Mapping of transcriptional regulatory domains of pseudorabies virus immediate-early protein

Effects of deletion of the early protein 0 gene of pseudorabies virus on the overall viral gene expression

Real-time RT-PCR analysis was applied to evaluate the impact of deletion of the early protein 0 (EP0) gene of pseudorabies virus (PRV) on the global expression of the viral transcripts during lytic infection in cultured porcine kidney cells. Our analysis showed that EP0 exerted an inhibitory effect on the transcription of the PRV genes in the early stage of infection, and alternating stimulatory and inhibitory effects on the viral gene expressions in the late stage of infection. The data also suggested that a general function of EP0 might be to reverse the kinetics of expression of early viral genes. We also observed that EP0 facilitated the development of correlations in the transcription kinetics between the immediate early 180 gene and the PRV transcripts, indicating that a major function of EP0 could be to modify the effects of the IE180 protein on the PRV transcriptome.

A wide extent of inter-strain diversity in virulent and vaccine strains of alphaherpesviruses

Alphaherpesviruses are widespread in the human population, and include herpes simplex virus 1 (HSV-1) and 2, and varicella zoster virus (VZV). These viral pathogens cause epithelial lesions, and then infect the nervous system to cause lifelong latency, reactivation, and spread. A related veterinary herpesvirus, pseudorabies (PRV), causes similar disease in livestock that result in significant economic losses. Vaccines developed for VZV and PRV serve as useful models for the development of an HSV-1 vaccine. We present full genome sequence comparisons of the PRV vaccine strain Bartha, and two virulent PRV isolates, Kaplan and Becker. These genome sequences were determined by high-throughput sequencing and assembly, and present new insights into the attenuation of a mammalian alphaherpesvirus vaccine strain. We find many previously unknown coding differences between PRV Bartha and the virulent strains, including changes to the fusion proteins gH and gB, and over forty other viral proteins. Inter-strain variation in PRV protein sequences is much closer to levels previously observed for HSV-1 than for the highly stable VZV proteome. Almost 20% of the PRV genome contains tandem short sequence repeats (SSRs), a class of nucleic acids motifs whose length-variation has been associated with changes in DNA binding site efficiency, transcriptional regulation, and protein interactions. We find SSRs throughout the herpesvirus family, and provide the first global characterization of SSRs in viruses, both within and between strains. We find SSR length variation between different isolates of PRV and HSV-1, which may provide a new mechanism for phenotypic variation between strains. Finally, we detected a small number of polymorphic bases within each plaque-purified PRV strain, and we characterize the effect of passage and plaque-purification on these polymorphisms. These data add to growing evidence that even plaque-purified stocks of stable DNA viruses exhibit limited sequence heterogeneity, which likely seeds future strain evolution.

Alphaherpesviruses such as herpes simplex virus (HSV) are ubiquitous in the human population. HSV causes oral and genital lesions, and has co-morbidities in acquisition and spread of human immunodeficiency virus (HIV). The lack of a vaccine for HSV hinders medical progress for both of these infections. A related veterinary alphaherpesvirus, pseudorabies virus (PRV), has long served as a model for HSV vaccine development, because of their similar pathogenesis, neuronal spread, and infectious cycle. We present here the first full genome characterization of a live PRV vaccine strain, Bartha, and reveal a spectrum of unique mutations that are absent from two divergent wild-type PRV strains. These mutations can now be examined individually for their contribution to vaccine strain attenuation and for potential use in HSV vaccine development. These inter-strain comparisons also revealed an abundance of short repetitive elements in the PRV genome, a pattern which is repeated in other herpesvirus genomes and even the unrelated Mimivirus. We provide the first global characterization of repeats in viruses, comparing both their presence and their variation among different viral strains and species. Repetitive elements such as these have been shown to serve as hotspots of variation between individuals or strains of other organisms, generating adaptations or even disease states through changes in length of DNA-binding sites, protein folding motifs, and other structural elements. These data suggest for the first time that similar mechanisms could be widely distributed in viral biology as well.

Cerebellar pathology in transgenic mice expressing the pseudorabies virus immediate-early protein IE180

Pseudorabies virus is an alphaherpesvirus causing fatal neurological diseases in animals. Pseudorabies virus carries a gene encoding immediate-early (IE) protein IE180, which controls the transcription of other viral and host cell genes. Previously, we reported that transgenic expression of IE180 in mice causes severe ataxia and cerebellar deformity. Here we identified profound abnormalities in adult IE180 transgenic mice, including malpositioning of Purkinje cells (PCs), granule cells (GCs) and Bergmann glia (BG), impaired dendritogenesis and synaptogenesis in PCs, disoriented BG fibers, absence of molecular layer interneurons, and increased apoptosis of neurons and glia. In accordance with the cellular defects, we found the expression of IE180 in PCs, GCs and astrocytes during cerebellar development. We next examined transgenic mice expressing truncated IE180 mutants: dlN132 lacking the acidic transcriptional active domain, dlC629 lacking the nuclear localization signal and dlC1081 having all known domains but lacking the carboxyl-terminal sequence. Despite similar expression levels of the transgenes, ataxia and cerebellar defects were only manifested in the dlC1081 transgenic mice but their phenotypes were milder compared with the IE180 transgenic mice. In the dlC1081 transgenic mice, cerebellar neurons and glia were normally positioned but cerebellar size was severely reduced due to GC deficits. Interestingly, dlC1081 was mainly expressed in the GCs with low expression in a few BG. Taken together, the present findings clarified a causal relationship between cerebellar pathology and cellular expression of IE180, and further afforded an experimental insight into different symptomatic severity as a consequence of different cellular defects caused by such cytotoxic viral agents.

Cell biological and molecular characteristics of pseudorabies virus infections in cell cultures and in pigs with emphasis on the respiratory tract

In the present review, several cell biological and molecular aspects of virus-cell and virus-host (pig) interactions are reviewed for pseudorabies (Aujeszky's disease) virus. Concerning the virus-cell interactions, the complex cascade of events in the virus replication cycle is given together with the different mechanisms of cell-to-cell spread. The pathogenesis of pseudorabies virus infections in pigs is concentrated on the sequence of events in the respiratory tract. Finally, a short overview is given on the control of the disease and eradication of the virus by the combination of marker vaccines and discriminating ELISA.

Molecular biology of pseudorabies virus: impact on neurovirology and veterinary medicine

Pseudorabies virus (PRV) is a herpesvirus of swine, a member of the Alphaherpesvirinae subfamily, and the etiological agent of Aujeszky's disease. This review describes the contributions of PRV research to herpesvirus biology, neurobiology, and viral pathogenesis by focusing on (i) the molecular biology of PRV, (ii) model systems to study PRV pathogenesis and neurovirulence, (iii) PRV transsynaptic tracing of neuronal circuits, and (iv) veterinary aspects of pseudorabies disease. The structure of the enveloped infectious particle, the content of the viral DNA genome, and a step-by-step overview of the viral replication cycle are presented. PRV infection is initiated by binding to cellular receptors to allow penetration into the cell. After reaching the nucleus, the viral genome directs a regulated gene expression cascade that culminates with viral DNA replication and production of new virion constituents. Finally, progeny virions self-assemble and exit the host cells. Animal models and neuronal culture systems developed for the study of PRV pathogenesis and neurovirulence are discussed. PRV serves asa self-perpetuating transsynaptic tracer of neuronal circuitry, and we detail the original studies of PRV circuitry mapping, the biology underlying this application, and the development of the next generation of tracer viruses. The basic veterinary aspects of pseudorabies management and disease in swine are discussed. PRV infection progresses from acute infection of the respiratory epithelium to latent infection in the peripheral nervous system. Sporadic reactivation from latency can transmit PRV to new hosts. The successful management of PRV disease has relied on vaccination, prevention, and testing.

Mapping of functional regions conferring nuclear localization and RNA-binding activity of pseudorabies virus early protein UL54

Pseudorabies virus (PRV) is an alphaherpesvirus, and its gene organization and regulation are similar to the well-characterized human herpes simplex virus (HSV). The PRV early protein UL54 consists of 363 amino acids with homology to the HSV ICP27 immediate-early protein. Previously, we have demonstrated the nuclear accumulation and poly(G) RNA-binding activity of UL54 protein. In the present study, we have identified further the functional regions within UL54 conferring for nuclear localization and RNA-binding activity. Several recombinant expression plasmids containing various coding regions of UL54 gene were constructed for producing a series of C-terminally truncated or internally deleted forms of UL54 mutants in Escherichia coli or porcine kidney (PK-15) cells. RNA-binding activity of E. coli-expressed UL54 mutants was characterized by the binding ability to poly(G) RNA homopolymer in dot blot hybridization assay and the results have shown that the N-terminal 83 residues were responsible for RNA-binding, and the region of residues 35-82 containing an RGG box was necessary for its function. Furthermore, the region responsible for nuclear localization was investigated by transient expression of various deletion mutants in PK-15 cells followed by detection of their subcellular distribution. The results showed that C-terminal deletion beyond the amino acid residue 83 or internal deletion containing the RGG box sequence could restrict UL54 mutants in the cytoplasm. The ability of the N-terminal 83 residues to target the green fluorescence protein to the nucleus confirmed further its role as a functional nuclear localization signal (NLS). The utmost N-terminal 83 residues portion of UL54 contains two important functional domains, NLS and RNA-binding, and thus it would play an indispensable role in UL54 regulatory function.

Characterization and expression of the pseudorabies virus early gene UL54

Pseudorabies virus (PRV) is an alphaherpesvirus, and its gene organization and regulation are similar to the well-characterized human simplex virus (HSV). Sequence analysis of the complete coding region of PRV UL54 gene revealed that the UL54 gene consisted of 1092 nucleotides encoding a protein of 363 amino acids and the gene showed homology to HSV immediate-early protein ICP27. Detection of the UL54 transcript in infected cells by reverse transcription-polymerase chain reaction (RT-PCR) demonstrated that the UL54 gene belonged to the early kinetic class based on sensitivity to cycloheximide and insensitivity to phosphonoacetic acid (PAA). To study the structure and function of UL54 protein, this gene was subcloned on Escherichia coli expression vector pET28b for overexpression, and the expressed product was applied to generate specific antibody against UL54 protein. The specificity of the mouse immuneserum was confirmed by its ability to react with a 40kDa viral protein present in the PRV infected cells in Western immunblotting assay, detected as early as 4h after infection. In addition, immunoperoxidasing staining of PRV infected cells undertaken with this antibody demonstrated mainly nuclear staining pattern. Furthermore, the RNA binding potential of UL54 protein was demonstrated by its binding activity to poly(G) RNA homopolymer in Northwestern blotting assay.

Negative regulation of herpes simplex virus type 1 ICP4 promoter by IE180 protein of pseudorabies virus

Recombinant pseudorabies viruses (PRVs) gIS8 and N1aHTK were constructed by the insertion of a chimeric gene (alpha4-TK) from herpes simplex virus type 1 (HSV-1) into wild-type PRV. HSV-1 TK expression by these recombinant viruses resulted in enhanced sensitivity to ganciclovir, compared to that of the wild-type PRV, and was similar to the sensitivity shown by HSV-1. Infection with gIS8 or N1aHTK recombinant viruses led to expression of HSV-1 TK mRNA as an immediate-early (IE) gene, observed by downregulation of the HSV-1 alpha4 promoter. This negative regulation was due to a PRV IE protein, IE180. IE180, however, does not have all the regulatory functions of the infected-cell protein ICP4, as it does not restore the growth of ICP4-deficient HSV-1 mutants.

Regulation of the vhs gene promoter of pseudorabies virus by IE180 and EP0, and the requirement of a Sp1 Site for the promoter function

The virion host shutoff (vhs) protein is a virion component of Alphaherpesviruses, including pseudorabies virus. In this work, the upstream sequences of vhs gene of pseudorabies virus (TNL strain) was cloned and sequenced. We linked the upstream sequences of vhs gene to the CAT reporter gene and examined the promoter function of this region. The immediate-early protein IE180 of Pseudorabies Virus (PRV) is expressed immediately after infection and plays a vital role in the regulation of other viral genes. Our results demonstrated that the vhs promoter was regulated by the IE180 in a dosage-dependent manner; the vhs promoter was stimulated by low concentration of IE180 but suppressed by high concentration of IE180. Mutational analysis indicated that the only IE180 binding site at the vhs promoter was not essential for its function; however, a Sp1 binding site (15 bp downstream to TATA box) was critical to its function. In addition, the result of cotransfection demonstrated that early protein 0 (EP0) of PRV, another protein with transcriptional function, inhibited the activity of the vhs promoter.

Impaired development of the cerebellum in transgenic mice expressing the immediate-early protein IE180 of pseudorabies virus

Pseudorabies virus (PRV) infection in animals other than its natural host almost always gives rise to fatal diseases in the central nervous system as a result of infection of peripheral neurons and subsequently to the brain. PRV immediate-early protein (IE180) activates transcription of the PRV early and late genes, and other viral and cellular genes, and represses its own transcription. To examine specific effects of IE180 in neuropathogenicity, we have generated four transgenic mouse lines expressing IE180 in a tetracycline-regulated system. In the transgenic mouse lines, cerebellar symptoms such as ataxic gait, tremor and motor discoordination were observed. Histopathology of the cerebella in the transgenic mouse lines showing severe symptoms was remarkable for a failure of layer formation and a reduction in cerebellar size. These findings suggest that IE180 affects the cascade of gene expression for development of the murine cerebellum, resulting in the impairment of the cerebellar development and differentiation.

A TEF-1-element is required for activation of the promoter of pseudorabies virus glycoprotein X gene by IE180

The pseudorabies virus (PRV) immediate-early regulatory protein IE180 is able to transactivate the viral early and late genes. Using chloramphenicol acetyltransferase (CAT) assay, we investigated the transactivation function of IE180 to the promoter of PRV glycoprotein X (gX) gene, and our results showed that IE180 could significantly increase the expression of CAT gene which was under the control of gX promoter. To further identify the activation domains of IE180 protein that interact with the gX promoter sequences, various truncated mutants of IE180 gene and gX promoter gene were constructed and analyzed by CAT and gel retardation assay. Results revealed that the N-terminal amino acid residues from 133 to 736 of IE180 could interact with the binding site of transcriptional enhancer factor-1 (TEF-1) that resides in the gX promoter. Formation of protein-DNA complexes between the IE180 protein and the TEF-1 element of the gX promoter was observed using electrophoretic mobility shift assay (EMSA) as well as Southwestern blot analysis. These results indicated that a direct interaction occurred between IE180 and the TEF-1 element; and this interaction was abolished if the TEF-1 element was mutated. The association of IE180 with the TEF-1 element was further confirmed by the supershift of EMSA complexes using IE180 specific antibody. Taken together, our results suggested that formation of a complex between the IE180 protein and TEF-1 element in the gX promoter region was involved in the transcriptional regulation of the gX gene.

Cloning and regulation of the promoter of pseudorabies virus (TNL strain) glycoprotein E gene

The nucleotide sequence upstream to the glycoprotein E (gE) gene of pseudorabies virus (PrV, TNL strain) was cloned from the genomic virus DNA by polymerase chain reaction (PCR) and its DNA sequences were determined. The DNA segment, which was supposed to contain the gE promoter, was subcloned into a chloramphenicol acetyltransferase (CAT) reporter gene and the resulting plasmid was named pgEp-B-CAT. To examine the promoter function of this upstream sequence of gE gene, we transfected pgEp-B-CAT DNA into L-M cells and the promoter activity was analyzed by CAT assay. Results showed that our DNA fragment could exhibit promoter activity. Furthermore, we transfected L-M cells with pgEp-B-CAT for 48 h, then superinfected cells with pseudorabies virus, and performed CAT assay. It was found that PrV superinfection could slightly enhance the activity of gE promoter, suggesting that factors produced during viral infection could stimulate the promoter. To explore the possible mechanism of regulation at transcriptional level, the pgEp-B-CAT plasmid were cotransfected with eukaryotic vectors expressing viral regulatory proteins IE or EP0, and results indicated that the gE promoter was activated by IE protein whereas it was inhibited by EP0 protein. Moreover, the effect of exogenous IE or EP0 on the protein level of gE in PrV-infected cells was examined; conclusion similar to that of CAT assay were obtained.

Analysis of regulatory functions for the region located upstream from the latency-associated transcript (LAT) promoter of pseudorabies virus in cultured cells

The latency-associated transcript (LAT) promoter of pseudorabies virus (PrV) is unique among the many promoters of the viral genome in that it remains active during the latent state. The regulatory mechanism of PrV LAT gene expression is complex and different between latency and lytic infection of cultured cells. Although two different sequences, LAP1 and LAP2, are thought to be involved in LAT gene expression, the function of the upstream region of the LAT promoter (LAP1 and LAP2) remains an enigma, even in cultured cells. To analyze the function of the upstream region, it is necessary to examine the effects of the upstream sequence on LAT gene expression in the absence of other viral proteins. Transient expression assays were performed by employing a series of reporter plasmids in which various sequences upstream of the LAT promoter (from nucleotide positions -592 to +423 relative to the transcriptional start site of the large latency transcript (LLT)) were linked to the chloramphenicol acetyltransferase (CAT) gene in cells of neuronal and non-neuronal origin. We identified a region (from nucleotide positions -3606 to -1386) that was capable of repressing the LAT promoter activity in Vero cells by analyzing CAT gene expression of the series of reporter plasmids. This effect was not observed in Neuro-2a cells. We have also shown that the LAT promoter activity of the reporter plasmid containing the upstream region was repressed by the immediate-early gene product IE180 in Vero cells, but not in Neuro-2a cells. These results suggest that the upstream region of the LAT promoter may have a role in repressing LAT gene expression in cultured non-neuronal cells.

Inhibition of pseudorabies virus replication by a dominant-negative mutant of early protein 0 expressed in a tetracycline-regulated system

Pseudorabies virus (PRV) early protein 0 (EP0) consisting of 410 amino acids is a transactivator of viral genes. A mutant consisting of amino acids 1-113 exhibits dominant-negative properties. In order to assess the antiviral potential of the EP0 mutant, Vero cells were transformed with the EP0 mutant gene expressed in a tetracycline-regulated system. The transformed cell lines showed marked resistance to PRV infection when expression of the EP0 mutant gene was induced. In the transformed cell line infected with PRV, synthesis of the immediate-early protein (IE180) and of EP0 was inhibited, whereas the levels of IE and EP0 messenger RNA (mRNA) were not decreased, as compared with those of the control cell line. The present results suggest that the EP0 mutant may not alter the efficiency of the viral gene transcription but rather translation efficiency of the viral mRNA.

Resistance to pseudorabies virus infection in transgenic mice expressing the chimeric transgene that represses the immediate-early gene transcription

A chimeric gene encoding a fusion protein consisting of the DNA-binding domain of the immediate-early (IE) protein of pseudorabies virus (PRV) and a tail-truncated VP16 of herpes simplex virus 1, lacking the transcription activation domain, has been shown to repress transcription of the PRV IE gene, resulting in the inhibition of PRV growth in vitro. To assess the antiviral potential of the fusion protein in vivo, transgenic mice containing the chimeric gene under the control of the virus- and interferon-inducible Mx 1 promoter were generated. A transgenic mouse line showed marked resistance to PRV infection when the mice were challenged intranasally with PRV. Inhibition of PRV replication was also observed in monolayers of embryonic cells prepared from the transgenic mice. In the cells infected with PRV, transcription of the PRV IE gene was repressed. The present results indicate that the chimeric gene is able to exert a significant antiviral effect against PRV infection in vivo.

Pseudorabies virus (PRV) early protein 0 activates PRV gene transcription in combination with the immediate-early protein IE180 and enhances the infectivity of PRV genomic DNA

Pseudorabies virus (PRV) early protein 0 (EP0) functions as a transactivator of the viral gene promoters. In transient expression assays employing chloramphenicol acetyl transferase (CAT) reporter constructs, EP0 and the immediate-early protein IE180 act in an additive manner to activate transcription from the thymidine kinase (TK) and glycoprotein G (gG) gene promoters. EP0 enhanced the synthesis of infectious virus in cotransfection experiments with the EP0-expression plasmid and PRV genomic DNA. EP0 was detected by Western blot analysis in the purified virions. These results may indicate that EP0 in the virions acts as an important transactivator to express the immediate-early gene efficiently in the first stage of infection, and IE180 and EP0 expressed after the infection cooperatively activate the early and late gene expression in the later stage of infection.

Promoter activity of sequence located upstream of the pseudorabies virus early protein 0 gene

Promoter activity of the 5'-flanking region of the pseudorabies virus (PRV) early protein 0 (EP0) gene was analysed by transient transfection assays employing chloramphenicol acetyl transferase (CAT) reporter constructs. We identified a 213 bp segment of the viral genome that was capable of efficiently driving expression of the EPO gene and a linked reporter gene upon transient transfection into Vero cells. This segment lacked the typical TATA element, and possessed an initiator element and the putative binding sites for the transcription factor Sp1 and immediate-early protein IE180, a strong transactivator of PRV. By analysing 5'-deletion mutants of the segment, a 48 bp segment (from nucleotide positions -65 to -17), which possessed three Sp1 binding sites, was identified to be critical for the promoter activity. Cotransfection of Vero cells with the mutant constructs and an IE180 expression plasmid resulted in transactivation of only those constructs in which the Sp1 sites were present. These results indicate that the EP0 gene may be transcribed from the TATA-less promoter that responds to Sp1.

Suppression of pseudorabies virus replication by a mutant form of immediate-early protein IE180 repressing the viral gene transcription

A mutant form of the immediate-early (IE) protein IE180 of pseudorabies virus (PRV), dIN454-C1081 is a strong repressor of the PRV IE gene promoter. In order to assess the antiviral potential of the IE180 mutant, HeLa cells were transformed with the mutant gene and then infected with PRV and herpes simplex virus type 1 (HSV-1). The transformed cell lines showed marked resistance to PRV infection, but were susceptible to infection with HSV-1, indicating that the IE180 mutant expressed in the stable cell line specifically inhibited PRV growth. In those cells infected with PRV, transcription of the PRV IE gene was repressed. In addition, the IE180 mutant exhibited a dominant-negative property in transient expression assay. The present results indicate that the resistance of the cells to PRV infection was due to repression of the IE gene transcription by the IE 180 mutant.

Negative regulation of immediate-early gene expression of pseudorabies virus by interferon-alpha

Pseudorabies rabies (PrV) replication in Vero cells was suppressed by treatment with human natural interferon-alpha (IFN-alpha). Messenger RNA transcribed from the PrV immediate-early (IE) gene was reduced in the IFN-alpha-treated cells. Transient expression assays showed that transcription from the PrV IE promoter was selectively inhibited in the IFN-alpha-treated cells. Analysis of deletion mutants of the PrV IE promoter sequence suggested that at least one element between the transcription initiation site (+1) and -90 in the PrV IE promoter was concerned with the negative regulation.

Mapping of transregulatory domains of pseudorabies virus early protein 0 and identification of its dominant-negative mutant

Pseudorabies virus (PRV) early protein 0 (EP0) is a transactivator containing the RING finger domain. Analysis of transactivating activity of truncated forms of the EP0 molecule consisting of 410 amino acids revealed that amino-terminal region containing the RING finger domain, amino acids 1 to 84, and the region between amino acids 114 to 242 containing acidic amino acid sequences were required for the transactivation. On the other hand, the mutant consisting of amino acids 1 to 113 exhibited a dominant-negative property.

Mapping of a functional region conferring nuclear localization of pseudorabies virus immediate-early protein

The immediate-early protein (IE180) of pseudorabies virus (PrV) is localized predominantly in the nuclei of infected cells. To define the nuclear localization signals within IE180, we prepared truncated mutants of IE180 and analyzed their localization in the transfected cells by indirect immunofluorescence. Analysis of mutants truncated from the carboxy-terminal end of the 1460-amino acid polypeptide showed that two regions including a short sequence of basic amino acid residues were associated with the nuclear localization of IE180. To assess whether these regions substantially function as signals for nuclear localization of the IE180 molecule, we then constructed two deletion mutants lacking each region. A mutant lacking amino acids 333 to 575 was detected in the nuclei of the transfected cells, whereas the other mutant lacking amino acids 900 to 950 was detected mainly in the cytoplasm. These results suggest that the region of amino acids 900 to 950 is responsible for nuclear localization of IE180.