Acidic pH Mediates Changes in Antigenic and Oligomeric Conformation of Herpes Simplex Virus gB and Is a Determinant of Cell-Specific Entry

Duck plague virus-encoded microRNA dev-miR-D28-3p inhibits viral replication via targeting UL27

Herpesviruses-encoded microRNAs (miRNAs) have been discovered to be essential regulators in viral life cycle, participating in viral replication, latent or lytic infection, and immunological escape. However, the roles of miRNAs encoded by duck plague virus (DPV) are still unknown. Dev-miR-D28-3p is a miRNA uniquely encoded by DPV CHv strain. The aim of this study was to explore the effect of dev-miR-D28-3p on DPV replication and explore the potential mechanisms involved. Our findings demonstrated that transfection of dev-miR-D28-3p mimic into duck embryo fibroblasts (DEFs) effectively suppressed viral copies, viral titers and viral protein expressions during DPV infection, while the results above were reversed after transfection with dev-miR-D28-3p inhibitor. Subsequently, we further discovered that dev-miR-D28-3p specifically bound to DPV-encoded UL27 and inhibited its expression, suggesting that UL27 was the target gene of dev-miR-D28-3p. Finally, we investigated the role of UL27 in DPV replication and found the overexpression of UL27 increased viral copies, viral titers, and viral protein expressions; whereas the opposite results appear when knockdown of UL27. Our findings illustrated a novel mechanism that DPV regulated itself replication via dev-miR-D28-3p, paving the way for exploring the role of DPV-encoded miRNAs.

Herpes Simplex Virus 1 Glycoprotein B from a Hyperfusogenic Virus Mediates Enhanced Cell-Cell Fusion

Herpes simplex virus 1 (HSV-1) causes significant morbidity and death in humans worldwide. Herpes simplex virus 1 has a complex fusion mechanism that is incompletely understood. The HSV-1 strain ANG has notable fusion and entry activities that distinguish it from wild type. HSV-1 ANG virions fused with the Vero cell surface at 4 °C and also entered cells more efficiently at 15 °C, relative to wild type HSV-1 strain KOS virions, consistent with a hyperfusogenic phenotype. Understanding the molecular basis for the unique entry and fusion activities of HSV-1 strain ANG will help decipher the HSV fusion reaction and entry process. Sequencing of HSV-1 ANG genes revealed multiple changes in gB, gC, gD, gH, and gL proteins relative to wild type HSV-1 strains. The ANG UL45 gene sequence, which codes for a non-essential envelope protein, was identical to wild type KOS. HSV-1 ANG gB, gD, and gH/gL were necessary and sufficient to mediate cell-cell fusion in a virus-free reporter assay. ANG gB, when expressed with wild type KOS gD and gH/gL, increased membrane fusion, suggesting that ANG gB has hyperfusogenic cell-cell fusion activity. Replacing the KOS gD, gH, or gL with the corresponding ANG alleles did not enhance cell-cell fusion. The novel mutations in the ANG fusion and entry glycoproteins provide a platform for dissecting the cascade of interactions that culminate in HSV fusion and entry.

Conformational Changes in Herpes Simplex Virus Glycoprotein C

Low endosomal pH facilitates herpesvirus entry in a cell-specific manner. Herpes simplex virus 1 (HSV-1) causes significant morbidity and death in humans worldwide. HSV-1 enters cells by low-pH and neutral-pH pathways. Low-pH-induced conformational changes in the HSV envelope glycoprotein B (gB) may mediate membrane fusion during viral entry. HSV-1 gC, a 511-amino acid, type I integral membrane glycoprotein, mediates HSV-1 attachment to host cell surface glycosaminoglycans, but this interaction is not essential for viral entry. We previously demonstrated that gC regulates low-pH viral entry independent of its known role in cell attachment. Low-pH-triggered conformational changes in gB occur at a lower pH when gC is absent, suggesting that gC positively regulates gB conformational changes. Here, we demonstrate that mildly acidic pH triggers conformational changes in gC itself. Low-pH treatment of virions induced antigenic changes in distinct gC epitopes, and those changes were reversible. One of these gC epitopes is recognized by a monoclonal antibody that binds to a linear sequence that includes residues within gC amino acids 33 to 123. This antibody inhibited low-pH entry of HSV, suggesting that its gC N-terminal epitope is particularly important. We propose that gC plays a critical role in HSV entry through a low-pH endocytosis pathway, which is a major entry route in human epithelial cells. IMPORTANCE Herpesviruses are ubiquitous pathogens that cause lifelong latent infections and are characterized by multiple entry pathways. The HSV envelope gC regulates HSV entry by a low-pH entry route. The fusion protein gB undergoes pH-triggered conformational changes that are facilitated by gC. Here, we report that gC itself undergoes a conformational change at low pH. A monoclonal antibody to gC that binds to a region that undergoes pH-induced changes also selectively inhibits HSV low-pH entry, corroborating the importance of gC in the low-pH entry pathway. This study illustrates the complex role of endosomal pH during HSV entry and provides novel insights into the functions of gC.

Cellular Receptors Involved in KSHV Infection

The process of Kaposi’s Sarcoma Herpes Virus’ (KSHV) entry into target cells is complex and engages several viral glycoproteins which bind to a large range of host cell surface molecules. Receptors for KSHV include heparan sulphate proteoglycans (HSPGs), several integrins and Eph receptors, cystine/glutamate antiporter (xCT) and Dendritic Cell-Specific Intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN). This diverse range of potential binding and entry sites allows KSHV to have a broad cell tropism, and entry into specific cells is dependent on the available receptor repertoire. Several molecules involved in KSHV entry have been well characterized, particularly those postulated to be associated with KSHV-associated pathologies such as Kaposi’s Sarcoma (KS). In this review, KSHV infection of specific cell types pertinent to its pathogenesis will be comprehensively summarized with a focus on the specific cell surface binding and entry receptors KSHV exploits to gain access to a variety of cell types. Gaps in the current literature regarding understanding interactions between KSHV glycoproteins and cellular receptors in virus infection are identified which will lead to the development of virus infection intervention strategies.

Herpes Simplex Virus Entry by a Nonconventional Endocytic Pathway

Herpes simplex virus 1 (HSV-1) causes significant morbidity and mortality in humans worldwide. HSV-1 enters epithelial cells via an endocytosis mechanism that is low-pH dependent. However, the precise intracellular pathway has not been identified, including the compartment where fusion occurs. In this study, we utilized a combination of molecular and pharmacological approaches to better characterize HSV entry by endocytosis. HSV-1 entry was unaltered in both cells treated with small interfering RNA (siRNA) to Rab5 or Rab7 and cells expressing dominant negative forms of these GTPases, suggesting entry is independent of the conventional endo-lysosomal network. The fungal metabolite brefeldin A (BFA) and the quinoline compound Golgicide A (GCA) inhibited HSV-1 entry via beta-galactosidase reporter assay and impaired incoming virus transport to the nuclear periphery, suggesting a role for trans-Golgi network (TGN) functions and retrograde transport in HSV entry. Silencing of Rab9 or Rab11 GTPases, which are involved in the retrograde transport pathway, resulted in only a slight reduction in HSV infection. Together, these results suggest that HSV enters host cells by an intracellular route independent of the lysosome-terminal endocytic pathway.IMPORTANCE Herpes simplex virus 1 (HSV-1), the prototype alphaherpesvirus, is ubiquitous in the human population and causes lifelong infection that can be fatal in neonatal and immunocompromised individuals. HSV enters many cell types by endocytosis, including epithelial cells, the site of primary infection in the host. The intracellular itinerary for HSV entry remains unclear. We probed the potential involvement of several Rab GTPases in HSV-1 entry and suggest that endocytic entry of HSV-1 is independent of the canonical lysosome-terminal pathway. A nontraditional endocytic route may be employed, such as one that intersects with the trans-Golgi network (TGN). These results may lead to novel targets for intervention.

Guanidine modifications enhance the anti-herpes simplex virus activity of (E,E)-4,6-bis(styryl)-pyrimidine derivatives in vitro and in vivo

BACKGROUND AND PURPOSE

The worldwide prevalence of herpes simplex virus (HSV) and shortage of efficient therapeutic strategies to counteract it are global concerns. In terms of treatment, the widely utilized anti-HSV drugs such as acyclovir have serious limitations, for example, drug resistance and side effects. Here, we have identified the guanidine-modified (E,E)-4,6-bis(styryl)-pyrimidine (BS-pyrimidine) derivative compound 5d as an inhibitor of HSV and further elucidated the anti-HSV mechanisms of compound 5d both in vitro and in vivo.

EXPERIMENTAL APPROACH

Cytopathic effect inhibition assay, plaque assay, and immunofluorescence assay were used to evaluate the anti-HSV effects of compound 5d in vitro. Membrane fusion assays, immunofluorescence assays, Western blotting assays, and pull-down assays were used to explore the anti-HSV mechanisms of compound 5d. HSV-1-infected mice, combined with haematoxylin-eosin staining and quantitative RT-PCR, were used to study the anti-HSV effects of compound 5d in vivo.

KEY RESULTS

The guanidine-modified compound 5d rather than the un-modified compound 3a effectively inhibited both HSV-1 and HSV-2 multiplication in different cell lines, more effectively than acyclovir. Compound 5d may block virus binding and post-binding processes such as membrane fusion, by targeting virus gB protein. In addition, compound 5d may also down-regulate the cellular PI3K/Akt signalling pathway to interfere with HSV replication. Treatment with compound 5d also markedly improved survival and decreased viral titres in HSV-infected mice.

CONCLUSIONS AND IMPLICATIONS

Thus, the guanidine-modified BS-pyrimidine derivatives have the potential to be developed into novel anti-HSV agents targeting both virus gB protein and cellular PI3K/Akt signalling pathways.

Conformational Change in Herpes Simplex Virus Entry Glycoproteins Detected by Dot Blot

Conformational changes in viral membrane proteins drive membrane fusion, a critical step in virus entry and infection. Here we describe a simple and rapid virus blotting immunoassay to define conformational changes with a panel of monoclonal antibodies to distinct sites across a viral glycoprotein. This dot blot technique has been utilized to define low pH-triggered changes in the prefusion form of the herpesviral fusogen gB. At pH of <6.2 there are specific changes in herpes simplex virus 1 gB domains I and V. This corresponds broadly to host cell endosomal pH. Many of the identified changes are at least partially reversible. This method can be adapted to document changes in viral proteins that are not fusion proteins, including those induced by alternate triggers such as receptor-binding or protease cleavage.

Towards Understanding KSHV Fusion and Entry

How viruses enter cells is of critical importance to pathogenesis in the host and for treatment strategies. Over the last several years, the herpesvirus field has made numerous and thoroughly fascinating discoveries about the entry of alpha-, beta-, and gamma-herpesviruses, giving rise to knowledge of entry at the amino acid level and the realization that, in some cases, researchers had overlooked whole sets of molecules essential for entry into critical cell types. Herpesviruses come equipped with multiple envelope glycoproteins which have several roles in many aspects of infection. For herpesvirus entry, it is usual that a collective of glycoproteins is involved in attachment to the cell surface, specific interactions then take place between viral glycoproteins and host cell receptors, and then molecular interactions and triggers occur, ultimately leading to viral envelope fusion with the host cell membrane. The fact that there are multiple cell and virus molecules involved with the build-up to fusion enhances the diversity and specificity of target cell types, the cellular entry pathways the virus commandeers, and the final triggers of fusion. This review will examine discoveries relating to how Kaposi's sarcoma-associated herpesvirus (KSHV) encounters and binds to critical cell types, how cells internalize the virus, and how the fusion may occur between the viral membrane and the host cell membrane. Particular focus is given to viral glycoproteins and what is known about their mechanisms of action.