The murine cytomegalovirus (MCMV) homolog of the HCMV phosphotransferase (UL97(pk)) gene

Virological, innate, and adaptive immune profiles shaped by variation in route and age of host in murine cytomegalovirus infection

Human cytomegalovirus (hCMV) is a ubiquitous facultative pathogen, which establishes a characteristic latent and reactivating lifelong infection in immunocompetent hosts. Murine CMV (mCMV) infection is widely used as an experimental model of hCMV infection, employed to investigate the causal nature and extent of CMV's contribution to inflammatory, immunological, and health disturbances in humans. Therefore, mimicking natural human infection in mice would be advantageous to hCMV research. To assess the role of route and age at infection in modeling hCMV in mice, we infected prepubescent and young sexually mature C57BL/6 (B6) mice intranasally (i.n., a likely physiological route in humans) and intraperitoneally (i.p., a frequently used experimental route, possibly akin to transplant-mediated infection). In our hands, both routes led to comparable early viral loads and tissue spreads. However, they yielded differential profiles of innate and adaptive systemic immune activation. Specifically, the younger, prepubescent mice exhibited the strongest natural killer cell activation in the blood in response to i.p. infection. Further, the i.p. infected animals (particularly those infected at 12 weeks) exhibited larger anti-mCMV IgG and greater expansion of circulating CD8+ T cells specific for both acute (non-inflationary) and latent phase (inflationary) mCMV epitopes. By contrast, tissue immune responses were comparable between i.n. and i.p. groups. Our results illustrate a distinction in the bloodborne immune response profiles across infection routes and ages and are discussed in light of physiological parameters of interaction between CMV, immunity, inflammation, and health over the lifespan.

IMPORTANCE

The majority of experiments modeling human cytomegalovirus (hCMV) infection in mice have been carried out using intraperitoneal infection in sexually mature adult mice, which stands in contrast to the large number of humans being infected with human CMV at a young age, most likely via bodily fluids through the nasopharyngeal/oral route. This study examined the impact of the choice of age and route of infection in modeling CMV infection in mice. By comparing young, prepubescent to older sexually mature counterparts, infected either via the intranasal or intraperitoneal route, we discovered substantial differences in deployment and response intensity of different arms of the immune system in systemic control of the virus; tissue responses, by contrast, appeared similar between ages and infection routes.

Cytomegalovirus infection induces Alzheimer's disease-associated alterations in tau

Alzheimer's disease (AD) manifests with loss of neurons correlated with intercellular deposition of amyloid (amyloid plaques) and intracellular neurofibrillary tangles of hyperphosphorylated tau. However, targeting AD hallmarks has not as yet led to development of an effective treatment despite numerous clinical trials. A better understanding of the early stages of neurodegeneration may lead to development of more effective treatments. One underexplored area is the clinical correlation between infection with herpesviruses and increased risk of AD. We hypothesized that similar to work performed with herpes simplex virus 1 (HSV1), infection with the cytomegalovirus (CMV) herpesvirus increases levels and phosphorylation of tau, similar to AD tauopathy. We used murine CMV (MCMV) to infect mouse fibroblasts and rat neuronal cells to test our hypothesis. MCMV infection increased steady-state levels of primarily high molecular weight forms of tau and altered the patterns of tau phosphorylation. Both changes required viral late gene products. Glycogen synthase kinase 3 beta (GSK3β) was upregulated in the HSVI model, but inhibition with lithium chloride suggested that this enzyme is unlikely to be involved in MCMV infection mediated tau phosphorylation. Thus, we confirm that MCMV, a beta herpes virus, like alpha herpes viruses (e.g., HSV1), can promote tau pathology. This suggests that CMV infection can be useful as another model system to study mechanisms leading to neurodegeneration. Since MCMV infects both mice and rats as permissive hosts, our findings from tissue culture can likely be applied to a variety of AD models to study development of abnormal tau pathology.

A viral kinase counteracts in vivo restriction of murine cytomegalovirus by SAMHD1

The deoxynucleotide triphosphate (dNTP) hydrolase SAMHD1 inhibits retroviruses in non-dividing myeloid cells. Although antiviral activity towards DNA viruses has also been demonstrated, the role of SAMHD1 during cytomegalovirus (CMV) infection remains unclear. To determine the impact of SAMHD1 on the replication of CMV, we used murine CMV (MCMV) to infect a previously established SAMHD1 knockout mouse model and found that SAMHD1 inhibits the replication of MCMV in vivo. By comparing the replication of MCMV in vitro in myeloid cells and fibroblasts from SAMHD1-knockout and control mice, we found that the viral kinase M97 counteracts SAMHD1 after infection by phosphorylating the regulatory residue threonine 603. The phosphorylation of SAMHD1 in infected cells correlated with a reduced level of dNTP hydrolase activity and the loss of viral restriction. Together, we demonstrate that SAMHD1 acts as a restriction factor in vivo and we identify the M97-mediated phosphorylation of SAMHD1 as a previously undescribed viral countermeasure.

Resolving the titer of murine cytomegalovirus by plaque assay using the M2-10B4 cell line and a low viscosity overlay

BACKGROUND

Murine cytomegalovirus (MCMV) is increasingly used as an infectious model to investigate host-pathogen interactions in mice. Detailed methods have been published for using primary murine embryonic fibroblasts (MEFs) for preparing stocks and determining viral titers of MCMV. For determining the titer of MCMV by plaque assay, these methods rely on a high viscosity media that restricts viral spreading through the supernatant of the culture, but is also usually too viscous to pipet. Moreover, MEFs must be repeatedly generated and can vary widely from batch-to-batch in purity, proliferation rates, and the development of senescence. In contrast, the M2-10B4 bone marrow stromal cell line (ATCC # CRL-1972), which is also permissive for MCMV, has been reported to produce high-titer stocks of MCMV and has the considerable advantages of growing rapidly and consistently. However, detailed methods using these cells have not been published.

METHODS

We modified existing protocols to use M2-10B4 cells for measuring MCMV titers by plaque assay.

RESULTS

We found that MCMV plaques could be easily resolved on monolayers of M2-10B4 cells. Moreover, plaques formed normally even when cultures of M2-10B4 cells were less than 50% confluent on the day of infection, as long as we also used a reduced viscosity overlay.

CONCLUSIONS

Overall, our protocol enabled us to use a consistent cell line to assess viral titers, rather than repeatedly producing primary MEFs. It also allowed us to start the assay with 4-fold fewer cells than would be required to generate a confluent monolayer, reducing the lead-time prior to the start of the assay. Finally, the reduced viscosity CMC could be handled by pipet and did not need to be pre-mixed with media, thus increasing its shelf-life and ease-of-use. We describe our results here, along with detailed protocols for the use of the M2-10B4 cell lines to determine the titer and grow stocks of MCMV.

Molecular investigation of the 7.2 kb RNA of murine cytomegalovirus

Background

HCMV encodes a stable 5 kb RNA of unknown function that is conserved across cytomegalovirus species. In vivo studies of the MCMV orthologue, a 7.2 kb RNA, demonstrated that viruses that do not express the RNA fail to establish efficient persistent replication in the salivary glands of mice. To gain further insight into the function and properties of this conserved locus, we characterized the MCMV intron in finer detail.

Methods

We performed multiple analyses to evaluate transcript expression kinetics, identify transcript termini and promoter elements. The half-lives of intron locus RNAs were quantified by measuring RNA levels following actinomycin D treatment in a qRT-PCR-based assay. We also constructed a series of recombinant viruses to evaluate protein coding potential in the locus and test the role of putative promoter elements. These recombinant viruses were tested in both in vitro and in vivo assays.

Results

We show that the 7.2 kb RNA is expressed with late kinetics during productive infection of mouse fibroblasts. The termini of the precursor RNA that is processed to produce the intron were identified and we demonstrate that the m106 open reading frame, which resides on the spliced mRNA derived from precursor processing, can be translated during infection. Mapping the 5′ end of the primary transcript revealed minimal promoter elements located upstream that contribute to transcript expression. Analysis of recombinant viruses with deletions in the putative promoter elements, however, revealed these elements exert only minor effects on intron expression and viral persistence in vivo. Low transcriptional output by the putative promoter element(s) is compensated by the long half-life of the 7.2 kb RNA of approximately 28.8 hours. Detailed analysis of viral spread prior to the establishment of persistence also showed that the intron is not likely required for efficient spread to the salivary gland, but rather enhances persistent replication in this tissue site.

Conclusions

This data provides a comprehensive transcriptional analysis of the MCMV 7.2 kb intron locus. Our studies indicate that the 7.2 kb RNA is an extremely long-lived RNA, a feature which is likely to be important in its role promoting viral persistence in the salivary gland.

Cytomegalovirus antivirals and development of improved animal models

INTRODUCTION

Cytomegalovirus (CMV) is a ubiquitous pathogen that establishes a lifelong asymptomatic infection in healthy individuals. Infection of immunesuppressed individuals causes serious illness. Transplant and AIDS patients are highly susceptible to CMV leading to life-threatening end-organ disease. Another vulnerable population is the developing fetus in utero, where congenital infection can result in surviving newborns with long-term developmental problems. There is no vaccine licensed for CMV and current antivirals suffer from complications associated with prolonged treatment. These include drug toxicity and emergence of resistant strains. There is an obvious need for new antivirals. Candidate intervention strategies are tested in controlled preclinical animal models but species specificity of human CMV precludes the direct study of the virus in an animal model.

AREAS COVERED

This review explores the current status of CMV antivirals and development of new drugs. This includes the use of animal models and the development of new improved models such as humanized animal CMV and bioluminescent imaging of virus in animals in real time.

EXPERT OPINION

Various new CMV antivirals are in development, some with greater spectrum of activity against other viruses. Although the greatest need is in the setting of transplant patients, there remains an unmet need for a safe antiviral strategy against congenital CMV. This is especially important as an effective CMV vaccine remains an elusive goal. In this regard, greater emphasis should be placed on suitable preclinical animal models and greater collaboration between industry and academia.

Function of human cytomegalovirus UL97 kinase in viral infection and its inhibition by maribavir

The serine/threonine kinase expressed by human cytomegalovirus from gene UL97 phosphorylates the antiviral drug ganciclovir, but its biological function is the phosphorylation of its natural viral and cellular protein substrates which affect viral replication at many levels. The UL97 kinase null phenotype is therefore complex, as is the mechanism of action of maribavir, a highly specific inhibitor of its enzymatic activity. Studies that utilise the drug corroborate results from genetic approaches and together have elucidated many functions of the UL97 kinase that are critical for viral replication. The kinase phosphorylates eukaryotic elongation factor 1delta, the carboxyl terminal domain of the large subunit of RNA polymerase II, the retinoblastoma tumour suppressor and lamins A and C. Each of these is also phosphorylated and regulated by cdc2/cyclin-dependent kinase 1, suggesting that the viral kinase may perform a similar function. These and other activities of the UL97 kinase appear to stimulate the cell cycle to support viral DNA synthesis, enhance the expression of viral genes, promote virion morphogenesis and facilitate the egress of mature capsids from the nucleus. In the absence of UL97 kinase activity, viral DNA synthesis is inefficient and structural proteins are sequestered in nuclear aggresomes, reducing the efficiency of virion morphogenesis. Mature capsids that do form fail to egress the nucleus as the nuclear lamina are not dispersed by the kinase. The critical functions performed by the UL97 kinase illustrate its importance in viral replication and confirm that the kinase is a target for the development of antiviral therapies.

Ly49h is necessary for genetic resistance to murine cytomegalovirus

Natural killer (NK) cells play critical roles in antiviral immunity. While the importance of effector mechanisms such as interferons has been demonstrated through knockout mice, specific mechanisms of how viruses are recognized and controlled by NK cells are less well defined. Previous genetic studies have mapped the resistance genes for murine cytomegalovirus (MCMV), herpes simplex virus-1 (HSV-1), and ectromelia virus to the NK gene complex on murine chromosome 6, a region containing the polymorphic Ly49 and Nkrp1 families. Genetic resistance to MCMV in C57BL/6 has been attributed to Ly49H, an activation receptor, through susceptibility of the recombinant inbred strain BXD-8 that lacks Ly49h (also known as Klra8) but derived about half of its genome from its DBA/2 progenitor. However, it remained possible that epigenetic effects could account for the MCMV phenotype in BXD-8 mice. Herein, we report the generation of a novel congenic murine strain, B6.BXD8-Klra8 ( Cmv1-del )/Wum, on the C57BL/6 genetic background to evaluate the effect of deletion of a single NK activation receptor, Ly49H. Deletion of Ly49H rendered mice much more susceptible to MCMV infection. This increase in susceptibility did not appear to be a result of a difference in NK cell expansion or interferon-gamma (IFN-gamma) production between the C57BL/6 and the B6.BXD8 strains. On the other hand, the deletion of Ly49h did not otherwise affect NK cell maturation or Ly49D expression and had no effect on susceptibility to HSV-1 or ectromelia virus. In conclusion, Ly49h is necessary for genetic resistance to MCMV, but not HSV-1 or ectromelia virus.

Expression of the human cytomegalovirus UL97 gene in a chimeric guinea pig cytomegalovirus (GPCMV) results in viable virus with increased susceptibility to ganciclovir and maribavir

In lieu of a licensed vaccine, antivirals are being considered as an intervention to prevent congenital human cytomegalovirus (HCMV) infection. Ideally, antiviral therapies should undergo pre-clinical evaluation in an animal model prior to human use. Guinea pig cytomegalovirus (GPCMV) is the only small animal model for congenital CMV. However, GPCMV is not susceptible to the most commonly used HCMV antiviral, ganciclovir (GCV), rendering in vivo study of this agent problematic in the guinea pig model. Human cytomegalovirus (HCMV) susceptibility to GCV is linked to the UL97 gene. We hypothesized that GPCMV susceptibility to GCV could be improved by inserting the HCMV (Towne) UL97 gene into the GPCMV genome in place of the homolog, GP97. A chimeric GPCMV (GPCMV::UL97) expressed UL97 protein, and replicated efficiently in cell culture, with kinetics similar to wild-type GPCMV. In contrast, deletion of GP97 resulted in a virus (GPCMVdGP97) that grew poorly in culture. GPCMV::UL97 had substantially improved susceptibility to the inhibitory effects of GCV in comparison to wild-type GPCMV. Additionally, GPCMV::UL97 exhibited improved susceptibility to another antiviral undergoing clinical trials, maribavir (MBV; benzimidazole riboside 1263W94), which also acts through UL97.

Coordinated function of murine cytomegalovirus genes completely inhibits CTL lysis

Murine CMV (MCMV) encodes three viral genes that interfere with Ag presentation (VIPRs) to CD8 T cells, m04, m06, and m152. Because the functional impact of these genes during normal infection of C57BL/6 mice is surprisingly modest, we wanted to determine whether the VIPRs are equally effective against the entire spectrum of H-2(b)-restricted CD8 T cell epitopes. We also wanted to understand how the VIPRs interact at a functional level. To address these questions, we used a panel of MCMV mutants lacking each VIPR in all possible combinations, and CTL specific for 15 H-2(b)-restricted MCMV epitopes. Only expression of all three MCMV VIPRs completely inhibited killing by CTL specific for all 15 epitopes, but removal of any one VIPR enabled lysis by at least some CTL. The dominant interaction between the VIPRs was cooperation: m06 increased the inhibition of lysis achieved by either m152 or m04. However, for 1 of 15 epitopes m04 functionally antagonized m152. There was little differential impact of any of the VIPRs on K(b) vs D(b), but a surprising degree of differential impact of the three VIPRs for different epitopes. These epitope-specific differences did not correlate with functional avidity, or with timing of VIPR expression in relation to Ag expression in the virus replication cycle. Although questions remain about the molecular mechanism and in vivo role of these genes, we conclude that the coordinated function of MCMV's three VIPRs results in a powerful inhibition of lysis of infected cells by CD8 T cells.

Murine cytomegalovirus resistant to antivirals has genetic correlates with human cytomegalovirus

Human cytomegalovirus (HCMV) resistance to antivirals is a significant clinical problem. Murine cytomegalovirus (MCMV) infection of mice is a well-described animal model for in vivo studies of CMV pathogenesis, although the mechanisms of MCMV antiviral susceptibility need elucidation. Mutants resistant to nucleoside analogues aciclovir, adefovir, cidofovir, ganciclovir, penciclovir and valaciclovir, and the pyrophosphate analogue foscarnet were generated by in vitro passage of MCMV (Smith) in increasing concentrations of antiviral. All MCMV antiviral resistant mutants contained DNA polymerase mutations identical or similar to HCMV DNA polymerase mutations known to confer antiviral resistance. Mapping of the mutations onto an MCMV DNA polymerase three-dimensional model generated using the Thermococcus gorgonarius Tgo polymerase crystal structure showed that the DNA polymerase mutations potentially confer resistance through changes in regions surrounding a catalytic aspartate triad. The ganciclovir-, penciclovir- and valaciclovir-resistant isolates also contained mutations within MCMV M97 identical or similar to recognized GCV-resistant mutations of HCMV UL97 protein kinase, and demonstrated cross-resistance to antivirals of the same class. This strongly suggests that MCMV M97 has a similar role to HCMV UL97 in the phosphorylation of nucleoside analogue antivirals. All MCMV mutants demonstrated replication-impaired phenotypes, with the lowest titre and plaque size observed for isolates containing mutations in both DNA polymerase and M97. These findings indicate DNA polymerase and protein kinase regions of potential importance for antiviral susceptibility and replication. The similarities between MCMV and HCMV mutations that arise under antiviral selective pressure increase the utility of MCMV as a model for in vivo studies of CMV antiviral resistance.

Identification of proteins associated with murine cytomegalovirus virions

Proteins associated with the murine cytomegalovirus (MCMV) viral particle were identified by a combined approach of proteomic and genomic methods. Purified MCMV virions were dissociated by complete denaturation and subjected to either separation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and in-gel digestion or treated directly by in-solution tryptic digestion. Peptides were separated by nanoflow liquid chromatography and analyzed by tandem mass spectrometry (LC-MS/MS). The MS/MS spectra obtained were searched against a database of MCMV open reading frames (ORFs) predicted to be protein coding by an MCMV-specific version of the gene prediction algorithm GeneMarkS. We identified 38 proteins from the capsid, tegument, glycoprotein, replication, and immunomodulatory protein families, as well as 20 genes of unknown function. Observed irregularities in coding potential suggested possible sequence errors in the 3'-proximal ends of m20 and M31. These errors were experimentally confirmed by sequencing analysis. The MS data further indicated the presence of peptides derived from the unannotated ORFs ORF(c225441-226898) (m166.5) and ORF(105932-106072). Immunoblot experiments confirmed expression of m166.5 during viral infection.

The UL97 protein kinase of human cytomegalovirus and homologues in other herpesviruses: impact on virus and host

The human herpesviruses, herpes simplex virus 1 (HSV-1), HSV-2, varicella zoster virus (VZV), Epstein-Barr virus (EBV), human cytomegalovirus (HCMV), human herpesvirus 6A (HHV-6A), HHV-6B, HHV-7 and HHV-8, establish persistent infections with possible recurrence during immunosuppression. HCMV replication is inhibited by the nucleoside analogue ganciclovir (GCV), the compound of choice for the treatment of HCMV diseases and preemptive treatment of infections. The viral UL97 protein (pUL97) which shares homologies with protein kinases and bacterial phosphotransferases is able to monophosphorylate GCV. Homologues of pUL97 are found in HSV (UL13), VZV (ORF47), EBV (BGLF4), HHV-6 (U69), HHV-8 (ORF36) as well as in murine CMV (M97) or rat CMV (R97). Several indolocarbazoles have been reported to be specific inhibitors of pUL97. The protein is important for efficient replication of the virus. Autophosphorylation of pUL97 was observed using different experimental systems. Most recently, it has been shown that pUL97 interacts with the DNA polymerase processivity factor pUL44. Indolocarbazole protein kinase inhibitors are promising lead compounds for the development of more specific inhibitors of HCMV.

Comparison between human cytomegalovirus pUL97 and murine cytomegalovirus (MCMV) pM97 expressed by MCMV and vaccinia virus: pM97 does not confer ganciclovir sensitivity

The UL97 protein (pUL97) of human cytomegalovirus (HCMV) is a protein kinase that also phosphorylates ganciclovir (GCV), but its biological function is not yet clear. The M97 protein (pM97) of mouse cytomegalovirus (MCMV) is the homolog of pUL97. First, we studied the consequences of genetic replacement of M97 by UL97. Using the infectious bacterial plasmid clone of the full-length MCMV genome (M. Wagner, S. Jonjic, U. H. Koszinowski, and M. Messerle, J. Virol. 73:7056-7060, 1999), we replaced the M97 gene with the UL97 gene and constructed an MCMV M97 deletion mutant and a revertant virus. In addition, pUL97 and pM97 were expressed by recombinant vaccinia virus to compare both for known functions. Remarkably, pM97 proved not to be the reason for the GCV sensitivity of MCMV. When expressed by the recombinant MCMV, however, pUL97 was phosphorylated and endowed MCMV with the capacity to phosphorylate GCV, thereby rendering MCMV more susceptible to GCV. We found that deletion of pM97, although it is not essential for MCMV replication, severely affected virus growth. This growth deficit was only partially amended by pUL97 expression. When expressed by recombinant vaccinia viruses, both proteins were phosphorylated and supported phosphorylation of GCV, but pUL97 was about 10 times more effective than pM97. One hint of the functional differences between the proteins was provided by the finding that pUL97 accumulates in the nucleus, whereas pM97 is predominantly located in the cytoplasm of infected cells. In vivo testing revealed that the UL97-MCMV recombinant should allow evaluation of novel antiviral drugs targeted to the UL97 protein of HCMV in mice.

[(11)C]FMAU and [(18)F]FHPG as PET tracers for herpes simplex virus thymidine kinase enzyme activity and human cytomegalovirus infections

[(11)C]-2'-Fluoro-5-methyl-1-beta-D-arabinofuranosyluracil ([(11)C]FMAU) and [(18)F]-9-[(3-fluoro-1-hydroxy-2-propoxy)methyl]guanine ([(18)F]FHPG), radiolabeled representatives of two classes of antiviral agents, were evaluated as tracers for measuring herpes simplex virus thymidine kinase (HSV-tk) enzyme activity after gene transfer and as tracers for localization of active human cytomegalovirus (HCMV) infections. In vitro accumulation experiments revealed that both [(11)C]FMAU and [(18)F]FHPG accumulated significantly more in HSV-tk expressing cells than they did in control cells. [(18)F]FHPG uptake in HSV-tk expressing cells, however, was found to depend strongly on the cell line used, which might be due to cell type dependent membrane transport or cell type dependent substrate specific susceptibility of the enzyme. In vitro, both tracers exhibited a good selectivity for accumulation in HCMV-infected human umbilical vein endothelial cells over uninfected cells. In contrast to [(18)F]FHPG, [(11)C]FMAU uptake in control cells was relatively high due to phosphorylation of the tracer by host kinases. Therefore, [(18)F]FHPG appears to be the more selective tracer not only to predict HSV-tk gene therapy outcome, but also to localize active HCMV infections with PET.

Characterization of rhesus cytomegalovirus genes associated with anti-viral susceptibility

Studies were initiated to determine whether rhesus cytomegalovirus (RhCMV)-infected macaques could serve as an animal model for evaluating anti-CMV compounds, as macaques have a naturally occurring CMV that is similar to human CMV (HCMV). Utilizing plaque reduction assays, RhCMV was tested to anti-viral susceptibility. By these assays. RhCMV displayed anti-viral susceptibility to ganciclovir at a 50% effective dose (ED50) of 0.8 microM, acyclovir at an ED50 of 15 microM, and foscarnet at an ED50 of 250 microM. By Southern blot analysis with HCMV-UL97 (phosphotransferase) and DNA polymerase (pol) genes as probes, we isolated viral DNA fragments that strongly hybridized. DNA sequence analysis of these DNA fragments revealed two open reading frames with homology to HCMV UL97 and DNA polymerase. Steady-state RNA analysis revealed that the RhCMV UL97 homologue and pol genes are transcribed as early late and early genes, respectively. Comparison against HCMV showed the RhCMV UL97 homologue exhibits 54.4% amino acid (aa) sequence identity to HCMV UL97 and the RhCMV DNA polymerase 59.2% aa sequence identity to HCMV DNA polymerase. Results from anti-viral assays and molecular characterization of these two viral genes suggest that RhCMV-infected rhesus macaques should serve as an excellent animal model for evaluating future anti-CMV compounds.