Alphaherpesvirus axon-to-cell spread involves limited virion transmission

A dual fluorescent herpes simplex virus type 1 recombinant reveals divergent outcomes of neuronal infection

Critical stages of lytic herpes simplex virus type 1 (HSV-1) replication are marked by the sequential expression of immediate early (IE) to early (E), then late (L) viral genes. HSV-1 can also persist in neuronal cells via a non-replicative, transcriptionally repressed infection called latency. The regulation of lytic and latent transcriptional profiles is critical to HSV-1 pathogenesis and persistence. We sought a fluorescence-based approach to observe the outcome of neuronal HSV-1 infection at the single-cell level. To achieve this goal, we constructed and characterized a novel HSV-1 recombinant that enables discrimination between lytic and latent infection. The dual reporter HSV-1 encodes a human cytomegalovirus-immediate early (hCMV-IE) promoter-driven enhanced yellow fluorescent protein (eYFP) to visualize the establishment of infection and an endogenous mCherry-VP26 fusion to report lytic replication. We confirmed that viral gene expression, replication, and spread of infection are not altered by the incorporation of the fluorescent reporters, and fluorescent protein (FP) detection virtuously reports the progression of lytic replication. We demonstrate that the outcome of HSV-1 infection of compartmentalized primary neurons is determined by viral inoculating dose: high-dose axonal inoculation proceeds to lytic replication, whereas low-dose axonal inoculation establishes a latent HSV-1 infection. Interfering with low-dose axonal inoculation via small molecule drugs reports divergent phenotypes of eYFP and mCherry reporter detection, correlating with altered states of viral gene expression. We report that the transcriptional state of neuronal HSV-1 infection is variable in response to changes in the intracellular neuronal environment.IMPORTANCEHerpes simplex virus type 1 (HSV-1) is a prevalent human pathogen that infects approximately 67% of the global human population. HSV-1 invades the peripheral nervous system, where latent HSV-1 infection persists within the host for life. Immunological evasion, viral persistence, and herpetic pathologies are determined by the regulation of HSV-1 gene expression. Studying HSV-1 gene expression during neuronal infection is challenging but essential for the development of antiviral therapeutics and interventions. We used a recombinant HSV-1 to evaluate viral gene expression during infection of primary neurons. Manipulation of cell signaling pathways impacts the establishment and transcriptional state of HSV-1 latency in neurons. The work here provides critical insight into the cellular and viral factors contributing to the establishment of latent HSV-1 infection.

A liquid chromatography-tandem mass spectrometry method development for the quantification of favipiravir drug and its related impurities in rat plasma and its application to pharmacokinetic studies

Favipiravir is an antiviral drug used for the treatment of virus-based diseases such as influenza. In this context, the development of a reliable liquid chromatography-tandem mass spectrometry method for the quantification of the drug and its impurities is necessary, particularly following the COVID-19 pandemic. Chromatographic separation was achieved on an inertial ODS column using gradient elution with a buffer containing triethylamine in high-performance liquid chromatography water and adjusting its pH with formic acid. The mixture of buffer and acetonitrile was used as a mobile phase with a flow rate of 1 ml/min at ambient temperature. The separation of favipiravir and its related impurities from remdesivir as an internal standard was achieved. The results indicated that all the variables, like precision, accuracy, linearity, matrix effect and stability, were successfully achieved within the limits of US Food and Drug Administration guidelines. This study could provide a new protocol for the development of new analytical methods for the detection of favipiravir and its impurities.

Antiviral potentials of garlic (Allium sativum) in poultry production: A mini review

Poultry enterprise is challenged with high economic losses due to viral infections. The outbreak of such infections, including Newcastle disease, avian influenza, infectious bronchitis and infectious bursal disease, could undermine poultry performance leading to decreased meat and egg production. The potency of vaccines in recent times has dropped with the rise in the virulence of antigens, which can interrupt vaccination defence. Natural herbs and phytochemicals have been extensively recommended because of their vast advantageous effects. Garlic and its bioactive organo-sulphur compounds have been proven to have antiviral, immunomodulatory and other pharmaceutical properties. Remarkable effects in poultry include a decrease in viral loads, an increase in antibody titres, lessening inflammatory cytokines and augmenting antiviral gene expression; however, methods of preparation, the dose of bioactive compounds and proportions administered may cause disparities in different reports. Therefore, this review highlights the potential of garlic against viral diseases, immunomodulatory, toxicity and pathological status in embryonated chicken eggs and poultry.

Transneuronal Circuit Analysis with Pseudorabies Viruses

Our ability to understand the function of the nervous system is dependent upon defining the connections of its constituent neurons. Development of methods to define connections within neural networks has always been a growth industry in the neurosciences. Transneuronal spread of neurotropic viruses currently represents the best means of defining synaptic connections within neural networks. The method exploits the ability of viruses to invade neurons, replicate, and spread through the intimate synaptic connections that enable communication among neurons. Since the method was first introduced in the 1970s, it has benefited from an increased understanding of the virus life cycle, the function of viral genomes, and the ability to manipulate the viral genome in support of directional spread of virus and the expression of transgenes. In this article, we review these advances in viral tracing technology and the ways in which they may be applied for functional dissection of neural networks. © 2023 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Retrograde infection of CNS circuits by peripheral injection of virus Basic Protocol 2: Transneuronal analysis by intracerebral injection Alternate Protocol 1: Transneuronal analysis with multiple recombinant strains Alternate Protocol 2: Conditional replication and spread of PRV Alternate Protocol 3: Conditional reporters of PRV infection and spread Alternate Protocol 4: Reporters of neural activity in polysynaptic circuits Support Protocol 1: Growing and titering a PRV viral stock Support Protocol 2: Immunohistochemical processing and detection Support Protocol 3: Dual-immunofluorescence localization.

A Single Herpes Simplex Virus 1 Genome Reactivates from Individual Cells

Latent infection is a characteristic feature of herpesviruses’ life cycle. Herpes simplex virus 1 is a common human pathogen that establishes lifelong latency in peripheral neurons. Symptomatic or asymptomatic periodic reactivations from the latent state allow the virus to replicate and spread among individuals. The latent viral genomes are found as several quiescent episomes inside the infected nuclei; however, it is not clear if and how many latent genomes are able to reactivate together. To address this question, we developed a quiescent infection assay, which provides a quantitative analysis of the number of genomes reactivating per cell, in cultured immortalized fibroblasts. We found that, almost always, only one viral genome reactivates per cell. We showed that different timing of entry to quiescence did not result in a significant change in the probability of reactivating. Reactivation from this quiescent state allowed only limited intergenomic recombination between two viral strains compared to lytic infection. Following coinfection with a mutant that is unable to reactivate, only coreactivation with a reactivation-proficient recombinant can provide the opportunity for the mutant to reactivate. We speculate that each individual quiescent viral genome has a low and stochastic chance to reactivate in each cell, an assumption that can explain the limited number of genomes reactivating per cell.

IMPORTANCE Herpesviruses are highly prevalent and cause significant morbidity in the human and animal populations. Most individuals who are infected with herpes simplex virus (HSV-1), a common human pathogen, will become lifelong carriers of the virus, as HSV-1 establishes latent (quiescent) infections in the host cells. Reactivation from the latent state leads to many of the viral symptoms and to the spread of the virus among individuals. While many triggers for reactivation were identified, how many genomes reactivate from an individual cell and how are these genomes selected remain understudied. Here, we identify that, in most cases, only one genome per cell reactivates. Mutated HSV-1 genomes require coinfection with another strain to allow coreactivation. Our findings suggest that the decision to reactivate is determined for each quiescent genome separately and support the notion that reactivation preferences occur at the single-genome level.

Recombinase polymerase amplification in the molecular diagnosis of microbiological targets and its applications

Since the introduction of the polymerase chain reaction (PCR) technique in 1983, nucleic acid amplification has permeated all fields of biological science, particularly clinical research. Despite its importance, PCR has been restricted to specialized centers and its use in laboratories with few resources is limited. In recent decades, there has been a notable increase in the development of new isothermal technologies for molecular diagnosis with the hope of overcoming the traditional limitations of the laboratory. Among these technologies, recombinase polymerase amplification (RPA) has a wide application potential because it does not require thermocyclers and has high sensitivity, specificity, simplicity, and detection speed. This technique has been used for DNA and RNA amplification in various pathogenic organisms such as viruses, bacteria, and parasites. In addition, RPA has been successfully implemented in different detection strategies, making it a promising alternative for performing diagnoses in environments with scarce resources and a high burden of infectious diseases. In this study, we present a review of the use of RPA in clinical settings and its implementation in various research areas.

Methods and Applications of Campenot Trichamber Neuronal Cultures for the Study of Neuroinvasive Viruses

The development of compartmentalized neuron culture systems has been invaluable in the study of neuroinvasive viruses, including the alpha herpesviruses Herpes Simplex Virus 1 (HSV-1) and Pseudorabies Virus (PRV). This chapter provides updated protocols for assembling and culturing rodent embryonic superior cervical ganglion (SCG) and dorsal root ganglion (DRG) neurons in Campenot trichamber cultures. In addition, we provide several illustrative examples of the types of experiments that are enabled by Campenot cultures: (1) Using fluorescence microscopy to investigate axonal outgrowth/extension through the chambers, and alpha herpesvirus infection, intracellular trafficking, and cell-cell spread via axons. (2) Using correlative fluorescence microscopy and cryo electron tomography to investigate the ultrastructure of virus particles trafficking in axons.

Mutational pressure by host APOBEC3s more strongly affects genes expressed early in the lytic phase of herpes simplex virus-1 (HSV-1) and human polyomavirus (HPyV) infection

Herpes-Simplex Virus 1 (HSV-1) infects most humans when they are young, sometimes with fatal consequences. Gene expression occurs in a temporal order upon lytic HSV-1 infection: immediate early (IE) genes are expressed, then early (E) genes, followed by late (L) genes. During this infection cycle, the HSV-1 genome has the potential for exposure to APOBEC3 (A3) proteins, a family of cytidine deaminases that cause C>U mutations on single-stranded DNA (ssDNA), often resulting in a C>T transition. We developed a computational model for the mutational pressure of A3 on the lytic cycle of HSV-1 to determine which viral kinetic gene class is most vulnerable to A3 mutations. Using in silico stochastic methods, we simulated the infectious cycle under varying intensities of A3 mutational pressure. We found that the IE and E genes are more vulnerable to A3 than L genes. We validated this model by analyzing the A3 evolutionary footprints in 25 HSV-1 isolates. We find that IE and E genes have evolved to underrepresent A3 hotspot motifs more so than L genes, consistent with greater selection pressure on IE and E genes. We extend this model to two-step infections, such as those of polyomavirus, and find that the same pattern holds for over 25 human Polyomavirus (HPyVs) genomes. Genes expressed earlier during infection are more vulnerable to mutations than those expressed later.

Viral infection of human neurons triggers strain-specific differences in host neuronal and viral transcriptomes

Infection with herpes simplex virus 1 (HSV-1) occurs in over half the global population, causing recurrent orofacial and/or genital lesions. Individual strains of HSV-1 demonstrate differences in neurovirulence in vivo, suggesting that viral genetic differences may impact phenotype. Here differentiated SH-SY5Y human neuronal cells were infected with one of three HSV-1 strains known to differ in neurovirulence in vivo. Host and viral RNA were sequenced simultaneously, revealing strain-specific differences in both viral and host transcription in infected neurons. Neuronal morphology and immunofluorescence data highlight the pathological changes in neuronal cytoarchitecture induced by HSV-1 infection, which may reflect host transcriptional changes in pathways associated with adherens junctions, integrin signaling, and others. Comparison of viral protein levels in neurons and epithelial cells demonstrated that a number of differences were neuron-specific, suggesting that strain-to-strain variations in host and virus transcription are cell type-dependent. Together, these data demonstrate the importance of studying virus strain- and cell-type-specific factors that may contribute to neurovirulence in vivo, and highlight the specificity of HSV-1-host interactions.

Mapping of spinal interneurons involved in regulation of the lower urinary tract in juvenile male rats

Coordination between the urinary bladder (BL) and external urethral sphincter (EUS) is necessary for storage and elimination of urine. In rats interneuronal circuits at two levels of the spinal cord (i.e., L6-S1 and L3-L4) play an important role in this coordination. In the present experiments retrograde trans-synaptic transport of pseudorabies virus (PRV) encoding fluorescent markers (GFP and RFP) was used to trace these circuits. To examine the relative localization of EUS-related and BL-related interneuronal populations we injected PRV-GFP into the EUS and PRV-RFP into the BL wall. The PRV infected populations of spinal interneurons were localized primarily in the dorsal commissure (DCM) of L6/S1 and in a hypothesized lumbar spinal coordinating center (LSCC) in L3/L4 above and lateral to central canal (CC). At both sites colocalization of markers occurred in a substantial number of labeled interneurons indicating concomitant involvement of these double-labelled neurons in the EUS- and BL-circuits and suggesting their role in EUS-BL coordination. Intense GFP or RFP fluorescent was detected in a subpopulation of cells at both sites suggesting that they were infected earlier and therefore likely to represent first order, primary interneurons that directly synapse with output neurons. Larger numbers of weakly fluorescent neurons that likely represent second order interneurons were also identified. Within the population of EUS-related first order interneurons only 3-8 % exhibited positive immunoreaction for an early transcription factor Pax2 specific to GABAergic and glycinergic inhibitory neurons suggesting that the majority of interneurons in DCM and LSCC projecting directly to the EUS motoneurons are excitatory.

Viral cell-to-cell spread: Conventional and non-conventional ways

A critical step in the life cycle of a virus is spread to a new target cell, which generally involves the release of new viral particles from the infected cell which can then initiate infection in the next target cell. While cell-free viral particles released into the extracellular environment are necessary for long distance spread, there are disadvantages to this mechanism. These include the presence of immune system components, the low success rate of infection by single particles, and the relative fragility of viral particles in the environment. Several mechanisms of direct cell-to-cell spread have been reported for animal viruses which would avoid the issues associated with cell-free particles. A number of viruses can utilize several different mechanisms of direct cell-to-cell spread, but our understanding of the differential usage by these pathogens is modest. Although the mechanisms of cell-to-cell spread differ among viruses, there is a common exploitation of key pathways and components of the cellular cytoskeleton. Remarkably, some of the viral mechanisms of cell-to-cell spread are surprisingly similar to those used by bacteria. Here we summarize the current knowledge of the conventional and non-conventional mechanisms of viral spread, the common methods used to detect viral spread, and the impact that these mechanisms can have on viral pathogenesis.

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.

Central sensory-motor crosstalk in the neural gut-brain axis

The neural gut-brain axis consists of viscerosensory and autonomic motor neurons innervating the gastrointestinal (GI) tract. Sensory neurons transmit nutrient-related and non-nutrient-related information to the brain, while motor neurons regulate GI motility and secretion. Previous research provides an incomplete picture of the brain nuclei that are directly connected with the neural gut-brain axis, and no studies have thoroughly assessed sensory-motor overlap in those nuclei. Our goal in this study was to comprehensively characterize the central sensory and motor circuitry associated with the neural gut-brain axis linked to a segment of the small intestine. We injected a retrograde (pseudorabies; PRV) and anterograde (herpes simplex virus 1; HSV) transsynaptic viral tracer into the duodenal wall of adult male rats. Immunohistochemical processing revealed single- and double-labeled cells that were quantified per nucleus. We found that across nearly all brain regions assessed, PRV + HSV immunoreactive neurons comprised the greatest percentage of labeled cells compared with single-labeled PRV or HSV neurons. These results indicate that even though sensory and motor information can be processed by separate neuronal populations, there is neuroanatomical evidence of direct sensory-motor feedback in the neural gut-brain axis throughout the entire caudal-rostral extent of the brain. This is the first study to exhaustively investigate the sensory-motor organization of the neural gut-brain axis, and is a step toward phenotyping the many central neuronal populations involved in GI control.

Local D2- to D1-neuron transmodulation updates goal-directed learning in the striatum

Extinction learning allows animals to withhold voluntary actions that are no longer related to reward and so provides a major source of behavioral control. Although such learning is thought to depend on dopamine signals in the striatum, the way the circuits that mediate goal-directed control are reorganized during new learning remains unknown. Here, by mapping a dopamine-dependent transcriptional activation marker in large ensembles of spiny projection neurons (SPNs) expressing dopamine receptor type 1 (D1-SPNs) or 2 (D2-SPNs) in mice, we demonstrate an extensive and dynamic D2- to D1-SPN transmodulation across the striatum that is necessary for updating previous goal-directed learning. Our findings suggest that D2-SPNs suppress the influence of outdated D1-SPN plasticity within functionally relevant striatal territories to reshape volitional action.

3D Printed Neural Regeneration Devices

Neural regeneration devices interface with the nervous system and can provide flexibility in material choice, implantation without the need for additional surgeries, and the ability to serve as guides augmented with physical, biological (e.g., cellular), and biochemical functionalities. Given the complexity and challenges associated with neural regeneration, a 3D printing approach to the design and manufacturing of neural devices could provide next-generation opportunities for advanced neural regeneration via the production of anatomically accurate geometries, spatial distributions of cellular components, and incorporation of therapeutic biomolecules. A 3D printing-based approach offers compatibility with 3D scanning, computer modeling, choice of input material, and increasing control over hierarchical integration. Therefore, a 3D printed implantable platform could ultimately be used to prepare novel biomimetic scaffolds and model complex tissue architectures for clinical implants in order to treat neurological diseases and injuries. Further, the flexibility and specificity offered by 3D printed in vitro platforms have the potential to be a significant foundational breakthrough with broad research implications in cell signaling and drug screening for personalized healthcare. This progress report examines recent advances in 3D printing strategies for neural regeneration as well as insight into how these approaches can be improved in future studies.

Abortive herpes simplex virus infection of nonneuronal cells results in quiescent viral genomes that can reactivate

Abortive viral infections are usually studied in populations of susceptible but nonpermissive cells. Single-cell studies of viral infections have demonstrated that even in susceptible and permissive cell populations, abortive infections can be detected in subpopulations of the infected cells. We have previously identified abortive infections in HeLa cells infected with herpes simplex virus 1 (HSV-1) at high multiplicity of infection (MOI). Here, we tested 4 additional human-derived nonneuronal cell lines (cancerous or immortalized) and found significant subpopulations that remain abortive. To characterize these abortive cells, we recovered cell populations that survived infection with HSV-1 at high MOI. The surviving cells retained proliferative potential and the ability to be reinfected. These recovered cell populations maintained the viral genomes in a quiescent state for at least 5 wk postinfection. Our results indicate that these viral genomes are maintained inside the nucleus, bound to cellular histones and occasionally reactivated to produce new progeny viruses. We conclude that abortive HSV-1 infection is a common feature during infection of nonneuronal cells and results in a latency-like state in the infected cells. Our findings question the longstanding paradigm that alphaherpesviruses can establish spontaneous latency only in neuronal cells and emphasize the stochastic nature of lytic versus latency decision of HSV-1 in nonneuronal cells.

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?

Trans-endocytosis elicited by nectins transfers cytoplasmic cargo, including infectious material, between cells

Here, we show that cells expressing the adherens junction protein nectin-1 capture nectin-4-containing membranes from the surface of adjacent cells in a trans-endocytosis process. We find that internalized nectin-1-nectin-4 complexes follow the endocytic pathway. The nectin-1 cytoplasmic tail controls transfer: its deletion prevents trans-endocytosis, while its exchange with the nectin-4 tail reverses transfer direction. Nectin-1-expressing cells acquire dye-labeled cytoplasmic proteins synchronously with nectin-4, a process most active during cell adhesion. Some cytoplasmic cargo remains functional after transfer, as demonstrated with encapsidated genomes of measles virus (MeV). This virus uses nectin-4, but not nectin-1, as a receptor. Epithelial cells expressing nectin-4, but not those expressing another MeV receptor in its place, can transfer infection to nectin-1-expressing primary neurons. Thus, this newly discovered process can move cytoplasmic cargo, including infectious material, from epithelial cells to neurons. We name the process nectin-elicited cytoplasm transfer (NECT). NECT-related trans-endocytosis processes may be exploited by pathogens to extend tropism. This article has an associated First Person interview with the first author of the paper.

Coalescing replication compartments provide the opportunity for recombination between coinfecting herpesviruses

Homologous recombination (HR) is considered a major driving force of evolution because it generates and expands genetic diversity. Evidence of HR between coinfecting herpesvirus DNA genomes can be found frequently both in vitro and in clinical isolates. Each herpes simplex virus type 1 (HSV-1) replication compartment (RC) derives from a single incoming genome and maintains a specific territory within the nucleus. This raises intriguing questions about where and when coinfecting viral genomes interact. To study the spatiotemporal requirements for intergenomic recombination, we developed an assay with dual-color FISH that enables detection of HR between different pairs of coinfecting HSV-1 genomes. Our results revealed that HR increases intermingling of RCs derived from different genomes. Furthermore, inhibition of RC movement reduces the rate of HR events among coinfecting viruses. Finally, we observed correlation between nuclear size and the number of RCs per nucleus. Our findings suggest that both viral replication and recombination are subject to nuclear spatial constraints. Other DNA viruses and cellular DNA are likely to encounter similar restrictions.-Tomer, E., Cohen, E. M., Drayman, N., Afriat, A., Weitzman, M. D., Zaritsky, A., Kobiler, O. Coalescing replication compartments provide the opportunity for recombination between coinfecting herpesviruses.

Infectious Herpes Simplex Virus in the Brain Stem Is Correlated with Reactivation in the Trigeminal Ganglia

Latent herpes simplex virus (HSV) DNA has been detected in the central nervous systems (CNS) of humans postmortem, and infection with HSV has been correlated with the development of neurodegenerative diseases. However, whether HSV can directly reactivate in the CNS and/or infectious virus can be transported to the CNS following reactivation in peripheral ganglia has been unclear. In this study, infectious virus was recovered from both the trigeminal ganglia and the brain stem of latently infected mice following a reactivation stimulus, but a higher frequency of reactivation and increased titers of infectious virus were recovered from the trigeminal ganglia. Viral proteins were detected in neurons of the trigeminal ganglia, but a cellular source of infectious virus could not be identified in the brain stem. These results suggest that infectious virus is transported from the ganglia to the CNS following reactivation but do not exclude the potential for direct reactivation in the CNS.

Herpes simplex virus (HSV) establishes latency in neurons of the peripheral and central nervous systems (CNS). Evidence is mounting that HSV latency and reactivation in the nervous system has the potential to promote neurodegenerative processes. Understanding how this occurs is an important human health goal. In the mouse model, in vivo viral reactivation in the peripheral nervous system, triggered by hyperthermic stress, has been well characterized with respect to frequency and cell type. However, characterization of in vivo reactivation in the CNS is extremely limited. Further, it remains unclear whether virus reactivated in the peripheral nervous system is transported to the CNS in an infectious form, how often this occurs, and what parameters underlie the efficiency and outcomes of this process. In this study, reactivation was quantified in the trigeminal ganglia (TG) and the brain stem from the same latently infected animal using direct assays of equivalent sensitivity. Reactivation was detected more frequently in the TG than in the brain stem and, in all but one case, the amount of virus recovered was greater in the TG than that detected in the brain stem. Viral protein positive neurons were observed in the TG, but a cellular source for reactivation in the brain stem was not identified, despite serially sectioning and examining the entire tissue (0/6 brain stems). These findings suggest that infectious virus detected in the brain stem is primarily the result of transport of reactivated virus from the TG into the brain stem.

IMPORTANCE Latent herpes simplex virus (HSV) DNA has been detected in the central nervous systems (CNS) of humans postmortem, and infection with HSV has been correlated with the development of neurodegenerative diseases. However, whether HSV can directly reactivate in the CNS and/or infectious virus can be transported to the CNS following reactivation in peripheral ganglia has been unclear. In this study, infectious virus was recovered from both the trigeminal ganglia and the brain stem of latently infected mice following a reactivation stimulus, but a higher frequency of reactivation and increased titers of infectious virus were recovered from the trigeminal ganglia. Viral proteins were detected in neurons of the trigeminal ganglia, but a cellular source of infectious virus could not be identified in the brain stem. These results suggest that infectious virus is transported from the ganglia to the CNS following reactivation but do not exclude the potential for direct reactivation in the CNS.

Application of droplet digital PCR to detect the pathogens of infectious diseases

Polymerase chain reaction (PCR) is a molecular biology technique used to multiply certain deoxyribonucleic acid (DNA) fragments. It is a common and indispensable technique that has been applied in many areas, especially in clinical laboratories. The third generation of polymerase chain reaction, droplet digital polymerase chain reaction (ddPCR), is a biotechnological refinement of conventional polymerase chain reaction methods that can be used to directly quantify and clonally amplify DNA. Droplet digital polymerase chain reaction is now widely used in low-abundance nucleic acid detection and is useful in diagnosis of infectious diseases. Here, we summarized the potential advantages of droplet digital polymerase chain reaction in clinical diagnosis of infectious diseases, including viral diseases, bacterial diseases and parasite infections, concluded that ddPCR provides a more sensitive, accurate, and reproducible detection of low-abundance pathogens and may be a better choice than quantitative polymerase chain reaction for clinical applications in the future.

Characterization of the neuroinvasive profile of a pseudorabies virus recombinant expressing the mTurquoise2 reporter in single and multiple injection experiments

BACKGROUND

Viral transneuronal tracing has become a well established technology used to define the synaptic architecture of polysynaptic neural networks.

NEW METHOD

In this report we define the neuroinvasive profile and reporter expression of a new recombinant of the Bartha strain of pseudorabies virus (PRV). The new recombinant, PRV-290, expresses the mTurquoise2 fluorophor and is designed to complement other isogenic recombinants of Bartha that express different reporters of infection. Results & Comparison with Existing Methods: PRV-290 was injected either alone or in combination with isogenic recombinants of PRV that express enhanced green fluorescent protein (EGFP; PRV-152) or monomeric red fluorescent protein (mRFP; PRV-614). Circuits previously defined using PRV-152 and PRV-614 were used for the analysis. The data demonstrate that PRV-290 is a retrograde transneuronal tracer with temporal kinetics similar to those of its isogenic recombinants. Stable expression of the diffusible mTurquoise2 reporter filled infected neurons, with the extent and intensity of labeling increasing with advancing post inoculation survival. In multiple injection experiments, PRV-290 established productive infections in neurons also replicating PRV-152 and/or PRV-614. This novel demonstration of three recombinants infecting individual neurons represents an important advance in the technology.

CONCLUSION

Collectively, these data demonstrate that PRV-290 is a valuable addition to the viral tracer toolbox for transneuronal tracing of neural circuitry.

Probabilistic Modeling of Pseudorabies Virus Infection in a Neural Circuit

Viral transneuronal tracing methods effectively label synaptically connected neurons in a time-dependent manner. However, the modeling of viral vectors has been largely absent. An objective of this article is to motivate and initiate a basis for computational modeling of viral labeling and the questions that can be investigated through modeling of pseudorabies virus (PRV) virion progression in a neural circuit. In particular, a mathematical model is developed for quantitative analysis of PRV infection. Probability expressions are presented to evaluate the progression of viral labeling along the neural circuit. The analysis brings forth various parameters, the numerical values of which must be attained through future experiments. This is the first computational model for PRV viral labeling of a neural circuit.

Visualizing Viral Infection In Vivo by Multi-Photon Intravital Microscopy

Viral pathogens have adapted to the host organism to exploit the cellular machinery for virus replication and to modulate the host cells for efficient systemic dissemination and immune evasion. Much of our knowledge of the effects that virus infections have on cells originates from in vitro imaging studies using experimental culture systems consisting of cell lines and primary cells. Recently, intravital microscopy using multi-photon excitation of fluorophores has been applied to observe virus dissemination and pathogenesis in real-time under physiological conditions in living organisms. Critical steps during viral infection and pathogenesis could be studied by direct visualization of fluorescent virus particles, virus-infected cells, and the immune response to viral infection. In this review, I summarize the latest research on in vivo studies of viral infections using multi-photon intravital microscopy (MP-IVM). Initially, the underlying principle of multi-photon microscopy is introduced and experimental challenges during microsurgical animal preparation and fluorescent labeling strategies for intravital imaging are discussed. I will further highlight recent studies that combine MP-IVM with optogenetic tools and transcriptional analysis as a powerful approach to extend the significance of in vivo imaging studies of viral pathogens.

Latent versus productive infection: the alpha herpesvirus switch

Alpha herpesviruses are common pathogens of mammals. They establish a productive infection in many cell types, but a life-long latent infection occurs in PNS neurons. A vast majority of the human population has latent HSV-1 infections. Currently, there is no cure to clear latent infections. Even though HSV-1 is among the best studied viral pathogens, regulation of latency and reactivation is not well understood due to several challenges including a lack of animal models that precisely recapitulate latency/reactivation episodes; a difficulty in modeling in vitro latency; and a limited understanding of neuronal biology. In this review, we discuss insights gained from in vitro latency models with a focus on the neuronal and viral factors that determine the mode of infection.

A New Approach to Assessing HSV-1 Recombination during Intercellular Spread

The neuroinvasive Herpes simplex virus type 1 (HSV-1) utilizes intergenomic recombination in order to diversify viral populations. Research efforts to assess HSV-1 recombination are often complicated by the use of attenuating mutations, which differentiate viral progeny but unduly influence the replication and spread. In this work, we generated viruses with markers that allowed for classification of viral progeny with limited attenuation of viral replication. We isolated viruses, harboring either a cyan (C) or yellow (Y) fluorescent protein (FP) expression cassette inserted in two different locations within the viral genome, in order to visually quantify the recombinant progeny based on plaque fluorescence. We found that the FP marked genomes had a limited negative affect on the viral replication and production of progeny virions. A co-infection of the two viruses resulted in recombinant progeny that was dependent on the multiplicity of infection and independent of the time post infection, at a rate that was similar to previous reports. The sequential passage of mixed viral populations revealed a limited change in the distribution of the parental and recombinant progeny. Interestingly, the neuroinvasive spread within neuronal cultures and an in vivo mouse model, revealed large, random shifts in the parental and recombinant distributions in viral populations. In conclusion, our approach highlights the utility of FP expressing viruses in order to provide new insights into mechanisms of HSV-1 recombination.

Spatiotemporal dynamics of HSV genome nuclear entry and compaction state transitions using bioorthogonal chemistry and super-resolution microscopy

We investigated the spatiotemporal dynamics of HSV genome transport during the initiation of infection using viruses containing bioorthogonal traceable precursors incorporated into their genomes (HSVEdC). In vitro assays revealed a structural alteration in the capsid induced upon HSVEdC binding to solid supports that allowed coupling to external capture agents and demonstrated that the vast majority of individual virions contained bioorthogonally-tagged genomes. Using HSVEdC in vivo we reveal novel aspects of the kinetics, localisation, mechanistic entry requirements and morphological transitions of infecting genomes. Uncoating and nuclear import was observed within 30 min, with genomes in a defined compaction state (ca. 3-fold volume increase from capsids). Free cytosolic uncoated genomes were infrequent (7-10% of the total uncoated genomes), likely a consequence of subpopulations of cells receiving high particle numbers. Uncoated nuclear genomes underwent temporal transitions in condensation state and while ICP4 efficiently associated with condensed foci of initial infecting genomes, this relationship switched away from residual longer lived condensed foci to increasingly decondensed genomes as infection progressed. Inhibition of transcription had no effect on nuclear entry but in the absence of transcription, genomes persisted as tightly condensed foci. Ongoing transcription, in the absence of protein synthesis, revealed a distinct spatial clustering of genomes, which we have termed genome congregation, not seen with non-transcribing genomes. Genomes expanded to more decondensed forms in the absence of DNA replication indicating additional transitional steps. During full progression of infection, genomes decondensed further, with a diffuse low intensity signal dissipated within replication compartments, but frequently with tight foci remaining peripherally, representing unreplicated genomes or condensed parental strands of replicated DNA. Uncoating and nuclear entry was independent of proteasome function and resistant to inhibitors of nuclear export. Together with additional data our results reveal new insight into the spatiotemporal dynamics of HSV genome uncoating, transport and organisation.

Promoting Simultaneous Onset of Viral Gene Expression Among Cells Infected with Herpes Simplex Virus-1

Synchronous viral infection facilitates the study of viral gene expression, viral host interactions, and viral replication processes. However, the protocols for achieving synchronous infections were hardly ever tested in proper temporal resolution at the single-cell level. We set up a fluorescence-based, time lapse microscopy assay to study sources of variability in the timing of gene expression during herpes simplex virus-1 (HSV-1) infection. We found that with the common protocol, the onset of gene expression within different cells can vary by more than 3 h. We showed that simultaneous viral genome entry to the nucleus can be achieved with a derivative of the previously characterized temperature sensitive mutant tsB7, however, this did not improve gene expression synchrony. We found that elevating the temperature in which the infection is done and increasing the multiplicity of infection (MOI) significantly promoted simultaneous onset of viral gene expression among infected cells. Further, elevated temperature result in a decrease in the coefficient of variation (a standardized measure of dispersion) of viral replication compartments (RCs) sizes among cells as well as a slight increment of viral late gene expression synchrony. We conclude that simultaneous viral gene expression can be improved by simple modifications to the infection process and may reduce the effect of single-cell variability on population-based assays.

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

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

Computational sensing of herpes simplex virus using a cost-effective on-chip microscope

Caused by the herpes simplex virus (HSV), herpes is a viral infection that is one of the most widespread diseases worldwide. Here we present a computational sensing technique for specific detection of HSV using both viral immuno-specificity and the physical size range of the viruses. This label-free approach involves a compact and cost-effective holographic on-chip microscope and a surface-functionalized glass substrate prepared to specifically capture the target viruses. To enhance the optical signatures of individual viruses and increase their signal-to-noise ratio, self-assembled polyethylene glycol based nanolenses are rapidly formed around each virus particle captured on the substrate using a portable interface. Holographic shadows of specifically captured viruses that are surrounded by these self-assembled nanolenses are then reconstructed, and the phase image is used for automated quantification of the size of each particle within our large field-of-view, ~30 mm². The combination of viral immuno-specificity due to surface functionalization and the physical size measurements enabled by holographic imaging is used to sensitively detect and enumerate HSV particles using our compact and cost-effective platform. This computational sensing technique can find numerous uses in global health related applications in resource-limited environments.

Microphysiological Human Brain and Neural Systems-on-a-Chip: Potential Alternatives to Small Animal Models and Emerging Platforms for Drug Discovery and Personalized Medicine

Translational challenges associated with reductionist modeling approaches, as well as ethical concerns and economic implications of small animal testing, drive the need for developing microphysiological neural systems for modeling human neurological diseases, disorders, and injuries. Here, we provide a comprehensive review of microphysiological brain and neural systems-on-a-chip (NSCs) for modeling higher order trajectories in the human nervous system. Societal, economic, and national security impacts of neurological diseases, disorders, and injuries are highlighted to identify critical NSC application spaces. Hierarchical design and manufacturing of NSCs are discussed with distinction for surface- and bulk-based systems. Three broad NSC classes are identified and reviewed: microfluidic NSCs, compartmentalized NSCs, and hydrogel NSCs. Emerging areas and future directions are highlighted, including the application of 3D printing to design and manufacturing of next-generation NSCs, the use of stem cells for constructing patient-specific NSCs, and the application of human NSCs to 'personalized neurology'. Technical hurdles and remaining challenges are discussed. This review identifies the state-of-the-art design methodologies, manufacturing approaches, and performance capabilities of NSCs. This work suggests NSCs appear poised to revolutionize the modeling of human neurological diseases, disorders, and injuries.

Quantitative Evaluation of Protein Heterogeneity within Herpes Simplex Virus 1 Particles

Several virulence genes have been identified thus far in the herpes simplex virus 1 genome. It is also generally accepted that protein heterogeneity among virions further impacts viral fitness. However, linking this variability directly with infectivity has been challenging at the individual viral particle level. To address this issue, we resorted to flow cytometry (flow virometry), a powerful approach we recently employed to analyze individual viral particles, to identify which tegument proteins vary and directly address if such variability is biologically relevant. We found that the stoichiometry of the UL37, ICP0, and VP11/12 tegument proteins in virions is more stable than the VP16 and VP22 tegument proteins, which varied significantly among viral particles. Most interestingly, viruses sorted for their high VP16 or VP22 content yielded modest but reproducible increases in infectivity compared to their corresponding counterparts containing low VP16 or VP22 content. These findings were corroborated for VP16 in short interfering RNA experiments but proved intriguingly more complex for VP22. An analysis by quantitative Western blotting revealed substantial alterations of virion composition upon manipulation of individual tegument proteins and suggests that VP22 protein levels acted indirectly on viral fitness. These findings reaffirm the interdependence of the virion components and corroborate that viral fitness is influenced not only by the genome of viruses but also by the stoichiometry of proteins within each virion.IMPORTANCE The ability of viruses to spread in animals has been mapped to several viral genes, but other factors are clearly involved, including virion heterogeneity. To directly probe whether the latter influences viral fitness, we analyzed the protein content of individual herpes simplex virus 1 particles using an innovative flow cytometry approach. The data confirm that some viral proteins are incorporated in more controlled amounts, while others vary substantially. Interestingly, this correlates with the VP16 trans-activating viral protein and indirectly with VP22, a second virion component whose modulation profoundly alters virion composition. This reaffirms that not only the presence but also the amount of specific tegument proteins is an important determinant of viral fitness.

Collective Infectious Units in Viruses

Increasing evidence indicates that viruses do not simply propagate as independent virions among cells, organs, and hosts. Instead, viral spread is often mediated by structures that simultaneously transport groups of viral genomes, such as polyploid virions, aggregates of virions, virion-containing proteinaceous structures, secreted lipid vesicles, and virus-induced cell-cell contacts. These structures increase the multiplicity of infection, independently of viral population density and transmission bottlenecks. Collective infectious units may contribute to the maintenance of viral genetic diversity, and could have implications for the evolution of social-like virus-virus interactions. These may include various forms of cooperation such as immunity evasion, genetic complementation, division of labor, and relaxation of fitness trade-offs, but also noncooperative interactions such as negative dominance and interference, potentially leading to conflict.

3D Printing of Tissue Engineered Constructs for In Vitro Modeling of Disease Progression and Drug Screening

2D cell culture and preclinical animal models have traditionally been implemented for investigating the underlying cellular mechanisms of human disease progression. However, the increasing significance of 3D vs. 2D cell culture has initiated a new era in cell culture research in which 3D in vitro models are emerging as a bridge between traditional 2D cell culture and in vivo animal models. Additive manufacturing (AM, also known as 3D printing), defined as the layer-by-layer fabrication of parts directed by digital information from a 3D computer-aided design file, offers the advantages of simultaneous rapid prototyping and biofunctionalization as well as the precise placement of cells and extracellular matrix with high resolution. In this review, we highlight recent advances in 3D printing of tissue engineered constructs that recapitulate the physical and cellular properties of the tissue microenvironment for investigating mechanisms of disease progression and for screening drugs.

Histone Deacetylase Inhibitors Reduce the Number of Herpes Simplex Virus-1 Genomes Initiating Expression in Individual Cells

Although many viral particles can enter a single cell, the number of viral genomes per cell that establish infection is limited. However, mechanisms underlying this restriction were not explored in depth. For herpesviruses, one of the possible mechanisms suggested is chromatinization and silencing of the incoming genomes. To test this hypothesis, we followed infection with three herpes simplex virus 1 (HSV-1) fluorescence expressing recombinants in the presence or absence of histone deacetylases inhibitors (HDACi's). Unexpectedly, a lower number of viral genomes initiated expression in the presence of these inhibitors. This phenomenon was observed using several HDACi: Trichostatin A (TSA), Suberohydroxamic Acid, Valporic Acid, and Suberoylanilide Hydroxamic Acid. We found that HDACi presence did not change the progeny outcome from the infected cells but did alter the kinetic of the gene expression from the viral genomes. Different cell types (HFF, Vero, and U2OS), which vary in their capability to activate intrinsic and innate immunity, show a cell specific basal average number of viral genomes establishing infection. Importantly, in all cell types, treatment with TSA reduced the number of viral genomes. ND10 nuclear bodies are known to interact with the incoming herpes genomes and repress viral replication. The viral immediate early protein, ICP0, is known to disassemble the ND10 bodies and to induce degradation of some of the host proteins in these domains. HDACi treated cells expressed higher levels of some of the host ND10 proteins (promyelocytic leukemia and ATRX), which may explain the lower number of viral genomes initiating expression per cell. Corroborating this hypothesis, infection with three HSV-1 recombinants carrying a deletion in the gene coding for ICP0, show a reduction in the number of genomes being expressed in U2OS cells. We suggest that alterations in the levels of host proteins involved in intrinsic antiviral defense may result in differences in the number of genomes that initiate expression.

Gene Expression Correlates with the Number of Herpes Viral Genomes Initiating Infection in Single Cells

Viral gene expression varies significantly among genetically identical cells. The sources of these variations are not well understood and have been suggested to involve both deterministic host differences and stochastic viral host interactions. For herpesviruses, only a limited number of incoming viral genomes initiate expression and replication in each infected cell. To elucidate the effect of this limited number of productively infecting genomes on viral gene expression in single cells, we constructed a set of fluorescence-expressing genetically tagged herpes recombinants. The number of different barcodes originating from a single cell is a good representative of the number of incoming viral genomes replicating (NOIVGR) in that cell. We identified a positive correlation between the NOIVGR and viral gene expression, as measured by the fluorescent protein expressed from the viral genome. This correlation was identified in three distinct cell-types, although the average NOIVGR per cell differed among these cell-types. Among clonal single cells, high housekeeping gene expression levels are not supportive of high viral gene expression, suggesting specific host determinants effecting viral infection. We developed a model to predict NOIVGR from cellular parameters, which supports the notion that viral gene expression is tightly linked to the NOIVGR in single-cells. Our results support the hypothesis that the stochastic nature of viral infection and host cell determinants contribute together to the variability observed among infected cells.

Single cell variation is of major interest in understanding key biological processes, like cancer, development and host pathogen interaction. During viral infection, these cell to cell variations can change the outcome of the whole organism infection. We suggested that differences in the number of parental viral genomes that initiate the replication process alter the outcome of infection among single cells. In this work we present a method based on genetically barcoded herpesvirus recombinants to identify the number of viral genomes initiating replication in individual cells. Our results indicate that viral gene expression is tightly linked to the number of viral genomes replicating per cell. Remarkably, we found that high cellular gene expression was an indicator for a lower viral gene expression in a given cell. We suggest that variations among single cells result from preexisting differences among cells, as well as from random viral host interactions.

Keeping it in check: chronic viral infection and antiviral immunity in the brain

It is becoming clear that the manner by which the immune response resolves or contains infection by a pathogen varies according to the tissue that is affected. Unlike many peripheral cell types, CNS neurons are generally non-renewable. Thus, the cytolytic and inflammatory strategies that are effective in controlling infections in the periphery could be damaging if deployed in the CNS. Perhaps for this reason, the immune response to some CNS viral infections favours maintenance of neuronal integrity and non-neurolytic viral control. This modified immune response - when combined with the unique anatomy and physiology of the CNS - provides an ideal environment for the maintenance of viral genomes, including those of RNA viruses. Therefore, it is possible that such viruses can reactivate long after initial viral exposure, contributing to CNS disease.

Interrelationship of Primary Virus Replication, Level of Latency, and Time to Reactivation in the Trigeminal Ganglia of Latently Infected Mice

We sought to determine the possibility of an interrelationship between primary virus replication in the eye, the level of viral DNA in the trigeminal ganglia (TG) during latency, and the amount of virus reactivation following ocular herpes simplex virus type 1 (HSV-1) infection. Mice were infected with virulent (McKrae) or avirulent (KOS and RE) strains of HSV-1, and virus titers in the eyes and TG during primary infection, level of viral gB DNA in TG on day 28 postinfection (p.i.), and virus reactivation on day 28 p.i. as measured by explant reactivation were calculated. Our results suggest that the avirulent strains of HSV-1, even after corneal scarification, had lower virus titers in the eye, had less latency in the TG, and took a longer time to reactivate than virulent strains of HSV-1. The time to explant reactivation of avirulent strains of HSV-1 was similar to that of the virulent LAT((-)) McKrae-derived mutant. The viral dose with the McKrae strain of HSV-1 affected the level of viral DNA and time to explant reactivation. Overall, our results suggest that there is no absolute correlation between primary virus titer in the eye and TG and the level of viral DNA in latent TG and time to reactivation.

IMPORTANCE

Very little is known regarding the interrelationship between primary virus replication in the eye, the level of latency in TG, and the time to reactivate in the mouse model. This study was designed to answer these questions. Our results point to the absence of any correlation between the level of primary virus replication and the level of viral DNA during latency, and neither was an indicator of how rapidly the virus reactivated following explant TG-induced reactivation.

3D printed nervous system on a chip

Bioinspired organ-level in vitro platforms are emerging as effective technologies for fundamental research, drug discovery, and personalized healthcare. In particular, models for nervous system research are especially important, due to the complexity of neurological phenomena and challenges associated with developing targeted treatment of neurological disorders. Here we introduce an additive manufacturing-based approach in the form of a bioinspired, customizable 3D printed nervous system on a chip (3DNSC) for the study of viral infection in the nervous system. Micro-extrusion 3D printing strategies enabled the assembly of biomimetic scaffold components (microchannels and compartmented chambers) for the alignment of axonal networks and spatial organization of cellular components. Physiologically relevant studies of nervous system infection using the multiscale biomimetic device demonstrated the functionality of the in vitro platform. We found that Schwann cells participate in axon-to-cell viral spread but appear refractory to infection, exhibiting a multiplicity of infection (MOI) of 1.4 genomes per cell. These results suggest that 3D printing is a valuable approach for the prototyping of a customized model nervous system on a chip technology.

gD-Independent Superinfection Exclusion of Alphaherpesviruses

Many viruses have the capacity to prevent a cell from being infected by a second virus, often termed superinfection exclusion. Alphaherpesviruses, including the human pathogen herpes simplex virus 1 (HSV-1) and the animal herpesvirus pseudorabies virus (PRV), encode a membrane-bound glycoprotein, gD, that can interfere with subsequent virion entry. We sought to characterize the timing and mechanism of superinfection exclusion during HSV-1 and PRV infection. To this end, we utilized recombinant viruses expressing fluorescent protein (FP) markers of infection that allowed the visualization of viral infections by microscopy and flow cytometry as well as the differentiation of viral progeny. Our results demonstrated the majority of HSV-1- and PRV-infected cells establish superinfection exclusion by 2 h postinfection. The modification of viral infections by virion inactivation and phosphonoacetic acid, cycloheximide, and actinomycin D treatments indicated new protein synthesis is needed to establish superinfection exclusion. Primary infection with gene deletion PRV recombinants identified that new gD expression is not required to establish superinfection exclusion of a secondary viral inoculum. We also identified the timing of coinfection events during axon-to-cell spread, with most occurring within a 2-h window, suggesting a role for cellular superinfection exclusion during neuroinvasive spread of infection. In summary, we have characterized a gD-independent mechanism of superinfection exclusion established by two members of the alphaherpesvirus family and identified a potential role of exclusion during the pathogenic spread of infection.

IMPORTANCE

Superinfection exclusion is a widely observed phenomenon initiated by a primary viral infection to prevent further viruses from infecting the same cell. The capacity for alphaherpesviruses to infect the same cell impacts rates of interviral recombination and disease. Interviral recombination allows genome diversification, facilitating the development of resistance to antiviral therapeutics and evasion of vaccine-mediated immune responses. Our results demonstrate superinfection exclusion occurs early, through a gD-independent process, and is important in the directed spread of infection. Identifying when and where in an infected host viral genomes are more likely to coinfect the same cell and generate viral recombinants will enhance the development of effective antiviral therapies and interventions.

Fluorescent Protein Approaches in Alpha Herpesvirus Research

In the nearly two decades since the popularization of green fluorescent protein (GFP), fluorescent protein-based methodologies have revolutionized molecular and cell biology, allowing us to literally see biological processes as never before. Naturally, this revolution has extended to virology in general, and to the study of alpha herpesviruses in particular. In this review, we provide a compendium of reported fluorescent protein fusions to herpes simplex virus 1 (HSV-1) and pseudorabies virus (PRV) structural proteins, discuss the underappreciated challenges of fluorescent protein-based approaches in the context of a replicating virus, and describe general strategies and best practices for creating new fluorescent fusions. We compare fluorescent protein methods to alternative approaches, and review two instructive examples of the caveats associated with fluorescent protein fusions, including describing several improved fluorescent capsid fusions in PRV. Finally, we present our future perspectives on the types of powerful experiments these tools now offer.

Cell entry mechanisms of HSV: what we have learned in recent years

HSV type-1 and -2 are widespread pathogens producing lifelong infection with multiple sequelae, including oral, ocular and genital disease. The process of herpesvirus entry is a highly complex process involving numerous viral and cellular factors. Entry begins with attachment of virus to the cell surface followed by interactions between viral glycoproteins and cellular receptors to facilitate capsid penetration. The nucleocapsid is then transported along microtubules to the nuclear membrane, where viral DNA is released for replication in the nucleus. The work reviewed here comprises the most recent advancements in our understanding of the mechanism involved in the herpesvirus entry process.

Measles Virus Defective Interfering RNAs Are Generated Frequently and Early in the Absence of C Protein and Can Be Destabilized by Adenosine Deaminase Acting on RNA-1-Like Hypermutations

Defective interfering RNAs (DI-RNAs) of the viral genome can form during infections of negative-strand RNA viruses and outgrow full-length viral genomes, thereby modulating the severity and duration of infection. Here we document the frequent de novo generation of copy-back DI-RNAs from independent rescue events both for a vaccine measles virus (vac2) and for a wild-type measles virus (IC323) as early as passage 1 after virus rescue. Moreover, vaccine and wild-type C-protein-deficient (C-protein-knockout [CKO]) measles viruses generated about 10 times more DI-RNAs than parental virus, suggesting that C enhances the processivity of the viral polymerase. We obtained the nucleotide sequences of 65 individual DI-RNAs, identified breakpoints and reinitiation sites, and predicted their structural features. Several DI-RNAs possessed clusters of A-to-G or U-to-C transitions. Sequences flanking these mutation sites were characteristic of those favored by adenosine deaminase acting on RNA-1 (ADAR1), which catalyzes in double-stranded RNA the C-6 deamination of adenosine to produce inosine, which is recognized as guanosine, a process known as A-to-I RNA editing. In individual DI-RNAs the transitions were of the same type and occurred on both sides of the breakpoint. These patterns of mutations suggest that ADAR1 edits unencapsidated DI-RNAs that form double-strand RNA structures. Encapsidated DI-RNAs were incorporated into virus particles, which reduced the infectivity of virus stocks. The CKO phenotype was dominant: DI-RNAs derived from vac2 with a CKO suppressed the replication of vac2, as shown by coinfections of interferon-incompetent lymphatic cells with viruses expressing different fluorescent reporter proteins. In contrast, coinfection with a C-protein-expressing virus did not counteract the suppressive phenotype of DI-RNAs.

IMPORTANCE

Recombinant measles viruses (MVs) are in clinical trials as cancer therapeutics and as vectored vaccines for HIV-AIDS and other infectious diseases. The efficacy of MV-based vectors depends on their replication proficiency and immune activation capacity. Here we document that copy-back defective interfering RNAs (DI-RNAs) are generated by recombinant vaccine and wild-type MVs immediately after rescue. The MV C protein interferes with DI-RNA generation and may enhance the processivity of the viral polymerase. We frequently detected clusters of A-to-G or U-to-C transitions and noted that sequences flanking individual mutations contain motifs favoring recognition by the adenosine deaminase acting on RNA-1 (ADAR1). The consistent type of transitions on the DI-RNAs indicates that these are direct substrates for editing by ADAR1. The ADAR1-mediated biased hypermutation events are consistent with the protein kinase R (PKR)-ADAR1 balancing model of innate immunity activation. We show by coinfection that the C-defective phenotype is dominant.

Viruses roll the dice: the stochastic behavior of viral genome molecules accelerates viral adaptation at the cell and tissue levels

Recent studies on evolutionarily distant viral groups have shown that the number of viral genomes that establish cell infection after cell-to-cell transmission is unexpectedly small (1-20 genomes). This aspect of viral infection appears to be important for the adaptation and survival of viruses. To clarify how the number of viral genomes that establish cell infection is determined, we developed a simulation model of cell infection for tomato mosaic virus (ToMV), a positive-strand RNA virus. The model showed that stochastic processes that govern the replication or degradation of individual genomes result in the infection by a small number of genomes, while a large number of infectious genomes are introduced in the cell. It also predicted two interesting characteristics regarding cell infection patterns: stochastic variation among cells in the number of viral genomes that establish infection and stochastic inequality in the accumulation of their progenies in each cell. Both characteristics were validated experimentally by inoculating tobacco cells with a library of nucleotide sequence-tagged ToMV and analyzing the viral genomes that accumulated in each cell using a high-throughput sequencer. An additional simulation model revealed that these two characteristics enhance selection during tissue infection. The cell infection model also predicted a mechanism that enhances selection at the cellular level: a small difference in the replication abilities of coinfected variants results in a large difference in individual accumulation via the multiple-round formation of the replication complex (i.e., the replication machinery). Importantly, this predicted effect was observed in vivo. The cell infection model was robust to changes in the parameter values, suggesting that other viruses could adopt similar adaptation mechanisms. Taken together, these data reveal a comprehensive picture of viral infection processes including replication, cell-to-cell transmission, and evolution, which are based on the stochastic behavior of the viral genome molecules in each cell.