Evidence that herpes simplex virus VP16 is required for viral egress downstream of the initial envelopment event

Multimodal Therapy Approaches for NUT Carcinoma by Dual Combination of Oncolytic Virus Talimogene Laherparepvec with Small Molecule Inhibitors

NUT (nuclear-protein-in-testis) carcinoma (NC) is a highly aggressive tumor disease. Given that current treatment regimens offer a median survival of six months only, it is likely that this type of tumor requires an extended multimodal treatment approach to improve prognosis. In an earlier case report, we could show that an oncolytic herpes simplex virus (T-VEC) is functional in NC patients. To identify further combination partners for T-VEC, we have investigated the anti-tumoral effects of T-VEC and five different small molecule inhibitors (SMIs) alone and in combination in human NC cell lines. Dual combinations were found to result in higher rates of tumor cell reductions when compared to the respective monotherapy as demonstrated by viability assays and real-time tumor cell growth monitoring. Interestingly, we found that the combination of T-VEC with SMIs resulted in both stronger and earlier reductions in the expression of c-Myc, a main driver of NC cell proliferation, when compared to T-VEC monotherapy. These results indicate the great potential of combinatorial therapies using oncolytic viruses and SMIs to control the highly aggressive behavior of NC cancers and probably will pave the way for innovative multimodal clinical studies in the near future.

Comprehensive analysis of liquid-liquid phase separation propensities of HSV-1 proteins and their interaction with host factors

In recent years, it has been shown that the liquid-liquid phase separation (LLPS) of virus proteins plays a crucial role in their life cycle. It promotes the formation of viral replication organelles, concentrating viral components for efficient replication and facilitates the assembly of viral particles. LLPS has emerged as a crucial process in the replication and assembly of herpes simplex virus-1 (HSV-1). Recent studies have identified several HSV-1 proteins involved in LLPS, including the myristylated tegument protein UL11 and infected cell protein 4; however, a complete proteome-level understanding of the LLPS-prone HSV-1 proteins is not available. We provide a comprehensive analysis of the HSV-1 proteome and explore the potential of its proteins to undergo LLPS. By integrating sequence analysis, prediction algorithms and an array of tools and servers, we identified 10 HSV-1 proteins that exhibit high LLPS potential. By analysing the amino acid sequences of the LLPS-prone proteins, we identified specific sequence motifs and enriched amino acid residues commonly found in LLPS-prone regions. Our findings reveal a diverse range of LLPS-prone proteins within the HSV-1, which are involved in critical viral processes such as replication, transcriptional regulation and assembly of viral particles. This suggests that LLPS might play a crucial role in facilitating the formation of specialized viral replication compartments and the assembly of HSV-1 virion. The identification of LLPS-prone proteins in HSV-1 opens up new avenues for understanding the molecular mechanisms underlying viral pathogenesis. Our work provides valuable insights into the LLPS landscape of HSV-1, highlighting potential targets for further experimental validation and enhancing our understanding of viral replication and pathogenesis.

Research Note: Duck plague virus pUL48 is a late protein that plays an important role in viral replication

Duck plague virus (DPV) pUL48 is a homologous of herpes simplex virus VP16, and some studies have shown that VP16 is essential for viral replication and proliferation, but there are few studies on DPV pUL48. Therefore, in order to study the function of pUL48 protein, we constructed a UL48-deleted mutant (DPV-BAC-∆UL48) that completely reemoved the UL48 gene from the DPV BAC genome and the revertant virus (DPV-BAC-∆UL48R) by using the 2-step red recombination system. Compared with the parental virus (DPV-BAC) and the revertant virus, the titer of UL48-deleted mutant was reduced by more than 38.2%, and the efficiency of producing infectious virions was significantly reduced. In addition, the average size of plaques produced by UL48-deleted mutant was about 30% smaller than that of the parental and revertant viruses, suggesting that pUL48 protein affected the cell-to-cell transmission of DPV. Finally, pharmacological inhibition assay showed that pUL48 is a late protein of DPV. In this study, we found that UL48, as a late gene, plays an important role in viral replication by affecting the formation of DPV infectious virion, virus cell-to-cell transmission, and viral genome transcription, which may provide some help for the study of the function of DPV pUL48 protein and the prevention and control of DPV.

Non-cytopathic herpes simplex virus type-1 isolated from acyclovir-treated patients with recurrent infections

Herpes simplex virus (HSV) usually produces cytopathic effect (CPE) within 24-72 h post-infection (P.I.). Clinical isolates from recurrent HSV infections in patients on Acyclovir therapy were collected between 2016 and 2019 and tested in cell cultures for cytopathic effects and further in-depth characterization. Fourteen such isolates did not show any CPE in A549 or Vero cell lines even at 120 h P.I. However, these cultures remained positive for HSV-DNA after several passages. Sequence analysis revealed that the non-CPE isolates were all HSV-1. Analysis of the thymidine kinase gene from the isolates revealed several previously reported and two novel ACV-resistant mutations. Immunofluorescence and Western blot data revealed a low-level expression of the immediate early protein, ICP4. Late proteins like ICP5 or capsid protein, VP16 were almost undetectable in these isolates. AFM imaging revealed that the non-CPE viruses had structural deformities compared to wild-type HSV-1. Our findings suggest that these strains are manifesting an unusual phenomenon of being non-CPE herpesviruses with low level of virus protein expressions over several passages. Probably these HSV-1 isolates are evolving towards a more "cryptic" form to establish chronic infection in the host thereby unraveling yet another strategy of herpesviruses to evade the host immune system.

Role of the arginine cluster in the disordered domain of Herpes Simplex Virus 1 UL34 for the recruitment of ESCRT-III for viral primary envelopment

During the nuclear export of nascent nucleocapsids of herpesviruses, the nucleocapsids bud through the inner nuclear membrane (INM) by acquiring the INM as a primary envelope (primary envelopment). We recently reported that herpes simplex virus 1 (HSV-1) nuclear egress complex (NEC), which consists of UL34 and UL31, interacts with an ESCRT-III adaptor ALIX and recruits ESCRT-III machinery to the INM for efficient primary envelopment. In this study, we identified a cluster of six arginine residues in the disordered domain of UL34 as a minimal region required for the interaction with ALIX as well as the recruitment of ALIX and an ESCRT-III protein CHMP4B to the INM in HSV-1-infected cells. Mutations in the arginine cluster exhibited phenotypes similar to those with ESCRT-III inhibition reported previously, including the mis-localization of NEC, induction of membranous invagination structures containing enveloped virions, aberrant accumulation of enveloped virions in the invaginations and perinuclear space, and reduction of viral replication. We also showed that the effect of the arginine cluster in UL34 on HSV-1 replication was dependent primarily on ALIX. These results indicated that the arginine cluster in the disordered domain of UL34 was required for the interaction with ALIX and the recruitment of ESCRT-III machinery to the INM to promote primary envelopment. IMPORTANCE Herpesvirus UL34 homologs contain conserved amino-terminal domains that mediate vesicle formation through interactions with UL31 homologs during primary envelopment. UL34 homologs also comprise other domains adjacent to their membrane-anchoring regions, which differ in length, are variable in herpesviruses and do not form distinguished secondary structures. However, the role of these disordered domains in infected cells remains to be elucidated. In this study, we present data suggesting that the arginine cluster in the disordered domain of HSV-1 UL34 mediates the interaction with ALIX, thereby leading to the recruitment of ESCRT-III machinery to the INM for efficient primary envelopment. This is the first study to report the role of the disordered domain of a UL34 homolog in herpesvirus infections.

Critical Involvement of TFIIB in Viral Pathogenesis

Viral infections and the harm they cause to their host are a perpetual threat to living organisms. Pathogenesis and subsequent spread of infection requires replication of the viral genome and expression of structural and non-structural proteins of the virus. Generally, viruses use transcription and translation machinery of the host cell to achieve this objective. The viral genome encodes transcriptional regulators that alter the expression of viral and host genes by manipulating initiation and termination steps of transcription. The regulation of the initiation step is often through interactions of viral factors with gene specific factors as well as general transcription factors (GTFs). Among the GTFs, TFIIB (Transcription Factor IIB) is a frequent target during viral pathogenesis. TFIIB is utilized by a plethora of viruses including human immunodeficiency virus, herpes simplex virus, vaccinia virus, Thogoto virus, hepatitis virus, Epstein-Barr virus and gammaherpesviruses to alter gene expression. A number of viral transcriptional regulators exhibit a direct interaction with host TFIIB in order to accomplish expression of their genes and to repress host transcription. Some viruses have evolved proteins with a three-dimensional structure very similar to TFIIB, demonstrating the importance of TFIIB for viral persistence. Upon viral infection, host transcription is selectively altered with viral transcription benefitting. The nature of viral utilization of TFIIB for expression of its own genes, along with selective repression of host antiviral genes and downregulation of general host transcription, makes TFIIB a potential candidate for antiviral therapies.

Characterization of Nucleocytoplasmic Shuttling of Pseudorabies Virus Protein UL46

Pseudorabies virus (PRV) is the etiological agent of Aujeszky's disease, which has caused severe economic loss in China since its re-emergence in 2011. UL46, a late gene of herpesvirus, codes for the abundant but non-essential viral phosphoproteins 11 and 12 (VP11/12). In this study, VP11/12 was found to localize inside both the nucleus and cytoplasm. The nuclear localization signal (NLS) of VP11/12 was identified as ³RRARGTRRASWKDASR¹⁸. Further research identified α5 and α7 to be the receptors for NLS and the chromosome region maintenance 1 (CRM1) to be the receptor for the nuclear export signal. Moreover, we found that PRV VP11/12 interacts with EP0 and the stimulator of interferon genes protein (STING), whereas the NLS of VP11/12 is the important part for VP11/12 to interact with UL48. To our knowledge, this is the first study to provide reliable evidence verifying the nuclear localization of VP11/12 and its role as an additional shuttling tegument protein for PRV. In addition, this is also the first study to elucidate the interactions between PRV VP11/12 and EP0 as well as between PRV VP11/12 and STING, while identifying the precise interaction sites of PRV VP11/12 and VP16.

HSV-1 Cytoplasmic Envelopment and Egress

Herpes simplex virus type 1 (HSV-1) is a structurally complex enveloped dsDNA virus that has evolved to replicate in human neurons and epithelia. Viral gene expression, DNA replication, capsid assembly, and genome packaging take place in the infected cell nucleus, which mature nucleocapsids exit by envelopment at the inner nuclear membrane then de-envelopment into the cytoplasm. Once in the cytoplasm, capsids travel along microtubules to reach, dock, and envelope at cytoplasmic organelles. This generates mature infectious HSV-1 particles that must then be sorted to the termini of sensory neurons, or to epithelial cell junctions, for spread to uninfected cells. The focus of this review is upon our current understanding of the viral and cellular molecular machinery that enables HSV-1 to travel within infected cells during egress and to manipulate cellular organelles to construct its envelope.

The Role of VP16 in the Life Cycle of Alphaherpesviruses

The protein encoded by the UL48 gene of alphaherpesviruses is named VP16 or alpha-gene-transactivating factor (α-TIF). In the early stage of viral replication, VP16 is an important transactivator that can activate the transcription of viral immediate-early genes, and in the late stage of viral replication, VP16, as a tegument, is involved in viral assembly. This review will explain the mechanism of VP16 acting as α-TIF to activate the transcription of viral immediate-early genes, its role in the transition from viral latency to reactivation, and its effects on viral assembly and maturation. In addition, this review also provides new insights for further research on the life cycle of alphaherpesviruses and the role of VP16 in the viral life cycle.

Functional Domains of the Herpes Simplex Virus Type 1 Tegument Protein pUL37: The Amino Terminus is Dispensable for Virus Replication in Tissue Culture

The herpes simplex virus type 1 (HSV-1) UL37 gene encodes for a multifunctional component of the virion tegument, which is necessary for secondary envelopment in the cytoplasm of infected cells, for motility of the viral particle, and for the first steps in the initiation of virus infection. This 120 kDa protein has several known viral interacting partners, including pUL36, gK/pUL20, pUS10, and VP26, and cellular interacting proteins which include TRAF6, RIG-I, and dystonin. These interactions are likely important for the functions of pUL37 at both early and late stages of infection. We employed a genetic approach to determine essential domains and amino acid residues of pUL37 and their associated functions in cellular localization and virion morphogenesis. Using marker-rescue/marker-transfer methods, we generated a library of GFP-tagged pUL37 mutations in the HSV-1 strain KOS genome. Through viral growth and ultra-structural analysis, we discovered that the C-terminus is essential for replication. The N-terminal 480 amino acids are dispensable for replication in cell culture, although serve some non-essential function as viral titers are reduced in the presence of this truncation. Furthermore, the C-terminal 133 amino acids are important in so much that their absence leads to a lethal phenotype. We further probed the carboxy terminal half of pUL37 by alanine scanning mutagenesis of conserved residues among alphaherpesviruses. Mutant viruses were screened for the inability to form plaques-or greatly reduced plaque size-on Vero cells, of which 22 mutations were chosen for additional analysis. Viruses discovered to have the greatest reduction in viral titers on Vero cells were examined by electron microscopy (EM) and by confocal light microscopy for pUL37-EGFP cellular localization. This genetic approach identified both essential and non-essential domains and residues of the HSV-1 UL37 gene product. The mutations identified in this study are recognized as significant candidates for further analysis of the pUL37 function and may unveil previously undiscovered roles and interactions of this essential tegument gene.

Innate Immune Evasion of Alphaherpesvirus Tegument Proteins

Alphaherpesviruses are a large family of highly successful human and animal DNA viruses that can establish lifelong latent infection in neurons. All alphaherpesviruses have a protein-rich layer called the tegument that, connects the DNA-containing capsid to the envelope. Tegument proteins have a variety of functions, playing roles in viral entry, secondary envelopment, viral capsid nuclear transportation during infection, and immune evasion. Recently, many studies have made substantial breakthroughs in characterizing the innate immune evasion of tegument proteins. A wide range of antiviral tegument protein factors that control incoming infectious pathogens are induced by the type I interferon (IFN) signaling pathway and other innate immune responses. In this review, we discuss the immune evasion of tegument proteins with a focus on herpes simplex virus type I.

Characteristics of herpes simplex virus infection and pathogenesis suggest a strategy for vaccine development

Herpes simplex virus (HSV) can cause oral or genital ulcerative lesions and even encephalitis in various age groups with high infection rates. More seriously, HSV may lead to a wide range of recurrent diseases throughout a lifetime. No vaccines against HSV are currently available. The accumulated clinical research data for HSV vaccines reveal that the effects of HSV interacting with the host, especially the host immune system, may be important for the development of HSV vaccines. HSV vaccine development remains a major challenge. Thus, we focus on the research data regarding the interactions of HSV and host immune cells, including dendritic cells (DCs), innate lymphoid cells (ILCs), macrophages, and natural killer (NK) cells, and the related signal transduction pathways involved in immune evasion and cytokine production. The aim is to explore possible strategies to develop new effective HSV vaccines.

Heat-shock protein 90α is involved in maintaining the stability of VP16 and VP16-mediated transactivation of α genes from herpes simplex virus-1

Background

Numerous host cellular factors are exploited by viruses to facilitate infection. Our previous studies and those of others have shown heat-shock protein 90 (Hsp90), a cellular molecular chaperone, is involved in herpes simplex virus (HSV)-1 infection. However, the function of the dominant Hsp90 isoform and the relationship between Hsp90 and HSV-1 α genes remain unclear.

Methods and results

Hsp90α knockdown or inhibition significantly inhibited the promoter activity of HSV-1 α genes and downregulated virion protein 16(VP16) expression from virus and plasmids. The Hsp90α knockdown-induced suppression of α genes promoter activity and downregulation of α genes was reversed by VP16 overexpression, indicating that Hsp90α is involved in VP16-mediated transcription of HSV-1 α genes. Co-immunoprecipitation experiments indicated that VP16 interacted with Hsp90α through the conserved core domain within VP16. Based on using autophagy inhibitors and the presence of Hsp90 inhibitors in ATG7^−/− (autophagy-deficient) cells, Hsp90 inhibition-induced degradation of VP16 is dependent on macroautophagy-mediated degradation but not chaperone-mediated autophagy (CMA) pathway. In vivo studies demonstrated that treatment with gels containing Hsp90 inhibitor effectively reduced the level of VP16 and α genes, which may contribute to the amelioration of the skin lesions in an HSV-1 infection mediated zosteriform model.

Conclusion

Our study provides new insights into the mechanisms by which Hsp90α facilitates the transactivation of HSV-1 α genes and viral infection, and highlights the importance of developing selective inhibitors targeting the interaction between Hsp90α and VP16 to reduce toxicity, a major challenge in the clinical use of Hsp90 inhibitors.

Electronic supplementary material

The online version of this article (10.1186/s10020-018-0066-x) contains supplementary material, which is available to authorized users.

Cellular Protein Kinase D Modulators Play a Role during Multiple Steps of Herpes Simplex Virus 1 Egress

The assembly of new herpes simplex virus 1 (HSV-1) particles takes place in the nucleus. These particles then travel across the two nuclear membranes and acquire a final envelope from a cellular compartment. The contribution of the cell to the release of the virus is, however, little known. We previously demonstrated, using a synchronized infection, that the host protein kinase D and diacylglycerol, a lipid that recruits the kinase to the trans-Golgi network (TGN), promote the release of the virus from that compartment. Given the role this cellular protein plays in the herpes simplex virus 1 life cycle and the many molecules that modulate its activity, we aimed to determine to what extent this virus utilizes the protein kinase D pathway during a nonsynchronized infection. Several molecular protein kinase D (PKD) regulators were targeted by RNA interference and viral production monitored. Surprisingly, many of these modulators negatively impacted the extracellular release of the virus. Overexpression studies, the use of pharmacological reagents, and assays to monitor intracellular lipids implicated in the biology of PKD suggested that these effects were oddly independent of total intracellular diacylglycerol levels. Instead, mapping of the viral intermediates by electron microscopy suggested that some of these modulators could regulate distinct steps along the viral egress pathway, notably nuclear egress. Altogether, this suggests a more complex contribution of PKD to HSV-1 egress than originally anticipated and new research avenues to explore.IMPORTANCE Viruses are obligatory parasites that highjack numerous cellular functions. This is certainly true when it comes to transporting viral particles within the cell. Herpesviruses share the unique property of traveling through the two nuclear membranes by subsequent budding and fusion and acquiring their final envelope from a cellular organelle. Albeit disputed, the overall evidence from many laboratories points to the trans-Golgi network (TGN) as the source of that membrane. Moreover, past findings revealed that the host protein kinase D (PKD) plays an important role at that stage, which is significant given the known implication of that protein in vesicular transport. The present findings suggest that the PKD machinery not only affects the late stages of herpes simplex virus I egress but also modulates earlier steps, such as nuclear egress. This opens up new means to control these viruses.

A bimodal switch in global protein translation coupled to eIF4H relocalisation during advancing cell-cell transmission of herpes simplex virus

We used the bioorthogonal protein precursor, homopropargylglycine (HPG) and chemical ligation to fluorescent capture agents, to define spatiotemporal regulation of global translation during herpes simplex virus (HSV) cell-to-cell spread at single cell resolution. Translational activity was spatially stratified during advancing infection, with distal uninfected cells showing normal levels of translation, surrounding zones at the earliest stages of infection with profound global shutoff. These cells further surround previously infected cells with restored translation close to levels in uninfected cells, reflecting a very early biphasic switch in translational control. While this process was dependent on the virion host shutoff (vhs) function, in certain cell types we also observed temporally altered efficiency of shutoff whereby during early transmission, naïve cells initially exhibited resistance to shutoff but as infection advanced, naïve target cells succumbed to more extensive translational suppression. This may reflect spatiotemporal variation in the balance of oscillating suppression-recovery phases. Our results also strongly indicate that a single particle of HSV-2, can promote pronounced global shutoff. We also demonstrate that the vhs interacting factor, eIF4H, an RNA helicase accessory factor, switches from cytoplasmic to nuclear localisation precisely correlating with the initial shutdown of translation. However translational recovery occurs despite sustained eIF4H nuclear accumulation, indicating a qualitative change in the translational apparatus before and after suppression. Modelling simulations of high multiplicity infection reveal limitations in assessing translational activity due to sampling frequency in population studies and how analysis at the single cell level overcomes such limitations. The work reveals new insight and a revised model of translational manipulation during advancing infection which has important implications both mechanistically and with regards to the physiological role of translational control during virus propagation. The work also demonstrates the potential of bioorthogonal chemistry for single cell analysis of cellular metabolic processes during advancing infections in other virus systems.

Virus Assembly and Egress of HSV

The assembly and egress of herpes simplex virus (HSV) is a complicated multistage process that involves several different cellular compartments and the activity of many viral and cellular proteins. The process begins in the nucleus, with capsid assembly followed by genome packaging into the preformed capsids. The DNA-filled capsids (nucleocapsids) then exit the nucleus by a process of envelopment at the inner nuclear membrane followed by fusion with the outer nuclear membrane. In the cytoplasm nucleocapsids associate with tegument proteins, which form a complicated protein network that links the nucleocapsid to the cytoplasmic domains of viral envelope proteins. Nucleocapsids and associated tegument then undergo secondary envelopment at intracellular membranes originating from late secretory pathway and endosomal compartments. This leads to assembled virions in the lumen of large cytoplasmic vesicles, which are then transported to the cell periphery to fuse with the plasma membrane and release virus particles from the cell. The details of this multifaceted process are described in this chapter.

A bovine herpesvirus 1 pUL51 deletion mutant shows impaired viral growth in vitro and reduced virulence in rabbits

Bovine herpesvirus 1 (BoHV-1) UL51 protein (pUL51) is a tegument protein of BoHV-1 whose function is currently unknown. Here, we aimed to illustrate the specific role of pUL51 in virion morphogenesis and its importance in BoHV-1 virulence. To do so, we constructed a BoHV-1 bacterial artificial chromosome (BAC). We used recombinant BAC and transgenic techniques to delete a major part of the UL51 open reading frame. Deletion of pUL51 resulted in severe viral growth defects, as evidenced by lower single and multi-step growth kinetics, reduced plaque size, and the accumulation of non-enveloped capsids in the cytoplasm of infected cells. Using tagged BoHV-1 recombinant viruses, it was determined that the pUL51 protein completely co-localized with the cis-Golgi marker protein GM-130. Taken altogether, pUL51 was demonstrated to play a critical role in BoHV-1 growth and it is involved in viral maturation and egress. Moreover, an in vivo analysis showed that the pUL51 mutant exhibited reduced virulence in rabbits, with no clinical signs, no nasal shedding of the virus, and no detectable serum neutralizing antibodies. Therefore, we conclude that the BoHV-1 pUL51 is indispensable for efficient viral growth in vitro and is essential for virulence in vivo.

Outer nuclear membrane fusion of adjacent nuclei in varicella-zoster virus-induced syncytia

Syncytia formation has been considered important for cell-to-cell spread and pathogenesis of many viruses. As a syncytium forms, individual nuclei often congregate together, allowing close contact of nuclear membranes and possibly fusion to occur. However, there is currently no reported evidence of nuclear membrane fusion between adjacent nuclei in wild-type virus-induced syncytia. Varicella-zoster virus (VZV) is one typical syncytia-inducing virus that causes chickenpox and shingles in humans. Here, we report, for the first time, an interesting observation of apparent fusion of the outer nuclear membranes from juxtaposed nuclei that comprise VZV syncytia both in ARPE-19 human epithelial cells in vitro and in human skin xenografts in the SCID-hu mouse model in vivo. This work reveals a novel aspect of VZV-related cytopathic effect in the context of multinucleated syncytia. Additionally, the information provided by this study could be helpful for future studies on interactions of viruses with host cell nuclei.

Herpes simplex virus 1 infection dampens the immediate early antiviral innate immunity signaling from peroxisomes by tegument protein VP16

Background

Herpes simplex virus 1 (HSV-1) is an archetypal member of the alphaherpesvirus subfamily with a large genome encoding over 80 proteins, many of which play a critical role in virus-host interactions and immune modulation. Upon viral infections, the host cells activate innate immune responses to restrict their replications. Peroxisomes, which have long been defined to regulate metabolic activities, are reported to be important signaling platforms for antiviral innate immunity. It has been verified that signaling from peroxisomal MAVS (MAVS-Pex) triggers a rapid interferon (IFN) independent IFN-stimulated genes (ISGs) production against invading pathogens. However, little is known about the interaction between DNA viruses such as HSV-1 and the MAVS-Pex mediated signaling.

Results

HSV-1 could activate the MAVS-Pex signaling pathway at a low multiplicity of infection (MOI), while infection at a high MOI dampens MAVS-Pex induced immediately early ISGs production. A high-throughput screen assay reveals that HSV-1 tegument protein VP16 inhibits the immediate early ISGs expression downstream of MAVS-Pex signaling. Moreover, the expression of ISGs was recovered when VP16 was knockdown with its specific short hairpin RNA.

Conclusion

HSV-1 blocks MAVS-Pex mediated early ISGs production through VP16 to dampen the immediate early antiviral innate immunity signaling from peroxisomes.

Cultured corneas show dendritic spread and restrict herpes simplex virus infection that is not observed with cultured corneal cells

Herpes simplex virus-1 (HSV-1) causes life-long morbidities in humans. While fever blisters are more common, occasionally the cornea is infected resulting in vision loss. A very intriguing aspect of HSV-1 corneal infection is that the virus spread is normally restricted to only a small fraction of cells on the corneal surface that connect with each other in a dendritic fashion. Here, to develop a comprehensive understanding of the susceptibility of human corneal epithelial (HCE) cells to HSV-1 infection, we infected HCE cells at three different dosages of HSV-1 and measured the outcomes in terms of viral entry, gene and protein expression, viral replication and cytokine induction. In cultured cells, infectivity and cytokine induction were observed even at the minimum viral dosage tested, while a more pronounced dose-restricted infectivity was seen in ex vivo cultures of porcine corneas. Use of fluorescent HSV-1 virions demonstrated a pattern of viral spread ex vivo that mimics clinical findings. We conclude that HCE cell cultures are highly susceptible to infection whereas the cultured corneas demonstrate a higher ability to restrict the infection even in the absence of systemic immune system. The restriction is helped in part by local interferon response and the unique cellular architecture of the cornea.

Herpes simplex virus Membrane Fusion

Herpes simplex virus mediates multiple distinct fusion events during infection. HSV entry is initiated by fusion of the viral envelope with either the limiting membrane of a host cell endocytic compartment or the plasma membrane. In the infected cell during viral assembly, immature, enveloped HSV particles in the perinuclear space fuse with the outer nuclear membrane in a process termed de-envelopment. A cell infected with some strains of HSV with defined mutations spread to neighboring cells by a fusion event called syncytium formation. Two experimental methods, the transient cell-cell fusion approach and fusion from without, are useful surrogate assays of HSV fusion. These five fusion processes are considered in terms of their requirements, mechanism, and regulation. The execution and modulation of these events require distinct yet often overlapping sets of viral proteins and host cell factors. The core machinery of HSV gB, gD, and the heterodimer gH/gL is required for most if not all of the HSV fusion mechanisms.

The Interaction between Herpes Simplex Virus 1 Tegument Proteins UL51 and UL14 and Its Role in Virion Morphogenesis

To investigate the molecular mechanism(s) by which herpes simplex virus 1 (HSV-1) tegument protein UL51 promotes viral replication, we screened for viral proteins that interact with UL51 in infected cells. Affinity purification of tagged UL51 in HSV-1-infected Vero cells was coupled with immunoblotting of the purified UL51 complexes with various antibodies to HSV-1 virion proteins. Subsequent analyses revealed that UL51 interacted with another tegument protein, UL14, in infected cells. Mutational analyses of UL51 showed that UL51 amino acid residues Leu-111, Ile-119, and Tyr-123 were required for interaction with UL14 in HSV-1-infected cells. Alanine substitutions of these UL51 amino acid residues reduced viral replication and produced an accumulation of unenveloped and partially enveloped nucleocapsids in the cytoplasm at levels comparable to those of UL51-null, UL14-null, and UL51/UL14 double-null mutations. In addition, although UL51 and UL14 colocalized at juxtanuclear domains in HSV-1-infected cells, the amino acid substitutions in UL51 produced aberrant localization of UL51 and UL14. The effects of these substitutions on localization of UL51 and UL14 were similar to those of the UL51-null and UL14-null mutations on localization of UL14 and UL51, respectively. These results suggested that the interaction between UL51 and UL14 was required for proper localization of these viral proteins in infected cells and that the UL51-UL14 complex regulated final viral envelopment for efficient viral replication.

IMPORTANCE

Herpesviruses contain a unique virion structure designated the tegument, which is a protein layer between the nucleocapsid and the envelope. HSV-1 has dozens of viral proteins in the tegument, which are thought to facilitate viral envelopment by interacting with other virion components. However, although numerous interactions among virion proteins have been reported, data on how these interactions facilitate viral envelopment is limited. In this study, we have presented data showing that the interaction of HSV-1 tegument proteins UL51 and UL14 promoted viral final envelopment for efficient viral replication. In particular, prevention of this interaction induced aberrant accumulation of partially enveloped capsids in the cytoplasm, suggesting that the UL51-UL14 complex acted in the envelopment process but not in an upstream event, such as transport of capsids to the site for envelopment. This is the first report showing that an interaction between HSV-1 tegument proteins directly regulated final virion envelopment.

HSV-1 tegument protein and the development of its genome editing technology

Herpes simplex virus 1 (HSV-1) is composed of complex structures primarily characterized by four elements: the nucleus, capsid, tegument and envelope. The tegument is an important viral component mainly distributed in the spaces between the capsid and the envelope. The development of viral genome editing technologies, such as the identification of temperature-sensitive mutations, homologous recombination, bacterial artificial chromosome, and the CRISPR/Cas9 system, has been shown to largely contribute to the rapid promotion of studies on the HSV-1 tegument protein. Many researches have demonstrated that tegument proteins play crucial roles in viral gene regulatory transcription, viral replication and virulence, viral assembly and even the interaction of the virus with the host immune system. This article briefly reviews the recent research on the functions of tegument proteins and specifically elucidates the function of tegument proteins in viral infection, and then emphasizes the significance of using genome editing technology in studies of providing new techniques and insights into further studies of HSV-1 infection in the future.

Novel Structure and Unexpected RNA-Binding Ability of the C-Terminal Domain of Herpes Simplex Virus 1 Tegument Protein UL21

Proteins forming the tegument layers of herpesviral virions mediate many essential processes in the viral replication cycle, yet few have been characterized in detail. UL21 is one such multifunctional tegument protein and is conserved among alphaherpesviruses. While UL21 has been implicated in many processes in viral replication, ranging from nuclear egress to virion morphogenesis to cell-cell spread, its precise roles remain unclear. Here we report the 2.7-Å crystal structure of the C-terminal domain of herpes simplex virus 1 (HSV-1) UL21 (UL21C), which has a unique α-helical fold resembling a dragonfly. Analysis of evolutionary conservation patterns and surface electrostatics pinpointed four regions of potential functional importance on the surface of UL21C to be pursued by mutagenesis. In combination with the previously determined structure of the N-terminal domain of UL21, the structure of UL21C provides a 3-dimensional framework for targeted exploration of the multiple roles of UL21 in the replication and pathogenesis of alphaherpesviruses. Additionally, we describe an unanticipated ability of UL21 to bind RNA, which may hint at a yet unexplored function.

IMPORTANCE

Due to the limited genomic coding capacity of viruses, viral proteins are often multifunctional, which makes them attractive antiviral targets. Such multifunctionality, however, complicates their study, which often involves constructing and characterizing null mutant viruses. Systematic exploration of these multifunctional proteins requires detailed road maps in the form of 3-dimensional structures. In this work, we determined the crystal structure of the C-terminal domain of UL21, a multifunctional tegument protein that is conserved among alphaherpesviruses. Structural analysis pinpointed surface areas of potential functional importance that provide a starting point for mutagenesis. In addition, the unexpected RNA-binding ability of UL21 may expand its functional repertoire. The structure of UL21C and the observation of its RNA-binding ability are the latest additions to the navigational chart that can guide the exploration of the multiple functions of UL21.

Kaposi's Sarcoma-Associated Herpesvirus Inhibitor of cGAS (KicGAS), Encoded by ORF52, Is an Abundant Tegument Protein and Is Required for Production of Infectious Progeny Viruses

Although Kaposi's sarcoma-associated herpesvirus (KSHV) ORF52 (also known as KSHV inhibitor of cGAS [KicGAS]) has been detected in purified virions, the roles of this protein during KSHV replication have not been characterized. Using specific monoclonal antibodies, we revealed that ORF52 displays true late gene expression kinetics and confirmed its cytoplasmic localization in both transfected and KSHV-infected cells. We demonstrated that ORF52 comigrates with other known virion proteins following sucrose gradient centrifugation. We also determined that ORF52 resides inside the viral envelope and remains partially associated with capsid when extracellular virions are treated with various detergents and/or salts. There results indicate that ORF52 is a tegument protein abundantly present in extracellular virions. To characterize the roles of ORF52 in the KSHV life cycle, we engineered a recombinant KSHV ORF52-null mutant virus and found that loss of ORF52 results in reduced virion production and a further defect in infectivity. Upon analysis of the virion composition of ORF52-null viral particles, we observed a decrease in the incorporation of ORF45, as well as other tegument proteins, suggesting that ORF52 is important for the packaging of other virion proteins. In summary, our results indicate that, in addition to its immune evasion function, KSHV ORF52 is required for the optimal production of infectious virions, likely due to its roles in virion assembly as a tegument protein.

IMPORTANCE

The tegument proteins of herpesviruses, including Kaposi's sarcoma-associated herpesvirus (KSHV), play key roles in the viral life cycle. Each of the three subfamilies of herpesviruses (alpha, beta, and gamma) encode unique tegument proteins with specialized functions. We recently found that one such gammaherpesvirus-specific protein, ORF52, has an important role in immune evasion during KSHV primary infection, through inhibition of the host cytosolic DNA sensing pathway. In this report, we further characterize ORF52 as a tegument protein with vital roles during KSHV lytic replication. We found that ORF52 is important for the production of infectious viral particles, likely through its role in virus assembly, a critical process for KSHV replication and pathogenesis. More comprehensive investigation of the functions of tegument proteins and their roles in viral replication may reveal novel targets for therapeutic interventions against KSHV-associated diseases.

Evasion of host antiviral innate immunity by HSV-1, an update

Herpes simplex virus type 1 (HSV-1) infection triggers a rapid induction of host innate immune responses. The type I interferon (IFN) signal pathway is a central aspect of host defense which induces a wide range of antiviral proteins to control infection of incoming pathogens. In some cases, viral invasion also induces DNA damage response, autophagy, endoplasmic reticulum stress, cytoplasmic stress granules and other innate immune responses, which in turn affect viral infection. However, HSV-1 has evolved multiple strategies to evade host innate responses and facilitate its infection. In this review, we summarize the most recent findings on the molecular mechanisms utilized by HSV-1 to counteract host antiviral innate immune responses with specific focus on the type I IFN signal pathway.

Tegument Assembly and Secondary Envelopment of Alphaherpesviruses

Alphaherpesviruses like herpes simplex virus are large DNA viruses characterized by their ability to establish lifelong latent infection in neurons. As for all herpesviruses, alphaherpesvirus virions contain a protein-rich layer called "tegument" that links the DNA-containing capsid to the glycoprotein-studded membrane envelope. Tegument proteins mediate a diverse range of functions during the virus lifecycle, including modulation of the host-cell environment immediately after entry, transport of virus capsids to the nucleus during infection, and wrapping of cytoplasmic capsids with membranes (secondary envelopment) during virion assembly. Eleven tegument proteins that are conserved across alphaherpesviruses have been implicated in the formation of the tegument layer or in secondary envelopment. Tegument is assembled via a dense network of interactions between tegument proteins, with the redundancy of these interactions making it challenging to determine the precise function of any specific tegument protein. However, recent studies have made great headway in defining the interactions between tegument proteins, conserved across alphaherpesviruses, which facilitate tegument assembly and secondary envelopment. We summarize these recent advances and review what remains to be learned about the molecular interactions required to assemble mature alphaherpesvirus virions following the release of capsids from infected cell nuclei.

Insights into the function of tegument proteins from the varicella zoster virus

Chickenpox (varicella) is caused by primary infection with varicella zoster virus (VZV), which can establish long-term latency in the host ganglion. Once reactivated, the virus can cause shingles (zoster) in the host. VZV has a typical herpesvirus virion structure consisting of an inner DNA core, a capsid, a tegument, and an outer envelope. The tegument is an amorphous layer enclosed between the nucleocapsid and the envelope, which contains a variety of proteins. However, the types and functions of VZV tegument proteins have not yet been completely determined. In this review, we describe the current knowledge on the multiple roles played by VZV tegument proteins during viral infection. Moreover, we discuss the VZV tegument protein-protein interactions and their impact on viral tissue tropism in SCID-hu mice. This will help us develop a better understanding of how the tegument proteins aid viral DNA replication, evasion of host immune response, and pathogenesis.

Cellular Protein WDR11 Interacts with Specific Herpes Simplex Virus Proteins at the trans-Golgi Network To Promote Virus Replication

It has recently been proposed that the herpes simplex virus (HSV) protein ICP0 has cytoplasmic roles in blocking antiviral signaling and in promoting viral replication in addition to its well-known proteasome-dependent functions in the nucleus. However, the mechanisms through which it produces these effects remain unclear. While investigating this further, we identified a novel cytoplasmic interaction between ICP0 and the poorly characterized cellular protein WDR11. During an HSV infection, WDR11 undergoes a dramatic change in localization at late times in the viral replication cycle, moving from defined perinuclear structures to a dispersed cytoplasmic distribution. While this relocation was not observed during infection with viruses other than HSV-1 and correlated with efficient HSV-1 replication, the redistribution was found to occur independently of ICP0 expression, instead requiring viral late gene expression. We demonstrate for the first time that WDR11 is localized to the trans-Golgi network (TGN), where it interacts specifically with some, but not all, HSV virion components, in addition to ICP0. Knockdown of WDR11 in cultured human cells resulted in a modest but consistent decrease in yields of both wild-type and ICP0-null viruses, in the supernatant and cell-associated fractions, without affecting viral gene expression. Although further study is required, we propose that WDR11 participates in viral assembly and/or secondary envelopment.

IMPORTANCE

While the TGN has been proposed to be the major site of HSV-1 secondary envelopment, this process is incompletely understood, and in particular, the role of cellular TGN components in this pathway is unknown. Additionally, little is known about the cellular functions of WDR11, although the disruption of this protein has been implicated in multiple human diseases. Therefore, our finding that WDR11 is a TGN-resident protein that interacts with specific viral proteins to enhance viral yields improves both our understanding of basic cellular biology as well as how this protein is co-opted by HSV.

Differential expression of Marek's disease virus (MDV) late proteins during in vitro and in situ replication: role for pUL47 in regulation of the MDV UL46-UL49 gene locus

Marek's disease virus (MDV) is a lymphotropic alphaherpesvirus that replicates in a highly cell-associated manner in vitro. Production of infectious cell-free virus only occurs in feather follicle epithelial (FFE) cells of infected chicken skins. Previously, we described differential expression for a core alphaherpesvirus protein, pUL47 that was found to be abundantly expressed in FFE cells of infected chickens, while barely detectable during in vitro propagation. Here, we further examined the dynamics of expression of four tegument proteins within the UL46-49 locus during in vitro and in situ replication. All four proteins examined were expressed abundantly in situ, whereas both pUL47 and pUL48 expression were barely detectable in vitro. Replacement of the putative UL47 and UL48 promoters with the minimal cytomegalovirus promoter enhanced mRNA and protein expression in vitro. Interestingly, enhanced expression of pUL47 resulted in increased UL46, UL48, and UL49 transcripts that resulted in increased pUL46 and pUL48 expression.

Structural analysis of herpes simplex virus by optical super-resolution imaging

Herpes simplex virus type-1 (HSV-1) is one of the most widespread pathogens among humans. Although the structure of HSV-1 has been extensively investigated, the precise organization of tegument and envelope proteins remains elusive. Here we use super-resolution imaging by direct stochastic optical reconstruction microscopy (dSTORM) in combination with a model-based analysis of single-molecule localization data, to determine the position of protein layers within virus particles. We resolve different protein layers within individual HSV-1 particles using multi-colour dSTORM imaging and discriminate envelope-anchored glycoproteins from tegument proteins, both in purified virions and in virions present in infected cells. Precise characterization of HSV-1 structure was achieved by particle averaging of purified viruses and model-based analysis of the radial distribution of the tegument proteins VP16, VP1/2 and pUL37, and envelope protein gD. From this data, we propose a model of the protein organization inside the tegument.

Antagonistic determinants controlling replicative and latent states of human cytomegalovirus infection

The mechanisms by which viruses persist and particularly those by which viruses actively contribute to their own latency have been elusive. Here we report the existence of opposing functions encoded by genes within a polycistronic locus of the human cytomegalovirus (HCMV) genome that regulate cell type-dependent viral fates: replication and latency. The locus, referred to as the UL133-UL138 (UL133/8) locus, encodes four proteins, pUL133, pUL135, pUL136, and pUL138. As part of the ULb' region of the genome, the UL133/8 locus is lost upon serial passage of clinical strains of HCMV in cultured fibroblasts and is therefore considered dispensable for replication in this context. Strikingly, we could not reconstitute infection in permissive fibroblasts from bacterial artificial chromosome clones of the HCMV genome where UL135 alone was disrupted. The loss of UL135 resulted in complex phenotypes and could ultimately be overcome by infection at high multiplicities. The requirement for UL135 but not the entire locus led us to hypothesize that another gene in this locus suppressed virus replication in the absence of UL135. The defect associated with the loss of UL135 was largely rescued by the additional disruption of the UL138 latency determinant, indicating a requirement for UL135 for virus replication when UL138 is expressed. In the CD34(+) hematopoietic progenitor model of latency, viruses lacking only UL135 were defective for viral genome amplification and reactivation. Taken together, these data indicate that UL135 and UL138 comprise a molecular switch whereby UL135 is required to overcome UL138-mediated suppression of virus replication to balance states of latency and reactivation.

IMPORTANCE

Mechanisms by which viruses persist in their host remain one of the most poorly understood phenomena in virology. Herpesviruses, including HCMV, persist in an incurable, latent state that has profound implications for immunocompromised individuals, including transplant patients. Further, the latent coexistence of HCMV may increase the risk of age-related pathologies, including vascular disease. The key to controlling or eradicating HCMV lies in understanding the molecular basis for latency. In this work, we describe the complex interplay between two viral proteins, pUL135 and pUL138, which antagonize one another in infection to promote viral replication or latency, respectively. We previously described the role of pUL138 in suppressing virus replication for latency. Here we demonstrate a role of pUL135 in overcoming pUL138-mediated suppression for viral reactivation. From this work, we propose that pUL135 and pUL138 constitute a molecular switch balancing states of latency and reactivation.

Nelfinavir inhibits maturation and export of herpes simplex virus 1

Nelfinavir (NFV) is an HIV-1 protease inhibitor with demonstrated antiviral activity against herpes simplex virus 1 (HSV-1) and several other herpesviruses. However, the stages of HSV-1 replication inhibited by NFV have not been explored. In this study, we investigated the effects of NFV on capsid assembly and envelopment. We confirmed the inhibitory effects of NFV on HSV-1 replication by plaque assay and found that treatment with NFV did not affect capsid assembly, activity of the HSV-1 maturational protease, or formation of DNA-containing capsids in the nucleus. Confocal and electron microscopy studies showed that these capsids were transported to the cytoplasm but failed to complete secondary envelopment and subsequent exit from the cell. Consistent with the microscopy results, a light-scattering band corresponding to enveloped virions was not evident following sucrose gradient rate-velocity separation of lysates from drug-treated cells. Evidence of a possibly related effect of NFV on viral glycoprotein maturation was also discovered. NFV also inhibited the replication of an HSV-1 thymidine kinase mutant resistant to nucleoside analogues such as acyclovir. Given that NFV is neither a nucleoside mimic nor a known inhibitor of nucleic acid synthesis, this was expected and suggests its potential as a coinhibitor or alternate antiviral therapeutic agent in cases of resistance.

IMPORTANCE

Nelfinavir (NFV) is a clinically important antiviral drug that inhibits production of infectious HIV. It was reported to inhibit herpesviruses in cell culture. Herpes simplex virus 1 (HSV-1) infections are common and often associated with several diseases. The studies we describe here confirm and extend earlier findings by investigating how NFV interferes with HSV-1 replication. We show that early steps in virus formation (e.g., assembly of DNA-containing capsids in the nucleus and their movement into the cytoplasm) appear to be unaffected by NFV, whereas later steps (e.g., final envelopment in the cytoplasm and release of infectious virus from the cell) are severely restricted by the drug. Our findings provide the first insight into how NFV inhibits HSV-1 replication and suggest that this drug may have applications for studying the herpesvirus envelopment process. Additionally, NFV may have therapeutic value alone or in combination with other antivirals in treating herpesvirus infections.

Elucidation of the block to herpes simplex virus egress in the absence of tegument protein UL16 reveals a novel interaction with VP22

UL16 is a tegument protein of herpes simplex virus (HSV) that is conserved among all members of the Herpesviridae, but its function is poorly understood. Previous studies revealed that UL16 is associated with capsids in the cytoplasm and interacts with the membrane protein UL11, which suggested a "bridging" function during cytoplasmic envelopment, but this conjecture has not been tested. To gain further insight, cells infected with UL16-null mutants were examined by electron microscopy. No defects in the transport of capsids to cytoplasmic membranes were observed, but the wrapping of capsids with membranes was delayed. Moreover, clusters of cytoplasmic capsids were often observed, but only near membranes, where they were wrapped to produce multiple capsids within a single envelope. Normal virion production was restored when UL16 was expressed either by complementing cells or from a novel position in the HSV genome. When the composition of the UL16-null viruses was analyzed, a reduction in the packaging of glycoprotein E (gE) was observed, which was not surprising, since it has been reported that UL16 interacts with this glycoprotein. However, levels of the tegument protein VP22 were also dramatically reduced in virions, even though this gE-binding protein has been shown not to depend on its membrane partner for packaging. Cotransfection experiments revealed that UL16 and VP22 can interact in the absence of other viral proteins. These results extend the UL16 interaction network beyond its previously identified binding partners to include VP22 and provide evidence that UL16 plays an important function at the membrane during virion production.

Mode of virus rescue determines the acquisition of VHS mutations in VP22-negative herpes simplex virus 1

It has been proposed that herpes simplex virus 1 with VP22 deleted requires secondary mutation of VHS for viability. Here we show that a replication-competent Δ22 virus constructed by homologous recombination maintains a wild-type (Wt) VHS gene and has no other gross mutations. By contrast, Δ22 viruses recovered from a bacterial artificial chromosome contain multiple amino acid changes within a conserved region of VHS. Hence, the mode of virus rescue influences the acquisition of secondary mutations.

Herpes simplex virus 1-encoded tegument protein VP16 abrogates the production of beta interferon (IFN) by inhibiting NF-κB activation and blocking IFN regulatory factor 3 to recruit its coactivator CBP

Host cells activate innate immune signaling pathways to defend against invading pathogens. To survive within an infected host, viruses have evolved intricate strategies to counteract host immune responses. Herpesviruses, including herpes simplex virus type 1 (HSV-1), have large genomes and therefore have the capacity to encode numerous proteins that modulate host innate immune responses. Here we define the contribution of HSV-1 tegument protein VP16 in the inhibition of beta interferon (IFN-β) production. VP16 was demonstrated to significantly inhibit Sendai virus (SeV)-induced IFN-β production, and its transcriptional activation domain was not responsible for this inhibition activity. Additionally, VP16 blocked the activation of the NF-κB promoter induced by SeV or tumor necrosis factor alpha treatment and expression of NF-κB-dependent genes through interaction with p65. Coexpression analysis revealed that VP16 selectively blocked IFN regulatory factor 3 (IRF-3)-mediated but not IRF-7-mediated transactivation. VP16 was able to bind to IRF-3 but not IRF-7 in vivo, based on coimmunoprecipitation analysis, but it did not affect IRF-3 dimerization, nuclear translocation, or DNA binding activity. Rather, VP16 interacted with the CREB binding protein (CBP) coactivator and efficiently inhibited the formation of the transcriptional complexes IRF-3-CBP in the context of HSV-1 infection. These results illustrate that VP16 is able to block the production of IFN-β by inhibiting NF-κB activation and interfering with IRF-3 to recruit its coactivator CBP, which may be important to the early events leading to HSV-1 infection.

Marek's disease virus expresses multiple UL44 (gC) variants through mRNA splicing that are all required for efficient horizontal transmission

Marek's disease (MD) is a devastating oncogenic viral disease of chickens caused by Gallid herpesvirus 2, or MD virus (MDV). MDV glycoprotein C (gC) is encoded by the alphaherpesvirus UL44 homolog and is essential for the horizontal transmission of MDV (K. W. Jarosinski and N. Osterrieder, J. Virol. 84:7911-7916, 2010). Alphaherpesvirus gC proteins are type 1 membrane proteins and are generally anchored in cellular membranes and the virion envelope by a short transmembrane domain. However, the majority of MDV gC is secreted in vitro, although secondary-structure analyses predict a carboxy-terminal transmembrane domain. In this report, two alternative mRNA splice variants were identified by reverse transcription (RT)-PCR analyses, and the encoded proteins were predicted to specify premature stop codons that would lead to gC proteins that lack the transmembrane domain. Based on the size of the intron removed for each UL44 (gC) transcript, they were termed gC104 and gC145. Recombinant MDV viruses were generated in which only full-length viral gC (vgCfull), gC104 (vgC104), or gC145 (vgC145) was expressed. Predictably, gCfull was expressed predominantly as a membrane-associated protein, while both gC104 and gC145 were secreted, suggesting that the dominant gC variants expressed in vitro are the spliced variants. In experimentally infected chickens, the expression of each of the gC variants individually did not alter replication or disease induction. However, horizontal transmission was reduced compared to that of wild-type or revertant viruses when the expression of only a single gC was allowed, indicating that all three forms of gC are required for the efficient transmission of MDV in chickens.

Probing of the nuclear import and export signals and subcellular transport mechanism of varicella-zoster virus tegument protein open reading frame 10

Varicella-zoster virus open reading frame 10 (ORF10), a tegument protein present in the virion, is a member of the alphaherpesvirus UL48 gene family that shares considerable amino acid sequence homology with the UL48 prototype, herpes simplex virus type 1 VP16. VP16 serves multiple functions, including transcriptional activation of viral immediate-early genes. Furthermore, VP16 has been shown to be involved in some aspects of virus assembly and/or maturation. However, little is known concerning the function of ORF10. Here, we found that transient expression of ORF10 fused to enhanced yellow fluorescent protein (EYFP) in COS-7 cells showed the predominantly nuclear localization in the absence of other viral proteins. By constructing a series of ORF10 variants fused to EYFP, a bona fide bipartite nuclear localization signal of ORF10 was, for the first time, determined and mapped to amino acids (aa) 302-347. Additionally, the nuclear export signal (NES) was identified and found to be in a leucine-rich region at aa 226-236. Finally, ORF10 was demonstrated to be targeted to the cytoplasm through the functional NES by chromosomal region maintenance 1-dependent pathway, and to the nucleus via Ran and importin β1-dependent pathway that does not require importin α5.

Analysis of the interaction between the essential herpes simplex virus 1 tegument proteins VP16 and VP1/2

The incorporation of tegument proteins into the herpes simplex virus 1 (HSV-1) virion during virion assembly is thought to be a complex, multistage process occurring via numerous interactions between the tegument and the capsid, within the tegument, and between the tegument and the envelope. Here, we set out to examine if the direct interaction between two essential tegument proteins VP1/2 and VP16 is required for connecting the inner tegument with the outer tegument. By using glutathione S-transferase (GST) pulldowns, we identified an essential role of lysine 343 in VP16, mutation of which to a neutral amino acid abrogated the interaction between VP1/2 and VP16. When the K343A substitution was inserted into the gene encoding VP16 (UL48) of the viral genome, HSV-1 replicated successfully although its growth was delayed, and final titers were reduced compared to titers of wild-type virus. Surprisingly, the mutated VP16 was incorporated into virions at levels similar to those of wild-type VP16. However, the analysis of VP16 on cytoplasmic capsids by fluorescence microscopy showed that VP16 associated with cytoplasmic capsids less efficiently when the VP16-VP1/2 interaction was inhibited. This implies that the direct interaction between VP1/2 and VP16 is important for the efficiency/timing of viral assembly but is not essential for HSV-1 replication in cell culture. These data also support the notion that the incorporation of tegument proteins into the herpesviruses is a very complex process with significant redundancy.

VP16 serine 375 is a critical determinant of herpes simplex virus exit from latency in vivo

Development of novel prevention and treatment strategies for herpes simplex virus (HSV) mediated diseases is dependent upon an accurate understanding of the central molecular events underlying the regulation of latency and reactivation. We have recently shown that the transactivation function of the virion protein VP16 is a critical determinant in the exit from latency in vivo. HSV-1 strain SJO2 carries a single serine to alanine substitution at position 375 in VP16 which disrupts its interaction with its essential co-activator Oct-1. Here we report that SJO2 is severely impaired in its ability to exit latency in vivo. This result reinforces our prior observations with VP16 transactivation mutant, in1814, in which VP16 interaction with Oct-1 is also disrupted and solidifies the importance of the VP16-Oct-1 interaction in the early steps in HSV-1 reactivation.

Small interfering RNA targeting for infected-cell polypeptide 4 inhibits herpes simplex virus type 1 replication in retinal pigment epithelial cells

Background:  This study sought to inhibit herpes simplex virus type 1 replication using small interfering RNA which targeting infected‐cell polypeptide 4 genes to mediate transcription of early and late viral genes in herpes simplex virus type 1 lytic (productive) infection in retina epithelial cells.

Methods:  After pre‐ or post‐infecting with herpes simplex virus type 1, small interfering RNAs were transfected into retina epithelial cells. The antiviral effects of small interfering RNA were evaluated by Western blot, plaque assays, indirect immunofluorescence and reverse transcription polymerase chain reaction. The viral titre was detected by the 50% tissue culture infective dose method.

Results:  Small interfering RNA decreased infected‐cell polypeptide 4 expression in retina epithelial cells that were infected with herpes simplex virus type 1 before or after small interfering RNA transfection. Compared with herpes simplex virus type 1 infection alone or transfection with negative control small interfering RNA, the viral titre and the retina epithelial cell cytopathic effect were significantly decreased in retina epithelial cells transfected with infected‐cell polypeptide 4‐targeting small interfering RNA (50 and 100 nM) (P < 0.05). The small interfering RNA effectively silenced herpes simplex virus type 1 infected‐cell polypeptide 4 expression on both mRNA and the protein levels (P < 0.05). The inhibition of infected‐cell polypeptide 4‐targeting small interfering RNA on infected‐cell polypeptide 4 protein expression was also verified by Western blot in herpes simplex virus type 1 infected human cornea epithelial cell, human trabecular meshwork cells and Vero cells.

Conclusions:  Infected‐cell polypeptide 4‐targeting small interfering RNA can inhibit herpes simplex virus type 1 replication in retina epithelial cells, providing a foundation for development of RNA interference as an antiviral therapy.

Immediate-early protein ME53 forms foci and colocalizes with GP64 and the major capsid protein VP39 at the cell membranes of Autographa californica multiple nucleopolyhedrovirus-infected cells

me53 is an immediate-early/late gene found in all lepidopteran baculoviruses sequenced to date. Deletion of me53 results in a greater-than-1,000-fold reduction in budded-virus production in tissue culture (J. de Jong, B. M. Arif, D. A. Theilmann, and P. J. Krell, J. Virol. 83:7440-7448, 2009). We investigated the localization of ME53 using an ME53 construct fused to green fluorescent protein (GFP). ME53:GFP adopted a primarily cytoplasmic distribution at early times postinfection and a primarily nuclear distribution at late times postinfection. Additionally, at late times ME53:GFP formed distinct foci at the cell periphery. These foci colocalized with the major envelope fusion protein GP64 and frequently with VP39 capsid protein, suggesting that these cell membrane regions may represent viral budding sites. Deletion of vp39 did not influence the distribution of ME53:GFP; however, deletion of gp64 abolished ME53:GFP foci at the cell periphery, implying an association between ME53 and GP64. Despite the association of ME53 and GP64, ME53 fractionated with the nucleocapsid only after budded-virus fractionation. Together these findings suggest that ME53 may be providing a scaffold that bridges the viral envelope and nucleocapsid.

Herpesviruses remodel host membranes for virus egress

Herpesviruses replicate their DNA and package this DNA into capsids in the nucleus. These capsids then face substantial obstacles to their release from cells. Unlike other DNA viruses, herpesviruses do not depend on disruption of nuclear and cytoplasmic membranes for their release. Enveloped particles are formed by budding through inner nuclear membranes, and then these perinuclear enveloped particles fuse with outer nuclear membranes. Unenveloped capsids in the cytoplasm are decorated with tegument proteins and then undergo secondary envelopment by budding into trans-Golgi network membranes, producing infectious particles that are released. In this Review, we describe the remodelling of host membranes that facilitates herpesvirus egress.

Role of tegument proteins in herpesvirus assembly and egress

Morphogenesis and maturation of viral particles is an essential step of viral replication. An infectious herpesviral particle has a multilayered architecture, and contains a large DNA genome, a capsid shell, a tegument and an envelope spiked with glycoproteins. Unique to herpesviruses, tegument is a structure that occupies the space between the nucleocapsid and the envelope and contains many virus encoded proteins called tegument proteins. Historically the tegument has been described as an amorphous structure, but increasing evidence supports the notion that there is an ordered addition of tegument during virion assembly, which is consistent with the important roles of tegument proteins in the assembly and egress of herpesviral particles. In this review we first give an overview of the herpesvirus assembly and egress process. We then discuss the roles of selected tegument proteins in each step of the process, i.e., primary envelopment, de-envelopment, secondary envelopment and transport of viral particles. We also suggest key issues that should be addressed in the near future.

Herpes simplex virus UL56 interacts with and regulates the Nedd4-family ubiquitin ligase Itch

Background

Herpes simplex virus type 2 (HSV-2) is one of many viruses that exploits and modifies the cellular ubiquitin system. HSV-2 expresses the tegument protein UL56 that has been implicated in cytoplasmic transport and/or release of virions, and is a putative regulatory protein of Nedd4 ubiquitin ligase. In order to elucidate the biological function of UL56, this study examined the interaction of UL56 with the Nedd4-family ubiquitin ligase Itch and its role in the regulation of Itch. Additionally, we assessed the similarity between UL56 and regulatory proteins of Itch and Nedd4, Nedd4-family-interactins proteins (Ndfip).

Results

UL56 interacted with Itch, independent of additional viral proteins, and mediated more striking degradation of Itch, compared to Nedd4. Moreover, it was suggested that the lysosome pathway as well as the proteasome pathway was involved in the degradation of Itch. Other HSV-2 proteins with PY motifs, such as VP5 and VP16, did not mediate the degradation of endogenous Itch. Ndfip1 and Ndfip2 were similar in subcellular distribution patterns to UL56 and colocalized with UL56 in co-transfected cells.

Conclusions

We believe that this is the first report demonstrating the interaction of a HSV-specific protein and Itch. Thus, UL56 could function as a regulatory protein of Itch. The mechanism, function and significance of regulating Itch in HSV-2 infection remain unclear and warrant further investigation.

Inner tegument protein pUL37 of herpes simplex virus type 1 is involved in directing capsids to the trans-Golgi network for envelopment

Secondary envelopment of herpes simplex virus type 1 has been demonstrated as taking place at the trans-Golgi network (TGN). The inner tegument proteins pUL36 and pUL37 and the envelope glycoproteins gD and gE are known to be important for secondary envelopment. We compared the cellular localizations of capsids from a virus mutant lacking the UL37 gene with those of a virus mutant lacking the genes encoding gD and gE. Although wild-type capsids accumulated at the TGN, capsids of the pUL37⁻ mutant were distributed throughout the cytoplasm and showed no association with TGN-derived vesicles. This was in contrast to capsids from a gD⁻gE⁻ mutant, which accumulated in the vicinity of TGN vesicles, but did not colocalize with them, suggesting that they were transported to the TGN but were unable to undergo envelopment. We conclude that the inner tegument protein pUL37 is required for directing capsids to the TGN, where secondary envelopment occurs.

The major determinant for addition of tegument protein pUL48 (VP16) to capsids in herpes simplex virus type 1 is the presence of the major tegument protein pUL36 (VP1/2)

In this study a number of herpes simplex virus type 1 (HSV-1) proteins were screened, using a yeast-two-hybrid assay, for interaction with the tegument protein pUL48 (VP16). This approach identified interactions between pUL48 and the capsid proteins pUL19 (VP5), pUL38 (VP19C), and pUL35 (VP26). In addition, the previously identified interaction of pUL48 with the major tegument protein pUL36 (VP1/2) was confirmed. All of these interactions, except that of pUL35 and pUL48, could be confirmed using an in vitro pulldown assay. A subsequent pulldown assay with intact in vitro-assembled capsids, consisting of pUL19, pUL38, and pUL18 (VP23) with or without pUL35, showed no binding of pUL48, suggesting that the capsid/pUL48 interactions initially identified were more then likely not biologically relevant. This was confirmed by using a series of HSV-1 mutants lacking the gene encoding either pUL35, pUL36, or pUL37. For each HSV-1 mutant, analysis of purified deenveloped C capsids indicated that only in the absence of pUL36 was there a complete loss of capsid-bound pUL48, as well as pUL37. Collectively, this study shows for the first time that pUL36 is a major factor for addition of both pUL48 and pUL37, likely through a direct interaction of both with nonoverlapping sites in pUL36, to unenveloped C capsids during assembly of HSV-1.