Axonal transport of herpes simplex virions to epidermal cells: evidence for a specialized mode of virus transport and assembly

Type II Grass Carp Reovirus Rapidly Invades Grass Carp (Ctenopharyngodon idella) via Nostril-Olfactory System-Brain Axis, Gill, and Skin on Head

Type II grass carp reovirus (GCRV-II) with high pathogenicity and infectivity causes severe hemorrhagic disease, which leads to extensive death in the grass carp and black carp aquaculture. However, the early invasion portal remains unclear. In this study, we explored the invasion portal, time, and pathway of GCRV-II by immersion infection in grass carp. Through the detection of the infected grass carp external body surface tissues, most of them could be detected to carry GCRV-II within 45 min except for the skin covered by scales. Further shortening the duration of infection, we proved that GCRV-II rapidly invades through the nostril (especially), gill, and skin on head at only 5 min post-immersion, rather than merely by adhesion. Subsequently, visual localization investigations of GCRV-II were conducted on the nostril, olfactory system (olfactory bulb and olfactory tract), and brain via immunofluorescence microscopy and transmission electron microscopy. We found that few viruses were located in the nostril at 5 min post-immersion infection, while a significantly increased quantity of viruses were distributed in all of the examined tissues at 45 min. Furthermore, the semi-qRT-PCR and Western blotting results of different infection times confirmed that GCRV-II invades grass carp via the nostril-olfactory system-brain axis and then viral replication unfolds. These results revealed the infection mechanism of GCRV-II in terms of the invasion portal, time, and pathway in grass carp. This study aims to understand the invasion mode of GCRV-II in grass carp, thus providing theoretical support for the prevention and control strategies of hemorrhagic disease.

Getting on the right track: Interactions between viruses and the cytoskeletal motor proteins

The cytoskeleton is an essential component of the cell and it is involved in multiple physiological functions, including intracellular organization and transport. It is composed of three main families of proteinaceous filaments; microtubules, actin filaments and intermediate filaments and their accessory proteins. Motor proteins, which comprise the dynein, kinesin and myosin superfamilies, are a remarkable group of accessory proteins that mainly mediate the intracellular transport of cargoes along with the cytoskeleton. Like other cellular structures and pathways, viruses can exploit the cytoskeleton to promote different steps of their life cycle through associations with motor proteins. The complexity of the cytoskeleton and the differences among viruses, however, has led to a wide diversity of interactions, which in most cases remain poorly understood. Unveiling the details of these interactions is necessary not only for a better comprehension of specific infections, but may also reveal new potential drug targets to fight dreadful diseases such as rabies disease and acquired immunodeficiency syndrome (AIDS). In this review, we describe a few examples of the mechanisms that some human viruses, that is, rabies virus, adenovirus, herpes simplex virus, human immunodeficiency virus, influenza A virus and papillomavirus, have developed to hijack dyneins, kinesins and myosins.

Invasion and Propagation of White Spot Syndrome Virus: Hijacking of the Cytoskeleton, Intracellular Transport Machinery, and Nuclear Import Transporters

The pathogenesis of white spot syndrome virus (WSSV) is largely unclear. In this study, we found that actin nucleation and clathrin-mediated endocytosis were recruited for internalization of WSSV into crayfish hematopoietic tissue (Hpt) cells. This internalization was followed by intracellular transport of the invading virions via endocytic vesicles and endosomes. After envelope fusion within endosomes, the penetrated nucleocapsids were transported along microtubules toward the periphery of the nuclear pores. Furthermore, the nuclear transporter CqImportin α1/β1, via binding of ARM repeat domain within CqImportin α1 to the nuclear localization sequences (NLSs) of viral cargoes and binding of CqImportin β1 to the nucleoporins CqNup35/62 with the action of CqRan for docking to nuclear pores, was hijacked for both targeting of the incoming nucleocapsids toward the nuclear pores and import of the expressed viral structural proteins containing NLS into the cell nucleus. Intriguingly, dysfunction of CqImportin α1/β1 resulted in significant accumulation of incoming nucleocapsids on the periphery of the Hpt cell nucleus, leading to substantially decreased introduction of the viral genome into the nucleus and remarkably reduced nuclear import of expressed viral structural proteins with NLS; both of these effects were accompanied by significantly inhibited viral propagation. Accordingly, the survival rate of crayfish post-WSSV challenge was significantly increased after dysfunction of CqImportin α1/β1, also showing significantly reduced viral propagation, and was induced either by gene silencing or by pharmacological blockade via dietary administration of ivermectin per os. Collectively, our findings improve our understanding of WSSV pathogenesis and support future antiviral designing against WSSV. IMPORTANCE As one of the largest animal DNA viruses, white spot syndrome virus (WSSV) has been causing severe economical loss in aquaculture due to the limited knowledge on WSSV pathogenesis for an antiviral strategy. We demonstrate that the actin cytoskeleton, endocytic vesicles, endosomes, and microtubules are hijacked for WSSV invasion; importantly, the nuclear transporter CqImportin α1/β1 together with CqRan were recruited, via binding of CqImportin β1 to the nucleoporins CqNup35/62, for both the nuclear pore targeting of the incoming nucleocapsids and the nuclear import of expressed viral structural proteins containing the nuclear localization sequences (NLSs). This is the first report that NLSs from both viral structure proteins and host factor are elaborately recruited together to facilitate WSSV infection. Our findings provide a novel explanation for WSSV pathogenesis involving systemic hijacking of host factors, which can be used for antiviral targeting against WSSV disease, such as the blockade of CqImportin α1/β1 with ivermectin.

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.

Hitchhiking on the neuronal highway: Mechanisms of transsynaptic specificity

Transsynaptic viral tracers are an invaluable neuroanatomical tool to define neuronal circuit connectivity across single or multiple synapses. There are variants that label either inputs or outputs of defined starter populations, most of which are based on the herpes and rabies viruses. However, we still have an incomplete understanding of the factors influencing specificity of neuron-neuron transmission and labeling of inputs vs. outputs. This article will touch on three topics: First, how specific are the directional transmission patterns of these viruses? Second, what are the properties that confer synaptic specificity of viral transmission? Lastly, what can we learn from this specificity, and can we use it to devise better transsynaptic tracers?

Herpes Simplex Virus, Alzheimer's Disease and a Possible Role for Rab GTPases

Herpes simplex virus (HSV) is a common pathogen, infecting 85% of adults in the United States. After reaching the nucleus of the long-lived neuron, HSV may enter latency to persist throughout the life span. Re-activation of latent herpesviruses is associated with progressive cognitive impairment and Alzheimer's disease (AD). As an enveloped DNA virus, HSV exploits cellular membrane systems for its life cycle, and thereby comes in contact with the Rab family of GTPases, master regulators of intracellular membrane dynamics. Knock-down and overexpression of specific Rabs reduce HSV production. Disheveled membrane compartments could lead to AD because membrane sorting and trafficking are crucial for synaptic vesicle formation, neuronal survival signaling and Abeta production. Amyloid precursor protein (APP), a transmembrane glycoprotein, is the parent of Abeta, the major component of senile plaques in AD. Up-regulation of APP expression due to HSV is significant since excess APP interferes with Rab5 endocytic trafficking in neurons. Here, we show that purified PC12-cell endosomes transport both anterograde and retrograde when injected into the squid giant axon at rates similar to isolated HSV. Intracellular HSV co-fractionates with these endosomes, contains APP, Rab5 and TrkA, and displays a second membrane. HSV infected PC12 cells up-regulate APP expression. Whether interference with Rabs has a specific effect on HSV or indirectly affects membrane compartment dynamics co-opted by virus needs further study. Ultimately Rabs, their effectors or their membrane-binding partners may serve as handles to reduce the impact of viral re-activation on cognitive function, or even as more general-purpose anti-microbial therapies.

Compartmentalized Devices as Tools for Investigation of Human Brain Network Dynamics

Neuropsychiatric disorders have traditionally been difficult to study due to the complexity of the human brain and limited availability of human tissue. Induced pluripotent stem (iPS) cells provide a promising avenue to further our understanding of human disease mechanisms, but traditional 2D cell cultures can only provide a limited view of the neural circuits. To better model complex brain neurocircuitry, compartmentalized culturing systems and 3D organoids have been developed. Early compartmentalized devices demonstrated how neuronal cell bodies can be isolated both physically and chemically from neurites. Soft lithographic approaches have advanced this approach and offer the tools to construct novel model platforms, enabling circuit-level studies of disease, which can accelerate mechanistic studies and drug candidate screening. In this review, we describe some of the common technologies used to develop such systems and discuss how these lithographic techniques have been used to advance our understanding of neuropsychiatric disease. Finally, we address other in vitro model platforms such as 3D culture systems and organoids and compare these models with compartmentalized models. We ask important questions regarding how we can further harness iPS cells in these engineered culture systems for the development of improved in vitro models. Developmental Dynamics 248:65-77, 2019. © 2018 Wiley Periodicals, Inc.

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.

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.

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.

The Olfactory Bulb: An Immunosensory Effector Organ during Neurotropic Viral Infections

In 1935, the olfactory route was hypothesized to be a portal for virus entry into the central nervous system (CNS). This hypothesis was based on experiments in which nasophayngeal infection with poliovirus in monkeys was prevented from spreading to their CNS via transection of olfactory tracts between the olfactory neuroepithelium (ONE) of the nasal cavity and the olfactory bulb (OB). Since then, numerous neurotropic viruses have been observed to enter the CNS via retrograde transport along axons of olfactory sensory neurons whose cell bodies reside in the ONE. Importantly, this route of infection can occur even after subcutaneous inoculation of arboviruses that can cause encephalitis in humans. While the olfactory route is now accepted as an important pathway for viral entry into the CNS, it is unclear whether it provides a way for infection to spread to other brain regions. More recently, studies of antiviral innate and adaptive immune responses within the olfactory bulb suggest it provides early virologic control. Here we will review the data demonstrating that neurotropic viruses gain access to the CNS initially via the olfactory route with emphasis on findings that suggest the OB is a critical immunosensory effector organ that effectively clears virus.

Dual Role of Herpes Simplex Virus 1 pUS9 in Virus Anterograde Axonal Transport and Final Assembly in Growth Cones in Distal Axons

The herpes simplex virus type 1 (HSV-1) envelope protein pUS9 plays an important role in virus anterograde axonal transport and spread from neuronal axons. In this study, we used both confocal microscopy and transmission electron microscopy (TEM) to examine the role of pUS9 in the anterograde transport and assembly of HSV-1 in the distal axon of human and rat dorsal root ganglion (DRG) neurons using US9 deletion (US9(-)), repair (US9R), and wild-type (strain F, 17, and KOS) viruses. Using confocal microscopy and single and trichamber culture systems, we observed a reduction but not complete block in the anterograde axonal transport of capsids to distal axons as well as a marked (∼90%) reduction in virus spread from axons to Vero cells with the US9 deletion viruses. Axonal transport of glycoproteins (gC, gD, and gE) was unaffected. Using TEM, there was a marked reduction or absence of enveloped capsids, in varicosities and growth cones, in KOS strain and US9 deletion viruses, respectively. Capsids (40 to 75%) in varicosities and growth cones infected with strain 17, F, and US9 repair viruses were fully enveloped compared to less than 5% of capsids found in distal axons infected with the KOS strain virus (which also lacks pUS9) and still lower (<2%) with the US9 deletion viruses. Hence, there was a secondary defect in virus assembly in distal axons in the absence of pUS9 despite the presence of key envelope proteins. Overall, our study supports a dual role for pUS9, first in anterograde axonal transport and second in virus assembly in growth cones in distal axons.

IMPORTANCE

HSV-1 has evolved mechanisms for its efficient transport along sensory axons and subsequent spread from axons to epithelial cells after reactivation. In this study, we show that deletion of the envelope protein pUS9 leads to defects in virus transport along axons (partial defect) and in virus assembly and egress from growth cones (marked defect). Virus assembly and exit in the neuronal cell body are not impaired in the absence of pUS9. Thus, our findings indicate that pUS9 contributes to the overall HSV-1 anterograde axonal transport, including a major role in virus assembly at the axon terminus, which is not essential in the neuronal cell body. Overall, our data suggest that the process of virus assembly at the growth cones differs from that in the neuronal cell body and that HSV-1 has evolved different mechanisms for virus assembly and exit from different cellular compartments.

Deciphering Human Cell-Autonomous Anti-HSV-1 Immunity in the Central Nervous System

Herpes simplex virus 1 (HSV-1) is a common virus that can rarely invade the human central nervous system (CNS), causing devastating encephalitis. The permissiveness to HSV-1 of the various relevant cell types of the CNS, neurons, astrocytes, oligodendrocytes, and microglia cells, as well as their response to viral infection, has been extensively studied in humans and other animals. Nevertheless, human CNS cell-based models of anti-HSV-1 immunity are of particular importance, as responses to any given neurotropic virus may differ between humans and other animals. Human CNS neuron cell lines as well as primary human CNS neurons, astrocytes, and microglia cells cultured/isolated from embryos or cadavers, have enabled the study of cell-autonomous anti-HSV-1 immunity in vitro. However, the paucity of biological samples and their lack of purity have hindered progress in the field, which furthermore suffers from the absence of testable primary human oligodendrocytes. Recently, the authors have established a human induced pluripotent stem cells (hiPSCs)-based model of anti-HSV-1 immunity in neurons, oligodendrocyte precursor cells, astrocytes, and neural stem cells, which has widened the scope of possible in vitro studies while permitting in-depth explorations. This mini-review summarizes the available data on human primary and iPSC-derived CNS cells for anti-HSV-1 immunity. The hiPSC-mediated study of anti-viral immunity in both healthy individuals and patients with viral encephalitis will be a powerful tool in dissecting the disease pathogenesis of CNS infections with HSV-1 and other neurotropic viruses.

The pathogenesis of 3 neurotropic flaviviruses in a mouse model depends on the route of neuroinvasion after viremia

Neurotropic flavivirus infection of humans results in viremia subsequently; in some cases, it causes meningitis encephalomyelitis, although the pathways from viremia to central nervous system (CNS) invasion are uncertain. Here, we intravenously infected BALB/c mice with 3 neurotropic flaviviruses, then examined the clinical manifestations and histopathologic changes. The Sofjin strain of tick-borne encephalitis virus-infected mice exhibited dose-dependent survival. The animals showed distention of the small intestine caused by peripheral neuritis because of infection of the myenteric plexus. Histopathologically, the strongly neurotropic Sofjin strain invaded the CNS of viremic mice via the autonomic nerves running from the plexus. The JaTH-160 strain of Japanese encephalitis virus was isolated from the lymph nodes during the preclinical phase of viral encephalitis. Therefore, this strain might infect the CNS via a hematogenous pathway, including through lymphoid tissues. The NY99-6922 strain of the West Nile virus caused clinical signs suggestive of intestinal, lymphoid, and/or neurologic involvement; the infected mice had prolonged viremia, suggesting that NY99-6922 may mainly use the hematogenous pathway; however, there was also histopathologic evidence of involvement of the autonomic nervous system pathway. In conclusion, the three neurotropic flaviviruses showed different pathogenesis, which were dependent upon overlapping but distinct pathways to CNS invasion after viremia.

Blocking ESCRT-mediated envelopment inhibits microtubule-dependent trafficking of alphaherpesviruses in vitro

Herpes simplex virus (HSV) and, as reported here, pseudorabies virus (PRV) utilize the ESCRT apparatus to drive cytoplasmic envelopment of their capsids. Here, we demonstrate that blocking ESCRT-mediated envelopment using the dominant-negative inhibitor Vps4A-EQ (Vps4A in which glutamate [E] at position 228 in the ATPase active site is replaced by a glutamine [Q]) reduced the ability of HSV and PRV particles to subsequently traffic along microtubules in vitro. HSV and PRV capsid-associated particles with bound green fluorescent protein (GFP)-labeled Vps4A-EQ were readily detected by fluorescence microscopy in cytoplasmic extracts of infected cells. These Vps4A-EQ-associated capsid-containing particles bound to microtubules in vitro but were unable to traffic along them. Using a PRV strain expressing a fluorescent capsid and a fluorescently tagged form of the envelope protein gD, we found that similar numbers of gD-positive and gD-negative capsid-associated particles accumulated in cytoplasmic extracts under our conditions. Both classes of PRV particle bound to microtubules in vitro with comparable efficiency, and similar results were obtained for HSV using anti-gD immunostaining. The gD-positive and gD-negative PRV capsids were both capable of trafficking along microtubules in vitro; however, motile gD-positive particles were less numerous and their trafficking was more sensitive to the inhibitory effects of Vps4A-EQ. We discuss our data in the context of microtubule-mediated trafficking of naked and enveloped alphaherpesvirus capsids.

IMPORTANCE

The alphaherpesviruses include several important human pathogens. These viruses utilize microtubule-mediated transport to travel through the cell cytoplasm; however, the molecular mechanisms of trafficking are not well understood. In this study, we have used a cell-free system to examine the requirements for microtubule trafficking and have attempted to distinguish between the movement of so-called "naked" and membrane-associated cytoplasmic alphaherpesvirus capsids.

A Mason-Pfizer Monkey virus Gag-GFP fusion vector allows visualization of capsid transport in live cells and demonstrates a role for microtubules

Immature capsids of the Betaretrovirus, Mason-Pfizer Monkey virus (M-PMV), are assembled in the pericentriolar region of the cell, and are then transported to the plasma membrane for budding. Although several studies, utilizing mutagenesis, biochemistry, and immunofluorescence, have defined the role of some viral and host cells factors involved in these processes, they have the disadvantage of population analysis, rather than analyzing individual capsid movement in real time. In this study, we created an M-PMV vector in which the enhanced green fluorescent protein, eGFP, was fused to the carboxyl-terminus of the M-PMV Gag polyprotein, to create a Gag-GFP fusion that could be visualized in live cells. In order to express this fusion protein in the context of an M-PMV proviral backbone, it was necessary to codon-optimize gag, optimize the Kozak sequence preceding the initiating methionine, and mutate an internal methionine codon to one for alanine (M100A) to prevent internal initiation of translation. Co-expression of this pSARM-Gag-GFP-M100A vector with a WT M-PMV provirus resulted in efficient assembly and release of capsids. Results from fixed-cell immunofluorescence and pulse-chase analyses of wild type and mutant Gag-GFP constructs demonstrated comparable intracellular localization and release of capsids to untagged counterparts. Real-time, live-cell visualization and analysis of the GFP-tagged capsids provided strong evidence for a role for microtubules in the intracellular transport of M-PMV capsids. Thus, this M-PMV Gag-GFP vector is a useful tool for identifying novel virus-cell interactions involved in intracellular M-PMV capsid transport in a dynamic, real-time system.

An In Vitro HSV-1 Reactivation Model Containing Quiescently Infected PC12 Cells

Advances in the understanding of the infection and reactivation process of herpes simplex type 1 (HSV-1) are generally gained by monolayer cultures or extensive and cost-intensive animal models. So far, no reliable in vitro skin model exists either to investigate the molecular mechanisms involved in controlling latency and virus reactivation or to test pharmaceuticals. Here we demonstrate the first in vitro HSV-1 reactivation model generated by using the human keratinocyte cell line HaCaT grown on a collagen substrate containing primary human fibroblasts. We integrated the unique feature of a quiescently infected neuronal cell line, the rat pheochromocytoma line PC12, within the dermal layer of the three-dimensional skin equivalent. Transmission electron microscopy, a cell-based TCID₅₀ assay, and polymerase chain reaction analysis were used to verify cell latency. Thereby viral DNA could be detected, whereas extracellular as well as intracellular virus activity could not be found. Further, the infected PC12 cells show no spontaneous reactivation within the in vitro skin equivalent. In order to simulate a physiologically comparable HSV-1 infection, we achieved a specific and pointed reactivation of quiescently HSV-1 infected PC12 cells by UVB irradiation at 1000 mJ/cm².

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.

3D-tissue model for herpes simplex virus-1 infections

Infection with herpes simplex virus type 1 (HSV-1) causes the most common skin disease. Various test systems have been established to recapitulate this cyclical pathway of productive infection, latency, and reactivation. Most studies of latency and reactivation are conducted in animal models. However, the small number of neurons which harbor the viral genome, the complexity of the in vivo setting, and ethical constraints place limits on animal studies. So far, no in vitro model which resembles natural latency exists. Here, we describe the first in vitro HSV-1 infection model based on a human skin equivalent. The 3D infection model is generated using the human keratinocyte cell line HaCaT grown on a collagen substrate containing human primary fibroblasts and in addition a quiescently HSV-1 infected neuronal component.

Combination of vinblastine and oncolytic herpes simplex virus vector expressing IL-12 therapy increases antitumor and antiangiogenic effects in prostate cancer models

Oncolytic herpes simplex virus (oHSV)-1-based vectors selectively replicate in tumor cells causing direct killing, that is, oncolysis, while sparing normal cells. The oHSVs are promising anticancer agents, but their efficacy, when used as single agents, leaves room for improvement. We hypothesized that combining the direct oncolytic and antiangiogenic activities of the interleukin (IL)-12-secreting NV1042 oHSV with microtubule disrupting agents (MDAs) would be an effective means to enhance antitumor efficacy. Vinblastine (VB) was identified among several MDAs screened, which displayed consistent and potent cytotoxic killing of both prostate cancer and endothelial cell lines. In matrigel tube-forming assays, VB was found to be highly effective at inhibiting tube formation of human umbilical vein endothelial cells. The combination of VB with NV1023 (the parental virus lacking IL-12) or NV1042 showed additive or synergistic activity against prostate cancer cell lines, and was not due to increased oHSV replication by VB. In athymic mice bearing CWR22 prostate tumors, VB in combination with NV1042 was superior to the combination of VB plus NV1023 in reducing tumor burden, appeared to be nontoxic and resulted in a statistically significant diminution in the number of CD31(+) cells as compared with other treatment groups. In human organotypic cultures using surgical samples from radical prostatectomies, both NV1023 and NV1042 were localized specifically to the epithelial cells of prostatic glands but not to the surrounding stroma. These data highlight the therapeutic advantage of combining the dual-acting antitumor and antiangiogenic activities of oHSVs and MDAs.

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.

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.

Herpesvirus transport to the nervous system and back again

Herpes simplex virus, varicella zoster virus, and pseudorabies virus are neurotropic pathogens of the Alphaherpesvirinae subfamily of the Herpesviridae. These viruses efficiently invade the peripheral nervous system and establish lifelong latency in neurons resident in peripheral ganglia. Primary and recurrent infections cycle virus particles between neurons and the peripheral tissues they innervate. This remarkable cycle of infection is the topic of this review. In addition, some of the distinguishing hallmarks of the infections caused by these viruses are evaluated in terms of their underlying similarities.

Entry of herpes simplex virus type 1 (HSV-1) into the distal axons of trigeminal neurons favors the onset of nonproductive, silent infection

Following productive, lytic infection in epithelia, herpes simplex virus type 1 (HSV-1) establishes a lifelong latent infection in sensory neurons that is interrupted by episodes of reactivation. In order to better understand what triggers this lytic/latent decision in neurons, we set up an organotypic model based on chicken embryonic trigeminal ganglia explants (TGEs) in a double chamber system. Adding HSV-1 to the ganglion compartment (GC) resulted in a productive infection in the explants. By contrast, selective application of the virus to distal axons led to a largely nonproductive infection that was characterized by the poor expression of lytic genes and the presence of high levels of the 2.0-kb major latency-associated transcript (LAT) RNA. Treatment of the explants with the immediate-early (IE) gene transcriptional inducer hexamethylene bisacetamide, and simultaneous co-infection of the GC with HSV-1, herpes simplex virus type 2 (HSV-2) or pseudorabies virus (PrV) helper virus significantly enhanced the ability of HSV-1 to productively infect sensory neurons upon axonal entry. Helper-virus-induced transactivation of HSV-1 IE gene expression in axonally-infected TGEs in the absence of de novo protein synthesis was dependent on the presence of functional tegument protein VP16 in HSV-1 helper virus particles. After the establishment of a LAT-positive silent infection in TGEs, HSV-1 was refractory to transactivation by superinfection of the GC with HSV-1 but not with HSV-2 and PrV helper virus. In conclusion, the site of entry appears to be a critical determinant in the lytic/latent decision in sensory neurons. HSV-1 entry into distal axons results in an insufficient transactivation of IE gene expression and favors the establishment of a nonproductive, silent infection in trigeminal neurons.

Upon primary infection of the oronasal mucosa, herpes simplex virus type 1 (HSV-1) rapidly reaches the ganglia of the peripheral nervous system via axonal transport and establishes lifelong latency in surviving neurons. Central to the establishment of latency is the ability of HSV-1 to reliably switch from productive, lytic spread in epithelia to nonproductive, latent infection in sensory neurons. It is not fully understood what specifically disposes incoming particles of a highly cytopathogenic, fast-replicating alphaherpesvirus to nonproductive, latent infection in sensory neurons. The present study shows that selective entry of HSV-1 into the distal axons of trigeminal neurons strongly favors the establishment of a nonproductive, latent infection, whereas nonselective infection of neurons still enables HSV-1 to induce lytic gene expression. Our data support a model of latency establishment in which the site of entry is an important determinant of the lytic/latent decision in the infected neuron. Productive infection of the neuron ensues if particles enter the soma of the neuron directly. In contrast, previous retrograde axonal transport of incoming viral particles creates a distinct scenario that abrogates VP16-dependent transactivation of immediate-early gene expression and precludes the expression of lytic genes to an extent sufficient to prevent the initiation of massive productive infection of trigeminal neurons.

Skin tissue engineering--in vivo and in vitro applications

Significant progress has been made over the years in the development of in vitro-engineered substitutes that mimic human skin, either to be used as grafts for the replacement of lost skin or for the establishment of human-based in vitro skin models. This review summarizes these advances in in vivo and in vitro applications of tissue-engineered skin. We further highlight novel efforts in the design of complex disease-in-a-dish models for studies ranging from disease etiology to drug development and screening.

Ocular herpes simplex virus: how are latency, reactivation, recurrent disease and therapy interrelated?

Most humans are infected with herpes simplex virus (HSV) type 1 in early childhood and remain latently infected throughout life. While most individuals have mild or no symptoms, some will develop destructive HSV keratitis. Ocular infection with HSV-1 and its associated sequelae account for the majority of corneal blindness in industrialized nations. Neuronal latency in the peripheral ganglia is established when transcription of the viral genome is repressed (silenced) except for the latency-associated transcripts and microRNAs. The functions of latency-associated transcripts have been investigated since 1987. Roles have been suggested relating to reactivation, establishment of latency, neuronal protection, antiapoptosis, apoptosis, virulence and asymptomatic shedding. Here, we review HSV-1 latent infections, reactivation, recurrent disease and antiviral therapies for the ocular HSV diseases.

The reovirus sigma1s protein is a determinant of hematogenous but not neural virus dissemination in mice

Nonstructural protein σ1s is a critical determinant of hematogenous dissemination by type 1 reoviruses, which reach the central nervous system (CNS) by a strictly blood-borne route. However, it is not known whether σ1s contributes to neuropathogenesis of type 3 reoviruses, which disseminate by both vascular and neural pathways. Using isogenic type 3 viruses that vary only in σ1s expression, we observed that mice survived at a higher frequency following hind-limb inoculation with σ1s-null virus than when inoculated with wild-type virus. This finding suggests that σ1s is essential for reovirus virulence when inoculated at a site that requires systemic spread to cause disease. Wild-type and σ1s-null viruses produced comparable titers in the spinal cord, suggesting that σ1s is dispensable for invasion of the CNS. Although the two viruses ultimately achieved similar peak titers in the brain, loads of wild-type virus were substantially greater than those of the σ1s-null mutant at early times after inoculation. In contrast, wild-type virus produced substantially higher titers than the σ1s-null virus in peripheral organs to which reovirus spreads via the blood, including the heart, intestine, liver, and spleen. Concordantly, viral titers in the blood were higher following infection with wild-type virus than following infection with the σ1s-null mutant. These results suggest that differences in viral brain titers at early time points postinfection are due to limited virus delivery to the brain by hematogenous pathways. Transection of the sciatic nerve prior to hind-limb inoculation diminished viral spread to the spinal cord. However, wild-type virus retained the capacity to disseminate to the brain following sciatic nerve transection, indicating that wild-type reovirus can spread to the brain by the blood. Together, these results indicate that σ1s is not required for reovirus spread by neural mechanisms. Instead, σ1s mediates hematogenous dissemination within the infected host, which is required for full reovirus neurovirulence.

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.

Apical transport of influenza A virus ribonucleoprotein requires Rab11-positive recycling endosome

Influenza A virus RNA genome exists as eight-segmented ribonucleoprotein complexes containing viral RNA polymerase and nucleoprotein (vRNPs). Packaging of vRNPs and virus budding take place at the apical plasma membrane (APM). However, little is known about the molecular mechanisms of apical transport of newly synthesized vRNP. Transfection of fluorescent-labeled antibody and subsequent live cell imaging revealed that punctate vRNP signals moved along microtubules rapidly but intermittently in both directions, suggestive of vesicle trafficking. Using a series of Rab family protein, we demonstrated that progeny vRNP localized to recycling endosome (RE) in an active/GTP-bound Rab11-dependent manner. The vRNP interacted with Rab11 through viral RNA polymerase. The localization of vRNP to RE and subsequent accumulation to the APM were impaired by overexpression of Rab binding domains (RBD) of Rab11 family interacting proteins (Rab11-FIPs). Similarly, no APM accumulation was observed by overexpression of class II Rab11-FIP mutants lacking RBD. These results suggest that the progeny vRNP makes use of Rab11-dependent RE machinery for APM trafficking.

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.

Skin tissue engineering--in vivo and in vitro applications

Significant progress has been made over the years in the development of in vitro-engineered substitutes that mimic human skin, either to be used as grafts for the replacement of lost skin or for the establishment of human-based in vitro skin models. This review summarizes these advances in in vivo and in vitro applications of tissue-engineered skin. We further highlight novel efforts in the design of complex disease-in-a-dish models for studies ranging from disease etiology to drug development and screening.

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.

Experimental models to study varicella-zoster virus infection of neurons

Varicella zoster virus (VZV) infection results in the establishment of latency in human sensory neurons. Reactivation of VZV leads to herpes zoster which can be followed by persistent neuropathic pain, termed post-herpetic neuralgia (PHN). Humans are the only natural host for VZV, and the strict species specificity of the virus has restricted the development of an animal model of infection which mimics all phases of disease. In order to elucidate the mechanisms which control the establishment of latency and reactivation as well as the effect of VZV replication on neuronal function, in vitro models of neuronal infection have been developed. Currently these models involve culturing and infecting dissociated human fetal neurons, with or without their supporting cells, an intact explant fetal dorsal root ganglia (DRG) model, neuroblastoma cell lines and rodent neuronal cell models. Each of these models has distinct advantages as well as disadvantages, and all have contributed towards our understanding of VZV neuronal infection. However, as yet none have been able to recapitulate the full virus lifecycle from primary infection to latency through to reactivation. The development of such a model will be a crucial step towards advancing our understanding of the mechanisms involved in VZV replication in neuronal cells, and the design of new therapies to combat VZV-related disease.

Completely assembled virus particles detected by transmission electron microscopy in proximal and mid-axons of neurons infected with herpes simplex virus type 1, herpes simplex virus type 2 and pseudorabies virus

The morphology of alphaherpesviruses during anterograde axonal transport from the neuron cell body towards the axon terminus is controversial. Reports suggest that transport of herpes simplex virus type 1 (HSV-1) nucleocapsids and envelope proteins occurs in separate compartments and that complete virions form at varicosities or axon termini (subassembly transport model), while transport of a related alphaherpesvirus, pseudorabies virus (PRV) occurs as enveloped capsids in vesicles (assembled transport model). Transmission electron microscopy of proximal and mid-axons of primary superior cervical ganglion (SCG) neurons was used to compare anterograde axonal transport of HSV-1, HSV-2 and PRV. SCG cell bodies were infected with HSV-1 NS and 17, HSV-2 2.12 and PRV Becker. Fully assembled virus particles were detected intracellularly within vesicles in proximal and mid-axons adjacent to microtubules after infection with each virus, indicating that assembled virions are transported anterograde within axons for all three alphaherpesviruses.

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."

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.

Molecular chaperone BiP interacts with Borna disease virus glycoprotein at the cell surface

Borna disease virus (BDV) is characterized by highly neurotropic infection. BDV enters its target cells using virus surface glycoprotein (G), but the cellular molecules mediating this process remain to be elucidated. We demonstrate here that the N-terminal product of G, GP1, interacts with the 78-kDa chaperone protein BiP. BiP was found at the surface of BDV-permissive cells, and anti-BiP antibody reduced BDV infection as well as GP1 binding to the cell surface. We also reveal that BiP localizes at the synapse of neurons. These results indicate that BiP may participate in the cell surface association of BDV.

Compartmented neuron cultures for directional infection by alpha herpesviruses

Compartmented neuronal cultures allow experimenters to establish separate fluid environments for neuronal axons and the soma from which they emanate. Physical isolation of cell bodies and axons is achieved by culturing neurons in tri-chambered Teflon rings. Dissociated ganglia are plated in one end compartment of the trichamber, and axonal growth is guided underneath watertight silicone grease barriers into a separate compartment. Since the axons and cell bodies are located in different compartments, they can be infected and assayed separately. We describe the assembly and use of compartmented neuronal cultures for in vitro study of directional infection of neurons by alpha herpesviruses. Selective application of viral inoculum to only one compartment ensures that the remainder of the neuron is not contaminated by input inoculum. This allows for quantification of viral spread, and unambiguous interpretation of immunofluorescence and electron microscopy images.

A herpesvirus encoded deubiquitinase is a novel neuroinvasive determinant

The neuroinvasive property of several alpha-herpesviruses underlies an uncommon infectious process that includes the establishment of life-long latent infections in sensory neurons of the peripheral nervous system. Several herpesvirus proteins are required for replication and dissemination within the nervous system, indicating that exploiting the nervous system as a niche for productive infection requires a specialized set of functions encoded by the virus. Whether initial entry into the nervous system from peripheral tissues also requires specialized viral functions is not known. Here we show that a conserved deubiquitinase domain embedded within a pseudorabies virus structural protein, pUL36, is essential for initial neural invasion, but is subsequently dispensable for transmission within and between neurons of the mammalian nervous system. These findings indicate that the deubiquitinase contributes to neurovirulence by participating in a previously unrecognized initial step in neuroinvasion.

Subsets of herpesviruses, such as herpes simplex virus and pseudorabies virus, are neuroinvasive pathogens. Upon infection, these viruses efficiently target peripheral nervous system tissue and establish a life-long infection for which there is no cure. Very few pathogens are known that invade the nervous system proficiently, and the mechanism by which herpesviruses achieve neuroinvasion is largely unknown. In this study, we demonstrate that a viral protease plays a critical and specific role allowing the virus to cross the threshold of the nervous system, but is dispensable for subsequent replication and encephalitic spread within the brain.

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.