Chimeric nectin1-poliovirus receptor molecules identify a nectin1 region functional in herpes simplex virus entry

TRPV4 channel is involved in HSV-2 infection in human vaginal epithelial cells through triggering Ca2+ oscillation

Herpes simplex virus (HSV) infection induces a rapid and transient increase in intracellular calcium concentration ([Ca2+]i), which plays a critical role in facilitating viral entry. T-type calcium channel blockers and EGTA, a chelate of extracellular Ca2+, suppress HSV-2 infection. But the cellular mechanisms mediating HSV infection-activated Ca2+ signaling have not been completely defined. In this study we investigated whether the TRPV4 channel was involved in HSV-2 infection in human vaginal epithelial cells. We showed that the TRPV4 channel was expressed in human vaginal epithelial cells (VK2/E6E7). Using distinct pharmacological tools, we demonstrated that activation of the TRPV4 channel induced Ca2+ influx, and the TRPV4 channel worked as a Ca2+-permeable channel in VK2/E6E7 cells. We detected a direct interaction between the TRPV4 channel protein and HSV-2 glycoprotein D in the plasma membrane of VK2/E6E7 cells and the vaginal tissues of HSV-2-infected mice as well as in phallic biopsies from genital herpes patients. Pretreatment with specific TRPV4 channel inhibitors, GSK2193874 (1-4 μM) and HC067047 (100 nM), or gene silence of the TRPV4 channel not only suppressed HSV-2 infectivity but also reduced HSV-2-induced cytokine and chemokine generation in VK2/E6E7 cells by blocking Ca2+ influx through TRPV4 channel. These results reveal that the TRPV4 channel works as a Ca2+-permeable channel to facilitate HSV-2 infection in host epithelial cells and suggest that the design and development of novel TRPV4 channel inhibitors may help to treat HSV-2 infections.

Implication of Soluble Forms of Cell Adhesion Molecules in Infectious Disease and Tumor: Insights from Transgenic Animal Models

Cell adhesion molecules (CAMs) are surface ligands, usually glycoproteins, which mediate cell-to-cell adhesion. They play a critical role in maintaining tissue integrity and mediating migration of cells, and some of them also act as viral receptors. It has been known that soluble forms of the viral receptors bind to the surface glycoproteins of the viruses and neutralize them, resulting in inhibition of the viral entry into cells. Nectin-1 is one of important CAMs belonging to immunoglobulin superfamily and herpesvirus entry mediator (HVEM) is a member of the tumor necrosis factor (TNF) receptor family. Both CAMs also act as alphaherpesvirus receptor. Transgenic mice expressing the soluble form of nectin-1 or HVEM showed almost complete resistance against the alphaherpesviruses. As another CAM, sialic acid-binding immunoglobulin-like lectins (Siglecs) that recognize sialic acids are also known as an immunoglobulin superfamily member. Siglecs play an important role in the regulation of immune cell functions in infectious diseases, inflammation, neurodegeneration, autoimmune diseases and cancer. Siglec-9 is one of Siglecs and capsular polysaccharide (CPS) of group B Streptococcus (GBS) binds to Siglec-9 on neutrophils, leading to suppress host immune response and provide a survival advantage to the pathogen. In addition, Siglec-9 also binds to tumor-produced mucins such as MUC1 to lead negative immunomodulation. Transgenic mice expressing the soluble form of Siglec-9 showed significant resistance against GBS infection and remarkable suppression of MUC1 expressing tumor proliferation. This review describes recent developments in the understanding of the potency of soluble forms of CAMs in the transgenic mice and discusses potential therapeutic interventions that may alter the outcomes of certain diseases.

HIV-associated disruption of tight and adherens junctions of oral epithelial cells facilitates HSV-1 infection and spread

Herpes simplex virus (HSV) types 1 and 2 are the most common opportunistic infections in HIV/AIDS. In these immunocompromised individuals, HSV-1 reactivates and replicates in oral epithelium, leading to oral disorders such as ulcers, gingivitis, and necrotic lesions. Although the increased risk of HSV infection may be mediated in part by HIV-induced immune dysfunction, direct or indirect interactions of HIV and HSV at the molecular level may also play a role. In this report we show that prolonged interaction of the HIV proteins tat and gp120 and cell-free HIV virions with polarized oral epithelial cells leads to disruption of tight and adherens junctions of epithelial cells through the mitogen-activated protein kinase signaling pathway. HIV-induced disruption of oral epithelial junctions facilitates HSV-1 paracellular spread between the epithelial cells. Furthermore, HIV-associated disruption of adherens junctions exposes sequestered nectin-1, an adhesion protein and critical receptor for HSV envelope glycoprotein D (gD). Exposure of nectin-1 facilitates binding of HSV-1 gD, which substantially increases HSV-1 infection of epithelial cells with disrupted junctions over that of cells with intact junctions. Exposed nectin-1 from disrupted adherens junctions also increases the cell-to-cell spread of HSV-1 from infected to uninfected oral epithelial cells. Antibodies to nectin-1 and HSV-1 gD substantially reduce HSV-1 infection and cell-to-cell spread, indicating that HIV-promoted HSV infection and spread are mediated by the interaction of HSV gD with HIV-exposed nectin-1. Our data suggest that HIV-associated disruption of oral epithelial junctions may potentiate HSV-1 infection and its paracellular and cell-to-cell spread within the oral mucosal epithelium. This could be one of the possible mechanisms of rapid development of HSV-associated oral lesions in HIV-infected individuals.

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.

Identification and characterization of two zebrafish nectin-1 genes that are differentially expressed in the developing eye and brain

Nectins are cell adhesion molecules of the immunoglobulin type that play important roles in the development of the nervous system. We have characterized two paralogous zebrafish nectin-1 genes, nectin-1a and nectin-1b, that differ in expression. Nectin-1a expression is first found in the anterior neural keel and later in the optic cup. In the retina, nectin-1a appears in the outer part and extends inwards, while nectin-1b starts in the inner part and spreads outwards. Only nectin-1a was detected in the cornea, the lens, and in the region of photoreceptor cell differentiation in the retina. Both genes were expressed in ganglion cells and inner nuclear neurons. In the brain, nectin-1a was restricted to the epiphysis and a cluster of cells in the posterior hindbrain, whereas nectin-1b was found in several brain areas. Zebrafish may, therefore, be a useful model for identifying different functions of nectin-1 in the developing eye and nervous system.

Fusion protein consisting of the first immunoglobulin-like domain of porcine nectin-1 and Fc portion of human IgG1 provides a marked resistance against pseudorabies virus infection to transgenic mice

Nectin-1 is a Ca2+-independent Ig-like cell-cell adhesion molecule and an alphaherpesvirus receptor that binds to virion glycoprotein D by the first Ig-like domain. We have investigated the antiviral potentials of soluble forms of porcine nectin-1 to PRV infection by generating transgenic mice expressing different types of fusion protein. Previously, we reported that mice transgenic for a chimera that carried the entire ectodomain of porcine nectin-1 fused to the Fc portion of porcine IgG1 were more resistant than those transgenic for a chimera that carried the first Ig-like domain fused to the Fc portion. Recently, we generated transgenic mice expressing a fusion protein made of the first Ig-like domain fused to the Fc portion of human IgG1, and reported that they showed a microphthalmia. Here, two transgenic mouse lines expressing the fusion protein were challenged with PRV for comparing their resistances with those of transgenic mice expressing different types of fusion protein. Surprisingly, both transgenic mouse lines showed a high resistance to the viral infection, especially via the i.n. route. Significant resistance of the embryonic fibroblasts was also observed. Altogether, these findings indicated that the fusion protein consisting of the first Ig-like domain fused to the human Fc portion provided a marked resistance against PRV infection to the transgenic mice.

Comparison of the antiviral potentials among the pseudorabies-resistant transgenes encoding different soluble forms of porcine nectin-1 in transgenic mice

Nectin-1 is an alphaherpesvirus receptor that binds to virion glycoprotein D by the first immunoglobulin (Ig)-like domain. The possibility of making animals resistant to pseudorabies virus (PRV) infection has been investigated by generating transgenic mice expressing soluble forms of porcine nectin-1. Previously, transgenic mice were generated that expressed a fusion protein made of the entire ectodomain of nectin-1 fused to the Fc portion of human IgG, or the first Ig-like domain fused to the Fc portion of porcine IgG. Here, the contribution of the second and third Ig-like domains of nectin-1 was analysed by generating transgenic mice expressing the entire ectodomain of nectin-1 fused to the porcine Fc portion. Transgenic mice expressing each of three different fusion proteins were challenged with PRV for comparison of their resistance. Altogether, mice transgenic for a chimera that carried the entire ectodomain were more resistant than those transgenic for a chimera that carried the first Ig-like domain.

The requirements for herpes simplex virus type 1 cell-cell spread via nectin-1 parallel those for virus entry

Herpes simplex virus type 1 (HSV-1) spreads from an infected cell to an uninfected cell by virus entry, virus-induced cell fusion, and cell-cell spread. The three forms of virus spread require the viral proteins gB, gD, and gH-gL, as well as a cellular gD receptor. The mutual requirement for the fusion glycoproteins and gD receptor suggests that virus entry, cell fusion, and cell-cell spread occur by a similar mechanism. The goals of this study were to examine the role of the nectin-1alpha transmembrane domain and cytoplasmic tail in cell-cell spread and to obtain a better understanding of the receptor-dependent events occurring at the plasma membrane during cell-cell spread. We determined that an intact nectin-1alpha V-like domain was required for cell-cell spread, while a membrane-spanning domain and cytoplasmic tail were not. Chimeric forms of nectin-1 that were non-functional for virus entry did not mediate cell-cell spread regardless of whether they could mediate cell fusion. Also, cell-cell spread of syncytial isolates was dependent upon nectin-1alpha expression and occurred through a nectin-1-dependent mechanism. Taken together, our results indicate that nectin-1-dependent events occurring at the plasma membrane during cell-cell spread were equivalent to those for virus entry.

Crystal structure of the V domain of human Nectin-like molecule-1/Syncam3/Tsll1/Igsf4b, a neural tissue-specific immunoglobulin-like cell-cell adhesion molecule

Nectins are Ca(2+)-independent immunoglobulin (Ig) superfamily proteins that participate in the organization of epithelial and endothelial junctions. Nectins have three Ig-like domains in the extracellular region, and the first one is essential in cell-cell adhesion and plays a central role in the interaction with the envelope glycoprotein D of several viruses. Five Nectin-like molecules (Necl-1 through -5) with similar domain structures to those of Nectins have been identified. Necl-1 is specifically expressed in neural tissue, has Ca(2+)-independent homophilic and heterophilic cell-cell adhesion activity, and plays an important role in the formation of synapses, axon bundles, and myelinated axons. Here we report the first crystal structure of its N-terminal Ig-like V domain at 2.4 A, providing insight into trans-cellular recognition mediated by Necl-1. The protein crystallized as a dimer, and the dimeric form was confirmed by size-exclusion chromatography and chemical cross-linking experiments, indicating this V domain is sufficient for homophilic interaction. Mutagenesis work demonstrated that Phe(82) is a key residue for the adhesion activity of Necl-1. A model for homophilic adhesion of Necl-1 at synapses is proposed based on its structure and previous studies.

Soluble V domain of Nectin-1/HveC enables entry of herpes simplex virus type 1 (HSV-1) into HSV-resistant cells by binding to viral glycoprotein D

Interaction of herpes simplex virus (HSV) glycoprotein D (gD) with specific cellular receptors is essential for HSV infection of susceptible cells. Virus mutants that lack gD can bind to the cell surface (attachment) but do not enter, implying that interaction of gD with its receptor(s) initiates the postattachment (entry) phase of HSV infection. In this report, we have studied HSV entry in the presence of the gD-binding variable (V) domain of the common gD receptor nectin-1/HveC to determine whether cell association of the gD receptor is required for HSV infection. In the presence of increasing amounts of the soluble nectin-1 V domain (sNec1(123)), increasing viral entry into HSV-resistant CHO-K1 cells was observed. At a multiplicity of 3 in the presence of optimal amounts of sNec1(123), approximately 90% of the cells were infected. The soluble V domain of nectin-2, a strain-specific HSV entry receptor, promoted entry of the HSV type 1 (HSV-1) Rid-1 mutant strain, but not of wild-type HSV-1. Preincubation and immunofluorescence studies indicated that free or gD-bound sNec1(123) did not associate with the cell surface. sNec1(123)-mediated entry was highly impaired by interference with the cell-binding activities of viral glycoproteins B and C. While gD has at least two functions, virus attachment to the cell and initiation of the virus entry process, our results demonstrate that the attachment function of gD is dispensable for entry provided that other means of attachment are available, such as gB and gC binding to cell surface glycosaminoglycans.

The nectin-1alpha transmembrane domain, but not the cytoplasmic tail, influences cell fusion induced by HSV-1 glycoproteins

Nectin-1 is a receptor for herpes simplex virus (HSV), a member of the immunoglobulin superfamily, and a cellular adhesion molecule. To study domains of nectin-1alpha involved in cell fusion, we measured the ability of nectin-1alpha/nectin-2alpha chimeras, nectin-1alpha/CD4 chimeras, and transmembrane domain and cytoplasmic tail mutants of nectin-1alpha to promote cell fusion induced by HSV-1 glycoproteins. Our results demonstrate that only chimeras and mutants containing the entire V-like domain and a link to the plasma membrane conferred cell-fusion activity. The transmembrane domain and cytoplasmic tail of nectin-1 were not required for any viral receptor or cell adhesion function tested. Cellular cytoplasmic factors that bind to the nectin-1alpha cytoplasmic tail, therefore, did not influence virus entry or cell fusion. Interestingly, the efficiency of cell fusion was reduced when membrane-spanning domains of nectin-1alpha and gD were replaced by glycosylphosphatidylinositol tethers, indicating that transmembrane domains may play a modulatory role in the gD/nectin-1alpha interaction in fusion.

Deletion of the second immunoglobulin-like domain of nectin-1 alters its intracellular processing and localization and ability to mediate entry of herpes simplex virus

Nectin-1 is an immunoglobulin (Ig)-like entry receptor for herpes simplex virus (HSV). Like other nectins, nectin-1 forms dimers and mediates cell adhesion through interactions with other nectins. We constructed a second-domain deletion mutant of nectin-1 (nectin-1-Delta2) to examine the role of the second Ig-like domain in HSV entry. Nectin-1-Delta2 exhibited a severely reduced ability to mediate HSV entry and accumulated in the endoplasmic reticulum but retained the ability to interact with its HSV ligand, gD. The failure of nectin-1-Delta2 to mediate HSV entry probably resulted from its failure to be transported to a membrane targeted by HSV for viral entry.

Possible Mechanism Underlying the Antiherpetic Activity of a Proteoglycan Isolated from the Mycelia of Ganoderma lucidum in Vitro

GLPG (Ganoderma lucidum proteoglycan) was a bioactive fraction obtained by the liquid fermentation of the mycelia of Ganoderma lucidum, EtOH precipitation, and DEAE-cellulose column chromatography.GLPG was a proteoglycan with a carbohydrate: protein ratio of 10.4: 1. Its antiviral activities against herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) were investigated using a cytopathic inhibition assay. GLPG inhibited cell death in a dose-dependent manner in HSV-infected cells. In addition, it had no cytotoxic effect even at 2 mg/ml. In order to study the mode of action of the antiviral activity of GLPG, cells were treated with GLPG before, during, and after infection, and viral titer in the supernatant of cell culture 48 h post-infection was determined using a TCID((50)) assay. The antiviral effects of GLPG were more remarkable before viral treatment than after treatment. Although the precise mechanism has yet to be defined, our work suggests that GLPG inhibits viral replication by interfering with the early events of viral adsorption and entry into target cells. Thus, this proteoglycan appears to be a candidate anti-HSV agent.

Possible mode of action of antiherpetic activities of a proteoglycan isolated from the mycelia of Ganoderma lucidum in vitro

A bioactive fraction (GLPG) was extracted and purified from the mycelia of Ganoderma lucidum by EtOH precipitation and DEAE-cellulose column chromatography. GLPG was a proteoglycan and had a carbohydrate:protein ratio of 10.4:1. Its antiviral activities against herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) were investigated by the cytopathic effect (CPE) inhibition assay in cell culture. This kind of polysaccharide inhibited the development of the cytopathic effect in dose-dependent manner in HSV-infected cells, moreover did not show any cytotoxic effects on cells even when a concentration was as high as 2000 microg/ml. In order to study the possible mode of action of the antiviral activity of GLPG, cells were treated with GLPG before, during and after infection, and the viral titers in the supernatant of cell culture 48 h post-infection were tested by TCID(50) assay. The antiviral effects in pre-treated and treated during virus infection with GLPG were more remarkable than the treatment of post-infection. Although the precise mechanism has yet to be defined, our work suggested that GLPG inhibits viral replication by interfering with the early events of viral adsorption and entry into target cells. Thus, this proteoglycan seems to be a potential candidate for anti-HSV agents.

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.

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.

Cellular localization of nectin-1 and glycoprotein D during herpes simplex virus infection

During viral entry, herpes simplex virus (HSV) glycoprotein D (gD) interacts with a specific cellular receptor such as nectin-1 (PRR1/HveC/CD111) or the herpesvirus entry mediator A (HVEM/HveA). Nectin-1 is involved in cell-to-cell adhesion. It is located at adherens junctions, where it bridges cells through homophilic or heterophilic interactions with other nectins. Binding of HSV gD prevents nectin-1-mediated cell aggregation. Since HSV gD affects the natural function of nectin-1, we further investigated the effects of gD expression on nectin-1 during HSV infection or in transfected cells. We also studied the importance of the interaction between nectin-1 and the cytoplasmic protein afadin for HSV entry and spread as well as the effects of infection on this interaction. In these investigations, we used a panel of cells expressing nectin-1 or nectin-1-green fluorescent protein fusions as the only mediators of HSV entry. During HSV infection, nectin-1 localization at adherens junction was dramatically altered in a manner dependent on gD expression. Nectin-1 and gD colocalized at cell contact areas between infected and noninfected cells and at the edges of plaques. This specific accumulation of gD at junctions was driven by expression of nectin-1 in trans on the surface of adjacent cells. Reciprocally, nectin-1 was maintained at junctions by the trans expression of gD in the absence of a cellular natural ligand. Our observations indicate that newly synthesized gD substitutes for nectin-1 of infected cells at junctions with noninfected cells. We propose that gD attracts and maintains the receptor at junctions where it can be used for virus spread.

The antiviral activity of naturally occurring proteins and their peptide fragments after chemical modification

Chemical modification of the proteins bovine serum albumin, alpha-lactalbumin, beta-lactoglobulin and chicken lysozyme by 3-hydroxyphthalic anhydride (3-HP) yielded compounds which exerted antiviral activity in vitro as compared with the native unmodified proteins. Of the three enveloped viruses tested, human herpes simplex virus type 1 (HSV-1), bovine parainfluenza virus type 3 and porcine respiratory corona virus, only HSV-1 proved sensitive to the 3-HP-proteins. All of the chemically modified proteins presented antiviral activity against HSV-1 when assayed before, during or after infection. However, to achieve HSV-1 inhibition, significantly higher concentrations of the modified proteins were required if present before infection as compared to during or after infection. Our results suggest that multiple mechanisms are involved in the inhibition of HSV-1 infection. Proteolytical digestion of albumin, alpha-lactalbumin, beta-lactoglobulin and lysozyme by trypsin, chymotrypsin and pepsin yielded several peptide fragments with antiherpetic activity. Chemical modification of these peptide fragments by 3-HP generated peptides with antiviral activity, however, this was almost always combined with a cytotoxic effect on the Vero cells. Overall, our results suggest that targeted chemical modification of some natural products might provide compounds effective against HSV-1 infection.

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.

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

Glycoprotein D (gD) interacts with two alternative protein receptors, nectin1 and HveA, to mediate herpes simplex virus (HSV) entry into cells. Fusion of the envelope with the plasma membrane requires, in addition to gD, glycoproteins gB, gH, and gL. Coexpression of the four glycoproteins (gD, gB, gH, and gL) promotes cell-cell fusion. gD delivered in trans is also capable of blocking the apoptosis induced by gD deletion viruses grown either in noncomplementing cells (gD(-/-)) or in complementing cells (gD(-/+)). While ectopic expression of cation-independent mannose-6 phosphate receptor blocks apoptosis induced by both stocks, other requirements differ. Thus, apoptosis induced by gD(-/-) virus is blocked by full-length gD (or two gD fragments reconstituting a full-length molecule), whereas ectopic expression of the gD ectodomain is sufficient to block apoptosis induced by gD(-/+) virus. In this report we took advantage of a set of gD insertion-deletion mutants to map the domains of gD required to block apoptosis by gD(-/-) and gD(-/+) viruses and those involved in cell-cell fusion. The mutations that resulted in failure to block apoptosis were the same for gD(-/-) and gD(-/+) viruses and were located in three sites, one within the immunoglobulin-type core region (residues 125, 126, and 151), one in the upstream connector region (residues 34 and 43), and one in the C-terminal portion of the ectodomain (residue 277). A mutant that carried amino acid substitutions at the three glycosylation sites failed to block apoptosis but behaved like wild-type gD in all other assays. The mutations that inhibited polykaryocyte formation were located in the upstream connector region (residues 34 and 43), at the alpha1 helix (residue 77), in the immunoglobulin core and downstream regions (residue 151 and 187), and at the alpha3 helix (residues 243 and 246). Binding of soluble nectin1-Fc to cells expressing the mutant gDs was generally affected by the same mutations that affected fusion, with one notable exception (Delta277-310), which affected fusion without hampering nectin1 binding. This deletion likely identifies a region of gD involved in fusion activity at a post-nectin1-binding step. We conclude that whereas mutations that affected all functions (e.g., upstream connector region and residue 151) may be detrimental to overall gD structure, the mutations that affect specific activities identify domains of gD involved in the interactions with entry receptors and fusogenic glycoproteins and with cellular proteins required to block apoptosis. The evidence that glycosylation of gD is required for blocking apoptosis supports the conclusion that the interacting protein is the mannose-6 phosphate receptor.

Mutations in the N-terminal domains of nectin-1 and nectin-2 reveal differences in requirements for entry of various alphaherpesviruses and for nectin-nectin interactions

Nectin-1 and nectin-2 are related molecules that can function with different specificities as entry receptors for mammalian alphaherpesviruses through interaction with viral glycoprotein D (gD). The normal function of members of the nectin family is to mediate cell-cell adhesion through homotypic and heterotypic nectin-nectin interactions in cadherin-based adherens junctions. We examined mutations in three equivalent regions of the N-terminal V-like domains of nectin-1 and nectin-2 to test the effects on entry of various alphaherpesviruses, nectin-nectin interactions, and interactions of the mutant nectins with gD. Mutations in region I previously shown to severely impair herpes simplex virus (HSV) entry activity, but not pseudorabies virus (PRV) or bovine herpesvirus 1 (BHV-1) entry, did not reduce homotypic trans interactions for either nectin-1 or nectin-2 or binding of nectin-3 to nectin-1. Mutations in region II, patterned after a reported single-nucleotide polymorphism in nectin-2, enhanced intracellular accumulation of both nectin-1 and nectin-2 and had a deleterious effect on all of the activities under study. Mutations in region III previously shown to reduce homotypic trans interactions of nectin-2 impaired the entry of PRV and BHV-1 when introduced into either nectin-1 or nectin-2, but only the nectin-2 mutation reduced HSV entry activity. Binding of nectin-1 to nectin-3 was not affected. Effects of the nectin-1 and nectin-2 mutations on interactions with gD did not necessarily correlate with entry activity of the mutant receptors. We can conclude that structural requirements for HSV entry, PRV and BHV-1 entry, and homotypic and heterotypic trans interactions are all different despite the previously reported ability of HSV and HSV gD to inhibit trans interactions.

Critical residues in the CC' ridge of the human nectin1 receptor V domain enable herpes simplex virus entry into the cell and act synergistically with the downstream region

The site on nectin1 receptor required for herpes simplex virus (HSV) entry into the cell was previously mapped to the 64-94 region, encompassing the predicted CC'C" region of the immunoglobulin V domain. Within it lies a minimal HSV entry site (residues 77-94). Here we transferred the 65-76 region (C strand and CC' loop) and portions, or single amino acids, thereof to nectin2, a homolog nonfunctional for wt HSV-1 entry. Replacement of the seven- or of three-amino-acid-long stretches from nectin1 to nectin2 (amino acids 69-75, 69-71, or 72-75) transferred wt HSV-1 and BHV-1 entry activity and enhanced HSV-2, PrV, and HSV-HSV(U21) entry to levels observed with nectin1. Thus, the CC' ridge is sufficient to mediate wt HSV entry at a reduced level and responsible for the wide virus range of the receptor. Altogether the HSV entry site appears to be composed of contiguous synergistic regions, 64-76 and 77-94, each independently capable of mediating virus entry at reduced efficiency.

Prominent role of the Ig-like V domain in trans-interactions of nectins. Nectin3 and nectin 4 bind to the predicted C-C'-C"-D beta-strands of the nectin1 V domain

Nectins form a family of integral molecules that belong to the immunoglobulin superfamily. Their ectodomain is made of three Ig-like domains (V, C, C). This family comprises at least five members, namely nectin1, -2, -3, -4, and poliovirus receptor (PVR), that are involved in different physiological and pathological processes. (i) Nectins are adhesion molecules localized at adherens junctions in epithelial cells. (ii) Some nectins act as poliovirus or alpha-herpesvirus receptors (nectin1). (iii) Nectin1 mutations are involved in orofacial developmental abnormalities in humans. Adhesion properties of nectins are mediated by Ca(2+)-independent homophilic and heterophilic processes through ectodomain trans-interactions. We have described a nectin trans-hetero-interaction network: nectin3 binds to nectin1, nectin2, and PVR; nectin1 also binds to nectin4. In the present study we compared the affinities of the different trans-interactions mediated by nectin1. We found that the K(D) of nectin1/nectin3 and nectin1/nectin4 interactions is 1 and 100 nm, respectively, whereas the K(D) of the nectin1-mediated homophilic interaction is 1 microm. We show that nectin1/nectin3 and nectin1/nectin4 trans-hetero-interactions were mediated through trans V to V domain interactions, whereas C domains contributed to increase the affinity of the interaction. Nectin3 and nectin4 share a common binding region in the nectin1 V domain: (i) nectin3 strongly competed with nectin4 binding, (ii) nectin3 and nectin4 binding to nectin1 was reduced by a number of monoclonal antibodies directed against the nectin1 V domain, and (iii) the glycoprotein D of herpes simplex virus-1 that binds to the V domain of nectin1 reduced nectin3 and nectin4 binding. Finally, using chimeric nectin1/PVR receptors where PVR V domain beta-strands were substituted with the corresponding regions of nectin1, the nectin3 and nectin4 minimal binding region on nectin1 V domain was mapped to the C-C'-C"-D beta-strands.

Amino acid substitutions in the V domain of nectin-1 (HveC) that impair entry activity for herpes simplex virus types 1 and 2 but not for Pseudorabies virus or bovine herpesvirus 1

The entry of herpes simplex virus (HSV) into cells requires the interaction of viral glycoprotein D (gD) with a cellular gD receptor to trigger the fusion of viral and cellular membranes. Nectin-1, a member of the immunoglobulin superfamily, can serve as a gD receptor for HSV types 1 and 2 (HSV-1 and HSV-2, respectively) as well as for the animal herpesviruses porcine pseudorabies virus (PRV) and bovine herpesvirus 1 (BHV-1). The HSV-1 gD binding domain of nectin-1 is hypothesized to overlap amino acids 64 to 104 of the N-terminal variable domain-like immunoglobulin domain. Moreover, the HSV-1 and PRV gDs compete for binding to nectin-1. Here we report that two amino acids within this region, at positions 77 and 85, are critical for HSV-1 and HSV-2 entry but not for the entry of PRV or BHV-1. Replacement of either amino acid 77 or amino acid 85 reduced HSV-1 and HSV-2 gD binding but had a lesser effect on HSV entry activity, suggesting that weak interactions between gD and nectin-1 are sufficient to trigger the mechanism of HSV entry. Substitution of both amino acid 77 and amino acid 85 in nectin-1 significantly impaired entry activity for HSV-1 and HSV-2 and eliminated binding to soluble forms of HSV-1 and HSV-2 gDs but did not impair the entry of PRV and BHV-1. Thus, amino acids 77 and 85 of nectin-1 form part of the interface with HSV gD or influence the conformation of that interface. Moreover, the binding sites for HSV and PRV or BHV-1 gDs on nectin-1 may overlap but are not identical.

Substitution in the murine nectin1 receptor of a single conserved amino acid at a position distal from the herpes simplex virus gD binding site confers high-affinity binding to gD

By analogy with its human nectin1 counterpart, murine nectin1 serves as a cellular receptor for the entry of herpes simplex virus (HSV) into murine cells. HSV entry mediated by either receptor is dependent on the viral glycoprotein D (gD). Whereas human nectin1 binds gD at high affinity and in a saturable manner, murine nectin1 binds gD in a barely detectable fashion, depending on the sensitivity of the assay. The immunoglobulin type V domain of murine nectin differs from its human counterpart in 11 amino acids. To identify the key residues responsible for the high-affinity binding of gD to human nectin1, we replaced each of the 11 divergent amino acids with the human counterparts singly or in groups in an incremental manner. Replacement in murine nectin1 of six amino acids that lie within the gD binding region of human nectin1 (previously mapped to residues 64 to 94, likely the CC'C" surface) increased the gD binding activity to a limited extent. In contrast, the single P138L substitution, which lies distal from the gD binding site, markedly increased gD binding. This substitution, when coupled with downstream substitutions, exerted the greatest effect. Three-dimensional modeling of the nectin1 V domain suggested that P138 in murine nectin1 might decrease the stability of the V domain by reducing the size of beta-strand G. The results support the notion that the overall structure of V nectin1 plays a pivotal role in its ability to bind HSV gD.

Effects of herpes simplex virus on structure and function of nectin-1/HveC

Herpes simplex virus (HSV) entry requires the interaction between the envelope glycoprotein D (gD) and a cellular receptor such as nectin-1 (also named herpesvirus entry mediator C [HveC]) or HveA/HVEM. Nectin-1 is a cell adhesion molecule found at adherens junctions associated with the cytoplasmic actin-binding protein afadin. Nectin-1 can act as its own ligand in a homotypic interaction to bridge cells together. We used a cell aggregation assay to map an adhesive functional site on nectin-1 and identify the effects of gD binding and HSV early infection on nectin-1 function. Soluble forms of nectin-1 and anti-nectin-1 monoclonal antibodies were used to map a functional adhesive site within the first immunoglobulin-like domain (V domain) of nectin-1. This domain also contains the gD-binding site, which appeared to overlap the adhesive site. Thus, soluble forms of gD were able to prevent nectin-1-mediated cell aggregation and to disrupt cell clumps in an affinity-dependent manner. HSV also prevented nectin-1-mediated cell aggregation by occupying the receptor. Early in infection, nectin-1 was not downregulated from the cell surface. Rather, detection of nectin-1 changed gradually over a 30-min period of infection, as reflected by a decrease in the CK41 epitope and an increase in the CK35 epitope. The level of detection of virion gD on the cell surface increased within 5 min of infection in a receptor-dependent manner. These observations suggest that cell surface nectin-1 and gD may undergo conformational changes during HSV entry as part of an evolving interaction between the viral envelope and the cell plasma membrane.

Nectin4/PRR4, a new afadin-associated member of the nectin family that trans-interacts with nectin1/PRR1 through V domain interaction

Nectins are adhesion molecules that participate in the organization of epithelial and endothelial junctions and serve as receptors for herpes simplex virus entry. They belong to the immunoglobulin superfamily, are homologues of the poliovirus receptor (PVR/CD155), and were also named poliovirus receptor-related (PRR) proteins. We identify a new member of the nectin family named nectin4. Peptide sequences of human and murine nectin4 share 92% identity, and as for other members, the ectodomain is made of three immunoglobulin-like domains of V, C, C types. In contrast to other nectin molecules, detection of nectin4 transcripts is mainly restricted to placenta in human tissues. Expression is broader in mouse, and interestingly nectin4 is detected at days 11, 15, and 17 during murine embryogenesis. Nectin4 interacts with afadin, a F-actin-associated molecule, via its carboxyl-terminal cytoplasmic sequence. Both molecules co-localize at cadherin-based adherens junctions in the MDCKII epithelial cell line. Nectins are homophilic adhesion molecules, and recently heterophilic interactions have been described between nectin3/nectin1 and nectin3/nectin2. We confirmed these trans-interactions and also described nectin3 as the PVR/CD155 ligand. By means of several approaches, we report on the identification of nectin4 as a new ligand for nectin1. First, a soluble chimeric recombinant nectin4 ectodomain (nectin4-Fc) trans-interacts with cells expressing nectin1 but not with cells expressing nectin2, nectin3, or PVR/CD155. Conversely, nectin1-Fc binds to cells expressing nectin4. Second, nectin1-Fc precipitates nectin4 expressed in COS cells. Third, reciprocal in vitro physical interactions were detected between nectin4-Fc and nectin1-Fc. The nectin4-Fc/nectin4-Fc interaction was detected suggesting that nectin4 exhibits both homophilic and heterophilic properties. Using the same approaches we demonstrate, for the first time, that the V domain of nectin1 acts as a major functional region involved in trans-heterointeraction with nectin4 and also nectin3.