Myosin Va enhances secretion of herpes simplex virus 1 virions and cell surface expression of viral glycoproteins

Rab27a GTPase and its effector Myosin Va are host factors required for efficient Oropouche virus cell egress

Oropouche fever, a debilitating illness common in South America, is caused by Oropouche virus (OROV), an arbovirus. OROV belongs to the Peribunyaviridae family, a large group of RNA viruses. Little is known about the biology of Peribunyaviridae in host cells, especially assembly and egress processes. Our research reveals that the small GTPase Rab27a mediates intracellular transport of OROV induced compartments and viral release from infected cells. We show that Rab27a interacts with OROV glycoproteins and colocalizes with OROV during late phases of the infection cycle. Moreover, Rab27a activity is required for OROV trafficking to the cell periphery and efficient release of infectious particles. Consistently, depleting Rab27a's downstream effector, Myosin Va, or inhibiting actin polymerization also hinders OROV compartments targeting to the cell periphery and infectious viral particle egress. These data indicate that OROV hijacks Rab27a activity for intracellular transport and cell externalization. Understanding these crucial mechanisms of OROV's replication cycle may offer potential targets for therapeutic interventions and aid in controlling the spread of Oropouche fever.

Current Insights into the Maturation of Epstein-Barr Virus Particles

The three subfamilies of herpesviruses (alphaherpesviruses, betaherpesviruses, and gammaherpesviruses) appear to share a unique mechanism for the maturation and egress of virions, mediated by several budding and fusion processes of various organelle membranes during replication, which prevents cellular membrane disruption. Newly synthesized viral DNA is packaged into capsids within the nucleus, which are subsequently released into the cytoplasm via sequential fusion (primary envelopment) and budding through the inner and outer nuclear membranes. Maturation concludes with tegumentation and the secondary envelopment of nucleocapsids, which are mediated by budding into various cell organelles. Intracellular compartments containing mature virions are transported to the plasma membrane via host vesicular trafficking machinery, where they fuse with the plasma membrane to extracellularly release mature virions. The entire process of viral maturation is orchestrated by sequential interactions between viral proteins and intracellular membranes. Compared with other herpesvirus subfamilies, the mechanisms of gammaherpesvirus maturation and egress remain poorly understood. This review summarizes the major findings, including recently updated information of the molecular mechanism underlying the maturation and egress process of the Epstein-Barr virus, a ubiquitous human gammaherpesvirus subfamily member that infects most of the population worldwide and is associated with a number of human malignancies.

MYO5A overexpression promotes invasion and correlates with low lymphocyte infiltration in head and neck squamous carcinoma

Head and neck squamous carcinoma (HNSC) poses a significant public health challenge due to its substantial morbidity. Nevertheless, despite advances in current treatments, the prognosis for HNSC remains unsatisfactory. To address this, single-cell RNA sequencing (RNA-seq) and bulk RNA-seq data combined with in vitro studies were conducted to examine the role of MYO5A (Myosin VA) in HNSC. Our investigation revealed an overexpression of MYO5A in HNSC that promotes HNSC migration in vitro. Remarkably, knockdown of MYO5A suppressed vimentin expression. Furthermore, analyzing the TCGA database evidenced that MYO5A is a risk factor for human papillomavirus positive (HPV+) HNSC (HR = 0.81, P < 0.001). In high MYO5A expression HNSC, there was a low count of tumor infiltrating lymphocytes (TIL), including activated CD4+ T cells, CD8+ T cells, and B cells. Of note, CD4+ T cells and B cells were positively associated with improved HPV+ HNSC outcomes. Correlation analysis demonstrated a decreased level of immunostimulators in high MYO5A-expressing HNSC. Collectively, these findings suggest that MYO5A may promote HNSC migration through vimentin and involve itself in the process of immune infiltration in HNSC, advancing the understanding of the mechanisms and treatment of HNSC.

Genomic and ultrastructural analysis of monkeypox virus in skin lesions and in human/animal infected cells reveals further morphofunctional insights into viral pathogenicity

Monkeypox (MPOX) is a zoonotic disease that affects humans and other primates, resulting in a smallpox-like illness. It is caused by monkeypox virus (MPXV), which belongs to the Poxviridae family. Clinically manifested by a range of cutaneous and systemic findings, as well as variable disease severity phenotypes based on the genetic makeup of the virus, the cutaneous niche and respiratory mucosa are the epicenters of MPXV pathogenicity. Herein, we describe the ultrastructural features of MPXV infection in both human cultured cells and cutaneous clinical specimens collected during the 2022-2023 MPOX outbreak in New York City that were revealed through electron microscopy. We observed typical enveloped virions with brick-shaped morphologies that contained surface protrusions, consistent with the classic ultrastructural features of MPXV. In addition, we describe morpho-functional evidence that point to roles of distinct cellular organelles in viral assembly during clinical MPXV infection. Interestingly, in skin lesions, we found abundant melanosomes near viral assembly sites, particularly in the vicinity of mature virions, which provides further insight into virus-host interactions at the subcellular level that contribute to MPXV pathogenesis. These findings not only highlight the importance of electron microscopic studies for further investigation of this emerging pathogen but also in characterizing MPXV pathogenesis during human infection.

Nectin-1 and Non-muscle Myosin Heavy Chain-IIB: Major Mediators of Herpes Simplex Virus-1 Entry Into Corneal Nerves

Herpes Simplex Virus 1 (HSV-1) invades corneal nerves upon its infection of the cornea and then establishes latency in the trigeminal ganglion (TG). The latent virus in TG is often reactivated and travels back to the cornea, causing recurrent herpes simplex keratitis (HSK). The entry of HSV-1 into the corneal nerve is considered the initial step of infection resulting in HSV-1 latency and HSK recurrence. Several gD and gB receptors have been identified, including nectin-1, herpes virus entry medium (HVEM) and 3-O-sulfated heparan sulfate (3-OS-HS) as gD receptors, and non-muscle myosin heavy chain IIA (NMHC-IIA), NMHC-IIB and myelin-associated glycoprotein (MAG) as gB receptors. However, which receptors contribute to the entry of HSV-1 into corneal nerves are yet to be determined. This study observed that receptors nectin-1, HVEM, 3-OS-HS, NMHC-IIA, and NMHC-IIB, not MAG, were expressed in healthy corneal nerves. Further, we cultured TG neurons extracted from mice in vitro to screen for functional gD/gB receptors. Both in vitro siRNA knockdown and in vivo antibody blocking of either nectin-1 or NMHC-IIB reduced the entry and the replication of HSV-1 as shown by qPCR analysis and immunofluorescence measure, respectively. Also, we observed that the re-localization and the upregulation expression of NMHC-IIB after HSV-1 exposure were inhibited when gD receptor nectin-1 was knocked down. These data suggest that nectin-1 was the main gD receptor and NMHC-IIB was the main gB receptor in mediating HSV-1 entry and hold promise as therapeutic targets for resolving HSV-1 latency and HSK recurrence.

Understanding the key functions of Myosins in viral infection

Myosins, a class of actin-based motor proteins existing in almost any organism, are originally considered only involved in driving muscle contraction, reshaping actin cytoskeleton, and anchoring or transporting cargoes, including protein complexes, organelles, vesicles. However, accumulating evidence reveals that myosins also play vital roles in viral infection, depending on viral species and infection stages. This review systemically summarizes the described various myosins, the performed functions, and the involved mechanisms or molecular pathways during viral infection. Meanwhile, the existing issues are also discussed. Additionally, the important technologies or agents, including siRNA, gene editing, and myosin inhibitors, would facilitate dissecting the actions and mechanisms for described and undescribed myosins, which could be adopted to prevent or control viral infection are also characterized.

Herpes simplex virus-1 utilizes the host actin cytoskeleton for its release from axonal growth cones

Herpes simplex virus type 1 (HSV-1) has evolved mechanisms to exploit the host cytoskeleton during entry, replication and exit from cells. In this study, we determined the role of actin and the molecular motor proteins, myosin II and myosin V, in the transport and release of HSV-1 from axon termini, or growth cones. Using compartmentalized neuronal devices, we showed that inhibition of actin polymerization, but not actin branching, significantly reduced the release of HSV-1 from axons. Furthermore, we showed that inhibition of myosin V, but not myosin II, also significantly reduced the release of HSV-1 from axons. Using confocal and electron microscopy, we determined that viral components are transported along axons to growth cones, despite actin or myosin inhibition. Overall, our study supports the role of actin in virus release from axonal growth cones and suggests myosin V as a likely candidate involved in this process.

Herpes simplex virus type 1 (HSV-1) is a ubiquitous human pathogen causing cold sores and genital herpes. HSV-1 infects sensory neurons of the peripheral nervous system where it establishes a lifelong infection and cannot be cured. Reactivation is common, with the virus transported back along sensory nerves, forming new lesions, or is shed asymptomatically. Antiviral resistance is emerging to current antivirals that target viral replication, indicating the need to identify new targets for future treatment. The host cell cytoskeleton plays an important role during transport of the virus. HSV-1 is transported along axons via microtubules; however, how the virus is released from axon termini, where actin predominates, is unknown. Here we show that an intact actin cytoskeleton is required for efficient virus release from axon termini. Furthermore, we show that myosin V, an actin based molecular motor that drives transport, is essential in virus release from axon termini. Together, this study defines the mechanisms behind HSV-1 release from axon termini which will guide future directions in identifying possible therapeutic targets for HSV-1.

Release of HSV-1 Cell-Free Virions: Mechanisms, Regulation, and Likely Role in Human-Human Transmission

Herpes simplex virus type 1, or HSV-1, is a widespread human pathogen that replicates in epithelial cells of the body surface and then establishes latent infection in peripheral neurons. When HSV-1 replicates, viral progeny must be efficiently released to spread infection to new target cells. Viral spread occurs via two major routes. In cell-cell spread, progeny virions are delivered directly to cellular junctions, where they infect adjacent cells. In cell-free release, progeny virions are released into the extracellular milieu, potentially allowing the infection of distant cells. Cell-cell spread of HSV-1 has been well studied and is known to be important for in vivo infection and pathogenesis. In contrast, HSV-1 cell-free release has received less attention, and its significance to viral biology is unclear. Here, I review the mechanisms and regulation of HSV-1 cell-free virion release. Based on knowledge accrued in other herpesviral systems, I argue that HSV-1 cell-free release is likely to be tightly regulated in vivo. Specifically, I hypothesize that this process is generally suppressed as the virus replicates within the body, but activated to high levels at sites of viral reactivation, such as the oral mucosa and skin, in order to promote efficient transmission of HSV-1 to new human hosts.

Motor Skills: Recruitment of Kinesins, Myosins and Dynein during Assembly and Egress of Alphaherpesviruses

The alphaherpesviruses are pathogens of the mammalian nervous system. Initial infection is commonly at mucosal epithelia, followed by spread to, and establishment of latency in, the peripheral nervous system. During productive infection, viral gene expression, replication of the dsDNA genome, capsid assembly and genome packaging take place in the infected cell nucleus, after which mature nucleocapsids emerge into the cytoplasm. Capsids must then travel to their site of envelopment at cytoplasmic organelles, and enveloped virions need to reach the cell surface for release and spread. Transport at each of these steps requires movement of alphaherpesvirus particles through a crowded and viscous cytoplasm, and for distances ranging from several microns in epithelial cells, to millimeters or even meters during egress from neurons. To solve this challenging problem alphaherpesviruses, and their assembly intermediates, exploit microtubule- and actin-dependent cellular motors. This review focuses upon the mechanisms used by alphaherpesviruses to recruit kinesin, myosin and dynein motors during assembly and egress.

Exploration of binary protein-protein interactions between tick-borne flaviviruses and Ixodes ricinus

Background

Louping ill virus (LIV) and tick-borne encephalitis virus (TBEV) are tick-borne flaviviruses that are both transmitted by the major European tick, Ixodes ricinus. Despite the importance of I. ricinus as an arthropod vector, its capacity to acquire and subsequently transmit viruses, known as vector competence, is poorly understood. At the molecular scale, vector competence is governed in part by binary interactions established between viral and cellular proteins within infected tick cells.

Methods

To investigate virus-vector protein–protein interactions (PPIs), the entire set of open reading frames for LIV and TBEV was screened against an I. ricinus cDNA library established from three embryonic tick cell lines using yeast two-hybrid methodology (Y2H). PPIs revealed for each viral bait were retested in yeast by applying a gap repair (GR) strategy, and notably against the cognate protein of both viruses, to determine whether the PPIs were specific for a single virus or common to both. The interacting tick proteins were identified by automatic BLASTX, and in silico analyses were performed to expose the biological processes targeted by LIV and TBEV.

Results

For each virus, we identified 24 different PPIs involving six viral proteins and 22 unique tick proteins, with all PPIs being common to both viruses. According to our data, several viral proteins (pM, M, NS2A, NS4A, 2K and NS5) target multiple tick protein modules implicated in critical biological pathways. Of note, the NS5 and pM viral proteins establish PPI with several tumor necrosis factor (TNF) receptor-associated factor (TRAF) proteins, which are essential adaptor proteins at the nexus of multiple signal transduction pathways.

Conclusion

We provide the first description of the TBEV/LIV-I. ricinus PPI network, and indeed of any PPI network involving a tick-borne virus and its tick vector. While further investigation will be needed to elucidate the role of each tick protein in the replication cycle of tick-borne flaviviruses, our study provides a foundation for understanding the vector competence of I. ricinus at the molecular level. Indeed, certain PPIs may represent molecular determinants of vector competence of I. ricinus for TBEV and LIV, and potentially for other tick-borne flaviviruses.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-021-04651-3.

A putative WAVE regulatory complex (WRC) interacting receptor sequence (WIRS) in the cytoplasmic tail of HSV-1 gE does not function in WRC recruitment or neuronal transport

HSV-1 envelope glycoprotein E (gE) is important for viral egress and cell-to-cell spread but the host protein(s) involved in these functions have yet to be determined. We aimed to investigate a role for the Arp2/3 complex and actin regulation in viral egress based on the identification of a WAVE Regulatory Complex (WRC) Interacting Receptor Sequence (WIRS) in the cytoplasmic tail (CT) of gE. A WIRS-dependent interaction between the gE(CT) and subunits of the WRC was demonstrated by GST-pulldown assay and a role for the Arp2/3 complex in cell-to-cell spread was also observed by plaque assay. Subsequent study of a recombinant HSV-1 gE WIRS-mutant found no significant changes to viral production and release based on growth kinetics studies, or changes to plaque and comet size in various cell types, suggesting no function for the motif in cell-to-cell spread. GFP-Trap pulldown and proximity ligation assays were unable to confirm a WIRS-dependent interaction between gE and the WRC in human cell lines though the WIRS-independent interaction observed in situ warrants further study. Confocal microscopy of infected cells of neuronal origin identified no impairment of gE WIRS-mutant HSV-1 anterograde transport along axons. We propose that the identified gE WIRS motif does not function directly in recruitment of the WRC in human cells, in cell-to-cell spread of virus or in anterograde transport along axons. Further studies are needed to understand how HSV-1 manipulates and traverses the actin cytoskeleton and how gE may contribute to these processes in a WIRS-independent manner.

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.

HSV-1 Cytoplasmic Envelopment and Egress

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

Epstein-Barr Virus Exploits the Secretory Pathway to Release Virions

Herpesvirus egress mechanisms are strongly associated with intracellular compartment remodeling processes. Previously, we and other groups have described that intracellular compartments derived from the Golgi apparatus are the maturation sites of Epstein-Barr virus (EBV) virions. However, the mechanism by which these virions are released from the host cell to the extracellular milieu is poorly understood. Here, I adapted two independent induction systems of the EBV lytic cycle in vitro, in the context of Rab GTPase silencing, to characterize the EBV release pathway. Immunofluorescence staining revealed that p350/220, the major EBV glycoprotein, partially co-localized with three Rab GTPases: Rab8a, Rab10, and Rab11a. Furthermore, the knockdown of these Rab GTPases promoted the intracellular accumulation of viral structural proteins by inhibiting its distribution to the plasma membrane. Finally, the knockdown of the Rab8a, Rab10, and Rab11a proteins suppressed the release of EBV infectious virions. Taken together, these findings support the hypothesis that mature EBV virions are released from infected cells to the extracellular milieu via the secretory pathway, as well as providing new insights into the EBV life cycle.

Current and Emerging Therapies for Ocular Herpes Simplex Virus Type-1 Infections

Herpes simplex virus type-1 (HSV-1) is a neurotropic, double-stranded DNA virus that can cause a wide variety of diseases, including many ocular pathologies. It is one of the leading causes of infectious blindness in the United States. Because of its ubiquitous nature and its potential to cause serious ocular maladies, there is a significant need for more effective antiviral therapies against ocular HSV-1. In this review, we discuss the lifecycle of HSV-1 as it pertains to corneal infections and the clinically approved as well as emerging treatments to combat HSV-1 infections. We also highlight some newly identified host targets for the antiviral drug development.

Proteomics Analysis Identifies IRSp53 and Fascin as Critical for PRV Egress and Direct Cell-Cell Transmission

Pseudorabies virus (PRV) has been widely used as a live trans-synaptic tracer for mapping neuronal circuits. Systematically identifying mature PRV virion proteomes and defining co-purified host proteins are necessary to fully understand the detailed mechanism underlying PRV transmission processes. Here, a PRV virion purification strategy based on sorting with flow cytometry is developed and the mature extracellular and intracellular PRV virion proteomes using LC coupled with MS/MS are characterized. In addition to viral proteins, a large number of host proteins are also identified, including proteins related to actin cytoskeletal dynamics and membrane protrusion. How many of these host proteins are true virion components are unknown and the majority of these may not be. Through functional analysis, it is found that IRSp53 and fascin are critical for the egress process and play a role in direct cell-cell transmission. Moreover, it is shown that CDC42 and Rac1 are also involved in the production of mature extracellular virions. The results suggest that the formation of the filopodia-like cytoskeleton and the rearrangement of the membrane, which are both associated with IRSp53 and fascin, may be important for the transmission of viruses used in neuronal tracing.

Differences in gene expression in whitefly associated with CYSDV-infected and virus-free melon, and comparison with expression in whiteflies fed on ToCV- and TYLCV-infected tomato

Background

Cucurbit yellow stunting disorder virus (CYSDV; genus Crinivirus, Closteroviridae) is transmitted in a semipersistent manner by the whitefly, Bemisia tabaci, and is efficiently transmitted by the widely prevalent B. tabaci cryptic species, MEAM1. In this study, we compared transcriptome profiles of B. tabaci MEAM1, after 24 h, 72 h and 7 days of acquisition feeding on melon plants infected with CYSDV (CYSDV-whiteflies) with those fed on virus-free melon, using RNA-Seq technology. We also compared transcriptome profiles with whiteflies fed on tomato plants separately infected with Tomato chlorosis virus (ToCV), a crinivirus closely related to CYSDV, and Tomato yellow leaf curl virus (TYLCV), a member of the genus Begomovirus, which has a distinctly different mode of transmission and their respective virus-free controls, to find common gene expression changes among viruliferous whiteflies feeding on different host plants infected with distinct (TYLCV) and related (CYSDV and ToCV) viruses.

Results

A total of 275 differentially expressed genes (DEGs) were identified in CYSDV-whiteflies, with 3 DEGs at 24 h, 221 DEGs at 72 h, and 51 DEGs at 7 days of virus acquisition. Changes in genes encoding orphan genes (54 genes), phosphatidylethanolamine-binding proteins (PEBP) (20 genes), and AAA-ATPase domain containing proteins (10 genes) were associated with the 72 h time point. Several more orphan genes (20 genes) were differentially expressed at 7 days. A total of 59 common DEGs were found between CYSDV-whiteflies and ToCV-whiteflies, which included 20 orphan genes and 6 lysosomal genes. A comparison of DEGs across the three different virus-host systems revealed 14 common DEGs, among which, eight showed similar and significant up-regulation in CYSDV-whiteflies at 72 h and TYLCV-whiteflies at 24 h, while down-regulation of the same genes was observed in ToCV-whiteflies at 72 h.

Conclusions

Dynamic gene expression changes occurred in CYSDV-whiteflies after 72 h feeding, with decreased gene expression changes associated with 7 days of CYSDV acquisition. Similarities in gene expression changes among CYSDV-whiteflies, ToCV-whiteflies and TYLCV-whiteflies suggest the possible involvement of common genes or pathways for virus acquisition and transmission by whiteflies, even for viruses with distinctly different modes of transmission.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5999-0) contains supplementary material, which is available to authorized users.

Kinetics of Mimivirus Infection Stages Quantified Using Image Flow Cytometry

Due to the heterogeneity of viruses and their hosts, a comprehensive view of viral infection is best achieved by analyzing large populations of infected cells. However, information regarding variation in infected cell populations is lost in bulk measurements. Motivated by an interest in the temporal progression of events in virally infected cells, we used image flow cytometry (IFC) to monitor changes in Acanthamoeba polyphaga cells infected with Mimivirus. This first use of IFC to study viral infection required the development of methods to preserve morphological features of adherent amoeba cells prior to detachment and analysis in suspension. It also required the identification of IFC parameters that best report on key events in the Mimivirus infection cycle. The optimized IFC protocol enabled the simultaneous monitoring of diverse processes including generation of viral factories, transport, and fusion of replication centers within the cell, accumulation of viral progeny, and changes in cell morphology for tens of thousands of cells. After obtaining the time windows for these processes, we used IFC to evaluate the effects of perturbations such as oxidative stress and cytoskeletal disruptors on viral infection. Accurate dose‐response curves could be generated, and we found that mild oxidative stress delayed multiple stages of virus production, but eventually infection processes occurred with approximately the same amplitudes. We also found that functional actin cytoskeleton is required for fusion of viral replication centers and later for the production of viral progeny. Through this report, we demonstrate that IFC offers a quantitative, high‐throughput, and highly robust approach to study viral infection cycles and virus–host interactions. © The Authors. Cytometry Part A published by Wiley Periodicals, Inc. on behalf of International Society for Advancement of Cytometry.

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.

Experimental Dissection of the Lytic Replication Cycles of Herpes Simplex Viruses in vitro

Herpes simplex viruses type 1 and type 2 (HSV-1 and HSV-2) produce lifelong infections and are highly prevalent in the human population. Both viruses elicit numerous clinical manifestations and produce mild-to-severe diseases that affect the skin, eyes, and brain, among others. Despite the existence of numerous antivirals against HSV, such as acyclovir and acyclovir-related analogs, virus variants that are resistant to these compounds can be isolated from immunosuppressed individuals. For such isolates, second-line drugs can be used, yet they frequently produce adverse side effects. Furthermore, topical antivirals for treating cutaneous HSV infections usually display poor to moderate efficacy. Hence, better or novel anti-HSV antivirals are needed and details on their mechanisms of action would be insightful for improving their efficacy and identifying specific molecular targets. Here, we review and dissect the lytic replication cycles of herpes simplex viruses, discussing key steps involved in cell infection and the processes that yield new virions. Additionally, we review and discuss rapid, easy-to-perform and simple experimental approaches for studying key steps involved in HSV replication to facilitate the identification of the mechanisms of action of anti-HSV compounds.

Remodeling of host membranes during herpesvirus assembly and egress

Many viruses, enveloped or non-enveloped, remodel host membrane structures for their replication, assembly and escape from host cells. Herpesviruses are important human pathogens and cause many diseases. As large enveloped DNA viruses, herpesviruses undergo several complex steps to complete their life cycles and produce infectious progenies. Firstly, herpesvirus assembly initiates in the nucleus, producing nucleocapsids that are too large to cross through the nuclear pores. Nascent nucleocapsids instead bud at the inner nuclear membrane to form primary enveloped virions in the perinuclear space followed by fusion of the primary envelopes with the outer nuclear membrane, to translocate the nucleocapsids into the cytoplasm. Secondly, nucleocapsids obtain a series of tegument proteins in the cytoplasm and bud into vesicles derived from host organelles to acquire viral envelopes. The vesicles are then transported to and fuse with the plasma membrane to release the mature virions to the extracellular space. Therefore, at least two budding and fusion events take place at cellular membrane structures during herpesviruses assembly and egress, which induce membrane deformations. In this review, we describe and discuss how herpesviruses exploit and remodel host membrane structures to assemble and escape from the host cell.

A Role for Myosin Va in Human Cytomegalovirus Nuclear Egress

Herpesviruses replicate and package their genomes into capsids in replication compartments within the nuclear interior. Capsids then move to the inner nuclear membrane for envelopment and release into the cytoplasm in a process called nuclear egress. We previously found that nuclear F-actin is induced upon infection with the betaherpesvirus human cytomegalovirus (HCMV) and is important for nuclear egress and capsid localization away from replication compartment-like inclusions toward the nuclear rim. Despite these and related findings, it has not been shown that any specific motor protein is involved in herpesvirus nuclear egress. In this study, we have investigated whether the host motor protein, myosin Va, could be fulfilling this role. Using immunofluorescence microscopy and coimmunoprecipitation, we observed associations between a nuclear population of myosin Va and the viral major capsid protein, with both concentrating at the periphery of replication compartments. Immunoelectron microscopy showed that nearly 40% of assembled nuclear capsids associate with myosin Va. We also found that myosin Va and major capsid protein colocalize with nuclear F-actin. Importantly, antagonism of myosin Va with RNA interference or a dominant negative mutant revealed that myosin Va is important for the efficient production of infectious virus, capsid accumulation in the cytoplasm, and capsid localization away from replication compartment-like inclusions toward the nuclear rim. Our results lead us to suggest a working model whereby human cytomegalovirus capsids associate with myosin Va for movement from replication compartments to the nuclear periphery during nuclear egress.IMPORTANCE Little is known regarding how newly assembled and packaged herpesvirus capsids move from the nuclear interior to the periphery during nuclear egress. While it has been proposed that an actomyosin-based mechanism facilitates intranuclear movement of alphaherpesvirus capsids, a functional role for any specific myosin in nuclear egress has not been reported. Furthermore, the notion that an actomyosin-based mechanism facilitates intranuclear capsid movement is controversial. Here we show that human cytomegalovirus capsids associate with nuclear myosin Va and F-actin and that antagonism of myosin Va impairs capsid localization toward the nuclear rim and nuclear egress. Together with our previous results showing that nuclear F-actin is induced upon HCMV infection and is also important for these processes, our results lend support to the hypothesis that nascent human cytomegalovirus capsids migrate to the nuclear periphery via actomyosin-based movement. These results shed light on a poorly understood viral process and the cellular machinery involved.

Infection and Transport of Herpes Simplex Virus Type 1 in Neurons: Role of the Cytoskeleton

Herpes simplex virus type 1 (HSV-1) is a neuroinvasive human pathogen that has the ability to infect and replicate within epithelial cells and neurons and establish a life-long latent infection in sensory neurons. HSV-1 depends on the host cellular cytoskeleton for entry, replication, and exit. Therefore, HSV-1 has adapted mechanisms to promote its survival by exploiting the microtubule and actin cytoskeletons to direct its active transport, infection, and spread between neurons and epithelial cells during primary and recurrent infections. This review will focus on the currently known mechanisms utilized by HSV-1 to harness the neuronal cytoskeleton, molecular motors, and the secretory and exocytic pathways for efficient virus entry, axonal transport, replication, assembly, and exit from the distinct functional compartments (cell body and axon) of the highly polarized sensory neurons.

Cytoskeletons in the Closet-Subversion in Alphaherpesvirus Infections

Actin filaments, microtubules and intermediate filaments form the cytoskeleton of vertebrate cells. Involved in maintaining cell integrity and structure, facilitating cargo and vesicle transport, remodelling surface structures and motility, the cytoskeleton is necessary for the successful life of a cell. Because of the broad range of functions these filaments are involved in, they are common targets for viral pathogens, including the alphaherpesviruses. Human-tropic alphaherpesviruses are prevalent pathogens carried by more than half of the world’s population; comprising herpes simplex virus (types 1 and 2) and varicella-zoster virus, these viruses are characterised by their ability to establish latency in sensory neurons. This review will discuss the known mechanisms involved in subversion of and transport via the cytoskeleton during alphaherpesvirus infections, focusing on protein-protein interactions and pathways that have recently been identified. Studies on related alphaherpesviruses whose primary host is not human, along with comparisons to more distantly related beta and gammaherpesviruses, are also presented in this review. The need to decipher as-yet-unknown mechanisms exploited by viruses to hijack cytoskeletal components—to reveal the hidden cytoskeletons in the closet—will also be addressed.

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.

Comparative protein profiling of B16 mouse melanoma cells susceptible and non-susceptible to alphavirus infection: Effect of the tumor microenvironment

Alphavirus vectors are promising tools for cancer treatment. However, relevant entry mechanisms and interactions with host cells are still not clearly understood. The first step toward a more effective therapy is the identification of novel intracellular alterations that could be associated with cancer aggressiveness and could affect the therapeutic potential of these vectors. In this study, we observed that alphaviruses efficiently infected B16 mouse melanoma tumors/tumor cells in vivo, whereas their transduction efficiency in B16 cells under in vitro conditions was blocked. Therefore, we further aimed to understand the mechanisms pertaining to the differential transduction efficacy of alphaviruses in B16 tumor cells under varying growth conditions. We hypothesized that the tumor microenvironment might alter gene expression in B16 cells, leading to an up-regulation of the expression of virus-binding receptors or factors associated with virus entry and replication. To test our hypothesis, we performed a proteomics analysis of B16 cells cultured in vitro and of B16 cells isolated from tumors, and we identified 277 differentially regulated proteins. A further in-depth analysis to identify the biological and molecular functions of the detected proteins revealed a set of candidate genes that could affect virus infectivity. Importantly, we observed a decrease in the expression of interferon α (IFN-α) in tumor-isolated cells that resulted in the suppression of several IFN-regulated genes, thereby abrogating host cell antiviral defense. Additionally, differences in the expression of genes that regulate cytoskeletal organization caused significant alterations in cell membrane elasticity. Taken together, our findings demonstrated favorable intracellular conditions for alphavirus transduction/replication that occurred during tumor transformation. These results pave the way for optimizing the development of strategies for the application of alphaviral vectors as a potent cancer therapy.

Genital Herpes: Insights into Sexually Transmitted Infectious Disease

Etiology, transmission and protection: Herpes simplex virus-2 (HSV-2) is a leading cause of sexually transmitted infections with recurring manifestations throughout the lifetime of infected hosts. Currently no effective vaccines or prophylactics exist that provide complete protection or immunity from the virus, which is endemic throughout the world. Pathology/Symptomatology: Primary and recurrent infections result in lesions and inflammation around the genital area and the latter accounts for majority of genital herpes instances. Immunocompromised patients including neonates are susceptible to additional systemic infections including debilitating consequences of nervous system inflammation. Epidemiology, incidence and prevalence: More than 500 million people are infected worldwide and most reported cases involve the age groups between 16-40 years, which coincides with an increase in sexual activity among this age group. While these numbers are an estimate, the actual numbers may be underestimated as many people are asymptomatic or do not report the symptoms. Treatment and curability: Currently prescribed medications, mostly nucleoside analogs, only reduce the symptoms caused by an active infection, but do not eliminate the virus or reduce latency. Therefore, no cure exists against genital herpes and infected patients suffer from periodic recurrences of disease symptoms for their entire lives. Molecular mechanisms of infection: The last few decades have generated many new advances in our understanding of the mechanisms that drive HSV infection. The viral entry receptors such as nectin-1 and HVEM have been identified, cytoskeletal signaling and membrane structures such as filopodia have been directly implicated in viral entry, host motor proteins and their viral ligands have been shown to facilitate capsid transport and many host and HSV proteins have been identified that help with viral replication and pathogenesis. New understanding has emerged on the role of autophagy and other innate immune mechanisms that are subverted to enhance HSV pathogenesis. This review summarizes our current understanding of HSV-2 and associated diseases and available or upcoming new treatments.

Tegument Assembly and Secondary Envelopment of Alphaherpesviruses

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

Nuclear actin and myosins in adenovirus infection

Adenovirus serotypes have been shown to cause drastic changes in nuclear organization, including the transcription machinery, during infection. This ability of adenovirus to subvert transcription in the host cell facilitates viral replication. Because nuclear actin and nuclear myosin I, myosin V and myosin VI have been implicated as direct regulators of transcription and important factors in the replication of other viruses, we sought to determine how nuclear actin and myosins are involved in adenovirus infection. We first confirmed reorganization of the host's transcription machinery to viral replication centers. We found that nuclear actin also reorganizes to sites of transcription through the intermediate but not the advanced late phase of viral infection. Furthermore, nuclear myosin I localized with nuclear actin and sites of transcription in viral replication centers. Intriguingly, nuclear myosins V and VI, which also reorganized to viral replication centers, exhibited different localization patterns, suggesting specialized roles for these nuclear myosins. Finally, we assessed the role of actin in adenovirus infection and found both cytoplasmic and nuclear actin likely play roles in adenovirus infection and replication. Together our data suggest the involvement of actin and multiple myosins in the nuclear replication and late viral gene expression of adenovirus.

Cellular mechanisms of alpha herpesvirus egress: live cell fluorescence microscopy of pseudorabies virus exocytosis

Egress of newly assembled herpesvirus particles from infected cells is a highly dynamic process involving the host secretory pathway working in concert with viral components. To elucidate the location, dynamics, and molecular mechanisms of alpha herpesvirus egress, we developed a live-cell fluorescence microscopy method to visualize the final transport and exocytosis of pseudorabies virus (PRV) particles in non-polarized epithelial cells. This method is based on total internal reflection fluorescence (TIRF) microscopy to selectively image fluorescent virus particles near the plasma membrane, and takes advantage of a virus-encoded pH-sensitive probe to visualize the precise moment and location of particle exocytosis. We performed single-particle tracking and mean squared displacement analysis to characterize particle motion, and imaged a panel of cellular proteins to identify those spatially and dynamically associated with viral exocytosis. Based on our data, individual virus particles travel to the plasma membrane inside small, acidified secretory vesicles. Rab GTPases, Rab6a, Rab8a, and Rab11a, key regulators of the plasma membrane-directed secretory pathway, are present on the virus secretory vesicle. These vesicles undergo fast, directional transport directly to the site of exocytosis, which is most frequently near patches of LL5β, part of a complex that anchors microtubules to the plasma membrane. Vesicles are tightly docked at the site of exocytosis for several seconds, and membrane fusion occurs, displacing the virion a small distance across the plasma membrane. After exocytosis, particles remain tightly confined on the outer cell surface. Based on recent reports in the cell biological and alpha herpesvirus literature, combined with our spatial and dynamic data on viral egress, we propose an integrated model that links together the intracellular transport pathways and exocytosis mechanisms that mediate alpha herpesvirus egress.

Pseudorabies virus, an alpha herpesvirus, is an important veterinary pathogen, and related to human varicella-zoster virus and herpes simplex viruses. New alpha herpesvirus particles are assembled inside an infected cell, and must exit from the infected cell by taking advantage of cellular mechanisms. How these virus particles are transported inside the infected cell and secreted at the cell surface is not understood in great detail. In particular, how this process unfolds over time is not easily observed using previous methods. In this study, we developed a new method to observe this egress process. Using this method, we described how virus particles move on their way out: individual virus particles travel to the cell surface, directly to the exit site, where they pause for several seconds before crossing out of the cell. We identified several cellular proteins that are involved in this process. After exiting, virus particles remained stuck to the outer cell surface. Finally, we draw connections between our observations and other recent studies to propose an integrated model of how alpha herpesvirus particles exit from infected cells.

Nonmuscle myosin heavy chain IIb mediates herpes simplex virus 1 entry

Nonmuscle myosin heavy chain IIA (NMHC-IIA) has been reported to function as a herpes simplex virus 1 (HSV-1) entry coreceptor by interacting with viral envelope glycoprotein B (gB). Vertebrates have three genetically distinct isoforms of the NMHC-II, designated NMHC-IIA, NMHC-IIB, and NMHC-IIC. COS cells, which are readily infected by HSV-1, do not express NMHC-IIA but do express NMHC-IIB. This observation prompted us to investigate whether NMHC-IIB might associate with HSV-1 gB and be involved in an HSV-1 entry like NMHC-IIA. In these studies, we show that (i) NMHC-IIB coprecipitated with gB in COS-1 cells upon HSV-1 entry; (ii) a specific inhibitor of myosin light chain kinase inhibited cell surface expression of NMHC-IIB in COS-1 cells upon HSV-1 entry as well as HSV-1 infection, as reported with NMHC-IIA; (iii) overexpression of mouse NMHC-IIB in IC21 cells significantly increased their susceptibility to HSV-1 infection; and (iv) knockdown of NMHC-IIB in COS-1 cells inhibited HSV-1 infection as well as cell-cell fusion mediated by HSV-1 envelope glycoproteins. These results supported the hypothesis that, like NMHC-IIA, NMHC-IIB associated with HSV-1 gB and mediated HSV-1 entry.

IMPORTANCE

Herpes simplex virus 1 (HSV-1) was reported to utilize nonmuscle myosin heavy chain IIA (NMHC-IIA) as an entry coreceptor associating with gB. Vertebrates have three genetically distinct isoforms of NMHC-II. In these isoforms, NMHC-IIB is of special interest since it highly expresses in neuronal tissue, one of the most important cellular targets of HSV-1 in vivo. In this study, we demonstrated that the ability to mediate HSV-1 entry appeared to be conserved in NMHC-II isoforms. These results may provide an insight into the mechanism by which HSV-1 infects a wide variety of cell types in vivo.

Influenza A virus uses intercellular connections to spread to neighboring cells

In the extracellular environment, cell-free virions seek out naive host cells over long distances and between organisms. This is the primary mechanism of spread for most viruses. Here we provide evidence for an alternative pathway previously undescribed for orthomyxoviruses, whereby the spread of influenza A virus (IAV) infectious cores to neighboring cells can occur within intercellular connections. The formation of these connections requires actin dynamics and is enhanced by viral infection. Connected cells have contiguous membranes, and the core infectious viral machinery (RNP and polymerase) was present inside the intercellular connections. A live-cell movie of green fluorescent protein (GFP)-tagged NS1 of IAV shows viral protein moving from one cell to another through an intercellular connection. The movement of tagged protein was saltatory but overall traveled only in one direction. Infectious virus cores can move from one cell to another without budding and release of cell-free virions, as evidenced by the finding that whereas a neuraminidase inhibitor alone did not inhibit the development of IAV microplaques, the presence of a neuraminidase inhibitor together with drugs inhibiting actin dynamics or the microtubule stabilizer paclitaxel (originally named taxol) precluded microplaque formation. Similar results were also observed with parainfluenza virus 5 (PIV5), a paramyxovirus, when neutralizing antibody was used to block spread by cell-free virions. Intercellular spread of infectious core particles was unaffected or enhanced in the presence of nocodazole for IAV but inhibited for PIV5. The intercellular connections have a core of filamentous actin, which hints toward transport of virus particles through the use of a myosin motor.

IMPORTANCE

Here we describe a new method by which influenza A virus (IAV) spreads from cell to cell: IAV uses intracellular connections. The formation of these connections requires actin dynamics and is enhanced by viral infection and the absence of microtubules. Connected cells appeared to have contiguous membranes, and the core infectious viral machinery (RNP and polymerase) was present inside the intercellular connections. Infectious virus cores can move from one cell to another without budding and release of cell-free virions. Similar results were also observed with parainfluenza virus 5 (PIV5).

Viral and Cellular Components of AAV2 Replication Compartments

Adeno-associated virus 2 (AAV2) is a helpervirus-dependent parvovirus with a bi-phasic life cycle comprising latency in absence and lytic replication in presence of a helpervirus, such as adenovirus (Ad) or herpes simplex virus type 1 (HSV-1). Helpervirus-supported AAV2 replication takes place in replication compartments (RCs) in the cell nucleus where virus DNA replication and transcription occur. RCs consist of a defined set of helper virus-, AAV2-, and cellular proteins. Here we compare the profile of cellular proteins recruited into AAV2 RCs or identified in Rep78-associated complexes when either Ad or HSV-1 is the helpervirus, and we discuss the potential roles of some of these proteins in AAV2 and helpervirus infection.

Herpes simplex virus 1 infection induces activation and subsequent inhibition of the IFI16 and NLRP3 inflammasomes

Inflammasomes are multiprotein complexes that recognize pathogens and pathogen- or danger-associated molecular patterns. They induce the maturation and secretion of powerful proinflammatory interleukin-1B (IL-1β), IL-18, and IL-33 cytokines, which in turn activate expression of other immune genes and lymphocyte recruitment to the site of primary infection, thereby controlling invading pathogens. Inflammasomes are comprised of cytoplasmic sensor molecules, such as NLRP3 and AIM2 or nuclear sensor IFI16, the adaptor protein ASC (apoptosis-associated speck-like protein containing CARD), and the effector protein procaspase-1. Herpes simplex virus 1 (HSV-1), a ubiquitous virus that infects humans and establishes life-long latency, has evolved numerous mechanisms to evade host detection and immune responses. Here, we show that early during in vitro infection of human foreskin fibroblasts (2 to 4 h), HSV-1 induced the activation of the IFI16 and NLRP3 inflammasomes and maturation of IL-1β. Independent of viral gene expression, IFI16 recognized the HSV-1 genome in infected cell nuclei, relocalized, and colocalized with ASC in the cytoplasm. However, HSV-1 specifically targeted IFI16 for rapid proteasomic degradation at later times postinfection, which was dependent on the expression of ICP0, an immediate early protein of HSV-1. In contrast, NLRP3, AIM2, and ASC levels were not decreased. Also, caspase-1 was "trapped" in actin clusters at later time points that likely blocked the NLRP3/IFI16 inflammasome activity. In addition, the secretion of mature IL-1β was inhibited. These results suggest that though the host cell responds to HSV-1 infection by IFI16 and NLRP3 inflammasomes early during infection, HSV-1 has evolved mechanisms to shut down these responses to evade the proinflammatory consequences.

Directional spread of alphaherpesviruses in the nervous system

Alphaherpesviruses are pathogens that invade the nervous systems of their mammalian hosts. Directional spread of infection in the nervous system is a key component of the viral lifecycle and is critical for the onset of alphaherpesvirus-related diseases. Many alphaherpesvirus infections originate at peripheral sites, such as epithelial tissues, and then enter neurons of the peripheral nervous system (PNS), where lifelong latency is established. Following reactivation from latency and assembly of new viral particles, the infection typically spreads back out towards the periphery. These spread events result in the characteristic lesions (cold sores) commonly associated with herpes simplex virus (HSV) and herpes zoster (shingles) associated with varicella zoster virus (VZV). Occasionally, the infection spreads transsynaptically from the PNS into higher order neurons of the central nervous system (CNS). Spread of infection into the CNS, while rarer in natural hosts, often results in severe consequences, including death. In this review, we discuss the viral and cellular mechanisms that govern directional spread of infection in the nervous system. We focus on the molecular events that mediate long distance directional transport of viral particles in neurons during entry and egress.

Analysis of herpes simplex virus type I nuclear particles by flow cytometry

Flow cytometry has been instrumental to characterize cell populations and examine their inner molecules and processes. In most instances, whole cells are analyzed, and hence, particle size is not an issue. Viruses are 2-3 orders of magnitude smaller than cells so flow cytometry has typically been used to study viral markers within whole infected cells. However, the ability to separate and purify viral particles representing different maturation stages within a viral life cycle would be a useful tool to analyze them in details and characterize the host proteins they associate with. Herpes simplex virus Type 1 is a 250 nm enveloped DNA virus that replicates in the nucleus where it assembles new viral particles called capsids. These capsids eventually travel to the cell surface and are modified along the way, producing several intermediate particles. In the nucleus, three types of stable nonenveloped 125 nm nuclear capsids exist that differ in protein composition and genome content. This includes so-called nuclear C-capsids that are the precursors of mature extracellular virions. We report that we can apply flow cytometry to sort these nuclear C-capsid intermediates by labeling the viral genome with Syto 13, a fluorescent marker that binds to nucleic acids. This is the first time flow cytometry has been used not only to detect but also to purify an intracellular viral maturation intermediate. This opens new research avenues in virology to study capsid assembly, maturation and egress, analyze mutant phenotypes, and define host factors associated with specific viral intermediates.

Herpesviruses exploit several host compartments for envelopment

Enveloped viruses acquire their host-derived membrane at a variety of intracellular locations. Herpesviruses are complex entities that undergo several budding and fusion events during an infection. All members of this large family are believed to share a similar life cycle. However, they seemingly differ in terms of acquisition of their mature envelope. Herpes simplex virus is often believed to bud into an existing intracellular compartment, while the related cytomegalovirus may acquire its final envelope from a novel virus-induced assembly compartment. This review focuses on recent advances in the characterization of cellular compartment(s) potentially contributing to herpes virion final envelopment. It also examines the common points between seemingly distinct envelopment pathways and highlights the dynamic nature of intracellular compartments in the context of herpesvirus infections.

A beta-herpesvirus with fluorescent capsids to study transport in living cells

Fluorescent tagging of viral particles by genetic means enables the study of virus dynamics in living cells. However, the study of beta-herpesvirus entry and morphogenesis by this method is currently limited. This is due to the lack of replication competent, capsid-tagged fluorescent viruses. Here, we report on viable recombinant MCMVs carrying ectopic insertions of the small capsid protein (SCP) fused to fluorescent proteins (FPs). The FPs were inserted into an internal position which allowed the production of viable, fluorescently labeled cytomegaloviruses, which replicated with wild type kinetics in cell culture. Fluorescent particles were readily detectable by several methods. Moreover, in a spread assay, labeled capsids accumulated around the nucleus of the newly infected cells without any detectable viral gene expression suggesting normal entry and particle trafficking. These recombinants were used to record particle dynamics by live-cell microscopy during MCMV egress with high spatial as well as temporal resolution. From the resulting tracks we obtained not only mean track velocities but also their mean square displacements and diffusion coefficients. With this key information, we were able to describe particle behavior at high detail and discriminate between particle tracks exhibiting directed movement and tracks in which particles exhibited free or anomalous diffusion.

The C terminus of the large tegument protein pUL36 contains multiple capsid binding sites that function differently during assembly and cell entry of herpes simplex virus

The largest tegument protein of herpes simplex virus type 1 (HSV1), pUL36, is a multivalent cross-linker between the viral capsids and the tegument and associated membrane proteins during assembly that upon subsequent cell entry releases the incoming capsids from the outer tegument and viral envelope. Here we show that pUL36 was recruited to cytosolic progeny capsids that later colocalized with membrane proteins of herpes simplex virus type 1 (HSV1) and the trans-Golgi network. During cell entry, pUL36 dissociated from viral membrane proteins but remained associated with cytosolic capsids until arrival at the nucleus. HSV1 UL36 mutants lacking C-terminal portions of increasing size expressed truncated pUL36 but could not form plaques. Cytosolic capsids of mutants lacking the C-terminal 735 of the 3,164 amino acid residues accumulated in the cytosol but did not recruit pUL36 or associate with membranes. In contrast, pUL36 lacking only the 167 C-terminal residues bound to cytosolic capsids and subsequently colocalized with viral and host membrane proteins. Progeny virions fused with neighboring cells, but incoming capsids did not retain pUL36, nor could they target the nucleus or initiate HSV1 gene expression. Our data suggest that residues 2430 to 2893 of HSV1 pUL36, containing one binding site for the capsid protein pUL25, are sufficient to recruit pUL36 onto cytosolic capsids during assembly for secondary envelopment, whereas the 167 residues of the very C terminus with the second pUL25 binding site are crucial to maintain pUL36 on incoming capsids during cell entry. Capsids lacking pUL36 are targeted neither to membranes for virus assembly nor to nuclear pores for genome uncoating.

Coupling viruses to dynein and kinesin-1

It is now clear that transport on microtubules by dynein and kinesin family motors has an important if not critical role in the replication and spread of many different viruses. Understanding how viruses hijack dynein and kinesin motors using a limited repertoire of proteins offers a great opportunity to determine the molecular basis of motor recruitment. In this review, we discuss the interactions of dynein and kinesin-1 with adenovirus, the α herpes viruses: herpes simplex virus (HSV1) and pseudorabies virus (PrV), human immunodeficiency virus type 1 (HIV-1) and vaccinia virus. We highlight where the molecular links to these opposite polarity motors have been defined and discuss the difficulties associated with identifying viral binding partners where the basis of motor recruitment remains to be established. Ultimately, studying microtubule-based motility of viruses promises to answer fundamental questions as to how the activity and recruitment of the dynein and kinesin-1 motors are coordinated and regulated during bi-directional transport.

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.

Subversion of the actin cytoskeleton during viral infection

Viral infection converts the normal functions of a cell to optimize viral replication and virion production. One striking observation of this conversion is the reconfiguration and reorganization of cellular actin, affecting every stage of the viral life cycle, from entry through assembly to egress. The extent and degree of cytoskeletal reorganization varies among different viral infections, suggesting the evolution of myriad viral strategies. In this Review, we describe how the interaction of viral proteins with the cell modulates the structure and function of the actin cytoskeleton to initiate, sustain and spread infections. The molecular biology of such interactions continues to engage virologists in their quest to understand viral replication and informs cell biologists about the role of the cytoskeleton in the uninfected cell.

Actin in herpesvirus infection

Actin is important for a variety of cellular processes, including uptake of extracellular material and intracellular transport. Several emerging lines of evidence indicate that herpesviruses exploit actin and actin-associated myosin motors for viral entry, intranuclear transport of capsids, and virion egress. The goal of this review is to explore these processes and to highlight potential future directions for this area of research.

Baculovirus VP80 protein and the F-actin cytoskeleton interact and connect the viral replication factory with the nuclear periphery

Recently, we showed that the Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) VP80 protein is essential for the formation of both virion types, budded virus (BV) and occlusion-derived virus (ODV). Deletion of the vp80 gene did not affect assembly of nucleocapsids. However, these nucleocapsids were not able to migrate from the virogenic stroma to the nuclear periphery. In the current paper, we constructed a baculovirus recombinant with enhanced-green fluorescent protein (EGFP)-tagged VP80, allowing visualization of the VP80 distribution pattern during infection. In baculovirus-infected cells, the EGFP-VP80 protein is entirely localized in nuclei, adjacent to the virus-triggered F-actin scaffold that forms a highly organized three-dimensional network connecting the virogenic stroma physically with the nuclear envelope. Interaction between VP80 and host actin was confirmed by coimmunoprecipitation. We further showed that VP80 is associated with the nucleocapsid fraction of both BVs and ODVs, typically at one end of the nucleocapsids. In addition, the presence of sequence motifs with homology to invertebrate paramyosin proteins strongly supports a role for VP80 in the polar transport of nucleocapsids to the periphery of the nucleus on their way to the plasma membrane to form BVs and for assembly in the nuclear periphery to form ODVs for embedding in viral occlusion bodies.

The taking of the cytoskeleton one two three: how viruses utilize the cytoskeleton during egress

The final assembly of nonlytic envelope viruses requires the coordinated transport of either subviral particles or fully formed virions to the plasma membrane for release from the cell. Recent research has delved into the mechanisms viruses employ to hijack the host cell's cytoskeletal system for active transport to the site of final assembly and release. This review will look at recent findings that relate to the transport of virions to the cell periphery and out of the cell.