Herpes simplex virus capsids are transported in neuronal axons without an envelope containing the viral glycoproteins

An ESCRT/VPS4 envelopment trap to examine the mechanism of alphaherpesvirus assembly and transport in neurons

The assembly and egress of alphaherpesviruses, including Herpes simplex virus type 1 (HSV-1) and Pseudorabies virus (PRV), within neurons is poorly understood. A key unresolved question is the structure of the viral particle that moves by anterograde transport along the axon, and two alternative mechanisms have been described. In the "Married" model capsids acquire their envelopes in the cell body, then traffic along axons as enveloped virions within a bounding organelle. In the "Separate" model non-enveloped capsids travel from the cell body into and along the axon, eventually encountering their envelopment organelles at a distal site such as the nerve cell terminal. Here we describe an "envelopment trap" to test these models using the dominant negative terminal ESCRT component VPS4-EQ. GFP-tagged VPS4-EQ was used to arrest HSV-1 or PRV capsid envelopment, inhibit downstream trafficking and GFP-label envelopment intermediates. We found that GFP-VPS4-EQ inhibited trafficking of HSV-1 capsids into and along the neurites and axons of mouse CAD cells and rat embryonic primary cortical neurons, consistent with egress via the married pathway. In contrast, transport of HSV-1 capsids was unaffected in the neurites of human SK-N-SH neuroblastoma cells, consistent with the separate mechanism. Unexpectedly, PRV (generally thought to utilize the married pathway) also appeared to employ the separate mechanism in SK-N-SH cells. We propose that apparent differences in the methods of HSV-1 and PRV egress are more likely a reflection of the host neuron in which transport is studied, rather than true biological differences between the viruses themselves. IMPORTANCE Alphaherpesviruses, including Herpes simplex virus type 1 (HSV-1) and Pseudorabies virus (PRV), are pathogens of the nervous system. They replicate in the nerve cell body then travel great distances along axons to reach nerve termini and spread to adjacent epithelial cells, however key aspects of how these viruses travel along axons remain controversial. Here we test two alternative mechanisms for transport, the married and separate models, by blocking envelope assembly, a critical step in viral egress. When we arrest formation of the viral envelope using a mutated component of the cellular ESCRT apparatus we find that entry of viral particles into axons is blocked in some types of neuron, but not others. This approach allows us to determine whether envelope assembly occurs prior to entry of viruses into axons, or afterwards, and thus to distinguish between the alternative models for viral transport.

Anterograde transport of α-herpesviruses in neuronal axons

α-herpesviruses have been very successful, principally because they establish lifelong latency in sensory ganglia. An essential piece of the lifecycle of α-herpesviruses involves the capacity to travel from sensory neurons to epithelial tissues following virus reactivation from latency, a process known as anterograde transport. Virus particles formed in neuron cell bodies hitchhike on kinesin motors that run along microtubules, the length of axons. Herpes simplex virus (HSV) and pseudorabies virus (PRV) have been intensely studied to elucidate anterograde axonal transport. Both viruses use similar strategies for anterograde transport, although there are significant differences in the form of virus particles transported in axons, the identity of the kinesins that transport viruses, and how certain viral membrane proteins, gE/gI and US9, participate in this process. This review compares the older models for HSV and PRV anterograde transport with recent results, which are casting a new light on several aspects of this process.

HSV-1 Cytoplasmic Envelopment and Egress

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

Microtubule-Dependent Trafficking of Alphaherpesviruses in the Nervous System: The Ins and Outs

The Alphaherpesvirinae include the neurotropic pathogens herpes simplex virus and varicella zoster virus of humans and pseudorabies virus of swine. These viruses establish lifelong latency in the nuclei of peripheral ganglia, but utilize the peripheral tissues those neurons innervate for productive replication, spread, and transmission. Delivery of virions from replicative pools to the sites of latency requires microtubule-directed retrograde axonal transport from the nerve terminus to the cell body of the sensory neuron. As a corollary, during reactivation newly assembled virions must travel along axonal microtubules in the anterograde direction to return to the nerve terminus and infect peripheral tissues, completing the cycle. Neurotropic alphaherpesviruses can therefore exploit neuronal microtubules and motors for long distance axonal transport, and alternate between periods of sustained plus end- and minus end-directed motion at different stages of their infectious cycle. This review summarizes our current understanding of the molecular details by which this is achieved.

Kinesin-1 Proteins KIF5A, -5B, and -5C Promote Anterograde Transport of Herpes Simplex Virus Enveloped Virions in Axons

Herpes simplex virus (HSV) and other alphaherpesviruses must spread from sites of viral latency in sensory ganglia to peripheral tissues, where the viruses can replicate to higher titers before spreading to other hosts. These viruses move in neuronal axons from ganglia to the periphery propelled by kinesin motors moving along microtubules. Two forms of HSV particles undergo this anterograde transport in axons: (i) unenveloped capsids that become enveloped after reaching axon tips and (ii) enveloped virions that are transported within membrane vesicles in axons. Fundamental to understanding this axonal transport is the question of which of many different axonal kinesins convey HSV particles. Knowing which kinesins promote axonal transport would provide clues to the identity of HSV proteins that tether onto kinesins. Prominent among axonal kinesins are the kinesin-1 (KIF5A, -5B, and -5C) and kinesin-3 (e.g., KIF1A and -1B) families. We characterized fluorescent forms of cellular cargo molecules to determine if enveloped HSV particles were present in the vesicles containing these cargos. Kinesin-1 cargo proteins were present in vesicles containing HSV particles, but not kinesin-3 cargos. Fluorescent kinesin-1 protein KIF5C extensively colocalized with HSV particles, while fluorescent kinesin-1 KIF1A did not. Silencing of kinesin-1 proteins KIF5A, -5B, and -5C or light chains KLC1 and KLC2 inhibited the majority of HSV anterograde transport, while silencing of KIF1A had little effect on HSV transport in axons. We concluded that kinesin-1 proteins are important in the anterograde transport of the majority of HSV enveloped virions in neuronal axons and kinesin-3 proteins are less important.IMPORTANCE Herpes simplex virus (HSV) and other alphaherpesviruses, such as varicella-zoster virus, depend upon the capacity to navigate in neuronal axons. To do this, virus particles tether onto dyneins and kinesins that motor along microtubules from axon tips to neuronal cell bodies (retrograde) or from cell bodies to axon tips (anterograde). Following reactivation from latency, alphaherpesviruses absolutely depend upon anterograde transport of virus particles in axons in order to reinfect peripheral tissues and spread to other hosts. Which of the many axonal kinesins transport HSV in axons is not clear. We characterized fluorescent cellular cargo molecules and kinesins to provide evidence that HSV enveloped particles are ferried by kinesin-1 proteins KIF5A, -5B, and -5C and their light chains, KLC1 and KLC2, in axons. Moreover, we obtained evidence that kinesin-1 proteins are functionally important in anterograde transport of HSV virions by silencing these proteins.

A VP26-mNeonGreen Capsid Fusion HSV-2 Mutant Reactivates from Viral Latency in the Guinea Pig Genital Model with Normal Kinetics

Fluorescent herpes simplex viruses (HSV) are invaluable tools for localizing virus in cells, permitting visualization of capsid trafficking and enhancing neuroanatomical research. Fluorescent viruses can also be used to study virus kinetics and reactivation in vivo. Such studies would be facilitated by fluorescent herpes simplex virus recombinants that exhibit wild-type kinetics of replication and reactivation and that are genetically stable. We engineered an HSV-2 strain expressing the fluorescent mNeonGreen protein as a fusion with the VP26 capsid protein. This virus has normal replication and in vivo recurrence phenotypes, providing an essential improved tool for further study of HSV-2 infection.

Differentiated Human SH-SY5Y Cells Provide a Reductionist Model of Herpes Simplex Virus 1 Neurotropism

Neuron-virus interactions that occur during herpes simplex virus (HSV) infection are not fully understood. Neurons are the site of lifelong latency and are a crucial target for long-term suppressive therapy or viral clearance. A reproducible neuronal model of human origin would facilitate studies of HSV and other neurotropic viruses. Current neuronal models in the herpesvirus field vary widely and have caveats, including incomplete differentiation, nonhuman origins, or the use of dividing cells that have neuropotential but lack neuronal morphology. In this study, we used a robust approach to differentiate human SH-SY5Y neuroblastoma cells over 2.5 weeks, producing a uniform population of mature human neuronal cells. We demonstrate that terminally differentiated SH-SY5Y cells have neuronal morphology and express proteins with subcellular localization indicative of mature neurons. These neuronal cells are able to support a productive HSV-1 infection, with kinetics and overall titers similar to those seen in undifferentiated SH-SY5Y cells and the related SK-N-SH cell line. However, terminally differentiated, neuronal SH-SY5Y cells release significantly less extracellular HSV-1 by 24 h postinfection (hpi), suggesting a unique neuronal response to viral infection. With this model, we are able to distinguish differences in neuronal spread between two strains of HSV-1. We also show expression of the antiviral protein cyclic GMP-AMP synthase (cGAS) in neuronal SH-SY5Y cells, which is the first demonstration of the presence of this protein in nonepithelial cells. These data provide a model for studying neuron-virus interactions at the single-cell level as well as via bulk biochemistry and will be advantageous for the study of neurotropic viruses in vitroIMPORTANCE Herpes simplex virus (HSV) affects millions of people worldwide, causing painful oral and genital lesions, in addition to a multitude of more severe symptoms such as eye disease, neonatal infection, and, in rare cases, encephalitis. Presently, there is no cure available to treat those infected or prevent future transmission. Due to the ability of HSV to cause a persistent, lifelong infection in the peripheral nervous system, the virus remains within the host for life. To better understand the basis of virus-neuron interactions that allow HSV to persist within the host peripheral nervous system, improved neuronal models are required. Here we describe a cost-effective and scalable human neuronal model system that can be used to study many neurotropic viruses, such as HSV, Zika virus, dengue virus, and rabies virus.

A Therapeutic Antiviral Antibody Inhibits the Anterograde Directed Neuron-to-Cell Spread of Herpes Simplex Virus and Protects against Ocular Disease

Herpes simplex virus (HSV) is a leading cause of blindness and viral encephalitis in the developed world. Upon reactivation from sensory neurons, HSV returns via axonal transport to peripheral tissues where it causes, e.g., severe, potentially blinding ocular diseases. In the present study we investigated whether the HSV-1/2 glycoprotein B-specific antibody mAb 2c or its humanized counterpart mAb hu2c can protect from ocular disease in a mouse model of HSV-1-induced acute retinal necrosis (ARN). In this model the viral spread from the initially infected to the contralateral eye resembles the routes taken in humans upon HSV reactivation. Systemic antibody treatment prior or early after infection effectively protected the mice from the development of ARN. These observations suggest that the antibody potently neutralized the infection and inhibited the viral transmission, since there was almost no virus detectable in the contralateral eyes and trigeminal ganglia of antibody treated mice. Besides of neutralizing free virus or limiting the infection via activating the complement or cellular effector functions, blocking of the anterograde directed neuron-to-cell spread of HSV represents a viable mode of action how mAb 2c protected the mice from ARN. We proved this hypothesis using a microfluidic chamber system. Neurons and epithelial cells were cultured in two separate compartments where the neurons sent axons via connecting microgrooves to the epithelial cells. Neurons were infected with a reporter HSV-1 strain expressing mCherry, and the co-culture was treated with neutralizing antibodies. In contrast to commercial polyclonal human HSV-neutralizing immunoglobulins, mAb 2c effectively blocked the anterograde directed neuron-to-cell transmission of the virus. Our data suggest that the humanized HSV-1/2-gB antibody protects mice from ocular disease by blocking the neuronal spread of HSV. Therefore, mAb hu2c may become a potent novel therapeutic option for severe ocular HSV infections.

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

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

Live cell imaging of alphaherpes virus anterograde transport and spread

Advances in live cell fluorescence microscopy techniques, as well as the construction of recombinant viral strains that express fluorescent fusion proteins have enabled real-time visualization of transport and spread of alphaherpes virus infection of neurons. The utility of novel fluorescent fusion proteins to viral membrane, tegument, and capsids, in conjunction with live cell imaging, identified viral particle assemblies undergoing transport within axons. Similar tools have been successfully employed for analyses of cell-cell spread of viral particles to quantify the number and diversity of virions transmitted between cells. Importantly, the techniques of live cell imaging of anterograde transport and spread produce a wealth of information including particle transport velocities, distributions of particles, and temporal analyses of protein localization. Alongside classical viral genetic techniques, these methodologies have provided critical insights into important mechanistic questions. In this article we describe in detail the imaging methods that were developed to answer basic questions of alphaherpes virus transport and spread.

Directional spread of alphaherpesviruses in the nervous system

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

Delivery of herpes simplex virus to retinal ganglion cell axon is dependent on viral protein Us9

PURPOSE

How herpes simplex virus (HSV) is transported from the infected neuron cell body to the axon terminal is poorly understood. Several viral proteins are candidates for regulating the process, but the evidence is controversial. We compared the results of Us9 deletions in two HSV strains (F and NS) using a novel quantitative assay to test the hypothesis that the viral protein Us9 regulates the delivery of viral DNA to the distal axon of retinal ganglion cells in vivo. We also deleted a nine-amino acid motif in the Us9 protein of F strain (Us9-30) to define the role of this domain in DNA delivery.

METHODS

The vitreous chambers of murine eyes were infected with equivalent amounts of F or NS strains of HSV. At 3, 4, or 5 days post infection (dpi), both optic tracts (OT) were dissected and viral genome was quantified by qPCR.

RESULTS

At 3 dpi, the F strain Us9- and Us9-30 mutants delivered less than 10% and 1%, respectively, of the viral DNA delivered after infection with the Us9R (control) strain. By 4 and 5 dpi, delivery of viral DNA had only partially recovered. Deletion of Us9 in NS-infected mice has a less obvious effect on delivery of new viral DNA to the distal OT. By 3 dpi the NS Us9-strain delivered 22% of the DNA that was delivered by the NS wt, and by 4 and 5 dpi the amount of Us9-viral DNA was 96% and 81%, respectively.

CONCLUSIONS

A highly conserved acidic cluster within the Us9 protein plays a critical role for genome transport to the distal axon. The transport is less dependent on Us9 expression in the NS than in the F strain virus. This assay can be used to compare transport efficiency in other neurotropic viral strains.

Endocytic tubules regulated by Rab GTPases 5 and 11 are used for envelopment of herpes simplex virus

Enveloped viruses employ diverse and complex strategies for wrapping at cellular membranes, many of which are poorly understood. Here, an ultrastructural study of herpes simplex virus 1 (HSV1)-infected cells revealed envelopment in tubular membranes. These tubules were labelled by the fluid phase marker horseradish peroxidase (HRP), and were observed to wrap capsids as early as 2 min after HRP addition, indicating that the envelope had recently cycled from the cell surface. Consistent with this, capsids did not colocalise with either the trans-Golgi network marker TGN46 or late endosomal markers, but showed coincidence with the transferrin receptor. Virus glycoproteins were retrieved from the plasma membrane (PM) to label wrapping capsids, a process that was dependent on both dynamin and Rab5. Combined depletion of Rab5 and Rab11 reduced virus yield to <1%, resulting in aberrant localisation of capsids. These results suggest that endocytosis from the PM into endocytic tubules provides the main source of membrane for HSV1, and reveal a new mechanism for virus exploitation of the endocytic pathway.

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

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

HSV, axonal transport and Alzheimer's disease: in vitro and in vivo evidence for causal relationships

HSV, a neurotropic virus, travels within neuronal processes by fast axonal transport. During neuronal infection HSV travels retrograde from the sensory nerve terminus to the neuronal cell body, where it replicates or enters latency. During replication HSV travels anterograde from the cell body to the nerve terminus. Postmortem studies find a high frequency of HSV DNA in the trigeminal ganglia as well as the brain. Studies correlating HSV with Alzheimer's disease (AD) have been controversial. Here we review clinical evidence supporting such a link. Furthermore, the author describes experimental data showing physical interactions between nascent HSV particles and host transport machinery implicated in AD. The author concludes that the complexity of this relationship has been insufficiently explored, although the relative ease and nontoxicity of a potential anti-HSV treatment for AD demands further study.

Improper tagging of the non-essential small capsid protein VP26 impairs nuclear capsid egress of herpes simplex virus

To analyze the subcellular trafficking of herpesvirus capsids, the small capsid protein has been labeled with different fluorescent proteins. Here, we analyzed the infectivity of several HSV1(17(+)) strains in which the N-terminal region of the non-essential small capsid protein VP26 had been tagged at different positions. While some variants replicated with similar kinetics as their parental wild type strain, others were not infectious at all. Improper tagging resulted in the aggregation of VP26 in the nucleus, prevented efficient nuclear egress of viral capsids, and thus virion formation. Correlative fluorescence and electron microscopy showed that these aggregates had sequestered several other viral proteins, but often did not contain viral capsids. The propensity for aggregate formation was influenced by the type of the fluorescent protein domain, the position of the inserted tag, the cell type, and the progression of infection. Among the tags that we have tested, mRFPVP26 had the lowest tendency to induce nuclear aggregates, and showed the least reduction in replication when compared to wild type. Our data suggest that bona fide monomeric fluorescent protein tags have less impact on proper assembly of HSV1 capsids and nuclear capsid egress than tags that tend to dimerize. Small chemical compounds capable of inducing aggregate formation of VP26 may lead to new antiviral drugs against HSV infections.

Cytoplasmic trafficking, endosomal escape, and perinuclear accumulation of adeno-associated virus type 2 particles are facilitated by microtubule network

Understanding adeno-associated virus (AAV) trafficking is critical to advance our knowledge of AAV biology and exploit novel aspects of vector development. Similar to the case for most DNA viruses, after receptor binding and entry, AAV traverses the cytoplasm and deposits the viral genome in the cell nucleus. In this study, we examined the role of the microtubule (MT) network in productive AAV infection. Using pharmacological reagents (e.g., nocodazole), live-cell imaging, and flow cytometry analysis, we demonstrated that AAV type 2 (AAV2) transduction was reduced by at least 2-fold in the absence of the MT network. Cell surface attachment and viral internalization were not dependent on an intact MT network. In treated cells at 2 h postinfection, quantitative three-dimensional (3D) microscopy determined a reproducible difference in number of intracellular particles associated with the nuclear membrane or the nucleus compared to that for controls (6 to 7% versus 26 to 30%, respectively). Confocal microscopy analysis demonstrated a direct association of virions with MTs, further supporting a critical role in AAV infection. To investigate the underling mechanisms, we employed single-particle tracking (SPT) to monitor the viral movement in real time. Surprisingly, unlike other DNA viruses (e.g., adenovirus [Ad] and herpes simplex virus [HSV]) that display bidirectional motion on MTs, AAV2 displays only unidirectional movement on MTs toward the nuclei, with peak instantaneous velocities at 1.5 to 3.5 μm/s. This rapid and unidirectional motion on MTs lasts for about 5 to 10 s and results in AAV particles migrating more than 10 μm in the cytoplasm reaching the nucleus very efficiently. Furthermore, electron microscopy analysis determined that, unlike Ad and HSV, AAV2 particles were transported on MTs within membranous compartments, and surprisingly, the acidification of AAV2-containing endosomes was delayed by the disruption of MTs. These findings together suggest an as-yet-undescribed model in which after internalization, AAV2 exploits MTs for rapid cytoplasmic trafficking in endosomal compartments unidirectionally toward the perinuclear region, where most acidification events for viral escape take place.

Making the case: married versus separate models of alphaherpes virus anterograde transport in axons

Alphaherpesvirus virions infect neurons and are transported in axons for long distance spread within the host nervous system. The assembly state of newly made herpesvirus particles during anterograde transport in axons is an essential question in alphaherpesvirus biology. The structure of the particle has remained both elusive and controversial for the past two decades, with conflicting evidence from EM, immunofluorescence, and live cell imaging studies. Two opposing models have been proposed-the Married and Separate Models. Under the Married Model, infectious virions are assembled in the neuronal cell body before sorting into axons and then traffic inside a transport vesicle. Conversely, the Separate Model postulates that vesicles containing viral membrane proteins are sorted into axons independent of capsids, with final assembly of mature virions occurring at a distant egress site. Recently, a complementary series of studies employing high-resolution EM and live cell fluorescence microscopy have provided evidence consistent with the Married Model, whereas other studies offer evidence supporting the Separate Model. In this review, we compare and discuss the published data and attempt to reconcile divergent findings and interpretations as they relate to these models.

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

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

Cryo electron tomography of herpes simplex virus during axonal transport and secondary envelopment in primary neurons

During herpes simplex virus 1 (HSV1) egress in neurons, viral particles travel from the neuronal cell body along the axon towards the synapse. Whether HSV1 particles are transported as enveloped virions as proposed by the ‘married’ model or as non-enveloped capsids suggested by the ‘separate’ model is controversial. Specific viral proteins may form a recruitment platform for microtubule motors that catalyze such transport. However, their subviral location has remained elusive. Here we established a system to analyze herpesvirus egress by cryo electron tomography. At 16 h post infection, we observed intra-axonal transport of progeny HSV1 viral particles in dissociated hippocampal neurons by live-cell fluorescence microscopy. Cryo electron tomography of frozen-hydrated neurons revealed that most egressing capsids were transported independently of the viral envelope. Unexpectedly, we found not only DNA-containing capsids (cytosolic C-capsids), but also capsids lacking DNA (cytosolic A-/B-capsids) in mid-axon regions. Subvolume averaging revealed lower amounts of tegument on cytosolic A-/B-capsids than on C-capsids. Nevertheless, all capsid types underwent active axonal transport. Therefore, even few tegument proteins on the capsid vertices seemed to suffice for transport. Secondary envelopment of capsids was observed at axon terminals. On their luminal face, the enveloping vesicles were studded with typical glycoprotein-like spikes. Furthermore, we noted an accretion of tegument density at the concave cytosolic face of the vesicle membrane in close proximity to the capsids. Three-dimensional analysis revealed that these assembly sites lacked cytoskeletal elements, but that filamentous actin surrounded them and formed an assembly compartment. Our data support the ‘separate model’ for HSV1 egress, i.e. progeny herpes viruses being transported along axons as subassemblies and not as complete virions within transport vesicles.

Herpes simplex virus 1 (HSV1) establishes lifelong latent infections in the peripheral nervous system. After reactivation, progeny viral particles travel within sensory neurons towards sites of initial infection. There are conflicting reports what type of viral structures are transported: some studies observed non-enveloped capsids traveling while others reported transport of fully enveloped viruses within vesicles. Here, we used cryo electron tomography to analyze the three-dimensional architecture of HSV1 in axons of hippocampal neurons. In mid-axonal regions we found predominantly non-enveloped capsids. Interestingly, we observed both genome-containing and empty capsids that differed significantly in the amount of bound proteins. Viral protein recruitment thus varied between the different cytosolic capsid types, but effective transport occurred despite these differences. Furthermore, we observed three-dimensional snapshots of secondary capsid envelopment in axon terminals. Altogether, this study provides valuable structural detail on axonal HSV1 particles supporting the notion that viral subassemblies are conveyed along the axons to be assembled only after axonal transport.

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.

Herpesviruses remodel host membranes for virus egress

Herpesviruses replicate their DNA and package this DNA into capsids in the nucleus. These capsids then face substantial obstacles to their release from cells. Unlike other DNA viruses, herpesviruses do not depend on disruption of nuclear and cytoplasmic membranes for their release. Enveloped particles are formed by budding through inner nuclear membranes, and then these perinuclear enveloped particles fuse with outer nuclear membranes. Unenveloped capsids in the cytoplasm are decorated with tegument proteins and then undergo secondary envelopment by budding into trans-Golgi network membranes, producing infectious particles that are released. In this Review, we describe the remodelling of host membranes that facilitates herpesvirus egress.

[Herpes simplex virus (HSV)]

Herpes simplex virus (HSV), the prototype of the herpesvirus family, causes a variety of diseases in human. In this review, I focus on the molecular mechanism of HSV infection including recent advance on this research field.

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

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

Anterograde transport of herpes simplex virus capsids in neurons by both separate and married mechanisms

Anterograde transport of herpes simplex virus (HSV) from neuronal cell bodies into, and down, axons is a fundamentally important process for spread to other hosts. Different techniques for imaging HSV in axons have produced two models for how virus particles are transported in axons. In the Separate model, viral nucleocapsids devoid of the viral envelope and membrane glycoproteins are transported in axons. In the Married model, enveloped HSV particles (with the viral glycoproteins) encased within membrane vesicles are transported in the anterograde direction. Earlier studies of HSV-infected human neurons involving electron microscopy (EM) and immunofluorescence staining of glycoproteins and capsids supported the Separate model. However, more-recent live-cell imaging of rat, chicken, and mouse neurons produced evidence supporting the Married model. In a recent EM study, a mixture of Married (75%) and Separate (25%) HSV particles was observed. Here, we studied an HSV recombinant expressing a fluorescent form of the viral glycoprotein gB and a fluorescent capsid protein (VP26), observing that human SK-N-SH neurons contained both Separate (the majority) and Married particles. Live-cell imaging of rat superior cervical ganglion (SCG) neuronal axons in a chamber system (which oriented the axons) also produced evidence of Separate and Married particles. Together, our results suggest that one can observe anterograde transport of both HSV capsids and enveloped virus particles depending on which neurons are cultured and how the neurons are imaged.

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

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

Ultrastructural analysis of virion formation and intraaxonal transport of herpes simplex virus type 1 in primary rat neurons

After primary replication at the site of entry into the host, alphaherpesviruses infect and establish latency in neurons. To this end, they are transported within axons retrograde from the periphery to the cell body for replication and in an anterograde direction to synapses for infection of higher-order neurons or back to the periphery. Retrograde transport of incoming nucleocapsids is well documented. In contrast, there is still significant controversy on the mode of anterograde transport. By high-resolution transmission electron microscopy of primary neuronal cultures from embryonic rat superior cervical ganglia infected by pseudorabies virus (PrV), we observed the presence of enveloped virions in axons within vesicles supporting the "married model" of anterograde transport of complete virus particles within vesicles (C. Maresch, H. Granzow, A. Negatsch, B.G. Klupp, W. Fuchs, J.P. Teifke, and T.C. Mettenleiter, J. Virol. 84:5528-5539, 2010). We have now extended these analyses to the related human herpes simplex virus type 1 (HSV-1). We have demonstrated that in neurons infected by HSV-1 strains HFEM, 17+ or SC16, approximately 75% of virus particles observed intraaxonally or in growth cones late after infection constitute enveloped virions within vesicles, whereas approximately 25% present as naked capsids. In general, the number of HSV-1 particles in the axons was significantly less than that observed after PrV infection.

Resolving the assembly state of herpes simplex virus during axon transport by live-cell imaging

Neurotropic herpesviruses depend on long-distance axon transport for the initial establishment of latency in peripheral ganglia (retrograde transport) and for viral spread in axons to exposed body surfaces following reactivation (anterograde transport). Images of neurons infected with herpes simplex virus type 1 (HSV-1), acquired using electron microscopy, have led to a debate regarding why different types of viral structures are seen in axons and which of these particles are relevant to the axon transport process. In this study, we applied time-lapse fluorescence microscopy to image HSV-1 virion components actively translocating to distal axons in primary neurons and neuronal cell lines. Key to these findings, only a small fraction of viral particles were engaged in anterograde transport during the egress phase of infection at any given time. By selective analysis of the composition of the subpopulation of actively transporting capsids, a link between transport of fully assembled HSV-1 virions and the neuronal secretory pathway was identified. Last, we have evaluated the seemingly opposing findings made in previous studies of HSV-1 axon transport in fixed cells and demonstrate a limitation to assessing the composition of individual HSV-1 particles using antibody detection methods.

Ultrastructural analysis of virion formation and anterograde intraaxonal transport of the alphaherpesvirus pseudorabies virus in primary neurons

A hallmark of alphaherpesviruses is their capacity to be neuroinvasive and establish latent infections in neurons. After primary replication in epithelial cells at the periphery, entry into nerve endings occurs, followed by retrograde transport of nucleocapsids to the nucleus where viral transcription, genome replication, and nucleocapsid formation take place. Translocation of nucleocapsids to the cytoplasm is followed by axonal transport to infect synaptically linked neurons. Two modes of intraaxonal anterograde herpesvirus transport have been proposed: transport of complete, enveloped virions within vesicles ("married model"), and separate transport of capsids and envelopes ("subassembly model"). To assess this in detail for the alphaherpesvirus pseudorabies virus (PrV), we used high-resolution transmission electron microscopy of primary neuronal cultures from embryonic rat superior cervical ganglia after infection with wild-type and gB-deficient PrV. Our data show that intranuclear capsid maturation, nuclear egress and cytoplasmic secondary envelopment occur as in cultured nonpolarized cells (H. Granzow, F. Weiland, A. Jöns, B. G. Klupp, A. Karger, and T. C. Mettenleiter, J. Virol. 71:2072-2082, 1997). PrV virions were present in axons as enveloped particles within vesicles associated with microtubules and apparently leave the neuron by exocytosis primarily at the growth cone. Only a few nonenveloped nucleocapsids were found in the axon. The same picture was observed after infection by phenotypically complemented gB-deficient PrV, which is able to complete only a single round of replication. Our data thus support intraaxonal anterograde transport of enveloped PrV virions within vesicles following the "married model."

Retrograde axon transport of herpes simplex virus and pseudorabies virus: a live-cell comparative analysis

Upon entry, neuroinvasive herpesviruses traffic from axon terminals to the nuclei of neurons resident in peripheral ganglia, where the viral DNA is deposited. A detailed analysis of herpes simplex virus type 1 (HSV-1) transport dynamics in axons following entry is currently lacking. Here, time lapse fluorescence microscopy was used to compare the postentry viral transport of two neurotropic herpesviruses: HSV-1 and pseudorabies virus (PRV). HSV-1 capsid transport dynamics were indistinguishable from those of PRV and did not differ in neurons of human, mouse, or avian origin. Simultaneous tracking of capsids and tegument proteins demonstrated that the composition of actively transporting HSV-1 is remarkably similar to that of PRV. This quantitative assessment of HSV-1 axon transport following entry demonstrates that HSV-1 and PRV share a conserved mechanism for postentry retrograde transport in axons and provides the foundation for further studies of the retrograde transport process.

Directional transneuronal spread of α-herpesvirus infection

Most α-herpesviruses are pantropic, neuroinvasive pathogens that establish a reactivateable, latent infection in the PNS of their natural hosts. Various manifestations of herpes disease rely on extent and direction of the spread of infection between the surface epithelia and the nervous system components that innervate that surface. One aspect of such controlled spread of infection is the capacity for synaptically defined, transneuronal spread, a property that makes α-herpesviruses useful tools for determining the connectivity of neural circuits. The current understanding of intra-axonal transport and transneuronal spread of α-herpesviruses is reviewed, focusing on work with herpes simplex virus and pseudorabies virus, the available in vitro technology used to study viral transport and spread is evaluated and how certain viral mutants can be used to examine neural circuit architecture is described in this article.

[Visualization of viruses in living cells]

Live-cell imaging of cells infected with recombinant viruses expressing fluorescently tagged structural viral proteins enables to visualize virion maturation pathways at a virion particle level in the same cells as infection progresses. This technology have gradually unveiled previously unreported aspects of the pathways. This article focuses on live-cell imaging technology of herpes simplex virus 1 (HSV-1) and reviews the up-to-date topics of HSV-1 maturation pathway including our recent progress.

Herpesvirus assembly: an update

The order Herpesvirales contains viruses infecting animals from molluscs to men with a common virion morphology which have been classified into the families Herpesviridae, Alloherpesviridae and Malacoherpesviridae. Herpes virions are among the most complex virus particles containing a multitude of viral and cellular proteins which assemble into nucleocapsid, envelope and tegument. After autocatalytic assembly of the capsid and packaging of the newly replicated viral genome, a process which occurs in the nucleus and resembles head formation and genome packaging in the tailed double-stranded DNA bacteriophages, the nucleocapsid is translocated to the cytoplasm by budding at the inner nuclear membrane followed by fusion of the primary envelope with the outer nuclear membrane. Viral and cellular proteins are involved in mediating this 'nuclear egress' which entails substantial remodeling of the nuclear architecture. For final maturation within the cytoplasm tegument components associate with the translocated nucleocapsid, with themselves, and with the future envelope containing viral membrane proteins in a complex network of interactions resulting in the formation of an infectious herpes virion. The diverse interactions between the involved proteins exhibit a striking redundancy which is still insufficiently understood. In this review, recent advances in our understanding of the molecular processes resulting in herpes virion maturation will be presented and discussed as an update of a previous contribution [Mettenleiter, T.C., 2004. Budding events in herpesvirus morphogenesis. Virus Res. 106, 167-180].

Herpes simplex virus utilizes the large secretory vesicle pathway for anterograde transport of tegument and envelope proteins and for viral exocytosis from growth cones of human fetal axons

Axonal transport of herpes simplex virus (HSV-1) is essential for viral infection and spread in the peripheral nervous system of the host. Therefore, the virus probably utilizes existing active transport and targeting mechanisms in neurons for virus assembly and spread from neurons to skin. In the present study, we used transmission immunoelectron microscopy to investigate the nature and origin of vesicles involved in the anterograde axonal transport of HSV-1 tegument and envelope proteins and of vesicles surrounding partially and fully enveloped capsids in growth cones. This study aimed to elucidate the mechanism of virus assembly and exit from axons of human fetal dorsal root ganglia neurons. We demonstrated that viral tegument and envelope proteins can travel in axons independently of viral capsids and were transported to the axon terminus in two types of transport vesicles, tubulovesicular membrane structures and large dense-cored vesicles. These vesicles and membrane carriers were derived from the trans-Golgi network (TGN) and contained key proteins, such as Rab3A, SNAP-25, GAP-43, and kinesin-1, involved in the secretory and exocytic pathways in axons. These proteins were also observed on fully and partially enveloped capsids in growth cones and on extracellular virions. Our findings provide further evidence to the subassembly model of separate transport in axons of unenveloped capsids from envelope and tegument proteins with final virus assembly occurring at the axon terminus. We postulate that HSV-1 capsids invaginate tegument- and envelope-bearing TGN-derived vesicles and utilize the large secretory vesicle pathway of exocytosis for exit from axons.

Development of recombinant varicella-zoster viruses expressing luciferase fusion proteins for live in vivo imaging in human skin and dorsal root ganglia xenografts

Varicella-zoster virus (VZV) is a host specific human pathogen that has been studied using human xenografts in SCID mice. Live whole-animal imaging is an emerging technique to measure protein expression in vivo using luminescence. Currently, it has only been possible to determine VZV protein expression in xenografts postmortem. Therefore, to measure immediate early (IE63) and late (glycoprotein E [gE]) protein expression in vivo viruses expressing IE63 or gE as luciferase fusion proteins were generated. Viable recombinant viruses pOka-63-luciferase and pOka-63/70-luciferase, which had luciferase genes fused to ORF63 and its duplicate ORF70, or pOka-gE-CBR were recovered that expressed IE63 or gE as fusion proteins and generated luminescent plaques. In contrast to pOka-63/70-luciferase viruses, the luciferase gene was rapidly lost in vitro when fused to a single copy of ORF63 or ORF68. IE63 expression was successfully measured in human skin and dorsal root ganglia xenografts infected with the genomically stable pOka-63/70-luciferase viruses. The progress of VZV infection in dorsal root ganglia xenografts was delayed in valacyclovir treated mice but followed a similar trend in untreated mice when the antiviral was withdrawn 28 days post-inoculation. Thus, IE63-luciferase fusion proteins were effective for investigating VZV infection and antiviral activity in human xenografts.

Two viral kinases are required for sustained long distance axon transport of a neuroinvasive herpesvirus

Axonal transport is essential for the successful establishment of neuroinvasive herpesvirus infections in peripheral ganglia (retrograde transport) and the subsequent spread to exposed body surfaces following reactivation from latency (anterograde transport). We examined two components of pseudorabies virus (US3 and UL13), both of which are protein kinases, as potential regulators of axon transport. Following replication of mutant viruses lacking kinase activity, newly assembled capsids displayed an increase in retrograde motion that prevented efficient delivery of capsids to the distal axon. The aberrant increase in retrograde motion was accompanied by loss of a viral membrane marker from the transported capsids, indicating that the viral kinases allow for efficient anterograde transport by stabilizing membrane-capsid interactions during the long transit from the neuron cell body to the distal axon.

Herpes simplex virus gE/gI and US9 proteins promote transport of both capsids and virion glycoproteins in neuronal axons

Following reactivation from latency, alphaherpesviruses replicate in sensory neurons and assemble capsids that are transported in the anterograde direction toward axon termini for spread to epithelial tissues. Two models currently describe this transport. The Separate model suggests that capsids are transported in axons independently from viral envelope glycoproteins. The Married model holds that fully assembled enveloped virions are transported in axons. The herpes simplex virus (HSV) membrane glycoprotein heterodimer gE/gI and the US9 protein are important for virus anterograde spread in the nervous systems of animal models. It was not clear whether gE/gI and US9 contribute to the axonal transport of HSV capsids, the transport of membrane proteins, or both. Here, we report that the efficient axonal transport of HSV requires both gE/gI and US9. The transport of both capsids and glycoproteins was dramatically reduced, especially in more distal regions of axons, with gE(-), gI(-), and US9-null mutants. An HSV mutant lacking just the gE cytoplasmic (CT) domain displayed an intermediate reduction in capsid and glycoprotein transport. We concluded that HSV gE/gI and US9 promote the separate transport of both capsids and glycoproteins. gE/gI was transported in association with other HSV glycoproteins, gB and gD, but not with capsids. In contrast, US9 colocalized with capsids and not with membrane glycoproteins. Our observations suggest that gE/gI and US9 function in the neuron cell body to promote the loading of capsids and glycoprotein-containing vesicles onto microtubule motors that ferry HSV structural components toward axon tips.

Deletion or green fluorescent protein tagging of the pUL35 capsid component of pseudorabies virus impairs virus replication in cell culture and neuroinvasion in mice

To facilitate tracing of virion movement, the non-essential capsid proteins pUL35 of herpes simplex virus type 1 and pseudorabies virus (PrV) have been tagged with green fluorescent protein (GFP). However, the biological relevance of PrV pUL35 and the functionality of the fusion proteins have not yet been investigated in detail. We generated PrV mutants either lacking the 12 kDa UL35 gene product, or expressing GFP fused to the N terminus of pUL35. Remarkably, both mutants exhibited significant replication defects in rabbit kidney cells, which could be corrected in pUL35-expressing cells. After intranasal infection of mice both mutants showed delayed neuroinvasion, and survival times of the animals were extended to 3 days, compared with 2 days after wild-type infection. Thus, fusion of pUL35 with GFP resulted in a non-functional protein, which has to be considered for the use of corresponding mutants in tracing studies.

Herpes simplex virus induces extensive modification and dynamic relocalisation of the nuclear mitotic apparatus (NuMA) protein in interphase cells

The nuclear mitotic apparatus (NuMA) protein is a component of the nuclear matrix in interphase cells and an essential protein for the formation of mitotic spindle poles. We used herpes simplex virus (HSV), an enveloped DNA virus that replicates in the nucleus, to study the intra-nuclear dynamics of NuMA in infected cells. This study shows that NuMA is extensively modified following HSV infection, including phosphorylation of an unidentified site(s), and that it depends to an extent on viral DNA synthesis. Although NuMA is insoluble in uninfected interphase cells, HSV infection induced solubilisation and dynamic relocalisation of NuMA, whereupon the protein became excluded from viral replication compartments -- sites of virus transcription and replication. Live cell, confocal imaging showed that NuMA localisation dramatically changed from the early stages (diffusely nuclear, excluding nucleoli) to late stages of infection (central diminuition, but remaining near the inner nuclear peripheries). In addition, NuMA knockdown using siRNA suggested that NuMA is important for efficient viral growth. In summary, we suggest that NuMA is required for efficient HSV infection, and identify further areas of research that address how the virus challenges host cell barriers.

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.

Pseudorabies virus Us9 directs axonal sorting of viral capsids

Pseudorabies virus (PRV) mutants lacking the Us9 gene cannot spread from presynaptic to postsynaptic neurons in the rat visual system, although retrograde spread remains unaffected. We sought to recapitulate these findings in vitro using the isolator chamber system developed in our lab for analysis of the transneuronal spread of infection. The wild-type PRV Becker strain spreads efficiently to postsynaptic neurons in vitro, whereas the Us9-null strain does not. As determined by indirect immunofluorescence, the axons of Us9-null infected neurons do not contain the glycoproteins gB and gE, suggesting that their axonal sorting is dependent on Us9. Importantly, we failed to detect viral capsids in the axons of Us9-null infected neurons. We confirmed this observation by using three different techniques: by direct fluorescence of green fluorescent protein-tagged capsids; by transmission electron microscopy; and by live-cell imaging in cultured, sympathetic neurons. This finding has broad impact on two competing models for how virus particles are trafficked inside axons during anterograde transport and redefines a role for Us9 in viral sorting and transport.

Spontaneous reactivation of herpes simplex virus type 1 in latently infected murine sensory ganglia

Careful studies of mouse trigeminal ganglia (TG) latently infected with herpes simplex virus type 1 (HSV-1) indicate the presence of productive cycle viral gene products and persistent immune response, suggesting ongoing spontaneous viral reactivation in these tissues. In the present study we set out to determine whether infectious virus is present in murine TG latently infected with HSV-1 (KOS). At 37 days after ocular inoculation we found a small amount of infectious virus in ca. 6% of latently infected murine TG. Furthermore, the amount of infectious virus that we detected (PFU per viral antigen-positive neuron) was similar to that detected in acutely infected ganglia. We conclude that spontaneous reactivation of infectious HSV-1 occurs in the mouse TG and is likely the principle cause of viral protein expression in these tissues. We next examined the role of latency-associated transcript (LAT) in spontaneous ganglionic reactivation by examining ganglia latently infected with KOS dlLAT1.8, a LAT deletion virus. Through the use of immunocytochemistry we found that KOS dlLAT1.8 had a rate of spontaneous ganglionic reactivation very similar to that of HSV-1 (KOS). Studying spontaneous ganglionic reactivation of HSV in the mouse TG allows a direct study of viral reactivation from latently infected neurons without the potential confounders and complicating downstream events that accompany the study of viral reactivation by explantation or peripheral viral shedding. Since most cases of human viral shedding and reactivation are not associated with a known trigger, spontaneous ganglionic reactivation of HSV-1 may be a better model of human disease than existing models.

A herpes simplex virus gD-YFP fusion glycoprotein is transported separately from viral capsids in neuronal axons

Two models describing how alphaherpesviruses exit neurons differ with respect to whether nucleocapsids and envelope glycoproteins travel toward axon termini separately or as assembled enveloped virions. Recently, a pseudorabies virus glycoprotein D (gD)-green fluorescent protein fusion was found to colocalize with viral capsids, supporting anterograde transport of enveloped virions. Previous antibody staining experiments demonstrated that herpes simplex virus (HSV) glycoproteins and capsids are separately transported in axons. Here, we generated an HSV expressing a gD-yellow fluorescent protein (YFP) fusion and found that gD-YFP and capsids were transported separately in neuronal axons. Anti-gD antibodies colocalized with gD-YFP, indicating that gD-YFP behaves like wild-type HSV gD.

Nanotechnology safety concerns revisited

Nanotechnology is an emerging science involving manipulation of matter at the nanometer scale. Due to concerns over nanomaterial risks, there has been a dramatic increase in focused safety research. The present review provides a summary of these published findings, identifying areas of agreement and discordance with regard to: (1) the potential for nanomaterial exposure, (2) the relative hazard nanomaterials pose to humans and the environment, and (3) the present deficits in our understanding of risk. Special attention is paid to study design and methodologies, offering valuable insight into the complexities encountered with nanomaterial safety assessment. Recent data highlight the impact of surface characteristics on nanomaterial biocompatibility and point to the inadequacy of the current size-dependent mechanistic paradigms, with nanoscale materials lacking unique or characteristic toxicity profiles. The available data support the ability of the lung, gastrointestinal tract, and skin to act as a significant barrier to the systemic exposure of many nanomaterials. Furthermore, the acute systemic toxicity of many nanomaterials appear to be low. By contrast, the potential pulmonary toxicity of certain nanomaterials, such as carbon nanotubes, is significant, requiring a better understanding of exposure to further evaluate their risk. While these findings arrive at an overall picture of material-specific rather than nanogeneralized risk, any conclusions should clearly be tempered by the fact that nanomaterial safety data are limited. Until such time as the exposures, hazards, and environmental life cycle of nanomaterials have been more clearly defined, cautious development and implementation of nanotechnology is the most prudent course.

Viral regulation of the long distance axonal transport of herpes simplex virus nucleocapsid

Many membranous organelles and protein complexes are normally transported anterograde within axons to the presynaptic terminal, and details of the motors, adaptors and cargoes have received significant attention. Much less is known about the transport in neurons of non-membrane bound particles, such as mRNAs and their associated proteins. We propose that herpes simplex virus type 1 (HSV) can be used to study the detailed mechanisms regulating long distance transport of particles in axons. A critical step in the transmission of HSV from one infected neuron to the next is the polarized anterograde axonal transport of viral DNA from the host infected nerve cell body to the axon terminal. Using the in vivo mouse retinal ganglion cell model infected with wild type virus or a mutant strain that lacks the protein Us9, we found that Us9 protein was necessary for long distance anterograde axonal transport of viral nucleocapsid (DNA surrounded by capsid proteins), but unnecessary for transport of virus envelope. Thus, we conclude that nucleocapsid can be transported independently down axons via a Us9-dependent mechanism.