Structural Basis of Vesicle Formation at the Inner Nuclear Membrane.

Vesicular nucleo-cytoplasmic transport is becoming recognized as a general cellular mechanism for translocation of large cargoes across the nuclear envelope. Cargo is recruited, enveloped at the inner nuclear membrane (INM), and delivered by membrane fusion at the outer nuclear membrane. To understand the structural underpinning for this trafficking, we investigated nuclear egress of progeny herpesvirus capsids where capsid envelopment is mediated by two viral proteins, forming the nuclear egress complex (NEC). Using a multi-modal imaging approach, we visualized the NEC in situ forming coated vesicles of defined size. Cellular electron cryo-tomography revealed a protein layer showing two distinct hexagonal lattices at its membrane-proximal and membrane-distant faces, respectively. NEC coat architecture was determined by combining this information with integrative modeling using small-angle X-ray scattering data. The molecular arrangement of the NEC establishes the basic mechanism for budding and scission of tailored vesicles at the INM.

Alpha herpesvirus exocytosis from neuron cell bodies uses constitutive secretory mechanisms, and egress and spread from axons is independent of neuronal firing activity.

Alpha herpesviruses naturally infect the peripheral nervous system, and can spread to the central nervous system, causing severe debilitating or deadly disease. Because alpha herpesviruses spread along synaptic circuits, and infected neurons exhibit altered electrophysiology and increased spontaneous activity, we hypothesized that alpha herpesviruses use activity-dependent synaptic vesicle-like regulated secretory mechanisms for egress and spread from neurons. Using live-cell fluorescence microscopy, we show that Pseudorabies Virus (PRV) particles use the constitutive Rab6 post-Golgi secretory pathway to exit from the cell body of primary neurons, independent of local calcium signaling. Some PRV particles colocalize with Rab6 in the proximal axon, but we did not detect colocalization/co-transport in the distal axon. Thus, the specific secretory mechanisms used for viral egress from axons remains unclear. To address the role of neuronal activity more generally, we used a compartmentalized neuron culture system to measure the egress and spread of PRV from axons, and pharmacological and optogenetics approaches to modulate neuronal activity. Using tetrodotoxin to silence neuronal activity, we observed no inhibition, and using potassium chloride or optogenetics to elevate neuronal activity, we also show no increase in virus spread from axons. We conclude that PRV egress from neurons uses constitutive secretory mechanisms: generally, activity-independent mechanisms in axons, and specifically, the constitutive Rab6 post-Golgi secretory pathway in cell bodies.

Neuronal expression of herpes simplex virus-1 VP16 protein induces pseudorabies virus escape from silencing and reactivation.

Alpha herpesvirus (α-HV) particles enter their hosts from mucosal surfaces and efficiently maintain fast transport in peripheral nervous system (PNS) axons to establish infections in the peripheral ganglia. The path from axons to distant neuronal nuclei is challenging to dissect due to the difficulty of monitoring early events in a dispersed neuron culture model. We have established well-controlled, reproducible, and reactivateable latent infections in compartmented rodent neurons by infecting physically isolated axons with a small number of viral particles. This system not only recapitulates the physiological infection route but also facilitates independent treatment of isolated cell bodies or axons. Consequently, this system enables study not only of the stimuli that promote reactivation but also the factors that regulate the initial switch from productive to latent infection. Adeno-associated virus (AAV)-mediated expression of herpes simplex-1 (HSV-1) VP16 alone in neuronal cell bodies enabled the escape from silencing of incoming pseudorabies virus (PRV) genomes. Furthermore, the expression of HSV VP16 alone reactivated a latent PRV infection in this system. Surprisingly, the expression of PRV VP16 protein supported neither PRV escape from silencing nor reactivation. We compared transcription transactivation activity of both VP16 proteins in primary neurons by RNA sequencing and found that these homolog viral proteins produce different gene expression profiles. AAV-transduced HSV VP16 specifically induced the expression of proto-oncogenes including c-Jun and Pim2. In addition, HSV VP16 induces phosphorylation of c-Jun in neurons, and when this activity is inhibited, escape of PRV silencing is dramatically reduced.IMPORTANCEDuring latency, alpha herpesvirus genomes are silenced yet retain the capacity to reactivate. Currently, host and viral protein interactions that determine the establishment of latency, induce escape from genome silencing or reactivation are not completely understood. By using a compartmented neuronal culture model of latency, we investigated the effect of the viral transcriptional activator, VP16 on pseudorabies virus (PRV) escape from genome silencing. This model recapitulates the physiological infection route and enables the study of the stimuli that regulate the initial switch from a latent to productive infection. We investigated the neuronal transcriptional activation profiles of two homolog VP16 proteins (encoded by HSV-1 or PRV) and found distinct gene activation signatures leading to diverse infection outcomes. This study contributes to understanding of how alpha herpesvirus proteins modulate neuronal gene expression leading to the initiation of a productive or a latent infection.

A Critical Analysis of the Evidence for the SARS-CoV-2 Origin Hypotheses.

When humans experience a new, devastating viral infection such as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), significant challenges arise. How should individuals as well as societies respond to the situation? One of the primary questions concerns the origin of the SARS-CoV-2 virus that infected and was transmitted efficiently among humans, resulting in a pandemic. At first glance, the question appears straightforward to answer. However, the origin of SARS-CoV-2 has been the topic of substantial debate primarily because we do not have access to some relevant data. At least two major hypotheses have been suggested: a natural origin through zoonosis followed by sustained human-to-human spread or the introduction of a natural virus into humans from a laboratory source. Here, we summarize the scientific evidence that informs this debate to provide our fellow scientists and the public with the tools to join the discussion in a constructive and informed manner. Our goal is to dissect the evidence to make it more accessible to those interested in this important problem. The engagement of a broad representation of scientists is critical to ensure that the public and policy-makers can draw on relevant expertise in navigating this controversy.

Field Research Is Essential to Counter Virological Threats.

The interface between humans and wildlife is changing and, with it, the potential for pathogen introduction into humans has increased. Avian influenza is a prominent example, with an ongoing outbreak showing the unprecedented expansion of both geographic and host ranges. Research in the field is essential to understand this and other zoonotic threats. Only by monitoring dynamic viral populations and defining their biology in situ can we gather the information needed to ensure effective pandemic preparation.

Virology under the Microscope-a Call for Rational Discourse.

Viruses have brought humanity many challenges: respiratory infection, cancer, neurological impairment and immunosuppression to name a few. Virology research over the last 60+ years has responded to reduce this disease burden with vaccines and antivirals. Despite this long history, the COVID-19 pandemic has brought unprecedented attention to the field of virology. Some of this attention is focused on concern about the safe conduct of research with human pathogens. A small but vocal group of individuals has seized upon these concerns - conflating legitimate questions about safely conducting virus-related research with uncertainties over the origins of SARS-CoV-2. The result has fueled public confusion and, in many instances, ill-informed condemnation of virology. With this article, we seek to promote a return to rational discourse. We explain the use of gain-of-function approaches in science, discuss the possible origins of SARS-CoV-2 and outline current regulatory structures that provide oversight for virological research in the United States. By offering our expertise, we - a broad group of working virologists - seek to aid policy makers in navigating these controversial issues. Balanced, evidence-based discourse is essential to addressing public concern while maintaining and expanding much-needed research in virology.

A Pseudorabies Virus Serine/Threonine Kinase, US3, Promotes Retrograde Transport in Axons via Akt/mToRC1.

Infection of peripheral axons by alpha herpesviruses (AHVs) is a critical stage in establishing a lifelong infection in the host. Upon entering the cytoplasm of axons, AHV nucleocapsids and associated inner-tegument proteins must engage the cellular retrograde transport machinery to promote the long-distance movement of virion components to the nucleus. The current model outlining this process is incomplete, and further investigation is required to discover all viral and cellular determinants involved as well as the temporality of the events. Using a modified trichamber system, we have discovered a novel role of the pseudorabies virus (PRV) serine/threonine kinase US3 in promoting efficient retrograde transport of nucleocapsids. We discovered that transporting nucleocapsids move at similar velocities in both the presence and absence of a functional US3 kinase; however, fewer nucleocapsids are moving when US3 is absent, and they move for shorter periods of time before stopping, suggesting that US3 is required for efficient nucleocapsid engagement with the retrograde transport machinery. This led to fewer nucleocapsids reaching the cell bodies to produce a productive infection 12 h later. Furthermore, US3 was responsible for the induction of local translation in axons as early as 1 h postinfection (hpi) through the stimulation of a phosphatidylinositol 3-kinase (PI3K)/Akt-mToRC1 pathway. These data describe a novel role for US3 in the induction of local translation in axons during AHV infection, a critical step in transport of nucleocapsids to the cell body. IMPORTANCE Neurons are highly polarized cells with axons that can reach centimeters in length. Communication between axons at the periphery and the distant cell body is a relatively slow process involving the active transport of chemical messengers. There is a need for axons to respond rapidly to extracellular stimuli. Translation of repressed mRNAs present within the axon occurs to enable rapid, localized responses independently of the cell body. AHVs have evolved a way to hijack local translation in the axons to promote their transport to the nucleus. We have determined the cellular mechanism and viral components involved in the induction of axonal translation. The US3 serine/threonine kinase of PRV activates Akt-mToRC1 signaling pathways early during infection to promote axonal translation. When US3 is not present, the number of moving nucleocapsids and their processivity are reduced, suggesting that US3 activity is required for efficient engagement of nucleocapsids with the retrograde transport machinery.

Are COVID-19 Vaccine Boosters Needed? The Science behind Boosters.

Waning vaccine-induced immunity coupled with the emergence of SARS-CoV-2 variants has led to increases in breakthrough infections, prompting consideration for vaccine booster doses. Boosters have been reported to be safe and increase SARS-CoV-2-specific neutralizing antibody levels, but how these doses impact the trajectory of the global pandemic and herd immunity is unknown. Information on immunology, epidemiology, and equitable vaccine distribution should be considered when deciding the timing and eligibility for COVID-19 vaccine boosters.

CRISPR/Cas9-Constructed Pseudorabies Virus Mutants Reveal the Importance of UL13 in Alphaherpesvirus Escape from Genome Silencing.

Latent and recurrent productive infection of long-living cells, such as neurons, enables alphaherpesviruses to persist in their host populations. Still, the viral factors involved in these events remain largely obscure. Using a complementation assay in compartmented primary peripheral nervous system (PNS) neuronal cultures, we previously reported that productive replication of axonally delivered genomes is facilitated by pseudorabies virus (PRV) tegument proteins. Here, we sought to unravel the role of tegument protein UL13 in this escape from silencing. We first constructed four new PRV mutants in the virulent Becker strain using CRISPR/Cas9-mediated gene replacement: (i) PRV Becker defective for UL13 expression (PRV ΔUL13), (ii) PRV where UL13 is fused to eGFP (PRV UL13-eGFP), and two control viruses (iii and iv) PRV where VP16 is fused with mTurquoise at either the N terminus (PRV mTurq-VP16) or the C terminus (PRV VP16-mTurq). Live-cell imaging of PRV capsids showed efficient retrograde transport after axonal infection with PRV UL13-eGFP, although we did not detect dual-color particles. However, immunofluorescence staining of particles in mid-axons indicated that UL13 might be cotransported with PRV capsids in PNS axons. Superinfecting nerve cell bodies with UV-inactivated PRV ΔUL13 failed to efficiently promote escape from genome silencing compared to UV-PRV wild type and UV-PRV UL13-eGFP superinfection. However, UL13 does not act directly in the escape from genome silencing, as adeno-associated virus (AAV)-mediated UL13 expression in neuronal cell bodies was not sufficient to provoke escape from genome silencing. Based on this, we suggest that UL13 may contribute to initiation of productive infection through phosphorylation of other tegument proteins.IMPORTANCE Alphaherpesviruses have mastered various strategies to persist in an immunocompetent host, including the induction of latency and reactivation in peripheral nervous system (PNS) ganglia. We recently discovered that the molecular mechanism underlying escape from latency by the alphaherpesvirus pseudorabies virus (PRV) relies on a structural viral tegument protein. This study aimed at unravelling the role of tegument protein UL13 in PRV escape from latency. First, we confirmed the use of CRISPR/Cas9-mediated gene replacement as a versatile tool to modify the PRV genome. Next, we used our new set of viral mutants and AAV vectors to conclude the indirect role of UL13 in PRV escape from latency in primary neurons, along with its spatial localization during retrograde capsid transport in axons. Based on these findings, we speculate that UL13 phosphorylates one or more tegument proteins, thereby priming these putative proteins to induce escape from genome silencing.

The axonal sorting activity of pseudorabies virus Us9 protein depends on the state of neuronal maturation.

Alpha-herpesviruses establish a life-long infection in the nervous system of the affected host; while this infection is restricted to peripheral neurons in a healthy host, the reactivated virus can spread within the neuronal circuitry, such as to the brain, in compromised individuals and lead to adverse health outcomes. Pseudorabies virus (PRV), an alpha-herpesvirus, requires the viral protein Us9 to sort virus particles into axons and facilitate neuronal spread. Us9 sorts virus particles by mediating the interaction of virus particles with neuronal transport machinery. Here, we report that Us9-mediated regulation of axonal sorting also depends on the state of neuronal maturation. Specifically, the development of dendrites and axons is accompanied with proteomic changes that influence neuronal processes. Immature superior cervical ganglionic neurons (SCGs) have rudimentary neurites that lack markers of mature axons. Immature SCGs can be infected by PRV, but they show markedly reduced Us9-dependent regulation of sorting, and increased Us9-independent transport of particles into neurites. Mature SCGs have relatively higher abundances of proteins characteristic of vesicle-transport machinery. We also identify Us9-associated neuronal proteins that can contribute to axonal sorting and subsequent anterograde spread of virus particles in axons. We show that SMPD4/nsMase3, a sphingomyelinase abundant in lipid-rafts, associates with Us9 and is a negative regulator of PRV sorting into axons and neuronal spread, a potential antiviral function.

A kinesin-3 recruitment complex facilitates axonal sorting of enveloped alpha herpesvirus capsids.

Axonal sorting, the controlled passage of specific cargoes from the cell soma into the axon compartment, is critical for establishing and maintaining the polarity of mature neurons. To delineate axonal sorting events, we took advantage of two neuroinvasive alpha-herpesviruses. Human herpes simplex virus 1 (HSV-1) and pseudorabies virus of swine (PRV; suid herpesvirus 1) have evolved as robust cargo of axonal sorting and transport mechanisms. For efficient axonal sorting and subsequent egress from axons and presynaptic termini, progeny capsids depend on three viral membrane proteins (Us7 (gI), Us8 (gE), and Us9), which engage axon-directed kinesin motors. We present evidence that Us7-9 of the veterinary pathogen pseudorabies virus (PRV) form a tripartite complex to recruit Kif1a, a kinesin-3 motor. Based on multi-channel super-resolution and live TIRF microscopy, complex formation and motor recruitment occurs at the trans-Golgi network. Subsequently, progeny virus particles enter axons as enveloped capsids in a transport vesicle. Artificial recruitment of Kif1a using a drug-inducible heterodimerization system was sufficient to rescue axonal sorting and anterograde spread of PRV mutants devoid of Us7-9. Importantly, biophysical evidence suggests that Us9 is able to increase the velocity of Kif1a, a previously undescribed phenomenon. In addition to elucidating mechanisms governing axonal sorting, our results provide further insight into the composition of neuronal transport systems used by alpha-herpesviruses, which will be critical for both inhibiting the spread of infection and the safety of herpesvirus-based oncolytic therapies.

Invasion of the Nervous System.

In vertebrates, the nervous system (NS) is composed of a peripheral collection of neurons (the peripheral nervous system, PNS), a central set found in the brain and spinal cord (the central nervous system, CNS). The NS is protected by rather complicated multi-layer barriers that allow access to nutrients and facilitate contact with the peripheral tissues, but block entry of pathogens and toxins. Virus infections usually begin in peripheral tissues and if these barriers are weakened, they can spread into the PNS and more rarely into the CNS. Most viral infections of the NS are opportunistic or accidental pathogens that gain access via the bloodstream (e.g., HIV and various arboviruses). But a few have evolved to enter the NS efficiently by invading neurons directly and by exploiting neuronal cell biology (e.g., rhabdoviruses and alphaherpesviruses). Most NS infections are devastating and difficult to manage. Remarkably, the alphaherpesviruses establish life-long quiescent infections in the PNS, with rare but often serious CNS pathology. In this review, we will focus on how alphaherpesviruses gain access to and spread in the NS, with particular emphasis on bidirectional transport and spread within and between neurons and neural circuits, which is regulated by complex viral-host protein interactions. Finally, we will describe the wide use of alphaherpesviruses as tools to study nerve connectivity and function in animal models.

Pseudorabies Virus Infection Accelerates Degradation of the Kinesin-3 Motor KIF1A.

Alphaherpesviruses, including pseudorabies virus (PRV), are neuroinvasive pathogens that establish lifelong latency in peripheral ganglia following the initial infection at mucosal surfaces. The establishment of latent infection and subsequent reactivations, during which newly assembled virions are sorted into and transported anterogradely inside axons to the initial mucosal site of infection, rely on axonal bidirectional transport mediated by microtubule-based motors. Previous studies using cultured peripheral nervous system (PNS) neurons have demonstrated that KIF1A, a kinesin-3 motor, mediates the efficient axonal sorting and transport of newly assembled PRV virions. Here we report that KIF1A, unlike other axonal kinesins, is an intrinsically unstable protein prone to proteasomal degradation. Interestingly, PRV infection of neuronal cells leads not only to a nonspecific depletion of KIF1A mRNA but also to an accelerated proteasomal degradation of KIF1A proteins, leading to a near depletion of KIF1A protein late in infection. Using a series of PRV mutants deficient in axonal sorting and anterograde spread, we identified the PRV US9/gE/gI protein complex as a viral factor facilitating the proteasomal degradation of KIF1A proteins. Moreover, by using compartmented neuronal cultures that fluidically and physically separate axons from cell bodies, we found that the proteasomal degradation of KIF1A occurs in axons during infection. We propose that the PRV anterograde sorting complex, gE/gI/US9, recruits KIF1A to viral transport vesicles for axonal sorting and transport and eventually accelerates the proteasomal degradation of KIF1A in axons.IMPORTANCE Pseudorabies virus (PRV) is an alphaherpesvirus related to human pathogens herpes simplex viruses 1 and 2 and varicella-zoster virus. Alphaherpesviruses are neuroinvasive pathogens that establish lifelong latent infections in the host peripheral nervous system (PNS). Following reactivation from latency, infection spreads from the PNS back via axons to the peripheral mucosal tissues, a process mediated by kinesin motors. Here, we unveil and characterize the underlying mechanisms for a PRV-induced, accelerated degradation of KIF1A, a kinesin-3 motor promoting the sorting and transport of PRV virions in axons. We show that PRV infection disrupts the synthesis of KIF1A and simultaneously promotes the degradation of intrinsically unstable KIF1A proteins by proteasomes in axons. Our work implies that the timing of motor reduction after reactivation would be critical because progeny particles would have a limited time window for sorting into and transport in axons for further host-to-host spread.

Alphaherpesvirus infection of mice primes PNS neurons to an inflammatory state regulated by TLR2 and type I IFN signaling.

Pseudorabies virus (PRV), an alphaherpesvirus closely related to Varicella-Zoster virus (VZV) and Herpes simplex type 1 (HSV1) infects mucosa epithelia and the peripheral nervous system (PNS) of its host. We previously demonstrated that PRV infection induces a specific and lethal inflammatory response, contributing to severe neuropathy in mice. So far, the mechanisms that initiate this neuroinflammation remain unknown. Using a mouse footpad inoculation model, we found that PRV infection rapidly and simultaneously induces high G-CSF and IL-6 levels in several mouse tissues, including the footpad, PNS and central nervous system (CNS) tissues. Interestingly, this global increase occurred before PRV had replicated in dorsal root ganglia (DRGs) neurons and also was independent of systemic inflammation. These high G-CSF and IL-6 levels were not caused by neutrophil infiltration in PRV infected tissues, as we did not detect any neutrophils. Efficient PRV replication and spread in the footpad was sufficient to activate DRGs to produce cytokines. Finally, by using knockout mice, we demonstrated that TLR2 and IFN type I play crucial roles in modulating the early neuroinflammatory response and clinical outcome of PRV infection in mice. Overall, these results give new insights into the initiation of virus-induced neuroinflammation during herpesvirus infections.

HSV-1 ICP27 targets the TBK1-activated STING signalsome to inhibit virus-induced type I IFN expression.

Herpes simplex virus (HSV) 1 stimulates type I IFN expression through the cGAS-STING-TBK1 signaling axis. Macrophages have recently been proposed to be an essential source of IFN during viral infection. However, it is not known how HSV-1 inhibits IFN expression in this cell type. Here, we show that HSV-1 inhibits type I IFN induction through the cGAS-STING-TBK1 pathway in human macrophages, in a manner dependent on the conserved herpesvirus protein ICP27. This viral protein was expressed de novo in macrophages with early nuclear localization followed by later translocation to the cytoplasm where ICP27 prevented activation of IRF3. ICP27 interacted with TBK1 and STING in a manner that was dependent on TBK1 activity and the RGG motif in ICP27. Thus, HSV-1 inhibits expression of type I IFN in human macrophages through ICP27-dependent targeting of the TBK1-activated STING signalsome.

Retrograde axonal transport of rabies virus is unaffected by interferon treatment but blocked by emetine locally in axons.

Neuroinvasive viruses, such as alpha herpesviruses (αHV) and rabies virus (RABV), initially infect peripheral tissues, followed by invasion of the innervating axon termini. Virus particles must undergo long distance retrograde axonal transport to reach the neuron cell bodies in the peripheral or central nervous system (PNS/CNS). How virus particles hijack the axonal transport machinery and how PNS axons respond to and regulate infection are questions of significant interest. To track individual virus particles, we constructed a recombinant RABV expressing a P-mCherry fusion protein, derived from the virulent CVS-N2c strain. We studied retrograde RABV transport in the presence or absence of interferons (IFN) or protein synthesis inhibitors, both of which were reported previously to restrict axonal transport of αHV particles. Using neurons from rodent superior cervical ganglia grown in tri-chambers, we showed that axonal exposure to type I or type II IFN did not alter retrograde axonal transport of RABV. However, exposure of axons to emetine, a translation elongation inhibitor, blocked axonal RABV transport by a mechanism that was not dependent on protein synthesis inhibition. The minority of RABV particles that still moved retrograde in axons in the presence of emetine, moved with slower velocities and traveled shorter distances. Emetine's effect was specific to RABV, as transport of cellular vesicles was unchanged. These findings extend our understanding of how neuroinvasion is regulated in axons and point toward a role for emetine as an inhibitory modulator of RABV axonal transport.

Intranasal Leptin Relieves Sleep-disordered Breathing in Mice with Diet-induced Obesity.

RATIONALE: Leptin treats upper airway obstruction and alveolar hypoventilation in leptin-deficient ob/ob mice. However, obese humans and mice with diet-induced obesity (DIO) are resistant to leptin because of poor permeability of the blood-brain barrier. We propose that intranasal leptin will bypass leptin resistance and treat sleep-disordered breathing in obesity. OBJECTIVES: To assess if intranasal leptin can treat obesity hypoventilation and upper airway obstruction during sleep in mice with DIO. METHODS: Male C57BL/6J mice were fed with a high-fat diet for 16 weeks. A single dose of leptin (0.4 mg/kg) or BSA (vehicle) were administered intranasally or intraperitoneally, followed by either sleep studies (n = 10) or energy expenditure measurements (n = 10). A subset of mice was treated with leptin daily for 14 days for metabolic outcomes (n = 20). In a separate experiment, retrograde viral tracers were used to examine connections between leptin receptors and respiratory motoneurons. MEASUREMENTS AND MAIN RESULTS: Acute intranasal, but not intraperitoneal, leptin decreased the number of oxygen desaturation events in REM sleep, and increased ventilation in non-REM and REM sleep, independently of metabolic effects. Chronic intranasal leptin decreased food intake and body weight, whereas intraperitoneal leptin had no effect. Intranasal leptin induced signal transducer and activator of transcription 3 phosphorylation in hypothalamic and medullary centers, whereas intraperitoneal leptin had no effect. Leptin receptor-positive cells were synaptically connected to respiratory motoneurons. CONCLUSIONS: In mice with DIO, intranasal leptin bypassed leptin resistance and significantly attenuated sleep-disordered breathing independently of body weight.

Functional Carboxy-Terminal Fluorescent Protein Fusion to Pseudorabies Virus Small Capsid Protein VP26.

Fluorescent protein fusions to herpesvirus capsids have proven to be a valuable method to study virus particle transport in living cells. Fluorescent protein fusions to the amino terminus of small capsid protein VP26 are the most widely used method to visualize pseudorabies virus (PRV) and herpes simplex virus (HSV) particles in living cells. However, these fusion proteins do not incorporate to full occupancy and have modest effects on virus replication and pathogenesis. Recent cryoelectron microscopy studies have revealed that herpesvirus small capsid proteins bind to capsids via their amino terminus, whereas the carboxy terminus is unstructured and therefore may better tolerate fluorescent protein fusions. Here, we describe a new recombinant PRV expressing a carboxy-terminal VP26-mCherry fusion. Compared to previously characterized viruses expressing amino-terminal fusions, this virus expresses more VP26 fusion protein in infected cells and incorporates more VP26 fusion protein into virus particles, and individual virus particles exhibit brighter red fluorescence. We performed single-particle tracking of fluorescent virus particles in primary neurons to measure anterograde and retrograde axonal transport, demonstrating the usefulness of this novel VP26-mCherry fusion for the study of viral intracellular transport.IMPORTANCE Alphaherpesviruses are among the very few viruses that are adapted to invade the mammalian nervous system. Intracellular transport of virus particles in neurons is important, as this process underlies both mild peripheral nervous system infection and severe spread to the central nervous system. VP26, the small capsid protein of HSV and PRV, was one of the first herpesvirus proteins to be fused to a fluorescent protein. Since then, these capsid-tagged virus mutants have become a powerful tool to visualize and track individual virus particles. Improved capsid tags will facilitate fluorescence microscopy studies of virus particle intracellular transport, as a brighter particle will improve localization accuracy of individual particles and allow for shorter exposure times, reducing phototoxicity and improving the time resolution of particle tracking in live cells.

Compartmented neuronal cultures reveal two distinct mechanisms for alpha herpesvirus escape from genome silencing.

Alpha herpesvirus genomes encode the capacity to establish quiescent infections (i.e. latency) in the peripheral nervous system for the life of their hosts. Multiple times during latency, viral genomes can reactivate to start a productive infection, enabling spread of progeny virions to other hosts. Replication of alpha herpesviruses is well studied in cultured cells and many aspects of productive replication have been identified. However, many questions remain concerning how a productive or a quiescent infection is established. While infections in vivo often result in latency, infections of dissociated neuronal cultures in vitro result in a productive infection unless lytic viral replication is suppressed by DNA polymerase inhibitors or interferon. Using primary peripheral nervous system neurons cultured in modified Campenot tri-chambers, we previously reported that reactivateable, quiescent infections by pseudorabies virus (PRV) can be established in the absence of any inhibitor. Such infections were established in cell bodies only when physically isolated axons were infected at a very low multiplicity of infection (MOI). In this report, we developed a complementation assay in compartmented neuronal cultures to investigate host and viral factors in cell bodies that prevent establishment of quiescent infection and promote productive replication of axonally delivered genomes (i.e. escape from silencing). Stimulating protein kinase A (PKA) signaling pathways in isolated cell bodies, or superinfecting cell bodies with either UV-inactivated PRV or viral light particles (LP) promoted escape from genome silencing and prevented establishment of quiescent infection but with different molecular mechanisms. Activation of PKA in cell bodies triggers a slow escape from silencing in a cJun N-terminal kinase (JNK) dependent manner. However, escape from silencing is induced rapidly by infection with UVPRV or LP in a PKA- and JNK-independent manner. We suggest that viral tegument proteins delivered to cell bodies engage multiple signaling pathways that block silencing of viral genomes delivered by low MOI axonal infection.