The herpes simplex virus 1 U(L)34 protein interacts with a cytoplasmic dynein intermediate chain and targets nuclear membrane

The precise function of alphaherpesvirus tegument proteins and their interactions during the viral life cycle

Alphaherpesvirus is a widespread pathogen that causes diverse diseases in humans and animals and can severely damage host health. Alphaherpesvirus particles comprise a DNA core, capsid, tegument and envelope; the tegument is located between the nuclear capsid and envelope. According to biochemical and proteomic analyses of alphaherpesvirus particles, the tegument contains at least 24 viral proteins and plays an important role in the alphaherpesvirus life cycle. This article reviews the important role of tegument proteins and their interactions during the viral life cycle to provide a reference and inspiration for understanding alphaherpesvirus infection pathogenesis and identifying new antiviral strategies.

Molecular plasticity of herpesvirus nuclear egress analysed in situ

The viral nuclear egress complex (NEC) allows herpesvirus capsids to escape from the nucleus without compromising the nuclear envelope integrity. The NEC lattice assembles on the inner nuclear membrane and mediates the budding of nascent nucleocapsids into the perinuclear space and their subsequent release into the cytosol. Its essential role makes it a potent antiviral target, necessitating structural information in the context of a cellular infection. Here we determined structures of NEC-capsid interfaces in situ using electron cryo-tomography, showing a substantial structural heterogeneity. In addition, while the capsid is associated with budding initiation, it is not required for curvature formation. By determining the NEC structure in several conformations, we show that curvature arises from an asymmetric assembly of disordered and hexagonally ordered lattice domains independent of pUL25 or other viral capsid vertex components. Our results advance our understanding of the mechanism of nuclear egress in the context of a living cell.

Microtubules and viral infection

Microtubules (MTs) form rapidly adaptable, complex intracellular networks of filaments that not only provide structural support, but also form the tracks along which motors traffic macromolecular cargos to specific sub-cellular sites. These dynamic arrays play a central role in regulating various cellular processes including cell shape and motility as well as cell division and polarization. Given their complex organization and functional importance, MT arrays are carefully controlled by many highly specialized proteins that regulate the nucleation of MT filaments at distinct sites, their dynamic growth and stability, and their engagement with other subcellular structures and cargoes destined for transport. This review focuses on recent advances in our understanding of how MTs and their regulatory proteins function, including their active targeting and exploitation, during infection by viruses that utilize a wide variety of replication strategies that occur within different cellular sub-compartments or regions of the cell.

Engagement of Neurotropic Viruses in Fast Axonal Transport: Mechanisms, Potential Role of Host Kinases and Implications for Neuronal Dysfunction

Much remains unknown about mechanisms sustaining the various stages in the life cycle of neurotropic viruses. An understanding of those mechanisms operating before their replication and propagation could advance the development of effective anti-viral strategies. Here, we review our current knowledge of strategies used by neurotropic viruses to undergo bidirectional movement along axons. We discuss how the invasion strategies used by specific viruses might influence their mode of interaction with selected components of the host’s fast axonal transport (FAT) machinery, including specialized membrane-bounded organelles and microtubule-based motor proteins. As part of this discussion, we provide a critical evaluation of various reported interactions among viral and motor proteins and highlight limitations of some in vitro approaches that led to their identification. Based on a large body of evidence documenting activation of host kinases by neurotropic viruses, and on recent work revealing regulation of FAT through phosphorylation-based mechanisms, we posit a potential role of host kinases on the engagement of viruses in retrograde FAT. Finally, we briefly describe recent evidence linking aberrant activation of kinase pathways to deficits in FAT and neuronal degeneration in the context of human neurodegenerative diseases. Based on these findings, we speculate that neurotoxicity elicited by viral infection may involve deregulation of host kinases involved in the regulation of FAT and other cellular processes sustaining neuronal function and survival.

Systematic profiling of protein complex dynamics reveals DNA-PK phosphorylation of IFI16 en route to herpesvirus immunity

Dynamically shifting protein-protein interactions (PPIs) regulate cellular responses to viruses and the resulting immune signaling. Here, we use thermal proximity coaggregation (TPCA) mass spectrometry to characterize the on-off behavior of PPIs during infection with herpes simplex virus 1 (HSV-1), a virus with an ancient history of coevolution with hosts. Advancing the TPCA analysis to infer associations de novo, we build a time-resolved portrait of thousands of host-host, virus-host, and virus-virus PPIs. We demonstrate that, early in infection, the DNA sensor IFI16 recruits the active DNA damage response kinase, DNA-dependent protein kinase (DNA-PK), to incoming viral DNA at the nuclear periphery. We establish IFI16 T149 as a substrate of DNA-PK upon viral infection or DNA damage. This phosphorylation promotes IFI16-driven cytokine responses. Together, we characterize the global dynamics of PPIs during HSV-1 infection, uncovering the co-regulation of IFI16 and DNA-PK functions as a missing link in immunity to herpesvirus infection.

Dynamic Dissection of Dynein and Kinesin-1 Cooperatively Mediated Intercellular Transport of Porcine Epidemic Diarrhea Coronavirus along Microtubule Using Single Virus Tracking

It is now clear that the intercellular transport on microtubules by dynein and kinesin-1 motors has an important role in the replication and spread of many viruses. Porcine epidemic diarrhea virus (PEDV) is an enveloped, single-stranded RNA virus of the Coronavirus family, which can infect swine of all ages and cause severe economic losses in the swine industry. Elucidating the molecular mechanisms of the intercellular transport of PEDV through microtubule, dynein and kinesin-1 will be crucial for understanding its pathogenesis. Here, we demonstrate that microtubule, dynein, and kinesin-1 are involved in PEDV infection and can influence PEDV fusion and accumulation in the perinuclear region but cannot affect PEDV attachment or internalization. Furthermore, we adopted a single-virus tracking technique to dynamically observe PEDV intracellular transport with five different types: unidirectional movement toward microtubule plus ends; unidirectional movement toward microtubule minus ends; bidirectional movement along the same microtubule; bidirectional movement along different microtubules and motionless state. Among these types, the functions of dynein and kinesin-1 in PEDV intercellular transport were further analyzed by single-virus tracking and found that dynein and kinesin-1 mainly transport PEDV to the minus and plus ends of the microtubules, respectively; meanwhile, they also can transport PEDV to the opposite ends of the microtubules different from their conventional transport directions and also coordinate the bidirectional movement of PEDV along the same or different microtubules through their cooperation. These results provided deep insights and references to understand the pathogenesis of PEDV as well as to develop vaccines and treatments.

Nuclear Egress

During viral replication, herpesviruses utilize a unique strategy, termed nuclear egress, to translocate capsids from the nucleus into the cytoplasm. This initial budding step transfers a newly formed capsid from within the nucleus, too large to fit through nuclear pores, through the inner nuclear membrane to the perinuclear space. The perinuclear enveloped virion must then fuse with the outer nuclear membrane to be released into the cytoplasm for further maturation, undergoing budding once again at the trans-Golgi network or early endosomes, and ultimately exit the cell non-lytically to spread infection. This first budding process is mediated by two conserved viral proteins, UL31 and UL34, that form a heterodimer called the nuclear egress complex (NEC). This review focuses on what we know about how the NEC mediates capsid transport to the perinuclear space, including steps prior to and after this budding event. Additionally, we discuss the involvement of other viral proteins in this process and how NEC-mediated budding may be regulated during infection.

Dynein Engages and Disassembles Cytosol-Localized Simian Virus 40 To Promote Infection

During entry, polyomavirus (PyV) is endocytosed and sorts to the endoplasmic reticulum (ER), where it penetrates the ER membrane to reach the cytosol. From the cytosol, the virus moves to the nucleus to cause infection. How PyV is transported from the cytosol into the nucleus, a crucial infection step, is unclear. We found that upon reaching the cytosol, the archetypal PyV simian virus 40 (SV40) recruits the cytoplasmic dynein motor, which disassembles the viral particle. This reaction enables the resulting disassembled virus to enter the nucleus to promote infection. Our findings reveal how a cytosolic motor can be hijacked to impart conformational changes to a viral particle, a process essential for successful infection.IMPORTANCE How a nonenveloped virus successfully traffics from the cell surface to the nucleus to cause infection remains enigmatic in many instances. In the case of the nonenveloped PyV, the viral particle is sorted from the plasma membrane to the ER and then the cytosol, from which it enters the nucleus to promote infection. The molecular mechanism by which PyV reaches the nucleus from the cytosol is not entirely clear. Here we demonstrate that the prototype PyV SV40 recruits dynein upon reaching the cytosol. Importantly, this cellular motor disassembles the viral particle during cytosol-to-nucleus transport to cause infection.

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.

Impacts of virus-mediated manipulation of host Dynein

In general viruses' modus operandi to propagate is achieved by the co-opting host cell components, membranes, proteins, and machineries to their advantage. This is true for virtually every aspect of a virus' replication cycle from virus entry to the budding or release of progeny virus particles. In this chapter, we will discuss new information on the impacts of virus-mediated manipulation of Dynein motor complexes and associated machineries and factors. We will highlight how these host cell components impact on pathogenicity and immune responses, as many of the virus-mediated hijacked components also play pivotal roles in immune responses to pathogen insult. There are several comprehensive reviews that define virus–Dynein interactions including the first edition of this book that describes how viruses manipulate the host cell machineries their advantage. An updated table is included to summarize these virus–host interactions. Notably, barriers to intracellular translocation represent major hurdles to viral components during de novo infection and during active replication and the generation of progeny virus particles. Clearly, the subversion of host cell molecular motor protein activities takes advantage of constitutive and regulated membrane trafficking events and will target virus components to intracytoplasmic locales and membrane assembly. Broadening our understanding of the interplay between viruses, Dynein and the cytoskeleton will likely inform on new types of therapies. Continual enhancement of the breadth of new information on how viruses manipulate host cell biology will inevitably aid in the identification of new targets that can be poisoned to block old, new, and emerging viruses alike in their tracks.

The Demographic and Adaptive History of the African Green Monkey

Relatively little is known about the evolutionary history of the African green monkey (genus Chlorocebus) due to the lack of sampled polymorphism data from wild populations. Yet, this characterization of genetic diversity is not only critical for a better understanding of their own history, but also for human biomedical research given that they are one of the most widely used primate models. Here, I analyze the demographic and selective history of the African green monkey, utilizing one of the most comprehensive catalogs of wild genetic diversity to date, consisting of 1,795,643 autosomal single nucleotide polymorphisms in 25 individuals, representing all five major populations: C. a. aethiops, C. a. cynosurus, C. a. pygerythrus, C. a. sabaeus, and C. a tantalus. Assuming a mutation rate of 5.9 × 10-9 per base pair per generation and a generation time of 8.5 years, divergence time estimates range from 523 to 621 kya for the basal split of C. a. aethiops from the other four populations. Importantly, the resulting tree characterizing the relationship and split-times between these populations differs significantly from that presented in the original genome paper, owing to their neglect of within-population variation when calculating between population-divergence. In addition, I find that the demographic history of all five populations is well explained by a model of population fragmentation and isolation, rather than novel colonization events. Finally, utilizing these demographic models as a null, I investigate the selective history of the populations, identifying candidate regions potentially related to adaptation in response to pathogen exposure.

The human cytomegalovirus nuclear egress complex unites multiple functions: Recruitment of effectors, nuclear envelope rearrangement, and docking to nuclear capsids

BACKGROUND

Nuclear replication represents a common hallmark of herpesviruses achieved by a number of sequentially unrolled regulatory processes. A rate-limiting step is provided by nucleo-cytoplasmic capsid export, for which a defined multiregulatory protein complex, namely, the nuclear egress complex (NEC), is assembled comprising both viral and cellular components. The NEC regulates at least 3 aspects of herpesviral nuclear replication: (1) multimeric recruitment of NEC-associated effector proteins, (2) reorganization of the nuclear lamina and membranes, and (3) the docking to nuclear capsids. Here, we review published data and own experimental work that characterizes the NEC of HCMV and other herpesviruses.

METHODS

A systematic review of information on nuclear egress of HCMV compared to selected alpha-, beta-, and gamma-herpesviruses: proteomics-based approaches, high-resolution imaging techniques, and functional investigations.

RESULTS

A large number of reports on herpesviral NECs have been published during the last two decades, focusing on protein-protein interactions, nuclear localization, regulatory phosphorylation, and functional validation. The emerging picture provides an illustrated example of well-balanced and sophisticated protein networking in virus-host interaction.

CONCLUSIONS

Current evidence refined the view about herpesviral NECs. Datasets published for HCMV, murine CMV, herpes simplex virus, and Epstein-Barr virus illustrate the marked functional consistency in the way herpesviruses achieve nuclear egress. However, this compares with only limited sequence conservation of core NEC proteins and a structural conservation restricted to individual domains. The translational use of this information might help to define a novel antiviral strategy on the basis of NEC-directed small molecules.

Virus-inspired nucleic acid delivery system: Linking virus and viral mimicry

Targeted delivery of nucleic acids into disease sites of human body has been attempted for decades, but both viral and non-viral vectors are yet to meet our expectations. Safety concerns and low delivery efficiency are the main limitations of viral and non-viral vectors, respectively. The structure of viruses is both ordered and dynamic, and is believed to be the key for effective transfection. Detailed understanding of the physical properties of viruses, their interaction with cellular components, and responses towards cellular environments leading to transfection would inspire the development of safe and effective non-viral vectors. To this goal, this review systematically summarizes distinctive features of viruses that are implied for efficient nucleic acid delivery but not yet fully explored in current non-viral vectors. The assembly and disassembly of viral structures, presentation of viral ligands, and the subcellular targeting of viruses are emphasized. Moreover, we describe the current development of cationic material-based viral mimicry (CVM) and structural viral mimicry (SVM) in these aspects. In light of the discrepancy, we identify future opportunities for rational design of viral mimics for the efficient delivery of DNA and RNA.

Tubulins interact with porcine and human S proteins of the genus Alphacoronavirus and support successful assembly and release of infectious viral particles

Coronavirus spike proteins mediate host-cell-attachment and virus entry. Virus replication takes place within the host cell cytosol, whereas assembly and budding occur at the endoplasmic reticulum-Golgi intermediate compartment. In this study we demonstrated that the last 39 amino acid stretches of Alphacoronavirus spike cytoplasmic domains of the human coronavirus 229E, NL63, and the porcine transmissible gastroenteritis virus TGEV interact with tubulin alpha and beta chains. In addition, a partial co-localization of TGEV spike proteins with authentic host cell β-tubulin was observed. Furthermore, drug-induced microtubule depolymerization led to changes in spike protein distribution, a reduction in the release of infectious virus particles and less amount of spike protein incorporated into virions. These data demonstrate that interaction of Alphacoronavirus spike proteins with tubulin supports S protein transport and incorporation into virus particles.

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The cytoplasmic domain of coronavirus S proteins interacts with tubulin.

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Microtubule depolymerization influences S protein distribution.

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Viral titers are reduced after microtubule depolymerization.

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S protein incorporation into virus particles depends on intact microtubule.

Microtubule plus end-associated CLIP-170 initiates HSV-1 retrograde transport in primary human cells

Herpes simplex virus particles that enter the cell do not randomly associate with microtubule filaments, but require plus end–binding proteins EB1, CLIP-170, and dynactin to initiate retrograde transport to the nucleus.

Dynamic microtubules (MTs) continuously explore the intracellular environment and, through specialized plus end–tracking proteins (+TIPs), engage a variety of targets. However, the nature of cargoes that require +TIP-mediated capture for their movement on MTs remains poorly understood. Using RNA interference and dominant-negative approaches, combined with live cell imaging, we show that herpes simplex virus particles that have entered primary human cells exploit a +TIP complex comprising end-binding protein 1 (EB1), cytoplasmic linker protein 170 (CLIP-170), and dynactin-1 (DCTN1) to initiate retrograde transport. Depletion of these +TIPs completely blocked post-entry long-range transport of virus particles and suppressed infection ∼5,000-fold, whereas transferrin uptake, early endosome organization, and dynein-dependent movement of lysosomes and mitochondria remained unaffected. These findings provide the first insights into the earliest stages of viral engagement of MTs through specific +TIPs, akin to receptors, with therapeutic implications, and identify herpesvirus particles as one of a very limited number of cargoes absolutely dependent on CLIP-170–mediated capture to initiate transport in primary human cells.

Herpesvirus nuclear egress: Pseudorabies Virus can simultaneously induce nuclear envelope breakdown and exit the nucleus via the envelopment-deenvelopment-pathway

Herpesvirus replication takes place in the nucleus and in the cytosol. After entering the cell, nucleocapsids are transported to nuclear pores where viral DNA is released into the nucleus. After gene expression and DNA replication new nucleocapsids are assembled which have to exit the nucleus for virion formation in the cytosol. Since nuclear pores are not wide enough to allow passage of the nucleocapsid, nuclear egress occurs by vesicle-mediated transport through the nuclear envelope. To this end, nucleocapsids bud at the inner nuclear membrane (INM) recruiting a primary envelope which then fuses with the outer nuclear membrane (ONM). In the absence of this regulated nuclear egress, mutants of the alphaherpesvirus pseudorabies virus have been described that escape from the nucleus after virus-induced nuclear envelope breakdown. Here we review these exit pathways and demonstrate that both can occur simultaneously under appropriate conditions.

A novel nonviral gene delivery system: multifunctional envelope-type nano device

In this review we introduce a new concept for developing a nonviral gene delivery system which we call "Programmed Packaging." Based on this concept, we succeeded in developing a multifunctional envelope-type nano device (MEND), which exerts high transfection activities equivalent to those of an adenovirus in a dividing cell. The use of MEND has been extended to in vivo applications. PEG/peptide/DOPE ternary conjugate (PPD)-MEND, a new in vivo gene delivery system for the targeting of tumor cells that dissociates surface-modified PEG in tumor tissue by matrix metalloproteinase (MMP) and exerts significant transfection activities, was developed. In parallel with the development of MEND, a quantitative gene delivery system, Confocal Image-assisted 3-dimensionally integrated quantification (CIDIQ), also was developed. This method identified the rate-limiting step of the nonviral gene delivery system by comparing it with adenoviral-mediated gene delivery. The results of this analysis provide a new direction for the development of rational nonviral gene delivery systems.

Highly acidic C-terminal region of cytomegalovirus pUL96 determines its functions during virus maturation independently of a direct pp150 interaction

Tegument proteins pp150 and pUL96 function at a late step in cytomegalovirus (CMV) maturation. Here, we show that pp150 interacts directly with pUL96; however, the N-terminal region of pp150 and the C-terminal region of pUL96, which are critical for these proteins to function, are not required for this interaction. Moreover, the largely dispensable C-terminal region of pp150 is critical for pp150-pUL96 interaction. To further study the role of pUL96, several point and clustered mutations were engineered into the CMV Towne bacterial artificial chromosome (Towne-BAC) genome, replacing the conserved negatively charged C-terminal residues of pUL96. Although individual point mutations (E122A, D124A, and D125A) reduced virus growth slightly, the clustered mutations of 122EVDDAV127 significantly reduced virus growth, produced small syncytial plaque phenotypes, and impacted a late stage of virus maturation. When the UL96 C-terminal alanine conversion mutant (B6-BAC) virus was serially passaged in cell culture, it gained a plaque size comparable to that of Towne-BAC, displayed an altered restriction fragment length pattern, and replicated with increased growth kinetics. Whole-genome sequencing of this passaged virus (UL96P10) and the similarly passaged Towne-BAC virus revealed major differences only in the RNA4.9 and UL96 regions. When one of the mutations in the UL96 coding region was engineered into the B6-BAC virus, it significantly increased the plaque size and rescued the virus growth rate. Thus, accumulation of compensatory mutations only in UL96 in this revertant and the specific involvement of functionally dispensable regions of pp150 in the pUL96-pp150 interaction point toward a role for pUL96 in virus maturation that does not depend upon pp150.

IMPORTANCE

Human cytomegalovirus causes significant medical problems in newborns, as well as in people with low immunity. In this study, we investigated the functions of two essential virus proteins, pp150 and pUL96, and determined the impact of their mutual interaction on virus replication. These studies provide valuable information that is critical for the development of targeted antiviral therapies.

Effect of microtubule disruption on neuronal spread and replication of demyelinating and nondemyelinating strains of mouse hepatitis virus in vitro

The isogenic host attachment spike protein recombinant demyelinating strain of mouse hepatitis virus (MHV) (RSA59) and the nondemyelinating strain (RSMHV2) differ in their abilities to infect distinct types of neural cells, spread from cell to cell, and induce subsequent demyelination and axonal loss. The differential demyelination properties of RSA59 and RSMHV2 may be a function of spike protein-mediated neuronal transport. Disruption of microtubules with colchicine and vinblastine significantly blocks neuronal transport and reduces the replication of RSA59, whereas RSMHV2 remains unaffected.

Viral interactions with microtubules: orchestrators of host cell biology?

Viral interaction with the microtubule (MT) cytoskeleton is critical to infection by many viruses. Most data regarding virus–MT interaction indicate key roles in the subcellular transport of virions/viral genomic material to sites of replication, assembly and egress. However, the MT cytoskeleton orchestrates diverse processes in addition to subcellular cargo transport, including regulation of signaling pathways, cell survival and mitosis, suggesting that viruses, expert manipulators of the host cell, may use the virus–MT interface to control multiple aspects of cell biology. Several lines of evidence support this idea, indicating that specific viral proteins can modify MT dynamics and/or structure and regulate processes such as apoptosis and innate immune signaling through MT-dependent mechanisms. Here, the authors review general aspects of virus–MT interactions, with emphasis on viral mechanisms that modify MT dynamics and functions to affect processes beyond virion transport. The emerging importance of discrete viral protein–MT interactions in pathogenic processes indicates that these interfaces may represent new targets for future therapeutics and vaccine development.

Mapping of sequences in Pseudorabies virus pUL34 that are required for formation and function of the nuclear egress complex

The nuclear egress complex (NEC) is required for efficient translocation of newly synthesized herpesvirus nucleocapsids from the nucleus to the cytosol. It consists of the type II membrane protein pUL34 which interacts with pUL31 at the inner nuclear membrane (INM). To map regions within pUL34 required for nuclear membrane targeting and pUL31 interaction, we constructed deletion/substitution mutations. Previously, we showed that 50 C-terminal amino acids (aa) of pseudorabies virus (PrV) pUL34, including the transmembrane domain, could be functionally replaced by cellular lamina-associated polypeptide 2β (Lap2β) sequences. In contrast, replacement of the C-terminal 100 aa abrogated complementation but not pUL31 interaction. To further delineate essential sequences within this region, C-terminal pUL34 truncations of 60, 70, 80, 85, and 90 aa fused to Lap2β sequences were generated. While truncations up to 85 aa were functional, deletion of the C-terminal 90 aa abrogated function, which indicates that the important region is located between aa 171 and 176. Amino acids 173 to 175 represent RQR, a motif suggested to mediate INM targeting. Mutagenesis to RQG revealed that the mutant protein exhibited pronounced Golgi localization, but a fraction still reached the INM. Deletion mutations in the N-terminal domain of pUL34 demonstrated that absence of the first 4 aa was tolerated, while removal of 9 or more residues resulted in a nonfunctional protein. In addition, mutation of three conserved cysteines did not abrogate pUL34 function, whereas alteration of a conserved glutamine/tyrosine sequence yielded a nonfunctional protein.

The way out: what we know and do not know about herpesvirus nuclear egress

Herpesvirus capsids are assembled in the nucleus of infected cells whereas final maturation occurs in the cytosol. To access the final maturation compartment, intranuclear capsids have to cross the nuclear envelope which represents a formidable barrier. They do so by budding at the inner nuclear membrane, thereby forming a primary enveloped particle residing in the perinuclear cleft. Formation of primary envelopes is driven by a heterodimeric complex of two conserved herpesviral proteins, designated in the herpes simplex virus nomenclature as pUL34, a tail-anchored transmembrane protein located in the nuclear envelope, and pUL31. This nuclear egress complex recruits viral and cellular kinases to soften the nuclear lamina and allowing access of capsids to the inner nuclear membrane. How capsids are recruited to the budding site and into the primary virus particle is still not completely understood, nor is the composition of the primary enveloped virion in the perinuclear cleft. Fusion of the primary envelope with the outer nuclear membrane then results in translocation of the capsid to the cytosol. This fusion event is clearly different from fusion during infectious entry of free virions into target cells in that it does not require the conserved essential core herpesvirus fusion machinery. Nuclear egress can thus be viewed as a vesicle (primary envelope)-mediated transport of cargo (capsids) through thenuclear envelope, a process which had been unique in cell biology. Only recently has a similar process been identified in Drosophila for nuclear egress of large ribonucleoprotein complexes. Thus, herpesviruses appear to subvert a hitherto cryptic cellular pathway for translocation of capsids from the nucleus to the cytosol.

Rotavirus viroplasm fusion and perinuclear localization are dynamic processes requiring stabilized microtubules

Rotavirus viroplasms are cytosolic, electron-dense inclusions corresponding to the viral machinery of replication responsible for viral template transcription, dsRNA genome segments replication and assembly of new viral cores. We have previously observed that, over time, those viroplasms increase in size and decrease in number. Therefore, we hypothesized that this process was dependent on the cellular microtubular network and its associated dynamic components. Here, we present evidence demonstrating that viroplasms are dynamic structures, which, in the course of an ongoing infection, move towards the perinuclear region of the cell, where they fuse among each other, thereby gaining considerably in size and, simultaneouly, explaining the decrease in numbers. On the viral side, this process seems to depend on VP2 for movement and on NSP2 for fusion. On the cellular side, both the temporal transition and the maintenance of the viroplasms are dependent on the microtubular network, its stabilization by acetylation, and, surprisingly, on a kinesin motor of the kinesin-5 family, Eg5. Thus, we provide for the first time deeper insights into the dynamics of rotavirus replication, which can explain the behavior of viroplasms in the infected cell.

Structural determinants for nuclear envelope localization and function of pseudorabies virus pUL34

Herpesvirus proteins pUL34 and pUL31 form a complex at the inner nuclear membrane (INM) which is necessary for efficient nuclear egress. Pseudorabies virus (PrV) pUL34 is a type II membrane protein of 262 amino acids (aa). The transmembrane region (TM) is predicted to be located between aa 245 and 261, leaving only one amino acid in the C terminus that probably extends into the perinuclear space. It is targeted to the nuclear envelope in the absence of other viral proteins, pointing to intrinsic localization motifs, and shows structural similarity to cellular INM proteins like lamina-associated polypeptide (Lap) 2ß and Emerin. To investigate which domains of pUL34 are relevant for localization and function, we constructed chimeric proteins by replacing parts of pUL34 with regions of cellular INM proteins. First the 18 C-terminal amino acids encompassing the TM were exchanged with TM regions and C-terminal domains of Lap2ß and Emerin or with the first TM region of the polytopic lamin B receptor (LBR), including the nine following amino acids. All resulting chimeric proteins complemented the replication defect of PrV-ΔUL34, demonstrating that the substitution of the TM and the extension of the C-terminal domain does not interfere with the function of pUL34. Complementation was reduced but not abolished when the C-terminal 50 aa were replaced by corresponding Lap2ß sequences (pUL34-LapCT50). However, replacing the C-terminal 100 aa (pUL34-LapCT100) resulted in a nonfunctional protein despite continuing pUL31 binding, pointing to an important functional role of this region. The replacement of the N-terminal 100 aa (pUL34-LapNT100) had no effect on nuclear envelope localization but abrogated pUL31 binding and function.

Reconstitution of the Kaposi's sarcoma-associated herpesvirus nuclear egress complex and formation of nuclear membrane vesicles by coexpression of ORF67 and ORF69 gene products

The Kaposi's sarcoma-associated herpesvirus nuclear egress complex is composed of two proteins, ORF67 and ORF69. In this study, we have recapitulated the KSHV complex by coexpression of these two proteins in insect cells using expression from recombinant baculoviruses. The proteins form a complex at the nuclear membrane as judged by live-cell analysis of protein fusions tagged with green fluorescent protein (GFP) and mCherry. Ultrastructural analysis of infected cells showed that ORF67 expression results in reduplication of the nuclear membrane. When the two proteins are expressed together, numerous virion-size nuclear membrane-derived vesicles were evident at the nuclear margins.

A physical link between the pseudorabies virus capsid and the nuclear egress complex

Following their assembly, herpesvirus capsids exit the nucleus by budding at the inner nuclear membrane. Two highly conserved viral proteins are required for this process, pUL31 and pUL34. In this report, we demonstrate that the pUL31 component of the pseudorabies virus nuclear egress complex is a conditional capsid-binding protein that is unmasked in the absence of pUL34. The interaction between pUL31 and capsids was confirmed through fluorescence microscopy and Western blot analysis of purified intranuclear capsids. Three viral proteins were tested for their abilities to mediate the pUL31-capsid interaction: the minor capsid protein pUL25, the portal protein pUL6, and the terminase subunit pUL33. Despite the requirement for each protein in nuclear egress, none of these viral proteins were required for the pUL31-capsid interaction. These findings provide the first formal evidence that a herpesvirus nuclear egress complex interacts with capsids and have implications for how DNA-containing capsids are selectively targeted for nuclear egress.

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.

Adenovirus recruits dynein by an evolutionary novel mechanism involving direct binding to pH-primed hexon

Following receptor-mediated uptake into endocytic vesicles and escape from the endosome, adenovirus is transported by cytoplasmic dynein along microtubules to the perinuclear region of the cell. How motor proteins are recruited to viruses for their own use has begun to be investigated only recently. We review here the evidence for a role for dynein and other motor proteins in adenovirus infectivity. We also discuss the implications of recent studies on the mechanism of dynein recruitment to adenovirus for understanding the relationship between pathogenic and physiological cargo recruitment and for the evolutionary origins of dynein-mediated adenovirus transport.

The association of viral proteins with host cell dynein components during virus infection

After fusion with the cellular plasma membrane or endosomal membranes, viral particles are generally too large to diffuse freely within the crowded cytoplasm environment. Thus, they will never reach the cell nucleus or the perinuclear areas where replication or reverse transcription usually takes place. It has been proposed that many unrelated viruses are transported along microtubules in a retrograde manner using the cellular dynein machinery or, at least, some dynein components. A putative employment of the dynein motor in a dynein‐mediated transport has been suggested from experiments in which viral capsid proteins were used as bait in yeast two‐hybrid screens using libraries composed of cellular proteins and dynein‐associated chains were retrieved as virus‐interacting proteins. In most cases DYNLL1, DYNLT1 or DYNLRB1 were identified as the dynein chains that interact with viral proteins. The importance of these dynein–virus interactions has been supported, in principle, by the observation that in some cases the dynein‐interacting motifs of viral proteins altered by site‐directed mutagenesis result in non‐infective virions. Furthermore, overexpression of p50 dynamitin, which blocks the dynein–dynactin interaction, or incubation of infected cells with peptides that compete with viral polypeptides for dynein binding have been shown to alter the viral retrograde transport. Still, it remains to be proved that dynein light chains can bind simultaneously to incoming virions and to the dynein motor for retrograde transport to take place. In this review, we will analyse the association of viral proteins with dynein polypeptides and its implications for viral infection.

A functional role for TorsinA in herpes simplex virus 1 nuclear egress

Herpes simplex virus 1 (HSV-1) capsids leave the nucleus by a process of envelopment and de-envelopment at the nuclear envelope (NE) that is accompanied by structural alterations of the NE. As capsids translocate across the NE, transient primary enveloped virions form in the perinuclear space. Here, we provide evidence that torsinA (TA), a ubiquitously expressed ATPase, has a role in HSV-1 nuclear egress. TA resides within the lumen of the endoplasmic reticulum (ER)/NE and functions in maintaining normal NE architecture. We show that perturbation of TA normal function by overexpressing torsinA wild type (TAwt) inhibits HSV-1 production. Ultrastructural analysis of infected cells overexpressing TAwt revealed reduced levels of surface virions in addition to accumulation of novel, double-membrane structures called virus-like vesicles (VLVs). Although mainly found in the cytoplasm, VLVs resemble primary virions in their size, by the appearance of the inner membrane, and by the presence of pUL34, a structural component of primary virions. Collectively, our data suggest a model in which interference of TA normal function by overexpression impairs de-envelopment of the primary virions leading to their accumulation in a cytoplasmic membrane compartment. This implies novel functions for TA at the NE.

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.

Identification of the dynein light chains required for human papillomavirus infection

Human papillomaviruses (HPVs) are a family of small non-enveloped DNA viruses. Some genital HPV types, including HPV type 16 (HPV16), are the causative agent for the development of cancer at the site of infection. HPVs encode two capsid proteins, L1 and L2. After endocytic cell entry and egress from endosomes, L2 accompanies the viral DNA to the nucleus where replication is initiated. For cytoplasmic transport, L2 interacts with the microtubule network via the motor protein complex dynein. We have performed yeast two-hybrid screening and identified the dynein light chain DYNLT1 (previously called Tctex1) as interaction partner of HPV16 L2. Using co-immunoprecipitation and immunofluorescence colocalization studies we confirmed the L2-DYNLT1 interaction in mammalian cells. Further studies revealed that DYNLT3, the second member of the Tctex-light chain family, also interacts with L2 in vitro and in vivo, whereas other constituents of the dynein complex were not found to associate with L2. Depletion of DYNLT1 and DYNLT3 by specific siRNAs or cytosolic delivery of light chain-specific antibodies inhibited infection of HPV16. Therefore, this work identified two host cell proteins involved in HPV16 infection that are most likely required for transport purposes towards the nucleus.

Light chain-dependent self-association of dynein intermediate chain

Dynein light chains are bivalent dimers that bind two copies of dynein intermediate chain IC to form a cargo attachment subcomplex. The interaction of light chain LC8 with the natively disordered N-terminal domain of IC induces helix formation at distant IC sites in or near a region predicted to form a coiled-coil. This fostered the hypothesis that LC8 binding promotes IC self-association to form a coiled-coil or other interchain helical structure. However, recent studies show that the predicted coiled-coil sequence partially overlaps the light chain LC7 recognition sequence on IC, raising questions about the apparently contradictory effects of LC8 and LC7. Here, we use NMR and fluorescence quenching to localize IC self-association to residues within the predicted coiled-coil that also correspond to helix 1 of the LC7 recognition sequence. LC8 binding promotes IC self-association of helix 1 from each of two IC chains, whereas LC7 binding reverses self-association by incorporating the same residues into two symmetrical, but distant, helices of the LC7-IC complex. Isothermal titration experiments confirm the distinction of LC8 enhancement of IC self-association and LC7 binding effects. When all three light chains are bound, IC self-association is shifted to another region. Such flexibility in association modes may function in maintaining a stable and versatile light chain-intermediate chain assembly under changing cellular conditions.

A hitchhiker's guide to the nervous system: the complex journey of viruses and toxins

To reach the central nervous system (CNS), pathogens have to circumvent the wall of tightly sealed endothelial cells that compose the blood-brain barrier. Neuronal projections that connect to peripheral cells and organs are the Achilles heels in CNS isolation. Some viruses and bacterial toxins interact with membrane receptors that are present at nerve terminals to enter the axoplasm. Pathogens can then be mistaken for cargo and recruit trafficking components, allowing them to undergo long-range axonal transport to neuronal cell bodies. In this Review, we highlight the strategies used by pathogens to exploit axonal transport during CNS invasion.

ICP0 dismantles microtubule networks in herpes simplex virus-infected cells

Infected-cell protein 0 (ICP0) is a RING finger E3 ligase that regulates herpes simplex virus (HSV) mRNA synthesis, and strongly influences the balance between latency and replication of HSV. For 25 years, the nuclear functions of ICP0 have been the subject of intense scrutiny. To obtain new clues about ICP0's mechanism of action, we constructed HSV-1 viruses that expressed GFP-tagged ICP0. To our surprise, both GFP-tagged and wild-type ICP0 were predominantly observed in the cytoplasm of HSV-infected cells. Although ICP0 is exclusively nuclear during the immediate-early phase of HSV infection, further analysis revealed that ICP0 translocated to the cytoplasm during the early phase where it triggered a previously unrecognized process; ICP0 dismantled the microtubule network of the host cell. A RING finger mutant of ICP0 efficiently bundled microtubules, but failed to disperse microtubule bundles. Synthesis of ICP0 proved to be necessary and sufficient to disrupt microtubule networks in HSV-infected and transfected cells. Plant and animal viruses encode many proteins that reorganize microtubules. However, this is the first report of a viral E3 ligase that regulates microtubule stability. Intriguingly, several cellular E3 ligases orchestrate microtubule disassembly and reassembly during mitosis. Our results suggest that ICP0 serves a dual role in the HSV life cycle, acting first as a nuclear regulator of viral mRNA synthesis and acting later, in the cytoplasm, to dismantle the host cell's microtubule network in preparation for virion synthesis and/or egress.

Role of the endoplasmic reticulum chaperone BiP, SUN domain proteins, and dynein in altering nuclear morphology during human cytomegalovirus infection

The process of assembly and egress of human cytomegalovirus (HCMV) virions requires significant morphological alterations of the nuclear and cytoplasmic architecture. In the studies presented we show that the nuclear periphery is dramatically altered, especially near the cytoplasmic assembly compartment, where the nuclear lamina is specifically rearranged, the outer nuclear membrane is altered, and the nucleus becomes permeable to large molecules. In addition, the tethering of the inner and outer nuclear membranes is lost during infection due to a decrease in levels of the SUN domain proteins. We previously demonstrated that the endoplasmic reticulum protein BiP functions as a component of the assembly compartment and disruption of BiP causes the loss of assembly compartment integrity. In this study we show that the depletion of BiP, and the loss of assembly compartment integrity, results in the loss of virally induced lamina rearrangement and morphology of the nucleus that is characteristic of HCMV infection. BiP functions in lamina rearrangement through its ability to affect lamin phosphorylation. Depletion of BiP and disruption of the assembly compartment result in the loss of lamin phosphorylation. The dependency of lamin phosphorylation on BiP correlates with an interaction between BiP and UL50. Finally, we confirm previous data (S. V. Indran, M. E. Ballestas, and W. J. Britt, J. Virol. 84:3162-3177, 2010) suggesting an involvement of dynein in assembly compartment formation and extend this observation by showing that when dynein is inhibited, the nuclear morphology characteristic of an HCMV infection is lost. Our data suggest a highly integrated assembly-egress continuum.

The crystal structure of dynein intermediate chain-light chain roadblock complex gives new insights into dynein assembly

The roadblock/LC7 dynein light chain is a ubiquitous component of all dyneins and is essential for many diverse processes including proper axonal transport and dendrite growth. In addition, LC7 functions in non-dynein transcriptional activation of the transforming growth factor-beta complex. Crystal structures of Drosophila melanogaster LC7 in the apo form and in complex with a segment of the disordered N-terminal domain of dynein intermediate chain (IC) provide the first definitive identification of the IC sequence recognized by LC7. The site, confirmed by isothermal titration calorimetry studies, overlaps the IC sequence considered in the literature to be an IC self-association domain. The IC peptide binds as two amphipathic helices that lie along an extensive hydrophobic cleft on LC7 and ends with a polar side-chain interaction network that includes conserved residues from both proteins. The LC7 recognition sequence on IC and its interface with LC7 are well conserved and are, thus, likely representative of all IC x LC7 structures. Interestingly, the position of bound IC in the IC x LC7 complex mimics a helix that is integrated into the primary structure in distantly related LC7 homologs. The IC x LC7 structure further shows that the naturally occurring robl(Z) deletion mutation contains the majority of the IC binding site and suggests that promotion of IC binding by phosphorylation of LC7 is an indirect effect.

Adenovirus transport via direct interaction of cytoplasmic dynein with the viral capsid hexon subunit

Early in infection, adenovirus travels to the nucleus as a naked capsid using the microtubule motor cytoplasmic dynein. How the dynein complex is recruited to viral cargo remains unclear. We find that cytoplasmic dynein and its associated proteins dynactin and NudE/NudEL, but not LIS1 or ZW10, colocalized with incoming, postendosomal adenovirus particles. However, in contrast to physiological cargos, dynein binding to adenovirus was independent of these dynein-associated proteins. Dynein itself directly interacted through its intermediate and light intermediate chains with the adenovirus capsid subunit hexon in a pH-dependent manner. Expression of hexon or injection of anti-hexon antibody inhibited virus transport but not physiological dynein function. These results identify hexon as a direct receptor for cytoplasmic dynein and demonstrate that hexon recruits dynein for transport to the nucleus by a mechanism distinct from that for physiological dynein cargo.

Early, active, and specific localization of herpes simplex virus type 1 gM to nuclear membranes

Thirteen different glycoproteins are incorporated into mature herpes simplex virus type 1 (HSV-1) virions. Five of them play important roles during entry, while others intervene during egress of the virus. Although HSV-1 gM is not essential in cell culture, its deletion reduces viral yields and promotes syncytium formation. Furthermore, gM is conserved among herpesviruses, is essential for several of them, and can redirect the gD and gH/gL viral glycoproteins from the cell surface to the trans-Golgi network, where gM presumably modulates final capsid envelopment. Late in infection, gM reaches the nuclear envelope and decorates perinuclear virions. This process seemingly requires U(L)31 and U(L)34 and occurs when several markers of the trans-Golgi network have relocalized to the nucleus. However, the precise mechanism of gM nuclear targeting is unclear. We now report that gM is quickly and specifically targeted to nuclear membranes in a virus-dependent manner. This occurs prior to the HSV-1-induced reorganization of the trans-Golgi network and before gM enters the secretory pathway. The presence of a high-mannose glycosylation pattern on gM further corroborated these findings. While gM was targeted to the inner nuclear membrane early in infection, its partners gD, gH, gN, VP22, U(L)31, and U(L)34 did not colocalize with gM. These data suggest that nuclear gM fulfills an early nuclear function that is independent of its known interaction partners and its function in viral egress.

Multivalency in the assembly of intrinsically disordered Dynein intermediate chain

Dynein light chains are thought to increase binding efficiency of dynein intermediate chain to both dynein heavy chain and dynactin, but their exact role is not clear. Isothermal titration calorimetry and x-ray crystallography reported herein indicate that multivalency effects underlie efficient dynein assembly and regulation. For a ternary complex of a 60-amino acid segment of dynein intermediate chain (IC) bound to two homodimeric dynein light chains Tctex1 and LC8, there is a 50-fold affinity enhancement for the second light chain binding. For a designed IC construct containing two LC8 sites, observed the 1000-fold enhancement reflects a remarkably pure entropic chelate effect of a magnitude commensurate with theoretical predictions. The lower enhancement in wild-type IC is attributed to unfavorable free energy changes associated with incremental interactions of IC with Tctex1. Our results show assembled dynein IC as an elongated, flexible polybivalent duplex, and suggest that polybivalency is an important general mechanism for constructing stable yet reversible and functionally versatile complexes.

Equine herpesvirus type 1 (EHV-1) utilizes microtubules, dynein, and ROCK1 to productively infect cells

To initiate infection, equine herpesvirus type 1 (EHV-1) attaches to heparan sulfate on cell surfaces and then interacts with a putative glycoprotein D receptor(s). After attachment, virus entry occurs either by direct fusion of the virus envelope with the plasma membrane or via endocytosis followed by fusion between the virus envelope and an endosomal membrane. Upon fusion, de-enveloped virus particles are deposited into the cytoplasm and travel to the nucleus for viral replication. In this report, we examined the mechanism of EHV-1 intracellular trafficking and investigated the ability of EHV-1 to utilize specific cellular components to efficiently travel to the nucleus post-entry. Using a panel of microtubule-depolymerizing drugs and inhibitors of microtubule motor proteins, we show that EHV-1 infection is dependent on both the integrity of the microtubule network and the minus-end microtubule motor protein, dynein. In addition, we show that EHV-1 actively induces the acetylation of tubulin, a marker of microtubule stabilization, as early as 15 min post-infection. Finally, our data support a role for the cellular kinase, ROCK1, in virus trafficking to the nucleus.

The U(L)31 and U(L)34 gene products of herpes simplex virus 1 are required for optimal localization of viral glycoproteins D and M to the inner nuclear membranes of infected cells

U(L)31 and U(L)34 of herpes simplex virus type 1 form a complex necessary for nucleocapsid budding at the inner nuclear membrane (INM). Previous examination by immunogold electron microscopy and electron tomography showed that pU(L)31, pU(L)34, and glycoproteins D and M are recruited to perinuclear virions and densely staining regions of the INM where nucleocapsids bud into the perinuclear space. We now show by quantitative immunogold electron microscopy coupled with analysis of variance that gD-specific immunoreactivity is significantly reduced at both the INM and outer nuclear membrane (ONM) of cells infected with a U(L)34 null virus. While the amount of gM associated with the nuclear membrane (NM) was only slightly (P = 0.027) reduced in cells infected with the U(L)34 null virus, enrichment of gM in the INM at the expense of that in the ONM was greatly dependent on U(L)34 (P < 0.0001). pU(L)34 also interacted directly or indirectly with immature forms of gD (species expected to reside in the endoplasmic reticulum or nuclear membrane) in lysates of infected cells and with the cytosolic tail of gD fused to glutathione S-transferase in rabbit reticulocyte lysates, suggesting a role for the pU(L)34/gD interaction in recruiting gD to the NM. The effects of U(L)34 on gD and gM localization were not a consequence of decreased total expression of gD and gM, as determined by flow cytometry. Separately, pU(L)31 was dispensable for targeting gD and gM to the two leaflets of the NM but was required for (i) the proper INM-versus-ONM ratio of gD and gM in infected cells and (ii) the presence of electron-dense regions in the INM, representing nucleocapsid budding sites. We conclude that in addition to their roles in nucleocapsid envelopment and lamina alteration, U(L)31 and U(L)34 play separate but related roles in recruiting appropriate components to nucleocapsid budding sites at the INM.

Translocation of incoming pseudorabies virus capsids to the cell nucleus is delayed in the absence of tegument protein pUL37

After fusion of the envelope of herpesvirus particles with the host cell plasma membrane, incoming nucleocapsids are transported to nuclear pores. Inner tegument proteins pUL36, pUL37, and pUS3 remain attached to the nucleocapsid after entry and therefore might mediate interactions between the nucleocapsid and cellular microtubule-associated motor proteins during transport. To assay for the role of pUL37 in this process, we constructed a pUL37-deleted pseudorabies virus mutant, PrV-DeltaUL37/UL35GFP, which expresses a fusion protein of green fluorescent protein (GFP) and the nonessential small capsid protein pUL35, resulting in the formation of fluorescently labeled capsids. Confocal laser-scanning microscopy of rabbit kidney cells infected with PrV-DeltaUL37/UL35GFP revealed that, whereas penetration was not affected in the absence of pUL37, nuclear translocation of incoming particles was delayed by approximately 1 h compared to PrV-UL35GFP, but not abolished. In contrast, phenotypically complemented pUL37-containing virions of PrV-DeltaUL37/UL35GFP exhibited wild type-like entry kinetics. Thus, the presence of pUL37 is required for rapid nuclear translocation of incoming nucleocapsids.

Effects of lamin A/C, lamin B1, and viral US3 kinase activity on viral infectivity, virion egress, and the targeting of herpes simplex virus U(L)34-encoded protein to the inner nuclear membrane

Previous results indicated that the U(L)34 protein (pU(L)34) of herpes simplex virus 1 (HSV-1) is targeted to the nuclear membrane and is essential for nuclear egress of nucleocapsids. The normal localization of pU(L)34 and virions requires the U(S)3-encoded kinase that phosphorylates U(L)34 and lamin A/C. Moreover, pU(L)34 was shown to interact with lamin A in vitro. In the present study, glutathione S-transferase/pU(L)34 was shown to specifically pull down lamin A and lamin B1 from cellular lysates. To determine the role of these interactions on viral infectivity and pU(L)34 targeting to the inner nuclear membrane (INM), the localization of pU(L)34 was determined in LmnA(-/-) and LmnB1(-/-) mouse embryonic fibroblasts (MEFs) by indirect immunofluorescence and immunogold electron microscopy in the presence or absence of U(S)3 kinase activity. While pU(L)34 INM targeting was not affected by the absence of lamin B1 in MEFs infected with wild-type HSV as viewed by indirect immunofluorescence, it localized in densely staining scalloped-shaped distortions of the nuclear membrane in lamin B1 knockout cells infected with a U(S)3 kinase-dead virus. Lamin B1 knockout cells were relatively less permissive for viral replication than wild-type MEFs, with viral titers decreased at least 10-fold. The absence of lamin A (i) caused clustering of pU(L)34 in the nuclear rim of cells infected with wild-type virus, (ii) produced extensions of the INM bearing pU(L)34 protein in cells infected with a U(S)3 kinase-dead mutant, (iii) precluded accumulation of virions in the perinuclear space of cells infected with this mutant, and (iv) partially restored replication of this virus. The latter observation suggests that lamin A normally impedes viral infectivity and that U(S)3 kinase activity partially alleviates this impediment. On the other hand, lamin B1 is necessary for optimal viral replication, probably through its well-documented effects on many cellular pathways. Finally, neither lamin A nor B1 was absolutely required for targeting pU(L)34 to the INM, suggesting that this targeting is mediated by redundant functions or can be mediated by other proteins.

UL36p is required for efficient transport of membrane-associated herpes simplex virus type 1 along microtubules

Microtubule-mediated anterograde transport is essential for the transport of herpes simplex virus type 1 (HSV-1) along axons, yet little is known regarding the mechanism and the machinery required for this process. Previously, we were able to reconstitute anterograde transport of HSV-1 on microtubules in an in vitro microchamber assay. Here we report that the large tegument protein UL36p is essential for this trafficking. Using a fluorescently labeled UL36 null HSV-1 strain, KDeltaUL36GFP, we found that it is possible to isolate a membrane-associated population of this virus. Although these viral particles contained normal amounts of tegument proteins VP16, vhs, and VP22, they displayed a 3-log decrease in infectivity and showed a different morphology compared to UL36p-containing virions. Membrane-associated KDeltaUL36GFP also displayed a slightly decreased binding to microtubules in our microchamber assay and a two-thirds decrease in the frequency of motility. This decrease in binding and motility was restored when UL36p was supplied in trans by a complementing cell line. These findings suggest that UL36p is necessary for HSV-1 anterograde transport.

Transduction of brain by herpes simplex virus vectors

An imposing obstacle to gene therapy is the inability to transduce all of the necessary cells in a target organ. This certainly applies to gene transfer to the brain, especially when one considers the challenges involved in scaling up transduction from animal models to use in the clinic. Non-neurotropic viral gene transfer vectors (e.g., adenovirus, adeno-associated virus, and lentivirus) do not spread very far in the nervous system, and consequently these vectors transduce brain regions mostly near the injection site in adult animals. This indicates that numerous, well-spaced injections would be required to achieve widespread transduction in a large brain with these vectors. In contrast, herpes simplex virus type 1 (HSV-1) is a promising vector for widespread gene transfer to the brain owing to the innate ability of the virus to spread through the nervous system and form latent infections in neurons that last for the lifetime of the infected individual. In this review, we summarize the published literature of the transduction patterns produced by attenuated HSV-1 vectors in small animals as a function of the injection site, and discuss the implications of the distribution for widespread gene transfer to the large animal brain.

Transport and egress of herpes simplex virus in neurons

The mechanisms of axonal transport of the alphaherpesviruses, HSV and pseudorabies virus (PrV), in neuronal axons are of fundamental interest, particularly in comparison with other viruses, and offer potential sites for antiviral intervention or development of gene therapy vectors. These herpesviruses are transported rapidly along microtubules (MTs) in the retrograde direction from the axon terminus to the dorsal root ganglion and then anterogradely in the opposite direction. Retrograde transport follows fusion and deenvelopment of the viral capsid at the axonal membrane followed by loss of most of the tegument proteins and then binding of the capsid via one or more viral proteins (VPs) to the retrograde molecular motor dynein. The HSV capsid protein pUL35 has been shown to bind to the dynein light chain Tctex1 but is likely to be accompanied by additional dynein binding of an inner tegument protein. The mechanism of anterograde transport is much more controversial with different processes being claimed for PrV and HSV: separate transport of HSV capsid/tegument and glycoproteins versus PrV transport as an enveloped virion. The controversy has not been resolved despite application, in several laboratories, of confocal microscopy (CFM), real-time fluorescence with viruses dual labelled on capsid and glycoprotein, electron microscopy in situ and immuno-electron microscopy. Different processes for each virus seem counterintuitive although they are the most divergent in the alphaherpesvirus subfamily. Current hypotheses suggest that unenveloped HSV capsids complete assembly in the axonal growth cones and varicosities, whereas with PrV unenveloped capsids are only found travelling in a retrograde direction.