Exploitation of Cellular Cytoskeletons and Signaling Pathways for Cell Entry by Kaposi's Sarcoma-Associated Herpesvirus and the Closely Related Rhesus Rhadinovirus

As obligate intracellular pathogens, viruses depend on the host cell machinery to complete their life cycle. Kaposi’s sarcoma-associated herpes virus (KSHV) is an oncogenicvirus causally linked to the development of Kaposi’s sarcoma and several other lymphoproliferative malignancies. KSHV entry into cells is tightly regulated by diverse viral and cellular factors. In particular, KSHV actively engages cellular integrins and ubiquitination pathways for successful infection. Emerging evidence suggests that KSHV hijacks both actin and microtubule cytoskeletons at different phases during entry into cells. Here, we review recent findings on the early events during primary infection of KSHV and its closely related primate homolog rhesus rhadinovirus with highlights on the regulation of cellular cytoskeletons and signaling pathways that are important for this phase of virus life cycle.

Inhibition of human respiratory syncytial virus infectivity by a dendrimeric heparan sulfate-binding peptide

Respiratory syncytial virus (RSV) interacts with cell surface heparan sulfate proteoglycans (HSPGs) to initiate infection. The interaction of RSV with HSPGs thus presents an attractive target for the development of novel inhibitors of RSV infection. In the present study, a minilibrary of linear, dimeric, and dendrimeric peptides containing clusters of basic amino acids was screened with the aim of identifying peptides able to bind HSPGs and thus block RSV attachment and infectivity. Of the compounds identified, the dendrimer SB105-A10 was the most potent inhibitor of RSV infectivity, with 50% inhibitory concentrations (IC(50)s) of 0.35 μM and 0.25 μM measured in Hep-2 and A549 cells, respectively. SB105-A10 was found to bind to both cell types via HSPGs, suggesting that its antiviral activity is indeed exerted by competing with RSV for binding to cell surface HSPGs. SB105-A10 prevented RSV infection when added before the viral inoculum, in line with its proposed HSPG-binding mechanism of action; moreover, antiviral activity was also exhibited when SB105-A10 was added postinfection, as it was able to reduce the cell-to-cell spread of the virus. The antiviral potential of SB105-A10 was further assessed using human-derived tracheal/bronchial epithelial cells cultured to form a pseudostratified, highly differentiated model of the epithelial tissue of the human respiratory tract. SB105-A10 strongly reduced RSV infectivity in this model and exhibited no signs of cytotoxicity or proinflammatory effects. Together, these features render SB105-A10 an attractive candidate for further development as a RSV inhibitor to be administered by aerosol delivery.

Specific interaction of the envelope glycoproteins E1 and E2 with liver heparan sulfate involved in the tissue tropismatic infection by hepatitis C virus

The first step in the process of infections by the hepatitis C virus (HCV) is attachment to the host cell, which is assumed to be mediated by interaction of the envelope glycoproteins E1 and E2 with cell surface glycosaminoglycans. In this study, a variety of glycosaminoglycans, heparan sulfate (HS) from various bovine tissues as well as chondroitin sulfate (CS)/dermatan sulfate from bovine liver, were used to examine the direct interaction with recombinant E1 and E2 proteins. Intriguingly, among HS preparations from various bovine tissues, only liver HS strongly bound to both E1 and E2. Since HS from liver, which is the target tissue of HCV, contains highly sulfated structures compared to HS from other tissues, the present results suggest that HS-proteoglycan on the liver cell surface appears to be one of the molecules that define the liver-specific tissue tropism of HCV infection. The interaction assay with chemically modified heparin derivatives provided evidence that the binding of the viral proteins to heparin/HS is not only mediated by simple ionic interactions, but that the 6-O-sulfation and N-sulfation are important. Heparin oligosaccharides equal to or larger than 10-mer were required to inhibit the binding. Notably, a highly sulfated CS-E preparation from squid cartilage also strongly interacted with both viral proteins and inhibited the entry of pseudotype HCV into the target cells, suggesting that the highly sulfated CS-E might be useful as an anti-HCV drug.

Herpes simplex virus glycoproteins gH/gL and gB bind Toll-like receptor 2, and soluble gH/gL is sufficient to activate NF-κB

A number of sentinels sense incoming herpes simplex virus (HSV) virions and initiate an immediate innate response. The first line of defense at the cell surface is TLR2 (Toll-like receptor 2), whose signature signaling activity leads to activation of the key transcription factor NF-κB. We report that the HSV pathogen-associated molecular patterns for TLR2 are the virion glycoproteins gH/gL and gB, which constitute the conserved fusion core apparatus across the members of the Herpesviridae family. Specifically, virions devoid singly of one of essential fusion glycoproteins (gD, gB, or gH null), able to attach to cells but defective in fusion/entry, were sufficient to elicit the first wave of NF-κB response to HSV. The most effective were the gD-null virions, positive for gH/gL and gB. A soluble form of gB, truncated upstream of the transmembrane sequence (gB(730t-st)), was produced in human cells and purified by means of a Strep tag. gH/gL and gB were each able to physically interact with TLR2 in coimmunoprecipitation assays, one independently of the other, yet gH(t-st)/gL, but not gB(730t-st), elicited an NF-κB response. Thus, whereas both HSV gH/gL and gB are ligands to TLR2, only gH/gL is sufficient to initiate a signaling cascade which leads to NF-κB activation.

A 3-O-sulfated heparan sulfate binding peptide preferentially targets herpes simplex virus 2-infected cells

Herpes simplex virus 2 (HSV-2) is the primary cause of genital herpes, which is one of the most common sexually transmitted viral infections worldwide and a major cofactor for human immunodeficiency virus infection. The lack of an effective vaccine or treatment and the emergence of drug-resistant strains highlight the need for developing new antivirals for HSV-2. Here, we demonstrate that a low-molecular-weight peptide isolated against 3-O-sulfated heparan sulfate (3-OS HS) can efficiently block HSV-2 infection. Treatment with the peptide inhibited viral entry and cell-to-cell spread both in vitro and in vivo using a mouse model of genital HSV-2 infection. Quite interestingly, the peptide showed a preferential binding to HSV-2-infected cells, with more than 200% increased binding compared to uninfected cells. Our additional results show that heparan sulfate expression is upregulated by 25% upon HSV-2 infection, which is a significant new finding that could be exploited for designing new diagnostic tests and treatment strategies against HSV-2-infected cells. In addition, our results also raise the possibility that 3-OS HS modifications within HS may be upregulated even more to accommodate for a significantly higher increase in the peptide binding to the infected cells.

Structure and orientation of the gH625-644 membrane interacting region of herpes simplex virus type 1 in a membrane mimetic system

Glycoprotein H (gH) of the herpes simplex virus type 1 is involved in the complex mechanism of membrane fusion of the viral envelope with host cells. The virus requires four glycoproteins (gB, gD, gH, gL) to execute fusion and the role played by gH remains mysterious. Mutational studies have revealed several regions of gH ectodomain required for fusion and identified the segment from amino acid 625 to 644 as the most fusogenic region. Here, we studied the behavior in a membrane-mimicking DPC micellar environment of a peptide encompassing this region (gH625-644) and determined its NMR solution structure and its orientation within the micelles.

Nonprofessional phagocytosis can facilitate herpesvirus entry into ocular cells

Phagocytosis is a major mechanism by which the mediators of innate immunity thwart microbial infections. Here we demonstrate that human herpesviruses may have evolved a common mechanism to exploit a phagocytosis-like entrapment to gain entry into ocular cells. While herpes simplex virus-1 (HSV-1) causes corneal keratitis, cytomegalovirus (CMV) is associated with retinitis in immunocompromised individuals. A third herpesvirus, human herpesvirus-8 (HHV-8), is crucial for the pathogenesis of Kaposi's sarcoma, a common AIDS-related tumor of eyelid and conjunctiva. Using laser scanning confocal microscopy, we show that successful infection of ocular cell types by all the three viruses, belonging to three divergent subfamilies of herpesviruses, is facilitated by induction of F-actin rich membrane protrusions. Inhibitors of F-actin polymerization and membrane protrusion formation, cytochalasin D and latrunculin B, were able to block infection by all three viruses. Similar inhibition was seen by blocking phosphoinositide 3 kinase signaling, which is required for microbial phagocytosis. Transmission electron microscopy data using human corneal fibroblasts for HSV-1, human retinal pigment epithelial cells for CMV, and human conjunctival epithelial cells for HHV-8 are consistent with the possibility that pseudopod-like membrane protrusions facilitate virus uptake by the ocular cells. Our findings suggest a novel mechanism by which the nonprofessional mediators of phagocytosis can be infected by human herpesviruses.

Herpes B virus utilizes human nectin-1 but not HVEM or PILRα for cell-cell fusion and virus entry

To investigate the requirements of herpesvirus entry and fusion, the four homologous glycoproteins necessary for herpes simplex virus (HSV) fusion were cloned from herpes B virus (BV) (or macacine herpesvirus 1, previously known as cercopithecine herpesvirus 1) and cercopithecine herpesvirus 2 (CeHV-2), both related simian simplexviruses belonging to the alphaherpesvirus subfamily. Western blots and cell-based enzyme-linked immunosorbent assay (ELISA) showed that glycoproteins gB, gD, and gH/gL were expressed in whole-cell lysates and on the cell surface. Cell-cell fusion assays indicated that nectin-1, an HSV-1 gD receptor, mediated fusion of cells expressing glycoproteins from both BV and CeHV-2. However, herpesvirus entry mediator (HVEM), another HSV-1 gD receptor, did not facilitate BV- and CeHV-2-induced cell-cell fusion. Paired immunoglobulin-like type 2 receptor alpha (PILRα), an HSV-1 gB fusion receptor, did not mediate fusion of cells expressing glycoproteins from either simian virus. Productive infection with BV was possible only with nectin-1-expressing cells, indicating that nectin-1 mediated entry while HVEM and PILRα did not function as entry receptors. These results indicate that these alphaherpesviruses have differing preferences for entry receptors. The usage of the HSV-1 gD receptor nectin-1 may explain interspecies transfer of the viruses, and altered receptor usage may result in altered virulence, tropism, or pathogenesis in the new host. A heterotypic cell fusion assay resulting in productive fusion may provide insight into interactions that occur to trigger fusion. These findings may be of therapeutic significance for control of deadly BV infections.

Semi-synthetic heparinoids

New chemical-enzymatic technology based on the modification of the bacterial polysaccharide K5 from Escherichia coli leads to the synthesis of a number of heparin/heparan sulfate-like molecules with different biological activities. With this technology, two families of sulfated compounds were synthesized, which differ in their uronic acid content. The first group contains only glucuronic acid, whereas the second group contains about 50% iduronic acid following epimerization by immobilized recombinant C5 epimerase. This has led to the development of various anticoagulant and nonanticoagulant K5 derivatives endowed with different - and sometimes highly specific - antitumor, antiviral, and/or anti-inflammatory activities.

Bovine herpesvirus type 4 glycoprotein L is nonessential for infectivity but triggers virion endocytosis during entry

The core entry machinery of mammalian herpesviruses comprises glycoprotein B (gB), gH, and gL. gH and gL form a heterodimer with a central role in viral membrane fusion. When archetypal alpha- or betaherpesviruses lack gL, gH misfolds and progeny virions are noninfectious. However, the gL of the rhadinovirus murid herpesvirus 4 (MuHV-4) is nonessential for infection. In order to define more generally what role gL plays in rhadinovirus infections, we disrupted its coding sequence in bovine herpesvirus 4 (BoHV-4). BoHV-4 lacking gL showed altered gH glycosylation and incorporated somewhat less gH into virions but remained infectious. However, gL(-) virions showed poor growth associated with an entry deficit. Moreover, a major part of their entry defect appeared to reflect impaired endocytosis, which occurs upstream of membrane fusion itself. Thus, the rhadinovirus gL may be more important for driving virion endocytosis than for incorporating gH into virions, and it is nonessential for membrane fusion.

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

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

Site-specific proteolytic cleavage of the amino terminus of herpes simplex virus glycoprotein K on virion particles inhibits virus entry

Herpes simplex virus 1 (HSV-1) glycoprotein K (gK) is expressed on virions and functions in entry, inasmuch as HSV-1(KOS) virions devoid of gK enter cells substantially slower than is the case for the parental KOS virus (T. P. Foster, G. V. Rybachuk, and K. G. Kousoulas, J. Virol. 75:12431-12438, 2001). Deletion of the amino-terminal 68-amino-acid (aa) portion of gK caused a reduction in efficiency and kinetics of virus entry similar to that of the gK-null virus in comparison to the HSV-1(F) parental virus. The UL20 membrane protein and gK were readily detected on double-gradient-purified virion preparations. Immuno-electron microscopy confirmed the presence of gK and UL20 on purified virions. Coimmunoprecipitation experiments using purified virions revealed that gK interacted with UL20, as has been shown in virus-infected cells (T. P. Foster, V. N. Chouljenko, and K. G. Kousoulas, J. Virol. 82:6310-6323, 2008). Scanning of the HSV-1(F) viral genome revealed the presence of a single putative tobacco etch virus (TEV) protease site within gD, while additional TEV predicted sites were found within the UL5 (helicase-primase helicase subunit), UL23 (thymidine kinase), UL25 (DNA packaging tegument protein), and UL52 (helicase-primase primase subunit) proteins. The recombinant virus gDΔTEV was engineered to eliminate the single predicted gD TEV protease site without appreciably affecting its replication characteristics. The mutant virus gK-V5-TEV was subsequently constructed by insertion of a gene sequence encoding a V5 epitope tag in frame with the TEV protease site immediately after gK amino acid 68. The gK-V5-TEV, R-gK-V5-TEV (revertant virus), and gDΔTEV viruses exhibited similar plaque morphologies and replication characteristics. Treatment of the gK-V5-TEV virions with TEV protease caused approximately 32 to 34% reduction of virus entry, while treatment of gDΔTEV virions caused slightly increased virus entry. These results provide direct evidence that the gK and UL20 proteins, which are genetically and functionally linked to gB-mediated virus-induced cell fusion, are structural components of virions and function in virus entry. Site-specific cleavage of viral glycoproteins on mature and fully infectious virions utilizing unique protease sites may serve as a generalizable method of uncoupling the roles of viral glycoproteins in virus entry and virion assembly.

Antibody-mediated targeted gene transfer of helper virus-free HSV-1 vectors to rat neocortical neurons that contain either NMDA receptor 2B or 2A subunits

Because of the numerous types of neurons in the brain, and particularly the forebrain, neuron type-specific expression will benefit many potential applications of direct gene transfer. The two most promising approaches for achieving neuron type-specific expression are targeted gene transfer to a specific type of neuron and using a neuron type-specific promoter. We previously developed antibody-mediated targeted gene transfer with Herpes Simplex Virus (HSV-1) vectors by modifying glycoprotein C (gC) to replace the heparin binding domain, which mediates the initial binding of HSV-1 particles to many cell types, with the Staphylococcus A protein ZZ domain, which binds immunoglobulin (Ig) G. We showed that a chimeric gC-ZZ protein is incorporated into vector particles and binds IgG. As a proof-of-principle for antibody-mediated targeted gene transfer, we isolated complexes of these vector particles and an anti-NMDA NR1 subunit antibody, and demonstrated targeted gene transfer to neocortical cells that contain NR1 subunits. However, because most forebrain neurons contain NR1, we obtained only a modest increase in the specificity of gene transfer, and this targeting specificity is of limited utility for physiological experiments. Here, we report efficient antibody-mediated targeted gene transfer to NMDA NR2B- or NR2A-containing cells in rat postrhinal cortex, and a neuron-specific promoter further restricted recombinant expression to neurons. Of note, because NR2A-containing neurons are relatively rare, these results show that antibody-mediated targeted gene transfer with HSV-1 vectors containing neuron type-specific promoters can restrict recombinant expression to specific types of forebrain neurons of physiological significance.

An important role for syndecan-1 in herpes simplex virus type-1 induced cell-to-cell fusion and virus spread

Herpes simplex virus type-1 (HSV-1) is a common human pathogen that relies heavily on cell-to-cell spread for establishing a lifelong latent infection. Molecular aspects of HSV-1 entry into host cells have been well studied; however, the molecular details of the spread of the virus from cell-to-cell remain poorly understood. In the past, the role of heparan sulfate proteoglycans (HSPG) during HSV-1 infection has focused solely on the role of HS chains as an attachment receptor for the virus, while the core protein has been assumed to perform a passive role of only carrying the HS chains. Likewise, very little is known about the involvement of any specific HSPGs in HSV-1 lifecycle. Here we demonstrate that a HSPG, syndecan-1, plays an important role in HSV-1 induced membrane fusion and cell-to-cell spread. Interestingly, the functions of syndecan-1 in fusion and spread are independent of the presence of HS on the core protein. Using a mutant CHO-K1 cell line that lacks all glycosaminoglycans (GAGs) on its surface (CHO-745) we demonstrate that the core protein of syndecan-1 possesses the ability to modulate membrane fusion and viral spread. Altogether, we identify a new role for syndecan-1 in HSV-1 pathogenesis and demonstrate HS-independent functions of its core protein in viral spread.

Virostatic potential of micro-nano filopodia-like ZnO structures against herpes simplex virus-1

Herpes simplex virus type-1 (HSV-1) entry into target cell is initiated by the ionic interactions between positively charged viral envelop glycoproteins and a negatively charged cell surface heparan sulfate (HS). This first step involves the induction of HS-rich filopodia-like structures on the cell surface that facilitate viral transport during cell entry. Targeting this initial first step in HSV-1 pathogenesis, we generated different zinc oxide (ZnO) micro-nano structures (MNSs) that were capped with multiple nanoscopic spikes mimicking cell induced filopodia. These MNSs were predicted to target the virus to compete for its binding to cellular HS through their partially negatively charged oxygen vacancies on their nanoscopic spikes, to affect viral entry and subsequent spread. Our results demonstrate that the partially negatively charged ZnO-MNSs efficiently trap the virions via a novel virostatic mechanism rendering them unable to enter into human corneal fibroblasts - a natural target cell for HSV-1 infection. The anti-HSV-1 activity of ZnO MNSs was drastically enhanced after creating additional oxygen vacancies under UV-light illumination. Our results provide a novel insight into the significance of ZnO MNSs as the potent HSV-1 inhibitor and rationalize their development as a novel topical agent for the prevention of HSV-1 infection.

Low-pH-dependent changes in the conformation and oligomeric state of the prefusion form of herpes simplex virus glycoprotein B are separable from fusion activity

The cellular requirements for activation of herpesvirus fusion and entry remain poorly understood. Low pH triggers change in the antigenic reactivity of the prefusion form of the herpes simplex virus (HSV) fusion protein gB in virions, both in vitro and during viral entry via endocytosis (S. Dollery et al., J. Virol. 84:3759-3766, 2010). However, the mechanism and magnitude of gB conformational change are not clear. Here we show that the conformation and oligomeric state of gB with mutations in the bipartite fusion loops were similarly altered despite the fusion-inactivating mutations. Together with previous studies, this suggests that fusion loop mutants undergo conformational changes but are defective for fusion because they fail to make productive contact with the outer leaflet of the host target membrane. A direct, reversible effect of low pH on the structure of gB was detected by fluorescence spectroscopy. A soluble form of gB containing cytoplasmic tail sequences (s-gB) was triggered by mildly acidic pH to undergo changes in tryptophan fluorescence emission, hydrophobicity, antigenic conformation, and oligomeric structure and thus resembled the prefusion form of gB in the virion. In contrast, soluble gB730, for which the postfusion crystal structure is known, was only marginally affected by pH using these measures. The results underscore the importance of using a prefusion form of gB to assess the activation and extent of conformation change. Further, acidic pH had little to no effect on the conformation or hydrophobicity of gD or on gD's ability to bind nectin-1 or HVEM receptors. Our results support a model in which endosomal low pH serves as a cellular trigger of fusion by activating conformational changes in the fusion protein gB.

Herpes simplex virus type-1 (HSV-1) entry into human mesenchymal stem cells is heavily dependent on heparan sulfate

Hematopoietic stem cells recipients remain susceptible to opportunistic viral infections including herpes simplex virus type-1 (HSV-1). The purpose of this investigation was to analyze susceptibility of human mesenchymal stem cells (hMSCs) to HSV-1 infection and identify the major entry receptor. Productive virus infection in hMSCs was confirmed by replication and plaque formation assays using a syncytial HSV-1 KOS (804) virus. To examine the significance of entry receptors, RT-PCR and antibody-blocking assays were performed. RT-PCR data showed the expression of gD receptors: nectin-1, 3-O sulfotransferase-3 (3-OST-3), and HVEM. Antibody-blocking assay together with heparinase treatment suggested an important role for HS and 3-O-sulfated heparan sulfate (3-OS HS), but not nectin-1 or HVEM, in mediating HSV-1 entry and spread in hMSCs. Taken together, our results provide strong evidence demonstrating that HSV-1 is capable of infecting hMSCs and HS and 3-OS HS serve as its entry receptors during this process.

Anti-heparan sulfate peptides that block herpes simplex virus infection in vivo

Heparan sulfate (HS) and its highly modified form, 3-O-sulfated heparan sulfate (3-OS HS), contribute strongly to herpes simplex virus type-1 (HSV-1) infection in vitro. Here we report results from a random M13-phage display library screening to isolate 12-mer peptides that bind specifically to HS, 3-OS HS, and block HSV-1 entry. The screening identified representative candidates from two-different groups of anti-HS peptides with high positive charge densities. Group 1, represented by G1 peptide (LRSRTKIIRIRH), belongs to a class with alternating charges (XRXRXKXXRXRX), and group 2, represented by G2 peptide (MPRRRRIRRRQK), shows repetitive charges (XXRRRRXRRRXK). Viral entry and glycoprotein D binding assays together with fluorescent microscopy data indicated that both G1 and G2 were potent in blocking HSV-1 entry into primary cultures of human corneal fibroblasts and CHO-K1 cells transiently expressing different glycoprotein D receptors. Interestingly, G2 peptide isolated against 3-OS HS displayed wider ability to inhibit entry of clinically relevant strains of HSV-1 and some divergent members of herpesvirus family including cytomegalovirus and human herpesvirus-8. To identify functional residues within G1 and G2, we performed point mutations and alanine-scanning mutagenesis. Several arginine and a lysine residues were needed for anti-HSV-1 activity, suggesting the importance of the positively charged residues in virus-cell binding and virus-induced membrane fusion. In vivo administration of G1 or G2 peptide as a prophylactic eye drop completely blocked HSV-1 spread in the mouse cornea as evident by immunohistochemistry. This result also highlights an in vivo significance of HS and 3-OS HS during ocular herpes infection.

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.

Glycosaminoglycans mediate retention of the poxvirus type I interferon binding protein at the cell surface to locally block interferon antiviral responses

Eradication of smallpox was accomplished 30 yr ago, but poxviral infections still represent a public health concern due to the potential release of variola virus or the emergence of zoonotic poxviruses, such as monkeypox virus. A critical determinant of poxvirus virulence is the inhibition of interferons (IFNs) by the virus-encoded type I IFN-binding protein (IFNα/βBP). This immunomodulatory protein is secreted and has the unique property of interacting with the cell surface in order to prevent IFN-mediated antiviral responses. However, the mechanism of its attachment to the cell surface remains unknown. Using surface plasmon resonance and cell-binding assays, we report that the IFNα/βBP from vaccinia virus, the smallpox vaccine, interacts with cell surface glycosaminoglycans (GAGs). Analysis of the contribution of different regions of the protein to cell surface binding demonstrated that clusters of basic residues in the first immunoglobulin domain mediate GAG interactions. Furthermore, mutation of the GAG-interaction motifs does not affect its IFN-binding and -blocking capacity. Functional conservation of GAG-binding sites is demonstrated for the IFNα/βBP from variola and monkeypox viruses, extending our understanding of immune modulation by the most virulent human poxviruses. These results are relevant for the design of improved vaccines and intervention strategies.

Syndecan-1 and syndecan-2 play key roles in herpes simplex virus type-1 infection

Herpes simplex virus type 1 (HSV-1) is an important human pathogen and a leading cause of infectious blindness in the developed world. HSV-1 exploits heparan sulfate proteoglycans (HSPG) for attachment to cells. While the significance of heparan sulphate (HS) moieties in HSV-1 infection is well established, the role of specific proteoglycan core proteins in the infection process remains poorly understood. The objective of this study was to assess the roles of syndecan-1 and syndecan-2 core proteins in HSV-1 infection, both of which are expressed by many HSV-1 target cell types. Our results demonstrate that syndecan-1 and syndecan-2 gene silencing by RNA interference reduces HSV-1 entry, plaque formation and facilitates cell survival. Furthermore, HSV-1 infection increases syndecan-1 and syndecan-2 protein synthesis and a resultant increase in cell surface expression of HS. Our observations suggest that changes in syndecan-1 and syndecan-2 expression levels may be related to active viral infection. Taken together, our findings provide new insights into HSPG functions during HSV-1 entry and spread.

Phosphoinositide 3 kinase signalling may affect multiple steps during herpes simplex virus type-1 entry

Early interactions of herpes simplex virus type-1 (HSV-1) with cells lead to cytoskeletal changes facilitating filopodia formation and membrane fusion. Here, we demonstrate that phosphoinositide 3 kinase (PI3K) signalling may affect multiple steps during HSV-1 entry. An inhibitor of PI3K (LY294002) blocked HSV-1 entry and the blockage was cell-type- and gD receptor-independent. Entry inhibition was also observed with primary cultures of the human corneal fibroblasts and unrelated β- and γ-herpesviruses. Immunofluorescence analysis demonstrated that LY294002 negatively affected HSV-1-induced filopodia formation. Similar effects of the inhibitor were seen on HSV-1 glycoprotein-induced cell-to-cell fusion. Cells expressing HSV-1 glycoproteins (gB, gD, gH and gL) showed significantly less fusion with target cells in the presence of the inhibitor. Expression of a dominant-negative PI3K mutant negatively affected both entry and fusion. We also show that inhibition of PI3K signalling also affected RhoA activation required for HSV-1 entry into certain cell types.

Zebrafish-encoded 3-O-sulfotransferase-3 isoform mediates herpes simplex virus type 1 entry and spread

Heparan sulfate proteoglycans modified by human glucosaminyl 3-O-sulfotransferase-3 (3-OST-3) isoform generates the cellular receptor for herpes simplex virus type 1 (HSV-1). Interestingly, the ability of zebrafish (ZF)-encoded 3-OST-3 isoform to modify heparan sulfate to mediate HSV-1 entry and cell-cell fusion has not been determined although it is predominantly expressed in ZF, a popular model organism to study viral infections. Here, we demonstrate that expression of ZF-encoded 3-OST-3 isoform renders the resistant Chinese hamster ovary (CHO-K1) cells to become susceptible for HSV-1 entry. The following lines of evidence support the important role of ZF-encoded 3-OST-3 isoform as the mediator of HSV-1 entry into CHO-K1 cells: (1) ZF 3-OST-3-expressing CHO-K1 cells were able to preferentially bind HSV-1 glycoprotein D, and (2) CHO-K1 cells expressing ZF-encoded 3-OST-3 acquire the ability to fuse with cells expressing HSV-1 glycoproteins. Finally, knocking down 3-OST-3 receptor by siRNA in ZF fibroblasts cells significantly reduced HSV-1 entry and glycoprotein D binding to cells. Taken together, our results provide novel insight into the significance of ZF 3-OST-3 isoform as an HSV-1 entry and fusion receptor and its potential involvement in the HSV-1 disease model of ZF.

In vitro antiviral activity of neem (Azardirachta indica L.) bark extract against herpes simplex virus type-1 infection

Herpes simplex virus type 1 (HSV-1) causes significant health problems from periodical skin and corneal lesions to encephalitis. We report here that an aqueous extract preparation from the barks of neem plant Azardirachta indica acts as a potent entry inhibitor against HSV-1 infection into natural target cells. The neem bark extract (NBE) significantly blocked HSV-1 entry into cells at concentrations ranging from 50 to 100 microg/ml. The blocking activity of NBE was observed when the extract was pre-incubated with the virus but not with the target cells, suggesting a direct antiHSV-1 property of the neem bark. Further, virions treated with NBE failed to bind the cells which implicate a role of NBE as an attachment step blocker. Cells treated with NBE also inhibited HSV-1 glycoprotein-mediated cell-cell fusion and polykaryocytes formation suggesting an additional role of NBE at the viral fusion step. These findings open a potential new avenue for the development of NBE as a novel antiherpetic microbicide.

A double mutation in glycoprotein gB compensates for ineffective gD-dependent initiation of herpes simplex virus type 1 infection

Herpes simplex virus (HSV) entry into cells is triggered by the binding of envelope glycoprotein D (gD) to a specific receptor, such as nectin-1 or herpesvirus entry mediator (HVEM), resulting in activation of the fusion effectors gB and gH and virus penetration. Here we report the identification of a hyperactive gB allele, D285N/A549T, selected by repeat passage of a gD mutant virus defective for nectin-1 binding through cells that express a gD-binding-impaired mutant nectin-1. The gB allele in a wild-type virus background enabled the use of other nectins as virus entry receptors. In addition, combination of the mutant allele with an epidermal growth factor receptor (EGFR)-retargeted gD gene yielded dramatically increased EGFR-specific virus entry compared to retargeted virus carrying wild-type gB. Entry of the gB mutant virus into nectin-1-bearing cells was markedly accelerated compared to that of wild-type virus, suggesting that the gB mutations affect a rate-limiting step in entry. Our observations indicate that ineffective gD activation can be complemented by hypersensitization of a downstream component of the entry cascade to gD signaling.

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.

Antibody-mediated targeted gene transfer to NMDA NR1-containing neurons in rat neocortex by helper virus-free HSV-1 vector particles containing a chimeric HSV-1 glycoprotein C-staphylococcus A protein

Because of the heterogeneous cellular composition of the brain, and especially the forebrain, cell type-specific expression will benefit many potential applications of direct gene transfer. The two prevalent approaches for achieving cell type-specific expression are using a cell type-specific promoter or targeting gene transfer to a specific cell type. Targeted gene transfer with Herpes Simplex Virus (HSV-1) vectors modifies glycoprotein C (gC) to replace the heparin binding domain, which binds to many cell types, with a binding activity for a specific cell surface protein. We previously reported targeted gene transfer to nigrostriatal neurons using chimeric gC-glial cell line-derived neurotrophic factor or gC-brain-derived neurotrophic factor protein. Unfortunately, this approach is limited to cells that express the cognate receptor for either neurotrophic factor. Thus, a general strategy for targeting gene transfer to many different types of neurons is desirable. Antibody-mediated targeted gene transfer has been developed for targeting specific virus vectors to specific peripheral cell types; a specific vector particle protein is modified to contain the Staphylococcus A protein ZZ domain, which binds immunoglobulin (Ig) G. Here, we report antibody-mediated targeted gene transfer of HSV-1 vectors to a specific type of forebrain neuron. We constructed a chimeric gC-ZZ protein, and showed this protein is incorporated into vector particles and binds Ig G. Complexes of these vector particles and an antibody to the NMDA receptor NR1 subunit supported targeted gene transfer to NR1-containing neocortical neurons in the rat brain, with long-term (2 months) expression.

Mediators and mechanisms of herpes simplex virus entry into ocular cells

The entry of herpes simplex virus into cells was once thought to be a general process. It is now understood that the virus is able to use multiple mechanisms for entry and spread, including the use of receptors and co-receptors that have been determined to be cell-type specific. This is certainly true for ocular cell types, which is important as the virus may use different mechanisms to gain access to multiple anatomic structures in close proximity, leading to various ocular diseases. There are some patterns that may be utilized by the virus in the eye and elsewhere, including surfing along filopodia in moving from cell to cell. There are common themes as well as intriguing differences in the entry mechanisms of herpes simplex virus into ocular cells. We discuss these issues in the context of conjunctivitis, keratitis, acute retinal necrosis, and other ocular diseases.

Identification of a dendrimeric heparan sulfate-binding peptide that inhibits infectivity of genital types of human papillomaviruses

Peptide dendrimers consist of a peptidyl branching core and/or covalently attached surface functional units. They show a variety of biological properties, including antiviral activity. In this study, a minilibrary of linear, dimeric, and dendrimeric peptides containing clusters of basic amino acids was evaluated for in vitro activity against human papillomaviruses (HPVs). The peptide dendrimer SB105-A10 was found to be a potent inhibitor of genital HPV types (i.e., types 16, 18, and 6) in pseudovirus-based neutralization assays. The 50% inhibitory concentration was between 2.8 and 4.2 μg/ml (0.59 and 0.88 μM), and no evidence of cytotoxicity was observed. SB105-A10 interacts with immobilized heparin and with heparan sulfates exposed on the cell surface, most likely preventing virus attachment. The findings from this study indicate SB105-A10 to be a leading candidate compound for further development as an active ingredient of a topical microbicide against HPV and other sexually transmitted viral infections.

The Ig-like v-type domain of paired Ig-like type 2 receptor alpha is critical for herpes simplex virus type 1-mediated membrane fusion

Paired immunoglobulin (Ig)-like type 2 receptor alpha (PILRalpha) and PILRbeta are paired receptors that are highly homologous to each other. When engaged by ligand, PILRalpha is inhibitory whereas PILRbeta is activating. PILRalpha is a newly identified herpes simplex virus type 1 (HSV-1) glycoprotein B (gB) receptor and is associated with membrane fusion and entry activity of HSV-1. PILRalpha is a 303-amino-acid protein with an Ig-like V (variable)-type domain from amino acid 31 to 150, whereas PILRbeta is a 217-amino-acid protein with an Ig-like V-type domain from amino acid 21 to 143. We report that PILRbeta is not a receptor for HSV-1 and HSV-2. Domain swaps between PILRalpha and PILRbeta reveal that the Ig-like V-type domain of PILRalpha, but not PILRbeta, plays a critical role in cell membrane fusion activity and the binding of PILRalpha to gB. Individual replacement of 13 amino acids in PILRalpha showed that most of these mutations had no effect on cell fusion activity. However, mutation of the tryptophan residue at amino acid 139 significantly impaired cell fusion activity for HSV-1 and eliminated binding to gB.

Expression of herpes virus entry mediator (HVEM) in the cornea and trigeminal ganglia of normal and HSV-1 infected mice

PURPOSE

Herpes virus entry mediator (HVEM) plays a critical role in the regulation of inflammation through interaction with its natural ligands LIGHT and lymphotoxin alpha and also serves as one of the entry receptors of herpes simplex virus (HSV). The purpose of this study was to better understand the expression of HVEM in the cornea and trigeminal ganglia (TG), which are important targets of HSV infection.

MATERIALS AND METHODS

Immunohistochemistry was used to define HVEM expression in the cornea and TG of normal and HSV-1 infected mice euthanized 2 to 5 days or 7 months following corneal inoculation of virus.

RESULTS

We found that HVEM is widely expressed in the normal corneal epithelium and endothelium, is weakly and focally expressed in the corneal stroma, and is expressed in a portion of neurons and non-neuronal cells in the TG. Acute HSV-1 keratitis and ganglionitis were associated with increased HVEM expression in the corneal epithelium and stroma and in neurons and non-neuronal cells of TG, and many inflammatory cells in these tissues also expressed HVEM. TG derived from mice 7 months after virus inoculation demonstrated latent HSV-1 infection that was associated with increased HVEM expression in neurons and non-neuronal cells relative to uninfected control tissues. Latent TG also contained focal infiltrates of mononuclear inflammatory cells, many of which expressed HVEM. Corneas derived from latently infected mice demonstrated chronic keratitis, with no evidence of virus replication or increased HVEM expression in the corneal epithelium, and inflammatory cells present showed only weak HVEM expression.

CONCLUSIONS

HVEM is expressed in the cornea and TG and therefore may serve as an HSV entry receptor in these tissues. Furthermore, these findings raise the possibility that changes in HVEM expression following ocular HSV-1 infection can modulate HSV spread and infection-induced inflammation in the cornea and TG.

Genome-wide identification of susceptibility alleles for viral infections through a population genetics approach

Viruses have exerted a constant and potent selective pressure on human genes throughout evolution. We utilized the marks left by selection on allele frequency to identify viral infection-associated allelic variants. Virus diversity (the number of different viruses in a geographic region) was used to measure virus-driven selective pressure. Results showed an excess of variants correlated with virus diversity in genes involved in immune response and in the biosynthesis of glycan structures functioning as viral receptors; a significantly higher than expected number of variants was also seen in genes encoding proteins that directly interact with viral components. Genome-wide analyses identified 441 variants significantly associated with virus-diversity; these are more frequently located within gene regions than expected, and they map to 139 human genes. Analysis of functional relationships among genes subjected to virus-driven selective pressure identified a complex network enriched in viral products-interacting proteins. The novel approach to the study of infectious disease epidemiology presented herein may represent an alternative to classic genome-wide association studies and provides a large set of candidate susceptibility variants for viral infections.

Viruses have represented a constant threat to human communities throughout their history, therefore, human genes involved in anti-viral response can be thought of as targets of virus-driven selective pressure. Here we utilized the marks left by selection to identify viral infection-associated allelic variants. We analyzed more than 660,000 single nucleotide polymorphisms (SNPs) genotyped in 52 human populations, and we used virus diversity (the number of different viruses in a geographic region) to measure virus-driven selective pressure. Results showed that genes involved in immune response and in the biosynthesis of glycan structures functioning as viral receptors display more variants associated with virus diversity than expected by chance. The same holds true for genes encoding proteins that directly interact with viral components. Genome-wide analysis identified 441 variants, mapping to 139 human genes, significantly associated with virus-diversity. We analyzed the functional relationships among genes subjected to virus-driven selective pressure and identified a complex interaction network enriched in viral products-interacting proteins. Therefore, we describe a novel approach for the identification of gene variants that may be involved in the susceptibility to viral infections.

Detection of anatid herpesvirus 1 gC gene by TaqMan fluorescent quantitative real-time PCR with specific primers and probe

BACKGROUND

Anatid herpesvirus 1 (AHV-1) is known for the difficulty of monitoring and controlling, because it has a long period of asymptomatic carrier state in waterfowls. Furthermore, as a significant essential agent for viral attachment, release, stability and virulence, gC (UL44) gene and its protein product (glycoprotein C) may play a key role in the epidemiological screening. The objectives of this study were to rapidly, sensitively, quantitatively detect gC gene of AHV-1 and provide the underlying basis for further investigating pcDNA3.1-gC DNA vaccine in infected ducks by TaqMan fluorescent quantitative real-time PCR assay (FQ-PCR) with pcDNA3.1-gC plasmid.

RESULTS

The repeatable and reproducible quantitative assay was established by the standard curve with a wide dynamic range (eight logarithmic units of concentration) and very good correlation values (1.000). This protocol was able to detect as little as 1.0 x 101 DNA copies per reaction and it was highly specific to AHV-1. The TaqMan FQ-PCR assay successfully detected the gC gene in tissue samples from pcDNA3.1-gC and AHV-1 attenuated vaccine (AHV-1 Cha) strain inoculated ducks respectively.

CONCLUSIONS

The assay offers an attractive method for the detection of AHV-1, the investigation of distribution pattern of AHV-1 in vivo and molecular epidemiological screening. Meanwhile, this method could expedite related AHV-1 and gC DNA vaccine research.

Proteoglycans in host-pathogen interactions: molecular mechanisms and therapeutic implications

Many microbial pathogens subvert proteoglycans for their adhesion to host tissues, invasion of host cells, infection of neighbouring cells, dissemination into the systemic circulation, and evasion of host defence mechanisms. Where studied, specific virulence factors mediate these proteoglycan-pathogen interactions, which are thus thought to affect the onset, progression and outcome of infection. Proteoglycans are composites of glycosaminoglycan (GAG) chains attached covalently to specific core proteins. Proteoglycans are expressed ubiquitously on the cell surface, in intracellular compartments, and in the extracellular matrix. GAGs mediate the majority of ligand-binding activities of proteoglycans, and many microbial pathogens elaborate cell-surface and secreted factors that interact with GAGs. Some pathogens also modulate the expression and function of proteoglycans through known virulence factors. Several GAG-binding pathogens can no longer attach to and invade host cells whose GAG expression has been reduced by mutagenesis or enzymatic treatment. Furthermore, GAG antagonists have been shown to inhibit microbial attachment and host cell entry in vitro and reduce virulence in vivo. Together, these observations underscore the biological significance of proteoglycan-pathogen interactions in infectious diseases.

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.

Diverse functions of glycosaminoglycans in infectious diseases

Glycosaminoglycans (GAGs) are complex carbohydrates that are expressed ubiquitously and abundantly on the cell surface and in the extracellular matrix (ECM). The extraordinary structural diversity of GAGs enables them to interact with a wide variety of biological molecules. Through these interactions, GAGs modulate various biological processes, such as cell adhesion, proliferation and migration, ECM assembly, tissue repair, coagulation, and immune responses, among many others. Studies during the last several decades have indicated that GAGs also interact with microbial pathogens. GAG-pathogen interactions affect most, if not all, the key steps of microbial pathogenesis, including host cell attachment and invasion, cell-cell transmission, systemic dissemination and infection of secondary organs, and evasion of host defense mechanisms. These observations indicate that GAG-pathogen interactions serve diverse functions that affect the pathogenesis of infectious diseases.

Insertion mutations in herpes simplex virus 1 glycoprotein H reduce cell surface expression, slow the rate of cell fusion, or abrogate functions in cell fusion and viral entry

Of the four required herpes simplex virus (HSV) entry glycoproteins, the precise role of gH-gL in fusion remains the most elusive. The heterodimer gH-gL has been proposed to mediate hemifusion after the interaction of another required glycoprotein, gD, with a receptor. To identify functional domains of HSV-1 gH, we generated 22 randomized linker-insertion mutants. Analyses of 22 gH mutants revealed that gH is relatively tolerant of insertion mutations, as 15 of 22 mutants permitted normal processing and transport of gH-gL to the cell surface. gH mutants that were not expressed well at the cell surface did not function in fusion or viral entry. The screening of gH mutants for function revealed the following: (i) for wild-type gH and some gH mutants, fusion with nectin-1-expressing target cells occurred more rapidly than with herpesvirus entry mediator (HVEM)-expressing target cells; (ii) some gH mutants reduced the rate of cell fusion without abrogating fusion completely, indicating that gH may play a role in governing the kinetics of fusion and may be responsible for a rate-limiting first stage in HSV-1 fusion; and (iii) only one gH mutant, located within the short cytoplasmic tail, completely abrogated function, indicating that the gH cytoplasmic tail is crucial for cell fusion and viral infectivity.

Alternate receptor usage of neuropilin-1 and glucose transporter protein 1 by the human T cell leukemia virus type 1

Recent studies have demonstrated that neuropilin 1 (NP-1) is involved in HTLV-1 entry; however, the role NP-1 plays in this process is not understood. We demonstrated that ectopic expression of human NP-1 but not NP-2 cDNA increased susceptibility to HTLV-1. SiRNA-mediated inhibition of NP-1 expression correlated with significant reduction of HTLV-1 Env-mediated fusion. The vascular endothelial growth factor (VEGF(165)) caused downmodulation of surface NP-1 and inhibited HTLV-1 infection of U87 cells. In contrast, VEGF(165) partially inhibited infection of primary astrocytes and had no significant effect on infection of HeLa cells. VEGF(165) and antibodies to the glucose transporter protein 1 (anti-GLUT-1) were both needed to block infection of primary astrocytes, however, only anti-GLUT-1 antibodies were sufficient to block infection of HeLa cells. HTLV-1 Env forms complexes with both NP-1 and GLUT-1 in primary human astrocytes. The alternate usage of these two cellular receptors may have important implications regarding HTLV-1 neuro-tropism.

A role for heparan sulfate in viral surfing

Heparan sulfate (HS) moieties on cell surfaces are known to provide attachment sites for many viruses including herpes simplex virus type-1 (HSV-1). Here, we demonstrate that cells respond to HSV-1 infection by enhancing filopodia formation. Filopodia express HS and are subsequently utilized for the transport of HSV-1 virions to cell bodies in a surfing-like phenomenon, which is facilitated by the underlying actin cytoskeleton and is regulated by transient activation of a small Rho GTPase, Cdc42. We also demonstrate that interaction between a highly conserved herpesvirus envelope glycoprotein B (gB) and HS is required for surfing. A HSV-1 mutant that lacks gB fails to surf and quantum dots conjugated with gB demonstrate surfing-like movements. Our data demonstrates a novel use of a common receptor, HS, which could also be exploited by multiple viruses and quite possibly, many additional ligands for transport along the plasma membrane.

Heparan sulfate proteoglycans are required for cellular binding of the hepatitis E virus ORF2 capsid protein and for viral infection

The hepatitis E virus (HEV), a nonenveloped RNA virus, is the causative agent of hepatitis E. The mode by which HEV attaches to and enters into target cells for productive infection remains unidentified. Open reading frame 2 (ORF2) of HEV encodes its major capsid protein, pORF2, which is likely to have the determinants for virus attachment and entry. Using an approximately 56-kDa recombinant pORF2 that can self-assemble as virus-like particles, we demonstrated that cell surface heparan sulfate proteoglycans (HSPGs), specifically syndecans, play a crucial role in the binding of pORF2 to Huh-7 liver cells. Removal of cell surface heparan sulfate by enzymatic (heparinase) or chemical (sodium chlorate) treatment of cells or competition with heparin, heparan sulfate, and their oversulfated derivatives caused a marked reduction in pORF2 binding to the cells. Syndecan-1 is the most abundant proteoglycan present on these cells and, hence, plays a key role in pORF2 binding. Specificity is likely to be dictated by well-defined sulfation patterns on syndecans. We show that pORF2 binds syndecans predominantly via 6-O sulfation, indicating that binding is not entirely due to random electrostatic interactions. Using an in vitro infection system, we also showed a marked reduction in HEV infection of heparinase-treated cells. Our results indicate that, analogous to some enveloped viruses, a nonenveloped virus like HEV may have also evolved to use HSPGs as cellular attachment receptors.

Infection of neurons and encephalitis after intracranial inoculation of herpes simplex virus requires the entry receptor nectin-1

Multiple entry receptors can mediate infection of cells by herpes simplex virus (HSV), permitting alternative pathways for infection and disease. We investigated the roles of two known entry receptors, herpesvirus entry mediator (HVEM) and nectin-1, in infection of neurons in the CNS and the development of encephalitis. Wild-type, HVEM KO, nectin-1 KO, and HVEM/nectin-1 double KO mice were inoculated with HSV into the hippocampus. The mice were examined for development of encephalitis or were killed at various times after inoculation for immunohistological analyses of brain slices. Nectin-1 KO mice showed no signs of disease after intracranial inoculation, and no HSV antigens were detectable in the brain parenchyma. However, HSV antigens were detected in non-parenchymal cells lining the ventricles. In the double KO mice, there was also no disease and no detectable expression of viral antigens even in non-parenchymal cells, indicating that infection of these cells in the nectin-1 KO mice was dependent on the expression of HVEM. Wild-type and HVEM KO mice rapidly developed encephalitis, and the patterns of HSV replication in the brain were indistinguishable. Thus, expression of nectin-1 is necessary for HSV infection via the intracranial route and for encephalitis; HVEM is largely irrelevant. These results contrast with recent findings that (i) either HVEM or nectin-1 can permit HSV infection of the vaginal epithelium in mice and (ii) nectin-1 is not the sole receptor capable of enabling spread of HSV infection from the vaginal epithelium to the PNS and CNS.

An unusual dependence of human herpesvirus-8 glycoproteins-induced cell-to-cell fusion on heparan sulfate

Human herpesvirus-8 (HHV-8) is known to interact with cell surface heparan sulfate (HS) for entry into a target cell. Here we investigated the role of HS during HHV-8 glycoproteins-induced cell fusion. Interestingly, the observed fusion demonstrated an unusual dependence on HS as evident from following lines of evidence: (1) a significant reduction in cell-to-cell fusion occurred when target cells were treated with heparinase; (2) in a competition assay, when the effector cells expressing HHV-8 glycoproteins were challenged with soluble HS, cell-to-cell fusion was reduced; and, (3) co-expression of HHV-8 glycoproteins gH-gL on target cells resulted in inhibition of cell surface HS expression. Taken together, our results indicate that cell surface HS can play an additional role during HHV-8 pathogenesis.

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.

Virus entry by macropinocytosis

As obligatory intracellular parasites, viruses rely on host-cell functions for most aspects of their replication cycle. This is born out during entry, when most viruses that infect vertebrate and insect cells exploit the endocytic activities of the host cell to move into the cytoplasm. Viruses belonging to vaccinia, adeno, picorna and other virus families have been reported to take advantage of macropinocytosis, an endocytic mechanism normally involved in fluid uptake. The virus particles first activate signalling pathways that trigger actin-mediated membrane ruffling and blebbing. Usually, this is followed by the formation of large vacuoles (macropinosomes) at the plasma membrane, internalization of virus particles and penetration by the viruses or their capsids into the cytosol through the limiting membrane of the macropinosomes. We review the molecular machinery involved in macropinocytosis and describe what is known about its role in virus entry.

Differential effects on cell fusion activity of mutations in herpes simplex virus 1 glycoprotein B (gB) dependent on whether a gD receptor or a gB receptor is overexpressed

Glycoprotein B (gB) of herpes simplex virus (HSV) is one of four glycoproteins essential for viral entry and cell fusion. Recently, paired immunoglobulin-like type 2 receptor (PILRalpha) was identified as a receptor for HSV type 1 (HSV-1) gB. Both PILRalpha and a gD receptor were shown to participate in HSV-1 entry into certain cell types. The purpose of this study was to determine whether insertional mutations in gB had differential effects on its function with PILRalpha and the gD receptor, nectin-1. Previously described gB mutants and additional newly characterized mutants were used in this study. We found that insertional mutations near the N terminus and C terminus of gB and especially in the central region of the ectodomain reduced cell fusion activity when PILRalpha was overexpressed much more than when nectin-1 was overexpressed. Most of the insertions reduced the binding of gB to PILRalpha, for at least some forms of gB, but this reduction did not necessarily correlate with the selective reduction in cell fusion activity with PILRalpha. These results suggest that the regions targeted by the relevant mutations are critical for functional activity with PILRalpha. They also suggest that, although both the binding of gB to a gB receptor and the binding of gD to a gD receptor may be required for HSV-induced cell fusion, the two receptor-binding activities may have unequal weights in triggering fusogenic activity, depending on the ratios of gB and gD receptors or other factors.

Mathematical modeling of herpes simplex virus distribution in solid tumors: implications for cancer gene therapy

PURPOSE

Although oncolytic viral vectors show promise for the treatment of various cancers, ineffective initial distribution and propagation throughout the tumor mass often limit the therapeutic response. A mathematical model is developed to describe the spread of herpes simplex virus from the initial injection site.

EXPERIMENTAL DESIGN

The tumor is modeled as a sphere of radius R. The model incorporates reversible binding, interstitial diffusion, viral degradation, and internalization and physiologic parameters. Three species are considered as follows: free interstitial virus, virus bound to cell surfaces, and internalized virus.

RESULTS

This analysis reveals that both rapid binding and internalization as well as hindered diffusion contain the virus to the initial injection volume, with negligible spread to the surrounding tissue. Unfortunately, increasing the dose to saturate receptors and promote diffusion throughout the tumor is not a viable option: the concentration necessary would likely compromise safety. However, targeted modifications to the virus that decrease the binding affinity have the potential to increase the number of infected cells by 1.5-fold or more. An increase in the effective diffusion coefficient can result in similar gains.

CONCLUSIONS

This analysis suggests criteria by which the potential response of a tumor to oncolytic herpes simplex virus therapy can be assessed. Furthermore, it reveals the potential of modifications to the vector delivery method, physicochemical properties of the virus, and tumor extracellular matrix composition to enhance efficacy.

In vivo importance of heparan sulfate-binding glycoproteins for murid herpesvirus-4 infection

Many herpesviruses bind to heparan sulfate (HS). Murid herpesvirus-4 (MuHV-4) does so via its envelope glycoproteins gp70 and gH/gL. MuHV-4 gp150 further regulates an HS-independent interaction to make that HS-dependent too. Cell binding by MuHV-4 virions is consequently strongly HS-dependent. Gp70 and gH/gL show some in vitro redundancy: an antibody-mediated blockade of HS binding by one is well tolerated, whereas a blockade of both severely impairs infection. In order to understand the importance of HS binding for MuHV-4 in vivo, we generated mutants lacking both gL and gp70. As expected, gL(-)gp70(-) MuHV-4 showed very poor cell binding. It infected mice at high dose but not at low dose, indicating defective host entry. But once entry occurred, host colonization, which for MuHV-4 is relatively independent of the infection dose, was remarkably normal. The gL(-)gp70(-) entry deficit was much greater than that of gL(-) or gp70(-) single knockouts. And gp150 disruption, which allows HS-independent cell binding, largely rescued the gL(-)gp70(-) cell binding and host entry deficits. Thus, it appeared that MuHV-4 HS binding is important in vivo, principally for efficient host entry.