Cellular proteins PML and Daxx mediate an innate antiviral defense antagonized by the adenovirus E4 ORF3 protein. J Virol. 2008;82:7325-7335. [PMID: 18480450 DOI: 10.1128/jvi.00723-08]

Viruses and Cajal Bodies: A Critical Cellular Target in Virus Infection?

Nuclear bodies (NBs) are dynamic structures present in eukaryotic cell nuclei. They are not bounded by membranes and are often considered biomolecular condensates, defined structurally and functionally by the localisation of core components. Nuclear architecture can be reorganised during normal cellular processes such as the cell cycle as well as in response to cellular stress. Many plant and animal viruses target their proteins to NBs, in some cases triggering their structural disruption and redistribution. Although not all such interactions have been well characterised, subversion of NBs and their functions may form a key part of the life cycle of eukaryotic viruses that require the nucleus for their replication. This review will focus on Cajal bodies (CBs) and the viruses that target them. Since CBs are dynamic structures, other NBs (principally nucleoli and promyelocytic leukaemia, PML and bodies), whose components interact with CBs, will also be considered. As well as providing important insights into key virus-host cell interactions, studies on Cajal and associated NBs may identify novel cellular targets for development of antiviral compounds.

The interactions between PML nuclear bodies and small and medium size DNA viruses

Promyelocytic leukemia nuclear bodies (PM NBs), often referred to as membraneless organelles, are dynamic macromolecular protein complexes composed of a PML protein core and other transient or permanent components. PML NBs have been shown to play a role in a wide variety of cellular processes. This review describes in detail the diverse and complex interactions between small and medium size DNA viruses and PML NBs that have been described to date. The PML NB components that interact with small and medium size DNA viruses include PML protein isoforms, ATRX/Daxx, Sp100, Sp110, HP1, and p53, among others. Interaction between viruses and components of these NBs can result in different outcomes, such as influencing viral genome expression and/or replication or impacting IFN-mediated or apoptotic cell responses to viral infection. We discuss how PML NB components abrogate the ability of adenoviruses or Hepatitis B virus to transcribe and/or replicate their genomes and how papillomaviruses use PML NBs and their components to promote their propagation. Interactions between polyomaviruses and PML NBs that are poorly understood but nevertheless suggest that the NBs can serve as scaffolds for viral replication or assembly are also presented. Furthermore, complex interactions between the HBx protein of hepadnaviruses and several PML NBs-associated proteins are also described. Finally, current but scarce information regarding the interactions of VP3/apoptin of the avian anellovirus with PML NBs is provided. Despite the considerable number of studies that have investigated the functions of the PML NBs in the context of viral infection, gaps in our understanding of the fine interactions between viruses and the very dynamic PML NBs remain. The complexity of the bodies is undoubtedly a great challenge that needs to be further addressed.

Human Adenovirus Gene Expression and Replication Is Regulated through Dynamic Changes in Nucleoprotein Structure throughout Infection

Human adenovirus (HAdV) is extremely common and can rapidly spread in confined populations such as daycare centers, hospitals, and retirement homes. Although HAdV usually causes only minor illness in otherwise healthy patients, HAdV can cause significant morbidity and mortality in certain populations, such as the very young, very old, or immunocompromised individuals. During infection, the viral DNA undergoes dramatic changes in nucleoprotein structure that promote the rapid expression of viral genes, replication of the DNA, and generation of thousands of new infectious virions-each process requiring a distinct complement of virus and host-encoded proteins. In this review, we summarize our current understanding of the nucleoprotein structure of HAdV DNA during the various phases of infection, the cellular proteins implicated in mediating these changes, and the role of epigenetics in HAdV gene expression and replication.

PML Body Component Sp100A Is a Cytosolic Responder to IFN and Activator of Antiviral ISGs

Promyelocytic leukemia protein (PML) bodies are implicated in one of the key pathways in the establishment of antiviral status in response to interferon (IFN), yet the molecular mechanisms bridging the cross talk remain elusive. Herein, we report that a major constitutive component of the PML body, Sp100A, is ubiquitously located in the cytosol of various cell types and is an immediate responder to multiple extracellular stimuli, including virus infection, IFN, epidermal growth factor (EGF), glial cell-derived nerve factor (GDNF), etc., signaling through the phosphatidylinositol 3-kinase (PI3K) pathway. IFN-β induces phosphorylation of Sp100A on Ser188, which fortifies the binding of Sp100A to pyruvate kinase 2 (PKM2) and facilitates its nuclear importation through the extracellular signal-regulated kinase 1/2 (ERK1/2)-PKM2-PIN1-importin axes. Blocking PI3K pathway signaling or interference with the ERK1/2-PKM2-PIN1-importin axes independently hampers nuclear translocation of Sp100A in response to IFN, reflecting a dual-regulation mechanism governing this event. In the nucleus, Sp100A is enriched in the promoter regions of essential antiviral interferon-stimulated genes (ISGs), such as those coding for IFI16, OAS2, and RIG-I, and activates their transcription. Importantly, nuclear importation of Sp100A, but not accumulation of a mutant Sp100A that failed to respond to IFN, during infection potently enhanced transcription of these antiviral ISGs and restricted virus propagation. These findings depict a novel IFN response mechanism by PML bodies in the cytosol and shed light on the complex sensing-regulatory network of PML bodies. IMPORTANCE PML bodies sit at the center stage of various important biological processes; however, the signal transduction networks of these macromolecular protein complexes remain enigmatic. The present study illustrates, in detail and for the first time, the course of signal receiving, processing, and implementation by PML bodies in response to IFN and virus infection. It shows that PML body constitutive component Sp100A was phosphorylated on Ser188 by IFN signaling through the PI3K pathway in the cytosol, cotranslocated into the nucleus with PKM2, enriched on the promoter regions of essential antiviral ISGs such as those coding for IFI16, RIG-I, OAS2, etc., and mediating their transcriptional activation.

Adenovirus infection controls processing bodies to stabilize AU-rich element-containing mRNA

In the adenovirus-infected cells, virus mRNAs are selectively exported to the cytoplasm by virus early gene products to facilitate virus replication. We previously showed AU-rich elements (AREs) containing mRNAs are exported to the cytoplasm and stabilized in infected cells. Here, we analyzed ribonucleoprotein (RNP) granules in the cytoplasm that are involved in mRNA degradation to elucidate the mechanism of ARE-mRNA stabilization in adenovirus infected cells. Our findings showed that processing bodies (PBs) aggregate, then almost all PBs are translocated to aggresomes formed by adenoviral gene products during the late phase of infection. Furthermore, E4orf3 was required for the PBs translocation, and the same domains of E4orf3-mutants required to change the form of promyelocytic leukemia bodies were also needed for PBs translocation. Luciferase activity showed that these domains were critical for miRNA- and ARE-mediated mRNA decay. These findings suggest that adenovirus changes the behavior of PBs to prevent ARE-mRNA downregulation.

Self-guarding of MORC3 enables virulence factor-triggered immunity

Pathogens use virulence factors to inhibit the immune system1. The guard hypothesis2,3 postulates that hosts monitor (or 'guard') critical innate immune pathways such that their disruption by virulence factors provokes a secondary immune response1. Here we describe a 'self-guarded' immune pathway in human monocytes, in which guarding and guarded functions are combined in one protein. We find that this pathway is triggered by ICP0, a key virulence factor of herpes simplex virus type 1, resulting in robust induction of anti-viral type I interferon (IFN). Notably, induction of IFN by ICP0 is independent of canonical immune pathways and the IRF3 and IRF7 transcription factors. A CRISPR screen identified the ICP0 target MORC34 as an essential negative regulator of IFN. Loss of MORC3 recapitulates the IRF3- and IRF7-independent IFN response induced by ICP0. Mechanistically, ICP0 degrades MORC3, which leads to de-repression of a MORC3-regulated DNA element (MRE) adjacent to the IFNB1 locus. The MRE is required in cis for IFNB1 induction by the MORC3 pathway, but is not required for canonical IFN-inducing pathways. As well as repressing the MRE to regulate IFNB1, MORC3 is also a direct restriction factor of HSV-15. Our results thus suggest a model in which the primary anti-viral function of MORC3 is self-guarded by its secondary IFN-repressing function-thus, a virus that degrades MORC3 to avoid its primary anti-viral function will unleash the secondary anti-viral IFN response.

Human IFIT3 Protein Induces Interferon Signaling and Inhibits Adenovirus Immediate Early Gene Expression

Interferons (IFNs) are one of the hallmarks of host antiviral immunity. IFNs exert their antiviral activities through the induction of IFN-stimulated genes (ISGs) and antiviral proteins; however, the mechanism by which ISGs inhibit adenovirus (Ad) replication is not clearly understood. IFNs repress Ad immediate early gene expression and, consequently, all subsequent aspects of the viral life cycle. In this study, we found that IFN-induced protein with tetratricopeptide repeats 3, IFIT3 (ISG60), restricts Ad replication. IFIT3 repressed Ad E1A immediate early gene expression but did not alter Ad genome entry into the nucleus. Expression of IFIT3 led to phosphorylation of TBK1, IRF3, and STAT1; increased expression of IFNβ and ISGs; and required IFIT1 and IFIT2 partner proteins. During RNA virus infections, it is known that IFIT3 stimulates IFN production through mitochondrial antiviral signaling (MAVS)-mediated activation of TBK1 which synergizes activation of IRF3 and NF-κB. MAVS or TBK1 depletion in cells expressing IFIT3 blocked IFN signaling and reversed the Ad replication restriction. In addition, STING depletion phenocopied the effect suggesting that IFIT3 activates the STING pathway with cross talk to the MAVS pathway. This occurs independently of viral pathogen-associated molecular patterns (PAMPs). These results demonstrate that the expression of a single ISG, IFIT3, activates IFN signaling and establishes a cellular antiviral state independent of viral PAMPs. IMPORTANCE IFITs belong to a family of IFN-induced proteins that have broad antiviral functions, primarily studied with RNA viruses leaving a gap of knowledge on the effects of these proteins on DNA viruses. In this study we show that IFIT3, with its partner proteins IFIT1 and IFIT2, specifically restricts replication of human Ad, a DNA virus, by stimulating IFNβ production via the STING and MAVS pathways. This effect enhanced the IFN response and is independent of viral PAMPs. These results reveal a novel mechanism of activation of IFN signaling to enhance cellular antiviral responses.

A Tale of Usurpation and Subversion: SUMO-Dependent Integrity of Promyelocytic Leukemia Nuclear Bodies at the Crossroad of Infection and Immunity

Promyelocytic leukemia nuclear bodies (PML NBs) are multi-protein assemblies representing distinct sub-nuclear structures. As phase-separated molecular condensates, PML NBs exhibit liquid droplet-like consistency. A key organizer of the assembly and dynamics of PML NBs is the ubiquitin-like SUMO modification system. SUMO is covalently attached to PML and other core components of PML NBs thereby exhibiting a glue-like function by providing multivalent interactions with proteins containing SUMO interacting motifs (SIMs). PML NBs serve as the catalytic center for nuclear SUMOylation and SUMO-SIM interactions are essential for protein assembly within these structures. Importantly, however, formation of SUMO chains on PML and other PML NB-associated proteins triggers ubiquitylation and proteasomal degradation which coincide with disruption of these nuclear condensates. To date, a plethora of nuclear activities such as transcriptional and post-transcriptional regulation of gene expression, apoptosis, senescence, cell cycle control, DNA damage response, and DNA replication have been associated with PML NBs. Not surprisingly, therefore, SUMO-dependent PML NB integrity has been implicated in regulating many physiological processes including tumor suppression, metabolism, drug-resistance, development, cellular stemness, and anti-pathogen immune response. The interplay between PML NBs and viral infection is multifaceted. As a part of the cellular antiviral defense strategy, PML NB components are crucial restriction factors for many viruses and a mutual positive correlation has been found to exist between PML NBs and the interferon response. Viruses, in turn, have developed counterstrategies for disarming PML NB associated immune defense measures. On the other end of the spectrum, certain viruses are known to usurp specific PML NB components for successful replication and disruption of these sub-nuclear foci has recently been linked to the stimulation rather than curtailment of antiviral gene repertoire. Importantly, the ability of invading virions to manipulate the host SUMO modification machinery is essential for this interplay between PML NB integrity and viruses. Moreover, compelling evidence is emerging in favor of bacterial pathogens to negotiate with the SUMO system thereby modulating PML NB-directed intrinsic and innate immunity. In the current context, we will present an updated account of the dynamic intricacies between cellular PML NBs as the nuclear SUMO modification hotspots and immune regulatory mechanisms in response to viral and bacterial pathogens.

Enhanced oncolytic adenoviral production by downregulation of death-domain associated protein and overexpression of precursor terminal protein

Adequate viral replication in tumor cells is the key to improving the anti-cancer effects of oncolytic adenovirus therapy. In this study, we introduced short hairpin RNAs against death-domain associated protein (Daxx), a repressor of adenoviral replication, and precursor terminal protein (pTP), an initiator of adenoviral genome replication, into adenoviral constructs to determine their contributions to viral replication. Both Daxx downregulation and pTP overexpression increased viral production in variety of human cancer cell lines, and the enhanced production of virus progeny resulted in more cell lysis in vitro, and tumor regression in vivo. We confirmed that increased virus production by Daxx silencing, or pTP overexpression, occurred using different mechanisms by analyzing levels of adenoviral protein expression and virus production. Specifically, Daxx downregulation promoted both virus replication and oncolysis in a consecutive manner by optimizing IVa2-based packaging efficiency, while pTP overexpression by increasing both infectious and total virus particles but their contribution to increased viral production may have been damaged to some extent by their another contribution to apoptosis and autophagy. Therefore, introducing both Daxx shRNA and pTP in virotherapy may be a suitable strategy to increase apoptotic tumor-cell death and to overcome poor viral replication, leading to meaningful reductions in tumor growth in vivo.

Double-edged role of PML nuclear bodies during human adenovirus infection

PML nuclear bodies are matrix-bound nuclear structures with a variety of functions in human cells. These nuclear domains are interferon regulated and play an essential role during virus infections involving accumulation of SUMO-dependent host and viral factors. PML-NBs are targeted and subsequently manipulated by adenoviral regulatory proteins, illustrating their crucial role during productive infection and virus-mediated oncogenic transformation. PML-NBs have a longstanding antiviral reputation; however, the genomes of Human Adenoviruses and initial sites of viral transcription/replication are found juxtaposed to these domains, resulting in a double-edged capacity of these nuclear multiprotein/multifunctional complexes. This enigma provides evidence that Human Adenoviruses selectively counteract antiviral responses, and simultaneously benefit from or even depend on proviral PML-NB associated components by active recruitment to PML track-like structures, that are induced during infection. Thereby, a positive microenvironment for adenoviral transcription and replication is created at these nuclear subdomains. Based on the available data, this review aims to provide a detailed overview of the current knowledge of Human Adenovirus crosstalk with nuclear PML body compartments as sites of SUMOylation processes in the host cells, evaluating the currently known principles and molecular mechanisms.

Antiviral Effects of Curcumin on Adenovirus Replication

Human adenovirus (HAdV) is a common pathogen that can cause severe morbidity and mortality in certain populations, including pediatric, geriatric, and immunocompromised patients. Unfortunately, there are no approved therapeutics to combat HAdV infections. Curcumin, the primary curcuminoid compound found in turmeric spice, has shown broad activity as an antimicrobial agent, limiting the replication of many different bacteria and viruses. In this study, we evaluated curcumin as an anti-HAdV agent. Treatment of cells in culture with curcumin reduced HAdV replication, gene expression, and virus yield, at concentrations of curcumin that had little effect on cell viability. Thus, curcumin represents a promising class of compounds for further study as potential therapeutics to combat HAdV infection.

Modification of Nuclear Compartments and the 3D Genome in the Course of a Viral Infection

The review addresses the question of how the structural and functional compartmentalization of the cell nucleus and the 3D organization of the cellular genome are modified during the infection of cells with various viruses. Particular attention is paid to the role of the introduced changes in the implementation of the viral strategy to evade the antiviral defense systems and provide conditions for viral replication. The discussion focuses on viruses replicating in the cell nucleus. Cytoplasmic viruses are mentioned in cases when a significant reorganization of the nuclear compartments or the 3D genome structure occurs during an infection with these viruses.

En Guard! The Interactions between Adenoviruses and the DNA Damage Response

Virus–host cell interactions include several skirmishes between the virus and its host, and the DNA damage response (DDR) network is one of their important battlegrounds. Although some aspects of the DDR are exploited by adenovirus (Ad) to improve virus replication, especially at the early phase of infection, a large body of evidence demonstrates that Ad devotes many of its proteins, including E1B-55K, E4orf3, E4orf4, E4orf6, and core protein VII, and utilizes varied mechanisms to inhibit the DDR. These findings indicate that the DDR would strongly restrict Ad replication if allowed to function efficiently. Various Ad serotypes inactivate DNA damage sensors, including the Mre11-Rad50-Nbs1 (MRN) complex, DNA-dependent protein kinase (DNA-PK), and Poly (ADP-ribose) polymerase 1 (PARP-1). As a result, these viruses inhibit signaling via DDR transducers, such as the ataxia-telangiectasia mutated (ATM) and ATM- and Rad3-related (ATR) kinases, to downstream effectors. The different Ad serotypes utilize both shared and distinct mechanisms to inhibit various branches of the DDR. The aim of this review is to understand the interactions between Ad proteins and the DDR and to appreciate how these interactions contribute to viral replication.

ATO (Arsenic Trioxide) Effects on Promyelocytic Leukemia Nuclear Bodies Reveals Antiviral Intervention Capacity

Human adenoviruses (HAdV) are associated with clinical symptoms such as gastroenteritis, keratoconjunctivitis, pneumonia, hepatitis, and encephalitis. In the absence of protective immunity, as in allogeneic bone marrow transplant patients, HAdV infections can become lethal. Alarmingly, various outbreaks of highly pathogenic, pneumotropic HAdV types have been recently reported, causing severe and lethal respiratory diseases. Effective drugs for treatment of HAdV infections are still lacking. The repurposing of drugs approved for other indications is a valuable alternative for the development of new antiviral therapies and is less risky and costly than de novo development. Arsenic trioxide (ATO) is approved for treatment of acute promyelocytic leukemia. Here, it is shown that ATO is a potent inhibitor of HAdV. ATO treatment blocks virus expression and replication by reducing the number and integrity of promyelocytic leukemia (PML) nuclear bodies, important subnuclear structures for HAdV replication. Modification of HAdV proteins with small ubiquitin-like modifiers (SUMO) is also key to HAdV replication. ATO reduces levels of viral SUMO-E2A protein, while increasing SUMO-PML, suggesting that ATO interferes with SUMOylation of proteins crucial for HAdV replication. It is concluded that ATO targets cellular processes key to HAdV replication and is relevant for the development of antiviral intervention strategies.

Viral and cellular interactions during adenovirus DNA replication

Adenoviruses represent ubiquitous and clinically significant human pathogens, gene-delivery vectors, and oncolytic agents. The study of adenovirus-infected cells has long been used as an excellent model to investigate fundamental aspects of both DNA virus infection and cellular biology. While many key details supporting a well-established model of adenovirus replication have been elucidated over a period spanning several decades, more recent findings suggest that we have only started to appreciate the complex interplay between viral genome replication and cellular processes. Here, we present a concise overview of adenovirus DNA replication, including the biochemical process of replication, the spatial organization of replication within the host cell nucleus, and insights into the complex plethora of virus-host interactions that influence viral genome replication. Finally, we identify emerging areas of research relating to the replication of adenovirus genomes.

Imaging the adenovirus infection cycle

Incoming adenoviruses seize control of cytosolic transport mechanisms to relocate their genome from the cell periphery to specialized sites in the nucleoplasm. The nucleus is the site for viral gene expression, genome replication, and the production of progeny for the next round of infection. By taking control of the cell, adenoviruses also suppress cell-autonomous immunity responses. To succeed in their production cycle, adenoviruses rely on well-coordinated steps, facilitated by interactions between viral proteins and cellular factors. Interactions between virus and host can impose remarkable morphological changes in the infected cell. Imaging adenoviruses has tremendously influenced how we delineate individual steps in the viral life cycle, because it allowed the development of specific optical markers to label these morphological changes in space and time. As technology advances, innovative imaging techniques and novel tools for specimen labeling keep uncovering previously unseen facets of adenovirus biology emphasizing why imaging adenoviruses is as attractive today as it was in the past. This review will summarize past achievements and present developments in adenovirus imaging centered on fluorescence microscopy approaches.

DAXX in cancer: phenomena, processes, mechanisms and regulation

DAXX displays complex biological functions. Remarkably, DAXX overexpression is a common feature in diverse cancers, which correlates with tumorigenesis, disease progression and treatment resistance. Structurally, DAXX is modular with an N-terminal helical bundle, a docking site for many DAXX interactors (e.g. p53 and ATRX). DAXX's central region folds with the H3.3/H4 dimer, providing a H3.3-specific chaperoning function. DAXX has two functionally critical SUMO-interacting motifs. These modules are connected by disordered regions. DAXX's structural features provide a framework for deciphering how DAXX mechanistically imparts its functions and how its activity is regulated. DAXX modulates transcription through binding to transcription factors, epigenetic modifiers, and chromatin remodelers. DAXX's localization in the PML nuclear bodies also plays roles in transcriptional regulation. DAXX-regulated genes are likely important effectors of its biological functions. Deposition of H3.3 and its interactions with epigenetic modifiers are likely key events for DAXX to regulate transcription, DNA repair, and viral infection. Interactions between DAXX and its partners directly impact apoptosis and cell signaling. DAXX's activity is regulated by posttranslational modifications and ubiquitin-dependent degradation. Notably, the tumor suppressor SPOP promotes DAXX degradation in phase-separated droplets. We summarize here our current understanding of DAXX's complex functions with a focus on how it promotes oncogenesis.

Adenoviral strategies to overcome innate cellular responses to infection

Viruses alter host cell processes to optimize their replication cycle. Human adenoviruses (Ad) encode proteins that promote viral macromolecular synthesis and counteract innate and adaptive responses to infection. The focus of this review is on how Ad evades innate cellular responses to infection, including an interferon (IFN) response and a DNA damage response (DDR). Ad blocks the IFN response by inhibiting cytoplasmic signaling pathways and the activation of IFN-stimulated genes (ISGs), as well as the functions of ISG products, such as PML. Ad also inhibits DDR sensors, for instance, the Mre11-Rad50-Nbs1 complex, and DDR effectors like DNA ligase IV. These innate cellular responses impact many different viruses, and studies on Ad have provided broad insight into these areas.

High levels of Daxx due to low cellular levels of HSP25 in murine cancer cells result in inefficient adenovirus replication

When the adenoviral protein E1B55K binds death domain-associated protein (Daxx), the proteasome-dependent degradation of Daxx is initiated, and adenoviral replication is effectively maintained. Here, we show that the cellular levels of Daxx differ between human and mouse cancer cell lines. Specifically, we observed higher cellular Daxx levels and the diminished replication of oncolytic adenovirus in mouse cancer cell lines, suggesting that cellular Daxx levels limit the replication of oncolytic adenoviruses that lack E1B55K in murine cells. Indeed, the replication of oncolytic adenoviruses that lack E1B55K was significantly increased following infection with oncolytic adenovirus expressing Daxx-specific shRNA. Cellular Daxx levels were decreased in mouse cells expressing heat shock protein 25 (HSP25; homolog of human HSP27) following heat shock or stable transfection with HSP25-bearing plasmids. Furthermore, Daxx expression in murine cell lines was primarily regulated at the transcriptional level via HSP25-mediated inhibition of the nuclear translocation of the signal transducer and activator of transcription 3 (stat3) protein, which typically upregulates Daxx transcription. Conversely, human HSP27 enhanced stat3 activity to increase Daxx transcription. Interestingly, human Daxx, but not mouse Daxx, was degraded as normal by ubiquitin-dependent lysosomal degradation; however, HSP27 downregulation induced the ubiquitin-independent proteasomal degradation of Daxx.

Cancer therapies that use a virus to kill tumor cells may get a boost by suppressing a common, ubiquitously expressed protein called Daxx. The relatively new field of virotherapy uses engineered adenoviruses, which usually cause fevers, coughs, or sore throats, to attack tumor cells, enabling treatment of advanced stage cancers, or those that have spread through the body. However, the immune system can attack the therapeutic virus, preventing it from replicating and reducing its effectiveness. Hye Jin Choi and Jae Song at Yonsei University, Seoul, South Korea, and coworkers have been investigating ways to maximize replication of the therapeutic virus. They found that suppressing Daxx improved viral replication; further testing showed that suppressing Daxx acted via different mechanisms in mouse and human cancer cells. These results will help develop more effective virus-based cancer therapies.

Mechanism of Adenovirus E4-ORF3-Mediated SUMO Modifications

Regulation of a variety of different cellular processes, including posttranslational modifications, is critical for the ability of many viruses to replicate efficiently within host cells. The adenovirus (Ad) E4-ORF3 protein assembles into polymers and forms a unique nuclear scaffold that leads to the relocalization and sequestration of cellular proteins, including small ubiquitin-like modifiers (SUMOs). Previously, we showed that E4-ORF3 functions as a SUMO E3 ligase of transcriptional intermediary factor-1 gamma (TIF-1γ) and promotes poly-SUMO chain formation. Here, we present cellular and biochemical data to further understand E4-ORF3 SUMO ligase activity. E4-ORF3 proteins from five different Ad species were found to possess SUMO E3 ligase activities in vitro In infected cells, SUMO modifications of target proteins occurred only when the proteins were recruited into E4-ORF3 polymeric structures. By analyzing SUMO-deficient TIF-1γ, we demonstrated that SUMO conjugations are not required for E4-ORF3-mediated relocalization of target proteins in infected cells, implying that sequestration is followed by SUMO modification. In vitro SUMO conjugation assays revealed the Ad E1B-55K oncoprotein as a new viral target of E4-ORF3-mediated SUMOylation. We also verified a direct function of E4-ORF3 as a SUMO ligase for multiple cellular proteins, including transcription factor II-I (TFII-I), Nbs1, and Mre11. Moreover, we discovered that E4-ORF3 associates with SUMO-bound UBC9, and E4-ORF3 polymerization is crucial for this ternary interaction. Together, our findings characterize E4-ORF3 as a novel polymer-type SUMO E3 ligase and provide mechanistic insights into the role of E4-ORF3 in SUMO conjugation.IMPORTANCE Viruses interplay with the host SUMOylation system to manipulate diverse cellular responses. The Ad E4-ORF3 protein forms a dynamic nuclear network to interfere with and exploit different host processes, including the DNA damage and interferon responses. We previously reported that E4-ORF3 is a SUMO E3 ligase. Here, we demonstrate that this activity is a conserved function of evolutionarily diverse human Ad E4-ORF3 proteins and that E4-ORF3 functions directly to promote SUMO conjugations to multiple cellular proteins. Recruitment of cellular substrates into E4-ORF3 nuclear inclusions is required for SUMO conjugation to occur in vivo We probed the mechanism by which E4-ORF3 functions as a SUMO E3 ligase. Only multimeric, but not dimeric, E4-ORF3 binds to the SUMO E2 conjugation enzyme UBC9 in vitro only in a trimeric complex with SUMO. These results reveal a novel mechanism by which a conserved viral protein usurps the cellular SUMO conjugation machinery.

ISG20 promotes local tumor immunity and contributes to poor survival in human glioma

Recent evidence has confirmed that a mutation of the isocitrate dehydrogenase (IDH) gene occurs early in gliomagenesis and contributes to suppressed immunity. The present study aimed to determine the candidate genes associated with IDH mutation status that could serve as biomarkers of immune suppression for improved prognosis prediction. Clinical information and RNA-seq gene expression data were collected for 932 glioma samples from the CGGA and TCGA databases, and differentially expressed genes in both lower-grade glioma (LGG) and glioblastoma (GBM) samples were identified according to IDH mutation status. Only one gene, interferon-stimulated exonuclease gene 20 (ISG20), with reduced expression in IDH mutant tumors, demonstrated significant prognostic value. ISG20 expression level significantly increased with increasing tumor grade, and its high expression was associated with a poor clinical outcome. Moreover, increased ISG20 expression was associated with increased infiltration of monocyte-derived macrophages and neutrophils, and suppressed adaptive immune response. ISG20 expression was also positively correlated with PD-1, PD-L1, and CTLA4 expression, along with the levels of several chemokines. We conclude that ISG20 is a useful biomarker to identify IDH-mediated immune processes in glioma and may serve as a potential therapeutic target.

Importance of Promyelocytic Leukema Protein (PML) for Kaposi's Sarcoma-Associated Herpesvirus Lytic Replication

Many DNA virus replication-related proteins are associated with promyelocytic leukemia protein (PML), a component of nuclear domain 10 (ND10), which has been investigated for its potential involvement in viral replication. In the case of Kaposi’s sarcoma-associated herpesvirus (KSHV) lytic gene products, K8 (K-bZIP), ORF59, and ORF75 have been shown to colocalize with PML, but its importance in KSHV lytic replication is still unclear. In this study, we analyzed the functional influence of PML on KSHV latency and lytic replication in KSHV-infected primary effusion lymphoma (PEL) cell lines. Stable PML-knockout (BC3-PML^KO) and PML-overexpressing BC3 cells (BC3^PML) were successfully generated and the latency and reactivation status were analyzed. The results demonstrated that neither KSHV latency nor the episome copy number was affected in BC3-PML^KO cells. In the reactivation phase, the expression dynamics of KSHV immediate-early or early lytic proteins such as RTA, K9 (vIRF1), K5, K3, ORF59, and K8 (K-bZIP) were comparable between wild-type, control BC3, and BC3-PML^KO cells. Interestingly, KSHV lytic replication, virion production, and expression of late genes were downregulated in BC3-PML^KO cells and upregulated in BC3^PML cells, compared to those in control or wild-type BC3 cells. Moreover, exogenous PML increased the size of the PML dots and recruited additional K8 (K-bZIP) to PML-NBs as dots. Therefore, PML would function as a positive regulator for KSHV lytic DNA replication by recruiting KSHV replication factors such as 8 (K-bZIP) or ORF59 to the PML-NBs.

E1B-55K-Mediated Regulation of RNF4 SUMO-Targeted Ubiquitin Ligase Promotes Human Adenovirus Gene Expression

Human adenovirus (HAdV) E1B-55K is a multifunctional regulator of productive viral replication and oncogenic transformation in nonpermissive mammalian cells. These functions depend on E1B-55K's posttranslational modification with the SUMO protein and its binding to HAdV E4orf6. Both early viral proteins recruit specific host factors to form an E3 ubiquitin ligase complex that targets antiviral host substrates for proteasomal degradation. Recently, we reported that the PML-NB-associated factor Daxx represses efficient HAdV productive infection and is proteasomally degraded via a SUMO-E1B-55K-dependent, E4orf6-independent pathway, the details of which remained to be established. RNF4, a cellular SUMO-targeted ubiquitin ligase (STUbL), induces ubiquitinylation of specific SUMOylated proteins and plays an essential role during DNA repair. Here, we show that E1B-55K recruits RNF4 to the insoluble nuclear matrix fraction of the infected cell to support RNF4/Daxx association, promoting Daxx PTM and thus inhibiting this antiviral factor. Removing RNF4 from infected cells using RNA interference resulted in blocking the proper establishment of viral replication centers and significantly diminished viral gene expression. These results provide a model for how HAdV antagonize the antiviral host responses by exploiting the functional capacity of cellular STUbLs. Thus, RNF4 and its STUbL function represent a positive factor during lytic infection and a novel candidate for future therapeutic antiviral intervention strategies.IMPORTANCE Daxx is a PML-NB-associated transcription factor that was recently shown to repress efficient HAdV productive infection. To counteract this antiviral measurement during infection, Daxx is degraded via a novel pathway including viral E1B-55K and host proteasomes. This virus-mediated degradation is independent of the classical HAdV E3 ubiquitin ligase complex, which is essential during viral infection to target other host antiviral substrates. To maintain a productive viral life cycle, HAdV E1B-55K early viral protein inhibits the chromatin-remodeling factor Daxx in a SUMO-dependent manner. In addition, viral E1B-55K protein recruits the STUbL RNF4 and sequesters it into the insoluble fraction of the infected cell. E1B-55K promotes complex formation between RNF4- and E1B-55K-targeted Daxx protein, supporting Daxx posttranslational modification prior to functional inhibition. Hence, RNF4 represents a novel host factor that is beneficial for HAdV gene expression by supporting Daxx counteraction. In this regard, RNF4 and other STUbL proteins might represent novel targets for therapeutic intervention.

Human adenovirus infections: update and consideration of mechanisms of viral persistence

PURPOSE OF REVIEW

To provide an update on recent studies of human adenoviral (HAdV) infections and to explore the mechanisms of viral persistence and the role of persistent infection in disseminated disease in immunocompromised patients.

RECENT FINDINGS

Human adenoviruses continue to be a problem in ophthalmology clinics and to cause periodic, limited, global outbreaks of respiratory disease. Ad14p1 remains in worldwide circulation and continues to result in miniepidemics of severe respiratory infections. New variants of Ad4 and Ad7 have emerged in both the United States and Asia. The severity of Ad4 infections in outbreaks appears to depend more on preexisting conditions in patients than on genetically determined, viral virulence factors, in contrast to limited evidence of Ad7 mutations that may convey increased viral pathogenesis. Reactivation of persistent adenovirus infection appears to be the primary source of disseminated infections in immunocompromised patients. New studies suggest that establishment of persistent infection and reactivation are related to variations in interferon-mediated control of viral replication.

SUMMARY

Innate immune responses can create a state of adenoviral persistence, and repression of these host defenses can result in reactivation and dissemination of infection. A better definition of the molecular mechanisms of immune-mediated control of viral replication might lead to new strategies for treatment of HAdV reactivation and dissemination.

The essential role of guinea pig cytomegalovirus (GPCMV) IE1 and IE2 homologs in viral replication and IE1-mediated ND10 targeting

Guinea pig cytomegalovirus (GPCMV) immediate early proteins, IE1 and IE2, demonstrated structural and functional homologies with human cytomegalovirus (HCMV). GPCMV IE1 and IE2 co-localized in the nucleus with each other, the viral polymerase and guinea pig ND10 components (gpPML, gpDaxx, gpSp100, gpATRX). IE1 showed direct interaction with ND10 components by immunoprecipitation unlike IE2. Additionally, IE1 protein disrupted ND10 bodies. IE1 mutagenesis mapped the nuclear localization signal to the C-terminus and identified the core domain for gpPML interaction. Individual knockout of GPCMV GP122 or GP123 (IE2 and IE1 unique exons respectively) was lethal to the virus. However, an IE1 mutant (codons 234-474 deleted), was viable with attenuated viral growth kinetics and increased susceptibility to type I interferon (IFN-I). In HCMV, the IE proteins are important T cell target antigens. Consequently, characterization of the homologs in GPCMV provides a basis for their evaluation in candidate vaccines against congenital infection.

The Role of Nuclear Antiviral Factors against Invading DNA Viruses: The Immediate Fate of Incoming Viral Genomes

In recent years, it has been suggested that host cells exert intrinsic mechanisms to control nuclear replicating DNA viruses. This cellular response involves nuclear antiviral factors targeting incoming viral genomes. Herpes simplex virus-1 (HSV-1) is the best-studied model in this context, and it was shown that upon nuclear entry HSV-1 genomes are immediately targeted by components of promyelocytic leukemia nuclear bodies (PML-NBs) and the nuclear DNA sensor IFI16 (interferon gamma inducible protein 16). Based on HSV-1 studies, together with limited examples in other viral systems, these phenomena are widely believed to be a common cellular response to incoming viral genomes, although formal evidence for each virus is lacking. Indeed, recent studies suggest that the case may be different for adenovirus infection. Here we summarize the existing experimental evidence for the roles of nuclear antiviral factors against incoming viral genomes to better understand cellular responses on a virus-by-virus basis. We emphasize that cells seem to respond differently to different incoming viral genomes and discuss possible arguments for and against a unifying cellular mechanism targeting the incoming genomes of different virus families.

Promyelocytic leukemia protein isoform II inhibits infection by human adenovirus type 5 through effects on HSP70 and the interferon response

Promyelocytic leukemia (PML) proteins have been implicated in antiviral responses but PML and associated proteins are also suggested to support virus replication. One isoform, PML-II, is required for efficient transcription of interferon and interferon-responsive genes. We therefore investigated the PML-II contribution to human adenovirus 5 (Ad5) infection, using shRNA-mediated knockdown. HelaΔII cells showed a 2-3-fold elevation in Ad5 yield, reflecting an increase in late gene expression. This increase was found to be due in part to the reduced innate immune response consequent upon PML-II depletion. However, the effect was minor because the viral E4 Orf3 protein targets and inactivates this PML-II function. The major benefit to Ad5 in HelaΔII cells was exerted via an increase in HSP70; depletion of HSP70 completely reversed this replicative advantage. Increased Ad5 late gene expression was not due either to the previously described inhibition of inflammatory responses by HSP70 or to effects of HSP70 on major late promoter or L4 promoter activity, but might be linked to an observed increase in E1B 55K, as this protein is known to be required for efficient late gene expression. The induction of HSP70 by PML-II removal was specific for the HSPA1B gene among the HSP70 gene family and thus was not the consequence of a general stress response. Taken together, these data show that PML-II, through its various actions, has an overall negative effect on the Ad5 lifecycle.

The Adenovirus E4-ORF3 Protein Stimulates SUMOylation of General Transcription Factor TFII-I to Direct Proteasomal Degradation

Modulation of host cell transcription, translation, and posttranslational modification processes is critical for the ability of many viruses to replicate efficiently within host cells. The human adenovirus (Ad) early region 4 open reading frame 3 (E4-ORF3) protein forms unique inclusions throughout the nuclei of infected cells and inhibits the antiviral Mre11-Rad50-Nbs1 DNA repair complex through relocalization. E4-ORF3 also induces SUMOylation of Mre11 and Nbs1. We recently identified additional cellular targets of E4-ORF3 and found that E4-ORF3 stimulates ubiquitin-like modification of 41 cellular proteins involved in a wide variety of processes. Among the proteins most abundantly modified in an E4-ORF3-dependent manner was the general transcription factor II-I (TFII-I). Analysis of Ad-infected cells revealed that E4-ORF3 induces TFII-I relocalization and SUMOylation early during infection. In the present study, we explored the relationship between E4-ORF3 and TFII-I. We found that Ad infection or ectopic E4-ORF3 expression leads to SUMOylation of TFII-I that precedes a rapid decline in TFII-I protein levels. We also show that E4-ORF3 is required for ubiquitination of TFII-I and subsequent proteasomal degradation. This is the first evidence that E4-ORF3 regulates ubiquitination. Interestingly, we found that E4-ORF3 modulation of TFII-I occurs in diverse cell types but only E4-ORF3 of Ad species C regulates TFII-I, providing critical insight into the mechanism by which E4-ORF3 targets TFII-I. Finally, we show that E4-ORF3 stimulates the activity of a TFII-I-repressed viral promoter during infection. Our results characterize a novel mechanism of TFII-I regulation by Ad and highlight how a viral protein can modulate a critical cellular transcription factor during infection.

IMPORTANCE

Adenovirus has evolved a number of mechanisms to target host signaling pathways in order to optimize the cellular environment during infection. E4-ORF3 is a small viral protein made early during infection, and it is critical for inactivating host antiviral responses. In addition to its ability to capture and reorganize cellular proteins, E4-ORF3 also regulates posttranslational modifications of target proteins, but little is known about the functional consequences of these modifications. We recently identified TFII-I as a novel target of E4-ORF3 that is relocalized into dynamic E4-ORF3 nuclear structures and subjected to E4-ORF3-mediated SUMO modification. Here, we show that TFII-I is targeted by E4-ORF3 for ubiquitination and proteasomal degradation and that E4-ORF3 stimulates gene expression from a TFII-I-repressed viral promoter. Our findings suggest that the specific targeting of TFII-I by E4-ORF3 is a mechanism to inactivate its antiviral properties. These studies provide further insight into how E4-ORF3 functions to counteract host antiviral responses.

E2F/Rb Family Proteins Mediate Interferon Induced Repression of Adenovirus Immediate Early Transcription to Promote Persistent Viral Infection

Interferons (IFNs) are cytokines that have pleiotropic effects and play important roles in innate and adaptive immunity. IFNs have broad antiviral properties and function by different mechanisms. IFNs fail to inhibit wild-type Adenovirus (Ad) replication in established cancer cell lines. In this study, we analyzed the effects of IFNs on Ad replication in normal human cells. Our data demonstrate that both IFNα and IFNγ blocked wild-type Ad5 replication in primary human bronchial epithelial cells (NHBEC) and TERT-immortalized normal human diploid fibroblasts (HDF-TERT). IFNs inhibited the replication of divergent adenoviruses. The inhibition of Ad5 replication by IFNα and IFNγ is the consequence of repression of transcription of the E1A immediate early gene product. Both IFNα and IFNγ impede the association of the transactivator GABP with the E1A enhancer region during the early phase of infection. The repression of E1A expression by IFNs requires a conserved E2F binding site in the E1A enhancer, and IFNs increased the enrichment of the E2F-associated pocket proteins, Rb and p107, at the E1A enhancer in vivo. PD0332991 (Pabociclib), a specific CDK4/6 inhibitor, dephosphoryles pocket proteins to promote their interaction with E2Fs and inhibited wild-type Ad5 replication dependent on the conserved E2F binding site. Consistent with this result, expression of the small E1A oncoprotein, which abrogates E2F/pocket protein interactions, rescued Ad replication in the presence of IFNα or IFNγ. Finally, we established a persistent Ad infection model in vitro and demonstrated that IFNγ suppresses productive Ad replication in a manner dependent on the E2F binding site in the E1A enhancer. This is the first study that probes the molecular basis of persistent adenovirus infection and reveals a novel mechanism by which adenoviruses utilize IFN signaling to suppress lytic virus replication and to promote persistent infection.

Interferons play important roles in both innate and adaptive immunity, and have broad antiviral properties. We demonstrate that type I (IFNα) and type II (IFNγ) IFNs inhibit the replication of divergent adenoviruses via an evolutionally conserved E2F binding site. IFNs augment the association of the tumor suppressors Rb and p107 with the E1A enhancer region in vivo to repress viral immediate early transcription. By comparing the properties of wild type and E2F site mutant viruses, we show that the IFN–E2F/Rb axis is critical for restriction of adenovirus replication to promote persistent viral infection. Relief of E2F/Rb repression counteracts IFN signaling whereas enforcement of E2F/Rb interaction mimics IFN signaling. These results reveal a novel mechanism by which adenoviruses utilize IFN signaling to suppress lytic virus replication and promote persistent infection.

The SUMOylation Pathway Restricts Gene Transduction by Adeno-Associated Viruses

Adeno-associated viruses are members of the genus dependoviruses of the parvoviridae family. AAV vectors are considered promising vectors for gene therapy and genetic vaccination as they can be easily produced, are highly stable and non-pathogenic. Nevertheless, transduction of cells in vitro and in vivo by AAV in the absence of a helper virus is comparatively inefficient requiring high multiplicity of infection. Several bottlenecks for AAV transduction have previously been described, including release from endosomes, nuclear transport and conversion of the single stranded DNA into a double stranded molecule. We hypothesized that the bottlenecks in AAV transduction are, in part, due to the presence of host cell restriction factors acting directly or indirectly on the AAV-mediated gene transduction. In order to identify such factors we performed a whole genome siRNA screen which identified a number of putative genes interfering with AAV gene transduction. A number of factors, yielding the highest scores, were identified as members of the SUMOylation pathway. We identified Ubc9, the E2 conjugating enzyme as well as Sae1 and Sae2, enzymes responsible for activating E1, as factors involved in restricting AAV. The restriction effect, mediated by these factors, was validated and reproduced independently. Our data indicate that SUMOylation targets entry of AAV capsids and not downstream processes of uncoating, including DNA single strand conversion or DNA damage signaling. We suggest that transiently targeting SUMOylation will enhance application of AAV in vitro and in vivo.

Adenovirus E4-ORF3 Targets PIAS3 and Together with E1B-55K Remodels SUMO Interactions in the Nucleus and at Virus Genome Replication Domains

Adenovirus E4-ORF3 and E1B-55K converge in subverting critical overlapping cellular pathways to facilitate virus replication. Here, we show that E1B-55K and E4-ORF3 induce sumoylation and the assembly of SUMO2/3 viral genome replication domains. Using a conjugation-deficient SUMO2 construct, we demonstrate that SUMO2/3 is recruited to E2A viral genome replication domains through noncovalent interactions. E1B-55K and E4-ORF3 have critical functions in inactivating MRN and ATM to facilitate viral genome replication. We show that ATM kinase inhibitors rescue ΔE1B-55K/ΔE4-ORF3 viral genome replication and that the assembly of E2A domains recruits SUMO2/3 independently of E1B-55K and E4-ORF3. However, the morphology and organization of SUMO2/3-associated E2A domains is strikingly different from that in wild-type Ad5-infected cells. These data reveal that E1B-55K and E4-ORF3 specify the nuclear compartmentalization and structure of SUMO2/3-associated E2A domains, which could have important functions in viral replication. We show that E4-ORF3 specifically targets and sequesters the cellular E3 SUMO ligase PIAS3 but not PIAS1, PIAS2, or PIAS4. The assembly of E4-ORF3 into a multivalent nuclear matrix is required to target PIAS3. In contrast to MRN, PIAS3 is targeted by E4-ORF3 proteins from disparate adenovirus subgroups. Our studies reveal that PIAS3 is a novel and evolutionarily conserved target of E4-ORF3 in human adenovirus infections. Furthermore, we reveal that viral proteins not only disrupt but also usurp SUMO2/3 to transform the nucleus and assemble novel genomic domains that could facilitate pathological viral replication.

IMPORTANCE

SUMO is a key posttranslational modification that modulates the function, localization, and assembly of protein complexes. In the ever-escalating host-pathogen arms race, viruses have evolved strategies to subvert sumoylation. Adenovirus is a small DNA tumor virus that is a global human pathogen and key biomedical agent in basic research and therapy. We show that adenovirus infection induces global changes in SUMO localization and conjugation. Using virus and SUMO mutants, we demonstrate that E1B-55K and E4-ORF3 disrupt and usurp SUMO2/3 interactions to transform the nucleus and assemble highly structured and compartmentalized viral genome domains. We reveal that the cellular E3 SUMO ligase PIAS3 is a novel and conserved target of E4-ORF3 proteins from disparate adenovirus subgroups. The induction of sumoylation and SUMO2/3 viral replication domains by early viral proteins could play an important role in determining the outcome of viral infection.

STAT2 Knockout Syrian Hamsters Support Enhanced Replication and Pathogenicity of Human Adenovirus, Revealing an Important Role of Type I Interferon Response in Viral Control

Human adenoviruses have been studied extensively in cell culture and have been a model for studies in molecular, cellular, and medical biology. However, much less is known about adenovirus replication and pathogenesis in vivo in a permissive host because of the lack of an adequate animal model. Presently, the most frequently used permissive immunocompetent animal model for human adenovirus infection is the Syrian hamster. Species C human adenoviruses replicate in these animals and cause pathology that is similar to that seen with humans. Here, we report findings with a new Syrian hamster strain in which the STAT2 gene was functionally knocked out by site-specific gene targeting. Adenovirus-infected STAT2 knockout hamsters demonstrated an accentuated pathology compared to the wild-type control animals, and the virus load in the organs of STAT2 knockout animals was 100- to 1000-fold higher than that in wild-type hamsters. Notably, the adaptive immune response to adenovirus is not adversely affected in STAT2 knockout hamsters, and surviving hamsters cleared the infection by 7 to 10 days post challenge. We show that the Type I interferon pathway is disrupted in these hamsters, revealing the critical role of interferon-stimulated genes in controlling adenovirus infection. This is the first study to report findings with a genetically modified Syrian hamster infected with a virus. Further, this is the first study to show that the Type I interferon pathway plays a role in inhibiting human adenovirus replication in a permissive animal model. Besides providing an insight into adenovirus infection in humans, our results are also interesting from the perspective of the animal model: STAT2 knockout Syrian hamster may also be an important animal model for studying other viral infections, including Ebola-, hanta-, and dengue viruses, where Type I interferon-mediated innate immunity prevents wild type hamsters from being effectively infected to be used as animal models.

Transcriptional analysis of immune-related gene expression in p53-deficient mice with increased susceptibility to influenza A virus infection

Background

p53 is a tumor suppressor that contributes to the host immune response against viral infections in addition to its well-established protective role against cancer development. In response to influenza A virus (IAV) infection, p53 is activated and plays an essential role in inhibiting IAV replication. As a transcription factor, p53 regulates the expression of a range of downstream responsive genes either directly or indirectly in response to viral infection. We compared the expression profiles of immune-related genes between IAV-infected wild-type p53 (p53WT) and p53-deficient (p53KO) mice to gain an insight into the basis of p53-mediated antiviral response.

Methods

p53KO and p53WT mice were infected with influenza A/Puerto Rico/8/1934 (PR8) strain. Clinical symptoms and body weight changes were monitored daily. Lung specimens of IAV-infected mice were collected for analysis of virus titers and gene expression profiles. The difference in immune-related gene expression levels between IAV-infected p53KO and p53WT mice was comparatively determined using microarray analysis and confirmed by quantitative real-time reverse transcription polymerase chain reaction.

Results

p53KO mice showed an increased susceptibility to IAV infection compared to p53WT mice. Microarray analysis of gene expression profiles in the lungs of IAV-infected mice indicated that the increased susceptibility was associated with significantly changed expression levels in a range of immune-related genes in IAV-infected p53KO mice. A significantly attenuated expression of Ifng (encoding interferon (IFN)-gamma), Irf7 (encoding IFN regulator factor 7), and antiviral genes, such as Mx2 and Eif2ak2 (encoding PKR), were observed in IAV-infected p53KO mice, suggesting an impaired IFN-mediated immune response against IAV infection in the absence of p53. In addition, dysregulated expression levels of proinflammatory cytokines and chemokines, such as Ccl2 (encoding MCP-1), Cxcl9, Cxcl10 (encoding IP-10), and Tnf, were detected in IAV-infected p53KO mice during early IAV infection, reflecting an aberrant inflammatory response.

Conclusion

Lack of p53 resulted in the impaired expression of genes involved in IFN signaling and the dysregulated expression of cytokine and chemokine genes in IAV-infected mice, suggesting an essential role of p53 in the regulation of antiviral and inflammatory responses during IAV infection.

Electronic supplementary material

The online version of this article (doi:10.1186/s12920-015-0127-8) contains supplementary material, which is available to authorized users.

Impact of Adenovirus E4-ORF3 Oligomerization and Protein Localization on Cellular Gene Expression

The Adenovirus E4-ORF3 protein facilitates virus replication through the relocalization of cellular proteins into nuclear inclusions termed tracks. This sequestration event disrupts antiviral properties associated with target proteins. Relocalization of Mre11-Rad50-Nbs1 proteins prevents the DNA damage response from inhibiting Ad replication. Relocalization of PML and Daxx impedes the interferon-mediated antiviral response. Several E4-ORF3 targets regulate gene expression, linking E4-ORF3 to transcriptional control. Furthermore, E4-ORF3 was shown to promote the formation of heterochromatin, down-regulating p53-dependent gene expression. Here, we characterize how E4-ORF3 alters cellular gene expression. Using an inducible, E4-ORF3-expressing cell line, we performed microarray experiments to highlight cellular gene expression changes influenced by E4-ORF3 expression, identifying over four hundred target genes. Enrichment analysis of these genes suggests that E4-ORF3 influences factors involved in signal transduction and cellular defense, among others. The expression of mutant E4-ORF3 proteins revealed that nuclear track formation is necessary to induce these expression changes. Through the generation of knockdown cells, we demonstrate that the observed expression changes may be independent of Daxx and TRIM33 suggesting that an additional factor(s) may be responsible. The ability of E4-ORF3 to manipulate cellular gene expression through the sequestration of cellular proteins implicates a novel role for E4-ORF3 in transcriptional regulation.

PML isoforms IV and V contribute to adenovirus-mediated oncogenic transformation by functionally inhibiting the tumor-suppressor p53

Although modulation of the cellular tumor-suppressor p53 is considered to have the major role in E1A/E1B-55K-mediated tumorigenesis, other promyelocytic leukemia nuclear body (PML-NB)/PML oncogenic domain (POD)-associated factors including SUMO, Mre11, Daxx, as well as the integrity of these nuclear bodies contribute to the transformation process. However, the biochemical consequences and oncogenic alterations of PML-associated E1B-55K by SUMO-dependent PML-IV and PML-V interaction have so far remained elusive. We performed mutational analysis to define a PML interaction motif within the E1B-55K polypeptide. Our results showed that E1B-55K/PML binding is not required for p53, Mre11 and Daxx interaction. We also observed that E1B-55K lacking subnuclear PML localization because of either PML-IV or PML-V-binding deficiency was no longer capable of mediating E1B-55K-dependent SUMOylation of p53, inhibition of p53-mediated transactivation or efficiently transforming primary rodent cells. These results together with the observation that E1B-55K-dependent SUMOylation of p53 is required for efficient cell transformation, provides evidence for the idea that the SUMO ligase activity of the E1B-55K viral oncoprotein is intimately linked to its growth-promoting oncogenic activities.

Promyelocytic Leukemia Protein Isoform II Promotes Transcription Factor Recruitment To Activate Interferon Beta and Interferon-Responsive Gene Expression

To trigger type I interferon (IFN) responses, pattern recognition receptors activate signaling cascades that lead to transcription of IFN and IFN-stimulated genes (ISGs). The promyelocytic leukemia (PML) protein has been implicated in these responses, although its role has not been defined. Here, we show that PML isoform II (PML-II) is specifically required for efficient induction of IFN-β transcription and of numerous ISGs, acting at the point of transcriptional complex assembly on target gene promoters. PML-II associated with specific transcription factors NF-κB and STAT1, as well as the coactivator CREB-binding protein (CBP), to facilitate transcriptional complex formation. The absence of PML-II substantially reduced binding of these factors and IFN regulatory factor 3 (IRF3) to IFN-β or ISGs promoters and sharply reduced gene activation. The unique C-terminal domain of PML-II was essential for its activity, while the N-terminal RBCC motif common to all PML isoforms was dispensable. We propose a model in which PML-II contributes to the transcription of multiple genes via the association of its C-terminal domain with relevant transcription complexes, which promotes the stable assembly of these complexes at promoters/enhancers of target genes, and that in this way PML-II plays a significant role in the development of type I IFN responses.

Impact of the adenoviral E4 Orf3 protein on the activity and posttranslational modification of p53

Our previous studies have established that the p53 populations that accumulate in normal human cells exposed to etoposide or infected by an E1B 55-kDa protein-null mutant of human adenovirus type 5 carry a large number of posttranslational modifications at numerous residues (C. J. DeHart, J. S. Chahal, S. J. Flint, and D. H. Perlman, Mol Cell Proteomics 13:1-17, 2014, http://dx.doi.org/10.1074/mcp.M113.030254). In the absence of this E1B protein, the p53 transcriptional program is not induced, and it has been reported that the viral E4 Orf3 protein inactivates p53 (C. Soria, F. E. Estermann, K. C. Espantman, and C. C. O'Shea, Nature 466:1076-1081, 2010, http://dx.doi.org/10.1038/nature09307). As the latter protein disrupts nuclear Pml bodies, sites at which p53 is modified, we used mass spectrometry to catalogue the posttranscriptional modifications of the p53 population that accumulates when neither the E1B 55-kDa nor the E4 Orf3 protein is made in infected cells. Eighty-five residues carrying 163 modifications were identified. The overall patterns of posttranslational modification of this population and p53 present in cells infected by an E1B 55-kDa-null mutant were similar. The efficiencies with which the two forms of p53 bound to a consensus DNA recognition sequence could not be distinguished and were lower than that of transcriptionally active p53. The absence of the E4 Orf3 protein increased expression of several p53-responsive genes when the E1B protein was also absent from infected cells. However, expression of these genes did not attain the levels observed when p53 was activated in response to etoposide treatment and remained lower than those measured in mock-infected cells.

IMPORTANCE

The tumor suppressor p53, a master regulator of cellular responses to stress, is inactivated and destroyed in cells infected by species C human adenoviruses, such as type 5. It is targeted for proteasomal degradation by the action of a virus-specific E3 ubiquitin ligase that contains the viral E1B 55-kDa and E4 Orf6 proteins, while the E4 Orf3 protein has been reported to block its ability to stimulate expression of p53-dependent genes. The comparisons reported here of the posttranslational modifications and activities of p53 populations that accumulate in infected normal human cells in the absence of both mechanisms of inactivation or of only the E3 ligase revealed little impact of the E4 Orf3 protein. These observations indicate that E4 Orf3-dependent disruption of Pml bodies does not have a major effect on the pattern of p53 posttranslational modifications in adenovirus-infected cells. Furthermore, they suggest that one or more additional viral proteins contribute to blocking p53 activation and the consequences that are deleterious for viral reproduction, such as apoptosis or cell cycle arrest.

Proteomic analysis of ubiquitin-like posttranslational modifications induced by the adenovirus E4-ORF3 protein

Viruses interact with and regulate many host metabolic pathways in order to advance the viral life cycle and counteract intrinsic and extrinsic antiviral responses. The human adenovirus (Ad) early protein E4-ORF3 forms a unique scaffold throughout the nuclei of infected cells and inhibits multiple antiviral defenses, including a DNA damage response (DDR) and an interferon response. We previously reported that the Ad5 E4-ORF3 protein induces sumoylation of Mre11 and Nbs1, which are essential for the DDR, and their relocalization into E4-ORF3-induced nuclear inclusions is required for this modification to occur. In this study, we sought to analyze a global change in ubiquitin-like (Ubl) modifications, with particular focus on SUMO3, by the Ad5 E4-ORF3 protein and to identify new substrates with these modifications. By a comparative proteome-wide approach utilizing immunoprecipitation/mass spectrometry, we found that Ubl modifications of 166 statistically significant lysine sites in 51 proteins are affected by E4-ORF3, and the proteome of modifications spans a diverse range of cellular functions. Ubl modifications of 92% of these identified sites were increased by E4-ORF3. We further analyzed SUMO3 conjugation of several identified proteins. Our findings demonstrated a role for the Ad5 E4-ORF3 protein as a regulator of Ubl modifications and revealed new SUMO3 substrates induced by E4-ORF3.

IMPORTANCE

The adenovirus E4-ORF3 protein induces dynamic structural changes in the nuclei of infected cells and counteracts host antiviral responses. One of the mechanisms that accounts for this process is the relocalization and sequestration of cellular proteins into an E4-ORF3 nuclear scaffold, but little is known about how this small viral protein affects diverse cellular responses. In this study, we analyzed for the first time the global pattern of ubiquitin-like (Ubl) modifications, with particular focus on SUMO3, altered by E4-ORF3 expression. The results suggest a role for the Ad5 E4-ORF3 protein as a regulator of Ubl modifications and reveal new SUMO3 substrates targeted by E4-ORF3. Our findings propose Ubl modifications as a new mechanism by which E4-ORF3 may modulate cellular protein functions in addition to subnuclear relocalization.

Viral and cellular subnuclear structures in human cytomegalovirus-infected cells

In human cytomegalovirus (HCMV)-infected cells, a dramatic remodelling of the nuclear architecture is linked to the creation, utilization and manipulation of subnuclear structures. This review outlines the involvement of several viral and cellular subnuclear structures in areas of HCMV replication and virus-host interaction that include viral transcription, viral DNA synthesis and the production of DNA-filled viral capsids. The structures discussed include those that promote or impede HCMV replication (such as viral replication compartments and promyelocytic leukaemia nuclear bodies, respectively) and those whose role in the infected cell is unclear (for example, nucleoli and nuclear speckles). Viral and cellular proteins associated with subnuclear structures are also discussed. The data reviewed here highlight advances in our understanding of HCMV biology and emphasize the complexity of HCMV replication and virus-host interactions in the nucleus.

Influence of ND10 components on epigenetic determinants of early KSHV latency establishment

We have previously demonstrated that acquisition of intricate patterns of activating (H3K4me3, H3K9/K14ac) and repressive (H3K27me3) histone modifications is a hallmark of KSHV latency establishment. The precise molecular mechanisms that shape the latent histone modification landscape, however, remain unknown. Promyelocytic leukemia nuclear bodies (PML-NB), also called nuclear domain 10 (ND10), have emerged as mediators of innate immune responses that can limit viral gene expression via chromatin based mechanisms. Consequently, although ND10 functions thus far have been almost exclusively investigated in models of productive herpesvirus infection, it has been proposed that they also may contribute to the establishment of viral latency. Here, we report the first systematic study of the role of ND10 during KSHV latency establishment, and link alterations in the subcellular distribution of ND10 components to a temporal analysis of histone modification acquisition and host cell gene expression during the early infection phase. Our study demonstrates that KSHV infection results in a transient interferon response that leads to induction of the ND10 components PML and Sp100, but that repression by ND10 bodies is unlikely to contribute to KSHV latency establishment. Instead, we uncover an unexpected role for soluble Sp100 protein, which is efficiently and permanently relocalized from nucleoplasmic and chromatin-associated fractions into the insoluble matrix. We show that LANA expression is sufficient to induce Sp100 relocalization, likely via mediating SUMOylation of Sp100. Furthermore, we demonstrate that depletion of soluble Sp100 occurs precisely when repressive H3K27me3 marks first accumulate on viral genomes, and that knock-down of Sp100 (but not PML or Daxx) facilitates H3K27me3 acquisition. Collectively, our data support a model in which non-ND10 resident Sp100 acts as a negative regulator of polycomb repressive complex-2 (PRC2) recruitment, and suggest that KSHV may actively escape ND10 silencing mechanisms to promote establishment of latent chromatin.

Innate immunity to adenovirus

Human adenoviruses are the most widely used vectors in gene medicine, with applications ranging from oncolytic therapies to vaccinations, but adenovirus vectors are not without side effects. In addition, natural adenoviruses pose severe risks for immunocompromised people, yet infections are usually mild and self-limiting in immunocompetent individuals. Here we describe how adenoviruses are recognized by the host innate defense system during entry and replication in immune and nonimmune cells. Innate defense protects the host and represents a major barrier to using adenoviruses as therapeutic interventions in humans. Innate response against adenoviruses involves intrinsic factors present at constant levels, and innate factors mounted by the host cell upon viral challenge. These factors exert antiviral effects by directly binding to viruses or viral components, or shield the virus, for example, soluble factors, such as blood clotting components, the complement system, preexisting immunoglobulins, or defensins. In addition, Toll-like receptors and lectins in the plasma membrane and endosomes are intrinsic factors against adenoviruses. Important innate factors restricting adenovirus in the cytosol are tripartite motif-containing proteins, nucleotide-binding oligomerization domain-like inflammatory receptors, and DNA sensors triggering interferon, such as DEAD (Asp-Glu-Ala-Asp) box polypeptide 41 and cyclic guanosine monophosphate-adenosine monophosphate synthase. Adenovirus tunes the function of antiviral autophagy, and counters innate defense by virtue of its early proteins E1A, E1B, E3, and E4 and two virus-associated noncoding RNAs VA-I and VA-II. We conclude by discussing strategies to engineer adenovirus vectors with attenuated innate responses and enhanced delivery features.

Death domain-associated protein 6 (Daxx) selectively represses IL-6 transcription through histone deacetylase 1 (HDAC1)-mediated histone deacetylation in macrophages

As a multifunctional nuclear protein, death domain-associated protein 6 (Daxx) regulates a wide range of biological processes, including cell apoptosis and gene transcription. However, the function of Daxx in innate immunity remains unclear. In our study, we show that Daxx is highly expressed in macrophages and localized in nucleus of macrophages. The expression of Daxx is significantly up-regulated by stimulation with TLR ligands LPS and poly(I:C). Silence of Daxx selectively represses IL-6 expression at transcription level in LPS-activated macrophages. Upon stimulation of LPS, Daxx specifically binds to the promoter of IL-6 and inhibits histone acetylation at IL-6 promoter region. Further mechanism analyses show that histone deacetylase 1 (HDAC1) interacts with Daxx and binds to the promoter of IL-6. Daxx silencing decreases the association of HDAC1 to IL-6 promoter. Therefore, our data reveal that Daxx selectively represses IL-6 transcription through HDAC1-mediated histone deacetylation in LPS-induced macrophages, acting as a negative regulator of IL-6 during innate immunity and potentially preventing inflammatory response because of overproduction of IL-6.

DNA virus replication compartments

Viruses employ a variety of strategies to usurp and control cellular activities through the orchestrated recruitment of macromolecules to specific cytoplasmic or nuclear compartments. Formation of such specialized virus-induced cellular microenvironments, which have been termed viroplasms, virus factories, or virus replication centers, complexes, or compartments, depends on molecular interactions between viral and cellular factors that participate in viral genome expression and replication and are in some cases associated with sites of virion assembly. These virus-induced compartments function not only to recruit and concentrate factors required for essential steps of the viral replication cycle but also to control the cellular mechanisms of antiviral defense. In this review, we summarize characteristic features of viral replication compartments from different virus families and discuss similarities in the viral and cellular activities that are associated with their assembly and the functions they facilitate for viral replication.

Induction and control of the type I interferon pathway by Bluetongue virus

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A general review describing the current knowledge on the type I IFN pathway.

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Description of several mechanisms evolved by viruses to counteract this antiviral response.

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An up-to-date review on the interaction of BTV and the type I IFN pathway in vivo and in vitro.

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Description of the cellular sensors involved in the induction of IFN-α/β synthesis upon BTV infection in haematopoietic and non-haematopoietic cells.

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Description of the strategies evolved by BTV to counteract this cellular antiviral response.

The innate immune response is the first line of defence against viruses, involving the production of type I IFN (IFN-α/β) and other pro-inflammatory cytokines that control the infection. It also shapes the adaptive immune response generated by both T and B cells. Production of type I IFN occurs both in vivo and in vitro in response to Bluetongue virus (BTV), an arthropod-borne virus. However, the mechanisms responsible for the production of IFN-β in response to BTV remained unknown until recently and are still not completely understood. In this review, we describe the recent advances in the identification of cellular sensors and signalling pathways involved in this process. The RNA helicases retinoic acid-inducible gene-I (RIG-I) and melanoma differentiation-associated gene 5 (MDA5) were shown to be involved in the expression of IFN-β as well as in the control of BTV infection in non-haematopoietic cells. In contrast, induction of IFN-α/β synthesis in sheep primary plasmacytoid dendritic cells (pDCs) required the MyD88 adaptor independently of the Toll-like receptor 7 (TLR7), as well as the kinases dsRNA-activated protein kinase (PKR) and stress-activated protein kinase (SAPK)/Jun N-terminal protein kinase (JNK). As type I IFN is essential for the establishment of an antiviral cellular response, most of viruses have elaborated counteracting mechanisms to hinder its action. This review also addresses the ability of BTV to interfere with IFN-β synthesis and the recent findings describing the non-structural viral protein NS3 as a powerful antagonist of the host cellular response.

Nuclear domain 10 of the viral aspect

Nuclear domain 10 (ND10) are spherical bodies distributed throughout the nucleoplasm and measuring around 0.2-1.0 μm. First observed under an electron microscope, they were originally described as dense bodies found in the nucleus. They are known by a number of other names, including Promyelocytic Leukemia bodies (PML bodies), Kremer bodies, and PML oncogenic domains. ND10 are frequently associated with Cajal bodies and cleavage bodies. It has been suggested that they play a role in regulating gene transcription. ND10 were originally characterized using human autoantisera, which recognizes Speckled Protein of 100 kDa, from patients with primary biliary cirrhosis. At the immunohistochemical level, ND10 appear as nuclear punctate structures, with 10 indicating the approximate number of dots per nucleus observed. ND10 do not colocalize with kinetochores, centromeres, sites of mRNA processing, or chromosomes. Resistance of ND10 antigens to nuclease digestion and salt extraction suggest that ND10 are associated with the nuclear matrix. They are often identified by immunofluorescent assay using specific antibodies against PML, Death domain-associated protein, nuclear dot protein (NDP55), and so on. The role of ND10 has long been the subject of investigation, with the specific connection of ND10 and viral infection having been a particular focus for almost 20 years. This review summarizes the relationship of ND10 and viral infection. Some future study directions are also discussed.

The role of chromatin in adenoviral vector function

Vectors based on adenovirus (Ad) are one of the most commonly utilized platforms for gene delivery to cells in molecular biology studies and in gene therapy applications. Ad is also the most popular vector system in human clinical gene therapy trials, largely due to its advantageous characteristics such as high cloning capacity (up to 36 kb), ability to infect a wide variety of cell types and tissues, and relative safety due to it remaining episomal in transduced cells. The latest generation of Ad vectors, helper‑dependent Ad (hdAd), which are devoid of all viral protein coding sequences, can mediate high-level expression of a transgene for years in a variety of species ranging from rodents to non-human primates. Given the importance of histones and chromatin in modulating gene expression within the host cell, it is not surprising that Ad, a nuclear virus, also utilizes these proteins to protect the genome and modulate virus- or vector‑encoded genes. In this review, we will discuss our current understanding of the contribution of chromatin to Ad vector function.

Control of human adenovirus type 5 gene expression by cellular Daxx/ATRX chromatin-associated complexes

Death domain-associated protein (Daxx) cooperates with X-linked α-thalassaemia retardation syndrome protein (ATRX), a putative member of the sucrose non-fermentable 2 family of ATP-dependent chromatin-remodelling proteins, acting as the core ATPase subunit in this complex, whereas Daxx is the targeting factor, leading to histone deacetylase recruitment, H3.3 deposition and transcriptional repression of cellular promoters. Despite recent findings on the fundamental importance of chromatin modification in host-cell gene regulation, it remains unclear whether adenovirus type 5 (Ad5) transcription is regulated by cellular chromatin remodelling to allow efficient virus gene expression. Here, we focus on the repressive role of the Daxx/ATRX complex during Ad5 replication, which depends on intact protein-protein interaction, as negative regulation could be relieved with a Daxx mutant that is unable to interact with ATRX. To ensure efficient viral replication, Ad5 E1B-55K protein inhibits Daxx and targets ATRX for proteasomal degradation in cooperation with early region 4 open reading frame protein 6 and cellular components of a cullin-dependent E3-ubiquitin ligase. Our studies illustrate the importance and diversity of viral factors antagonizing Daxx/ATRX-mediated repression of viral gene expression and shed new light on the modulation of cellular chromatin remodelling factors by Ad5. We show for the first time that cellular Daxx/ATRX chromatin remodelling complexes play essential roles in Ad gene expression and illustrate the importance of early viral proteins to counteract cellular chromatin remodelling.

The repression domain of the E1B 55-kilodalton protein participates in countering interferon-induced inhibition of adenovirus replication

To begin to investigate the mechanism by which the human adenovirus type 5 E1B 55-kDa protein protects against the antiviral effects of type 1 interferon (IFN) (J. S. Chahal, J. Qi, and S. J. Flint, PLoS Pathog. 8:e1002853, 2012 [doi:10.1371/journal.ppat.1002853]), we examined the effects of precise amino acid substitution in this protein on resistance of viral replication to the cytokine. Only substitution of residues 443 to 448 of E1B for alanine (E1B Sub19) specifically impaired production of progeny virus and resulted in a large defect in viral DNA synthesis in IFN-treated normal human fibroblasts. Untreated or IFN-treated cells infected by this mutant virus (AdEasyE1Sub19) contained much higher steady-state concentrations of IFN-inducible GBP1 and IFIT2 mRNAs than did wild-type-infected cells and of the corresponding newly transcribed pre-mRNAs, isolated exploiting 5'-ethynyluridine labeling and click chemistry. These results indicated that the mutations created by substitution of residues 443 to 448 for alanine (Sub19) impair repression of transcription of IFN-inducible genes, by the E1B, 55-kDa protein, consistent with their location in a segment required for repression of p53-dependent transcription. However, when synthesized alone, the E1B 55-kDa protein inhibited expression of the p53-regulated genes BAX and MDM2 but had no impact whatsoever on induction of IFIT2 and GBP1 expression by IFN. These observations correlate repression of transcription of IFN-inducible genes by the E1B 55-kDa protein with protection against inhibition of viral genome replication and indicate that the E1B 55-kDa protein is not sufficient to establish such transcriptional repression.

Retroviral DNA methylation and epigenetic repression are mediated by the antiviral host protein Daxx

Integrated retroviral DNA is subject to epigenetic transcriptional silencing at different frequencies. This process is mediated by repressive DNA methylation and histone modifications on viral chromatin. However, the detailed mechanisms by which retroviral silencing is initiated and maintained are not well understood. Using a model system in which avian sarcoma virus (ASV) DNA is epigenetically repressed in mammalian cells, we previously found that a cellular scaffolding protein, Daxx, acts as an antiretroviral factor that promotes epigenetic repression through recruitment of histone deacetylases (HDACs). Here we show that human Daxx protein levels are increased in response to retroviral infection and that Daxx acts at the time of infection to initiate epigenetic repression. Consistent with a rapid and active antiviral epigenetic response, we found that repressive histone marks and long terminal repeat (LTR) DNA methylation could be detected within 12 h to 3 days postinfection, respectively. Daxx was also found to be required for long-term ASV silencing maintenance and full viral DNA methylation, and it was physically associated with both viral DNA and DNA methyltransferases (DNMTs). These findings support a model in which incoming retroviral protein-DNA complexes are detected by Daxx, and the integrated provirus is rapidly chromatinized and repressed by DNA methylation and histone modification as part of an antiviral response. These results uncover a possible direct and active antiviral mechanism by which DNMTs can be recruited to retroviral DNA.

The adenoviral oncogene E1A-13S interacts with a specific isoform of the tumor suppressor PML to enhance viral transcription

PML nuclear bodies (PML NBs), also called ND10, are matrix-bound nuclear structures that have been implicated in a variety of functions, including DNA repair, transcriptional regulation, protein degradation, and tumor suppression. These domains are also known for their potential to mediate an intracellular defense mechanism against many virus types. This is likely why they are targeted and subsequently manipulated by numerous viral proteins. Paradoxically, the genomes of various DNA viruses become associated with PML NBs, and initial sites of viral transcription/replication centers are often juxtaposed to these domains. The question is why viruses start their transcription and replication next to their supposed antagonists. Here, we report that PML NBs are targeted by the adenoviral (Ad) transactivator protein E1A-13S. Alternatively spliced E1A isoforms (E1A-12S and E1A-13S) are the first proteins expressed upon Ad infection. E1A-13S is essential for activating viral transcription in the early phase of infection. Coimmunoprecipitation assays showed that E1A-13S preferentially interacts with only one (PML-II) of at least six nuclear human PML isoforms. Deletion mapping located the interaction site within E1A conserved region 3 (CR3), which was previously described as the transcription factor binding region of E1A-13S. Indeed, cooperation with PML-II enhanced E1A-mediated transcriptional activation, while deleting the SUMO-interacting motif (SIM) of PML proved even more effective. Our results suggest that in contrast to PML NB-associated antiviral defense, PML-II may help transactivate viral gene expression and therefore play a novel role in activating Ad transcription during the early viral life cycle.