Herpes simplex virus type 1 glycoprotein H binds to αvβ3 integrins

Virion endocytosis is a major target for murid herpesvirus-4 neutralization

Herpesviruses consistently transmit from immunocompetent carriers, implying that their neutralization is hard to achieve. Murid herpesvirus-4 (MuHV-4) exploits host IgG Fc receptors to bypass blocks to cell binding, and pH-dependent protein conformation changes to unveil its fusion machinery only after endocytosis. Nevertheless, neutralization remains possible by targeting the virion glycoprotein H (gH)–gL heterodimer, and the neutralizing antibody responses of MuHV-4 carriers are improved by boosting with recombinant gH–gL. We analysed here how gH–gL-directed neutralization works. The MuHV-4 gH–gL binds to heparan sulfate. However, most gH–gL-specific neutralizing antibodies did not block this interaction; neither did they act directly on fusion. Instead, they blocked virion endocytosis and transport to the late endosomes, where membrane fusion normally occurs. The poor endocytosis of gH–gL-neutralized virions was recapitulated precisely by virions genetically lacking gL. Therefore, driving virion uptake appears to be an important function of gH–gL that provides a major target for antibody-mediated neutralization.

Early events in herpes simplex virus lifecycle with implications for an infection of lifetime

Affecting a large percentage of human population herpes simplex virus (HSV) types -1 and -2 mainly cause oral, ocular, and genital diseases. Infection begins with viral entry into a host cell, which may be preceded by viral “surfing” along filopodia. Viral glycoproteins then bind to one or more of several cell surface receptors, such as herpesvirus entry mediator (HVEM), nectin-1, 3-O sulfated heparan sulfate (3-OS HS), paired immunoglobulin-like receptor α, and non-muscle myosin-IIA. At least five viral envelope glycoproteins participate in entry and these include gB, gC, gD and gH-gL. Post-entry, these glycoproteins may also facilitate cell-to-cell spread of the virus, which helps in the evasion of physical barriers as well as several components of the innate and adaptive immune responses. The spread may be facilitated by membrane fusion, movement across tight junctions, transfer across neuronal synapses, or the recruitment of actin-containing structures. This review summarizes some of the recent advances in our understanding of HSV entry and cell-to-cell spread.

The KGD motif of Epstein-Barr virus gH/gL is bifunctional, orchestrating infection of B cells and epithelial cells

Epstein-Barr virus (EBV), a member of the herpesvirus family, is the causative agent of common human infections and specific malignancies. EBV entry into target cells, including B cells and epithelial cells, requires the interaction of multiple virus-encoded glycoproteins. Glycoproteins H and L (gH/gL) cooperate with glycoprotein B (gB) to mediate fusion of the viral envelope with target cell membranes. Both the gH/gL complex and gB are required for fusion, whereas glycoprotein 42 (gp42) acts as a tropism switch and is required for B cell infection and inhibits epithelial cell infection. Our previous studies identified a prominent KGD motif located on the surface of gH/gL. In the current study, we found that this motif serves as a bifunctional domain on the surface of gH/gL that directs EBV fusion of B cells and epithelial cells. Mutation of the KGD motif to AAA decreased fusion with both epithelial and B cells and reduced the binding of gH/gL to epithelial cells and to gp42. We also demonstrate that deletion of amino acids 62 to 66 of gp42 selectively reduces binding to wild-type gH/gL, but not the KGD mutant, suggesting that the KGD motif of gH/gL interacts with the N-terminal amino acids 62 to 66 of gp42.

Epithelial and B cells are the major targets of Epstein-Barr virus (EBV) infection in the human host. EBV utilizes different glycoprotein complexes to enter these cell types. For B cell fusion, EBV uses complexes containing gp42, gH/gL, and gB, whereas just gH/gL and gB are required for epithelial cell fusion. In the current study, a bifunctional domain consisting of a prominent KGD motif on the surface of the gH/gL structure was identified; this domain affects interactions with gp42 or epithelial receptors, ultimately dictating with which cell type virus-induced fusion can occur. These studies will lead to a better understanding of the mechanism of EBV-induced membrane fusion and herpesvirus-induced membrane fusion in general.

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

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

Structure-function analysis of varicella-zoster virus glycoprotein H identifies domain-specific roles for fusion and skin tropism

Enveloped viruses require membrane fusion for cell entry and replication. For herpesviruses, this event is governed by the multiprotein core complex of conserved glycoproteins (g)B and gH/gL. The recent crystal structures of gH/gL from herpes simplex virus 2, pseudorabies virus, and Epstein-Barr virus revealed distinct domains that, surprisingly, do not resemble known viral fusogens. Varicella-zoster virus (VZV) causes chicken pox and shingles. VZV is an α-herpesvirus closely related to herpes simplex virus 2, enabling prediction of the VZV gH structure by homology modeling. We have defined specific roles for each gH domain in VZV replication and pathogenesis using structure-based site-directed mutagenesis of gH. The distal tip of domain (D)I was important for skin tropism, entry, and fusion. DII helices and a conserved disulfide bond were essential for gH structure and VZV replication. An essential (724)CXXC(727) motif was critical for DIII structural stability and membrane fusion. This assignment of domain-dependent mechanisms to VZV gH links elements of the glycoprotein structure to function in herpesvirus replication and virulence.

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

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

Structure of a core fragment of glycoprotein H from pseudorabies virus in complex with antibody

Compared with many well-studied enveloped viruses, herpesviruses use a more sophisticated molecular machinery to induce fusion of viral and cellular membranes during cell invasion. This essential function is carried out by glycoprotein B (gB), a class III viral fusion protein, together with the heterodimer of glycoproteins H and L (gH/gL). In pseudorabies virus (PrV), a porcine herpesvirus, it was shown that gH/gL can be substituted by a chimeric fusion protein gDgH, containing the receptor binding domain (RBD) of glycoprotein D fused to a truncated version of gH lacking its N-terminal domain. We report here the 2.1-Å resolution structure of the core fragment of gH present in this chimera, bound to the Fab fragment of a PrV gH-specific monoclonal antibody. The structure strongly complements the information derived from the recently reported structure of gH/gL from herpes simplex virus type 2 (HSV-2). Together with the structure of Epstein-Barr virus (EBV) gH/gL reported in parallel, it provides insight into potentially functional conserved structural features. One feature is the presence of a syntaxin motif, and the other is an extended "flap" masking a conserved hydrophobic patch in the C-terminal domain, which is closest to the viral membrane. The negative electrostatic surface potential of this domain suggests repulsive interactions with the lipid heads. The structure indicates the possible unmasking of an extended hydrophobic patch by movement of the flap during a receptor-triggered conformational change of gH, exposing a hydrophobic surface to interact with the viral membrane during the fusion process.

{alpha}V{beta}3-integrin routes herpes simplex virus to an entry pathway dependent on cholesterol-rich lipid rafts and dynamin2

HSVs enter cells in a receptor-dependent [nectin1 or herpesviruses entry mediator (HVEM)] fashion by fusion of the viral envelope with plasma membrane (neutral pH compartment), by endocytosis into neutral or acidic compartments, or by macropinocytosis/phagocytosis. The cellular determinants of the route of entry are unknown. Here, we asked what cellular factors determine the pathway of HSV entry. CHO cells lack β(3)-integrin and the respective α-subunits' heterodimers. We report that, in the absence of α(V)β(3)-integrin, HSV enters CHO-nectin1 cells through a pathway independent of cholesterol-rich rafts and dynamin2. In the presence of α(V)β(3)-integrin, HSV enters CHO-nectin1 cells through a pathway dependent on cholesterol-rich rafts and dynamin2. HSV enters J-nectin1 and 293T cells through a neutral compartment independent of cholesterol-rich rafts and dynamin2. α(V)β(3)-integrin overexpression in these cells modifies the route of entry to an acidic compartment dependent on cholesterol-rich rafts and dynamin2, hence similar to that in α(V)β(3)-integrin-positive CHO-nectin1 cells. In some cells, the diversion of entry from an integrin- and raft-independent pathway to an acidic compartment requiring cholesterol-rich lipids rafts and dynamin2 is irreversible. Indeed, HSV cannot infect CHO-nectin1-α(V)β(3) cells through any compartment when the αvβ3-integrin-dependent pathway is blocked by anti-integrin antibody, anti-dynamin2, or anti-acidification drugs. We conclude that the αvβ3-integrin is a determinant in the choice of HSV entry pathway into cells. Because the pathway dictated by αvβ3-integrin is through lipid rafts, the platforms for a number of Toll-like receptors, current findings raise the possibility that αvβ3-integrin acts as a sentinel of innate immunity.

Reevaluating herpes simplex virus hemifusion

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

Binding of herpes simplex virus type-1 virions leads to the induction of intracellular signalling in the absence of virus entry

The envelope of HSV-1 contains a number of glycoproteins, four of which are essential for virus entry. Virus particles lacking gB, gD, gH or gL are entry-defective, although these viruses retain the ability to bind to the plasma membrane via the remaining glycoproteins. Soluble forms of gD have been shown to trigger the nuclear translocation of the NF-kappaB transcriptional complex in addition to stimulating the production of Type I interferon. By taking advantage of the entry-defective phenotype of glycoprotein-deficient HSV-1 virus particles, the results presented here show that binding of virions to cellular receptors on the plasma membrane is sufficient to stimulate a change in cellular gene expression. Preliminary microarray studies, validated by quantitative real-time PCR, identified the differential expression of cellular genes associated with the NF-kappaB, PI3K/Akt, Jak/Stat and related Jak/Src pathways by virions lacking gB or gH but not gD. Gene induction occurred at a few particles per cell, corresponding to physiological conditions during primary infection. Reporter assay studies determined that NF-kappaB transcriptional activity is stimulated within an hour of HSV-1 binding, peaks between two and three hours post-binding and declines to background levels by five hours after induction. The immediate, transient nature of these signalling events suggests that HSV-1 glycoproteins, particularly gD, may alter the cellular environment pre-entry so as to condition the cell for viral replication.

Herpes simplex virus glycoproteins H/L bind to cells independently of {alpha}V{beta}3 integrin and inhibit virus entry, and their constitutive expression restricts infection

Herpes simplex virus (HSV) fusion with cells requires the gD, gB, and gH/gL glycoprotein quartet. gD serves as a receptor binding glycoprotein. gB and gH/gL execute fusion in an as-yet-unclear manner. To better understand the role of gH/gL in HSV entry, we produced a soluble version of gH/gL carrying a One-STrEP tag (gH(t.st)/gL). Previous findings implicated integrins as possible ligands to gH/gL (C. Parry et al., J. Gen. Virol. 86:7-10, 2005). We report that (i) gH(t.st)/gL bound a number of cells in a dose-dependent manner at concentrations similar to those required for the binding of soluble gB or gD. (ii) gH(t.st)/gL inhibited HSV entry at the same concentrations required for binding. It also inhibited cell-cell fusion in transfected cells. (iii) The absence of beta3 integrin did not prevent the binding of gH(t.st)/gL to CHO cells and infection inhibition. Conversely, integrin-negative K562 cells did not acquire the ability to bind gH(t.st)/gL when hyperexpressing alphaVbeta3 integrin. (iv) Constitutive expression of wild-type gH/gL (wt-gH/gL) restricted infection in all of the cell lines tested, a behavior typical of glycoproteins which bind cellular receptors. The extent of restriction broadly paralleled the efficiency of gH/gL transfection. RGD motif mutant gH/gL could not be differentiated from wt-gH with respect to restriction of infection. Cumulatively, the present results provide several lines of evidence that HSV gH/gL interacts with a cell surface cognate protein(s), that this protein is not necessarily an alphaVbeta3 integrin, and that this interaction is required for the process of virus entry/fusion.

Interaction of herpes simplex virus type 2 (HSV-2) glycoprotein D with the host cell surface is sufficient to induce Chlamydia trachomatis persistence

When presented with certain unfavourable environmental conditions, Chlamydia trachomatis reticulate bodies (RBs) enter into a viable, yet non-cultivable state called persistence. Previously, we established an in vitro C. trachomatis and herpes simplex virus type 2 (HSV-2) co-infection model. These data indicate that (i) viral co-infection stimulates chlamydial persistence, (ii) productive HSV replication is not required for persistence induction, and (iii) HSV-induced persistence is not mediated by any currently characterized anti-chlamydial pathway or persistence inducer. In this study we demonstrated that chlamydial infectivity, though initially suppressed, recovered within 44 h of co-infection with UV-inactivated HSV-2, demonstrating that HSV-induced persistence is reversible. Co-incubation of chemically fixed, HSV-2-infected inducer cells with viable, C. trachomatis-infected responder cells both suppressed production of infectious chlamydial progeny and stimulated formation of swollen, aberrantly shaped RBs. In addition, pre-incubation of viral particles with viral glycoprotein D (gD)-specific neutralizing antibody prevented co-infection-induced persistence. Finally, exposure of C. trachomatis-infected cells to a soluble, recombinant HSV-2 gD : Fc fusion protein decreased production of infectious EBs to a degree similar to that observed in co-infected cultures. Thus, we conclude that interaction of HSV gD with the host cell surface is sufficient to trigger a novel host anti-chlamydial response that restricts chlamydial development.

Glycoprotein D actively induces rapid internalization of two nectin-1 isoforms during herpes simplex virus entry

Entry of herpes simplex virus (HSV) occurs either by fusion at the plasma membrane or by endocytosis and fusion with an endosome. Binding of glycoprotein D (gD) to a receptor such as nectin-1 is essential in both cases. We show that virion gD triggered the rapid down-regulation of nectin-1 with kinetics similar to those of virus entry. In contrast, nectin-1 was not constitutively recycled from the surface of uninfected cells. Both the nectin-1alpha and beta isoforms were internalized in response to gD despite having different cytoplasmic tails. However, deletion of the nectin-1 cytoplasmic tail slowed down-regulation of nectin-1 and internalization of virions. These data suggest that nectin-1 interaction with a cytoplasmic protein is not required for its down-regulation. Overall, this study shows that gD binding actively induces the rapid internalization of various forms of nectin-1. We suggest that HSV activates a nectin-1 internalization pathway to use for endocytic entry.

Viral entry mechanisms: cellular and viral mediators of herpes simplex virus entry

Herpes simplex virus type-1 and type-2 are highly prevalent human pathogens causing life-long infections. The process of infection begins when the virions bind heparan sulfate moieties present on host cell surfaces. This initial attachment then triggers a cascade of molecular interactions involving multiple viral and host cell proteins and receptors, leading to penetration of the viral nucleocapsid and tegument proteins into the cytoplasm. The nucleocapsid is then transported to the nuclear membrane and the viral DNA is released for replication in the nucleus. Recent studies have revealed that herpes simplex virus entry or penetration into cells may be a highly complex process and the mechanism of entry may demonstrate unique cell-type specificities. Although specificities clearly exist, past and ongoing studies demonstrate that herpes simplex virus may share certain common receptors and pathways that are also used by many other human viruses. This minireview helps to shed light on recent revelations on the herpes simplex virus entry process.

Fusion of epithelial cells by Epstein-Barr virus proteins is triggered by binding of viral glycoproteins gHgL to integrins alphavbeta6 or alphavbeta8

Epstein-Barr virus (EBV) is a ubiquitous human herpesvirus that is causally implicated in the development of lymphoid and epithelial tumors. Entry of virus requires fusion of virus envelopes and cell membranes. Fusion with B lymphocytes requires virus glycoprotein gB and a 3-part complex of glycoproteins, gHgLgp42. It is triggered by interactions between glycoprotein 42 (gp42) and HLA class II. However, fusion with epithelial cells is impeded by gp42 and instead is triggered by interactions between an unknown epithelial protein and a 2-part complex of gHgL. We report here that gHgL binds with high affinity to epithelial cells and that affinity of binding is increased by 3 orders of magnitude in the presence of Mn(2+). Binding and infection can be reduced by fibronectin and vitronectin, by down-regulation of integrin alphav, or by a peptide corresponding to 13 aa of gH which include a KGDE motif. Fusion of cells expressing gB and gHgL can be blocked by vitronectin or triggered by addition of soluble truncated integrins alphavbeta6 and alphavbeta8. We conclude that the direct interaction between EBV gHgL and integrins alphavbeta6 and alphavbeta8 can provide the trigger for fusion of EBV with an epithelial cell.

Targeting human glioma cells using HSV-1 amplicon peptide display vector

Targeting cell infection using herpes simplex virus type 1 (HSV-1) vectors is a complicated issue as the process involves multiple interactions of viral envelope glycoproteins and cellular host surface proteins. In this study, we have inserted a human glioma-specific peptide sequence (denoted as MG11) into a peptide display HSV-1 amplicon vector replacing the heparan sulfate-binding domain of glycoprotein C (gC). The modified MG11:gC envelope recombinant vectors were subsequently packaged into virions in the presence of helper virus deleted for gC. Our results showed that the tropism of these HSV-1 recombinant virions was increased for human glioma cells in culture as compared with wild-type virions. The binding of these recombinant virions could also be blocked effectively by pre-incubating the cells with the glioma-specific peptide, indicating that MG11 peptide and the recombinant virions competed for the same or similar receptor-binding sites on the cell surface of human glioma cells. Furthermore, preferential homing of these virions was shown in xenograft glioma mouse model following intravascular delivery. Taken together, these results validated the hypothesis that HSV-1 binding to cells can be redirected to human gliomas through the incorporation of MG11 peptide sequence to the virions.

Insertional mutations in herpes simplex virus type 1 gL identify functional domains for association with gH and for membrane fusion

Glycoprotein L (gL) is one of four glycoproteins required for the entry of herpes simplex virus (HSV) into cells and for virus-induced cell fusion. This glycoprotein oligomerizes with gH to form a membrane-bound heterodimer but can be secreted when expressed without gH. Twelve unique gL linker-insertion mutants were generated to identify regions critical for gH binding and gH/gL processing and regions essential for cell fusion and viral entry. All gL mutants were detected on the cell surface in the absence of gH, suggesting incomplete cleavage of the signal peptide or the presence of a cell surface receptor for secreted gL. Coexpression with gH enhanced the levels of cell surface gL detected by antibodies for all gL mutants except those that were defective in their interactions with gH. Two insertions into a conserved region of gL abrogated the binding of gL to gH and prevented gH expression on the cell surface. Three other insertions reduced the cell surface expression of gH and/or altered the properties of gH/gL heterodimers. Altered or absent interaction of gL with gH was correlated with reduced or absent cell fusion activity and impaired complementation of virion infectivity. These results identify a conserved domain of gL that is critical for its binding to gH and two noncontiguous regions of gL, one of which contains the conserved domain, that are critical for the gH/gL complex to perform its role in membrane fusion.

Role of nectin-1, HVEM, and PILR-alpha in HSV-2 entry into human retinal pigment epithelial cells

PURPOSE

Herpes simplex virus-type 2 (HSV-2) can cause acute retinal necrosis (ARN), which can lead to exudative and rhegmatogenous retinal detachment, yet little is known about the cellular and molecular mechanisms of HSV-2 entry into retinal pigment epithelial (RPE) cells. The goal of this study was to establish the identity of the critical receptors used by the virus for infection.

METHODS

A reporter HSV-2 virus, which expresses beta-galactosidase, was used to quantify entry into RPE cells, and viral replication was ascertained using a plaque assay. Flow cytometry and immunocytochemistry were used to determine cellular expression of entry receptors. Localization of these receptors to the apical or basal surface of RPE cells was determined with immunocytochemistry. The necessity of these receptors, individually and in combination, for viral entry was established using receptor-specific antibodies and siRNAs.

RESULTS

RPE cells are highly susceptible to HSV-2 entry and replication. Several assays demonstrated the expression of the entry receptors nectin-1, HVEM, and PILR-alpha and their localization primarily to the apical surfaces of RPE cells. Receptor-specific antibodies and siRNA knockdown of receptors significantly reduced viral entry and implicated nectin-1 as an important receptor, with HVEM and PILR-alpha potentially also contributing to entry.

CONCLUSIONS

HSV-2 is capable of developing a productive infection in RPE cells by using nectin-1 as an important entry receptor. To lesser degrees, HVEM and PILR-alpha may also contribute to HSV-2 entry into RPE cells.

Kaposi's sarcoma-associated herpesvirus gH/gL: glycoprotein export and interaction with cellular receptors

The attachment, entry, and fusion of Kaposi's sarcoma-associated herpesvirus (KSHV) with target cells are mediated by complex machinery containing, among others, viral glycoprotein H (gH) and its alleged chaperone, gL. We observed that KSHV gH, in contrast to its homologues in several other herpesviruses, is transported to the cytoplasm membrane independently from gL, but not vice versa. Mutational analysis revealed that the N terminus of gH is sufficient for gL interaction. However, the entire extracellular part of gH is required for efficient gL secretion. The soluble ectodomain of gH was sufficient to interact with the surfaces of potential target cells in a heparin-dependent manner, and binding was further enhanced by coexpression of gL. Surface plasmon resonance revealed a remarkably high affinity of gH for glycosaminoglycans. Heparan sulfate (HS) proteoglycans of the syndecan family act as cellular receptors for the gH/gL complex. They promoted KSHV infection, and expression of gH/gL on target cells inhibited subsequent KSHV infection. Whereas gH alone was able to bind to HS, we observed that only the gH/gL complex adhered to heparan sulfate-negative cells at lamellipodium-like structures.

Equine herpesvirus 1 entry via endocytosis is facilitated by alphaV integrins and an RSD motif in glycoprotein D

Equine herpesvirus 1 (EHV-1) is a member of the Alphaherpesvirinae, and its broad tissue tropism suggests that EHV-1 may use multiple receptors to initiate virus entry. EHV-1 entry was thought to occur exclusively through fusion at the plasma membrane, but recently entry via the endocytic/phagocytic pathway was reported for Chinese hamster ovary cells (CHO-K1 cells). Here we show that cellular integrins, and more specifically those recognizing RGD motifs such as alphaVbeta5, are important during the early steps of EHV-1 entry via endocytosis in CHO-K1 cells. Moreover, mutational analysis revealed that an RSD motif in the EHV-1 envelope glycoprotein D (gD) is critical for entry via endocytosis. In addition, we show that EHV-1 enters peripheral blood mononuclear cells predominantly via the endocytic pathway, whereas in equine endothelial cells entry occurs mainly via fusion at the plasma membrane. Taken together, the data in this study provide evidence that EHV-1 entry via endocytosis is triggered by the interaction between cellular integrins and the RSD motif present in gD and, moreover, that EHV-1 uses different cellular entry pathways to infect important target cell populations of its natural host.

Integrin-dependent phagocytosis: spreading from microadhesion to new concepts

By linking actin dynamics to extracellular components, integrins are involved in a wide range of cellular processes that are associated with or require cytoskeletal remodelling and cell-shape changes. One such function is integrin-dependent phagocytosis, a process that several integrins are capable of mediating and that allows the binding and clearance of particles. Integrin-dependent phagocytosis is involved in a wide range of physiological processes, from the clearance of microorganisms and apoptotic-cell removal to extracellular-matrix remodelling. Integrin signalling is also exploited by microbial pathogens for entry into host cells. Far from being a particular property of specific integrins and specialised cells, integrin-dependent uptake is emerging as a general, intrinsic ability of most integrins that is associated with their capacity to signal to the actin cytoskeleton. Integrin-mediated phagocytosis can therefore be used as a robust model in which to study integrin regulation and signalling.

The murid herpesvirus-4 gH/gL binds to glycosaminoglycans

The first contact a virus makes with cells is an important determinant of its tropism. Murid Herpesvirus-4 (MuHV-4) is highly dependent on glycosaminoglycans (GAGs) for cell binding. Its first contact is therefore likely to involve a GAG-binding virion glycoprotein. We have previously identified two such proteins, gp70 and gp150. Gp70 binds strongly to GAGs. However, deleting it makes little difference to MuHV-4 cell binding or GAG-dependence. Deleting gp150, by contrast, frees MuHV-4 from GAG dependence. This implies that GAGs normally displace gp150 to allow GAG-independent cell binding. But the gp150 GAG interaction is weak, and so would seem unlikely to make an effective first contact. Since neither gp70 nor gp150 matches the expected profile of a first contact glycoprotein, our understanding of MuHV-4 GAG interactions must be incomplete. Here we relate the seemingly disconnected gp70 and gp150 GAG interactions by showing that the MuHV-4 gH/gL also binds to GAGs. gH/gL-blocking and gp70-blocking antibodies individually had little effect on cell binding, but together were strongly inhibitory. Thus, there was redundancy in GAG binding between gp70 and gH/gL. Gp150-deficient MuHV-4 largely resisted blocks to gp70 and gH/gL binding, consistent with its GAG independence. The failure of wild-type MuHV-4 to do the same argues that gp150 is normally engaged only down-stream of gp70 or gH/gL. MuHV-4 GAG dependence is consequently two-fold: gp70 or gH/gL binding provides virions with a vital first foothold, and gp150 is then engaged to reveal GAG-independent binding.

Characterization of the human cytomegalovirus gH/gL/UL128-131 complex that mediates entry into epithelial and endothelial cells

The entry of human cytomegalovirus (HCMV) into biologically relevant epithelial and endothelial cells involves endocytosis followed by low-pH-dependent fusion. This entry pathway is facilitated by the HCMV UL128, UL130, and UL131 proteins, which form one or more complexes with the virion envelope glycoprotein gH/gL. gH/gL/UL128-131 complexes appear to be distinct from the gH/gL/gO complex, which likely facilitates entry into fibroblasts. In order to better understand the assembly and protein-protein interactions of gH/gL/UL128-131 complexes, we generated HCMV mutants lacking UL128-131 proteins and nonreplicating adenovirus vectors expressing gH, gL, UL128, UL130, and UL131. Our results demonstrate that UL128, UL130, and UL131 can each independently assemble onto gH/gL scaffolds. However, the binding of individual UL128-131 proteins onto gH/gL can significantly affect the binding of other proteins; for example, UL128 increased the binding of both UL130 and UL131 to gH/gL. Direct interactions between gH/UL130, UL130/UL131, gL/UL128, and UL128/UL130 were also observed. The export of gH/gL complexes from the endoplasmic reticulum (ER) to the Golgi apparatus and cell surface was dramatically increased when all of UL128, UL130, and UL131 were coexpressed with gH/gL (with or without gO expression). Incorporation of gH/gL complexes into the virion envelope requires transport beyond the ER. Thus, we concluded that UL128, UL130, and UL131 must all bind simultaneously onto gH/gL for the production of complexes that can function in entry into epithelial and endothelial cells.

Herpes simplex virus type 2 entry into cultured human corneal fibroblasts is mediated by herpesvirus entry mediator

Herpes simplex virus type 2 (HSV-2) infections in the eye are becoming increasingly common in adults. The most likely point of entry for HSV-2 into the eye is through the cornea. By using primary cultures of human corneal fibroblasts (CFs), a natural target-cell type for infection, it was demonstrated that CFs are highly susceptible to HSV-2 entry and replication. RT-PCR and flow-cytometry analyses demonstrated expression of herpesvirus entry mediator (HVEM), a known mediator for HSV-2 entry into cells. Blocking of virus entry into CFs by anti-HVEM antibody implicated HVEM as a potential receptor for HSV-2 infection. These results indicate that HVEM may play a crucial role in HSV-2-induced corneal infections.

Equine herpesvirus 1 enters cells by two different pathways, and infection requires the activation of the cellular kinase ROCK1

Equine herpesvirus type 1 (EHV-1), a member of the Alphaherpesviridae, displays a broad host range in vitro, allowing for detailed study of the mechanisms of productive infection, including attachment and entry, in various cell culture systems. Previously, we showed that EHV-1 infects Chinese hamster ovary (CHO-K1) cells even though these cells do not express a known alphaherpesvirus entry receptor. In this report, we show by electron microscopy and an infectious recovery assay that entry into CHO-K1 cells occurs via an endocytic or phagocytic mechanism, while entry into equine dermal (ED) or rabbit kidney (RK13) cells occurs by direct fusion at the cell surface. In both cases (endocytic/phagocytic or direct fusion), entry leads to productive infection. Using drugs that inhibit clathrin-dependent or caveola-dependent endocytosis, we showed that EHV-1 entry into CHO-K1 cells does not require clathrin or caveolae. We also show that EHV-1 infection requires the activation of cell signaling molecules. In particular, we demonstrate that activation of the serine/threonine Rho kinase ROCK1 is critical for infection. Inhibition of this kinase by drugs or overexpression of a negative regulator of ROCK1 significantly blocked EHV-1 infection. These results show that EHV-1 can enter disparate cell types by at least two distinct mechanisms and that productive infection is dependent upon the activation of ROCK1.

Multiple receptor interactions trigger release of membrane and intracellular calcium stores critical for herpes simplex virus entry

Herpes simplex viruses (HSV) harness cellular calcium signaling pathways to facilitate viral entry. Confocal microscopy and small interfering RNA (siRNA) were used to identify the source of the calcium and to dissect the requisite viral-cell interactions. Binding of HSV to human epithelial cells induced no calcium response, but shifting the cells to temperatures permissive for penetration triggered increases in plasma membrane calcium followed by a global release of intracellular calcium. Transfection with siRNA targeting the proteoglycan syndecan-2 blocked viral binding and abrogated any calcium response. Transfection with siRNA targeting nectin-1, a glycoprotein D receptor, also prevented both membrane and intracellular calcium responses. In contrast, the membrane response was preserved after transfection with siRNA targeting integrinalphav, a novel glycoprotein H receptor. The membrane response, however, was not sufficient for viral entry, which required interactions with integrinalphav and release of inositol-triphosphate receptor-dependent intracellular calcium stores. Thus, calcium plays a critical, complex role in HSV entry.

Sialic acid on herpes simplex virus type 1 envelope glycoproteins is required for efficient infection of cells

Herpes simplex virus type 1 (HSV-1) envelope proteins are posttranslationally modified by the addition of sialic acids to the termini of the glycan side chains. Although gC, gD, and gH are sialylated, it is not known whether sialic acids on these envelope proteins are functionally important. Digestion of sucrose gradient purified virions for 4 h with neuraminidases that remove both alpha2,3 and alpha2,6 linked sialic acids reduced titers by 1,000-fold. Digestion with a alpha2,3-specific neuraminidase had no effect, suggesting that alpha2,6-linked sialic acids are required for infection. Lectins specific for either alpha2,3 or alpha2,6 linkages blocked attachment and infection to the same extent. In addition, the mobility of gH, gB, and gD in sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels was altered by digestion with either alpha2,3 specific neuraminidase or nonspecific neuraminidases, indicating the presence of both linkages on these proteins. The infectivity of a gC-1-null virus, DeltagC2-3, was reduced to the same extent as wild-type virus after neuraminidase digestion, and attachment was not altered. Neuraminidase digestion of virions resulted in reduced VP16 translocation to the nucleus, suggesting that the block occurred between attachment and entry. These results show for the first time that sialic acids on HSV-1 virions play an important role in infection and suggest that targeting virion sialic acids may be a valid antiviral drug development strategy.

Glycoprotein L disruption reveals two functional forms of the murine gammaherpesvirus 68 glycoprotein H

The herpesvirus glycoprotein H (gH) and gL associate to form a heterodimer that plays a central role in virus-driven membrane fusion. When archetypal alpha- or betaherpesviruses lack gL, gH misfolds and progeny virions are noninfectious. In order to define the role that gL plays in gamma-2 herpesvirus infections, we disrupted its coding sequence in murine gammaherpesvirus-68 (MHV-68). MHV-68 lacking gL folded gH into a conformation antigenically distinct from the form that normally predominates on infected cells. gL-deficient virions bound less well than the wild type to epithelial cells and fibroblasts. However, they still incorporated gH and remained infectious. The cell-to-cell spread of gL-deficient viruses was remarkably normal, as was infection, dissemination, and latency establishment in vivo. Viral membrane fusion was therefore gL independent. The major function of gL appeared to be allowing gH to participate in cell binding prior to membrane fusion. This function was most important for the entry of MHV-68 virions into fibroblasts and epithelial cells.

A novel role for phagocytosis-like uptake in herpes simplex virus entry

It is becoming increasingly clear that herpesviruses can exploit the endocytic pathway to infect cells, yet several important features of this process remain poorly defined. Using herpes simplex virus-1 (HSV-1) as a model, we demonstrate that endocytosis of the virions mimic many features of phagocytosis. During entry, HSV-1 virions associated with plasma membrane protrusions followed by a phagocytosis-like uptake involving rearrangement of actin cytoskeleton and trafficking of the virions in large phagosome-like vesicles. RhoA GTPase was activated during this process and the mode of entry was cell type-specific. Clathrin-coated vesicles had no detectable role in virion trafficking as Eps15 dominant-negative mutants failed to affect HSV-1 uptake. Binding and fusion of the virion envelope with the phagosomal membrane is likely facilitated by clustering of nectin-1 (or HVEM) in phagosomes, which was observed in infected cells. Collectively, our data suggests a novel mode of uptake by which the virus can infect both professional and nonprofessional phagocytes.

Role for 3-O-sulfated heparan sulfate as the receptor for herpes simplex virus type 1 entry into primary human corneal fibroblasts

Herpes simplex virus type 1 (HSV-1) infection of the corneal stroma remains a major cause of blindness. Primary cultures of corneal fibroblasts (CF) were tested and found susceptible to HSV-1 entry, which was confirmed by deconvolution imaging of infected cells. Plaque assay and real-time PCR demonstrated viral replication and hence a productive infection of CF by HSV-1. A role for glycoprotein D (gD) receptors in cultured CF was determined by gD interference assay. Reverse transcription-PCR analysis indicated expression of herpesvirus entry mediator and 3-O-sulfated (3-OS) heparan sulfate (HS)-generating enzyme 3-O sulfotransferase 3 (3-OST-3) but not nectin-1 or nectin-2. Subsequently, HS isolated from these cells was found to contain two distinct disaccharides (IdoUA2S-AnMan3S and IdoUA2S-AnMan3S6S) that are representative of 3-OST-3 activity. The following lines of evidence supported the important role of 3-OS HS as the mediator of HSV-1 entry into CF. (i) Blockage of entry was observed in CF treated with heparinases. The same enzymes had significantly less effect on HeLa cells that use nectin-1 as the entry receptor. (ii) Enzymatic removal of cell surface HS also removed the major gD-binding receptor, as evident from the reduced binding of gD to cells. (iii) Spinoculation assay demonstrated that entry blockage by heparinase treatment included the membrane fusion step. (iv) HSV-1 glycoprotein-induced cell-to-cell fusion was inhibited by either prior treatment of cells with heparinases or by HS preparations enriched in 3-OS HS. Taken together, the data in this report provide novel information on the role of 3-OS HS in mediating infection of CF, a natural target cell type.

Novel role for integrin-linked kinase in modulation of coxsackievirus B3 replication and virus-induced cardiomyocyte injury

Viral myocarditis is a major cause of sudden cardiac death in children and young adults. Among viruses, coxsackievirus B3 (CVB3) is the most common agent for myocarditis. Recently, more consideration has been given to the role of signaling pathways in pathogenesis of enteroviral myocarditis, providing new platform for identifying a new potential therapeutic target for this, so far, incurable disease. Previously, we reported on the role of the protein kinase-B/Akt in CVB3 replication and virus-induced cell injury. Here, we report on regulation of virus-induced Akt activation by the integrin-linked kinase in infected mouse cardiomyocytes and HeLa cells. This study also presents the first observation that inhibition of ILK in CVB3-infected cells significantly improves the viability of infected cells, while blocking viral replication and virus release. Complementary experiments using a constitutively active form of Akt1 revealed that the observed protective effect of ILK inhibition is dependent on the associated downregulation of virus-induced Akt activation. To our knowledge, this is the first report of such beneficial effects of ILK inhibition in a viral infection model and conveys new insights in our efforts to characterize a novel therapeutic target for treatment of enteroviral myocarditis.

Murine gammaherpesvirus-68 glycoprotein H-glycoprotein L complex is a major target for neutralizing monoclonal antibodies

Herpesviruses characteristically persist in immune hosts as latent genomes, but to transmit infection they must reactivate and replicate lytically. The interaction between newly formed virions and pre-existing antibody is therefore likely to be a crucial determinant of viral fitness. Murine gammaherpesvirus-68 (MHV-68) behaves as a natural pathogen of conventional, inbred mice and consequently allows such interactions to be analysed experimentally in a relatively realistic setting. Here, monoclonal antibodies (mAbs) were derived from MHV-68-infected mice and all those recognizing infected-cell surfaces were tested for their capacity to neutralize MHV-68 virions. All of the neutralizing mAbs identified were specific for the viral glycoprotein H (gH)-gL heterodimer and required both gH and gL to reproduce their cognate epitopes. Based on antibody interference, there appeared to be two major neutralization epitopes on gH-gL. Analysis of a representative mAb indicated that it blocked infection at a post-binding step--either virion endocytosis or membrane fusion.

A role for herpesvirus entry mediator as the receptor for herpes simplex virus 1 entry into primary human trabecular meshwork cells

The human eye is an important target for infection with herpes simplex virus 1 (HSV-1). Damage to cells forming the trabeculum of the eye by HSV-1 infection could contribute to the development of glaucoma, a major blinding disease. Primary cultures of human trabecular meshwork cells were used as an in vitro model to demonstrate the ability of HSV-1 to enter into and establish a productive infection of the trabeculum. Blocking of entry by anti-herpesvirus entry mediator (HVEM) antibody implicated HVEM as the major receptor for HSV-1 infection.

The amino terminus of Epstein-Barr virus glycoprotein gH is important for fusion with epithelial and B cells

Epstein-Barr virus (EBV) infects B lymphocytes and epithelial cells. While the glycoproteins required for entry into these two cell types differ, the gH/gL glycoprotein complex is essential for entry into both epithelial and B cells. Analysis of gH protein sequences from three gammaherpesviruses (EBV, marmoset, and rhesus) revealed a potential coiled-coil domain in the N terminus. Four leucines located in this region in EBV gH were replaced by alanines by site-directed mutagenesis and analyzed for cell-cell membrane fusion with B cells and epithelial cells. Reduction in fusion activity was observed for mutants containing L65A and/or L69A mutations, while substitutions in L55 and L74 enhanced the fusion activity of the mutant gH/gL complexes with both cell types. All of the mutants displayed levels of cell surface expression similar to those of wild-type gH and interacted with gL and gp42. The observation that a conservative mutation of leucine to alanine in the N terminus of EBV gH results in fusion-defective mutant gH/gL complexes is striking and points to an important role for this region in EBV-mediated membrane fusion with B lymphocytes and epithelial cells.

Spinoculation of heparan sulfate deficient cells enhances HSV-1 entry, but does not abolish the need for essential glycoproteins in viral fusion

Cell surface heparan sulfate functions as a co-receptor in HSV-1 entry. In order to study its significance in context with specific gD receptors (nectin-1, HVEM, and 3-O-sulfated heparan sulfate) a low speed centrifugation based virus inoculation (spinoculation) method was used. The experiments were performed at 1200 x g using glycosylaminoglycan positive (GAG+) or deficient (GAG-) cells expressing gD receptors. Clearly, spinoculation of GAG- nectin-1 or HVEM cells enhanced significantly viral entry compared to similar but unspun cells. The enhanced entry was due to increased virus deposition at the cell surface and not due to pelleting of the virus. Among the gD receptors, spinoculated GAG- HVEM cells showed restoration of HSV-1 entry compared to unspinoculated GAG+ HVEM cells. In contrast, spinoculated GAG- nectin-1 cells showed less entry than unspinoculated GAG+ nectin-1 cells. GAG- 3-O-sulfotransferase-expressing cells or heparinase treated GAG+ 3-O-sulfated heparan sulfate cells, in contrast, remained resistant to entry even after spinoculation. To investigate further, any potential effects of centrifugation on membrane fusion, a virus-free cell fusion assay was performed. Clearly, spinning had no effects on cell fusion, nor could it replace the need for all four essential glycoproteins. Taken together these results suggest that heparan sulfate plays a role of an attachment receptor, which could be substituted by spinoculation. This effect, however, varies with the gD receptor used, which in turn, could be used as a means for identifying gD receptor usage for entry into a cell type.

Focal Adhesion Kinase Plays a Pivotal Role in Herpes Simplex Virus Entry

Development of strategies to prevent herpes simplex virus (HSV) infection requires knowledge of cellular pathways harnessed by the virus for invasion. This study demonstrates that HSV induces rapid phosphorylation of focal adhesion kinase (FAK) in several human target cells and that phosphorylation is important for entry post-binding. Nuclear transport of the viral tegument protein VP16, transport of viral capsids to the nuclear pore, and downstream events (including expression of immediate-early genes and viral plaque formation) were substantially reduced in cells transfected with dominant-negative mutants of FAK or small interfering RNA designed to inhibit FAK expression. These observations were substantiated using mouse embryonic fibroblast cells derived from embryonic FAK-deficient mice. Infection was reduced by >90% in knockout cells relative to control cells and was further reduced if the knockout cells were transfected with small interfering RNA targeting proline-rich tyrosine kinase-2, which was also phosphorylated in response to HSV. The knockout cells were permissive for viral binding, and virus triggered an intracellular calcium response, but nuclear transport was inhibited. Together, these results support a novel model for invasion that implicates FAK phosphorylation as important for delivery of viral capsids to the nuclear pore.