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

Enhancement of HSV-1 cell-free virion release by the envelope protein gC

Glycoprotein C (gC), one of ∼12 HSV-1 envelope glycoproteins, carries out several important functions during infection, including the enhancement of virion attachment by binding to host cell heparan sulfate proteoglycans (HSPG). Here we report that gC can also enhance the release of cell-free progeny virions at the end of the infectious cycle. This activity was observed in multiple cellular contexts including Vero cells and immortalized human keratinocytes. In the absence of gC, progeny virions bound more tightly to infected cells, suggesting that gC promotes the detachment of virions from the infected cell surface. Given this finding, we analyzed the biochemical interactions that tether progeny virions to cells and report evidence for two distinct modes of binding. One is consistent with a direct interaction between gC and HSPG, whereas the other is gC-independent and likely does not involve HSPG. Together, our results i) identify a novel function for a long-studied HSV-1 glycoprotein, and ii) demonstrate that the extracellular release of HSV-1 virions is a dynamic process involving multiple viral and host components.

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.

Comprehensive analysis of liquid-liquid phase separation propensities of HSV-1 proteins and their interaction with host factors

In recent years, it has been shown that the liquid-liquid phase separation (LLPS) of virus proteins plays a crucial role in their life cycle. It promotes the formation of viral replication organelles, concentrating viral components for efficient replication and facilitates the assembly of viral particles. LLPS has emerged as a crucial process in the replication and assembly of herpes simplex virus-1 (HSV-1). Recent studies have identified several HSV-1 proteins involved in LLPS, including the myristylated tegument protein UL11 and infected cell protein 4; however, a complete proteome-level understanding of the LLPS-prone HSV-1 proteins is not available. We provide a comprehensive analysis of the HSV-1 proteome and explore the potential of its proteins to undergo LLPS. By integrating sequence analysis, prediction algorithms and an array of tools and servers, we identified 10 HSV-1 proteins that exhibit high LLPS potential. By analysing the amino acid sequences of the LLPS-prone proteins, we identified specific sequence motifs and enriched amino acid residues commonly found in LLPS-prone regions. Our findings reveal a diverse range of LLPS-prone proteins within the HSV-1, which are involved in critical viral processes such as replication, transcriptional regulation and assembly of viral particles. This suggests that LLPS might play a crucial role in facilitating the formation of specialized viral replication compartments and the assembly of HSV-1 virion. The identification of LLPS-prone proteins in HSV-1 opens up new avenues for understanding the molecular mechanisms underlying viral pathogenesis. Our work provides valuable insights into the LLPS landscape of HSV-1, highlighting potential targets for further experimental validation and enhancing our understanding of viral replication and pathogenesis.

Dual impacts of a glycan shield on the envelope glycoprotein B of HSV-1: evasion from human antibodies in vivo and neurovirulence

Identification of the mechanisms of viral evasion from human antibodies is crucial both for understanding viral pathogenesis and for designing effective vaccines. Here we show in cell cultures that an N-glycan shield on the herpes simplex virus 1 (HSV-1) envelope glycoprotein B (gB) mediated evasion from neutralization and antibody-dependent cellular cytotoxicity due to pooled γ-globulins derived from human blood. We also demonstrated that the presence of human γ-globulins in mice and immunity to HSV-1 induced by viral infection in mice significantly reduced replication in their eyes of a mutant virus lacking the glycosylation site but had little effect on the replication of its repaired virus. These results suggest that an N-glycan shield on a specific site of HSV-1 envelope gB mediated evasion from human antibodies in vivo and from HSV-1 immunity induced by viral infection in vivo. Notably, we also found that an N-glycan shield on a specific site of HSV-1 gB was significant for HSV-1 neurovirulence and replication in the central nervous system of naïve mice. Thus, we have identified a critical N-glycan shield on HSV-1 gB that has dual impacts, namely evasion from human antibodies in vivo and viral neurovirulence. IMPORTANCE Herpes simplex virus 1 (HSV-1) establishes lifelong latent and recurrent infections in humans. To produce recurrent infections that contribute to transmission of the virus to new human host(s), the virus must be able to evade the antibodies persisting in latently infected individuals. Here, we show that an N-glycan shield on the specific site of the envelope glycoprotein B (gB) of HSV-1 mediates evasion from pooled γ-globulins derived from human blood both in cell cultures and mice. Notably, the N-glycan shield on the specific site of gB was also significant for HSV-1 neurovirulence in naïve mice. Considering the clinical features of HSV-1 infection, these results suggest that the glycan shield not only facilitates recurrent HSV-1 infections in latently infected humans by evading antibodies but is also important for HSV-1 pathogenesis during the initial infection.

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.

Spatiotemporally Orchestrated Interactions between Viral and Cellular Proteins Involved in the Entry of African Swine Fever Virus

African swine fever (ASF) is a highly contagious hemorrhagic disease in domestic pigs and wild boars with a mortality of up to 100%. The causative agent, African swine fever virus (ASFV), is a member of the Asfarviridae family of the nucleocytoplasmic large DNA viruses. The genome size of ASFV ranges from 170 to 194 kb, encoding more than 50 structural and 100 nonstructural proteins. ASFV virions are 260–300 nm in diameter and composed of complex multilayered structures, leading to an intricate internalization pathway to enter host cells. Currently, no commercial vaccines or antivirals are available, due to the insufficient knowledge of the viral receptor(s), the molecular events of ASFV entry into host cells, and the functions of virulence-associated genes. During the early stage of ASFV infection, the fundamental aspects of virus-host interactions, including virus internalization, intracellular transport through the endolysosomal system, and membrane fusion with endosome, are precisely regulated and orchestrated via a series of molecular events. In this review, we summarize the currently available knowledge on the pathways of ASFV entry into host cells and the functions of viral proteins involved in virus entry. Furthermore, we conclude with future perspectives and highlight areas that require further investigation. This review is expected to provide unique insights for further understanding ASFV entry and facilitate the development of vaccines and antivirals.

Receptors and ligands for herpes simplex viruses: Novel insights for drug targeting

Human herpes simplex viruses (HSVs) belong to the Herpesviridae family. At present, no vaccine or curative treatment is available for the prevention of HSV infections. Here, we review the cell surface receptors that are recognized by HSV's glycoprotein B, glycoprotein C, glycoprotein D, and the glycoprotein H - glycoprotein L complex to facilitate entry into host cells. These receptors include heparan sulfate (HS), herpesvirus entry mediator (HVEM), and nectin-1/-2, 3-O-sulfated heparan sulfate (3-OS HS).

Antibodies Elicited in Response to a Single Cycle Glycoprotein D Deletion Viral Vaccine Candidate Bind C1q and Activate Complement Mediated Neutralization and Cytolysis

Herpes simplex virus (HSV) prevention is a global health priority but, despite decades of research, there is no effective vaccine. Prior efforts focused on generating glycoprotein D (gD) neutralizing antibodies, but clinical trial outcomes were disappointing. The deletion of gD yields a single-cycle candidate vaccine (∆gD-2) that elicits high titer polyantigenic non-gD antibodies that exhibit little complement-independent neutralization but mediate antibody-dependent cellular cytotoxicity (ADCC) and phagocytosis (ADCP). Active or passive immunization with DgD-2 completely protects mice from lethal disease and latency following challenge with clinical isolates of either serotype. The current studies evaluated the role of complement in vaccine-elicited protection. The immune serum from the DgD-2 vaccinated mice exhibited significantly greater C1q binding compared to the serum from the gD protein vaccinated mice with infected cell lysates from either serotype as capture antigens. The C1q-binding antibodies recognized glycoprotein B. This resulted in significantly greater antibody-mediated complement-dependent cytolysis and neutralization. Notably, complete protection was preserved when the DgD-2 immune serum was passively transferred into C1q knockout mice, suggesting that ADCC and ADCP are sufficient in mice. We speculate that the polyfunctional responses elicited by DgD-2 may prove more effective in preventing HSV, compared to the more restrictive responses elicited by adjuvanted gD protein vaccines.

Proteomic analysis of host cellular proteins co-immunoprecipitated with duck enteritis virus gC

Duck enteritis virus (DEV), the causative agent of duck viral enteritis, causes a contagious, lethal viral disease in Anseriformes (waterfowls). In virus infection, host-virus interaction plays a crucial role in virus replication and pathogenesis. In our previous study, mRFP was fused with the C-terminus of DEV glycoprotein C (gC) to construct a fluorescent-tag DEV virus rgCRFP. In the current study, fluorescent fusion protein (gC-mRFP) was used as the proteomic probe. Co-immunoprecipitation and mass spectrometric analysis of proteins from rgCRFP-infected chicken embryo fibroblasts using commercial anti-RFP antibody led to the identification of a total of 21 gC interacting host proteins. Out of these 21 proteins, the interaction of seven host proteins (GNG2, AR1H1, PPP2CA, UBE2I, MCM5, NUBP1, HN1) with DEV gC protein was validated using membrane-bound split-ubiquitin yeast two-hybrid system (MbYTH) and bimolecular fluorescence complementation (BiFC) analyses. It indicated direct interaction between these proteins with DEV gC protein. This study has furthered the current understanding of DEV virus infection and pathogenesis. SIGNIFICANCE: gC is an crucial glycoprotein of duck enteritis virus that plays an important role in the viral life cycle. Uncovering the interaction between virus-host is very important to elucidate the pathogenic mechanism of the virus. In this study, host factors interacting with DEV gC have been discerned. And seven host proteins (GNG2, AR1H1, PPP2CA, UBE2I, MCM5, NUBP1, HN1) have been further validated to interact with DEV gC using MbYTH and BiFC analyses. These outcomes could shed light on how DEV manipulates the cellular machinery, which could further our understanding of DEV pathogenesis.

Herpes Simplex Virus 1 Infection of Neuronal and Non-Neuronal Cells Elicits Specific Innate Immune Responses and Immune Evasion Mechanisms

Alphaherpesviruses (α-HV) are a large family of double-stranded DNA viruses which cause many human and animal diseases. There are three human α-HVs: Herpes Simplex Viruses (HSV-1 and HSV-2) and Varicella Zoster Virus (VZV). All α-HV have evolved multiple strategies to suppress or exploit host cell innate immune signaling pathways to aid in their infections. All α-HVs initially infect epithelial cells (primary site of infection), and later spread to infect innervating sensory neurons. As with all herpesviruses, α-HVs have both a lytic (productive) and latent (dormant) stage of infection. During the lytic stage, the virus rapidly replicates in epithelial cells before it is cleared by the immune system. In contrast, latent infection in host neurons is a life-long infection. Upon infection of mucosal epithelial cells, herpesviruses immediately employ a variety of cellular mechanisms to evade host detection during active replication. Next, infectious viral progeny bud from infected cells and fuse to neuronal axonal terminals. Here, the nucleocapsid is transported via sensory neuron axons to the ganglion cell body, where latency is established until viral reactivation. This review will primarily focus on how HSV-1 induces various innate immune responses, including host cell recognition of viral constituents by pattern-recognition receptors (PRRs), induction of IFN-mediated immune responses involving toll-like receptor (TLR) signaling pathways, and cyclic GMP-AMP synthase stimulator of interferon genes (cGAS-STING). This review focuses on these pathways along with other mechanisms including autophagy and the complement system. We will summarize and discuss recent evidence which has revealed how HSV-1 is able to manipulate and evade host antiviral innate immune responses both in neuronal (sensory neurons of the trigeminal ganglia) and non-neuronal (epithelial) cells. Understanding the innate immune response mechanisms triggered by HSV-1 infection, and the mechanisms of innate immune evasion, will impact the development of future therapeutic treatments.

The requirement of glycoprotein C (gC) for interindividual spread is a conserved function of gC for avian herpesviruses

We have formerly shown that glycoprotein C (gC) of Gallid alphaherpesvirus 2, better known as Marek’s disease (MD) alphaherpesvirus (MDV), is required for interindividual spread in chickens. Since gC is conserved within the Alphaherpesvirinae subfamily, we hypothesized gC was important for interindividual spread of other alphaherpesviruses. To test this hypothesis, we first generated a fluorescent protein tagged clone of Gallid alphaherpesvirus 3 MD vaccine strain 301B/1 to track virus replication in cell culture and chickens using fluorescent microscopy. Following validation of this system, we removed the open reading frame of 301B/1 gC from the genome and determined whether it was required for interindividual spread using experimental and natural infection studies. Interindividual spread of MD vaccine 301B/1 was abrogated by removal of 301B/1 gC. Rescuent virus in which 301B/1 gC was inserted back into the genome efficiently spread among chickens. To further study the conserved function of gC, we replaced 301B/1 gC with MDV gC and this virus also efficiently spread in chickens. These data suggest the essential function of alphaherpesvirus gC proteins is conserved and can be exploited during the generation of future vaccines against MD that affects the poultry industry worldwide.

Targeting m6A modification inhibits herpes virus 1 infection

The latent infection by herpes virus type 1 (HSV-1) may be lifelong in trigeminal ganglia and a suspected cause of Alzheimer's Disease (AD) and Amyotrophic lateral sclerosis (ALS). Whether and how N6-methyladenosine (m6A) modification of viral RNAs affects virus infection are poorly understood. Here, we report that HSV-1 infection enhanced the expression of m6A writers (METTL3, METTL14) and readers (YTHDF1/2/3) at the early infection stage and decreased their expression later on, while suppressed the erasers' (FTO, ALBKH5) expression immediately upon infection to facilitate viral replication. Inhibiting m6A modification by 3-deazaadenosine (DAA) significantly decreased viral replication and reduced viral reproduction over 1000 folds. More interestingly, depleting the writers and readers by siRNAs inhibited virus replication and reproduction; whereas depleting the erasers promoted viral replication and reproduction. Silencing YTHDF3 strikingly decreased viral replication by up to 90%, leading to reduction of up to 10-fold viral replication and over 100-fold virus reproduction, respectively. Depletion of m6A initiator METTL3 (by 60%–70%) by siRNA correlatedly decreased viral replication 60%–70%, and reduced virus yield over 30-fold. Consistently, ectopic expression of METTL3 largely increased virus yield. METTL3 knockdown suppressed the HSV-1 intermediate early and early genes (ICP0, ICP8 and UL23) and late genes (VP16, UL44, UL49 and ICP47); while ectopic expression of METTL3 upregulated these gene expression. Results from our study shed the lights on the importance for m6A modification to initiate HSV-1 early replication. The components of m6A modification machinery, particularly m6A initiator METTL3 and reader YTHDF3, would be potential important targets for combating HSV-1 infections.

The Role of VP16 in the Life Cycle of Alphaherpesviruses

The protein encoded by the UL48 gene of alphaherpesviruses is named VP16 or alpha-gene-transactivating factor (α-TIF). In the early stage of viral replication, VP16 is an important transactivator that can activate the transcription of viral immediate-early genes, and in the late stage of viral replication, VP16, as a tegument, is involved in viral assembly. This review will explain the mechanism of VP16 acting as α-TIF to activate the transcription of viral immediate-early genes, its role in the transition from viral latency to reactivation, and its effects on viral assembly and maturation. In addition, this review also provides new insights for further research on the life cycle of alphaherpesviruses and the role of VP16 in the viral life cycle.

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.