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.

Pseudorabies virus infection alters neuronal activity and connectivity in vitro.

Alpha-herpesviruses, including human herpes simplex virus 1 & 2, varicella zoster virus and the swine pseudorabies virus (PRV), infect the peripheral nervous system of their hosts. Symptoms of infection often include itching, numbness, or pain indicative of altered neurological function. To determine if there is an in vitro electrophysiological correlate to these characteristic in vivo symptoms, we infected cultured rat sympathetic neurons with well-characterized strains of PRV known to produce virulent or attenuated symptoms in animals. Whole-cell patch clamp recordings were made at various times after infection. By 8 hours of infection with virulent PRV, action potential (AP) firing rates increased substantially and were accompanied by hyperpolarized resting membrane potentials and spikelet-like events. Coincident with the increase in AP firing rate, adjacent neurons exhibited coupled firing events, first with AP-spikelets and later with near identical resting membrane potentials and AP firing. Small fusion pores between adjacent cell bodies formed early after infection as demonstrated by transfer of the low molecular weight dye, Lucifer Yellow. Later, larger pores formed as demonstrated by transfer of high molecular weight Texas red-dextran conjugates between infected cells. Further evidence for viral-induced fusion pores was obtained by infecting neurons with a viral mutant defective for glycoprotein B, a component of the viral membrane fusion complex. These infected neurons were essentially identical to mock infected neurons: no increased AP firing, no spikelet-like events, and no electrical or dye transfer. Infection with PRV Bartha, an attenuated circuit-tracing strain delayed, but did not eliminate the increased neuronal activity and coupling events. We suggest that formation of fusion pores between infected neurons results in electrical coupling and elevated firing rates, and that these processes may contribute to the altered neural function seen in PRV-infected animals.

Gene editing and elimination of latent herpes simplex virus in vivo.

We evaluate gene editing of HSV in a well-established mouse model, using adeno-associated virus (AAV)-delivered meganucleases, as a potentially curative approach to treat latent HSV infection. Here we show that AAV-delivered meganucleases, but not CRISPR/Cas9, mediate highly efficient gene editing of HSV, eliminating over 90% of latent virus from superior cervical ganglia. Single-cell RNA sequencing demonstrates that both HSV and individual AAV serotypes are non-randomly distributed among neuronal subsets in ganglia, implying that improved delivery to all neuronal subsets may lead to even more complete elimination of HSV. As predicted, delivery of meganucleases using a triple AAV serotype combination results in the greatest decrease in ganglionic HSV loads. The levels of HSV elimination observed in these studies, if translated to humans, would likely significantly reduce HSV reactivation, shedding, and lesions. Further optimization of meganuclease delivery and activity is likely possible, and may offer a pathway to a cure for HSV infection.

A conserved alpha-herpesvirus protein necessary for axonal localization of viral membrane proteins.

Pseudorabies virus, an alpha-herpesvirus, is capable of infecting the nervous system and spreading between synaptically connected neurons in diverse hosts. At least three viral membrane proteins (gE, gI, and Us9) are necessary for the spread of infection from presynaptic to postsynaptic neurons (anterograde spread) in infected rodents. To understand how these proteins effect anterograde spread between neurons, we analyzed the subcellular localization of viral proteins after infection of cultured rat sympathetic neurons with wild-type or mutant viruses. After Us9-null mutant infections but not gE-null mutant infections, only a subset of the viral structural proteins had entered axons. Surprisingly, capsid and tegument proteins but not viral membrane proteins were detected in axons. The spread of Us9 missense mutants in the rodent nervous system correlated with the amount of viral membrane proteins localized to axons. We conclude that the Us9 membrane protein controls axonal localization of diverse viral membrane proteins but not that of capsid or tegument proteins. The data support a model where virion subassemblies but not complete virions are transported in the axon. Our results provide new insight into the process of virion assembly and exit from neurons that leads to directional spread of herpesviruses in the nervous system.

Neuronal propagation of HSV1 from the oral mucosa to the eye.

PURPOSE: To identify possible neuronal pathways leading to herpetic ocular disease after primary oral infection in mice. METHODS: The SC16 strain of herpes simplex virus (HSV)-1 (10(6) plaque-forming units) was injected into the mucocutaneous border of the left upper lip. Animals were killed 2 to 10 days postinoculation (DPI). Spread of the virus in neural structures was studied by immunochemistry. RESULTS: HSV1 first replicated at the site of inoculation and then at the superior cervical ganglion (at 2 DPI). The trigeminal ganglion and the facial nerve fibers were infected by 4 DPI. Infection of the ciliary body and iris occurred at 6 DPI, together with several brain stem nuclei belonging to the autonomic or sensory pathways. Between 8 and 10 DPI, the neural infection gradually cleared up, except for the ipsilateral sympathetic ganglion, and ipsilateral keratitis appeared in some animals. CONCLUSIONS: The pattern of viral dissemination in this mouse model suggests that infection of iris and ciliary body results from transfer of virus in the superior cervical ganglion from sympathetic neurons innervating the lip to neighboring neurons innervating the anterior uvea. Later, zosteriform spread of virus from the trigeminal system may have contributed to the clinical and histologic findings.

Transport of a synaptotagmin-YFP fusion protein in sympathetic neurons during early neurite outgrowth in vitro after transfection in vivo.

Developing neurons are engaged in neurite outgrowth as well as the synthesis and transport of proteins involved in synaptic transmission. Very little is known about when transport is established in these rudimentary neurites. We used a novel technique to visualize protein transport during the early hours of neurite outgrowth in culture. Recombinant adenoviruses were used to express a synaptotagmin-YFP fusion protein in the superior cervical ganglia of neonatal rats in vivo and protein transport was examined in neuronal cultures established from the superior cervical ganglions (SCGs). We find that, as early as 4h in culture, synaptotagmin-YFP was present in the cytoplasm, lamellipodia, filopodia and growth cones. Protein expression appeared punctate in neurites at 8h in vitro and is consistent with a vesicular localization. These results indicate that the machinery to transport synapse-specific proteins is functional in rudimentary neurites at this time and indicates that this technique can be used to study early neuronal development.

Immunohistochemical analysis of pseudorabies virus spread through neurons innervating the eyeball.

Pseudorabies virus (PRV) was inoculated intraocularly into BALB/c mice, and the distribution pattern of cells positive for several neurotransmitters and viral antigens in the eyeball, trigeminal nerve ganglia, and superior cervical ganglia was examined immunohistochemically to clarify the neural route of the virus spread. In the eyeball, substance P (SP)- and calcitonin gene-related peptide (CGRP)-positive cells were detected in the ipsilateral iris and ciliary body, neuropeptide tyrosine (NPY)-positive cells were detected in the choloid membrane, and tyrosine hydroxylase (TH)-positive cells were detected in the ipsilateral inner nuclear layer of the retina; all these cells contained viral antigens. In the superior cervical ganglia, viral antigen-positive cells containing TH or NPY were found at bilateral sites. In the trigeminal nerve ganglia, viral antigen-positive cells containing SP or CGRP were found at bilateral sites. These findings indicated that the SP- and CGRP-positive ganglion cells of the trigeminal nerve ganglia innervating the iris or ciliary body, and the NPY-positive ganglion cells of the superior cervical ganglia innervating the iris, ciliary body, and choroid membrane served as the route for the virus spread. These findings also suggested that dopaminergic neurons, such as the TH-positive retinal cells and TH-positive ganglion cells of the superior cervical ganglia, served as the route for virus spread.

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.

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.

Axonal transport mediates West Nile virus entry into the central nervous system and induces acute flaccid paralysis.

West Nile virus (WNV) has emerged as a significant cause of epidemic viral encephalitis and flaccid limb paralysis, yet the mechanism by which it enters the CNS remains uncertain. We used compartmentalized neuron cultures to demonstrate that WNV spreads in both retrograde and anterograde directions via axonal transport. Transneuronal spread of WNV required axonal release of viral particles and was blocked by addition of a therapeutic neutralizing antibody. To test the physiologic significance of axonal transport in vivo, we directly inoculated the sciatic nerve of hamsters with WNV. Intrasciatic infection resulted in paralysis of the hind limb ipsilateral but not contralateral to the injection site. Limb paralysis was blocked either by surgical transection of the sciatic nerve or treatment with the therapeutic neutralizing antibody. Collectively, these studies establish that WNV undergoes bidirectional spread in neurons and that axonal transport promotes viral entry into the CNS and acute limb paralysis. Moreover, antibody therapeutics directly inhibit transneuronal spread of WNV infection and prevent the development of paralysis in vivo.

PKR and RNase L contribute to protection against lethal West Nile Virus infection by controlling early viral spread in the periphery and replication in neurons.

West Nile virus (WNV) is a neurotropic, mosquito-borne flavivirus that can cause lethal meningoencephalitis. Type I interferon (IFN) plays a critical role in controlling WNV replication, spread, and tropism. In this study, we begin to examine the effector mechanisms by which type I IFN inhibits WNV infection. Mice lacking both the interferon-induced, double-stranded-RNA-activated protein kinase (PKR) and the endoribonuclease of the 2',5'-oligoadenylate synthetase-RNase L system (PKR(-/-) x RL(-/-)) were highly susceptible to subcutaneous WNV infection, with a 90% mortality rate compared to the 30% mortality rate observed in congenic wild-type mice. PKR(-/-) x RL(-/-) mice had increased viral loads in their draining lymph nodes, sera, and spleens, which led to early viral entry into the central nervous system (CNS) and higher viral burden in neuronal tissues. Although mice lacking RNase L showed a higher CNS viral burden and an increased mortality, they were less susceptible than the PKR(-/-) x RL(-/-) mice; thus, we also infer an antiviral role for PKR in the control of WNV infection. Notably, a deficiency in both PKR and RNase L resulted in a decreased ability of type I IFN to inhibit WNV in primary macrophages and cortical neurons. In contrast, the peripheral neurons of the superior cervical ganglia of PKR(-/-) x RL(-/-) mice showed no deficiency in the IFN-mediated inhibition of WNV. Our data suggest that PKR and RNase L contribute to IFN-mediated protection in a cell-restricted manner and control WNV infection in peripheral tissues and some neuronal subtypes.

Efficient axonal localization of alphaherpesvirus structural proteins in cultured sympathetic neurons requires viral glycoprotein E.

Pseudorabies virus (PRV) glycoprotein E (gE) is a type I viral membrane protein that facilitates the anterograde spread of viral infection from the peripheral nervous system to the brain. In animal models, a gE-null mutant infection spreads inefficiently from presynaptic neurons to postsynaptic neurons (anterograde spread of infection). However, the retrograde spread of infection from post- to presynaptic neurons remains unaffected. Here we show that gE is required for wild-type localization of viral structural proteins in axons of infected neurons. During a gE-null PRV infection, a subset of viral glycoproteins, capsids, and tegument proteins enter and localize to the axon inefficiently. This defect is most obvious in the distal axon and growth cones. However, axonal entry and localization of other viral membrane proteins and endogenous cellular proteins remains unaffected. Neurons infected with gE-null mutants produce wild-type levels of viral structural proteins and infectious virions in the cell body. Our results indicate that reduced axonal targeting of viral structural proteins is a compelling explanation for the lack of anterograde spread in neural circuits following infection by a gE-null mutant.

Herpes simplex virus type 1 glycoprotein e is required for axonal localization of capsid, tegument, and membrane glycoproteins.

Herpes simplex virus type 1 (HSV-1) glycoprotein E (gE) promotes cell-to-cell spread at basolateral surfaces of epithelial cells, but its activity in neurons is less clear. We used the mouse retina infection model and neuronal cell cultures to define the spread phenotype of gE mutant viruses. Wild-type (WT) and gE-null (NS-gEnull) viruses both infected retina ganglion cell neurons; however, NS-gEnull viral antigens failed to reach the optic nerve, which indicates a defect in axonal localization. We evaluated two Fc receptor-negative gE mutant viruses containing four amino acid inserts in the gE ectodomain. One mutant virus failed to spread from the retina into the optic nerve, while the other spread normally. Therefore, the gE ectodomain is involved in axonal localization, and the Fc receptor and neuronal spread are mediated by overlapping but distinct gE domains. In the retina infection model, virus can travel to the brain via the optic nerve from presynaptic to postsynaptic neurons (anterograde direction) or via nerves that innervate the iris and ciliary body from postsynaptic to presynaptic neurons (retrograde direction). WT virus infected the brain by anterograde and retrograde routes, whereas NS-gEnull virus failed to travel by either pathway. The site of the defect in retrograde spread remains to be determined; however, infection of rat superior cervical ganglia neurons in vitro indicates that gE is required to target virion components to the axon initial segment. The requirement for gE in axonal targeting and retrograde spread highlights intriguing similarities and differences between HSV-1 and pseudorabies virus gE.

Nerve growth factor antibody stimulates reactivation of ocular herpes simplex virus type 1 in latently infected rabbits.

Anti-nerve growth factor (anti-NGF) antibody has been shown to induce reactivation of latent herpes simplex virus type 1 (HSV-1) in vitro. We found that systemically administered anti-NGF induces ocular shedding of HSV-1 in vivo in rabbits harboring latent virus. Rabbits in which HSV-1 latency had been established were given intravenous injections of goat anti-NGF serum daily for 10 days beginning 42 days after primary viral infection. Tears were assayed for virus for 12 days beginning on the day of the first injection. All eight rabbits given high titer anti-NGF had infectious virus in their tears at least once during the 12-day period. Fifteen of 16 eyes were positive and the average duration of viral shedding for these eyes was 4.0 days. Latently infected rabbits receiving daily injections of nonimmune goat serum or saline for 10 consecutive days were controls. Only six of the 16 (38%) eyes from rabbits receiving nonimmune goat serum shed virus. Only one of 12 eyes from untreated rabbits shed virus. Sera from control rabbits had no detectable anti-NGF activity; titers in anti-NGF-treated rabbits ranged between 1:1000 and 1:10,000. NGF deprivation may act as a neuronal stressor and may share a common second messenger pathway with heat- or cold-stress induced reactivation of latent HSV-1.

The distribution of P2X receptor clusters on individual neurons in sympathetic ganglia and their redistribution on agonist activation.

The distribution of P2X receptors on neurons in rat superior cervical ganglia and lability of P2X receptors on exposure to agonists were determined. Antibody labeling of each P2X subtype P2X(1)-P2X(7) showed neurons isolated into culture possessed primarily P2X(2) subunits with others occurring in order P2X(7) > P2X(6) > P2X(3) > P2X(1) > P2X(5) > P2X(4). Application of ATP and alpha,beta-meATP to neurons showed they possessed a predominantly nondesensitizing P2X receptor type insensitive to alpha,beta-meATP, consistent with immunohistochemical observations. P2X(1)-green fluorescent protein (GFP) was used to study the time course of P2X(1) receptor clustering in plasma membranes of neurons and internalization of receptors following prolonged exposure to ATP. At 12-24 h after adenoviral infection, P2X(1)-GFP formed clusters about 1 microm diameter in the neuron membrane. Application of ATP and alpha,beta-meATP showed these neurons possessed a predominantly desensitizing P2X receptor type sensitive to alpha,beta-meATP. Infection converted the major functional P2X receptor type in the membrane to P2X(1). Exposure of infected neurons to alpha,beta-meATP for less than 60 s led to the disappearance of P2X(1)-GFP fluorescence from the cell surface that was blocked by monensin, indicating the chimera is normally endocytosed into these organelles on exposure to agonist.

A pseudo-revertant of a Sindbis virus 6K protein mutant, which corrects for aberrant particle formation, contains two new mutations that map to the ectodomain of the E2 glycoprotein.

Most site-directed mutations in the gene encoding the small, membrane-associated 6K protein of Sindbis virus interfere selectively with virus assembly and budding. Particles are released that are aberrant in structure, with a single membrane enclosing multiple nucleocapsids. A revertant for the mutation that inserted a serine for a cysteine at position 39 in the 6K protein was isolated and found to correct for the defective budding so that normal particles were formed. Genetic analysis of this revertant showed that two additional mutations, which were mapped to the ectodomain of the E2 virus glycoprotein, were present in addition to the original 6K substitution. The phenotype of the revertant differed from the wild-type strain and the original mutation with regard to plaque size, thermostability, and growth in neuronal cells. Five new virus genetic constructs were prepared by insertion of these mutations into the wild-type virus. Phenotypes of these constructs confirmed that the mutations in the E2 ectodomain were responsible for both correcting the original defect in budding as well as imparting changes in cell tropism, plaque size, and thermolability on the virus. These results indicate that 6K may play an indirect role in the packing of the virus spike glycoproteins, which allows for membrane deformation and bending during the budding process.