A second-site revertant of a defective herpes simplex virus ICP4 protein with restored regulatory activities and impaired DNA-binding properties

Gene Therapy Tools for Brain Diseases

Neurological disorders affecting the central nervous system (CNS) are still incompletely understood. Many of these disorders lack a cure and are seeking more specific and effective treatments. In fact, in spite of advancements in knowledge of the CNS function, the treatment of neurological disorders with modern medical and surgical approaches remains difficult for many reasons, such as the complexity of the CNS, the limited regenerative capacity of the tissue, and the difficulty in conveying conventional drugs to the organ due to the blood-brain barrier. Gene therapy, allowing the delivery of genetic materials that encodes potential therapeutic molecules, represents an attractive option. Gene therapy can result in a stable or inducible expression of transgene(s), and can allow a nearly specific expression in target cells. In this review, we will discuss the most commonly used tools for the delivery of genetic material in the CNS, including viral and non-viral vectors; their main applications; their advantages and disadvantages. We will discuss mechanisms of genetic regulation through cell-specific and inducible promoters, which allow to express gene products only in specific cells and to control their transcriptional activation. In addition, we will describe the applications to CNS diseases of post-transcriptional regulation systems (RNA interference); of systems allowing spatial or temporal control of expression [optogenetics and Designer Receptors Exclusively Activated by Designer Drugs (DREADDs)]; and of gene editing technologies (CRISPR/Cas9, Zinc finger proteins). Particular attention will be reserved to viral vectors derived from herpes simplex type 1, a potential tool for the delivery and expression of multiple transgene cassettes simultaneously.

Inhibition of HSV-1 proliferation by decoy phosphodiester oligonucleotides containing ICP4 recognition sequences

Transcriptional control in eukaryotes results from the interplay between DNA sequences in promoters, enhancers, or silencers and transcription factors. Selective control of gene expression can thus be achieved by inhibiting specific transcription factor/DNA interactions. Transcriptional activity of DNA binding transcription factors can be inhibited by competition with double-stranded oligonucleotides (decoys) that contain their specific recognition sequences. The immediate early protein ICP4 of herpes simplex virus type 1 (HSV-1) is a sequence-specific DNA binding protein that is essential for viral replication. We synthesized double-stranded hairpin phosphodiester oligonucleotides carrying ICP4 sites and demonstrated their ability to specifically titrate ICP4. Upon addition to Vero cells, ICP4 hairpin decoys significantly reduced HSV-1 titers (IC50 = 0.3 microM), whereas a control hairpin oligonucleotide had no activity. Antiviral activity of ICP4 hairpin decoys was correlated to their relative binding affinities. These results show that phosphodiester oligonucleotides can compete for binding of specific transcription factors within cells, thus providing a potential therapeutic tool to control disease-causing genes.

An Essential Viral Transcription Activator Modulates Chromatin Dynamics

Although ICP4 is the only essential transcription activator of herpes simplex virus 1 (HSV-1), its mechanisms of action are still only partially understood. We and others propose a model in which HSV-1 genomes are chromatinized as a cellular defense to inhibit HSV-1 transcription. To counteract silencing, HSV-1 would have evolved proteins that prevent or destabilize chromatinization to activate transcription. These proteins should act as HSV-1 transcription activators. We have shown that HSV-1 genomes are organized in highly dynamic nucleosomes and that histone dynamics increase in cells infected with wild type HSV-1. We now show that whereas HSV-1 mutants encoding no functional ICP0 or VP16 partially enhanced histone dynamics, mutants encoding no functional ICP4 did so only minimally. Transient expression of ICP4 was sufficient to enhance histone dynamics in the absence of other HSV-1 proteins or HSV-1 DNA. The dynamics of H3.1 were increased in cells expressing ICP4 to a greater extent than those of H3.3. The dynamics of H2B were increased in cells expressing ICP4, whereas those of canonical H2A were not. ICP4 preferentially targets silencing H3.1 and may also target the silencing H2A variants. In infected cells, histone dynamics were increased in the viral replication compartments, where ICP4 localizes. These results suggest a mechanism whereby ICP4 activates transcription by disrupting, or preventing the formation of, stable silencing nucleosomes on HSV-1 genomes.

The nuclear-replicating DNA viruses of the family herpesviridae cause a variety of diseases. Eight herpesviruses infect humans. Three of them, including herpes simplex virus 1 (HSV-1), belong to the alpha-herpesvirus sub-family. Viruses in this family have the fastest replication cycles of all herpesviruses, producing acute symptoms. During lytic infection, the genomes of HSV-1 associate with histones in more dynamic chromatin than those of the beta- and gamma- herpesviruses. The transcription activator ICP4 is conserved only among alpha-herpesviruses. Although ICP4 is essential, relatively little is known about its mechanisms of action. We have shown that histone dynamics are enhanced in HSV-1 lytically infected cells. Here we show that HSV-1 mutants in ICP4 are deficient in their ability to enhance histone dynamics. ICP4 was sufficient to enhance histone dynamics in the absence of other HSV-1 proteins or DNA. The dynamics of histones were greater in the viral replication compartments, where ICP4 localizes, than in the cellular chromatin. ICP4 may thus mobilize histones away from HSV-1 genomes to activate transcription. Such a mechanism of transcription activation would result in the highly dynamic nature of the viral chromatin and the fast replication cycles, and the acute pathologies, of the alpha-herpesviruses.

Effect of herpes simplex virus vector-mediated interleukin-4 gene therapy on bladder overactivity and nociception

We investigated the effects of replication-defective herpes simplex virus (HSV) vector expression of interleukin-4 (IL-4) on bladder overactivity and nociception. HSV vector expressing murine interleukin-4 (S4IL4) or the control vector expressing β-galactosidase (SHZ) were injected to the rat bladder wall. At 1 week after viral injection, in cystometry performed under urethane anesthesia, the S4IL4-treated group did not show the intercontraction intervals reduction during intravesical administration of 10 nM resiniferatoxin (RTx). At 2 weeks after viral injection, behavioral studies were performed on vector-injected animals in an awakened state. Freezing behavior induced by 3 μM RTx, administered for 1 min into the bladder, was significantly suppressed in the S4IL4 group compared with the SHZ group. Murine IL-4 levels examined by ELISA were significantly increased in bladder and bladder afferent dorsal root ganglia at 2 weeks after viral injection. The expression of IL-1β and IL-2 and bladder inflammatory responses were significantly suppressed in the RTx-irritated bladder of S4IL4-injected rats. These results indicate that HSV vector-mediated interleukin-4 expression in the bladder and bladder afferent pathways reduces the inflammatory response, bladder overactivity and nociceptive behavior induced by bladder irritation in the rat model. Therefore, IL-4 gene therapy could be a new strategy for treating urinary frequency and/or bladder pain.

HSV-mediated expression of interleukin-4 in dorsal root ganglion neurons reduces neuropathic pain

BACKGROUND

To examine the role of inflammatory mediators in neuropathic pain, we used a replication-defective genomic herpes simplex virus (HSV)-based vector containing the coding sequence for the anti-inflammatory peptide interleukin (IL)-4 under the transcriptional control of the HSV ICP4 immediate early promoter, vector S4IL4, to express IL-4 in dorsal root ganglion (DRG) neurons in vivo.

RESULTS

Subcutaneous inoculation of S4IL4 in the foot transduced lumbar DRG to produce IL-4. Transgene-mediated expression of IL-4 did not alter thermal latency or tactile threshold in normal animals, but inoculation of S4IL4 1 week after spinal nerve ligation (SNL) reduced mechanical allodynia and reversed thermal hyperalgesia resulting from SNL. Inoculation of S4IL4 1 week before SNL delayed the development of thermal hyperalgesia and tactile allodynia, but did not prevent the ultimate development of these manifestations of neuropathic pain. S4IL4 inoculation suppressed non-noxious-induced expression of c-Fos immunoreactivity in dorsal horn of spinal cord and reversed the upregulation of spinal IL-1beta, PGE2, and phosphorylated-p38 MAP kinase, characteristic of neuropathic pain.

CONCLUSION

HSV-mediated expression of IL-4 effectively reduces the behavioral manifestations of neuropathic pain, and reverses some of the biochemical and histologic correlates of neuropathic pain at the spinal level.

Viral activation of macrophages through TLR-dependent and -independent pathways

Induction of cytokine production is important for activation of an efficient host defense response. Macrophages constitute an important source of cytokines. In this study we have investigated the virus-cell interactions triggering induction of cytokine expression in macrophages during viral infections. We found that viral entry and viral gene products produced inside the cell are responsible for activation of induction pathways leading to IFN-alphabeta expression, indicating that virus-cell interactions on the cell surface are not enough. Moreover, by the use of cell lines expressing dominant negative versions of TLR-associated adaptor proteins we demonstrate that Toll/IL-1 receptor domain-containing adaptor inducing IFN-beta is dispensable for all virus-induced cytokine expression examined. However, a cell line expressing dominant negative MyD88 revealed the existence of distinct induction pathways because virus-induced expression of RANTES and TNF-alpha was totally blocked in this cell line whereas IFN-alphabeta expression was much less affected in the absence of signaling via MyD88. In support of this, we also found that inhibitory CpG motifs, which block TLR9 signaling inhibited early HSV-2-induced TNF-alpha and RANTES expression dramatically whereas IFN-alphabeta induction was only slightly affected. This suggests that virus activates macrophages through distinct pathways, of which some are dependent on TLRs signaling through MyD88, whereas others seem to be independent of TLR signaling. Finally we demonstrate that IFN-alphabeta induction in HSV-2-infected macrophages requires a functional dsRNA-activated protein kinase molecule because cells expressing a dsRNA-dependent protein kinase version unable to bind dsRNA do not express IFN-alphabeta on infection.

Second site mutations in the N-terminus of the major capsid protein (VP5) overcome a block at the maturation cleavage site of the capsid scaffold proteins of herpes simplex virus type 1

VP5, the major capsid protein of herpes simplex virus type 1 (HSV-1), interacts with the C-terminal residues of the scaffold molecules encoded by the overlapping UL26 and UL26.5 open reading frames. Scaffold molecules are cleaved by a UL26 encoded protease (VP24) as part of the normal capsid assembly process. In this study, residues of VP5 have been identified that alter its interaction with the C-terminal residues of the scaffold proteins. A previously isolated virus (KUL26-610/611) was used that encoded a lethal mutation in the UL26 and UL26.5 open reading frames and required a transformed cell line that expresses these proteins for virus growth. The scaffold maturation cleavage site between amino acids 610 and 611 was blocked by changing Ala-Ser to Glu-Phe, which generated a new EcoRI restriction site. Revertant viruses, that formed small plaques on nontransformed cells, were detected at a frequency of 1:3800. Nine revertants were isolated, and all of them retained the EcoRI site and therefore were due to mutations at a second site. The second site mutations were extragenic. Using marker-transfer techniques, the mutation in one of the revertants was mapped to the 5' region of the gene encoding VP5. DNA sequence analysis was performed for the N-terminal 571 codons encoding VP5 for all of the revertant viruses. Six of the nine revertants showed a single base pair change that caused an amino acid substitution between residues 30 and 78 of VP5. Three of these were identical and changed Ala to Val at residue 78. The data provide a partial map of residues of VP5 that alter its interaction with scaffold proteins blocked at their normal cleavage site. The yeast two-hybrid system was used as a measure of the interaction between mutant VP5 and scaffold molecules and varied from 11% to nearly 100%, relative to wild-type VP5. One revertant gave no detectable interaction by this assay. The amount of UL26 encoded protease (VP24) in B capsids for KUL26-610/611 and for revertants was 7% and 25%, respectively, relative to the amount in capsids for wild-type virus. The lack of retention of the viral protease in the mutant virus and a fourfold increase for the revertants suggest an additional essential function for VP24 in capsid maturation, and a role in DNA packaging is indicated.

Modulation of mucosal and systemic immunity by enteric administration of nonreplicating herpes simplex virus expressing cytokines

In this report the ability of enteric immunization with recombinant replication deficient (ICP4-/-) HSV expressing IFN gamma to generate protection and modulate mucosal and systemic immunity was evaluated. ICP4-/-HSV, ICP4-/-HSV expressing IL4, live replicating, and uv HSV were used as controls. Following enteric administration of live HSV, a Th1 cytokine response was induced in the spleen, while both Th1 and notable Th2 cytokine production were detected at mucosal sites. Modulation of mucosal and systemic immune response was achieved when nonreplicating recombinant HSV viruses expressing cytokines were used. Compared to the control replication defective viruses, decreased frequency of Th2 cytokine producing cells in Peyer's patches was observed following enteric administration of nonreplicating HSV expressing IFN gamma. When IFN gamma expressing virus was given enterically, modulation was observed at the systemic level, measured by ELISPOT for cytokine producing cells, ELISA from the in vitro restimulated splenic cell cultures, and by the increase of the IgG2a/IgG1 ratio in the serum. This report provides evidence that replication defective viruses expressing cytokine genes in contrast to uv HSV, are immunogenic when administered enterically and can generate significant immunomodulatory effects at the mucosal and systemic levels.

Association of herpes simplex virus regulatory protein ICP22 with transcriptional complexes containing EAP, ICP4, RNA polymerase II, and viral DNA requires posttranslational modification by the U(L)13 proteinkinase

The expression of herpes simplex virus 1 gamma (late) genes requires functional alpha proteins (gamma1 genes) and the onset of viral DNA synthesis (gamma2 genes). We report that late in infection after the onset of viral DNA synthesis, cell nuclei exhibit defined structures which contain two viral regulatory proteins (infected cell proteins 4 and 22) required for gamma gene expression, RNA polymerase II, a host nucleolar protein (EAP or L22) known to be associated with ribosomes and to bind small RNAs, including the Epstein-Barr virus small nuclear RNAs, and newly synthesized progeny DNA. The formation of these complexes required the onset of viral DNA synthesis. The association of infected cell protein 22, a highly posttranslationally processed protein, with these structures did not occur in cells infected with a viral mutant deleted in the genes U(L)13 and U(S)3, each of which specifies a protein kinase known to phosphorylate the protein.

The herpes simplex virus major regulatory protein ICP4 blocks apoptosis induced by the virus or by hyperthermia

Cells infected with herpes simplex virus 1 (HSV-1) undergo productive or latent infection without exhibiting features characteristic of apoptosis. In this report, we show that HSV-1 induces apoptosis but has evolved a function that blocks apoptosis induced by infection as well as by other means. Specifically, (i) Vero cells infected with a HSV-1 mutant deleted in the regulatory gene alpha 4 (that encodes repressor and transactivating functions), but not those infected with wild-type HSV-1(F), exhibit cytoplasmic blebbing, chromatin condensation, and fragmented DNA detected as a ladder in agarose gels or by labeling free DNA ends with terminal transferase; (ii) Vero cells infected with wild-type HSV-1(F) or cells expressing the alpha 4 gene and infected with the alpha 4- virus did not exhibit apoptosis; (iii) fragmentation of cellular DNA was observed in Vero cells that were mock-infected or infected with the alpha 4- virus and maintained at 39.5 degrees C, but not in cells infected with wild-type virus and maintained at the same temperature. Wild-type strains of HSV-1 with limited extrahuman passages, such as HSV-1 (F), carry a temperature-sensitive lesion in the alpha 4 gene and at 39.5 degrees C only alpha genes are expressed. These results indicate that the product of the alpha 4 gene is able to suppress apoptosis induced by the virus as well by other means.

Functional interaction and colocalization of the herpes simplex virus 1 major regulatory protein ICP4 with EAP, a nucleolar-ribosomal protein

The herpes simplex virus 1 infected cell protein 4 (ICP4) binds to DNA and regulates gene expression both positively and negatively. EAP (Epstein-Barr virus-encoded small nuclear RNA-associated protein) binds to small nonpolyadenylylated nuclear RNAs and is found in nucleoli and in ribosomes, where it is also known as L22. We report that EAP interacts with a domain of ICP4 that is known to bind viral DNA response elements and transcriptional factors. In a gel-shift assay, a glutathione S-transferase (GST)-EAP fusion protein disrupted the binding of ICP4 to its cognate site on DNA in a dose-dependent manner. This effect appeared to be specifically due to EAP binding to ICP4 because (i) GST alone did not alter the binding of ICP4 to DNA, (ii) GST-EAP did not bind to the probe DNA, and (iii) GST-EAP did not influence the binding of the alpha gene trans-inducing factor (alphaTIF or VP16) to its DNA cognate site. Early in infection, ICP4 was dispersed throughout the nucleoplasm, whereas EAP was localized to the nucleoli. Late in infection, EAP was translocated from nucleoli and colocalized with ICP4 in small, dense nuclear structures. The formation of dense structures and the colocalization of EAP and ICP4 did not occur if virus DNA synthesis and late gene expression were prevented by the infection of cells at the nonpermissive temperature with a mutant virus defective in DNA synthesis, or in cells infected and maintained in the presence of phosphonoacetate, which is an inhibitor of viral DNA synthesis. These results suggest that the translocation of EAP from the nucleolus to the nucleoplasm is a viral function and that EAP plays a role in the regulatory functions expressed by ICP4.

Role of protein kinase A and the serine-rich region of herpes simplex virus type 1 ICP4 in viral replication

Efficient expression of herpes simplex virus genes requires the synthesis of functional ICP4, a nuclear phosphoprotein that contains a prominent serine-rich region between amino acids 142 and 210. Residues in this region not only are potential sites for phosphorylation but also are involved in the functions of ICP4. By comparing the growth of a virus in which this region is deleted (d8-10) with wild-type virus (KOS) in PC12 cells or PC12 cells that are deficient in cyclic AMP-dependent protein kinase (PKA), two observations were made: (i) the growth of wild-type virus was impaired by 1 to 2 orders of magnitude in the PKA-deficient cells, indicating the involvement of PKA in the growth cycle of herpes simplex virus type 1, and (ii) while the growth of d8-10 was impaired by almost 2 orders of magnitude in wild-type cells, it was not further impaired (as was that of wild-type virus) in PKA-deficient cells, implicating the region deleted in d8-10 as a possible target for cellular PKA. In trigeminal'ganglia of mice, the d8-10 mutant virus grew poorly; however, it established latency in nearly 90% of ganglia tested. Studies of the phosphorylation of wild-type and d8-10 ICP4 proteins revealed that the serine-rich region is a major determinant for phosphorylation of ICP4 in vivo and that the phosphorylation state could change as a function of the PKA activity. Consistent with this observation, the serine-rich region of ICP4 was shown to be a target for PKA in vitro. While intact ICP4 was readily phosphorylated by ICP4 in vitro, the d8-10 mutant ICP4 was not. Moreover, a synthethic peptide representing a sequence in the serine tract that is predicted to be a substrate for PKA was phosphorylated by PKA in vitro, having a Km within the physiological range. These data suggest that PKA plays a role in viral growth through phosphorylation of one or more sites on the ICP4 molecule.

Functional interactions between herpes simplex virus immediate-early proteins during infection: gene expression as a consequence of ICP27 and different domains of ICP4

Two of the five immediate-early gene products, ICP4 and ICP27, expressed by herpes simplex virus type 1 have profound effects on viral gene expression and are absolutely essential for virus replication. Functional interactions between ICP4 and ICP27 may contribute to establishing the program of viral gene expression that ensues during lytic infection. To evaluate this possibility, viral mutants simultaneously deleted for ICP27 and defined functional domains of ICP4 were constructed. These mutant viruses allowed a comparison of gene expression as a function of different domains of ICP4 in the presence and absence of ICP27. Gene expression in the absence of both ICP4 and ICP27 was also examined. The results of this study demonstrate a clear involvement for ICP27 in the induction of early genes, in addition to its known role in enhancing late gene expression during viral infection. In the absence of both ICP4 and ICP27, viral early gene expression, as measured by the accumulation of thymidine kinase and ICP6 messages was dramatically reduced relative to the amounts of these messages seen in the absence of only ICP4. Therefore, elevated levels of early gene expression as a consequence of ICP27 occurred in the absence of any ICP4 activity. Evidence is also presented regarding the modulation of the ICP4 repression function by ICP27. When synthesized in the absence of ICP27, a mutant ICP4 protein was impaired in its ability to repress transcription from the L/ST promoter in the context of viral infection and in vitro. This defect correlated with the loss of the ability of this mutant protein to bind to its recognition sequence when produced in infected cells in the absence of ICP27. These observations indicate that ICP27 can regulate the activity of at least one domain of the ICP4 protein as well as contribute to elevated early gene expression independently of ICP4.

Requirements for activation of the herpes simplex virus glycoprotein C promoter in vitro by the viral regulatory protein ICP4

During infection with herpes simplex virus, infected-cell polypeptide 4 (ICP4) activates transcription of most herpes simplex virus genes. In the present study, the mechanism of activation of transcription by ICP4 was investigated by using a reconstituted in vitro system with fractionated and purified general transcription factors, coupled with DNA-binding assays. The templates used in the reactions included regions of the gC and thymidine kinase (tk) promoters in plasmids, and on isolated fragments, allowing for the evaluation of the potential function of naturally occurring and inserted ICP4-binding sites and elements of the core promoter. ICP4 efficiently activated transcription of the gC promoter by facilitating the formation of transcription initiation complexes. ICP4 could not substitute for any of the basal transcription factors. Moreover, TATA-binding protein (TBP) could not substitute for TFIID in activation, suggesting a requirement for TBP-associated factors. Interactions between ICP4 and DNA 3' to the start site was necessary for activation of the gC promoter. The requirement for DNA-protein contacts could be met either by the presence of an ICP4-binding site in the gC leader, by the presence of a site more than 150 nucleotides further downstream, by an inserted site that normally acts to repress transcription, or by the addition of sufficient non-site-containing DNA. The gC TATA box and start site, or initiator element (inr), were individually sufficient for activation by ICP4 and together contributed to optimal activation. In contrast to gC, the tk promoter was poorly activated in the reconstituted system. However, the tk TATA box was efficiently activated when the tk start site region was replaced with the gC inr, suggesting that activation was mediated through the inr and inr-binding proteins. In addition, mutation of the inr core resulted in a gC promoter that was very poorly activated by ICP4. The results of this and previous studies demonstrate that ICP4 activates transcription in a complex manner involving contacts with DNA 3' to the start site, TBP, TFIIB, TBP-associated factors, and possibly proteins functioning at the start site of transcription.

Mutation of a single lysine residue severely impairs the DNA recognition and regulatory functions of the VZV gene 62 transactivator protein

The product of varicella-zoster virus gene 62 (VZV 140k) is a potent transactivator protein. We have identified a region within the DNA binding domain of VZV 140k that shows a striking similarity to the DNA recognition helix of the homeodomain, with an especially highly conserved quartet of residues, WLQN. The 140k protein has functional counterparts within the other alphaherpesviruses, which include the major transcriptional regulatory protein of HSV-1, (ICP4), and the WLQN region is highly conserved among the members of this family of viral transactivators. Substitution of VZV 140k residue lysine 548, just adjacent to the WLQN region, drastically reduces the DNA binding activity of the 140k DNA binding domain and the intact 140k mutant protein fails to activate gene expression. Substitutions of two other VZV 140k residues in this conserved WLQN region result in alterations to the DNA binding interaction and reduced transactivation activities. All three mutations act at the level of DNA recognition, as they have no apparent effect on the dimerization state, solubility or efficiency of expression of the mutant peptides.

Amino acid substitution mutations in the herpes simplex virus ICP27 protein define an essential gene regulation function

ICP27 is an essential herpes simplex virus type 1 (HSV-1) alpha protein that is required for the transition from the beta to the gamma phase of infection. To identify functional regions of ICP27, we constructed 16 plasmids that contain nucleotide substitution mutations in the ICP27 gene. The mutations created XhoI restriction sites, altered one or two codons, and were spaced at semiregular intervals throughout the coding region. Three mutations completely inactivated an essential function of ICP27, as demonstrated by the inability of the transfected plasmids to complement the growth of an HSV-1 ICP27 deletion mutant. These mutations, M11, M15, and M16, mapped in the carboxyl-terminal one-third of ICP27 at residues 340 and 341, 465 and 466, and 488, respectively. In cotransfection assays, all three defective-plasmid mutants retained the transrepression function of ICP27 but were defective at transactivation. To define the lytic functions that are mediated by the transactivation activity of ICP27, we engineered HSV-1 recombinants containing the M11, M15, or M16 mutation. All three viral mutants failed to grow in Vero cells and possessed similar phenotypes. The viral mutants replicated their DNA similarly to the wild-type virus but showed several defects in viral gene expression. These were a failure to down-regulate alpha and beta genes at late times after infection and an inability to induce certain gamma-2 genes. Our results demonstrate that the transactivation function of ICP27 (as it is defined in cotransfection assays) mediates an essential gene regulation function during the HSV-1 infection. This activity is not required for ICP27-dependent enhancement of viral DNA replication. Our work supports and extends previous studies which suggest that ICP27 carries out two distinct regulatory activities during the HSV-1 infection.

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.

ICP4, the major transcriptional regulatory protein of herpes simplex virus type 1, forms a tripartite complex with TATA-binding protein and TFIIB

The ICP4 protein of herpes simplex virus can either increase or decrease the rate of transcription mediated by RNA polymerase II, depending on the target promoter. The interplay of DNA-protein and protein-protein contacts determining ICP4 function has yet to be characterized, and consequently the molecular mechanism by which the protein acts remains unclear. ICP4 can transactivate minimal promoters containing only TATA homologies, and therefore it is reasonable to hypothesize that ICP4 works by influencing the TATA-dependent assembly of general transcription factors via specific protein-protein interactions. This study directly addresses this hypothesis by determining whether ICP4 affects the assembly of general transcription factors on templates bearing a TATA box and an ICP4-binding site. Using gel retardation and footprinting assays, we found that ICP4 forms a tripartite complex with TFIIB and either the TATA-binding protein (TBP) or TFIID. The formation of this complex was not the result of simple tripartite occupancy of the DNA but the consequence of protein-protein interactions. In the presence of all three proteins, the affinity of ICP4 and TBP for their respective binding sites was substantially increased. Using mutant derivatives of ICP4 and defective versions of promoters, we also demonstrated that the ability of ICP4 to regulate gene expression correlated with its ability to form a tripartite complex with TFIIB and TBP in vitro.

The acidic amino-terminal region of herpes simplex virus type 1 alpha protein ICP27 is required for an essential lytic function

The herpes simplex virus type 1 (HSV-1) alpha protein ICP27 regulates the transition between the delayed-early and late phases of the viral infection. Previous genetic analyses have suggested that the important functional domains of ICP27 map to its carboxyl-terminal half. One striking feature of the primary sequence of ICP27, however, is an extremely acidic region near its amino terminus. To determine whether this region is required for ICP27 function, we deleted the sequences in the ICP27 gene which encode it (codons 12 through 63). In transient expression assays, the deletion mutant was unable to efficiently repress the expression of a cotransfected reporter gene or to efficiently complement the growth of d27-1, an HSV-1 ICP27 null mutant. These results suggested that the acidic region of ICP27 is involved in a regulatory function required for lytic growth. To test this possibility further, we introduced the mutant allele into the HSV-1 genome by marker transfer. Two independently derived isolates of the mutant virus, designated d1-2a and d1-2b, were recovered and analyzed. Both isolates were defective for growth in Vero cells, exhibiting a 100-fold reduction in virus yield compared with the wild-type infection. Vero cells infected with the d1-2 isolates showed a three- to eightfold reduction in viral DNA replication, a moderate reduction in the expression of viral gamma genes, and a delay in the repression of beta genes. The phenotype of the d1-2 isolates differs substantially from the phenotypes of previously isolated ICP27 mutants, which show much more severe defects in viral gene expression. Our results demonstrate that the amino-terminal half of ICP27 participates in its regulatory activities in both infected and transfected cells.

Intragenic complementation of herpes simplex virus ICP8 DNA-binding protein mutants

The major DNA-binding protein, or infected-cell protein 8 (ICP8), of herpes simplex virus is required for viral DNA synthesis and normal regulation of viral gene expression. Previous genetic analysis has indicated that the carboxyl-terminal 28 residues are the only portion of ICP8 capable of acting independently as a nuclear localization signal. In this study, we constructed a mutant virus (n11SV) in which the carboxyl-terminal 28 residues of ICP8 were replaced by the simian virus 40 large-T-antigen nuclear localization signal. The n11SV ICP8 localized into the nucleus and bound to single-stranded DNA in vitro as tightly as wild-type ICP8 did but was defective for viral DNA synthesis and viral growth in Vero cells. Two mutant ICP8 proteins (TL4 and TL5) containing amino-terminal alterations could complement the n11SV mutant but not ICP8 gene deletion mutants. Cell lines expressing TL4 and TL5 ICP8 were isolated, and in these cells, complementation of n11SV was observed at the levels of both viral DNA replication and viral growth. Therefore, complementation between n11SV ICP8 and TL4 or TL5 ICP8 reconstituted wild-type ICP8 functions. Our results demonstrate that (i) the carboxyl-terminal 28 residues of ICP8 are required for a function(s) involved in viral DNA replication, (ii) this function can be supplied in trans by another mutant ICP8, and (iii) ICP8 has multiple domains possessing different functions, and at least some of these functions can complement in trans.

Transdominant inhibition of herpes simplex virus growth in transgenic mice

A mutant allele (X25) of an essential regulatory protein, ICP4, encoded by herpes simplex virus (HSV) has been shown to have a transdominant, negative effect on the activity of the wild-type protein, resulting in the inhibition of virus growth in vitro. The X25 protein appears to exert its transdominant effect by sequestering functional ICP4 monomers into nonfunctional, heterodimeric complexes (A. Shepard, P. Tolentino, and N. A. DeLuca, 1990, J. Virol. 64, 3916-3926). In order to assess the antiviral potential of X25 in vivo, four transgenic mouse lines were generated bearing 1 to 10 copies of a DNA fragment encoding the mutant allele. Monolayers of embryonic cells prepared from each of the lines expressed the transgenic X25 protein. When challenged via the eye, every line exhibited at least some enhanced resistance to HSV infection. In the best line, transgenic animals exhibited a statistically significant (> 95% confidence) 5- to 13-fold lower eye swab titer relative to their nontransgenic littermates at Day 1 postinfection. A similar reduction in titer was observed in the trigeminal ganglia at Day 3 postinfection. These results indicate that the X25 protein is able to exert a significant antiviral effect in vivo.

Substitution of a TATA box from a herpes simplex virus late gene in the viral thymidine kinase promoter alters ICP4 inducibility but not temporal expression

The role of cis-acting promoter elements associated with herpes simplex virus type 1 (HSV-1) early and late genes was evaluated during productive infection with regard to activation of gene expression by the HSV-1 transactivator ICP4 and control of temporal regulation. A set of recombinant viruses was constructed such that expression of an HSV-1 early gene, thymidine kinase (tk), was placed under the control of either the tk TATA box or the TATA box from the late gene, glycoprotein C (gC), in the presence or absence of the upstream Sp1 and CCAAT sites normally found in the tk promoter. The presence of Sp1 sites in the promoter or replacement of the tk TATA box with the gC TATA box resulted in a decreased activation of tk mRNA expression by ICP4. Substitution of the A + T-rich region from the gC TATA box in the context of the remainder of the surrounding tk sequences resulted in a promoter that bound recombinant TATA-binding protein (TBP) better at lower concentrations than the wild-type tk promoter did. These results indicate that tk promoters that are better able to utilize TBP are less responsive to ICP4 activation and suggest that activation by ICP4 involves the general transcription factors that interact with TBP or TBP itself. Additionally, all of the viruses expressed tk at early times postinfection, indicating that cis-acting promoter elements that control the level of expression of HSV-1 early and late genes do not determine temporal regulation.

The herpes simplex virus type 1 immediate-early polypeptide ICP4 is required for expression of globin genes located in the viral genome

We infected Vero cells with ICP4-deficient herpes simplex virus recombinants bearing the rabbit beta-globin and human alpha 2-globin genes under the control of their own promoters and found that globin gene expression occurred only when ICP4 was provided in trans. These results demonstrate that ICP4 is required for the activity of globin promoters located in the viral genome and support the hypothesis that these cellular promoters are functionally equivalent to HSV early regulatory regions.

Complete nucleotide sequence of the Marek's disease virus ICP4 gene

The Marek's disease virus (MDV) gene encoding a homologue to the ICP4 protein of herpes simplex virus has been mapped to BamHl fragment A based on the physical map of the MDV genome (Fukuchi et al., 1984). The gene lies completely within the inverted repeat flanking the unique short region of the genome. The complete nucleotide sequence of the MDV ICP4 gene has been determined. The coding region is 4245 nucleotides long and has an overall G+C content of 52%. The MDV ICP4 protein is predicted to have a structure similar to that of ICP4-like proteins of other herpesviruses in that it has five distinct regions, the second and fourth of which are highly conserved. In addition, the protein contains the characteristic run of serine residues located toward its amino terminus. The MDV ICP4 gene is expressed in MDV-infected chicken embryo fibroblasts.

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.

The ICP4 binding sites in the herpes simplex virus type 1 glycoprotein D (gD) promoter are not essential for efficient gD transcription during virus infection

Activation of the early and late genes of herpes simplex virus type 1 during infection in tissue culture requires functional immediate-early regulatory protein ICP4. ICP4 is a specific DNA-binding protein which recognizes a variety of DNA sequences, many of which contain the consensus ATCGTC. In general, mutations which impair the ability of ICP4 to bind to DNA also eliminate its ability to activate viral early and late promoters both in transfection assays and in the infected cell. However, the role of ICP4 binding sites in the viral genome is unclear; many early and late promoters do not contain consensus binding sites in their vicinity. The glycoprotein D (gD) gene contains two well-characterized ICP4 binding sites upstream of its promoter and a third downstream of the transcription start site. Multimerization of one of these sites has been shown to increase the response of the gD promoter to ICP4 in transfection assays, while their removal reduces stimulation of the gD promoter by ICP4 in vitro. To assess the role of these binding sites during virus infection, we have constructed a recombinant viral genome which has mutations affecting all three. Comparison of the amounts of gD RNA synthesized by the recombinant and wild-type viruses indicated that the mutations had little or no effect on the activity of the gD promoter. Therefore, either the sites have no essential role in gD promoter regulation in the presence of all of the herpes simplex virus type 1 IE polypeptides during a normal infection or they can be functionally substituted by other ICP4 binding sites elsewhere in the genome.

Analysis of the specificity and mechanism of transcriptional activation of the human hsp70 gene during infection by DNA viruses

We have examined the transcriptional regulation of the 70-kDa (70K) heat shock gene family following infection of human and monkey cells with four different DNA viruses: adenovirus type 5 (Ad5), herpes simplex virus type 1 (HSV-1), simian virus 40, and vaccinia virus. Our results indicate that induction of these genes is not a general response to the stress of viral infection but is instead a highly specific response, both with regard to the inducing virus and with regard to the target gene. Of three 70K heat shock genes examined, only hsp70 was induced during viral infection, and induction occurred only after infection by Ad5 and HSV-1. As revealed by genomic footprinting analysis, the mechanism of transcriptional activation of hsp70 during Ad5 or HSV-1 infection does not involve changes in the avidity of binding of basal transcription factors to the hsp70 promoter. In HSV-1-infected HeLa cells, transcriptional activation of hsp70 was quite transient, following which transcription was rapidly repressed; this was accompanied by the release of bound factors from the hsp70 promoter. In addition to the selectivity which characterizes the viral activation of hsp70 transcription, our results indicate that the consequences of this activation, as measured by changes in hsp70 mRNA levels and protein synthesis, are also virus specific.

Purification of the DNA binding domain of herpes simplex virus type 1 immediate-early protein Vmw175 as a homodimer and extensive mutagenesis of its DNA recognition site

The herpes simplex virus type 1 (HSV-1) Immediate-Early (IE) polypeptide Vmw175 is essential for the activation of transcription from viral early and late promoters. Vmw175 also reduces the activity of its own (IE-3) promoter in transfection assays. Both transactivation and repression mediated by Vmw175 require the integrity of a conserved domain of the polypeptide which has been shown to bind to specific DNA sequences. We have investigated the DNA sequence requirements for Vmw175 binding using a randomly mutated target. The results indicate that the binding site covers a region of 13 nucleotides divided into proximal and distal parts which are consistent with the consensus ATCGTNNNNNYSG. We have also expressed several different constructs encompassing the DNA binding domain of Vmw175 in bacteria, and obtained preparations of greater than 90% purity. The DNA binding domain is a dimer in solution, and binds DNA with a specificity similar to that of the intact protein, although the smallest DNA binding competent protein has a slightly reduced specificity.

ICP4, the major regulatory protein of herpes simplex virus, shares features common to GTP-binding proteins and is adenylated and guanylated

Infected cell protein 4 (ICP4), the product of the alpha 4 gene, regulates herpes simplex virus 1 and herpes simplex virus 2 gene expression at the transcriptional level both positively and negatively. Previous studies have shown that ICP4 is extensively modified posttranslationally. We report that ICP4 was labeled in isolated nuclei of infected cells by [alpha-32P]GTP or [alpha-32P]ATP. The labeling of ICP4 by [alpha-32P]GTP or [alpha-32P]ATP required excess GTP, ATP, GDP, and ADP and occurred also in the presence of excess GTP(gamma)S. While GDP and ADP activated the labeling process, only GTP and ATP labeled ICP4. Accumulation of labeled ICP4 was favored at temperatures from 15 to 27 degrees C and in the presence of okadaic acid. The conditions for labeling ICP4 with [alpha-32P]GTP or [alpha-32P]ATP and the stability of the labeled protein were different from those of ICP4 labeled with [gamma-32P]ATP. Labeling studies with tritiated ATP and GTP showed that ICP4 is nucleotidylated, and chemical degradation of ICP4 labeled with [alpha-32P]GTP yielded ribose-5-phosphate. Pulse-chase experiments indicated that the adenylation and guanylation are independent processes. These results, and the observation that ICP4 contains four regions which possess consensus GTP-binding elements, suggest that ICP4 may belong to a class of GTP-binding proteins which function in transcriptional transactivation.