Alpha-herpesvirus infection induces the formation of nuclear actin filaments

Nuclear actin filaments - a historical perspective

The view on nuclear filaments formed by non-skeletal β-actin has significantly changed over the decades. Initially, filamentous actin was observed in amphibian oocyte nuclei and only under specific cell stress conditions in mammalian cell nuclei. Improved labeling and imaging technologies have permitted insights into a transient but microscopically apparent filament network that is relevant for chromatin organization, biomechanics of the mammalian cell nucleus, gene expression, and DNA damage repair. Here, we will provide a historical perspective on the developing insight into nuclear actin filaments.

The actin cytoskeleton is important for pseudorabies virus infection

Viruses are dependent on the host factors for their replication and survival. Therefore, identification of host factors that druggable for antiviral development is crucial. The actin cytoskeleton plays an important role in the virus infection. The dynamics change of actin and its function are regulated by multiple actin-associated proteins (AAPs). However, the role and mechanism of various AAPs in the life cycle of virus are still enigmatic. In this study, we analyzed the roles of actin and AAPs in the replication of pseudorabies virus (PRV). Using a library of compounds targeting AAPs, our data found that multiple AAPs, such as Rho-GTPases, Rock, Myosin and Formin were involved in PRV infection. Besides, our result demonstrated that the actin-binding protein Drebrin was also participated in PRV infection. Further studies are necessary to elucidate the molecular mechanism of AAPs in the virus life cycle, in the hope of mining host factors for antiviral developments.

Myosin motors on the pathway of viral infections

Molecular motors are microscopic machines that use energy from adenosine triphosphate (ATP) hydrolysis to generate movement. While kinesins and dynein are molecular motors associated with microtubule tracks, myosins bind to and move on actin filaments. Mammalian cells express several myosin motors. They power cellular processes such as endo- and exocytosis, intracellular trafficking, transcription, migration, and cytokinesis. As viruses navigate through cells, they may take advantage or be hindered by host components and machinery, including the cytoskeleton. This review delves into myosins' cell roles and compares them to their reported functions in viral infections. In most cases, the previously described myosin functions align with their reported role in viral infections, although not in all cases. This opens the possibility that knowledge obtained from studying myosins in viral infections might shed light on new physiological roles for myosins in cells. However, given the high number of myosins expressed and the variety of viruses investigated in the different studies, it is challenging to infer whether the interactions found are specific to a single virus or can be applied to other viruses with the same characteristics. We conclude that the participation of myosins in viral cycles is still a largely unexplored area, especially concerning unconventional myosins.

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.

Nuclear Cytoskeleton in Virus Infection

The nuclear lamina is the main component of the nuclear cytoskeleton that maintains the integrity of the nucleus. However, it represents a natural barrier for viruses replicating in the cell nucleus. The lamina blocks viruses from being trafficked to the nucleus for replication, but it also impedes the nuclear egress of the progeny of viral particles. Thus, viruses have evolved mechanisms to overcome this obstacle. Large viruses induce the assembly of multiprotein complexes that are anchored to the inner nuclear membrane. Important components of these complexes are the viral and cellular kinases phosphorylating the lamina and promoting its disaggregation, therefore allowing virus egress. Small viruses also use cellular kinases to induce lamina phosphorylation and the subsequent disruption in order to facilitate the import of viral particles during the early stages of infection or during their nuclear egress. Another component of the nuclear cytoskeleton, nuclear actin, is exploited by viruses for the intranuclear movement of their particles from the replication sites to the nuclear periphery. This study focuses on exploitation of the nuclear cytoskeleton by viruses, although this is just the beginning for many viruses, and promises to reveal the mechanisms and dynamic of physiological and pathological processes in the nucleus.

Human Cytomegalovirus Egress: Overcoming Barriers and Co-Opting Cellular Functions

The assembly of human cytomegalovirus (HCMV) and other herpesviruses includes both nuclear and cytoplasmic phases. During the prolonged replication cycle of HCMV, the cell undergoes remarkable changes in cellular architecture that include marked increases in nuclear size and structure as well as the reorganization of membranes in cytoplasm. Similarly, significant changes occur in cellular metabolism, protein trafficking, and cellular homeostatic functions. These cellular modifications are considered integral in the efficient assembly of infectious progeny in productively infected cells. Nuclear egress of HCMV nucleocapsids is thought to follow a pathway similar to that proposed for other members of the herpesvirus family. During this process, viral nucleocapsids must overcome structural barriers in the nucleus that limit transit and, ultimately, their delivery to the cytoplasm for final assembly of progeny virions. HCMV, similar to other herpesviruses, encodes viral functions that co-opt cellular functions to overcome these barriers and to bridge the bilaminar nuclear membrane. In this brief review, we will highlight some of the mechanisms that define our current understanding of HCMV egress, relying heavily on the current understanding of egress of the more well-studied α-herpesviruses, HSV-1 and PRV.

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.

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.

Development of a reliable bovine neuronal cell culture system and labeled recombinant bovine herpesvirus type-1 for studying virus-host cell interactions

Bovine herpesvirus type 1 (BoHV-1) is the viral causative agent of infectious bovine rhinotracheitis and a component of the bovine respiratory complex commonly referred to as shipping fever in calves. BoHV-1 is also responsible for losses of aborted calves and reductions in dairy productivity. BoHV-1 belongs to the neurotropic alphaherpesviruses which have a predilection to infect and establish latency in sensory neurons. Neuronal cell cultures provide a useful platform for experiments investigating neuronal entry, retrograde and anterograde transport, and the establishment of latency. Rodent neuronal cell lines and primary rabbit neuronal cells have been utilized for BoHV-1, though a reliable host-specific neuronal cell culture system has not been developed. In this study, BoHV-1 readily infected bovine-derived immortalized neuronal progenitor cells (FBBC-1) differentiated in cell culture producing neurite-like projections and exhibiting neuronal cell markers NeuN and L1CAM. FBBC-1 cells expressed both nectin-1 and nectin-2 alphaherpesvirus receptors on their cell surfaces, however, nectin-2 was detected in much greater abundance than nectin-1. To facilitate investigations of BoHV-1 infection, a recombinant BoHV-1 virus expressing the green fluorescent protein (GFP) cloned into a bacterial artificial chromosome (BAC) was used to generate an mCherry-VP26 fusion protein. The BoHV-1 GFP expressing VP26mCherry labeled virus infected differentiated FBBC-1 cells as evidenced by the production of infectious virions and the expression of both GFP and mCherry fluorophores. Time-lapse live cell microscopy revealed the presence of mCherry fluorescent capsids in neuronal projections immediately after virus entry moving retrograde in a saltatory manner. Proximity ligation assays (PLA) using MDBK cells demonstrated that BoHV-1 glycoprotein D (gD) interacted more efficiently with nectin-1 than nectin-2. However, the gD interaction with nectin-2 predominated in differentiated FBBC-1 cells in comparison to the gD nectin-1 interaction. The efficiently differentiated FBBC-1 neuronal cell line and fluorescently labeled BoHV-1 virions will assist experimentation aiming to elucidate specific mechanisms of virus entry and transport in a homologous bovine neuronal cell culture system.

Identification of A functional region in Bombyx mori nucleopolyhedrovirus VP39 that is essential for nuclear actin polymerization

Nuclear actin polymerization plays an indispensable role in the nuclear assembly of baculovirus nucleocapsid, but the underlying viral infection-mediated mechanism remains unclear. VP39 is the major protein in baculovirus capsid, which builds the skeleton of the capsid tubular structure. VP39 is suggested in previous studies to interact with cellular actin and mediate actin polymerization. However, it is unclear about the role of VP39 in mediating nuclear actin polymerization. Results in this study indicated that vp39 deletion abolished nuclear actin polymerization, which was recovered after vp39 repair, revealing the essential part of VP39 in nuclear actin polymerization. Furthermore, a series of mutants with vp39 deletions were constructed to analyze the important region responsible for nuclear actin polymerization. In addition, intracellular localization analysis demonstrated that the amino acids 192-286 in VP39 C-terminal are responsible for nuclear actin polymerization.

Cytoplasmic control of intranuclear polarity by human cytomegalovirus

Despite its size and rigidity, the cell nucleus can be moved or reorganized by cytoskeletal filaments under various conditions (for example, during viral infection)1-11. Moreover, whereas chromatin organizes into non-random domains12, extensive heterogeneity at the single-cell level13 means that precisely how and why nuclei reorganize remains an area of intense investigation. Here we describe convolutional neural network-based automated cell classification and analysis pipelines, which revealed the extent to which human cytomegalovirus generates nuclear polarity through a virus-assembled microtubule-organizing centre. Acetylation of tubulin enables microtubules emanating from this centre to rotate the nucleus by engaging cytoplasmically exposed dynein-binding domains in the outer nuclear membrane protein nesprin-2G, which polarizes the inner nuclear membrane protein SUN1. This in turn creates intranuclear polarity in emerin, and thereby controls nuclear actin filaments that spatially segregate viral DNA from inactive histones and host DNA, maximizing virus replication. Our findings demonstrate the extent to which viruses can control the nucleus from the cytoplasm.

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.

Herpes Simplex Virus: The Hostile Guest That Takes Over Your Home

Alpha (α)-herpesviruses (HSV-1 and HSV-2), like other viruses, are obligate intracellular parasites. They hijack the cellular machinery to survive and replicate through evading the defensive responses by the host. The viral genome of herpes simplex viruses (HSVs) contains viral genes, the products of which are destined to exploit the host apparatus for their own existence. Cellular modulations begin from the entry point itself. The two main gateways that the virus has to penetrate are the cell membrane and the nuclear membrane. Changes in the cell membrane are triggered when the glycoproteins of HSV interact with the surface receptors of the host cell, and from here, the components of the cytoskeleton take over. The rearrangement in the cytoskeleton components help the virus to enter as well as transport to the nucleus and back to the cell membrane to spread out to the other cells. The entire carriage process is also mediated by the motor proteins of the kinesin and dynein superfamily and is directed by the viral tegument proteins. Also, the virus captures the cell’s most efficient cargo carrying system, the endoplasmic reticulum (ER)–Golgi vesicular transport machinery for egress to the cell membrane. For these reasons, the host cell has its own checkpoints where the normal functions are halted once a danger is sensed. However, a cell may be prepared for the adversities from an invading virus, and it is simply commendable that the virus has the antidote to these cellular strategies as well. The HSV viral proteins are capable of limiting the use of the transcriptional and translational tools for the cell itself, so that its own transcription and translation pathways remain unhindered. HSV prefers to constrain any self-destruction process of the cell—be it autophagy in the lysosome or apoptosis by the mitochondria, so that it can continue to parasitize the cell for its own survival. This review gives a detailed account of the significance of compartmentalization during HSV pathogenesis. It also highlights the undiscovered areas in the HSV cell biology research which demand attention for devising improved therapeutics against the infection.

Rho-Associated Coiled-Coil Kinase 1 Translocates to the Nucleus and Inhibits Human Cytomegalovirus Propagation

Rho-associated coiled-coil kinase (ROCK) protein is a central kinase that regulates numerous cellular functions, including cellular polarity, motility, proliferation, and apoptosis. Here, we demonstrate that ROCK has antiviral properties, and inhibition of its activity results in enhanced propagation of human cytomegalovirus (HCMV). We show that during HCMV infection, ROCK1 translocates to the nucleus and concentrates in the nucleolus, where it colocalizes with the stress-related chaperone heat shock cognate 71-kDa protein (Hsc70). Gene expression measurements show that inhibition of ROCK activity does not seem to affect the cellular stress response. We demonstrate that inhibition of myosin, one of the central targets of ROCK, also increases HCMV propagation, implying that the antiviral activity of ROCK might be mediated by activation of the actomyosin network. Finally, we demonstrate that inhibition of ROCK results in increased levels of the tegument protein UL32 and of viral DNA in the cytoplasm, suggesting ROCK activity hinders the efficient egress of HCMV particles out of the nucleus. Altogether, our findings illustrate ROCK activity restricts HCMV propagation and suggest this inhibitory effect may be mediated by suppression of capsid egress out of the nucleus.IMPORTANCE ROCK is a central kinase in cells that regulates numerous cellular functions, including cellular polarity, motility, proliferation, and apoptosis. Here we reveal a novel antiviral activity of ROCK during infection with HCMV, a prevalent pathogen infecting most of the population worldwide. We reveal ROCK1 is translocated to the nucleus, where it mainly localizes to the nucleolus. Our findings suggest that ROCK's antiviral activity may be related to activation of the actomyosin network and inhibition of capsid egress out of the nucleus.

Herpesvirus acts with the cytoskeleton and promotes cancer progression

The cytoskeleton is a complex fibrous reticular structure composed of microfilaments, microtubules and intermediate filaments. These components coordinate morphology support and intracellular transport that is involved in a variety of cell activities, such as cell proliferation, migration and differentiation. In addition, the cytoskeleton also plays an important role in viral infection. During an infection by a Herpesvirus, the virus utilizes microfilaments to enter cells and travel to the nucleus by microtubules; the viral DNA replicates with the help of host microfilaments; and the virus particles start assembling with a capsid in the cytoplasm before egress. The cytoskeleton changes in cells infected with Herpesvirus are made to either counteract or obey the virus, thereby promote cell transforming into cancerous ones. This article aims to clarify the interaction between the virus and cytoskeleton components in the process of Herpesvirus infection and the molecular motor, cytoskeleton-associated proteins and drugs that play an important role in the process of a Herpesvirus infection and carcinogenesis process.

Intracellular dynamics of actin affects Borna disease virus replication in the nucleus

Borna disease virus (BoDV) is a nonsegmented, negative-strand RNA virus that uniquely replicates and establishes persistent infection in cell nucleus. Recent studies have demonstrated the presence of actin in the nucleus and its role in intranuclear phenomena such as transcription and DNA repair. Although nuclear actin is involved in the life cycle of some intranuclear DNA viruses, the interaction between BoDV and nuclear actin has not been reported. In this study, we show that the inhibition of the nucleocytoplasmic transport of actin affects the replication of BoDV in the nucleus. The knockdown of a nuclear export factor of actin, exportin 6, results in the induction of structural aberration in intranuclear viral factories of BoDV. Furthermore, the inhibition of the nuclear export of actin promotes accumulation of viral matrix protein in the cytoplasm and periphery of the infected cells. These results suggest that the dynamics of actin affect the replication of BoDV by disturbing the structure of viral factories in the nucleus.

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.

Venture from the Interior-Herpesvirus pUL31 Escorts Capsids from Nucleoplasmic Replication Compartments to Sites of Primary Envelopment at the Inner Nuclear Membrane

Herpesviral capsid assembly is initiated in the nucleoplasm of the infected cell. Size constraints require that newly formed viral nucleocapsids leave the nucleus by an evolutionarily conserved vescular transport mechanism called nuclear egress. Mature capsids released from the nucleoplasm are engaged in a membrane-mediated budding process, composed of primary envelopment at the inner nuclear membrane and de-envelopment at the outer nuclear membrane. Once in the cytoplasm, the capsids receive their secondary envelope for maturation into infectious virions. Two viral proteins conserved throughout the herpesvirus family, the integral membrane protein pUL34 and the phosphoprotein pUL31, form the nuclear egress complex required for capsid transport from the infected nucleus to the cytoplasm. Formation of the nuclear egress complex results in budding of membrane vesicles revealing its function as minimal virus-encoded membrane budding and scission machinery. The recent structural analysis unraveled details of the heterodimeric nuclear egress complex and the hexagonal coat it forms at the inside of budding vesicles to drive primary envelopment. With this review, I would like to present the capsid-escort-model where pUL31 associates with capsids in nucleoplasmic replication compartments for escort to sites of primary envelopment thereby coupling capsid maturation and nuclear egress.

Getting to and through the inner nuclear membrane during herpesvirus nuclear egress

Herpesviruses, like most DNA viruses, replicate and package their genomes into capsids in the host cell nucleus. Capsids then transit to the cytoplasm in a fascinating process called nuclear egress, which includes several unusual steps: Movement of capsids from the nuclear interior to the periphery, disruption of the nuclear lamina, capsid budding through the inner nuclear membrane, and fusion of enveloped particles with the outer nuclear membrane. Here, we review recent advances and emerging questions relating to herpesvirus nuclear egress, emphasizing controversies regarding mechanisms for capsid trafficking to the nuclear periphery, and implications of recent structures of the two-subunit, viral nuclear egress complex for the process, particularly at the step of budding through the inner nuclear membrane.

Autographa californica Multiple Nucleopolyhedrovirus Ac34 Protein Retains Cellular Actin-Related Protein 2/3 Complex in the Nucleus by Subversion of CRM1-Dependent Nuclear Export

Actin, nucleation-promoting factors (NPFs), and the actin-related protein 2/3 complex (Arp2/3) are key elements of the cellular actin polymerization machinery. With nuclear actin polymerization implicated in ever-expanding biological processes and the discovery of the nuclear import mechanisms of actin and NPFs, determining Arp2/3 nucleo-cytoplasmic shuttling mechanism is important for understanding the function of nuclear actin. A unique feature of alphabaculovirus infection of insect cells is the robust nuclear accumulation of Arp2/3, which induces actin polymerization in the nucleus to assist in virus replication. We found that Ac34, a viral late gene product encoded by the alphabaculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV), is involved in Arp2/3 nuclear accumulation during virus infection. Further assays revealed that the subcellular distribution of Arp2/3 under steady-state conditions is controlled by chromosomal maintenance 1 (CRM1)-dependent nuclear export. Upon AcMNPV infection, Ac34 inhibits CRM1 pathway and leads to Arp2/3 retention in the nucleus.

Viruses That Exploit Actin-Based Motility for Their Replication and Spread

The actin cytoskeleton is a crucial part of the eukaryotic cell. Viruses depend on host cells for their replication, and, as a result, many have developed ways of manipulating the actin network to promote their spread. This chapter reviews the various ways in which viruses utilize the actin cytoskeleton at discrete steps in their life cycle, from entry into the host cell, replication, and assembly of new progeny to virus release. Various actin inhibitors that function in different ways to affect proper actin dynamics can be used to parse the role of actin at these steps.

Host cytoskeleton in respiratory syncytial virus assembly and budding

Respiratory syncytial virus (RSV) is one of the major pathogens responsible for lower respiratory tract infections (LRTI) in young children, the elderly, and the immunosuppressed. Currently, there are no antiviral drugs or vaccines available that effectively target RSV infections, proving a significant challenge in regards to prevention and treatment. An in-depth understanding of the host-virus interactions that underlie assembly and budding would inform new targets for antiviral development.Current research suggests that the polymerised form of actin, the filamentous or F-actin, plays a role in RSV assembly and budding. Treatment with cytochalasin D, which disrupts F-actin, has been shown to inhibit virus release. In addition, the actin cytoskeleton has been shown to interact with the RSV matrix (M) protein, which plays a central role in RSV assembly. For this reason, the interaction between these two components is hypothesised to facilitate the movement of viral components in the cytoplasm and to the budding site. Despite increases in our knowledge of RSV assembly and budding, M-actin interactions are not well understood. In this review, we discuss the current literature on the role of actin cytoskeleton during assembly and budding of RSV with the aim to integrate disparate studies to build a hypothetical model of the various molecular interactions between actin and RSV M protein that facilitate RSV assembly and budding.

A Role for Nuclear F-Actin Induction in Human Cytomegalovirus Nuclear Egress

Herpesviruses, which include important pathogens, remodel the host cell nucleus to facilitate infection. This remodeling includes the formation of structures called replication compartments (RCs) in which herpesviruses replicate their DNA. During infection with the betaherpesvirus, human cytomegalovirus (HCMV), viral DNA synthesis occurs at the periphery of RCs within the nuclear interior, after which assembled capsids must reach the inner nuclear membrane (INM) for translocation to the cytoplasm (nuclear egress). The processes that facilitate movement of HCMV capsids to the INM during nuclear egress are unknown. Although an actin-based mechanism of alphaherpesvirus capsid trafficking to the INM has been proposed, it is controversial. Here, using a fluorescently-tagged, nucleus-localized actin-binding peptide, we show that HCMV, but not herpes simplex virus 1, strongly induced nuclear actin filaments (F-actin) in human fibroblasts. Based on studies using UV inactivation and inhibitors, this induction depended on viral gene expression. Interestingly, by 24 h postinfection, nuclear F-actin formed thicker structures that appeared by super-resolution microscopy to be bundles of filaments. Later in infection, nuclear F-actin primarily localized along the RC periphery and between the RC periphery and the nuclear rim. Importantly, a drug that depolymerized nuclear F-actin caused defects in production of infectious virus, capsid accumulation in the cytoplasm, and capsid localization near the nuclear rim, without decreasing capsid accumulation in the nucleus. Thus, our results suggest that for at least one herpesvirus, nuclear F-actin promotes capsid movement to the nuclear periphery and nuclear egress. We discuss our results in terms of competing models for these processes.

The mechanisms underlying herpesvirus nuclear egress have not been fully determined. In particular, how newly assembled capsids move to the inner nuclear membrane for envelopment is uncertain and controversial. In this study, we show that HCMV, an important human pathogen, induces actin filaments in the nuclei of infected cells and that an inhibitor of nuclear F-actin impairs nuclear egress and capsid localization toward the nuclear periphery. Herpesviruses are widespread pathogens that cause or contribute to an array of human diseases. A better understanding of how herpesvirus capsids traffic in the nucleus may uncover novel targets for antiviral intervention and elucidate aspects of the nuclear cytoskeleton, about which little is known.

Cycloheximide Inhibits Actin Cytoskeletal Dynamics by Suppressing Signaling via RhoA

Genome-wide screening of the yeast Saccharomyces cerevisiae knockout collection was used to characterize chemical-genetic interactions of cycloheximide (CHX). The results showed that while the act1Δ mutant was the only deletion mutant in the heterozygous essential gene deletion collection that showed hypersensitivity to sub-inhibitory concentrations of CHX, deletion of nonessential genes that work in concert with either cytoplasmic or nuclear actin in the homozygous deletion collection also highly sensitized yeast to CHX. Fluorescence microscopy analysis revealed that CHX disrupts filamentous actin structures and fluid phase endocytosis in the yeast cell. It also showed that CHX disrupts transforming growth factor-β1 (TGF-β1)-induced actin reorganization and polygonal architecture of microfilaments in mammalian cells. This inhibitory effect is mediated, at least in part, through the actin dynamics signaling pathway via suppression of activation of the small GTPase RhoA. J. Cell. Biochem. 117: 2886-2898, 2016. © 2016 Wiley Periodicals, Inc.

The diffusive way out: Herpesviruses remodel the host nucleus, enabling capsids to access the inner nuclear membrane

Herpesviruses are large DNA viruses that utilize the host nucleus for genome replication as well as capsid assembly. After maturation, these 125 nm large capsid assemblies must cross the nucleoplasm to engage the nuclear envelope and bud into the cytoplasm. Here we summarize our recent findings how this motility is facilitated. We suggest that herpesvirus induced nuclear remodeling allows capsids to move by diffusion in the nucleus and not by motor-dependent transport.

Actin, actin-binding proteins, and actin-related proteins in the nucleus

Extensive research in the past decade has significantly broadened our view about the role actin plays in the life of the cell and added novel aspects to actin research. One of these new aspects is the discovery of the existence of nuclear actin which became evident only recently. Nuclear activities including transcriptional activation in the case of all three RNA polymerases, editing and nuclear export of mRNAs, and chromatin remodeling all depend on actin. It also became clear that there is a fine-tuned equilibrium between cytoplasmic and nuclear actin pools and that this balance is ensured by an export-import system dedicated to actin. After over half a century of research on conventional actin and its organizing partners in the cytoplasm, it was also an unexpected finding that the nucleus contains more than 30 actin-binding proteins and new classes of actin-related proteins which are not able to form filaments but had evolved nuclear-specific functions. The actin-binding and actin-related proteins in the nucleus have been linked to RNA transcription and processing, nuclear transport, and chromatin remodeling. In this paper, we attempt to provide an overview of the wide range of information that is now available about actin, actin-binding, and actin-related proteins in the nucleus.

Nuclear Egress of Herpesviruses: The Prototypic Vesicular Nucleocytoplasmic Transport

Herpesvirus particles mature in two different cellular compartments. While capsid assembly and packaging of the genomic linear double-stranded DNA occur in the nucleus, virion formation takes place in the cytoplasm by the addition of numerous tegument proteins as well as acquisition of the viral envelope by budding into cellular vesicles derived from the trans-Golgi network containing virally encoded glycoproteins. To gain access to the final maturation compartment, herpesvirus nucleocapsids have to cross a formidable barrier, the nuclear envelope (NE). Since the ca. 120 nm diameter capsids are unable to traverse via nuclear pores, herpesviruses employ a vesicular transport through both leaflets of the NE. This process involves proteins which support local dissolution of the nuclear lamina to allow access of capsids to the inner nuclear membrane (INM), drive vesicle formation from the INM and mediate inclusion of the capsid as well as scission of the capsid-containing vesicle (also designated as "primary virion"). Fusion of the vesicle membrane (i.e., the "primary envelope") with the outer nuclear membrane subsequently results in release of the nucleocapsid into the cytoplasm for continuing virion morphogenesis. While this process has long been thought to be unique for herpesviruses, a similar pathway for nuclear egress of macromolecular complexes has recently been observed in Drosophila. Thus, herpesviruses may have coopted a hitherto unrecognized cellular mechanism of vesicle-mediated nucleocytoplasmic transport. This could have far reaching consequences for our understanding of cellular functions as again unraveled by the study of viruses.

Remodeling nuclear architecture allows efficient transport of herpesvirus capsids by diffusion

The nuclear chromatin structure confines the movement of large macromolecular complexes to interchromatin corrals. Herpesvirus capsids of approximately 125 nm assemble in the nucleoplasm and must reach the nuclear membranes for egress. Previous studies concluded that nuclear herpesvirus capsid motility is active, directed, and based on nuclear filamentous actin, suggesting that large nuclear complexes need metabolic energy to escape nuclear entrapment. However, this hypothesis has recently been challenged. Commonly used microscopy techniques do not allow the imaging of rapid nuclear particle motility with sufficient spatiotemporal resolution. Here, we use a rotating, oblique light sheet, which we dubbed a ring-sheet, to image and track viral capsids with high temporal and spatial resolution. We do not find any evidence for directed transport. Instead, infection with different herpesviruses induced an enlargement of interchromatin domains and allowed particles to diffuse unrestricted over longer distances, thereby facilitating nuclear egress for a larger fraction of capsids.

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.

The Herpes Simplex Virus Protein pUL31 Escorts Nucleocapsids to Sites of Nuclear Egress, a Process Coordinated by Its N-Terminal Domain

Progeny capsids of herpesviruses leave the nucleus by budding through the nuclear envelope. Two viral proteins, the membrane protein pUL34 and the nucleo-phosphoprotein pUL31 form the nuclear egress complex that is required for capsid egress out of the nucleus. All pUL31 orthologs are composed of a diverse N-terminal domain with 1 to 3 basic patches and a conserved C-terminal domain. To decipher the functions of the N-terminal domain, we have generated several Herpes simplex virus mutants and show here that the N-terminal domain of pUL31 is essential with basic patches being critical for viral propagation. pUL31 and pUL34 entered the nucleus independently of each other via separate routes and the N-terminal domain of pUL31 was required to prevent their premature interaction in the cytoplasm. Unexpectedly, a classical bipartite nuclear localization signal embedded in this domain was not required for nuclear import of pUL31. In the nucleus, pUL31 associated with the nuclear envelope and newly formed capsids. Viral mutants lacking the N-terminal domain or with its basic patches neutralized still associated with nucleocapsids but were unable to translocate them to the nuclear envelope. Replacing the authentic basic patches with a novel artificial one resulted in HSV1(17⁺)Lox-UL31-hbpmp1mp2, that was viable but delayed in nuclear egress and compromised in viral production. Thus, while the C-terminal domain of pUL31 is sufficient for the interaction with nucleocapsids, the N-terminal domain was essential for capsid translocation to sites of nuclear egress and a coordinated interaction with pUL34. Our data indicate an orchestrated sequence of events with pUL31 binding to nucleocapsids and escorting them to the inner nuclear envelope. We propose a common mechanism for herpesviral nuclear egress: pUL31 is required for intranuclear translocation of nucleocapsids and subsequent interaction with pUL34 thereby coupling capsid maturation with primary envelopment.

Herpesviral capsid assembly is initiated in the host nucleus. Due to size constraints, newly formed nucleocapsids are unable to leave the nucleus through the nuclear pore complex. Instead herpesviruses apply an evolutionarily conserved mechanism for nuclear export of capsids called nuclear egress. This process is initiated by docking of capsids at the inner nuclear membrane, budding of enveloped capsids into the perinuclear space followed by de-envelopment and release of capsids to the cytoplasm where further maturation occurs. Two viral proteins conserved throughout the herpesvirus family, the membrane protein pUL34 and the phosphoprotein pUL31 form the nuclear egress complex that is critical for primary envelopment. We show here that pUL31 and pUL34 enter the nucleus independently of each other. pUL31 is targeted to the nucleoplasm where it binds to nucleocapsids via the conserved C-terminal domain, while its N-terminal domain is important for capsid translocation to the nuclear envelope and for a coordinated interaction with pUL34. Our data suggest a mechanism that is apparently conserved among all herpesviruses with pUL31 escorting nucleocapsids to the nuclear envelope in order to couple capsid maturation with primary envelopment.

Pathogenic microbes manipulate cofilin activity to subvert actin cytoskeleton

Actin-depolymerizing factor (ADF)/cofilin proteins are key players in controlling the temporal and spatial extent of actin dynamics, which is crucial for mediating host-pathogen interactions. Pathogenic microbes have evolved molecular mechanisms to manipulate cofilin activity to subvert the actin cytoskeletal system in host cells, promoting their internalization into the target cells, modifying the replication niche and facilitating their intracellular and intercellular dissemination. The study of how these pathogens exploit cofilin pathways is crucial for understanding infectious disease and providing potential targets for drug therapies.

Pseudorabies virus US3 leads to filamentous actin disassembly and contributes to viral genome delivery to the nucleus

The conserved alphaherpesvirus US3 tegument protein induces rearrangements of the actin cytoskeleton, consisting of protrusion formation and stress fiber breakdown. Although US3 does not affect levels of total actin protein, it remains unclear whether US3 modulates the total levels of filamentous (F) actin. In this report, we show that the pseudorabies virus (PRV) US3 protein, via its kinase activity, leads to disassembly of F-actin in porcine ST cells. F-actin disassembly has been reported before to contribute to host cell entry of HIV. In line with this, in the current study, we report that US3 has a previously uncharacterized role in viral genome delivery to the nucleus, since quantitative polymerase chain reaction (qPCR) assays on nuclear fractions demonstrated a reduced nuclear delivery of US3null PRV compared to wild type PRV genomes. Treatment of cells with the actin depolymerizing drug cytochalasin D enhanced virus genome delivery to the nucleus, particularly of US3null PRV, supporting a role for F-actin disassembly during certain aspects of viral entry. In conclusion, the US3 kinase of PRV leads to F-actin depolymerization, and US3 and F-actin disassembly contribute to viral genome delivery to the nucleus.

Nuclear herpesvirus capsid motility is not dependent on F-actin

A considerable part of the herpesvirus life cycle takes place in the host nucleus. While much progress has been made to understand the molecular processes required for virus replication in the nucleus, much less is known about the temporal and spatial dynamics of these events. Previous studies have suggested that nuclear capsid motility is directed and dependent on actin filaments (F-actin), possibly using a myosin-based, ATP-dependent mechanism. However, the conclusions from these studies were indirect. They either relied on the effects of F-actin depolymerizing drugs to deduce an F-actin dependency or they visualized nuclear F-actin but failed to show a direct link to capsid motility. Moreover, no direct link between nuclear capsid motility and a molecular motor has been established. In this report, we reinvestigate the involvement of F-actin in nuclear herpesvirus capsid transport. We show for representative members of all three herpesvirus subfamilies that nuclear capsid motility is not dependent on nuclear F-actin and that herpesvirus infection does not induce nuclear F-actin in primary fibroblasts. Moreover, in these cells, three F-actin-inhibiting drugs failed to effect capsid motility. Only latrunculin A treatment stalled nuclear capsids but did so by an unexpected effect: the drug induced actin rods in the nucleus. Immobile capsids accumulated around actin rods, and immunoprecipitation experiments suggested that capsid motility stopped because latrunculin-induced actin rods nonspecifically bind nuclear capsids. Interestingly, capsid motility was unaffected in cells that do not induce actin rods. Based on these data, we conclude that herpesvirus nuclear capsid motility is not dependent on F-actin.

Herpesviruses are large DNA viruses whose replication is dependent on the host nucleus. However, we do not understand how key nuclear processes, including capsid assembly, genome replication, capsid packaging, and nuclear egress, are dynamically connected in space and time. Fluorescence live-cell microscopy revealed that nuclear capsids are highly mobile early in infection. Two studies suggested that this motility might be due to active myosin-based transport of capsids on nuclear F-actin. However, direct evidence for such motor-based transport is lacking. We revisited this phenomenon and found no evidence that nuclear capsid motility depended on F-actin. Our results reopen the question of how nuclear herpesvirus capsids move in the host nucleus.

Varicella-zoster virus induces the formation of dynamic nuclear capsid aggregates

The first step of herpesviruses virion assembly occurs in the nucleus. However, the exact site where nucleocapsids are assembled, where the genome and the inner tegument are acquired, remains controversial. We created a recombinant VZV expressing ORF23 (homologous to HSV-1 VP26) fused to the eGFP and dually fluorescent viruses with a tegument protein additionally fused to a red tag (ORF9, ORF21 and ORF22 corresponding to HSV-1 UL49, UL37 and UL36). We identified nuclear dense structures containing the major capsid protein, the scaffold protein and maturing protease, as well as ORF21 and ORF22. Correlative microscopy demonstrated that the structures correspond to capsid aggregates and time-lapse video imaging showed that they appear prior to the accumulation of cytoplasmic capsids, presumably undergoing the secondary egress, and are highly dynamic. Our observations suggest that these structures might represent a nuclear area important for capsid assembly and/or maturation before the budding at the inner nuclear membrane.

Equine herpesvirus type 1 (EHV-1)-induced rearrangements of actin filaments in productively infected primary murine neurons

Equine herpesvirus type 1 (EHV-1) causes respiratory disease, abortion and neurological disorders in horses. In the present study, we investigated reorganization of the cytoskeleton in neurons infected with two EHV-1 strains: Jan-E (wild-type strain) and Rac-H (attenuated strain). The studies were performed on primary murine neurons, which are an excellent model for studying neurotropism and neurovirulence of EHV-1. We have demonstrated for the first time that EHV-1 infection causes rearrangements in the actin network of neurons that are dependent on the virus strain and its adaptation to cell culture in vitro. Immunofluorescent labeling and confocal microscopy revealed the formation of long, thin projections in neurons infected with the Jan-E strain, which was probably associated with enhanced intracellular spread of the virus. The EHV-1 Rac-H strain caused disruption of the microfilaments system and general depolymerization of actin, but treatment of neurons with cytochalasin D or latrunculin A resulted in limitation of viral replication. It can therefore be assumed that actin filaments are required only at the early stages of infection. Our results allow us to suggest that the actin cytoskeleton participates in EHV-1 infection of primary murine neurons but is not essential, and that other components of the cytoskeleton and/or cellular mechanisms may be also involved during EHV-1 infection.

Ubiquitin-proteasome-dependent slingshot 1 downregulation in neuronal cells inactivates cofilin to facilitate HSV-1 replication

Actin and its regulators are critical for neuronal function. Infection with herpes simplex virus 1 (HSV-1) remodels neuronal cell actin dynamics, which may relate virus-induced pathological processes in the nervous system. We previously demonstrated that cofilin is an actin regulator that participates in HSV-1-induced actin dynamics in neuronal cells, but how HSV-1 regulates cofilin has remained unclear. In the present study, we demonstrated the HSV-1-induced the inactivation of cofilin and the accumulation of phosphorylated cofilin in the nucleus, which together benefited viral replication. This consistent cofilin inactivation was achieved by the downregulation of slingshot 1 (SSH1). Notably, virus-induced SSH1 downregulation depended on the ubiquitin-proteasome system. Cofilin inactivation is therefore critical for HSV-1 replication during neuronal infection and is maintained by SSH1 downregulation. Moreover, these results provide new insight into the HSV-1-induced neurological pathogenesis and suggest potential new strategies to inhibit HSV-1 replication.

Imaging the transport dynamics of single alphaherpesvirus particles in intact peripheral nervous system explants from infected mice

ABSTRACT Alphaherpesvirus particles travel long distances in the axons of neurons using host microtubule molecular motors. The transport dynamics of individual virions in neurons have been assessed in cultured neurons, but imaging studies of single particles in tissue from infected mice have not been reported. We developed a protocol to image explanted, infected peripheral nervous system (PNS) ganglia and associated innervated tissue from mice infected with pseudorabies virus (PRV). This ex vivo preparation allowed us to visualize and track individual virions over time as they moved from the salivary gland into submandibular ganglion neurons of the PNS. We imaged and tracked hundreds of virions from multiple mice at different time points. We quantitated the transport velocity, particle stalling, duty cycle, and directionality at various times after infection. Using a PRV recombinant that expressed monomeric red fluorescent protein (mRFP)-VP26 (red capsid) and green fluorescent protein (GFP)-Us9 (green membrane protein), we corroborated that anterograde transport in axons occurs after capsids are enveloped. We addressed the question of whether replication occurs initially in the salivary gland at the site of inoculation or subsequently in the neurons of peripheral innervating ganglia. Our data indicate that significant amplification of infection occurs in the peripheral ganglia after transport from the site of infection and that these newly made particles are transported back to the salivary gland. It is likely that this reseeding of the infected gland contributes to massive invasion of the innervating PNS ganglia. We suggest that this "round-trip" infection process contributes to the characteristic peripheral neuropathy of PRV infection. IMPORTANCE Much of our understanding of molecular mechanisms of alphaherpesvirus infection and spread in neurons comes from studying cultured primary neurons. These techniques enabled significant advances in our understanding of the viral and neuronal components needed for efficient replication and directional spread between cells. However, in vitro systems cannot recapitulate the environment of innervated tissue in vivo with associated defensive properties, such as innate immunity. Therefore, in this report, we describe a system to image the progression of infection by single virus particles in tissue harvested from infected animals. We explanted intact innervated tissue from infected mice and imaged fluorescent virus particles in infected axons of the specific ganglionic neurons. Our measurements of virion transport dynamics are consistent with published in vitro results. Importantly, this system enabled us to address a fundamental biological question about the amplification of a herpesvirus infection in a peripheral nervous system circuit.

3D reconstruction of VZV infected cell nuclei and PML nuclear cages by serial section array scanning electron microscopy and electron tomography

Varicella-zoster virus (VZV) is a human alphaherpesvirus that causes varicella (chickenpox) and herpes zoster (shingles). Like all herpesviruses, the VZV DNA genome is replicated in the nucleus and packaged into nucleocapsids that must egress across the nuclear membrane for incorporation into virus particles in the cytoplasm. Our recent work showed that VZV nucleocapsids are sequestered in nuclear cages formed from promyelocytic leukemia protein (PML) in vitro and in human dorsal root ganglia and skin xenografts in vivo. We sought a method to determine the three-dimensional (3D) distribution of nucleocapsids in the nuclei of herpesvirus-infected cells as well as the 3D shape, volume and ultrastructure of these unique PML subnuclear domains. Here we report the development of a novel 3D imaging and reconstruction strategy that we term Serial Section Array-Scanning Electron Microscopy (SSA-SEM) and its application to the analysis of VZV-infected cells and these nuclear PML cages. We show that SSA-SEM permits large volume imaging and 3D reconstruction at a resolution sufficient to localize, count and distinguish different types of VZV nucleocapsids and to visualize complete PML cages. This method allowed a quantitative determination of how many nucleocapsids can be sequestered within individual PML cages (sequestration capacity), what proportion of nucleocapsids are entrapped in single nuclei (sequestration efficiency) and revealed the ultrastructural detail of the PML cages. More than 98% of all nucleocapsids in reconstructed nuclear volumes were contained in PML cages and single PML cages sequestered up to 2,780 nucleocapsids, which were shown by electron tomography to be embedded and cross-linked by an filamentous electron-dense meshwork within these unique subnuclear domains. This SSA-SEM analysis extends our recent characterization of PML cages and provides a proof of concept for this new strategy to investigate events during virion assembly at the single cell level.

Varicella-zoster virus (VZV), the cause of varicella and zoster, is a human herpesvirus that replicates in the host cell nucleus where viral genomes are packaged into virion nucleocapsids. We have recently identified antiviral PML (promyelocytic leukemia) nuclear cages that sequester VZV nucleocapsids and inhibit formation of infectious particles. Here we developed a novel three-dimensional (3D) imaging and reconstruction strategy, termed Serial Section Array-Scanning Electron Microscopy (SSA-SEM) that together with electron tomography made it possible to derive 3D reconstructions of complete herpesvirus infected host cell nuclei and of PML cages with ultrastructural precision for the first time. We determined the 3D distribution of several thousand nucleocapsids within reconstructed volumes of single host cell nuclei and in PML cages as well as their sequestration efficiency and sequestration capacity: more than 98% of nucleocapsids were entrapped within PML cages and individual PML cages could sequester nearly 3,000 nucleocapsids which were cross-linked by an irregular electron-dense meshwork within the PML cages. This 3D analysis provides a proof of concept for using SSA-SEM to investigate virion assembly at the whole cell level and further elucidates our observation that PML cages are antiviral nuclear domains which block VZV nucleocapsid egress from the infected cell nucleus.

Deciphering novel host-herpesvirus interactions by virion proteomics

Over the years, a vast array of information concerning the interactions of viruses with their hosts has been collected. However, recent advances in proteomics and other system biology techniques suggest these interactions are far more complex than anticipated. One particularly interesting and novel aspect is the analysis of cellular proteins incorporated into mature virions. Though sometimes considered purification contaminants in the past, their repeated detection by different laboratories suggests that a number of these proteins are bona fide viral components, some of which likely contribute to the viral life cycles. The present mini review focuses on cellular proteins detected in herpesviruses. It highlights the common cellular functions of these proteins, their potential implications for host–pathogen interactions, discusses technical limitations, the need for complementing methods and probes potential future research avenues.

Nuclear actin and lamins in viral infections

Lamins are the best characterized cytoskeletal components of the cell nucleus that help to maintain the nuclear shape and participate in diverse nuclear processes including replication or transcription. Nuclear actin is now widely accepted to be another cytoskeletal protein present in the nucleus that fulfills important functions in the gene expression. Some viruses replicating in the nucleus evolved the ability to interact with and probably utilize nuclear actin for their replication, e.g., for the assembly and transport of capsids or mRNA export. On the other hand, lamins play a role in the propagation of other viruses since nuclear lamina may represent a barrier for virions entering or escaping the nucleus. This review will summarize the current knowledge about the roles of nuclear actin and lamins in viral infections.

Nuclear actin and myosins: life without filaments

Actin and myosin are major components of the cell cytoskeleton, with structural and regulatory functions that affect many essential cellular processes. Although they were traditionally thought to function only in the cytoplasm, it is now well accepted that actin and multiple myosins are found in the nucleus. Increasing evidence on their functional roles has highlighted the importance of these proteins in the nuclear compartment.