The receptor-binding domain of pseudorabies virus glycoprotein gC is composed of multiple discrete units that are functionally redundant

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.

Characterization of new strains of Pseudorabies virus in Argentina: Detection of interspecies transmission

Background

Aujeszky's disease is mainly a swine disease, still endemic worldwide. It can infect other mammalians, including human beings, and it is usually fatal with nervous symptoms. Ever since the disease was detected in 1988 in Argentina, many outbreaks have been reported involving both feral swine and dogs.

Aim

At present, in Argentina, Pseudorabies virus (PRV) cases are sporadically reported; however, clinical cases are informed. This study aims to obtain information about the seroprevalence of PRV in wild boars and to isolate and characterize PRV from clinical samples.

Methods

From 2018 to 2019, 78 wild boars' serum samples from Bahía de Samborombón natural reserve were analyzed for antibodies to PRV using a virus neutralization test. Clinical samples from 17 pigs, 2 wild boars, 1 dog, and 1 cat were collected from 2013 to 2019 for viral isolation and detection of the presence of the gD gene by PCR. For sequence analysis, the gC partial gene was amplified.

Results

Five strains were isolated from the dog, cat, and swine samples. The new PRV strains identified were confirmed by BLAST analysis, which revealed between 99.74% and 100% of similarity to the NIA-3 strain and phylogenetic analysis of the partial gene encoding the gC protein revealed that the PRV strains have divided into two main clades, clade 1 and clade 2.

Conclusion

This report informed that most new cases of PRV were detected in the central regions of Argentina, where pig production is concentrated. The study in Bahía de Samborombón revealed a high percentage of detection but, the sampling is not representative of that of the rest of the country. Therefore, a systematic sampling effort of wild boar throughout the country should be included in the national program control. Although in Argentina only the inactivated Bartha vaccine is allowed, recombination risk should not be ignored if attenuated vaccines are incorporated into the National control plan. The two strains, one from the cat and one from the dog sample, are directly related to infected swine. The information about clinical cases and molecular characterization of new strains is important for a better understanding of the dynamics of PRV and to promote preventive measures.

Recombinant Pseudorabies Virus Usage in Vaccine Development against Swine Infectious Disease

Pseudorabies virus (PRV) is the pathogen of pseudorabies (PR), which belongs to the alpha herpesvirus subfamily with a double stranded DNA genome encoding approximately 70 proteins. PRV has many non-essential regions for replication, has a strong capacity to accommodate foreign genes, and more areas for genetic modification. PRV is an ideal vaccine vector, and multivalent live virus-vectored vaccines can be developed using the gene-deleted PRV. The immune system continues to be stimulated by the gene-deleted PRVs and maintain a long immunity lasting more than 4 months. Here, we provide a brief overview of the biology of PRV, recombinant PRV construction methodology, the technology platform for efficiently constructing recombinant PRV, and the applications of recombinant PRV in vaccine development. This review summarizes the latest information on PRV usage in vaccine development against swine infectious diseases, and it offers novel perspectives for advancing preventive medicine through vaccinology.

Glycoproteins C and D of PRV Strain HB1201 Contribute Individually to the Escape From Bartha-K61 Vaccine-Induced Immunity

The newly emerged pseudorabies virus (PRV) novel variants can escape from the immunity induced by the classical vaccine Bartha-K61. Here we investigated the underlying mechanisms by constructing chimeric mutants between epidemic strain HB1201 and the Bartha-K61 vaccine. Our analyses focused on three viral envelope glycoproteins, namely gB, gC, and gD, as they exhibit remarkable genetic variations and are also involved in induction of protective immunity. The corresponding genes were swapped reciprocally either individually or in combination by using CRISPR/Cas9 technology and homologous recombination. The rescued chimeric viruses exhibited differential sensitivity to neutralizing antibodies in vitro, and gC was found to be the major contributor to inefficient neutralization against HB1201 by anti-Bartha-K61 serum. When tested in the 4-week-piglet model, substitution with HB1201 gC enabled Bartha-K61 to induce a protective immunity against HB1201 at a high challenge dose of 10⁷ TCID₅₀. Interestingly, despite a relatively lower cross-neutralization ability, the gD exchange also enabled Bartha-K61 to protect piglets from lethal challenge. In both cases, clinical signs and microscopic lesions were eased, and so was the viral tissue load with the exception of brain. A better protection could be achieved when both gC and gD were swapped in terms of reducing viral load in brain and virus-induced microscopic lesions. Thus, our studies not only revealed individual roles of gC and gD variations in the immune escape and also suggested a synergistic effect of both proteins on induction of protective immunity. These findings have important implications in novel vaccine development for PRV control in China.

Genome Characteristics and Evolution of Pseudorabies Virus Strains in Eastern China from 2017 to 2019

Since late 2011, outbreaks of pseudorabies virus (PRV) have occurred in southern China causing major economic losses to the pig industry. We previously reported that variant PRV forms and recombination in China could be the source of continued epidemics. Here, we analyzed samples from intensive pig farms in eastern China between 2017 and 2019, and sequenced the main glycoproteins (gB, gC, gD, and gE) to study the evolution characteristics of PRV. Based on the gC gene, we found that PRV variants belong to clade 2 and detected a founder effect during by the PRV epidemic. In addition, we detected inter- and intra-clade recombination; in particular, inter-clade recombination in the gB genes of strains FJ-ZXF and FJ-W2, which were recombinant with clade 1 strains. We also found specific amino-acid changes and positively selected sites, possibly associated with functional changes. This analysis of the emergence of PRV in China illustrates the need for continuous monitoring and the development of vaccines against specific variants of PRV.

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.

The effects of viral load on pseudorabies virus gene expression

Background

Herpesvirus genes are classified into distinct kinetic groups on the basis of their expression dynamics during lytic growth of the virus in cultured cells at a high, typically 10 plaque-forming units/cell multiplicity of infection (MOI). It has been shown that both the host response and the success of a pathogen are dependent on the quantity of particles infecting an organism. This work is a continuation of an earlier study [1], in which we characterized the overall expression of PRV genes following low-MOI infection. In the present study, we have addressed the question of whether viral gene expressions are dependent on the multiplicity of infection by comparing gene expressions under low and high-MOI conditions.

Results

In the present study, using a real-time RT-PCR assay, we address the question of whether the expression properties of the pseudorabies virus (PRV) genes are dependent on the number of virion particles infecting a single cell in a culture. Our analysis revealed a significant dependence of the gene expression on the MOI in most of these genes. Specifically, we found that most of the examined viral genes were expressed at a lower level at a low MOI (0.1) than at a high MOI (10) experiment in the early stage of infection; however, this trend reversed by six hour post-infection in more than half of the genes. Furthermore, in the high-MOI infection, several PRV genes substantially declined within the 4 to 6-h infection period, which was not the case in the low-MOI infection. In the low-MOI infection, the level of antisense transcript (AST), transcribed from the antiparallel DNA strand of the immediate-early 180 (ie180) gene, was comparable to that of ie180 mRNA, while in the high-MOI experiment (despite the 10 times higher copy number of the viral genome in the infected cells) the amount of AST dropped by more than two log values at the early phase of infection. Furthermore, our analysis suggests that adjacent PRV genes are under a common regulation. This is the first report on the effect of the multiplicity of infection on genome-wide gene expression of large DNA viruses, including herpesviruses.

Conclusion

Our results show a strong dependence of the global expression of PRV genes on the MOI. Furthermore, our data indicate a strong interrelation between the expressions of ie180 mRNA and AST, which determines the expression properties of the herpesvirus genome and possibly the replication strategy (lytic or latent infection) of the virus in certain cell types.

Identification and characterization of duck plague virus glycoprotein C gene and gene product

BACKGROUND

Viral envelope proteins have been proposed to play significant roles in the process of viral infection.

RESULTS

In this study, an envelope protein gene, gC (NCBI GenBank accession no. EU076811), was expressed and characterized from duck plague virus (DPV), a member of the family herpesviridae. The gene encodes a protein of 432 amino acids with a predicted molecular mass of 45 kDa. Sequence comparisons, multiple alignments and phylogenetic analysis showed that DPV gC has several features common to other identified herpesvirus gC, and was genetically close to the gallid herpervirus.Antibodies raised in rabbits against the pET32a-gC recombinant protein expressed in Escherichia coli BL21 (DE3) recognized a 45-KDa DPV-specific protein from infected duck embryo fibroblast (DEF) cells. Transcriptional and expression analysis, using real-time fluorescent quantitative PCR (FQ-PCR) and Western blot detection, revealed that the transcripts encoding DPV gC and the protein itself appeared late during infection of DEF cells. Immunofluorescence localization further demonstrated that the gC protein exhibited substantial cytoplasm fluorescence in DPV-infected DEF cells.

CONCLUSIONS

In this work, the DPV gC protein was successfully expressed in a prokaryotic expression system, and we presented the basic properties of the DPV gC product for the first time. These properties of the gC protein provided a prerequisite for further functional analysis of this gene.

Plasmodium falciparum BAEBL binds to heparan sulfate proteoglycans on the human erythrocyte surface

Erythrocyte invasion is critical to the pathogenesis and survival of the malarial parasite, Plasmodium falciparum. This process is partly mediated by proteins that belong to the Duffy binding-like family, which are expressed on the merozoite surface. One of these proteins, BAEBL (also known as EBA-140), is thought to bind to glycophorin C in a sialic acid-dependent manner. In this report, by the binding assay between recombinant BAEBL protein and enzyme-treated erythrocytes, we show that the binding of BAEBL to erythrocytes is mediated primarily by sialic acid and partially through heparan sulfate (HS). Because BAEBL binds to several kinds of HS proteoglycans or purified HS, the BAEBL-HS binding was found to be independent of the HS proteoglycan peptide backbone and the presence of sialic acid moieties. Furthermore, both the sialic acid- and HS-dependent binding were disrupted by the addition of soluble heparin. This inhibition may be the result of binding between BAEBL and heparin. Invasion assays demonstrated that HS-dependent binding was related to the efficiency of merozoite invasion. These results suggest that HS functions as a factor that promotes the binding of BAEBL and merozoite invasion. Moreover, these findings may explain the invasion inhibition mechanisms observed following the addition of heparin and other sulfated glycoconjugates.

Characterization of the gene encoding glycoprotein C of duck enteritis virus

A total of 2,718 bp of DNA fragment was amplified from the C-KCE strain of duck enteritis virus (DEV) genome using thermal asymmetric interlaced PCR. This newly identified viral DNA fragment contained two non-overlapping open reading frames (ORFs) oriented from the 5' to 3' direction. The first ORF was comprised of 43.5% G + C and contained the full-length genomic sequence of the UL44 gene (1,296 bp) encoding 431 amino acid residues of DEV glycoprotein C (gC). The second ORF encoded a partial peptide of the UL43 gene. The sequences of DNA and deduced amino acids of the DEV gC gene shared high homology with other members of known herpesviruses, supporting the classification of DEV. Phylogenetic analysis of the DEV gC gene revealed that the gC gene had a close evolutionary relationship with the subfamily of Alphaherpesvirinae.

Effective protection of pigs against lethal Pseudorabies virus infection after a single injection of low-dose Sindbis-derived plasmids encoding PrV gB, gC and gD glycoproteins

This study compared protection of pigs against lethal Pseudorabies virus (PrV) infection induced by a single injection of various quantities of Sindbis virus-derived plasmids encoding PrV glycoproteins gB, gC and gD. Pigs were injected with 340, 68 or 13 microg of each plasmid. Few immune differences were observed after DNA injection and more importantly the pigs of the three groups were equally protected against virulent PrV infection. Single-shot injection of 13 microg of each PrV glycoprotein encoding Sindbis virus-derived plasmid is able to effectively protect pigs from PrV infection.

A novel strategy for defining critical amino acid residues involved in protein/glycosaminoglycan interactions

The binding of proteins to glycosaminoglycans (GAGs) is the prerequisite for a large number of cellular processes and regulatory events and is associated to many pathologies. However, progress in the understanding of these mechanisms has been hampered by the lack of simple and comprehensive analytical tools for the identification of the structural attributes involved in protein/saccharide interaction. Characterization of GAG binding motifs on proteins has so far relied on site-directed mutagenesis studies, protein sequence mapping using synthetic peptides, molecular modeling, or structural analysis. Here, we report the development of a novel approach for identifying protein residues involved in the binding to heparin, the archetypal member of the GAG family. This method, which uses native proteins, is based on the formation of cross-linked complexes of the protein of interest with heparin beads, the proteolytic digestion of these complexes, and the subsequent identification of the heparin binding containing peptides by N terminus sequencing. Analysis of the CC chemokine regulated on activation, normal T-cell expressed, and secreted (RANTES), the envelope glycoprotein gC from pseudorabies virus and the laminin-5 alpha 3LG4/5 domain validated the techniques and provided novel information on the heparin binding motifs present within these proteins. Our results highlighted this method as a fast and valuable alternative to existing approaches. Application of this technique should greatly contribute to facilitate the structural study of protein/GAG interactions and the understanding of their biological functions.

Heparan sulphate mediates swine vesicular disease virus attachment to the host cell

Heparan sulphate (HS) has been found to serve as receptor for initial cell binding of numerous viruses. Different glycosaminoglycans (GAGs), including heparin and HS, were analysed for their ability to bind swine vesicular disease virus (SVDV), a picornavirus with close homology to human coxsackie B5 virus. Binding of SVDV was established by heparin-affinity chromatography. In addition, infection of IB-RS-2 epithelial porcine cells was inhibited by treating the virus with soluble HS, heparin, and chondroitin sulphate B (CS-B), as well as by enzymic digestion of cell surface GAGs. Analysis of the infection course showed that SVDV uses cellular HS for its binding to the cell surface and that this interaction occurs during attachment of the virus, prior to its internalization into the cell. Sequence analysis of SVDV variants selected for their lack of sensitivity to heparin inhibition in vitro led to the identification of two residues (A2135V and I1266K) potentially involved in heparin/HS interaction. The location of these residues in a three-dimensional model shows that they are clustered in a well-exposed region of the capsid, providing a physical mechanism that could account for the heparin-binding phenotype.

A role for glycoprotein C in pseudorabies virus entry that is independent of virus attachment to heparan sulfate and which involves the actin cytoskeleton

Glycoprotein C (gC) of pseudorabies virus, a swine herpesvirus, initiates virus attachment by binding to heparan sulfate (HS) linked to proteoglycans (HSPGs) on the cell surface. This interaction facilitates a required step in virus entry, the binding to a non-HS coreceptor, likely by another viral glycoprotein, gD. We demonstrate that gC has an even more direct role in virus entry than simply promoting adhesion strengthening. A porcine cell line expressing gC trans-complemented the penetration, but not attachment, defect of gC null mutants. In addition, gC promoted the colocalization of cell surface HSPGs and the actin cytoskeleton, suggesting a role for filamentous actin in virus entry. This was supported by results showing that both the engagement of a non-HS coreceptor and entry events subsequent to coreceptor binding were impaired if cells were first treated with an actin depolymerizing agent, cytochalasin D. Our results suggest a model in which gC-HS interactions promote not only virus attachment but also virus entry by usurping the normal properties of HSPGs.

Identification of the glycosaminoglycan-binding site on the glycoprotein E(rns) of bovine viral diarrhoea virus by site-directed mutagenesis

Bovine viral diarrhoea virus (BVDV) envelope glycoprotein E(rns) interacts with highly sulphated heparin-like glycosaminoglycans (GAGs) located on the cell surface as an early step in virus infection of cells. Site-directed mutagenesis of recombinant E(rns) was undertaken and analysis of mutants by heparin-affinity chromatography and cell surface binding showed that a cluster of basic amino acids (480KKLENKSK487) near the C terminus of E(rns) was essential for binding. Mutants with amino acid substitutions of lysine residues 481 and 485 in E(rns) reduced the binding of E(rns) to immobilized heparin and cellular GAGs but retained ribonuclease activity. In contrast to normal E(rns), E(rns) that was unable to bind to cells also failed to inhibit BVDV infection of cells when the cells were pre-incubated with E(rns). It is proposed that the cluster of basic residues (480KKLENKSK487) localized at the C-terminal end of E(rns) constitutes a GAG-binding site.

Characterization of pseudorabies virus glycoprotein C attachment to heparan sulfate proteoglycans

Pseudorabies virus first attaches to cells through an interaction between the envelope glycoprotein C (gC) and the cell surface heparan sulfate (HS) that is linked to proteoglycans (HSPGs). The HS-binding domain of gC is composed of three discrete heparin-binding domains (HBDs), designated HBD1, -2 and -3 for their proximity to the amino terminus of gC. Each HBD can independently mediate virus attachment to HS, yet each also exhibits a distinct binding preference for differentially sulfated derivatives of heparin. To demonstrate this, affinity columns composed of wild-type gC or mutant gC retaining a single HBD to capture several HSPGs from cultured pig and bovine kidney cells were used. The wild-type gC column bound all of the HSPGs well and, overall, bound more than 90% of the total sample applied to the column. Columns composed of either HBD2 or -3 bound intermediate amounts (40%) of the total sample applied, while the HBD1 column bound low amounts of HSPGs. HBD2 and -3 columns did not uniformly bind all of the HSPGs from bovine kidney cells, but the same HSPGs were bound with equal efficiency on each column. Thus, despite their different preferences for sulfation patterns on HS side-chains, HBD2 and -3 appear to bind the same proteoglycan cores. These results established a hierarchy of HBD2=HBD3>HBD1 in importance for HSPG binding. These in vitro-binding results correlated with the attachment phenotype of virus strains expressing gC with a single HBD in their envelopes.

Human herpesvirus 8 envelope glycoprotein K8.1A interaction with the target cells involves heparan sulfate

Human herpesvirus-8 (HHV-8) or Kaposi's sarcoma-associated herpesvirus K8.1 gene encodes for two immunogenic glycoproteins, gpK8.1A and gpK8.1B, originating from spliced messages. The 228-amino-acid (aa) gpK8.1A is the predominant form associated with the virion envelope, consisting of a 167-aa region identical to gpK8.1B and a 61-aa unique region (L. Zhu, V. Puri, and B. Chandran, Virology 262:237-249, 1999). HHV-8 has a broad in vivo and in vitro cellular tropism, and our studies showed that this may be in part due to HHV-8's interaction with the ubiquitous host cell surface molecule, heparan sulfate (HS). Since HHV-8 K8.1 gene is positionally colinear to the Epstein-Barr virus (EBV) gene encoding the gp350/gp220 protein involved in EBV binding to the target cells, gpK8.1A's ability to interact with the target cells was examined. The gpK8.1A without the transmembrane and carboxyl domains (DeltaTMgpK8.1A) was expressed in a baculovirus system and purified. Radiolabeled purified DeltaTMgpK8.1A protein bound to the target cells, which was blocked by unlabeled DeltaTMgpK8.1A. Unlabeled DeltaTMgpK8.1A blocked the binding of [(3)H]thymidine-labeled purified HHV-8 to the target cells. Binding of radiolabeled DeltaTMgpK8.1A to the target cells was inhibited in a dose-dependent manner by soluble heparin, a glycosaminoglycan (GAG) closely related to HS, but not by other GAGs such as chondroitin sulfate A and C, N-acetyl heparin and de-N-sulfated heparin. Cell surface absorbed DeltaTMgpK8.1A was displaced by soluble heparin. Radiolabeled DeltaTMgpK8.1A also bound to HS expressing Chinese hamster ovary (CHO-K1) cells, and binding to mutant CHO cell lines deficient in HS was significantly reduced. The DeltaTMgpK8.1A specifically bound to heparin-agarose beads, which was inhibited by HS and heparin, but not by other GAGs. Virion envelope-associated gpK8.1A was specifically precipitated by heparin-agarose beads. These findings suggest that gpK8.1A interaction with target cells involves cell surface HS-like moieties, and HHV-8 interaction with HS could be in part mediated by virion envelope-associated gpK8.1A.

Mapping of a dengue virus neutralizing epitope critical for the infectivity of all serotypes: insight into the neutralization mechanism

Dengue virus infections are a growing public health concern and strategies to control the spread of the virus are urgently needed. The murine monoclonal antibody 4E11 might be of interest, since it neutralizes dengue viruses of all serotypes by binding to the 296-400 segment of the major dengue virus envelope glycoprotein (DE). When phage-displayed peptide libraries were screened by affinity for 4E11, phage clone C1 was selected with a 50% frequency. C1 shared three of nine residues with DE(306-314) and showed significant reactivity to 4E11 in ELISA. C1-induced antibodies cross-reacted with DE(296-400) in mice, suggesting that it was a structural equivalent of the native epitope of 4E11 on DE. Accordingly, 4E11 bound to the DE(306-314) synthetic peptide and this reaction was inhibited by DE(296-400). Moreover, DE(306-314) could block dengue virus infection of target cells in an in vitro assay. A three-dimensional model of DE revealed that the three amino acids shared by DE(296-400) and C1 were exposed to the solvent and suggested that most of the amino acids comprising the 4E11 epitope were located in the DE(306-314) region. Since 4E11 blocked the binding of DE(296-400) to heparin, which is a highly sulfated heparan sulfate (HSHS) molecule, 4E11 may act by neutralizing the interaction of DE(306-314) with target cell-displayed HSHS. Our data suggest that the DE(306-314) segment is critical for the infectivity of all dengue virus serotypes and that molecules that block the binding of DE(306-314) to HSHS may be antiviral reagents of therapeutic interest.

Mutational analysis of the major heparan sulfate-binding domain of herpes simplex virus type 1 glycoprotein C

Heparan sulfate (HS) has been identified as a receptor molecule for numerous microbial pathogens, including herpes simplex virus type 1 (HSV-1). To further define the major HS-binding domain of the HSV-1 attachment protein, i.e. glycoprotein C (gC), virus mutants carrying alterations of either two neighbouring basic amino acid residues or a single hydrophobic amino acid residue within the N-terminal domain of the protein (residues 26-227) were constructed. In addition, a mutant lacking the Asn148 glycosylation site was included in the study. Binding of purified mutated gC proteins to isolated HS chains showed that viruses with mutations at residues Arg(129,130), Ile142, Arg(143,145), Arg(145,147), Arg(151,155) and Arg(155,160) had significantly impaired HS binding, in contrast to the other mutations, including Asn148. Impairment of the HS-binding activity of gC by these mutations had profound consequences for virus attachment and infection of cells in which amounts of HS exposed on the cell surface had been reduced. It is suggested that basic and hydrophobic residues localized at the Cys127-Cys144 loop of HSV-1 gC constitute a major HS-binding domain, with the most active amino acids situated near the C-terminal region of the two cysteines.

Human herpesvirus 8 envelope-associated glycoprotein B interacts with heparan sulfate-like moieties

Cell-surface heparan sulfate (HS) serves as an initial attachment receptor for several herpesviruses. The gamma2-human herpesvirus-8 (HHV-8) or Kaposi's sarcoma associated herpesvirus DNA and transcripts have been detected in B cells, endothelial cells, macrophages, and epithelial cells. HHV-8 infects a variety of human and animal cell lines leading to latent or abortive infection. Our studies showed that this broad cellular tropism may be in part due to HHV-8's interaction with the ubiquitous host cell-surface HS-like molecules. HHV-8 binding to the target cells and the infection were inhibited by soluble heparan, a glycosaminoglycan (GAG) closely related to HS. Since HHV-8 gB possess a putative heparan-binding domain (HBD) in the extracellular domain, the interaction of gB with HS-like moieties was examined. Unlike gB of gamma1-Epstein-Barr virus and gamma2-murine herpesvirus 68, HHV-8 gB was expressed on the surface of the infected cell membranes and virion envelopes. Envelope-associated gB was made up of 75 and 54 kDa polypeptides forming disulfide-linked heterodimers and multimers. Rabbit anti-gB antibodies neutralized HHV-8 infection. Virion envelope-associated gB specifically bound to heparan-agarose, which was eluted by high concentration of soluble heparan, but not by chondroitin sulfates. In vitro transcribed and translated products of gB gene specifically bound to heparan-agarose beads, which was blocked by HS and heparan, but not by other GAGs such as chondroitin sulfates (A, B, and C), N-acetyl heparan, and de-N-sulfated heparan. Biotinylated gB peptide corresponding to the putative HBD also bound to heparan. These results suggest that gB plays an important role in the infectious process of HHV-8 and virus interaction with cell-surface HS-like moieties could be in part mediated by the envelope-associated gB.

Identification and analysis of a novel heparin-binding glycoprotein encoded by human herpesvirus 7

Human herpesvirus 6 (HHV-6) and HHV-7 are closely related betaherpesviruses that encode a number of genes with no known counterparts in other herpesviruses. The product of one such gene is the HHV-6 glycoprotein gp82-105, which is a major virion component and a target for neutralizing antibodies. A 1.7-kb cDNA clone from HHV-7 was identified which contains a large open reading frame capable of encoding a predicted primary translational product of 468 amino acids (54 kDa) with 13 cysteine residues and 9 potential N-linked glycosylation sites. This putative protein, which we have termed gp65, was homologous to HHV-6 gp105 (30% identity) and contained a single potential membrane-spanning domain located near its amino terminus. Comparison of the cDNA sequence with that of the viral genome revealed that the gene encoding gp65 contains eight exons, spanning almost 6 kb of the viral genome at the right (3') end of the HHV-7 genome. Northern (RNA) blot analysis with poly(A)(+) RNA from HHV-7-infected cells revealed that the cDNA insert hybridized to a single major RNA species of 1.7 kb. Antiserum raised against a purified, recombinant form of gp65 recognized a protein of roughly 65 kDa in sucrose density gradient-purified HHV-7 preparations; treatment with PNGase F reduced this glycoprotein to a putative precursor of approximately 50 kDa. Gp65-specific antiserum also neutralized the infectivity of HHV-7, while matched preimmune serum did not do so. Finally, analysis of the biochemical properties of recombinant gp65 revealed a specific interaction with heparin and heparan sulfate proteoglycans and not with closely related molecules such as N-acetylheparin and de-N-sulfated heparin. At least two domains of the protein were found to contribute to heparin binding. Taken together, these findings suggest that HHV-7 gp65 may contribute to viral attachment to cell surface proteoglycans.

The porcine humoral immune response against pseudorabies virus specifically targets attachment sites on glycoprotein gC

High titers of virus-neutralizing antibodies directed against glycoprotein gC of Pseudorabies virus (PRV) (Suid herpesvirus 1) are generally observed in the serum of immunized pigs. A known function of the glycoprotein gC is to mediate attachment of PRV to target cells through distinct viral heparin-binding domains (HBDs). Therefore, it was suggested that the virus-neutralizing activity of anti-PRV sera is directed against HBDs on gC. To address this issue, sera with high virus-neutralizing activity against gC were used to characterize the anti-gC response. Epitope mapping demonstrated that amino acids of HBDs are part of an antigenic antibody binding domain which is located in the N-terminal part of gC. Binding of antibodies to this antigenic domain of gC was further shown to interfere with the viral attachment. Therefore, these results show that the viral HBDs are accessible targets for the humoral anti-PRV response even after tolerance induction against self-proteins, which utilize similar HBDs to promote host protein-protein interactions. The findings indicate that the host's immune system can specifically block the attachment function of PRV gC. Since HBDs promote the attachment of a number of herpesviruses, the design of future antiherpesvirus vaccines should aim to induce a humoral immune response that prevents HBD-mediated viral attachment.

Lysine residue 117 of the FasG adhesin of enterotoxigenic Escherichia coli is essential for binding of 987P fimbriae to sulfatide

The FasG subunit of the 987P fimbriae of enterotoxigenic strains of Escherichia coli was previously shown to mediate fimbrial binding to a glycoprotein and a sulfatide receptor on intestinal brush borders of piglets. Moreover, the 987P adhesin FasG is required for fimbrial expression, since fasG null mutants are nonfimbriated. In this study, fasG was modified by site-directed mutagenesis to study its sulfatide binding properties. Twenty single mutants were generated by replacing positively charged lysine (K) or arginine (R) residues with small, nonpolar alanine (A) residues. Reduced levels of binding to sulfatide-containing liposomes correlated with reduced fimbriation and FasG surface display in four fasG mutants (R27A, R286A, R226A, and R368). Among the 16 remaining normally fimbriated mutants with wild-type levels of surface-exposed FasG, only one mutant (K117A) did not interact at all with sulfatide-containing liposomes. Four mutants (K117A, R116A, K118A, and R200A) demonstrated reduced binding to such liposomes. Since complete phenotypic dissociation between the structure and specific function of 987P was observed only with mutant K117A, this residue is proposed to play an essential role in the FasG-sulfatide interaction, possibly communicating with the sulfate group of sulfatide by hydrogen bonding and/or salt bridge formation. Residues K17, R116, K118, and R200 may stabilize this interaction.

Identification of functional domains in the 14-kilodalton envelope protein (A27L) of vaccinia virus

The mechanism of entry of vaccinia virus (VV) into cells is still a poorly understood process. A 14-kDa protein (encoded by the A27L gene) in the envelope of intracellular mature virus (IMV) has been implicated in virus-cell attachment, virus-cell fusion, and virus release from cells. We have previously described the structural organization of the VV 14-kDa protein, consisting of a triple-stranded coiled-coil region responsible for oligomer formation and a predicted Leu zipper-like third alpha helix with an important role in the interaction with a 21-kDa membrane protein (encoded by the A17L gene) thought to anchor the 14-kDa protein to the envelope of IMV (M.-I. Vázquez, G. Rivas, D. Cregut, L. Serrano, and M. Esteban, J. Virol. 72:10126-10137, 1998). To identify the functional domains important for virus entry and release, we have generated VV recombinants containing a copy of the A27L gene regulated by the lacI operator-repressor system of Escherichia coli (VVIndA27L) in the thymidine kinase locus and a mutant form of the A27L gene in the hemagglutinin locus but expressed constitutively under the control of an early-late VV promoter. Cells infected with a VV recombinant that expresses a mutant 14-kDa form lacking the first 29 amino acids at the N terminus failed to form extracellular enveloped virus (EEV). Fusion-from-without assays with purified virus confirmed that the fusion process was mediated by the 14-kDa protein and the fusion domain to be contained within amino acids 29 to 43 of the N-terminal region. Competitive inhibition of the infection process with soluble heparin and synthetic peptides and in vitro experiments with purified mutant proteins identified the heparin binding domain within amino acids 21 to 33, suggesting that this domain is involved in virus-cell binding via heparan sulfate. Thus, the N terminus of the 14-kDa protein contains a heparin binding domain, a fusion domain, and a domain responsible for interacting with proteins or lipids in the Golgi stacks for EEV formation and virus spread.

Neuroinvasiveness of pseudorabies virus injected intracerebrally is dependent on viral concentration and terminal field density

Pseudorabies virus (PRV), a neurotropic swine alpha herpesvirus, has been used extensively for transneuronal analysis ofmultisynaptic circuitry after peripheral injection. In the present analysis, we examined the influence of viral concentration and neuronal architecture on the invasiveness, replication, and transynaptic passage of an attenuated strain of PRV (PRV-Bartha) injected into rat striatum. Different concentrations of PRV-Bartha were injected into the striatum at a constant rate of infusion (10 nl/minute), and animals were killed 50 hours later. Viral concentration was manipulated by either altering the volume of the inoculum (100, 50, 20 nl) or by diluting the inoculum within a constant volume of 100 nl. Immunohistochemical localization of infected neurons revealed dramatic differences in the progression of infection that were dependent directly on the concentration of injected virus. In every case, the pattern of infection was consistent with preferential uptake of virions by axon terminals and retrograde transynaptic passage of virus from the injection site. The known topographically organized corticostriatal projections permitted a precise definition of the zone of viral uptake. This analysis demonstrated that the "effective zone of viral uptake" (i.e., the zone within which viral uptake led to productive replication of virus) varied in relation to the concentration of injected virus, with the highest concentration of PRV invading terminals within a 500 microm radius of the canula. Concentration-dependent changes in the progression of retrograde transynaptic infection also were observed. The highest concentration of virus produced the most extensive infection. The distribution of infected neurons in these cases included those with known afferent projections to striatum as well as those that became infected by retrograde transynaptic infection. Lesser concentrations of PRV-Bartha produced an increasingly restricted infection of the same circuitry within the same postinoculation interval. It is noteworthy that neurons known to elaborate dense striatal terminal fields were less sensitive to reduction in viral concentration than those giving rise to terminal fields of lesser density. Collectively, the data indicate that the onset of viral replication after intracerebral injection of PRV is directly dependent on virus concentration and terminal field density at the site of virus injection.

Baculovirus vector requires electrostatic interactions including heparan sulfate for efficient gene transfer in mammalian cells

BACKGROUND

Recently, several reports have described the ability of recombinant baculoviruses to transduce a variety of mammalian cells. Yet, mechanisms involved in baculovirus entry in those cells remain largely unexplored, particularly at the primary binding step of the virions to the cell membrane.

METHODS

This report focused on the primary virus-cell interactions that lead to in vitro transduction of human 293 cells using a polyhedrin-deleted baculovirus harboring a CMV-driven beta-galactosidase gene (BacLacZ).

RESULTS

Infection rate monitored for 8 h and transduction rate with a multiplicity of infection of up to 800 were, both, non-saturable. Temperatures from 37 degrees C to 4 degrees C dramatically impaired BacLacZ but not adenovirus cell attachment. Competitive infections performed with an excess of a non LacZ-expressing baculovirus hardly competed at a 1/1 ratio. Consistent with an adsorptive binding process onto the cell surface, interactions through electrostatic charges between both viral and cell membranes appeared to be critical for BacLacZ transduction. The addition of polybrene to the cells prior to or during the infection prevented both virus binding and LacZ gene transfer, suggesting the involvement of negatively charged epitopes exposed at the cell surface. The simultaneous presence of the highly charged heparin abrogated BacLacZ binding to the cell surface and subsequent gene transfer. Lastly, direct in vitro binding of BacLacZ to heparin but not BSA columns could be demonstrated after elution of infectious BacLacZ virus in high salt molarity.

CONCLUSION

Electrostatic charges play a critical role during the first step in mammalian cell transduction mediated by a recombinant baculovirus.

Transneuronal tracing of neural pathways controlling an abdominal muscle, rectus abdominis, in the ferret

Abdominal muscles participate in generating a large number of behaviors and reflex responses, including expiration, coughing, sneezing, vomiting, postural control, production of speech, straining, facilitation of venous return to the heart, and reaction to vestibular stimulation. However, the only premotor neurons that have been conclusively shown to influence abdominal motoneurons are located in nucleus retroambiguus, the expiratory region of the caudal ventral respiratory group. In the present study, the neural circuitry controlling the activity of one abdominal muscle, rectus abdominis, was mapped using the transneuronal tracer pseudorabies virus (PRV) in the ferret. Injections of PRV into rectus abdominis labeled large presumed motoneurons in the ventral horn of T12-L4, and smaller presumed interneurons that were scattered in laminae VII, VIII, IX, and X of T4-L4. In addition, neurons in several areas of the medulla and caudal pons, including the retroambigual nucleus, medial and ventromedial reticular formation, nucleus prepositus hypoglossi, vestibular nuclei, and raphe nuclei, were infected by transynaptic passage of PRV from rectus abdominis motoneurons. Thus, the multifunctional roles of abdominal muscles appear to be coordinated by premotor neurons located in both the spinal cord and several regions of the brainstem.

Adaptation of Sindbis virus to BHK cells selects for use of heparan sulfate as an attachment receptor

Attachment of Sindbis virus to the cell surface glycosaminoglycan heparan sulfate (HS) and the selection of this phenotype by cell culture adaptation were investigated. Virus (TR339) was derived from a cDNA clone representing the consensus sequence of strain AR339 (K. L. McKnight, D. A. Simpson, S. C. Lin, T. A. Knott, J. M. Polo, D. F. Pence, D. B. Johannsen, H. W. Heidner, N. L. Davis, and R. E. Johnston, J. Virol. 70:1981-1989, 1996) and from mutant clones containing either one or two dominant cell culture adaptations in the E2 structural glycoprotein (Arg instead of Ser at E2 position 1 [designated TRSB]) or this mutation plus Arg for Ser at E2 114 [designated TRSB-R114]). The consensus virus, TR339, bound to baby hamster kidney (BHK) cells very poorly. The mutation in TRSB increased binding 10- to 50-fold, and the additional mutation in TRSB-R114 increased binding 3- to 5-fold over TRSB. The magnitude of binding was positively correlated with the degree of cell culture adaptation and with attenuation of these viruses in neonatal mice. HS was identified as the attachment receptor for the mutant viruses by the following experimental results. (i) Low concentrations of soluble heparin inhibited plaque formation on and binding of mutant viruses to BHK cells by >95%. In contrast, TR339 showed minimal inhibition at high concentrations. (ii) Binding and infectivity of TRSB-R114 was sensitive to digestion of cell surface HS with heparinase III, and TRSB was sensitive to both heparinase I and heparinase III. TR339 infectivity was only slightly affected by either digestion. (iii) Radiolabeled TRSB and TRSB-R114 attached efficiently to heparin-agarose beads in binding assays, while TR339 showed virtually no binding. (iv) Binding and infectivity of TRSB and TRSB-R114, but not TR339, were greatly reduced on Chinese hamster ovary cells deficient in HS specifically or all glycosaminoglycans. (v) High-multiplicity-of-infection passage of TR339 on BHK cell cultures resulted in rapid coselection of high-affinity binding to BHK cells and attachment to heparin-agarose beads. Sequencing of the passaged virus population revealed a mutation from Glu to Lys at E2 70, a mutation common to many laboratory strains of Sindbis virus. These results suggest that TR339, the most virulent virus tested, attaches to cells through a low-affinity, primarily HS-independent mechanism. Adaptive mutations, selected during cell culture growth of Sindbis virus, enhance binding and infectivity by allowing the virus to attach by an alternative mechanism that is dependent on the presence of cell surface HS.

Interaction between pseudorabies virus and heparin/heparan sulfate. Pseudorabies virus mutants differ in their interaction with heparin/heparan sulfate when altered for specific glycoprotein C heparin-binding domain

Cell surface heparan sulfate serves as an initial receptor for a number of herpesviruses including pseudorabies virus (PrV). It has been demonstrated that the heparan sulfate-binding domain of PrV glycoprotein C is composed of three discrete clusters of basic residues corresponding to amino acids 76-RRKPPR-81, 96-HGRKR-100, and 133-RFYRRGRFR-141, respectively, and that these clusters are functionally redundant, i.e. each of them could independently support PrV attachment to cells (Flynn, S. J., and Ryan, P. (1996) J. Virol. 70, 1355-1364). To evaluate the functional significance of each of these clusters we have used PrV mutants in which, owing to specific alterations in glycoprotein C, the heparan sulfate-binding site is dominated by a single specific cluster. These mutants exhibited different patterns of susceptibility to selectively N-, 2-O-, and 6-O-desulfated heparin preparations in virus attachment/infectivity assay. Moreover PrV mutants differed as regard to efficiency of their attachment to and infection of cells pretreated with relatively low amounts of heparan sulfate-degrading enzymes. Furthermore glycoprotein C species, purified from respective mutants, bound heparin oligosaccharide fragments of different minimum size. These differences suggest that specific clusters of basic amino acids of the heparan sulfate-binding domain of glycoprotein C may support PrV binding to different structural features/stretches within the heparan sulfate chain.

Attenuation of the vaccine Oka strain of varicella-zoster virus and role of glycoprotein C in alphaherpesvirus virulence demonstrated in the SCID-hu mouse

The SCID-hu mouse implanted with human fetal tissue is a novel model for investigating human viral pathogenesis. Infection of human skin implants was used to investigate the basis for the clinical attenuation of the varicella-zoster virus (VZV) strain, V-Oka, from which the newly licensed vaccine is made. The pathogenicity of V-Oka was compared with that of its parent, P-Oka, another low-passage clinical isolate, strain Schenke (VZV-S), and VZV-Ellen, a standard laboratory strain. The role of glycoprotein C (gC) in infectivity for human skin was assessed by using gC-negative mutants of V-Oka and VZV-Ellen. Whereas all of these VZV strains replicated well in tissue culture, only low-passage clinical isolates were fully virulent in skin, as shown by infectious virus yields and analysis of implant tissues for VZV DNA and viral protein synthesis. The infectivity of V-Oka in skin was impaired compared to that of P-Oka, providing the first evidence of a virologic basis for the clinical attenuation of V-Oka. The infectivity of V-Oka was further diminished in the absence of gC expression. All strains except gC-Ellen retained some capacity to replicate in human skin, but cell-free virus was recovered only from implants infected with P-Oka or VZV-S. Although VZV is closely related to herpes simplex virus type 1 (HSV-1) genetically, experiments in the SCID-hu model revealed differences in tropism for human cells that correlated with differences in VZV and HSV-1 disease. VZV caused extensive infection of epidermal and dermal skin cells, while HSV-1 produced small, superficial lesions restricted to the epidermis. As in VZV, gC expression was a determinant for viral replication in skin. VZV infects human CD4+ and CD8+ T cells in thymus/liver implants, but HSV-1 was detected only in epithelial cells, with no evidence of lymphotropism. These SCID-hu mouse experiments show that the clinical attenuation of the varicella vaccine can be attributed to decreased replication of V-Oka in skin and that tissue culture passage alone reduces the ability of VZV to infect human skin in vivo. Furthermore, gC, which is dispensable for replication in tissue culture, plays a critical role in the virulence of the human alphaherpesviruses VZV and HSV-1 for human skin.

Glycosaminoglycan-protein interactions: definition of consensus sites in glycosaminoglycan binding proteins

Although interactions of proteins with glycosaminoglycans (GAGs), such as heparin and heparan sulphate, are of great biological importance, structural requirements for protein-GAG binding have not been well-characterised. Ionic interactions are important in promoting protein-GAG binding. Polyelectrolyte theory suggests that much of the free energy of binding comes from entropically favourable release of cations from GAG chains. Despite their identical charges, arginine residues bind more tightly to GAGs than lysine residues. The spacing of these residues may determine protein-GAG affinity and specificity. Consensus sequences such as XBBBXXBX, XBBXBX and a critical 20 A spacing of basic residues are found in some protein sites that bind GAG. A new consensus sequence TXXBXXTBXXXTBB is described, where turns bring basic interacting amino acid residues into proximity. Clearly, protein-GAG interactions play a prominent role in cell-cell interaction and cell growth. Pathogens including virus particles might target GAG-binding sites in envelope proteins leading to infection.

Transmission, species specificity, and pathogenicity of Aujeszky's disease virus

Aujeszky's disease virus (ADV), also known as pseudorabies virus (PrV), is an alphaherpesvirus that causes fatal infections in a wide range of animal species. The virus shares a variety of biological properties with human pathogenic herpesviruses like herpes simplex virus or varicella-zoster virus. Although only limited data are available, it seems unlikely that PrV causes disease in immunocompetent humans, but may pose a risk for immunocompromised patients.