Regulation of glycoprotein D synthesis: does alpha 4, the major regulatory protein of herpes simplex virus 1, regulate late genes both positively and negatively?

The HIV Integrase Inhibitor Raltegravir Inhibits Felid Alphaherpesvirus 1 Replication by Targeting both DNA Replication and Late Gene Expression

Alphaherpesvirus-associated ocular infections in humans caused by human alphaherpesvirus 1 (HHV-1) remain challenging to treat due to the frequency of drug application required and the potential for the selection of drug-resistant viruses. Repurposing on-the-market drugs is a viable strategy to accelerate the pace of drug development. It has been reported that the human immunodeficiency virus (HIV) integrase inhibitor raltegravir inhibits HHV-1 replication by targeting the DNA polymerase accessory factor and limits terminase-mediated genome cleavage of human betaherpesvirus 5 (HHV-5). We have previously shown, both in vitro and in vivo, that raltegravir can also inhibit the replication of felid alphaherpesvirus 1 (FeHV-1), a common ocular pathogen of cats with a pathogenesis similar to that of HHV-1 ocular disease. In contrast to what was reported for HHV-1, we were unable to select for a raltegravir-resistant FeHV-1 strain in order to define any basis for drug action. A candidate-based approach to explore the mode of action of raltegravir against FeHV-1 showed that raltegravir did not impact FeHV-1 terminase function, as described for HHV-5. Instead, raltegravir inhibited DNA replication, similarly to HHV-1, but by targeting the initiation of viral DNA replication rather than elongation. In addition, we found that raltegravir specifically repressed late gene expression independently of DNA replication, and both activities are consistent with inhibition of ICP8. Taken together, these results suggest that raltegravir could be a valuable therapeutic agent against herpesviruses.IMPORTANCE The rise of drug-resistant herpesviruses is a longstanding concern, particularly among immunocompromised patients. Therefore, therapies targeting viral proteins other than the DNA polymerase that may be less likely to lead to drug-resistant viruses are urgently needed. Using FeHV-1, an alphaherpesvirus closely related to HHV-1 that similarly causes ocular herpes in its natural host, we found that the HIV integrase inhibitor raltegravir targets different stages of the virus life cycle beyond DNA replication and that it does so without developing drug resistance under the conditions tested. This shows that the drug could provide a viable strategy for the treatment of herpesvirus infections.

The ectodomain of a novel member of the immunoglobulin subfamily related to the poliovirus receptor has the attributes of a bona fide receptor for herpes simplex virus types 1 and 2 in human cells

We report on the functional cloning of a hitherto unknown member of the immunoglobulin (Ig) superfamily selected for its ability to confer susceptibility to herpes simplex virus (HSV) infection on a highly resistant cell line (J1.1-2 cells), derived by exposure of BHKtk- cells to a recombinant HSV-1 expressing tumor necrosis factor alpha (TNF-alpha). The sequence of herpesvirus Ig-like receptor (HIgR) predicts a transmembrane protein with an ectodomain consisting of three cysteine-bracketed domains, one V-like and two C-like. HIgR shares its ectodomain with and appears to be an alternative splice variant of the previously described protein PRR-1 (poliovirus receptor-related protein). Both HIgR and PRR-1 conferred on J1.1-2 cells susceptibility to HSV-1, HSV-2, and bovine herpesvirus 1. The viral ligand of HIgR and PRR-1 is glycoprotein D, a constituent of the virion envelope long known to mediate viral entry into cells through interaction with cellular receptor molecules. Recently, PRR-1, renamed HveC (herpesvirus entry mediator C), and the related PRR-2, renamed HveB, were reported to mediate the entry of HSV-1, HSV-2, and bovine herpesvirus 1, and the homologous poliovirus receptor was reported to mediate the entry of pseudorabies virus (R. J. Geraghty, C. Krummenacher, G. H. Cohen, R. J. Eisenberg, and P. G. Spear, Science 280:1618-1620, 1998; M. S. Warner, R. J. Geraghty, W. M. Martinez, R. I. Montgomery, J. C. Whitbeck, R. Xu, R. J. Eisenberg, G. H. Cohen, and P. G. Spear, Virology 246:179-189, 1998). Here we further show that HIgR or PRR-1 proteins detected by using a monoclonal antibody to PRR-1 are widely distributed among human cell lines susceptible to HSV infection and commonly used for HSV studies. The monoclonal antibody neutralized virion infectivity in cells transfected with HIgR or PRR-1 cDNA, as well as in the human cell lines, indicating a direct interaction of virions with the receptor molecule, and preliminarily mapping this function to the ectodomain of HIgR and PRR-1. Northern blot analysis showed that HIgR or PRR-1 mRNAs were expressed in human tissues, with the highest expression being detected in nervous system samples. HIgR adds a novel member to the cluster of Ig superfamily members able to mediate the entry of alphaherpesviruses into cells. The wide distribution of HIgR or PRR-1 proteins among human cell lines susceptible to HSV infection, coupled with the neutralizing activity of the antibody in the same cells, provides direct demonstration of the actual use of this cluster of molecules as HSV-1 and HSV-2 entry receptors in human cell lines. The high level of expression in samples from nervous system makes the use of these proteins in human tissues very likely. This cluster of molecules may therefore be considered to constitute bona fide receptors for HSV-1 and HSV-2.

Herpes simplex virus recombination vectors designed to allow insertion of modified promoters into transcriptionally "neutral" segments of the viral genome

The use of recombinant viruses has been essential in investigation of the biology of herpes simplex virus (HSV). In this communication we describe a number of viral recombination vectors that we have generated for use in promoter structure/function analysis within the context of the HSV-1 genome. We have utilized two regions of the HSV genome that contain genes nonessential for replication in cultured cells--the glycoprotein C (gC or UL44) locus in the UL of the genome and the area encompassing the promoter and 5' portion of the latency associated transcript (LAT) within the RL factual influence on promoters due to the site of insertion. Two different kinetic promoters were analyzed, those controlling expression of the gamma UL 38 and the beta dUTPase genes, in both loci. All constructs tested displayed reporter gene mRNA expression with expected kinetics, and we conclude that there are no neighboring cryptic promoter elements that could interfere with expression studies using the vectors described.

Transformed cells producing the glycoprotein D of HSV-1 are resistant to infection with clinical strains of HSV

The generality of the resistance exhibited by gD producing cells to HSV-1 infection was tested. We tested three different cell lines producing various amounts of gD for resistance against three HSV-1 strains. The strains used were the prototype laboratory F strain and two recently isolated low passage local clinical strains, VG and VD. The results indicate that: (i) the resistance of the cell lines is directly related to the amount of gD they produce, (ii) the cell lines showed greater resistance against the two local clinical HSV-1 strains than against the laboratory strain, and (iii) the resistance is not mediated at the level of virus adsorption to the cell membranes.

Mapping of herpes simplex virus 1 genes with mutations which overcome host restrictions to infection

Earlier studies have shown that the thymidine kinase-negative baby hamster kidney (BHKTK-) cell lines expressing constitutively the herpes simplex virus 1 (HSV-1) glycoprotein D (gD), designated BJ, restrict infection by HSV-1 at the level of virus entry. U10, a HSV-1 mutant not restricted by the BJ cells, carried the substitution of proline for Leu25 in the gD gene, suggesting that gD encodes a specialized domain which precludes virus entry into cells expressing gD. Analyses of a new series of 36 unrestricted viral mutants showed the following. (i) Only two mutants contained mutations at a site which did not overlap with the previously reported mutation. A representative of a previously mapped mutant and one of the two new mutants were examined in detail. Thus, in the gD of mutant U30 Ala185 was replaced by threonine, whereas in gD of U21, Ala185 and Leu25 were replaced with threonine and proline, respectively. U30 and U21 multiplied better than the wild-type parent virus in the parental BHKTK- cells. (ii) Transfer of the gD gene from U21 or U30 to wild-type parent virus or to the gD- virus FgD beta yielded recombinants which, while capable of infecting BJ cells, were considerably less efficient than the parent unrestricted mutants, suggesting that the latter contained additional mutations which were responsible in part for the unrestricted phenotype. Conversely, marker rescue of mutant viruses with wild-type gD reduced but did not abrogate entirely the unrestricted phenotype. (iii) Mutations in gD which conferred the unrestricted phenotype were not random. (iv) gD plays a role in the restriction, inasmuch as preincubation of cells expressing gD with antibodies to gD abolished restriction. (v) In mutant R5000, the gD substitution Ser140 to Asn was capable of overcoming a restriction of a BHKTK- clonal line which does not express gD but conferred very low ability to replicate on BJ cells. We conclude that (a) uncloned stocks of BHKTK- cells exhibit a low level restriction to infection with wild-type virus, (b) clonal lines of BHKTK- cells which vary with respect to the stringency of restriction express either allelic genes differing in the properties of their products or products of different genes, and (c) both the restricted and unrestricted phenotypes reflect the interactions of gD with these cellular products. The implications of these conclusions with respect to the restriction imposed on BHK cells by the expression of gD are discussed.

Binding sites for the herpes simplex virus immediate-early protein ICP4 impose an increased dependence on viral DNA replication on simple model promoters located in the viral genome

We examined the ability of binding sites for the herpes simplex virus immediate-early protein ICP4 to alter the regulation of closely linked promoters by placing strong ICP4 binding sites upstream or downstream of simple TATA promoters in the intact viral genome. We found that binding sites strongly reduced the levels of expression at early times postinfection and that this effect was partially overcome after the onset of viral DNA replication. These data confirm that DNA-bound ICP4 can inhibit the activity of a closely linked promoter and raise the possibility that ICP4 binding sites contribute to temporal regulation during infection.

Mapping of the functional domains of the alpha 4 protein of herpes simplex virus 1

Truncated alpha 4 genes were introduced into BHK tk- cells along with the neomycin phosphotransferase gene, that confers resistance to the eukaryotic antibiotic G418, driven by the HSV-1 beta tk promoter (beta tk- neor). Stably transformed cell lines were obtained and studied for the ability of the resident truncated alpha 4 genes to regulate the expression of the beta tk- neor, and for the ability of the truncated alpha 4 polypeptides to localize to the nuclei of transformed cells. The results indicated that the domain(s) for beta gene induction and for nuclear localization of the alpha 4 protein are located within the N-terminal 288 amino acids of the protein.

Regulation of glycoprotein D synthesis of herpes simplex virus 1 by alpha 4 protein, the major regulatory protein of the virus, in stably transformed cell lines: effect of the relative gene copy numbers

Earlier studies concerning gamma 1 gene regulation by the alpha 4 protein, the major regulatory protein of herpes simplex virus 1 (HSV-1), in stably transformed cell lines, reported conflicting results, i.e., alpha 4 protein positively regulated the gamma 1 gB gene in alpha 4/gB cells, while it negatively regulated the gamma 1 gD gene in alpha 4/BJ cells. Both cell lines were derived from a common parental cell line alpha 4/c 113 that contains 1 copy of the alpha 4 gene, and the only apparent difference between them was the relative copy number of the gB and gD sequences (1 and 30-50, respectively) resident in the cell genome. We investigated this disparity by constructing a cell line (BA 4) that contains one copy each of the alpha 4 and gamma 1 gD sequences, by fusion of alpha 4/c 113 and BJt cells, containing and expressing respectively 1 copy of the alpha 4 and gD genes. BA 4 cells constitutively expressed both the alpha 4, gD genes inherited from the parental cell lines (alpha 4/c 113 and BJt). In BA 4 cells that alpha 4 protein positively regulates the gD gene as evidenced from (i) higher levels of gD expression than the parental BJt cells lacking the alpha 4 gene, and (ii) significant decrease in gD expression under conditions that render the alpha 4 protein produced in BA 4 cells non-functional. In addition the gamma 2gG gene contained within the DNA fragment encoding the gD gene, is also expressed in BA 4 cells. On the basis of these data, we propose that gamma gene regulation by the alpha 4 protein is affected by the relative copy number of these genes, resident in the cell genome.

Mutational analysis of the ICP4 binding sites in the 5' transcribed noncoding domains of the herpes simplex virus 1 UL 49.5 gamma 2 gene

A previous report (P. Mavromara-Nazos and B. Roizman, Proc. Natl. Acad. Sci. USA 86:4071-4075, 1989) demonstrated that substitution of sequences of the thymidine kinase (tk) gene, a beta gene, extending from -16 to +51 with sequences extending from -12 to +104 of the gamma 2 UL 49.5 gene in viral recombinant R3820 conferred upon the chimeric gene gamma 2 attributes in the context of the viral genome in a productive infection. The UL49.5 gene sequences extending from -179 to +104 contain four DNA binding sites for the major regulatory protein ICP4. Of these sites, two map between nucleotides +20 and +80 within the sequence which confers gamma 2 regulation upon the chimeric gene. To determine the role of these ICP4 binding sites in conferring the gamma 2 gene attributes, sequences comprising the two ICP4 binding sites were mutagenized and used to reconstruct the R3820 recombinant virus. In addition, a new recombinant virus (R8023) was constructed in which tk sequences extending from -240 to +51 were replaced with wild-type or mutated sequences contained between nucleotides -179 to +104 of the UL 49.5 gene. Vero cells infected with the recombinant viruses in the presence or absence of phosphonoacetate, a specific inhibitor of viral DNA synthesis, were then tested for accumulation of tk RNA by using an RNase protection assay. The results indicate that in the recombinant R3820, a mutation which destroyed one of the two UL49.5 ICP4 DNA binding sites significantly reduced the accumulation of tk RNA at both early and late times after infection. The effect of this mutation was less pronounced in cells infected with the R8023 virus, whose chimeric tk gene contains the two upstream UL49.5 ICP4 binding sites. None of the mutations affected the sensitivity of the chimeric genes to phosphonoacetate. The mutated site appears to be involved in the accumulation of RNA.

Herpes simplex virus infection can occur without involvement of the fibroblast growth factor receptor

Basic fibroblast growth factor (bFGF) has been reported to block uptake of herpes simplex virus type 1 (HSV-1) and plaque formation on arterial smooth muscle cells, suggesting a role for the bFGF receptor in HSV entry (R. J. Kaner, A. Baird, A. Mansukhani, C. Basilico, B. D. Summers, R. Z. Florkiewicz, and D. P. Hajjar, Science 248:1410-1413, 1990). We confirmed the effect of bFGF on infection of this cell type with HSV-1 and HSV-2 and found the same result with umbilical vein endothelial cells. However, bFGF does not inhibit plaque formation on any other cell type we tested. Furthermore, there is no correlation between the level of expression of the bFGF receptor and the effect of bFGF. HEp-2 and A431 cells express barely detectable levels of receptor, and yet they are fully permissive for HSV infection in a bFGF-insensitive manner. Thus, interaction of virus with the bFGF receptor is not required for infection of many cell types. In addition, infection of smooth muscle cells is not prevented by incubation of virus with an anti-bFGF antibody, arguing against the hypothesis that virion-associated bFGF acts as a bridge between virus and receptor (A. Baird, R. Z. Florkiewicz, P. A. Maher, R. J. Kaner, and D. P. Hajjar, Nature [London] 348:344-346, 1990).

Origin of unenveloped capsids in the cytoplasm of cells infected with herpes simplex virus 1

In cells infected with herpes simplex viruses the capsids acquire an envelope at the nuclear membrane and are usually found in the cytoplasm in structures bound by membranes. Infected cells also accumulate unenveloped capsids alone or juxtaposed to cytoplasmic membranes. The juxtaposed capsids have been variously interpreted as either undergoing terminal deenvelopment resulting from fusion of the envelope with the membrane of the cytoplasmic vesicles or undergoing sequential envelopment and deenvelopment as capsids transit the cytoplasm into the extracellular space. Recent reports have shown that (i) wild-type virus attaches to but does not penetrate cells expressing glycoprotein D (G. Campadelli-Fiume, M. Arsenakis, F. Farabegoli, and B. Roizman, J. Virol. 62:159-167, 1988) and that (ii) a mutation in glycoprotein D enables the mutant virus to productively infect cells expressing the wild-type glycoprotein (G. Campadelli-Fiume, S. Qi, E. Avitabile, L. Foa-Tomasi, R. Brandimarti, and B. Roizman, J. Virol. 64:6070-6079, 1990). If the unenveloped capsids in the cytoplasm result from fusion of the cytoplasmic membranes with the envelopes of viruses transiting the cytoplasm, cells infected with virus carrying the mutation in glycoprotein D should contain many more unenveloped capsids in the cytoplasm inasmuch as there would be little or no restriction in the fusion of the envelope with cytoplasmic membranes. Comparison of thin sections of baby hamster kidney cells infected with wild-type and mutant viruses indicated that this was the case. Moreover, in contrast to the wild-type parent, the mutant virus was not released efficiently from infected cells. The conclusion that the unenveloped capsids are arrested forms of deenveloped capsids is supported by the observation that the unenveloped capsids were unstable in that they exhibited partially extruded DNA.

Herpes simplex virus (HSV) glycoprotein H is partially processed in a cell line that expresses the glycoprotein and fully processed in cells infected with deletion or ts mutants in the known HSV glycoproteins

Cell lines that constitutively express herpes simplex virus 1 (HSV-1) glycoprotein H (gH-1) failed to synthesize the mature form of gH and accumulated a precursor-like form of the glycoprotein, which was retained intracellularly, most likely in RER. Fine-structure analysis of the oligosaccharides present in recombinant gH revealed oligosaccharides processed by RER enzymes; sialylated complex-type and biantennary oligosaccharides, which are assembled in the trans-Golgi, were absent. A small fraction had the characteristics of oligosaccharides processed by the early mannosidases of the Golgi. These findings suggest that a defect in the transport out of RER to the Golgi may account for the intracellular retention of the immature form of gH in cells that express the glycoprotein constitutively. Upon superinfection of cells expressing gH-1 with HSV-2, recombinant gH-1 underwent maturation, indicating that a viral function is required to attain full processing of gH. The known HSV glycoproteins do not appear to carry out this function, since in cells infected with deletion mutants in gD, gG, gE, and gE-gI, with a spontaneous gC- mutant, or with a temperature-sensitive mutant in gB, maturation of gH occurred independently of the presence or of the maturation of the single glycoproteins tested. The present findings together with previous observations on HSV, human CMV, and the EBV homologue of gH suggest that inability of gH to undergo full processing in the absence of viral protein(s) is a property of gH.

Glycoprotein D of herpes simplex virus encodes a domain which precludes penetration of cells expressing the glycoprotein by superinfecting herpes simplex virus

Earlier studies have shown that herpes simplex viruses adsorb to but do not penetrate permissive baby hamster kidney clonal cell lines designated the BJ series and constitutively expressing the herpes simplex virus 1 (HSV-1) glycoprotein D (gD). To investigate the mechanism of the restriction, the following steps were done. First, wild-type HSV-1 strain F [HSV-1(F)] virus was passaged blindly serially on clonal line BJ-1 and mutant viruses [HSV-1(F)U] capable of penetration were selected. The DNA fragment capable of transferring the capacity to infect BJ cells by marker transfer contains the gD gene. The mutant gD, designated gDU, differed from wild-type gD only in the substitution of Leu-25 by proline. gDU reacted with monoclonal antibodies which neutralize virus and whose epitopes encompass known functional domains involved in virus entry into cells. It did not react with the monoclonal antibody AP7 previously shown to react with an epitope which includes Leu-25. Second, cell lines expressing gDU constitutively were constructed and cloned. Unlike the clonal cell lines constitutively expressing gD (e.g., the BJ cell line), those expressing gDU were infectable by both HSV-1(F) and HSV-1(F)U. Lastly, exposure of BJ cells to monoclonal antibody AP7 rendered the cells capable of being infected with HSV-1(F). The results indicate that (i) gD expresses a specific function, determined by sequences at or around Leu-25, which blocks entry of virus into cells synthesizing gD, (ii) the gD which blocks penetration by superinfecting virus is located in the plasma membrane, (iii) the target of the restriction to penetration is the identical domain of the gD molecule contained in the envelope of the superinfecting virus, and (iv) the molecular basis of the restriction does not involve competition for a host protein involved in entry, as was previously thought.

Bovine cells expressing bovine herpesvirus 1 (BHV-1) glycoprotein IV resist infection by BHV-1, herpes simplex virus, and pseudorabies virus

We expressed the bovine herpesvirus 1 (BHV-1) glycoprotein IV (gIV) in bovine cells. The protein expressed was identical in molecular mass and antigenic reactivity to the native gIV protein but was localized in the cytoplasm. Expressing cells were partially resistant to BHV-1, herpes simplex virus, and pseudorabies virus, as shown by a 10- to 1,000-fold-lower number of plaques forming on these cells than on control cells. The level of resistance depended on the level of gIV expression and the type and amount of challenge virus. These data are consistent with previous reports by others that cellular expression of the BHV-1 gIV homologs, herpes simplex virus glycoprotein D, and pseudorabies virus glycoprotein gp50 provide partial resistance against infection with these viruses. We have extended these findings by showing that once BHV-1 enters gIV-expressing cells, it replicates and spreads normally, as shown by the normal size of BHV-1 plaques and the delayed but vigorous synthesis of viral proteins. Our data are consistent with the binding of BHV-1 gIV to a cellular receptor required for initial penetration by all three herpesviruses and interference with the function of that receptor molecule.

Identification of a site on herpes simplex virus type 1 glycoprotein D that is essential for infectivity

Herpes simplex virus glycoprotein D (gD) plays an essential role during penetration of the virus into cells. There is evidence that it recognizes a specific receptor after initial attachment of virions to cell surface heparan sulfate and also that gD-1, gD-2, and gp50 (the pseudorabies virus gD homolog) bind to the same receptor. Although the antigenic structure of gD has been studied intensively, little is known about functional regions of the protein. Antigenic site I is a major target for neutralizing antibodies and has been partially mapped by using deletion mutants and neutralization-resistant viruses. Working on the assumption that such a site may overlap with a functional region of gD, we showed previously that combining two or more amino acid substitutions within site I prevents gD-1 from functioning and is therefore lethal. We have now used a complementation assay to measure the functional activity of a panel of deletion mutants and compared the results with an antigenic analysis. Several mutations cause gross changes in protein folding and destroy functional activity, whereas deletions at the N and C termini have little or no effect on either. In contrast, deletion of residues 234 to 244 has only localized effects on antigenicity but completely abolishes functional activity. This region, which is part of antigenic site Ib, is therefore essential for gD-1 function. The complementation assay was also used to show that a gD-negative type 1 virus can be rescued by gD-2 and by two gD-1-gD-2 hybrids but not by gp50, providing some support for the existence of a common receptor for herpes simplex virus types 1 and 2 but not pseudorabies virus. Alternatively, gp50 may lack a signal for incorporation into herpes simplex virions.

Codons 262 to 490 from the herpes simplex virus ICP4 gene are sufficient to encode a sequence-specific DNA binding protein

The HSV-1 immediate early (IE) protein ICP4 (alpha 4, IE175, Vmw175) is an oligomeric molecule which activates transcription of viral early genes, represses transcription of viral IE genes, and binds to specific sequences in certain viral promoters. The extent to which these functions are interrelated has not been fully established. We have expressed truncated portions of the ICP4 gene in E. coli as trpE fusion proteins. DNA-binding studies with these hybrid proteins revealed that ICP4 residues 262 to 490 are sufficient for sequence-specific DNA-binding. DNA-binding was not detected with polypeptides extending from residue 262 to 464 or from residue 306 to 490. Multiple bands of protein-DNA complexes observed in gel mobility shift assays indicate that residues 262 to 490 may also contribute to the oligomerization of ICP4.

Effect of 2',5'-oligoadenylate on herpes simplex virus-infected cells and preventive action of 2',5'-oligoadenylate on the lethal effect of HSV-2

Antiviral effects of ppp(A2'p)nA, (2-5A) on herpes simplex virus type 1 or type 2 (HSV-1 or HSV-2)-infected baby hamster kidney fibroblasts (BHK cells) and HSV-2-infected female guinea pigs were examined. 2-5A was introduced into BHK cells in the form of a calcium phosphate precipitate and as an ointment of polyethylene glycol (PEG) into guinea pig vagina. Under optimum conditions, 2-5A trimer and other 2-5A derivatives inhibited over 90% of HSV-1 syncytia formation and over 50% of HSV-2 plaque formation. The growth of uninfected cells was only slightly and transiently inhibited under these conditions. Northern analysis of viral immediate early mRNAs and cellular mRNAs showed that all transcripts determined were reduced in amount by the 2-5A trimer treatment. The reduction in level of viral mRNAs (ICP4, ICP22, and ICP47) by 2-5A trimer was significantly more than that of cellular mRNAs (represented by beta-actin). HSV-2 (strain 186) inoculation into the vagina of female guinea pigs causes severe symptoms in the genital area and high mortality. Topically applied 2-5A trimer almost completely prevented the lethal effect of HSV-2. These data are discussed from the viewpoint of mechanism of interferon action.

Antigenic properties and cellular localization of herpes simplex virus glycoprotein H synthesized in a mammalian cell expression system

Herpes simplex virus type 1 glycoprotein H (HSV-1 gH) was synthesized in an inducible mammalian cell expression system, and its properties were examined. The gH coding sequence, together with the stable 5' untranslated leader sequence from xenopus beta-globin, was placed under control of the strong promoter from the human cytomegalovirus major immediate-early gene in an amplifiable plasmid which contains the simian virus 40 (SV40) virus origin for replication (ori). This expression vector was transfected into ts COS cells constitutively expressing a temperature-sensitive SV40 T antigen which allows utilization of the SV40 ori at permissive temperatures. The results of transient expression assays at the permissive temperature showed that HSV-1 gH could be synthesized in greater amounts than those produced by a high-multiplicity virus infection. The proteins produced were detected in Western blots (immunoblots) with a HSV-1 gH-specific polyclonal serum raised against a TrpE-gH fusion protein. The transfected gH had an apparent molecular weight of approximately 105,000, intermediate in size to those of the precursor (100,000) and fully processed forms (110,000) of HSV-1 gH from infections. Antigenicity was investigated by reactions with three virus-neutralizing monoclonal antibodies specific for conformational epitopes on gH. Only one of these monoclonal antibodies could immunoprecipitate the synthesized gH. However, equal recognition of the transfected gH was achieved by superinfection with virus. In addition, detectable amounts of gH were not expressed on the cell surface unless the cells were superinfected with virus. Studies with a temperature-sensitive mutant, ts1201, defective in encapsidation showed that the changes in antigenic structure and cell surface expression caused by superinfection with virus were not due simply to incorporation of gH into virions. These results suggest that gH requires additional virus gene products for cell surface localization and formation of an antigenic structure important for its function in mediating infectivity.

Use of a glucocorticoid-inducible promoter for expression of herpes simplex virus type 1 glycoprotein gC1, a cytotoxic protein in mammalian cells

Abundant expression of herpes simplex virus type 1 glycoprotein gC (gC1) in transfected mammalian cells has not previously been achieved, possibly because gC1 protein is toxic to cells. To approach this problem, the gC1 coding sequence was placed under the control of the weak but inducible glucocorticoid-responsive promoter from the mouse mammary tumor virus (MMTV) long terminal repeat (LTR). As controls to evaluate for gC1 cytotoxicity, the MMTV LTR promoter was used to express glycoprotein gD1, and a strong, constitutive promoter from the Moloney murine sarcoma virus LTR was used to express gC1. L cells were transfected with these constructs, and a clone expressing gC1 from the inducible MMTV LTR promoter was analyzed. In the absence of glucocorticoid (dexamethasone) stimulation, only a low level of gC1 mRNA expression was detected; after overnight stimulation with dexamethasone, transcription increased approximately 200-fold. Abundant gC1 protein that was functionally active in that it bound complement component C3b, was produced. From passages 5 through 26 (70 cell population doublings), the gC1-producing clone became less responsive to overnight dexamethasone stimulation. The block to gC1 expression occurred at the level of transcription and was associated with hypermethylation of the MMTV LTR DNA. Treatment of the clone with 5-aza-2'-deoxycytidine partially reversed the block in gC1 protein production. Late-passage cells assumed a gC1-negative phenotype that appeared to offer a selective growth advantage, which suggested that gC1 was cytotoxic. Several findings support this view: (i) some cells expressing gC1 after overnight stimulation with dexamethasone assumed bizarre, syncytial shapes; (ii) continuous stimulation with dexamethasone for 5 weeks resulted in death of most cells; (iii) cells transfected with gC1 under the control of the strong Moloney murine sarcoma virus promoter assumed bizarre shapes, and stable gC1-expressing clones could not be established; and (iv) cells induced to express gD1 retained a normal appearance after overnight stimulation or 15 weeks of continuous stimulation with dexamethasone. The inducible MMTV LTR promoter is useful for expressing gC1 and may have applications for expressing other cytotoxic proteins.

Use of a glucocorticoid-inducible promoter for expression of herpes simplex virus type 1 glycoprotein gC1, a cytotoxic protein in mammalian cells

Abundant expression of herpes simplex virus type 1 glycoprotein gC (gC1) in transfected mammalian cells has not previously been achieved, possibly because gC1 protein is toxic to cells. To approach this problem, the gC1 coding sequence was placed under the control of the weak but inducible glucocorticoid-responsive promoter from the mouse mammary tumor virus (MMTV) long terminal repeat (LTR). As controls to evaluate for gC1 cytotoxicity, the MMTV LTR promoter was used to express glycoprotein gD1, and a strong, constitutive promoter from the Moloney murine sarcoma virus LTR was used to express gC1. L cells were transfected with these constructs, and a clone expressing gC1 from the inducible MMTV LTR promoter was analyzed. In the absence of glucocorticoid (dexamethasone) stimulation, only a low level of gC1 mRNA expression was detected; after overnight stimulation with dexamethasone, transcription increased approximately 200-fold. Abundant gC1 protein that was functionally active in that it bound complement component C3b, was produced. From passages 5 through 26 (70 cell population doublings), the gC1-producing clone became less responsive to overnight dexamethasone stimulation. The block to gC1 expression occurred at the level of transcription and was associated with hypermethylation of the MMTV LTR DNA. Treatment of the clone with 5-aza-2'-deoxycytidine partially reversed the block in gC1 protein production. Late-passage cells assumed a gC1-negative phenotype that appeared to offer a selective growth advantage, which suggested that gC1 was cytotoxic. Several findings support this view: (i) some cells expressing gC1 after overnight stimulation with dexamethasone assumed bizarre, syncytial shapes; (ii) continuous stimulation with dexamethasone for 5 weeks resulted in death of most cells; (iii) cells transfected with gC1 under the control of the strong Moloney murine sarcoma virus promoter assumed bizarre shapes, and stable gC1-expressing clones could not be established; and (iv) cells induced to express gD1 retained a normal appearance after overnight stimulation or 15 weeks of continuous stimulation with dexamethasone. The inducible MMTV LTR promoter is useful for expressing gC1 and may have applications for expressing other cytotoxic proteins.

Analysis of viral proteins in human cytomegalovirus-infected cells during impaired lytic replication of herpes simplex virus

Herpes simplex virus (HSV) latency can be established in vitro following arrest of virus replication and survival of infected cells in culture. Human cytomegalovirus (HCMV) has been shown to interact with HSV, resulting in reactivation of latent HSV. In addition, impaired replication of superinfecting HSV occurs in HCMV-infected human cells. HCMV-infected human embryonic lung cells inhibit production of infectious HSV despite replication of HSV DNA at levels comparable to those in control cultures infected only with HSV. Using radioimmunoprecipitation techniques, we found that the synthesis of HSV type 1 proteins of the alpha, beta/gamma, and gamma kinetic classes was impaired during the restricted replication of HSV in HCMV-infected HEL cells. However, synthesis of the HSV beta protein ICP-8 and HCMV alpha and beta proteins was not significantly affected in superinfected cell cultures.

Direct correlation between a negative autoregulatory response element at the cap site of the herpes simplex virus type 1 IE175 (alpha 4) promoter and a specific binding site for the IE175 (ICP4) protein

In transient-expression assays, the IE175 (alpha 4) promoter region of herpes simple virus is down-regulated after cotransfection with DNA encoding its own protein product (IE175 or ICP4). The inhibition by IE175 proved to be highly specific for its own promoter region and did not act on either the herpes simplex virus type 1 IE110 (alpha 0) or human cytomegalovirus major immediate-early promoters. Furthermore, the inhibition was still exhibited by IE175 effector plasmids driven by strong heterologous promoters and therefore must be a direct autoregulatory response that cannot be explained by promoter competition effects. In gel mobility retardation assays with infected-cell nuclear extracts, a prominent and specific DNA-protein complex was formed with DNA fragments containing sequences from -108 to +30 in the IE175 promoter region. This activity was not present in mock-infected samples. Even stronger binding occurred with a fragment containing sequences from -128 to +120 in the IE110 promoter, but this second locus was not associated with any detectable response phenotype in cotransfection assays. Supershift experiments with an anti-IE175 monoclonal antibody confirmed the presence of the IE175 protein in both DNA-protein complexes. In the IE175 promoter, specific binding correlated closely with the presence of an intact autoregulatory signal near the cap site as judged by the loss of both activities in a 3'-deleted promoter fragment lacking sequences from -7 to +30. Insertion of a cloned 30-mer synthetic oligonucleotide sequence from positions -8 to +18 in IE175 restored both IE175 binding activity and the down-regulation phenotype. Direct shift-up assays with a similar 30-base-pair (bp) oligonucleotide containing 21 bp from positions -75 to -55 of IE110 (which encompasses a consensus ATCGTC motif) also produced a specific DNA-protein complex containing the IE175 protein. This ATCGTC motif proved to be a necessary component of both the IE110 and IE175 binding sites, but was insufficient on its own for complex formation. Finally, deletion of 2 bp from positions -3 and -4 within the ATCGTC sequence in the IE175 cap site region abolished both binding activity and the IE175-dependent autoregulation phenotype.

Herpes simplex virus glycoprotein D is sufficient to induce spontaneous pH-independent fusion in a cell line that constitutively expresses the glycoprotein

Spontaneous small polykaryocytes were detected in a cell line designated BJ-o that harbors the BamHI J fragment of herpes simplex virus 1 DNA and expresses constitutively glycoprotein D (gD). The fusion activity of BJ-o cells correlated with gD production and was drastically reduced following exposure of the cells to monoclonal antibody HD1 to gD. Studies on the characteristics and requirements of cell fusion dependent on gD led to the conclusion that the characteristics and requirements for gD-mediated fusion activity of BJ-o cells are similar to those previously reported for cell fusion induced by the virus in that (i) polykaryocytosis was not augmented by exposure to medium of low pH with or without prior exposure to trypsin, (ii) the number of polykaryocytes was reduced following removal of terminal sialic acid residues by neuraminidase, and (iii) the number of polykaryocytes was augmented by masking of high-mannose N-linked oligosaccharides with concanavalin A or with its reduced form, succinyl concanavalin A. This effect was reversed by competition with mannose.

Individual herpes simplex virus 1 glycoproteins display characteristic rates of maturation from precursor to mature form both in infected cells and in cells that constitutively express the glycoproteins

Pulse-chase experiments in conjunction with quantitative immunoprecipitation have been used to study the time-course of conversion from precursor to mature form of herpes simplex virus 1 glycoproteins C, D and B (gC, gD, and gB). The experimental systems employed were two infected cell lines and cells that constitutively express gD or gB. The relative rates of conversion among the glycoproteins did not vary in the systems used; the rate of maturation of gC was about two-fold higher than that of gD which, in turn, was about one and a half-fold higher than that of gB. Treatment with phosphonoacetate which inhibits viral DNA synthesis and hence virion morphogenesis induced a striking increase in the time course of conversion of immature gC, gD, and gB to fully glycosylated forms when measured late in the infection. The model of HSV glycoproteins maturation as integral components of the virion envelope is discussed.

Entry of herpes simplex virus 1 in BJ cells that constitutively express viral glycoprotein D is by endocytosis and results in degradation of the virus

The BJ cell line which constitutively expresses herpes simplex virus 1 glycoprotein D is resistant to infection with herpes simplex viruses. Analysis of clonal lines indicated that resistance to superinfecting virus correlates with the expression of glycoprotein D. Resistance was not due to a failure of attachment to cells, since the superinfecting virus absorbed to the BJ cells. Electron microscopic studies showed that the virions are juxtaposed to coated pits and are then taken up into endocytic vesicles. The virus particles contained in the vesicles were in various stages of degradation. Viral DNA that reached the nucleus was present in fewer copies per BJ cell than that in the parental BHKtk- cells infected at the same multiplicity. Moreover, unlike the viral DNA in BHKtk- cells which was amplified, that in BJ cells decreased in copy number. The results suggest that the glycoprotein D expressed in the BJ cell line interfered with fusion of the virion envelope with the plasma membrane but not with the adsorption of the virus to cells and that the viral proteins that mediate adsorption to and fusion of membranes appear to be distinct.