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

Exploring gastric cancer genetics: A turning point in common variable immunodeficiency

Background

Gastric cancer (GC) stands as a prominent cause of cancer-related mortality and ranks second among the most frequently diagnosed malignancies in individuals with common variable immunodeficiency (CVID).

Objective

We sought to conduct a comprehensive, large-scale genetic analysis to explore the CVID-associated germline variant landscape within gastric adenocarcinoma samples and to seek to delineate the transcriptomic similarities between GC and CVID.

Methods

We investigated the presence of CVID-associated germline variants in 1591 GC samples and assessed their impact on tumor mutational load. The progression of GC was evaluated in patients with and without these variants. Transcriptomic similarities were explored by matching differentially expressed genes in GC to healthy gastric tissue with a CVID transcriptomic signature.

Results

CVID-associated germline variants were found in 60% of GC samples. Our analysis revealed a significant association between the presence of CVID-related genetic variants and higher tumor mutational load in GC (P < .0001); high GC mutational load seems to be linked to immunotherapy response and worse prognosis. Transcriptomic similarities unveiled key genes and pathways implicated in innate immune responses and tumorigenesis. We identified upregulated genes related to oncogene drivers, inflammation, tumor suppression, DNA repair, and downregulated immunomodulatory genes shared between GC and CVID.

Conclusions

Our findings contribute to a deeper understanding of potential molecular modulators of GC and shed light on the intricate interplay between immunodeficiency and cancer. This study underscores the clinical relevance of CVID-related variants in influencing GC progression and opens avenues for further exploration into novel therapeutic approaches.

Viral regulation of organelle membrane contact sites

At the core of organelle functions lies their ability and need to form dynamic organelle-organelle networks that drive intracellular communication and coordination of cellular pathways. These networks are facilitated by membrane contact sites (MCSs) that promote both intra-organelle and inter-organelle communication. Given their multiple functions, MCSs and the proteins that form them are commonly co-opted by viruses during infection to promote viral replication. This Essay discusses mechanisms acquired by diverse human viruses to regulate MCS functions in either proviral processes or host defense. It also examines techniques used for examining MCSs in the context of viral infections.

Individual herpes simplex virus 1 (HSV-1) particles exit by exocytosis and accumulate at preferential egress sites

The human pathogen herpes simplex virus 1 (HSV-1) produces a lifelong infection in the majority of the world's population. While the generalities of alpha herpesvirus assembly and egress pathways are known, the precise molecular and spatiotemporal details remain unclear. In order to study this aspect of HSV-1 infection, we engineered a recombinant HSV-1 strain expressing a pH-sensitive reporter, gM-pHluorin. Using a variety of fluorescent microscopy modalities, we can detect individual virus particles undergoing intracellular transport and exocytosis at the plasma membrane. We show that particles exit from epithelial cells individually, not bulk release of many particles at once, as has been reported for other viruses. In multiple cell types, HSV-1 particles accumulate over time at the cell periphery and cell-cell contacts. We show that this accumulation effect is the result of individual particles undergoing exocytosis at preferential sites and that these egress sites can contribute to cell-cell spread. We also show that the viral membrane proteins gE, gI, and US9, which have important functions in intracellular transport in neurons, are not required for preferential egress and clustering in non-neuronal cells. Importantly, by comparing HSV-1 to a related alpha herpesvirus, pseudorabies virus, we show that this preferential exocytosis and clustering effect are cell type dependent, not virus dependent. This preferential egress and clustering appear to be the result of the arrangement of the microtubule cytoskeleton, as virus particles co-accumulate at the same cell protrusions as an exogenous plus end-directed kinesin motor.IMPORTANCEAlpha herpesviruses produce lifelong infections in their human and animal hosts. The majority of people in the world are infected with herpes simplex virus 1 (HSV-1), which typically causes recurrent oral or genital lesions. However, HSV-1 can also spread to the central nervous system, causing severe encephalitis, and might also contribute to the development of neurodegenerative diseases. Many of the steps of how these viruses infect and replicate inside host cells are known in depth, but the final step, exiting from the infected cell, is not fully understood. In this study, we engineered a novel variant of HSV-1 that allows us to visualize how individual virus particles exit from infected cells. With this imaging assay, we investigated preferential egress site formation in certain cell types and their contribution to the cell-cell spread of HSV-1.

Biological roles of plant synaptotagmins

Plant synaptotagmins (SYTs) are resident proteins of the endoplasmic reticulum (ER). They are characterized by an N-terminal transmembrane region and C2 domains at the C-terminus, which tether the ER to the plasma membrane (PM). In addition to their tethering role, SYTs contain a lipid-harboring SMP domain, essential for shuttling lipids between the ER and the PM. There is now abundant literature on Arabidopsis SYT1, the best-characterized family member, which link it to biotic and abiotic responses as well as to ER morphology. Here, we review the current knowledge of SYT members, focusing on their role in stress, and discuss how these roles can be related to their tethering and lipid transport functions. Finally, we contextualize this information about SYTs with their homologs, the yeast tricalbins and the mammalian extended synaptotagmins.

N-Linked Glycosylation and Expression of Duck Plague Virus pUL10 Promoted by pUL49.5

Duck plague virus (DPV) is a member of the alphaherpesvirus subfamily, and its genome encodes a conserved envelope protein, protein UL10 (pUL10). pUL10 plays complex roles in viral fusion, assembly, cell-to-cell spread, and immune evasion, which are closely related to its protein characteristics and partners. Few studies have been conducted on DPV pUL10. In this study, we identified the characteristics of pUL10, such as the type of glycosylation modification and subcellular localization. The characteristic differences in pUL10 in transfection and infection suggest that there are other viral proteins that participate in pUL10 modification and localization. Therefore, pUL49.5, the interaction partner of pUL10, was explored. We found that pUL10 interacts with pUL49.5 during transfection and infection. Their interaction entailed multiple interaction sites, including noncovalent forces in the pUL49.5 N-terminal domains and C-terminal domains and a covalent disulfide bond between two conserved cysteines. pUL49.5 promoted pUL10 expression and mature N-linked glycosylation modification. Moreover, deletion of UL49.5 in DPV caused the molecular mass of pUL10 to decrease by approximately3 to 10 kDa, which suggested that pUL49.5 was the main factor affecting the N-linked glycosylation of DPV pUL10 during infection. This study provides a basis for future exploration of the effect of pUL10 glycosylation on virus proliferation. IMPORTANCE Duck plague is a disease with high morbidity and mortality rates, and it causes great losses for the duck breeding industry. Duck plague virus (DPV) is the causative agent of duck plague, and DPV UL10 protein (pUL10) is a homolog of glycoprotein M (gM), which is conserved in herpesviruses. pUL10 plays complex roles in viral fusion, assembly, cell-to-cell spread, and immune evasion, which are closely related to its protein characteristics and partners. In this study, we systematically explored whether pUL49.5 (a partner of pUL10) plays roles in the localization, modification, and expression of pUL10.

LIVE-CELL FLUORESCENCE MICROSCOPY OF HSV-1 CELLULAR EGRESS BY EXOCYTOSIS

The human pathogen Herpes Simplex Virus 1 (HSV-1) produces a lifelong infection in the majority of the world's population. While the generalities of alpha herpesvirus assembly and egress pathways are known, the precise molecular and spatiotemporal details remain unclear. In order to study this aspect of HSV-1 infection, we engineered a recombinant HSV-1 strain expressing a pH-sensitive reporter, gM-pHluorin. Using a variety of fluorescent microscopy modalities, we can detect individual virus particles undergoing intracellular transport and exocytosis at the plasma membrane. We show that particles exit from epithelial cells individually, not bulk release of many particles at once, as has been reported for other viruses. In multiple cell types, HSV-1 particles accumulate over time at the cell periphery and cell-cell contacts. We show that this accumulation effect is the result of individual particles undergoing exocytosis at preferential sites and that these egress sites can contribute to cell-cell spread. We also show that the viral membrane proteins gE, gI, and US9, which have important functions in intracellular transport in neurons, are not required for preferential egress and clustering in non-neuronal cells. Importantly, by comparing HSV-1 to a related alpha herpesvirus, pseudorabies virus, we show that this preferential exocytosis and clustering effect is cell type-dependent, not virus dependent. This preferential egress and clustering appears to be the result of the arrangement of the microtubule cytoskeleton, as virus particles co-accumulate at the same cell protrusions as an exogenous plus end-directed kinesin motor.

SUN2 Modulates the Propagation of HSV-1

Herpesviruses assemble new viral particles in the nucleus. These nucleocapsids bud through the inner nuclear membrane to produce enveloped viral particles in the perinuclear space before fusing with the outer nuclear membrane to reach the cytoplasm. This unusual route is necessary since viral capsids are too large to pass through nuclear pores. However, the transient perinuclear nucleocapsids (250 nm in diameter) are also larger than the width of the perinuclear space (30 to 50 nm). Interestingly, linker of the nucleoskeleton and cytoskeleton (LINC) components SUN and KASH connect the inner and outer nuclear membranes and regulate their spacing. Previous work by others on the related pseudorabies virus and human cytomegalovirus showed that they functionally interact with SUN proteins. To clarify the role of SUN proteins, we explored their impact on herpes simplex virus 1 (HSV-1), another herpesvirus. Using dominant negative SUN mutants and RNA interference, we show that HSV-1 propagation is dependent on the LINC complex. In contrast to pseudorabies virus, SUN2 disruption by either approach led to increased HSV-1 extracellular viral yields. This SUN2 dependency may be linked to its greater impact on perinuclear spacing in infected cells compared to SUN1. Finally, the virus itself seems to modulate perinuclear spacing. IMPORTANCE The large size of herpesviruses prevents them from travelling across the nuclear pores, and they instead egress across the two nuclear membranes, generating short-lived enveloped perinuclear virions. This poses a challenge as the perinuclear space is smaller than the virions. This implies the separation (unzipping) of the two nuclear membranes to accommodate the viral particles. The LINC complex bridges the two nuclear membranes and is an important regulator of perinuclear spacing. Work by others hint at its functional implication during pseudorabies virus and cytomegalovirus propagation. The present study probes the importance for HSV-1 of the SUN proteins, the LINC components found in the inner nuclear membrane. Using dominant negative constructs and RNA interference (RNAi), the data reveal that SUN2 exhibits antiviral propriety toward HSV-1, as disrupting the protein leads to increased viral yields. This is in contrast with that reported for pseudorabies and suggests that differences among herpesviruses may, once again, prevail.

Synaptotagmin 5 Controls SYP132-VAMP721/722 Interaction for Arabidopsis Immunity to Pseudomonas syringae pv tomato DC3000

Vesicle-associated membrane proteins 721 and 722 (VAMP721/722) are secretory vesicle-localized arginine-conserved soluble N-ethylmaleimide-sensitive factor attachment protein receptors (R-SNAREs) to drive exocytosis in plants. They are involved in diverse physiological processes in plants by interacting with distinct plasma membrane (PM) syntaxins. Here, we show that synaptotagmin 5 (SYT5) is involved in plant defense against Pseudomonas syringae pv tomato (Pst) DC3000 by regulating SYP132-VAMP721/722 interactions. Calcium-dependent stimulation of in vitro SYP132-VAMP722 interaction by SYT5 and reduced in vivo SYP132-VAMP721/722 interaction in syt5 plants suggest that SYT5 regulates the interaction between SYP132 and VAMP721/722. We interestingly found that disease resistance to Pst DC3000 bacterium but not to Erysiphe pisi fungus is compromised in syt5 plants. Since SYP132 plays an immune function to bacteria, elevated growth of surface-inoculated Pst DC3000 in VAMP721/722-deficient plants suggests that SYT5 contributes to plant immunity to Pst DC3000 by promoting the SYP132-VAMP721/722 immune secretory pathway.

Role of Lipid Transfer Proteins (LTPs) in the Viral Life Cycle

Viruses are obligate parasites that depend on the host cell machinery for their replication and dissemination. Cellular lipids play a central role in multiple stages of the viral life cycle such as entry, replication, morphogenesis, and egress. Most viruses reorganize the host cell membranes for the establishment of viral replication complex. These specialized structures allow the segregation of replicating viral RNA from ribosomes and protect it from host nucleases. They also facilitate localized enrichment of cellular components required for viral replication and assembly. The specific composition of the lipid membrane governs its ability to form negative or positive curvature and possess a rigid or flexible form, which is crucial for membrane rearrangement and establishment of viral replication complexes. In this review, we highlight how different viruses manipulate host lipid transfer proteins and harness their functions to enrich different membrane compartments with specific lipids in order to facilitate multiple aspects of the viral life cycle.

The Roles of Envelope Glycoprotein M in the Life Cycle of Some Alphaherpesviruses

The envelope glycoprotein M (gM), a surface virion component conserved among alphaherpesviruses, is a multiple-transmembrane domain-containing glycoprotein with a complex N-linked oligosaccharide. The gM mediates a diverse range of functions during the viral life cycle. In this review, we summarize the biological features of gM, including its characterization and function in some specicial alphaherpesviruses. gM modulates the virus-induced membrane fusion during virus invasion, transports other proteins to the appropriate intracellular membranes for primary and secondary envelopment during virion assembly, and promotes egress of the virus. The gM can interact with various viral and cellular components, and the focus of recent research has also been on interactions related to gM. And we will discuss how gM participates in the life cycle of alphaherpesviruses.

Endoplasmic Reticulum-Plasma Membrane Contact Sites: Regulators, Mechanisms, and Physiological Functions

The endoplasmic reticulum (ER) forms direct membrane contact sites with the plasma membrane (PM) in eukaryotic cells. These ER-PM contact sites play essential roles in lipid homeostasis, ion dynamics, and cell signaling, which are carried out by protein-protein or protein-lipid interactions. Distinct tethering factors dynamically control the architecture of ER-PM junctions in response to intracellular signals or external stimuli. The physiological roles of ER-PM contact sites are dependent on a variety of regulators that individually or cooperatively perform functions in diverse cellular processes. This review focuses on proteins functioning at ER-PM contact sites and highlights the recent progress in their mechanisms and physiological roles.

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

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

The XPO6 Exportin Mediates Herpes Simplex Virus 1 gM Nuclear Release Late in Infection

The glycoprotein M of herpes simplex virus 1 (HSV-1) is dynamically relocated from nuclear membranes to the trans-Golgi network (TGN) during infection, but molecular partners that promote this relocalization are unknown. Furthermore, while the presence of the virus is essential for this phenomenon, it is not clear if this is facilitated by viral or host proteins. Past attempts to characterize glycoprotein M (gM) interacting partners identified the viral protein gN by coimmunoprecipitation and the host protein E-Syt1 through a proteomics approach. Interestingly, both proteins modulate the activity of gM on the viral fusion machinery. However, neither protein is targeted to the nuclear membrane and consequently unlikely explains the dynamic regulation of gM nuclear localization. We thus reasoned that gM may transiently interact with other molecules. To resolve this issue, we opted for a proximity-dependent biotin identification (BioID) proteomics approach by tagging gM with a BirA* biotinylation enzyme and purifying BirA substrates on a streptavidin column followed by mass spectrometry analysis. The data identified gM and 170 other proteins that specifically and reproducibly were labeled by tagged gM at 4 or 12 h postinfection. Surprisingly, 35% of these cellular proteins are implicated in protein transport. Upon testing select candidate proteins, we discovered that XPO6, an exportin, is required for gM to be released from the nucleus toward the TGN. This is the first indication of a host or viral protein that modulates the presence of HSV-1 gM on nuclear membranes.IMPORTANCE The mechanisms that enable integral proteins to be targeted to the inner nuclear membrane are poorly understood. Herpes simplex virus 1 (HSV-1) glycoprotein M (gM) is an interesting candidate, as it is dynamically relocalized from nuclear envelopes to the trans-Golgi network (TGN) in a virus- and time-dependent fashion. However, it was, until now, unclear how gM was directed to the nucleus or evaded that compartment later on. Through a proteomic study relying on a proximity-ligation assay, we identified several novel gM interacting partners, many of which are involved in vesicular transport. Analysis of select proteins revealed that XPO6 is required for gM to leave the nuclear membranes late in the infection. This was unexpected, as XPO6 is an exportin specifically associated with actin/profilin nuclear export. This raises some very interesting questions about the interaction of HSV-1 with the exportin machinery and the cargo specificity of XPO6.

Sticking With It: ER-PM Membrane Contact Sites as a Coordinating Nexus for Regulating Lipids and Proteins at the Cell Cortex

Membrane contact sites between the cortical endoplasmic reticulum (ER) and the plasma membrane (PM) provide a direct conduit for small molecule transfer and signaling between the two largest membranes of the cell. Contact is established through ER integral membrane proteins that physically tether the two membranes together, though the general mechanism is remarkably non-specific given the diversity of different tethering proteins. Primary tethers including VAMP-associated proteins (VAPs), Anoctamin/TMEM16/Ist2p homologs, and extended synaptotagmins (E-Syts), are largely conserved in most eukaryotes and are both necessary and sufficient for establishing ER-PM association. In addition, other species-specific ER-PM tether proteins impart unique functional attributes to both membranes at the cell cortex. This review distils recent functional and structural findings about conserved and species-specific tethers that form ER-PM contact sites, with an emphasis on their roles in the coordinate regulation of lipid metabolism, cellular structure, and responses to membrane stress.

Functional interactions between herpes simplex virus pUL51, pUL7 and gE reveal cell-specific mechanisms for epithelial cell-to-cell spread

Herpes simplex virus spread between epithelial cells is mediated by virus tegument and envelope protein complexes including gE/gI and pUL51/pUL7. pUL51 interacts with both pUL7 and gE/gI in infected cells. We show that amino acids 30-90 of pUL51 mediate interaction with pUL7. We also show that deletion of amino acids 167-244 of pUL51, or ablation of pUL7 expression both result in failure of gE to concentrate at junctional surfaces of Vero cells. We also tested the hypothesis that gE and pUL51 function on the same pathway for cell-to-cell spread by analyzing the phenotype of a double gE/UL51 mutant. In HaCaT cells, pUL51 and gE function on the same spread pathway, whereas in Vero cells they function on different pathways. Deletion of the gE gene strongly enhanced virus release to the medium in Vero cells, suggesting that the gE-dependent spread pathway may compete with virion release to the medium.

Single nucleotide polymorphisms in candidate genes are significantly associated with resistance to Haemonchus contortus infection in goats

BACKGROUND

Haemonchosis is a major economic problem in goat production in humid, tropical and subtropical regions. The disease is caused by an abomasal nematode, Haemonchus contortus, which is highly pathogenic in small ruminants. The aim of this study was to identifying single-nucleotide polymorphisms (SNP) that were associated with fecal egg counts (FEC) and could be used as markers to identify resistance to H. contortus in goats.

RESULTS

Ten novel variants in the CIITA, ATP2A3, HSPA8, STAT5B, ESYT1, and SERPING1 genes were associated with FEC in goats with a nominal significance level of P < 0.05. Two missense mutation in the exon region of the caprine CIITA gene resulted in replacement of arginine with cysteine at position 9473550 (R9473550C) and aspartic acid with glutamic acid at position 9473870 (D9473870E). Chinese goat breeds had significantly higher FEC than Bangladeshi goat breeds within their respective genotypes. Polymorphism information content (PIC), effective allele number (Ne), and heterozygosity (He) were greatest for the STAT5B_197_A > G SNP locus in all goat breeds. Pairwise coefficients of linkage disequilibrium (D´, r 2) revealed complete LD (r 2 = 1) between significant SNP polymorphisms in CIITA and SERPING1 and strong LD (r 2 = 0.93 and 0.98) between polymorphisms in HSPA8 and ATP2A3, respectively. Correlation coefficient (r) between FEC and body weight (BW) was significantly positive (r = 0.56***, P < 0.001) but that between FEC and packed cell volume (PCV) was negatively significant (r = - 0.47**, P < 0.01) in the total population of goats. On the other hand, correlation coefficient (r) between BW and PCV was not significant in total population of goats. Association analysis revealed that haplotypes within ATP2A3, HSPA8, and SERPING1 were significantly associated with FEC. Quantitative real-time PCR revealed that the relative expression of mRNA was higher (P < 0.001) for resistant, compared to susceptible, groups of goats for all candidate genes except CIITA.

CONCLUSIONS

This study identified SNP markers that can potentially be used in marker-assisted selection programs to develop goat breeds that are resistant to H. contortus.

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.

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.