The domains of glycoprotein D required to block apoptosis induced by herpes simplex virus 1 are largely distinct from those involved in cell-cell fusion and binding to nectin1

Caspase-8 is required for HSV-1-induced apoptosis and promotes effective viral particle release via autophagy inhibition

Regulated cell death (RCD) plays an important role in the progression of viral replication and particle release in cells infected by herpes simplex virus-1 (HSV-1). However, the kind of RCD (apoptosis, necroptosis, others) and the resulting cytopathic effect of HSV-1 depends on the cell type and the species. In this study, we further investigated the molecular mechanisms of apoptosis induced by HSV-1. Although a role of caspase-8 has previously been suggested, we now clearly show that caspase-8 is required for HSV-1-induced apoptosis in a FADD-/death receptor-independent manner in both mouse embryo fibroblasts (MEF) and human monocytes (U937). While wild-type (wt) MEFs and U937 cells exhibited increased caspase-8 and caspase-3 activation and apoptosis after HSV-1 infection, respective caspase-8-deficient (caspase-8−/−) cells were largely impeded in any of these effects. Unexpectedly, caspase-8−/− MEF and U937 cells also showed less virus particle release associated with increased autophagy as evidenced by higher Beclin-1 and lower p62/SQSTM1 levels and increased LC3-I to LC3-II conversion. Confocal and electron microscopy revealed that HSV-1 stimulated a strong perinuclear multivesicular body response, resembling increased autophagy in caspase-8−/− cells, entrapping virions in cellular endosomes. Pharmacological inhibition of autophagy by wortmannin restored the ability of caspase-8−/− cells to release viral particles in similar amounts as in wt cells. Altogether our results support a non-canonical role of caspase-8 in both HSV-1-induced apoptosis and viral particle release through autophagic regulation.

Viral-mediated activation and inhibition of programmed cell death

Viruses are ubiquitous intracellular genetic parasites that heavily rely on the infected cell to complete their replication life cycle. This dependency on the host machinery forces viruses to modulate a variety of cellular processes including cell survival and cell death. Viruses are known to activate and block almost all types of programmed cell death (PCD) known so far. Modulating PCD in infected hosts has a variety of direct and indirect effects on viral pathogenesis and antiviral immunity. The mechanisms leading to apoptosis following virus infection is widely studied, but several modalities of PCD, including necroptosis, pyroptosis, ferroptosis, and paraptosis, are relatively understudied. In this review, we cover the mechanisms by which viruses activate and inhibit PCDs and suggest perspectives on how these affect viral pathogenesis and immunity.

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).

Oncolytic herpes simplex virus immunotherapy for brain tumors: current pitfalls and emerging strategies to overcome therapeutic resistance

Malignant tumors of the central nervous system (CNS) continue to be a leading cause of cancer-related mortality in both children and adults. Traditional therapies for malignant brain tumors consist of surgical resection and adjuvant chemoradiation; such approaches are often associated with extreme morbidity. Accordingly, novel, targeted therapeutics for neoplasms of the CNS, such as immunotherapy with oncolytic engineered herpes simplex virus (HSV) therapy, are urgently warranted. Herein, we discuss treatment challenges related to HSV virotherapy delivery, entry, replication, and spread, and in so doing focus on host antiviral immune responses and the immune microenvironment. Strategies to overcome such challenges including viral re-engineering, modulation of the immunoregulatory microenvironment and combinatorial therapies with virotherapy, such as checkpoint inhibitors, radiation, and vaccination are also examined in detail.

Analysis of herpes simplex type 1 gB, gD, and gH/gL on production of infectious HIV-1: HSV-1 gD restricts HIV-1 by exclusion of HIV-1 Env from maturing viral particles

BACKGROUND

We previously showed that the gM of HSV-1 could restrict the release of infectious HIV-1 from cells. In this study, we analyzed if the four HSV-1 glycoproteins (gD, gB, and gH/gL), which are the minimum glycoproteins required for HSV-1 entry, restricted the release of infectious HIV-1.

RESULTS

Of these four glycoproteins, gD and gH/gL restricted the production of infectious HIV-1 from cells transfected with an infectious molecular clone of HIV-1 (strain NL4-3) while gB had no significant effect. Pulse-chase analyses indicated that gD did not affect the biosynthesis and processing of gp160 into gp120/gp41, the transport of the gp120/gp41 to the cell surface, or the release of HIV-1 particles from the cell surface. Our analyses revealed that gD was incorporated into HIV-1 virus particles while gp120/gp41 was excluded from released virus particles. Truncated mutants of gD revealed that the cytoplasmic domain was dispensable but that a membrane bound gD was required for the restriction of release of infectious HIV-1. Finally, cell lines expressing gD also potently restricted the release of infectious virus.

CONCLUSIONS

Due to its ability to exclude HIV-1 gp120/gp41 from maturing virus, gD may provide a useful tool in deciphering mechanisms of Env incorporation into maturing virus particles.

Inhibition of apoptosis in BHV-1-infected cells depends on Us3 serine/threonine kinase and its enzymatic activity

Us3 protein is a serine/threonine kinase conserved within the Alphaherpesvirinae subfamily of herpesviruses. The Us3 homologs of herpes simplex virus, pseudorabies virus, and bovine herpesvirus type 5 have been shown to block apoptosis triggered by viral infection or exogenous inducers. To determine whether these characteristics are shared by bovine herpesvirus type 1 Us3, we constructed two viral mutants: BHV-1 Us3 deletion mutant (BHV-1ΔUs3) and a kinase-dead mutant (BHV-1KD). Flow cytometry analysis and TUNEL assay clearly demonstrated, that only BHV-1 wild type virus suppressed infection-induced apoptosis and protected cells from apoptosis triggered by exogenous factors: sorbitol or staurosporine. Us3 of BHV-1 was directly capable of blocking apoptosis without the presence of other viral proteins. The presence of Us3 correlated with phosphorylation of BAD, a pro-apoptotic Bcl-2 family member. Our results clearly indicate that BHV-1 Us3 is necessary for efficient blocking of apoptosis triggered by viral infection and exogenous factors.

The suppression of apoptosis by α-herpesvirus

Apoptosis, an important innate immune mechanism that eliminates pathogen-infected cells, is primarily triggered by two signalling pathways: the death receptor pathway and the mitochondria-mediated pathway. However, many viruses have evolved various strategies to suppress apoptosis by encoding anti-apoptotic factors or regulating apoptotic signalling pathways, which promote viral propagation and evasion of the host defence. During its life cycle, α-herpesvirus utilizes an elegant multifarious anti-apoptotic strategy to suppress programmed cell death. This progress article primarily focuses on the current understanding of the apoptosis-inhibition mechanisms of α-herpesvirus anti-apoptotic genes and their expression products and discusses future directions, including how the anti-apoptotic function of herpesvirus could be targeted therapeutically.

Manipulation of apoptosis and necroptosis signaling by herpesviruses

Like apoptosis, necroptosis is an innate immune mechanism that eliminates pathogen-infected cells. Receptor-interacting protein kinase (RIP)3 (also called RIPK3) mediates necrotic death by phosphorylating an executioner protein, MLKL, leading to plasma membrane leakage. The pathway is triggered against viruses that block caspase 8. In murine CMV, the viral inhibitor of caspase 8 activation prevents extrinsic apoptosis but also has the potential to unleash necroptosis. This virus encodes the viral inhibitor of RIP activation to prevent RIP homotypic interaction motif (RHIM)-dependent signal transduction and necroptosis. Recent investigations reveal a similar mechanism at play in the human alpha-herpesviruses, herpes simplex virus (HSV)1 and HSV2, where RHIM competitor function and caspase 8 suppression are carried out by the virus-encoded large subunit of ribonucleotide reductase (R1). In human cells, R1 inhibition of caspase 8 prevents TNF-induced apoptosis, but sensitizes to TNF-induced necroptosis. The RHIM and caspase 8 interaction domains of R1 collaborate to prevent RIP3-dependent steps and enable both herpesviruses to deflect host cell death machinery that would cut short infection. In mouse cells, HSV1 infection by itself triggers necroptosis by driving RIP3 protein kinase activity. HSV1 R1 contributes to the activation of RIP3 adaptor function in mice, a popular host animal for experimental infection. Based on these studies, infection of RIP3-kinase inactive mice should be explored in models of pathogenesis and latency. The necrotic death pathway that is suppressed during infection in the natural host becomes a cross-species barrier to infection in a non-natural host.

Necroptosis: The Trojan horse in cell autonomous antiviral host defense

Herpesviruses suppress cell death to assure sustained infection in their natural hosts. Murine cytomegalovirus (MCMV) encodes suppressors of apoptosis as well as M45-encoded viral inhibitor of RIP activation (vIRA) to block RIP homotypic interaction motif (RHIM)-signaling and recruitment of RIP3 (also called RIPK3), to prevent necroptosis. MCMV and human cytomegalovirus encode a viral inhibitor of caspase (Casp)8 activation to block apoptosis, an activity that unleashes necroptosis. Herpes simplex virus (HSV)1 and HSV2 incorporate both RHIM and Casp8 suppression strategies within UL39-encoded ICP6 and ICP10, respectively, which are herpesvirus-conserved homologs of MCMV M45. Both HSV proteins sensitize human cells to necroptosis by blocking Casp8 activity while preventing RHIM-dependent RIP3 activation and death. In mouse cells, HSV1 ICP6 interacts with RIP3 and, surprisingly, drives necroptosis. Thus, herpesviruses have illuminated the contribution of necoptosis to host defense in the natural host as well as its potential to restrict cross-species infections in nonnatural hosts.

Structure of herpes simplex virus glycoprotein D bound to the human receptor nectin-1

Binding of herpes simplex virus (HSV) glycoprotein D (gD) to a cell surface receptor is required to trigger membrane fusion during entry into host cells. Nectin-1 is a cell adhesion molecule and the main HSV receptor in neurons and epithelial cells. We report the structure of gD bound to nectin-1 determined by x-ray crystallography to 4.0 Å resolution. The structure reveals that the nectin-1 binding site on gD differs from the binding site of the HVEM receptor. A surface on the first Ig-domain of nectin-1, which mediates homophilic interactions of Ig-like cell adhesion molecules, buries an area composed by residues from both the gD N- and C-terminal extensions. Phenylalanine 129, at the tip of the loop connecting β-strands F and G of nectin-1, protrudes into a groove on gD, which is otherwise occupied by C-terminal residues in the unliganded gD and by N-terminal residues in the gD/HVEM complex. Notably, mutation of Phe129 to alanine prevents nectin-1 binding to gD and HSV entry. Together these data are consistent with previous studies showing that gD disrupts the normal nectin-1 homophilic interactions. Furthermore, the structure of the complex supports a model in which gD-receptor binding triggers HSV entry through receptor-mediated displacement of the gD C-terminal region.

HERPES SIMPLEX VIRUS INFECTION AND APOPTOSIS

Consequences of human herpes simplex virus (HSV) infection include the induction of apoptosis and the concomitant synthesis of proteins which act to block this process from killing the infected cell. Recent data has clarified our current understanding of the mechanisms of induction and prevention of apoptosis by HSV. These findings emphasize the fact that modulation of apoptosis by HSV during infection is a multicomponent phenomenon. We review recent evidence showing how this important human pathogen modulates the fundamental cell death process.

Complexes between herpes simplex virus glycoproteins gD, gB, and gH detected in cells by complementation of split enhanced green fluorescent protein

The interactions between herpes simplex virus gD and its nectin1 receptor or between gD, gB, and gH were analyzed by complementation of the N and C portions of split enhanced green fluorescent protein (EGFP) fused to the glycoproteins. The gD(N)-Nect(C) complex was readily detected; the gD(N)-gC(C) complex was undetectable, highlighting the specificity of the assay. Split EGFP complementation was detected between proteins designated gD(N)+gH(C), gD(N)+gB(C), and gH(N)+gB(C)+wtgD (gB was deleted of endocytosis motifs), both in cells transfected with two-tree glycoproteins and in syncytia. The in situ assay provides evidence that gD interacts with gH and gB independently of each other and supports a model whereby gH and gB in complex exert their activities to gD.

Herpes simplex virus blocks Fas-mediated apoptosis independent of viral activation of NF-kappaB in human epithelial HEp-2 cells

The goal of our study was to characterize the apoptotic response of herpes simplex virus (HSV)-infected, human epithelial HEp-2 cells to extrinsic treatments through the Fas receptor. Initially, we defined the Fas response of these cells. We found the following: (1) Treatment of HEp-2 cells with anti-Fas antibody or Fas ligand (FasL) alone did not induce apoptosis. (2) In addition, these inducers did not activate NF-kappaB in these cells. (3) The addition of cycloheximide (CHX) during these treatments caused a dramatic increase in programmed cell death. (4) HEp-2 cells infected with HSV for 6 h prior to anti-Fas plus CHX treatment were nonapoptotic, and (5) these cells possessed nuclear NFkappaB. (6) HSV blocked anti-Fas or FasL plus CHX-induced apoptosis in HEp-2 cells that stably expressed a dominant-negative form of IkappaBalpha. These results indicate that HSV infection can block the process of Fas-mediated apoptosis through a mechanism that is independent of viral activation of NFkappaB. Our findings help define the molecular mechanisms involved in HSV evasion of the cytokine-driven, innate immune response in human epithelial cells.

A herpes simplex virus recombinant that exhibits a single-chain antibody to HER2/neu enters cells through the mammary tumor receptor, independently of the gD receptors

The human epidermal growth factor receptor 2/neuregulin (HER2/neu) receptor is overexpressed in highly malignant mammary and ovarian tumors and correlates with a poor prognosis. It is a target for therapy; humanized monoclonal antibodies to HER2 have led to increased survival of patients with HER2/neu-positive breast cancer. As a first step in the design of an oncolytic herpes simplex virus able to selectively infect HER2/neu-positive cells, we constructed two recombinants, R-LM11 and R-LM11L, that carry a single-chain antibody (scFv) against HER2 inserted at residue 24 of gD. The inserts were 247 or 256 amino acids long, and the size of the gD ectodomain was almost doubled by the insertion. We report the following. R-LM11 and R-LM11L infected derivatives of receptor-negative J or CHO cells that expressed HER2/neu as the sole receptor. Entry was dependent on HER2/neu, since it was inhibited in a dose-dependent manner by monoclonal antibodies to HER2/neu and by a soluble form of the receptor. The scFv insertion in gD disrupted the ability of the virus to enter cells through HVEM but maintained the ability to enter through nectin1. This report provides proof of principle that gD can tolerate fusion to a heterologous protein almost as large as the gD ectodomain itself without loss of profusion activity. Because the number of scFv's to a variety of receptors is continually increasing, this report makes possible the specific targeting of herpes simplex virus to a large collection of cell surface molecules for both oncolytic activity and visualization of tumor cells.

Separation of receptor-binding and profusogenic domains of glycoprotein D of herpes simplex virus 1 into distinct interacting proteins

The 369-residue glycoprotein D (gD) is the entry, receptor-binding protein of herpes simplex virus 1. The common receptors for viral entry are nectin-1, HveA, and a specific O-linked sulfated proteoglycan. The major receptor-binding sites of gD are at the N terminus, whereas the domain required for fusion of viral envelope with the plasma membrane is at the C terminus of the ectodomain (residues 260-310). In the course of retargeting gD to the urokinase plasminogen activator (uPA) receptor for potential therapeutic applications, we obtained a genetically engineered infectious virus in which the receptor-binding domain consisting of the N-terminal domain of uPA fused to residues 33-60 of gD was separated from an independently expressed C-terminal domain of gD containing residues 219-369. The intervening sequences (residues 62-218) were replaced by a stop codon and a promoter for the C-terminal domain of gD. The physical interaction of the two components was reconstructed by coimmunoprecipitation of the N-terminal domain of uPA with the C-terminal domain of gD. These results indicate that codons 61-218 of gD do not encode executable functions required for viral entry into cells and suggest that the receptor-binding ligand must interact with but need not alter the structure of the residual portion of gD to effect virus entry. This finding opens the way for the development of a family of recombinant viruses in which the profusion domain of gD and independently furnished, interacting receptor-binding domains effect entry of the virus via a range of receptors.

The multipartite system that mediates entry of herpes simplex virus into the cell

The multipartite entry-fusion system of herpes simplex virus is made of a quartet of glycoproteins-gD, gB, gH.gL-and three alternative gD receptors, herpesvirus entry mediator (HVEM), nectin1 and modified sites on heparan sulphate. This multipartite system recapitulates the basic steps of virus-cell fusion, i.e. receptor recognition, triggering of fusion and fusion execution. Specifically, in addition to serving as the receptor-binding glycoprotein, gD triggers fusion through a specialised domain, named pro-fusion domain (PFD), located C-terminally in the ectodomain. In the unliganded gD the C-terminal region folds around the N-terminal region, such that gD adopts a closed autoinhibited conformation. In HVEM- and nectin1-bound gD the C-terminal region is displaced (opened conformation). gD is the tool for modification of HSV tropism, through insertion of ligands to heterologous tumour-specific receptors. It is discussed whether gD responds to the interaction with the natural and the heterologous receptors by adopting similar conformations, and whether the closed-to-open switch in conformation is a generalised mechanism of activation. A peculiar recombinant highlighted that the central Ig-folded core of gD may not encode executable functions for entry and that the 219-314 aa segment may be sufficient to trigger fusion. With respect to fusion execution, gB appears to be a prospective fusogen based on its coiled-coil trimeric structure, similar to that of another fusion glycoprotein. On the other hand, gH exhibits molecular elements typical of class 1 fusion glycoproteins, in particular heptad repeats and strong tendency to interact with lipids. Whether fusion execution is carried out by gB or gH.gL, or both glycoproteins in complex or sequentially remains to be determined.

Apoptosis and antigen receptor function in T and B cells following exposure to herpes simplex virus

T cells are an essential component of the immune response against herpes simplex virus (HSV) infection. We previously reported that incubation of T cells with HSV-infected fibroblasts inhibits subsequent T cell antigen receptor signal transduction. In the current study, we found that incubation of T cells with HSV-infected fibroblasts also leads to apoptosis in exposed T cells. Apoptosis was observed in Jurkat cells, a T cell leukemia line, and also in CD4(+) cells isolated from human peripheral blood mononuclear cells. Direct infection of these cells with HSV also resulted in apoptosis. Clinical isolates of both HSV type 1 and 2 induced apoptosis in infected T cells at comparable levels to cells infected with laboratory strains of HSV, suggesting an immune evasion mechanism that may be clinically relevant. Further understanding of these viral immune evasion mechanisms could be exploited for better management of HSV infection.

Construction and properties of a herpes simplex virus 1 designed to enter cells solely via the IL-13alpha2 receptor

Current design of genetically engineered viruses for selective destruction of cancer cells is based on the observation that attenuated viruses replicate better in tumor cells than in normal cells. The ideal virus, however, is one that can infect only cancer cells by virtue of altered host range. Such a virus can be made more robust than the highly attenuated viruses used in clinical trials. Earlier, we reported the construction of a recombinant herpes simplex virus 1 (R5111) in which the capacity to bind heparan sulfate was disabled and which contained a chimeric IL-13-glycoprotein D that enabled the virus to infect cells expressing the IL-13alpha2 receptor (IL-13Ralpha2) commonly found on the surface of malignant glioblastomas or high-grade astrocytomas. In the earlier report, we showed that the recombinant R5111 was able to enter and infect cells via the interaction of the chimeric glycoprotein D with IL-13Ralpha2 but that the virus retained the capacity to bind and replicate in cells expressing the natural viral receptors HveA or nectin-1. Here, we report the construction of a recombinant virus (R5141) that can only enter and replicate in cells that express the IL-13Ralpha2. The recombinant R5141 does not depend on endocytosis to infect cells. It does not infect cells expressing HveA or nectin-1 receptors or cells expressing IL-13Ralpha2 that had been exposed to soluble IL-13 before infection. The studies described here show that the host range of herpes simplex viruses can be altered by genetic manipulation to specifically target cancer cells.

Varicella-zoster virus ORF63 inhibits apoptosis of primary human neurons

Virus-encoded modulation of apoptosis may serve as a mechanism to enhance cell survival and virus persistence. The impact of productive varicella-zoster virus (VZV) infection on apoptosis appears to be cell type specific, as infected human sensory neurons are resistant to apoptosis, yet human fibroblasts readily become apoptotic. We sought to identify the viral gene product(s) responsible for this antiapoptotic phenotype in primary human sensory neurons. Treatment with phosphonoacetic acid to inhibit viral DNA replication and late-phase gene expression did not alter the antiapoptotic phenotype, implicating immediate-early (IE) or early genes or a virion component. Compared to the parental VZV strain (rOKA), a recombinant virus unable to express one copy of the diploid IE gene ORF63 (rOka deltaORF63) demonstrated a significant induction of apoptosis in infected neurons, as determined by three methods: annexin V staining, deoxynucleotidyltransferase-mediated dUTP-biotin nick end label staining, and transmission electron microscopy. Furthermore, neurons transfected with a plasmid expressing ORF63 resisted apoptosis induced by nerve growth factor withdrawal. These results show that ORF63 can suppress apoptosis of neurons and provide the first identification of a VZV gene encoding an antiapoptotic function. As ORF63 is expressed in neurons during both productive and latent infection, it may play a significant role in viral pathogenesis by promoting neuron survival during primary and reactivated infections.

Apoptosis During Herpes Simplex Virus Infection

Herpes simplex virus (HSV) infection triggers apoptosis in infected cells. However, proteins synthesized later in infected cells prevent apoptotic cell death from ensuing. In vivo data showing that apoptosis accompanies herpes stromal keratitis and encephalitis suggest that apoptotic modulation plays a role in the development of herpetic disease. Tremendous progress has been made toward identifying the viral factors that are responsible for inducing and inhibiting apoptosis during infection. However, the mechanisms whereby they act are still largely unknown. Recent studies have illustrated a wide diversity in the cellular response to HSV-triggered apoptosis, emphasizing the importance of host factors in this process. Together, these findings indicate that apoptosis during HSV infection represents an important virus-host interaction process, which likely influences viral pathogenesis.

Structure of unliganded HSV gD reveals a mechanism for receptor-mediated activation of virus entry

Herpes simplex virus (HSV) entry into cells requires binding of the envelope glycoprotein D (gD) to one of several cell surface receptors. The 50 C-terminal residues of the gD ectodomain are essential for virus entry, but not for receptor binding. We have determined the structure of an unliganded gD molecule that includes these C-terminal residues. The structure reveals that the C-terminus is anchored near the N-terminal region and masks receptor-binding sites. Locking the C-terminus in the position observed in the crystals by an intramolecular disulfide bond abolished receptor binding and virus entry, demonstrating that this region of gD moves upon receptor binding. Similarly, a point mutant that would destabilize the C-terminus structure was nonfunctional for entry, despite increased affinity for receptors. We propose that a controlled displacement of the gD C-terminus upon receptor binding is an essential feature of HSV entry, ensuring the timely activation of membrane fusion.

Baculovirus as versatile vectors for protein expression in insect and mammalian cells

Today, many thousands of recombinant proteins, ranging from cytosolic enzymes to membrane-bound proteins, have been successfully produced in baculovirus-infected insect cells. Yet, in addition to its value in producing recombinant proteins in insect cells and larvae, this viral vector system continues to evolve in new and unexpected ways. This is exemplified by the development of engineered insect cell lines to mimic mammalian cell glycosylation of expressed proteins, baculovirus display strategies and the application of the virus as a mammalian-cell gene delivery vector. Novel vector design and cell engineering approaches will serve to further enhance the value of baculovirus technology.

A new class of receptor for herpes simplex virus has heptad repeat motifs that are common to membrane fusion proteins

We isolated a human cDNA by expression cloning and characterized its gene product as a new human protein that enables entry and infection of herpes simplex virus (HSV). The gene, designated hfl-B5, encodes a type II cell surface membrane protein, B5, that is broadly expressed in human primary tissue and cell lines. It contains a high-scoring heptad repeat at the extracellular C terminus that is predicted to form an alpha-helix for coiled coils like those in cellular SNAREs or in some viral fusion proteins. A synthetic 30-mer peptide that has the same sequence as the heptad repeat alpha-helix blocks HSV infection of B5-expressing porcine cells and human HEp-2 cells. Transient expression of human B5 in HEp-2 cells results in increased polykarocyte formation even in the absence of viral proteins. The B5 protein fulfills all criteria as a receptor or coreceptor for HSV entry. Use by HSV of a human cellular receptor, such as B5, that contains putative membrane fusion domains provides an example where a pathogenic virus with broad tropism has usurped a widely expressed cellular protein to function in infection at events that lead to membrane fusion.

The pro-fusion domain of herpes simplex virus glycoprotein D (gD) interacts with the gD N terminus and is displaced by soluble forms of viral receptors

Entry of herpes simplex virus into the cell requires the interaction of gD with one of its receptors, herpesvirus entry mediator or nectin 1, and the intervention of gB, gH, or gL, required to execute fusion of the virion envelope with cell membranes. The gD ectodomain is organized in two structurally and functionally differentiated regions. The N terminus (residues 1-260) carries the receptor binding sites, and the C terminus (residues 260-310) functions as the pro-fusion domain (PFD), which is required for viral infectivity and fusion but not for receptor binding. The objective of our studies is to elucidate how gD links receptor recognition to the triggering of fusion. Here, we show that PFD is made of subdomains 1 and 2 (amino acids 260-285 and 285-310). Each one partially contributed to herpes simplex virus infectivity. By means of glutathione S-transferase (GST) fusion proteins, we show that PFD bound soluble forms of gD, truncated at residue 260 (gD260t) or downstream. Both PFD subdomains bound gD260t, highlighting multiple contact sites between the N and C termini of gD. When gD260t was in complex with either receptor, it failed to bind GST-PFD. In turn, the receptors did not bind GST-PFD, irrespective of whether they were in complex with gD. Thus, gD260t interacted with the C terminus only if unbound to the receptor. We propose that (i) before receptor binding, gD adopts a "closed" conformation in which the N and C termini interact; and (ii) on encounter with a receptor, gD modifies its conformation and the N and C termini are released from reciprocal interactions ("opened" conformation) and enabled to trigger fusion.

Potential nectin-1 binding site on herpes simplex virus glycoprotein d

Four glycoproteins (gD, gB, gH, and gL) are essential for herpes simplex virus (HSV) entry into cells. An early step of fusion requires gD to bind one of several receptors, such as nectin-1 or herpesvirus entry mediator (HVEM). We hypothesize that a conformational change in gD occurs upon receptor binding that triggers the other glycoproteins to mediate fusion. Comparison of the crystal structures of gD alone and gD bound to HVEM reveals that upon HVEM binding, the gD N terminus transitions from a flexible stretch of residues to a hairpin loop. To address the contribution of this transition to the ability of gD to trigger fusion, we attempted to "lock" the gD N terminus into a looped conformation by engineering a disulfide bond at its N and C termini. The resulting mutant (gD-A3C/Y38C) failed to trigger fusion in the absence of receptor, suggesting that formation of the loop is not the sole fusion trigger. Unexpectedly, although gD-A3C/Y38C bound HVEM, it failed to bind nectin-1. This was due to the key role played by Y38 in interacting with nectin-1. Since tyrosines are often "hot spot" residues at the center of protein-protein interfaces, we mutated residues that surround Y38 on the same face of gD and tested their binding and functional properties. Our results suggest that this region of gD is important for nectin-1 interaction and is distinct from but partially overlaps the site of HVEM binding. Unique gD mutants with altered receptor usage generated in this study may help dissect the roles played by various HSV receptors during infection.

Herpes simplex virus and varicella-zoster virus: why do these human alphaherpesviruses behave so differently from one another?

Members of the Herpesviridae family of viruses are classified into the alpha, beta and gamma subfamilies. The alpha subfamily is estimated to have diverged from the beta and gamma subfamilies 200-220 million years ago. The ancestors of the herpes simplex virus (HSV) and the varicella-zoster virus (VZV), two ubiquitous and clinically important human pathogens, appeared 70-80 million years ago. As these viruses coevolved with their specific primate hosts, genetic rearrangements led to the development of the contemporary alphaherpesviruses and their distinct complement of genes. Here the distinct features of HSV and VZV are discussed in terms of their transmissibility, clinical picture, tissue tropism, establishment of latency/reactivation and immune evasion, which can, at least in part, be explained by differences in their genomes.

Efficient replication by herpes simplex virus type 1 involves activation of the IkappaB kinase-IkappaB-p65 pathway

Infection by herpes simplex virus type 1 (HSV-1) induces a persistent nuclear translocation of NFkappaB. To identify upstream effectors of NFkappaB and their effect on virus replication, we employed mouse embryo fibroblast (MEF)-derived cell lines with deletions of either IKK1 or IKK2, the catalytic subunits of the IkappaB kinase (IKK) complex. Infected MEFs were assayed for virus yield, loss of IkappaBalpha, nuclear translocation of p65, and NFkappaB DNA-binding activity. Absence of either IKK1 or IKK2 resulted in an 86 to 94% loss of virus yield compared to that of normal MEFs, little or no loss of IkappaBalpha, and greatly reduced NFkappaB nuclear translocation. Consistent with reduced virus yield, accumulation of the late proteins VP16 and gC was severely depressed. Infection of normal MEFs, Hep2, or A549 cells with an adenovirus vector expressing a dominant-negative (DN) IkappaBalpha, followed by superinfection with HSV, resulted in a 98% drop in virus yield. These results indicate that the IKK-IkappaB-p65 pathway activates NFkappaB after virus infection. Analysis of NFkappaB activation and virus replication in control and double-stranded RNA-activated protein kinase-null MEFs indicated that this kinase plays no role in the NFkappaB activation pathway. Finally, in cells where NFkappaB was blocked because of DNIkappaB expression, HSV failed to suppress two markers of apoptosis, cell surface Annexin V staining and PARP cleavage. These results support a model in which activation of NFkappaB promotes efficient replication by HSV, at least in part by suppressing a host innate response to virus infection.

Entry of herpes simplex virus mediated by chimeric forms of nectin1 retargeted to endosomes or to lipid rafts occurs through acidic endosomes

Herpes simplex virus (HSV) enters cells by fusion with target membranes, commonly the plasma membrane. In some cells, including CHO cells expressing the nectin1 or herpesvirus entry mediator receptors, entry occurs through an endocytic route. We report the following results. (i) When expressed in J cells, nectin1 and HVEM mediated a pathway of entry insensitive to endosome acidification inhibitors. (ii) A chimeric nectin1 receptor competent for endosomal uptake by fusion of the nectin1 ectodomain with the transmembrane sequence and cytoplasmic tail of the epidermal growth factor receptor (EGFR1) (nectin1-EGFR1) and chimeric nectin1 sorted to lipid rafts by a glycosylphosphatidylinositol anchor mediated endocytic entry blocked by the early endosome inhibitor wortmannin and by the endosome acidification inhibitors bafilomycin and NH(4)Cl. (iii) Entry mediated by nectin1-EGFR1 was selectively inhibited by AG1478, a tyrosine phosphorylation inhibitor that targets the EGFR1 cytoplasmic tail and blocks the signaling pathway that culminates in clathrin-dependent uptake of the receptor into endosomes. We draw the following conclusions. (i) The same receptor may initiate different routes of infection, depending on the cell in which it is expressed. Hence, the cell is a determinant that controls whether a given receptor initiates a plasma membrane or an endocytic route of entry. (ii) Receptors whose physiology involves uptake into endosomes or sorting to lipid rafts are suitable to serve as HSV receptors. (iii) Structural features of the receptors are additional determinants that control whether HSV entry occurs at the plasma membrane or at endosomes. These findings are relevant to studies of HSV retargeting to specific receptors.

Discovery of mammalian genes that participate in virus infection

Background

Viruses are obligate intracellular parasites that rely upon the host cell for different steps in their life cycles. The characterization of cellular genes required for virus infection and/or cell killing will be essential for understanding viral life cycles, and may provide cellular targets for new antiviral therapies.

Results

Candidate genes required for lytic reovirus infection were identified by tagged sequence mutagenesis, a process that permits rapid identification of genes disrupted by gene entrapment. One hundred fifty-one reovirus resistant clones were selected from cell libraries containing 2 × 10⁵ independently disrupted genes, of which 111 contained mutations in previously characterized genes and functionally anonymous transcription units. Collectively, the genes associated with reovirus resistance differed from genes targeted by random gene entrapment in that known mutational hot spots were under represented, and a number of mutations appeared to cluster around specific cellular processes, including: IGF-II expression/signalling, vesicular transport/cytoskeletal trafficking and apoptosis. Notably, several of the genes have been directly implicated in the replication of reovirus and other viruses at different steps in the viral lifecycle.

Conclusions

Tagged sequence mutagenesis provides a rapid, genome-wide strategy to identify candidate cellular genes required for virus infection. The candidate genes provide a starting point for mechanistic studies of cellular processes that participate in the virus lifecycle and may provide targets for novel anti-viral therapies.

Coexpression of UL20p and gK inhibits cell-cell fusion mediated by herpes simplex virus glycoproteins gD, gH-gL, and wild-type gB or an endocytosis-defective gB mutant and downmodulates their cell surface expression

Syncytium formation in cells that express herpes simplex virus glycoprotein B (gB), gD, gH, and gL is blocked by gK (E. Avitabile, G. Lombardi, and G. Campadelli-Fiume, J. Virol. 77:6836-6844, 2003). Here, we report the results of two series of experiments. First, UL20 protein (UL20p) expression weakly inhibited cell-cell fusion. Coexpression of UL20p and gK drastically reduced fusion in a cell-line-dependent manner, with the highest inhibition in BHK cells. Singly expressed UL20p and gK localized at the endoplasmic reticulum and nuclear membranes. When they were coexpressed, both proteins relocalized to the Golgi apparatus. Remarkably, in cells that coexpressed UL20p and gK, the antifusion activity correlated with a downmodulation of gD, gB, gH, and gL cell surface expression. Second, gB(Delta867) has a partial deletion in the cytoplasmic tail that removed endocytosis motifs. Whereas wild-type (wt) gB was internalized in vesicles lined with the endosomal marker Rab5, gB(delta867) was not internalized, exhibited enhanced cell surface expression, and was more efficient in mediating cell-cell fusion than wt gB. The antifusion activity of UL20p and gK was also exerted when gB(delta867) replaced wt gB in the cell fusion assay. These studies show that the gB C tail carries a functional endocytosis motif(s) and that the removal of the motif correlated with increased gB surface expression and increased fusion activity. We conclude that cell-cell fusion in wt-virus-infected cells is negatively controlled by at least two mechanisms. The novel mechanism described here involves the concerted action of UL20p and gK and correlates with a moderate but consistent reduction in the cell surface expression of the fusion glycoproteins. This mechanism is independent of the one exerted through endocytosis-mediated downmodulation of gB from the plasma membrane.

The herpes simplex virus JMP mutant enters receptor-negative J cells through a novel pathway independent of the known receptors nectin1, HveA, and nectin2

The herpes simplex virus type 1(JMP) [HSV-1(JMP)] mutant was selected for its ability to grow and form plaques in receptor-negative J cells. It enters J cells through a novel gD-dependent pathway, independent of all known HSV receptors, nectin1, nectin2, and HveA. Evidence that the pathway is dependent on a nectin3 binding site on HSV-1(JMP) and requires three mutations in gD rests on the following. We derived monoclonal antibodies to nectin3 and show that J cells express nectin3. HSV-1(JMP) entry and cell-to-cell spread were inhibited by soluble nectin3-Fc, demonstrating that virions carry a binding site for nectin3. The site is either directly involved in HSV-1(JMP) entry, or nectin3 binding to its site affects the gD domains involved in entry (entry site). HSV-1(JMP) entry and cell-to-cell spread in J cells were also inhibited by soluble nectin1-Fc, showing that the nectin1 binding site on gD(JMP) overlaps with the entry site or that nectin1 binding to gD affects the entry site. gD(JMP) carries three mutations, S140N, R340H, and Q344R. The latter two lie in the C tail and are present in the parental HSV-1(MP). HSV-1 strain R5000 carrying the S140N substitution was not infectious in J cells, indicating that this substitution was not sufficient. We constructed two recombinants, one carrying the three substitutions and the other carrying the two C-tail substitutions. Only the first recombinant infected J cells with an efficiency similar to that of HSV-1(JMP), indicating that the three mutations are required for the novel entry pathway. The results highlight plasticity in gD which accounts for changes in receptor usage.

The soluble ectodomain of herpes simplex virus gD contains a membrane-proximal pro-fusion domain and suffices to mediate virus entry

Entry of herpes simplex virus (HSV) 1 into cells requires the interaction of HSV gD with herpesvirus entry mediator or nectin1 receptors, and fusion with cell membrane mediated by the fusion glycoproteins gB, gH, and gL. We report that the gD ectodomain in soluble form (amino acids 1-305) was sufficient to rescue the infectivity of a gD-null HSV mutant, indicating that gD does not need to be anchored to the virion envelope to mediate entry. Entry mediated by soluble gD required, in addition to the receptor-binding sites contained within residues 1-250, a discrete downstream portion (amino acids 261-305), located proximal to the transmembrane segment in full-length gD. We named it as profusion domain. The pro-fusion domain was required for entry mediated by virion-bound gD, because its substitution with the corresponding region of CD8 failed to complement the infectivity of gD(-/+) HSV. Furthermore, a receptor-negative gD (gD(Delta6-259)) inhibited virus infectivity when coexpressed with wild-type gD; i.e., it acted as a dominant-negative gD mutant. The pro-fusion domain is proline-rich, which is characteristic of regions involved in protein-protein interactions. P291L-P292A substitutions diminished the gD capacity to complement gD(-/+) HSV infectivity. We propose that gD forms a tripartite complex with its receptor and, by way of the proline-rich pro-fusion domain, with the fusion glycoproteins, or with one of them. The tripartite complex would serve to recruit/activate the fusion glycoproteins and bring them from a fusion-inactive to a fusion-active state, such that they execute fusion of the virion envelope with cell membrane.

Herpes simplex virus type 1 immediate-early gene expression is required for the induction of apoptosis in human epithelial HEp-2 cells

Wild-type herpes simplex virus type 1 (HSV-1) induces apoptosis in human epithelial HEp-2 cells, but infected cell proteins produced later in infection block the process from killing the cells. Thus, HSV-1 infection in the presence of the translational inhibitor cycloheximide (CHX) results in apoptosis. Our specific goal was to gain insight as to the viral feature(s) responsible for triggering apoptosis during HSV-1 infection. We now report the following. (i) No viral protein synthesis or death factor processing was detected after infection with HSV-1(HFEMtsB7) at 39.5 degrees C; this mutant virus does not inject its virion DNA into the nucleus at this nonpermissive temperature. (ii) No death factor processing or apoptotic morphological changes were detected following infection with UV-irradiated, replication-defective viruses possessing transcriptionally active incoming VP16. (iii) Addition of the transcriptional inhibitor actinomycin D prevented death factor processing upon infection with the apoptotic, ICP27-deletion virus HSV-1(vBSDelta27). (iv) Apoptotic morphologies and death factor processing were not observed following infection with HSV-1(d109), a green fluorescent protein-expressing recombinant virus possessing deletions of all five immediate-early (IE) (or alpha) genes. (v) Finally, complete death factor processing was observed upon infection with the VP16 transactivation domain-mutant HSV-1(V422) in the presence of CHX. Based on these findings, we conclude that (vi) the expression of HSV-1 alpha/IE genes is required for the viral induction of apoptosis and (vii) the transactivation activity of VP16 is not necessary for this induction.

Effects of linker-insertion mutations in herpes simplex virus 1 gD on glycoprotein-induced fusion with cells expressing HVEM or nectin-1

Several cell surface molecules, including HVEM and nectin-1, can serve as entry receptors for herpes simplex virus (HSV) and as receptors for virus-induced or viral glycoprotein-induced cell fusion. The viral ligand for these receptors is the HSV envelope glycoprotein gD. A set of linker-insertion and deletion mutants of HSV type 1 (HSV-1) gD was analyzed for effects of the mutations on binding of gD to HVEM and nectin-1, on viral glycoprotein-induced cell fusion with target cells expressing HVEM or nectin-1 and on complementation of infectivity of a gD-null HSV-1 viral mutant. Insertions after amino acid 151 or 225 or deletion of amino acids 234-244 disrupted (i) binding of the mutant forms of gD to both receptors and (ii) functional interactions (cell fusion and complementation) with both receptors, but were without effect on cell surface expression. Insertions in the N-terminal domain of gD (after amino acid 12, 34 or 43) disrupted binding to HVEM and functional activities with HVEM, as expected from a previously reported X-ray structure of a gD-HVEM complex, but were without effect in the case of nectin-1. These and other results indicate that the mutations disruptive of interactions with both receptors probably affect conformations of contact sites that are different for each receptor.

Herpes simplex virus glycoprotein K, but not its syncytial allele, inhibits cell-cell fusion mediated by the four fusogenic glycoproteins, gD, gB, gH, and gL

A Myc epitope was inserted at residue 283 of herpes simplex virus type 1 (HSV-1) glycoprotein K (gK), a position previously shown not to interfere with gK activity. The Myc-tagged gK localized predominantly to the endoplasmic reticulum, both in uninfected and in HSV-infected cells. gK, coexpressed with the four HSV fusogenic glycoproteins, gD, gB, gH, and gL, inhibited cell-cell fusion. The effect was partially dose dependent and was observed both in baby hamster kidney (BHK) and in Vero cells, indicating that the antifusion activity of gK may be cell line independent. The antifusion activity of gK did not require viral proteins other than the four fusogenic glycoproteins. A syncytial (syn) allele of gK (syn-gK) carrying the A40V substitution present in HSV-1(MP) did not block fusion to the extent seen with the wild-type (wt) gK, indicating that the syn mutation ablated, at least in part, the antifusogenic activity of wt gK. We conclude that gK is part of the mechanism whereby HSV negatively regulates its own fusion activity. Its effect accounts for the notion that cells infected with wt HSV do not fuse with adjacent, uninfected cells into multinucleated giant cells or syncytia. gK may also function to preclude fusion between virion envelope and the virion-encasing vesicles during virus transport to the extracellular compartment, thus preventing nucleocapsid de-envelopment in the cytoplasm.