Molecular basis for resistance of herpes simplex virus type 1 mutants to the sulfated oligosaccharide inhibitor PI-88

Cellular Chondroitin Sulfate and the Mucin-like Domain of Viral Glycoprotein C Promote Diffusion of Herpes Simplex Virus 1 While Heparan Sulfate Restricts Mobility

The diffusion of viruses at the cell membrane is essential to reach a suitable entry site and initiate subsequent internalization. Although many viruses take advantage of glycosaminoglycans (GAG) to bind to the cell surface, little is known about the dynamics of the virus–GAG interactions. Here, single-particle tracking of the initial interaction of individual herpes simplex virus 1 (HSV-1) virions reveals a heterogeneous diffusive behavior, regulated by cell-surface GAGs with two main diffusion types: confined and normal free. This study reports that different GAGs can have competing influences in mediating diffusion on the cells used here: chondroitin sulfate (CS) enhances free diffusion but hinders virus attachment to cell surfaces, while heparan sulfate (HS) promotes virus confinement and increases entry efficiency. In addition, the role that the viral mucin-like domains (MLD) of the HSV-1 glycoprotein C plays in facilitating the diffusion of the virus and accelerating virus penetration into cells is demonstrated. Together, our results shed new light on the mechanisms of GAG-regulated virus diffusion at the cell surface for optimal internalization. These findings may be extendable to other GAG-binding viruses.

Physicochemical tools for studying virus interactions with targeted cell membranes in a molecular and spatiotemporally resolved context

The objective of this critical review is to provide an overview of how emerging bioanalytical techniques are expanding our understanding of the complex physicochemical nature of virus interactions with host cell surfaces. Herein, selected model viruses representing both non-enveloped (simian virus 40 and human norovirus) and enveloped (influenza A virus, human herpes simplex virus, and human immunodeficiency virus type 1) viruses are highlighted. The technologies covered utilize a wide range of cell membrane mimics, from supported lipid bilayers (SLBs) containing a single purified host membrane component to SLBs derived from the plasma membrane of a target cell, which can be compared with live-cell experiments to better understand the role of individual interaction pairs in virus attachment and entry. These platforms are used to quantify binding strengths, residence times, diffusion characteristics, and binding kinetics down to the single virus particle and single receptor, and even to provide assessments of multivalent interactions. The technologies covered herein are surface plasmon resonance (SPR), quartz crystal microbalance with dissipation (QCM-D), dynamic force spectroscopy (DFS), total internal reflection fluorescence (TIRF) microscopy combined with equilibrium fluctuation analysis (EFA) and single particle tracking (SPT), and finally confocal microscopy using multi-labeling techniques to visualize entry of individual virus particles in live cells. Considering the growing scientific and societal needs for untangling, and interfering with, the complex mechanisms of virus binding and entry, we hope that this review will stimulate the community to implement these emerging tools and strategies in conjunction with more traditional methods. The gained knowledge will not only contribute to a better understanding of the virus biology, but may also facilitate the design of effective inhibitors to block virus entry.

Herpes Simplex Virus Type 2 Mucin-Like Glycoprotein mgG Promotes Virus Release from the Surface of Infected Cells

The contribution of virus components to liberation of herpes simplex virus type 2 (HSV-2) progeny virions from the surface of infected cells is poorly understood. We report that the HSV-2 mutant deficient in the expression of a mucin-like membrane-associated glycoprotein G (mgG) exhibited defect in the release of progeny virions from infected cells manifested by ~2 orders of magnitude decreased amount of infectious virus in a culture medium as compared to native HSV-2. Electron microscopy revealed that the mgG deficient virions were produced in infected cells and present at the cell surface. These virions could be forcibly liberated to a nearly native HSV-2 level by the treatment of cells with glycosaminoglycan (GAG)-mimicking oligosaccharides. Comparative assessment of the interaction of mutant and native virions with surface-immobilized chondroitin sulfate GAG chains revealed that while the mutant virions associated with GAGs ~fourfold more extensively, the lateral mobility of bound virions was much poorer than that of native virions. These data indicate that the mgG of HSV-2 balances the virus interaction with GAG chains, a feature critical to prevent trapping of the progeny virions at the surface of infected cells.

Regulatory Mechanisms of the Mucin-Like Region on Herpes Simplex Virus during Cellular Attachment

Mucin-like regions, characterized by a local high density of O-linked glycosylation, are found on the viral envelope glycoproteins of many viruses. Herpes simplex virus type 1 (HSV-1), for example, exhibits a mucin-like region on its glycoprotein gC, a viral protein involved in initial recruitment of the virus to the cell surface via interaction with sulfated glycosaminoglycans. So far, this mucin-like region has been proposed to play a key role in modulating the interactions with cellular glycosaminoglycans, and in particular to promote release of HSV-1 virions from infected cells. However, the molecular mechanisms and the role as a pathogenicity factor remains unclear. Using single virus particle tracking, we show that the mobility of chondroitin sulfate-bound HSV-1 virions is decreased in absence of the mucin-like region. This decrease in mobility correlates with an increase in HSV-1-chondroitin sulfate binding forces as observed using atomic force microscopy-based force spectroscopy. Our data suggest that the mucin-like region modulates virus-glycosaminoglycan interactions by regulating the affinity, type, and number of glycoproteins involved in the virus-glycosaminoglycan interaction. This study therefore presents new evidence for a role of the mucin-like region in balancing the interaction of HSV-1 with glycosaminoglycans and provides further insights into the molecular mechanisms used by the virus to ensure both successful cell entry and release from the infected cell.

Cell Membrane Derived Platform To Study Virus Binding Kinetics and Diffusion with Single Particle Sensitivity

Discovery and development of new antiviral therapies essentially rely on two key factors: an in-depth understanding of the mechanisms involved in viral infection and the development of fast and versatile drug screening platforms. To meet those demands, we present a biosensing platform to probe virus-cell membrane interactions on a single particle level. Our method is based on the formation of supported lipid bilayers from cell membrane material. Using total internal reflection fluorescence microscopy, we report the contribution of viral and cellular components to the interaction kinetics of herpes simplex virus type 1 with the cell membrane. Deletion of glycoprotein C (gC), the main viral attachment glycoprotein, or deletion of heparan sulfate, an attachment factor on the cell membrane, leads to an overall decrease in association of virions to the membrane and faster dissociation from the membrane. In addition to this, we perform binding inhibition studies using the antiviral compound heparin to estimate its IC₅₀ value. Finally, single particle tracking is used to characterize the diffusive behavior of the virus particles on the supported lipid bilayers. Altogether, our results promote this platform as a complement to existing bioanalytical assays, being at the interface between simplified artificial membrane models and live cell experiments.

The Cholestanol-Conjugated Sulfated Oligosaccharide PG545 Disrupts the Lipid Envelope of Herpes Simplex Virus Particles

Herpes simplex virus (HSV) and many other viruses, including HIV, initiate infection of host cells by binding to glycosaminoglycan (GAG) chains of cell surface proteoglycans. Although GAG mimetics, such as sulfated oligo- and polysaccharides, exhibit potent antiviral activities in cultured cells, the prophylactic application of these inhibitors as vaginal microbicides failed to protect women upon their exposure to HIV. A possible explanation for this failure is that sulfated oligo- and polysaccharides exhibit no typical virucidal activity, as their interaction with viral particles is largely electrostatic and reversible and thereby vulnerable to competition with GAG-binding proteins of the genital tract. Here we report that the cholestanol-conjugated sulfated oligosaccharide PG545, but not several other sulfated oligosaccharides lacking this modification, exhibited virucidal activity manifested as disruption of the lipid envelope of HSV-2 particles. The significance of the virus particle-disrupting activity of PG545 was also demonstrated in experimental animals, as this compound, in contrast to unmodified sulfated oligosaccharide, protected mice against genital infection with HSV-2. Thus, PG545 offers a novel prophylaxis option against infections caused by GAG-binding viruses.

Mucin-like Region of Herpes Simplex Virus Type 1 Attachment Protein Glycoprotein C (gC) Modulates the Virus-Glycosaminoglycan Interaction

Glycoprotein C (gC) mediates the attachment of HSV-1 to susceptible host cells by interacting with glycosaminoglycans (GAGs) on the cell surface. gC contains a mucin-like region located near the GAG-binding site, which may affect the binding activity. Here, we address this issue by studying a HSV-1 mutant lacking the mucin-like domain in gC and the corresponding purified mutant protein (gCΔmuc) in cell culture and GAG-binding assays, respectively. The mutant virus exhibited two functional alterations as compared with native HSV-1 (i.e. decreased sensitivity to GAG-based inhibitors of virus attachment to cells and reduced release of viral particles from the surface of infected cells). Kinetic and equilibrium binding characteristics of purified gC were assessed using surface plasmon resonance-based sensing together with a surface platform consisting of end-on immobilized GAGs. Both native gC and gCΔmuc bound via the expected binding region to chondroitin sulfate and sulfated hyaluronan but not to the non-sulfated hyaluronan, confirming binding specificity. In contrast to native gC, gCΔmuc exhibited a decreased affinity for GAGs and a slower dissociation, indicating that once formed, the gCΔmuc-GAG complex is more stable. It was also found that a larger number of gCΔmuc bound to a single GAG chain, compared with native gC. Taken together, our data suggest that the mucin-like region of HSV-1 gC is involved in the modulation of the GAG-binding activity, a feature of importance both for unrestricted virus entry into the cells and release of newly produced viral particles from infected cells.

O-linked glycosylation of the mucin domain of the herpes simplex virus type 1-specific glycoprotein gC-1 is temporally regulated in a seed-and-spread manner

The herpes simplex virus type 1 (HSV-1) glycoprotein gC-1, participating in viral receptor interactions and immunity interference, harbors a mucin-like domain with multiple clustered O-linked glycans. Using HSV-1-infected diploid human fibroblasts, an authentic target for HSV-1 infection, and a protein immunoaffinity procedure, we enriched fully glycosylated gC-1 and a series of its biosynthetic intermediates. This fraction was subjected to trypsin digestion and a LC-MS/MS glycoproteomics approach. In parallel, we characterized the expression patterns of the 20 isoforms of human GalNAc transferases responsible for initiation of O-linked glycosylation. The gC-1 O-glycosylation was regulated in an orderly manner initiated by synchronous addition of one GalNAc unit each to Thr-87 and Thr-91 and one GalNAc unit to either Thr-99 or Thr-101, forming a core glycopeptide for subsequent additions of in all 11 GalNAc residues to selected Ser and Thr residues of the Thr-76-Lys-107 stretch of the mucin domain. The expression patterns of GalNAc transferases in the infected cells suggested that initial additions of GalNAc were carried out by initiating GalNAc transferases, in particular GalNAc-T2, whereas subsequent GalNAc additions were carried out by followup transferases, in particular GalNAc-T10. Essentially all of the susceptible Ser or Thr residues had to acquire their GalNAc units before any elongation to longer O-linked glycans of the gC-1-associated GalNAc units was permitted. Because the GalNAc occupancy pattern is of relevance for receptor binding of gC-1, the data provide a model to delineate biosynthetic steps of O-linked glycosylation of the gC-1 mucin domain in HSV-1-infected target cells.

Involvement of viral glycoprotein gC-1 in expression of the selectin ligand sialyl-Lewis X induced after infection with herpes simplex virus type 1

Several herpesviruses induce expression of the selectin receptor sialyl-Lewis X (sLe(x) ) by activating transcription of one or more of silent host FUT genes, each one encoding a fucosyltransferase that catalyses the rate-limiting step of sLe(x) synthesis. The aim here was to identify the identity of the glycoconjugate associated with sLe(x) glycoepitope in herpes simplex virus type 1 (HSV-1) infected human diploid fibroblasts, using immunofluorescence confocal microscopy. Cells infected with all tested HSV-1 strains analysed demonstrated bright sLe(x) fluorescence, except for two mutant viruses that were unable to induce proper expression of viral glycoprotein gC-1: One gC-1 null mutant and another mutant expressing gC-1 devoid of its major O-glycan-containing region (aa 33-116). The sLe(x) reactivity of HSV-1 infected cells was abolished by mild alkali treatment. Altogether the results indicated that the detectable sLe(x) was associated with O-linked glycans, situated in the mucin region of gC-1. No evidence for sLe(x) (i) in other HSV-1 glycoproteins with mucin domains such as gI-1 or (ii) in host cell glycoproteins/glycolipids was found. Thus, the mucin domain of HSV-1 gC-1 may support expression of selectin ligands such as sLe(x) and other larger O-linked glycans in cell types lacking endogenous mucin domain-containing glycoproteins, optimized for O-glycan expression, provided that the adequate host glycosyltransferase genes are activated.

Potent anti-respiratory syncytial virus activity of a cholestanol-sulfated tetrasaccharide conjugate

A number of different viruses including respiratory syncytial virus (RSV) initiate infection of cells by binding to cell surface glycosaminoglycans and sulfated oligo- and polysaccharide mimetics of these receptors exhibit potent antiviral activity in cultured cells. We investigated whether the introduction of different lipophilic groups to the reducing end of sulfated oligosaccharides would modulate their anti-RSV activity. Our results demonstrate that the cholestanol-conjugated tetrasaccharide (PG545) exhibited ∼5- to 16-fold enhanced anti-RSV activity in cultured cells compared with unmodified sulfated oligosaccharides. Furthermore, PG545 displayed virus-inactivating (virucidal) activity, a feature absent in sulfated oligosaccharides. To inhibit RSV infectivity PG545 had to be present during the initial steps of viral infection of cells. The anti-RSV activity of PG545 was due to both partial inhibition of the virus attachment to cells and a more profound interference with some post-attachment steps as PG545 efficiently neutralized infectivity of the cell-adsorbed virus. The anti-RSV activity of PG545 was reduced when tested in the presence of human nasal secretions. Serial passages of RSV in the presence of increasing concentrations of PG545 selected for weakly resistant viral variants that comprised the F168S and the P180S amino acid substitutions in the viral G protein. Altogether we identified a novel and potent inhibitor of RSV, which unlike sulfated oligo- and polysaccharide compounds, could irreversibly inactivate RSV infectivity.

A highly lipophilic sulfated tetrasaccharide glycoside related to muparfostat (PI-88) exhibits virucidal activity against herpes simplex virus

Although sulfated polysaccharides potently inhibit the infectivity of herpes simplex virus (HSV) and human immunodeficiency virus in cultured cells, these compounds fail to show protective effects in humans, most likely due to their poor virucidal activity. Herein we report on sulfated oligosaccharide glycosides related to muparfostat (formerly known as PI-88) and their assessment for anti-HSV activity. Chemical modifications based on the introduction of specific hydrophobic groups at the reducing end of a sulfated oligosaccharide chain enhanced the compound's capability to inhibit the infection of cells by HSV-1 and HSV-2 and abrogated the cell-to-cell transmission of HSV-2. Furthermore, modification with a highly lipophilic cholestanyl group provided a compound with virucidal activity against HSV. This glycoside targeted the viral particle and, to a lesser degree, the cell, and exhibited an antiviral mode of action typical for sulfated polysaccharides and virucides, i.e., interference with the virus attachment to cells and irreversible inactivation of virus infectivity, respectively. The virucidal activity was decreased in the presence of human cervical secretions suggesting that higher doses of this glycoside might be needed for in vivo application. Altogether, the sulfated oligosaccharide-cholestanyl glycoside exhibits potent anti-HSV activity and is, therefore, a good candidate for development as a virucide.

Divergence and genotyping of human alpha-herpesviruses: an overview

Herpesviruses are large DNA viruses that are highly disseminated among animals. Of the eight herpesviruses identified in humans, three are classified into the alpha-herpesvirus subfamily: herpes simplex virus types 1 (HSV-1) and 2 (HSV-2), which are typically associated with mucocutaneous lesions, and varicella-zoster virus (VZV), which is the cause of chicken pox and herpes zoster. All three viruses establish lifelong infections and may also induce more severe symptoms, such as neurological manifestations and fatal neonatal infections. Despite thorough investigation of the genetic variability among circulating strains of each virus in recent decades, little is known about possible associations between the genetic setups of the viruses and clinical manifestations in human hosts. This review focuses mainly on evolutionary studies of and genotyping strategies for these three human alpha-herpesviruses, emphasizing the ambiguities induced by a high frequency of circulating recombinant strains. It also aims to shed light on the challenges of establishing a uniform genotyping strategy for all three viruses.

Inhibition of herpes simplex virus type 1 infection by silver nanoparticles capped with mercaptoethane sulfonate

Interactions between biomolecules and nanoparticles suggest the use of nanoparticles for various medical interventions. The attachment and entry of herpes simplex virus type 1 (HSV-1) into cells involve interaction between viral envelope glycoproteins and cell surface heparan sulfate (HS). Based on this mechanism, we designed silver nanoparticles that are capped with mercaptoethane sulfonate (Ag-MES). These nanoparticles are predicted to target the virus and to compete for its binding to cellular HS through their sulfonate end groups, leading to the blockage of viral entry into the cell and to the prevention of subsequent infection. Structurally defined Ag-MES nanoparticles that are readily redispersible in water were sonochemically synthesized. No toxic effects of these nanoparticles on host cells were observed. Effective inhibition of HSV-1 infection in cell culture by the capped nanoparticles was demonstrated. However, application of the soluble surfactant MES failed to inhibit viral infection, implying that the antiviral effect of Ag-MES nanoparticles is imparted by their multivalent nature and spatially directed MES on the surface. Our results suggest that capped nanoparticles may serve as useful topical agents for the prevention of infections with pathogens dependent on HS for entry.

Herpes simplex virus type 2 glycoprotein G is targeted by the sulfated oligo- and polysaccharide inhibitors of virus attachment to cells

Variants of herpes simplex virus type 2 (HSV-2) generated by virus passage in GMK-AH1 cells in the presence of the sulfated oligosaccharide PI-88 were analyzed. Many of these variants were substantially resistant to PI-88 in their initial infection of cells and/or their cell-to-cell spread. The major alteration detected in all variants resistant to PI-88 in the initial infection of cells was a frameshift mutation(s) in the glycoprotein G (gG) gene that resulted in the lack of protein expression. Molecular transfer of the altered gG gene into the wild-type background confirmed that the gG-deficient recombinants were resistant to PI-88. In addition to PI-88, all gG-deficient variants of HSV-2 were resistant to the sulfated polysaccharide heparin. The gG-deficient virions were capable of attaching to cells, and this activity was relatively resistant to PI-88. In addition to having a drug-resistant phenotype, the gG-deficient variants were inefficiently released from infected cells. Purified gG bound to heparin and showed the cell-binding activity which was inhibited by PI-88. Many PI-88 variants produced syncytia in cultured cells and contained alterations in gB, including the syncytium-inducing L792P amino acid substitution. Although this phenotype can enhance the lateral spread of HSV in cells, it conferred no virus resistance to PI-88. Some PI-88 variants also contained occasional alterations in gC, gD, gE, gK, and UL24. In conclusion, we found that glycoprotein gG, a mucin-like component of the HSV-2 envelope, was targeted by sulfated oligo- and polysaccharides. This is a novel finding that suggests the involvement of HSV-2 gG in interactions with sulfated polysaccharides, including cell surface glycosaminoglycans.