Herpes simplex virus gE/gI sorts nascent virions to epithelial cell junctions, promoting virus spread

The C-terminus of varicella-zoster virus glycoprotein M contains trafficking motifs that mediate skin virulence in the SCID-human model of VZV pathogenesis

Knowledge about the function of varicella-zoster virus glycoprotein M is limited; the requirement of gM for skin and neural tropism are unknown. VZV gM contains two predicted YXXΦ trafficking motifs and a dileucine motif in the carboxyl-terminus. We constructed a recombinant VZV with gM truncated from the first YXXΦ and five additional viruses with YXXΦ tyrosine substitutions, alone and in combination with dileucine substitution. All recombinant viruses grew to high titer but mutation of the membrane-proximal YXXΦ motif reduced plaque size in cultured cells and altered gM localization. C-terminus truncation had a pronounced effect on virion morphogenesis and plaque size, but not on overall replication kinetics in vitro. Mutation of gM trafficking motifs and truncation attenuated replication in human skin xenografts in vivo; gM truncation did not alter neurotropism. Our results demonstrate that the gM C-terminus is dispensable for virus replication in cultured cells but is important for skin pathogenesis.

The HSV-1 mechanisms of cell-to-cell spread and fusion are critically dependent on host PTP1B

All herpesviruses have mechanisms for passing through cell junctions, which exclude neutralizing antibodies and offer a clear path to neighboring, uninfected cells. In the case of herpes simplex virus type 1 (HSV-1), direct cell-to-cell transmission takes place between epithelial cells and sensory neurons, where latency is established. The spreading mechanism is poorly understood, but mutations in four different HSV-1 genes can dysregulate it, causing neighboring cells to fuse to produce syncytia. Because the host proteins involved are largely unknown (other than the virus entry receptor), we were intrigued by an earlier discovery that cells infected with wild-type HSV-1 will form syncytia when treated with salubrinal. A biotinylated derivative of this drug was used to pull down cellular complexes, which were analyzed by mass spectrometry. One candidate was a protein tyrosine phosphatase (PTP1B), and although it ultimately proved not to be the target of salubrinal, it was found to be critical for the mechanism of cell-to-cell spread. In particular, a highly specific inhibitor of PTP1B (CAS 765317-72-4) blocked salubrinal-induced fusion, and by itself resulted in a dramatic reduction in the ability of HSV-1 to spread in the presence of neutralizing antibodies. The importance of this phosphatase was confirmed in the absence of drugs by using PTP1B^-/- cells. Importantly, replication assays showed that virus titers were unaffected when PTP1B was inhibited or absent. Only cell-to-cell spread was altered. We also examined the effects of salubrinal and the PTP1B inhibitor on the four Syn mutants of HSV-1, and strikingly different responses were found. That is, both drugs individually enhanced fusion for some mutants and reduced fusion for others. PTP1B is the first host factor identified to be specifically required for cell-to-cell spread, and it may be a therapeutic target for preventing HSV-1 reactivation disease.

It is estimated that 67% of the global population is infected with herpes simplex virus type 1 (HSV-1). This virus resides in sensory neurons in a quiescent state but periodically reactivates, producing virus particles that travel down the axon to infect epithelial cells of the skin, where it can be transmitted to additional people. To avoid neutralizing antibodies, herpesviruses have evolved mechanisms for moving directly from one cell to another through their sites of intimate contact; however, the mechanism of cell-to-cell spread is poorly understood. Studies of HSV-1 mutants have implicated numerous viral proteins, but the necessary cellular factors are unknown except for the one that the virus uses to enter cells. Our experiments have identified a cellular enzyme (PTP1B, a tyrosine phosphatase) that is dispensable for the production of infectious virions but is critically important for the cell-to-cell spreading mechanism. Promising drugs targeting PTP1B have already been tested in early clinical trials for possible treatment of obesity and type-2 diabetes, and thus, our study may have immediate utility for attenuating HSV-1 reactivation disease in immunocompromised patients.

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.

Extended Synaptotagmin 1 Interacts with Herpes Simplex Virus 1 Glycoprotein M and Negatively Modulates Virus-Induced Membrane Fusion

Enveloped viruses typically encode their own fusion machinery to enter cells. Herpesviruses are unusual, as they fuse with a number of cellular compartments throughout their life cycles. As uncontrolled fusion of the host membranes should be avoided in these events, tight regulation of the viral fusion machinery is critical. While studying herpes simplex virus 1 (HSV-1) glycoprotein gM, we identified the cellular protein E-Syt1 (extended synaptotagmin 1) as an interaction partner. The interaction took place in both infected and transfected cells, suggesting other viral proteins were not required for the interaction. Most interestingly, E-Syt1 is a member of the synaptotagmin family of membrane fusion regulators. However, the protein is known to promote the tethering of the endoplasmic reticulum (ER) to the plasma membrane. We now show that E-Syt1, along with the related E-Syt3, negatively modulates viral release into the extracellular milieu, cell-to-cell viral spread, and viral entry, all processes that implicate membrane fusion events. Similarly, these E-Syt proteins impacted the formation of virus-induced syncytia. Altogether, these findings hint at the modulation of the viral fusion machinery by the E-Syt family of proteins.IMPORTANCE Viruses typically encode their own fusion apparatus to enable them to enter cells. For many viruses, this means a single fusogenic protein. However, herpesviruses are large entities that express several accessory viral proteins to regulate their fusogenic activity. The present study hints at the additional participation of cellular proteins in this process, suggesting the host can also modulate viral fusion to some extent. Hence E-Syt proteins 1 and 3 seem to negatively modulate the different viral fusion events that take place during the HSV-1 life cycle. This could represent yet another innate immunity response to the virus.

In Vitro Replication of Chelonid Herpesvirus 5 in Organotypic Skin Cultures from Hawaiian Green Turtles (Chelonia mydas)

Fibropapillomatosis (FP) is a tumor disease of marine turtles associated with chelonid herpesvirus 5 (ChHV5), which has historically been refractory to growth in tissue culture. Here we show, for the first time, de novo formation of ChHV5-positive intranuclear inclusions in cultured green turtle cells, which is indicative of active lytic replication of the virus. The minimal requirements to achieve lytic replication in cultured cells included (i) either in vitro cultures of ChHV5-positive tumor biopsy specimens (plugs) or organotypic cultures (rafts) consisting of ChHV5-positive turtle fibroblasts in collagen rafts seeded with turtle keratinocytes and (ii) keratinocyte maturation induced by raising raft or biopsy cultures to the air-liquid interface. Virus growth was confirmed by detailed electron microscopic studies that revealed intranuclear sun-shaped capsid factories, tubules, various stages of capsid formation, nuclear export by budding into the perinuclear space, tegument formation, and envelopment to complete de novo virus production. Membrane synthesis was also observed as a sign of active viral replication. Interestingly, cytoplasmic particles became associated with keratin filaments, a feature not seen in conventional monolayer cell cultures, in which most studies of herpesvirus replication have been performed. Our findings draw a rich and realistic picture of ChHV5 replication in cells derived from its natural host and may be crucial not only to better understand ChHV5 circulation but also to eventually complete Koch's postulates for FP. Moreover, the principles described here may serve as a model for culture of other viruses that are resistant to replication in conventional cell culture.IMPORTANCE A major challenge in virology is the study of viruses that cannot be grown in the laboratory. One example is chelonid herpesvirus 5 (ChHV5), which is associated with fibropapillomatosis, a globally distributed, debilitating, and fatal tumor disease of endangered marine turtles. Pathological examination shows that ChHV5 is shed in skin. Here we show that ChHV5 will grow in vitro if we replicate the complex three-dimensional structure of turtle skin. Moreover, lytic virus growth requires a close interplay between fibroblasts and keratinocytes. Finally, the morphogenesis of herpesviral growth in three-dimensional cultures reveals a far richer, and likely more realistic, array of capsid morphologies than that encountered in traditional monolayer cell cultures. Our findings have applications to other viruses, including those of humans.

Herpes Simplex Virus gE/gI and US9 Promote both Envelopment and Sorting of Virus Particles in the Cytoplasm of Neurons, Two Processes That Precede Anterograde Transport in Axons

Herpes simplex virus (HSV) anterograde transport in neuronal axons is vital, allowing spread from latently infected ganglia to epithelial tissues, where viral progeny are produced in numbers allowing spread to other hosts. The HSV membrane proteins gE/gI and US9 initiate the process of anterograde axonal transport, ensuring that virus particles are transported from the cytoplasm into the most proximal segments of axons. These proteins do not appear to be important once HSV is inside axons. We previously described HSV double mutants lacking both gE and US9 that failed to transport virus particles into axons. Here we show that gE^- US9^- double mutants accumulate large quantities of unenveloped and partially enveloped capsids in neuronal cytoplasm. These defects in envelopment can explain the defects in axonal transport of enveloped virions. In addition, the unenveloped capsids that accumulated were frequently bound to cytoplasmic membranes, apparently immobilized in intermediate stages of envelopment. A gE-null mutant produced enveloped virions, but these accumulated in large numbers in the neuronal cytoplasm rather than reaching cell surfaces as wild-type HSV virions do. Thus, in addition to the defects in envelopment, there was missorting of capsids and enveloped particles in the neuronal cytoplasm, which can explain the reduced anterograde transport of unenveloped capsids and enveloped virions. These mechanisms differ substantially from existing models suggesting that gE/gI and US9 function by tethering HSV particles to kinesin microtubule motors. The defects in assembly of gE^- US9^- mutant virus particles were novel because they were neuron specific, in keeping with observations that US9 is neuron specific.IMPORTANCE Herpes simplex virus (HSV) and other alphaherpesviruses, such as varicella-zoster virus, depend upon the capacity to navigate in neuronal axons. To do this, virus particles tether themselves to dyneins and kinesins that motor along microtubules from axon tips to neuronal cell bodies (retrograde transport) or from cell bodies to axon tips (anterograde transport). This transit in axons is essential for alphaherpesviruses to establish latency in ganglia and then to reactivate and move back to peripheral tissues for spread to other hosts. Anterograde transport of HSV requires two membrane proteins: gE/gI and US9. Our studies reveal new mechanisms for how gE/gI and US9 initiate anterograde axonal transport. HSV mutants lacking both gE and US9 fail to properly assemble enveloped virus particles in the cytoplasm, which blocks anterograde transport of enveloped particles. In addition, there are defects in the sorting of virus particles such that particles, when formed, do not enter proximal axons.

Deletion of the us7 and us8 genes of pseudorabies virus exerts a differential effect on the expression of early and late viral genes

The pseudorabies virus (PRV; also known as Suid herpesvirus-1) is a neurotropic herpesvirus of swine. The us7 and us8 genes of this virus encode the glycoprotein I and E membrane proteins that form a heterodimer that is known to control cell-to-cell spread in tissue culture and in animals. In this study, we investigated the effect of the deletion of the PRV us7 and us8 genes on the genome-wide transcription and DNA replication using a multi-time-point quantitative reverse transcriptase-based real-time PCR technique. Abrogation of the us7/8 gene function was found to exert a drastic but differential effect on the expression of PRV genes during lytic infection. In the mutant virus, all kinetic classes of viral genes were significantly down-regulated at the first 6 h of infection, while having been upregulated later. The level of upregulation was the highest in the immediate-early (IE) and the early (E) genes; lower in the early-late (E/L) genes; and the lowest in the late (L) genes. The relative contribution of the L transcripts to the global transcriptome became lower, while the rest of the transcripts were expressed at a higher level in the mutant than in the wild-type virus.

The US3 Protein of Pseudorabies Virus Drives Viral Passage across the Basement Membrane in Porcine Respiratory Mucosa Explants

Passage of the basement membrane (BM), which forms a barrier between the epithelium and the underlying lamina propria, represents an important step in the early pathogenesis of different alphaherpesviruses. Rho GTPase signaling plays an important role in transmigration of cells across the BM during physiological and pathological processes. We reported earlier that the US3 protein kinase of the alphaherpesvirus pseudorabies virus (PRV) interferes with Rho GTPase signaling and causes a reorganization of the host cell cytoskeleton, which as a consequence, enhances viral cell-to-cell spread in epithelial cell cultures. Here, using an ex vivo system of porcine nasal respiratory mucosa explants that allows to study PRV invasion through the BM, we found that a PRV strain that lacks US3 expression (ΔUS3 PRV) showed a reduced spread in mucosal epithelium and was virtually unable to breach the BM, in contrast to isogenic wild-type (WT) or US3 rescue PRV strains. Interestingly, addition of IPA3, an inhibitor of p21-activated kinases that blocks the effects of US3 on the cytoskeleton, suppressed the ability of WT PRV to spread across the BM. In addition, artificial suppression of RhoA signaling using CPC3 (cell-permeable C3 transferase) to mimic the effects of US3 on Rho GTPase signaling, significantly increased passage of ΔUS3 PRV through the BM, whereas it did not significantly affect BM passage of WT or US3 rescue PRV. In conclusion, these data indicate that US3 plays an important role in PRV mucosal invasion across the BM, which involves its interference with Rho GTPase signaling. This is the first report describing an alphaherpesvirus protein that drives viral BM passage.

IMPORTANCE

Many viruses, including alphaherpesviruses, primarily replicate in epithelial cells of surface mucosae, such as the respiratory mucosa. Some of these viruses breach the basement membrane underlying these epithelial cells to reach underlying connective tissue and blood vessels and invade the host. Hence, epithelial spread and basement membrane passage represent crucial but still poorly understood early steps in (alphaherpes)virus pathogenesis. Here, using ex vivo porcine respiratory mucosa explants, we show that the conserved US3 protein of the porcine alphaherpesvirus pseudorabies virus (PRV) is critical for passage of PRV across the basement membrane and contributes to efficient viral epithelial spread. In addition, we show that US3-mediated viral epithelial spread and passage across the basement membrane depend at least in part on the ability of this viral protein to modulate cellular Rho GTPase signaling. This is the first report that identifies an alphaherpesvirus protein that drives viral basement membrane passage.

Novel therapeutic investigational strategies to treat severe and disseminated HSV infections suggested by a deeper understanding of in vitro virus entry processes

The global burden of herpes simplex virus (HSV) legitimates the critical need to develop new prevention strategies, such as drugs and vaccines that are able to fight either primary HSV infections or reactivations. Moreover, the ever-growing number of patients receiving transplants increases the number of severe HSV infections that are unresponsive to current therapies. Finally, the high global incidence of genital HSV-2 infection increases the risk of perinatal transmission to newborns, in which disseminated infection or central nervous system (CNS) involvement is frequent, with associated high morbidity and mortality rates. There are several key features shared by novel anti-HSV drugs, from currently available optimized drugs to small molecules able to interfere with various virus replication steps. However, several virological aspects of the disease and associated clinical needs highlight why an ideal anti-HSV drug has yet to be developed.

Anti-herpetic and anti-dengue activity of abietane ferruginol analogues synthesized from (+)-dehydroabietylamine

The abietane-type diterpenoid (+)-ferruginol (1), a bioactive compound isolated from several plants, has attracted much attention as consequence of its pharmacological properties, which includes antibacterial, antifungal, antimicrobial, cardioprotective, anti-oxidative, anti-plasmodial, leishmanicidal, anti-ulcerogenic, anti-inflammatory and antitumor actions. In this study, we report on the antiviral evaluation of ferruginol (1) and several analogues synthesized from commercial (+)-dehydroabietylamine. Thus, the activity against Human Herpesvirus type 1, Human Herpesvirus type 2 and Dengue Virus type 2, was studied. Two ferruginol analogues showed high antiviral selectivity index and reduced viral plaque-size in post-infection stages against both Herpes and Dengue viruses. A promising lead, compound 8, was ten-fold more potent (EC50 = 1.4 μM) than the control ribavirin against Dengue Virus type 2. Our findings suggest that the 12-hydroxyabieta-8,11,13-triene skeleton, which is characteristic of the diterpenoid ferruginol (1), is an interesting molecular scaffold for development of novel antivirals. In addition, the cytotoxic and antifungal activities of the synthesized ferruginol analogues have also been investigated. (©)20155 Elsevier Science. All rights reserved.

CD2v Interacts with Adaptor Protein AP-1 during African Swine Fever Infection

African swine fever virus (ASFV) CD2v protein is believed to be involved in virulence enhancement, viral hemadsorption, and pathogenesis, although the molecular mechanisms of the function of this viral protein are still not fully understood. Here we describe that CD2v localized around viral factories during ASFV infection, suggesting a role in the generation and/or dynamics of these viral structures and hence in disturbing cellular traffic. We show that CD2v targeted the regulatory trans-Golgi network (TGN) protein complex AP-1, a key element in cellular traffic. This interaction was disrupted by brefeldin A even though the location of CD2v around the viral factory remained unchanged. CD2v-AP-1 binding was independent of CD2v glycosylation and occurred on the carboxy-terminal part of CD2v, where a canonical di-Leu motif previously reported to mediate AP-1 binding in eukaryotic cells, was identified. This motif was shown to be functionally interchangeable with the di-Leu motif present in HIV-Nef protein in an AP-1 binding assay. However, we demonstrated that it was not involved either in CD2v cellular distribution or in CD2v-AP-1 binding. Taken together, these findings shed light on CD2v function during ASFV infection by identifying AP-1 as a cellular factor targeted by CD2v and hence elucidate the cellular pathways used by the virus to enhance infectivity.

Evasion of early antiviral responses by herpes simplex viruses

Besides overcoming physical constraints, such as extreme temperatures, reduced humidity, elevated pressure, and natural predators, human pathogens further need to overcome an arsenal of antimicrobial components evolved by the host to limit infection, replication and optimally, reinfection. Herpes simplex virus-1 (HSV-1) and herpes simplex virus-2 (HSV-2) infect humans at a high frequency and persist within the host for life by establishing latency in neurons. To gain access to these cells, herpes simplex viruses (HSVs) must replicate and block immediate host antiviral responses elicited by epithelial cells and innate immune components early after infection. During these processes, infected and noninfected neighboring cells, as well as tissue-resident and patrolling immune cells, will sense viral components and cell-associated danger signals and secrete soluble mediators. While type-I interferons aim at limiting virus spread, cytokines and chemokines will modulate resident and incoming immune cells. In this paper, we discuss recent findings relative to the early steps taking place during HSV infection and replication. Further, we discuss how HSVs evade detection by host cells and the molecular mechanisms evolved by these viruses to circumvent early antiviral mechanisms, ultimately leading to neuron infection and the establishment of latency.

Non-redundant and redundant roles of cytomegalovirus gH/gL complexes in host organ entry and intra-tissue spread

Herpesviruses form different gH/gL virion envelope glycoprotein complexes that serve as entry complexes for mediating viral cell-type tropism in vitro; their roles in vivo, however, remained speculative and can be addressed experimentally only in animal models. For murine cytomegalovirus two alternative gH/gL complexes, gH/gL/gO and gH/gL/MCK-2, have been identified. A limitation of studies on viral tropism in vivo has been the difficulty in distinguishing between infection initiation by viral entry into first-hit target cells and subsequent cell-to-cell spread within tissues. As a new strategy to dissect these two events, we used a gO-transcomplemented ΔgO mutant for providing the gH/gL/gO complex selectively for the initial entry step, while progeny virions lack gO in subsequent rounds of infection. Whereas gH/gL/gO proved to be critical for establishing infection by efficient entry into diverse cell types, including liver macrophages, endothelial cells, and hepatocytes, it was dispensable for intra-tissue spread. Notably, the salivary glands, the source of virus for host-to-host transmission, represent an exception in that entry into virus-producing cells did not strictly depend on either the gH/gL/gO or the gH/gL/MCK-2 complex. Only if both complexes were absent in gO and MCK-2 double-knockout virus, in vivo infection was abolished at all sites.

The herpes simplex virus 1 UL51 protein interacts with the UL7 protein and plays a role in its recruitment into the virion

The alphaherpesvirus UL51 protein is a tegument component that interacts with the viral glycoprotein E and functions at multiple steps in virus assembly and spread in epithelial cells. We show here that pUL51 forms a complex in infected cells with another conserved tegument protein, pUL7. This complex can form in the absence of other viral proteins and is largely responsible for recruitment of pUL7 to cytoplasmic membranes and into the virion tegument. Incomplete colocalization of pUL51 and pUL7 in infected cells, however, suggests that a significant fraction of the population of each protein is not complexed with the other and that they may accomplish independent functions.

IMPORTANCE

The ability of herpesviruses to spread from cell to cell in the face of an immune response is critical for disease and shedding following reactivation from latency. Cell-to-cell spread is a conserved ability of herpesviruses, and the identification of conserved viral genes that mediate this process will aid in the design of attenuated vaccines and of novel therapeutics. The conserved UL51 gene of herpes simplex virus 1 plays important roles in cell-to-cell spread and in virus assembly in the cytoplasm, both of which likely depend on specific interactions with other viral and cellular proteins. Here we identify one of those interactions with the product of another conserved herpesvirus gene, UL7, and show that formation of this complex mediates recruitment of UL7 to membranes and to the virion.

Functional comparison of herpes simplex virus 1 (HSV-1) and HSV-2 ICP27 homologs reveals a role for ICP27 in virion release

Numerous studies have focused on the regulatory functions of ICP27, an immediate-early (IE) protein of herpes simplex virus 1 (HSV-1). However, its homolog in HSV-2, termed ICP27t2, has been little studied. Here, we used two different approaches to functionally compare ICP27t2 and ICP27. In transfection-based assays, ICP27t2 closely resembled ICP27 in its capacity to enhance HSV-1 late gene expression, suppress the splicing of a viral intron, and complement the growth of an HSV-1 ICP27 null mutant. To study ICP27t2 in the context of viral infection, we engineered K2F1, an HSV-1 mutant that encodes ICP27t2 in place of ICP27. In Vero cells, K2F1 replicated with wild-type (WT) kinetics and yields, expressed delayed-early and late proteins normally, and was fully capable of activating several cellular signal transduction pathways that are ICP27 dependent. Thus, we conclude that ICP27t2 and ICP27 are functionally very similar and that ICP27t2 can mediate all ICP27 activities that are required for HSV-1 replication in cell culture. Surprisingly, however, we found that K2F1 forms plaques that are morphologically different from those of WT HSV-1. Investigation of this trait demonstrated that it results from the decreased release of progeny virions into the culture medium. This appears to be due to a reduction in the detachment of K2F1 progeny from the extracellular surface of the infected cell. We identified two HSV-1 ICP27 amino-terminal deletion mutants with a similar release defect. Together, these results demonstrate that ICP27 plays a heretofore-unappreciated role in modulating the efficiency of progeny virion release.

IMPORTANCE

ICP27 is an essential, multifunctional regulatory protein that has a number of critical roles in the HSV-1 life cycle. Although ICP27 homologs are encoded by all known members of the Herpesviridae, previous work with several of these homologs has shown that they cannot substitute for ICP27 in the context of HSV-1-infected cells. Here, we identify ICP27t2 as the first homolog that can efficiently replace ICP27 in HSV-1 infection. Unexpectedly, our results also reveal that the sequence of the ICP27 gene can affect the release of HSV-1 progeny virions from the infected cell. Thus, our comparative study has revealed a novel function for ICP27 in the regulation of virus release.

The unusual fold of herpes simplex virus 1 UL21, a multifunctional tegument protein

UL21 is a conserved protein in the tegument of alphaherpesviruses and has multiple important albeit poorly understood functions in viral replication and pathogenesis. To provide a roadmap for exploration of the multiple roles of UL21, we determined the crystal structure of its conserved N-terminal domain from herpes simplex virus 1 to 2.0-Å resolution, which revealed a novel sail-like protein fold. Evolutionarily conserved surface patches highlight residues of potential importance for future targeting by mutagenesis.

The highly conserved proline at position 438 in pseudorabies virus gH is important for regulation of membrane fusion

Membrane fusion in herpesviruses requires viral glycoproteins (g) gB and gH/gL. While gB is considered the actual fusion protein but is nonfusogenic per se, the function of gH/gL remains enigmatic. Crystal structures for different gH homologs are strikingly similar despite only moderate amino acid sequence conservation. A highly conserved sequence motif comprises the residues serine-proline-cysteine corresponding to positions 437 to 439 in pseudorabies virus (PrV) gH. The PrV-gH structure shows that proline(438) induces bending at the end of an alpha-helix, thereby placing cysteine(404) and cysteine(439) in juxtaposition to allow formation of a strictly conserved disulfide bond. However, PrV vaccine strain Bartha unexpectedly carries a serine at this conserved position. To test the influence of this substitution, we constructed different gH chimeras carrying proline or serine at position 438 in gH derived from either PrV strain Kaplan or strain Bartha. Mutants expressing gH with serine(438) showed reduced fusion activity in transient-fusion assays and during infection, with delayed penetration kinetics and a small-plaque phenotype which indicates that proline(438) is important for efficient fusion. A more drastic effect was observed when disulfide bond formation was completely blocked by mutation of cysteine(404) to serine. Although PrV expressing gHC(404)S was viable, plaque size and penetration kinetics were drastically reduced. Alteration of serine(438) to proline in gH of strain Bartha enhanced cell-to-cell spread and penetration kinetics, but restoration of full activity required additional alteration of aspartic acid to valine at position 59.

IMPORTANCE

The role of the gH/gL complex in herpesvirus membrane fusion is still unclear. Structural studies predicted a critical role for proline(438) in PrV gH to allow the formation of a conserved disulfide bond and correct protein folding. Functional analyses within this study corroborated these structural predictions: mutation of this residue resulted in a drastic impairment of membrane fusion kinetics not only in vitro in transient transfection-fusion assays but also during virus infection. Elimination of formation of the disulfide bond yielded the same phenotype in transient assays but had a more drastic effect on virus replication. Thus, our studies add important information to structure-function analyses of herpesvirus gH.

Prospects and perspectives for development of a vaccine against herpes simplex virus infections

Herpes simplex viruses 1 and 2 are human pathogens that lead to significant morbidity and mortality in certain clinical settings. The development of effective antiviral medications, however, has had little discernible impact on the epidemiology of these pathogens, largely because the majority of infections are clinically silent. Decades of work have gone into various candidate HSV vaccines, but to date none has demonstrated sufficient efficacy to warrant licensure. This review examines developments in HSV immunology and vaccine development published since 2010, and assesses the prospects for improved immunization strategies that may result in an effective, licensed vaccine in the near future.

Herpes simplex virus gE/gI extracellular domains promote axonal transport and spread from neurons to epithelial cells

Following reactivation from latency, there are two distinct steps in the spread of herpes simplex virus (HSV) from infected neurons to epithelial cells: (i) anterograde axonal transport of virus particles from neuron bodies to axon tips and (ii) exocytosis and spread of extracellular virions across cell junctions into adjacent epithelial cells. The HSV heterodimeric glycoprotein gE/gI is important for anterograde axonal transport, and gE/gI cytoplasmic domains play important roles in sorting of virus particles into axons. However, the roles of the large (∼400-residue) gE/gI extracellular (ET) domains in both axonal transport and neuron-to-epithelial cell spread have not been characterized. Two gE mutants, gE-277 and gE-348, contain small insertions in the gE ET domain, fold normally, form gE/gI heterodimers, and are incorporated into virions. Both gE-277 and gE-348 did not function in anterograde axonal transport; there were markedly reduced numbers of viral capsids and glycoproteins compared with wild-type HSV. The defects in axonal transport were manifest in neuronal cell bodies, involving missorting of HSV capsids before entry into proximal axons. Although there were diminished numbers of mutant gE-348 capsids and glycoproteins in distal axons, there was efficient spread to adjacent epithelial cells, similar to wild-type HSV. In contrast, virus particles produced by HSV gE-277 spread poorly to epithelial cells, despite numbers of virus particles similar to those for HSV gE-348. These results genetically separate the two steps in HSV spread from neurons to epithelial cells and demonstrate that the gE/gI ET domains function in both processes.

IMPORTANCE

An essential phase of the life cycle of herpes simplex virus (HSV) and other alphaherpesviruses is the capacity to reactivate from latency and then spread from infected neurons to epithelial tissues. This spread involves at least two steps: (i) anterograde transport to axon tips followed by (ii) exocytosis and extracellular spread from axons to epithelial cells. HSV gE/gI is a glycoprotein that facilitates this virus spread, although by poorly understood mechanisms. Here, we show that the extracellular (ET) domains of gE/gI promote the sorting of viral structural proteins into proximal axons to begin axonal transport. However, the gE/gI ET domains also participate in the extracellular spread from axon tips across cell junctions to epithelial cells. Understanding the molecular mechanisms involved in gE/gI-mediated sorting of virus particles into axons and extracellular spread to adjacent cells is fundamentally important for identifying novel targets to reduce alphaherpesvirus disease.

The herpes simplex virus 1 UL51 gene product has cell type-specific functions in cell-to-cell spread

The herpes simplex virus 1 (HSV-1) UL51 gene encodes a 244-amino-acid (aa) palmitoylated protein that is conserved in all herpesviruses. The alphaherpesvirus UL51 (pUL51) protein has been reported to function in nuclear egress and cytoplasmic envelopment. No complete deletion has been generated because of the overlap of the UL51 coding sequence 5' end with the UL52 promoter sequences, but partial deletions generated in HSV and pseudorabies virus (PrV) suggest an additional function in epithelial cell-to-cell spread. Here we show partial uncoupling of the replication, release, and cell-to-cell spread functions of HSV-1 pUL51 in two ways. Viruses in which aa 73 to 244 were deleted from pUL51 or in which a conserved YXXΦ motif near the N terminus was altered showed cell-specific defects in spread that cannot be accounted for by defects in replication and virus release. Also, a cell line that expresses C-terminally enhanced green fluorescent protein (EGFP)-tagged pUL51 supported normal virus replication and release into the medium but the formation of only small plaques. This cell line also failed to support normal localization of gE to cell junctions. gE and pUL51 partially colocalized in infected cells, and these two proteins could be coimmunoprecipitated from infected cells, suggesting that they can form a complex during infection. The cell-to-cell spread defect associated with the pUL51 mutation was more severe than that associated with gE-null virus, suggesting that pUL51 has gE-independent functions in epithelial cell spread.

IMPORTANCE

Herpesviruses establish and reactivate from lifelong latency in their hosts. When they reactivate, they are able to spread within their hosts despite the presence of a potent immune response that includes neutralizing antibody. This ability is derived in part from a specialized mechanism for virus spread between cells. Cell-to-cell spread is a conserved property of herpesviruses that likely relies on conserved viral genes. An understanding of their function may aid in the design of vaccines and therapeutics. Here we show that one of the conserved viral genes, UL51, has an important role in cell-to-cell spread in addition to its previously demonstrated role in virus assembly. We find that its function depends on the type of cell that is infected, and we show that it interacts with and modulates the function of another viral spread factor, gE.

Different modes of herpes simplex virus type 1 spread in brain and skin tissues

Herpes simplex virus type 1 (HSV-1) initially infects the skin and subsequently spreads to the nervous system. To investigate and compare HSV-1 mode of propagation in the two clinically relevant tissues, we have established ex vivo infection models, using native tissues of mouse and human skin, as well as mouse brain, maintained in organ cultures. HSV-1, which is naturally restricted to the human, infects and spreads in the mouse and human skin tissues in a similar fashion, thus validating the mouse model. The spread of HSV-1 in the skin was concentric to form typical plaques of limited size, predominantly of cytopathic cells. By contrast, HSV-1 spread in the brain tissue was directed along specific neuronal networks with no apparent cytopathic effect. Two additional differences were noted following infection of the skin and brain tissues. First, only a negligible amount of extracellular progeny virus was produced of the infected brain tissues, while substantial quantity of infectious progeny virus was released to the media of the infected skin. Second, antibodies against HSV-1, added following the infection, effectively restricted viral spread in the skin but have no effect on viral spread in the brain tissue. Taken together, these results reveal that HSV-1 spread within the brain tissue mostly by direct transfer from cell to cell, while in the skin the progeny extracellular virus predominates, thus facilitating the infection to new individuals.

Glycoproteins gE and gI are required for efficient KIF1A-dependent anterograde axonal transport of alphaherpesvirus particles in neurons

Alphaherpesviruses, including pseudorabies virus (PRV), spread directionally within the nervous systems of their mammalian hosts. Three viral membrane proteins are required for efficient anterograde-directed spread of infection in neurons, including Us9 and a heterodimer composed of the glycoproteins gE and gI. We previously demonstrated that the kinesin-3 motor KIF1A mediates anterograde-directed transport of viral particles in axons of cultured peripheral nervous system (PNS) neurons. The PRV Us9 protein copurifies with KIF1A, recruiting the motor to transport vesicles, but at least one unidentified additional viral protein is necessary for this interaction. Here we show that gE/gI are required for efficient anterograde transport of viral particles in axons by mediating the interaction between Us9 and KIF1A. In the absence of gE/gI, viral particles containing green fluorescent protein (GFP)-tagged Us9 are assembled in the cell body but are not sorted efficiently into axons. Importantly, we found that gE/gI are necessary for efficient copurification of KIF1A with Us9, especially at early times after infection. We also constructed a PRV recombinant that expresses a functional gE-GFP fusion protein and used affinity purification coupled with mass spectrometry to identify gE-interacting proteins. Several viral and host proteins were found to associate with gE-GFP. Importantly, both gI and Us9, but not KIF1A, copurified with gE-GFP. We propose that gE/gI are required for efficient KIF1A-mediated anterograde transport of viral particles because they indirectly facilitate or stabilize the interaction between Us9 and KIF1A.

Polarized cell migration during cell-to-cell transmission of herpes simplex virus in human skin keratinocytes

In addition to transmission involving extracellular free particles, a generally accepted model of virus propagation is one wherein virus replicates in one cell, producing infectious particles that transmit to the next cell via cell junctions or induced polarized contacts. This mechanism of spread is especially important in the presence of neutralizing antibody, and the concept underpins analysis of virus spread, plaque size, viral and host functions, and general mechanisms of virus propagation. Here, we demonstrate a novel process involved in cell-to-cell transmission of herpes simplex virus (HSV) in human skin cells that has not previously been appreciated. Using time-lapse microscopy of fluorescent viruses, we show that HSV infection induces the polarized migration of skin cells into the site of infection. In the presence of neutralizing antibody, uninfected skin cells migrate to the initial site of infection and spread over infected cells to become infected in a spatially confined cluster containing hundreds of cells. The cells in this cluster do not undergo cytocidal cell lysis but harbor abundant enveloped particles within cells and cell-free virus within interstitial regions below the cluster surface. Cells at the base and outside the cluster were generally negative for virus immediate-early expression. We further show, using spatially separated monolayer assays, that at least one component of this induced migration is the paracrine stimulation of a cytotactic response from infected cells to uninfected cells. The existence of this process changes our concept of virus transmission and the potential functions, virus, and host factors involved.

Entry of herpesviruses into cells: the enigma variations

The entry of herpesviruses into their target cells is complex at many levels. Virus entry proceeds by a succession of interactions between viral envelope glycoproteins and molecules on the cell membrane. The process is divided into distinct steps: attachment to the cell surface, interaction with a specific entry receptor, internalization of the particle (optional and cell specific), and membrane fusion. Several viral envelope glycoproteins are involved in one or several of these steps. The most conserved entry glycoproteins in the herpesvirus family (gB, gH/gL) are involved in membrane fusion. Around this functional core, herpesviruses have a variety of receptor binding glycoproteins, which interact with cell surface proteins often from different families. This interaction activates and controls the actual fusion machinery. Interactions with cellular receptors and between viral glycoproteins have to be tightly coordinated and regulated to guarantee successful entry. Although additional entry receptors for herpesviruses continue to be identified, the molecular interactions between viral glycoproteins remain mostly enigmatic. This chapter will review our current understanding of the molecular interactions that occur during herpesvirus entry from attachment to fusion. Particular emphasis will be placed on structure-based representation of receptor binding as a trigger of fusion during herpes simplex virus entry.

Function of glycoprotein E of herpes simplex virus requires coordinated assembly of three tegument proteins on its cytoplasmic tail

Glycoprotein E (gE) of HSV plays a key role in cell-to-cell spread and virus-induced cell fusion. Here, we report that this function of gE requires the cooperation of tegument proteins UL11, UL16, and UL21. We found that the four proteins come together with very high efficiency to form a complex in transfected cells and in a manner that is regulated and coordinated. In particular, the inefficient interaction of UL16 with each membrane protein (UL11 and gE) observed in pairwise transfections became efficient when other binding partners were present. The significance of these interactions was revealed in studies of viral mutants, which showed that each of these tegument proteins is critical for processing, transport, and biological activity of gE. These findings provide insights into the mechanisms of how gE executes its function and also have implications in understanding HSV assembly and budding.

Herpes simplex virus membrane proteins gE/gI and US9 act cooperatively to promote transport of capsids and glycoproteins from neuron cell bodies into initial axon segments

Herpes simplex virus (HSV) and other alphaherpesviruses must move from sites of latency in ganglia to peripheral epithelial cells. How HSV navigates in neuronal axons is not well understood. Two HSV membrane proteins, gE/gI and US9, are key to understanding the processes by which viral glycoproteins, unenveloped capsids, and enveloped virions are transported toward axon tips. Whether gE/gI and US9 function to promote the loading of viral proteins onto microtubule motors in neuron cell bodies or to tether viral proteins onto microtubule motors within axons is not clear. One impediment to understanding how HSV gE/gI and US9 function in axonal transport relates to observations that gE(-), gI(-), or US9(-) mutants are not absolutely blocked in axonal transport. Mutants are significantly reduced in numbers of capsids and glycoproteins in distal axons, but there are less extensive effects in proximal axons. We constructed HSV recombinants lacking both gE and US9 that transported no detectable capsids and glycoproteins to distal axons and failed to spread from axon tips to adjacent cells. Live-cell imaging of a gE(-)/US9(-) double mutant that expressed fluorescent capsids and gB demonstrated >90% diminished capsids and gB in medial axons and no evidence for decreased rates of transport, stalling, or increased retrograde transport. Instead, capsids, gB, and enveloped virions failed to enter proximal axons. We concluded that gE/gI and US9 function in neuron cell bodies, in a cooperative fashion, to promote the loading of HSV capsids and vesicles containing glycoproteins and enveloped virions onto microtubule motors or their transport into proximal axons.

Rho-ROCK and Rac-PAK signaling pathways have opposing effects on the cell-to-cell spread of Marek's Disease Virus

Marek's Disease Virus (MDV) is an avian alpha-herpesvirus that only spreads from cell-to-cell in cell culture. While its cell-to-cell spread has been shown to be dependent on actin filament dynamics, the mechanisms regulating this spread remain largely unknown. Using a recombinant BAC20 virus expressing an EGFPVP22 tegument protein, we found that the actin cytoskeleton arrangements and cell-cell contacts differ in the center and periphery of MDV infection plaques, with cells in the latter areas showing stress fibers and rare cellular projections. Using specific inhibitors and activators, we determined that Rho-ROCK pathway, known to regulate stress fiber formation, and Rac-PAK, known to promote lamellipodia formation and destabilize stress fibers, had strong contrasting effects on MDV cell-to-cell spread in primary chicken embryo skin cells (CESCs). Inhibition of Rho and its ROCKs effectors led to reduced plaque sizes whereas inhibition of Rac or its group I-PAKs effectors had the adverse effect. Importantly, we observed that the shape of MDV plaques is related to the semi-ordered arrangement of the elongated cells, at the monolayer level in the vicinity of the plaques. Inhibition of Rho-ROCK signaling also resulted in a perturbation of the cell arrangement and a rounding of plaques. These opposing effects of Rho and Rac pathways in MDV cell-to-cell spread were validated for two parental MDV recombinant viruses with different ex vivo spread efficiencies. Finally, we demonstrated that Rho/Rac pathways have opposing effects on the accumulation of N-cadherin at cell-cell contact regions between CESCs, and defined these contacts as adherens junctions. Considering the importance of adherens junctions in HSV-1 cell-to-cell spread in some cell types, this result makes of adherens junctions maintenance one potential and attractive hypothesis to explain the Rho/Rac effects on MDV cell-to-cell spread. Our study provides the first evidence that MDV cell-to-cell spread is regulated by Rho/Rac signaling.

Role of adherens junction proteins in differential herpes simplex virus type 2 infectivity in communication-competent and -deficient cell lines

BACKGROUND

Gap junctional intercellular communication decreases with HSV-2 infection. To determine the importance of functional gap junctions for infectivity, we compared HSV-2 growth in communication-competent and -deficient cell lines.

METHODS

HSV-2 infectivity was tested in five cell lines: WB rat liver epithelial cells (communication-competent), WB-aB1 (communication-deficient), WB-a/32-10 (communication-rescued), HeLa (communication-deficient), and Cx43-transfected HeLa (communication-rescued) cells. HSV-2 growth curves and indirect immunofluorescence labeling of viral and cell proteins were performed in wild-type and mutant WB cells.

RESULTS

Although wild-type WB cells were highly permissive for HSV-2 infection, virus production was significantly attenuated in communication-deficient and -rescued mutant WB cells. HeLa exhibited no difference in infectivity between communication-competent and -deficient cell lines. Tight and adherens junction proteins, including zonula occludens-1 and nectin-1, were not different in the WB cell lines. However, E-cadherin levels were elevated and β-catenin was found to co-localize with glycoprotein E, a viral glycoprotein associated with cell-to-cell spread, in the mutant WB cells.

CONCLUSIONS

These results suggest that attenuated viral production in mutant WB cells is due to viral protein co-localization with adherens junction proteins rather than the loss or restoration of functional gap junctions.

Structure-based mutational analysis of the highly conserved domain IV of glycoprotein H of pseudorabies virus

Glycoprotein H (gH) is an envelope protein conserved in the Herpesviridae. Together with glycoprotein B (gB), the heterodimeric complex of gH and glycoprotein L (gL) mediates penetration and direct viral cell-to-cell spread. In herpes simplex and pseudorabies virus (PrV), coexpression of gH/gL, gB, and gD induces membrane fusion to form polykaryocytes. The recently determined crystal structure of a core fragment of PrV gH revealed marked structural similarity to other gH proteins (M. Backovic et al., Proc. Natl. Acad. Sci. U. S. A. 107:22635-22640, 2010). Within the membrane-proximal part (domain IV), a conserved negatively charged surface loop (flap) is flanked by intramolecular disulfide bonds. Together with an N-linked carbohydrate moiety, this flap covers an underlying patch of hydrophobic residues. To investigate the functional relevance of these structures, nonconservative amino acid substitutions were introduced by site-directed mutagenesis. The mutated proteins were tested for correct expression, fusion activity, and functional complementation of gH-deleted PrV. Several single amino acid changes within the flap and the hydrophobic patch were tolerated, and deletion of the glycosylation site had only minor effects. However, multiple alanine substitutions within the flap or the hydrophobic patch led to significant defects. gH function was also severely affected by disruption of the disulfide bond at the C terminus of the flap and after introduction of cysteine pairs designed to bridge the central part of the flap with the hydrophobic patch. Interestingly, all mutated gH proteins were able to complement gH-deleted PrV, but fusion-deficient gH mutants resulted in a pronounced delay in virus entry.

Replication of herpes simplex virus: egress of progeny virus at specialized cell membrane sites

In the final stages of the herpes simplex virus 1 (HSV-1) life cycle, a viral nucleocapsid buds into a vesicle of trans-Golgi network (TGN)/endosome origin, acquiring an envelope and an outer vesicular membrane. The virus-containing vesicle then traffics to the plasma membrane where it fuses, exposing a mature virion. Although the process of directed egress has been studied in polarized epithelial cell lines, less work has been done in nonpolarized cell types. In this report, we describe a study of HSV-1 egress as it occurs in nonpolarized cells. The examination of infected Vero cells by electron, confocal, and total internal reflection fluorescence (TIRF) microscopy revealed that HSV-1 was released at specific pocket-like areas of the plasma membrane that were found along the substrate-adherent surface and cell-cell-adherent contacts. Both the membrane composition and cytoskeletal structure of egress sites were found to be modified by infection. The plasma membrane at virion release sites was heavily enriched in viral glycoproteins. Small glycoprotein patches formed early in infection, and virus became associated with these areas as they expanded. Glycoprotein-rich areas formed independently from virion trafficking as confirmed by the use of a UL25 mutant with a defect in capsid nuclear egress. The depolymerization of the cytoskeleton indicated that microtubules were important for the trafficking of virions and glycoproteins to release sites. In addition, the actin cytoskeleton was found to be necessary for maintaining the integrity of egress sites. When actin was depolymerized, the glycoprotein concentrations dispersed across the membrane, as did the surface-associated virus. Lastly, viral glycoprotein E appeared to function in a different manner in nonpolarized cells compared to previous studies of egress in polarized epithelial cells; the total amount of virus released at egress sites was slightly increased in infected Vero cells when gE was absent. However, gE was important for egress site formation, as Vero cells infected with gE deletion mutants formed glycoprotein patches that were significantly reduced in size. The results of this study are interpreted to indicate that the egress of HSV-1 in Vero cells is directed to virally induced, specialized egress sites that form along specific areas of the cell membrane.

Parallels and distinctions in the direct cell-to-cell spread of the plant and animal viruses

The paradigm that viruses can move directly, and in some cases covertly, between contacting target cells is now well established for several virus families. The underlying mechanisms of cell-to-cell spread, however, remain to be fully elucidated and may differ substantially depending on the viral exit/entry route and the cellular tropism. Here, two divergent cell-to-cell spread mechanisms are exemplified: firstly by human retroviruses, which rely upon transient adhesive structures that form between polarized immune cells termed virological synapses, and secondly by herpesviruses that depend predominantly on pre-existing stable cellular contacts, but may also form virological synapses. Plant viruses can also spread directly between contacting cells, but are obliged by the rigid host cell wall to move across pore structures termed plasmodesmata. This review will focus primarily on recent advances in our understanding of animal virus cell-to-cell spread using examples from these two virus families to highlight differences and similarities, and will conclude by comparing and contrasting the cell-to-cell spread of animal and plant viruses.

Effects of Ilex paraguariensis A. St. Hil. (yerba mate) on herpes simplex virus types 1 and 2 replication

The antiherpes effects of the crude extract obtained from Ilex paraguariensis leaves (yerba mate) and their purified fractions were investigated. The most active fraction was selected and assayed to determine the viral multiplication steps upon which it acted. In order to detect the major components of this fraction, thin layer chromatography (TLC) analysis was performed. The antiviral activity was evaluated against HSV-1 and HSV-2 by a viral plaque number reduction assay (IC(50) ) and the cytotoxicity by a MTT assay (CC(50) ). According to the obtained results, all tested samples showed antiherpes activity at noncytotoxic concentrations, and the ethyl acetate fraction was the most active (SI = CC(50) /IC(50)  = 188.7 and 264.7 for HSV-1 and HSV-2, respectively). The results also demonstrated that this fraction exerts antiviral activity by the reduction of viral infectivity, the inhibition of virus entry into cells and cell-to-cell virus spread, as well as by the impaired levels of ICP27, ICP4, gD and gE proteins of HSV-1. The TLC analysis showed that this fraction contains monodesmosidic triterpenoid saponins, matesaponin-1 (a bidesmosidic one), caffeic and chlorogenic acids and rutin, which suggests that they could act synergistically and be responsible for the detected antiherpes activity.

Direct and specific binding of the UL16 tegument protein of herpes simplex virus to the cytoplasmic tail of glycoprotein E

The UL16 tegument protein of herpes simplex virus (HSV) is conserved throughout all of the herpesvirus families. Previous studies have shown that the binding of HSV to heparan sulfate molecules on the host cell triggers the release of UL16 from the capsid, but the mechanism by which the signal is sent from the virion surface into the tegument is unknown. Here, we report that a glutathione S-transferase chimera bearing the cytoplasmic tail of viral glycoprotein E (gE) is capable of binding to UL16 in lysates of eukaryotic cells or purified from bacteria. Moreover, mass spectrometry studies of native-UL16 complexes purified from infected cells also revealed the presence of gE. Proof that UL16-gE can interact within cells required the fortuitous discovery of a mutant possessing only the first 155 residues of UL16. Confocal microscopy of cotransfected cells revealed that this mutant colocalized with gE in the cytoplasm, whereas it was found throughout the cytoplasm and nucleus when expressed alone. In contrast, the full-length UL16 molecule was very poorly capable of finding gE. Moreover, membrane flotation assays showed that UL16(1-155) was able to float to the top of sucrose step gradients when coexpressed with gE, whereas full-length UL16 was not. Thus, the discovery of the UL16(1-155) mutant confirmed the specific in vitro interaction with gE and provides evidence that a binding domain at the N terminus of UL16 may be controlled by a regulatory domain within the C terminus. These findings suggest the possibility that the UL16-gE interaction may play roles in the tegument signaling mechanism, virus budding, and the gE-mediated mechanism of cell-to-cell spread.

Alphaherpesviruses and the cytoskeleton in neuronal infections

Following infection of exposed peripheral tissues, neurotropic alphaherpesviruses invade nerve endings and deposit their DNA genomes into the nuclei of neurons resident in ganglia of the peripheral nervous system. The end result of these events is the establishment of a life-long latent infection. Neuroinvasion typically requires efficient viral transmission through a polarized epithelium followed by long-distance transport through the viscous axoplasm. These events are mediated by the recruitment of the cellular microtubule motor proteins to the intracellular viral particle and by alterations to the cytoskeletal architecture. The focus of this review is the interplay between neurotropic herpesviruses and the cytoskeleton.

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.

Expression and characterization of duck enteritis virus gI gene

BACKGROUND

At present, alphaherpesviruses gI gene and its encoding protein have been extensively studied. It is likely that gI protein and its homolog play similar roles in virions direct cell-to-cell spread of alphaherpesviruses. But, little is known about the characteristics of DEV gI gene. In this study, we expressed and presented the basic properties of the DEV gI protein.

RESULTS

The special 1221-bp fragment containing complete open reading frame(ORF) of duck enteritis virus(DEV) gI gene was extracted from plasmid pMD18-T-gI, and then cloned into prokaryotic expression vector pET-32a(+), resulting in pET-32a(+)-gI. After being confirmed by PCR, restriction endonuclease digestion and sequencing, pET-32a(+)-gI was transformed into E.coli BL21(DE3) competent cells for overexpression. DEV gI gene was successfully expressed by the addition of isopropyl-β-D-thiogalactopyranoside(IPTG). SDS-PAGE showed that the recombinant protein His6-tagged gI molecular weight was about 61 kDa. Subsequently, the expressed product was applied to generate specific antibody against gI protein. The specificity of the rabbit immuneserum was confirmed by its ability to react with the recombinant protein His6-tagged gI. In addition, real time-PCR was used to determine the the levels of the mRNA transcripts of gI gene, the results showed that the DEV gI gene was transcribed most abundantly during the late phase of infection. Furthermore, indirect immunofluorescence(IIF) was established to study the gI protein expression and localization in DEV-infected duck embryo fibroblasts (DEFs), the results confirmed that the protein was expressed and located in the cytoplasm of the infected cells, intensively.

CONCLUSIONS

The recombinant prokaryotic expression vector of DEV gI gene was constructed successfully. The gI protein was successfully expressed by E.coli BL21(DE3) and maintained its antigenicity very well. The basic information of the transcription and intracellular localization of gI gene were presented, that would be helpful to assess the possible role of DEV gI gene. The research will provide useful clues for further functional analysis of DEV gI gene.

Expression and immunohistochemical distribution of duck plague virus glycoprotein gE in infected ducks

To determine the distribution of duck plague virus (DPV) gE protein in paraformaldehyde-fixed, paraffin-embedded tissues of experimentally DPV-infected ducks, an indirect immunoperoxidase assay was established to detect glycoprotein E (gE) protein for the first time. The rabbit anti-His-gE serum, raised against the recombinant His-gE fusion protein expressed in Escherichia coli BL21 (DE3), was prepared and purified. Western blotting and indirect immunofluorescence analysis showed that the anti-His-gE serum had a high level of reactivity and specificity and could be used as the first antibody for further experiments to study the distribution of DPV gE protein in DPV-infected tissues. A number of DPV gE proteins were distributed in the bursa of Fabricius, thymus, spleen, liver, esophagus, duodenum, jejunum, ileum, and kidney of DPV-infected ducks and a few DPV gE were distributed in the Harders glands, myocardium, cerebrum, and lung, whereas the gE was not seen in the skin, muscle, and pancreas. Moreover, DPV gE was expressed abundantly in the cytoplasm of lymphocytes, reticulum cells, macrophages, epithelial cells, and hepatocytes. The present study may be useful not only for describing the characteristics of gE expression and distribution in infected ducks but also for understanding the pathogenesis of DPV.

Herpes simplex virus dances with amyloid precursor protein while exiting the cell

Herpes simplex type 1 (HSV1) replicates in epithelial cells and secondarily enters local sensory neuronal processes, traveling retrograde to the neuronal nucleus to enter latency. Upon reawakening newly synthesized viral particles travel anterograde back to the epithelial cells of the lip, causing the recurrent cold sore. HSV1 co-purifies with amyloid precursor protein (APP), a cellular transmembrane glycoprotein and receptor for anterograde transport machinery that when proteolyzed produces A-beta, the major component of senile plaques. Here we focus on transport inside epithelial cells of newly synthesized virus during its transit to the cell surface. We hypothesize that HSV1 recruits cellular APP during transport. We explore this with quantitative immuno-fluorescence, immuno-gold electron-microscopy and live cell confocal imaging. After synchronous infection most nascent VP26-GFP-labeled viral particles in the cytoplasm co-localize with APP (72.8+/−6.7%) and travel together with APP inside living cells (81.1+/−28.9%). This interaction has functional consequences: HSV1 infection decreases the average velocity of APP particles (from 1.1+/−0.2 to 0.3+/−0.1 µm/s) and results in APP mal-distribution in infected cells, while interplay with APP-particles increases the frequency (from 10% to 81% motile) and velocity (from 0.3+/−0.1 to 0.4+/−0.1 µm/s) of VP26-GFP transport. In cells infected with HSV1 lacking the viral Fc receptor, gE, an envelope glycoprotein also involved in viral axonal transport, APP-capsid interactions are preserved while the distribution and dynamics of dual-label particles differ from wild-type by both immuno-fluorescence and live imaging. Knock-down of APP with siRNA eliminates APP staining, confirming specificity. Our results indicate that most intracellular HSV1 particles undergo frequent dynamic interplay with APP in a manner that facilitates viral transport and interferes with normal APP transport and distribution. Such dynamic interactions between APP and HSV1 suggest a mechanistic basis for the observed clinical relationship between HSV1 seropositivity and risk of Alzheimer's disease.

Herpes simplex virus glycoproteins gB and gD function in a redundant fashion to promote secondary envelopment

Egress of herpes simplex virus (HSV) and other herpesviruses from cells involves extensive modification of cellular membranes and sequential envelopment and deenvelopment steps. HSV glycoproteins are important in these processes, and frequently two or more glycoproteins can largely suffice in any step. Capsids in the nucleus undergo primary envelopment at the inner nuclear membrane (INM), and then enveloped virus particles undergo deenvelopment by fusing with the outer nuclear membrane (ONM). Capsids delivered into the cytoplasm then undergo secondary envelopment, involving trans-Golgi network (TGN) membranes. The deenvelopment step involves HSV glycoproteins gB and gH/gL acting in a redundant fashion. This fusion has features common to the fusion that occurs between the virion envelope and cellular membranes when HSV enters cells, a process requiring gB, gD, and gH/gL. Whether HSV gD also participates (in a redundant fashion with gB or gH/gL) in deenvelopment has not been characterized. Secondary envelopment in the cytoplasm is known to involve HSV gD and gE/gI, also acting in a redundant fashion. Whether gB might also contribute to secondary envelopment, collaborating with gD and gE/gI, is also not clear. To address these questions, we constructed an HSV double mutant lacking gB and gD. The HSV gB(-)/gD(-) mutant exhibited no substantial defects in nuclear egress. In contrast, secondary envelopment was markedly reduced, and there were numerous unenveloped capsids that accumulated in the cytoplasm, as well as increased numbers of partially enveloped capsids and morphologically aberrant enveloped particles with thicker, oblong tegument layers. These defects were different from those observed with HSV gD(-)/gE(-)/gI(-) mutants, which accumulated capsids in large, aggregated masses in the cytoplasm. Our results suggest that HSV gB functions in secondary envelopment, apparently acting downstream of gE/gI.

Immunofluorescence analysis of duck plague virus gE protein on DPV-infected ducks

BACKGROUND

In previous studies, the expression and localization characteristics of duck plague virus (DPV) gE protein have been described in cultured cells, but the properties of DPV gE protein have not been reported in vivo. Immunofluorescence analysis had been used for the detection of virus antigen, but there was no report on the use of this technique for the detection of DPV gE. In this study, we investigated the distribution of DPV gE protein on DPV-infected ducks using polyclonal antibody raised against the recombinant His-gE fusion protein by indirect immunofluorescence assay (IFA).

RESULTS

The recombinant gE protein was highly immunogenicity by ELISA, and the gE was used as an antigen for the preparation of polyclonal antibody, which could be used the first antibody for further experiment to study the distribution of DPV gE protein in DPV-infected tissues by indirect immunofluorescence assay. DPV gE protein were distributed in the immune organs (thymus, bursa of fabricius (BF), Harders glands, spleen), the digestive organs (liver, duodenum, jejunum, ileum), and the other parenchymatous organs (kidney, myocardium, cerebrum, and lung) of DPV-infected ducks, but the positive immunofluorescence signal was not seen in the muscle and pancreas. The lymphocytes, reticulum cells, macrophages, epithelial cells, and hepatocytes served as the principal site for the localization of DPV gE antigen. Moreover, the intensity of fluorescence increased sharply from 12 to 216 h post-infection (p.i.).

CONCLUSIONS

In this work, the immunogenicity of the recombinant gE protein was analyzed by ELISA, and we presented the distribution properties of DPV gE antigen in infected ducks for the first time, which may be useful for understanding the pathogenesis of DPV. These properties of the gE protein provided the prerequisite for further functional analysis.

Herpes simplex virus type 1/adeno-associated virus hybrid vectors

Herpes simplex virus type 1 (HSV-1) amplicons can accommodate foreign DNA of any size up to 150 kbp and, therefore, allow extensive combinations of genetic elements. Genomic sequences as well as cDNA, large transcriptional regulatory sequences for cell type-specific expression, multiple transgenes, and genetic elements from other viruses to create hybrid vectors may be inserted in a modular fashion. Hybrid amplicons use genetic elements from HSV-1 that allow replication and packaging of the vector DNA into HSV-1 virions, and genetic elements from other viruses that either direct integration of transgene sequences into the host genome or allow episomal maintenance of the vector. Thus, the advantages of the HSV-1 amplicon system, including large transgene capacity, broad host range, strong nuclear localization, and availability of helper virus-free packaging systems are retained and combined with those of heterologous viral elements that confer genetic stability to the vector DNA. Adeno-associated virus (AAV) has the unique capability of integrating its genome into a specific site, designated AAVS1, on human chromosome 19. The AAV rep gene and the inverted terminal repeats (ITRs) that flank the AAV genome are sufficient for this process. HSV-1 amplicons have thus been designed that contain the rep gene and a transgene cassette flanked by AAV ITRs. These HSV/AAV hybrid vectors direct site-specific integration of transgene sequences into AAVS1 and support long-term transgene expression.

Cloning, expression and characterization of gE protein of duck plague virus

BACKGROUND

The gE protein of duck plague virus is the important membrane glycoprotein, its protein characterization has not been reported. In this study, we expressed and presented the characterization of the DPV gE product.

RESULTS

According to the sequence of the gE gene, a pair of primers were designed, and the DNA product with 1490bp in size was amplified by using the polymerase chain reaction (PCR). The PCR product was cloned into pMD18-T vector, and subcloned into pET32a(+), generating the recombinant plasmid pET32a/DPV-gE. SDS-PAGE analysis showed that the fusion pET32a/DPV-gE protein was highly expressed after induction by 0.2 mM IPTG at 30 degrees C for 4.5 h in Rosseta host cells. Over expressed 6xHis-gE fusion protein was purified by nickel affinity chromatography, and used to immunize the rabbits for the preparation of polyclonal antibody. The result of the intracellular localization revealed that the gE protein was appeared to be in the cytoplasm region. The real time PCR, RT-PCR analysis and Western blotting revealed that the gE gene was produced most abundantly during the late phase of replication in DPV-infected cells.

CONCLUSIONS

In this work, the DPV gE protein was successfully expressed in a prokaryotic expression system, and we presented the basic properties of the DPV gE product for the first time. These properties of the gE protein provided a prerequisite for further functional analysis of this gene.

Canine distemper virus persistence in demyelinating encephalitis by swift intracellular cell-to-cell spread in astrocytes is controlled by the viral attachment protein

The mechanism of viral persistence, the driving force behind the chronic progression of inflammatory demyelination in canine distemper virus (CDV) infection, is associated with non-cytolytic viral cell-to-cell spread. Here, we studied the molecular mechanisms of viral spread of a recombinant fluorescent protein-expressing virulent CDV in primary canine astrocyte cultures. Time-lapse video microscopy documented that CDV spread was very efficient using cell processes contacting remote target cells. Strikingly, CDV transmission to remote cells could occur in less than 6 h, suggesting that a complete viral cycle with production of extracellular free particles was not essential in enabling CDV to spread in glial cells. Titration experiments and electron microscopy confirmed a very low CDV particle production despite higher titers of membrane-associated viruses. Interestingly, confocal laser microscopy and lentivirus transduction indicated expression and functionality of the viral fusion machinery, consisting of the viral fusion (F) and attachment (H) glycoproteins, at the cell surface. Importantly, using a single-cycle infectious recombinant H-knockout, H-complemented virus, we demonstrated that H, and thus potentially the viral fusion complex, was necessary to enable CDV spread. Furthermore, since we could not detect CD150/SLAM expression in brain cells, the presence of a yet non-identified glial receptor for CDV was suggested. Altogether, our findings indicate that persistence in CDV infection results from intracellular cell-to-cell transmission requiring the CDV-H protein. Viral transfer, happening selectively at the tip of astrocytic processes, may help the virus to cover long distances in the astroglial network, “outrunning” the host’s immune response in demyelinating plaques, thus continuously eliciting new lesions.

EBV-positive human sera contain antibodies against the EBV BMRF-2 protein

We previously showed that the EBV glycoprotein BMRF-2 contains a functional integrin-binding Arg-Gly-Asp (RGD) domain that plays an important role in viral infection and cell-to-cell spread of progeny virions in oral epithelial cells. In this study, we found that EBV-seropositive human sera contain antibodies against BMRF-2. The inhibitory effect of EBV-positive sera on EBV infection of oral epithelial cells was substantially reduced by pre-incubation of serum samples with the BMRF-2 RGD peptide, suggesting that anti-BMRF-2 human antibodies possess neutralizing activity. EBV-specific sera reacted strongly with the BMRF-2 extracellular domain (170-213 aa) containing the RGD motif, whereas they reacted only weakly or not at all with a mutated form of the BMRF-2 extracellular domain containing AAA instead of RGD. These data indicate that RGD motif of BMRF-2 is part of an immunodominant antigenic determinant within the extracellular domain of BMRF-2 that may contribute to EBV neutralization during EBV reactivation.

EBV BMRF-2 facilitates cell-to-cell spread of virus within polarized oral epithelial cells

We previously reported that the Epstein-Barr virus (EBV) BMRF-2 protein plays an important role in EBV infection of polarized oral epithelial cells by interacting with beta1 and alphav family integrins. Here we show that infection of polarized oral epithelial cells with B27-BMRF-2(low) recombinant virus, expressing a low level of BMRF-2, resulted in significantly smaller plaques compared with infection by parental B95-8 virus. BMRF-2 localized in the trans-Golgi network (TGN) and basolateral sorting vesicles and was transported to the basolateral membranes of polarized epithelial cells. Mutation of the tyrosine- and dileucine-containing basolateral sorting signal, YLLV, in the cytoplasmic domain of BMRF-2 led to the failure of its accumulation in the TGN and its basolateral transport. These data show that BMRF-2 may play an important role in promoting the spread of EBV progeny virions through lateral membranes of oral epithelial cells.

Herpes simplex virus type 1 glycoprotein E mediates retrograde spread from epithelial cells to neurites

In animal models of infection, glycoprotein E (gE) is required for efficient herpes simplex virus type 1 (HSV-1) spread from the inoculation site to the cell bodies of innervating neurons (retrograde direction). Retrograde spread in vivo is a multistep process, in that HSV-1 first spreads between epithelial cells at the inoculation site, then infects neurites, and finally travels by retrograde axonal transport to the neuron cell body. To better understand the role of gE in retrograde spread, we used a compartmentalized neuron culture system, in which neurons were infected in the presence or absence of epithelial cells. We found that gE-deleted HSV-1 (NS-gEnull) retained retrograde axonal transport activity when added directly to neurites, in contrast to the retrograde spread defect of this virus in animals. To better mimic the in vivo milieu, we overlaid neurites with epithelial cells prior to infection. In this modified system, virus infects epithelial cells and then spreads to neurites, revealing a 100-fold retrograde spread defect for NS-gEnull. We measured the retrograde spread defect of NS-gEnull from a variety of epithelial cell lines and found that the magnitude of the spread defect from epithelial cells to neurons correlated with epithelial cell plaque size defect, indicating that gE plays a similar role in both types of spread. Therefore, gE-mediated spread between epithelial cells and neurites likely explains the retrograde spread defect of gE-deleted HSV-1 in vivo.

Inhibition of cyclooxygenase activity blocks cell-to-cell spread of human cytomegalovirus

Human cytomegalovirus has previously been shown to induce the accumulation of cyclooxygenase-2 RNA, protein, and enzyme activity. High doses of cyclooxygenase inhibitors substantially block viral replication in cultured fibroblasts. However, doses corresponding to the level of drug achieved in the plasma of patients have little effect on the replication of human cytomegalovirus in cultured cells. Here, we demonstrate that two nonsteroidal anti-inflammatory drugs, tolfenamic acid and indomethacin, markedly reduce direct cell-to-cell spread of human cytomegalovirus in cultured fibroblasts. The block is reversed by addition of prostaglandin E2, proving that it results from the action of the drugs on cyclooxygenase activity. Because direct cell-to-cell spread likely contributes importantly to pathogenesis of the virus, we suggest that nonsteroidal anti-inflammatory drugs might help to control human cytomegalovirus infections in conjunction with other anti-viral treatments.

Deletion of the first cysteine-rich region of the varicella-zoster virus glycoprotein E ectodomain abolishes the gE and gI interaction and differentially affects cell-cell spread and viral entry

Varicella-zoster virus (VZV) glycoprotein E (gE) is the most abundant glycoprotein in infected cells and, in contrast to those of other alphaherpesviruses, is essential for viral replication. The gE ectodomain contains a unique N-terminal region required for viral replication, cell-cell spread, and secondary envelopment; this region also binds to the insulin-degrading enzyme (IDE), a proposed VZV receptor. To identify new functional domains of the gE ectodomain, the effect of mutagenesis of the first cysteine-rich region of the gE ectodomain (amino acids 208 to 236) was assessed using VZV cosmids. Deletion of this region was compatible with VZV replication in vitro, but cell-cell spread of the rOka-DeltaCys mutant was reduced significantly. Deletion of the cysteine-rich region abolished the binding of the mutant gE to gI but not to IDE. Preventing gE binding to gI altered the pattern of gE expression at the plasma membrane of infected cells and the posttranslational maturation of gI and its incorporation into viral particles. In contrast, deletion of the first cysteine-rich region did not affect viral entry into human tonsil T cells in vitro or into melanoma cells infected with cell-free VZV. These experiments demonstrate that gE/gI heterodimer formation is essential for efficient cell-cell spread and incorporation of gI into viral particles but that it is dispensable for infectious varicella-zoster virion formation and entry into target cells. Blocking gE binding to gI resulted in severe impairment of VZV infection of human skin xenografts in SCIDhu mice in vivo, documenting the importance of cell fusion mediated by this complex for VZV virulence in skin.