretreatment is a hallmark of viruses that utilize a low-pH-mediated entry pathway. Exposure of PRV virions to pH 5.0 in the absence of host cell membranes reduced entry into PK15 and Vero cells by >80%. Together, these findings suggest that endocytosis followed by fusion with host membranes triggered by low endosomal pH is an important route of entry for PRV.IMPORTANCE PRV is a pathogen of great economic and animal welfare importance in many parts of the world. PRV causes neurological, respiratory, and reproductive disorders, often resulting in mortality of young and immunocompromised animals. Mortality, decreased production, and trade restrictions result in significant financial losses for the agricultural industry. Understanding the molecular mechanisms utilized by PRV to enter host cells is an important step in identifying novel strategies to prevent infection and spread. A thorough understanding of these mechanisms will contribute to a broader understanding of alphaherpesvirus entry. Here, we demonstrate PRV entry into multiple model cell lines via a low-pH endocytosis pathway. Together, these results provide a framework for elucidating the early events of the PRV replicative cycle.

Bovine Herpesvirus 1 Entry by a Low-pH Endosomal Pathway

Bovine herpesvirus 1 (BoHV-1) is an alphaherpesvirus that poses a significant challenge to health and welfare in the cattle industry. We investigated the cellular entry route utilized by BoHV-1. We report that BoHV-1 enters Madin Darby bovine kidney (MDBK) cells, bovine turbinate cells, and African green monkey kidney (Vero) cells via a low-pH-mediated endocytosis pathway. Treatment of MDBK cells with hypertonic medium, which inhibits receptor-mediated endocytosis, prevented infection as measured by a beta-galactosidase reporter assay. Treatment of cells with noncytotoxic concentrations of the lysosomotropic agents ammonium chloride and monensin, which block the acidification of endosomes, inhibited BoHV-1 entry in a concentration-dependent fashion. The kinetics of endocytic uptake of BoHV-1 from the cell surface was rapid (50% uptake by ∼5 min). Time-of-addition experiments indicated that the lysosomotropic agents acted at early times postinfection, consistent with entry. Inactivation of virions by pretreatment with mildly acidic pH is a hallmark characteristic of viruses that utilize a low-pH-activated entry pathway. When BoHV-1 particles were exposed to pH 5.0 in the absence of target membrane, infectivity was markedly reduced. Lastly, treatment of cells with the proteasome inhibitor MG132 inhibited BoHV-1 entry in a concentration-dependent manner. Together, these results support a model of BoHV-1 infection in which low endosomal pH is a critical host trigger for fusion of the viral envelope with an endocytic membrane and necessary for successful infection of the target cell.IMPORTANCE BoHV-1 is a ubiquitous pathogen affecting cattle populations worldwide. Infection can result in complicated, polymicrobial infections due to the immunosuppressive properties of the virus. While there are vaccines on the market, they only limit disease severity and spread but do not prevent infection. The financial and animal welfare ramifications of this virus are significant, and in order to develop more effective prevention and treatment regimens, a more complete understanding of the initial steps in viral infection is necessary. This research establishes the initial entry pathway of BoHV-1, which provides a foundation for future development of effective treatments and preventative vaccines. Additionally, it allows comparisons to the entry pathways of other alphaherpesviruses, such as HSV-1.

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.

Herpes Simplex Virus 1 Envelope Cholesterol Facilitates Membrane Fusion

Methyl beta-cyclodextrin (MβCD) treatment of herpes simplex virus 1 (HSV-1) reduced envelope cholesterol levels and inhibited viral entry and infectivity in several cell types, regardless of the dependence of entry on endocytosis or low pH. Viral protein composition was similar in MβCD-treated and untreated virions, and ultrastructural analysis by electron microscopy revealed that cholesterol removal did not grossly affect virion structure or integrity. Removal of envelope cholesterol greatly reduced virion fusion activity as measured by fusion-from-without, suggesting that virion cholesterol is critical for the step of membrane fusion. MβCD-treatment of HSV-1 did not reduce viral attachment to the cells nor endocytic uptake of HSV-1 from the cell surface. The pre-fusion form of gB present in the HSV-1 envelope undergoes conformational changes in response to mildly acidic pH. These gB changes occurred independently of envelope cholesterol. Removal of cholesterol compromised virion stability as measured by recovery of infectivity following cycles of freeze-thaw. Taken together, the data suggest that HSV-1 envelope cholesterol is important for viral entry and infectivity due to a critical role in membrane fusion.

Mildly Acidic pH Triggers an Irreversible Conformational Change in the Fusion Domain of Herpes Simplex Virus 1 Glycoprotein B and Inactivation of Viral Entry

Herpes simplex virus (HSV) entry into a subset of cells requires endocytosis and endosomal low pH. Preexposure of isolated virions to mildly acidic pH of 5 to 6 partially inactivates HSV infectivity in an irreversible manner. Acid inactivation is a hallmark of viruses that enter via low-pH pathways; this occurs by pretriggering conformational changes essential for fusion. The target and mechanism(s) of low-pH inactivation of HSV are unclear. Here, low-pH-treated HSV-1 was defective in fusion activity and yet retained normal levels of attachment to cell surface heparan sulfate and binding to nectin-1 receptor. Low-pH-triggered conformational changes in gB reported to date are reversible, despite irreversible low-pH inactivation. gB conformational changes and their reversibility were measured by antigenic analysis with a panel of monoclonal antibodies and by detecting changes in oligomeric conformation. Three-hour treatment of HSV-1 virions with pH 5 or multiple sequential treatments at pH 5 followed by neutral pH caused an irreversible >2.5 log infectivity reduction. While changes in several gB antigenic sites were reversible, alteration of the H126 epitope was irreversible. gB oligomeric conformational change remained reversible under all conditions tested. Altogether, our results reveal that oligomeric alterations and fusion domain changes represent distinct conformational changes in gB, and the latter correlates with irreversible low-pH inactivation of HSV. We propose that conformational change in the gB fusion domain is important for activation of membrane fusion during viral entry and that in the absence of a host target membrane, this change results in irreversible inactivation of virions.IMPORTANCE HSV-1 is an important pathogen with a high seroprevalence throughout the human population. HSV infects cells via multiple pathways, including a low-pH route into epithelial cells, the primary portal into the host. HSV is inactivated by low-pH preexposure, and gB, a class III fusion protein, undergoes reversible conformational changes in response to low-pH exposure. Here, we show that low-pH inactivation of HSV is irreversible and due to a defect in virion fusion activity. We identified an irreversible change in the fusion domain of gB following multiple sequential low-pH exposures or following prolonged low-pH treatment. This change appears to be separable from the alteration in gB quaternary structure. Together, the results are consistent with a model by which low pH can have an activating or inactivating effect on HSV depending on the presence of a target membrane.

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.

The Hsp60 protein of helicobacter pylori displays chaperone activity under acidic conditions

The heat shock protein, Hsp60, is one of the most abundant proteins in Helicobacter pylori. Given its sequence homology to the Escherichia coli Hsp60 or GroEL, Hsp60 from H. pylori would be expected to function as a molecular chaperone in this organism. H. pylori is an organism that grows on the gastric epithelium, where the pH can fluctuate between neutral and 4.5 and the intracellular pH can be as low as 5.0. This study was performed to test the ability of Hsp60 from H. pylori to function as a molecular chaperone under mildly acidic conditions. We report here that Hsp60 could suppress the acid-induced aggregation of alcohol dehydrogenase (ADH) in the 7.0-5.0 pH range. Hsp60 was found to undergo a conformational change within this pH range. It was also found that exposure of hydrophobic surfaces of Hsp60 is significant and that their exposure is increased under acidic conditions. Although, alcohol dehydrogenase does not contain exposed hydrophobic surfaces, we found that their exposure is triggered at low pH. Our results demonstrate that Hsp60 from H. pylori can function as a molecular chaperone under acidic conditions and that the interaction between Hsp60 and other proteins may be mediated by hydrophobic interactions.

Herpesvirus Entry into Host Cells Mediated by Endosomal Low pH

Herpesviral pathogenesis stems from infection of multiple cell types including the site of latency and cells that support lytic replication. Herpesviruses utilize distinct cellular pathways, including low pH endocytic pathways, to enter different pathophysiologically relevant target cells. This review details the impact of the mildly acidic milieu of endosomes on the entry of herpesviruses, with particular emphasis on herpes simplex virus 1 (HSV-1). Epithelial cells, the portal of primary HSV-1 infection, support entry via low pH endocytosis mechanisms. Mildly acidic pH triggers reversible conformational changes in the HSV-1 class III fusion protein glycoprotein B (gB). In vitro treatment of herpes simplex virions with a similar pH range inactivates infectivity, likely by prematurely activating the viral entry machinery in the absence of a target membrane. How a given herpesvirus mediates both low pH and pH-independent entry events is a key unresolved question.

Liposome-Mediated Herpes Simplex Virus Uptake Is Glycoprotein-D Receptor-Independent but Requires Heparan Sulfate

Cationic liposomes are widely used to facilitate introduction of genetic material into target cells during transfection. This study describes a non-receptor mediated herpes simplex virus type-1 (HSV-1) entry into the Chinese hamster ovary (CHO-K1) cells that naturally lack glycoprotein D (gD)-receptors using a commercially available cationic liposome: lipofectamine. Presence of cell surface heparan sulfate (HS) increased the levels of viral entry indicating a potential role of HS in this mode of entry. Loss of viral entry in the presence of actin de-polymerizing or lysosomotropic agents suggests that this mode of entry results in the endocytosis of the lipofectamine-virus mixture. Enhancement of HSV-1 entry by liposomes was also demonstrated in vivo using a zebrafish embryo model that showed stronger infection in the eyes and other tissues. Our study provides novel insights into gD receptor independent viral entry pathways and can guide new strategies to enhance the delivery of viral gene therapy vectors or oncolytic viruses.

Polyethylene glycol-mediated fusion of herpes simplex type 1 virions with the plasma membrane of cells that support endocytic entry

BACKGROUND

Mouse B78 cells and Chinese hamster ovary (CHO) cells are important to the study of HSV-1 entry because both are resistant to infection at the level of viral entry. When provided with a gD-receptor such as nectin-1, these cells support HSV-1 entry by an endocytosis pathway. Treating some viruses bound to cells with the fusogen polyethylene glycol (PEG) mediates viral fusion with the cell surface but is insufficient to rescue viral entry. It is unclear whether PEG-mediated fusion of HSV with the plasma membrane of B78 or CHO cells results in successful entry and infection.

FINDINGS

Treating HSV-1 bound to B78 or CHO cells with PEG allowed viral entry as measured by virus-induced beta-galactosidase activity. Based on the mechanism of PEG action, we propose that entry likely proceeds by direct fusion of HSV particles with the plasma membrane. Under the conditions tested, PEG-mediated infection of CHO cells progressed to the level of HSV late gene expression, while B78 cells supported HSV DNA replication. We tested whether proteolysis or acidification of cell-bound virions could trigger HSV fusion with the plasma membrane. Under the conditions tested, mildly acidic pH of 5-6 or the protease trypsin were not capable of triggering HSV-1 fusion as compared to PEG-treated cell-bound virions.

CONCLUSIONS

B78 cells and CHO cells, which typically endocytose HSV prior to viral penetration, are capable of supporting HSV-1 entry via direct penetration. HSV capsids delivered directly to the cytosol at the periphery of these cells complete the entry process. B78 and CHO cells may be utilized to screen for factors that trigger entry as a consequence of fusion of virions with the cell surface, and PEG treatment can provide a necessary control.

Entry of Oncolytic Herpes Simplex Virus into Human Squamous Cell Carcinoma Cells by Ultrasound

Low-intensity ultrasound is a useful method to introduce materials into cells due to the transient formation of micropores, called sonoporations, on the cell membrane. Whether oncolytic herpes simplex virus type 1 (HSV-1) can be introduced into oral squamous cell carcinoma (SCC) cells through membrane pores remains undetermined. Human SCC cell line SAS and oncolytic HSV-1 RH2, which was deficient in the 134.5 gene and fusogenic, were used. Cells were exposed to ultrasound in the presence or absence of microbubbles. The increase of virus entry was estimated by plaque numbers. Viral infection was hardly established without the adsorption step, but plaque number was increased by the exposure of HSV-1-inoculated cells to ultrasound. Plaque number was also increased even if SAS cells were exposed to ultrasound and inoculated with RH2 without the adsorption step. This effect was abolished when the interval from ultrasound exposure to virus inoculation was prolonged. Scanning electron microscopy revealed depressed spots on the cell surface after exposure to ultrasound. These results suggest that oncolytic HSV-1 RH2 can be introduced into SAS cells through ultrasound-mediated pores of the cell membrane that are resealed after an interval.

Forces and Structures of the Herpes Simplex Virus (HSV) Entry Mechanism

This paper discusses physical and structural aspects of the mechanisms herpes simplex virus (HSV) uses for membrane fusion. Calculations show that herpes simplex virus glycoprotein D has such avidity for its receptors that it can hold the virion against the plasma membrane of a neuron strongly enough for glycoprotein B (gB) to disrupt both leaflets of the bilayer. The strong electric field generated by the cell potential across perforations at this disruption would break the hydrogen bonds securing the gB fusion loops, leading to fusion of the plasma and viral membranes. This mechanism agrees with the high stability of the tall trimeric spike structure of gB and is consistent with the probable existence of a more compact initial conformation that would allow it to closely approach the plasma membrane. The release of the fusion domains by disruption of hydrogen bonds is shared with the endocytotic entry pathway where, for some cell types not punctured by gB, the virus is able to induce inward forces that cause endocytosis and the fusion loops are released by acidification. The puncture-fusion mechanism requires low critical strain or high tissue strain, matching primary tropism of neural processes at the vermillion border. In support of this mechanism, this paper proposes a functional superstructure of the antigens essential to entry and reviews its consistency with experimental evidence.

Widely Used Herpes Simplex Virus 1 ICP0 Deletion Mutant Strain dl1403 and Its Derivative Viruses Do Not Express Glycoprotein C Due to a Secondary Mutation in the gC Gene

Herpes simplex virus 1 (HSV-1) ICP0 is a multi-functional phosphoprotein expressed with immediate early kinetics. An ICP0 deletion mutant, HSV-1 dl1403, has been widely used to study the roles of ICP0 in the HSV-1 replication cycle including gene expression, latency, entry and assembly. We show that HSV-1 dl1403 virions lack detectable levels of envelope protein gC, and that gC is not synthesized in infected cells. Sequencing of the gC gene from HSV-1 dl1403 revealed a single amino acid deletion that results in a frameshift mutation. The HSV-1 dl1403 gC gene is predicted to encode a polypeptide consisting of the original 62 N-terminal amino acids of the gC protein followed by 112 irrelevant, non-gC residues. The mutation was also present in a rescuant virus and in two dl1403-derived viruses, D8 and FXE, but absent from the parental 17+, suggesting that the mutation was introduced during the construction of the dl1403 virus, and not as a result of passage in culture.

Nipah virion entry kinetics, composition, and conformational changes determined by enzymatic virus-like particles and new flow virometry tools

Virus-cell membrane fusion is essential for enveloped virus infections. However, mechanistic viral membrane fusion studies have predominantly focused on cell-cell fusion models, largely due to the low availability of technologies capable of characterizing actual virus-cell membrane fusion. Although cell-cell fusion assays are valuable, they do not fully recapitulate all the variables of virus-cell membrane fusion. Drastic differences between viral and cellular membrane lipid and protein compositions and curvatures exist. For biosafety level 4 (BSL4) pathogens such as the deadly Nipah virus (NiV), virus-cell fusion mechanistic studies are notably cumbersome. To circumvent these limitations, we used enzymatic Nipah virus-like-particles (NiVLPs) and developed new flow virometric tools. NiV's attachment (G) and fusion (F) envelope glycoproteins mediate viral binding to the ephrinB2/ephrinB3 cell receptors and virus-cell membrane fusion, respectively. The NiV matrix protein (M) can autonomously induce NiV assembly and budding. Using a β-lactamase (βLa) reporter/NiV-M chimeric protein, we produced NiVLPs expressing NiV-G and wild-type or mutant NiV-F on their surfaces. By preloading target cells with the βLa fluorescent substrate CCF2-AM, we obtained viral entry kinetic curves that correlated with the NiV-F fusogenic phenotypes, validating NiVLPs as suitable viral entry kinetic tools and suggesting overall relatively slower viral entry than cell-cell fusion kinetics. Additionally, the proportions of F and G on individual NiVLPs and the extent of receptor-induced conformational changes in NiV-G were measured via flow virometry, allowing the proper interpretation of the viral entry kinetic phenotypes. The significance of these findings in the viral entry field extends beyond NiV to other paramyxoviruses and enveloped viruses.

IMPORTANCE

Virus-cell membrane fusion is essential for enveloped virus infections. However, mechanistic viral membrane fusion studies have predominantly focused on cell-cell fusion models, largely due to the low availability of technologies capable of characterizing actual virus-cell membrane fusion. Although cell-cell fusion assays are valuable, they do not fully recapitulate all the variables of virus-cell membrane fusion. For example, drastic differences between viral and cellular membrane lipid and protein compositions and curvatures exist. For biosafety level 4 (BSL4) pathogens such as the deadly Nipah virus (NiV), virus-cell fusion mechanistic studies are especially cumbersome. To circumvent these limitations, we used enzymatic Nipah virus-like-particles (NiVLPs) and developed new flow virometric tools. Our new tools allowed us the high-throughput measurement of viral entry kinetics, glycoprotein proportions on individual viral particles, and receptor-induced conformational changes in viral glycoproteins on viral surfaces. The significance of these findings extends beyond NiV to other paramyxoviruses and enveloped viruses.

Epstein-Barr virus glycoprotein gB and gHgL can mediate fusion and entry in trans, and heat can act as a partial surrogate for gHgL and trigger a conformational change in gB

Epstein-Barr virus (EBV) fusion with an epithelial cell requires virus glycoproteins gHgL and gB and is triggered by an interaction between gHgL and integrin αvβ5, αvβ6, or αvβ8. Fusion with a B cell requires gHgL, gp42, and gB and is triggered by an interaction between gp42 and human leukocyte antigen class II. We report here that, like alpha- and betaherpesviruses, EBV, a gammaherpesvirus, can mediate cell fusion if gB and gHgL are expressed in trans. Entry of a gH-null virus into an epithelial cell is possible if the epithelial cell expresses gHgL, and entry of the same virus, which phenotypically lacks gHgL and gp42, into a B cell expressing gHgL is possible in the presence of a soluble integrin. Heat is capable of inducing the fusion of cells expressing only gB, and the proteolytic digestion pattern of gB in virions changes in the same way following the exposure of virus to heat or to soluble integrins. It is suggested that the Gibbs free energy released as a result of the high-affinity interaction of gHgL with an integrin contributes to the activation energy required to cause the refolding of gB from a prefusion to a postfusion conformation.

IMPORTANCE

The core fusion machinery of herpesviruses consists of glycoproteins gB and gHgL. We demonstrate that as in alpha- and betaherpesvirus, gB and gHgL of the gammaherpesvirus EBV can mediate fusion and entry when expressed in trans in opposing membranes, implicating interactions between the ectodomains of the proteins in the activation of fusion. We further show that heat and exposure to a soluble integrin, both of which activate fusion, result in the same changes in the proteolytic digestion pattern of gB, possibly representing the refolding of gB from its prefusion to its postfusion conformation.

Prospects and perspectives for development of a vaccine against herpes simplex virus infections

Herpes simplex viruses 1 and 2 are human pathogens that lead to significant morbidity and mortality in certain clinical settings. The development of effective antiviral medications, however, has had little discernible impact on the epidemiology of these pathogens, largely because the majority of infections are clinically silent. Decades of work have gone into various candidate HSV vaccines, but to date none has demonstrated sufficient efficacy to warrant licensure. This review examines developments in HSV immunology and vaccine development published since 2010, and assesses the prospects for improved immunization strategies that may result in an effective, licensed vaccine in the near future.

Mechanism of neutralization of herpes simplex virus by antibodies directed at the fusion domain of glycoprotein B

Glycoprotein B (gB), the fusogen of herpes simplex virus (HSV), is a class III fusion protein with a trimeric ectodomain of known structure for the postfusion state. Seen by negative-staining electron microscopy, it presents as a rod with three lobes (base, middle, and crown). gB has four functional regions (FR), defined by the physical location of epitopes recognized by anti-gB neutralizing monoclonal antibodies (MAbs). Located in the base, FR1 contains two internal fusion loops (FLs) and is the site of gB-lipid interaction (the fusion domain). Many of the MAbs to FR1 are neutralizing, block cell-cell fusion, and prevent the association of gB with lipid, suggesting that these MAbs affect FL function. Here we characterize FR1 epitopes by using electron microscopy to visualize purified Fab-gB ectodomain complexes, thus confirming the locations of several epitopes and localizing those of MAbs DL16 and SS63. We also generated MAb-resistant viruses in order to localize the SS55 epitope precisely. Because none of the epitopes of our anti-FR1 MAbs mapped to the FLs, we hyperimmunized rabbits with FL1 or FL2 peptides to generate polyclonal antibodies (PAbs). While the anti-FL1 PAb failed to bind gB, the anti-FL2 PAb had neutralizing activity, implying that the FLs become exposed during virus entry. Unexpectedly, the anti-FL2 PAb (and the anti-FR1 MAbs) bound to liposome-associated gB, suggesting that their epitopes are accessible even when the FLs engage lipid. These studies provide possible mechanisms of action for HSV neutralization and insight into how gB FR1 contributes to viral fusion.

IMPORTANCE

For herpesviruses, such as HSV, entry into a target cell involves transfer of the capsid-encased genome of the virus to the target cell after fusion of the lipid envelope of the virus with a lipid membrane of the host. Virus-encoded glycoproteins in the envelope are responsible for fusion. Antibodies to these glycoproteins are important biological tools, providing a way of examining how fusion works. Here we used electron microscopy and other techniques to study a panel of anti-gB antibodies. Some, with virus-neutralizing activity, impair gB-lipid association. We also generated a peptide antibody against one of the gB fusion loops; its properties provide insight into the way the fusion loops function as gB transits from its prefusion form to an active fusogen.

Development of a stable virus-like particle vaccine formulation against Chikungunya virus and investigation of the effects of polyanions

Chikungunya virus (CHIKV) is an alphavirus that infects millions of people every year, especially in the developing world. The selective expression of recombinant CHIKV capsid and envelope proteins results in the formation of self-assembled virus-like particles (VLPs) that have been shown to protect nonhuman primates against infection from multiple strains of CHIKV. This study describes the characterization, excipient screening, and optimization of CHIKV VLP solution conditions toward the development of a stable parenteral formulation. The CHIKV VLPs were found to be poorly soluble at pH 6 and below. Circular dichroism, intrinsic fluorescence, and static and dynamic light scattering measurements were therefore performed at neutral pH, and results consistent with the formation of molten globule structures were observed at elevated temperatures. A library of generally recognized as safe excipients was screened for their ability to physically stabilize CHIKV VLPs using a high-throughput turbidity-based assay. Sugars, sugar alcohols, and polyanions were identified as potential stabilizers and the concentrations and combinations of select excipients were optimized. The effects of polyanions were further studied, and while all polyanions tested stabilized CHIKV VLPs against aggregation, the effects of polyanions on conformational stability varied.

Conformational modifications of gB from herpes simplex virus type 1 analyzed by synthetic peptides

Entry of enveloped viruses requires fusion of viral and cellular membranes, driven by conformational changes of viral glycoproteins. The crystallized trimeric glycoprotein gB of herpes simplex virus has been described as a postfusion conformation, and several studies prove that like other class III fusion proteins, gB undergoes a pH-dependent switch between the pre- and postfusion conformations. Using several biophysical techniques, we show that peptides corresponding to the long helix of the gB postfusion structure interfere with the membrane fusion event, likely hampering the conformational rearrangements from the pre- to the postfusion structures. Those peptides represent good candidates for further design of peptidomimetic antagonists capable of blocking the fusion process.

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

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

Analysis of herpes simplex virion tegument ICP4 derived from infected cells and ICP4-expressing cells

ICP4 is the major transcriptional regulatory protein of herpes simplex virus (HSV). It is expressed in infected cells with immediate early kinetics and is essential for viral growth. ICP4 is also a structural component of the virion tegument layer. Herpesviral tegument proteins exert regulatory functions important for takeover of the host cell. Tegument ICP4 has not been well characterized. We examined the ICP4 present in HSV-1 virions that were either derived from wild type infected cells or from ICP4-expressing (E5) cells infected with ICP4 deletion virus d120. Limited proteolysis demonstrated that virion-associated ICP4 from particles derived from E5 cells was indeed an internal component of the virion. A similar subset of virion structural proteins was detected in viral particles regardless of the cellular origin of ICP4. Genotypically ICP4-negative virions complemented with tegument ICP4 entered cells via a proteasome-dependent, pH-dependent pathway similar to wild type virions. In infected cells, ICP4 was distributed predominantly in intranuclear replication compartments regardless of whether it was expressed from a transgene or from the HSV genome.

Detection of receptor-induced glycoprotein conformational changes on enveloped virions by using confocal micro-Raman spectroscopy

Conformational changes in the glycoproteins of enveloped viruses are critical for membrane fusion, which enables viral entry into cells and the pathological cell-cell fusion (syncytia) associated with some viral infections. However, technological capabilities for identifying viral glycoproteins and their conformational changes on actual enveloped virus surfaces are generally scarce, challenging, and time-consuming. Our model, Nipah virus (NiV), is a syncytium-forming biosafety level 4 pathogen with a high mortality rate (40 to 75%) in humans. Once the NiV attachment glycoprotein (G) (NiV-G) binds the cell receptor ephrinB2 or -B3, G triggers conformational changes in the fusion glycoprotein (F) that result in membrane fusion and viral entry. We demonstrate that confocal micro-Raman spectroscopy can, within minutes, simultaneously identify specific G and F glycoprotein signals and receptor-induced conformational changes in NiV-F on NiV virus-like particles (VLPs). First, we identified reproducible G- and F-specific Raman spectral features on NiV VLPs containing M (assembly matrix protein), G, and/or F or on NiV/vesicular stomatitis virus (VSV) pseudotyped virions via second-derivative transformations and principal component analysis (PCA). Statistical analyses validated our PCA models. Dynamic temperature-induced conformational changes in F and G or receptor-induced target membrane-dependent conformational changes in F were monitored in NiV pseudovirions in situ in real time by confocal micro-Raman spectroscopy. Advantageously, Raman spectroscopy can identify specific protein signals in relatively impure samples. Thus, this proof-of-principle technological development has implications for the rapid identification and biostability characterization of viruses in medical, veterinary, and food samples and for the analysis of virion glycoprotein conformational changes in situ during viral entry.

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.

Residues within the C-terminal arm of the herpes simplex virus 1 glycoprotein B ectodomain contribute to its refolding during the fusion step of virus entry

Herpesvirus entry into cells requires coordinated interactions among several viral glycoproteins. The final membrane fusion step of entry is executed by glycoprotein B (gB), a class III viral fusion protein that is conserved across all herpesviruses. Fusion proteins are metastable proteins that mediate fusion by inserting into a target membrane and refolding from a prefusion to postfusion conformation to bring the viral and cell membranes together. Although the structure of gB has been solved in a conformation that likely represents its postfusion form, its prefusion structure and the details of how it refolds to execute fusion are unknown. The postfusion gB structure contains a trimeric coiled-coil at its core and a long C-terminal arm within the ectodomain packs against this coil in an antiparallel manner. This coil-arm complex is reminiscent of the six-helix bundle that provides the energy for fusion in class I fusogens. To determine the role of the coil-arm complex, we individually mutated residues in the herpes simplex virus 1 gB coil-arm complex to alanine and assessed the contribution of each residue to cell-cell and virus-cell fusion. Several coil mutations resulted in a loss of cell surface expression, indicating that the coil residues are important for proper processing of gB. Three mutations in the arm region (I671A, H681A, and F683A) reduced fusion without affecting expression. Combining these three arm mutations drastically reduced the ability of gB to execute fusion; however, fusion function could be restored by adding known hyperfusogenic mutations to the arm mutant. We propose that the formation of the coil-arm complex drives the gB transition to a postfusion conformation and the coil-arm complex performs a function similar to that of the six-helix bundle in class I fusion. Furthermore, we suggest that these specific mutations in the arm may energetically favor the prefusion state of gB.

Vaccinia mature virus fusion regulator A26 protein binds to A16 and G9 proteins of the viral entry fusion complex and dissociates from mature virions at low pH

Vaccinia mature virus enters cells through either endocytosis or plasma membrane fusion, depending on virus strain and cell type. Our previous results showed that vaccinia virus mature virions containing viral A26 protein enter HeLa cells preferentially through endocytosis, whereas mature virions lacking A26 protein enter through plasma membrane fusion, leading us to propose that A26 acts as an acid-sensitive fusion suppressor for mature virus (S. J. Chang, Y. X. Chang, R. Izmailyan R, Y. L. Tang, and W. Chang, J. Virol. 84:8422-8432, 2010). In the present study, we investigated the fusion suppression mechanism of A26 protein. We found that A26 protein was coimmunoprecipitated with multiple components of the viral entry-fusion complex (EFC) in infected HeLa cells. Transient expression of viral EFC components in HeLa cells revealed that vaccinia virus A26 protein interacted directly with A16 and G9 but not with G3, L5 and H2 proteins of the EFC components. Consistently, a glutathione S-transferase (GST)-A26 fusion protein, but not GST, pulled down A16 and G9 proteins individually in vitro. Together, our results supported the idea that A26 protein binds to A16 and G9 protein at neutral pH contributing to suppression of vaccinia virus-triggered membrane fusion from without. Since vaccinia virus extracellular envelope proteins A56/K2 were recently shown to bind to the A16/G9 subcomplex to suppress virus-induced fusion from within, our results also highlight an evolutionary convergence in which vaccinia viral fusion suppressor proteins regulate membrane fusion by targeting the A16 and G9 components of the viral EFC complex. Finally, we provide evidence that acid (pH 4.7) treatment induced A26 protein and A26-A27 protein complexes of 70 kDa and 90 kDa to dissociate from mature virions, suggesting that the structure of A26 protein is acid sensitive.

Herpesvirus glycoproteins undergo multiple antigenic changes before membrane fusion

Herpesvirus entry is a complicated process involving multiple virion glycoproteins and culminating in membrane fusion. Glycoprotein conformation changes are likely to play key roles. Studies of recombinant glycoproteins have revealed some structural features of the virion fusion machinery. However, how the virion glycoproteins change during infection remains unclear. Here using conformation-specific monoclonal antibodies we show in situ that each component of the Murid Herpesvirus-4 (MuHV-4) entry machinery—gB, gH/gL and gp150—changes in antigenicity before tegument protein release begins. Further changes then occurred upon actual membrane fusion. Thus virions revealed their final fusogenic form only in late endosomes. The substantial antigenic differences between this form and that of extracellular virions suggested that antibodies have only a limited opportunity to block virion membrane fusion.

Herpes simplex virus infects most cell types in vitro: clues to its success

Herpes simplex virus (HSV) type-1 and type-2 have evolved numerous strategies to infect a wide range of hosts and cell types. The result is a very successful prevalence of the virus in the human population infecting 40-80% of people worldwide. HSV entry into host cell is a multistep process that involves the interaction of the viral glycoproteins with various cell surface receptors. Based on the cell type, HSV enter into host cell using different modes of entry. The combination of various receptors and entry modes has resulted in a virus that is capable of infecting virtually all cell types. Identifying the common rate limiting steps of the infection may help the development of antiviral agents that are capable of preventing the virus entry into host cell. In this review we describe the major features of HSV entry that have contributed to the wide susceptibility of cells to HSV infection.

Autographa californica multiple nucleopolyhedrovirus GP64 protein: roles of histidine residues in triggering membrane fusion and fusion pore expansion

The Autographa californica multiple nucleopolyhedrovirus (AcMNPV) GP64 protein mediates membrane fusion during entry. Fusion results from a low-pH-triggered conformational change in GP64 and subsequent interactions with the membrane bilayers. The low-pH sensor and trigger of the conformational change are not known, but histidine residues are implicated because the pK(a) of histidine is near the threshold for triggering fusion by GP64. We used alanine substitutions to examine the roles of all individual and selected clusters of GP64 histidine residues in triggering and mediating fusion by GP64. Three histidine residues (H152, H155, and H156), located in fusion loop 2, were identified as important for membrane fusion. These three histidine residues were important for efficient pore expansion but were not required for the pH-triggered conformational change. In contrast, a cluster of three histidine residues (H245, H304, and H430) located near the base of the central coiled coil was identified as a putative sensor for low pH. Three alanine substitutions in cluster H245/H304/H430 resulted in dramatically reduced membrane fusion and the apparent loss of the prefusion conformation at neutral pH. Thus, the H245/H304/H430 cluster of histidines may function or participate as a pH sensor by stabilizing the prefusion structure of GP64.

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.

A mechanistic basis for potent, glycoprotein B-directed gammaherpesvirus neutralization

Glycoprotein B (gB) is a conserved, essential component of gammaherpes virions and so potentially vulnerable to neutralization. However, few good gB-specific neutralizing antibodies have been identified. Here, we show that murid herpesvirus 4 is strongly neutralized by mAbs that recognize an epitope close to one of the gB fusion loops. Antibody binding did not stop gB interacting with its cellular ligands or initiating its fusion-associated conformation change, but did stop gB resolving stably to its post-fusion form, and so blocked membrane fusion to leave virions stranded in late endosomes. The conservation of gB makes this mechanism a possible general route to gammaherpesvirus neutralization.

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.