Virion factors that target Daxx to overcome intrinsic immunity

Strong selection signatures for Aleutian disease tolerance acting on novel candidate genes linked to immune and cellular responses in American mink (Neogale vison)

Aleutian disease (AD) is a multi-systemic infectious disease in American mink (Neogale vison) caused by Aleutian mink disease virus (AMDV). This study aimed to identify candidate regions and genes underlying selection for response against AMDV using whole-genome sequence (WGS) data. Three case-control selection signatures studies were conducted between animals (N = 85) producing high versus low antibody levels against AMDV, grouped by counter immunoelectrophoresis (CIEP) test and two enzyme-linked immunosorbent assays (ELISA). Within each study, selection signals were detected using fixation index (FST) and nucleotide diversity (θπ ratios), and validated by cross-population extended haplotype homozygosity (XP-EHH) test. Within- and between-studies overlapping results were then evaluated. Within-studies overlapping results indicated novel candidate genes related to immune and cellular responses (e.g., TAP2, RAB32), respiratory system function (e.g., SPEF2, R3HCC1L), and reproduction system function (e.g., HSF2, CFAP206) in other species. Between-studies overlapping results identified three large segments under strong selection pressure, including two on chromosome 1 (chr1:88,770-98,281 kb and chr1:114,133-120,473) and one on chromosome 6 (chr6:37,953-44,279 kb). Within regions with strong signals, we found novel candidate genes involved in immune and cellular responses (e.g., homologous MHC class II genes, ITPR3, VPS52) in other species. Our study brings new insights into candidate regions and genes controlling AD response.

Host Cell Restriction Factors Blocking Efficient Vector Transduction: Challenges in Lentiviral and Adeno-Associated Vector Based Gene Therapies

Gene therapy relies on the delivery of genetic material to the patient's cells in order to provide a therapeutic treatment. Two of the currently most used and efficient delivery systems are the lentiviral (LV) and adeno-associated virus (AAV) vectors. Gene therapy vectors must successfully attach, enter uncoated, and escape host restriction factors (RFs), before reaching the nucleus and effectively deliver the therapeutic genetic instructions to the cell. Some of these RFs are ubiquitously expressed in mammalian cells, while others are cell-specific, and others still are expressed only upon induction by danger signals as type I interferons. Cell restriction factors have evolved to protect the organism against infectious diseases and tissue damage. These restriction factors can be intrinsic, directly acting on the vector, or related with the innate immune response system, acting indirectly through the induction of interferons, but both are intertwined. The innate immunity is the first line of defense against pathogens and, as such cells derived from myeloid progenitors (but not only), are well equipped with RFs to detect pathogen-associated molecular patterns (PAMPs). In addition, some non-professional cells, such as epithelial cells, endothelial cells, and fibroblasts, play major roles in pathogen recognition. Unsurprisingly, foreign DNA and RNA molecules are among the most detected PAMPs. Here, we review and discuss identified RFs that block LV and AAV vector transduction, hindering their therapeutic efficacy.

PML Alternative Splice Products Differentially Regulate HAdV Productive Infection

Promyelocytic leukemia nuclear bodies (PML-NBs) were considered to maintain antiviral capacity, as these spherical complexes are antagonized by viruses. Actual work provides evidence, that PML-NB-associated factors might also be beneficial for distinct viral processes indicating why genomes and replication centers of nuclear replicating viruses are often found juxtaposed to PML-NBs. Several early HAdV proteins target PML-NBs, such as E4orf3 that promotes redistribution into track-like structures. PML-associated dependency factors that enhance viral gene expression, such as Sp100A remain in the nuclear tracks while restrictive factors, such as Daxx, are inhibited by either proteasomal degradation or relocalization to repress antiviral functions. Here, we did a comprehensive analysis of nuclear PML isoforms during HAdV infection. Our results show cell line specific differences as PML isoforms differentially regulate productive HAdV replication and progeny production. Here, we identified PML-II as a dependency factor that supports viral progeny production, while PML-III and PML-IV suppress viral replication. In contrast, we identified PML-I as a positive regulator and PML-V as a restrictive factor during HAdV infection. Solely PML-VI was shown to repress adenoviral progeny production in both model systems. We showed for the first time, that HAdV can reorganize PML-NBs that contain PML isoforms other then PML-II. Intriguingly, HAdV was not able to fully disrupt PML-NBs composed out of the PML isoforms that inhibit viral replication, while PML-NBs composed out of PML isoforms with beneficial influence on the virus formed tracks in all examined cells. In sum, our findings clearly illustrate the crucial role of PML-track formation in efficient viral replication. IMPORTANCE Actual work provides evidence that PML-NB-associated factors might also be beneficial for distinct viral processes indicating why genomes and replication centers of nuclear replicating viruses are often found juxtaposed to PML-NBs. Alternatively spliced PML isoforms I-VII are expressed from one single pml gene containing nine exons and their transcription is tightly controlled and stimulated by interferons and p53. Several early HAdV proteins target PML-NBs, such as E4orf3, promoting redistribution into track-like structures. Our comprehensive studies indicate a diverging role of PML isoforms throughout the course of productive HAdV infection in either stably transformed human lung (H1299) or liver (HepG2) cells, in which we observed a multivalent regulation of HAdV by all six PML isoforms. PML-I and PML-II support HAdV-mediated track formation and efficient formation of viral replication centers, thus promoting HAdV productive infection. Simultaneously, PML-III, -IV,-V, and -VI antagonize viral gene expression and particle production.

Intracellular trafficking of adeno-associated virus (AAV) vectors: challenges and future directions

In the last two decades, recombinant adeno-associated virus has emerged as the most popular gene therapy vector. Recently AAV gene therapy has been approved by the FDA for the treatment of two rare genetic disorders, namely the early childhood blindness disease Leber congenital amaurosis and spinal muscular atrophy (SMA). As is the case for the treatment of SMA, if the AAV vector must be administered systemically, very high vector doses are often required for therapeutic efficacy. But higher vector doses inevitably increase the risk of adverse events. The tragic death of three children in a clinical trial to treat X-linked myotubular myopathy with an AAV vector has thrown this limitation into sharp relief. Regardless of the precise cause(s) that led to the death of the two children, it is critical that we develop better AAV vectors to achieve therapeutic levels of expression with lower vector doses. To transduce successfully a target cell, AAV has to overcome both systemic as well as cellular roadblocks. In this review, we discuss some of the most prominent cellular roadblocks that AAV must get past to deliver successfully its therapeutic payload. We also highlight recent advancements in our knowledge of AAV biology that can potentially be harnessed to improve AAV vector performance and thereby make AAV gene therapy safer.

Rapid Degradation Pathways of Host Proteins During HCMV Infection Revealed by Quantitative Proteomics

Human cytomegalovirus (HCMV) is an important pathogen in immunocompromised individuals and neonates, and a paradigm for viral immune evasion. We previously developed a quantitative proteomic approach that identified 133 proteins degraded during the early phase of HCMV infection, including known and novel antiviral factors. The majority were rescued from degradation by MG132, which is known to inhibit lysosomal cathepsins in addition to the proteasome. Global definition of the precise mechanisms of host protein degradation is important both to improve our understanding of viral biology, and to inform novel antiviral therapeutic strategies. We therefore developed and optimized a multiplexed comparative proteomic analysis using the selective proteasome inhibitor bortezomib in addition to MG132, to provide a global mechanistic view of protein degradation. Of proteins rescued from degradation by MG132, 34-47 proteins were also rescued by bortezomib, suggesting both that the predominant mechanism of protein degradation employed by HCMV is via the proteasome, and that alternative pathways for degradation are nevertheless important. Our approach and data will enable improved mechanistic understanding of HCMV and other viruses, and provide a shortlist of candidate restriction factors for further analysis.

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.

SUMOylation Targets Adeno-associated Virus Capsids but Mainly Restricts Transduction by Cellular Mechanisms

Adeno-associated virus (AAV) has proven to be a promising candidate for gene therapy due to its nonpathogenic nature, ease of production, and broad tissue tropism. However, its transduction capabilities are not optimal due to the interaction with various host factors within the cell. In a previous study, we identified members of the small ubiquitin-like modifier (SUMO) pathway as significant restriction factors in AAV gene transduction. In the present study, we explored the scope of this restriction by focusing on the AAV capsid and host cell proteins as targets. We show that during vector production, the capsid protein VP2 becomes SUMOylated, as indicated by deletion and point mutations of VP2 or the obstruction of its N terminus via the addition of a tag. We observed that SUMOylated AAV capsids display higher stability than non-SUMOylated capsids. Prevention of capsid SUMOylation by VP2 mutations did not abolish transduction restriction by SUMOylation; however, it reduced activation of gene transduction by shutdown of the cellular SUMOylation pathway. This indicates a link between capsid SUMOylation and SUMOylation of cellular proteins in restricting gene transduction. Infection with AAV triggers general SUMOylation of cellular proteins. In particular, the DAXX protein, a putative host cell restriction factor that can become SUMOylated, is able to restrict AAV gene transduction by reducing the intracellular accumulation of AAV vectors. We also observe that the coexpression of a SUMOylation inhibitor with an AAV2 reporter gene vector increased gene transduction significantly.IMPORTANCE Host factors within the cell are the major mode of restriction of adeno-associated virus (AAV) and keep it from fulfilling its maximum potential as a gene therapy vector. A better understanding of the intricacies of restriction would enable the engineering of better vectors. Via a genome-wide short interfering RNA screen, we identified that proteins of the small ubiquitin-like modifier (SUMO) pathway play an important role in AAV restriction. In this study, we investigate whether this restriction is targeted to the AAV directly or indirectly through host cell factors. The results indicate that both targets act in concert to restrict AAV.

Quantitative Temporal Proteomic Analysis of Vaccinia Virus Infection Reveals Regulation of Histone Deacetylases by an Interferon Antagonist

Vaccinia virus (VACV) has numerous immune evasion strategies, including multiple mechanisms of inhibition of interferon regulatory factor 3 (IRF-3), nuclear factor κB (NF-κB), and type I interferon (IFN) signaling. Here, we use highly multiplexed proteomics to quantify ∼9,000 cellular proteins and ∼80% of viral proteins at seven time points throughout VACV infection. A total of 265 cellular proteins are downregulated >2-fold by VACV, including putative natural killer cell ligands and IFN-stimulated genes. Two-thirds of these viral targets, including class II histone deacetylase 5 (HDAC5), are degraded proteolytically during infection. In follow-up analysis, we demonstrate that HDAC5 restricts replication of both VACV and herpes simplex virus type 1. By generating a protein-based temporal classification of VACV gene expression, we identify protein C6, a multifunctional IFN antagonist, as being necessary and sufficient for proteasomal degradation of HDAC5. Our approach thus identifies both a host antiviral factor and a viral mechanism of innate immune evasion.

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Temporal proteomic analysis quantifies host and viral dynamics during vaccinia infection

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Host protein families are proteasomally degraded over the course of vaccinia infection

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Vaccinia protein C6 targets HDAC5 for proteasomal degradation

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HDAC5 is a host antiviral factor that restricts different families of DNA viruses

Daxx Inhibits HIV-1 Reverse Transcription and Uncoating in a SUMO-Dependent Manner

Death domain-associated protein 6 (Daxx) is a multifunctional, ubiquitously expressed and highly conserved chaperone protein involved in numerous cellular processes, including apoptosis, transcriptional repression, and carcinogenesis. In 2015, we identified Daxx as an antiretroviral factor that interfered with HIV-1 replication by inhibiting the reverse transcription step. In the present study, we sought to unravel the molecular mechanism of Daxx-mediated restriction and, in particular, to identify the protein(s) that Daxx targets in order to achieve its antiviral activity. First, we show that the SUMO-interacting motif (SIM) located at the C-terminus of the protein is strictly required for Daxx to inhibit HIV-1 reverse transcription. By performing a quantitative proteomic screen combined with classical biochemical analyses, we found that Daxx associated with incoming HIV-1 cores through a SIM-dependent interaction with cyclophilin A (CypA) and capsid (CA). Daxx was found to reside within a multiprotein complex associated with viral capsids, also containing TNPO3, TRIM5α, and TRIM34. Given the well-known influence of these cellular factors on the stability of HIV-1 cores, we investigated the effect of Daxx on the cytoplasmic fate of incoming cores and found that Daxx prevented HIV-1 uncoating in a SIM-dependent manner. Altogether, our findings suggest that, by recruiting TNPO3, TRIM5α, and TRIM34 and possibly other proteins onto incoming HIV-1 cores through a SIM-dependent interaction with CA-bound CypA, Daxx increases their stability, thus preventing uncoating and reverse transcription. Our study uncovers a previously unknown function of Daxx in the early steps of HIV-1 infection and further illustrates how reverse transcription and uncoating are two tightly interdependent processes.

Epigenetics and the dynamics of chromatin during adenovirus infections

The DNA genome of eukaryotic cells is compacted by histone proteins within the nucleus to form chromatin. Nuclear-replicating viruses such as adenovirus have evolved mechanisms of chromatin manipulation to promote infection and subvert host defenses. Epigenetic factors may also regulate persistent adenovirus infection and reactivation in lymphoid tissues. In this review, we discuss the viral proteins E1A and protein VII that interact with and alter host chromatin, as well as E4orf3, which separates host chromatin from sites of viral replication. We also highlight recent advances in chromatin technologies that offer new insights into virus-directed chromatin manipulation. Beyond the role of chromatin in the viral replication cycle, we discuss the nature of persistent viral genomes in lymphoid tissue and cell lines, and the potential contribution of epigenetic signals in maintaining adenovirus in a quiescent state. By understanding the mechanisms through which adenovirus manipulates host chromatin, we will understand new aspects of this ubiquitous virus and shed light on previously unknown aspects of chromatin biology.

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.

Enhancing safety of cytomegalovirus-based vaccine vectors by engaging host intrinsic immunity

Rhesus cytomegalovirus (RhCMV)-based vaccines maintain effector memory T cell responses (TEM) that protect ~50% of rhesus monkeys (RMs) challenged with simian immunodeficiency virus (SIV). Because human CMV (HCMV) causes disease in immunodeficient subjects, clinical translation will depend upon attenuation strategies that reduce pathogenic potential without sacrificing CMV's unique immunological properties. We demonstrate that "intrinsic" immunity can be used to attenuate strain 68-1 RhCMV vectors without impairment of immunogenicity. The tegument proteins pp71 and UL35 encoded by UL82 and UL35 of HCMV counteract cell-intrinsic restriction via degradation of host transcriptional repressors. When the corresponding RhCMV genes, Rh110 and Rh59, were deleted from 68-1 RhCMV (ΔRh110 and ΔRh59), we observed only a modest growth defect in vitro, but in vivo, these modified vectors manifested little to no amplification at the injection site and dissemination to distant sites, in contrast to parental 68-1 RhCMV. ΔRh110 was not shed at any time after infection and was not transmitted to naïve hosts either by close contact (mother to infant) or by leukocyte transfusion. In contrast, ΔRh59 was both shed and transmitted by leukocyte transfusion, indicating less effective attenuation than pp71 deletion. The T cell immunogenicity of ΔRh110 was essentially identical to 68-1 RhCMV with respect to magnitude, TEM phenotype, epitope targeting, and durability. Thus, pp71 deletion preserves CMV vector immunogenicity while stringently limiting vector spread, making pp71 deletion an attractive attenuation strategy for HCMV vectors.

Virome and Inflammasomes, a Finely Tuned Balance with Important Consequences for the Host Health

BACKGROUND

The virome is a network of viruses normally inhabiting humans. It forms a conspicuous portion of the so-called microbiome, once generically referred to as resident flora. Indeed, viruses infecting humans without leading to clinical disease are increasingly recognized as part of the microbiome and have an impact on the development of our immune system. In addition, they activate inflammasomes, multiprotein complexes that assemble in cells and that are responsible for the downstream effects of sensing pathogens.

OBJECTIVE

This review aims at summarizing the evidence on the role of the virome in modulating inflammation and emphasizes evidence for Anelloviruses as useful molecular markers to monitor inflammatory processes and immune system competence.

METHOD

We carried out a review of the literature published in the last 5 years and summarized older literature to take into account ground-breaking discoveries concerning inflammasome assembly and virome.

RESULTS

A massive amount of data recently emerging demonstrate that the microbiome closely reflects what we eat, and many other unexpected variables. Composition, location, and amount of the microbiome have an impact on innate and adaptive immune defences. Viruses making up the virome contribute to shaping the immune system. Anelloviruses, the best known of such viruses, are present in most human beings, persistently without causing apparent disease. Depending on their interplay with such viruses, inflammasomes instruct host defences to tolerate or forfeit a specific microorganism.

CONCLUSION

The virome plays an important role in shaping human immune defences and contributes to inflammatory processes by quenching or increasing them.

Baculovirus IE2 Interacts with Viral DNA through Daxx To Generate an Organized Nuclear Body Structure for Gene Activation in Vero Cells

Upon virus infection of a cell, the uncoated DNA is usually blocked by the host intrinsic immune system inside the nucleus. Although it is crucial for the virus to counteract the host intrinsic immune system and access its genome, little is known about how viruses can knock down host restriction and identify their blocked genomes for later viral gene activation and replication. We found that upon baculovirus transduction into Vero E6 cells, the invading viral DNA is trapped by the cellular death domain-associated protein (Daxx) and histone H3.3 in the nucleus, resulting in gene inactivation. IE2, a baculovirus transactivator, targets host Daxx through IE2 SUMO-interacting motifs (SIMs) to indirectly access viral DNA and forms unique nuclear body structures, which we term clathrate cage-like apparatus (CCLAs), at the early transduction stage. At the later transduction stage, CCLAs gradually enlarge, and IE2 continues to closely interact with viral DNA but no longer associates with Daxx. The association with Daxx is essential for IE2 CCLA formation, and the enlarged CCLAs are capable of transactivating viral but not chromosomal DNA of Vero E6 cells. Our study reveals that baculovirus IE2 counteracts the cellular intrinsic immune system by specifically targeting Daxx and H3.3 to associate with viral DNA indirectly and efficiently. IE2 then utilizes this association with viral DNA to establish a unique CCLA cellular nanomachinery, which is visible under light microscopy as an enclosed environment for proper viral gene expression.IMPORTANCE The major breakthrough of this work is that viral protein IE2 localizes and transactivates its own viral DNA through a most unlikely route, i.e., host proteins Daxx and H3.3, which are designed to efficiently restrict viral DNA from expression. By interacting with these host intrinsic immune factors, IE2 can thus target the viral DNA and then form a unique spherical nuclear body, which we name the CCLA, to enclose the viral DNA and necessary factors to assist in high-level transactivation. Our study represents one of the most complete investigations of nuclear body formation. In addition, so far only RNA or protein molecules have been reported as potential nucleators for initiating nuclear body formation; our study may represent the first example showing that DNA can be a nucleator for a new class of nuclear body formation.

High-Definition Analysis of Host Protein Stability during Human Cytomegalovirus Infection Reveals Antiviral Factors and Viral Evasion Mechanisms

Human cytomegalovirus (HCMV) is an important pathogen with multiple immune evasion strategies, including virally facilitated degradation of host antiviral restriction factors. Here, we describe a multiplexed approach to discover proteins with innate immune function on the basis of active degradation by the proteasome or lysosome during early-phase HCMV infection. Using three orthogonal proteomic/transcriptomic screens to quantify protein degradation, with high confidence we identified 35 proteins enriched in antiviral restriction factors. A final screen employed a comprehensive panel of viral mutants to predict viral genes that target >250 human proteins. This approach revealed that helicase-like transcription factor (HLTF), a DNA helicase important in DNA repair, potently inhibits early viral gene expression but is rapidly degraded during infection. The functionally unknown HCMV protein UL145 facilitates HLTF degradation by recruiting the Cullin4 E3 ligase complex. Our approach and data will enable further identifications of innate pathways targeted by HCMV and other viruses.

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Multiplexed viral screens uncover host proteins targeted by early HCMV infection

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Finding host proteins targeted for degradation by HCMV reveals immune evasion strategies

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A screen of HCMV deletion mutants discovers viral factors that target >250 host proteins

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HLTF is an antiviral restriction factor that is targeted for degradation by HCMV UL145

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.

HSV-1 ICP27 represses NF-κB activity by regulating Daxx sumoylation

Herpes simplex virus type 1 ICP27 is a multifunctional protein responsible for viral replication, late gene expression, and reactivation from latency. ICP27 interacts with various cellular proteins, including Daxx. However, the role of interaction between ICP27 and Daxx is largely unknown. Since Daxx is known to repress NF-κB activity, there is a possibility that ICP27 may influence the inhibitory effect of Daxx on NF-κB activity. In this study, we tested whether ICP27 affects the NF-κB activity through its interaction with Daxx. Interestingly, ICP27 enhanced the Daxx-mediated repression of NF-κB activity. In addition, we found that sumoylation of Daxx regulates its interaction with p65. ICP27 binds to Daxx, inhibits Daxx sumoylation, and enhances p65 deacetylation induced by Daxx. Consequently, ICP27 represses the NF-κB activity, by elevating the inhibitory effect of Daxx on NF-κB activity through desumoylation of Daxx.

Potential mechanisms facilitating herpesvirus-induced nuclear remodeling: how are herpesvirus capsids able to leave the nucleus?

Herpesviruses replicate their DNA, assemble and package their capsids in the host nucleus. How capsids transverse the nuclear space to reach nuclear egress sites at the inner nuclear membrane has been a matter of some debate. We recently showed that HSV-1 and pseudorabies virus rely on the large-scale remodeling of host chromatin to allow intranuclear capsids to cross the nucleoplasm by diffusion. Which molecular pathways induce large-scale chromatin remodeling is currently not known. In this perspective, we propose a four-step speculative model that bridges the gap between known virus–host interactions and large-scale chromatin remodeling. We hope that this hypothetical framework will be used as a basis to elucidate how herpesviruses remodel the host nucleus and enable capsids to escape the nucleus.

A Temporal Proteomic Map of Epstein-Barr Virus Lytic Replication in B Cells

Epstein-Barr virus (EBV) replication contributes to multiple human diseases, including infectious mononucleosis, nasopharyngeal carcinoma, B cell lymphomas, and oral hairy leukoplakia. We performed systematic quantitative analyses of temporal changes in host and EBV proteins during lytic replication to gain insights into virus-host interactions, using conditional Burkitt lymphoma models of type I and II EBV infection. We quantified profiles of >8,000 cellular and 69 EBV proteins, including >500 plasma membrane proteins, providing temporal views of the lytic B cell proteome and EBV virome. Our approach revealed EBV-induced remodeling of cell cycle, innate and adaptive immune pathways, including upregulation of the complement cascade and proteasomal degradation of the B cell receptor complex, conserved between EBV types I and II. Cross-comparison with proteomic analyses of human cytomegalovirus infection and of a Kaposi-sarcoma-associated herpesvirus immunoevasin identified host factors targeted by multiple herpesviruses. Our results provide an important resource for studies of EBV replication.

The Epstein-Barr virus miR-BHRF1-1 targets RNF4 during productive infection to promote the accumulation of SUMO conjugates and the release of infectious virus

Post-translational modification by the Small Ubiquitin-like Modifier (SUMO) regulates a variety of cellular functions, and is hijacked by viruses to remodel the host cell during latent and productive infection. Here we have monitored the activity of the SUMO conjugation machinery in cells productively infected with Epstein-Barr virus (EBV). We found that SUMO2/3 conjugates accumulate during the late phase of the productive virus cycle, and identified several viral proteins as bone fide SUMOylation substrates. Analysis of the mechanism involved in the accumulation of SUMOylated proteins revealed upregulation of several components of the SUMO-conjugation machinery and post-transcriptional downregulation of the SUMO-targeted ubiquitin ligase RNF4. The latter effect was mediated by selective inhibition of RNF4 protein expression by the viral miR-BHRF1-1. Reconstitution of RNF4 in cells expressing an inducible miR-BHRF1-1 sponge or a miR-BHRF1-1 resistant RNF4 was associated with reduced levels of early and late viral proteins and impaired virus release. These findings illustrate a novel strategy for viral interference with the SUMO pathway, and identify the EBV miR-BHRF1-1 and the cellular RNF4 as regulators of the productive virus cycle.

We have investigated the activity of the SUMOylation machinery in cells infected with Epstein-Barr virus (EBV), a human herpesvirus that infects B-lymphocytes and is associated with malignancies. We found that activation of the productive virus cycle is accompanied by accumulation of SUMO conjugates, upregulation of components of the SUMO conjugation machinery, and downregulation of the SUMO-targeted ubiquitin ligase RNF4. The decrease of RNF4 is due to post-transcriptional downregulation by miR-BHRF1-1, a member of the BHRF1 microRNA cluster that is upregulated during productive infection. The effect of miR-BHRF1-1 was confirmed in luciferase reported assays, by mutation of the RNF4 3’UTR seed site, by transfection of a synthetic miR-BHRF1-1 mimic, by ectopic expression of miR-BHRF1-1 and by the reversal of RNF4 downregulation in cells expressing a miR-BHRF1-1 sponge. We also found that several early and late viral proteins are bona fide SUMOylation substrates. Reconstitution of RNF4 in productively infected cells was accompanied by proteasome-dependent degradation of the SUMOylated viral protein and by a significantly reduced virus yield. These findings illustrate a new strategy for viral interference with the SUMO pathway, an unexpected contribution of miR-BHRF1-1 to the productive cycle of EBV and a previously unrecognized role of the RNF4 ligase in the regulation of virus production.

Host cell restriction factors that limit transcription and replication of human papillomavirus

The life cycle of human papillomaviruses (HPV) is tightly regulated by the differentiation state of mucosal and cutaneous keratinocytes. To counteract viral infection, constitutively expressed cellular factors, which are defined herein as restriction factors, directly mitigate viral gene expression and replication. In turn, some HPV gene products target these restriction factors and abrogate their anti-viral effects to establish efficient gene expression and replication programs. Ironically, in certain circumstances, this delicate counterbalance between viral gene products and restriction factors facilitates persistent infection by HPVs. This review serves to recapitulate the current knowledge of nuclear restriction factors that directly affect the HPV infectious cycle.

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.

Stimulation of the Replication of ICP0-Null Mutant Herpes Simplex Virus 1 and pp71-Deficient Human Cytomegalovirus by Epstein-Barr Virus Tegument Protein BNRF1

It is now well established that several cellular proteins that are components of promyelocytic leukemia nuclear bodies (PML NBs, also known as ND10) have restrictive effects on herpesvirus infections that are countered by viral proteins that are either present in the virion particle or are expressed during the earliest stages of infection. For example, herpes simplex virus 1 (HSV-1) immediate early (IE) protein ICP0 overcomes the restrictive effects of PML-NB components PML, Sp100, hDaxx, and ATRX while human cytomegalovirus (HCMV) IE protein IE1 targets PML and Sp100, and its tegument protein pp71 targets hDaxx and ATRX. The functions of these viral regulatory proteins are in part interchangeable; thus, both IE1 and pp71 stimulate the replication of ICP0-null mutant HSV-1, while ICP0 increases plaque formation by pp71-deficient HCMV. Here, we extend these studies by examining proteins that are expressed by Epstein-Barr virus (EBV). We report that EBV tegument protein BNRF1, discovered by other investigators to target the hDaxx/ATRX complex, increases the replication of both ICP0-null mutant HSV-1 and pp71-deficient HCMV. In addition, EBV protein EBNA-LP, which targets Sp100, also augments ICP0-null mutant HSV-1 replication. The combination of these two EBV regulatory proteins had a greater effect than each one individually. These findings reinforce the concept that disruption of the functions of PML-NB proteins is important for efficient herpesvirus infections.

IMPORTANCE

Whether a herpesvirus initiates a lytic infection in a host cell or establishes quiescence or latency is influenced by events that occur soon after the viral genome has entered the host cell nucleus. Certain cellular proteins respond in a restrictive manner to the invading pathogen's DNA, while viral functions are expressed that counteract the cell-mediated repression. One aspect of cellular restriction of herpesvirus infections is mediated by components of nuclear structures known as PML nuclear bodies (PML NBs), or ND10. Members of the alpha-, beta-, and gammaherpesvirus families all express proteins that interact with, degrade, or otherwise counteract the inhibitory effects of various PML NB components. Previous work has shown that there is the potential for a functional interchange between the viral proteins expressed by alpha- and betaherpesviruses, despite a lack of obvious sequence similarity. Here, this concept is extended to include a member of the gammaherpesviruses.

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.

Components of Adenovirus Genome Packaging

Adenoviruses (AdVs) are icosahedral viruses with double-stranded DNA (dsDNA) genomes. Genome packaging in AdV is thought to be similar to that seen in dsDNA containing icosahedral bacteriophages and herpesviruses. Specific recognition of the AdV genome is mediated by a packaging domain located close to the left end of the viral genome and is mediated by the viral packaging machinery. Our understanding of the role of various components of the viral packaging machinery in AdV genome packaging has greatly advanced in recent years. Characterization of empty capsids assembled in the absence of one or more components involved in packaging, identification of the unique vertex, and demonstration of the role of IVa2, the putative packaging ATPase, in genome packaging have provided compelling evidence that AdVs follow a sequential assembly pathway. This review provides a detailed discussion on the functions of the various viral and cellular factors involved in AdV genome packaging. We conclude by briefly discussing the roles of the empty capsids, assembly intermediates, scaffolding proteins, portal vertex and DNA encapsidating enzymes in AdV assembly and packaging.

Structural basis underlying viral hijacking of a histone chaperone complex

The histone H3.3 chaperone DAXX is implicated in formation of heterochromatin and transcription silencing, especially for newly infecting DNA virus genomes entering the nucleus. Epstein-Barr virus (EBV) can efficiently establish stable latent infection as a chromatinized episome in the nucleus of infected cells. The EBV tegument BNRF1 is a DAXX-interacting protein required for the establishment of selective viral gene expression during latency. Here we report the structure of BNRF1 DAXX-interaction domain (DID) in complex with DAXX histone-binding domain (HBD) and histones H3.3-H4. BNRF1 DID contacts DAXX HBD and histones through non-conserved loops. The BNRF1-DAXX interface is responsible for BNRF1 localization to PML-nuclear bodies typically associated with host-antiviral resistance and transcriptional repression. Paradoxically, the interface is also required for selective transcription activation of viral latent cycle genes required for driving B-cell proliferation. These findings reveal molecular details of virus reprogramming of an antiviral histone chaperone to promote viral latency and cellular immortalization.

The Epstein-Barr virus tegument protein BNRF1 is required for the establishment of selective viral gene expression during latency and interacts with the histone chaperone DAXX. Here the authors provide structural insight into how BNRF1 hijacks the DAXX-histone H3.3-H4 complex.

Imaging analysis of nuclear antiviral factors through direct detection of incoming adenovirus genome complexes

Recent studies involving several viral systems have highlighted the importance of cellular intrinsic defense mechanisms through nuclear antiviral proteins that restrict viral propagation. These factors include among others components of PML nuclear bodies, the nuclear DNA sensor IFI16, and a potential restriction factor PHF13/SPOC1. For several nuclear replicating DNA viruses, it was shown that these factors sense and target viral genomes immediately upon nuclear import. In contrast to the anticipated view, we recently found that incoming adenoviral genomes are not targeted by PML nuclear bodies. Here we further explored cellular responses against adenoviral infection by focusing on specific conditions as well as additional nuclear antiviral factors. In line with our previous findings, we show that neither interferon treatment nor the use of specific isoforms of PML nuclear body components results in co-localization between incoming adenoviral genomes and the subnuclear domains. Furthermore, our imaging analyses indicated that neither IFI16 nor PHF13/SPOC1 are likely to target incoming adenoviral genomes. Thus our findings suggest that incoming adenoviral genomes may be able to escape from a large repertoire of nuclear antiviral mechanisms, providing a rationale for the efficient initiation of lytic replication cycle.

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 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.

PML/TRIM19-Dependent Inhibition of Retroviral Reverse-Transcription by Daxx

PML (Promyelocytic Leukemia protein), also known as TRIM19, belongs to the family of tripartite motif (TRIM) proteins. PML is mainly expressed in the nucleus, where it forms dynamic structures known as PML nuclear bodies that recruit many other proteins, such as Sp100 and Daxx. While the role of PML/TRIM19 in antiviral defense is well documented, its effect on HIV-1 infection remains unclear. Here we show that infection by HIV-1 and other retroviruses triggers the formation of PML cytoplasmic bodies, as early as 30 minutes post-infection. Quantification of the number and size of PML cytoplasmic bodies revealed that they last approximately 8 h, with a peak at 2 h post-infection. PML re-localization is blocked by reverse-transcription inhibitors and is not observed following infection with unrelated viruses, suggesting it is specifically triggered by retroviral reverse-transcription. Furthermore, we show that PML interferes with an early step of retroviral infection since PML knockdown dramatically increases reverse-transcription efficiency. We demonstrate that PML does not inhibit directly retroviral infection but acts through the stabilization of one of its well-characterized partners, Daxx. In the presence of PML, cytoplasmic Daxx is found in the vicinity of incoming HIV-1 capsids and inhibits reverse-transcription. Interestingly, Daxx not only interferes with exogenous retroviral infections but can also inhibit retrotransposition of endogenous retroviruses, thus identifying Daxx as a broad cellular inhibitor of reverse-transcription. Altogether, these findings unravel a novel antiviral function for PML and PML nuclear body-associated protein Daxx.

Among the several protection mechanisms raised by mammalian organisms against viral infections, an early line of defense consists of cellular proteins known as restriction factors that interfere with viral replication. PML, also known as TRIM19, is one of these proteins and has been shown to inhibit diverse viruses. PML is mainly expressed in the nucleus, where it forms dynamic structures known as PML nuclear bodies that recruit many other proteins, such as Sp100 and Daxx. While the role of PML/TRIM19 in antiviral defense is well documented, its effect on HIV-1 infection remains unclear. Here we show that PML is rapidly re-localized in the cytoplasm of cells infected by HIV-1 and other retroviruses and interferes with reverse-transcription of incoming retroviral RNA. We were able to demonstrate that PML does not inhibit retroviral infection directly but through the stabilization of one of its well-characterized partners, Daxx. In the presence of PML, Daxx inhibits an early step of reverse-transcription thereby interfering with retroviral infections. Our findings unravel a novel antiviral function for PML and its partner Daxx.

Molecular cloning and expression analysis of an apoptosis-associated gene Daxx from zebrafish, Danio rerio

The death domain-associated protein Daxx exerts many functions including the induction and inhibition of apoptosis, regulation of chromatin remodeling and gene transcription. In this report, we have cloned and characterized a Daxx ortholog from the zebrafish, Danio rerio. The bioinformatics analysis results indicated that the open reading frame (ORF) of zebrafish Daxx is 2,151bp long and encodes a putative protein of 716 amino acids containing Daxx domain. Though quantitative PCR analyses, Daxx mRNA was detected in embryonic development from 6 h to 120 h and in all 11 selected zebrafish tissues, and the expression of Daxx was increased first and then decreased during megalocytivirus infectious spleen and kidney necrosis virus (ISKNV) infection. Fluorescence microscopy indicated that the full-length protein was located in the nuclei of the tested Hela cells uniformly but punctiform distribution in HEK293T. In the luciferase report assays, the GAL4-Daxx fusion protein inhibited the transcriptional activity of L8G5-Luc reporter gene showed that Daxx might act as a transcriptional repressor, following the over-expression in HEK293T, the activation of NF-κB-Luc and p53/p21-Luc reporter genes were repressed by the protein. These results suggested that Daxx might play definite role in apoptosis and innate immunity in zebrafish.

Positive role of promyelocytic leukemia protein in type I interferon response and its regulation by human cytomegalovirus

Promyelocytic leukemia protein (PML), a major component of PML nuclear bodies (also known as nuclear domain 10), is involved in diverse cellular processes such as cell proliferation, apoptosis, gene regulation, and DNA damage response. PML also acts as a restriction factor that suppresses incoming viral genomes, therefore playing an important role in intrinsic defense. Here, we show that PML positively regulates type I interferon response by promoting transcription of interferon-stimulated genes (ISGs) and that this regulation by PML is counteracted by human cytomegalovirus (HCMV) IE1 protein. Small hairpin RNA-mediated PML knockdown in human fibroblasts reduced ISG induction by treatment of interferon-β or infection with UV-inactivated HCMV. PML was required for accumulation of activated STAT1 and STAT2, interacted with them and HDAC1 and HDAC2, and was associated with ISG promoters after HCMV infection. During HCMV infection, viral IE1 protein interacted with PML, STAT1, STAT2, and HDACs. Analysis of IE1 mutant viruses revealed that, in addition to the STAT2-binding domain, the PML-binding domain of IE1 was necessary for suppression of interferon-β-mediated ISG transcription, and that IE1 inhibited ISG transcription by sequestering interferon-stimulated gene factor 3 (ISGF3) in a manner requiring its binding of PML and STAT2, but not of HDACs. In conclusion, our results demonstrate that PML participates in type I interferon-induced ISG expression by regulating ISGF3, and that this regulation by PML is counteracted by HCMV IE1, highlighting a widely shared viral strategy targeting PML to evade intrinsic and innate defense mechanisms.

For productive viral infection, virus needs to overcome successive host defenses including intrinsic defense and innate and acquired immunity. Promyelocytic leukemia protein (PML) has been shown to play an important role in intrinsic defense by acting as a nuclear restriction factor that suppresses incoming viral genomes. In this study, we demonstrate that PML also positively regulates type I interferon response by promoting transcription of interferon-stimulated genes (ISGs). Therefore, PML is a key player in both intrinsic and innate host defenses. We further show that this regulation by PML in type I interferon response is inhibited by human cytomegalovirus (HCMV) IE1 protein, which forms a complex with PML, STAT1, STAT2, and HDACs in virus-infected cells. By analyzing mutant viruses, we demonstrate that IE1 inhibits ISG transcription by sequestering interferon-stimulated gene factor 3 (ISGF3) in a manner requiring its binding of PML and STAT2, but not of HDACs. Our findings reveal that PML is a regulator of ISGF3 in type I interferon response and that this PML activity is counteracted by HCMV IE1. Our study explains why PML targeting activity is widely conserved among many viruses.

Analysis of the functional interchange between the IE1 and pp71 proteins of human cytomegalovirus and ICP0 of herpes simplex virus 1

Human cytomegalovirus (HCMV) immediate early protein IE1 and the tegument protein pp71 are required for efficient infection. These proteins have some functional similarities with herpes simplex virus 1 (HSV-1) immediate early protein ICP0, which stimulates lytic HSV-1 infection and derepresses quiescent HSV-1 genomes. All three proteins counteract antiviral restriction mediated by one or more components of promyelocytic leukemia (PML) nuclear bodies, and IE1 and pp71, acting together, almost completely complement ICP0 null mutant HSV-1. Here, we investigated whether ICP0 might substitute for IE1 or pp71 during HCMV infection. Using human fibroblasts that express ICP0, IE1, or pp71 in an inducible manner, we found that ICP0 stimulated replication of both wild-type (wt) and pp71 mutant HCMV while IE1 increased wt HCMV plaque formation and completely complemented the IE1 mutant. Although ICP0 stimulated IE2 expression from IE1 mutant HCMV and increased the number of IE2-positive cells, it could not compensate for IE1 in full lytic replication. These results are consistent with previous evidence that both IE1 and IE2 are required for efficient HCMV gene expression, but they also imply that IE2 functionality is influenced specifically by IE1, either directly or indirectly, and that IE1 may include sequences that have HCMV-specific functions. We discovered a mutant form of IE1 (YL2) that fails to stimulate HCMV infection while retaining 30 to 80% of the activity of the wt protein in complementing ICP0 null mutant HSV-1. It is intriguing that the YL2 mutation is situated in the region of IE1 that is shared with IE2 and which is highly conserved among primate cytomegaloviruses.

IMPORTANCE

Herpesvirus gene expression can be repressed by cellular restriction factors, one group of which is associated with structures known as ND10 or PML nuclear bodies (PML NBs). Regulatory proteins of several herpesviruses interfere with PML NB-mediated repression, and in some cases their activities are transferrable between different viruses. For example, the requirement for ICP0 during herpes simplex virus 1 (HSV-1) infection can be largely replaced by ICP0-related proteins expressed by other alphaherpesviruses and even by a combination of the unrelated IE1 and pp71 proteins of human cytomegalovirus (HCMV). Here, we report that ICP0 stimulates gene expression and replication of wt HCMV but cannot replace the need for IE1 during infection by IE1-defective HCMV mutants. Therefore, IE1 includes HCMV-specific functions that cannot be replaced by ICP0.

Adenovirus E1A targets the DREF nuclear factor to regulate virus gene expression, DNA replication, and growth

The adenovirus E1A gene is the first gene expressed upon viral infection. E1A remodels the cellular environment to maximize permissivity for viral replication. E1A is also the major transactivator of viral early gene expression and a coregulator of a large number of cellular genes. E1A carries out its functions predominantly by binding to cellular regulatory proteins and altering their activities. The unstructured nature of E1A enables it to bind to a large variety of cellular proteins and form new molecular complexes with novel functions. The C terminus of E1A is the least-characterized region of the protein, with few known binding partners. Here we report the identification of cellular factor DREF (ZBED1) as a novel and direct binding partner of E1A. Our studies identify a dual role for DREF in the viral life cycle. DREF contributes to activation of gene expression from all viral promoters early in infection. Unexpectedly, it also functions as a growth restriction factor for adenovirus as knockdown of DREF enhances virus growth and increases viral genome copy number late in the infection. We also identify DREF as a component of viral replication centers. E1A affects the subcellular distribution of DREF within PML bodies and enhances DREF SUMOylation. Our findings identify DREF as a novel E1A C terminus binding partner and provide evidence supporting a role for DREF in viral replication.

IMPORTANCE

This work identifies the putative transcription factor DREF as a new target of the E1A oncoproteins of human adenovirus. DREF was found to primarily localize with PML nuclear bodies in uninfected cells and to relocalize into virus replication centers during infection. DREF was also found to be SUMOylated, and this was enhanced in the presence of E1A. Knockdown of DREF reduced the levels of viral transcripts detected at 20 h, but not at 40 h, postinfection, increased overall virus yield, and enhanced viral DNA replication. DREF was also found to localize to viral promoters during infection together with E1A. These results suggest that DREF contributes to activation of viral gene expression. However, like several other PML-associated proteins, DREF also appears to function as a growth restriction factor for adenovirus infection.

The interferon-inducible antiviral protein Daxx is not essential for interferon-mediated protection against avian sarcoma virus

Background

The antiviral protein Daxx acts as a restriction factor of avian sarcoma virus (ASV; Retroviridae) in mammalian cells by promoting epigenetic silencing of integrated proviral DNA. Although Daxx is encoded by a type I (α/β) interferon-stimulated gene, the requirement for Daxx in the interferon anti-retroviral response has not been elucidated. In this report, we describe the results of experiments designed to investigate the role of Daxx in the type I interferon-induced anti-ASV response.

Findings

Using an ASV reporter system, we show that type I interferons are potent inhibitors of ASV replication. We demonstrate that, while Daxx is necessary to silence ASV gene expression in the absence of interferons, type I interferons are fully-capable of inducing an antiviral state in the absence of Daxx.

Conclusions

These results provide evidence that Daxx is not essential for the anti-ASV interferon response in mammalian cells, and that interferons deploy multiple, redundant antiviral mechanisms to protect cells from ASV.

Adenovirus precursor pVII protein stability is regulated by its propeptide sequence

Adenovirus encodes for the pVII protein, which interacts and modulates virus DNA structure in the infected cells. The pVII protein is synthesized as the precursor protein and undergoes proteolytic processing by viral proteinase Avp, leading to release of a propeptide sequence and accumulation of the mature VII protein. Here we elucidate the molecular functions of the propeptide sequence present in the precursor pVII protein. The results show that the propeptide is the destabilizing element targeting the precursor pVII protein for proteasomal degradation. Our data further indicate that the propeptide sequence and the lysine residues K26 and K27 regulate the precursor pVII protein stability in a co-dependent manner. We also provide evidence that the Cullin-3 E3 ubiquitin ligase complex alters the precursor pVII protein stability by association with the propeptide sequence. In addition, we show that inactivation of the Cullin-3 protein activity reduces adenovirus E1A gene expression during early phase of virus infection. Collectively, our results indicate a novel function of the adenovirus propeptide sequence and involvement of Cullin-3 in adenovirus gene expression.