Bovine herpesvirus 1 glycoprotein B does not productively interact with cell surface heparan sulfate in a pseudorabies virion background

The mutations on the envelope glycoprotein D contribute to the enhanced neurotropism of the pseudorabies virus variant

The pseudorabies virus (PRV) TJ strain, a variant of PRV, induces more severe neurological symptoms and higher mortality in piglets and mice than the PRV SC strain isolated in 1980. However, the mechanism underlying responsible for the discrepancy in virulence between these strains remains unclear. Our study investigated the differences in neurotropism between PRV TJ and PRV SC using both in vitro and in vivo models. We discovered that PRV TJ enters neural cells more efficiently than PRV SC. Furthermore, we found that PRV TJ has indistinguishable genomic DNA replication capability and axonal retrograde transport dynamics compared to the PRV SC. To gain deeper insights into the mechanisms underlying these differences, we constructed gene-interchanged chimeric virus constructs and assessed the affinity between envelope glycoprotein B, C, and D (gD) and corresponding receptors. Our findings confirmed that mutations in these envelope proteins, particularly gD, significantly contributed to the heightened attachment and penetration capabilities of PRV TJ. Our study revealed the critical importance of the gDΔR278/P279 and gDV338A in facilitating viral invasion. Furthermore, our observations indicated that mutations in envelope proteins have a more significant impact on viral invasion than on virulence in the mouse model. Our findings provide valuable insights into the roles of natural mutations on the PRV envelope glycoproteins in cell tropism, which sheds light on the relationship between cell tropism and clinical symptoms and offers clues about viral evolution.

Common characteristics and unique features: A comparison of the fusion machinery of the alphaherpesviruses Pseudorabies virus and Herpes simplex virus

Membrane fusion is a fundamental biological process that allows different cellular compartments delimited by a lipid membrane to release or exchange their respective contents. Similarly, enveloped viruses such as alphaherpesviruses exploit membrane fusion to enter and infect their host cells. For infectious entry the prototypic human Herpes simplex viruses 1 and 2 (HSV-1 and -2, collectively termed HSVs) and the porcine Pseudorabies virus (PrV) utilize four different essential envelope glycoproteins (g): the bona fide fusion protein gB and the regulatory heterodimeric gH/gL complex that constitute the "core fusion machinery" conserved in all members of the Herpesviridae; and the subfamily specific receptor binding protein gD. These four components mediate attachment and fusion of the virion envelope with the host cell plasma membrane through a tightly regulated sequential activation process. Although PrV and the HSVs are closely related and employ the same set of glycoproteins for entry, they show remarkable differences in the requirements for fusion. Whereas the HSVs strictly require all four components for membrane fusion, PrV can mediate cell-cell fusion without gD. Moreover, in contrast to the HSVs, PrV provides a unique opportunity for reversion analyses of gL-negative mutants by serial cell culture passaging, due to a limited cell-cell spread capacity of gL-negative PrV not observed in the HSVs. This allows a more direct analysis of the function of gH/gL during membrane fusion. Unraveling the molecular mechanism of herpesvirus fusion has been a goal of fundamental research for years, and yet important mechanistic details remain to be uncovered. Nevertheless, the elucidation of the crystal structures of all key players involved in PrV and HSV membrane fusion, coupled with a wealth of functional data, has shed some light on this complex puzzle. In this review, we summarize and discuss the contemporary knowledge on the molecular mechanism of entry and membrane fusion utilized by the alphaherpesvirus PrV, and highlight similarities but also remarkable differences in the requirements for fusion between PrV and the HSVs.

Glycoprotein B plays a predominant role in mediating herpes simplex virus type 2 attachment and is required for entry and cell-to-cell spread

Heparan sulfate moieties serve as receptors for initial binding of herpes simplex virus types 1 and 2 (HSV-1 and -2) to cells. Deletion of HSV-1 glycoprotein C (gC-1) but not HSV-2 gC (gC-2) results in virions with reduced specific binding activity (virus particles bound per cell) and specific infectivity (p.f.u. per particle), suggesting that for HSV-1, but not HSV-2, gC plays a major role in mediating virus attachment. To test the hypothesis that glycoprotein B (gB), the other heparin-binding glycoprotein, mediates HSV-2 attachment, HSV-2 viruses deleted in gB-2 alone or deleted in both gB-2 and gC-2 were constructed. These viruses were grown on complementing or non-complementing cells and were compared with parental HSV-2(G) or a gC-2-deleted HSV-2 mutant (with respect to ability to bind and infect cells). At equivalent input concentrations of purified virions, significantly fewer gB-2-deleted virions bound to cells compared to parental HSV-2(G) or virus grown on complementing cells. In addition, viruses deleted in gB-2 were non-infectious. No immediate early proteins were detected in cells infected with gB-2-deleted virus harvested from non-complementing Vero cells, whereas these proteins were readily detected 4 h post-infection in cells infected with virus grown on complementing cells or with parental viruses. Viruses deleted in gB-2 failed to spread cell to cell, as evidenced by the inability to form plaques. Together these studies demonstrate that gB-2 plays a key role in mediating HSV-2 attachment and is required for entry and cell-to-cell spread. This glycoprotein is an important target for development of novel antiviral drugs.

Heparan sulfate: anchor for viral intruders?

Heparan sulfates (HS) are ubiquitous, polyanionic carbohydrate chains linked to core proteins in cell membranes and extracellular matrices of all eukaryotes. Due to the complex nature of the HS-biosynthesis, a wealth of different structures are produced. These seem to have a well defined distribution in different tissues and cells throughout development. Binding of endogenous proteins with different functional properties such as growth factors, adhesion molecules or enzymes, is one of the functions of HS. Besides interaction with endogenous factors, glycosaminoglycans (GAG) and especially HS have also been demonstrated to function as receptors for a number of different pathogens. What roles may HS play in the pathogenesis and tropism of different intruders like parasites or viruses? What implications does binding of viruses to HS have for the development of drugs or the application of viral vectors for gene targeting? In this review an attempt is made to collect our present knowledge on viral usage of HS and the implications that follow.

Pseudorabies virus expressing bovine herpesvirus 1 glycoprotein B exhibits altered neurotropism and increased neurovirulence

Herpesvirus glycoproteins play dominant roles in the initiation of infection of target cells in culture and thus may also influence viral tropism in vivo. Whereas the relative contribution of several nonessential glycoproteins to neurovirulence and neurotropism of Pseudorabies virus (PrV), an alphaherpesvirus which causes Aujeszky's disease in pigs, has recently been uncovered in studies using viral deletion mutants, the importance of essential glycoproteins is more difficult to assess. We isolated an infectious PrV mutant, PrV-9112C2, which lacks the gene encoding the essential PrV glycoprotein B (gB) but stably carries in its genome and expresses the homologous gene of bovine herpesvirus 1 (BHV-1) (A. Kopp and T. C. Mettenleiter, J. Virol. 66:2754-2762, 1992). Apart from exhibiting a slight delay in penetration kinetics, PrV-9112C2 was similar in its growth characteristics in cell culture to wild-type PrV. To analyze the effect of the exchange of these homologous glycoproteins in PrV's natural host, swine, 4-week-old piglets were intranasally infected with 10(6) PFU of either wild-type PrV strain Kaplan (PrV-Ka), PrV-9112C2, or PrV-9112C2R, in which the PrV gB gene was reinserted instead of the BHV-1 gB gene. Animals infected with PrV-Ka and PrV-9112C2R showed a similar course of disease, i.e., high fever, marked respiratory symptoms but minimal neurological disorders, and excretion of high amounts of virus. All animals survived the infection. In contrast, animals infected with PrV-9112C2 showed no respiratory symptoms and developed only mild fever. However, on day 5 after infection, all piglets developed severe central nervous system (CNS) symptoms leading to death within 48 to 72 h. Detailed histological analyses showed that PrV-9112C2R infected all regions of the nasal mucosa and subsequently spread to the CNS preferentially by the trigeminal route. In contrast, PrV-9112C2 primarily infected the olfactory epithelium and spread via the olfactory route. In the CNS, more viral antigen and significantly more pronounced histological changes resulting in more severe encephalitis were found after PrV-9112C2 infection. Thus, our results demonstrate that replacement of PrV gB by the homologous BHV-1 glycoprotein resulted in a dramatic increase in neurovirulence combined with an alteration in the route of neuroinvasion, indicating that the essential gB is involved in determining neurotropism and neurovirulence of PrV.

Construction and characterization of an equine herpesvirus 1 glycoprotein C negative mutant

An equine herpesvirus 1 (EHV-1) strain RacL 11 mutant was constructed that carries the Escherichia coli LacZ gene instead of the open reading frame encoding glycoprotein C (gC). The engineered virus mutant (L11(delta)gC) lacked codons 46-440 of the 1404 bp gene. On rabbit kidney cell line Rk13 and equine dermal cell line Edmin337, the L11(delta)gC virus grew to titers which were reduced by approximately 5- to 10-fold compared with wild-type RacL11 virus or a repaired virus (R-L11(delta)gC). However, when L11(delta)gC growth properties were analyzed on primary equine cells a decrease of viral titers was observed such that extracellular L11(delta)gC titers were reduced by 48- to 210-fold compared with those of wild-type or repaired virus. Heparin sensitive and heparin resistant attachment was assessed by binding studies using radiolabeled virion preparations. These studies revealed that EHV-1 gC is important for heparin sensitive attachment to the target cell. Similar results were obtained when cellular glycosaminoglycan (GAG) synthesis was inhibited by chlorate treatment or when cells defective in GAG synthesis were used. L11(delta)gC also exhibited significantly delayed penetration kinetics on Rk13 and primary equine cells. Infection of mice with L11(delta)gC did not cause EHV-1-related disease, whereas mice infected with either RacL11 or R-L11(delta)gC exhibited massive bodyweight losses, high virus titers in the lungs, and viremia. Taken together, EHV-1 gC was shown to play important roles in the early steps of infection and in release of virions, especially in primary equine cells, and contributes to EHV-1 virulence.

Infectivity of a pseudorabies virus mutant lacking attachment glycoproteins C and D

Initiation of herpesvirus infection requires attachment of virions to the host cell followed by fusion of virion envelope and cellular cytoplasmic membrane during penetration. In several alphaherpesviruses, glycoprotein C (gC) is the primary attachment protein, interacting with cell-surface heparan sulfate proteoglycans. Secondary binding is mediated by gD, which, normally, is also required for penetration. Recently, we described the isolation of a gD-negative infectious pseudorabies virus (PrV) mutant, PrV gD- Pass (J. Schmidt, B. G. Klupp, A. Karger, and T. C. Mettenleiter, J. Virol. 71:17-24, 1997). In PrV gD- Pass, attachment and penetration occur in the absence of gD. To assess the importance of specific attachment for infectivity of PrV gD- Pass, the gene encoding gC was deleted, resulting in mutant PrV gCD- Pass. Deletion of both known attachment proteins reduced specific infectivity compared to wild-type PrV by more than 10,000-fold. Surprisingly, the virus mutant still retained significant infectivity and could be propagated on normal noncomplementing cells, indicating the presence of another receptor-binding virion protein. Selection of bovine kidney (MDBK) cells resistant to infection by PrV gCD- Pass resulted in the isolation of a cell clone, designated NB, which was susceptible to infection by wild-type PrV but refractory to infection by either PrV gCD- Pass or PrV gD- Pass, a defect which could partially be overcome by polyethylene glycol (PEG)-induced membrane fusion. However, even after PEG-induced infection plaque formation of PrV gCD- Pass or PrV gD- Pass did not ensue in NB cells. Also, phenotypic gD complementation of PrV gCD- Pass or PrV gD- Pass rescued the defect in infection of NB cells but did not restore plaque formation. Glycosaminoglycan analyses of MDBK and NB cells yielded identical results, and NB cells were normally susceptible to infection by other alphaherpesviruses as well as vesicular stomatitis virus. Infectious center assays after PEG-induced infection of NB cells with PrV gD- Pass on MDBK cells indicated efficient exit of virions from infected NB cells. Together, our data suggest the presence of another receptor and receptor-binding virion protein which can mediate PrV entry and cell-to-cell spread in MDBK cells.