Role for centromeric heterochromatin and PML nuclear bodies in the cellular response to foreign DNA

PML-NB-dependent type I interferon memory results in a restricted form of HSV latency

Herpes simplex virus (HSV) establishes latent infection in long-lived neurons. During initial infection, neurons are exposed to multiple inflammatory cytokines but the effects of immune signaling on the nature of HSV latency are unknown. We show that initial infection of primary murine neurons in the presence of type I interferon (IFN) results in a form of latency that is restricted for reactivation. We also find that the subnuclear condensates, promyelocytic leukemia nuclear bodies (PML-NBs), are absent from primary sympathetic and sensory neurons but form with type I IFN treatment and persist even when IFN signaling resolves. HSV-1 genomes colocalize with PML-NBs throughout a latent infection of neurons only when type I IFN is present during initial infection. Depletion of PML prior to or following infection does not impact the establishment latency; however, it does rescue the ability of HSV to reactivate from IFN-treated neurons. This study demonstrates that viral genomes possess a memory of the IFN response during de novo infection, which results in differential subnuclear positioning and ultimately restricts the ability of genomes to reactivate.

Modulation of H3.3 chromatin assembly by PML: A way to regulate epigenetic inheritance

Although the promyelocytic leukemia (PML) protein is renowned for regulating a wide range of cellular processes and as an essential component of PML nuclear bodies (PML-NBs), the mechanisms through which it exerts its broad physiological impact are far from fully elucidated. Here, we review recent studies supporting an emerging view that PML's pleiotropic effects derive, at least partially, from its role in regulating histone H3.3 chromatin assembly, a critical epigenetic mechanism. These studies suggest that PML maintains heterochromatin organization by restraining H3.3 incorporation. Examination of PML's contribution to H3.3 chromatin assembly in the context of the cell cycle and PML-NB assembly suggests that PML represses heterochromatic H3.3 deposition during S phase and that transcription and SUMOylation regulate PML's recruitment to heterochromatin. Elucidating PML' s contributions to H3.3-mediated epigenetic regulation will provide insight into PML's expansive influence on cellular physiology and open new avenues for studying oncogenesis linked to PML malfunction.

PML nuclear bodies and chromatin dynamics: catch me if you can!

Eukaryotic cells compartmentalize their internal milieu in order to achieve specific reactions in time and space. This organization in distinct compartments is essential to allow subcellular processing of regulatory signals and generate specific cellular responses. In the nucleus, genetic information is packaged in the form of chromatin, an organized and repeated nucleoprotein structure that is a source of epigenetic information. In addition, cells organize the distribution of macromolecules via various membrane-less nuclear organelles, which have gathered considerable attention in the last few years. The macromolecular multiprotein complexes known as Promyelocytic Leukemia Nuclear Bodies (PML NBs) are an archetype for nuclear membrane-less organelles. Chromatin interactions with nuclear bodies are important to regulate genome function. In this review, we will focus on the dynamic interplay between PML NBs and chromatin. We report how the structure and formation of PML NBs, which may involve phase separation mechanisms, might impact their functions in the regulation of chromatin dynamics. In particular, we will discuss how PML NBs participate in the chromatinization of viral genomes, as well as in the control of specific cellular chromatin assembly pathways which govern physiological mechanisms such as senescence or telomere maintenance.

Nuclear domain 10 components upregulated via interferon during human cytomegalovirus infection potently regulate viral infection

Human cytomegalovirus (HCMV) is a ubiquitous betaherpesvirus that causes life-threatening disease in immunocompromised and immunonaïve individuals. Type I interferons (IFNs) are crucial molecules in the innate immune response to HCMV and are also known to upregulate several components of the interchromosomal multiprotein aggregates collectively referred to as nuclear domain 10 (ND10). In the context of herpesvirus infection, ND10 components are known to restrict gene expression. This raises the question as to whether key ND10 components (PML, Sp100 and hDaxx) act as anti-viral IFN-stimulated genes (ISGs) during HCMV infection. In this study, analysis of ND10 component transcription during HCMV infection demonstrated that PML and Sp100 were significantly upregulated whilst hDaxx expression remained unchanged. In cells engineered to block the production of, or response to, type I IFNs, upregulation of PML and Sp100 was not detected during HCMV infection. Furthermore, pre-treatment with an IFN-β neutralizing antibody inhibited upregulation of PML and Sp100 during both infection and treatment with HCMV-infected cell supernatant. The significance of ND10 components functioning as anti-viral ISGs during HCMV infection was determined through knockdown of PML, Sp100 and hDaxx. ND10 knockdown cells were significantly more permissive to HCMV infection, as previously described but, in contrast to control cells, could support HCMV plaque formation following IFN-β pre-treatment. This ability of HCMV to overcome the potently anti-viral effects of IFN-β in ND10 expression deficient cells provides evidence that ND10 component upregulation is a key mediator of the anti-viral activity of IFN-β.

Herpesvirus Latency: On the Importance of Positioning Oneself

The nucleus is composed of multiple compartments and domains, which directly or indirectly influence many cellular processes including gene expression, RNA splicing and maturation, protein post-translational modifications, and chromosome segregation. Nuclear-replicating viruses, especially herpesviruses, have co-evolved with the cell, adopting strategies to counteract and eventually hijack this hostile environment for their own benefit. This allows them to persist in the host for the entire life of an individual and to ensure their maintenance in the target species. Herpesviruses establish latency in dividing or postmitotic cells from which they can efficiently reactivate after sometimes years of a seemingly dormant state. Therefore, herpesviruses circumvent the threat of permanent silencing by reactivating their dormant genomes just enough to escape extinction, but not too much to avoid life-threatening damage to the host. In addition, herpesviruses that establish latency in dividing cells must adopt strategies to maintain their genomes in the daughter cells to avoid extinction by dilution of their genomes following multiple cell divisions. From a biochemical point of view, reactivation and maintenance of viral genomes in dividing cells occur successfully because the viral genomes interact with the nuclear architecture in a way that allows the genomes to be transmitted faithfully and to benefit from the nuclear micro-environments that allow reactivation following specific stimuli. Therefore, spatial positioning of the viral genomes within the nucleus is likely to be essential for the success of the latent infection and, beyond that, for the maintenance of herpesviruses in their respective hosts.

An Adenovirus DNA Replication Factor, but Not Incoming Genome Complexes, Targets PML Nuclear Bodies

Promyelocytic leukemia protein nuclear bodies (PML-NBs) are subnuclear domains implicated in cellular antiviral responses. Despite the antiviral activity, several nuclear replicating DNA viruses use the domains as deposition sites for the incoming viral genomes and/or as sites for viral DNA replication, suggesting that PML-NBs are functionally relevant during early viral infection to establish productive replication. Although PML-NBs and their components have also been implicated in the adenoviral life cycle, it remains unclear whether incoming adenoviral genome complexes target PML-NBs. Here we show using immunofluorescence and live-cell imaging analyses that incoming adenovirus genome complexes neither localize at nor recruit components of PML-NBs during early phases of infection. We further show that the viral DNA binding protein (DBP), an early expressed viral gene and essential DNA replication factor, independently targets PML-NBs. We show that DBP oligomerization is required to selectively recruit the PML-NB components Sp100 and USP7. Depletion experiments suggest that the absence of one PML-NB component might not affect the recruitment of other components toward DBP oligomers. Thus, our findings suggest a model in which an adenoviral DNA replication factor, but not incoming viral genome complexes, targets and modulates PML-NBs to support a conducive state for viral DNA replication and argue against a generalized concept that PML-NBs target incoming viral genomes.

IMPORTANCE

The immediate fate upon nuclear delivery of genomes of incoming DNA viruses is largely unclear. Early reports suggested that incoming genomes of herpesviruses are targeted and repressed by PML-NBs immediately upon nuclear import. Genome localization and/or viral DNA replication has also been observed at PML-NBs for other DNA viruses. Thus, it was suggested that PML-NBs may immediately sense and target nuclear viral genomes and hence serve as sites for deposition of incoming viral genomes and/or subsequent viral DNA replication. Here we performed a detailed analyses of the spatiotemporal distribution of incoming adenoviral genome complexes and found, in contrast to the expectation, that an adenoviral DNA replication factor, but not incoming genomes, targets PML-NBs. Thus, our findings may explain why adenoviral genomes could be observed at PML-NBs in earlier reports but argue against a generalized role for PML-NBs in targeting invading viral genomes.

The histone deacetylase inhibitor Entinostat enhances polymer-mediated transgene expression in cancer cell lines

Eukaryotic cells maintain an immense amount of genetic information by tightly wrapping their DNA around positively charged histones. While this strategy allows human cells to maintain more than 25,000 genes, histone binding can also block gene expression. Consequently, cells express histone acetyl transferases (HATs) to acetylate histone lysines and release DNA for transcription. Conversely, histone deacetylases (HDACs) are employed for restoring the positive charge on the histones, thereby silencing gene expression by increasing histone-DNA binding. It has previously been shown that histones bind and silence viral DNA, while hyperacetylation of histones via HDAC inhibition restores viral gene expression. In this study, we demonstrate that treatment with Entinostat, an HDAC inhibitor, enhances transgene (luciferase) expression by up to 25-fold in human prostate and murine bladder cancer cell lines when used with cationic polymers for plasmid DNA delivery. Entinostat treatment altered cell cycle progression, resulting in a significant increase in the fraction of cells present in the G0/G1 phase at low micromolar concentrations. While this moderate G0/G1 arrest disappeared at higher concentrations, a modest increase in the fraction of apoptotic cells and a decrease in cell proliferation were observed, consistent with the known anticancer effects of the drug. DNase accessibility studies revealed no significant change in plasmid transcriptional availability with Entinostat treatment. However, quantitative PCR studies indicated that Entinostat treatment, at the optimal dose for enhancing transgene expression, led to an increase in the amount of plasmid present in the nucleus in two cancer cell lines. Taken together, our results show that Entinostat enhances polymer- mediated transgene expression and can be useful in applications related to transient protein expression in mammalian cells. Biotechnol. Bioeng. 2016;113: 1345-1356. © 2015 Wiley Periodicals, Inc.

Alterations of Nuclear Architecture and Epigenetic Signatures during African Swine Fever Virus Infection

Viral interactions with host nucleus have been thoroughly studied, clarifying molecular mechanisms and providing new antiviral targets. Considering that African swine fever virus (ASFV) intranuclear phase of infection is poorly understood, viral interplay with subnuclear domains and chromatin architecture were addressed. Nuclear speckles, Cajal bodies, and promyelocytic leukaemia nuclear bodies (PML-NBs) were evaluated by immunofluorescence microscopy and Western blot. Further, efficient PML protein knockdown by shRNA lentiviral transduction was used to determine PML-NBs relevance during infection. Nuclear distribution of different histone H3 methylation marks at lysine’s 9, 27 and 36, heterochromatin protein 1 isoforms (HP1α, HPβ and HPγ) and several histone deacetylases (HDACs) were also evaluated to assess chromatin status of the host. Our results reveal morphological disruption of all studied subnuclear domains and severe reduction of viral progeny in PML-knockdown cells. ASFV promotes H3K9me3 and HP1β foci formation from early infection, followed by HP1α and HDAC2 nuclear enrichment, suggesting heterochromatinization of host genome. Finally, closeness between DNA damage response factors, disrupted PML-NBs, and virus-induced heterochromatic regions were identified. In sum, our results demonstrate that ASFV orchestrates spatio-temporal nuclear rearrangements, changing subnuclear domains, relocating Ataxia Telangiectasia Mutated Rad-3 related (ATR)-related factors and promoting heterochromatinization, probably controlling transcription, repressing host gene expression, and favouring viral replication.

Analysis of essential viral gene functions after highly efficient adenofection of cells with cloned human cytomegalovirus genomes

Human cytomegalovirus (HCMV) has a large 240 kb genome that may encode more than 700 gene products with many of them remaining uncharacterized. Mutagenesis of bacterial artificial chromosome (BAC)-cloned CMV genomes has greatly facilitated the analysis of viral gene functions. However, the roles of essential proteins often remain particularly elusive because their investigation requires the cumbersome establishment of suitable complementation systems. Here, we show that HCMV genomes can be introduced into cells with unprecedented efficiency by applying a transfection protocol based on replication-defective, inactivated adenovirus particles (adenofection). Upon adenofection of several permissive cell types with HCMV genomes carrying mutations in essential genes, transfection rates of up to 60% were observed and viral proteins of all kinetic classes were found expressed. This enabled further analyses of the transfected cells by standard biochemical techniques. Remarkably, HCMV genomes lacking elements essential for viral DNA replication, such as the lytic origin of replication, still expressed several late proteins. In conclusion, adenofection allows the study of essential HCMV genes directly in BAC-transfected cells without the need for sophisticated complementation strategies.

Differential type I interferon-dependent transgene silencing of helper-dependent adenoviral vs. adeno-associated viral vectors in vivo

We previously dissected the components of the innate immune response to Helper-dependent adenoviral vectors (HDAds) using genetic models, and demonstrated that multiple pattern recognition receptor signaling pathways contribute to this host response to HDAds in vivo. Based on analysis of cytokine expression profiles, type I interferon (IFN) mRNA is induced in host mouse livers at 1 hour post-injection. This type I IFN signaling amplifies cytokine expression in liver independent of the nature of vector DNA sequences after 3 hours post-injection. This type I IFN signaling in response to HDAds administration contributes to transcriptional silencing of both HDAd prokaryotic and eukaryotic DNA in liver. This silencing occurs early and is mediated by epigenetic modification as shown by in vivo chromatin immunoprecipitation (ChIP) with anti-histone deacetylase (HDAC) and promyelocytic leukemia protein (PML). In contrast, self-complementary adeno-associated viral vectors (scAAVs) showed significantly lower induction of type I IFN mRNA in liver compared to HDAds at both early and late time points. These results show that the type I IFN signaling dependent transgene silencing differs between AAV and HDAd vectors after liver-directed gene transfer.

Histone deacetylase inhibition activates transgene expression from integration-defective lentiviral vectors in dividing and non-dividing cells

Integration-defective lentiviral vectors (IDLVs) are being increasingly deployed in both basic and preclinical gene transfer settings. Often, however, the IDLV transgene expression profile is muted when compared to that of their integration-proficient counterparts. We hypothesized that the episomal nature of IDLVs turns them into preferential targets for epigenetic silencing involving chromatin-remodeling histone deacetylation. Therefore, vectors carrying an array of cis-acting elements and transcriptional unit components were assembled with the aid of packaging constructs encoding either the wild-type or the class I mutant D116N integrase moieties. The transduction levels and transgene-product yields provided by each vector class were assessed in the presence and absence of the histone deacetylase (HDAC) inhibitors sodium butyrate and trichostatin A. To investigate the role of the target cell replication status, we performed experiments in growth-arrested human mesenchymal stem cells and in post-mitotic syncytial myotubes. We found that IDLVs are acutely affected by HDACs regardless of their genetic makeup or target cell replication rate. Interestingly, the magnitude of IDLV transgene expression rescue due to HDAC inhibition varied in a vector backbone- and cell type-dependent manner. Finally, investigation of histone modifications by chromatin immunoprecipitation followed by quantitative PCR (ChIP-qPCR) revealed a paucity of euchromatin marks distributed along IDLV genomes when compared to those measured on isogenic integration-competent vector templates. These findings support the view that IDLVs constitute preferential targets for epigenetic silencing involving histone deacetylation, which contributes to dampening their full transcriptional potential. Our data provide leads on how to most optimally titrate and deploy these promising episomal gene delivery vehicles.

Hepatitis B virus X protein stimulates gene expression selectively from extrachromosomal DNA templates

Chronic hepatitis B virus (HBV) infection is a major risk factor for liver cancer development. HBV encodes the hepatitis B virus X (HBx) protein that promotes transcription of the viral episomal DNA genome by the host cell RNA polymerase II. Here we provide evidence that HBx accomplishes this task by a conserved and unusual mechanism. Thus, HBx strongly stimulates expression of transiently transfected reporter constructs, regardless of the enhancer and promoter sequences. This activity invariably requires HBx binding to the cellular UV-damaged DDB1 E3 ubiquitin ligase, suggesting a common mechanism. Unexpectedly, none of the reporters tested is stimulated by HBx when integrated into the chromosome, despite remaining responsive to their cognate activators. Likewise, HBx promotes gene expression from the natural HBV episomal template but not from a chromosomally integrated HBV construct. The same was observed with the HBx protein of woodchuck HBV. HBx does not affect nuclear plasmid copy number and functions independently of CpG dinucleotide methylation.

CONCLUSION

We propose that HBx supports HBV gene expression by a conserved mechanism that acts specifically on episomal DNA templates independently of the nature of the cis-regulatory sequences. Because of its uncommon property and key role in viral transcription, HBx represents an attractive target for new antiviral therapies.

HSV-1 genome subnuclear positioning and associations with host-cell PML-NBs and centromeres regulate LAT locus transcription during latency in neurons

Major human pathologies are caused by nuclear replicative viruses establishing life-long latent infection in their host. During latency the genomes of these viruses are intimately interacting with the cell nucleus environment. A hallmark of herpes simplex virus type 1 (HSV-1) latency establishment is the shutdown of lytic genes expression and the concomitant induction of the latency associated (LAT) transcripts. Although the setting up and the maintenance of the latent genetic program is most likely dependent on a subtle interplay between viral and nuclear factors, this remains uninvestigated. Combining the use of in situ fluorescent-based approaches and high-resolution microscopic analysis, we show that HSV-1 genomes adopt specific nuclear patterns in sensory neurons of latently infected mice (28 days post-inoculation, d.p.i.). Latent HSV-1 genomes display two major patterns, called “Single” and “Multiple”, which associate with centromeres, and with promyelocytic leukemia nuclear bodies (PML-NBs) as viral DNA-containing PML-NBs (DCP-NBs). 3D-image reconstruction of DCP-NBs shows that PML forms a shell around viral genomes and associated Daxx and ATRX, two PML partners within PML-NBs. During latency establishment (6 d.p.i.), infected mouse TGs display, at the level of the whole TG and in individual cells, a substantial increase of PML amount consistent with the interferon-mediated antiviral role of PML. “Single” and “Multiple” patterns are reminiscent of low and high-viral genome copy-containing neurons. We show that LAT expression is significantly favored within the “Multiple” pattern, which underlines a heterogeneity of LAT expression dependent on the viral genome copy number, pattern acquisition, and association with nuclear domains. Infection of PML-knockout mice demonstrates that PML/PML-NBs are involved in virus nuclear pattern acquisition, and negatively regulate the expression of the LAT. This study demonstrates that nuclear domains including PML-NBs and centromeres are functionally involved in the control of HSV-1 latency, and represent a key level of host/virus interaction.

After an initial lytic infection, many viruses establish a lifelong latent infection that hides them from the host immune system activity until reactivation. To understand the resurgence of the associated diseases, it is indispensable to acquire a better knowledge of the different mechanisms involved in the antiviral defense. During latency, viral genomes of nuclear-replicative viruses, such as herpes simplex virus type 1 (HSV-1), are stored in the nucleus of host cells in a non-integrated form. Latency establishment is associated with a drastic change in HSV-1 gene expression program that is maintained until reactivation occurs. The last two decades of research has revealed that the functional organization of the cell nucleus, so-called nuclear architecture, is a major factor of regulation of cellular genes expression. Nonetheless, the role of nuclear architecture on HSV-1 gene expression has been widely overlooked. Here we describe that the genome of HSV-1 selectively interacts with two major nuclear structures, the promyelocytic nuclear bodies (PMLNBs or ND10) and the centromeres. We provide evidence supporting that these nuclear domains directly influence the behavior of latent viral genomes and their transcriptional activity. Overall, this study demonstrates that nuclear architecture is a major parameter driving the highly complex HSV-1 latency process.

Recognition and suppression of transfected plasmids by protein ZNF511-PRAP1, a potential molecular barrier to transgene expression

Nonviral vectors present considerable advantages over viral counterparts in gene transfer. However, the poor expression efficiency of the transfected genes poses a challenge for their use in gene therapy, primarily due to the inability of these vectors to overcome various barriers, including the biological barriers. Here, we report that ZNF511-PRAP1 may be involved in the recognition and inactivation of transfected plasmids. ZNF511-PRAP1 is induced by transfection of plasmid DNA and suppresses the transcription of transfected plasmids. It binds directly to the p21 promoter in transfected plasmids but not the endogenous counterpart. Similarly, ZNF511-PRAP1 suppresses the expression of the green fluorescent protein reporter gene on transiently transfected plasmids but not an integrated red fluorescence reporter gene with the same cytomegalovirus (CMV) promoter. Therefore, ZNF511-PRAP1 is able to differentiate between exogenous/nonintegrated and endogenous/integrated DNA. The suppression by ZNF511-PRAP1 is independent of DNA methylation and can be abolished by trichostatin A (TSA) treatment and knockdown of HDAC2 and/or ZNF511-PRAP1. Furthermore, ZNF511-PRAP1 interacts directly with HDAC2. Our results revealed that transfected plasmids are recognized by ZNF511-PRAP1 and suppressed by a repressor complex comprising ZNF511-PRAP1 and HDAC2 and suggest that ZNF511-PRAP1 could play a role as a potential molecular barrier in nonviral transgene expression.

Functional nuclear topography of transcriptionally inducible extra-chromosomal transgene clusters

A new experimental approach was designed to test different predictions of current models of the nuclear architecture with respect to the topography of transcription. We constructed a plasmid, termed pIndi, which carries a reporter gene coding for a red cytoplasmic fluorescent reporter protein. Transcription of the reporter gene is regulated by the inducible promoter of the human immunodeficiency virus (HIV) and is strongly dependent on the HIV-1 Tat protein. Expressing the red fluorescent reporter protein allowed us to distinguish between cells with active and silent reporter genes. Importantly, transient transfection resulted in the clustering of plasmids, forming one or several extra-chromosomal pIndi bodies. Repetitive lac operator sequences in pIndi allowed us to visualize these bodies in living cells by the binding of LacI proteins tagged with a fluorescent protein. Using this model, we analyzed the three-dimensional nuclear topography of pIndi bodies with active or silent reporter genes. Our results are compatible with predictions of the chromosome territory-interchromatin compartment (CT-IC) model. We demonstrate that pIndi bodies localize in the IC, both in the silent and active state. Activation of transgene transcription resulted in the recruitment of RNA polymerase II and NFkappaB and a closer positioning to splicing speckles.

Quantitation of human seroresponsiveness to Merkel cell polyomavirus

Merkel cell carcinoma (MCC) is a relatively uncommon but highly lethal form of skin cancer. A majority of MCC tumors carry DNA sequences derived from a newly identified virus called Merkel cell polyomavirus (MCV or MCPyV), a candidate etiologic agent underlying the development of MCC. To further investigate the role of MCV infection in the development of MCC, we developed a reporter vector-based neutralization assay to quantitate MCV-specific serum antibody responses in human subjects. Our results showed that 21 MCC patients whose tumors harbored MCV DNA all displayed vigorous MCV-specific antibody responses. Although 88% (42/48) of adult subjects without MCC were MCV seropositive, the geometric mean titer of the control group was 59-fold lower than the MCC patient group (p<0.0001). Only 4% (2/48) of control subjects displayed neutralizing titers greater than the mean titer of the MCV-positive MCC patient population. MCC tumors were found not to express detectable amounts of MCV VP1 capsid protein, suggesting that the strong humoral responses observed in MCC patients were primed by an unusually immunogenic MCV infection, and not by viral antigen expressed by the MCC tumor itself. The occurrence of highly immunogenic MCV infection in MCC patients is unlikely to reflect a failure to control polyomavirus infections in general, as seroreactivity to BK polyomavirus was similar among MCC patients and control subjects. The results support the concept that MCV infection is a causative factor in the development of most cases of MCC. Although MCC tumorigenesis can evidently proceed in the face of effective MCV-specific antibody responses, a small pilot animal immunization study revealed that a candidate vaccine based on MCV virus-like particles (VLPs) elicits antibody responses that robustly neutralize MCV reporter vectors in vitro. This suggests that a VLP-based vaccine could be effective for preventing the initial establishment of MCV infection.

PML has a predictive role in tumor cell permissiveness to interferon-sensitive oncolytic viruses

The oncotropic phenotypes of several viruses correlate with tumor-associated deficiencies within interferon (IFN) signaling pathways. This observation formed the conceptual basis for developing oncolytic viruses deleted for viral proteins that inhibit the host IFN-dependent antiviral response, such as herpes simplex virus type-1 infected cell protein-0 (ICP0) and vesicular stomatitis virus matrix protein. Many viruses have evolved means to disrupt promyelocytic leukemia protein (PML) nuclear bodies. For example, ICP0 promotes PML degradation to inhibit the antiviral activities of this IFN-stimulated gene. As PML is downregulated in a variety of tumors, we hypothesized ICP0-null herpes simplex type-1 viruses are selectively oncolytic in tumors with impaired PML expression. We illustrate that ICP0-null herpes simplex type-1 viruses target tumor cells that either possess impaired PML signaling or cannot upregulate PML because of impaired IFN responsiveness. Disruption of PML signaling through overexpression of the dominant-negative protein PML-retinoic acid receptor alpha in PML-positive cells renders them sensitive to oncolysis by ICP0-null herpes simplex virus type-1 and vesicular stomatitis virus M protein mutant viruses, whereas PML overexpression reverses this phenomenon. Together, these data illustrate that PML mediates an antiviral mechanism that predicts the tropism of IFN-sensitive oncolytic viruses. To our knowledge, these viruses are the first examples of anti-cancer therapeutics capable of targeting deficiencies in PML expression.

New insights into the role of the subnuclear structure ND10 for viral infection

Nuclear domains 10 (ND10), alternatively termed PML nuclear bodies (PML-NBs) or PML oncogenic domains (PODs), have been discovered approximately 15 years ago as a nuclear substructure that is targeted by a variety of viruses belonging to different viral families. This review will summarize the most important structural and functional characteristics of ND10 and its major protein constituents followed by a discussion of the current view regarding the role of this subnuclear structure for various DNA and RNA viruses with an emphasis on herpesviruses. It is concluded that accumulating evidence argues for an involvement of ND10 in host antiviral defenses either via mediating an intrinsic immune response against specific viruses or via acting as a component of the cellular interferon pathway.

New insights into the role of PML in tumour suppression

The PML gene is involved in the t(15;17) translocation of acute promyelocytic leukaemia (APL), which generates the oncogenic fusion protein PML (promyelocytic leukaemia protein)-retinoic acid receptor alpha. The PML protein localises to a subnuclear structure called the PML nuclear domain (PML-ND), of which PML is the essential structural component. In APL, PML-NDs are disrupted, thus implicating these structures in the pathogenesis of this leukaemia. Unexpectedly, recent studies indicate that PML and the PML-ND play a tumour suppressive role in several different types of human neoplasms in addition to APL. Because of PML's extreme versatility and involvement in multiple cellular pathways, understanding the mechanisms underlying its function, and therefore role in tumour suppression, has been a challenging task. In this review, we attempt to critically appraise the more recent advances in this field and propose new avenues of investigation.

Imaging single virus particles on the surface of cell membranes by high-resolution scanning surface confocal microscopy

We have developed a high-resolution scanning surface confocal microscopy technique capable of imaging single virus-like particles (VLPs) on the surfaces of cells topographically and by fluorescence. The technique combines recently published single-molecule-resolution ion-conductance microscopy that acquires topographical data with confocal microscopy providing simultaneous fluorescent imaging. In our experiments we have demonstrated that the cell membrane exhibits numerous submicrometer-sized surface structures that could be topographically confused with virus particles. However, simultaneous acquisition of confocal images allows the positions of fluorescently tagged particles to be identified. Using this technique, we have, for the first time, visualized single polyoma VLPs adsorbed onto the cell membrane. Observed VLPs had a mean width of 108 +/- 16 nm. The particles were randomly distributed across the cell membrane, and no specific interactions were seen with cell membrane structures such as microvilli. These experiments demonstrate the utility of this new microscope for imaging the interactions of nanoparticles with the cell surface to provide novel insights into the earliest interactions of viruses and other nanoparticles such as gene therapy vectors with the cell.

PML and PML nuclear bodies: implications in antiviral defence

The establishment of an intracellular antiviral state is the defining activity of interferons (IFNs) as well as the property that permitted their discovery. Several pathways have been implicated in resistance to viral infection in IFN-treated cells, one of which implicates the ProMyelocytic Leukaemia (PML) protein and PML nuclear bodies (NBs, also known as ND10). PML NBs are dynamic intranuclear structures that require PML for their formation and which harbour numerous other transiently or permanently localised proteins. PML is expressed as a family of isoforms (PML I-VII) as a result of alternative splicing, most of which are found in the nucleus. IFN treatment directly induces transcription of the genes encoding both PML and Sp100, (another major component of PML NBs), resulting in higher levels of expression of these proteins and increases in both the size and number of PML NBs. These and other observations have encouraged the hypothesis that PML, PML NBs and a number of other constituents of these structures are involved in host antiviral defences. For example, exogenous expression of PML III or PML VI can impede infection by a number of RNA and DNA viruses, and certain viral proteins accumulate in PML NBs then cause their disruption by a variety of mechanisms. Although there are many other functions of PML NBs in a wide range of cellular pathways, there is accumulating evidence that they represent preferential targets for viral infections and that PML plays a role in the mechanism of the antiviral action of IFN. This article reviews the potential antiviral activities of PML NB constituent proteins, how RNA and DNA viruses overcome these defences, and the connections between these events and IFN pathways.

Histone modifications are associated with the persistence or silencing of vector-mediated transgene expression in vivo

One of the major obstacles to success in non-viral gene therapy is transcriptional silencing of the DNA vector. The mechanisms underlying gene silencing/repression in mammalian cells are complex and remain unclear. Because changes in chromatin structure and, in particular, histone modifications are involved in transcriptional regulation of endogenous genes, we hypothesized that changes in the pattern of histone modifications were related to the observed transcriptional silencing of exogenous DNA vectors. We used antibodies against specific modified histones to perform chromatin immunoprecipitation (ChIP) analyses on liver lysates from mice transfected with two types of plasmids: (i) DNA minicircles (MCs) devoid of bacterial plasmid backbone DNA, which showed marked persistence of transgene expression, and (ii) their parental plasmids, which were silenced over time. Silencing of the transgene from the parental vectors was accompanied by an increase in heterochromatin-associated histone modifications and a decrease in modifications typically associated with euchromatin. Conversely, the pattern of histone modifications on the MC DNA was consistent with euchromatin. Our data indicates that (i) episomal vectors undergo chromatinization in vivo, and (ii) both persistence and silencing of transgene expression are associated with specific histone modifications.

Human papillomavirus-like particles mediate functional delivery of plasmid DNA to antigen presenting cells in vivo

Because recombinant empty viral capsids are potentially attractive vectors for gene therapy, here we examined the ability of human papillomavirus (HPV) virus-like particles (VLPs) to mediate delivery and expression of DNA plasmids in vitro and in vivo. VLP-mediated delivery and expression of a GFP reporter construct in vitro was found to be highly dependent upon the presence of full-length L2 protein within the VLPs. Similarly, expression of GFP and luciferase reporter plasmids in vivo was strongly enhanced by co-administration of L1/L2 VLPs. Interestingly, in these experiments we routinely observed GFP expression in migrating antigen presenting cells (APC) recovered from mice inoculated with GFP plasmid in combination with VLPs, but not in APC recovered from mice inoculated with the plasmid alone. Additional evidence to support this concept was generated in experiments in which co-administration of VLPs with a plasmid designed to express HPV16 E6 oncoprotein was associated with significant enhancement of plasmid-encoded E6-specific cellular immune responses. These findings have implications for the design of vaccines for combined prophylaxis and therapy of HPV-associated diseases, and for other vaccines that rely on the administration of DNA-based immunogens, adjuvants, and/or other factors.

Induction of a subnuclear structure by the simultaneous expression of baculovirus proteins, IE1, LEF3, and P143 in the presence of hr

Baculoviruses elicit the formation of a nuclear domain, called the virogenic stroma, in which viral DNA replication and nucleocapsid assembly occur. We had previously reported that nuclear focus formation of a transcriptional activator, IE1, is triggered by its binding to a viral DNA element, hr, and predicted that this hr-induced IE1 focus is an initial scaffold for the virogenic stroma. However, LEF3, a component of the virogenic stroma, did not localize to the IE1 foci. In exploring a mediator for its localization, we found that a baculovirus DNA helicase (P143), in combination with IE1 and hr, induced a subnuclear structure to which LEF3 localized and also that another component of the virogenic stroma, DBP, is able to localize to this structure. These results reveal that only four viral molecules are necessary to establish a nuclear domain which possesses a recruiting ability for a component of the virogenic stroma.