The major tegument structural protein VP22 targets areas of dispersed nucleolin and marginalized chromatin during productive herpes simplex virus 1 infection

Mechanism of herpesvirus UL24 protein regulating viral immune escape and virulence

Herpesviruses have evolved a series of abilities involved in the process of host infection that are conducive to virus survival and adaptation to the host, such as immune escape, latent infection, and induction of programmed cell death for sustainable infection. The herpesvirus gene UL24 encodes a highly conserved core protein that plays an important role in effective viral infection. The UL24 protein can inhibit the innate immune response of the host by acting on multiple immune signaling pathways during virus infection, and it also plays a key role in the proliferation and pathogenicity of the virus in the later stage of infection. This article reviews the mechanism by which the UL24 protein mediates herpesvirus immune escape and its effects on viral proliferation and virulence by influencing syncytial formation, DNA damage and the cell cycle. Reviewing these studies will enhance our understanding of the pathogenesis of herpesvirus infection and provide evidence for new strategies to combat against viral infection.

Microscopy deep learning predicts virus infections and reveals mechanics of lytic-infected cells

Imaging across scales reveals disease mechanisms in organisms, tissues, and cells. Yet, particular infection phenotypes, such as virus-induced cell lysis, have remained difficult to study. Here, we developed imaging modalities and deep learning procedures to identify herpesvirus and adenovirus (AdV) infected cells without virus-specific stainings. Fluorescence microscopy of vital DNA-dyes and live-cell imaging revealed learnable virus-specific nuclear patterns transferable to related viruses of the same family. Deep learning predicted two major AdV infection outcomes, non-lytic (nonspreading) and lytic (spreading) infections, up to about 20 hr prior to cell lysis. Using these predictive algorithms, lytic and non-lytic nuclei had the same levels of green fluorescent protein (GFP)-tagged virion proteins but lytic nuclei enriched the virion proteins faster, and collapsed more extensively upon laser-rupture than non-lytic nuclei, revealing impaired mechanical properties of lytic nuclei. Our algorithms may be used to infer infection phenotypes of emerging viruses, enhance single cell biology, and facilitate differential diagnosis of non-lytic and lytic infections.

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Artificial intelligence identifies HSV- and AdV-infected cells without specific probes.

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Imaging lytic-infected cells reveals nuclear envelope rupture and AdV dissemination.

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Live cell imaging and neural networks presciently pinpoint lytic-infected cells.

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Lytic-infected cell nuclei have mechanical properties distinct from non-lytic nuclei.

"Non-Essential" Proteins of HSV-1 with Essential Roles In Vivo: A Comprehensive Review

Viruses encode for structural proteins that participate in virion formation and include capsid and envelope proteins. In addition, viruses encode for an array of non-structural accessory proteins important for replication, spread, and immune evasion in the host and are often linked to virus pathogenesis. Most virus accessory proteins are non-essential for growth in cell culture because of the simplicity of the infection barriers or because they have roles only during a state of the infection that does not exist in cell cultures (i.e., tissue-specific functions), or finally because host factors in cell culture can complement their absence. For these reasons, the study of most nonessential viral factors is more complex and requires development of suitable cell culture systems and in vivo models. Approximately half of the proteins encoded by the herpes simplex virus 1 (HSV-1) genome have been classified as non-essential. These proteins have essential roles in vivo in counteracting antiviral responses, facilitating the spread of the virus from the sites of initial infection to the peripheral nervous system, where it establishes lifelong reservoirs, virus pathogenesis, and other regulatory roles during infection. Understanding the functions of the non-essential proteins of herpesviruses is important to understand mechanisms of viral pathogenesis but also to harness properties of these viruses for therapeutic purposes. Here, we have provided a comprehensive summary of the functions of HSV-1 non-essential proteins.

Alphaherpesvirus Major Tegument Protein VP22: Its Precise Function in the Viral Life Cycle

Alphaherpesviruses are zoonotic pathogens that can cause a variety of diseases in humans and animals and severely damage health. Alphaherpesvirus infection is a slow and orderly process that can lie dormant for the lifetime of the host but may be reactivated when the immune system is compromised. All alphaherpesviruses feature a protein layer called the tegument that lies between the capsid and the envelope. Virus protein (VP) 22 is one of the most highly expressed tegument proteins; there are more than 2,000 copies of this protein in each viral particle. VP22 can interact with viral proteins, cellular proteins, and chromatin, and these interactions play important roles. This review summarizes the latest literature and discusses the roles of VP22 in viral gene transcription, protein synthesis, virion assembly, and viral cell-to-cell spread with the purpose of enhancing understanding of the life cycle of herpesviruses and other pathogens in host cells. The molecular interaction information herein provides important reference data.

ORF9p phosphorylation by ORF47p is crucial for the formation and egress of varicella-zoster virus viral particles

The role of the tegument during the herpesvirus lytic cycle is still not clearly established, particularly at the late phase of infection, when the newly produced viral particles need to be fully assembled before being released from the infected cell. The varicella-zoster virus (VZV) protein coded by open reading frame (ORF) 9 (ORF9p) is an essential tegument protein, and, even though its mRNA is the most expressed during the productive infection, little is known about its functions. Using a GalK positive/negative selection technique, we modified a bacterial artificial chromosome (BAC) containing the complete VZV genome to create viruses expressing mutant versions of ORF9p. We showed that ORF9p is hyperphosphorylated during the infection, especially through its interaction with the viral Ser/Thr kinase ORF47p; we identified a consensus site within ORF9p recognized by ORF47p and demonstrated its importance for ORF9p phosphorylation. Strikingly, an ultrastructural analysis revealed that the mutation of this consensus site (glutamate 85 to arginine) strongly affects viral assembly and release, reproducing the ORF47 kinase-dead VZV phenotype. It also slightly diminishes the infectivity toward immature dendritic cells. Taken together, our results identify ORF9p as a new viral substrate of ORF47p and suggest a determinant role of this phosphorylation for viral infectivity, especially during the process of viral particle formation and egress.

NC-mediated nucleolar localization of retroviral gag proteins

The assembly and release of retrovirus particles from the cell membrane is directed by the Gag polyprotein. The Gag protein of Rous sarcoma virus (RSV) traffics through the nucleus prior to plasma membrane localization. We previously reported that nuclear localization of RSV Gag is linked to efficient packaging of viral genomic RNA, however the intranuclear activities of RSV Gag are not well understood. To gain insight into the properties of the RSV Gag protein within the nucleus, we examined the subnuclear localization and dynamic trafficking of RSV Gag. Restriction of RSV Gag to the nucleus by mutating its nuclear export signal (NES) in the p10 domain or interfering with CRM1-mediated nuclear export of Gag by leptomycin B (LMB) treatment led to the accumulation of Gag in nucleoli and discrete nucleoplasmic foci. Retention of RSV Gag in nucleoli was reduced with cis-expression of the 5' untranslated RU5 region of the viral RNA genome, suggesting the psi (Ψ) packaging signal may alter the subnuclear localization of Gag. Fluorescence recovery after photobleaching (FRAP) demonstrated that the nucleolar fraction of Gag was highly mobile, indicating that there was rapid exchange with Gag proteins in the nucleoplasm. RSV Gag is targeted to nucleoli by a nucleolar localization signal (NoLS) in the NC domain, and similarly, the human immunodeficiency virus type 1 (HIV-1) NC protein also contains an NoLS consisting of basic residues. Interestingly, co-expression of HIV-1 NC or Rev with HIV-1 Gag resulted in accumulation of Gag in nucleoli. Moreover, a subpopulation of HIV-1 Gag was detected in the nucleoli of HeLa cells stably expressing the entire HIV-1 genome in a Rev-dependent fashion. These findings suggest that the RSV and HIV-1 Gag proteins undergo nucleolar trafficking in the setting of viral infection.

The nucleolus and herpesviral usurpation

The nucleolus is a distinct subnuclear compartment known as the site for ribosome biogenesis in eukaryotes. Consequently, the nucleolus is also proposed to function in cell-cycle control, stress sensing and senescence, as well as in viral infection. An increasing number of viral proteins have been found to localize to the nucleolus. In this article, we review the current understanding of the functions of the nucleolus, the molecular mechanism of cellular and viral protein targeting to the nucleolus and the functional roles of the nucleolus during viral infection with a specific focus on the herpesvirus family.

Purification of full-length VP22 from cells infected with HSV-1: A two-pronged approach for the solubilization and purification of viral proteins for use in biochemical studies

VP22, encoded by the U(L)49 gene, is one of the most abundant proteins of the herpes simplex virus type 1 (HSV-1) tegument and has been shown to be important for virus replication and spread. However, the exact role(s) played by VP22 in the HSV-1 replication cycle have yet to be delineated. The lack of a procedure to purify full-length VP22 has limited molecular studies on VP22 function. A procedure was developed for the purification of soluble, full-length VP22 from cells infected with HSV-1. A recombinant virus encoding His-tagged VP22 was generated and found to express VP22 at levels comparable to the wild type virus upon infection of Vero cells. By experimenting with a wide variety of cell lysis buffer conditions, several buffers that promote the solubility of full-length VP22 were identified. Buffers that gave the highest levels of solubility were then used in immobilized metal ion affinity chromatography experiments to identify conditions that provided the greatest level of VP22 binding and recovery from cobalt and nickel affinity resins. Using this strategy soluble, full-length VP22 was purified from cells infected with HSV-1.

Investigating the intercellular spreading properties of the foamy virus Gag protein

Small regions called protein transduction domains (PTDs), identified in cellular and viral proteins, have been reported to efficiently cross biological membranes. Here we show that the structural Gag protein of the prototypic foamy virus (PFV) is apparently able to move from cell to cell and to transport the green fluorescent protein (GFP) from few transfected cells to the nuclei of the entire monolayer. Deletion studies showed that this property lies within the second glycine/arginine (GRII) box in the C-terminus of the protein. We also found that uptake and nuclear accumulation of Gag GRII expressed as GFP-fusion protein in recipient cells was observed only following methanol fixation, but never in living cells or when cells were fixed with glutaraldehyde or treated with trichloroacetic acid prior to methanol fixation. Absence of intercellular spreading in vivo was further confirmed using a sensitive luciferase activity assay based on transactivation of the PFV long terminal repeats. Thus, we conclude that intercellular spreading of PFV Gag represents an artificial diffusion event occurring during cell fixation, followed by nuclear retention mediated by the chromatin-binding sequence within the Gag GRII box. In light of these results, we advise caution before defining a peptide as PTD on the basis of intercellular spreading observed by fluorescence microscopy.

Nucleocapsid protein VP15 of White spot syndrome virus colocalizes with the nucleolar proteins nucleolin and fibrillarin

The core nucleocapsid protein VP15 of White spot syndrome virus (WSSV) was shown to interact with DNA and predicted to be involved in the packaging of the WSSV genome. In the present study, we explored the colocalization of VP15 with several nuclear proteins in insect cells. The results showed that the VP15 completely colocalized with nucleolin and fibrillarin, suggesting that VP15 is a nucleolar localization protein and plays an important role in the life cycle of WSSV in host cells.

Involvement of the UL24 protein in herpes simplex virus 1-induced dispersal of B23 and in nuclear egress

UL24 of herpes simplex virus 1 (HSV-1) is widely conserved within the Herpesviridae family. Herein, we tested the hypothesis that UL24, which we have previously shown to induce the redistribution of nucleolin, also affects the localization of the nucleolar protein B23. We found that HSV-1-induced dispersal of B23 was dependent on UL24. The conserved N-terminal portion of UL24 was sufficient to induce the redistribution of B23 in transient transfection assays. Mutational analysis revealed that the endonuclease motif of UL24 was important for B23 dispersal in both transfected and infected cells. Nucleolar protein relocalization during HSV-1 infection was also observed in non-immortalized cells. Analysis of infected cells by electron microscopy revealed a decrease in the ratio of cytoplasmic versus nuclear viral particles in cells infected with a UL24-deficient strain compared to KOS-infected cells. Our results suggest that UL24 promotes nuclear egress of nucleocapsids during HSV-1 infection, possibly though effects on nucleoli.

Molecular anatomy of subcellular localization of HSV-1 tegument protein US11 in living cells

The herpes simplex virus type I (HSV-1) US11 protein is an RNA-binding multifunctional regulator that specifically and stably associates with nucleoli. Although the C-terminal part of US11 was responsible for its nucleolar localization, the precise nucleolar localization signal (NoLS) and nuclear export signal (NES) of US11 and its nuclear import and export mechanisms are still elusive. In this study, fluorescence microscopy was employed to investigate the subcellular localization of US11 and characterize its transport mechanism in living cells. By constructing a series of deletion mutants fused with enhanced yellow fluorescent protein (EYFP), three novel NoLSs of US11 were for the first time mapped to amino acids 84-125, 126-152, and 89-146, respectively. Additionally, the NES was identified to locate between amino acids 89 and 119. Furthermore, the US11 protein was demonstrated to target to the cytoplasm through the NES by chromosomal region maintenance 1 (CRM1)-independent pathway, and to the nucleolus through Ran and importin beta-dependent mechanism that does not require importin alpha 5.

Relocalization of upstream binding factor to viral replication compartments is UL24 independent and follows the onset of herpes simplex virus 1 DNA synthesis

Herpes simplex virus 1 (HSV-1) induces relocalization of several nucleolar proteins. We have found that, as for fibrillarin, the HSV-1-induced redistribution of two RNA polymerase I components, upstream binding factor (UBF) and RPA194, was independent of the viral protein UL24, which affects nucleolin localization. Nevertheless, the kinetics and sites of redistribution for fibrillarin and UBF differed. Interestingly, UBF remained associated with RPA194 during infection. Although UBF is redistributed to viral replication compartments during infection, we did not detect foci of UBF at early sites of viral DNA synthesis, suggesting that it may not be directly involved in this process at early times.

Nucleolin is required for efficient nuclear egress of herpes simplex virus type 1 nucleocapsids

Herpesvirus nucleocapsids assemble in the nucleus and must cross the nuclear membrane for final assembly and maturation to form infectious progeny virions in the cytoplasm. It has been proposed that nucleocapsids enter the perinuclear space by budding through the inner nuclear membrane, and these enveloped nucleocapsids then fuse with the outer nuclear membrane to enter the cytoplasm. Little is known about the mechanism(s) for nuclear egress of herpesvirus nucleocapsids and, in particular, which, if any, cellular proteins are involved in the nuclear egress pathway. UL12 is an alkaline nuclease encoded by herpes simplex virus type 1 (HSV-1) and has been suggested to be involved in viral DNA maturation and nuclear egress of nucleocapsids. Using a live-cell imaging system to study cells infected by a recombinant HSV-1 expressing UL12 fused to a fluorescent protein, we observed the previously unreported nucleolar localization of UL12 in live infected cells and, using coimmunoprecipitation analyses, showed that UL12 formed a complex with nucleolin, a nucleolus marker, in infected cells. Knockdown of nucleolin in HSV-1-infected cells reduced capsid accumulation, as well as the amount of viral DNA resistant to staphylococcal nuclease in the cytoplasm, which represented encapsidated viral DNA, but had little effect on these viral components in the nucleus. These results indicated that nucleolin is a cellular factor required for efficient nuclear egress of HSV-1 nucleocapsids in infected cells.

Conserved residues in the UL24 protein of herpes simplex virus 1 are important for dispersal of the nucleolar protein nucleolin

The UL24 family of proteins is widely conserved among herpesviruses. We demonstrated previously that UL24 of herpes simplex virus 1 (HSV-1) is important for the dispersal of nucleolin from nucleolar foci throughout the nuclei of infected cells. Furthermore, the N-terminal portion of UL24 localizes to nuclei and can disperse nucleolin in the absence of any other viral proteins. In this study, we tested the hypothesis that highly conserved residues in UL24 are important for the ability of the protein to modify the nuclear distribution of nucleolin. We constructed a panel of substitution mutations in UL24 and tested their effects on nucleolin staining patterns. We found that modified UL24 proteins exhibited a range of subcellular distributions. Mutations associated with a wild-type localization pattern for UL24 correlated with high levels of nucleolin dispersal. Interestingly, mutations targeting two regions, namely, within the first homology domain and overlapping or near the previously identified PD-(D/E)XK endonuclease motif, caused the most altered UL24 localization pattern and the most drastic reduction in its ability to disperse nucleolin. Viral mutants corresponding to the substitutions G121A and E99A/K101A both exhibited a syncytial plaque phenotype at 39 degrees C. vUL24-E99A/K101A replicated to lower titers than did vUL24-G121A or KOS. Furthermore, the E99A/K101A mutation caused the greatest impairment of HSV-1-induced dispersal of nucleolin. Our results identified residues in UL24 that are critical for the ability of UL24 to alter nucleoli and further support the notion that the endonuclease motif is important for the function of UL24 during infection.

Involvement of the nucleolus in replication of human viruses

Viruses are intracellular pathogens that have to usurp some of the cellular machineries to provide an optimal environment for their own replication. An increasing number of reports reveal that many viruses induce modifications of nuclear substructures including nucleoli, whether they replicate or not in the nucleus of infected cells. Indeed, during infection of cells with various types of human viruses, nucleoli undergo important morphological modifications. A large number of viral components traffic to and from the nucleolus where they interact with different cellular and/or viral factors, numerous host nucleolar proteins are redistributed in other cell compartments or are modified and some cellular proteins are delocalised in the nucleolus of infected cells. Well‐documented studies have established that several of these nucleolar modifications play a role in some steps of the viral cycle, and also in fundamental cellular pathways. The nucleolus itself is the place where several essential steps of the viral cycle take place. In other cases, viruses divert host nucleolar proteins from their known functions in order to exert new unexpected role(s). Copyright © 2009 John Wiley & Sons, Ltd.

A conserved carboxy-terminal domain in the major tegument structural protein VP22 facilitates virion packaging of a chimeric protein during productive herpes simplex virus 1 infection

Recombinant virus HSV-1(RF177) was previously generated to examine tegument protein VP22 function by inserting the GFP gene into the gene encoding VP22. During a detailed analysis of this virus, we discovered that RF177 produces a novel fusion protein between the last 15 amino acids of VP22 and GFP, termed GCT-VP22. Thus, the VP22 carboxy-terminal specific antibody 22-3 and two anti-GFP antibodies reacted with an approximately 28 kDa protein from RF177-infected Vero cells. GCT-VP22 was detected at 1 and 3 hpi. Examination of purified virions indicated that GCT-VP22 was incorporated into RF177 virus particles. These observations imply that at least a portion of the information required for virion targeting is located in this domain of VP22. Indirect immunofluorescence analyses showed that GCT-VP22 also localized to areas of marginalized chromatin during RF177 infection. These results indicate that the last fifteen amino acids of VP22 participate in virion targeting during HSV-1 infection.

Enhancement of adeno-associated virus infection by mobilizing capsids into and out of the nucleolus

Adeno-associated virus (AAV) serotypes are being tailored for numerous therapeutic applications, but the parameters governing the subcellular fate of even the most highly characterized serotype, AAV2, remain unclear. To understand how cellular conditions control capsid trafficking, we have tracked the subcellular fate of recombinant AAV2 (rAAV2) vectors using confocal immunofluorescence, three-dimensional infection analysis, and subcellular fractionation. Here we report that a population of rAAV2 virions enters the nucleus and accumulates in the nucleolus after infection, whereas empty capsids are excluded from nuclear entry. Remarkably, after subcellular fractionation, virions accumulating in nucleoli were found to retain infectivity in secondary infections. Proteasome inhibitors known to enhance transduction were found to potentiate nucleolar accumulation. In contrast, hydroxyurea, which also increases transduction, mobilized virions into the nucleoplasm, suggesting that two separate pathways influence vector delivery in the nucleus. Using a small interfering RNA (siRNA) approach, we then evaluated whether nucleolar proteins B23/nucleophosmin and nucleolin, previously shown to interact with AAV2 capsids, affect trafficking and transduction efficiency. Similar to effects observed with proteasome inhibition, siRNA-mediated knockdown of nucleophosmin potentiated nucleolar accumulation and increased transduction 5- to 15-fold. Parallel to effects from hydroxyurea, knockdown of nucleolin mobilized capsids to the nucleoplasm and increased transduction 10- to 30-fold. Moreover, affecting both pathways simultaneously using drug and siRNA combinations was synergistic and increased transduction over 50-fold. Taken together, these results support the hypothesis that rAAV2 virions enter the nucleus intact and can be sequestered in the nucleolus in stable form. Mobilization from the nucleolus to nucleoplasmic sites likely permits uncoating and subsequent gene expression or genome degradation. In summary, with these studies we have refined our understanding of AAV2 trafficking dynamics and have identified cellular parameters that mobilize virions in the nucleus and significantly influence AAV infection.