Changes in nuclear basic proteins during pseudorabies virus infection

How to control an infectious bead string: nucleosome-based regulation and targeting of herpesvirus chromatin

Herpesvirus infections of humans can cause a broad variety of symptoms ranging from mild afflictions to life-threatening disease. During infection, the large double-stranded DNA genomes of all herpesviruses are transcribed, replicated and encapsidated in the host cell nucleus, where DNA is typically structured and manoeuvred through nucleosomes. Nucleosomes individually assemble DNA around core histone octamers to form 'beads-on-a-string' chromatin fibres. Herpesviruses have responded to the advantages and challenges of chromatin formation in biologically unique ways. Although herpesvirus DNA is devoid of histones within nucleocapsids, nuclear viral genomes most likely form irregularly arranged or unstable nucleosomes during productive infection, and regular nucleosomal arrays resembling host cell chromatin in latently infected cells. Besides variations in nucleosome density, herpesvirus chromatin 'bead strings' undergo dynamic changes in histone composition and modification during the different stages of productive replication, latent infection and reactivation from latency, raising the likely possibility that epigenetic processes may dictate, at least in part, the outcome of infection and ensuing pathogenesis. Here, we summarise and discuss several new and important aspects regarding the nucleosome-based mechanisms that regulate herpesvirus chromatin structure and function in infected cells. Special emphasis is given to processes of histone deposition, histone variant exchange and covalent histone modification in relation to the transcription from the viral genome during productive and latent infections by human cytomegalovirus and herpes simplex virus type 1. We also present an overview on emerging histone-directed antiviral strategies that may be developed into 'epigenetic therapies' to improve current prevention and treatment options targeting herpesvirus infection and disease.

Transcriptomic analysis of the dialogue between Pseudorabies virus and porcine epithelial cells during infection

Background

Transcriptomic approaches are relevant for studying virus-host cell dialogues to better understand the physiopathology of infection and the immune response at the cellular level. Pseudorabies virus (PrV), a porcine Alphaherpesvirus, is a good model for such studies in pig. Since PrV displays a strong tropism for mucous epithelial cells, we developed a kinetics study of PrV infection in the porcine PK15 epithelial cell line. To identify as completely as possible, viral and cellular genes regulated during infection, we simultaneously analyzed PrV and cellular transcriptome modifications using two microarrays i.e. a laboratory-made combined SLA/PrV microarray, consisting of probes for all PrV genes and for porcine genes contained in the Swine Leukocyte Antigen (SLA) complex, and the porcine generic Qiagen-NRSP8 oligonucleotide microarray. We confirmed the differential expression of a selected set of genes by qRT-PCR and flow cytometry.

Results

An increase in the number of differentially expressed cellular genes and PrV genes especially from 4 h post-infection (pi) was observed concomitantly with the onset of viral progeny while no early global cellular shutoff was recorded. Many cellular genes were down-regulated from 4 h pi and their number increased until 12 h pi. UL41 transcripts encoding the virion host shutoff protein were first detected as differentially expressed at 8 h pi. The viral gene UL49.5 encoding a TAP inhibitor protein was differentially expressed as soon as 2 h pi, indicating that viral evasion via TAP inhibition may start earlier than the cellular gene shutoff. We found that many biological processes are altered during PrV infection. Indeed, several genes involved in the SLA class I antigenic presentation pathway (SLA-Ia, TAP1, TAP2, PSMB8 and PSMB9), were down-regulated, thus contributing to viral immune escape from this pathway and other genes involved in apoptosis, nucleic acid metabolism, cytoskeleton signaling as well as interferon-mediated antiviral response were also modulated during PrV infection.

Conclusion

Our results show that the gene expression of both PrV and porcine cells can be analyzed simultaneously with microarrays, providing a chronology of PrV gene transcription, which has never been described before, and a global picture of transcription with a direct temporal link between viral and host gene expression.

Restricted replication of herpes simplex virus in spinal ganglia of resistant mice is accompanied by an early infiltration of immunoglobulin G-bearing cells

In an attempt to define the nature of the difference in the susceptibility of C57BL/6 (resistant) and A/J (susceptible) mice to herpes simplex virus type 1, we initiated a study of virus progression through the nervous system. After inoculation of virus in a rear footpad, C57BL/6 mice were found to be more than 500-fold more resistant, but resistance did not extend to pseudorabies virus. In additional investigations, it was found that the virus was selectively restricted at the level of spinal ganglia in C57BL/6 mice. No intrinsic difference in the ability of this tissue from either mouse strain to replicate virus was found. However, by 4 days after infection, morphological investigations indicated that a mononuclear cell infiltrate was present surrounding infected neurons and satellite cells both earlier and in greater numbers in the ganglia of C57BL/6 mice. Immunohistochemical methods showed that most of these cells did not express Thy 1.2 antigen, but the vast majority bore immunoglobulin G. The mechanism by which these infiltrating cells could restrict virus spread is discussed.

Isoelectric focusing of infectious particles of porcine pseudorabies virus strains in granulated dextran gels

Isoelectric focusing of infectious particles of four strains of porcine pseudorabies viruses (Indiana, Iowa, Shope and an avirulent live-virus vaccine strains) are described. The pseudorabies virus strains exhibited great mobility in the electric field typical of viruses of the herpes group. Strains were considered electrophoretically homogeneous based on their respective isoelectric points. Maximal virus infectivity was concentrated in reproducible, stationary zones representing 73 to 86% of the total virus infectivity initially applied throughout the gels. All pseudorabies virus strains during processing and after isoelectric focusing retained their ability as whole complete particles to typically infect porcine kidney cell cultures. Virus from gel fractions produces foci in cell cultures that specifically reacted with pseudorabies virus fluorescent antibody conjugate. Prevention of foci could be demonstrated by neutralizing with pseudorabies virus monospecific antiserum. Maximal infectivity titers were demonstrated to be directly related to isoelectric points. Strain differences, in relation to virulence in swine, apparently is not related to isoelectric points.

Characteristics of chromatin preparations from herpes simplex virus infected cells

Chromatin isolated from herpes simplex virus type 1-infected baby hamster kidney cells contains a number of tightly associated virus-induced polypeptides. A subset of these proteins bind to immobilized DNA in vitro (Vmw 175, 155, 130, 63, 43, 38/39). Virus-induced polypeptides extractable with acid from infected cell chromatin include Vmw 155, the major capsid protein of herpes simplex virus type 1 virions, and Vmw 63 and 38/39 which are heterogeneous with respect to charge and are phosphorylated. These chromatin preparations, in the presence of deoxynucleoside triphosphates and MgCl2 were capable of synthesizing viral and cell DNA in a reaction which was stimulated by the addition of ATP, riboNTPs and potassium acetate. In vitro synthesized viral DNA co-sedimented with prelabelled parental DNA but the single-stranded product was smaller than parental DNA. Density labelling indicated that extensive synthesis was taking place and all BamHI fragments of viral DNA were represented by the DNA synthesized in vitro.

Structural analysis of the capsid polypeptides of herpes simplex virus types 1 and 2

Capsids of herpes simplex virus (HSV) types 1 and 2 contain seven polypeptides ranging in molecular weight from 154,000 to 12,000 (termed NC-1 through NC-7 in order of descending molecular weight). Antibodies prepared to HSV-1 capsid polypeptides isolated from sodium dodecyl sulfate-polyacrylamide gels reacted in an immunofluorescence assay against HSV-1-infected KB cells. Three of the antibodies (anti-NC-1, anti-NC-2, and anti-NC-3,4) also reacted with HSV-2-infected cells. Tryptic peptide analysis showed that each of the HSV-1 capsid polypeptides had a unique methionine peptide profile, and none appeared to be derived from the major capsid polypeptide. Comparative peptide analysis of HSV-1 and HSV-2 showed that one polypeptide (NC-7, 12,000 molecular weight) had an identical methionine peptide profile and a very similar arginine peptide profile in both virus types. The arginine peptide profile of NC-7 of HSV-1 was very different from the arginine profile of KB histone H4. Although there were certain intertypic similarities in the methionine peptide profiles of the other capsid components especially in NC-1 (the major capsid protein), there was no case where the tryptic peptides were identical in the two virus types.

Virus-induced proteins in pseudorabies-infected cells. I. Acid-extractable proteins of the nucleus

Three basic proteins have been identified in chromatin preparations from pseudorabies virus-infected cells. They appear to be virus specified and are similar in size and charge to host histones; one difference however is that they contain tryptophan. All are produced by 3 h postinfection, and two (IP II and III) seem to be arginine rich. Three similar proteins are also found in herpes simplex MP 17-infected cells, and two of these co-electrophorese with two of the pseudorabies proteins. Partially purified preparations of pseudorabies virus contain low amounts of all three proteins.

Pathogenesis of herpetic neuritis and ganglionitis in mice: evidence for intra-axonal transport of infection

The pathogenesis of acute herpetic infection in the nervous system has been studied following rear footpad inoculation of mice. Viral assays performed on appropriate tissues at various time intervals indicated that the infection progressed sequentially from peripheral to the central nervous system, with infectious virus reaching the sacrosciatic spinal ganglia in 20 to 24 hr. The infection also progressed to ganglia in mice given high levels of anti-viral antibody. Immunofluorescent techniques demonstrated that both neurons and supporting cells produced virus-specific antigens. By electron microscopy, neurons were found to produce morphologically complete virions, but supporting cells replicated principally nucleocapsids. These results are discussed in the context of possible mechanisms by which herpes simplex virus might travel in nerve trunks. They are considered to offer strong support for centripetal transport in axons.

Transformation of mammalian cells by crude histones

A baby hamster kidney cell line (BHK21), maintained in the presence of crude histone preparations for 3 days, has been shown to undergo morphological and behavioural transformations similar in nature to those obtained with viruses and mycoplasmas.

Restriction of herpes simplex virus by macrophages. An analysis of the cell-virus interaction

Macrophages restrict herpes simplex virus replication and can prevent the development of herpetic disease in mice. In an attempt to define the nature of this restriction, an analysis of virus-specified macromolecular syntheses in infected macrophages was undertaken. The significant results were the following: All cells were killed, but the infection was considered to be abortive since the level of infectious virus in macrophage cultures dropped steadily to a level beyond detection by 25 hr after infection. This restriction appeared to be specific for macrophages; the virus replicated efficiently in other mouse cells. DNA with a density characteristic for herpes simplex virus DNA was extracted from infected cultures, and the proportion of macrophages synthesizing DNA increased from less than 1% to greater than 50% by 6 hr after infection. Studies employing polyacrylamide gel electrophoresis indicated that the major viral-specific proteins were induced in macrophage cultures. In addition, all cells showing cytopathic changes characteristic of herpes virus infection also contained viral antigens which could be detected by fluorescent antibody techniques and, by 15 hr after infection, most contained nascent capsids lacking central dense cores. It is suggested that an error in DNA metabolism may be the primary cause of restriction.

Incorporation of Cystine by Yeast Cells of Histoplasma capsulatum

This report deals with the incorporation of cystine into macromolecules by yeast cells of Histoplasma capsulatum . The results show that at saturating concentrations of cystine in a rich medium, total uptake of the cystine occurs in 10 hr, whereas the amount of label in the cold trichloroacetic acid-soluble material reaches a maximum at 3 to 4 hr and remains at this value. The amount of label in the cold acid-insoluble material accumulates linearly for 4 to 5 hr and reaches a plateau at 7 to 9 hr. A chemical fractionation of labeled cells shows that 25% of the cystine taken up remains as low-molecular-weight components, of which cystine comprises 60 to 75%. Approximately 30% of the total label incorporated is ethanol-soluble, and polyacrylamide gel electrophoresis of this material shows a rather uniform incorporation of the amino acid into many proteins. The combined hot KOH fractions account for 40% of the total label incorporated. The amount of hot KOH-insoluble material almost doubles in a 10-hr pulse, whereas there is an increase in hot KOH-soluble material. Hence, the greatest amount of label from cells pulsed with 14 C-cystine is recovered from cell wall material.